i$73'] Condition of the Moon's Surface. 37 



due to meteoric rain falling when the moon was in a plastic 

 condition. Indeed, it is somewhat remarkable how strikingly 

 certain parts of the moon resemble a surface which has been 

 rained upon while sufficiently plastic to receive the im- 

 pressions, but not too soft to retain them. Nor is it any 

 valid objection to this supposition, that the rings left by 

 meteoric downfall would only be circular when the falling 

 matter chanced to strike the moon's surface squarely; for it 

 is far more probable that even when the surface was struck 

 very obliquely and the opening first formed by the meteoric 

 mass or cloud of bodies was therefore markedly elliptic, the 

 plastic surface would close in round the place of impact 

 until the impression actually formed had assumed a nearly 

 circular shape. 



Before passing from this part of my subject, I would in- 

 vite attention to the aspect of the half moon as presented in 

 the photograph illustrating this paper (see Frontispiece).* It 

 will be seen that the multitudinous craters near the top of 

 the picture (the southern part of the moont) are strongly 

 suggestive of the kind of process I have referred to, and 

 that, in fact, if one judged solely by appearances, one would 

 be disposed to adopt somewhat confidently the theory that 

 the moon had had her present surface craters chiefly formed 

 by meteoric downfalls during the period of her existence 

 when she was plastic to impressions from without. I am, 

 however, sensible that the great craters under close telescopic 

 scrutiny by no means correspond in appearance to what we 

 should expect if they were formed by the downfall of great 

 masses from without. The regular, and we may almost say 

 battlemented, aspect of some of these craters, the level 

 floor, and the central peaks so commonly recognised, seem 

 altogether different from what we should expect if a great 

 mass fell from outer space upon the moon's surface. It is 

 indeed just possible that under the tremendous heat 



* This photograph is interesting as the work of the Great Melbourne re- 

 flector. It was taken directly of its present size, and in this respect differs 

 from all others of the same size, since, hitherto the negatives taken have been 

 small. 



t Owing to the fact that this photograph has been taken with a Newtonian 

 reflector, we have not the same kind of inversion as in the case of photographs 

 taken with refractors. In the latter case all that is necessary to cause the 

 picture to represent the moon as we see her, is simply to hold the picture up- 

 side down ; but the photograph illustrating this paper will only resemble the 

 half moon as she actually appears (at the time of first quarter, the epoch 

 of the photograph) by holding the picture inverted before a looking-glass. 

 The picture would also show rightly if inverted and then looked at from behind, 

 supposing the method of mounting such that the picture can be seen from 

 behind when held up between the eye and the light. At present I do not know 

 whether this will be the case or not. 



