ell try to get a
steam-engine which would produce enough heat by friction not only to
supply its own boilers, but to satisfy all the thermal wants of the
whole parish. We must therefore adopt the other alternative. The tides
do not draw their energy from the moon; they draw it from the store
possessed by the earth in virtue of its rotation.

We can now state the end of this rather long discussion in a very
simple and brief manner. Energy can only be yielded by the earth at
the expense of some of the speed of its rotation. The tides must
therefore cause the earth to revolve more slowly; in other words, _the
tides are increasing the length of the day_.

The earth therefore loses some of its velocity of rotation;
consequently it does less than its due share of the total quantity of
spin, and an increased quantity of spin must therefore be accomplished
by the moon; but this can only be done by an enlargement of its orbit.
Thus there are two great consequences of the tides in the earth-moon
system--the days are getting longer, the moon is receding further.

These points are so important that I shall try and illustrate them in
another way, which will show, at all events, that one and both of
these tidal phenomena commend themselves to our common sense. Have we
not shown how the tides in their ebb and flow are incessantly
producing friction, and have we not also likened the earth to a great
wheel? When the driver wants to stop a railway train the brakes are
put on, and the brake is merely a contrivance for applying friction to
the circumference of a wheel for the purpose of checking its motion.
Or when a great weight is being lowered by a crane, the motion is
checked by a band which applies friction on the circumference of a
wheel, arranged for the special purpose. Need we then be surprised
that the friction of the tides acts like a brake on the earth, and
gradually tends to check its mighty rotation? The progress of
lengthening the day by the tides is thus readily intelligible. It is
not quite so easy to see why the ebbing and the flowing of the tide on
the earth should actually have the effect of making the moon to
retreat; this phenomenon is in deference to a profound law of nature,
which tells us that action and reaction are equal and opposite to each
other. If I might venture on a very homely illustration, I may say
that the moon, like a troublesome fellow, is constantly annoying the
earth by dragging its waters backwa