513 Prof. Helinholtz on the Interaction of Natural Forces. 



One of these waves has its crest on the quarter of the earth's 

 surface which is turned towards the moon, the other is at the 

 opposite side. Both these quarters possess the flow of the tide, 

 while the regions which he between have the ebb. Although in 

 the open sea the height of the tide amounts to only about three 

 feet, and only in certain narrow channels, where the moving water 

 is squeezed together, rises to thirty feet, the might of the phse- 

 nomenon is nevertheless manifest from the calculation of Bessel, 

 according to which a quarter of the earth covered by the sea 

 possesses, during the flow of the tide, about 25,000 cubic miles of 

 water more than during the ebb, and that therefore such a mass 

 of water must, in G]; hours, flow from one quarter of the earth 

 to the other. 



The phaenomenon of the ebb and flow, as already recognized 

 by Mayer, combined with the law of the conservation 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 periodically, and can never exceed certain 

 limits, so that by this cause the eternal existence of the planetary 

 system is unendaugered. 



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 observations 

 reach, for they have never been able to detect any i-etardation 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 corresponds to that through 

 a resisting medium, be actually due to the existence 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 imagination to conceive of 

 it. But even should the existence of a resisting medium appear 

 doubtful to us, there is no doubt that the planets are not 

 wholly composed of solid matei'ials which are inseparably bound 



