632 
Climate  of  the  British  Islands 
the  autumn  than  the  spring'.  There  is  always  much  more  mois- 
ture in  south  and  westerly  winds  than  in  north  or  easterly,  yet  to 
the  eastern  shores  an  easterly  wind  may  bring  more  moisture  than 
a western,  for  western  winds  have  always  a great  portion  of  their 
vapour  condensed  in  passing  over  the  mountainous  district  on  the 
west  of  our  islands. 
Average  Dew-point  on  the  Eastern  and  Western  parts  of  the  British 
Islands  for  the  several  Months  of  the  Year. 
Eastern. 
Western. 
North. 
South. 
North. 
South. 
o 
o 
o 
o 
January 
30 
31 
31 
40 
February  . 
30 
31 
31 
40 
March  . 
33 
35 
34 
44 
April  . 
40 
43 
41 
47 
May  . 
41 
46 
41 
47 
June 
48 
51 
50 
56 
July  . . 
50 
53 
51 
58 
August 
51 
56 
52 
58 
September  • 
49 
54 
50 
56 
October  * 
43 
47 
43 
50 
November  . 
39 
43 
39 
46 
December  . 
32 
34 
32 
42 
Intimately  connected  with  this  subject  is  the  next  subject  pro- 
posed, viz. — 4th.  Different  amount  of  sensible  moisture  or  fogs. 
With  every  respiration  men  and  animals  give  out  a quantity  of 
moisture,  which  is  generally  invisible  ; but  if  the  air  be  very  cold, 
or  the  animal  heat  much  increased  by  active  exertion,  then  this 
moisture  is  visible.  The  cause  of  this  is  in  both  cases  the  same  : 
in  the  former  case,  when  the  atmosphere  is  very  cold,  men  and 
animals  give  out  vapour  whose  tension  being  greater  than  can  lie 
maintained  by  the  temperature,  it  is  of  necessity  partially  con- 
densed and  made  visible  : in  the  latter  case  also  the  heat  of  the 
body  is  raised  by  exertion,  so  that  the  moisture  given  out  is 
equally  condensed.  With  respect  to  mists  or  fogs  and  clouds, 
they  are  every  way  similar,  except  in  their  height  from  the 
ground.  When  vapour  rises  from  moist  earth  or  water,  it 
ascends  on  account  of  its  specific  levity,  till  it  is  condensed  by 
the  cold  of  the  upper  regions  and  forms  clouds;  but  should  the 
temperature  of  the  air  be  on  the  surface  lower  than  that  of  the 
earth  or  water  on  which  it  rests,  then  condensation  must  immedi- 
ately take  place,  and  the  result  be  the  formation  of  fog  or  mist ; 
the  former  term  being  generally  applied  to  vapour  from  the  land, 
the  latter  to  vapour  from  the  water.  Fog  may  also  be  formed  by 
the  air  at  some  distance  from  the  ground  being  cooled  below  the 
dew  point,  especially  if  this  cooling  is  rapid,  for  then  the  air 
