88  M.  E.  EdlumTs  Researches  on  the  Electromotive 
for  this  purpose,  is  hollowed  out  at  its  two  extremities  into  the 
form  of  a  fork,  between  the  branches  of  which  the  cylinders  are 
placed,  and  on  which  the  tubes  c,  c,  c'}  c'  rest  and  can  be  fixed. 
The  board  d  d  is  moveable  on  the  horizontal  axis  e,  fixed  to  its 
centre,  so  as  to  make  different  angles  with  the  horizontal  plane. 
The  inclination  of  the  board  to  the  horizontal  is  read  on  the 
graduated  plate  ft  by  means  of  which  the  board  can  also  be  fixed 
in  the  position  desired.  In  one  of  the  ends  of  each  cylinder,  at 
h,  a  brass  tube  is  fixed.  These  tubes,  which  form  a  right  angle 
with  the  ends,  are  themselves  bent  at  a  right  angle,  so  as  to  run 
parallel  with  the  surface  of  the  ends,  and  then  rise  in  the  man- 
ner indicated  in  fig.  2.  The  brass  tubes  are  put  in  communica- 
tion with  each  other  through  the  glass  tube  k,  the  extremities  of 
which  are  curved  upwards  in  order  to  adapt  them  to  the  extre- 
mities of  the  brass  tubes.  The  glass  tube  is  joined  to  the  brass 
ones  by  caoutchouc  tubes,  which  are  tightly  bound  in  order  to 
render  the  joint  impermeable  to  the  air.  Into  the  part  of  the 
brass  tubes  parallel  with  the  ends  of  the  cylinders  the  brass  cocks 
m,  m  are  fitted,  their  plugs  being  pierced  in  the  form  of  a  T. 
Each  tube  has  a  lateral  aperture  at  right  angles  with  the  longi- 
tudinal axis  of  the  tube  and  opposite  to  an  aperture  of  the  cock 
when  one  of  the  others  opens  the  communication  with  the  cylin- 
der or  with  the  glass  tube.  It  clearly  follows  from  this,  that, 
by  giving  the  cock  a  suitable  position,  one  can  either  put  the 
glass  tube  in  communication  with  the  copper  cylinder  and  close 
them  both  to  the  external  air,  close  the  cylinder  and  put  the 
glass  tube  in  communication  with  the  air,  close  the  tube  and 
open  the  cylinder,  or  at  the  same  time  put  in  reciprocal  commu- 
nication the  tube,  the  cylinder,  and  the  air.  The  glass  tube, 
having  an  internal  diameter  of  2*5  millims.,  rests  on  a  brass 
scale  which  is  fixed  to  the  mahogany  board  and  divided  into 
millimetres. 
For  the  purpose  of  protecting  the  copper  cylinders  against 
the  variations  of  the  temperature  of  the  laboratory,  they  were 
each  entirely  covered  with  a  jacket  (g,g)  of  sheet  zinc.  These 
jackets  had  double  walls,  and  would  each  contain  5*8  kilo- 
grammes of  water.  Their  internal  diameter  was  140  millims. ; 
so  that,  the  copper  cylinder  measuring  80,  the  annular  space 
filled  with  air  which  separated  them  had  a  thickness  of  30  mil- 
lims. In  order  that  the  jackets  might  be  easily  taken  off  and 
replaced,  they  could  be  separated  into  four  pieces  accurately 
fitting  one  another.  In  fig.  1  the  upper  halves  as  well  as  the 
ends  of  the  jackets,  and  in  fig.  2  the  ends  alone,  are  removed. 
The  two  jackets  were  exactly  equal  in  dimensions  and  alike  in 
shape ;  and  their  outer  surfaces,  as  well  as  those  toward  the 
copper  cylinders,  had  been  polished,  and  then  varnished  to  pre- 
