The leaden pieces, D and E, are cast in their places, andhave no packing what. ever. They move very easily; and if at any time they should become loose, they may be spread out by a few blows with a proper instrument, without taking them out of their place. On the side of the two brass cylinders, in which D and E move, there are square holes communicating towards G, with a horizontal trunk, or square pipe, four inches wide, and three inches deep. All the other pipes, G, G, and R, are six inches in diameter, except the principal cylinder wherein the piston, H, moves ; and this cylinder is ten inches in diameter, and admits a nine foot stroke.
The piston rod works through a stuf fing-box above, and is attached to M N, which is the pit rod, or a perpendicular piece divided into two, so as to allow its alternate motion up and down, and leave a space between, without touching the fixed apparatus, or great cylinder. The pit rod is prolonged clown in the mine, where it is employed to work the pump ; or, if the engine was applied to mill-work, or any other use, this rod would be the communication of the first mover. K L, is a tumbler, or tumbling bob, capable of being moved on the gudgeons, V, from its present position to another, in which the weight L, shall hang over with the same inclination on the opposite side of the perpendicular, and consequently the end, K, will then be as much depressed as it is now elevated.
The pipe, R S, has its lower end im mersed in a cistern, by which means it delivers its water without the posibility of the external air introducing itself; so that it constitutes a Torricellian column, or water barometer, and renders the whole column from A to S effectual, as we shall see in our view of the opera tion.
The operation. Let us suppose the lower bar, K V, of the tumbler to be horizontal, and the rod, P 0, so situated, as that the plugs, or leaden pistons, D and E, shall lie opposite to each other, and stop the water ways, G and F. In this state of the engine, though each of these pistons is pressed by a force equi valent to more than a thousand pounds, they will remain motionless, because these actions being contrary to each other, they are constantly in equilibrio. The great piston, H, being at the bottom of its cylinder, the tumbler is to be thrown by hand into the position here de lineated. Its action upon 0 P, and con sequently upon the wheel, Q, draws up the plug E, and depresses D, so that the water way, F, becomes open from A B, and that of G to the pipe It: the water consequently descends from A to C, thence to F, until it acts above the piston F. This pressure forces down the pis ton, and if there be any water below the piston, it causes it to pass through G G G into R: during the fall of the piston, which carries the pit rod, M N, along with it, a sliding block of wood, I, (dot ted) fixed to this rod, is brought into con tact with the tail, K, of the tumbler, and lowers it to the horizontal position, be yond which it oversets by the acquired motion of the weight L.
The mere rising of the piston, if there was no additional motion in the tumbler, would only bring the two plugs, D and E, to the position of rest, namely, to close G and F, and then the engine would stop ; but the fall of' the tumbler carries the plug, D, upwards, quite clear of the hole, F, and the other plug, E, down wards, quite clear of the hole, G : these motions require no consumption of power, because the plugs are in equilibrio, as was just observed. In this new situation the column, A B, no longer communi• eates with F, but acts through G upon the lower part of the piston 11, and raises it ; while the contents of the great cylin der above that piston are driven out through F, and pass through the open ing at D into R. It may be observed, that the column which acts against the piston is assisted by the pressure of the atmosphere, rendered active by the co lumn of water hanging in 1I, to swhich that assisting pressure is equivalent, as has already been noticed. When the piston has ascended through a certain length, another slide or block upon the pit-rod (not seen) applies against the tail, K, of the tumbler, which it raises and again oversets, producing once more the posi tion of the plugs D E, here delineated, and the consequent descent of the great piston H, as before described. The de scent produced the former effect on the tumbler and plugs, and in this manner it is evident that the alternations will go on without limit, or until the manager shall think fit to place the tumbler and plugs, D E, in the positions of rest, namely, so as to stop the passages, F and G. The length of the stroke may be varied by altering the positions of the pieces, I, and the other lower down, which will shorten the stroke, the nearer they are together; as in that case they will sooner alternate upon the tail, K. As the sudden stoppage of the descent of the column, A B, at the instant when the two plugs were both in the water way, might jar and shake the apparatus, those plugs are made half an inch shorter than the depth of the side holes, so that in that case the water can escape directly through both the small cylinders to R. This gives a moment of time for the ge neration of the contrary motion in the piston, and the water in G G G, and greatly deadens the concussion which might else be produced. See STEAM, ENGINE.
Some former attempts to make pres sure engines upon the principle of the steam-engine have failed ; because water, not being elastic, could not be made to carry the piston onwards a little, so as completely to shut one set of valves and open another; in the present judicious construction, the tumbler performs the office of the expansive force of steam at the end of the stroke.