INERTIA

(Lat. iners.) This term is used to 697 INFANTRY. denote the principle or law of the material world that all bodies are absolutely pa ive or indifferent to a state of rest or motion, and would continue for ever at rest, or persevere in the same uniform and rectilinear motion, unle disturbed by the action of some extrinsic force. The ancients attributed to matter a certain inaptitude, reluctance, or renitency to motion; but that a body in motion required the operation of an extrinsic cause to bring it to rest was first discovered by Galileo. Kepler, conceiving the disposition of a body to maintain its motion as indicating an exertion of power, prefixed the word vis; and the compound expre ion, vis inertia, though le accurate, has been generally retained. Inertia is one of the inherent properties of matter, and unceasingly recalled to our observation in every incident of life. [s. 610]