26 
On  the  Climate  of  the  British  Islands 
do  not,  however,  precisely  represent  the  comparative  dryness  of 
the  air  at  the  places  named,  in  consequence  of  the  greater  amount 
of  vapour  which  the  air  is  able  to  sustain  at  a higher  temperature. 
The  difference  in  July  between  Helston  and  Greenwich  is  given 
as  5 *2  to  4*  8 grains ; but  in  consequence  of  the  higher  tempe- 
rature of  the  latter  place  in  July,  the  air  is  much  less  charged 
with  vapour  than  the  figures  represent.  With  this  general  cor- 
rection, which  will  not  materially  affect  the  results,  especially  as 
showing  the  mean  of  the  year,  the  table  gives  the  hygrometric 
condition  of  the  air  at  the  places  mentioned. 
62.  But  again  ; the  greater  dryness  of  the  air  of  the  eastern 
counties  must  necessarily  result  from  the  daily  range  of  the  ther- 
mometer being  greater  there  than  on  the  western  coast.  Dryness 
depends  on  cold  nights  preceding  warm  days  ; the  low  tempe- 
rature of  the  night  air  causing  a precipitation  of  moisture,  which 
the  ordinary  evaporation  cannot  readily  replace,  and  the  returning 
heat  of  the  day  is  combined  with  a dry  clear  atmosphere.  Let  us 
apply  this  rule  to  two  places  on  the  opposite  sides  of  England,  and 
it  will  furnish  results  which  may  be  relied  on  when  not  violated 
by  accidental  causes. 
A mean  of  ten  years  at  Truro  gives  65° '6  for  the  temperature 
of  a July  day,  and  52°  for  that  of  the  night ; then — 
Vapour  sustainable  at  65° ’6  = ‘629  inches. 
Ditto  ditto  at  52°  = -400  „ 
Dryness,  or  capacity  for  ad- 
ditional vapour  . 
} 
•229  inches. 
A mean  of  the  same  ten  years  at  Chiswick  gives  72° ‘8  for  the 
day,  and  51°  '1  for  the  night  in  July  : — 
Vapour  sustainable  at  72° ’8  = 800  inches. 
Ditto  ditto  at5l0,7  = ‘395  ,, 
Dryness  . . *405  inches. 
In  the  winter  months  the  difference  between  the  humidity  of  the 
air  at  these  places  is  in  a much  lower  ratio. 
Truro,  in  January,  vapour  sustainable  at  46°  *3  = *330  inches. 
Ditto  ditto  ditto  at  39° ’4  = *260  ,, 
Dryness,  or  capacity  for  additional  vapour  ‘070  inches. 
Chiswick,  in  January,  vapour  sustainable  at  43° ' 1 = ‘294 inches. 
Ditto  ditto  ditto  at31°-24  = -194  „ 
Dryness  . 100 
Thus  between  Truro  and  Chiswick  the  dryness  in  the  warmest 
month  is  represented  by  *229  and  *405  ; and  the  coldest  month 
by  • 070  and  • 100 
