188 0^1 THE IXTEUACTION OF NATtmAL FOHCES. 



due to the existence of such a medium 5 a time will come 

 when the comet will strike the sun ; and a similar end 

 threatens all the planets, although after a time, the 

 length of which baffles our imagination to conceive 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 undoubtedly a fluid portion on its 

 sm'face, 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 

 diminishes 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 in- 

 creased since the time of Hipparchus by the gi^ 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 



