I873-] 



Condition of the Moon's Surface. 



489 



position in latitude, and in every possible mean position as 

 well as with every possible combination of tilt in longitude 

 and latitude. In fine, if o (Fig. 1) be its mean and central 

 position, then this point occupies in turn (and in the course 

 of time) every part of the area abcd. 



Now this has only been stated to show the actual 

 librational sway of the moon, not to indicate the impor- 

 tance of the effects due to such libration. For it is 

 manifest that the region about o cannot be very much 

 affected in appearance by being shifted even to the point 

 a, or to b, or to c, or to D, that is, to its maximum amount. 

 If we were looking at the summit and slopes of any hills or 



Fig. 1. 



craters, when the central region was at o, we should also 

 be looking at those summits and slopes when the region was 

 at A, b, c, or d, — unless, indeed, the slopes were exceedingly 

 steep. 



Of course the two opposite slopes of a ridge, suppose, 

 would be seen in different proportion at A and c, or at D 

 and b, and if they were differently tinted, a different effect 

 would be produced, whether we could see such slopes 

 separately or not. 



Thus if such a ridge as abc (Fig. 2) were looked at 

 directly when at o, we should see the slopes A B, B c, of 

 apparently equal width, as shown by the equality of D E 



