642 
Climate  of  the  British  Islands 
ground  will  have  a greater  quantity  of  rain  in  it  than  the  one  at 
a higher  level.  On  elevated  places  the  drops  are  small,  and  rain 
assumes  the  form  generally  of  a drizzling  haze.  Every  traveller  on 
the  mountains  of  the  west  of  England,  and  in  Scotland  and 
Wales,  is  familiar  with  the  fact,  that  there  it  is  rarely  quite  fair, 
neither  do  the  drops  generally  fall  of  a large  size,  but  in  a small 
fine  haze,  yet  in  descending  into  the  valleys  the  drops  are  always 
felt  to  be  sensibly  larger. 
Prognostications  of  the  weather,  especially  with  respect  to  the 
approach  of  rain,  are  generally  formed  from  the  barometer,  winds, 
clouds,  and  the  actions  of  animals,  in  addition  to  the  opening  and 
shutting  of  the  leaves  and  flowers  of  plants;  but  with  respect  to 
the  last,  the  signs  of  rain  derived  from  the  movements  of  plants 
are  so  near  the  weather  they  foretell,  that  they  are  generally  of 
little  use.  Of  all  instruments  used  in  foretelling  the  weather 
none  is  more  general  than  the  barometer  ; yet  whilst  many  are 
inclined  to  trust  entirely  to  its  indications,  others  look  upon  it 
with  contempt,  and  deem  its  indications  of  no  use  whatever.  In 
the  following  remarks  I shall  endeavour  to  steer  clear  of  both 
these  extremes. 
The  barometer,  as  its  name  implies,  is  a measure  of  the  weight 
of  the  atmosphere  ; and  this  indicated  truly,  is  all  it  can  do.  If 
correctly  made  and  graduated,  if  the  index  point  to  30,  it  proves 
that  a column  of  air,  extending  from  its  place  to  the  limits  of  our 
atmosphere,  is  of  equal  weight  to  a column  of  mercury  of  the 
height  of  30  inches — if  it  point  to  29,  that  the  column  of  air  is 
equal  in  weight  to  a column  of  mercury  29  inches  in  height,  &c. 
It  is  evident  from  this,  that  if  one  barometer  be  placed  at  the 
level  of  the  sea,  and  another  in  a higher  situation,  their  indications 
will  be  different,  and  therefore  all  persons  living  at  any  height 
above  the  level  of  the  sea  must  make  allowance  for  that  difference. 
But  when  the  weight  of  the  atmosphere  is  ascertained,  how  is  this 
to  influence  the  fall  of  rain  ? This  is  an  important  subject,  for 
it  is  certain  that  directly  it  has  nothing  to  do  with  it,  the  atmos- 
phere may  be  very  light,  and  hence  the  barometer  low,  yet  no 
rain  fall ; or  the  atmosphere  very  heavy,  and  hence  the  barometer 
very  high,  and  yet  rain  may  descend  in  torrents.  This  in  our 
climate  is  especially  so  in  March  and  February  ; also,  in  tropical 
regions,  the  barometer  scarcely  ever  varies  through  the  year,  and 
yet  at  some  times  the  rains  are  excessive,  and  at  others  the  land 
is  parched  by  continual  drought.  The  reason  why  the  barometer 
is  there  so  uniform  is  because  the  winds  are  so  unvarying  ; and  the 
reason  it  here  varies  so  much  (the  variation  being  in  the  British 
Islands  about  3 inches  of  mercury)  is  because  the  winds  are  so 
changeable.  The  barometer  then  is  most  influenced  by  the  wind, 
and  is  almost  uniformly  lower  with  a south  or  south-west  wind  than 
