The Lunar Mare Serenitatis. 293 



for two or three days before and after the Full Moon. I have 

 never succeeded in seeing it decidedly green. My attention 

 having been called to the point, I can detect some difference in 

 the tint from that of other plains, but the contrast is not great : 

 I should perhaps call the hue, as it appears to me, a greenish 

 yellow. The boundary between the lighter centre and its 

 deeper fringe is very distinct, especially to the S., where it 

 passes alike over plain and ridge : in other directions it 

 coincides with slight elevations. 



The plain is nearly equally divided by an almost straight 

 whitish streak, with a breadth of about 12 or 14 miles, a lumi- 

 nosity of 4° to 4^°, and a direction not much differing from 

 that of the meridian. It becomes less conspicuous as the 

 terminator approaches it, and vanishes entirely before the 

 lunar sunset, exhibiting not the slightest elevation or depres- 

 sion, though concurring in some places with the direction of 

 slight ridges, which Schr. mistook as an indication of height 

 throughout. Our guides remark that in so favourable a posi- 

 tion banks of 60 or even 40 feet might be detected, though 

 scarcely capable of delineation ; and that every elevation, how- 

 ever insignificant, or of whatever tint, becomes more distin- 

 guishable in the immediate vicinity of the terminator ; while on 

 the contrary this, and the other streaks in general, comport 

 themselves in the very reverse way; there being not one on the 

 whole lunar surface that is perceptible in that situation. 



This beautiful expanse will afford to the observer an excel- 

 lent opportunity of studying the more familiar characteristics 

 of these grey plains. He will not indeed meet with many of 

 those insulated mountains and craters which give so great a 

 charm to the scenery of the M. Crisium, the M. Imbrium, and 

 others, nor will he find those roughened tracts which charac- 

 terize the neighbouring Lacus Somniorum and similar regions. 

 But the broad sweep of undisturbed level can scarcely any- 

 where be studied to greater advantage, and the low ridges 

 which may possibly have a greater significance in selenology 

 than might at first have been supposed, are here very clearly 

 developed. A small bright crater named Taquet, between 

 Menelaus (15) and the Promontorium Acherusia (previously 

 described), is the starting point of the ridge-system of the 

 plain ; and we have here an exemplification of a fact to which 

 Schr. has repeatedly invited our attention — that these low 

 banks form lines of communication between objects of more 

 apparent importance. Though in many of them no such 

 arrangement can be made out, yet the instances are innumer- 

 able in which they are found to connect distant craters or head- 

 lands, or to serve as the bases of rocks or small hills, much 

 resembling, if so homely an illustration may be permitted, the 



