DYNAMICS OF SUNLIGHT. 241 



and heat-giving rays of the sun ; and you see in this a re- 

 markable example, how Proteus-like the effects of a single 

 cause, under altered external conditions, may exhibit itself in 

 nature. Besides these, the earth experiences an action 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 quar- 

 ters 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 cer- 

 tain 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, accord- 

 ing to which a quarter of the earth covered by the sea pos- 

 sesses, 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 sta- 

 bility 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 

 11 



