THE TIDES. 155 



subtleties, and with some of the mere elements, of 

 physical astronomy. I will not enter into details, 

 as it would be useless for those who already 

 understand the tidal theory, and unintelligible to 

 those who do not. 



I may just say that the moon attracts a piece of 

 matter, for example a pound-weight, here on the 

 earth, with a force which we compare with the 

 earth's attraction thus. Her mass is 1/80 of the 

 earth's, and she is sixty times as far away from the 

 earth's centre as we are here. Newton's theory 

 of gravitation shows, that when you get outside 

 the mass of the earth the resultant attraction of 

 the earth on the pound weight, is the same as if 

 the whole mass of the earth were collected at 

 the centre, and that it varies inversely as the 

 square of the distance from the centre. The same 

 law is inferred regarding the moon's attraction 



from the general theory. The moon's attraction 



i 



on this pound weight is therefore 8 , or 



60x60 



288000 ^ ^ e at ti"action f tne earth on the 

 same mass. But that is not the tide-generating 



