iscoverable difference as three-fourths of an inch
or an inch. This, or the reverse, which we believe might happen any year,
and could certainly not be detected without far more accurate
observations and calculations for the mean sea-level than any hitherto
made, would slacken or quicken the earth's rate as a timekeeper by one-
tenth of a second per year."[19]

[Footnote 19: _Ibid._]

I do not presume to throw the slightest doubt upon the accuracy of any of
the calculations made by such distinguished mathematicians as those who
have made the suggestions I have 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 process 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 upon 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.[20] And yet this is the key of the whole position.

[Footnote 20: It will be understood that I do not wish to deny that the
earth's rotation _may be_ undergoing retardation.]

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