extends  over  nearly  all  the  torrid  zone  because  there  the
annual  variation  of  insolation  is  small.  It  also  extends  over
large  parts  of  the  oceans  of  the  southern  hemisphere  even  up
to  quite  high  latitudes  because  the  waters  are  only  able  to  change
their  temperature  with  difficulty.
The  areas  of  the  most  extreme  ranges,  over  70°  F.,  are  only
found  in  the  large  land  masses  remote  from  the  equator.  Thus
they  are  limited  to  the  northern  hemisphere  as  no  great  land
masses  are  so  situated  in  the  southern.  The  greatest  annual
range  occurs  in  the  centre  of  Asiatic  Siberia,  where  it  approxi-
mates to  120°  F.  This  is  all  in  accord  with  what  would  be
expected  from  what  has  been  previously  said  with  regard  to
absorption  and  radiation.
24
CHAPTER  VI.
Water  Vapour  in  the  Atmosphere.
26.  General.* — Of  all  the  constituents  of  the  lower  atmos-
phere water  vapour   is   the   most  variable.     At   any  time   the
total  mass  of  it  is  very  small  compared  with  the  mass  of  dry
air,  but  the  results  accruing  from  that  small  amount  are,  from
the  meteorological  point  of  view,  amazingly  great.
27.  Evaporation. — The   Molecular   Theory  of  Matter  states
that  matter  is   discrete,  and   not   continuous,  being  made   up
of  minute  particles  termed  molecules.     Further,  the   molcules
are  assumed  to  be  in  a  state  of  incessant  motion.
The  results  of  the  incessant  motion  vary,  however,  according
to  whether  the  matter  is  in  the  gaseous,  liquid,  or  solid  state.
