1 873.] Condition of the Moon's Surface. 485 



Gruithuisen went further even than Schroter had done ; 

 but his chief reliance was placed, not upon changes in the 

 moon, but on the existence of certain regular formations. 

 " He collected with the utmost diligence," says Crampton, 

 in his interesting little treatise on the Moon, " all the 

 objects which he beheld on the lunar surface having the 

 appearance of regularity, such as would be shown in the 

 works of man here;" and, strange to say, his search, with 

 the aid of a somewhat wild imagination to help it, was by 

 no means unsuccessful, at least as far as " finding such 

 objects" was concerned. Yet objects of the kind appear 

 regular only when examined with low telescopic power. 

 In instruments of considerable size they show manifest 

 signs of being simply natural formations. 



However, it is not my present purpose to consider those 

 features of the moon which present an aspect suggestive 

 of artificial construction, but to deal specially with the 

 instances or supposed instances of change. 



No one can study the moon for any considerable time 

 without being led to the conclusion that her general aspect 

 is unvarying, save, of course, as respects the changing 

 amount of illumination as the lunar month proceeds. It 

 becomes clearer and clearer, the longer the study of the 

 moon is continued, that it is by the careful scrutiny of small 

 portions of the moon's surface that any signs of change are 

 to be recognised. 



But this general constancy is in itself a most important 

 point in our subject. For supposing we should recognise 

 signs of change in any small portion of the lunar surface, 

 we shall have to enquire into the probable cause of such 

 change ; and in making such an enquiry, it will' be most 

 important to have determined beforehand whether any 

 atmosphere surrounds the moon's globe, and if so, what 

 is its probable nature and extent. I do not propose to 

 discuss this point at any length here, because it is one 

 which .has been already discussed in full elsewhere ; but 

 I would invite the student of selenography to notice that 

 all the direct evidence tends to show that there is scarcely 

 any appreciable atmosphere round the moon, not in quantity 

 (the absolute quantity may be and probably is very con- 

 siderable), but as respects actual density at the mean level 

 of the moon's surface. Now, in the enquiry into changes 

 taking place on the moon as the result of mechanical 

 processes, whether from the contraction and expansion 

 of the surface, or from sublunarian forces which may 

 still be at work, we are not compelled to consider the 



VOL. III. (N.S.) 3 R 



