in  its  Effect  on  Cultivation. 
29 
are  drained  of  a large  portion  of  their  moisture,  and  with 
diminished  humidity  and  bulk  they  pass  on  to  water  the  low 
eastern  lands.  Thus  in  Cumberland,  and  Cornwall,  November  is 
a very  wet  month  ; but  at  Chiswick,  Boston,  and  Durham,  less 
rain  falls  in  November  than  in  the  summer  months. 
Table  IX. — Showing  the  Mean  Amount  of  Cloud  for  each  Month  at  the 
places  mentioned,  in  1848,  0 to  10  (Truro  and  Exeter  give  the  mean  of  10 
and  9 years,  the  other  places  are  reduced  by  the  mean  thus  obtained)  : — 
Place. 
Jan. 
Feb. 
Mar. 
Apr. 
May. 
June 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
Greenwich  Observatory 
6-6 
7-6 
5-6 
7*3 
3-0 
7*4 
6-6 
7-6 
5'6 
7-3 
6*7 
6-7 
Walworth  .... 
4-a 
4-8 
2-G 
8-2 
1*3 
7-5 
4-5 
4-8 
2-6 
Aylesbury  .... 
G'7 
7-1 
4-5 
6-8 
2'3 
8-5 
6-7 
7*1 
4-5 
7-3 
5*7 
7'4 
Saffron  Waldren 
4*7 
5-4 
3-9 
G-3 
2-7 
6-0 
4-7 
5 4 
3-9 
4-9 
5-7 
5-9 
Leicester  • . . • 
5-5 
5-9 
4-5 
8-1 
2-2 
7-0 
5-5 
5-9 
4-5 
6-5 
5-5 
7-0 
Durham 
5-7 
5-5 
6-2 
G'3 
t-4 
6*8 
5-7 
5-5 
6-2 
7-3 
5-4 
6*7 
Stou ey hurst  College 
7-7 
7-1 
7-4 
7-1 
5-4 
7-8 
7-7 
7-1 
7-4 
7-2 
7-0 
7-7 
Liverpool  .... 
7-0 
6-4 
6‘7 
5-6 
3-7 
8-5 
7-0 
6-4 
6-7 
6-9 
7*1 
7'0 
Southampton  . • . 
c-o 
7*0 
7-0 
7-0 
2-5 
7-3 
6-0 
7-0 
7-0 
6-0 
6-5 
70 
Exeter 
4-2 
4'6 
4-1 
2-8 
1-0 
3'5 
4*2 
4-6 
4-1 
4*4 
50 
6'0 
Truro  
6-2 
6-8 
5-8 
6-5 
2-4 
7-1 
6-2 
6*8 
5-8 
6-7 
7-5 
7-5 
The  table  shows  a greater  amount  of  obscuration  for  the  eastern 
land  than  is  generally  supposed  to  prevail  ; but  the  clouds  are 
there  found  at  a higher  altitude,  and  less  affect  the  surface  soil. 
May  month  is  remarkably  open  and  unclouded,  so  that  the  spring 
crops  receive  much  benefit  from  the  solar  ray,  when  light,  heat, 
and  moisture  are  required  to  promote  a rapid  growth.  The 
cloudy  state  of  the  western  sky  materially  prevents  the  radiation  of 
heat  from  the  soil  during  the  night,  so  that  warm  close  nights, 
without  any  deposition  of  dew,  often  occur;  this  leads  to  the 
consideration  of 
68.  Dew. — In  a calm  clear  night  the  radiation  of  heat  from  the 
soil  is  so  rapid  that  the  surface  of  a grass-plot  is  often  10°  or  15° 
cooler  than  the  air  a few  feet  above  it.  The  lower  stratum  of 
air  which  immediately  rests  on  the  grass  is  cooled  down  below 
the  dew  point,  when  its  excess  of  moisture  is  deposited  on  the 
herbage. 
My  observations  show  that,  in  September  and  October,  1849, 
the  warm  cloudy  nights  in  the  south-west  of  England  were 
accompanied  with  very  little  dew,  on  some  mornings  the  grass 
was  quite  dry ; but  with  a calm  night,  and  a clear  sky,  the  dew 
was  most  abundant.  On  the  eastern  coast  the  clearer  nights  and 
the  greater  diurnal  range  of  the  thermometer  tend  to  produce  a 
larger  deposition  of  dew. 
Dr.  Dalton  estimates  the  annual  deposit  in  this  country  to  be 
5 inches,  or  about  22,000,000  of  tons. 
69.  Hoar-frost,  the  ice  of  dew  and  mist,  is  formed  when  the 
temperature  of  the  lower  stratum  of  air,  which  rests  immediately 
