NATURAL PHILOSOPHY. 137 



tide, while the regions which lie 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 phenomena 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 six and a quarter hours, flow from one quarter of the earth 

 to the other. 



The phenomena of the ebb and flow, as already recognized by Mayer, 

 combined with the law of the conservation of force, stands in remarkable 

 connection 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 bodv in absolute vacua, attracted by the sun, move around 



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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 dis- 

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

 Mecanique Celeste, has proved that in our planetary system all these disturb- 

 ances increase and diminish periodically, and can neA'cr exceed certain limits, 

 so that by this cause the eternal existence of the planetary system is nnen- 

 dangercd. 



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 

 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 ob- 

 served 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 materials which are 

 inseparably bound together. Signs of the existence of an atmosphere arc 

 observed on the Sun, on Venus, Mars, Jupiter, and Saturn. Signs of wat'.T 

 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 mechanical 

 force of the system ; and as a consequence of this, the rotation of the planets 

 ui question round their axes must become more slow, they must therefore 



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