the act of rubbing or grating the surface of one body against that of another, called also attrition. The phenomena arising upon the friction of divers bodies, under different circumstances, are very numerous and considerable. Mr Hawkbee gives us a number of experiments of this kind; particularly of the attrition or friction of glass, under various circumstances, the result of which was, that it yielded light and became electrical. All bodies by friction produce heat; many of them emit light; particularly a cat's back, sugar, beaten sulphur, mercury, sea water, gold, copper, &c. but, above all, diamonds, which, when briskly rubbed against glass, gold, or the like, yield a light equal to that of a live coal when blowed by the bellows. See ELECTRICS and ELECTRICITY.

in Mechanics, denotes the resistance a moving body meets with from the surface on which it moves. Friction arises from the roughness or alperity 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 broke 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, cæteris paribus, 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.

in Medicine and Surgery, denotes the act of rubbing a diseased part with oils, unguents, or other matters, in order to ease, relieve, and cure it. Frictions are much used of late in veneral cases. They prefer the applying of mercury externally by way of friction, to that of giving it internally, to raise a salivation.

There are also frictions with the flesh brush, a linen cloth, or the hand only. These frictions are a sort of exercise which contributes greatly to health; as they excite and stir up the natural warmth, divert defluxions, promote perspiration, open the pores of the skin, and carry off stagnant humours.

The flesh brush (Dr Cheyne observes) is an exercise extremely useful for promoting a full and free perspiration and circulation. Every body knows the effect of currying horses; that it makes them fleck, gay, lively, and active; so as even to be judged equivalent to half the feeding. This it can no otherwise effect, but by affluing nature to throw off the recrements of the juices, which stop the free circulation, and, by constant friction, irritation, and stimulation, to bring the blood and spirits to the parts most dilatant from the seat of heat and motion; and so plump up the superficial muscles. And the same effect it would have in other creatures, and man himself, if managed in the same manner, and with the same care and regularity. Perls, Persons, therefore, of weak nerves and sedentary lives, would do well to supply the want of other exercise with spending half an hour, morning and night, in currying and rubbing their whole body, especially their limbs, with a flesh bruth. But this means of health is most advantageously used when the prime vae are most empty.