in  its  Effect  on  Cultivation. 
II 
heavier,  and  a stratum  of  clay  generally  forms  the  subsoil.  At  a 
moderate  altitude,  and  especially  where  trap  rocks  have  been 
intruded,  this  soil  will  bear  as  much  as  40  inches  of  rain  annually 
without  injury  to  the  crops;  but  on  high  lands,  where  a much 
larger  quantity  of  rain  falls,  it  cannot  percolate  through  the  clayey 
subsoil,  and  either  runs  in  torrents  off  the  mountain-side,  or 
gathers  into  pools  and  lakes  on  the  flat  and  basin-shaped  lands. 
It  is  to  the  heavy  rains  on  elevated  lands  and  the  inability  of  the 
rock  to  absorb  this  excess  of  moisture,  that  the  extensive  peat- 
mosses of  Scotland  and  England,  and  the  bogs  of  Ireland,  owe 
their  origin.  This  wet  state  of  the  soil  tends  to  lower  the  summer 
temperature  of  such  lands  below  what  might  be  expected  from 
their  situation  ; the  rays  of  heat  do  not  effectually  penetrate  the  soil, 
and  the  constant  evaporation  tends  to  chill  both  the  soil  and  the  air. 
The  valuable  observations  of  Mr.  Parkes  show,  that  in  the 
month  of  June  the  temperature  of  a bog  at  7 inches  below  the 
surface  was  47°,  whilst  that  of  worked  soil  at  the  same  depth 
varied  from  49°  to  66°.*  On  the  16th  of  September,  the  air  in 
the  shade  being  65°,  I ascertained  that  the  temperature  of  the 
upper  two  inches  of  garden-mould  exposed  to  the  sun’s  rays  was 
84°.  At  4 inches  deep  the  thermometer  stood  at  69°.  There  is, 
therefore,  a great  difference  between  the  reception  and  consequent 
radiation  of  heat  by  the  surface  of  a bog  and  by  worked  soil. 
29.  The  low  summer  temperature  of  the  great  central  plain  of 
Ireland  arises  chiefly  from  this  cause.  Its  undulating  surface  is 
only  from  100  to  200  feet  above  the  level  of  the  sea,  but  the 
compact  structure  of  the  mountain  limestone  retains  the  rain- 
water on  its  surface  ; hence  the  extensive  bogs  and  low  summer 
temperature  of  this  region.  Partial  and  detached  efforts  to  drain 
and  cultivate  this  land  will  be  attended  by  many  disadvantages, 
but  a general  and  effective  system  of  drainage  would  not  only 
reclaim  the  soil,  but  materially  improve  the  climate. 
30.  Rain. — The  evaporation  constantly  arising  from  the  surface 
of  the  earth  and  the  sea,  on  meeting  with  a colder  stratum  of  air, 
becomes  visible  in  the  form  of  clouds,  which  on  a further  decrease 
of  temperature  form  small  globules  of  water,  and  ultimately  falls 
in  rain.  The  general  amount  of  rain  is  therefore  in  proportion  to 
the  quantity  of  vapour  raised  into  the  air,  the  direction  in  which 
the  clouds  are  carried  by  the  prevailing  winds,  and  the  change  of 
temperature  necessary  for  condensation.  Equatorial  regions  have, 
therefore,  a much  larger  quantity  of  rain  than  falls  in  temperate 
climates;  and  according  to  the  scale  laid  down  by  Humboldt, 
and  which  decreases  from  the  equator  northward,  the  amount 
which  falls  in  this  country  should  be  about  22  inches  annually. 
* Journal  of  Royal  Agricultural  Society  of  England,  vol.  v.  p.  142. 
