To  illustrate  this,  let  us  suppose  a  gossamer  sack,  capable  of  being  bermoticallT
sealed;  that  it  is  impervious  to  heat,  and  elastic  as  the  oir  itself;  that  with  ^
barometer  at  30  in.,  tne  temperature  at  6CP,  and  the  dew-point  the  same,  this  sack
be  filled  with  air ;  that  then  it  be  attached  to  a  balloon  and  sent  up  in  the  sky,  to
a  height  where  the  barometric  pressure  is  only  15  in.,  and  where  tlie  temp»atui«  of
tibe  air  in  the  sack,  by  reason  of  this  diminished  pressure,  and  by  virtue  of  the  ex-
pansion of  the  air  within  and  its  consequent  cooling,  is  reducea  to  zero.  Bj  thii
process,  the  vapour  with  which  the  air  was  loaded  when  it  was  admitted  into  the
sack  has,  let  it  be  assumed,  been  condensed,  and  consequently  its  latent  heat  w.
free  in  the  sack.
Suppose  now  the  sack  be  hauled  down  to  the  surface  again,  where  the  iMiroxnetnc
pressure  is  30  in.,  as  beforo,  and  ^hat  have  we  ?  The  sack  is  reduced  to  its  former
dimensions  you  will  perceive,  but  instead  of  damp  air  we  now  have  it  filled  wiili
dry ;  moreover,  there  is  at  the  bottom  a  meosuro  of  water — ^the  condensed  vapoor.
