Neighbourhood of the Lunar Spot, Mare Crisium. 367 



ultimately compensate themselves under the eye of a patient 

 observer: — the epoch of mean libration returning almost 

 exactly at the end of every three years. So that at present 

 the decision of this curious question may perhaps be considered 

 as in abeyance : if we are forced to conclude that Schroter 

 was often mistaken from not sufficiently adverting to the 

 effects of libration, or from his imperfect mode of measuring it, 

 we may ourselves occasionally err on the other side, and only 

 impede fair and full investigation, by ascribing everything 

 which we do not know how to account for to this cause alone. 



To return in conclusion to Gleomedes, the original source of 

 all this discussion. The objects in the interior specified by 

 B. and M. are, a central hill, 6° bright in the full moon, but not 

 very distinguishable under oblique illumination : it is not 

 evident whether this is the same with the hill described by 

 Schroter, or whether what he saw was a part of the ring of a 

 small crater (B in the map of B. and M.), a little S. of it : 

 three deep craters equally luminous in the darker S. part, of 

 which the map shows but two, that furthest S. lying, where 

 Schroter once perceived it, at the foot of the wall : — and in the 

 N. part three somewhat larger, that in the W. 8° bright and very 

 conspicuous, but, as even these advocates of unchangeableness 

 are obliged to admit, not always equally defined, in the full moon. 



Gruithuisen tells us that, 1825, April 6, he saw distinctly in 

 the W. part of the interior of Gleomedes elevations resembling 

 long straight hills, including several rhombus-shaped spaces 

 between them. These rapidly became invisible, and he 

 thought the appearance indicative of something artificial, per- 

 haps connected with cultivation. His figure, as engraved by 

 Bode, represents between the W. wall and the central hill, 

 which is double and casts a long shadow across half the plain 

 to the WSW. (thus showing the libration at the time), an 

 object of considerable size, like a lozenge in heraldry ; in fact, 

 a foreshortened square lying obliquely, subdivided into four 

 similar spaces by a cross-bar each way. But Gruithuisen him- 

 self condemns it as an unsuccessful representation. 



In an old sketch of my own, 1849, April 26, the central 

 hill is also represented as distinctly double ; and there are two 

 craters to the N. as in Schroter, the larger one having, as he 

 has described it, a small elevation in the midst of its grey 

 inteiior. I have not yet examined Cleomedcs with my present 

 instrument. 



On a mountain plateau, a little to the left of this great 

 ring, B. and M. describe a crater between four and five miles 

 across, on whose wall another of one-third its diameter has 

 encroached. This latter is worthy of notice, as being appa- 

 rently deeper than it is broad. 



