488 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



which therefore demand the attention of the physicist, although, 

 from their nature, they must probably long remain subjects of 

 speculation. Of these the spiriform nebulae, discovered by Lord 

 Rosse, have been already referred to from this Chair, as indicating 

 changes in the more distant regions of the universe, to which there 

 is nothing entirely analogous in our own system. These appear- 

 ances are accounted for, by an able anonymous writer, by the 

 action of gravitating forces combined with the effects of a resisting 

 medium, the resistance being supposed to bear a sensible propor- 

 tion to the gravitating action. 



The constitution of the central body of our own System pre- 

 sents a nearer and more interesting subject of speculation. To- 

 wards the close of the last century many hypotheses were advanced 

 regarding the nature and constitution of the Sun, all of which 

 agreed in considering it to be an opaque body, surrounded at some 

 distance by a luminous envelope. But the only certain fact which 

 has been added to Science in this department is the proof given by 

 Arago, that the light of the Sun emanated not from an incandes- 

 cent solid but from a gaseous atmosphere; the light of incan- 

 descent solid bodies being polarized by refraction, while the light of 

 the Sun, and that emitted by gaseous bodies, is -unpolarized. 



According to the observations of Schwabe, which have been 

 continued without intermission for more than thirty years, the 

 magnitude of the solar surface obscured by spots increases and 

 decreases periodically, the length of the period being 11 years and 

 40 days. This remarkable fact, and the relation which it appears 

 to bear to certain phenomena of terrestrial magnetism, have at- 

 tracted fresh interest to the study of the solar surface ; and upon 

 the suggestion of Sir John Herschel, a photo-heliographic appa- 

 ratus has lately been established at Kew, for the purpose of de- 

 picting the actual macular state of the Sun's surface from time 

 to time. 



It is well known that Sir "William Herschel accounted for the 

 solar spots by currents of an elastic fluid, ascending from the body 

 of the Sun, and penetrating the exterior luminous envelope. A 

 somewhat different speculation of the same kind has been recently 

 advanced by Mosotti, who has endeavoured to connect the phe- 

 nomena of the solar spots with those of the red protuberances which 

 appear to issue from the body of the Sun in a total eclipse, and which 

 so much interested astronomers in the remarkable eclipse of 1842. 



