Whence is
this energy derived with which the tides do their work? If the tides are
caused by the moon, the energy they possess must also be derived from
the moon. This looks plain enough, but unfortunately it is not true.
Would it be true to assert that the finger of the rifleman which pulls
the trigger supplies the energy with which the rifle bullet is animated?
Of course it would not. The energy is derived from the explosion of
gunpowder, and the pulling of the trigger is merely the means by which
that energy is liberated.
In somewhat similar manner the tidal wave produced by the moon is the
means whereby a part of the energy stored in the earth is compelled to
expend itself in work. Let me illustrate this by a comparison between
the earth rotating on its axis and the fly-wheel of an engine: The fly
wheel is a sort of reservoir, into which the engine pours its power at
each stroke of the piston. The various machines in the mill merely draw
off the power from the store accumulated in the fly-wheel. The earth
is like a gigantic fly-wheel detached from the engine, though still
connected with the machines in the mill. In that mighty fly-wheel a
stupendous quantity of energy is stored up, and a stupendous quantity of
energy would be given out before that fly-wheel would come to rest.
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