SOME  EFFECTS  OF  THE
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these  objects  to  the  eye,  is  disturbed  a  little  from  its  straight  course,
by  passing  through  the  streams  of  warm  air  ;  and  coming,  therefore,
to  the  eye,  with  fuccessive  changes  of  its  direction,  occasions  an  un
steadiness  in  the  apparent  place  of  the  object,  or  parts  of  the  object,
from  which  it  comes.
We  also  learn  from  this  effect  of  heat  upon  air,  the  reason  of  the
common  opinion,  that  heat,  or  fire,  has  a  tendency  upwards.  This
opinion  has  been  founded  on  the  stream  of  warm  air  which  may  be
felt  ascending  from  any  hot  body  exposed  to  the  atmosphere  ;  and
the  appearance  and  properties  of  flame  have  also  contributed  to  the
support  of  such  an  opinion.  Flame,  in  consequence  of  its  being  composed ¬
  of  a  luminous  fluid,  rarer  than  air,  is  driven  upwards,  in  this
laſt,  on  every  occation.  To  be  convinced,  however,  that  heat  has
not  a  prevalent  tendency  upwards,  we  need  only  ſhut  up  a  hot  body
in  a  glass  receiver,  and  exhauft  the  air  :  We  fhall  then  find  the  heat
penetrating  equally  through  every  part  of  the  receiver  ;  or,  if  it  af
fects  some  more  than  others,  it  is  those  that  are  nearest  the  hot  body.
But,  let  the  air  be  admitted  into  the  receiver,  the  uppermost  part  of
the  glaſs  will  soon  be  the  hottest,  although  the  most  distant  from  the
hot  body.
Heat,  therefore,  by  rarefying  air,  and  thereby  producing  à  motion
in  the  parts  of  this  fluid,  is  more  quickly  and  equally  dispersed
through  it  than  through  any  solid  quantity  of  matter,  and  occasions
the  appearances  above  described.
There  is  one  other  fact  that  depends  on  this  principle,  and  which
deserves  particular  notice,  on  account  of  its  importance.  This  fact  is
the  ventilation  of  mining  works  that  are  properly  planned,  and  the
proper  effect  of  fire-places  and  vents  in  houses  rightly  constructed.
In  mines  that  are  properly  planned,  a  stream  of  air  passes  constant
ly  through  the  mine,  and  with  this  remarkable  circumstance,  that  it
has  one  direction  in  summer,  and  another  in  winter.
To  understand  this,  it  is  necessary  to  know,  that  in  beginning  to