96 The Lunar Clefts — Mare Vaporum. 



occasioned by foreshortening, the general parallelism of all 

 chains of hills and lines of valleys is peculiarly evident : this 

 bearing — c c geognostically " towards lOh. (of a dial divided 

 into 24h.) is visible throughout the Apennines, the Hosmus, the 

 M. Vaporum and its neighbourhood, Ukert, Pallas, Bode (28), 

 Julius Ocesar, Agrippa, Godin, Dionysius, and in part the M. 

 Tranquillitatis itself : in many parts it exceeds any other direc- 

 tion as 30 : 1, and between Mt. Hcemus and the cleft of 

 Ariadosus, it is almost the exclusive one. They point out also 

 another peculiarity, the great variety of local shading, or 

 " colour," in this region under varied illumination. In Full 

 Moon the higher of the ridges N. of Hyginus come out with a 

 reflective power of 4° or 5°, the interjacent valleys, with 3° or 

 3^°, brighter than the levels of the M. Vaporum, and the darkest 

 part of the neighbourhood does not lie here, but at the foot of 

 some higher mountains E. of the cleft ; and even these are 

 less dark than the spot Boscovich. On the other hand, about 

 the time of the quadratures, a large blackish somewhat unde- 

 fined spot comes out just between the two higher chains of 

 mountains near Hyginus y (the two N. of that crater in our 

 diagram) covering these ridges, whose summits are with diffi- 

 culty visible in it, and contrasting singularly with the sur- 

 rounding landscape. It is evident, on inspection, as B. and 

 M. remark, that it cannot be shadow; nor can it be merely 

 a feebler illumination, which could not in this way reduce to 

 blackness the yellow, yellowish grey, and pale greenish hues 

 of the lunar surface ; so that we have here a colour variable 

 with the phases, which appears to require some further ex- 

 planation than merely varied reflection. The day and night are 

 also the summer and winter of the moon ; and consequently a 

 change of colour whose period is that of a lunation may as 

 well be the result of temperature as of light ; and a careful 

 examination, of such regions seems especially suited to lead to 

 some discoveries as to the physical economy of our neighbour- 

 world. Had this been SchVs. conclusion, it would have been 

 entirely in character with his general ideas, but proceeding 

 from observers so little disposed to receive any evidence of 

 change, it is worth)' - of pointed attention. Fortunately the 

 district is one very well marked, easily found, and unaffected 

 in appearance by libration. 



The second great cleft, that of Arladceus, lies W. of the 

 preceding, and is described by B. and M. the reverse way — 

 from W. to E. It was discovered by Schr., like the other, 

 which it exceeds in length, breadth, and probably depth, 

 though less easy to find on account of its more mountainous 

 surroundings. It begins at a mountain Ariadteus <y (see the 

 diagram), and after a course of about 18 miles, receives another 



