1 66 THE SOLAR SPOTS. PART i. 



light of these flames is so inferior to its photographic 

 power, that Mr. De la Rue photographed one of the pro- 

 tuberances, although it was invisible to the naked eye. 



The sun spots which are situated in that region of 

 the sun which lies below the photosphere consist of a 

 central darkness or umbra, surrounded by a penumbra 

 which is less dark. Professor Wilson, of Glasgow, 

 proved that the spots are cavities, of which the umbra 

 or darkest part forms the bottom, and the penumbra 

 the sloping sides, by observing that the umbra en- 

 croaches on that side of the penumbra which is next 

 the visual centre of the sun. Hence the umbra of a spot 

 is at a lower level than the penumbra ; and since lumi- 

 nous ridges and sometimes detached portions of luminous 

 matter cross over the spots, it is concluded that the 

 whole phenomenon is below the surface. The spots 

 have an apparent motion from east to west, due to the 

 rotation of the sun ; and Mr. Carrington discovered that 

 they have a proper motion also from east to west, those 

 nearest the solar equator moving fastest. They are 

 confined to the equatorial regions. 



No reason has yet been assigned for the periodicity 

 of the spots, which go through a cycle of maxima and 

 minima every ten years nearly. They are singularly 

 connected with terrestrial magnetism ; the maximum 

 of the spots coincides with the period of the greatest 

 disturbance of terrestrial magnetism. The spots seem 

 to be influenced by the planet Venus in such a manner 

 that when a spot comes round by rotation to the eclip- 

 tical neighbourhood of this planet, it has a tendency to 

 dissolve ; and, on the other hand, as the sun's surface 

 recedes from the planet it has a tendency to break out 

 into spots. 1 



1 On the latest discoveries concerning the sun's surface, by Balfour 

 Stewart, Esq., in the ' Chemical News and Journal of Physical Science ' of 

 April 1865, 



