Theory  of  Magnetism.  419 
ence  of  the  horizontal  terrestrial  current  with  a  vertical  galvanic 
current  produces  an  action  on  the  rheophore  tending  icestward 
or  eastward  according  as  the  vertical  current  is  ascending  or  de- 
scending. Now  on  the  hypotheses  that  the  earth's  current  is 
southward,  and  the  galvanic  current  in  the  direction  from  the 
zinc  terminal  to  the  copper  terminal,  these  facts  are  explainable 
only  on  the  supposition  that  the  spiral  motion  is  dextrorsum,  that 
term  being  taken  in  the  sense  defined  in  art.  27.  For  under 
these  circumstances  the  magnetic  current  and  the  circular  part 
of  an  ascending  current  are  opposed  to  each  other  on  the  east 
side  of  the  wire  and  concur  on  the  west  side,  and  consequently, 
by  the  hydrodynamics  of  steady  motion,  the  result  is  an  excess 
of  pressure  westward-,  and  similarly,  the  magnetic  current  and 
the  circular  part  of  a  descending  galvanic  current  are  opposed  on 
the  west  side  of  the  wire  and  concur  on  the  east  side,  and  the 
consequent  action  is  eastward.  These  inferences  agree  with  the 
experimental  facts;  and  accordingly  this  argument  is  decisive  as 
to  the  direction  of  the  spiral  motion  on  the  above  hypotheses 
respecting  the  directions  of  the  currents. 
33.  Supposing,  in  accordance  with  this  conclusion,  that  the 
spiral  motion  is  always  dextrorsum,  we  may  proceed  next  to  ac- 
count for  the  results  of  Oersted's  experiment  on  the  same  hypo- 
theses. The  horizontal  component  of  the  earth's  current  flowing 
southward,  it  will  follow  that  the  end  into  which  the  proper 
streams  of  a  magnet  flow  in  converging  courses  is  that  which, 
when  the  magnet  is  suspended  horizontally,  points  northward; 
for  it  is  under  these  conditions  that  the  terrestrial  current  gives 
that  end  a  northward  direction,  as  may  be  thus  shown.  Con- 
ceive the  north  end  to  deviate  from  the  magnetic  meridian 
through  a  certain  angle  icestward,  and  let  the  earth's  current 
and  the  above-mentioned  conver^-ins;  streams  be  both  resolved 
perpendicularly  to  the  axis  of  the  needle.  Then  the  resolved 
parts  will  be  in  opposite  directions  on  the  ivest  side  of  the  north 
end,  and  in  the  same  direction  on  the  east  side,  and  thus  the 
north  end  will  be  driven  eastward.  Similarly,  since  the  needle's 
streams  are  divergent  from  the  south  end,  this  end  will  be  driven 
westward.  Just  the  opposite  effects  would  take  place  if  the  de- 
viation of  the  north  end  were  eastward.  Accordingly  the  needle 
is  in  a  position  of  stable  equilibrium  when  it  points  northward 
and  the  marked  end  is  in  the  magnetic  meridian.  This  theory 
accounts  for  the  directive  action  of  the  earth's  magnetism. 
34.  New  let  a  galvanic  current  be  caused  to  pass  from  south 
to  north  along  a  conducting-wire  placed  over  the  needle  in  the 
magnetic  meridian,  as  in  one  case  of  Oersted's  experiment. 
(See  the  figure  in  art.  627  of  '  Ganot,'  which  is  the  same  as  that 
in  art.  GfJ8  of  Atkinson's  edition.)     Then  supposing  the  con- 
2E2 
