Sciences of Meteorology and Terrestrial Magnetism. 171 



atmosphere, but at a great altitude ; aud that just on the limit of the 

 atmosphere the temperature will be intermediate between this minimum 

 and that of planetary space, which he would make higher than either, 

 the thermometer being of course supposed to be placed in shelter of all 

 solar radiation, but open to every other influence. The paper is a specu- 

 lation upon the degrees of heat which may exist on the surfaces of the 

 planets ; and the matter now referred to is introduced somewhat inci- 

 dentally. He does not attempt to fix any temperature for the celestial 

 spaces. 



(12.) The sun, it is now considered highly probable, may act as an 

 agent in meteorological phenomena in another way than by the direct 

 radiation before described. There seems to be a periodicity in its 

 emission of heat, in connection with the number of spots at any time 

 visible on the disc. The year 1856 has been remarkable for an 

 almost total absence of spots ; a like phenomenon was noticed about 

 eleven years ago ; and, in fact, from continued observations, it would 

 seem that the spots recur in the same order and magnitude in ten 

 or eleven years, giving about nine periods to a century — that is, from 

 the minimum display to the mitiimum again is ten or eleven years. 

 The spots are now universally agreed to be owing to vast openings 

 in the luminous envelope, which display dark clouds within, or the 

 opaque substance of a solid globe. This theory was first proposed 

 by Dr. Alexander Wilson of the Glasgow Observatory and College in 

 1774, and adopted by Sir W. Herschel, without, however, any refer- 

 ence to the source whence it was derived. The paper appeared in 

 the PhUosophical Transactions, and must have been known to Sir 

 William. We do not know that his son has anywhere acknowledged 

 Dr. Wilson, or explained the omission of his father in adopting the 

 hypothesis without mentioning Dr. Wilson's name. Most probably 

 he took it for granted that all interested in the subject would know the 

 source ; and it seems to have been his constant practice in all his papers 

 to refrain from allusions to the past history of the inquiries, and to 

 record merely his own observations, leaving it to others afterwards to 

 weave the wliole into a connected history. Arago, in his late work, 

 has done ample justice to Dr. Wilson, and imputes no blame to either 

 Herschel. Solar spots have been observed many times larger than 

 the whole surface of the earth, some even as great as one-tenth the 

 sun's diameter. The least that could be seen by our existing instru- 

 ments, must have a diameter of 461 miles, and an area of 167,000 

 square miles. It is obvious, therefore, that a great development of 

 spots may considerably diminish the amount of heat emitted by the 

 photosphere of the sun. Respecting the mechanical value of sunlight, 



