568  PEOFESSOE  PLtrCKEE  ON  THE  MAGNETIC  INHrCTION  OE  CETSTALS. 
51.  In  the  four  suspensions  along  25°,  155°,  205°,  335°,  the  torsion-Tvire  being  attached 
to  the  ring  in  the  corresponding  division,  beginning  firom  one  extremity  of  the  longest 
axis  and  going  round  from  0°  to  360°,  the  mean  axis  2B  pointed  axially,  the  longest 
axis  2A  being  within  the  equatorial  plane.  In  the  four  suspensions  along  30°,  150°, 
210°,  330°,  the  mean  axis  2B  pointed  equatorialLy,  the  longest  axis  2A  being  within 
the  axial  plane ; but  here  the  directing  power  appeared  not  quite  so  strong  as  before, 
and  most  weak  in  the  second  and  fourth  of  these  suspensions.  TMien  suspended  along 
27|°,  152-|°,  207-|°,  332|-°  the  ellipsoid  set  its  mean  axis  2B  axially.  Hence  the  angle, 
denoted  before  by  sy,  is  between  27^°  and  30°,  about  29°,  Therefore  the  angle  between 
the  two  sets  of  ch’cular  sections  of  the  auxiliary  elhpsoid,  i.  e.  the  angle  between  the 
two  magnetic  axes,  is  about  58°,  while  the  two  circular  sections  of  the  induced  ellipsoid 
of  iron  are  perpendicular  to  each  other. 
The  same  experiment  has  been  repeated  exactly  in  the  same  way  with  the  smaller  ellip- 
soid of  iron.  The  resulting  magnetic  axis  had  nearly  the  same  position  as  in  the  former 
case,  the  angle  a being  fully  30°  instead  of  29°.  This  angle  was  not  sensibly  changed 
when  the  distance  from  the  point  midway  between  the  centres  of  the  flat  poles  of  the 
electro-magnet  was  increased  from  30-74  to  39-37  inches. 
52.  2nd.  Then  we  proceeded  to  a second  series  of  experiments,  in  order,  first,  to  verify 
the  equation (30.),  which  maybe  considered  as  the  test  of  Poissox’s  theory;  secondly,  to 
determine  by  means  of  the  method  of  oscillations  the  same  angle  u which  we  formerly 
obtained  by  dh-ect  observation.  We  made  use  of  the  smaller  ellipsoid,  attached  without 
the  divided  ring  by  means  of  very  fine  copper  wires  to  the  silk  thread  of  the  torsion- 
balance,  having  its  centre  always  hi  the  same  place,  39-37  inches  distant  from  the  equa- 
torial plane  of  the  electro-magnet.  In  three  different  suspensions  the  axes  2A,  2B,  2C 
were  successively  vertical,  the  corresponding  principal  planes,  marked  by*  white  paint, 
oscillating  horizontally,  always  at  the  same  height.  It  had  been  ascertained  that  the 
copper  wire  did  not  sensibly  increase  the  moment  of  inertia  of  the  fron  ellipsoid,  nor 
had  the  torsion  of  the  carefully  selected  silk  thread  any  sensible  influence  on  the  number 
of  oscillations.  The  current  being  interrupted,  the  longer  axis  of  the  horizontal  section 
of  the  ellipsoid  was,  before  oscillating,  directed  by  means  of  the  torsion-balance  towai-ds 
the  poles.  The  observed  numbers  of  oscillations  corresponding  to  difierent  magnetic 
powers  were  in  full  accordance.  Here  I shall  refer  only  to  the  last  series  of  obseiwations, 
where  the  current  was  excited  by  twelve  elements,  the  acids  not  ha-sing  been  used 
before,  the  zinc  being  newly  amalgamated.  Immediately  after  each  observation  we 
repeated  the  experiment,  -with  this  difference,  that  that  extremity  of  the  vertical  axis 
which  had  pointed  downwards  was  now  directed  upwards.  Thus  we  got  two  numbers 
relating  to  each  of  the  three  cases.  After  having  determined  the  second  number  of  the 
first  case,  the  ellipsoid  was  brought  again  into  the  primitive  position : thus  was  obtained 
a third  number  equal  to  the  first. 
