630 
Climate  of  the  British  Islands 
the  other  of  vapour,  each  of  these  being  governed  bv  its  own 
laws,  and  exerting  no  effect  upon  the  other  save  that  of  a 
mechanical  resistance  when  in  motion  in  opposite  directions. 
Every  one  is  familiar  with  the  fact  that  water  when  sufficiently 
heated  has  a tendency  to  fly  off  in  steam,  and  also  with  the  fact 
that  this  tendency  is  greater  as  the  temperature  is  increased. 
That  this  tendency  is  very  great  in  some  cases  is  proved  by  the 
fact  that  if  this  vapour  is  not  permitted  to  escape  with  sufficient 
rapidity,  its  force  is  sufficient  to  burst  asunder  the  strongest 
boilers.  But  it  has  only  recently  become  known  that  water  at 
all  temperatures,  even  below  the  ireezing  point,  has  a tendency 
to  give  off  vapour  or  steam.  Now  it  being  known  that  water  has 
this  tendency  to  form  and  throw  off  vapour,  it  follows  that  it  will 
always  do  so  unless  prevented  by  some  cause.  Thus  at  the 
temperature  of  60°  water  has  a tendency  ^o  throw  off  vapour  of 
an  elastic  force  equal  to  0 524  ; and  if  nothing  counteracted  this 
tendency,  wrater  of  that  temperature  would  continue  to  give  off 
vapour  of  that  force,  and  in  a still  atmosphere  would,  from  a 
vessel  6 inches  in  diameter,  convert  into  vapour  2T  grains  of 
water  every  minute.  But  if,  instead  of  there  being  nothing 
present  in  the  atmosphere  to  counteract  this  tendency,  there 
was  already  vapour  in  the  atmosphere  having  an  elastic  force 
of  0-263 ; then  the  tendency  to  form  vapour,  viz.,  0 524, 
would  be  opposed  by  a force  of  0'263,  and  the  real  resulting 
tendency  would  be  the  difference,  or  0*261,  and  the  water 
evaporated  from  a similar  vessel,  instead  of  2T  grains,  would 
be  only  1-045  grain  per  minute.  Again,  it  is  evident  that  if  the 
force  of  the  vapour  in  the  air  was  equal  to  the  evaporating 
force  of  water,  then  no  evaporation  would  take  place,  and  the  air 
would  be  what  is  termed  saturated  with  moisture,  that  is,  unable, 
without  an  increase  of  temperature,  to  receive  any  more.  It  is 
also  evident  that  were  the  temperature  by  any  means  lowered,  the 
force  of  the  vapour  would  be  greater  than  the  force  of  water  to 
form  it,  and  to  restore  the  equilibrium  a portion  of  the  vapour 
would  again  be  precipitated  on  the  surface.  The  point  at  which 
this  condensation  takes  place  is  called  the  dew  point.  This 
point,  when  the  air  is  saturated  with  moisture,  is  evidently  the 
same  as  the  temperature  of  the  air  ; but  when  the  air  is  not  thus 
saturated,  it  will  be  below  that  temperature.  Thus,  supposing 
the  heat  of  the  air  60°,  then,  as  before,  the  force  of  vapour  is 
0-524  ; but  if  vapour  exist  in  the  air  of  an  elastic  force  equal  to 
0-375,  then,  as  we  find  from  a table  calculated  for  the  purpose, 
that  it  requires  only  a temperature  of  50°  to  give  off  vapour 
having  this  force,  the  air  might  be  cooled  10°,  or  from  60Q  to  50J, 
without  any  condensation  of  vapour  taking  place,  but  the  least 
decrease  below  this  point  would  be  followed  by  condensation,  or 
the  conversion  of  a part  of  the  vapour  into  water. 
