communicates with F, but acts through G upon the lower part of the piston H, and raises it; while the contents of the great cylinder above that piston are driven out through F, and pass through the opening 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 column of water hanging in R, to which 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 position of the plugs, DE, here delineated, and the consequent descent of the great piston, H, as before described. The descent 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 waterway 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 ease the water can escape directly through both the small cylinders to R. This gives a moment of time for the generation 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 pressure 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.

ENGINE for driving piles, used at building Westminster bridge, is constructed as follows: A, (Plate V. Miscel. fig. 3.) is the great shaft, on which are the great wheel and drum: B, the great wheel with cogs, that turns a trundle head with a fly, to prevent

the horse's falling when the ram is discharged; C, the drum on which the great rope is wound; D, the follower (with a roller at one corner) in which are contained the tongs, to take hold of the ram, and are fastened to the other end of the great rope, which passes over the pulley, near the upper end of the guides between which the ram falls; E, the inclined planes, which serve to open the tongs, and discharge the ram; F, the spiral barrel that is fixed to the drum, on which is wound a rope with a counterpoise, to hinder the follower from accelerating, when it falls down to take up the ram; G, the great bolt which locks the drum to the great wheel; H, the small lever, which has a weight fixed at one end, passes through the great shaft below the great wheel, and always tends to push the great bolt upwards, and lock the drum to the great wheel; I, the forcing bar, which passes through the hollow axis of the great shaft, bears upon the small lever, and has near the upper end a catch by which the crooked lever keeps it down; K, the great lever, which presses down the forcing bar, and discharges the great bolt at the time the long end is lifted up by the follower; L, the crooked lever, one end of which has a roller, that is pressed upon by the great rope, the other end bears upon the catch of the forcing bar during the time the follower is descending; M, the spring that presses against the crooked lever, and discharges it from the catch of the forcing bar as soon as the great rope slackens and gives liberty to the small lever to push up the bolt.

By the horse's going round, the great rope is wound about the drum, and the ram is drawn up, till the tongs come between the inclined planes, where they are opened, and the ram is discharged.

Immediately after the ram is discharged, the roller, which is at one end of the follower, takes hold of the rope that is fastened to the long end of the great lever, and lifts it up; the other end presses down the forcing bar, unlocks the drum, and the follower comes down by its own weight.

As soon as the follower touches the ram, the great rope slackens, and the spring, M, discharges the crooked lever from the eatch of the forcing bar, and gives liberty to the small lever to push up the great bolt, and to lock the drum to the great wheel, and the ram is drawn up again as before.

ENGINEER, in the military art, an able, expert man, who by a perfect knowledge in mathematics, delineates upon pa