THE 
LONDON,  EDINBURGH,   and   DUBLIN 
PHILOSOPHICAL     MAGAZINE 
AND 
JOURNAL   OP   SCIENCE. 
[FOURTH   SERIES.] 
JUNE   1872. 
LI.  A  new  discussion  of  the  Hydrodynamical  Theory  of  Mag- 
netism.    By  Professor  Challis,  M.A.,  LL.D.,  FM.S* 
A  THEORY  of  Magnetism  founded  on  propositions  in  Hy- 
drodynamics was  originally  proposed  by  me  in  the  Num- 
bers of  the  Philosophical  Magazine  for  January  and  February 
1861.  The  same  theory,  considerably  modified,  is  given  in  my 
work  '  On  the  Principles  of  Mathematics  and  Physics ;'  and  the 
Number  of  the  Philosophical  Magazine  for  July  1869  contains 
an  article  the  purpose  of  which  is  to  make  the  mathematical  part 
of  the  theory  more  complete.  In  the  present  communication  I 
propose  to  discuss  anew  the  principles  on  which  magnetic  phe- 
nomena are  explained  according  to  this  theory,  with  the  view  of 
correcting  or  extending  the  results  previously  obtained.  The 
discussion  will  necessarily  involve  to  a  considerable  extent  the 
hydrodynamical  theory  of  galvanic  currents. 
1.  I  assume,  as  I  have  already  done,  (1)  that  all  the  active 
forces  in  nature  are  different  modes  of  pressure,  under  different 
circumstances,  of  a  universal  elastic  sether,  which  maybe  mathe- 
matically treated  as  a  continuous  substance  pressing  always  pro- 
portionally to  its  density ;  (2)  that  all  visible  and  tangible  bodies 
consist  of  inert  spherical  atoms  of  constant  magnitude,  held, 
when  undisturbed,  in  positions  of  equilibrium  by  attractive  and 
repulsive  forces,  the  laws  of  which  result  both  from  the  active 
pressure  of  the  sether  and  the  passive  resistance  of  the  atoms  due 
to  the  constancy  of  their  form  and  magnitude.  The  sether  at 
rest  is  accordingly  assumed  to  be  everywhere  of  the  same  den- 
:':  Communicated  by  the  Author. 
Phil.  Mag.  S.  4.  Vol.  43.  No.  288.  June  1872.  2  D 
