The Lunar Clefts — Mare Vaporum. 95 



THE LTJNAK CLEFTS— MAKE VAPOEUM— JUPITER'S 



SATELLITES— OCCULTATIONS. 



BY THE EEV. T. W. WEBB, A.M., E.R.A.S. 



Tn our 'last paper we followed the Great Cleft as far as the 

 crater Hyginus : we now proceed to give the results of the 

 observations of B. and M. upon its future course ; referring 1 

 at the same time to the diagram in our last number. This 

 crater, 3-f- miles in diani., and tolerably deep, with 6° or 7° of 

 brightness, has its ring split by the cleft, which passes across 

 the interior, with elevated edges, in such a manner that the 

 continuity of its banks is nowhere interrupted. Of this they 

 had an interesting proof upon one occasion in the waning 

 moon, when the steady air admitted of a power of 800 : the 

 interior of the crater was wholly in shade, with the exception 

 of two minute but very brilliant lines of light in the position 

 of the cleft, while the wall on the N.E. and W., where the 

 cleft encountered it, was interrupted by a very narrow but 

 perfectly black shadow. From the E. side of Hyginus issues 

 an extremely small cleft, 4 or 5 miles in length, which was 

 detected by Lohxmann, and seen, once only, by B. and M. 

 After leaving this spot, the cleft, in its westward course, 

 touches on five extremely small craters, or possibly only " long*ish 

 circular widenings/' and is bordered on the S. by two broad flat 

 hills; all these being objects of an exceedingly difficult character. 

 Beyond the tenth crater the cleft becomes wider, flatter, and 

 less regular, and ends as it began, at a long hill, at whose 

 S.W. extremity a little crater (Agrvppah) is faintly perceptible. 

 Its whole length is about 106 miles, its average breadth nearly 

 1 mile : the steeper are the more reflective parts ; the light 

 of the flatter ends losing itself in that of the surrounding sur- 

 face. With the great achromatic at Dorpat, M. subsequently 

 found that a great part of this cleft consisted of a chain of con- 

 fluent roundish cavities. The plain S. of the cleft contains a 

 few small craters of various sizes, two small dark spots and 

 one larger, and one of a green hue, about 3° in brightness, 

 and extending over about 3,600 square miles. On the 1ST. the 

 cleft is attended by a number of very low parallel chains of 

 hills; one mass immediately 1ST. of Hyginus shows a curious 

 spiral arrangement; but generally they point S.W. or S.S.W., 

 and this bearing is visible over a considerable area in this 

 direction, overspread with minute banks and ridges, invisible 

 excepting near the terminator, and usually only from 40 to 60 

 yards in height. Our authorities especially direct attention 

 to the fact that in this region, where no deception can be 



