xpend it in doing work, that
energy is not replaced. The consequence is irresistible: the energy in
the rotation of the earth must be decreasing. This leads to a
consequence of the utmost significance. If the engine be cut off from
the fly-wheel, then, as everyone knows, the massive fly-wheel may still
give a few rotations, but it will speedily come to rest. A similar
inference must be made with regard to the earth; but its store of energy
is so enormous, in comparison with the demands which are made upon it,
that the earth is able to hold out. Ages of countless duration must
elapse before the energy of the earth's rotation can be completely
exhausted by such drafts as the tides are capable of making.
Nevertheless, it is necessarily true that the energy is decreasing; and
if it be decreasing, then the speed of the earth's rotation must be
surely, if slowly, abating. Now we have arrived at a consequence of the
tides which admits of being stated in the simplest language. If the
speed of rotation be abating, then the length of the day must be
increasing; and hence we are conducted to the following most important
statement: that the _tides are increasing the length of the day_.

To-day is longer than yesterday--to-morrow will be longer than to-day.
The difference is so small that even in the course of ages it can hardly
be said to have been distinctly established by observation. We do not
pretend to say how many centuries have elapsed since the day was even
one second shorter than it is at present; but centuries are not the
units which we employ in tidal evolution. A million years ago it is
quite probable that the divergence of the length of the day from its
present value may have been very considerable. Let us take a glance back
into the profound depths of times past, and see what the tides have to
tell us. If the present order of things has lasted, the day must have
been shorter and shorter the farther we look back into the dim past. The
day is now twenty-four hours; it was once twenty hours, once ten hours;
it was once six hours. How much farther can we go? Once the six hours is
past, we begin to approach a limit which must at some point bound our
retrospect. The shorter the day the more is the earth bulged at the
equator; the more the earth is bulged at the equator the greater is the
strain put upon the materials of the earth by the centrifugal force of
its rotation. If the earth were to go too fast it would be unable to