An engine of this kind, of large dimensions, is re presented in Fig. 1. of Plate LXIII. It is wrought by a steam engine of thirty-five horse power, with a steam cylinder of thirty.three inches diameter, acting with a seven feet stroke. On the opposite end of the beam from the steam'cylinder is jointed the rod D, which is turned exceedingly true, so as to move through the stuffing box without allowing any air to escape, and without any unnecessary friction. A quantity of hemp is placed round the rod in the box a a, which forms part of the lid 'of the cylinder, and is held tight by the iron nuts bb. The piston is' fit ted to the lower end of the rod D, and is packed with leather so as to fill exactly the internal diameter of the cylinder AA.. To this cylinder are fixed four necks, B, F, G, H ; two of which, B, F, contain the suction valves, by which the air enters the cylin der, while the other two contains the forcing valves, through which the air is expelled at every elevation and depression of the piston into the chambers 1, K, and through the pipes L, M, into the regulating re ceiver OP, which is of the form of a parallelopipedon, or an inverted box without the lid, and is immersed in a cistern RS, filled with water.
Let us now suppose that the piston is at the bot tom of the cylinder A A, and begins to be raised by the engine. The air above the piston will obviously be condensed, and forcing open the hanging valves in the neck G, will rush through them into the pipe L, and thence into the receiver 0 P. While the piston thus rises and condenses the air above it, there is a vacuum below the piston, and the external air rushes through the valves in the neck F, and fills the space below the piston. When the piston descends from the top to the bottom of the cylinder, the air below it is condensed, and forced through the valves in the neck H into the pipe L, and thence into the receiver 0 P, while the space above the piston is a vacuum, and is instantly filled by the rush of the ex ternal air through the valves in the neck B. This operation is repeated at every stroke of the engine ; the cylinder-full of air, which is inhaled at the necks B and F, being forced through the opposite necks G and H. When the piston reaches the top or bot tom of the cylinder, there would evidently be a short cessation in the blast of air that passes into the fur nace, were it not for the regulating receiver 0 P. When the air is forced into this receiver, the water within it is pushed out or displaced, and rises in the cistern, so that the surface of the water in the cistern is often six, seven, or eight feet higher than the sur face of the water in the receiver. The air in the re ceiver, therefore, is pressed upwards by a column of water, six, seven, or eight feet high, so that if there should be any intermission in the supply of air from the cylinder, the blast will be kept up by the extru sion of the air in the receiver. The receiver O P, as shesTii in the Figure, is composed of a great number of cast iron plates, united by screws and !launches. Its size in the drawing is purposely diminished, in order to comprise it within the limits of the Plate. The general size is forty feet in length, twelve feet in depth, and twelve feet in breadth. The water cis tern is then forty-seven feet long, fourteen feet deep, and nineteen feet broad. The receiver is supported upon blocks of wood and masonry ; its lower edge being two feet from the floor of the cistern, to allow a free passage for the water. The buoyancy of the
receiver is overcome by a great quantity of masonry placed upon the top of it ; but we have omitted this in the figure, for the purpose of shewing the manner of uniting the plates of which it is composed.
A valve, loaded with a weight, is placed at T in the horizontal pipe. The weight is sufficient to keep the valve shut when the engine works with a proper velocity ; but when it works too hard, the excess of air will escape through the valve. When this hap, pens, the velocity of the engine must be diminished.
The horizontal pipe N M, after bending down wards, is divided into two branches X, Y, which, by a series of pipes, convey the air round the furnace, so as to introduce the blast at opposite sides of the hearth ; a practice which is now pretty generally followed.
In the construction .of a blowing machine the greatest caution is necessary. The pipes should be carried at 'such a height above the cistern, that there is no possibility of the water, when at its highest point, being forced through the pipe, into the fur risme.
The cylinder A A is made of cast iron, with a flannel; at each end. The upper necks G, B are cast in the same piece with it, hut the lower ones, H, F, are screwed to the under haunch of the cylinder. The valves within the necks B and F open inwards. They are made of leather, covered with plates of iron, and are screwed, by a projecting part of the leather, against the external plate a of the chamber, so as to cover three corresponding apertures in the plate (See Fig. 2.) s y, which is screwed to the neck by a num ber of bolts, shcwn in tile Figure. This plate is re moved when the valves require any material repairs ; but any trifling adjustments may be made, by the workman's thrusting his hand through one of the valves to repair the adjacent one. The plates which carry the valves in the chambers I, K, arc rot move able ; but apertures arc left above to give access to the valves. These apertures, when the engine is at work, are covered by the lids h h, which are fixed down by screws at each end.
The piston is rendered air tight by means of a ring of leather screwed on the upper and one on the under side of the piston, which, in consequence of their elasticity, press gently against the inside of the cylin der. In order to renew these rings when worn out,. there is a hole in the lid, and another in the bottom of the cylinder, sufficient to admit a man for that purpose. In some cases a moveable lid is made it the piston.
The cylinder is held down by four large bolts, two of which are seen in the figure at d passing through a massive pier of brickwork or mas fficiently stable to keep the cylinder steadily in s place. The cistern R S is placed at a much greater distance from the cylinder than is represented in the Figure, lest the tremulous motion produced by the violent concus sion of the included air should make the cistern leak. An accident of this nature ought to be carefully pre vented; as the water which escapes may insinuate it self into the sand of the casting-house, and occasion the most perilnus explosions, when the hot metal is introduced into the moulds.
The internal diameter of the cylinder A A is five • feet two inches, and the stroke seven feet. It is ca pable of blowing one furnace, when working at the rate of six strokes per minute. (Q)