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Reverting now to the earth-moon system, the energy which that system contains consists essentially of two parts the moon-energy, whose composite character I have already explained, and the earth-energy, which has its origin solely in the rotation of the earth on its axis.
On the whole, we may represent the total energy of the moon as a single quantity, which increases when the distance of the moon from the earth increases, and lessens when the distance from the earth to the moon lessens. For simplicity we may speak of this as moon-energy. But the most important constituent of the store of energy in the earth-moon system is that contributed by the earth itself.
The energy which this work has necessitated can only have been drawn from the existing store in the system; that energy consists of two parts the moon-energy and the earth's rotation energy. The problem therefore for us to consider is, which of these two banks the tides have drawn on to meet their constant expenditure.
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