634 
Climate  of  the  British  Islands 
of  saturation,  when  a portion  of  the  vapour  becomes  condensed 
and  forms  fog  or  mist.  This  is  more  common,  too,  in  places 
elevated  above  the  level  of  the  sea,  and  on  mountains  than  on 
plains  ; and,  as  far  as  I have  been  able  to  determine,  more  dense 
generally  where  places  rise  300  to  600  feet,  than  on  the  low 
ground,  or  at  places  whose  elevation  is  greater.  This  kind  of  fog 
abounds  most  in  the  western  and  northern  parts  of  our  island, 
and,  being  caused  by  the  same  influences  as  cause  rain,  is  most 
common  where  most  rain  falls  ; and  hence  a very  accurate  con- 
clusion may  be  come  to  respecting  the  prevalence  of  this  kind  of 
fog,  by  perusing  and  marking  the  distribution  of  rain  as  recorded 
in  that  part  of  this  Essay  relating  to  that  subject.  I may  also 
remark  that  by  far  the  greatest  quantity  of  the  rain  that  falls 
during  the  autumnal  and  early  winter  months  falls  during  the 
night  and  early  part  of  the  morning ; for  the  vapour  being  then 
condensed  into  fog,  if  any  cause  tending  to  a further  condensation 
occur,  rain  is  sure  to  follow  ; whilst,  when  the  moisture  is  invisible, 
and  the  capacity  of  the  air  for  moisture  is  increasing,  it  requires  a 
greater  depression  of  temperature  to  cause  rain,  and  consequently 
less  rain  falls  during  the  day  than  night. 
In  the  western  parts  of  England  and  Ireland,  during  the  months 
of  October,  November,  and  December,  this  kind  of  fog  occurs  on 
an  average  two  nights  out  of  three,  less  or  more.  On  the  eastern 
parts  about  half  the  nights  may  be  free  from  fog;  but  it  occurs 
also  more  frequently  in  the  northern  than  the  southern  parts  of 
our  islands. 
The  second  kind  of  fog  is  different  in  its  origin  from  the  pre- 
ceding kind,  and  different  laws  govern  its  distribution  in  any 
locality.  If  its  source  be  a river  or  lake,  it  will  abound  in  the 
immediate  locality  of  its  cause,  and  will  follow  the  windings  of 
the  river,  or  extend  along  the  low  ground  adjoining  the  lake,  and 
will  always  exist  more  or  less  whenever  the  temperature  of  the 
u'ater  is  much  higher  than  that  of  the  air.  If  this  fog  arise  from 
the  sea,  it  may  extend  much  farther,  and  in  particular  seasons  be 
much  more  regular  in  its  recurrence,  than  when  its  source  is  more 
confined.  A remarkable  instance  of  this  occurs  in  what  is  termed 
the  Eastern  haars,  which  occur  during  the  spring  and  early  sum- 
mer months  on  the  eastern  coast  of  Scotland  and  the  northern 
parts  of  England.  The  German  Ocean,  being-  comparatively 
narrow,  is  raised  in  temperature  more  easily  than  the  Atlantic, 
and  hence  during  the  early  part  of  summer  and  spring  a rapid 
evaporation  takes  place  from  its  waters.  Now  the  same  cause 
which  tends  to  raise  the  temperature  of  the  German  Ocean  also 
tends  to  raise  that  of  the  land.  But  the  land  is  here  narrow  and 
elevated,  so  that  the  vapour  is  condensed  on  crossing  the  land. 
