6 
On  the  Climate  of  the  British  Islands 
16.  The  mean  of  the  whole  corresponds  with  the  decrement 
of  Snowdon,  viz.  37-100ths  of  a degree  for  every  100  feet,  or  1° 
decrease  for  every  270  feet  of  altitude. 
If  the  mean  annual  temperature  of  Leith  is  compared  with 
that  of  Bonally,  in  the  same  latitude,  at  an  increased  elevation  of 
1 100  feet,  there  is  a difference  in  the  year  of  4°  15' ; which  gives 
for  a decrease  of  1D  of  temperature,  an  ascent  of  264  feet,  nearly 
agreeing  with  the  previous  result. 
It  is  also  satisfactory  to  find  that  the  decrease  of  temperature 
observed  by  Lagrange  for  small  heights,  is  stated  to  be  1°  for  270 
feet  of  altitude.*  This  result  may,  therefore,  be  taken  as  an 
approximation  near  enough  for  all  practical  purposes.! 
17.  These  observations  tend  to  fix  the  height  of  the  snow-line 
on  the  south  of  England  at  5400  feet,  and  on  the  north  of  Scot- 
land at  about  4000  feet.  In  the  month  of  January  the  snow- 
line descends  in  South  Wales,  Devon,  and  Cornwall  to  2700 
feet  above  the  sea;  in  Scotland  to  1620  feet ; and  on  the  eastern 
coast  of  England  to  1080  feet.  It  is  obvious,  therefore,  that  the 
high  lands  of  Wales,  the  north  of  England,  and  Scotland,  must 
be  exposed  to  a severe  and  rigorous  winter — the  fluctuating  cha- 
racter of  which  renders  it  still  more  detrimental  to  vegetable  and 
animal  existence. 
18.  The  effect  of  elevation  on  the  climate  of  these  islands  is, 
however,  not  so  injurious  to  agriculture  by  increasing  the  winter’s 
cold,  as  it  is  by  diminishing  the  summer’s  heat.  Our  grain- 
producing  plants  are  natives  of  a warmer  climate ; wheat,  in 
particular,  not  only  bears,  but  comes  most  profitably  to  perfection 
where  in  England  the  highest  summer  temperature  exists ; what- 
ever tends  to  diminish  that  temperature  has  a corresponding 
injurious  effect  on  the  products  of  the  harvest. 
J 9.  One  of  the  most  important  agricultural  elements  of  climate 
is  the  amount  of  summer’s  heat.  A certain  quantity  of  heat 
distributed  through  the  summer  months  is  requisite  for  the  per- 
fect maturity  of  the  white  crops;  where  the  temperature  barely 
reaches  the  standard,  there  will  often  be  a deficiency  in  the 
amount  of  produce,  and  more  often  a defect  in  the  quality  of  the 
grain.  And  where  the  summer  temperature  falls  below  the 
standard,  it  is  in  vain  for  the  husbandman  to  struggle  against  a 
destiny  he  cannot  withstand — inferior  grain  crops,  roots  and 
pasture,  are  then  his  best  reliance.  The  summer  temperature  of 
the  cultivated  lands  of  Great  Britain  varies  from  64  to  54  de- 
grees ; and  on  mountainous  districts  it  is  still  lower.  These 
limits  include  a climate  well  adapted  to  bring  the  wheat-crop 
* First  Report  of  British  Association,  p.  220. 
f The  decrease  of  temperature  on  the  Pentland  Hills,  as  observed  by  Professor 
Forbes,  is  1°  for  every  230  feet  of  altitude. — ‘Transactions  of  Edinburgh  Royal 
Society,  1840.’ 
