BELLOWS, a machine so contrived as to agitate the air with great briskness, ex piring and inspiring it by turns, and that only from enlarging and contracting its capacity.
This machine is used in chambers and kitchens, in forges, furnaces, and feund rieS, to blow up the fire ; it serves also for organs and other pneumatic instru ments, to give them a proper degree of air. All these are of various construc tions, according to their different pur poses, but in general they are composed of two flat boards, sometimes of an oval, sometimes of a triangular figure. Two or more hoops, bent according to the figure of the boards, are placed between them ; a piece of leather, broad in the middle, and narrow at both ends, is nail ed on the edges of the boards, which it thus unites together : as also on the hoops which separate the boards, that the leather may the easier open and fold again; a tube of iron, brass, or copper, is fastened to the undermost board, and there is a valve within that covers the holes in the under board, to keep in the air.
The action and effect of bellows of eve ry kind, whether constructed of leather or wood, wrought by men, by steam, or by water, depends on this, that the air which enters them, and which they con tain when raised, is again compressed in to a narrower space when they are clos ed. As the air flows to that place where it meets with the least resistance, it must of necessity fly out of the pipe with a ve locity proportional to the force by which it is compressed, and must therefore blow stronger or weaker, as the velocity with which the top and bottom of the bellows meet is greater or less. The blast will
last in proportion to the quantity of air that was drawn into the bellows through the valve. The action of the bellows bears a near affinity to that of the lungs, and what is called blowing in the latter affords an illustration of what is called re spiring in the former : hence bellows have been employed in restoring suspend ed animation. See DROWNING.
The bellows of smiths, founders, &c. are worked by means of a rocker, with a string fastened to it, and pulled by the workman. One of the boards is fixed so as not to play at all. By drawing down the handle of the rocker, the moveable board rises, and by means of a weight on the top of the upper board sinks again. Large bellows used in founderies, &c. ceive their motion from water wheels or steam : others that are small are worked by the feet of the men using them, as is the case with enamellers, jewellers, &c. The bellows of an organ are six feet long, and four feet broad, each having an ture of four inches, that the valve may play easily. To blow an organ of sixteen feet, there are required four pair of these bellows.