in  its  Effect  on  Cultivation. 
35 
From  this  Table  the  connexion  between  the  temperature  of 
summer,  and  the  produce  of  the  harvest  is  most  obvious  and 
instructive.  An  average  amount  of  heat  occasionally  produced 
a good  crop,  as  in  1791  ; but  then  experience  shows  that  it  must 
be  accompanied  by  a dry  season.  In  1840,  the  summer  tem- 
perature at  Truro  was  59°,  or  1°  below  the  mean,  and  not  much 
above  half  the  usual  quantity  of  rain  fell.  A finer  harvest  since 
1818  never  occurred  in  the  West  of  England  ; the  wheat  crop  was 
above  the  average,  and  the  yield  excellent.  But,  though  the 
harvest  will  succeed  in  England  at  an  ordinary  summer’s  tem- 
perature, or  even  at  1°  below,  other  things  being  equal,  Scotland 
is  extremely  sensitive  to  the  least  depression  of  summer  heat,  as 
shown  by  the  effects  of  the  summers  of  1800  and  1836,  when 
the  wheat  was  saved  in  tolerable  condition  in  England,  while  a 
large  portion  of  the  crop  was  completely  lost  in  Scotland. 
76.  The  table  shows  that  a cold  wet  summer  is  followed  by 
the  most  lamentable  consequences  ; a deficiency  of  only  2 ’ of 
heat  falls  with  a withering  influence  on  the  harvest  prospects, 
but  when  it  amounts  to  3°  or  4°,  dearth  and  famine  follow  in  its 
train.  Happily,  this  depression  of  temperature  is  of  rare  occur- 
rence, for  in  a period  of  65  years  the  summer  temperature  fell 
only  seven  times  below  the  mean,  from  2°  to  3°  ; and  only  three 
times  during  these  years  did  it  fall  lower  than  3° — in  1810, 
1812,  and  1816.  On  the  other  hand,  for  the  same  period,  the 
summer  temperature  rose  above  the  mean  eight  times  from  2°  to 
3°,  and  five  times  it  exceeded  3°. 
77.  Those  summers  which  have  been  more  than  usually  cold 
have  generally  been  accompanied  by  a large  quantity  of  rain. 
In  fact,  the  rain  has  rather  been  the  cause  than  the  effect  of  the 
low  state  of  the  thermometer.  Such  a season  tends  to  produce 
a large  amount  of  straw,  which,  on  a continuance  of  damp 
weather,  is  peculiarly  liable  to  be  attacked  by  disease  ; hence 
in  the  wet  years  of  1809,  10,  11,  much  damage  was  done  by 
mildew. 
78.  Turning  our  attention  to  the  years  when  the  summer 
temperature  has  been  in  excess,  it  is  found  that  the  abundance 
of  the  crop,  and  the  perfection  of  the  grain  have  almost  invariably 
been  of  the  most  cheering  character.  An  occasional  high  wind 
during  the  flowering  time,  or  a sudden  change  of  weather  at 
harvest,  may  in  some  years  have  done  injury  ; but,  generally,  the 
height  of  the  thermometer  is  a good  indication  of  the  productive- 
ness of  the  harvest. 
These  islands  seldom  suffer  (in  respect  to  the  wheat  crop) 
from  an  excess  of  heat,  and  it  is  of  great  national  importance 
that  a year  of  drought  followed  by  deficiency  scarce  ever  occurs. 
The  year  1818  was  that  which  approached  the  nearest  to  this 
d 2 
