24 THE TIDES. 



and least strongly those that are farthest off. And 

 the difference between nearest and farthest in this 

 case is by no means inconsiderable, for the mean 

 distance of the moon from the earth's centre is 

 only sixty times the length of the radius of the 

 earth ; so that the distances from the moon, of the 

 nearest and farthest parts of the earth, are to each 

 other as fifty-nine to sixty-one. 



The whirling or centrifugal force due to the 

 actual monthly revolution of the moon round the 

 earth acts upon the latter as an outward pull; 

 just as a weight tied to a string, which you whirl 

 round with your hand, is always felt to exert upon 

 the hand a strain in the direction in which it lies 

 at any moment. Now we must not suppose the 

 centre of the earth, to which the moon is tied by 

 gravity, and round which it turns, to be absolutely 

 still. On the contrary, it must be continually 

 yielding to this strain ; and the only reason that 

 its slight movements (with regard to the common 

 centre of gravity of the two planets), which are 

 thus produced, do not affect the general motion 

 of the earth, is, that in one complete revolution 

 of the moon they must mutually compensate each 

 other. While it is thus being moved regularly 

 to and fro by the moon, the earth itself is turning 

 once a day round its own axis, and is thereby 

 always bringing fresh parts of its liquid surface 

 into the places nearest to and farthest from its 



