is released from the sun's influence she revolves faster round the earth.
When it was seen how completely the difficulties in ancient observations were explained away by the calculations of Laplace, all doubt was considered to be at an end, and astronomers supposed that the whole truth was known. But, in 1853, it occurred to Prof. Adams to recalculate Laplace's investigations, and the result was the detection of a material error, which vitiated the whole series of observations. The results of Prof. Adams's calculations were submitted to the Royal Society[1] in a paper, the explanatory part of which is very short indeed, occupying but a couple of pages of the "Proceedings." The brief statement is followed by a corroborative sea of high mathematics, into which we have no intention of asking the reader to plunge. The result, roughly stated, was to halve the amount of acceleration calculated by Laplace, and thus to leave half of the acceleration of the moon necessary for his explanation of ancient eclipses to be found in some other way. Astronomers were now in a condition almost as bad as that from which they had been rescued by Laplace.
Adams communicated his final result to M. Delaunay, one of the great French mathematicians; and it seems to have been during the investigations which that astronomer undertook to verify the calculations of Adams that it occurred to him to inquire whether our measure of time itself remains unchanged? in other words, whether the earth itself may not be rotating more slowly, instead of the moon more quickly, than in by-gone ages? It is plain that the moon will appear to be moving more quickly round the earth, if the earth itself—which is furnishing the standard by which the moon's revolution is to be measured—is rotating more and more slowly from age to age.
Newton laid it down in his first law of motion that motion unresisted remains uniform forever; and he gave as an instance of constant motion, unaffected by any external causes, this very rotation of the earth about its axis. But M. Delaunay remembered that Kant had pointed out the resistance which the earth must incur from the tide-wave, and had even approximately calculated its amount. The tidal wave is lifted up toward the moon, and on the side of the earth opposite the moon; so that, as Prof. Tait puts it, the earth has always to revolve within a friction-brake. Adams adopted this theory of tidal friction; and, in conjunction with Prof. Tait and Sir William Thomson, assigned twenty-two seconds per century as the error by which the earth would, in the course of a century, get behind a thoroughly-perfect clock (if such a machine were possible).
It may be asked, If the earth's movement be diminishing gradually in rapidity, will it eventually stop altogether? No; if ever the earth shall so far yield to the action of the tidal wave as to rotate not more rapidly than the moon, she will present to the moon always the
- ↑ June 16, 1853.
