in *Mechanics*, denotes the resistance which a moving body meets with from the surface on which it moves. Friction arises from the roughness or asperity of the surface of the body moved on and that of the body moving; for such surfaces consisting alternately of eminences and cavities, either the eminences of the one must be raised over those of the other, or they must be both broken and worn off; but neither can happen without motion, nor can motion be produced without a force impressed. Hence the force applied to move the body is either wholly or partly spent on this effect; and consequently there arises a resistance or friction, which will be greater, *ceteris paribus*, according as the eminences are the greater and the substance the harder; and as the body, by continual friction, becomes more and more polished, the friction diminishes. See *Mechanics*.