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On  the  Climate  of  the  British  Islands 
districts  sinks  by  night,  and  the  vapour  becomes  visible  and 
forms  a fog.  The  eastern  counties  and  lands  along  the  coast  are 
more  subject  to  fogs  of  this  nature  than  on  the  western  lands. 
In  November,  after  the  exhalations  of  summer,  and  when  the 
chill  nights  of  winter  begin  to  appear,  these  fogs  are  the  most 
abundant ; the  air  sinks  beneath  the  temperature  of  the  dew- 
point, and  vapour  is  immediately  formed.  The  low  marsh-lands 
adjoining  the  Wash,  and  the  coast  of  Lincolnshire,  being  bordered 
by  hills  and  open  to  the  sea  on  one  side,  are  much  subject  to  fog. 
Such  also  is  the  origin  and  nature  of  the  fogs  in  the  valley  of  the 
Thames,  on  the  lowlands  of  the  Humber  and  Holderness,  and  on 
the  north-east  coast  of  Scotland,  especially  in  Aberdeenshire,  and 
from  thence  to  Orkney. 
66.  On  the  western  coasts  the  fogs  are  chiefly  confined  to  the 
narrow  valleys,  which  are  often  filled  to  a height  of  from  50  to 
100  feet  with  a cold  stratum  of  fog,  which  is  drawn  up  by  the 
morning  sun.  During  the  past  winter  I took  a set  of  observations 
on  the  temperature  of  the  water  in  the  tidal  part  of  the  river  Fal. 
I found  that  on  cold  nights  when  the  temperature  of  the  air  fell 
below  that  of  the  water,  a dense  fog  was  formed  along  the  valley; 
but  when  in  the  morning  the  air  became  heated  to  the  same 
degree  as  the  water,  the  fog  gradually  rose  and  was  dispersed 
over  the  hills. 
Along  the  north  coast  of  Devon  and  Cornwall,  in  the  summer 
of  1847,  a heavy  fog  would  roll  in  at  night  from  the  Atlantic  and 
cover  the  land ; on  the  return  of  day  it  would  again  recede,  and 
might  be  seen  hanging  over  the  sea  along  the  horizon  throughout 
the  day. 
67.  Clouds. — Mists  and  fogs  are  principally  due  to  local  causes, 
and  affect  peculiar  districts,  and  like  clouds  are  masses  of  visible 
vapour,  differing  only  in  their  mode  of  formation.  The  vapours 
which  so  abundantly  rise  from  the  warm  water  of  the  Atlantic, 
become  visible  in  the  higher  regions  of  the  atmosphere,  and  are 
carried  on  by  the  prevailing  south-west  wind  to  these  islands,  in 
accumulated  masses  of  heavy  cloud.  The  north  and  north-east 
winds  on  the  contrary  generally  bring  a clear  sky.  As  it  was  in 
the  days  of  the  earliest  philosopher  so  it  is  now.  “ Fair  weather 
cometh  out  of  the  North,”  says  Job. 
During  rainy  weather  the  clouds  are  but  little  elevated  above 
the  western  lands,  and  may  often  be  observed  below  the  tops  of 
the  hills.  After  continued  rain  from  the  south-west  for  many 
hours  I found  that  the  temperature  of  the  air  was  but  3°  above 
that  of  the  rain-water,  giving  a height  of  about  700  feet  for  the 
greatest  amount  of  saturation. 
In  autumn  and  winter  the  rain-clouds  are  placed  more  fully 
within  the  condensing  power  of  the  western  hills,  by  which  they 
