in  its  Effect  on  Cultivation. 
G31 
Now  as  the  air  is  rarely  saturated  with  moisture,  and  conse- 
quently, from  what  has  been  stated,  the  elasticity  of  the  vapour 
existing  in  the  atmosphere  being  below  that  due  to  the  tempera- 
ture of  the  air,  but  always  equal  to  the  elastic  force  of  vapour  at 
the  dew  point,  to  determine  the  dew  point  becomes  necessary. 
An  instrument  for  effecting  this  was  contrived  by  Dalton,  ex- 
tremely simple  in  its  arrangement.  He  poured  water  of  a tem- 
perature below  that  of  the  atmosphere  into  a very  thin  glass 
tumbler;  if  dew  immediately  formed  on  the  outside  of  the  glass, 
he  poured  in  a little  warmer  water,  till  dew  just  began  to  form. 
All  that  was  then  necessary  was  to  determine  the  temperature  of 
the  water  by  the  thermometer,  and  hence  the  dew  point;  for  the 
point  at  which  dew  was  formed  was  evidently  the  temperature  at 
which  the  vapour  had  assumed  the  aeriform  or  vaporous  state. 
I need  scarcely  add,  that  the  difference  between  the  dew 
point  and  the  temperature  of  the  air  is  a measure  of  its 
dryness,  and  that  the  greater  the  difference  between  these  points 
the  less  probability  of  rain  : and  in  the  months  of  May  and 
June,  when  the  advantage  of  a moist  atmosphere  is  so  im- 
portant to  the  success  of  the  turnip  crop,  if  the  dew  point  is  not 
more  than  5°  or  6’  below  the  temperature  of  the  atmosphere, 
turnips  may  safely  be  sown  ; for  though  rain  may  not  fall,  yet 
the  soil  will  imbibe  sufficient  moisture  from  the  air  for  the  suc- 
cessful germination  of  the  plant.  I shall  now  proceed  to  give 
the  dew  point,  on  the  average,  for  the  eastern  and  western  parts  of 
the  British  Islands  ; but  as  these  results  can  only  be  general, 
I would  strongly  urge  all  farmers  to  make  use  of  the  above  simple 
process  to  determine  it  for  themselves.  I may  add,  that  a dew 
point  6°  below  the  temperature  of  the  air  has  been  considered  an 
average;  therefore  a greater  difference  than  this  indicates  dryness, 
a less  wetness  of  the  air.  I may  also  add,  that  the  quantity 
of  evaporation  increases  with  a decrease  of  pressure,  and  conse- 
quently at  a height  of  about  15,000  feet  is  increased  by  one-half, 
or  water  will  at  the  same  temperature  give  off  twice  as  much 
vapour  as  at  the  level  of  the  sea.  Mountains  are  always  more 
humid  than  valleys,  and  the  air  nearer  the  point  of  saturation. 
This  is  one  reason  why  much  more  rain  falls  in  elevated  places 
than  at  the  level  of  the  sea. 
From  the  following  table,  which  may  give  a general  idea  on  the 
subject,  and  which  I have  made  as  correct  as  possible,  it  will  be 
seen,  by  comparing  the  figures  with  the  temperature  of  the  different 
months,  that  the  air  is  much  more  saturated  with  moisture  durinjr 
the  months  of  November  and  December  than  in  those  of  corre- 
sponding temperature,  viz.,  February  and  March.  Also,  that 
generally  there  is,  as  had  been  before  inferred,  much  more 
vapour  present  in  the  air  during  summer  than  winter,  and  during 
