FOR THE ADVANCEMENT OF SCIENCE, 1857. 489 



Next to the Sun, our own satellite has always claimed the 

 attention of astronomers, 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. 

 Eobinson, and Professor 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 these eminent observers 

 has been directed to the subject, may be inferred from the commu- 

 nication since made by Professor Phillips to the Eoyal Society on 

 the lunar mountain, Grassendi, 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 indi- 

 cations the most delicate, perhaps, are those afforded by the occul- 

 tation of a planet by the Moon. The occultation of Jupiter, which 

 took place on the 2nd of January last, was observed with this refer- 

 ence, and it 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 proba- 

 bly 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 obser- 

 vations have been repeated recently with any of the larger teles- 

 copes. 



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 Go- 

 vernment the scientific importance of establishing a large reflector at 

 some elevated station in the Southern Hemisphere. In the mean- 

 time, and to gain (as it were) a sample of the results which might 

 be expected from a more systematic search, Professor Piazzi Smyth 

 undertook, last summer, the task of transporting a large collection of 



