COMMUNICATION  OF  HEAT.
92
veral  yards,  and  even  of  fathoms.  Thus  nature,  which  has  clothed
the  animals  of  thofe  regions  with  the  richest  and  warmest  furs,  throws
over  the  ground  this  thick  and  downy  covering,  which  has  a  wonderful ¬
  effect  in  preserving  its  heat.  It  cannot,  it  is  true,  preserve  it
at  a  higher  degree  than  the  freezing  point  ;  for  unless  the  furface  be
cooled  to  this  degree,  the  fnow  cannot  be  on  it.  But  this  degree  is
a  great  degree  of  heat,  when  compared  with  the  terrible  and  deadly
cold  which  is  common  during  the  winter  in  the  air  of  those  climates.
In  countries  exposed  to  fevere  winters,  the  thermometer  commonly
falls  to  the  beginning  of  Fahrenheit’s  scale,  or  32  of  his  degrees  below ¬
  froſt  :  a  degree  of  cold  which  is  just  as  much  colder  than  simple
frost,  as  frost  is  colder  than  the  warmth  of  our  summer  weather.  And,
in  the  countries  the  most  remarkable  for  fevere  winters,  a  cold  has  often ¬
  been  observed,  which  was  more  than  twice  as  far  below  frost.  In
Siberia,  at  Krafnojark,  east  of  Siberia,  and  in  some  of  our  settlements -
  in  Hudson’s  Bay,  the  thermometer  has  often  fallen  to  35  and
45  degrees  below  o  of  Fahrenheit:  a  degree  of  cold  in  which  quickfilver -
  was  congealed  by  simply  exposing  it  to  the  cold  air.
In  such  dreadful  colds,  a  great  depth  of  snow  lying  on  the  ground
has  much  effect  in  preserving  the  vegetables.
It  is  proper,  however,  in  this  place  to  remark,  that  this  effect  of
the  rarity,  or  sponginess  of  bodies,  this  retardation  of  communication, ¬
  or  transition  of  heat  through  them,  does  not  take  place  in  fluids
equally  as  in  folids.  Fluids  generally  receive  heat,  and  transmit  it
through  them  more  quickly,  than  the  greater  number  of  solids.  Of
this  we  have  an  obvious  example  in  air,  which,  though  excessively
rare  and  light,  cools  the  bodies  exposed  to  it  remarkably  fast  ;  much
faster  than  we  might  expect  from  our  knowledge  of  its  rarity.
This  depends,  partly,  on  the  capacity  which  such  fluids  may  have
for  the  matter  of  heat,  but  more  on  their  great  expansibility  by  heat,
and  on  the  easy  mobility  of  their  parts  among  one  another;  which
two  last  qualities,  combined  in  the  same  matter,  produce  a  particular