360     M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
explanation  of  the  observed  decrease  in  horizontal  intensity  and 
increase  in  dip. 
The  increasing  strength  of  magnetic  disturbances  with  in- 
creasing latitude  is  mentioned  by  Lamont  in  the  following 
words  (/.  c.  p.  271) : — "  The  magnitude  of  the  movements  in- 
creases gradually  from  the  equator  towards  the  north  and  south 
pole;  in  the  equatorial  zone  only  slight  movements  are  observed." 
Another  remark  about  the  magnitude  of  the  movements  of  the 
magneticneedlein  disturbances  is  tobefoundonthe  following  page. 
"  I  have  seen  myself  changes  in  declination  of  10  minutes  of 
arc  taking  place  in  one  minute's  time.  In  our  neighbour- 
hood such  rapid  movements  are  extremely  rare ;  in  the  polar 
regions,  however,  it  is  frequently  the  case  that  the  movement  of 
the  instruments  cannot  any  more  be  accurately  measured.  Bra- 
vais  and  his  companions,  in  the  North-pole  Expedition,  1838-39, 
had  several  times  opportunity  to  convince  themselves  of  this 
fact.  Already  in  Petersburg  and  Sitka  very  rapid  movements 
are  not  rare." 
11. 
It  is  easily  seen  that  the  beginning  of  a  disturbance  must  be 
observed  simultaneously  in  all  stations  on  the  earth's  surface; 
for  the  influence  of  the  earth  on  the  needle  is  the  resultant  of 
the  whole  magnetic  action  of  the  earth.  Every  change  in  any 
one  of  the  components  must  make  itself  perceptible  in  our  ob- 
servations as  a  change  in  the  resultant.  It  is  otherwise  with  the 
strength  and  nature  of  a  magnetic  disturbance ;  for  its  intensity 
must  evidently  be  greatest  at  a  point  which  is  nearest  to  the 
seat  of  the  cause.  According  to  our  theory,  such  a  cause  would 
consist  in  the  tearing  away  or  settling  down  of  a  piece  of  rock, 
which  modifies  the  velocity  of  the  glowing  streams  at  the  parti- 
cular spot,  and  hence  also  the  magnetic  action  on  the  needle. 
If  we  call  to  our  minds  the  movements  which,  according  to 
the  laws  of  hydrodynamics  and  the  observations  taken  under 
analogous  conditions,  can  be  noticed  in  liquids,  it  is  easy  to  ac- 
count, from  our  point  of  view,  for  the  propagation  of  these  mo- 
tions. 
Suppose  a  piece  of  rock  breaks  off  from  the  solid  crust  of  the 
earth  just  below  our  feet.  It  would,  following  the  law  of  gravi- 
tation, sink  into  the  liquid,  and  then,  according  to  its  specific 
gravity,  either  rise  again  or  sink  down  into  deeper  parts  of  the 
liquid  nucleus,  where  under  the  influence  of  the  greater  heat  it 
would  dissolve. 
In  both  cases  the  hydrostatic  equilibrium  which  has  been  sud- 
denly disturbed  will  restore  itself  in  the  form  of  an  undulatory 
motion,  which,  starting  from  the  point  where  the  disturbance 
has  taken  place,  will  propagate  itself  in  all  directions  with  a  ve- 
