From  the  wings  of  birds,  the  transition  is  easy
to  the^^ns  of  fish.  They  are  both,  to  their  re-
spective tribes,  the  instruments  of  their  motion ;
but,  in  the  work  which  they  have  to  do,  there  is
a  considerable  difierence,  founded  in  this  circum-
stance. Fish,  unlike  birds,  have  very  nearly  the
same  specific  gravity  with  the  element  in  which
they  move.  In  the  case  of  fish,  therefore,  there
is  little  or  no  weight  to  bear  up  ;  what  is  wanted,
is  only  an  impulse  sufficient  to  carr)^  the  body
through  a  resisting  medium,  or  to  maintain  the
posture,  or  to  support  or  restore  the  balance  of  the
body,  which  is  always  the  most  unsteady  where
there  is  no  weight  to  sink  it.  For  these  offices,
the  fins  are  as  large  as  necessary,  though  much
smaller  than  wings,  their  action  mechanical,  their
position,  and  the  muscles  by  which  they  are
moved,  in  the  highest  degree  convenient.  The
following  short  account  of  some  experiments  upon
fish,  made  for  the  purpose  of  ascertaining  the
use  of  their  fins,  will  be  the  best  confirmation  of
what  we  assert.  In  most  fish,  beside  the  great
fin  the  tail,  we  find  two  pairs  of  fins  upon  the
sides,  two  single  fhis  upon  the  back,  and  one  upon
the  belly,  or  rather  between  the  belly  and  the  tail.
