292 The Lunar Mare Serenitatis. 



THE LUNAR MARE SERENITATIS.— DOUBLE 



STARS.— OCCULTATIONS. 



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



We come now to a grey level of very interesting character, as 

 well as peculiarly well situated for study, the Mare Serenitatis 

 (lettered E in our map). It is of an approximately circular 

 form, extending 442 miles in a N. and S. direction, and 423 

 from E. to W. ; its longest axis lying from SW. to NE. Some 

 foreshortening of course results from its N. latitude, but not 

 sufficient to distort it materially ; the telescopic view, however, 

 gives but little idea of the convexity of so considerable an 

 area upon the comparatively small lunar globe. Its outline is 

 not much indented, and is more distinctly marked out than in 

 the majority of these plains : its circumference, including bays, 

 equals about 1840 miles, three-fourths of which consist of the 

 cliffs of Hcemus, Taurus, Caucasus, and the Apennines. Broad 

 passages unite it on the S. with the Mare Tranauillitatis, on 

 the N. with the Lacus Somniorum, and on the E. with the 

 Palus Putredinis. When the M. Tranauillitatis and L. Som- 

 niorum lie with it on the same terminator, it is evidently on a 

 lower level than the other two ; and this is confirmed by the 

 circumstance that the slopes of nearly all its mountain borders 

 are much more steep on their inner than their outer sides, and 

 when there is any perceptible difference in the slopes of the 

 low ridges in the interior, it is always in the same direction. 

 This great plain presents in many phases a fine telescopic 

 object, and when the terminator passes through the E. edge, 

 the whole may be seen at once in beautiful projection. I have 

 noticed it thus some hours before quadrature, but, of course, 

 libration makes a material difference in all such epochs. The 

 outer edge, on nearly every side, for a breadth of 28 to 83 

 miles, shows usually a dark uniform grey, of IF to 2° of 

 reflective power. The whole interior, comprising nearly two- 

 thirds of the entire area, exhibits at the Eull Moon, according 

 to B. and M., a beautiful clear uniform light green of 3°, which, 

 as they state, was remarked neither by Schroter nor Lohr- 

 mann, and which they freely admit may be so much matter of 

 ' ' personal chromatic equation " (to borrow an expression of 

 later invention), that they should not feel surprise at its not 

 being perceived by others : they are well assured that the con- 

 trast between the light of the plain and its border is a difference 

 not merely in degree but in kind ; but they allow that it is not 

 easy to be satisfied of it, and that it is chiefly discoverable by 

 comparison with other levels of similar brightness, and only 



