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 have an accelerating cause which covers all the
loss from tidal action, and leaves a balance of 4/5 of a second per annum
in the way of acceleration.

If tidal retardation can be thus checked and overthrown by other
temporary conditions, what becomes of the confident assertion, based upon
the assumed uniformity of tidal retardation, that ten thousand million
years ago the earth must have been rotating more than twice as fast as at
present, and, therefore, that we geologists are "in direct opposition to
the principles of Natural Philosophy" if we spread geological history
over that time?

II. The second argument is thus stated by Sir W. Thomson:--"An article,
by myself, published in 'Macmillan's Magazine' for March 1862, on the age
of the sun's heat, explains results of investigation into various
questions as to possibilities regarding the amount of heat that the sun
could have, dealing with it as you would with a stone, or a piece of
matter, only taking into account the sun's dimensions, which showed it to
be possible that the sun may have already illuminated the earth for as
many as one hundred million years, but at the same time rendered it
almost certain that he had not illuminated the earth for five hundred
millions of years. The estimates here are necessarily very vague; but
yet, vague as they are, I do not know that it is possible, upon any
reasonable estimate founded on known properties of matter, to say that we
can believe the sun has really illuminated the earth for five hundred
million years."[21]

[Footnote 21: _Loc. cit._ p. 20.]

I do not wish to "Hansardise" Sir William Thomson by laying much stress
on the fact that, only fifteen years ago he entertained a totally
different view of the origin of the sun's heat, and believed th