464 BRITISH SCIENTIFIC ASSOCIATION* 



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 lias been added 

 to science in this department is the proof given by Arago that the light of the sun 

 emanated (not from an incandescent solid, but) from a gaseous atmosphere, the 

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

 tire sun, and that emitted by gaseous bodies, is unpolarized. According to the 

 observations of Sehwabe, -which have been continued without intermission for niore- 

 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 pheno- 

 mena of terrestrial magnetism, have attracted fresh interest to the study of the 

 solar surface; and, upon the suggestion of Sir John Herscliel, a photoheliographie 

 apparatus has lately been established at Kew, for the purpose of depicting 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 phenomena 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. 



" Next to the sun, our own satellite has always claimed the attention of astrono- 

 mers, while the comparative smallness of its distance inspired the hope that some 

 knowledge of its physical structure could be attained with the large instrumental 

 means now available. Accordingly, at the meeting of the Association held at 

 Belfast in 1852, it was proposed that the Earl of Rosse, Dr. Robinson, and Prof. 

 Phillips, be requested to draw up a Report on the physical character of the moon's- 

 surface, as compared with that of the earth. That the attention of those eminent, 

 observers has been directed to the subject, may be inferred from the communication 

 lately made by Prof. Phillips to the Royal Society on the mountain Gassendi, and 

 the surrounding region. But I am not aware that the subject is yet ripe for a 

 Report. I need not remind you that the moon possesses neither sea nor atmosphere- 

 of appreciable extent. Still, as a negative, in such case, is relative only to the 

 capabilities of the instruments employed, the search for the indications of a lunar 

 atmosphere has been renewed with every fresh augmentation of telescopic power. 

 Of such indications, the most delicate, perhaps, are those afforded by the occulta, 

 tion of a planet by the moon. The occultation of Jupiter, which took place on 

 the 2nd of January last, was observed with this reference, and is said to have 

 exhibited no hesitation, or change of form or brightness, such as would be produced 

 by the refraction or absorption of an atmosphere. As respects the sea, the mode 

 of examination long since suggested by Sir David Brewster is probably the most 

 effective. If water existed on the moon's surface, the sun's light reflected from it 

 should be completely polarized at a certain elongation of the moon from the sun. 

 No traces of such light have been observed; but I am not aware that the observa- 

 tions have been repeated recently with any of the larger telescopes. It is now 

 well understood that the path of astronomical discovery is obstructed much more 

 by the earth's atmosphere than by the limitation of telescopic powers. Impressed 

 with this conviction, the Association has, for some time past, urged upon Her 

 Majesty's Government the scientific importance of establishing a large reflector at 



