the two should be filled with powdered fire-brick or broken-up crucibles (fig. 3). BELLOWS AND BLOWING MACHINES. Illustration: FIG. 1. (‡ BLACKSMITH BELLOW.) Illustration: FIG. 2. (‡ DOUBLE-BELLOW.) Illustration: FIG. 3. (‡ FAN WHEEL.) Illustration: FIG. 4. (‡ FAN WHEEL HOUSING.) The common bellows is the most familiar form of blowing machine. It consists of two boards bound together with leather, having folds so arranged that the upper board may be raised or depressed, and the whole is made air-tight; in the lower board is a hole with a leather flap-valve opening inwards. When the upper board is raised, the air rushes in at the hole, pushing up the valve, and when the board is lowered the air presses the valve down, and so shuts it close, it has therefore no exit but at the nose of the bellows, from which it passes out. Blacksmiths’ bellows (fig. 1) are made double, for the purpose of keeping up a continuous stream of air, instead of the separate puffs produced by the common single bellows. The arrangement of the double bellows is as follows:--There are three boards bound together with leather folded as in the common house bellows; the board in the middle is fixed, and to this the nose is fastened, but it opens only into the space above; the upper and lower boards are united to the middle one by a hinge, and are capable of being moved up and down; the middle and lower ones have each holes and valves opening upwards as in the common bellows, and when the lower board is raised it presses the air in the space between it and the middle board through the hole in the latter, into the space between it and the upper one, and so raises it; this has a heavy iron weight placed on it which makes it sink down and force the air out through the nose. While this weight is sinking the lower board is pushed down, and is ready to force a fresh quantity of air into the upper space, so that one continuous stream of air issues at the nose of the bellows. The handle is fixed to the lower board, and generally has a cord uniting it to a wooden handle, which is worked like a pump-handle (fig. 2). For large furnaces, blowing machines of various kinds are used, generally consisting of a pair of large cylinders having pistons worked in them by steam power, and pumping air into a large air-chamber, from which it proceeds in three or four pipes to the furnace, or sometimes to numerous furnaces, each having a tube and stop-cock by which the “blast” may be turned on, similarly to gas or water, the air-chamber being always kept filled at a great pressure by the cylinders, and furnished with a safety-valve to prevent the pressure bursting it. There is another kind of blowing-machine, consisting of a fan wheel turning very rapidly in a round box (figs. 3 and 4), from which a tube proceeds, and having holes in the sides to admit the air, which is thrown forwards by the fans of the wheel. SCREW PROPELLERS.