358     M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
of  the  globe  will  present  the  same  general  appearances  as  a  magnet 
on  the  surface  of  the  earth.  Suppose,  now,  that  in  one  of  the 
circular  wires  surrounding  the  globe  an  alteration  in  the  electric 
current  takes  place ;  the  needle  will  then  be  influenced  in  the 
same  way  in  which  that  particular  turn  of  the  wire  would  influence 
it  if  acting  by  itself  alone.  Suppose  the  turn  of  the  wire  in  ques- 
tion is  just  below  the  freely  suspended  needle  and  the  current  is 
increased.  We  see  at  once  that  the  horizontal  intensity  will  be  di- 
minished and  the  dip  increased.  The  direction  of  the  disturbance 
will  always  be  the  same,  if  those  currents  are  increased  which  are 
running  through  wires  whose  distance  from  the  equator  is  smaller 
than  that  of  the  needle  (i.  e.,  speaking  of  the  earth,  have  a  lower 
geographical  latitude).  A  disturbance  taking  place  in  higher 
latitudes  than  that  of  the  needle,  must  consist  in  a  decrease  of 
the  current  if  the  needle  be  deflected  in  the  same  direction  as 
before. 
If,  therefore,  the  above  rule  given  by  Lamont,  as  a  consequence 
of  his  observations  in  the  temperate  zone,  is  on  the  average  to 
be  considered  correct,  we  may  conclude  from  the  developed 
theory  that,  if  the  cause  of  a  magnetic  disturbance  is  in  higher 
latitudes,  then  the  disturbance  must  consist  in  a  decrease  of  the 
intensity  of  the  subterranean  currents ;  if  the  cause  takes  place 
in  lower  latitudes,  it  must  consist  in  an  acceleration  of  these 
currents. 
10. 
The  probability  of  this  assumption  is  considerably  increased  if 
we  express  mathematically  the  relative  velocities  of  the  glowing 
streams  for  different  geographical  latitudes. 
As  we  have  only  to  consider  the  equatorial  upper  current  of 
the  liquid  mass,  which  by  the  rotation  of  the  earth  and  friction 
against  the  inner  surface  of  the  crust  is  deflected  analogous  to 
the  upper  trade-winds,  the  same  theoretical  considerations  may 
be  used,  mutatis  mutandis,  which  I  have  employed  for  the  deduc- 
tion of  the  law  of  the  sun's  rotation*.  The  conditions  of  the 
problem  agree  in  this  case  even  better  with  the  reality,  inasmuch 
as  we  have  here  to  do  with  a  solid  and  rigid  surface,  which  in- 
fluences by  friction  the  current  of  the  liquids. 
Designating  the  loss  of  velocity  which  a  liquid  particle  suffers 
moving  from  lat.  <j>  to  lat.  <f>-\-8(j>  by  dv,  and  putting  this  loss 
proportional  to 
1.  The  difference  in  velocity  of  two  points  in  the  concave 
surface  of  the  sphere,  of  which  the  difference  in  latitude 
is  d<f>, 
2.  The  area  of  the  surface  of  friction  for  unity  of  mass, 
*  Proceedings  of  the  Royal  Saxon  Society  of  Sciences,  Feb.  11,  p.  54. 
