in  its  Effect  on  Cultivation. 
633 
immediately  in  contact  with  the  land  will  form  dew;  but  the  air 
not  thus  in  actual  contact  with  the  land  will  have  its  moisture 
condensed,  and  a mist  be  formed,  which  will  appear  to  rise  gra- 
dually as  the  air  is  cooled  farther  from  the  ground.  It  will  thus 
be  evident  that  fogs  will  be  more  frequent  when  the  air  is  nearly 
saturated  with  moisture,  a slight  cooling  thereof  then  causing 
condensation.  They  will,  for  the  same  reason,  be  more  common 
in  marshes  and  damp  situations,  on  account  of  more  vapour  arising 
from  these  than  from  drier  localities.  Hence  the  reason  of  their 
frequency  in  autumn,  for  then  the  air  is  near  the  point  of  satura- 
tion, and  during  the  night  generally  sinks  below  that  point. 
Hence  also  their  frequency,  especially  during  autumn,  in  the 
neighbourhood  of  rivers  and  lakes;  for  the  water,  not  cooling  so 
quickly  in  the  evening  as  the  land  or  the  air,  gives  off  vapour 
which  is  immediately  condensed  and  forms  mist.  I have  fre- 
quently watched  its  formation  and  seen  it  gradually  arise  as  a 
slight  haze  on  the  surface  of  the  water,  and  slowly  rise  higher 
and  spread  along  the  course  of  the  river  as  far  as  the  eye  could 
reach.  I lived  for  many  years  near  a large  lake  surrounded  by 
hills,  and  I have  watched  it  thus  by  the  ligfht  of  the  moon  °ra- 
dually  rise  and  spread  through  the  entire  valley,  and  in  the 
morning  I have  frequently  followed  it  as  it  retired  up  the  hill 
and  cleared  away  when  the  temperature  of  the  air  rose  higher 
than  the  temperature  of  the  lake  at  the  time  of  its  formation.  I 
remember  well  that,  not  being  then  acquainted  with  the  subject 
so  well  as  now,  I wondered  why  the  vapour  did  not  first  disappear 
where  it  had  been  last  to  form,  viz.,  on  the  upper  part,  instead  of 
the  lower  ; but  the  thermometer  assigned  the  true  reason — it  dis- 
appeared at  any  place  when  the  temperature  reached  the  dew 
point,  and  this  was  reached  naturally  in  the  lower  sooner  than 
the  higher  situation,  unless  a current  of  wind  interfered  to  disturb 
the  natural  tendency. 
Fogs  and  mists  may  then  naturally  be  divided  into  two  kinds, 
essentially  different  in  their  origin.  The  first  kind  arises  from 
the  cooling  of  air  saturated  with  moisture.  The  second  arises 
from  vapour  ascending  from  water  of  a higher  temperature  than 
the  air,  which  hence  becomes  condensed  as  it  is  given  off  from  the 
water,  and  spreads  more  or  less  over  the  low  grounds  adjoining 
the  rivers,  seas,  or  lakes,  which  supply  the  vapour. 
The  first  kind  occurs  most  frequently  during  the  night,  and 
continues  till  dispersed  by  the  sun  in  the  morning.  It  is  more 
common  from  September  to  January  than  during  any  other  season, 
November  being  the  month  it  most  prevails  in;  the  reason  of  this 
being,  that  during  these  months  the  air  is  most  saturated  with 
moisture,  and  during  the  night  is  generally  cooled  below  the  point 
