But the most perfect blowing machines are those in which the blast is pro duced by the motion of pistons in a cylinder. The annexed engraving represents a blowing machine of this description, erected by Mr. Paterson, of Lanark. It consists of two double acting force pumps, placed at right angles to each other, to equalize the draft; they are driven by a water-wheel of 5-horse power. a is the vertical cylinder ; b the horizontal cylinder; c c two connecting rods united to the crank d; e the working beam ; f the parallel motion ; g the pipe for con veying the blast to the cupola or furnace h; i a small wheel, running in a groove in a cast iron plate ; j frame supporting the vertical cylinder, between which the lowermost connecting rod c passes. At k k are placed valves, to admit the air into the vertical cylinder; similar valves are placed at the ends of the horizontal cylinder into it. The operation is simply this: by the revolution of the crank the air is drawn in at each end alternately of both cylinders, and at the same time it is forced out at the opposite extremity along the pipe g into the furnace; and the cylinders being placed at right angles, one piston will be moving with its greatest velocity whilst the other is moving with its least velocity, by which means the blast is rendered nearly uniform, and an air chamber or reservoir rendered unnecessary. The first cylinders of magnitude used as blowing
machines, were erected by Mr. Smeaton, in 1760, at the Carron Iron Works, the cylinders being four in number, 4 feet 6 inches in diameter, and the piston snaking a stroke of 4 feet 6 inches in length ; but the blowing machine lately erected at the Smithery, in the Royal Dockyard at Woolwich, is perhaps the most powerful and the most complete in the kingdom.
In this machine there are three blowing cylinders, of 4 feet 8 inches diameter, with a stroke of 4 feet 8 inches, and each cylinder making 20 strokes per minute, expelling near 5000 feet of air per minute. Over the wind chest is fixed a regulating cylinder, which has no bottom, being open to the wind chest; and its piston, which weighs 700 lbs. serves only to regulate the pressure, which amounts to about }lb. per square inch. When the pressure exceeds this, the piston rises, and opens an escape valve at the back of the cylinder.