Theory  of  Magnetism,  407 
the  opposite  direction.  To  the  velocity  (V)  in  these  outstanding 
vibrations  it  is  reasonable  to  attribute  the  generation,  in  the 
manner  explained  above,  of  the  magnetic  streams  of  the  theory. 
In  fact  the  generation  of  such  streams  may  be  regarded  as  a  re- 
action arising  from  the  state  of  constraint  into  which  the  sub- 
stance is  put  by  the  abnormal  relative  positions  of  its  atoms. 
According  to  these  views  the  magnetism  of  a  steel  bar  is  in  no 
sensible  degree  due  to  the  earth's  motions  relative  to  the  aether, 
but  results  from  vibratory  motions  of  the  aether  of  the  order  of 
those  by  which,  in  previous  researches,  I  have  endeavoured  to 
account  theoretically  for  intrinsic  molecular  forces.  This  is  an 
important  correction  of  the  principles  I  have  hitherto  adopted 
in  the  hydrodynamical  theory  of  magnetism. 
10.  It  is  a  general  law  of  magnetic  streams  that  they  are  re- 
entering. The  streams,  for  instance,  which  issue  from  that  which 
is  assumed  to  be  the  denser  end  of  a  magnet  are  turned  back, 
and,  after  flowing  in  the  direction  of  the  magnet's  length,  enter 
it  at  the  other  end.  This  general  law  admits  of  being  accounted 
for  theoretically  as  follows.  It  is  evident  that  the  acceleration 
of  a  mass  of  unlimited  dimensions  by  the  action  of  a  finite  pres- 
sure on  a  finite  surface  is  an  infinitesimal  quantity  of  the  third 
order,  and  that,  consequently,  if  the  mass  be  a  fluid  as  nearly 
incompressible  as  the  sether  is  assumed  to  be,  such  pressure  pro- 
duces absolutely  no  movement  of  the  whole  mass  in  either  a  finite 
or  an  infinite  interval  of  time.  Hence  the  displacement,  by  the 
pressure,  of  any  portion  of  the  fluid  in  the  direction  of  its  action 
must  immediately  give  rise  to  the  displacement  of  an  equal  por- 
tion in  the  contrary  direction.  In  other  words,  there  can  be  no 
permanent  flow  of  the  fluid  across  any  plane  perpendicular  to 
the  direction  of  the  impulses.  To  satisfy  this  condition  the  mo- 
tion must  take  place  in  reentering  courses  or  circuits.  Thus 
the  existence  of  complete  circuits  as  a  necessary  condition  of  mag- 
netic, as  also  of  galvanic  phenomena,  is  accounted  for  by  the 
hydrodynamical  theory.  I  am  not  aware  that  any  other  a  priori 
explanation  of  this  very  general  and  prominent  characteristic  of 
physical  currents  has  been  given. 
11.  Another  general  law  relating  to  galvanic  and  magnetic 
circuits  may  be  referred  to  the  hydrodynamical  fact  that  the 
currents  of  a  fluid  always  take  the  easiest  course — that  is,  the 
course  in  which  the  least  resistance  from  the  inertia  of  the  fluid 
is  to  be  overcome.  In  a  bar  magnet  there  is  very  little  magne- 
tism about  its  middle  part  in  directions  transverse  to  its  length, 
the  magnetic  action  taking  place  chiefly  about  the  two  ends,  as 
is  shown  by  immersing  the  magnet  in  iron-filings.  The  hydro- 
dynamical explanation  of  these  facts  is  as  follows.  About  the 
middle  of  the  magnet  there  is  no  transverse  impulse  capable  of 
