m 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 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 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 moon," find the earth may lose twenty-two
seconds of time in a century from this cause.[17]

[Footnote 17: Sir W. Thomson, _loc. cit_. p. 14.]

(_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.[18]

[Footnote 18: _Ibid._ p. 27.]

(_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 und