460     M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
face.  Most  likely  the  observations  of  the  next  few  years  will 
place  this  relation  beyond  all  doubt. 
We  must  now  ask  whether  the  moon  can  also  have,  according 
to  my  theory,  a  magnetic  influence  on  the  earth,  and  of  what  kind 
this  influence  must  be. 
The  magnetic  influence  at  a  distance  of  the  hearenly  bodies  is, 
according  to  the  theory  developed  here,  connected  with  two  con- 
ditions : — 
1.  The  existence  of  liquids  flowing  according  to  definite  laws, 
either  on  the  surface  or  in  the  interior  of  the  bodies. 
2.  The  bodies  must  have  such  dimensions  that  their  size  is 
not  too  small  in  comparison  with  their  distance,  so  that  an  incli- 
nation of  their  magnetic  axis  may  cause  a  difference  in  the  action 
of  the  two  poles. 
The  first  of  these  conditions  may  be  regarded  as  not  fulfilled 
in  the  moon,  if  we  consider  its  time  of  rotation  as  well  as  its 
general  exterior  appearance.  But  it  is  known  that  the  second 
condition  is  the  cause  of  the  tides. 
If,  therefore,  the  moon  wrere  of  a  magnetic  mass  or  of  one 
liable  to  electric  induction,  i.  e.  if  its  chief  constituents  were  good 
conductors,  the  varying  position  of  the  earth's  axis  with  respect 
to  the  moon  would  necessarily  have  an  influence  upon  the  earth's 
magnetism.  This  influence,  according  to  our  theory,  is  incon- 
ceivable without  a  simultaneous  reaction  on  the  inner  streams  of 
the  earth.  It  follows  that  the  influence  which  the  moon  or  any 
heavenly  body  exerts  on  the  earth  by  induction  cannot  be  so 
simple  as  the  inductive  actions  we  observe  in  our  laboratories  in 
solid  masses  of  small  dimensions.  Therefore  we  can  in  general 
only  expect  to  find  the  duration  of  the  period  which  depends  on 
the  declination  and  hour-angle,  while  the  times  of  maximum  and 
minimum  can  only  be  found  theoretically  by  taking  account  of  the 
mechanical  reaction  necessarily  accompanying  magnetic  induction. 
I  take  the  liberty  of  stating  in  the  following  the  facts  proved 
by  the  observations  of  reliable  philosophers,  and  I  leave  to  the 
reader  to  form  an  independent  judgment  as  to  how  far  these  ob- 
servations can  be  taken  as  confirmations  of  the  above  conse- 
quences of  my  theory. 
As  regards  the  historical  question  of  the  discovery  of  a  mag- 
netic influence  of  the  moon  upon  the  earth,  I  cite  the  words  of 
Lamont*. 
"  Kreil  was  the  first  who  deduced  from  his  observations  an  in- 
fluence of  the  moon  on  the  declination,  and  then  also  on  the  in- 
tensity ;  his  results  were  confirmed  by  Brown,  who  also  found  an 
influence,  which,  however,  was  not  the  same  in  all  seasons;  re- 
cently Airy  has  occupied  himself  with  this  problem,  and  arrived 
*  Berliner  Berichte  der  physikalischen  Gesellschaft,  1861,  p.  558. 
