PEOFESSOE  PLIJCKEE  ON  THE  MAONETIC  INDUCTION  OF  CETSTALS.  573 
torial  one  takes  place  gradually,  the  crystal  passing  through  all  intermediate  positions 
while  it  rotates  through  several  degrees  round  the  mean  axis  of  rotation.  This  mode  of 
determining  a admits  therefore  of  no  great  accuracy. 
The  inchnation  of  the  cleavage  plane  to  the  horizontal  plane  was  determined,  when 
the  symmetrical  plane  pointed  45°,  bisecting  the  angle  between  the  axial  and  the  equa- 
torial plane.  We  got  for  this  angle,  by  inclining  the  cleavage  plane  on  both  sides,  on 
one  side  21°,  on  the  other  26°,  whence  iy=23^°,  and  the  angle  between  the  normal 
to  the  cleavage  plane  and  the  mean  axis  2^°.  [Instead  of  23^°,  we  got  formerly  25° 
(art.  21) ; instead  of  2^°,  by  direct  measure  3°.] 
62.  2nd.  Then  the  number  of  small  oscillations  was  counted  which  the  same  sphere 
performed  in  one  second,  when  successively  suspended  along  its  three  axes  of  induction. 
In  one  series  of  observations  six,  in  another  twelve  of  Grove’s  elements  were  used.  In 
both  series  the  crystal  was  tvrice  suspended  along  each  axis.  The  following  Table  fur- 
nishes the  numbers  thus  obtained : — 
Suspended  along 
The  greatest  axis 
The  mean  axis  . 
The  least  axis  . 
The  greatest  axis 
The  mean  axis 
Numbers  of  oscillations. 
Six  elements. 
22i 
53 
49 
Twelve  elements. 
....  31 
The  least  axis 
From  the  first  series  of  observations  we  get 
from  the  second. 
i+i=5166, 
73 
67 
^-2809; 
7^=5329. 
23 
53 
49 
31i 
73 
67 
1 
'^/// 
@ 
1^ 
^ HI 
The  differences  between  the  two  numbers  thus  obtained  in  both  cases  (109  and  — 163), 
are  so  small,  with  regard  to  the  limits  of  error,  that  the  theorem  of  article  58  can  be 
considered  confirmed  by  experiment. 
In  order  to  eliminate  these  small  differences,  let  us  remember  that  the  ratio  of  any 
two  corresponding  numbers  in  the  two  series  of  observations  is  to  be  the  same,  this  ratio 
equahng  the  ratio  of  the  inducing  powers  of  the  magnet.  Again,  in  the  two  series  of 
observations,  the  numbers  corresponding  to  the  same  mode  of  suspension  are  likewise  in 
the  same  ratio.  These  two  conditions  are  fulfilled  with  regard  to  the  suspensions  along 
