The Lunar Clefts — Mare Vcvporum. 99 



operation in utilizing a natural feature. The alteration is 

 much more probably due in part to libration, exposing to the 

 eye more or less, at different times, of a very narrow line of 

 shade, and partly to the change of seasons on the moon, 

 which, slight as it is, is sufficient, by varying a little the bear- 

 ing of the sunrise, to cause the same edge of a cleft running 

 E. and W. to be at one time illuminated, at another in shade. 



We will now turn to Lohrmann, and hear what he has to 

 say — premising that the date of his iC Sections," though 

 earlier, did not differ much from that of B. and M. And 

 here again we find discrepancy enough to be unpleasant. 

 His first cleft, to begin with, is much more sharply bent at 

 Hyginus than in our diagram : and though he agrees with 

 B. and M. as to the wider commencement, and the four craters 

 in its course before reaching Hyginus, yet it is remarkable, 

 that while he makes the first and fourth very minute, he 

 enlarges the two intermediate ones to two-thirds of the size of 

 Hyginus, saying also in his text that there are two larger and 

 two smaller deep cavities, which under favourable circum- 

 stances he has several times distinctly seen. One of these 

 little craters, he adds, lies on the edge of Hyginus ; his draw- 

 ing shows that it is additional to the other four. He remarks 

 that Hyginus has no visible ring, and has drawn none round 

 the two larger of the four craters. Another minute crater is 

 drawn, but not described, just W. of Hyginus. He carries the 

 end of the cleft as far as the site of B. and M/s Agrijppa b, 

 stopping it against a chain of hills running up to the wall of 

 the great crater. 



The cleft of Ariaclwus he begins (from E.) with a small 

 crater, as B. and M., and has shown their two minute pits on 

 either side of it, further W. He recognizes Schr/s D as a flat 

 crater, the ring of which is a mere bank on the N. : he shows 

 traces of v, and like Schr. places the smaller crater opposite 

 Silberschlag at some little distance from the N. bank. After 

 several failures he found the mountain measured by Schr. 

 He carries the cleft through everything in its course, expand- 

 ing it a little at its W. end. It is less luminous, he says, than 

 that of Hyginus, but still visible in Full. Of the cleft-system 

 near Triesnecker he perceived nothing. Chacornac perfectly 

 recognizes Schr/s D and v, and makes the cleft divide a, but 

 not /3, W. of Silberschlag. Schmidt has added considerably to 

 the number of clefts in this district ; but as the descriptions in 

 his catalogue are very brief, and without designs, they are 

 little more than memoranda, which will become fully intelligible, 

 we trust, hereafter, by comparison with his promised lunar 

 map. 



As the present is our best opportunity of studying* these 



