and  the  Magnetic  Relations  of  the  Heavenly  Bodies.       353 
It  is  known  that  the  assumption  of  electrical  currents  so 
directed  accounts  for  the  most  general  appearances  presented  by 
the  earth's  magnetism ;  at  the  same  time  its  origin  is  assumed 
to  be  by  this  theory  at  a  considerable  depth  below  our  feet — a 
circumstance  very  remarkable  with  reference  to  the  theory  of 
Gauss.  Lamont,  to  whom  we  are  so  much  indebted  for  the 
closer  examination  of  the  earth's  magnetism,  and  who  probably 
amongst  living  philosophers  has  the  most  competent  judgment 
on  this  subject,  makes  the  following  remarks  : — 
"  Gauss  (and  this  is  doubtless  the  most  remarkable  part  of  his 
theory)  has  directed  our  attention  to  a  circumstance  which 
must  become  of  great  importance  for  future  researches ;  for 
under  certain  conditions  we  can  draw  from  it  conclusions  which 
will  allow  us  to  say  whether  the  seat  of  the  magnetic  force 
is  above  us  in  the  atmosphere  or  below  us  within  the  earth,  or 
on  its  surface.  If  we  apply  the  rules-  flowing  from  this  to  the 
formulae  of  Gauss,  we  shall  find  that  the  seat  of  the  earth's  mag- 
netism, if  not  wholly,  at  any  rate  in  its  chief  part  is  situated  below 
the  surface  of  the  earth  "*. 
It  follows  moreover,  from  the  assumptions  justified  above  for 
the  physical  cause  of  the  earth's  magnetism,  that  all  causes  and 
circumstances  modifying  the  direction  and  intensity  of  the  glow- 
ing liquid  streams  in  the  earth  must  necessarily  also  change  the 
direction  and  intensity  of  the  earth's  magnetism. 
Such  causes  are  not  only  possible ;  they  are  necessary.  In  fact, 
if  the  streams  alter  the  inner  side  of  the  earth's  crust  in  a  similar 
manner  as  the  currents  of  water  and  air  alter  the  side  we  in- 
habit, the  form  of  the  bed  in  which  the  glowing  masses  flow 
must  change,  and  hence  the  intensity  and  direction  of  the  cur- 
rents must  vary. 
I  believe  we  observe  these  continual  changes  in  the  so-called 
secular  variations  of  the  earth's  magnetism. 
About  the  attempts  to  explain  these  secular  variations,  Lamont 
expresses  himself  as  follows  : — 
"  If  there  are  many  difficulties  in  reconciling  with  each  other 
the  different  peculiarities  mentioned,  so  as  to  form  a  theory  of 
the  earth's  magnetism,  this  becomes  quite  impossible  as  soon  as 
we  add  the  fact  proved  by  observation,  that  the  distribution  of 
the  magnetism  within  the  earth  changes  sensibly  from  year  to 
year,  and  not  by  starts,  but  by  a  continuous  movement/" 
"  In  order  to  explain  these  secular  variations,  some  have  said 
that  some  parts  in  the  earth's  liquid  nucleus  are  solidifying  and 
so  modify  the  distribution  of  the  magnetism ;  others  have  as- 
sumed the  existence  of  an  invisible  magnetic  planet  revolving  in 
*  Lamont,  Astronomy  and  Magnetism  of  the  Earth.  Stuttgart,  1851, 
p.  260. 
Phil.  Mag.  S.  4.  Vol.  43.  No.  287.  May  1872.  2  A 
