rpetual motion. It is needless to enter into
details of any proposed contrivance of wheels, of pumps, of pulleys;
it is sufficient to say that nothing in the shape of mechanism can
work without friction, that friction produces heat, that heat is a
form of energy, and that to replace the energy consumed in producing
the heat there must be some source from which the machine is
replenished if its motion is to be continued indefinitely.

Hence, as the tides may be regarded as a machine doing work, we have
to ascertain the origin of that energy which they are continually
expending. It is at this point that we first begin to feel the
difficulties inherent in the theory of tidal evolution. I do not mean
difficulties in the sense of doubts, for up to the present I have
mentioned no doubtful point. When I come to such I shall give due
warning. By difficulties I now mean points which it is not easy to
understand without a little dynamical theory; but we must face these
difficulties, and endeavour to elucidate them as well as we can.

Let us first see what the sources of energy can possibly be on which
the tides are permitted to draw. Our course is simplified by the fact
that the energy of which we have to speak is of a mechanical
description, that is to say, not involving heat or other more obscure
forms of energy. A simple type of energy is that possessed by a
clock-weight after the clock has been wound. A store of power is thus
laid up which is gradually doled out during the week in small
quantities, second by second, to sustain the motion of the pendulum.
The energy in this case is due to the fact that the weight is
attracted by the earth, and is yielded according as the weight sinks
downwards. In the separation between two mutually attracting bodies, a
store of energy is thus implied. What we learn from an ordinary clock
may be extended to the great bodies of the universe. The moon is a
gigantic globe separated from our earth by a distance of 240,000
miles. The attraction between these two bodies always tends to bring
them together. No doubt the moon is not falling towards the earth as
the descending clock-weight is doing. We may, in fact, consider the
moon, so far as our present object is concerned, to be revolving
almost in a circle, of which the earth is the centre. If the moon,
however, were to be stopped, it would at once commence to rush down
towards the earth, whither it would arrive with an awful crash in the
course