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FLYING MACHINES TODAY




"Hitherto aviation has been almost monopolized by that much-overpraised
and much-overtrusted person, 'the practical man.' It is much in need
of the services of the theorist--the engineer with his mathematical
calculations of how a flying machine ought to be built and of how the
material used in its construction should be distributed to give the
greatest possible amount of strength and efficiency."

                         --From the _New York Times_, January 16, 1911.


  [Illustration: THE FALL OF ICARUS]




FLYING MACHINES TODAY

 BY
 WILLIAM DUANE ENNIS

 _Professor of Mechanical Engineering in the Polytechnic
  Institute of Brooklyn_

_123 ILLUSTRATIONS_

  [Illustration: VAN NOSTRAND LOGO]

  NEW YORK
  D. VAN NOSTRAND COMPANY
  23 MURRAY AND          1911          27 WARREN STS.




  _Copyright, 1911, by_
  D. VAN NOSTRAND COMPANY


THE · PLIMPTON · PRESS · NORWOOD · MASS · U · S · A




 To
 MY MOTHER




PREFACE


Speaking with some experience, the writer has found that instruction
in the principles underlying the science and sport of aviation must be
vitalized by some contemporaneous study of what is being accomplished
in the air. No one of the revolutionizing inventions of man has
progressed as rapidly as aerial navigation. The "truths" of today are
the absurdities of tomorrow.

The suggestion that some grasp of the principles and a very fair
knowledge of the current practices in aeronautics may be had without
special technical knowledge came almost automatically. If this book
is comprehensible to the lay reader, and if it conveys to him even a
small proportion of the writer's conviction that flying machines are to
profoundly influence our living in the next generation, it will have
accomplished its author's purpose.

  POLYTECHNIC INSTITUTE OF BROOKLYN,
   NEW YORK, April, 1911.




CONTENTS


                                                          PAGE
 THE DELIGHTS AND DANGERS OF FLYING.--Dangers of
 Aviation.--What it is Like to Fly                           1

 SOARING FLIGHT BY MAN.--What Holds it Up?--Lifting
 Power.--Why so Many Sails?--Steering                       17

 TURNING CORNERS.--What Happens when Making a Turn.--
 Lateral Stability.--Wing Warping.--Automatic Control.--
 The Gyroscope.--Wind Gusts                                 33

 AIR AND THE WIND.--Sailing Balloons.--Field and Speed      43

 GAS AND BALLAST.--Buoyancy in Air.--Ascending and
 Descending.--The Ballonet.--The Equilibrator               57

 DIRIGIBLE BALLOONS AND OTHER KINDS.--Shapes.--
 Dimensions.--Fabrics.--Framing.--Keeping the Keel
 Horizontal.--Stability.--Rudders and Planes.--Arrangement
 and Accessories.--Amateur Dirigibles.--The Fort
 Omaha Plant.--Balloon Progress                             71

 THE QUESTION OF POWER.--Resistance of Aeroplanes.--
 Resistance of Dirigibles.--Independent Speed and
 Time-Table.--The Cost of Speed.--The Propeller            101

 GETTING UP AND DOWN; MODELS AND GLIDERS;
 AEROPLANE DETAILS.--Launching.--Descending.--
 Gliders.--Models.--Balancing.--Weights.--
 Miscellaneous.--Things to Look After                      121

 SOME AEROPLANES.--SOME ACCOMPLISHMENTS                    143

 THE POSSIBILITIES IN AVIATION.--The Case of the
 Dirigible.--The Orthopter.--The Helicopter.--Composite
 Types.--What is Promised                                  170

 AERIAL WARFARE                                            189




 LIST OF ILLUSTRATIONS


                                                         PAGE

  The Fall of Icarus                           _Frontispiece_
  The Aviator                                               3
  The Santos-Dumont "_Demoiselle_"                          4
  View from a Balloon                                       9
  Anatomy of a Bird's Wing                                 10
  Flight of a Bird                                         11
  In a Meteoric Shower                                     13
  How a Boat Tacks                                         15
  Octave Chanute                                           18
  Pressure of the Wind                                     19
  Forces Acting on a Kite                                  20
  Sustaining Force in the Aeroplane                        23
  Direct Lifting and Resisting Forces                      24
  Shapes of Planes                                         26
  Balancing Sail                                           28
  Roe's Triplane at Wembley                                30
  Action of the Steering Rudder                            31
  Recent Type of Wright Biplane                            31
  Circular Flight                                          33
  The Aileron                                              35
  Wing Tipping                                             36
  Wing Warping                                             37
  The Gyroscope                                            39
  Diurnal Temperatures at Different Heights                45
  Seasonal Variation in Wind Velocities                    47
  The Wind Rose for Mt. Weather, Va.                       49
  Diagram of Parts of a Drifting Balloon                   51
  Glidden and Stevens Getting Away in the "_Boston_"       52
  Relative and Absolute Balloon Velocities                 53
  Field and Speed                                          53
  Influence of Wind on Possible Course                     54
  Count Zeppelin                                           55
  Buoyant Power of Wood                                    57
  One Cubic Foot of Wood Loaded in Water                   58
  Buoyant Power of Hydrogen                                59
  Lebaudy's "_Jaune_"                                      60
  Air Balloon                                              62
  Screw Propeller for Altitude Control                     66
  Balloon with Ballonets                                   67
  Construction of the Zeppelin Balloon                     68
  The Equilibrator                                         69
  Henry Giffard's Dirigible                                71
  Dirigible of Dupuy de Lome                               72
  Tissandier Brothers' Dirigible Balloon                   73
  The "_Baldwin_"                                          74
  The "_Zeppelin_" on Lake Constance                       75
  The "_Patrie_"                                           77
  Manufacturing the Envelope of a Balloon                  79
  Andrée's Balloon, "_L'Oernen_"                           80
  Wreck of the "_Zeppelin_"                                82
  Car of the "_Zeppelin_"                                  84
  Stern View of the "_Zeppelin_"                           86
  The "_Clément-Bayard_"                                   87
  The "_Ville de Paris_"                                   88
  Car of the "_Liberté_"                                   89
  The "_Zodiac No. 2_"                                     92
  United States Signal Corps Balloon Plant at Fort Omaha   93
  The "_Caroline_"                                         94
  The Ascent at Versailles, 1783                           95
  Proposed Dirigible                                       96
  The "_République_"                                       97
  The First Flight for the Gordon-Bennett Cup              99
  The Gnome Motor                                         102
  Screw Propeller                                         103
  One of the Motors of the "_Zeppelin_"                   104
  The Four-Cycle Engine                                   105
  Action of Two-Cycle Engine                              106
  Motor and Propeller                                     108
  Two-Cylinder Opposed Engine                             110
  Four-Cylinder Vertical Engine                           110
  Head End Shapes                                         113
  The Santos-Dumont Dirigible No. 2                       115
  In the Bay of Monaco: Santos-Dumont                     117
  Wright Biplane on Starting Rail                         121
  Launching System for Wright Aeroplane                   122
  The Nieuport Monoplane                                  124
  A Biplane                                               125
  Ely at Los Angeles                                      126
  Trajectory During Descent                               127
  Descending                                              128
  The Witteman Glider                                     130
  French Monoplane                                        132
  A Problem in Steering                                   133
  Lejeune Biplane                                         134
  Tellier Monoplane                                       135
  A Monoplane                                             137
  Cars and Framework                                      139
  Some Details                                            139
  Recent French Machines                                  141
  Orville Wright at Fort Myer                             143
  The First Flight Across the Channel                     144
  Wright Motor                                            145
  Voisin-Farman Biplane                                   147
  The Champagne Grand Prize Flight                        148
  Farman's First Biplane                                  149
  The "_June Bug_"                                        150
  Curtiss Biplane                                         151
  Curtiss' Hydro-Aeroplane at San Diego Bay               152
  Flying Over the Water                                   153
  Blériot-Voisin Cellular Biplane with Pontoons           154
  Latham's "_Antoinette_"                                 155
  James J. Ward at Lewiston Fair                          156
  Marcel Penot in the "_Mohawk_"                          157
  Santos-Dumont's "_Demoiselle_"                          159
  Blériot Monoplane                                       160
  Latham's Fall into the Channel                          161
  De Lesseps Crossing the Channel                         163
  The Maxim Aeroplane                                     164
  Langley's Aeroplane                                     165
  Robart Monoplane                                        166
  Vina Monoplane                                          167
  Blanc Monoplane                                         170
  Melvin Vaniman Triplane                                 171
  Jean de Crawhez Triplane                                171
  A Triplane                                              172
  Giraudon's Wheel Aeroplane                              175
  Bréguet Gyroplane (Helicopter)                          177
  Wellman's "_America_"                                   181
  The German Emperor Watching the Progress of Aviation    189
  Automatic Gun for Attacking Airships                    193
  Gun for Shooting at Aeroplanes                          197
  Santos-Dumont Circling the Eiffel Tower                 199
  Latham, Farman and Paulhan                              202




FLYING MACHINES TODAY




THE DELIGHTS AND DANGERS OF FLYING


Few things have more charm for man than flight. The soaring of a bird
is beautiful and the gliding of a yacht before the wind has something
of the same beauty. The child's swing; the exercise of skating on
good ice; a sixty-mile-an-hour spurt on a smooth road in a motor car;
even the slightly passé bicycle: these things have all in their time
appealed to us because they produce the illusion of flight--of progress
through the intangible air with all but separation from the prosaic
earth.

But these sensations have been only illusions. To actually leave the
earth and wander at will in aerial space--this has been, scarcely a
hope, perhaps rarely even a distinct dream. From the days of Dædalus
and Icarus, of Oriental flying horses and magic carpets, down to
"Darius Green and his flying machine," free flight and frenzy were
not far apart. We were learnedly told, only a few years since, that
sustention by heavier-than-air machines was impossible without the
discovery, first, of some new matter or some new force. It is now
(1911) only eight years since Wilbur Wright at Kitty Hawk, with the
aid of the new (?) matter--aluminum--and the "new" force--the gasoline
engine--in three successive flights proved that a man could travel
through the air and safely descend, in a machine weighing many times
as much as the air it displaced. It is only five years since two
designers--Surcouf and Lebaudy--built dirigible balloons approximating
present forms, the _Ville de Paris_ and _La Patrie_. It is only now
that we average people may confidently contemplate the prospect of an
aerial voyage for ourselves before we die. A contemplation not without
its shudder, perhaps; but yet not altogether more daring than that of
our grandsires who first rode on steel rails behind a steam locomotive.


The Dangers of Aviation

We are very sure to be informed of the fact when an aviator is
killed. Comparatively little stir is made nowadays over an automobile
fatality, and the ordinary railroad accident receives bare mention. For
instruction and warning, accidents to air craft cannot be given too
much publicity; but if we wish any accurate conception of the danger we
must pay regard to factors of proportion. There are perhaps a thousand
aeroplanes and about sixty dirigible balloons in the world. About 500
men--amateurs and professionals--are continuously engaged in aviation.
The Aero Club of France has issued in that country nearly 300 licenses.
In the United States, licenses are held by about thirty individuals.
We can form no intelligent estimate as to the number of unlicensed
amateurs of all ages who are constantly experimenting with gliders at
more or less peril to life and limb.

A French authority has ascertained the death rate among air-men to
have been--to date--about 6%. This is equivalent to about one life for
4000 miles of flight: but we must remember that accidents will vary
rather with the number of ascents and descents than with the mileage.
Four thousand miles in 100 flights would be much less perilous, under
present conditions, than 4000 miles in 1000 flights.

  [Illustration: THE AVIATOR]

There were 26 fatal aeroplane accidents between September 17, 1908,
and December 3, 1910. Yet in that period there were many thousands of
ascents: 1300 were made in one week at the Rheims tournament alone.
Of the 26 accidents, 1 was due to a wind squall, 3 to collision,
6 (apparently) to confusion of the aviator, and 12 to mechanical
breakage. An analysis of 40 British accidents shows 13 to have been
due to engine failures, 10 to alighting on bad ground, 6 to wind
gusts, 5 to breakage of the propeller, and 6 to fire and miscellaneous
causes. These casualties were not all fatal, although the percentage of
fatalities in aeronautic accidents is high. The most serious results
were those due to alighting on bad ground; long grass and standing
grain being very likely to trip the machine and throw the occupant.
French aviators are now strapping themselves to their seats in order to
avoid this last danger.

  [Illustration: THE SANTOS-DUMONT "DEMOISELLE"
  (From _The Aeroplane_, by Hubbard, Ledeboer and Turner)]

Practically all of the accidents occur to those who are flying; but
spectators may endanger themselves. During one of the flights of
Mauvais at Madrid, in March of the present year, the bystanders rushed
through the barriers and out on the field before the machine had well
started. A woman was decapitated by the propeller, and four other
persons were seriously injured.

Nearly all accidents result from one of three causes: bad design,
inferior mechanical construction, and the taking of unnecessary risks
by the operator. Scientific design at the present writing is perhaps
impossible. Our knowledge of the laws of air resistance and sustention
is neither accurate nor complete. Much additional study and experiment
must be carried on; and some better method of experimenting must be
devised than that which sends a man up in the air and waits to see what
happens. A thorough scientific analysis will not only make aviation
safer, it will aid toward making it commercially important. Further
data on propeller proportions and efficiencies, and on strains in the
material of screws under aerial conditions, will do much to standardize
power plant equipment. The excessive number of engine breakdowns
is obviously related to the extremely light weight of the engines
employed: better design may actually increase these weights over those
customary at present. Great weight reduction is no longer regarded
as essential at present speeds in aerial navigation: we have perhaps
already gone too far in this respect.

Bad workmanship has been more or less unavoidable, since no one has
yet had ten years' experience in building aeroplanes. The men who have
developed the art have usually been sportsmen rather than mechanics,
and only time is necessary to show the impropriety of using "safety
pins" and bent wire nails for connections.

The taking of risks has been an essential feature. When one man earns
$100,000 in a year by dare-devil flights, when the public flocks in
hordes--and pays good prices--to see a man risk his neck, he will
usually aim to satisfy it. This is not developing aerial navigation:
this is circus riding--looping-the-loop performances which appeal to
some savage instinct in us but lead us nowhere. Men have climbed two
miles into the clouds, for no good purpose whatever. All that we need
to know of high altitude conditions is already known or may be learned
by ascents in anchored balloons. Records up to heights of sixteen miles
have been obtained by sounding balloons.

If these high altitudes may under certain conditions be desirable for
particular types of balloon, they are essentially undesirable for the
aeroplane. The supporting power of a heavier-than-air machine decreases
in precisely inverse ratio with the altitude. To fly high will then
involve either more supporting surface and therefore a structurally
weaker machine, or greater speed and consequently a larger motor. It
is true that the resistance to propulsion decreases at high altitudes,
just as the supporting power decreases: and on this account, given only
a sufficient margin of supporting power, we might expect a standard
machine to work about as well at a two-mile elevation as at a height of
200 feet; but rarefaction of the air at the higher altitudes decreases
the weight of carbureted mixture drawn into the motor, and consequently
its output. Any air-man who attempts to reach great heights in a
machine not built for such purpose is courting disaster.

Flights over cities, spectacular as they are, and popular as they are
likely to remain, are doubly dangerous on account of the irregular air
currents and absence of safe landing places. They have at last been
officially discountenanced as not likely to advance the sport.

All flights are exhibition flights. The day of a quiet,
mind-your-own-business type of aerial journey has not yet arrived.
Exhibition performances of any sort are generally hazardous. There
were nine men killed in one recent automobile meet. If the automobile
were used exclusively for races and contests, the percentage of
fatalities might easily exceed that in aviation. It is claimed that no
inexperienced aviator has ever been killed. This may not be true, but
there is no doubt that the larger number of accidents has occurred to
the better-known men from whom the public expects something daring.

Probably the best summing up of the danger of aviation may be obtained
from the insurance companies. The courts have decided that an
individual does not forfeit his life insurance by making an occasional
balloon trip. Regular classified rates for aeroplane and balloon
operators are in force in France and Germany. It is reported that Mr.
Grahame-White carries a life insurance policy at 35% premium--about
the same rate as that paid by a "crowned head." Another aviator of a
less professional type has been refused insurance even at 40% premium.
Policies of insurance may be obtained covering damage to machines by
fire or during transportation and by collisions with other machines;
and covering liability for injuries to persons other than the aviator.

On the whole, flying is an ultra-hazardous _occupation_; but an
_occasional_ flight by a competent person or by a passenger with a
careful pilot is simply a thrilling experience, practically no more
dangerous than many things we do without hesitation. Nearly all
accidents have been due to preventable causes; and it is simply a
matter of science, skill, perseverance, and determination to make an
aerial excursion under proper conditions as safe as a journey in a
motor car. Men who for valuable prizes undertake spectacular feats will
be killed as frequently in aviation as in bicycle or even in automobile
racing; but probably not very much more frequently, after design and
workmanship in flying machines shall have been perfected. The total
number of deaths in aviation up to February 9, 1911, is stated to have
been forty-two.


What It Is Like to Fly

We are fond of comparing flying machines with birds, with fish,
and with ships: and there are useful analogies with all three. A
drifting balloon is like a becalmed ship or a dead fish. It moves at
the speed of the aerial fluid about it and the occupants perceive no
movement whatever. The earth's surface below appears to move in the
opposite direction to that in which the wind carries the balloon. With
a dirigible balloon or flying machine, the sensation is that of being
exposed to a violent wind, against which (by observation of landmarks)
we find that we progress. It is the same experience as that obtained
when standing in an exposed position on a steamship, and we wonder if a
bird or a fish gradually gets so accustomed to the opposing current as
to be unconscious of it. But in spite of jar of motors and machinery,
there is a freedom of movement, a detachment from earth-associations,
in air flight, that distinguishes it absolutely from the churning of a
powerful vessel through the waves.

  [Illustration: VIEW FROM A BALLOON]

  [Illustration: ANATOMY OF A BIRD'S WING
  (From Walker's _Aerial Navigation_)]

  [Illustration: FLIGHT OF A BIRD]

Birds fly in one of three ways. The most familiar bird flight is
by a rapid wing movement which has been called oar-like, but which
is precisely equivalent to the usual movement of the arms of a man
in swimming. The edge of the wing moves forward, cutting the air; on
the return stroke the leading edge is depressed so as to present a
nearly flat surface to the air and thus propel the bird forward. A
slight downward direction of this stroke serves to impel the flight
sufficiently upward to offset the effect of gravity. Any man can learn
to swim, but no man can fly, because neither in his muscular frame nor
by any device which he can attach thereto can he exert a sufficient
pressure to overcome his own weight against as imponderable a fluid
as air. If air were as heavy as water, instead of 700 times lighter,
it would be as easy to fly as to swim. The bird can fly because of
the great surface, powerful construction, and rapid movement of its
wings, in proportion to the weight of its body. But compared with
the rest of the animal kingdom, flying birds are all of small size.
Helmholz considered that the vulture represented the heaviest body that
could possibly be raised and kept aloft by the exercise of muscular
power, and it is understood that vultures have considerable difficulty
in ascending; so much so that unless in a position to take a short
preliminary run they are easily captured.

Every one has noticed a second type of bird flight--soaring. It is this
flight which is exactly imitated in a glider. An aeroplane differs
from a soaring bird only in that it carries with it a producer of
forward impetus--the propeller--so that the soaring flight may last
indefinitely: whereas a soaring bird gradually loses speed and descends.

  [Illustration: IN A METEORIC SHOWER]

A third and rare type of bird flight has been called _sailing_. The
bird faces the wind, and with wings outspread and their forward edge
elevated rises while being forced backward under the action of the
breeze. As soon as the wind somewhat subsides, the bird turns and
_soars_ in the desired direction. Flight is thus accomplished without
muscular effort other than that necessary to properly incline the
wings and to make the turns. It is practicable only in squally winds,
and the birds which practice "sailing"--the albatross and frigate
bird--are those which live in the lower and more disturbed regions of
the atmosphere. This form of flight has been approximately imitated in
the man[oe]uvering of aeroplanes.

Comparison of flying machines and ships suggests many points of
difference. Water is a fluid of great density, with a definite upper
surface, on which marine structures naturally rest. A vessel in the air
may be at any elevation in the surrounding rarefied fluid, and great
attention is necessary to keep it at the elevation desired. The air has
no surface. The air ship is like a submarine--the dirigible balloon
of the sea--and perhaps rather more safe. An ordinary ship is only
partially immersed; the resistance of the fluid medium is exerted over
a portion only of its head end: but the submarine or the flying machine
is wholly exposed to this resistance. The submarine is subjected to
ocean currents of a very few miles per hour, at most; the currents to
which the flying machine may be exposed exceed a mile a minute. Put a
submarine in the Whirlpool Rapids at Niagara and you will have possible
air ship conditions.

A marine vessel may _tack_, _i.e._, may sail partially against the
wind that propels it, by skillful utilization of the resistance to
sidewise movement of the ship through the water: but the flying machine
is wholly immersed in a single fluid, and a head wind is nothing else
than a head wind, producing an absolute subtraction from the proper
speed of the vessel.

  [Illustration: HOW A BOAT TACKS
  The wind always exerts a pressure, perpendicular to the sail,
  which tends to drift the boat sidewise (R) and also to propel
  it forward (L). Sidewise movement is resisted by the hull.
  An air ship cannot tack because there is no such resistance
  to drift.]

Aerial navigation is thus a new art, particularly when heavier-than-air
machines are used. We have no heavier-than-water _ships_. The flying
machine must work out its own salvation.




SOARING FLIGHT BY MAN


Flying machines have been classified as follows:--

     Lighter than Air
  Fixed balloon,
  Drifting balloon,
  Sailing balloon,
  Dirigible balloon
        rigid (Zeppelin),
        ballonetted.

     Heavier than Air
  Orthopter,
  Helicopter,
  Aeroplane
        monoplane,
        multiplane.

We will fall in with the present current of popular interest and
consider the aeroplane--that mechanical grasshopper--first.


What Holds It Up?

  [Illustration: OCTAVE CHANUTE (DIED 1910)]

To the researches of Chanute and Langley must be ascribed much of
American progress in aviation.

When a flat surface like the side of a house is exposed to the breeze,
the velocity of the wind exerts a force or pressure directly against
the surface. This principle is taken into account in the design of
buildings, bridges, and other structures. The pressure exerted per
square foot of surface is equal (approximately) to the square of the
wind velocity in miles per hour, divided by 300. Thus, if the wind
velocity is thirty miles, the pressure against a house wall on which it
acts directly is 30 × 30 ÷ 300 = 3 pounds per square foot: if the wind
velocity is sixty miles, the pressure is 60 × 60 ÷ 300 = 12 pounds:
if the velocity is ninety miles, the pressure is 90 × 90 ÷ 300 = 27
pounds, and so on.

  [Illustration: PRESSURE OF THE WIND]

If the wind blows obliquely toward the surface, instead of directly,
the pressure at any given velocity is reduced, but may still be
considerable. Thus, in the sketch, let _ab_ represent a wall, toward
which we are looking downward, and let the arrow _V_ represent the
direction of the wind. The air particles will follow some such paths
as those indicated, being deflected so as to finally escape around
the ends of the wall. The result is that a pressure is produced which
may be considered to act along the dotted line _P_, perpendicular to
the wall. This is the invariable law: that no matter how oblique the
surface may be, with reference to the direction of the wind, there is
always a pressure produced against the surface by the wind, and this
pressure always acts _in a direction perpendicular to the surface_. The
amount of pressure will depend upon the wind velocity and the obliquity
or inclination of the surface (_ab_) with the wind (_V_).

Now let us consider a kite--the "immediate ancestor" of the aeroplane.
The surface _ab_ is that of the kite itself, held by its string _cd_.
We are standing at one side and looking at the _edge_ of the kite. The
wind is moving horizontally against the face of the kite, and produces
a pressure _P_ directly against the latter. The pressure tends both to
move it toward the left and to lift it. If the tendency to move toward
the left be overcome by the string, then the tendency toward lifting
may be offset--and in practice _is_ offset--by the weight of the kite
and tail.

  [Illustration: FORCES ACTING ON A KITE]

We may represent the two tendencies to movement produced by the force
_P_, by drawing additional dotted lines, one horizontally to the left
(_R_) and the other vertically (_L_); and it is known that if we let
the length of the line _P_ represent to some convenient scale the
amount of direct pressure, then the lengths of _R_ and _L_ will also
represent to the same scale the amounts of horizontal and vertical
force due to the pressure. If the weight of kite and tail exceeds the
vertical force _L_, the kite will descend: if these weights are less
than that force, the kite will ascend. If they are precisely equal to
it, the kite will neither ascend nor descend. The ratio of _L_ to _R_
is determined by the slope of _P_; and this is fixed by the slope of
_ab_; so that we have the most important conclusion: _not only does
the amount of direct pressure (P) depend upon the obliquity of the
surface with the breeze (as has already been shown), but the relation
of vertical force (which sustains the kite) to horizontal force also
depends on the same obliquity_. For example, if the kite were flying
almost directly above the boy who held the string, so that _ab_ became
almost horizontal, _P_ would be nearly vertical and _L_ would be much
greater than _R_. On the other hand, if _ab_ were nearly vertical, the
kite flying at low elevation, the string and the direct pressure would
be nearly horizontal and _L_ would be much less than _R_. The force _L_
which lifts the kite seems to increase while _R_ decreases, as the kite
ascends: but _L_ may not actually increase, because it depends upon the
amount of direct pressure, _P_, as well as upon the direction of this
pressure; and the amount of direct pressure steadily decreases during
ascent, on account of the increasing obliquity of _ab_ with _V_. All of
this is of course dependent on the assumption that the kite always has
the same inclination to the string, and the described resolution of the
forces, although answering for illustrative purposes, is technically
incorrect.

It seems to be the wind velocity, then, which holds up the kite: but
in reality the string is just as necessary as the wind. If there is
no string, and the wind blows the kite with it, the kite comes down,
because the pressure is wholly due to a relative velocity as between
kite and wind. The wind exerts a pressure against the rear of a railway
train, if it happens to be blowing in that direction, and if we
stood on the rear platform of a stationary train we should feel that
pressure: but if the train is started up and caused to move at the same
speed as the wind there would be no pressure whatever.

One of the very first heavier-than-air flights ever recorded is said
to have been made by a Japanese who dropped bombs from an immense
man-carrying kite during the Satsuma rebellion of 1869. The kite as
a flying machine has, however, two drawbacks: it needs the wind--it
cannot fly in a calm--and it stands still. One early effort to improve
on this situation was made in 1856, when a man was towed in a sort of
kite which was hauled by a vehicle moving on the ground. In February
of the present year, Lieut. John Rodgers, U.S.N., was lifted 400 feet
from the deck of the cruiser _Pennsylvania_ by a train of eleven large
kites, the vessel steaming at twelve knots against an eight-knot
breeze. The aviator made observations and took photographs for about
fifteen minutes, while suspended from a tail cable about 100 feet
astern. In the absence of a sufficient natural breeze, an artificial
wind was thus produced by the motion imparted to the kite; and the
device permitted of reaching some destination. The next step was
obviously to get rid of the tractive vehicle and tow rope by carrying
propelling machinery on the kite. This had been accomplished by Langley
in 1896, who flew a thirty-pound model nearly a mile, using a steam
engine for power. The gasoline engine, first employed by Santos-Dumont
(in a dirigible balloon) in 1901, has made possible the present day
_aeroplane_.

  [Illustration: SUSTAINING FORCE IN THE AEROPLANE]

What "keeps it up", in the case of this device, is likewise its
velocity. Looking from the side, _ab_ is the sail of the aeroplane,
which is moving toward the right at such speed as to produce the
equivalent of an air velocity _V_ to the left. This velocity causes the
direct pressure _P_, equivalent to a lifting force _L_ and a retarding
force _R_. The latter is the force which must be overcome by the motor:
the former must suffice to overcome the whole weight of the apparatus.
Travel in an aeroplane is like skating rapidly over very thin ice: the
air literally "doesn't have time to get away from underneath."

  [Illustration: DIRECT, LIFTING, AND RESISTING FORCES
  If the pressure is 10 lbs. when the wind blows directly toward the
  surface (at an angle of 90 degrees), then the forces for other angles
  of direction are as shown on the diagram. The _amounts_ of all forces
  depend upon the wind velocity: that assumed in drawing the diagram
  was about 55 miles per hour. But the _relations_ of the forces are
  the same for the various angles, no matter what the velocity.]

If we designate the angle made by the wings (_ab_) with the horizontal
(_V_) as _B_, then _P_ increases as _B_ increases, while (as has been
stated) the ratio of _L_ to _R_ decreases. When the angle _B_ is a
right angle, the wings being in the position _a´b´_, _P_ has its
maximum value for direct wind--1/300 of the square of the velocity,
in pounds per square foot; but _L_ is zero and _R_ is equal to _P_.
The plane would have no lifting power. When the angle _B_ becomes
zero, position _a´´b´´_, wings being horizontal, _P_ becomes zero
and (so far as we can now judge) the plane has neither lifting power
nor retarding force. At some intermediate position, like _ab_, there
will be appreciable lifting and retarding forces. The chart shows the
approximate lifting force, in pounds per square foot, for various
angles. This force becomes a maximum at an angle of 45° (half a
right angle). We are not yet prepared to consider why in all actual
aeroplanes the angle of inclination is much less than this. The reason
will be shown presently. At this stage of the discussion we may note
that the lifting power per square foot of sail area varies with

  the square of the velocity, _and_
  the angle of inclination.

The total lifting power of the whole plane will also vary with its
area. As we do not wish this whole lifting power to be consumed in
overcoming the dead weight of the machine itself, we must keep the
parts light, and in particular must use for the wings a fabric of light
weight per unit of surface. These fabrics are frequently the same as
those used for the envelopes of balloons.

Since the total supporting power varies both with the sail area and
with the velocity, we may attain a given capacity either by employing
large sails or by using high speed. The size of sails for a given
machine varies inversely as the square of the speed. The original
Wright machine had 500 square feet of wings and a speed of forty
miles per hour. At eighty miles per hour the necessary sail area for
this machine would be only 125 square feet; and at 160 miles per hour
it would be only 31-1/4 square feet: while if we attempted to run the
machine at ten miles per hour we should need a sail area of 8000 square
feet. This explains why the aeroplane cannot go slowly.

It would seem as if when two or more superposed sails were used, as in
biplanes, the full effect of the air would not be realized, one sail
becalming the other. Experiments have shown this to be the case; but
there is no great reduction in lifting power unless the distance apart
is considerably less than the width of the planes.

In all present aeroplanes the sails are concaved on the under side.
This serves to keep the air from escaping from underneath as rapidly
as it otherwise would, and increases the lifting power from one-fourth
to one-half over that given by our 1/300 rule: the divisor becoming
roughly about 230 instead of 300.

  [Illustration: SHAPES OF PLANES]

Why are the wings placed crosswise of the machine, when the other
arrangement--the greatest dimension in the line of flight--would seem
to be stronger? This is also done in order to "keep the air from
escaping from underneath." The sketch shows how much less easily the
air will get away from below a wing of the bird-like spread-out form
than from one relatively long and narrow but of the same area.

A sustaining force of two pounds per square foot of area has been
common in ordinary aeroplanes and is perhaps comparable with the
results of bird studies: but this figure is steadily increasing as
velocities increase.


Why so Many Sails?

Thus far a single wing or pair of wings would seem to fully answer
for practicable flight: yet every actual aeroplane has several small
wings at various points. The necessity for one of these had already
been discovered in the kite, which is built with a balancing tail. In
the sketch on page 18 it appears that the particles of air which are
near the upper edge of the surface are more obstructed in their effort
to get around and past than those near the lower edge. They have to
turn almost completely about, while the others are merely deflected.
This means that on the whole the upper air particles will exert more
pressure than the lower particles and that the "center of pressure"
(the point where the entire force of the wind may be assumed to act)
will be, not at the center of the surface, but at a point some distance
_above_ this center. This action is described as the "displacement of
the center of pressure." It is known that the displacement is greatest
for least inclinations of surface (as might be surmised from the
sketch already referred to), and that it is always proportional to the
dimension of the surface in the direction of movement; _i.e._, to the
length of the line _ab_.

  [Illustration: BALANCING SAIL]

If the weight _W_ of the aeroplane acts downward at the center of the
wing (at _o_ in the accompanying sketch), while the direct pressure
_P_ acts at some point _c_ farther along toward the upper edge of
the wing, the two forces _W_ and _P_ tend to revolve the whole wing
in the direction indicated by the curved arrow. This rotation, in an
aeroplane, is resisted by the use of a tail plane or planes, such as
_mn_. The velocity produces a direct pressure _P´_ on the tail plane,
which opposes, like a lever, any rotation due to the action of _P_.
It may be considered a matter of rather nice calculation to get the
area and location of the tail plane just right: but we must remember
that the amount of pressure _P´_ can be greatly varied by changing
the inclination of the surface _mn_. This change of inclination is
effected by the operator, who has access to wires which are attached to
the pivoted tail plane. It is of course permissible to place the tail
plane _in front_ of the main planes--as in the original Wright machine
illustrated: but in this case, with the relative positions of _W_ and
_P_ already shown, the forward edge of the tail plane would have to be
depressed instead of elevated. The illustration shows the tail built as
a biplane, just as are the principal wings (page 141).

Suppose the machine to be started with the tail plane in a horizontal
position. As its speed increases, it rises and at the same time (if
the weight is suspended from the center of the main planes) tilts
backward. The tilting can be stopped by swinging the tail plane on its
pivot so as to oppose the rotative tendency. If this control is not
carried too far, the main planes will be allowed to maintain some of
their excessive inclination and ascent will continue. When the desired
altitude has been attained, the inclination of the main planes will, by
further swinging of the tail plane, be reduced to the normal amount,
at which the supporting power is precisely equal to the load; and the
machine will be in vertical equilibrium: an equilibrium which demands
at every moment, however, the attention of the operator.

In many machines, ascent and tilting are separately controlled by using
two sets of transverse planes, one set placed forward, and the other
set aft, of the main planes. In any case, quick ascent can be produced
only by an increase in the lifting force _L_ (see sketch, page 24) of
the main planes: and this force is increased by enlarging the angle of
inclination of the main planes, that is, by a controlled and partial
tilting. The forward transverse wing which produces this tilting is
therefore called the _elevating rudder_ or elevating plane. The rear
transverse plane which checks the tilting and steadies the machine is
often described as the _stabilizing plane_. _Descent_ is of course
produced by _decreasing_ the angle of inclination of the main planes.

  [Illustration: ROE'S TRIPLANE AT WEMBLEY
  (From Brewer's _Art of Aviation_)]


Steering

If we need extra sails for stability and ascent or descent, we need
them also for changes of horizontal direction. Let _ab_ be the top view
of the main plane of a machine, following the course _xy_. At _rs_
is a vertical plane called the _steering rudder_. This is pivoted,
and controlled by the operator by means of the wires _t_, _u_. Let
the rudder be suddenly shifted to the position _r´s´_. It will then
be subjected to a pressure _P´_ which will swing the whole machine
into the new position shown by the dotted lines, its course becoming
_x´y´_. The steering rudder may of course be double, forming a vertical
biplane, as in the Wright machine shown below.

  [Illustration: ACTION OF THE STEERING RUDDER]

Successful steering necessitates lateral resistance to drift, _i.e._,
a fulcrum. This is provided, to some extent, by the stays and frame of
the machine; and in a much more ample way by the vertical planes of the
original Voisin cellular biplane. A recent Wright machine had vertical
planes forward probably intended for this purpose.

  [Illustration: RECENT TYPE OF WRIGHT BIPLANE]

It now begins to appear that the aviator has a great many things to
look after. There are many more things requiring his attention than
have yet been suggested. No one has any business to attempt flying
unless he is superlatively cool-headed and has the happy faculty of
instinctively doing the right thing in an emergency. Give a chauffeur
a high power automobile running at maximum speed on a rough and
unfamiliar road, and you have some conception of the position of the
operator of an aeroplane. It is perhaps not too much to say that to
make the two positions fairly comparable we should _blindfold_ the
chauffeur.

Broadly speaking, designers may be classed in one of two groups--those
who, like the Wrights, believe in training the aviator so as to
qualify him to properly handle his complicated machine; and those
who aim to simplify the whole question of control so that to acquire
the necessary ability will not be impossible for the average man. If
aviation is to become a popular sport, the latter ideal must prevail.
The machines must be more automatic and the aviator must have time
to enjoy the scenery. In France, where amateur aviation is of some
importance, progress has already been made in this direction. The
universal steering head, for example, which not only revolves like that
of an automobile, but is hinged to permit of additional movements,
provides for simultaneous control of the steering rudder and the main
plane warping, while scarcely demanding the conscious thought of the
operator.




TURNING CORNERS


A year elapsed after the first successful flight at Kitty Hawk before
the aviator became able to describe a circle in the air. A later date,
1907, is recorded for the first European half-circular flight: and the
first complete circuit, on the other side of the water, was made a year
after that; by both biplane and monoplane. It was in the same year that
Louis Blériot made the pioneer cross-country trip of twenty-one miles,
stopping at will _en route_ and returning to his starting point.


What Happens When Making a Turn

  [Illustration: CIRCULAR FLIGHT]

We are looking downward on an aeroplane _ab_ which has been moving
along the straight path _cd_. At _d_ it begins to describe the circle
_de_, the radius of which is _od_, around the center _o_. The outer
portion of the plane, at the edge _b_, must then move faster than the
inner edge _a_. We have seen that the direct air pressure on the plane
is proportional to the square of the velocity. The direct pressure
_P_ (see sketch on page 22) will then be greater at the outer than at
the inner limb; the lifting force _L_ will also be greater and the
outer limb will tend to rise, so that the plane (viewed from the rear)
will take the inclined position shown in the lower view: and this
inclination will increase as long as the outer limb travels faster than
the inner limb; that is, as long as the orbit continues to be curved.
Very soon, then, the plane will be completely tipped over.

Necessarily, the two velocities have the ratio _om_:_om´_; the
respective lifting forces must then be proportional to the squares of
these distances. The difference of lifting forces, and the tendency to
overturn, will be more important as the distances most greatly differ:
which is the case when the distance _om_ is small as compared with
_mm´_. The shorter the radius of curvature, the more dangerous, for a
given machine, is a circling flight: and in rounding a curve of given
radius the most danger is attached to the machine of greatest spread of
wing.


Lateral Stability

This particular difficulty has considerably delayed the development
of the aeroplane. It may, however, be overcome by very simple
methods--simple, at least as far as their mechanical features are
concerned. If the outer limb of the plane is tilted upward, it is
because the wind pressure is greater there. The wind pressure is
greater because the velocity is greater. We have only to increase the
wind pressure at the inner limb, in order to restore equilibrium. This
cannot be done by adjusting the velocity, because the velocity is fixed
by the curvature of path required: but the total wind pressure depends
upon the _sail area_ as well as the velocity; so that by increasing the
surface at the inner limb we may equalize the value of _L_, the lifting
force, at the two ends of the plane. This increase of surface must be
a temporary affair, to be discontinued when moving along a straight
course.

  [Illustration: THE AILERON]

Let us stand in the rear of an aeroplane, the main wing of which is
represented by _ab_. Let the small fan-shaped wings _c_ and _d_ be
attached near the ends, and let the control wires, _e_, _f_, passing
to the operator at _g_, be employed to close and unclasp the fans. If
these fans are given a forward inclination at the top, as indicated in
the end view, they will when spread out exert an extra lifting force.
A fan will be placed at each end. They will be ordinarily folded up:
but when rounding a curve the aviator will open the fan on the inner or
more slowly moving limb of the main plane. This represents one of the
first forms of the _aileron_ or wing-tip for lateral control.

The more common present form of aileron is that shown in the lower
sketch, at _s_ and _t_. The method of control is the same.

  [Illustration: WING TIPPING]

The cellular Voisin biplanes illustrate an attempt at self-sufficing
control, without the interposition of the aviator. Between the upper
and lower sails of the machine there were fore and aft vertical
partitions. The idea was that when the machine started to revolve, the
velocity of rotation would produce a pressure against these partitions
which would obstruct the tipping. But rotation may take place slowly,
so as to produce an insufficient pressure for control, and yet be amply
sufficient to wreck the apparatus. The use of extra vertical rudder
planes, hinged on a horizontal longitudinal axis, is open to the same
objection.


Wing Warping

In some monoplanes with the inverted _V_ wing arrangement, a dipping
of one wing answers, so to speak, to increase its concavity and thus
to augment the lifting force on that side. The sketch shows the normal
and distorted arrangement of wings: the inner limb being the one bent
down in rounding a curve. An equivalent plan was to change the angle
of inclination of one-half the sail by swinging it about a horizontal
pivot at the center or at the rear edge: some machines have been built
with sails divided in the center. The obvious objection to both of
these plans is that too much mechanism is necessary in order to distort
what amounts to nearly half the whole machine. They remind one of
Charles Lamb's story of the discovery of roast pig.

  [Illustration: WING WARPING]

The distinctive feature of the Wright machines lies in the warping
or distorting of the _ends only_ of the main planes. This is made
possible, not by hinging the wings in halves, but by the flexibility
of the framework, which is sufficiently pliable to permit of a
considerable bending without danger. The operator, by pulling on a
stout wire linkage, may tip up (or down) the corners _cc´_ of the sails
at one limb, thus decreasing or increasing the effective surface acted
on by the wind, as the case may require. The only objection is that
the scheme provides one more thing for the aviator to think about and
manipulate.


Automatic Control

Let us consider again the condition of things when rounding a curve,
as in the sketch on page 32. As long as the machine is moving forward
in a straight line, the operator sits upright. When it begins to tip,
he will unconsciously tip himself the other way, as represented by
the line _xy_ in the rear view. Any bicyclist will recognize this as
plausible. Why not take advantage of this involuntary movement to
provide a stabilizing force? If operating wires are attached to the
aviator's belt and from thence connected with ailerons or wing-warping
devices, then by a proper proportioning of levers and surfaces to the
probable swaying of the man, the control may become automatic. The idea
is not new; it has even been made the subject of a patent.


The Gyroscope

  [Illustration: THE GYROSCOPE]

This device for automatic control is being steadily developed and may
ultimately supersede all others. It uses the inertia of a fast-moving
fly wheel for control, in a manner not unlike that contemplated in
proposed methods of automatic balancing by the action of a suspended
pendulum. Every one has seen the toy gyroscope and perhaps has wondered
at its mysterious ways. The mathematical analysis of its action fills
volumes: but some idea of what it does, and why, may perhaps be
gathered at the expense of a very small amount of careful attention.
The wheel _acbd_, a thin disc, is spinning rapidly about the axle _o_.
In the side view, _ab_ shows the edge of the wheel, and _oo´_ the
axle. This axle is not fixed, but may be conceived as held in some
one's fingers. Now suppose the right-hand end of the axle (_o´_) to
be suddenly moved toward us (away from the paper) and the left-hand
(_o_) to be moved away. The wheel will now appear in both views as an
ellipse, and it has been so represented, as _afbe_. Now, any particle,
like _x_, on the rim of the wheel, will have been regularly moving
in the circular orbit _cb_. The tendency of any body in motion is to
move indefinitely in a straight line. The cohesion of the metal of
the disc prevents the particle _x_ from flying off at a straight line
tangent, _xy_, and it is constrained, therefore, to move in a circular
orbit. Unless some additional constraint is imposed, it will at least
remain in this orbit and will try to remain in its plane of rotation.
When the disc is tipped, the plane of rotation is changed, and the
particle is required, instead of (so to speak) remaining in the plane
of the paper--in the side view--to approach and pass through that plane
at _b_ and afterward to continue receding from us. Under ordinary
circumstances, this is just what it would do: but if, as in the
gyroscope, the axle _oo´_ is perfectly free to move in any direction,
the particle _x_ will refuse to change its direction of rotation.
Its position has been shifted: it no longer lies in the plane of the
paper: but it will at least persist in rotating in a parallel plane:
and this persistence forces the revolving disc to swing into the new
position indicated by the curve _hg_, the axis being tipped into the
position _pq_. The whole effect of all particles like _x_ in the entire
wheel will be found to produce precisely this condition of things: if
we undertake to change the plane of rotation by shifting the axle in
a horizontal plane, the device itself will (if not prevented) make
a further change in the plane of rotation by shifting the axle in a
vertical plane.

A revolving disc mounted on the gyroscopic framework therefore resists
influences tending to change its plane of rotation. If the device
is placed on a steamship, so that when the vessel rolls a change of
rotative plane is produced, the action of the gyroscope will resist
the rolling tendency of the vessel. All that is necessary is to have
the wheel revolving in a fore and aft plane on the center line of the
vessel, the axle being transverse and firmly attached to the vessel
itself. A small amount of power (consumed in revolving the wheel) gives
a marked steadying effect. The same location and arrangement on an
aeroplane will suffice to overcome tendencies to transverse rotation
when rounding curves. The device itself is automatic, and requires no
attention, but it does unfortunately require power to drive it and it
adds some weight.

The gyroscope is being tested at the present time on some of the
aeroplanes at the temporary army camps near San Antonio, Texas.


Wind Gusts

This feature of aeronautics is particularly important, because any
device which will give automatic stability when turning corners will go
far toward making aviation a safe amusement. Inequalities of velocity
exist not only on curves, but also when the wind is blowing at anything
but uniform velocity across the whole front of the machine. The
slightest "flaw" in the wind means an at least temporary variation in
lifting force of the two arms. Here is a pregnant source of danger, and
one which cannot be left for the aviator to meet by conscious thought
and action. It is this, then, that blindfolds him: he cannot see the
wind conditions in advance. The conditions are upon him, and may have
done their destructive work, before he can prepare to control them. We
must now study what these conditions are and what their influence may
be on various forms of aerial navigation: after which, a return to our
present subject will be possible.




AIR AND THE WIND

The air that surrounds us weighs about one-thirteenth of a pound per
cubic foot and exerts a pressure, at sea level, of nearly fifteen
pounds per square inch. Its temperature varies from 30° below to 100°
above the Fahrenheit zero. The pressure of the air decreases about
one-half pound for each thousand feet of altitude; at the top of Mt.
Blanc it would be, therefore, only about six pounds per square inch.
The temperature also decreases with the altitude. The weight of a cubic
foot, or _density_, which, as has been stated, is one-thirteenth of a
pound ordinarily, varies with the pressure and with the temperature.
The variation with pressure may be described by saying that the
_quotient_ of the pressure by the density is constant: one varies in
the same ratio as the other. Thus, at the top of Mt. Blanc (if the
temperature were the same as at sea level), the density of air would be
about 6/15 × 1/13 = 2/65: less than half what it is at sea level. As to
temperature, if we call our Fahrenheit zero 460°, and correspondingly
describe other temperatures--for instance, say that water boils at
672°--then (pressure being unchanged) the _product_ of the density
and the temperature is constant. If the density at sea level and zero
temperature is one-thirteenth pound, then that at sea level and 460°
Fahrenheit would be

  (0 + 460)/(460 + 460) × 1/13 = 1/26.

These relations are particularly important in the design of all
balloons, and in computations relating to aeroplane flight at
high altitudes. We shall be prepared to appreciate some of their
applications presently.

Generally speaking, the atmosphere is always in motion, and moving air
is called wind. Our meteorologists first studied winds near the surface
of the ground: it is only of late years that high altitude measurements
have been considered practically desirable. Now, records are obtained
by the aid of kites up to a height of nearly four miles: estimates of
cloud movements have given data on wind velocities at heights above six
miles: and much greater heights have been obtained by free balloons
equipped with instruments for recording temperatures, pressures,
altitude, time, and other data.

When the Eiffel Tower was completed, it was found that the average wind
velocity at its summit was about four times that at the base. Since
that time, much attention has been given to the contrasting conditions
of surface and upper breezes as to direction and velocity.

Air is easily impeded in its movement, and the well-known uncertainties
of the weather are closely related to local variations in atmospheric
pressure and temperature. When near the surface of the ground,
impingement against irregularities therein--hills, cliffs, and
buildings--makes the atmospheric currents turbulent and irregular.
Where there are no surface irregularities, as on a smooth plain or
over water, the friction of the air particles passing over the surface
still results in a stratification of velocities. Even on a mountain
top, the direction and speed of the wind are less steady than in the
open where measured by a captive balloon. The stronger the wind,
the greater, relatively, is the irregularity produced by surface
conditions. Further, the earth's surface and its features form a vast
sponge for sun heat, which they transfer in turn to the air in an
irregular way, producing those convectional currents peculiar to low
altitudes, the upper limit of which is marked by the elevation of the
cumulus clouds. Near the surface, therefore, wind velocities are lowest
in the early morning, rising to a maximum in the afternoon.

  [Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS
  (From Rotch's _The Conquest of the Air_)]

Every locality has its so-called "prevailing winds." Considering the
compass as having eight points, one of those points may describe
as many as 40% of all the winds at a given place. The direction of
prevalence varies with the season. The range of wind velocities is
also a matter of local peculiarity. In Paris, the wind speed exceeds
thirty-four miles per hour on only sixty-eight days in the average
year, and exceeds fifty-four miles on only fifteen days. Observations
at Boston show that the velocity of the wind exceeds twenty miles per
hour on half the days in winter and on only one-sixth the days in
summer. Our largest present dirigible balloons have independent speeds
of about thirty-four miles per hour and are therefore available (at
some degree of effectiveness) for nearly ten months of the year, in the
vicinity of Paris. In a region of low wind velocities--like western
Washington, in this country--they would be available a much greater
proportion of the time. To make the dirigible able to at least move
nearly every day in the average year--in Paris--it must be given a
speed of about fifty-five miles per hour.

Figures as to wind velocity mean little to one unaccustomed to using
them. A five-mile breeze is just "pleasant." Twelve miles means a brisk
gale. Thirty miles is a high wind: fifty miles a serious storm (these
are the winds the aviator constantly meets): one hundred miles is
perhaps about the maximum hurricane velocity.

As we ascend from the surface of the earth, the wind velocity steadily
increases; and the excess velocity of winter winds over summer winds
is as steadily augmented. Thus, Professor Rotch found the following
variations:

  Altitude in Feet      Annual Average
                Wind Velocity, Feet per Second
          656               23.15
        1,800               32.10
        3,280               35.
        8,190               41.
       11,440               50.8
       17,680               81.7
       20,970               89.
       31,100              117.5

                          Average Wind Velocities,
  Altitude in Feet          Feet per Second
                            Summer          Winter
     656 to 3,280            24.55           28.80
   3,280 to 9,810            26.85           48.17
   9,810 to 16,400           34.65           71.00
  16,400 to 22,950           62.60          161.5
  22,950 to 29,500           77.00          177.0

  [Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS]

These results are shown in a more striking way by the chart. At a five
or six mile height, double-barreled hurricanes at speeds exceeding 200
miles per hour are not merely possible; they are part of the regular
order of things, during the winter months.

The winds of the upper air, though vastly more powerful, are far less
irregular than those near the surface: and the directions of prevailing
winds are changed. If 50% of the winds, at a given location on the
surface, are from the southwest, then at as moderate an elevation
as even 1000 feet, the prevailing direction will cease to be from
southwest; it may become from west-southwest; and the proportion of
total winds coming from this direction will not be 50%. These factors
are represented in meteorological papers by what is known as the _wind
rose_. From the samples shown, we may note that 40% of the surface
winds at Mount Weather are from the northwest; while at some elevation
not stated the most prevalent of the winds (22% of the total) are
westerly. The direction of prevalence has changed through one-eighth
of the possible circle, and in a _counter-clockwise_ direction. This
is contrary to the usual variation described by the so-called Broun's
Law, which asserts that as we ascend the direction of prevalence
rotates around the circle like the hands of a watch; being, say, from
northwest at the surface, from north at some elevation, from northeast
at a still higher elevation, and so on. At a great height, the change
in direction may become total: that is, the high altitude winds blow
in the exactly opposite direction to that of the surface winds. In
the temperate regions, most of the high altitude winds are from the
west: in the tropics, the surface winds blow _toward_ the west and
toward the equator; being northeasterly in the northern hemisphere
and southeasterly in the southern: and there are undoubtedly equally
prevalent high-altitude counter-trades.

  [Illustration: THE WIND ROSE FOR MOUNT WEATHER, VA.
  (From the _Bulletin_ of the Mount Weather Observatory, II, 6)]

The best flying height for an aeroplane over a flat field out in the
country is perhaps quite low--200 or 300 feet: but for cross-country
trips, where hills, rivers, and buildings disturb the air currents, a
much higher elevation is necessary; perhaps 2000 or 3000 feet, but in
no case more than a mile. The same altitude is suitable for dirigible
balloons. At these elevations we have the conditions of reasonable
warmth, dryness, and moderate wind velocities.


Sailing Balloons

In classifying air craft, the sailing balloon was mentioned as a type
intermediate between the drifting balloon and the dirigible. No such
type has before been recognized: but it may prove to have its field,
just as the sailing vessel on the sea has bridged the gap between the
raft and the steamship. It is true that tacking is impossible, so that
our sailing balloons must always run before the wind: but they possess
this great advantage over marine sailing craft, that by varying their
altitude they may always be able to find a favorable wind. This implies
adequate altitude control, which is one of the problems not yet solved
for lighter-than-air flying machines: but when it has been solved we
shall go far toward attaining a dirigible balloon without motor or
propeller; a true sailing craft.

  [Illustration: DIAGRAM OF PARTS OF A DRIFTING BALLOON]

This means more study and careful utilization of stratified atmospheric
currents. Professor Rotch suggests the utilization of the upper
westerly wind drift across the American continent and the Atlantic
Ocean, which would carry a balloon from San Francisco to southern
Europe at a speed of about fifty feet per second--thirty-four miles
per hour. Then by transporting the balloon to northern Africa, the
northeast surface trade wind would drive it back to the West Indies at
twenty-five miles per hour. This without any motive power: and since
present day dirigibles are all short of motive power for complete
dirigibility, we must either make them much more powerful or else adopt
the sailing principle, which will permit of actually decreasing present
sizes of motors, or even possibly of omitting them altogether. Our next
study is, then, logically, one of altitude control in balloons.

  [Illustration: GLIDDEN AND STEVENS GETTING AWAY IN THE "BOSTON"
  (Leo Stevens, N.Y.)]


Field and Speed

  [Illustration: RELATIVE AND ABSOLUTE BALLOON VELOCITIES]

  [Illustration: FIELD AND SPEED]

An _aerostat_ (non-dirigible balloon), unless anchored, drifts at the
speed of the wind. To the occupants, it seems to stand still, while
the surface of the earth below appears to move in a direction opposite
to that of the wind. In the sketch, if the independent velocity of a
_dirigible_ balloon be _PB_, the wind velocity _PV_, then the actual
course pursued is _PR_, although the balloon always points in the
direction _PB_, as shown at 1 and 2. If the speed of the wind exceed
that of the balloon, there will be some directions in which the latter
cannot progress. Thus, let _PV_ be the wind velocity and _TV_ the
independent speed of the balloon. The tangents _PX_, _PX´_, include the
whole "field of action" possible. The wind direction may change during
flight, so that the initial objective point may become unattainable,
or an initially unattainable point may be brought within the field. The
present need is to increase independent speeds from thirty or forty
to fifty or sixty miles per hour, so that the balloon will be truly
dirigible (even if at low effectiveness) during practically the whole
year.

  [Illustration: INFLUENCE OF WIND ON POSSIBLE COURSE]

Suppose a dirigible to start on a trip from New York to Albany,
150 miles away. Let the wind be a twenty-five mile breeze from the
southwest. The wind alone tends to carry the balloon from New York to
the point _d_ in four hours. If the balloon meanwhile be headed due
west, it would need an independent velocity of its own having the same
ratio to that of the wind as that of _de_ to _fd_, or about seventeen
and one-half miles per hour. Suppose its independent speed to be only
twelve and one-half miles; then after four hours it will be at the
position _b_, assuming it to have been continually headed due west, as
indicated at _a_. It will have traveled northward the distance _fe_,
apparently about sixty-nine miles.

  [Illustration: COUNT ZEPPELIN]

After this four hours of flight, the wind suddenly changes to
south-southwest. It now tends to carry the balloon to _g_ in the next
four hours. Meanwhile the balloon, heading west, overcomes the easterly
drift, and the balloon actually lands at _c_. Unless there is some
further favorable shift of the wind it cannot reach Albany. If, during
the second four hours, its independent speed could have been increased
to about fifteen and a half miles it would have just made it. The
actual course has been _fbc_: a drifting balloon would have followed
the course _fdh_, _dh_ being a course parallel to _bg_.




GAS AND BALLAST


A cubical block of wood measuring twelve inches on a side floats on
water because it is lighter than water; it weighs, if yellow pine,
thirty-eight pounds, whereas the same volume of water weighs about
sixty-two pounds. Any substance weighing more than sixty-two pounds to
the cubic foot would sink in water.

  [Illustration: BUOYANT POWER OF WOOD]

If our block of wood be drilled, and _lead_ poured in the hole, the
total size of wood-and-lead block being kept constantly at one cubic
foot, the block will sink as soon as its whole weight exceeds sixty-two
pounds. Ignoring the wood removed by boring (as, compared with the lead
which replaces it, an insignificant amount), the weight of lead plugged
in may reach twenty-four pounds before the block will sink.

This figure, twenty-four pounds, the difference between sixty-two and
thirty-eight pounds, then represents the maximum buoyant power of a
cubic foot of wood in water. It is the difference between the weight of
the wood block and the weight of the water it displaces. If any weight
less than this is added to that of the wood, the block will float,
projecting above the water's surface more or less, according to the
amount of weight buoyed up. It will not rise entirely from the water,
because to do this it would need to be lighter, not only than water,
but than air.

  [Illustration: ONE CUBIC FOOT OF WOOD LOADED IN WATER]


Buoyancy in Air

There are _gases_, if not woods, lighter than air: among them, coal gas
and hydrogen. A "bubble" of any of these gases, if isolated from the
surrounding atmosphere, cannot sink but must rise. At the same pressure
and temperature, hydrogen weighs about one-fifteenth as much as air;
coal gas, about one-third as much. If a bubble of either of these gases
be isolated in the atmosphere, it must continually rise, just as wood
immersed in water will rise when liberated. But the wood will stop
when it reaches the surface of the water, while there is no reason
to suppose that the hydrogen or coal gas bubbles will ever stop. The
hydrogen bubble can be made to remain stationary if it is weighted down
with something of about fourteen times its own weight (thirteen and
one-half times, accurately). Perhaps it would be better to say that it
would still continue to rise slowly because that additional something
would itself displace some additional air; but if the added weight is a
solid body, its own buoyancy in air is negligible.

  [Illustration: BUOYANT POWER OF HYDROGEN]

Our first principle is, then, that at the same pressure and
temperature, any gas lighter than air, if properly confined, will exert
a net lifting power of (_n_-1) times its own weight, where _n_ is the
ratio of weights of air and gas per cubic foot.

  [Illustration: LEBAUDY'S "JAUNE"]

If the pressures and temperatures are different, this principle is
modified. In a balloon, the gas is under a pressure slightly in
excess of that of the external atmosphere: this decreases its lifting
power, because the weight of a given volume of gas is greater as the
pressure to which it is subjected is increased. The weight of a given
volume we have called the _density_: and, as has been stated, if the
temperature be unchanged, the density varies directly as the pressure.

The pressure in a balloon is only about 1% greater than that of
the atmosphere at sea level, so that this factor has only a slight
influence on the lifting power. That it leads to certain difficulties
in economy of gas will, however, soon be seen.

The temperature of the gas in a balloon, one might think, would
naturally be the same as that of the air outside: but the surface
of the balloon envelope has an absorbing capacity for heat, and on
a bright sunny day the gas may be considerably warmed thereby. This
action increases the lifting power, since increase of temperature (the
pressure remaining fixed) decreases the density of a gas. To avoid
this possibly objectionable increase in lifting power, balloons are
sometimes painted with a non-absorbent color. One of the first Lebaudy
balloons received a popular nickname in Paris on account of the yellow
hue of its envelope.

Suppose we wish a balloon to carry a total weight, including that of
the envelope itself, of a ton. If of hydrogen, it will have to contain
one fifteenth of this weight or about 133 pounds of that gas, occupying
a space of about 23,000 cubic feet. If coal gas is used, the size
of the balloon would have to be much greater. If hot air is used--as
has sometimes been the case--let us assume the temperature of the air
inside the envelope such that the density is just half that of the
outside air. This would require a temperature probably about 500°.
The air needed would be just a ton, and the balloon would be of about
52,000 cubic feet. It would soon lose its lifting power as the air
cooled; and such a balloon would be useful only for short flights.

  [Illustration: AIR BALLOON
  (Photo by Paul Thompson, N.Y.)
  Built by some Germans in the backwoods of South Africa]

The 23,000 cubic foot hydrogen balloon, designed to carry a ton, would
just answer to sustain the weight. If anchored at sea level, it would
neither fall to the ground nor tug upward on its holding-down ropes.
In order to ascend, something more is necessary. This "something more"
might be some addition to the size and to the amount of hydrogen. Let
us assume that we, instead, drop one hundred pounds of our load. Thus
relieved of so much ballast, the balloon starts upward, under the net
lifting force of one hundred pounds. It is easy to calculate how far
it will go. It will not ascend indefinitely, because, as the altitude
increases, the pressure (and consequently the density) of the external
atmosphere decreases. At about a 2000-foot elevation, this decrease
in density will have been sufficient to decrease the buoyant power of
the hydrogen to about 1900 pounds, and the balloon will cease to rise,
remaining at this level while it moves before the wind.

There are several factors to complicate any calculations. Any
expansion of the gas bag--stretching due to an increase in internal
pressure--would be one; but the envelope fabrics do not stretch much;
there is indeed a very good reason why they must not be allowed to
stretch. The pressure in the gas bag is a factor. If there is no
stretching of the bag, this pressure will vary directly with the
temperature of the gas, and might easily become excessive when the sun
shines on the envelope.

A more serious matter is the increased difference between the internal
pressure of the gas and the external pressure of the atmosphere at high
altitudes. Atmospheric pressure decreases as we ascend. The difference
between gas pressure and air pressure thus increases, and it is this
difference of pressure which tends to burst the envelope. Suppose the
difference of pressure at sea level to have been two-tenths of a pound.
For a balloon of twenty feet diameter, this would give a stress on
the fabric, per lineal inch, of twenty-four pounds. At an altitude of
2000 feet, the atmospheric pressure would decrease by one pound, the
difference of pressures would become one and two-tenths pounds, and the
stress on the fabric would be 144 pounds per lineal inch--an absolutely
unpermissible strain. There is only one remedy: to allow some of the
gas to escape through the safety valve; and this will decrease our
altitude.


Ascending and Descending

To ascend, then, we must discard ballast: and we cannot ascend beyond
a certain limit on account of the limit of allowable pressure on the
envelope fabric. To again descend, we must discharge some of the gas
which gives us lifting power. Every change of altitude thus involves
a loss either of gas or of ballast. Our vertical field of control
may then be represented by a series of oscillations of gradually
decreasing magnitude until finally all power to ascend is gone. And
even this situation, serious as it is, is made worse by the gradual but
steady leakage of gas through the envelope fabric. Here, in a word,
is the whole problem of altitude regulation. Air has no surface of
equilibrium like water. Some device supplementary to ballast and the
safety valve is absolutely necessary for practicable flight in any
balloon not staked to the ground.

A writer of romance has equipped his aeronautic heroes with a complete
gas-generating plant so that all losses might be made up; and in
addition, heating arrangements were provided so that when the gas
supply had been partially expended its lifting power could be augmented
by warming it so as to decrease its density below even the normal.
There might be something to say in favor of this latter device, if used
in connection with a collapsible gas envelope.

Methods of mechanically varying the size of the balloon, so as by
compressing the gas to cause descent and by giving it more room to
increase its lifting power and produce ascent, have been at least
suggested. The idea of a vacuum balloon, in which a rigid hollow shell
would be exhausted of its contents by a continually working pump, may
appear commendable. Such a balloon would have maximum lifting power for
its size; but the weight of any rigid shell would be considerable, and
the pressure tending to rupture it would be about 100 times that in
ordinary gas balloons.

It has been proposed to carry stored gas at high pressure (perhaps
in the liquefied condition) as a supplementary method of prolonging
the voyage while facilitating vertical movements: but hydrogen gas
at a pressure of a ton to the square inch in steel cylinders would
give an ultimate lifting power of only about one-tenth the weight of
the cylinders which contain it. These cylinders might be regarded as
somewhat better than ordinary ballast: but to throw them away, with
their gas charge, as ballast, would seem too tragic. Liquefied gas
might possibly appear rather more desirable, but would be altogether
too expensive.

  [Illustration: SCREW PROPELLER FOR ALTITUDE CONTROL]

If a screw propeller can be used on a steamship, a dirigible balloon,
or an aeroplane to produce forward motion, there is no reason why it
could not also be used to produce upward motion in any balloon; and the
propeller with its operating machinery would be a substitute for twice
its equivalent in ballast, since it could produce motion either upward
or downward. Weight for weight, however, the propeller and engine give
only (in one computed case) about half the lifting power of hydrogen.
If we are to use the screw for ascent, we might well use a helicopter,
heavier than air, rather than a balloon.


The Ballonet

The present standard method of improving altitude regulation involves
the use of the ballonet, or compartment air bag, inside the main
envelope. For stability and effective propulsion, it is important that
the balloon preserve its shape, no matter how much gas be allowed to
escape. Dirigible balloons are divided into two types, according to
the method employed for maintaining the shape. In the Zeppelin type, a
rigid internal metal framework supports the gas envelope. This forms
a series of seventeen compartments, each isolated from the others. No
matter what the pressure of gas, the shape of the balloon is unchanged.

In the more common form of balloon, the internal air ballonet is empty,
or nearly so, when the main envelope is full. As gas is vented from the
latter, air is pumped into the former. This compresses the remaining
gas and thus preserves the normal form of the balloon outline.

  [Illustration: BALLOON WITH BALLONETS]

But the air ballonet does more than this. It provides an opportunity
for keeping the balloon on a level keel, for by using a number of
compartments the air can be circulated from one to another as the case
may require, thus altering the distribution of weights. Besides this,
if the pressure in the air ballonet be initially somewhat greater
than that of the external atmosphere, a considerable ascent may be
produced by merely venting this air ballonet. This involves no loss of
gas; and when it is again desired to descend, air may be pumped into
the ballonet. If any considerable amount of gas should be vented, to
produce quick and rapid descent, the pumping of air into the ballonet
maintains the shape of the balloon and also facilitates the descent.

  [Illustration: CONSTRUCTION OF THE ZEPPELIN BALLOON]


The Equilibrator

  [Illustration: THE EQUILIBRATOR IN NEUTRAL POSITION]

Suppose a timber block of one square foot area, ten feet long, weighing
380 pounds, to be suspended from the balloon in the ocean, and let
mechanism be provided by which this block may be raised or lowered
at pleasure. When completely immersed in water it exerts an upward
pressure (lifting force) of 240 pounds, which may be used to supplement
the lifting power of the balloon. If wholly withdrawn from the water,
it pulls down the balloon with its weight of 380 pounds. It seems to be
equivalent, therefore, to about 620 pounds of ballast. When immersed a
little over six feet--the upper four feet being out of the water--it
exerts neither lifting nor depressing effect. The amount of either
may be perfectly adjusted between the limits stated by varying the
immersion.

In the Wellman-Vaniman equilibrator attached to the balloon _America_,
which last year carried six men (and a cat) a thousand miles in three
days over the Atlantic Ocean, a string of tanks partly filled with fuel
was used in place of the timber block. As the tanks were emptied, the
degree of control was increased; and this should apparently have given
ideal results, equilibration being augmented as the gas supply was lost
by leakage: but the unsailorlike disregard of conditions resulting from
the strains transferred from a choppy sea to the delicate gas bag led
to disaster, and it is doubtful whether this method of control can ever
be made practicable. The _America's_ trip was largely one of a drifting
rather than of a dirigible balloon. The equilibrator could be used only
in flights over water in any case: and if we are to look to water for
our buoyancy, why not look wholly to water and build a ship instead of
a balloon?




DIRIGIBLE BALLOONS AND OTHER KINDS


Shapes

  [Illustration: HENRY GIFFARD'S DIRIGIBLE
  (The first with steam power)]

The cylindrical Zeppelin balloon with approximately conical ends
has already been shown (page 68). Those balloons in which the shape
is maintained by internal pressure of air are usually _pisciform_,
that is, fish-shaped. Studies have actually been made of the contour
lines of various fishes and equivalent symmetrical forms derived,
the outline of the balloon being formed by a pair of approximately
parabolic curves.

  [Illustration: DIRIGIBLE OF DUPUY DE LOME
  (Man Power)]

The first flight in a power driven balloon was made by Giffard in 1852.
This balloon had an independent speed of about ten feet per second, but
was without appliances for steering. A ballonetted balloon of 120,000
cubic feet capacity was directed by man power in 1872: eight men turned
a screw thirty feet in diameter which gave a speed of about seven miles
per hour. Electric motors and storage batteries were used for dirigible
balloons in 1883-'84: in the latter year, Renard and Krebs built the
first fish-shaped balloon. The first dirigible driven by an internal
combustion motor was used by Santos-Dumont in 1901.

  [Illustration: TISSANDIER BROTHERS' DIRIGIBLE BALLOON
  (Electric Motor)]


Dimensions

The displacements of present dirigibles vary from 20,000 cubic feet (in
the United States Signal Corps airship) up to 460,000 cubic feet (in
the Zeppelin). The former balloon has a carrying capacity only about
equivalent to that of a Wright biplane. While anchored or drifting
balloons are usually spherical, all dirigibles are elongated, with
a length of from four to eleven diameters. The Zeppelin represents
an extreme elongation, the length being 450 feet and the diameter
forty-two feet. At the other extreme, some of the English military
dirigibles are thirty-one feet in diameter and only 112 feet long.
Ballonet capacities may run up to one-fifth the gas volume. All present
dirigibles have gasoline engines driving propellers from eight to
twenty feet in diameter. The larger propellers are connected with
the motors by gearing, and make from 250 to 700 turns per minute.
The smaller propellers are direct connected and make about 1200
revolutions. Speeds are usually from fifteen to thirty miles per hour.

  [Illustration: THE BALDWIN
  Dirigible of the United States Signal Corps]

The present-day elongated shape is the result of the effort to
decrease the proportion of propulsion resistance due to the pressure
of the air against the head of the balloon. This has led also to the
pointed ends now universal; and to avoid eddy resistance about the
rear it is just as important to point the stern as the bow. As far as
head end resistance alone is concerned, the longer the balloon the
better: but the friction of the air along the side of the envelope
also produces resistance, so that the balloon must not be too much
elongated. Excessive elongation also produces structural weakness. From
the standpoint of stress on the fabric of the envelope, the greatest
strain is that which tends to break the material along a longitudinal
line, and this is true no matter what the length, as long as the seams
are equally strong in both directions and the load is so suspended as
not to produce excessive bending strain on the whole balloon. In the
_Patrie_ (page 77), some distortion due to loading is apparent. The
stress per lineal inch of fabric is obtained by multiplying the net
pressure by half the diameter of the envelope (in inches).

  [Illustration: THE ZEPPELIN ENTERING ITS HANGAR ON LAKE CONSTANCE]

Ample steering power (provided by vertical planes, as in
heavier-than-air machines) is absolutely necessary in dirigibles: else
the head could not be held up to the wind and the propelling machinery
would become ineffective.


Fabrics

The material for the envelope and ballonets should be light, strong,
unaffected by moisture or the atmosphere, non-cracking, non-stretching,
and not acted upon by variations in temperature. The same
specifications apply to the material for the wings of an aeroplane. In
addition, for use in dirigible balloons, fabrics must be impermeable,
resistent to chemical action of the gas, and not subject to spontaneous
combustion. The materials used are vulcanized silk, gold beater's
skin, Japanese silk and rubber, and cotton and rubber compositions.
In many French balloons, a middle layer of rubber has layers of
cotton on each side, the whole thickness being the two hundred and
fiftieth part of an inch. In the _Patrie_, this was supplemented by
an outside non-heat-absorbent layer of lead chromate and an inside
coating of rubber, all rubber being vulcanized. The inner rubber layer
was intended to protect the fabric against the destructive action of
impurities in the gas.

  [Illustration: THE "PATRIE." DESTROYED BY A STORM]

Fabrics are obtainable in various colors, painted, varnished, or wholly
uncoated. The rubber and cotton mixtures are regularly woven in France
and Germany for aeroplanes and balloons. The cars and machinery are
frequently shielded by a fabricated wall. Weights of envelope materials
range from one twenty-third to one-fourteenth pound per square foot,
and breaking stresses from twenty-eight to one hundred and thirty
pounds. Pressures (net) in the main envelope are from three-fifths to
one and a quarter _ounces_ per square inch, those in the ballonets
being somewhat less. The _Patrie_ of 1907 had an envelope guaranteed
not to allow the leakage of more than half a cubic inch of hydrogen per
square foot of surface per twenty-four hours.

  [Illustration: MANUFACTURING THE ENVELOPE OF A BALLOON]

  [Illustration: INSPECTING THE ENVELOPE OF ANDRÉE'S BALLOON "L'OERNEN"]

The best method of cutting the fabric is to arrange for building up the
envelope by a series of strips about the circumference, the seams being
at the bottom. The two warps of the cloth should cross at an angle so
as to localize a rip or tear. Bands of cloth are usually pasted over
the seams, inside and out, with a rubber solution; this is to prevent
leakage at the stitches.


Framing

In the _Zeppelin_, the rigid aluminum frame is braced every forty-five
feet by transverse diametral rods which make the cross-sections
resemble a bicycle wheel (page 68). This cross-section is not circular,
but sixteen-sided. The pressure is resisted by the framework itself,
the envelope being required to be impervious only. The seventeen
compartments are separated by partitions of sheet aluminum. There is a
system of complete longitudinal bracing between these partitions. Under
the main framework, the cars and machinery are carried by a truss about
six feet deep which runs the entire length. The cars are boat-shaped,
twenty feet long and six feet wide, three and one-half feet high,
enclosed in aluminum sheathing. These cars, placed about one hundred
feet from the ends, are for the operating force and machinery. The
third car, carrying passengers, is built into the keel.

  [Illustration: WRECK OF THE "ZEPPELIN"]

In non-rigid balloons like the _Patrie_, the connecting frame must be
carefully attached to the envelope. In this particular machine, cloth
flaps were sewed to the bag, and nickel steel tubes then laced in
the flaps. With these tubes as a base, a light framework of tubes
and wires, covered with a laced-on waterproof cloth, was built up
for supporting the load. Braces ran between the various stabilizing
and controlling surfaces and the gas bag; these were for the most
part very fine wire cables. The weight of the car was concentrated
on about seventy feet of the total length of 200 feet. This accounts
for the deformation of the envelope shown in the illustration (page
77). The frame and car of this balloon were readily dismantled for
transportation.

In some of the English dirigibles the cars were suspended by network
passing over the top of the balloon.


Keeping the Keel Horizontal

In the _Zeppelin_, a sliding weight could be moved along the keel so as
to cause the center of gravity to coincide with the center of upward
pressure in spite of variations in weight and position of gas, fuel,
and ballast. In the German balloon _Parseval_, the car itself was
movable on a longitudinal suspending cable which carried supporting
sheaves. This balloon has figured in recent press notices. It was
somewhat damaged by a collision with its shed in March: the sixteen
passengers escaped unharmed. A few days later, emergency deflation by
the rip-strip was made necessary during a severe storm. In the ordinary
non-rigid balloon, the pumping of air between the ballonets aids in
controlling longitudinal equilibrium. The pump may be arranged for
either hand or motor operation: that in the _Clément-Bayard_ had a
capacity of 1800 liters per minute against the pressure of a little
over three-fifths of an ounce. The _Parseval_ has two ballonets. Into
the rear of these air is pumped at starting. This raises the bow and
facilitates ascent on the principle of the inclined surface of an
aeroplane. After some elevation is attained, the forward ballonet is
also filled.

  [Illustration: CAR OF THE ZEPPELIN
  (From the _Transactions_ of the American Society of
  Mechanical Engineers)]


Stability

Besides proper distribution of the loads, correct vertical location of
the propeller is important if the balloon is to travel on a level keel.
In some early balloons, two envelopes side by side had the propeller
at the height of the axes of the gas bags and midway between them. The
modern forms carry the car, motor, and propeller below the balloon
proper. The air resistance is mostly that of the bow of the envelope:
but there is some resistance due to the car, and the propeller shaft
should properly be at the equivalent center of all resistance, which
will be between car and axis of gas bag and nearer the latter than
the former. With a single envelope and propeller, this arrangement
is impracticable. By using four (or even two) propellers, as in the
_Zeppelin_ machine (page 68), it can be accomplished. If only one
propeller is employed, horizontal rudder planes must be disposed at
such angles and in such positions as to compensate for the improper
position of the tractive force. Even on the _Zeppelin_, such planes
were employed with advantage (pages 66 and 73).

Perfect stability also involves freedom from rolling. This is usually
inherent in a balloon, because the center of mass is well below the
center of buoyancy: but in machines of the non-rigid type the absence
of a ballonet might lead to both rolling and pitching when the gas was
partially exhausted.

  [Illustration: STERN VIEW OF THE ZEPPELIN]

What is called "route stability" describes the condition of straight
flight. The balloon must point directly in its (independent) course.
This involves the use of a steering rudder, and, in addition, of fixed
vertical planes, which, on the principle of the vertical partitions
of Voisin, probably give some automatic steadiness to the course. To
avoid the difficulty or impossibility of holding the head up to the
wind at high speeds, an _empennage_ or feathering tail is a feature of
all present balloons. The empennage of the _Patrie_ (page 77) consisted
of pairs of vertical and horizontal planes at the extreme stern. In
the _France_, thirty-two feet in maximum diameter and nearly 200 feet
long, empennage planes aggregating about 400 square feet were placed
somewhat forward of the stern. In the _Clément-Bayard_, the empennage
consisted of cylindro-conical ballonets projecting aft from the stern.
A rather peculiar grouping of such ballonets was used about the
prolonged stern of the _Ville de Paris_.

  [Illustration: THE "CLÉMENT-BAYARD"]

  [Illustration: THE "VILLE DE PARIS"]


Rudders and Planes

The dirigible has thus several air-resisting or gliding surfaces.
The approximately "horizontal" (actually somewhat inclined) planes
permit of considerable ascent and descent by the expenditure of power
rather than gas, and thus somewhat influence the problem of altitude
control. Each of the four sets of horizontal rudder planes on the
_Zeppelin_, for example, has, at thirty-five miles per hour, with an
inclination equal to one-sixth a right angle, a lifting power of nearly
a ton; about equal to that of all of the gas in one of the sixteen
compartments.

  [Illustration: CAR OF THE "LIBERTÉ"]

Movable rudders may be either hand or motor-operated. The double
vertical steering rudder of the _Ville de Paris_ had an area of 150
square feet. The horizontally pivoted rudders for vertical direction
had an area of 130 square feet.


Arrangement and Accessories

The motor in the _Ville de Paris_ was at the front of the car, the
operator behind it. This car had the excessive weight of nearly 700
pounds. The _Patrie_ employed a non-combustible shield over the motor,
for the protection of the envelope: its steering wheel was in front and
the motor about in the middle of the car. The gasoline tank was under
the car, compressed air being used to force the fuel up to the motor,
which discharged its exhaust downward at the rear through a spark
arrester. Motors have battery and magneto ignition and decompression
cocks, and are often carried on a spring-supported chassis. The
interesting _Parseval_ propeller has four cloth blades which hang limp
when not revolving. When the motor is running, these blades, which are
weighted with lead at the proper points, assume the desired form.

Balloons usually carry guide ropes at head and stern, the aggregate
weight of which may easily exceed a hundred pounds. In descending, the
bow rope is first made fast, and the airship then stands with its head
to the wind, to be hauled in by the stern rope. For the large French
military balloons, this requires a force of about thirty men. The
_Zeppelin_ descends in water, being lowered until the cars float, when
it is docked like a ship (see page 84). Landing skids are sometimes
used, as with aeroplanes.

The balloon must have escape valves in the main envelope and ballonets.
In addition it has a "rip-strip" at the bottom by which a large cut
can be made and the gas quickly vented for the purpose of an emergency
descent. Common equipment includes a siren, megaphone, anchor pins,
fire extinguisher, acetylene search light, telephotographic apparatus,
registering and indicating gages and other instruments, anemometer,
possibly carrier pigeons; besides fuel, oil and water for the motor,
and the necessary supplies for the crew. The glycerine floated compass
of Moisant must now also be included if we are to contemplate genuine
navigation without constant recourse to landmarks.


Amateur Dirigibles

The French Zodiac types of "aerial runabout" displace 700 cubic
meters, carrying one passenger with coal gas or two passengers with a
mixture of coal gas and hydrogen. The motor is four-cylinder, sixteen
horse-power, water-cooled. The stern screw, of seven feet diameter,
makes 600 turns per minute, giving an independent speed of nineteen
miles per hour. The machine can remain aloft three hours with 165
pounds of supplies. It costs $5000. Hydrogen costs not far from a cent
per cubic foot (twenty cents per cubic meter) so that the question
of gas leakage may be at least as important as the tire question with
automobiles.

  [Illustration: THE ZODIAC NO. 2
  May be deflated and easily transported]


The Fort Omaha Plant

The Signal Corps post at Fort Omaha has a plant comprising a steel
balloon house of size sufficient to house one of the largest dirigibles
built, an electrolytic plant for generating hydrogen gas, having a
capacity of 3000 cubic feet per hour, a 50,000 cubic foot gas storage
tank, and the compressing and carrying equipment involved in preparing
gas for shipment at high pressure in steel cylinders.

  [Illustration: UNITED STATES SIGNAL CORPS BALLOON PLANT AT
  FORT OMAHA, NEB.
  (From the _Transactions_ of the American Society of
  Mechanical Engineers)]


Balloon Progress

  [Illustration: THE "CAROLINE" OF ROBERT BROTHERS, 1784
  The ascent terminated tragically]

The first aerial buoy of Montgolfier brothers, in 1783, led to the
suggestion of Meussier that two envelopes be used; the inner of an
impervious material to prevent gas leakage, and the outer for strength.
There was perhaps a foreshadowing of the Zeppelin idea. Captive and
drifting balloons were used during the wars of the French Revolution:
they became a part of standard equipment in our own War of Secession
and in the Franco-Prussian conflict. The years 1906 to 1908 recorded
rapid progress in the development of the dirigible: the record-breaking
_Zeppelin_ trip was in 1909 and Wellman's _America_ exploit in
October, 1910. Unfortunately, dirigibles have had a bad record for
stanchness: the _Patrie_, _République_, _Zeppelin_ (_I_ and _II_),
_Deutschland_, _Clément-Bayard_--all have gone to that bourne whence no
balloon returns.

  [Illustration: THE ASCENT AT VERSAILLES, 1783
  The first balloon carrying living beings in the air]

  [Illustration: PROPOSED DIRIGIBLE
  Investors were lacking to bring about the realization of this project]

It is gratifying to record that Count Zeppelin's latest machine, the
_Deutschland II_, is now in operation. During the present month (April,
1911), flights have been made covering 90 miles and upward at speeds
exceeding 20 miles per hour with the wind unfavorable. This balloon
is intended for use as a passenger excursion vehicle during the coming
summer, under contract with the municipality of Düsseldorf.

  [Illustration: THE "RÉPUBLIQUE"]

At the present moment, Neale, in England, is reported to be
building a dirigible for a speed of a hundred miles per hour. The
Siemens-Schuckart non-rigid machine, nearly 400 feet long and of 500
horse-power, is being tried out at Berlin: it is said to carry fifty
passengers.[A] Fabrice, of Munich, is experimenting with the _Inchard_,
with a view to crossing the Atlantic at an early date. Mr. Vaniman,
partner of Wellman on the _America_ expedition, is planning a new
dirigible which it is proposed to fly across the ocean before July 4.
The engine, according to press reports, will develop 200 horse-power,
and the envelope will be more elongated than that of the _America_.
And meanwhile a Chicago despatch describes a projected fifty-passenger
machine, to have a gross lifting power of twenty-five tons!

  [Illustration: THE FIRST FLIGHT FOR THE GORDON-BENNET CUP.
  Won by Lieut. Frank P. Lahm, U.S.A., 1906. Figures on the map denote
  distances in kilometers. The cup has been offered annually by Mr.
  James Gordon-Bennet for international competition under such
  conditions as may be prescribed by the International Aeronautic
  Federation.]

Germany has a slight lead in number of dirigible balloons--sixteen
in commission and ten building. France follows closely with fourteen
active and eleven authorized. This accounts for two-thirds of all the
dirigible balloons in the world. Great Britain, Italy, and Russia
rank in the order named. The United States has one balloon of the
smallest size. Spain has, or had, one dirigible. As to aeroplanes,
however, the United States and England rank equally, having each about
one-fourth as many machines as France (which seems, therefore, to
maintain a "four-power standard"). Germany, Russia, and Italy follow,
in order, the United States. These figures include all machines,
whether privately or nationally owned. Until lately, our own government
operated but one aeroplane. A recent appropriation by Congress of
$125,000 has led to arrangements for the purchase of a few additional
biplanes of the Wright and Curtiss types; and a training school for
army officers has been regularly conducted at San Diego, Cal., during
the past winter. The Curtiss machine to be purchased is said to carry
700 pounds of dead weight with a sail area of 500 square feet. It is
completely demountable and equipped with pontoons.




THE QUESTION OF POWER


In the year 1810, a steam engine weighed something over a ton to
the horse-power. This was reduced to about 200 pounds in 1880. The
steam-driven dirigible balloon of Giffard, in 1852, carried a complete
power plant weighing a little over 100 pounds per horse-power; about
the weight of a modern locomotive. The unsuccessful Maxim flying
machine of 1894 brought this weight down to less than 20 pounds. The
gasoline engine on the original Wright machines weighed about 5 pounds
to the horse-power; those on some recent French machines not far from 2
pounds.

Pig iron is worth perhaps a cent a pound. An ordinary steam or gas
engine may cost eight cents a pound; a steam turbine, perhaps forty
cents. A high grade automobile or a piano may sell for a dollar a
pound; the Gnome aeroplane motor is priced at about twenty dollars a
pound. This is considerably more than the price of silver. The motor
and accessories account for from two-thirds to nine-tenths of the total
cost of an aeroplane.

A man weighing 150 pounds can develop at the outside about one-eighth
of a horse-power. It would require 1200 pounds of man to exert one
horse-power. Considered as an engine, then, a man is (weight for
weight) only one six-hundredth as effective as a Gnome motor. In the
original Wright aeroplane, a weight of half a ton was sustained at the
expenditure of about twenty-five horse-power. The motor weight was
about one-eighth of the total weight. If traction had been produced by
man-power, 30,000 pounds of man would have been necessary: thirty times
the whole weight supported.

  [Illustration: THE GNOME MOTOR
  (Aeromotion Company of America)]

Under the most favorable conditions, to support his own weight of
150 pounds (at very high gliding velocity and a slight angle of
inclination, disregarding the weight of sails necessary), a man would
need to have the strength of about fifteen men. No such thing as an
aerial bicycle, therefore, appears possible. The man can not emulate
the bird.

  [Illustration: SCREW PROPELLER (American Propeller Company)]

The power plant of an air craft includes motor, water and water tank,
radiator and piping, shaft and bearings, propeller, controlling wheels
and levers, carbureter, fuel, lubricating oil and tanks therefor. Some
of the weight may eventually be eliminated by employing a two-cycle
motor (which gives more power for its size) or by using rotary
air-cooled cylinders. Propellers are made light by employing wood or
skeleton construction. One eight-foot screw of white oak and spruce,
weighing from twelve to sixteen pounds, is claimed to give over 400
pounds of propelling force at a thousand turns per minute.

  [Illustration: ONE OF THE MOTORS OF THE ZEPPELIN]

The cut shows the action of the so-called "four-cycle" motor. Four
strokes are required to produce an impulse on the piston and return
the parts to their original positions. On the first, or suction
stroke, the combustible mixture is drawn into the cylinder, the inlet
valve being open and the outlet valve closed. On the second stroke,
both valves are closed and the mixture is highly compressed. At about
the end of this stroke, a spark ignites the charge, a still greater
pressure is produced in consequence, and the energy of the gas now
forces the piston outward on its third or "working" stroke, the valves
remaining closed. Finally, the outlet valve is opened and a fourth
stroke sweeps the burnt gas out of the cylinder.

  [Illustration: ACTION OF THE FOUR-CYCLE ENGINE]

In the "two-cycle" engine, the piston first moves to the left,
compressing a charge already present in the cylinder at _F_, and
meanwhile drawing a fresh supply through the valve _A_ and passages
_C_ to the space _D_. On the return stroke, the exploded gas in _F_
expands, doing its work, while that in _D_ is slightly compressed, the
valve _A_ being now closed. When the piston, moving toward the right,
opens the passage _E_, the burnt gas rushes out. A little later, when
the passage _I_ is exposed, the fresh compressed gas in _D_ rushes
through _C_, _B_, and _I_ to _F_. The operation may now be repeated.
Only two strokes have been necessary. The cylinder develops power twice
as rapidly as before: but at the cost of some waste of gas, since the
inlet (_I_) and outlet (_E_) passages are for a brief interval _both
open at once_: a condition not altogether remedied by the use of a
deflector at _G_. A two-cycle cylinder should give nearly twice the
power of a four-cycle cylinder of the same size, and the two-cycle
engine should weigh less, per horse-power; but it requires from 10 to
30% more fuel, and fuel also counts in the total weight.

  [Illustration: ACTION OF TWO-CYCLE ENGINE]

The high temperatures in the cylinder would soon make the cast-iron
walls red-hot, unless the latter where artificially cooled. The
usual method of cooling is to make the walls hollow and circulate
water through them. This involves a pump, a quantity of water, and a
"radiator" (cooling machine) so that the water can be used over and
over again. To cool by air blowing over the surface of the cylinder is
relatively ineffective: but has been made possible in automobiles by
building fins on the cylinders so as to increase the amount of cooling
surface. When the motors are worked at high capacity, or when two-cycle
motors are used, the heat is generated so rapidly that this method of
cooling is regarded as inapplicable. By rapidly rotating the cylinders
themselves through the air, as in motors like the Gnome, air cooling is
made sufficiently adequate, but the expenditure of power in producing
this rotation has perhaps not been sufficiently regarded.

  [Illustration: MOTOR AND PROPELLER
  (Detroit Aeronautic Construction Co.)]

Possible progress in weight economy is destined to be limited by the
necessity for reserve motor equipment.

The engine used is usually the four-cycle, single-acting, four-cylinder
gasoline motor of the automobile, designed for great lightness. The
power from each cylinder of such a motor is approximately that obtained
by dividing the square of the diameter in inches by the figure 2-1/2.
Thus a five-inch cylinder should give ten horse-power--at normal piston
speed. On account of friction losses and the wastefulness of a screw
propeller, not more than half this power is actually available for
propulsion.

The whole power plant of the _Clément-Bayard_ weighed about eleven
pounds to the horse-power. This balloon was 184 feet long and 35
feet in maximum diameter, displacing about 100,000 cubic feet. It
carried six passengers, about seventy gallons of fuel, four gallons of
lubricating oil, fifteen gallons of water, 600 pounds of ballast, and
130 pounds of ropes. The motor developed 100 horse-power at a thousand
revolutions per minute. About eight gallons of fuel and one gallon of
oil were consumed per hour when running at the full independent speed
of thirty-seven miles per hour.

The Wellman balloon _America_ is said to have consumed half a ton of
gasoline per twenty-four hours: an eight days' supply was carried. The
gas leakage in this balloon was estimated to have been equivalent to a
loss of 500 pounds of lifting power per day.

The largest of dirigibles, the _Zeppelin_, had two motors of 170
horse-power each. It made, in 1909, a trip of over 800 miles in
thirty-eight hours.

The engine of the original Voisin cellular biplanes was an
eight-cylinder Antoinette of fifty horse-power, set near the rear
edge of the lower of the main planes. The Wright motors are placed near
the front edge. A twenty-five horse-power motor at 1400 revolutions
propelled the Fort Myer machine, which was built to carry two
passengers, with fuel for a 125 mile flight: the total weight of the
whole flying apparatus being about half a ton.

  [Illustration: TWO-CYLINDER OPPOSED ENGINE.
  (From _Aircraft_)]

  [Illustration: FOUR-CYLINDER VERTICAL ENGINE
  (The Dean Manufacturing Co.)]

The eight-cylinder Antoinette motor on a Farman biplane, weighing 175
pounds, developed thirty-eight horse-power at 1050 revolutions. The
total weight of the machine was nearly 1200 pounds, and its speed
twenty-eight miles per hour.

The eight-cylinder Curtiss motor on the _June Bug_ was air cooled. This
aeroplane weighed 650 pounds and made thirty-nine miles per hour, the
engine developing twenty-five horse-power at 1200 turns.


Resistance of Aeroplanes

The chart on page 24 (see also the diagram of page 23) shows that the
lifting power of an aeroplane increases as the angle of inclination
increases, up to a certain limit. The resistance to propulsion also
increases, however: and the ratio of lifting power to resistance is
greatest at a very small angle--about five or six degrees. Since the
motor power and weight are ruling factors in design, it is important to
fly at about this angle. The supporting force is then about two pounds,
and the resistance about three-tenths of a pound, per square foot of
sail area, if the velocity is that assumed in plotting the chart:
namely, about fifty-five miles per hour.

But the resistance _R_ indicated on pages 23 and 24 is not the
only resistance to propulsion. In addition, we have the frictional
resistance of the air sliding along the sail surface. The amount of
this resistance is independent of the angle of inclination: it depends
directly upon the area of the planes, and in an indirect way on their
dimensions in the direction of movement. It also varies nearly with
the square of the velocity. At any velocity, then, the addition of
this frictional resistance, which does not depend on the angle of
inclination, modifies our views as to the desirable angle: and the
total resistance reaches a minimum (in proportion to the weight
supported) when the angle is about three degrees and the velocity about
fifty miles per hour.

This is not quite the best condition, however. The skin friction does
not vary exactly with the square of the velocity: and when the true law
of variation is taken into account, it is found that the _horse-power_
is a minimum at an angle of about five degrees and a speed of about
forty miles per hour. The weight supported per horse-power may then be
theoretically nearly a hundred pounds: and the frictional resistance is
about one-third the direct pressure resistance. This must be regarded
as the approximate condition of best effectiveness: not the exact
condition, because in arriving at this result we have regarded the
sails as square flat planes whereas in reality they are arched and of
rectangular form.

At the most effective condition, the resistance to propulsion is only
about one-tenth the weight supported. Evidently the air is helping the
motor.


Resistance of Dirigibles

If the bow of a balloon were cut off square, its head end resistance
would be that given by the rule already cited (page 19): one
three-hundredth pound per square foot, multiplied by the square of
the velocity. But by pointing the bow an enormous reduction of this
pressure is possible. If the head end is a hemisphere (as in the
English military dirigible), the reduction is about one-third. If it
is a sharp cone, the reduction may be as much as four-fifths. Unless
the stern is also tapered, however, there will be a considerable eddy
resistance at that point.

  [Illustration: HEAD END SHAPES]

If head end resistance were the only consideration, then for a balloon
of given diameter and end shape it would be independent of the length
and capacity. The longer the balloon, the better. Again, since the
volume of any solid body increases more rapidly than its surface (as
the linear dimensions are increased), large balloons would have a
distinct advantage over small ones. The smallest dirigible ever built
was that of Santos-Dumont, of about 5000 cubic feet.

Large balloons, however, are structurally weak: and more is lost by
the extra bracing necessary than is gained by reduction of head end
resistance. It is probable that the Zeppelin represents the limit of
progress in this direction; and even in that balloon, if it had not
been that the adoption of a rigid type necessitated great structural
strength, it is doubtful if as great a length would have been fixed
upon, in proportion to the diameter.

The frictional resistance of the air gliding along the surface of the
envelope, moreover, invalidates any too arbitrary conclusions. This,
as in the aeroplane, varies nearly as the square of the velocity, and
is usually considerably greater than the direct head end resistance.
Should the steering gear break, however, and the wind strike the _side_
of the balloon, the pressure of the wind against this greatly increased
area would absolutely deprive it of dirigibility.

A stationary, drifting, or "sailing" balloon may as well have the
spherical as well as any other shape: it makes the wind a friend
instead of a foe and requires nothing in the way of control other than
regulation of altitude.


Independent Speed and Time Table

The air pressure, direct and frictional resistances, and power depend
upon the _relative_ velocity of flying machine and air. It is this
relative velocity, not the velocity of the balloon as compared with a
point on the earth's surface, that marks the limit of progression.
Hence the speed of the wind is an overwhelming factor to be reckoned
with in developing an aerial time table. If we wish to travel east at
an effective speed of thirty miles per hour, while the wind is blowing
due west at a speed of ten miles, our machine must have an independent
speed of forty miles. On the other hand, if we wish to travel west, an
independent speed of twenty miles per hour will answer.

  [Illustration: THE SANTOS-DUMONT DIRIGIBLE NO. 2 (1909)]

Again, if the wind is blowing north at thirty miles per hour, and the
minimum (relative) velocity at which an aeroplane will sustain its load
is forty miles per hour, we cannot progress northward any more slowly
than at seventy miles' speed. And we have this peculiar condition of
things: suppose the wind to be blowing north at fifty miles per hour.
The aeroplane designed for a forty mile speed may then face this wind
and sustain itself while actually moving backward at an absolute speed
(as seen from the earth) of ten miles per hour.

We are at the mercy of the wind, and wind velocities may reach a
hundred miles an hour. The inherent disadvantage of aerial flight is
in what engineers call its "low load factor." That is, the ratio of
normal performance required to possible abnormal performance necessary
under adverse conditions is extremely low. To make a balloon truly
dirigible throughout the year involves, at Paris, for example, as
we have seen, a speed exceeding fifty-four miles per hour: and even
then, during one-tenth the year, the _effective_ speed would not
exceed twenty miles per hour. A time table which required a schedule
speed reduction of 60% on one day out of ten would be obviously
unsatisfactory.

  [Illustration: IN THE BAY OF MONACO SANTOS-DUMONT'S NO. 6
  The flights terminated with a fall into the sea,
  happily without injury to the operator]

Further, if we aim at excessively high independent speeds for our
dirigible balloons, in order to become independent of wind conditions,
we soon reach velocities at which the gas bag is unnecessary: that
is, a simple wing surface would at those speeds give ample support.
The increased difficulty of maintaining rigidity of the envelope, and
of steering, at the great pressures which would accompany these high
velocities would also operate against the dirigible type.

With the aeroplane, higher speed means less sail area for a given
weight and a stronger machine. Much higher speeds are probable. We have
already a safe margin as to weight per horse-power of motor, and many
aeroplane motors are for stanchness purposely made heavier than they
absolutely need to be.


The Cost of Speed

Since the whole resistance, in either type of flying machine, is
approximately proportional to the square of the velocity; and since
horse-power (work) is the product of resistance and velocity, the
horse-power of an air craft of any sort varies about as the cube of the
speed. To increase present speeds of dirigible balloons from thirty to
sixty miles per hour would then mean eight times as much horse-power,
eight times as much motor weight, eight times as rapid a rate of fuel
consumption, and (since the speed has been doubled) four times as rapid
a consumption of fuel in proportion to the distance traveled. Either
the radius of action must be decreased, or the weight of fuel carried
must be greatly increased, if higher velocities are to be attained.
Present (independent) aeroplane speeds are usually about fifty miles
per hour, and there is not the necessity for a great increase which
exists with the lighter-than-air machines. We have already succeeded in
carrying and propelling fifty pounds of total load or fifteen pounds
of passenger load per horse-power of motor, with aeroplanes; the ratio
of net load to horse-power in the dirigible is considerably lower; but
the question of weight in relation to power is of relatively smaller
importance in the latter machine, where support is afforded by the gas
and not by the engine.


The Propeller

Very little effort has been made to utilize paddle wheels for aerial
propulsion; the screw is almost universally employed. Every one knows
that when a bolt turns in a stationary nut, it moves forward a distance
equal to the _pitch_ (lengthwise distance between two adjacent threads)
at every revolution. A screw propeller is a bolt partly cut away for
lightness, and the "nut" in which it works is water or air. It does
not move forward quite as much as its pitch, at each revolution,
because any fluid is more or less slippery as compared with a nut of
solid metal. The difference between the pitch and the actual forward
movement of the vessel at each revolution is called the "slip," or
"slip ratio." It is never less than ten or twelve per cent in marine
work, and with aerial screws is much greater. Within certain limits,
the less the slip, the greater the efficiency of the propeller. Small
screws have relatively greater slips and less efficiency, but are
lighter. The maximum efficiency of a screw propeller in water is under
80%. According to Langley's experiments, the usual efficiency in air
is only about 50%. This means that only half the power of the motor
will be actually available for producing forward movement--a conclusion
already foreshadowed.

In common practice, the pitch of aerial screws is not far from equal
to the diameter. The rate of forward movement, if there were no slip,
would be proportional to the pitch and the number of revolutions per
minute. If the latter be increased, the former may be decreased.
Screws direct-connected to the motors and running at high speeds will
therefore be of smaller pitch and diameter than those run at reduced
speed by gearing, as in the machine illustrated on page 134. The number
of blades is usually two, although this gives less perfect balance than
would a larger number. The propeller is in many monoplanes placed in
_front_: this interferes, unfortunately, with the air currents against
the supporting surfaces.

There is always some loss of power in the bearings and
power-transmitting devices between the motor and propeller. This may
decrease the power usefully exerted even to _less_ than half that
developed by the motor.




GETTING UP AND DOWN: MODELS AND GLIDERS: AEROPLANE DETAILS


Launching

The Wright machines (at least in their original form) have usually
been started by the impetus of a falling weight, which propels them
along skids until the velocity suffices to produce ascent. The
preferred designs among French machines have contemplated self-starting
equipment. This involves mounting the machine on pneumatic-tired
bicycle wheels so that it can run along the ground. If a fairly long
stretch of good, wide, straight road is available, it is usually
possible to ascend. The effect of altitude and atmospheric density on
sustaining power is forcibly illustrated by the fact that at Salt Lake
City one of the aviators was unable to rise from the ground.

  [Illustration: WRIGHT BIPLANE ON STARTING RAIL, SHOWING PYLON AND
  WEIGHT]

To accelerate a machine from rest to a given velocity in a given time
or distance involves the use of propulsive force additional to that
necessary to maintain the velocity attained. Apparently, therefore, any
self-starting machine must have not only the extra weight of framework
and wheels but also extra motor power.

  [Illustration: LAUNCHING SYSTEM FOR WRIGHT AEROPLANE
  (From Brewer's _Art of Aviation_)]

Upon closer examination of the matter, we may find a particularly
fortunate condition of things in the aeroplane. Both sustaining power
and resistance vary with the inclination of the planes, as indicated
by the chart on page 24. It is entirely possible to start with no such
inclination, so that the direct wind resistance is eliminated. The
motor must then overcome only air friction, in addition to providing
an accelerating force. The machine runs along the ground, its velocity
rapidly increasing. As soon as the necessary speed (or one somewhat
greater) is attained, the planes are tilted and the aeroplane rises
from the ground.

  [Illustration: THE NIEUPORT MONOPLANE
  Self-Starting with an 18 hp. motor (From _The Air Scout_)]

The velocity necessary to just sustain the load at a given angle of
inclination is called the _critical_ or _soaring_ velocity. For a given
machine, there is an angle of inclination (about half a right angle)
at which the minimum speed is necessary. This speed is called the
"least soaring velocity." If the velocity is now increased, the angle
of inclination may be reduced and the planes will soar through the
air almost edgewise, apparently with diminished resistance and power
consumption. This decrease in power as the speed increases is called
_Langley's Paradox_, from its discoverer, who, however, pointed out
that the rule does not hold in practice when frictional resistances
are included. We cannot expect to actually save power by moving more
rapidly than at present; but we should have to provide much more power
if we tried to move much more slowly.

  [Illustration: A BIPLANE
  (From _Aircraft_)]

  [Illustration: ELY AT LOS ANGELES
  (Photo by American Press Association)]

Economical and practicable starting of an aeroplane thus requires a
free launching space, along which the machine may accelerate with
nearly flat planes: a downward slope would be an aid. When the planes
are tilted for ascent, after attaining full speed, quick control is
necessary to avoid the possibility of a back-somersault. A fairly
wide launching platform of 200 feet length would ordinarily suffice.
The flight made by Ely in January of this year, from San Francisco
to the deck of the cruiser _Pennsylvania_ and back, demonstrated the
possibility of starting from a limited area. The wooden platform built
over the after deck of the warship was 130 feet long, and sloped. On
the return trip, the aeroplane ran down this slope, dropped somewhat,
and then ascended successfully.

If the effort is made to ascend at low velocities, then the motor
power must be sufficient to propel the machine at an extreme angle of
inclination--perhaps the third of a right angle, approximating to the
angle of least velocity for a given load. According to Chatley, this
method of starting by Farman at Issy-les-Moulineaux involved the use of
a motor of fifty horse-power: while Roe's machine at Brooklands rose,
it is said, with only a six horse-power motor.


Descending

  [Illustration: TRAJECTORY DURING DESCENT]

What happens when the motor stops? The velocity of the machine
gradually decreases: the resistance to forward movement stops its
forward movement and the excess of weight over upward pressure due to
velocity causes it to descend. It behaves like a projectile, but the
details of behavior are seriously complicated by the variation in head
resistance and sustaining force due to changes in the angle of the
planes. The "angle of inclination" is now not the angle made by the
planes with the horizontal, but the angle which they make with the path
of flight. Theory indicates that this should be about two-thirds the
angle which the path itself makes with the horizontal: that is, the
planes themselves are inclined downward toward the front. The forces
which determine the descent are fixed by the velocity and the angle
between the planes and the path of flight. Manipulation of the rudders
and main planes or even the motor may be practised to ensure lancing
to best advantage; but in spite of these (or perhaps on account of
these) scarcely any part of aviation offers more dangers, demands more
genius on the part of the operator, and has been less satisfactorily
analyzed than the question of "getting down." It is easy to stay up
and not very hard to "get up," weather conditions being favorable; but
it is an "all-sufficient job" to _come down_. Under the new rules of
the International Aeronautic Federation, a test flight for a pilot's
license must terminate with a descent (motor stopped) in which the
aviator is to land within fifty yards of the observers and come to
a full stop inside of fifty yards therefrom. The elevation at the
beginning of descent must be at least 150 feet.

  [Illustration: DESCENDING]


Gliders

If the motor and its appurtenances, and some of the purely auxiliary
planes, be omitted, we have a _glider_. The glider is not a toy; some
of the most important problems of balancing may perhaps be some day
solved by its aid. Any boy may build one and fly therewith, although
a large kite promises greater interest. The cost is trifling, if the
framework is of bamboo and the surfaces are cotton. Areas of glider
surfaces frequently exceed 100 square feet. This amount of surface
is about right for a person of moderate weight if the machine itself
does not weigh over fifty pounds. By running down a slope, sufficient
velocity may be attained to cause ascent; or in a favorable wind (up
the slope) a considerable backward flight may be experienced. Excessive
heights have led to fatal accidents in gliding experiments.

  [Illustration: THE WITTEMAN GLIDER]


Models

The building of flying models has become of commercial importance.
It is not difficult to attain a high ratio of surface to weight,
but it is almost impossible to get motor power in the small units
necessary without exceeding the permissible limit of motor weight. No
gasoline engine or electric motor can be made sufficiently light for
a toy model. Clockwork springs, if especially designed, may give the
necessary power for short flights, but no better form of power is known
just now than the twisted rubber band. For the small boy, a biplane
with sails about eighteen inches by four feet, eighteen inches apart,
anchored under his shoulders by six-foot cords while he rides his
bicycle, will give no small amount of experience in balancing and will
support enough of a load to make the experiment interesting.


Some Details: Balancing

  [Illustration: FRENCH MONOPLANE
  (From _Aircraft_)]

It is easily possible to compute the areas, angles, and positions
of auxiliary planes to give desired controlling or stabilizing
effects; but the computation involves the use of accurate data as to
positions of the various weights, and on the whole it is simpler to
correct preliminary calculations by actually supporting the machine
at suitable points and observing its balance. Stability is especially
uncertain at very small angles of inclination, and such angles are
to be avoided whether in ordinary operation or in descent. The
necessity for rotating main planes in order to produce ascent is
disadvantageous on this ground; but the proposed use of sliding or
jockey weights for supplementary balancing appears to be open to
objections no less serious. Steering may be perceptibly assisted, in
as delicately a balanced device as the aeroplane, by the inclination
of the body of the operator, just as in a bicycle. The direction of
the wind in relation to the required course may seriously influence
the steering power. Suppose the course to be northeast, the wind east,
the independent speed of the machine and that of the wind being the
same. The car will head due north. By bringing the rudder in position
(_a_), the course may be changed to north, or nearly so, the wind
exerting a powerful pressure on the rudder; but if a more easterly or
east-northeast course be desired, and the rudder be thrown into the
usual position therefor (_b_), it will exert no influence whatever,
because it is moving before the wind and precisely at the speed of the
wind.

  [Illustration: A PROBLEM IN STEERING]

It might be thought that, following analogies of marine engineering,
the center of gravity of an aeroplane should be kept low. The effect
of any unbalanced pressure or force against the widely extended sails
of the machine is to rotate the whole apparatus about its center of
gravity. The further the force from the center of gravity, the
more powerful is the force in producing rotation. The defect in most
aeroplanes (especially biplanes) is that the center of gravity is _too_
low. If it could be made to coincide with the center of disturbing
pressure, there would be no unbalancing effect from the latter. It is
claimed that the steadiest machines are those having a high center of
gravity; and the claim, from these considerations, appears reasonable.

  [Illustration: LEJEUNE BIPLANE (385 LBS., 10-12 HP.)]


Weights

It has been found not difficult to keep down the weight of framework
and supporting surfaces to about a pound per square foot. The most
common ratio of surface to total weight is about one to two: so that
the machinery and operator will require one square foot of surface
for each pound of their weight. On this basis, the smallest possible
man-carrying aeroplane would have a surface scarcely below 250 square
feet. Most biplanes have twice this surface: a thousand square feet
seems to be the limit without structural weakness. Some recent French
machines, designed for high speeds, show a greatly increased ratio
of weight to surface. The _Hanriot_, a monoplane with wings upwardly
inclined toward the outer edge, carries over 800 pounds on less than
300 square feet. The Farman monoplane of only 180 square feet sustains
over 600 pounds. The same aviator's racing biplane is stated to support
nearly 900 pounds on less than 400 square feet.

  [Illustration: THE TELLIER TWO-SEAT SIX-CYLINDER MONOPLANE AT
  THE PARIS SHOW
  One of this type has been sold to the Russian Government
  (From _Aircraft_)]

Motor weights can be brought down to about two pounds per horse-power,
but such extreme lightness is not always needed and may lead to
unreliability of operation. The effect of an accumulation of ice,
sleet, snow, rain, or dew might be serious in connection with flights
in high altitudes or during bad weather. After one of his last year's
flights at Étampes Mr. Farman is said to have descended with an extra
load of nearly 200 pounds on this account. With ample motor power,
great flexibility in weight sustention is made possible by varying the
inclination of the planes. In January of this year, Sommer at Douzy
carried six passengers in a large biplane on a cross-country flight:
and within the week afterward a monoplane operated by Le Martin flew
for five minutes with the aeronaut and seven passengers, at Pau. The
total weight lifted was about half a ton, and some of the passengers
must have been rather light. The two-passenger Fort Myer biplane of
the Wright brothers is understood to have carried about this total
weight. These records have, however, been surpassed since they were
noted. Bréguet, at Douai, in a deeply-arched biplane of new design,
carried eleven passengers, the total load being 2602 pounds, and that
of aeronaut and passengers alone 1390 pounds. The flight was a short
one, at low altitude; but the same aviator last year made a long flight
with five passengers, and carried a load of 1262 pounds at 62 miles per
hour. And as if in reply to this feat, Sommer carried a live load of
1436 pounds (13 passengers) for nearly a mile, a day or two later, at
Mouzon. One feels less certain than formerly, now, in the snap judgment
that the heavier-than-air machine will never develop the capacity for
heavy loads.

  [Illustration: A MONOPLANE
  (From _Aircraft_)]


Miscellaneous

French aviators are fond of employing a carefully designed car for the
operator and control mechanism. The Wright designs practically ignore
the car: the aviator sits on the forward edge of the lower plane with
his legs hanging over.

It has been found that auxiliary planes must not be too close to the
main wings: a gap of a distance about 50% greater than the width of
the widest adjacent plane must be maintained if interference with the
supporting air currents is to be avoided. Main planes are now always
arched; auxiliary planes, not as universally. The concave under surface
of supporting wings has its analogy in the wing of the bird and had
long years since been applied in the parachute.

  [Illustration: CARS AND FRAMEWORK]

The car (if used) and all parts of the framework should be of "wind
splitter" construction, if useless resistance is to be avoided. The
ribs and braces of the frame are of course stronger, weight for weight,
in this shape, since a narrow deep beam is always relatively stronger
than one of square or round section. Excessive frictional resistance is
to be avoided by using a smoothly finished fabric for the wings, and
the method of attaching this fabric to the frame should be one that
keeps it as flat as possible at all joints.

  [Illustration: SOME DETAILS]

The sketches give the novel details of some machines recently exhibited
at the Grand Central Palace in New York. The stabilizing planes were
invariably found in the rear, in all machines exhibited.


The Things to Look After

The operator of an aeroplane has to do the work of at least two men. No
vessel in water would be allowed to attain such speeds as are common
with air craft, unless provided with both pilot and engineer. The
aviator is his own pilot and his own engineer. He must both manage his
propelling machinery and steer. Separate control for vertical rudders,
elevating rudders and ailerons, for starting the engine; the adjustment
of the carbureter, the spark, and the throttle to get the best results
from the motor; attention to lubrication and constant watchfulness of
the water-circulating system: these are a few of the things for him to
consider; to say nothing of the laying of his course and the necessary
anticipation of wind and altitude conditions.

These things demand great resourcefulness, but--for their best
control--involve also no small amount of scientific knowledge. For
example, certain adjustments at the motor may considerably increase its
power, a possibly necessary increase under critical conditions: but if
such adjustments also decrease the motor efficiency there must be a
nice analysis of the two effects so that extra power may not be gained
at too great a cost in radius of action.

  [Illustration: SOME RECENT FRENCH MACHINES (From _Aircraft_)]

The whole matter of flight involves both sportsman's and engineers
problems. Wind gusts produce the same effects as "turning corners";
or worse--rapidly changing the whole balance of the machines and
requiring immediate action at two or three points of control. Both
ascent and descent are influenced by complicated laws and are scarcely
rendered safe--under present conditions--by the most ample experience.
A lateral air current bewilders the steering and also demands special
promptness and skill. To avoid disturbing surface winds, even over open
country, a minimum flying height of 300 feet is considered necessary.
This height, furthermore, gives more choice in the matter of landing
ground than a lower elevation.

When complete and automatic balance shall have been attained--as it
must be attained--we may expect to see small amateur aeroplanes flying
along country roads at low elevations--perhaps with a guiding wheel
actually in contact with the ground. They will cost far less than even
a small automobile, and the expense for upkeep will be infinitely less.
The grasshopper will have become a water-spider.




SOME AEROPLANES--SOME ACCOMPLISHMENTS


  [Illustration: ORVILLE WRIGHT AT FORT MYER, VA., 1908]

  [Illustration: THE FIRST BALLOON FLIGHT ACROSS THE BRITISH CHANNEL
  More than a century before Blériot's feat, Blanchard crossed from
  Dover to Calais]

The Wright biplane has already been shown (see pages 31, 37, 121, 122).
It was distinguished by the absence of a wheel frame or car and by the
wing-warping method of stabilizing. Later Wright machines have the
spring frame and wheels for self-starting. The best known aeroplane of
this design was built to meet specifications of the United States
Signal Corps issued in 1907. It was tried out during 1908 at Fort
Myer, Va., while one of the Wright brothers was breaking all records
in Europe: making over a hundred flights in all, first carrying a
passenger and attaining the then highest altitude (360 feet) and
greatest distance of flight (seventy-seven miles).

  [Illustration: WRIGHT MOTOR. DIMENSIONS IN MILLIMETERS
  (From Petit's _How to Build an Aeroplane_)]

The ownership of the Wrights in the wing-warping method of control is
still the subject of litigation. The French infringers, it is stated,
concede priority of application to the Wright firm, but maintain that
such publicity was given the device that it was in general use before
it was patented.

The Fort Myer machine had sails of forty feet spread, six and one-half
feet deep, with front elevating planes three by sixteen feet. It made
about forty miles per hour with two passengers. The apparatus was
specified to carry a passenger weight of 350 pounds, with fuel for a
125-mile flight. The main planes were six feet apart. The steering
rudder (double) was of planes one foot deep and nearly six feet high.
The four-cylinder-four-cycle, water-cooled motor developed twenty-five
horse-power at 1400 revolutions. The two propellers, eight and one-half
feet in diameter, made 400 revolutions.

The flight by Mr. Wilbur Wright from the Statue of Liberty to the tomb
of General Grant, in New York, 1909, and the exploits of his brother
in the same year, when a new altitude record of 1600 feet was made
and H.R.H. the Crown Prince of Germany was taken up as a passenger,
are only specimens of the later work done by these pioneers in aerial
navigation.

Like the Wrights, the Voisin firm from the beginning adhered firmly to
the biplane type of machine. The sketch gives dimensions of one of the
early cellular forms built for H. Farman (see illustration, page 147).
The metal screw makes about a thousand revolutions. The wings are of
india rubber sheeting on an ash frame, the whole frame and car body
being of wood, the latter covered with canvas and thirty inches wide by
ten feet long. The engine weighed 175 pounds. The whole weight of this
machine was nearly 1200 pounds; that built later for Delagrange was
brought under a thousand pounds. The ratio of weight to main surface in
the Farman aeroplane was about 2-3/4 to 1.

A modified cellular biplane also built for Farman had a main wing area
of 560 square feet, the planes being seventy-nine inches wide and only
fifty-nine inches apart. The tail was an open box, seventy-nine inches
wide and of about ten feet spread. The cellular partitions in this tail
were pivoted along the vertical front edges so as to serve as steering
rudders. The elevating rudder was in front. The total weight was about
the same as that of the first machine and the usual speed twenty-eight
miles per hour.

  [Illustration: VOISIN-FARMAN BIPLANE]

Henry Farman has been flying publicly since 1907. He made the first
circular flight of one kilometer, and attained a speed of about a mile
a minute, in the year following. In 1909 he accomplished a trip of
nearly 150 miles, remaining four hours in the air. Farman was probably
the first man to ascend with two passengers.

  [Illustration: THE CHAMPAGNE GRAND PRIZE WON BY HENRY FARMAN
  80 Kilometers in 3 hours]

  [Illustration: FARMAN'S FIRST BIPLANE AT ISSY-LES-MOULINEAUX
  Returning to the Hangar After a Flight]

The _June Bug_, one of the first Curtiss machines, is shown below. This
was one of the lightest of biplanes, having a wing spread of forty-two
feet and an area of 370 square feet. The wings were transversely
arched, being furthest apart at the center: an arrangement which has
not been continued. It had a box tail, with a steering rudder of about
six square feet area, _above_ the tail. The horizontal rudder, in
front, had a surface of twenty square feet. Four triangular ailerons
were used for stability. The machine had a landing frame and wheels,
made about forty miles per hour, and weighed, in operation, 650 pounds.

  [Illustration: THE "JUNE BUG"]

Mr. Curtiss first attained prominence in aviation circles by winning
the _Scientific American_ cup by his flight at the speed of fifty-seven
miles per hour, in 1908. In the following year he exhibited intricate
curved flights at Mineola, and circled Governor's Island in New York
harbor. In 1910 he made his famous flight from Albany to New York,
stopping _en route_, as prearranged. At Atlantic City he flew fifty
miles over salt water. A flight of seventy miles over Lake Erie was
accomplished in September of the same year, the return trip being
made the following day. On January 26, 1911, Curtiss repeatedly
ascended and descended, with the aid of hydroplanes, in San Diego bay,
California: perhaps one of the most important of recent achievements.
It is understood that Mr. Curtiss is now attempting to duplicate some
of these performances under the high-altitude conditions of Great
Salt Lake. According to press reports, he has been invited to give a
similar demonstration before the German naval authorities at Kiel.

  [Illustration: CURTIS BIPLANE
  (Photo by Levick, N.Y.)]

  [Illustration: CURTISS' HYDRO-AEROPLANE AT SAN DIEGO GETTING UNDER WAY
  (From the _Columbian Magazine_)]

The _aeroscaphe_ of Ravard was a machine designed to move either on
water or in air. It was an aeroplane with pontoons or floaters. The
supporting surface aggregated 400 square feet, and the gross weight
was about 1100 pounds. A fifty horse-power Gnome seven-cylinder motor
at 1200 revolutions drove two propellers of eight and ten and one-half
feet diameter respectively: the propellers being mounted one behind
the other on the same shaft.

  [Illustration: FLYING OVER THE WATER AT FIFTY MILES PER HOUR
  Curtiss at San Diego Bay
  (From the _Columbian Magazine_)]

Ely's great shore-to-warship flight was made without the aid of the
pontoons which he carried. Ropes were stretched across the landing
platform, running over sheaves and made fast to heavy sand bags. As a
further precaution, a canvas barrier was stretched across the forward
end of the platform. The descent brought the machine to the platform at
a distance of forty feet from the upper end: grappling hooks hanging
from the framework of the aeroplane then caught the weighted ropes, and
the speed was checked (within about sixty feet) so gradually that "not
a wire or bolt of the biplane was injured."

  [Illustration: BLÉRIOT-VOISIN CELLULAR BIPLANE WITH PONTOONS
  Hauled by a Motor Boat]

  [Illustration: LATHAM'S "ANTOINETTE"]

  [Illustration: JAMES J. WARD AT LEWISTON FAIR, IDAHO
  (Photo copyright 1910 by Burns)
  Flying Machine Mfg. Co. Biplane (30 hp. Motor)]

  [Illustration: MARCEL PENOT IN THE MOHAWK BIPLANE,
  Mineola to Hicksville, L. I.
  26 miles cross-country in 30 minutes (50 hp. Harriman Engine)]

Recent combinations of aeroplane and automobile, and aeroplane with
motor boat, have been exhibited. One of the latter devices is like any
monoplane, except that the lower part is a water-tight aluminum boat
body carrying three passengers. It is expected to start of itself from
the water and to fly at a low height like a flying fish at a speed of
about seventy-five miles per hour. Should anything go wrong, it is
capable of floating on the water.

In the San Diego Curtiss flights, the machine skimmed along the surface
of the bay, then rose to a height of a hundred feet, moved about two
miles through the air in a circular course, and finally alighted
close to its starting-point in the water. Turns were made in water as
well as in air, a speed of forty miles per hour being attained while
"skimming." The "hydroplanes" used are rigid flat surfaces which
utilize the pressure of the water for sustention, just as the main
wings utilize air pressure. On account of the great density of water,
no great amount of surface is required: but it must be so distributed
as to balance the machine. The use of pontoons makes it possible to
rest upon the water and to start from rest. A trip like Ely's could
be made without a landing platform, with this type of machine; the
aeroplane could either remain alongside the war vessel or be hoisted
aboard until ready to venture away again.

There are various other biplanes attracting public attention in this
country. In France the tendency is all toward the monoplane form, and
many of the "records" have, during the past couple of years, passed
from the former to the latter type of machine. The monoplane is simpler
and usually cheaper. The biplane may be designed for greater economy
in weight and power. Farman has lately experimented with the monoplane
type of machine: the large number of French designs in this class
discourages any attempt at complete description.

  [Illustration: SANTOS-DUMONT'S "DEMOISELLE"]

The smallest of aeroplanes is the Santos-Dumont _Demoiselle_. The
original machine is said to have supported 260 pounds on 100 square
feet of area, making a speed of sixty miles per hour. Its proprietor
was the first aviator in Europe of the heavier-than-air class. After
having done pioneer work with dirigible balloons, he won the Deutsch
prize for a hundred meter aeroplane flight (the first outside of the
United States) in 1906; the speed being twenty-three miles per hour.
His first flight, of 400 feet, in a monoplane was made in 1907.

  [Illustration: BLÉRIOT MONOPLANE]

The master of the monoplane has been Louis Blériot. Starting in
1907 with short flights in a Langley type of machine, he made his
celebrated cross-country run, and the first circling flights ever
achieved in a monoplane, the following year. On July 25, 1909, he
crossed the British Channel, thirty-two miles, in thirty-seven minutes.

  [Illustration: LATHAM'S FALL INTO THE CHANNEL]

The Channel crossing has become a favorite feat. Mr. Latham, only two
days after Blériot, all but completed it in his Antoinette monoplane.
De Lesseps, in a Blériot machine, was more fortunate. Sopwith, last
year, won the de Forest prize of $20,000 by a flight of 174 miles
from England into Belgium. The ill-fated Rolls made the round trip
between England and France. Grace, contesting for the same prize,
reached Belgium, was driven back to Calais, started on the return
voyage, and vanished--all save some few doubtful relics lately found.
Moisant reached London from Paris--the first trip on record between
these cities without change of conveyance: and one which has just been
duplicated by Pierre Prier, who, on April 12, made the London to Paris
journey, 290 miles, in 236 minutes, without a stop. This does not,
however, make the record for a continuous flight: which was attained by
Tabuteau, who at Buc, on Dec. 30, 1910, flew around the aerodrome for
465 minutes at the speed of 48-1/2 miles per hour.

Other famous crossings include those of the Irish Sea, 52 miles, by
Loraine; Long Island Sound, 25 miles, by Harmon; and Lake Geneva, 40
miles, by Defaux.

It was just about a century ago that Cayley first described a soaring
machine, heavier than air, of a form remarkably similar to that of the
modern aeroplane. Aside from Henson's unsuccessful attempt to build
such a machine, in 1842, and Wenham's first gliding experiments with a
triplane in 1857, soaring flight made no real progress until Langley's
experiments. That investigator, with Maxim and others, ascertained
those laws of aerial sustention the application of which led to success
in 1903.

  [Illustration: DE LESSEPS IN A BLÉRIOT CROSSING THE CHANNEL
  (Photo by Levick, N.Y.)]

The eight years since have held the crowded hours of aviation. Before
this book is printed, it may be rendered obsolete by new developments.
The exploits of Paulhan, of R. E. Peltèrie since 1907, Bell's work with
his tetrahedral kites--all have been either stimulating or directly
fruitful. Delagrange began to break speed records in 1908. A year
later he attained a speed of fifty miles. The first woman to enjoy an
aeroplane voyage was Mme. Delagrange, in Turin, in 1908.

  [Illustration: THE MAXIM AEROPLANE]

  [Illustration: LANGLEY'S AEROPLANE (1896)
  Steam driven]

The first flight in England by an English-built machine was made in
January, 1909. That year, Count de Lambert flew over Paris, and in
1910 Grahame-White circled his machine over the city of Boston. The
year 1910 surpassed all its predecessors in increasing the range and
control of aeroplanes; over 1500 ascents were made by Wright machines
alone; but 1911 promises to show even greater results. Three men made
cross-country flights from Belmont Park to the Statue of Liberty and
back, in New York;[B] at least five men attained altitudes exceeding
9,000 feet. Hamilton made the run from New York to Philadelphia and
return, in June. The unfortunate Chavez all but abolished the fames of
Hannibal and Napoleon by crossing the icy barrier of the Alps, from
Switzerland to Italy--in forty minutes!

  [Illustration: ROBART MONOPLANE.]

Tabuteau, almost on New Year's eve, broke all distance records by a
flight of 363 miles in less than eight hours; while Barrier at Memphis
probably reached a speed of eighty-eight miles per hour (timing
unofficial). With the new year came reports of inconceivable speeds by
a machine skidding along the ice of Lake Erie; the successful receipt
by Willard and McCurdy of wireless messages from the earth to their
aeroplanes; and the proposal by the United States Signal Corps for the
use of flying machines for carrying Alaskan mails.

  [Illustration: VINA MONOPLANE.]

McCurdy all but succeeded in his attempt to fly from Key West to
Havana, surpassing previous records by remaining aloft above salt water
while traveling eighty miles. Lieutenant Bague, in March, started from
Antibes, near Nice, for Corsica. After a 124-mile flight, breaking all
records for sea journeys by air, he reached the islet of Gorgona, near
Leghorn, Italy, landing on bad ground and badly damaging his machine.
The time of flight was 5-1/2 hours. Bellinger completed the 500-mile
"accommodation train" flight from Vincennes to Pau; Vedrine, on April
12, by making the same journey in 415 minutes of actual flying time,
won the Béarn prize of $4000; Say attained a speed of 74 miles per hour
in circular flights at Issy-les-Moulineaux. Aeroplane flights have been
made in Japan, India, Peru, and China.

One of the most spectacular of recent achievements is that of Renaux,
competing for the Michelin Grand Prize. A purse of $20,000 was offered
in 1909 by M. Michelin, the French tire manufacturer, for the first
successful flight from Paris to Clermont-Ferrand--260 miles--in less
than six hours. The prize was to stand for ten years. It was prescribed
that the aviator must, at the end of the journey, circle the tower of
the Cathedral and alight on the summit of the Puy de Dome--elevation
4500 feet--on a landing place measuring only 40 by 100 yards,
surrounded by broken and rugged ground and usually obscured by fog.

The flight was attempted last year by Weymann, who fell short of the
goal by only a few miles. Leon Morane met with a serious accident,
a little later, while attempting the trip with his brother as a
passenger. Renaux completed the journey with ease in his Farman
biplane, carrying a passenger, his time being 308 minutes.

This Michelin Grand Prize is not to be confused with the Michelin
Trophy of $4000 offered yearly for the longest flight in a closed
circuit.

Speeds have increased 50% during the past year; even with passengers,
machines have moved more than a mile a minute: average motor capacities
have been doubled or tripled. The French men and machines hold the
records for speed, duration, distance, and (perhaps) altitude. The
highest altitude claimed is probably that attained by Garros at Mexico
City, early this year--12,052 feet above sea level. The world's speed
record for a two-man flight appears to be that of Foulois and Parmalee,
made at Laredo, Texas, March 3, 1911: 106 miles, cross-country, in 127
minutes. Three-fourths of all flights made up to this time have been
made in France--a fair proportion, however, in American machines.


NOTE

The rapidity with which history is made in aeronautics is forcibly
suggested by the revision of text made necessary by recent news.
The new _Deutschland_ has met the fate of its predecessors; the
Paris-Rome-Turin flight is at this moment under way; and Lieutenant
Bayne, attempting once more his France-to-Corsica flight, has--for the
time being at least--disappeared.




THE POSSIBILITIES IN AVIATION


Men now fly and will probably keep on flying; but aviation is still
too hazardous to become the popular sport of the average man. The
overwhelmingly important problem with the aeroplane is that of
stability. These machines must have a better lateral balance when
turning corners or when subjected to wind gusts: and the balance must
be automatically, not manually, produced.

  [Illustration: BLANC MONOPLANE]

Other necessary improvements are of minor urgency and in some cases
will be easy to accomplish. Better mechanical construction, especially
in the details of attachments, needs only persistence and common sense.
Structural strength will be increased; the wide spread of wing presents
difficulties here, which may be solved either by increasing the number
of superimposed surfaces, as in triplanes, or in some other manner.
Greater carrying capacity--two men instead of one--may be insisted
upon: and this leads to the difficult question of motor weights.
The revolving air-cooled motor may offer further possibilities: the
two-cycle idea will help if a short radius of action is permissible:
but a weight of less than two pounds to the horse-power seems
to imply, almost essentially, a lack of ruggedness and surety of
operation. A promising field for investigation is in the direction of
increasing propeller efficiencies. If such an increase can be effected,
the whole of the power difficulty will be greatly simplified.

  [Illustration: MELVIN VANIMAN TRIPLANE]

  [Illustration: JEAN DE CRAWHEZ TRIPLANE]

  [Illustration: A TRIPLANE]

This same motor question controls the proposal for increased speed.
The use of a reserve motor would again increase weights; though not
necessarily in proportion to the aggregate engine capacity. Perhaps
something may be accomplished with a gasoline turbine, when one
is developed. In any case, no sudden increase in speeds seems to
be probable; any further lightening of motors must be undertaken
with deliberation and science. If much higher maximum speeds are
attained, there will be an opportunity to vary the speed to suit the
requirements. Then clutches, gears, brakes, and speed-changing devices
of various sorts will become necessary, and the problem of weights of
journal bearings--already no small matter--will be made still more
serious. And with variable speed must probably come variable sail
area--in preference to tilting--so that the fabric must be reefed on
its frame. Certainly two men, it would seem, will be needed!

Better methods for starting are required. The hydroplane idea promises
much in this respect. With a better understanding and control of the
conditions associated with successful and safe descent--perhaps with
improved appliances therefor--the problem of ascent will also be partly
solved. If such result can be achieved, these measures of control must
be made automatic.

The building of complete aeroplanes to standard designs would be
extremely profitable at present prices, which range from $2500 to
$5000. Perhaps the most profitable part would be in the building of the
motor. The framing and fabric of an ordinary monoplane could easily
be constructed at a cost below $300. The propeller may cost $50 more.
The expense for wires, ropes, etc., is trifling; and unless special
scientific instruments and accessories are required, all of the rest
of the value lies in the motor and its accessories. Within reasonable
limits, present costs of motors vary about with the horse-power. The
amateur designer must therefore be careful to keep down weight and
power unless he proposes to spend money quite freely.


The Case of the Dirigible

Not very much is being heard of performances of dirigible balloons just
at present. They have shown themselves to be lacking in stanchness
and effectiveness under reasonable variations of weather. We must
have fabrics that are stronger for their weight and more impervious.
Envelopes must be so built structurally as to resist deformation at
high speeds, without having any greatly increased weight. A cheap way
of preparing pure hydrogen gas is to be desired.

Most important of all, the balloon must have a higher speed, to make it
truly dirigible. This, with sufficient steering power, will protect it
against the destructive accidents that have terminated so many balloon
careers. Here again arises the whole question of power in relation
to motor weight, though not as formidably as is the case with the
aeroplane. The required higher speeds are possible now, at the cost
merely of careful structural design, reduced radius of action, and
reduced passenger carrying capacity.

Better altitude control will be attained with better fabrics and
the use of plane fin surfaces at high speeds. The employment of a
vertically-acting propeller as a somewhat wasteful but perhaps finally
necessary measure of safety may also be regarded as probable.

  [Illustration: GIRAUDON'S WHEEL AEROPLANE]


The Orthopter

The _aviplane_, _ornithoptère_ or _orthopter_ is a flying machine with
bird-like flapping wings, which has received occasional attention
from time to time, as the result of a too blind adherence to Nature's
analogies. Every mechanical principle is in favor of the screw as
compared with any reciprocating method of propulsion. There have been
few actual examples of this type: a model was exhibited at the Grand
Central Palace in New York in January of this year.

The mechanism of an orthopter would be relatively complex, and the
flapping wings would have to "feather" on their return stroke. The
flapping speed would have to be very high or the surface area very
great. This last requirement would lead to structural difficulties.
Propulsion would not be uniform, unless additional complications were
introduced. The machine would be the most difficult of any type to
balance. The motion of a bird's wing is extremely complicated in its
details--one that it would be as difficult to imitate in a mechanical
device as it would be for us to obtain the structural strength of an
eagle's wing in fabric and metal, with anything like the same extent of
surface and limit of weight. According to Pettigrew, the efficiency of
bird and insect flight depends largely upon the elasticity of the wing.
Chatley gives the ratio of area to weight as varying from fifty (gnat)
to one-half (Australian crane) square feet per pound. The usual ratio
in aeroplanes is from one-third to one-half.

About the only advantages perceptible with the orthopter type of
machine would be, first, the ability "to start from rest without
a preliminary surface glide"; and second, more independence of
irregularity in air currents, since the propulsive force is exerted
over a greater extent than is that of a screw propeller.


The Helicopter

The _gyroplane_ or _helicopter_ was the type of flying machine
regarded by Lord Kelvin as alone likely to survive. It lifts itself
by screw propellers acting vertically. This form was suggested in
1852. When only a single screw was used, the whole machine rotated
about its vertical axis. It was attempted to offset this by the use
of vertical fin-planes: but these led to instability in the presence
of irregular air currents. One early form had two oppositely-pitched
screws driven by a complete steam engine and boiler plant. One of the
Cornu helicopters had adjustable inclined planes under the two large
vertically propelling screws. The air which slipped past the screws
imposed a pressure on the inclined planes which was utilized to produce
horizontal movement in any desired direction--if the wind was not too
adverse. A gasoline engine was carried in a sort of well between the
screws.

  [Illustration: BRÉGUET GYROPLANE DURING CONSTRUCTION
  (Helicopter type)]

The helicopter may be regarded as the limiting type of aeroplane, the
sail area being reduced nearly to zero; the wings becoming mere fins,
the smaller the better. It therefore requires maximum motor power and
is particularly dependent upon the development of an excessively light
motor. It is launched and descends under perfect control, without
regard to horizontal velocity. It has very little exposed surface and
is therefore both easy to steer and independent of wind conditions. By
properly arranging the screws it can be amply balanced: but it must
have a particularly stout and strong frame.

The development of this machine hinges largely on the propeller. It is
not only necessary to develop _power_ (which means force multiplied by
velocity) but actual propulsive vertical _force_: and this must exceed
or at least equal the whole weight of the machine. From ten to forty
pounds of lifting force per horse-power have been actually attained:
and with motors weighing less than five pounds there is evidently some
margin. The propellers are of special design, usually with very large
blades. Four are commonly used: one, so to speak, at each "corner" of
the machine. The helicopter is absolutely dependent upon its motors.
It cannot descend safely if the power fails. If it is to do anything
but ascend and descend it must have additional propulsive machinery for
producing horizontal movement.


Composite Types

The aeroplane is thus particularly weak as to stability, launching,
and descending: but it is economical in power because it uses the
air to hold itself up. The dirigible balloon is lacking in power and
speed, but can ascend and descend safely, even if only by wasteful
methods; and it can carry heavy weights, which are impossible with the
structurally fragile aeroplane. The helicopter is wasteful in power,
but is stable and sure in ascending and descending, providing only that
the motor power does not fail.

Why, then, not combine the types? An aeroplane-dirigible would
be open to only one objection: on the ground of stability. The
dirigible-helicopter would have as its only disadvantage a certain
wastefulness of power, while the aeroplane-helicopter would seem to
have no drawback whatever.

All three combinations have been, or are being, tried. An Italian
engineer officer has designed a balloon-aeroplane. The balloon is
greatly flattened, or lens-shaped, and floats on its side, presenting
its edge to the horizon--if inclination be disregarded. With some
inclination, the machine acts like an aeroplane and is partially
self-sustaining at any reasonable velocity.

The use of a vertically-acting screw on a dirigible combines the
features of that type and the helicopter. This arrangement has also
been the subject of design (as in Captain Miller's flexible balloon)
if not of construction. The combination of helicopter and aeroplane
seems especially promising: the vertical propellers being employed for
starting and descending, as an emergency safety feature and perhaps for
aid in stabilizing. The fact that composite types of flying machine
have been suggested is perhaps, however, an indication that the
ultimate type has not yet been established.


What is Promised

The flying machine will probably become the vehicle of the explorer.
If Stanley had been able to use a small high-powered dirigible in
the search for Livingstone, the journey would have been one of hours
as compared with months, the food and general comfort of the party
would have been equal in quality to those attainable at home, and the
expense in money and in human life would have been relatively trifling.

  [Illustration: WELLMAN'S AMERICA
  (From Wellman's _Aerial Age_)]

Most readers will remember the fate of Andrée, and the projected polar
expeditions of Wellman in 1907 and 1909. Misfortune accompanied both
attempts; but one has only to read Peary's story of the dogged tramp
over the Greenland ice blink to realize that danger and misfortune
in no less degree have accompanied other plans of Arctic pioneering.
With proper design and the right men, it does not seem unreasonable to
expect that a hundred flying machines may soar above Earth's invisible
axial points during the next dozen years.[C]

The report of Count Zeppelin's Spitzbergen expedition of last year
has just been made public. This was undertaken to ascertain the
adaptability of flying machines for Arctic navigation. Besides speed
and radius of action, the conclusive factors include that of freedom
from such breakdowns as cannot be made good on the road.

For exploration in other regions, the balloon or the aeroplane is
sure to be employed. Rapidity of progress without fatigue or danger
will replace the floundering through swamps, shivering with ague, and
bickering with hostile natives now associated with tropical and other
expeditions. The stereoscopic camera with its scientific adjuncts will
permit of almost automatic map-making, more comprehensive and accurate
than any now attempted in other than the most settled sections. It
is not too much to expect that arrangements will be perfected for
conducting complete topographical surveys without more than occasional
descents. If extremely high altitudes must be attained--over a
mile--the machines will be of special design; but as far as can now be
anticipated, there will be no insurmountable difficulties. The virgin
peaks of Ruwenzori and the Himalayas may become easily accessible--even
to women and children if they desire it. We may obtain direct evidence
as to the contested ascent of Mt. McKinley. A report has been current
that a Blériot monoplane has been purchased for use in the inspection
of construction work for an oil pipe line across the Persian desert;
the aeroplane being regarded as "more expeditious and effectual" than
an automobile.

The flying machine is the only land vehicle which requires no
"permanent way." Trains must have rails, bicycles and automobiles
must have good roads. Even the pedestrian gets along better on a
path. The ships of the air and the sea demand no improvement of the
fluids in which they float. To carry mails, parcels, persons, and even
light freight--these applications, if made commercially practicable
tomorrow,[D] would surprise no one; their possibility has already
been amply demonstrated. With the dirigible as the transatlantic
liner and the aeroplane as the naphtha launch of the air, the whole
range of applications is commanded. Hangars and landing stages--the
latter perhaps on the roofs of buildings, revolutionizing our domestic
architecture--may spring up as rapidly as garages have done. And the
aeroplane is potentially (with the exception of the motorcycle) the
cheapest of self-propelled vehicles.

Governments have already considered the possibilities of aerial
smuggling. Perhaps our custom-house officers will soon have to watch a
fence instead of a line: to barricade in two dimensions instead of one.
They will need to be provided with United States Revenue aeroplanes.
But how are aerial frontiers to be marked? And does a nation own the
air above it, or is this, like the high seas, "by natural right, common
to all"? Can a flying-machine blockade-runner above the three-mile
height claim extraterritoriality?

The flying machine is no longer the delusion of the "crank," because
it has developed a great industry. A now antiquated statement put
the capitalization of aeroplane manufactories in France at a million
dollars, and the development expenditure to date at six millions.
There are dozens of builders, in New York City alone, of monoplanes,
biplanes, gliders, and models. A permanent exhibition of air craft is
just being inaugurated. We have now even an aeronautic "trust," since
the million-dollar capitalization of the Maxim, Blériot, Grahame-White
firm.

According to the New York _Sun_, over $500,000 has been subscribed for
aviation prizes in 1911. The most valuable prizes are for new records
in cross-country flights. The Paris _Journal_ has offered $70,000 for
the best speed in a circling race from Paris to Berlin, Brussels,
London, and back to Paris--1500 miles. Supplementary prizes from other
sources have increased the total stake in this race to $100,000. A
purse of $50,000 is offered by the London _Daily Mail_ for the "Circuit
of Britain" race, from London up the east coast to Edinburgh, across
to Glasgow, and home by way of the west coast, Exeter, and the Isle of
Wight; a thousand miles, to be completed in two weeks, beginning July
22, with descents only at predetermined points. This contest will be
open (at an entrance fee of $500) to any licensee of the International
Federation. A German circuit, from Berlin to Bremen, Magdeburg,
Düsseldorf, Aix-la-Chapelle, Dresden, and back to the starting point,
is proposed by the _Zeitung am Mittag_ of Berlin, a prize of $25,000
having been offered. In this country, a comparatively small prize
has been established for a run from San Francisco to New York, _via_
Chicago. Besides a meet at Bridgeport, May 18-20, together with those
to be held by several of the colleges and the ones at Bennings and
Chicago, there will be, it is still hoped, a national tournament at
Belmont Park at the end of the same month. Here probably a dozen
aviators will contest in qualification for the international meet in
England, to which three American representatives should be sent as
competitors for the championship trophy now held by Mr. Grahame-White.
It is anticipated that the chances in the international races favor
the French aviators, some of whom--in particular, Leblanc--have been
making sensational records at Pau. Flights between aviation fields in
different cities are the leading feature in the American program for
the year. A trip is proposed from Washington to Belmont Park, _via_
Atlantic City, the New Jersey coast, and lower New York bay. The
distance is 250 miles and the time will probably be less than that of
the best passenger trains between Washington and New York. If held,
this race will probably take place late in May. It is wisely concluded
that the advancement of aviation depends upon cross-country runs under
good control and at reasonable speeds and heights rather than upon
exhibition flights in enclosures. It is to be hoped that commercial
interests will not be sufficiently powerful to hinder this development.

We shall of course have the usual international championship balloon
race, preceded by elimination contests. From present indications Omaha
is likely to be chosen as the point of departure.

The need for scientific study of aerial problems is recognized. The
sum of $350,000 has been offered the University of Paris to found an
aeronautic institute. In Germany, the university at Göttingen has for
years maintained an aerodynamic laboratory. Lord Rayleigh, in England,
is at the head of a committee of ten eminent scientists and engineers
which has, under the authority of Parliament, prepared a program of
necessary theoretical and experimental investigations in aerostatics
and aerodynamics. Our American colleges have organized student aviation
societies and in some of them systematic instruction is given in the
principles underlying the art. A permanent aeronautic laboratory, to be
located at Washington, D.C., is being promoted.

Aviation as a sport is under the control of the International
Aeronautic Federation, having its headquarters at Paris. Bodies
like the Royal Aero Club of England and the Aero Club of America
are subsidiaries to the Federation. In addition, we have in this
country other clubs, like the Aeronautic Society, the United States
Aeronautical Reserve, etc. The National Council of the Aero Clubs of
America is a sort of supreme court for all of these, having control of
meets and contests; but it has no affiliation with the International
body, which is represented here by the Aero Club of America. The
Canadian Auto and Aero Club supervises aviation in the Dominion.

Aviation has developed new legal problems: problems of liability for
accidents to others; the matter of supervision of airship operators.
Bills to license and regulate air craft have been introduced in at
least two state legislatures.

Schools for instruction in flying as an art or sport are being
promoted. It is understood that the Wright firm is prepared to organize
classes of about a dozen men, supplying an aeroplane for their
instruction. Each man pays a small fee, which is remitted should he
afterward purchase a machine. Mr. Grahame-White, at Pau, in the south
of France, conducts a school of aviation, and the arrangements are now
being duplicated in England. Instruction is given on Blériot monoplanes
and Farman biplanes, at a cost of a hundred guineas for either. The
pupil is coached until he can make a three-mile flight; meanwhile, he
is held partially responsible for damage and is required to take out a
"third-party" insurance policy.

There is no lack of aeronautic literature. Major Squier's paper in the
_Transactions_ of the American Society of Mechanical Engineers, 1908,
gave an eighteen-page list of books and magazine articles of fair
completeness up to its date; Professor Chatley's book, _Aeroplanes_,
1911, discusses some recent publications; the Brooklyn Public Library
in New York issued in 1910 (misdated 1909) a manual of fourteen pages
critically referring to the then available literature, and itself
containing a list of some dozen bibliographies.


Aerial Warfare

  [Illustration: THE GERMAN EMPEROR WATCHING THE PROGRESS OF AVIATION]

The use of air craft as military auxiliaries is not new. As early
as 1812 the Russians, before retreating from Moscow, attempted to
drop bombs from balloons: an attempt carried to success by Austrian
engineers in 1849. Both contestants in our own War of Secession
employed captive and drifting balloons. President Lincoln organized a
regular aeronautic auxiliary staff in which one Lowe held the official
rank of chief aeronaut. This same gentleman (who had accomplished a
reconnaissance of 350 miles in eight hours in a 25,000 cubic foot
drifting balloon) was subjected to adverse criticism on account of a
weakness for making ascents while wearing the formal "Prince Albert"
coat and silk hat! A portable gas-generating plant was employed by
the Union army. We are told that General Stoneman, in 1862, directed
artillery fire from a balloon, which was repeatedly fired at by the
enemy, but not once hit. The Confederates were less amply equipped.
Their balloon was a patchwork of silk skirts contributed (one doubts
not, with patriotic alacrity) by the daughters of the Confederacy.

It is not forgotten that communication between besieged Paris and the
external world was kept up for some months during 1870-71 by balloons
exclusively. Mail was carried on a truly commercial scale: pet animals
and--the anticlimax is unintended--164 persons, including M. Gambetta,
escaped in some sixty-five flights. Balloons were frequently employed
in the Franco-Prussian contest; and they were seldom put _hors de
combat_ by the enemy.

During our war with Spain, aerial craft were employed in at least
one instance, namely, at San Juan, Porto Rico, for reconnoitering
entrenchments. Frequent ascents were made from Ladysmith, during the
Boer war. The balloons were often fired at, but never badly damaged.
Cronje's army was on one occasion located by the aid of a British
scout-balloon. Artillery fire was frequently directed from aerial
observations. Both sides employed balloons in the epic conflict between
Russia and Japan.

A declaration introduced at the second international peace conference
at the Hague proposed to prohibit, for a limited period, the discharge
of projectiles or explosives from flying machines of any sort. The
United States was the only first-class power which endorsed the
declaration. It does not appear likely, therefore, that international
law will discountenance the employment of aerial craft in international
disputes. The building of airships goes on with increasing eagerness.
Last year the Italian chamber appropriated $5,000,000 for the
construction and maintenance of flying machines.

A press report dated February 4 stated that a German aeronaut had
been spending some weeks at Panama, studying the air currents of the
Canal Zone. No flying machine may in Germany approach more closely
than within six miles of a fort, unless specially licensed. At the
Krupp works in Essen there are being tested two new guns for shooting
at aeroplanes and dirigibles. One is mounted on an armored motor
truck. The other is a swivel-mounted gun on a flat-topped four-wheeled
carriage.

The United States battleship _Connecticut_ cost $9,000,000. It
displaces 18,000 tons, uses 17,000 horse-power and 1000 men, and makes
twenty miles an hour. An aeroplane of unusual size with nearly three
times this speed, employing from one to three men with an engine of 100
horse-power, would weigh one ton and might cost $5000. A Dreadnought
costs $16,000,000, complete, and may last--it is difficult to say, but
few claim more than ten years. It depreciates, perhaps, at the rate
of $2,000,000 a year. Aeroplanes built to standard designs in large
quantities would cost certainly not over $1000 each. The ratio of cost
is 16,000 to 1. Would the largest Dreadnought, exposed unaided to the
attack of 16,000 flying machines, be in an entirely enviable situation?

An aeroplane is a fragile and costly thing to hazard at one blow: but
not more fragile or costly than a Whitehead torpedo. The aeroplane
soldier takes tremendous risks; but perhaps not greater risks than
those taken by the crew of a submarine. There is never any lack of
daring men when daring is the thing needed.

All experience goes to show that an object in the air is hard to
hit. The flying machine is safer from attack where it works than it
is on the ground. The aim necessary to impart a crippling blow to an
aeroplane must be one of unprecedented accuracy. The dirigible balloon
gives a larger mark, but could not be immediately crippled by almost
any projectile. It could take a good pounding and still get away.
Interesting speculations might be made as to the outcome of an aerial
battle between the two types of craft. The aeroplane might have a
sharp cutting beak with which to ram its more cumbersome adversary,
but this would involve some risk to its own stability: and the balloon
could easily escape by a quick ascent. It has been suggested that each
dirigible would need an aeroplane escort force for its defense against
ramming. Any collision between two opposing heavier-than-air machines
could not, it would seem, be other than disastrous: but perhaps
the dirigible could rescue the wrecks. Possibly gas-inflated life
buoys might be attached to the individual combatants. In the French
man[oe]uvers, a small aeroplane circled the dirigible with ease, flying
not only around it, but in vertical circles over and under it.

  [Illustration: 7.5 CENTIMETER GERMAN AUTOMATIC GUN FOR ATTACKING
  AIRSHIPS
  (From Brewer's _Art of Aviation_)]

The French war office has exploited both types of machine. In
Germany, the dirigible has until recently received nearly all the
attention of strategists: but the results of a recent aerial war
game have apparently suggested a change in policy, and the Germans
are now, without neglecting the balloon, actively developing its
heavier-than-air competitor. England seems to be muddled as to its
aerial policy, while the United States has been waiting and for the
most part doing nothing. Now, however, the mobilizations in Texas have
been associated with a considerable amount of aeroplane enthusiasm.
A half-dozen machines, it is expected, will soon be housed in the
aerodrome at San Antonio. Experiments are anticipated in the carrying
of light ammunition and emergency supplies, and one of the promised
man[oe]uvers is to be the locating of concealed bodies of troops by air
scouts. Thirty army officers are to be detailed for aeroplane service
this year; five training schools are to be established.

If flying machines are relatively unsusceptible to attack, there is
also some question as to their effectiveness _in_ attack. Rifles have
been discharged from moving balloons with some degree of accuracy in
aim; but long-range marksmanship with any but hand weapons involves the
mastery of several difficult factors additional to those present in
gunnery at sea. The recoil of guns might endanger stability; and it is
difficult to estimate the possible effects of a powerful concussion,
with its resulting surges of air, in the immediate vicinity of a
delicately balanced aerial vessel.

But aside from purely combative functions, air craft may be
superlatively useful as messengers. To send despatches rapidly and
without interference, or to carry a general 100 miles in as many
minutes--these accomplishments would render impossible the romance
of a "Sheridan's Ride," but might have a romance of their own. With
the new sense added to human equipment by wireless communication,
the results of observations may be signaled to friends over miles of
distance without intervening permanent connections of however fragile a
nature.

Flying machines would seem to be the safest of scouts. They could pass
over the enemy's country with as little direct danger--perhaps as
unobserved--as a spy in disguise; yet their occupants would scarcely
be subjected to the penalty accompanying discovery of a spy. They
could easily study the movements of an opposing armed force: a study
now frequently associated with great loss of life and hampering of
effective handling of troops. They could watch for hostile fleets with
relatively high effectiveness (under usual conditions), commanding
distant approaches to a long coast line at slight cost. From their
elevated position, they could most readily detect hostile submarines
threatening their own naval fleet. Maximum effective reconnaissance in
minimum time would be their chief characteristic: in fact, the high
speeds might actually constitute an objection, if they interfered with
thorough observation. But if air craft had been available at Santiago
in 1898, Lieutenant Blue's expedition would have been unnecessary, and
there would have been for no moment any doubt that Admiral Cervera's
fleet was actually bottled up behind the Morro. No besieged fortress
need any longer be deprived of communication with--or even some
medical or other supplies from--its friends. Suppose that Napoleon had
been provided with a flying machine at Elba--or even at St. Helena!

The applications to rapid surveying of unknown ground that have been
suggested as possible in civil life would be equally possible in time
of war. Even if the scene of conflict were in an unmapped portion of
the enemy's territory, the map could be quickly made, the location of
temporary defenses and entrenchments ascertained, and the advantage
of superior knowledge of the ground completely overcome prior to an
engagement. The searchlight and the compass for true navigation on long
flights over unknown country would be the indispensable aids in such
applications.

During the current mobilization of the United States Army at Texas, a
dispatch was carried 21 miles on a map-and-compass flight, the round
trip occupying less than two hours and being made without incident. The
machine flew at a height of 1500 feet and was sighted several miles off.

A dirigible balloon, it has been suggested, is comparatively safe while
moving in the air, but is subjected to severe strains when anchored to
the ground, if exposed. It must have either safe harbors of refuge or
actual shelter buildings--dry docks, so to speak. In an enemy's country
a ravine or even a deep railway cut might answer in an emergency, but
the greatest reliance would have to be placed on quick return trips
from a suitable base. The balloon would be, perhaps, a more effective
weapon in defense than in attack. Major Squier regards a flying height
of one mile as giving reasonable security against hostile projectiles
in the daytime. A lower elevation would be sufficient at night. Given a
suitable telephotographic apparatus, all necessary observations could
easily be made from this altitude. Even in the enemy's territory,
descent to the earth might be possible at night under reasonably
favorable conditions. Two sizes of balloon would seem to be indicated:
the scouting work described would be done by a small machine having the
greatest possible radius of action. Frontiers would be no barrier to
it. Sent from England in the night it could hover over a Kiel canal or
an island of Heligoland at sunrise, there to observe in most leisurely
fashion an enemy's mobilizations.

  [Illustration: GERMAN GUN FOR SHOOTING AT AEROPLANES
  (From Brewer's _Art of Aviation_)]

At the London meeting of the Institute of Naval Architects, in April,
1911, the opinion was expressed that the only effective way of meeting
attack from a flying machine at sea would be by a counter-attack from
the same type of craft. The ship designers concluded that the aeroplane
would no more limit the sizes of battleships than the torpedo has
limited them.

For the more serious work of fighting, larger balloons would be needed,
with net carrying capacities perhaps upward from one ton. Such a
machine could launch explosives and combustibles against the enemy's
forts, dry docks, arsenals, magazines, and battleships. It could easily
and completely destroy his railroads and bridges; perhaps even his
capital itself, including the buildings housing his chief executive
and war office staff. Nothing--it would seem--could effectually combat
it save air craft of its own kind. The battles of the future may be
battles of the air.

There are of course difficulties in the way of dropping missiles
of any great size from flying machines. Curtiss and others have
shown that accuracy of aim is possible. Eight-pound shrapnel shells
have been dropped from an aeroplane with measurably good effect,
without upsetting the vessel; but at best the sudden liberation of
a considerable weight will introduce stabilizing and controlling
difficulties. The passengers who made junketing trips about Paris on
the _Clément-Bayard_ complained that they were not allowed to throw
even a chicken-bone overboard! But it does not seem too much to expect
that these purely mechanical difficulties will be overcome by purely
mechanical remedies. An automatic venting of a gas ballonet of just
sufficient size to compensate for the weight of the dropped shell would
answer in a balloon: a similar automatic change in propeller speed and
angle of planes would suffice with the aeroplane. There is no doubt
but that air craft may be made efficient agents of destruction on a
colossal scale.

  [Illustration: SANTOS-DUMONT CIRCLING THE EIFFEL TOWER
  (From Walker's _Aerial Navigation_)]

A Swedish engineer officer has invented an aerial torpedo,
automatically propelled and balanced like an ordinary submarine
torpedo. It is stated to have an effective radius of three miles while
carrying two and one-half pounds of explosive at the speed of a bullet.
One can see no reason why such torpedoes of the largest size are not
entirely practicable: though much lower speeds than that stated should
be sufficient.

According to press reports, the Krupps have developed a non-recoiling
torpedo, having a range exceeding 5000 yards. The percussion device is
locked at the start, to prevent premature explosion: unlocking occurs
only after a certain velocity has been attained.

Major Squier apparently contends that the prohibition of offensive
aerial operations is unfair, unless with it there goes the reciprocal
provision that a war balloon shall not be fired at from below. Again,
there seems to be no good reason why aerial mines dropped from above
should be forbidden, while submarine mines--the most dangerous naval
weapons--are allowed. Modern strategy aims to capture rather than to
destroy: the man[oe]uvering of the enemy into untenable situations by
the rapid mobilization of troops being the end of present-day highly
organized staffs. Whether the dirigible (certainly not the aeroplane)
will ever become an effective vehicle for transport of large bodies of
troops cannot yet be foreseen.

Differences in national temper and tradition, and the conflict of
commercial enterprise, perhaps the very recentness of the growth of a
spirit of national unity on the one hand, are rapidly bringing the two
foremost powers of Europe into keen competition: a competition which
is resulting in a bloodless revolution in England, necessitated by the
financial requirements of its naval program. Germany, by its strategic
geographical position, its dominating military organization, and the
enforced frugality, resourcefulness, and efficiency of its people,
possesses what must be regarded as the most invincible army in the
world. Its avowed purpose is an equally invincible navy. Whether the
Gibraltar-Power can keep its ascendancy may well be doubted. The one
doubtful--and at the same time perhaps hopeful--factor lies in the
possibilities of aerial navigation.

  [Illustration: LATHAM, FARMAN, AND PAULHAN]

If one battleship, in terms of dollars, represents 16,000 airships,
and if one or a dozen of the latter can destroy the former--a feat
not perhaps beyond the bounds of possibility--if the fortress that
represents the skill and labor of generations may be razed by twoscore
men operating from aloft, then the nations may beat their spears into
pruning-hooks and their swords into plowshares: then the battle ceases
to hinge on the power of the purse. Let war be made so costly that
nations can no more afford it than sane men can wrestle on the brink
of a precipice. Let armed international strife be viewed as it really
is--senseless as the now dying duello. Let the navy that represents
the wealth, the best engineering, the highest courage and skill, of
our age, be powerless at the attack of a swarm of trifling gnats like
Gulliver bound by Lilliputians--what happens then? It is a _reductio ad
absurdum_. Destructive war becomes so superlatively destructive as to
destroy itself.

There is only one other way. Let the two rival Powers on whom the peace
of the world depends settle their difficulties--surely the earth must
be big enough for both!--and then as one would gently but firmly take
away from a small boy his too destructive toy rifle, spike the guns
and scuttle the ships, their own and all the rest, leaving to some
unambitious and neutral power the prosaic task of policing the world.
Here is a work for red blood and national self-consciousness. If war
were ever needed for man's best development, other things will answer
now. The torn bodies and desolated homes of millions of men have paid
the price demanded. No imaged hell can surpass the unnamed horrors that
our fathers braved.

"Enforced disarmament!" Why not? Force (and public opinion) have
abolished private duels. Why not national duels as well? Civilization's
control of savagery always begins with compulsion. For a generation,
no first-class power has had home experience in a serious armed
conflict. We should not willingly contemplate such experience now. We
have too much to do in the world to fight.

       *       *       *       *       *

The writer has felt some hesitancy in letting these words stand as
the conclusion of a book on flying machines: but as with the old
Roman who terminated every oration with a defiance of Carthage, the
conviction prevails that no other question of the day is of comparable
importance; and on a matter of overwhelming consequence like this
no word can ever be out of place. The five chief powers spent for
war purposes (officially, as Professor Johnson puts it, for the
"preservation of peace") about $1,000,000,000 in the year 1908. In
the worst period of the Napoleonic operations the French military and
naval budget was less than $100,000,000 annually. Great Britain, on
the present peace footing, is spending for armament more rapidly than
from 1793 to 1815. The gigantic "War of the Spanish Succession" (which
changed the map of Europe) cost England less than a present year's
military expenditure. Since the types for these pages have been set,
the promise of international peace has been distinctly strengthened.
President Taft has suggested that as, first, questions of individual
privilege, and, finally, even those of individual honor, have been
by common consent submitted to adjudication, so also may those
so-called "issues involving national honor" be disposed of without
dishonor by international arbitration. Sir Edward Grey, who does not
hesitate to say that increase of armaments may end in the destruction
of civilization unless stopped by revolt of the masses against the
increasing burdens of taxation, has electrified Europe by his reception
of the Taft pronouncement. England and the United States rule one-third
the inhabitants of the earth. It is true that a defensive alliance
might be more advantageous to the former and disagreeably entangling to
the latter; but a binding treaty of arbitration between these powers
would nevertheless be a worthy climax to our present era. And if it
led to alliance against a third nation which had refused to arbitrate
(led--as Sir Edward Grey suggests--by the logic of events and not by
subterranean device) would not such be the fitting and conclusive
outcome?

The Taft-Grey program--one would wish to call it that--has had all
reputable endorsement; in England, no factional opposition may be
expected. Our own jingoes are strangely silent. Mr. Dillon's fear that
compulsory disarmament would militate against the weaker nations is
offset by the hearty adherence of Denmark. A resolution in favor of the
establishment of an international police force has passed the House of
Commons by a heavy majority. It looks now as if we might hope before
long to re-date our centuries. We have had Olympiads and Years of Rome,
B.C. and A.D. Perhaps next the dream of thoughtful men may find its
realization in the new (and, we may hope, English) prefix, Y.P.--Year
of Peace.




FOOTNOTES

[A] According to press reports, temporary water ballast will
be taken on during the daytime, to offset the ascensional effect of the
hot sun on the envelope.

[B] The contestants for the Ryan prize of $10,000 were
Moisant, Count de Lesseps, and Grahame-White. Owing to bad weather,
there was no general participation in the preliminary qualifying
events, and some question exists as to whether such qualification was
not tacitly waived; particularly in view of the fact that the prize was
awarded to the technically unqualified competitor, Mr. Moisant, who
made the fastest time. This award was challenged by Mr. Grahame-White,
and upon review by the International Aeronautic Federation the prize
was given to de Lesseps, the slowest of the contestants, Grahame-White
being disqualified for having fouled a pylon at the start. This
gentleman has again appealed the case, and a final decision cannot be
expected before the meeting of the Federation in October, 1911.

[C] The high wind velocities of the southern circumpolar
regions may be an insurmountable obstacle in the Antarctic. Yet Mawson
expects to take with him a 2-passenger monoplane having a 180-mile
radius of action on the expedition proposed for this year.

[D] It seems that tomorrow has come; for an aeroplane is being
regularly used (according to a reported interview with Dr. Alexander
Graham Bell) for carrying mails in India.




Books on Aeronautics


 =FLYING MACHINES TO-DAY.= By WILLIAM D. ENNIS, M. E., Professor of
   Mechanical Engineering, Polytechnic Institute, Brooklyn. 12mo.,
   cloth, 218 pp., 123 illustrations =$1.50 net=
   =CONTENTS=: THE DELIGHTS AND DANGERS OF FLYING--Dangers of
   Aviation--What it is Like to Fly. SOARING FLIGHT BY MAN--What Holds
   it Up. Lifting Power. Why so Many Sails. Steering. TURNING
   CORNERS--What Happens When Making a Turn. Lateral Stability. Wing
   Warping. Automatic Control. The Gyroscope. Wind Gusts. AIR AND THE
   WIND--Sailing Balloons. Field and Speed. GAS AND BALLAST--Buoyancy in
   Air. Ascending and Descending. The Ballonet. The Equilibrator.
   DIRIGIBLE BALLOONS AND OTHER KINDS--Shapes. Dimensions. Fabrics.
   Framing. Keeping the Keel Horizontal. Stability. Rudders and Planes.
   Arrangement and Accessories. Amateur Dirigibles. Fort Omaha Plant.
   Balloon Progress. QUESTION OF POWER--Resistance of Aeroplanes.
   Resistance of Dirigibles. Independent Speed and Timetable. Cost of
   Speed. Propeller. GETTING UP AND DOWN; MODELS AND GLIDERS; AEROPLANE
   DETAILS--Launching. Descending. Gliders. Models. Balancing. Weights.
   Miscellaneous. Things to Look After. SOME AEROPLANES--SOME
   ACCOMPLISHMENTS. THE POSSIBILITIES IN AVIATION--Case of the
   Dirigible. The Orthopter. The Helicopter. Composite Types. What is
   Promised. AERIAL WARFARE.

 =AERIAL FLIGHT. Vol. 1. Aerodynamics.= By F. W. LANCHESTER. 8vo.,
   cloth, 438 pp., 162 illustrations =$6.00 net=
   =CONTENTS=: Fluid Resistance and Its Associated Phenomena. Viscosity
   and Skin Friction. The Hydrodynamics of Analytical Theory. Wing
   Form and Motion in the Periphery. The Aeroplane. The Normal Plane.
   The Inclined Aeroplane. The Economics of Flight. The Aerofoil. On
   Propulsion, the Screw Propeller, and the Power Expended in Flight.
   Experimental Aerodynamics. Glossary. Appendices.
 =Vol. II. Aerodonetics.= By F. W. LANCHESTER. 8vo., cloth, 433 pp.,
   208 illustrations =$6.00 net=
   =CONTENTS=: Free Flight. General Principles and Phenomena. The Phugoid
   Theory--The Equations of the Flight Path. The Phugoid 1852-1872.
   Dirigible Balloons from 1883-1897; 1898-1906. Flying Machine
   Theory--The Flight Path Plotted. Elementary Deductions from the
   Phugoid Theory. Stability of the Flight Path as Affected by Resistance
   and Moment of Inertia. Experimental Evidence and Verification of the
   Phugoid Theory. Lateral and Directional Stability. Review of Chapters
   I to VII and General Conclusions. Soaring. Experimental. Aerodonetics.

 =AERIAL NAVIGATION. A practical handbook on the construction of
  dirigible balloons, aerostats, aeroplanes and aeromotors=, by
  FREDERICK WALKER. 12mo., cloth, 151 pp., 100 illustrations
  =$3.00 net.=
  =CONTENTS=: Laws of Flight. Aerostatics. Aerostats. Aerodynamics.
  Screw Propulsion. Paddles and Aeroplanes. Motive Power. Structure of
  Airships and Materials. Airships. Appendix.

 =AEROPLANE PATENTS.= By ROBERT M. NEILSON. 8vo., cloth, 101 pp., 77
  illustrations =$2.00 net=
  =CONTENTS=: Advice to Inventors. Review of British Patents. British
  Patents and Applications for Patents from 1860 to 1910, arranged in
  Order of Application. British Patentees, arranged alphabetically.
  United States Patents from 1896 to 1909, arranged in order of issue.
  United States Patentees, arranged alphabetically.

 =THE PRINCIPLES OF AEROPLANE CONSTRUCTION.= By RANKIN KENNEDY, C. E.
  8vo., cloth, 145 pp., 51 diagrams =$1.50 net=
  =CONTENTS=: Elementary Mechanics and Physics. Principles of Inclined
  Planes. Air and Its Properties. Principles of the Aeroplane. The
  Curves of the Aeroplane. Centers of Gravity: Balancing; Steering. The
  Propeller. The Hélicoptère. The Wing Propeller. The Engine. The Future
  of the Aeroplane.

 =HOW TO DESIGN AN AEROPLANE.= By HERBERT CHATLEY. 16mo., boards, 109
  pp., illustrated (Van Nostrand's Science Series) =50 cents=
  =CONTENTS=: The Aeroplane. Air Pressure. Weight. Propellers and
  Motors. Balancing. Construction. Difficulties. Future Developments.
  Cost. Other Flying-Machines (Gyroplane and Orinthoptere).

 =HOW TO BUILD AN AEROPLANE.= By ROBERT PETIT. Translated from the
  French by T. O'B. Hubbard and J. H. Ledeboer. 8vo., cloth, 131 pp., 93
  illustrations =$1.50 net=
  =CONTENTS=: General Principles of Aeroplane Design. Theory and
  Calculation. Resistance, Lift, Power, Calculations for the Design of
  an Aeroplane, Application of Power, Design of Propeller, Arrangements
  of Surfaces, Stability, Center of Gravity, etc. Materials.
  Construction of Propellers. Arrangements for Starting and Landing.
  Controls. Placing Motor. The Planes. Curvatures. Motors.

 =AIRSHIPS, PAST AND PRESENT. Together with chapters on the use of
  balloons in connection with meteorology, photography, and the carrier
  pigeon.= By A. HILDEBRANDT, Captain and Instructor in the Prussian
  Balloon Corps. Translated by W. H. Story. 8vo., cloth, 361 pp., 222
  illustrations =$3.50 net=
  =CONTENTS=: Early History of the Art. Invention of the Air Balloon.
  Montgolfieres, Charlieres, and Rozieres. Theory of the Balloon.
  Development of the Dirigible Balloon. History of the Dirigible
  Balloon, 1852-1872. Dirigible Balloons from 1883-1897; 1898-1906.
  Flying Machines. Kites. Parachutes. Development of Military
  Ballooning. Ballooning in Franco-Prussian War. Modern Organization
  of Military Ballooning in France, Germany, England and Russia.
  Military Ballooning in Other Countries. Balloon Construction and the
  Preparation of the Gas. Instruments. Ballooning as a Sport. Scientific
  Ballooning. Balloon Photography. Photographic Outfit for Balloon Work.
  Interpretation of Photographs. Hectography by Means of Kites and
  Rockets. Carrier Pigeons for Balloons. Balloon Law.

  [Illustration: VAN NOSTRAND LOGO]


D. VAN NOSTRAND CO., Publishers

23 MURRAY and 27 WARREN STREETS, NEW YORK




Transcriber's Note: Italics are delimited by underscores; bold by equal
signs. Four occurrences of the oe-ligature in the word man[oe]uver
are left as [oe]. The four footnotes have been moved to the end of the
book. A few words were judged to be printer errors and were changed.
These include two occurrences of horse-power in the unhyphenated form,
the spelling of Tabuteau as Tabuteaw on p. 162, and the spelling of
hélicoptère as helicoptéré on p.208. On a few of the
figure captions, missing accents were added to some French names.







End of Project Gutenberg's Flying Machines Today, by William Duane Ennis



