lowing very ingenious method, illustrated by Figures 17 and 18. The steam is conveyed to the cylinder from the boiler by the large steam-pipe, t S. The quantity of steam admitted is regulated according to the demand, by the throttle-valve t, which, as will be easily understood, admits more steam in proportion as it is more inclined. The action of this valve will be explained under the head GOVERNOR. It is supposed to quit this pipe at the point S, and is then directed upwards to the top of the cylinder, or in an opposite course to the bottom of the cylinder, according to the position of the valve. Cy is the cylinder, having two apertures, one above and one below, by each of which steam may be admitted to the cylinder from the steampipe, or withdrawn from it to the condenser. P is the piston, to be moved alternately up and down; ei o u is a box at the side of the cylinder, in which the valve works, and into which the steam enters first; and a bed is the valve, which is a tube, capable of being moved up and down from the position in Fig. 17, to that shewn in Fig. 18. These two figures are alike in all respects, except the position of the moveable bodies the valve and piston. The valve is moved by the engine itself, in the manner that will be explained afterwards: it is by the rod v v that it is worked. There are many kinds of valves. That shewn in these figures is the D valve, so called because it resembles that letter in its horizontal section. It is one of those called slide-valves, and is now very generally employed. By the tube m n, at the lower part of the valve-box e io u the steam passes to the condenser, after it has performed its office in the cylinder. The condenser, &c., is shewn in Fig. 19, page 129, but had better be disregarded at present, confining our attention to what passes on in the valve and cylinder, and simply bearing in mind that there is a constant vacuum in the condenser,

and, consequently, in that part of the cylinder which communicates with it.

265. Let us suppose, then, that the steam has just pushed the piston up to the top of the cylinder; the object now is to remove the steam which fills the cylinder, cause a vacuum in the cylinder, and admit steam above the piston, which steam not being resisted by any force below the piston, will easily press that body to the bottom of the cylinder. For this purpose, the valve is raised to the position shewn in Fig. 17. In this position of the valve, the communication between the lower part of the cylinder and the condenser (by the tube m n) is free, and the steam rushes to the condenser, as shewn by the course of the arrows; thus the vacuum is formed in the cylinder below the piston; at the same time, it will be seen that, from the construction of the valve, the passage from S to the cylinder, by its upper aperture, is now open, so that steam enters the cylinder above, and, exerting its elastic force on the upper surface of the piston, while there is a vacuum below it, presses it down to the bottom of the cylinder. Thus, the downwards motion is produced; steam being the moving power, and steam, by its condensation, the means of forming a vacuum to give effect to this moving power.

266. The manner in which the upwards motion is effected, will be easily understood, with the aid of Fig. 18. The piston is there represented as it would be, after being acted on by the steam with the valve in the position shewn in Fig. 17. To raise the piston, let the valve be brought down to the position given in Fig. 18. Then, the steam in the cylinder above the piston will rush to the condenser, passing out by the upper opening in the cylinder, and through the tube of the valve, which communicates freely with the condenser, following the course shewn by the arrows in

the figure. Thus, a vacuum is formed in the cylinder above the piston. This new position of the valve, at the same time admits steam from S to the lower opening of the cylinder; it enters, and presses up the piston to the top of the cylinder. Thus, by the movements of the valve, steam is admitted alternately on each side of the piston; while the steam on the other side is removed by a communication being at the same time opened with the condenser; and, by this beautiful adjustment, a steady alternate rectilinear motion is produced. We cannot help remarking here how perfectly the action of the steam, in producing motions of the same body alternately, in two directly opposite directions, illustrates Worcester's graphic description of the powers of this versatile agent, (page 80.) The connection of the piston-rod, the medium by which the motion is transmitted from the cylinder, with the other parts of the engine, will be explained afterwards. We shall now trace the course of the steam after it leaves the cylinder.

267. Fig. 19 represents the condenser and apparatus connected with it. C d is the condenser: above,

it communicates with the cylinder by the pipe n m, and, at its lower part, with the air-pump A, by the valve between them. This valve, as will be seen in the figure, is of such a construction as to permit the passage of fluids from the condenser to the air-pump; but not from the air-pump to the condenser. The condenser and air-pump are surrounded with cold water, a jet of which is continually playing into the interior of the condenser. The piston (p) of the airpump, is alternately raised and depressed by the beam of the engine, to which its rod is attached, and thus draws the fluids out of the condenser; the air-pump piston and cylinder piston moving simultaneously in same direction. At the right of the air-pump, is the the hot-well (wh), into which the piston of the air-pump throws the liquids which it draws from the condenser. To the extreme right of the figure, is the pump (worked by the engine) which draws cold water to surround and supply the condenser. At the left of the figure, is a tube leading from the condenser, with a plug permitting the exit of any fluids; but not the entrance. This is the snifting valve.

268. Let us now suppose that the engine is to be started, (set a-going.) All the valves are opened, and steam driven through the engine to expel the air, which is driven out at the snifting valve s, Fig. 19. The injection-cock i, Fig. 19, is then adjusted so as to pour in the necessary quantity of water into the condenser. The steam, after acting on the piston, rushes to the condenser, where it is instantly reduced to the liquid state. The condenser would soon be filled with the injection water, condensed steam, and air which entered along with the steam; but the airpump removes these. The valves in the air-pump piston open upwards only. Accordingly, when the piston is raised, as nothing can pass through the pis