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When force is manifested by the motion of bodies from each other, it is called repulsion. Thus, if a piece of glass, having been briskly rubbed with a silk handkerchief, touch successively two feathers, these feathers, if brought near each other, will move asunder. This effect is called repulsion, and the feathers are said to repel each other.

(9.) The influence which forces have upon the form, state, arrangement, and motions of material substances is the principal object of physical science. In its strict sense, Mechanics is a term of very extensive signification. According to the more popular usage, however, it has been generally applied to that part of physical science which includes the investigation of the phenomena of motion and rest, pressure and other effects developed by the mutual action of solid masses. The consideration of similar phenomena, exhibited in bodies of the liquid form, is consigned to Hydrostatics, and that of aeriform fluids to Pneumatics.