Under  the  law  of  Mariotte  and  Boyle,  the  theory  of  co.iciition  or  tiin
atmospheric  equilibrium  falls  into  the  department  of  pi<">i'"n-
Rational  Mechanics.  We  see  at  once  that  the  air  can  no  more  be
in  a  state  of  real  equilibrium  than  the  ocean ;  and  so  much  the
further  from  it  as  heat  expands  air  more  than  water.  Yet  we  must
conceive  of  the  partial  equilibrium  of  a  vei-y  narrow  atmospheric
column,  to  form  a  just  general  idea  of  the  mode  of  diminution  in
regard  to  the  density  and  pressure  of  the  different  strata.  Putting
aside  considerations  of  heat,  and  the  small  effects  of  gravitation  in
such  a  case,  we  see  that  density  and  pressure  must  diminish  in  a
geometrical  progression  for  altitudes  increasing  in  arithmetical
progression :  but  this  abstract  variation  is  retarded  by  tlie  diminish-
ing heat  of  the  loftier  atmospheric  strata,  which  makes  each  stra-
tum more  dense  than  it  would  be  from  its  position.  Here,  there-
fore, we  are  stopped  by  the  intervention  of  a  new  element  which
we  do  not  understand, — the  law  of  the  vertical  variation  of  atmo-
spheric temperatures — our  ignorance  of  which  can  be  supplied  only
by  inexact  and  uncertain  expedients.  Great  caution  is  necessary
in  using  Bouguer's  method  of  measuring  altitudes  by  the  barometer ;
a  method  very  ingenious,  but  depending  on  such  complex  and  un-
certain conditions,  and  requiring  sometimes  so  much  delay,  that  it
is  even  preferable,  when  circumstances  permit,  to  enter  upon  a  ge-
ometrical measurement,  which  has  so  greatly  the  advantage  in  cer-
tainty. Yet,  considered  by  itself,  the  method  of  measurement  by
the  barometer  is  valuable  for  its  contributions  to  our  Isnowledge  of
the  surface  reliefs  of  our  globe.
SECTION  II.
DYNAMICS.
