ON THE INTERACTION OF NATURAL FORCES. 167 



the sun ; and a similar end threatens all the planets, although 

 after a time, the length of which baffles our imagination to con- 

 ceive 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 are 

 observed on the Sun, on Venus, Mars, Jupiter, and Saturn. 

 Signs of water and ice upon Mars ; and our earth has undoubt- 

 edly 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 dimi- 

 nishes the store of mechanical force of the system ; and as a 

 consequence of this, the rotation of the planets in question 

 round their axes must become more slow. The recent careful 

 investigations of the moon's motion made by Hansen, Adams, 

 and Delaunay, have proved that the earth does experience such 

 a retardation. According to the former, the length of each 

 sidereal day has increased since the time of Hipparchus by the 

 J- f part of a second, and the duration of a century by half a 

 quarter of an hour ; according to Adams and Sir W. Thomson, 

 the increase has been almost twice as great. A clock which 

 went right at the beginning of a century, would be twenty-two 

 seconds in advance of the earth at the end of the century. 

 Laplace had denied the existence of such a retardation in the 

 case of the earth ; to ascertain the amount, the theory of lunar 

 motion required a greater development than was possible in his 

 time. The final consequence would be, but after millions of 

 years, if in the meantime the ocean did not become frozen, 

 that one side of the earth would be constantly turned towards 

 the sun, and enjoy a perpetual day, whereas the opposite side 

 would be involved in eternal night. Such a position we observe 

 in our moon with regard to the earth, and also in the case 

 of the satellites as regards their planets ; it is, perhaps, 

 due to the action of the mighty ebb and flow to which these 



