620 
Climate  of  the  British  Islands 
London  is  63° ' 14..  and  in  Edinburgh  5S0,28.  We  now  come  to 
speak  of  the  next  agent  in  the  modification  of  climate,  viz. : — 
Thirdly. — Prevailing  winds.  And  this  subject,  were  the  Bri- 
tish Islands  situated  either  in  the  centre  of  some  vast  continent 
or  ocean,  might  be  dismissed  very  briefly  ; but  situated  as  thev 
are  near  to  a great  continent  on  the  east,  whilst  from  a point  of 
the  compass,  south  by  west,  round  the  west  to  north  by  east, 
these  islands  are  surrounded  by  a vast  expanse  of  ocean,  in  con- 
sidering their  climate  the  direction  of  the  winds  is  of  the  utmost 
importance : for  were  these  winds  generally  from  the  north- 
east, east,  or  south  east,  then,  notwithstanding  our  proximity 
to  the  ocean,  our  climate  would  be  cold  in  winter ; and  the  winds 
having  passed  over  a large  tract  of  land  would  be  dry,  though 
a portion  of  moisture  might  be  absorbed  by  them  on  their  pas- 
sage across  the  German  Ocean  : yet  owing  to  their  previous  dry- 
ness they  would  impart  but  little  of  either  rain  or  dew  to  our 
island.  Whereas,  were  our  winds  continually  from  the  south- 
west, west,  or  north,  charged  as  they  would  be  by  the  vast 
quantitv  of  moisture  raised  by  evaporation  during  their  passage 
over  a great  extent  of  ocean,  they  would  impart  a moistness  to 
our  atmosphere,  which,  though  eminently  favourable  to  the  pro- 
duction of  a luxuriant  vegetation,  and  to  the  growth  of  grasses, 
trees,  &c.,  would  yet  prevent  the  ripening  of  grain,  and  the 
general  growth  of  those  plants  cultivated  for  the  value  of  their 
seeds,  or  the  ripening  of  their  fruits. 
Wind  is  air  in  motion,  and  this  motion  arises  from  two  causes 
— elasticity  and  gravity  * Every  one  is  familiar  with  the  fact  that 
•heated  air  rises,  and  colder  air  rushes  in  to  supply  its  place. 
Thus,  as  the  heated  air  rushes  up  the  chimney,  carrying  with  it 
the  smoke,  a current  rushes  in  through  the  chinks  of  the  doors 
and  windows  to  supply  its  place,  and  it  was  quaintly  remarked 
bv  a writer  on  the  great  fire  of  London  in  1666,  that  even  the 
winds  seemed  to  rush  in  from  every  quarter  to  augment  its  force. 
It  is  on  the  same  principle  that  the  cold  air  from  the  poles 
" It  may  be  useful  to  say  that  elasticity  is  the  force  with  which  air  tends  to  expand 
itself,  and  it  is  increased  by  heat;  as  if  any  one  fill  a bladder  with  air  and  then  place 
It  near  the  fire,  it  will  soon  heat  so  that  the  elastic  force  of  the  air  will  burst  the 
bladder.  Now,  it  is  evident  that  this  elastic  force  of  air  will  be  greatest  where  it  is 
the  most  heated,  and  consequently  increases  from  the  poles  to  the  equator.  But  heat 
thus  expanding  air  and  making  it  occupy  a larger  space,  necessarily  causes  it  to  be 
lighter  bulk  for  bulk,  and  therefore  its  specific  gravity  is  the  greatest  at  the  poles  and 
decreases  to  the  equator.  Now  in  the  free  atmosphere  each  of  these  forces  has  its  un- 
limited actiou,  the  elasticity  tending  to  force  the  air  from  the  equator,  and  the  force  of 
gravity  tending  to  press  it  from  the  poles,  and,  did  no  cause  interfere,  there  would 
always  be  a current  of  air  near  the  surface  of  the  earth  from  the  poles  to  the  equator 
owing  to  the  force  of  gravity,  and  one  from  the  equator  to  the  poles  in  the  higher 
regions  owing  to  the  force  of  elasticity.  But  the  polar  or  north  wind  by  the  rotation 
of  the  earth  is  converted  into  a north-eastern,  and  the  equatorial  or  southern  is  also 
changed  into  a south-western. 
