BLOWING-MACHINES. The earliest blowing-machine was, doubtless, some form of the common bellows, the idea of which is supposed to have been derived from the lungs. A very primitive form of this instrument is still in use in some eastern countries, con fisting simply of the skin of some animal sewed into a rude bag with a valve and nozzle. The older forms of domestic bellows are all constructed on the same principle—viz., a chamber formed of two boards with flexible leather sides, having at one end a nozzle with a narrow meutli; and in the lower board, a valve of considerably larger area for the admission of air. When the bellows are distended by drawing the boards apart, air is sucked in by the valve, to replace the vacuum which would otherwise be formed; and then, when the boards are being closed. the valve, which only opens inwards, is shut by the compressed air; and the latter, having no other escape, is forced out at the nozzle.
The great fault of the common bellows is, that it gives a succession of puffs, and not a continuous blast. One remedy for this was to use two bellows, so that one was blow ing while the other was filling; but it was afterwards found that the double-bellows secured a still more uniform blast. This machine is merely the common bellows with a third board of the same shape as the other two placed between them, so as to form two tthambers instead of one. The middle 'board it fixed, and both it nod the lower or lia+e valves placed in them opening inward. A weight on the lower board keeps the under chamber filled with air; and when this board Is raised by a lever or otherwise, the air which it contains is forced into the upper chamber. The exit-pipe is attached to the latter, and a weight is placed on the upper board sufficiently heavy to press the air out iu a continuous stream, the continuity being maintained by the large quantity of air always present in the upper chamber, and the uniform pressure of the weight. Some times a spring is used instead of a weight to press out the air. Even with the double
bellows, however, the constant refilling of the upper portion from the lower prevents the blast from being quite regular.
For such purposes as the supplying of a continuous stream of air to a flame for glass blowing or soldering, a very convenient form of apparatus has been constructed by Mr. P. Stevenson of Edinburgh. By means of the common bellows worked by a treadle, air is blown into the lower portion of a small cylinder containing a quantity of water, and having a diaphragm in the middle of the height, with a wide pipe nearly to the bottom. When the apparatus is at rest, the water remains below the diaphragm; but when air is blown in, it gradually rises through the pipe. The water as it descends then presses out the air in a steady stream by the exit-pipe, as a valve prevents it return ing to the bellows.
.Bellows made entirely of wood except the nozzle, first made in Germany in the 13:1 o., are in use in some continental countries. They are usually of large size, and the contrivance consists in having two boxes, of which the sides of the upper inclose those of the lower, so that the former can move up and down on the latter without admittit air except by a valve, as in the common bellows, of which, in fact, they are only a modi fication.
The Chinese have a very simple form of bellows, which is not only interesting hi itself, but also because its action is almost the same as the blowing-engine. It is merely a square chamber of wood, with a close-fitting piston, which, when drawn from the nozzle, opens the valves to admit air, and when pushed in the opposite direction, shuts these valves, and forces the air out by the nozzle.
For blowing is domestic fire in a chimney, the most effective contrivance is a meLal screen to close the front of the aperture above • the grate, so that the supply of air must all pass through the fire. This kind of blower, however, will only act when the fire is already producing as much heat as to cause a sensible draught up the chimney.