exterior; figure 6 is a horizontal section of the cylinder, piston, and valve-box; figure 7 is a plan, or a view of the engine as you look directly down upon it.

The cylinder is shown at A, figures 5, 6, and 7; but 5 the construction is best seen from the section, figure 6. The cylinder is of cast iron, with the ends flanged, so that the cylinder covers, or end plates, aa, and the

The piston, shown at

frame, PP, may be bolted to it. B, is a circular cast iron disc, fitting the cylinder so Io tightly that no steam can pass it. Into the piston is

fixed the piston-rod, C, which passes through the front cylinder cover: the opening where it passes is made steam-tight by the stuffing-box, D. The further end of the piston-rod is fastened to the cross-head, E, figure 5; 15 this cross-head is a joint for connecting the pistonrod to the connecting-rod, F, so that the latter can swing up and down, as the piston and piston-rod travel backward and forward. The cross-head is also provided with two slides, ee, figure 5, which move between the guide-bars, ff, figures 5 and 6, and which keep the piston-rod from being bent, and from moving otherwise than in a straight line. The connecting-rod, F, joins

20

the end of the piston-rod to the crank-pin, G. This pin turns about an axle, the crank-axle, which is shown in section at H, figure 5, but is seen more clearly in the plan, figure 7, where the axle passes through the 5 two bearings, LL.

When the steam is allowed to flow from the boiler into the cylinder, so as to obtain admission into the space to the left of the piston in figure 6, the piston begins to move, carrying with it the piston-rod, the 10 cross-head, and the connecting-rod. Since, however, the further end of the connecting-rod is fastened to the crank-pin, G, figure 5, this further end can move only in the circle which the pin must describe about its axle. Consequently, when the motion of the piston 15 drives the connecting-rod away from the cylinder, the crank-pin will be pushed through a part of the circle indicated by the dotted line—say from G to G' in figure 5; then the momentum of the wheel to which the crank-pin is attached will carry the pin a little 20 further down along the same dotted line. If at this moment the steam is cut off from the side of the piston on which it has been pressing, and at the same instant is allowed to rush into the other end of the cylinder, then the piston will be driven in the contrary 25 direction, and the crank-pin, together with the wheel to which it is fastened, will be pulled through the complete circle. This operation of impelling the piston backward and forward may be repeated as often as we like, and the piston, working through the connecting30 rod and crank-pin, may thus turn a wheel continuously and drive machinery, provided only that we have a suitable contrivance for admitting the steam to the

sides of the piston alternately, and alternately letting it escape into the open air or into a condenser.

The admission of the steam is regulated as follows: In figure 6 is shown a box-like casing, MM, cast in one piece with the cylinder, and on one side of it. This 5 box contains the valve, V, which controls the flow of the steam. For the sake of clearness, the following diagram, figure 8, is given to exhibit the valve and the side of the cylinder drawn to a larger scale. The side of the cylinder next the valve-box has two passages, 10

ss, called the steam-ports, because through them the steam gains access to the cylinder and escapes from it. When the engine is at work, the cast iron box containing the valve is always filled with steam from the boiler. When the valve occupies the position shown in figure 15 8, the steam cannot enter the cylinder at all, because both ports are covered by the valve. If the valve, however, be moved a little to the right, so as to uncover the steam-port, s, two things will happen: first, the steam will pass through the port,s, into the cylinder, and 20 will push the piston from left to right; and secondly,