PEOFESSOE  PLUCKEE  ON  THE  MAGNETIC  INDUCTION  OP  CETSTALS.  549 
same  deviation  takes  place  in  the  same  way  on  the  other  side  of  the  axial  line.  Hence, 
during  an  entire  revolution  of  the  cyhnder  round  its  horizontal  axis,  this  axis  passes 
twice  through  the  axial  line  joining  the  two  poles. 
8.  II.  Let  the  axis  of  the  circular  cyhnder  lie  in  the  principal  plane  aX,  making  with 
the  crystallographic  axis  a any  angle  <r. 
a.  Whatever  may  be  the  angle  c,  the  cylinder,  when  oscillating  round  its  axis,  this 
axis  being  vertical,  assumes  always  such  a position  that  the  vertical  plane  containing  the 
axes  a and  X becomes  equatorial,  the  axis  x,  axial  (fig.  10). 
h.  When  suspended  in  such  a way  that  its  axis  is  horizontal,  the  cylinder  sets  it  gene- 
rally neither  axially  nor  equatorially.  When  it  rotates  round  its  horizontal  axis,  the 
angle  of  de\iation  from  the  equatorial  line  varies  in  a way  similar  to  that  in  which  in  the 
former  case  the  angle  of  deviation  from  the  axial  line  varied.  There  are  two  positions  of 
the  rotating  cylinder  where  its  axis  is  directed  equatorially.  In  this  case  the  plane  aX 
becomes  vertical  (fig.  11).  The  maximum  of  the  angle  of  deviation  from  the  equatorial 
line  equals  <7,  and  in  this  case  the  plane  aX  is  horizontal  (fig.  12).  During  an  entire 
revolution  of  the  circular  cylinder  round  its  horizontal  axis,  this  axis  passes  twice 
through  the  equatorial  line. 
9.  III.  Let  the  axis  of  the  circular  cylinder  lie  in  the  principal  plane  a»,  making  any 
angle  r with  the  crystallographic  axis  x. 
a.  'WTien  suspended,  its  axis  being  vertical  and  X being  horizontal,  the  cylinder  will, 
according  to  the  value  of  the  angle  r,  set  the  axis  X either  axially  or  equatorially.  By 
varying  the  angle  r you  will  find  a certain  value  of  it,  which  we  may  denote  by  oo, 
for  which  the  circular  cylinder  will  not  be  directed  at  all,  no  more  than  any  amorphous 
paramagnetic  body  of  the  same  shape  and  suspended  in  the  same  manner.  The  angle  a, 
after  some  trials,  was  found  to  be  about  70°.  When  such  a circular  cylinder  (or  plate), 
unaffected  by  the  magnet,  is  gently  inclined,  by  turning  it  round  the  horizontal  axis  X, 
in  one  direction  and  the  contrary,  the  cylinder  (or  plate)  will  again  take  a certain 
direction ; passing  through  the  indifferent  state,  it  will  rotate  round  the  vertical  line  of 
suspension  through  an  angle  of  90°.  When  T<a),  the  axis  X points  axially ; when  r^aj, 
equatorially.  The  directive  power  of  the  poles  has  its  two  maxima  at  t=0  and  r=90°. 
