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



Alhazen at first sight appeared to have somewhat the 

 appearance of a crater, the west edge being high but the 

 east much lower. " Upon attentively considering it," says 

 Mr. Birt, "I have some reason to think it consists of two 

 nearly parallel ranges of mountains, just on the borders of 

 the Mare, the eastern range forming a part of the actual 

 border. The shadow of the western range is, under this 

 illumination, terminated at a line west of the eastern range, 

 the western slope of which is glowing in bright sunshine." 

 On January 4, Mr. Birt remarks as follows, — " At times 

 the definition has been very fine, and the real character of 

 Schroter's Alhazen well seen ; the southern terminations of 

 the two mountain ranges were seen to be quite separate the 

 one from the other, and the level surface passing between 

 them. It is not surprising that the two combined should 

 have been regarded as a crater, especially if viewed by a low 

 power; for now, haze coming on, it is impossible to distin- 

 guish the two from a crater. In the earlier part of the 

 evening, the independence of the two ranges, especially 

 on the south, was very apparent ; the shorter shadow brought 

 out very distinctly the mountain character, and the recess of 

 the shadow of the eastern range revealed the existence of 

 two (?) small craters lying at the foot of the eastern slope, 

 upon the very border of the Mare Crisium. Beer and 

 Madler figure two mountain ranges in the locality, but very 

 unlike the mountain ranges described above." 



All this is very instructive. It shows what effects the 

 varying visual angle and illumination will produce where 

 most effective, and therefore the effects which they tend to 

 produce under other circumstances. 



I pass to the consideration of two lunar regions, to which 

 in recent times attention has been specially directed, — the 

 crater Linne and the Floor of Plato, — proposing to discuss 

 the evidence of change (mechanical change in the one case, 

 and systematic variation recurring each lunar day in the 

 other), with careful reference to all the known circumstances 

 of each case. 



On November 17, 1866, Mr. Birt received a letter from 

 Dr. Schmidt, of Athens, to the following effect : — " Depuis 

 quelque terns je trouve qu'un cratere de la lune situe dans le 

 plaine du Mare Serenitatis, n'est plus visible. Cette cratere 

 nomme par Madler ' Linne ' se trouve dans la quatrieme 

 section de Lohrmann sous le signe A. Je connais ce cratere 

 depuis 1841, et meme en pleine lune il n'etait pas difficile 

 de l'apercevoir; en Octobre et Novembre, 1866, a l'epoque 

 du maximum de son apparence, c'est a dire un jour avant le 



