limitation of the period during which living beings have inhabited this planet to one, two, or three hundred million years requires a complete revolution in geological speculation, the onus prohandi rests on the maker of the assertion, who brings forward not a shadow of evidence in its support.
Thus, if we accept the limitation of time placed before us by Sir W. Thomson, it is not obvious, on the face of the matter, that we shall have to alter, or reform, our ways in any appreciable degree ; and we may therefore proceed with much calmness, and, indeed, much indifference to the result, to inquire whether that limitation is justified by the arguments employed in its support. These arguments are three in number : —
I. The first is based upon the undoubted fact that the tides tend to retard the rate of the earth's rotation upon its axis. That this must be so is obvious, if one considers roughly that the tides result from the pull which the sun and the moon exert upon the sea, causing it to act as a sort of break upon the rotating solid earth.
Kant, who was by no means a mere " abstract philosopher," but a good mathematician and well versed in the physical science of his time, not only proved this in an essay of exquisite clearness and intelligibility, now more than a century old *, but deduced from it some of its more important consequences, such as the constant turning of one face of the moon towards the earth.
But there is a long step from the demonstration of a tendency to the estimation of the practical value of that tendency, which is all with which we are at present concerned. The facts bearing on this point appear to stand as follows : —
It is a matter of observation that the moon's mean motion is (and has for the last 3000 years been) undergoing an acceleration relatively to the rotation of the earth. Of course this may result from one of two causes : the moon may really have been moving more swiftly in its orbit; or the earth may have been rotating more slowly on its axis.
Laplace believed he had accounted for this phenomenon by the fact that the eccentricity of the earth's orbit has been diminishing throughout these 3000 years. This would produce a diminution of the mean attraction of the sun on the moon, or, in other words, an increase in the attraction of the earth on the moon, and, consequently, an increase in the rapidity of the orbital motion of the latter body. Laplace, therefore, laid the responsibility of the acceleration upon the moon ; and if his views were correct, the tidal retardation must either be insignificant in amount, or be counteracted by some other agency.
Our great astronomer Adams, however, appears to have found a flaw in Laplace's calculation, and to have shown that only half the
- " Untersuchung der Frage ob die Erde in ihrer Umdrehung um die Achse,
wodurch sie die Abwechselung des Tages und der Nacht hervorbringt, einige Veranderung seit den ersten Zeiten ihres Ursprunges erlitten habe, &c." — Kant's ' Sammtliche Werke,' Bd. i. p. 178.
