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 observed
retardation could be accounted for in the way he had suggested. There
remains, therefore, the other half to be accounted for; and here, in the
absence of all positive knowledge, three sets of hypotheses have been
suggested.

(a) M. Delaunay suggests that the earth is at fault, in consequence
of the tidal retardation. Messrs. Adams, Thomson, and Tait work out this
suggestion, and, "on a certain assumption as to the proportion of
retardations due to the sun and the moon," find the earth may lose
twenty-two seconds of time in a century from this cause.[55]

(b) But M. Dufour suggests that the retardation of the earth (which
is hypothetically assumed to exist) may be due in part, or wholly, to
the increase of the moment of inertia of the earth by meteors falling
upon its surface. This suggestion also meets with the entire approval of
Sir W. Thomson, who shows that meteor-dust, accumulating at the rate of
one foot in 4,000 years, would account for the remainder of
retardation.[56]

(c) Thirdly, Sir W. Thomson brings forward an hypothesis of his own
with respect to the cause of the hypothetical retardation of the earth's
rotation:--

"Let us suppose ice to melt from the polar regions (20 deg. round each
pole, we may say) to the extent of something more than a foot thick, enough
to give 1.1 foot of water over those areas, or 0.006 of a foot of water if
spread over the whole globe, which would, in reality, raise the
sea-level by only some such undiscoverable 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."[57]

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