18. Description of Trevethick's Temporary Forcer.
The object of this contrivance, which we owe to Mr. Trevethick, is to produce a constant stream in a common pump, and it consists in attaching to any pump AB, Fig. 19, an additional barrel CD, communicating with the space between the two valves of the pump; and in fixing the two pistons so that they may be wrought at the same time. When the pistons are raised, the space BD below them is filled by the pressure of the atmosphere, while the water above the piston of the pump flows out at E. But when the pistons descend, the valve v shuts, and consequently the water driven by the piston b being unable to descend through v, must ascend through the valve in the piston a, and consequently produce a continued discharge from E during the downward stroke of the pistons. See Nichol son's Journal, vol. ii. p. 216.
19. Description of .Vewsham's Fire Engine.
As the engines for extinguishing fire are nothing more than pumps for forcing out water in a continued stream, we shall make no apology for describing, in this place, the fire engine of Mr. R. Newsham, and some other con trivances of the same kind.
This engine consists of two pumps, A, B, Plate CCCCLXXI. Fig. 1, 2, which are both sucking and forcing ones, whose pistons are wrought by a double lever LL, fixed on the centre C of two arched heads, upon which the chains wind and unwind, as the pistons a, b, to which they are attached, rise or fall. When one of the pistons is raised, the water from the reservoir R follows it into the barrel through the valve v. But when the same piston is depressed the valve v shuts, and the water in the barrel is forced through the valve x into the air vessel NV NV, into which a pipe P P' is inserted, and reaches near to the bot tom of the vessel. The same effect is produced by the other piston, with this difference only, that while the one piston is rising and sucking water from the reservoir, the other is forcing into the air vessel the water which it had raised into the barrel by its previous ascent. As soon as
the water has risen in the air vessel above the end P' of the pipe P P', it is obvious that the air included in the vessel must be compressed by every pew quantity of water that is forced into it, and when the water has risen to a considerable height in it, the elasticity or spring of the air reacts powerfully on the surface m n of the water in the air vessel, and compels it to ascend through the pipe P P, through a long flexible leather pipe called the hose, screwed in at P, from the extremity of which it moves with great force, and may be directed, in consequence of the flexibility of the leather pipe, to any part of a house on fire. When more than two men are to work the levers LL, a sort of frame work is attached to the engine, by the cross-bars G H, and treddles are also added, by which the workmen act with their weight in treading. The side trough, into which the water is first poured, is shown at Z. It enters the reservoir R through a copper grating c d, leaving any sand, dirt, or stones with which it may be mixed in the pump. At X Y is seen the handle of a cock E, which may be turned into three different situations. The first is used when there is water near at hand to work the engine by the sucking pipe S, in which case the water enters at S, and rises through the valve v, and there is no communication between the barrels and the reservoir R. In the second position of the cock, there is no communi cation between the barrels and the end S of the sucking pipe ; but the water from the reservoir R enters the cock E sidewise, and turning at right angles through the cock towards v, enters the pumps. The third position is that in which there is no communication, either with the suck ing pipe S, or with the reservoir R, but only a communi cation between the reservoir R and the sucking pipe S, which is the position when the engine is done working, to empty the water left in the cavity of the cistern.