202 The Moon. 



that under many angles of illumination it assumes the appear- 

 ance of a long mountain ridge. Such is the explanation 

 offered by B. and M. (after Kunowsky) of the singular 

 changes in form and shading which long perplexed Schroter, 

 and which led him to infer atmospheric if not volcanic changes ; 

 and it seems probable that in this instance the more modern 

 astronomers have the best of the argument. The question 

 however is not altogether clear. Schroter' s observations upon 

 this group of mountains are too numerous to be recited here ; 

 they refer chiefly to the varied appearance of shadows, longer, 

 shorter, imperceptible, or unusually directed, at different times, 

 but under nearly similar angles of illumination ; to unaccount- 

 able changes in the forms of mountains ; and to the discovery, 

 and subsequent invisibility, of ridges or hillocks in well- 

 known and often observed situations. There can be no doubt 

 that, as he was himself aware, a slight difference in the con- 

 ditions of illumination and reflection may produce a very dis- 

 proportionate change of aspect; still, there is much weight in 

 his remark that this must not be pushed too far, or we should 

 find similar variations occurring from the moon's progress 

 during the course of a single observation extended through 

 several hours, which has never yet been found to take place. 

 To one source of error he was perhaps not sufficiently alive — 

 the increased perception of the true nature of a distant or 

 obscure object, in proportion as it becomes familiar to the eye. 

 It certainly does not seem at all likely that the crater e was 

 ever seen in actual eruption by Schroter, as he was inclined to 

 suppose ; but we must bear in mind that, as eruptions of some 

 kind — whatever that kind may be — must have taken place upon 

 the moon times without number, there is no antecedent impos- 

 sibility in such an idea. It must be admitted that the region 

 is a curious one, and well adapted for an inquiry which may bo 

 worth the while of future observers, whether all these variations 

 are due solely to differences of illumination and libration, or 

 whether there may be, as Schroter supposed, occasional modi- 

 fications of a lunar amosphere, capable under certain cireum- 

 . bailees of impeding or perverting our view; and it would be 

 anphilosophicaJ and unwise to allow the greater probability of 

 (lie former alternative to stifle the inquiry. In order to bring 

 out of it any successful result, Schroter's observations would 

 lead us, not merely to note the aspect of the craters in all 

 positions relative to the sun and earth, but also to examine the 

 proportionate lengths of the shadows of the other mountains 

 In the group, and the firsi and last appearance of their summits 



at the time of lunar sunrise or sunset. My own observations 

 have not been sufficiently consecutive to be of real service in 

 establishing anything. To the general reader much that has, 



