REACTION

A term used in Mechanics to denote the reciprocality of force exerted by two bodies which act mutually on each other; or the gene>-al fact, collected from observation, that any two bodies repelling or attracting each other are made to recede or approach with equal momenta. Newton's third law of motion is, that " reaction is always contrary and equal to action, or that the mutual actions of two bodies are always equal, exerted in opposite directions." In the mathematical consideration of mechanics, this principle must be a imied as a nece ary axiom or law; and, in fact, as is remarked by Dr. Young {Nat.P/n'l), there would be something peculiar, and almost inconceivable, in a force which could affect unequally the similar particles of matter; or in the particles themselves, if they could be supposed of such different degrees of mobility as to be equally moveable with respect to one force, and unequally with respect to another. The principle may, therefore, as justly be termed a nece ary law as an experimental fact. [s. 1038]