27 8 EIGHTEENTH CENTURY. PT. IIL 



Many people find it very difficult to understand how the 

 moon can be always turning round upon her own axis, as a 

 top spins, and yet always keep the same side towards us ; 

 therefore, it will be as well to make a simple experiment 

 which explains it quite clearly. Take a round ball and stick 

 a pin in one side of it, then turn the ball slowly round like a 

 teetotum, and notice as it goes round that the pin points 

 successively to each of the sides of the room one after the 

 other ; then sew a piece of cotton to the side of the ball 

 opposite the pin, and fasten the other end down to the 



table (as at E, Fig. 55). If 

 you now roll the ball round 

 the table, you will observe 

 FlG S5> that the pin points to each 



Diagram showing why one side of the ide Of the TOOni in SUCCCS- 



Moon is always turned towards the s [ Qn as it did before, showing 



Earth. , , ... 



M, Ball representing the moon. E, Point that it has been turning slowly 



representing the centre of the earth. Qnce fl fa Qwn ^ while 



/, Pin to mark the side of the moon 



which is never turned towards the going OnCC TOUnd the point E, 



and that, for this reason, the 

 same side has been facing E all the time. 



This is the case with the moon as she travels round our 

 earth, and Lagrange proved mathematically that it must be 

 so, as Newton had already suggested, if the moon's equator 

 is not circular, but in the form of an ellipsoid, as in that 

 case its longer axis would be acted on by the attraction of 

 the earth so as to keep it always in the same direction. 

 But Lagrange also showed that as the moon moves in an 

 ellipse round the earth, and therefore goes at one time a 

 little faster, and at another a little slower, while her rotation 

 on her own axis does not vary, she does not keep always 

 exactly the same face towards us, but we catch little glimpses 

 farther round her globe, sometimes on one side and some- 



