580 THE POPULAR SCIENCE MONTHLY. 



of Nature held such a sway over scientific opinion that even slight 

 accession of foreign matter to the earth by falling meteors would not 

 be admitted. As, even at the present day, the origin of meteoric stones 

 and shooting-stars cannot be said to be entirely free from doubt, I 

 shall not introduce them for evidence in this stage of my inquiry, as 

 items of the greatest certainty must claim the first attention. There 

 is, indeed, no doubt that, if the mass of the sun were increased by fall- 

 ing meteors, his greater attractive power would make the planets de- 

 scribe smaller ellipses and occupy less time in their revolutions. Were 

 the large worlds also to have their attraction increased by similar acces- 

 sions of meteoric matter, they would reduce the orbits and quicken the 

 speed of their satellites. Analogous results should be indirectly occa- 

 sioned to both classes of planets by the resistance of the medium sup- 

 posed to pervade space. But tidal friction is the best known impedi- 

 ment to planetary motion, and the changes which it slowly occasions 

 in the condition and the career of the celestial bodies come more 

 decidedly within the range of the investigations of modern science. 



This retarding influence has been studied much in modern times, in 

 so far as it affects the rotation of the earth, and even the movements of 

 the moon. More than a hundred years ago Kant maintained that the 

 tide-wave, in rolling from east to west, would reduce the earth's diurnal 

 motion ; but no positive proof of this conclusion could be given for a 

 long time, in consequence of the peculiar difficulties of the inquiry and 

 the imperfect condition of the tidal theory. Laplace, in dealing with 

 the problem, concluded that within the last 2,000 years the length 

 of the day has not been perceptibly affected by the alternate rise 

 and fall of our oceans. But about thirty years ago, when science was 

 enriched by the development of the doctrines respecting the conser- 

 vation and the transformation of energy, the question of the effects 

 of tidal friction was again opened for discussion, and Mayer was able 

 to reproduce and to maintain on new grounds the almost forgotten 

 doctrine of Kant. The evidence on which Mayer based his convictions 

 was subsequently strengthened by a discovery which Prof. Adams 

 made, of an error in the investigations of Laplace ; and soon afterward 

 Delaunay, in taking up the inquiry and repeating the previous opera- 

 tions with great care, found that the earth's diurnal motion is reduced 

 on a scale corresponding to the waste of power involved in tidal move- 

 ments. Although the amount of energy thus wasted has been esti- 

 mated as over two thousand times greater than that of the united labor 

 of the entire human population, yet so great is the stock of working 

 force embodied in terrestrial rotation that 20,000,000 years must 

 elapse before the length of our day is increased one per cent, by tidal 

 friction. 



The alteration which the moon occasions in terrestrial gravity is too 

 small to be detected by the most delicate experiment; and it might re- 

 main forever unknown if our watery domain were not so sensitive to 



