496     M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
the  case  of  the  moon,  and  -jig-  in  that  of  the  sun.  But,  as  already- 
stated,  the  exterior  conditions  under  which  the  observations 
have  been  made  are  very  unfavourable  for  the  purpose  in  ques- 
tion ;  so  that  in  mines,  under  the  supposition  that  the  reactions 
of  the  glowing  liquid  nucleus  of  the  earth  are  not  magnitudes  of 
the  same  order,  the  sensitiveness  of  the  instrument  would  be 
considerably  increased.  We  thus  may  even  expect  to  determine 
the  magnitudes  on  which  the  just-mentioned  influences  depend 
by  a  much  extended  and  statistically  treated  series  of  observations, 
— that  is  to  say,  the  masses  and  distances  of  sun  and  moon  in 
units  of  the  mass  and  radius  of  the  earth. 
Another  subject  of  interest  is  connected  theoretically  with  the 
observations  of  the  horizontal  pendulum.  Supposing  the  instru- 
ment is  set  up  as  mentioned  in  the  meridian,  the  pendulum,  if 
moving  only  under  the  influence  of  the  sun,  would  pass  in  twenty- 
four  hours  four  times  through  its  position  of  equilibrium  in  the 
meridian — at  sunrise,  at  sunset,  and  at  the  upper  and  under 
passage  of  the  sun  through  the  meridian.  As  the  movement  of 
the  pendulum  is  not  an  effect  of  summation  as  that  of  the  sea  in 
the  tides,  but  is  generated  directly  by  attractive  action  at  a  dis- 
tance, it  must  take  place  simultaneously  with  the  corresponding 
true  position  of  the  sun.  But  if  gravity,  as  light,  takes  a  time 
of  about  eight  minutes  in  arriving  from  the  sun  to  the  earth, 
the  above  positions  of  equilibrium  would  take  place  so  much  later. 
If,  therefore,  we  only  succeed  in  determining  these  positions 
to  within  one  minute  of  accuracy,  the  question  whether  gravity 
needs  time  for  its  propagation  could  be  decided,  even  if  this  ve- 
locity were  ten  times  that  of  light. 
The  future,  and  persevering  observation,  must  decide  whether 
the  rigidity  of  the  crust  of  our  earth*  is  sufficiently  great,  in  pro- 
portion to  the  above-mentioned  influences,  to  allow  such  obser- 
vations to  be  made  with  success.     At  any  rate,  systematical  ob- 
*  Several  attempts  have  been  made,  chiefly  by  Hopkins,  Sir  W.  Thom- 
son, and  Pratt,  to  decide  the  question  whether  the  nucleus  of  the  earth  is 
solid  or  liquid,  whether  its  crust  is  thick  or  thin,  moveable  or  immove- 
able. The  researches  relating  to  this  question  have,  on  the  one  hand,  taken 
into  consideration  the  influences  of  these  properties  on  the  constant  of  pre- 
cession, aud,  on  the  other,  transferred  directly  the  observations  made  on 
the  rigidity  of  solid  bodies  to  the  conditions  of  cohesion  of  the  earth.  It 
follows  directly  from  the  phenomena  of  the  tides,  that  the  crust  of  the 
earth  is  not  to  the  fluid  nucleus  as  a  swimming  island  or  a  covering  of  ice 
to  the  sea.  But  to  determine  the  degree  of  rigidity  of  the  crust  of  the 
earth  (which  is  compressed  by  immense  forces  according  to  the  principles 
of  cellar-vaults)  from  the  forces  which  the  moon  exerts  on  these  masses 
is,  I  think,  just  as  hazardous  as  to  draw  a  conclusion  on  the  inner  condi- 
tion of  an  egg  from  its  deformation  by  outward  forces.  As  long  as  the  forces 
are  not  strong  enough  to  break  the  egg,  we  shall  hardly  ascertain  in  this  way 
whether  the  egg  is  raw  or  boiled. 
