The North-West Lunar Limb. 259 



"which we have been describing, divided by a short, protuberant 

 luminous ridge, which interrupted the limb at about an equal 

 distance between Struve and Endymion. This happened 

 6d. 7h. before the greatest SW. libration. On the following 

 night the appearance remained much the same, and the straight- 

 ness of the outline in these two places was proved by the 

 application of a micrometer thread to be no illusion : the flat- 

 tening nearest the N". pole ended in that direction with a 

 magnificent mountain, having its steepest slope facing N. This 

 I thought must lie towards the N. end of the Mare Hum- 

 boldtianum. 1864, Oct. 22, 4d. 21h. before greatest SW. 

 libration, I saw one of these appearances very well again, 

 cutting off the limb for 10° of arc, and ending with the same 

 great peak. It seemed nearly to correspond with the position 

 of the M. Humboldtianum. Oct. 26, 19h. before greatest 

 SW. libration, this flattening had nearly disappeared. 



I have not subsequently paid much attention to this curious 

 phenomenon, but point out its general position, in the hope 

 that some of my readers may be led to determine it more pre- 

 cisely. A careful examination of the limb, night after night, 

 is sure at length to concur with the state of libration in which 

 it is most perceptible. We have nothing analogous to it on 

 the surface of the earth, unless a mountain chain exists any- 

 where in a position to show itself in profile upon the earth's 

 limb, loftier at each end than in the centre, but so regularly 

 curved, as we should think it, viewing it from the earth, that 

 it would be straight as seen from the moon. For we must 

 bear in mind the apparent paradox, that all that we consider 

 straight, or level, on the surface of the earth, is really curved, 

 in couformity with the figure of our globe ; and that/ therefore, 

 a line really straight, or a surface really plane, would seem to 

 us curved, lower in the centre than at either end (or on all 

 sides) . And it would be virtually so in effect ; for, strange as 

 it may seem, were a slice taken in a truly level direction from 

 any part of the earth, since the centre of the flattened area 

 would be nearer than the sides to the centre of the earth, 

 gravity would be strongest at that point ; — a pedestrian leav- 

 ing it in any possible direction would have the impression of 

 walking up-hill, a locomotive engine on a railway diametrically 

 crossing it would run down there from either side, and all the 

 water in the flattened space would collect and form a lake in 

 the centre, whose surface would not be level, but convex. The 

 lunar depression of which we speak cannot be a portion of the 

 area of the Mare Humboldtianum,, as that would follow the con- 

 vexity of the globe; but its connection with the great level so 

 named ought to be made out by observation. 



VOL. VI. — NO. iv. s 



