492 Condition of the Moon's Surface, [October, 



one other six-yearly period then brings it into all but perfect 

 coincidence with its first position. But 186 years have then 

 elapsed, and though the conditions are nearly reproduced 

 so far as libration is concerned, the astronomer who made a 

 first series of observations at the beginning of this period is 

 not alive to repeat them under like conditions, even if like 

 conditions existed. Even this, however, is not the case 

 absolutely, since the lunation would be in another part of its 

 progress at any given season of the year, at the end of the 

 long period named. 



Of course I would not have it understood that there is 

 not, within much shorter periods, an approach to the 

 restoration of given relations. Two or three times, perhaps, 

 in ten years, any given feature in the moon may be seen 

 under conditions so nearly alike as to produce a great 

 similarity of aspect ; and if the weather is favourable on 

 such occasions, a legitimate comparison may be instituted 

 for the purpose of ascertaining if change has taken place. 

 But it may safely be asserted that the opportunities pre- 

 sented during the life of any single astronomer for a trust- 

 worthy investigation of any portion of the moon's surface 

 under like conditions are few and far between, and the whole 

 time so employed must be brief even though the astronomer 

 devote many more years than usual to observational 

 research. 



I shall now proceed to indicate a remarkable instance of 

 the effects produced by libration on the aspect of a lunar 

 region lying not very far from the limb, in order that the 

 student of the subject may duly recognise the importance of 

 position in this matter. 



Mr. Webb, in his " Catalogue of Lunar Objects," makes 

 the following remarks respecting a supposed lunar crater: — 

 " On the western edge of the Mare Crisium Schroter 

 delineated a crater, called by him Alhazen, which he 

 employed to measure the existing libration ; he saw in it, 

 after a time, unaccountable changes; and now, it is said, it 

 cannot be found. Beer and Madler confounded it with a 

 crater lying further south ; the question, however, which in 

 the interim was debated between Kunowsky and Kohler, is 

 not quite cleared up." In January, 1862, Mr. Birt observed 

 two objects where Schroter had seen a single crater, and 

 addressed a letter on the subject to Mr. Webb, who then 

 gave him the following interesting and instructive history of 

 the region: — "Schroter had watched Alhazen and measured 

 from it for years, and found it too varying in aspect to be 

 accounted for by the varying angle of illumination. At first 



