and  the  Magnetic  Relations  of  the  Heavenly  Bodies.        363 
reaching  an  easterly  maximum  at  180°  from  the  point  where  the 
westerly  maximum  appears."" 
12. 
The  modifications  of  a  magnetic  disturbance  in  the  direction 
of  the  meridian  are  explained  in  the  same  satisfactory  manner  in 
their  general  features  by  the  developed  theory,  as  are  the  modi- 
fications for  the  same  parallel. 
If  we  remember  that  as  the  wave  proceeds  towards  the  poles  it 
must  be  necessarily  hemmed  in  between  its  borders,  and  that  the 
velocity  of  the  current  becomes  larger  with  increasing  equatorial 
distance,  it  is  clear  that  through  the  first  circumstance  the  height 
of  the  wave  proceeding  towards  the  poles  must  increase,  as  the 
height  of  the  tidal  wave  is  increased  by  like  causes  on  the 
surface  of  the  earth.  By  the  second  circumstance  the  intensity 
of  the  disturbance  is  increased  as  well ;  so  that  a  disturbance 
beginning  in  our  latitudes  will  make  itself  more  perceptible 
towards  the  north,  and  less  perceptible  towards  the  south,  with- 
out changing  its  direction  in  the  same  hemisphere. 
If  a  wave  crosses  the  equator  and  propagates  itself  in  the 
southern  hemisphere,  it  is  evident  that  it  must  here  cause  the 
opposite  movement  of  the  needle.  For  as  such  a  wave  can  only 
increase  friction,  and  by  it  the  electrical  current,  the  direction 
of  which  is  determined  by  other  circumstances,  the  same  cause 
which  induces  the  north  end  of  the  needle  to  sink  in  the 
northern  hemisphere  will  in  the  southern  hemisphere  produce  a 
sinking  of  the  south  end. 
All  these  consequences  are  confirmed  by  observation.  La- 
mont,  comparing  the  graphical  representations  of  observations 
made  simultaneously  in  the  same  meridian,  remarks,  on  a  dis- 
turbance beginning  on  August  28,  1841,  at  1  o'clock  in  the 
morning  (p.  273) : — 
"  If  we  go  from  the  equator  towards  the  north  or  south,  the 
movements  are  continually  increasing  in  size ;  the  form  remains 
in  essentials  the  same. 
"  The  southern  and  northern  stations  seem  at  first  sight  not 
to  agree ;  but  there  is  a  perfect  harmony  if  we  observe  that  in 
the  south  the  movements  are  in  an  opposite  direction. 
"  It  is  to  be  noticed  that  the  further  we  remove  from  the 
equator  the  more  discrepancies  in  form  appear.  In  this  obser- 
vation no  stations  situated  far  to  the  north  or  to  the  south  have 
taken  part,  and  we  do  not  know  what  form  the  disturbance  has 
taken  towards  the  poles ;  however,  it  is  known  from  other  obser- 
vations that  in  the  polar  regions  the  disturbances  reach  an  ex- 
traordinary magnitude,  and  change  entirely  their  form." 
"As  regards  the  magnitude  of  the  disturbances,  I  have  de- 
