in  its  Effect  on  Cultivation. 
627 
fluencing  the  distribution  of  heat  in  our  climate,  that  the  less 
difference  there  is  between  the  summer  and  winter  temperature 
of  any  place,  the  less  difference  there  will  be  between  its  high- 
est and  its  lowest  daily  temperature.  For  the  general  pur- 
poses of  meteorology,  the  temperature  is  determined  by  thermo- 
meters in  the  shade,  but  the  direct  radiation  of  heat  by  the  sun 
has  a powerful  influence  in  maturing  fruits  and  the  seed  of  plants, 
and  this  direct  influence  of  the  sun  is  greatest  at  the  time  when 
the  sun  is  farthest  north  of  the  equator,  viz.,  the  21st  of  June, 
and  least  on  the  21st  of  December.  The  temperature  of  the  soil 
for  two  or  three  feet  below  the  surface  influences  the  growth  of 
plants,  for  the  juices  of  plants  are  absorbed  by  the  roots,  and 
they  take  up  their  food  much  more  readily  when  the  temperature 
is  high,  just  as  sugar  or  salt  will  melt  in  warm  much  sooner  than 
in  cold  water ; and  when  the  ground  and  the  moisture  near  the 
roots  of  plants  are  frozen,  they  must  cease  to  vegetate,  as  no 
moisture  can  then  be  imbibed.  The  surface  of  the  soil  generally 
averages  through  the  year  2°  to  3°  more  than  the  air,  and  arrives 
at  its  maximum  in  August,  and  its  minimum  in  January.  The  soil 
is  rarely  frozen  more  than  two  or  three  inches  below  the  surface. 
In  giving  the  mean  temperature  of  each  month  it  will  be 
necessary  to  state  the  greatest  variations  in  the  temperature  of 
that  month  in  different  seasons,  letting  it  always  be  borne  in  mind 
that  this  variation  is  greater  on  the  eastern  than  the  western 
coast.  January  has  a mean  temperature  varying  from  42°  in  the 
south-west  of  Ireland  and  Cornwall  to  34°  on  the  eastern  coast, 
and  varies  during  different  seasons  as  much  as  6°,  being  some- 
times 3£°  belowr,  and  sometimes  2^°  above  the  mean.  The 
coldest  instances  within  my  recollection  occurred  in  1838  and 
1820,  and  the  warmest  in  1834  and  1846. 
February  has  a mean  temperature,  varying  from  45°  in  the 
south-west  to  38°  in  the  north-east.  In  1846  this  month  was 
generally  3°  to  4°  above  the  average.  In  1847  and  1838  it  was 
nearly  2°  below  the  average.  Hence  its  range  of  temperature 
from  the  mean  appears  to  be  4°  above,  and  2°  below  the  general 
average  temperature. 
March  has  an  average  of  mean  temperature  varying  from  48° 
to  40°  in  our  islands,  and  varies  from  its  mean  temperature  as 
much  as  4°.  It  was  warmest  in  1841  and  1846,  and  coldesUin 
1837  and  1845.  About  the  middle  of  the  month,  in  1845,  the 
thermometer  fell  to  6°,  the  lowest  I ever  observed  in  this  month. 
April  is  an  important  month  with  respect  to  vegetation,  the 
revival  of  nature  usually  taking  place  at  this  season.  The  tem- 
perature of  this  month  generally  approaches  nearly  to  the  mean 
of  the  whole  year.  In  the  south-west  of  Ireland  and  Cornwall 
its  mean  temperature  is  50°  ; in  Lancashire  and  Cheshire  51°  ; but 
at  London  49°  : and  at  Edinburgh  only  42°.  Its  temperature  varies 
