464       M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism. 
daily  variations  of  the  earth's  magnetism  cannot  be  accounted 
for  by  a  direct  magnetic  influence  of  sun  and  moon." 
I  believe  I  can  show  still  clearer  by  the  following  the  in- 
sufficiency of  the  supposition  of  a  direct  magnetic  influence  of 
sun  and  moon  as  the  only  cause  of  the  action  of  these  bodies. 
By  whatever  cause  a  heavenly  body,  as  for  instance  the 
sun,  acquires  magnetic  polarity,  we  must  think  it  probable, 
according  to  the  phenomena  presented  to  us  by  earthly  bodies, 
that  the  potentials  of  the  two  opposite  and  separated  magne- 
tisms are  equally  great.  If  the  sun  possessed  two  magnetic 
poles,  the  earth  in  its  yearly  course  round  the  sun  would 
necessarily  come  twice  into  a  position  in  which  the  magnetic 
induction  of  the  two  poles  would  destroy  each  other.  At 
these  two  days  of  the  year  the  daily  variation  would  disappear 
if  it  were  only  produced  by  direct  magnetic  induction.  We  have 
here  supposed  that  the  two  magnetic  poles  coincide  with  the  geo- 
graphical poles.  But  if,  as  in  the  earth,  this  is  not  the  case*,  and 
if  the  mutual  distance  and  the  intensity  of  the  two  magnetic  poles 
are  sufficiently  great  compared  with  their  distance  from  the  earth, 
the  daily  variations,  if  only  produced  by  the  changes  of  position 
of  these  two  poles  by  the  sun's  rotation,  would  necessarily  disap- 
pear about  every  thirteen  days,  and  then  reach  a  positive  or  ne- 
gative maximum  corresponding  to  the  induction  of  the  sun's 
north  or  south  pole.  By  this  consideration,  therefore,  we  obtain 
an  empirical  criterion  for  deciding  the  question  whether  the  sun 
exerts  an  appreciable  magnetic  induction  upon  the  earth,  and 
whether  the  daily  variations  are  only  produced  by  this  induc- 
tion, or  whether  other  causes  come  into  play. 
19. 
Bearing  in  mind  the  magnetic  influence  of  the  moon  which  we 
have  discussed  above,  it  is  clearly  the  most  probable  supposition 
that  the  sun  exerts  a  double  influence — a  direct  magnetical  one, 
and,  secondly,  as  in  the  case  of  the  moon,  a  dynamical  one, 
through  the  generation  of  a  wave  of  pressure  in  the  glowing  liquid 
nucleus  of  the  earth.  This  dynamical  influence  alone  would,  ac- 
cording to  my  theory,  be  sufficient  to  produce  a  variation  of  the 
magnetic  constants.  It  is  to  be  seen,  from  the  difference  pointed 
out  by  Lloyd  between  the  daily  periods  of  the  sun  and  moon, 
that  the  latter  bear  the  character  of  a  tidal  wave  with  its  two 
maxima  and  minima  in  one  day. 
Mr.  Carl  Hornstein,  the  Director  of  the  observatory  in  Prague, 
in  a  paper  "  On  the  Dependence  of  Terrestrial  Magnetism  upon 
the  Sun's  Rotation,"  has  confirmed  the  fact  that  the  sun  exerts 
*  On  the  necessity  of  this  non-coincidence,  and  its  physical  cause,  com- 
pare what  has  been  said  above,  §  8. 
