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64. the last chapter we introduced our readers to two varieties of energy, one of them visible, and the other invisible or molecular; and it will now be our duty to search through the whole field of physical science for other varieties. Here it is well to bear in mind that all energy consists of two kinds, that of position and that of actual motion, and also that this distinction holds for invisible molecular energy just as truly as it does for that which is visible. Now, energy of position implies a body in a position of advantage with respect to some force, and hence we may with propriety begin our search by investigating the various forces of nature.

65. The most general, and perhaps the most important, of these forces is gravitation, and the law of action of this force may be enunciated as follows:—Every particle of the universe attracts every other particle with a force