Accordingly  1000  fathoms  of  depth  would  produce  a  diminution  by  ij  ^Vff^^  P^
of  the  fbtal  gravitation— a  differpnce  eo  small  that  it  appears  at  first  sight  impossible
to  constnict  an  instrument  capable  of  indicating  it  witu  sufficient  accuracy.
The  second  part  of  the  paper  described  the  instrument  designed  for  this  purpose,
which  consists  of  a  tube  containing  mercury,  diluted  spirits  of  wine,  and  coloured
iuniper  oil.  The  mercury  column,  about  SO  inches  high,  ascends  in  a  tube  from  tho
bottom  of  a  lanre  bulb  containing  imprisoned  air,  and  terminates  in  the  middle  of  a
second  bulb.  The  remainder  of  the  second  bulb  is  filled  with  the  dUuted  spirits,
which  reach  upward  into  a  narrow  tube  provided  with  a  scale.  Upon  this  rests  a
column  of  the  coloured  oil,  which  terminates  in  a  third  bulb, — the  remaining  space
being  vacuous,  or  nearly  so.  This  gauge  is  enclosed  in  a  glass  tube  fiU^  with
distmcd  water,  which  in  its  turn  is  surrounded  with  ice  contained  in  an  outer
casing*    The  latter  is  suspended  by  a  universal  joint.    The  air  in  the  lower  bulb


74  BBPOBV — 1861

beuig  nudntained  in  Uub  wajn  at  a  pexfectly  nnifiom  temnenton,  wpl  oppoae  s
uniKsm  elastic  force  against  the  colmnn  of  meieiuy^  whicn  lattei^  being  lemovei
from  all  atmoroheric  iimuenoes,  fairly  represents  the  graritation  ox  the  earth.
