ON THE INTERACTION OF NATURAL FORCES. 187 



which a quarter of the earth covered by the sea possesses, 

 during the flow of the tide, about 22,000 cubic miles of 

 water more than during the ebb, and that therefore such 

 a mass of water must, in 6 J hours, flow from one quarter 

 of the earth to the other. 



The phsenomenon of the ebb and flow, as already recog- 

 nised by Mayer, combined with the law of the conserva- 

 tion of force, stands in remarkable connexion with the 

 question of the stability of our planetary system. The 

 mechanical theory of the planetary motions discovered 

 by Newton teaches, that if a solid body in absolute vacuo, 

 attracted by the sun, move around him in the same 

 manner as the planets, this motion will endure unchanged 

 through all eternity. 



Now we have actually not only one, but several such 

 planets, which move around the sun, and by their mutual 

 attraction create little changes and disturbances in each 

 other's paths. Nevertheless Laplace, in his great work, 

 the ' Mecanique celeste,' has proved that in our planetary 

 system all these disturbances increase and diminish peri- 

 odically, and can never exceed certain limits, so that by 

 this cause the eternal existence of the planetary system ia 

 unendangered. 



But I have already named two assumptions which must 

 be made : first, that the celestial spaces must be abso- 

 lutely empty ; and secondly, that the sun and planets 

 must be solid bodies. The first is at least the case as 

 far as astronomical observations reach, for they have 

 never been able to detect any retardation of the planets, 

 such as would occur if they moved in a resisting medium. 

 But on a body of less mass, the comet of Encke, changes 

 are observed of such a nature : this comet describes 

 ellipses round the sun which are becoming gradually 

 smaller. If this kind of motion, which certainly corre- 

 sponds to that through a resisting medium, be actually 



