4-4,000 cubic feet per minute, at a pressure of 3¼ Ib to the
square inch.Where it is desirable to make small blast engines do the work of large ones, compensating smallness of size with velocity, it becomes necessary that the air valves be moved otherwise than by the simple action of the air itself. The best form of such an arrangement is that devised by Mr Slate, in which there is an annular slide valve placed outside the blast cylinder ; it receives its motion from a crank connected with the fly-wheel shaft. Thus, with lap and lead of the valve properly proportioned, a high velocity can be attained, and the tremor and jar that are observable in some of the larger engines are entirely absent. Two such engines working together, with their cranks at right angles, give such a uniform blast that no regulator of any kind is needed. In Fossey s engine, which appeared in the Exhibition of 1862, the slide valves are replaced by discs with radial perforations, which are put in slow rotatory motion by gearing connected with the main shaft.
The blast engines with slide valves, however, have not proved so advantageous in practice as was anticipated, owing to the large amount of friction on the valve surfaces, greater liability to derangement, and the wear and tear resulting from such rapid motion.
As a recent example of engines of the vertical type, with steam and air cylinders in one line (which have now come a ^ood deal into use in the north of England) we may brL.ly notice the compound cylinder blowing engines at the Lackenby Iron-Works, Middlesborough. These engines were described by Mr Alfred Hill before the Institution of Mechanical Engineers in 1871. Fig 4 (copied from the drawings by permission of the Institute and of Mr Hill) presents them in vertical section.
They consist of a high pressure non-condensing engine and a low pressure condensing engine, the latter supplied by steam from the former, this arrangement being adopted for economical reasons. A is the high pressure cylinder (32 inches in diameter) and C the low pressure (80 inches). Both engines have a stroke of 54 inches ; and a peculiarity is that they are coupled by cranks placed directly opposite each other instead of, as usual, at right angles, a light fly-wheel being relied on to carry them over the dead centres. This secures a better balance of the engines, and expansion of the steam in both cylinders in the most advantageous manner ; it also obviates the danger of breakages common in the case of right-angle cranks, which probably arises from the tendency to sudden accelera tion of one engine over the other at the commencement of each stroke, full steam pressure being then upon both pistons simultaneously, whilst the resistance of the blast pressure is acting against only one of the blowing pistons. In the blowing cylinders B, the inlet valves in the bottom are circular disc valves of leather, eighteen in number. The inlet valves T on the top of the cylinder are arranged in ten rectangular boxes, having openings in their vertical sides, inside which are hung leather flap valves. The box covers are made hollow, and are carried clown between the backs of the leathern flaps (so as to diminish the air-space as much as possible). The outlet valves o for air are ten in number, at each end of the cylinders, and are hung against flat gratings, which are fixed round the circumference of the cylinder. Enclosing each cylinder is an air-tight wrought-iron case M, into which the blast is delivered, and a branch at one side (not shown in figure) conveys the blast to the main. The area of the inlet valves is 860 square inches, or about -th the area of the piston ; that of the outlet valves is about th. For details of the balanced slide valves of the steam cylinders, the surface condenser D, the circulating pump E, the air-pump F, the feed pumps G, &c., we must refer to Mr Hill s paper.
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FIG. 4. Vertical Section of Lackenby Blowing Engines.
The capacity of each blowing cylinder is 157 cubic feet; consequently, the total quantity of blast supplied from both cylinders at the regular speed of 24 revolutions per minute is 15,072 feet per minute, measured at atmospheric pressure; thus the supply of blast, including loss by leakage, amounts to 190,000 cubic feet per ton of iron made. The pressure of blast in the blast-main is very free from fluctuations, owing, doubtless, to its large size, 12 times ths joint capacity of the two blowing cylinders. The indicated power of the engine is found to give a total of 290 horse ; that of the two blowing cylinders 258.
Among the more powerful blowing engines of piston and cylinder type at present in use, may be mentioned, besides that at Dowlais referred to above, those of Woolwich dockyard, employed for supplying air to forty forge fires, the Kirkless Hall engines, constructed from Robert Wilson s designs for the Wigan Iron and Coal Company, and the seven engines of Schneider and Co. at Creusot, three of which are horizontal engines of an old type, and the other four direct-acting vertical engines. Descriptions of these will be found in various standard works on metallurgy and engineering. For a description of the large blowing and exhausting engines lately constructed for the new Post- Office in London, see Engineering, 20th February 1874.
Savoy, Carniola, and in some parts of America ; it is the trompe or water-blowing engine. A flow of a few yards of water is required. From the bottom of a reservoir water is admitted, by removal of a plug from a conical- shaped aperture, into a large vertical wooden pipo, which
terminates below in a wind chest. The water, falling
