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 falling meteors, his greater attractive power would make the planets describe smaller ellipses and occupy less time in their revolutions. Were the large worlds also to have their attraction increased by similar accessions of meteoric matter, they would reduce the orbits and quicken the speed of their satellites. Analogous results should be indirectly occasioned to both classes of planets by the resistance of the medium supposed to pervade space. But tidal friction is the best known impediment 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 conservation 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 operations with great care, found that the earth's diurnal motion is reduced on a scale corresponding to the waste of power involved in tidal movements. Although the amount of energy thus wasted has been estimated 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 remain forever unknown if our watery domain were not so sensitive to