cited. On the contrary, it is
necessary to my argument to assume that they are all correct. But I
desire to point out that this seems to be one of the many cases in which
the admitted accuracy of mathematical processes is allowed to throw a
wholly inadmissible appearance of authority over the results obtained by
them. Mathematics may be compared to a mill of exquisite workmanship,
which grinds you stuff of any degree of fineness; but, nevertheless,
what you get out depends on what you put in; and as the grandest mill in
the world will not extract wheat-flour from peascods, so pages of
formulae will not get a definite result out of loose data.

In the present instance it appears to be admitted:--

1. That it is not absolutely certain, after all, whether the moon's mean
motion is undergoing acceleration, or the earth's rotation
retardation.[58] And yet this is the key of the whole position.

2. If the rapidity of the earth's rotation is diminishing, it is not
certain how much of that retardation is due to tidal friction,--how much
to meteors,--how much to possible excess of melting over accumulation of
polar ice, during the period covered by observation, which amounts, at
the outside, to not more than 2,600 years.

3. The effect of a different distribution of land and water in modifying
the retardation caused by tidal friction, and of reducing it, under some
circumstances, to a minimum, does not appear to be taken into account.

4. During the Miocene epoch the polar ice was certainly many feet
thinner than it has been during, or since, the Glacial epoch. Sir W.
Thomson tells us that the accumulation of something more than a foot of
ice around the poles (which implies the withdrawal of, say, an inch of
water from the general surface of the sea) will cause the earth to
rotate quicker by one-tenth of a second per annum. It would appear,
therefore, that the earth may have been rotating, throughout the whole
period which has elapsed from the commencement of the Glacial epoch down
to the present time, one, or more, seconds per annum quicker than it
rotated during the Miocene epoch.

But, according to Sir W. Thomson's calculation, tidal retardation will
only account for a retardation of 22" in a century, or 22/100 (say 1/5)
of a second per annum.

Thus, assuming that the accumulation of polar ice since the Miocene
epoch has only been sufficient to produce ten times the effect of a coat
of ice one foot thick, we shall hav