110 THE EVOLUTION OF SATELLITES. 



they are the feeble residue of iufluences which have probably exercised 

 a predomiiiaDt control over the history of the earth and the moon since 

 an indeterminate but remote epoch in tlie past, and will continue that 

 control into the distant future. 



Newton was tlie first to jjrove that the tides are caused by the attrac- 

 tions of the moon and the sun. It would need much space to explain 

 fully the manner in which those attractions operate, yet it is possible 

 to give in a few words a rough sketch of the mode in which the tide- 

 generating forces arise. It will suffice for this purpose to confine our 

 attention to the more important of the two bodies, the moon, since the 

 action of the sun will then follow by parity of reasoning. According to 

 the law of universal gravitation, the moon attracts matter which stands 

 near to her more strongly than that which is more remote. It follows 

 that the attraction on the ocean, at the side of the earth which is near- 

 est to the moon must be greater than that exercised on the solid earth 

 itself. Hence there is a tendency for tlie sea to depart from its natural 

 spherical shape, and to bulge outward toward the moon. So far the 

 matter is simple, but it is perplexing to many that the moon should 

 apparently repel the water lying on the farther side of the earth. This 

 action, however, is not due to any ideal repulsion from the moon, but 

 results from the fact that on the farther side the moon must attract the 

 solid earth more strongly than she does the water. On the nearer side 

 the moon j)ulls the water away from the earth, and on the farther side 

 she pulls the earth away from the water, thus producing an apj^arent 

 repulsion of the water to an extent equal to the attraction on the other 

 side. In this way there arises a tendency for the ocean to bulge equally 

 toward and away from the moon, and to assume an egg-like shape, with 

 the length of the egg pointed toward the moon. 



If the whole planet were fluid, instead of being i)artly fluid and 

 partly solid, the same tendency would still exist, but the tide-generat- 

 ing force would have the whole mass of the planet as its field of opera- 

 tion, instead of merely the superficial oceaii. The fact that the earth, 

 the moon, and the planets are all nearly spherical proves that in early 

 times they were molten and plastic, and that they assumed their pres- 

 ent round sha^ie under the influence of gravitation. When the material 

 of which any planet is formed was semiliquid through heat, its satel- 

 lites, or at any rate the sun, must have produced tidal oscillations in the 

 molten rock, just as the sun and the moon now raise tides in our oceans. 



INIolteu rock and molten iron are rather sticky or viscous substances, 

 and any movement which agitates them must be subject to much fric- 

 tion. Even water, which is a very good lubricant, is not entirely free 

 from friction, and so our present oceanic tides must be influenced by 

 fluid friction, although to a far less extent than the molten solid just 

 referred to. Now, all moving systems which are subject to friction 

 gradually come to rest. A train will run a long way when the steam is 

 turned off, but it stops at last, and a fly wheel will continue to spin for 

 only a limited time. This general law renders it certain that the fric- 



