268 EIGHTEENTH CENTURY. pt. hi. 



side of her surface, so long as she has the same movement 

 as at present. 



In 1764 the Academie des Sciences offered a prize for a 

 complete explanation of this curious fact, and Lagrange was 

 thus led to study the question, which he solved quite satis- 

 factorily in 1780. 



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 

 Pj^ g other; then sew a piece ol 



cotton to the side of the ball 

 opposite the pin, and fasten 



\^J^ J the other end down to the 



Diagram showing why one side of the table (aS at E, Fig. 46). If yOU 

 Moon is always turned towards the 



Earth. now roll the ball round the 



M, Ball representing: the moon. E, Point . •11 'n i, ^i. j. 



representing the centre of the earth, table, yOU Will obsCrve that 



A Pin to mark the side of the moon , , • . , , 1 • j c 



which is never turned towards the the pm pOmtS tO eacll SIQC OI 



earth. ■. 



the room m succession, as it 

 did before, showing that it has been turning slowly once 

 upon its own axis while going once round 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, on account of the at- 

 traction of the earth upon the bulge at the moon's equator. 

 But I^grange also showed that as the moon moves in an 



