in  its  Effect  on  Cultivation. 
63/ 
Again,  during  the  night  it  is  not  to  be  expected  that,  unless  for 
other  purposes,  observers  will  be  found  sufficiently  numerous  to 
determine  the  cloudiness  of  the  sky  with  sufficient  accuracy  to  be 
useful  for  the  proper  determination  of  the  question.  Yet  a rude 
approximation  may  be  come  to  by  any  one  who  carefully  studies 
the  subject;  and  so  far  as  1 personally  have  had  opportunity  of 
judging,  the  results  1 have  given  are  quite  in  accordance  with 
those  arising  from  observation.  Taking  the  whole  of  the  British 
Islands,  there  is  scarcely  a day  cloudless  in  the  year  ; and,  as  far 
as  I have  been  able  to  decide,  1 calculate  that  on  an  average  fully 
one-third  part  of  the  heavens  is  always  obscured  by  clouds ; that 
on  the  w est  side  of  our  islands  three  fifths  of  the  heavens  are  on  an 
average  clouded,  and  on  the  eastern  four-fifteenths  ; but  during  the 
latter  part  of  autumn  and  winter,  on  the  western  side  one-half  of 
the  heavens  is  thus  obscured,  and  on  the  eastern  rather  more 
than  one-third.  The  preceding  remarks  apply  more  to  England 
and  Ireland  than  to  Scotland,  especially  its  northern  parts.  In 
many  elevated  places  in  the  north  of  Scotland,  the  Orkney  and 
Shetland  islands,  the  sky  is  rarely  clear,  and  the  cloudiness  is 
greater  than  in  any  other  parts  of  the  British  Islands.  The  effect 
of  the  cloudiness  of  the  atmosphere  is  to  equalize  the  temperature 
of  day  and  night ; hence  I showed,  in  stating  the  temperature  of 
the  different  months  at  p.  626,  that  those  least  cloudy  varied 
more  than  those  having  a greater  quantity  of  vapour  in  the  air. 
The  reason  of  this  is  evident : the  clouds  prevent  the  radiation  of 
heat  from  the  earth.  The  experiments  of  Glaisher,  as  detailed 
in  the  Philosophical  Transactions  for  1847,  have  fully  settled  this 
point ; but  they  not  only  prevent  the  radiation  of  heat  by  the 
earth  during  the  night,  they  also  prevent  the  access  of  the  sun’s 
rays  to  the  earth  during  the  day ; and  every  one  is  familiar  with 
the  greater  heat  of  the  air  when  the  sun  shines  than  that  when  the 
sun  is  obscured  by  clouds.  The  cloudiness  of  our  climate,  then, 
has  a great  effect  on  its  cultivation ; for  that  cloudiness  is  unfavour- 
able to  the  ripening  of  fruit  and  grain,  but  well  adapted  for  the 
growth  of  grasses  and  plants  of  all  descriptions  cultivated  for  their 
fibre. 
The  direct  rays  of  the  summer  sun  tend  to  scorch  up  pastures, 
but  to  perfect  the  seeds  of  grain  and  the  fruits  of  trees.  The  effect  of 
the  cloudiness  of  the  sky  on  cultivation  is  more  marked  from  being 
added  to  another  of  the  same  tendency.  Thus,  in  places  having 
an  equal  temperature,  more  rain  falls  where  the  air  is  the  most 
cloudy  and  the  evaporation  least.  Hence  on  the  Continent  they 
have  not  only  less  rain,  but  less  clouds  to  protect  the  earth  from 
the  force  of  the  sun’s  rays  ; therefore  we  see  at  once  a reason  for 
the  perpetual  verdure  and  the  luxuriant  growth  of  grass  in  our 
VOL.  xi.  2 t 
