242 INTERACTION OF NATURAL FORCES. 



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 Mecan- 

 ique Celeste, has proved that in our planetary system all these 

 disturbances increase and diminish periodically, and can never 

 exceed certain limits, so that by this cause the eternal exist- 

 ence of the planetary system is unendangered. 



But I have already named two assumptions which must be 

 made : first that the celestial spaces must be absolutely empty ; 

 and secondly, that the sun and planets must be solid bodies. 

 The first is at least the case as far as astronomical observa- 

 tions reach, for they have never been able to detect any retar- 

 dation 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 de- 

 scribes ellipses round the sun which are becoming gradually 

 smaller. If this kind of motion, which certainly corresponds 

 to that through a resisting medium, be actually due to the ex- 

 istence of such a medium, a time will come when the comet 

 will strike the sun ; and a similar end threatens all the planets, 

 although after a time, the length of which baffles our imagina- 

 tion to conceive of it. But even should the existence of a re- 

 sisting medium appear doubtful to us, there is no doubt that 

 the planets are not wholly composed of solid materials which 

 are inseparably bound together. Signs of the existence of an 

 atmosphere are observed on the Sun, on Venus, Mars, Jupi- 

 ter, and Saturn. Signs of water and ice upon Mars ; and 

 our earth has undoubtedly a fluid portion on its surface, 

 and perhaps a still greater portion of fluid within it. The 

 motions of the tides, however, produce friction, all friction 

 destroys vis viva, and the loss in this case can only affect the 

 vis viva of the planetary system. We come thereby to the 

 unavoidable conclusion, that every tide, although with infinite 

 slowness, still with certainty, diminishes the store of mechani- 

 cal force of the system ; and as a consequence of this, the ro- 



