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.