166 ON THE INTERACTION OF NATUEAL FORCES. 



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 phenomenon 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-22,000 cubic miles of water more than during the 

 ebb, and that therefore such a mass of water must, in 6 hours, 

 flow from one quarter of the earth to the other. 



The phenomenon of the ebb and flow, as already recognised 

 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 plane- 

 tary motions discovered by Newton teaches, that if a solid body 

 in absolute vacua, attracted by the sun, move around him in 

 the same manner as the planets, this motion will endure un- 

 changed 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 plane- 

 tary system is unendangered. 



But 1 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 rst 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 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 



