484     M.  F.  Zollner  on  the  Origin  of  the  Earth7 s  Magnetism, 
First  minimum     .     . 
10-0130  C.  form: 
=   15  14 
Absolute  maximum    . 
10-7857 
109  15 
Absolute  minimum    . 
9-4955 
246  47 
Second  maximum 
10-2682 
310  48 
"  It  is  remarkable  that  already  Nervander  (Bulletins,  vol.  iii, 
p.  14)  suspected  the  existence  of  at  least  two  minima  and  two 
maxima.  If  we  must  content  ourselves  to  have  proved  the  exist- 
ence of  a  variation,  and,  as  it  seems,  a  variation  without  equal 
decrease  and  increase,  while  the  coefficient  of  the  inequality 
cannot  be  determined  accurately  from  the  observations  at  our 
disposal — still,  if  the  sun  belongs  to  the  variable  stars,  the  ex- 
istence of  two  greatest  and  two  smallest  values  corresponds  to 
the  phenomena  of  light  observed  on  /3  Lyrse"*. 
It  is  evident  that  the  hope  to  determine  accurately  the  position 
of  the  poles  of  cold  of  the  sun  (which,  according  to  my  theory, 
must  coincide  with  the  magnetic  poles)  can  much  more  easily  be 
realized  by  actinometric  than  by  thermometric  observations  of  the 
temperature  of  the  air.  These  observations  can  be  made  simply 
by  comparing  two  thermometers  put  very  near  together,  one  of 
which  has  its  bulb  exposed  to  the  sun  for  a  certain  number  of 
seconds,  whilst  the  bulb  of  the  other  is  in  the  shade.  The  dif- 
ference in  the  expansion  of  the  mercury  in  the  two  bulbs  fur- 
nishes us  the  means  for  a  relative  determination  of  the  sun's 
radiation  of  heat.  The  height  of  the  sun  will,  of  course,  have 
to  be  noticed  and  eliminated  in  a  similar  manner  as  in  photome- 
tric observations.  In  this  way  we  shall  be  able  to  prove  in  a 
comparatively  short  period  the  variations  in  the  heating  energy 
of  the  sun  caused  by  the  periodicity  of  the  sun-spots. 
21. 
Partly  from  the  recent  observations  of  Secchi,  and  partly  from 
not  yet  published  communications  of  Professor  Sporer  in  Anclam 
and  Dr.  Vogel  in  Bothkamp,  it  follows  that  all  phenomena  de- 
duced theoretically  by  me  find  an  unexpectedly  speedy  empirical 
confirmation.  This  regards  as  well  the  fact  that  the  poles  of  the 
currents  on  the  sun's  surface  do  not  coincide  with  the  poles  of 
*  If  Airy,  in  the  'Astronomical  Notices/  No.  934,  does  not  come  to  any 
decisive  result  as  regards  the  variations  of  temperature  caused  by  the  sun's 
rotation,  making  use  of  the  observations  made  at  Greenwich  during  a  pe- 
riod of  only  six  years  (1848-1853;,  this  is  partly  explained  by  the  different 
manner  of  reduction,  and  partly  by  climatic  conditions.  It  is  clear  that  the 
effect  will  be  the  less  perceptible  in  the  temperature  of  the  air  the  more  the 
air  is  filled  with  aqueous  vapour  and  the  sky  covered  with  clouds  and  fog. 
The  choice  of  D'Arrest,  therefore  (only  to  make  use  of  the  observations  at 
noon,  when  the  sun's  radiation  has  the  greatest  effect),  seems  to  me  by  far 
more  rational  than  the  mean  daily  temperature  used  by  Airy. 
