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s are generally of great importance in physical problems. Consider, for example, the , but are subject to no other forces. Except for . Yet these s have ten independent simple algebraic integrals, seven of which do not depend upon the time. Six of the ten are the mathematical expressions for the fact that the of the three bodies moves with respect to the axes of reference in a with uniform speed. Three of the integrals express the fact that the projections of the of the system upon the three planes of reference are constant. And one integral expresses the fact that the sum of the and the of the system is constant. The integrals describe s of the system, and their importance is easily seen.
— FOREST RAY MOULTON