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278   Art of Travel.

aim must be just twice as accurate, neither more nor less, as would suffice to point a rifle at the sun when it was sufficiently obscured by a cloud to bear being looked at : for the object of the aim is of the same apparent size, but a movement of a mirror causes the ray reflected from it to move through a double angle.

The power of these sun-signals is extraordinarily great. The result of several experiments that I made in England showed that the smallest mirror visible under atmospheric conditions such that the signaller's station was discernible, but dim, subtended an angle of only one-tenth of a second of a degree. It is very important that the mirror should be of truly plane and parallel glass, such as instrument-makers . procure; the index glass of a full-sized sextant is very suitable for this purpose : there is a loss of power when there is any imperfection in the glass. A plane mirror only three inches across, reflects as much of the sun as a globe of 120 feet diameter ; it looks like a dazzling star at ten miles' distance.

To direct the flash of the Mirror.-There are makeshift ways
of directing the flash of the mirror ; as, by observing its play
on an object some paces off, nearly in a line with the station it
is wished to communicate with. In doing this, two cautions
are requisite : first, the distance of the object must be so large
compared to the diameter of the mirror that the play of the
flash shall appear truly circular and exactly like a faint sun
(see preceding paragraph) : secondly, be careful to bring the
eye to the very edge of the mirror ; there should be as little
" dispart " as possible, as artillerymen would say. Unless
these cautions be attended to very strictly, the flash will never
be seen at the distant station.
An object, in reality of a white
colour but apparently dark,
owing to its being shaded, shows
the play of a mirror's flash bet
ter than any other. The play
of a flash, sent through an open
window, on the walls of a
room, can be seen at upwards
of 100 yards, It is a good

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