ch. v.] PLANETARY EVOLUTION. 393 



draw them towards the centre. Let us see which set must 

 prevail in the end. 



Hitherto, in all probability, the first set of circumstances 

 has had the advantage. There is little reason to doubt that 

 all the planetary orbits, both primary and secondary, are 

 somewhat larger now than they were originally. This i~ an 

 indirect consequence of the slow loss of rotatory momentum 

 due to tidal action. The calculation by which Laplace 

 thought he had proved that the terrestrial day had not 

 lengthened since the time of Hipparchos, has been shown 

 by Prof. Adams to be vitiated by the inclusion of an er- 

 roneous datum; and the theory involved is no longer 

 tenable. It has been proved that the tidal wave which 

 the moon draws twice a day around the earth, in the op- 

 posite direction to the terrestrial rotation, acts upon the 

 earth like a brake on a carriage- wheel. Owing to this cir- 

 cumstance, the day is now one eighty-fourth part of a second 

 longer than at the beginning of the Christian era ; and it is 

 destined to continue lengthening until in the remote future 

 there will be from three to four hundred hours between 

 sunrise and sunset. But the rotatory momentum thus 

 lost by the earth is not destroyed. In conformity with 

 a well-known principle of dynamics, it is added to the 

 tangential momentum of the moon, and thus lengthens the 

 radius of the moon's orbit. The more slowly our planet 

 rotates, the farther the moon retires from us. A similar 

 relation holds good in the case of the planets and the sun. 

 Not only is it demonstrable d priori that the planets must 

 cause tides upon the surface of the sun, but the tides caused 

 by all the primary planets, save Mars, Uranus, and Neptune, 

 have been actually detected by a minute comparison of 

 the variations in the solar spots. These tidal waves are 

 drawn around the sun in the direction opposite to that of 

 his rotation, and must therefore exert a retarding effect. 

 A.nd the rotatory momentum thus stolen from the sun is 



