and  the  Magnetic  Relations  of  the  Heavenly  Bodies.        489 
So  far  as  these  changes  are  generated  by  exterior  causes, 
we  have  only  to  consider  the  sun  and  moon.  It  has  been  shown 
that  these  bodies  may  exert  a  double  influence  on  the  mechanical 
reaction  of  the  liquid  nucleus  against  the  solid  crust  of  the  earth, 
viz. : — 
1.  A  direct  one,  by  generating  a  pressure-  or  tidal  wave. 
2.  An  indirect  one,  by  magnetical  induction. 
The  direct  influence  will  appear  most  strongly  with  the  moon, 
the  indirect  one  with  the  sun. 
The  great  incompleteness  in  which  our  instruments  for  exact 
measurement  for  sudden  changes  in  the  level  of  the  ground  are 
at  present,  reduces  the  mechanical  reactions  able  to  be  observed 
to  oscillations  of  the  ground  strong  enough  to  make  themselves 
perceptible  to  our  senses  in  the  form  of  earthquakes. 
It  is  therefore  to  be  asked  whether  we  can  find  any  connexion 
between  the  frequency  of  these  phenomena  and  the  path  of  the 
moon  which  would  correspond  to  the  influence  discussed  theo- 
retically above.  The  influence  of  the  moon  on  the  moveable  parts 
of  the  earth  must  be  greatest  when  its  distance  from  the  earth 
is  smallest  or  in  the  perigee,  and  then  in  the  two  positions  in 
which  the  moon  adds  its  attractive  action  to  that  of  the  sun  or 
in  the  two  syzygies.  The  minimum  must  fall  on  the  two  qua- 
dratures. 
The  most  complete  and  detailed  collection  of  communications 
regarding  volcanoes  and  earthquakes  is  to  be  found  in  A.  Perrey's 
Bibliographic  Se'ismique*.  In  a  separate  momoirf  the  author 
developes  the  relations  of  earthquakes  and  volcanic  eruptions 
which  are  indicated  by  his  collections,  and  advances  numerous 
theorems  regarding  the  nature  of  these  phenomena.  I  take 
the  liberty  to  add  here  the  parts  which  are  most  important  for 
our  present  purpose,  in  the  words  of  the  reporter  of  the  Berlin 
Physical  Society  (1863),  pp.  718  &c.  :— 
"The  phenomenon  of  earthquakes  is  a  complex  one,  for  which 
wTe  can  with  difficulty  assume  only  one  cause.  Certain  shocks, 
or  series  of  shocks  on  a  given  place,  lead  to  particular  causes,  of 
which  a  certain  number  may  be  independent  of  the  general 
cause,  and  may  have  a  modifying  influence  upon  this. 
ft  We  observe  a  certain  periodical  return  for  which  we  recognize 
an  influence  of  the  moon ;  for  in  the  month  two  maxima  and  two 
minima  appear.  The  moon  attracts  the  glowing  liquid  nucleus 
of  the  earth,  and  generates  a  pressure  against  its  solid  crust,  which 
*  Memoires  de  VAcademie  de  Dijon,  vol.  iv.  pp.  1-112,  vol.  v.  pp.  183- 
253,  vol.  ix.  pp.  87-192,  and  vol.  x.  pp.  1-53.  The  first  register  appeared 
in  1855,  the  latter  in  1863. 
t  "  Propositions  sur  les  tremblements  de  terre  et  les  volcans,  adressees 
a  M.  Lame,  Paris,  1863,"  Silliman's  Journal  (2)  vol.  xxxvii.  pp.  1-10. 
