490 



Condition of the Moon's Surface. 



[October, 



and E F drawn square across the parallels from A, B, and c, 

 towards the eye ; whereas, if the base A c were inclined to 

 the position ac or a'c', we should in one case see be the 

 wider, and in the other see b' c' the wider, as shown by the 

 inequality of the lines d e, ef, and d' e\ e'f. Such changes 

 would necessarily produce some effect ; and the effect might 

 be deceptive, and indicate change where there had been no 



Fig. 2. 



&' 



f 



change, if a surface were covered with ridges such as a b c, 

 too minute to be individually discernible, and having the 

 side towards c darker or lighter than the side towards A. 

 The same would hold also if the slopes a b and b c were 

 not, as in the imagined case, inclined equally to the base AC 

 But it will be manifest that these effects, though they 

 might be appreciable, would be insignificant compared with 

 those which libration might produce, in certain circum- 

 stances, on points at a considerable distance from the centre 

 of the disc. Thus, take such a case as is illustrated in 

 Fig. 3> where a ridge, ABC, instead of being looked at 

 squarely, is viewed at an angle of 40 , corresponding to 



Fig. 3. 



/A 



/f / / N/ 



/ «//' 



X e ' 



^tH^ 



?r 



a position removed 50 from o, in Fig. 1. Then lines being 

 drawn from ABC, and, at an angle of 40 , to a c, and 

 parallel lines from the corresponding points of the same 

 ridge tilted on either side of its mean position as before, we 

 see that the inequality between D E and E F is much less 

 than that between d e and ef, and much greater than that 

 between d' e' and e'f. The effects of libration may thus be 



