of  Animals  and  Vegetables . 3 3 
ing  recourfe  to  deep  water,  all  which  places  are  gene- 
rally above  the  freezing  point  in  our  hardeft  frofts: 
however,  our  frofts  are  fometimes  fo  fevere  as  to  kill 
many  whole  habitations  are  not  very  fecure.  « 
When  the  froft  is  more  intenfe  and  of  longer  Hand- 
ing than  common,  or  in  countries  where  the  winters  are 
always  fevere,  there  is  generally  i'now,  and  the  water 
freezes : the  advantage  arifing  from  thefe  two  circurn- 
ftances  are  great;  the  fnow  ferving  as  a blanket  to  the 
earth,  and  the  ice  to  the  water^k 
(e)  Snow  and  ice  are  perhaps  the  worfl  conductors  of  heat  of  any  fubftance 
yet  known.  In  the  firft  place,  they  never  allow  their  own  heat  to  rife  above 
the  freezing  point,  fo  that  no  heat  can  pafs  through  ice  or  fnow  when  at  32°, 
by  which  means  they  become  an  abfolute  barrier  to  all  heat  that  is  at  or  above 
that  degree;  fo  that  the  heat  of  the  earth,  or  whatever  fubftance  they  cover,  is 
retained:  but  they  are  conductors  of  heat  below  320.  Perhaps  that  power 
decreafes  in  proportion  as  the  heat  decreafes  under  that  part. 
In  the  winter  1776  a froft  came  on,  the  furface  of  the  ground  was  frozen; 
but  a conftderable  fall  of  fnow  all'o  came  on,  and  continued  feveral  weeks;  the 
atmofphere  at  this  time  was  often  at  15°,  but  it  was  not  allowed  to  affeCt  the 
furface  of  the  earth  conftderably,  fo  that  the  furface  of  the  ground  thawed, 
and  the  earth  retained  the  heat  of  340,  in  which  beans  and  peas  grow. 
The  fame  thing  took  place  in  water,  in  a pond  where  the  water  was  frozen 
on  the  furface  to  a conftderable  thicknefs ; a large  quantity  of  fnow  fell  and 
covered  the  ice ; the  heat  of  the  water  was  preferved  and  thawed  the  ice,  and 
the  fnow  at  its  under  furface  was  found  mixed  with  the  water. 
The  heat  of  the  water  under  the  fnow  was  at  35°,  in  which  the  fifh  lived 
very  well. 
It  would  be  worthy  of  the  attention  of  the  philofopher,  to  inveftigate  the 
caufe  of  the  heat  of  the  earth,  upon  what  principle  it  is  preferved,  &c. 
Vol.  LXVIII. 
F 
As 
