260 REPORT— 1872. 



remarks relative to an unusual exhalation of vapour causing an extension of 

 lucid area (see post, p. 261) or a flowing together of neighbouring light-streaks 

 is obvious. On the 12th of May, 1870 (the brightening of the floor was ob- 

 served on the 9th of May), it had so far subsided as to allow of the north-west 

 streak being seen, one observer (Mr. Gledhill) recording it as the brightest 

 on the floor, another (Mr. Elger) registering the part east of spot No. 16 as 

 very bright and well defined. This was from 96 to lOS hours after suniise. 

 That this increase of light was independent of either illumination or reflec- 

 tion, except as transmitting agents, is evident from the fact that at the same 

 interval from sunrise, 96 to 108 hours on the 14th of March, 1870, the eastern 

 arm of the " trident " was recorded as the brightest marking. 



Mr. Pratt, writing under dates October 17 and 18, 1871, says : — " A year or 

 two since I was reading Kirchhoff's ' Memoirs,' Roscoe's ' Spectrum Analysis,' 

 and several other works on the subject, and at the same time frequently spent 

 an evening in Mr. Mayall's laboratory with his splendid spectroscope. About 

 that time I often considered the possibility of vapours rising from the heated 

 surface of the moon, and wondered if the dark spaces were in any way 

 absorptive clouds, and became of the opinion, and am so still, that both the 

 darkest and the lightest markings on the surface may be but the appearances 



of vapours Looking at Proclus, Aristarchus, and the interior of Tycho, 



I can never feel certain that their brilliancy is merely the dazzling reflection 

 from naked rocks ; and that great valley running N.E. from Tycho suggests 

 other causes for its Avhiteuess than merely different incident and reflecting 

 angles and different materials of its soil. The Mare Frigoris has very 

 often suggested to me (and I have mentioned the same to you more than 

 once) both the possibility and probability of its being covered with some- 

 thing very foggy in its nature. It has frequently had that appearance in my 

 telescope; and while objects on the heights of the rim of Plato have been 

 well defined, a general haziness has belonged to those on the Mare below, — 

 full proof, I think, that the obscuration not only belonged to the moon itself, 

 but was confined to its lower levels. That under the circumstances the 

 intense heat must produce vapours from the surface, even if small in quantity, 

 and that, once produced, they must act as your theory supposes, seems incon- 

 trovertible. Some visible effects of their production, both in absorption and 

 reflection of solar rays, must follow as a natural consequence ; and if those 

 instances you have thought proper to adduce are not real observations of 

 those visible eff"ects, it I'cmains for some more assiduous observers to bring 

 forward more complete proof of their presence ; but I cannot then see that 

 even the supposed presence of such vapours is in the least degree negatived. 

 The proof of their presence by observation of their efl'ects would only be in 

 abeyance. Does not the softened margins of light-streaks generally on the 

 moon suggest a vaporous origin 'i Surely, in the case of Liune, it is as reason- 

 able as that the white spot arises from reflection from a sui-face of shivered 

 glass. The past observations of Linne and your present theory fit well to- 

 gether ; and it seems to me that if one may be forgiven for supposing the 

 presence of a small quantity of moisture on the moon, then the liotter the 

 surface the whiter the spots and streaks woidd become to our view." 



[In reference to Mr. Pratt's remai-k on the ivhiteness of the heated surface, 

 I may call to mind the appearance of slated roofs under a July or August sun 

 which I have noticed. It is just as if the slates had been coated with a ivliite 

 pigment. Instead of a dark slaty hue they have presented a strongly decided 

 white, so as to induce the belief that the roof had been whitewashed or 

 painted white. Having given some attention lately to the so-called irregular 



