iii  its  Effect  on  Cultivation. 
37 
method  may  be  the  most  definite,  hut  it  appears  to  give  very 
dubious  results,  and  would  be  of  little  practical  utility  to  the 
British  farmer.  I shall  therefore  continue  the  same  line  of 
inquiry  which  I have  thus  far  marked  out,  and  which  admits  of 
easy  reference  to  the  indications  of  the  thermometer,  and  to  the 
tables  of  temperature  which  accompany  this  essay,  by  examining 
the  summer  temperature  requisite  for  the  production  of  wheat 
and  the  modifying  effects  of  cloud  and  moisture. 
The  wheat-plant  is  distributed  over  a wide  range  of  climate, 
from  the  hot,  fertile  valley  of  the  Nile  to  the  glens  of  Norway ; 
it  will  therefore  bear,  or  rather  requires  a higher  temperature  than 
is  usually  found  in  the  British  Isles.  This  limits  our  inquiry  to 
the  minimum  amount  of  heat  necessary  for  the  ripening  process 
under  these  cloudy  skies. 
The  geological  survey  of  Sir  Henry  De  la  Beche,  and  the 
numerous  railway  sections  taken  through  the  west  of  England 
form  a good  basis  as  to  altitude ; and  from  a long  acquaint- 
ance with  the  state  of  the  crops  in  this  part  of  England  I have 
ascertained  that  wheat  in  general  fails  to  come  to  that  perfection 
which  justifies  cultivation,  at  a height  of  600  feet  above  the  sea  ; 
allowing  for  elevation,  this  leaves  a summer  temperature  of  58°. 
In  confirmation  of  this  position  it  is  found  that  a deficiency  of 
2°  of  summer  heat  is  followed  by  a great  deficiency  of  crop  in  the 
lower  lands,  where  the  average  temperature  of  this  season  is  60°. 
On  the  eastern  plain  of  England  there  are  few  hills  to  which 
the  scale  laid  down  (16)  can  be  applied.  But  when  the  summer 
temperature  has  fallen  to  58°  the  crop  has  invariably  been  most 
defective;  in  the  year  1812  it  fell  to  570-2,  and  the  nation  was  on 
the  verge  of  famine. 
The  hills  and  lowlands  of  Scotland  afford  for  that  district  a 
much  better  criterion.  At  Hopetoun  House,  in  West  Lothian, 
where  wheat  has  been  most  successfully  cultivated,  the  average 
summer  heat  of  26  years  is  somewhat  below  58°.  The  crop  has 
been  pushed  so  far  to  the  North  of  Scotland,*  that  the  mean  heat 
of  the  season  is  only  56°.  But  then  it  is  only  in  warm,  sheltered 
vales  where  the  harvest  can  be  relied  on. 
83.  Thus  in  Scotland  the  minimum  summer  temperature  required 
is  from  56i°  to  57°.  On  the  south  of  England,  where  the  summer 
days  are  shorter,  and  other  things  being  equal,  the  amount  of  solar 
heat  in  the  same  time  is  less,  58°  are  requisite. 
It  has  appeared  to  me  that  the  extra  humidity  of  the  western 
coast,  and  the  greater  amount  of  cloud  supposed  to  exist  there, 
requires  to  be  compensated  by  an  amount  of  atmospheric  heat 
somewhat  higher  than  on  the  eastern  coast,  but  I do  not  find 
* Journal  of  Highland  Agricultural  Society,  No.  14,  p.  471. 
