298 CLASSICS OF MODERN SCIENCE 

 of another kind from its central luminary, as well as from its satellite 

 the moon, which exhibits itself in the remarkable phenomenon of the 

 ebb and flow of the tide. 



Each of these bodies excites, by its attraction upon the waters of 

 the sea, two gigantic waves, which flow in the same direction round 

 the world, as the attracting bodies themselves apparently do. The 

 two waves of the moon, on account of her greater nearness, are about 

 three and a half times as large as those excited by the sun. 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 

 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 cov- 

 ered 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, stand 

 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 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 La- 

 place, in his great work, the Mccaniqiie Celeste, has proved that in our 

 planetary system all these disturbances increase and diminish period- 

 ically, and can never exceed certain limits, so that by this cause 

 the external existence 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 soHd bodies. The first is at least the 



