The spiral springs, used to give elasticity to the seats, &c., are tempered by heating them in a close vessel with bone dust or animal charcoal, and, when thoroughly heated, cooled in a bath of oil. They are tempered by putting them into an iron pan with tallow or oil, and shaking them about over a brisk fire. The tallow will soon blaze, and keeping them on the move will cause them to heat evenly. The steel springs for fire-arms are tempered in this way, and are literally “fried in oil.” If a long slender spring is needed with a low temper, it can be made by simply beating the soft forging on a smooth anvil with a smooth-faced hammer. _Setting and Tempering old Springs._ In setting up old springs where they are inclined to settle, first take the longest plate (having separated all the plates) and bring it into shape; then heat it for about 2 feet in the centre to a cherry red, and cool it off in cold water as quick as possible. This will give the steel such a degree of hardness that it will be liable to break if dropped on the floor. To draw the temper hold it over the blaze, carrying backward and forward through the fire until it is so hot that it will sparkle when the hammer is drawn across it, and then cool off. Another mode is to harden the steel, as before stated, and draw the temper with oil or tallow—tallow is the best. Take a candle, carry the spring as before through the fire, and occasionally draw the candle over the length hardened, until the tallow will burn off in a blaze, and then cool. Each plate is served in the same way. _Varieties of Springs._ The names given to springs are numerous, but the simple forms are few, the greater part of the varieties being combinations of the simple forms. [Illustration: Fig. 25.] [Illustration: Fig. 26.] The simple forms are the elliptic spring, the straight spring, and the regular curve or C spring (Fig. 25). There are also one or two forms of spring which have become obsolete. Such are the whip spring (Fig. 26), and the reverse curved spring, which was superseded by the last. The elliptic spring is the one most commonly used at the present day. Fig. 27, _b_, shows two of these united at the extremities by means of a bolt; this is called a double elliptic spring. The elliptic spring is sometimes used single in what are called under-spring carriages, where the spring rests on the axle and is connected with the framework of the body with an imitation spring or dumb iron to complete the ellipse. Its technical name is an “under-spring.” When four pairs of these springs are hinged together so as to form four ellipses they constitute a set, and are used in carriages without perches. Their technical name is “nutcracker spring.” The straight springs are used in phaetons and tilburies, and are called “single-elbow springs.” The double straight spring is used in omnibuses, carts, &c., where it is fixed across the angle at right angles. It is called a “double-elbow spring.”