do is to modify his notions of the rapidity of change accordingly. And I
venture to point out that, when we are told that the 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 probandi_ 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, as
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,[54] 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 follow:--

It is a matter of observation that the moon's mean motion is (and has
for the last 3,000 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 phaenomenon by the fact that
the eccentricity of the earth's orbit has been diminishing throughout
these 3,000 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