424  Prof.  Challis  on  the  Hydro  dynamical 
same  principles  as  those  adopted  and  exemplified  in  the  foregoin 
case ;  and  their  phenomena  may  be  similarly  accounted  for. 
41.  To  complete  this  review  of  the  Hydrodynamical  Theory  of 
Magnetism,  it  remains  to  discuss  and  correct  the  theoretical  ex- 
planations I  have  proposed  respecting  certain  phenom en  a  of  Terres- 
trial Magnetism.  At  the  endof  the  article  "  On  Atmospheric  Tides" 
in  the  Number  of  the  Philosophical  Magazine  for  January  1872, 
I  have  intimated  my  abandonment  of  the  hypothesis  that  the 
lunar- diurnal  variation  of  terrestrial  magnetism  is  attributable 
to  gradations  of  density  of  the  atmosphere  produced  by  the  moon's 
gravitational  attraction.  Having  in  thaf  article  succeeded  in  sol- 
ving with  sufficient  generality  the  problem  of  the  disturbance  of 
the  atmosphere  by  the  moon,  I  found  that  neither  the  law  nor  the 
amount  of  the  gradation  of  density  due  to  the  moon's  attraction 
could  account  for  the  facts  of  the  lunar-diurnal  variation.  Being 
thus  compelled  to  seek  for  another  explanation,  I  reconsidered  the 
views  advanced  in  my  first  essay  towards  a  theory  of  magnetism 
published  in  the  Numbers  of  the  Philosophical  Magazine  for 
January  and  February  1861,  and  have  come  to  the  conclusion  that 
the  gyratory  motions  of  the  sether  there  attributed  to  the  rotations 
of  the  bodies  of  the  sun,  moon,  and  planets  about  axes  are  strictly 
deduced  from  the  physical  principles  on  which  the  theory  is 
founded,  and  must  accordingly  be  regarded  as  necessary  conse- 
quences of  the  hypotheses  of  the  theory.  In  fact  I  do  not 
think  that  I  could  say  on  this  part  of  the  subject  any  thing  dif- 
ferent from  what  is  said  in  arts.  27-30  contained  in  the  February 
Number,  which,  after  giving  this  reference,  I  consider  it  unneces- 
cessary  to  reproduce  here. 
42.  With  respect  to  all  magnetism  which  has  a  cosmical origin, 
the  view  that  I  now  take  is  that  it  is  due  to  gyrations  of  the 
sether  produced  by  the  impulses  it  receives  from  the  motions  of 
the  constituent  atoms  of  the  bodies  of  the  solar  system.  The 
gyrations  may  either  be  immediately  generated  by  the  rotations 
of  the  bodies  about  their  axes,  or  indirectly  result  from  disturb- 
ances of  the  sether  caused  by  their  motions  of  translation.  Ac- 
cording to  hydrodynamics,  the  motion  of  translation  of  a  mass 
constituted  atomically  (as  stated  in  art.  1)  will  continually  im- 
press on  the  sether  motions  whose  mean  direction  is  at  the  first 
instant  directly  contrary  to  that  of  the  motion  of  the  mass. 
This  impressed  motion  will  be  subsequently  converted  into  cir- 
culating or  gyratory  motion,  because,  according  to  the  argument 
in  art.  10,  there  can  be  no  permanent  transfer  of  any  portion  of 
the  setherial  fluid  across  any  fixed  unlimited  plane.  Such  cir- 
culating motions  will  necessarily  partake  of  the  motions  of  trans- 
lation of  the  bodies  which  generate  them,  so  as  to  have  always 
the  same  geometrical  relations  to  these  bodies,  provided  their 
motions  be  uniform. 
