A (_fig._ 82.) is the boiler, with the furnace beneath it; C C are two cylinders with solid pistons P P′, connected with the working-beams B B′, to which are attached the pump-rods R R′, of two forcing pumps F F′, which communicate with a great force-pipe S; G is a _four-way cock_ (66.) already described. In the position in which it stands in the figure, the steam issues from below the piston P into the atmosphere, and the piston is descending by its own weight; steam from the boiler is at the same time pressing up the piston P′, with a force equal to the difference between the pressure of the steam and that of the atmosphere. Thus the piston R of the forcing-pump is being drawn up, and the piston P′ is forcing the piston R′ down, and thereby driving water into the force-pipe [Pg324] S. On the arrival of the piston P at the bottom of the cylinder C, and P′ at the top of the cylinder C′, the position of the cock is changed as represented in _fig._ 83. The steam, which has just pressed up the piston P′, is allowed to escape into the atmosphere, while the steam, passing from the boiler below the piston P, presses it up, and thus P ascends by the steam pressure, and P′ descends by its own weight. By these means the piston R is forced down, driving before it the water in the pump-cylinder into the force-pipe S, and the piston R′ is drawn up to allow the other pump-cylinder to be re-filled; and so the process is continued. [Illustration: _Fig._ 83.] A valve is placed in the bottom of the force-pipes, to prevent the water which has been driven into it from returning. This valve opens upwards; and, consequently, the weight of the water pressing upon it only keeps it more effectually closed. On each descent of the piston, the pressure transmitted to the valve acting upwards being greater than the weight of the water resting upon it, forces it open, and an increased quantity of water is introduced. (182.) From the date of the improvement of Watt until the commencement of the present century, non-condensing engines were altogether neglected in these countries. In the year 1802, Messrs. Trevethick and Vivian constructed the first non-condensing engine of this kind which was ever brought into extensive practical use in this kingdom. A section of this machine, made by a vertical plane, is represented in _fig._ 84.