HEAT BY FRICTION.
Friction as a source of Heat is well known: we rub our hands to warm them, and we grease the axles of carriage-wheels to prevent their setting fire to the wood. Count Rumford has established the extraordinary fact, that an unlimited supply of heat may be derived from friction by the same materials: he made great quantities of water boil by causing a blunt borer to rub against a mass of metal immersed in the water. Savages light their fires by rubbing two pieces of wood: the modus operandi, as practised by the Kaffirs of South Africa, is thus described by Captain Drayton:
Two dry sticks, one being of hard and the other of soft wood, were the materials used. The soft stick was laid on the ground, and held firmly down by one Kaffir, whilst another employed himself in scooping out a little hole in the centre of it with the point of his assagy: into this little hollow the end of the hard wood was placed, and held vertically. These two men sat face to face, one taking the vertical stick between the palms of his hands, and making it twist about very quickly, while the other Kaffir held the lower stick firmly in its place; the friction caused by the end of one piece of wood revolving upon the other soon made the two pieces smoke. When the Kaffir who twisted became tired, the respective duties were exchanged. These operations having continued about a couple of minutes, sparks began to appear, and when they became numerous, were gathered into some dry grass, which was then swung round at arm’s length until a blaze was established; and a roaring fire was gladdening the hearts of the Kaffirs with the anticipation of a glorious feast in about ten minutes from the time that the operation was first commenced.