508 Changes in the Moon's Surface. . [October, 



may be discussed as though the surface really were optically 

 smooth. In point of fact, if a surface were covered over 

 with minute cones a quarter of an inch in height, it would 

 present the same peculiarities of general illumination as 

 though it were covered with conical hills several hundred 

 feet in height. Now it may be said that as no such 

 irregularities, whether large or small, can be detected in the 

 Floor of Plato, it is not admissible to make the possible 

 existence of such peculiarities a basis of reasoning. That is 

 perfectly just ; but it is equally inadmissible to make the 

 possible smoothness of the Floor of Plato a basis of 

 reasoning. We have no direct evidence either way. As to 

 probabilities, it seems at least as likely that the floor is 

 covered with minute elevations as that it is smooth ; nay, if 

 we remember that the floors of the lunar craters present all 

 the appearance of having once been fluid with intensity of 

 heat, it seems more reasonable to infer that their surfaces 

 now have a crystalline structure than that they are smooth. 



We may, indeed, sum up the evidence obtained by 

 Mr. Birt in this way : — It implies either real changes, 

 or surface peculiarities, probably of the nature of minute 

 irregularities, such as result from crystallisation. If real 

 changes be regarded as very unlikely, we have strong 

 probable evidence in favour of surface peculiarities, a result 

 of considerable interest. If surface irregularities bethought 

 very improbable, then we have strong evidence in favour of 

 real changes, a result also of considerable interest. Whether 

 it is more unlikely that real changes so affect these spots as 

 to make their whole surface change in brightness (though no 

 large surfaces ever show such changes), or that the once 

 fluid surfaces within the craters should have a granulated 

 surface different in different parts, is a question which will 

 be answered according to the general views of the lunar 

 student. I have no hesitation in adopting the second view 

 as far the more probable. 



I would not, however, have it understood by any means 

 that I think it unlikely that change is taking place on the 

 moon's surface. On the contrary, when one considers the 

 wide variations of temperature to which the surface of our 

 satellite is exposed, it seems altogether probable that (i) a 

 process of disintegration must be in progress, which should 

 at times, one could suppose, produce even remarkable catas- 

 trophes on the moon's surface ; and (2) that the lunar 

 atmosphere, tenuous though it undoubtedly is, may be 

 affected by changes of condition detectible under certain 

 conditions from our distant standpoint. 



