Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/566

550 The blast here is, of course, not continuous, but in puffs, a certain interval being needed for refilling the bellows after each discharge. This drawback was remedied by the invention of double bellows. To understand their action, it is only necessary to conceive an additional board with valve, like the lower board of the single bellows, attached by leather under this lower board. Thus two similar cavities are obtained, separated by the lower board of what was the single bellows. The lowest board is held down by a weight, aud another weight presses the top board. When the lowest board is raised it forces air into the upper cavity, and the valve of the middle board prevents return of this air. The lowest board being then depressed, air enters the lower cavity from without, and this in its turn is next forced into the upper cavity. The weighted top board is meanwhile continuously pressing the air of the upper cavity through the nozzle. While the blast thus obtained is continuous, it is not wholly free from irregularities. FIGS. 1 and 2. Common Smiths Bellows. The common smiths bellows, made on the principle just indicated, are generally of circular form, as shown in tigs. 1 and 2. A is the blast pipe, B the movable lowest board, C the fixed middle board (into which the pipe is inserted), and D the movable upper board pressed by a weight. The lowest board is moved by means of the lever L and the chain H working on the roller R. The weight required to produce a certain force of blast is easily deter mined ; if the diameter of the bellows be 1 foot, the area will be 113-19 inches, and the upper board will require a weight of 56 5 Ib for a blast equivalent to a pressure of ? ft on the square inch, or a velocity of 207 feet per second, which is well suited for a smith s forge. By a simple arrangement for altering the diameter of the pipe the force of the blast may be varied. It may be noted that in some parts of the Continent a simple form of bellows is made of two wooden boxes, each open on one side, and the one just fitting into the other. The open sides being opposed to each other, the upper enclosing box is made to move up and down over the other, with which it is jointed at one part, and which is provided with a nozzle, and a valve opening inwards. The changa of capacity produces a blast. There is con siderable loss of air, however, from the boxes not exactly fitting. The blowing-machines now almost exclusively used for blast furnaces are of the cylinder and piston type (which is the principle adopted, it may be remarked, in a small hand bellows used by the Chinese). At first the blowing cylinders were single-acting, that is to say, they had the power of propelling a blast only when the piston was moving in one direction. With two or more of these blowing cylinders attached to one crank-shah, worked by a water-wheel, a tolerably steady pressure of air was obtained. But in these and other respects considerable progress has been realized. The cylinder-engines of the present day (which are generally driven by steam) may be classed in two chief systems, according as the cylinder is placed horizontally or vertically. In the former case the steam and blast cylinders are usually in one line, the same rod carrying the pistons of both, and being guided on both sides, while a fly-wheel is employed as regulator. In the vertical systems the steam and blowing cylinders are sometimes similarly connected, but, in the larger engines, they are generally placed one at each end of a beam connecting their pistons. The vertical engines have been most popular in England and in some parts of the Continent (as Silesia), but the other type (almost exclusively used in Westphalia and on the Rhine) is now adopted in several English works. FIG. 3. Section of Cylinder of Blowing Engine. The general action of many of these machines may be illustrated by the large blowing engine at the Dowlais iron-works, erected in 1851. Fig. 3 is a representation of its blast cylinder, the piston of which, made air-tight by packing, is moved by the oscillating beam of the engine. The cover of the cylinder, and also its bottom, have several openings, furnished with valves v. which open inwards Other valves v, above and below, open into a lateral chamber B, which is connected by the aperture to the different tuyeres of the furnaces. Suppose, now, the piston is at the top and begins to be forced down. The air in the upper part of the cylinder becomes more and more rarefied, and the difference of density between it and that of the blast in chamber B, causes the upper valve v to be applied firmly to the metallic surface before which it is hung. The upper valves v, on the other hand, will be raised by the external air which enters to compensate the rarefaction. The same motion of the piston compresses the air below it, causing the lower valves v (which open inwards) to be firmly closed, while the valve v will be raised and admit the air into chamber B, whence it passes to the furnace. When the piston is raised the reverse takes place ; the lower portion of the cylinder receives air from without, and the upper discharges its air through the pipes leading to the furnace. Thus a nearly continuous flow is obtained. To ensure regularity the pipe is made to communicate with a closed reservoir of wrought iron, where the variations are destroyed by the elasticity of the air itself. The cylinder here figured is 144 inches in. diameter, with a stroke of 12 feet, and discharges about