418  Prof.  Challis  on  the  Hydrodynamical 
as/(r)  is  greater;  and  since  bf(r)=rE(r),  for  a  given  value  of  r 
f(r)  is  greater  as  the  circular  motion  F(r)  is  greater.  The  latter 
function  may  be  considered  to  be  the  exponent  of  the  capacity 
of  the  substance  for  generating  spiral  motion,  and  thereby  con- 
ducting galvanic  currents.  This  property,  which  exists  in  very 
different  degrees  in  different  substances,  seems  to  be  possessed 
in  an  eminent  degree  by  copper,  wires  of  this  metal  being  gene- 
rally used  for  conducting  galvanic  currents.  I  shall  have  occa- 
sion to  advert  again  to  the  origination  of  spiral  motion  by  the 
agency  of  copper. 
30.  The  mathematical  theory  being  incapable  of  determining 
by  itself  whether  the  spiral  motion  is  dextrorsum  or  sinistrorsum, 
it  remains  to  inquire  how  far  this  question  may  be  decided  by 
combining  experiment  with  the  theory.  Experiment,  in  the 
first  place,  may  be  taken  to  indicate  that  the  direction  of  the 
theoretical  circular  motion  about  a  rheophore  is  related  in  a 
constant  manner  to  the  directfon  of  the  current  along  the  rheo- 
phore, so  that  when  the  latter  is  given  the  other  should  admit 
of  being  inferred.  In  order  to  determine  that  relation  I  make, 
the  provisional  hypothesis  that  currents  flow,  or  tend  to  flow, 
out  of  the  poles  which  experimenters  call  positive,  and  into  those 
which  they  call  negative.  That  pole  of  a  horizontal  magnetic 
needle  which  is  southward  is  called  the  positive  pole.  Hence, 
by  the  hypothesis,  magnetic  streams  flow  out  of  it,  and  enter  into 
the  pole  which  is  northward ;  whence  it  follows,  by  the  hydro- 
dynamical  theory  of  the  magnetic  needle  (see  infra,  art.  33),  that 
the  horizontal  components  of  the  terrestrial  streams  flow  south- 
ward both  in  the  northern  and  southern  magnetic  hemispheres, 
and  by  consequence,  as  the  dipping-needle  show's,  that  the  ver- 
tical components  flow  upwards.  Thus  the  total  terrestrial  streams 
issue  from  the  earth  at  and  about  the  north  magnetic  pole,  and 
enter  into  it  at  and  about  the  south  magnetic  pole. 
31.  Again,  that  pole  of  a  voltaic  pile  or  galvanic  battery  which 
has  the  zinc  terminal  plate  is  called  positive,  and  that  which  has 
the  copper  terminal  plate  negative.  Hence,  by  the  hypothesis, 
the  galvanic  current  along  the  connecting  rheophore  outside  the 
pile  or  battery  flows  from  the  zinc  to  the  copper. 
[I  have  previously  taken  the  current  to  be  in  the  contrary  di- 
rection, having  had  difficulty  in  deciding  between  the  discordant 
indications  of  experimenters  on  this  point.  The  direction  shown 
by  arrows  and  the  signs  +  and  —  in  the  figure  in  art.  682  of 
Atkinson's  edition  of  '  Ganot  *■  does  not  accord  with  the  present 
hypothesis ;  neither,  in  fact,  is  it  consistent  with  directions  simi-  ' 
larly  indicated  in  other  cases  in  the  same  work.] 
32.  By  referring  to  two  experiments  described  in  arts.  732 
and  734  of  Atkinson's  '  Ganot/  it  will  be  seen  that  the  coexist- 
