The
earth's rotation is a reservoir from whence the tides draw the energy
they require for doing work. Hence it is that though the tides are
caused by the moon, yet whenever they require energy they draw on the
supply ready to hand in the rotation of the earth. The earth differs
from the fly-wheel of an engine in a very important point. As the energy
is withdrawn from the fly-wheel by the machines in the mill, so it is
restored thereto by the power of the steam engine, and the fly runs
uniformly. But the earth is merely the fly-wheel without the engine.
When the work by the tides withdraws energy from the earth, that energy
is never restored. It, therefore, follows that the earth's rotation
must be decreasing. This leads to a consequence of the most wonderful
importance. It tells us that the speed with which the earth rotates on
its axis is diminishing. We can state the result in a manner which has
the merits of simplicity and brevity. The tides are increasing the
length of the day. At present, no doubt, the effect of the tides
in changing the length of the day is very small. A day now is not
appreciably longer than a day a hundred years ago.
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