BELLOWS, an appliance to produce a current of air by compressing a collapsible bag or receptacle into which air has been admitted. The O.E. word for bellows was bldstbaelig; i.e., "blow-bag." By the 11th century baelig, bag, is found in early glossaries. Baelig became in M.E. bely, the plural "bellies" being found till the 16th century, when "bellows" appears, and the word in the singular ceases to be used.
Among the earliest contrivances employed for producing the 'movement of air under a small pressure were those used in Egypt during the Greek occupation. These depended upon the heating of the air, which, being raised in pressure and bulk, was made to force water out of closed vessels, the water being afterwards em ployed for moving some kind of mechanism. In the process of iron smelting there is still used in some parts of India an artificial blast, produced by a simple form of bellows made from the skins of goats; bellows of this kind probably represent one of the earliest contrivances used for producing currents of air.
The bellows now in use consists, in its simplest form, of two flat boards, of rectangular, circular or pear shape, connected round their edges by a wide band of leather so as to include an air chamber, which can be increased or diminished in volume by separating the boards or bringing them nearer together. The leather is kept from collapsing, on the separation of the boards, by wire rings which act like the ribs of animals. The lower board has a hole in the centre, covered inside by a leather flap or valve which can only open inwards ; there is also an open outlet, generally in the form of a pipe or nozzle, the aperture of which is much smaller than that of the valve. When the upper board is raised air rushes into the cavity through the valve to fill up the partial vacuum produced; on again de pressing the upper board the valve is closed by the air seeking an outlet, and this air is discharged through the open nozzle with a velocity depending on the pressure exerted.
Double Bellows.—It is evident that the current of air pro duced is not continuous but intermittent or in puffs, because an interval is needed to refill the cavity after each discharge. In order to remedy this drawback the double bellows are used. To understand their action it is only necessary to conceive an addi tional board with valve, like the lower board of the single bellows, attached in the same way by leather below this lower board. Thus there are three boards, forming two cavities, the two lower boards being fitted with air-valves. The lowest board is held down by a weight and another weight rests on the top board. In working these double bellows the lowest board is raised, driving the air from the lower cavity into the upper. On lowering the bottom board again a fresh supply of air is drawn in through the bottom valve, to be discharged again when the board is raised. As the air passes from the lower to the upper cavity it is kept from returning by the valve in the middle board, and in this way a quantity of air is sent into the upper cavity each time the lowest board is raised. The weight on the top board pro vides the necessary pressure for the blast, and at the same time causes the current of air delivered to be fairly continuous. When the air is being forced into the upper cavity the weight is being raised, and, during the interval when the lowest board is des ccending, the weight is slowly forcing the top board down and thus keeping up a continuous flow of air.
Hand-bellows for domestic use are generally shaped like a pear, with the hinge at the narrow end. The same shape was adopted for the older forms of smiths' bellows, with the difference that two bellows were used superposed, in a manner similar to that just described, so as to provide for a continuous blast. In the later form of smiths' bellows the same principle is employed, but the boards are made circular in shape and are always maintained roughly parallel to one another. These are shown in the figure. Here A is the blast pipe, B the movable lowest board, C the fixed middle board, close to which the pipe A is inserted, and D is the movable uppermost board pressed upon by the weight shown. The board B is raised by means of a hand lever L, through either a chain or a connecting rod, and lowered by a weight. The size of the weight on D depends on the air pressure required. For instance, if a blast pressure of half a pound per square inch is wanted and the boards are 18 in. in diameter, and therefore have an area of sq. in., on each of the 254 sq. in. there is to be a pressure of half a pound, so that the weight to balance this must be halt of or 127 pounds. The diameter of the air-pipe can be varied to suit the required conditions. Instead of bellows with flexible sides, a sliding arrangement is sometimes used; this consists of what are really two boxes fitting into one another with the open sides both facing inwards, as if one were acting as a lid to the other. By having a valve and outlet pipe fitted as in the bellows and sliding them alternately apart and together, an intermittent blast is produced. The chief defect of this arrangement is the leakage of air caused by the difficulty in making the joint a sufficiently good fit to be air-tight.