Page:Motors and motor-driving (1902).djvu/297

Rh In fig. 19 the piston has descended further, and the top steam port is fully open to it, but as the slide valve has also descended further, the bottom steam port is open, and the steam which had forced the piston upward in the previous stroke is now, having done its work, pushed out by the descending piston through the bottom steam port into the box portion of the slide valve, and as it will be seen that this is also open to the exhaust port, the steam passes away to the chimney.

In fig. 20 the piston has reached the bottom of its stroke, and is just commencing to ascend, steam being gradually admitted to the bottom steam port while the top steam port is

being opened by the upward movement of the slide valve, so that the steam on the top of the piston can escape into the exhaust. These actions are continued while the engine runs, and, as we have already seen how the up-and-down motion of the piston turns the crank, it is necessary for us to find out how the slide valve is moved backward and forward, so as to perform the operations we have described at the proper time with relation to the piston.

Figs 23, 24, and 25, although mainly for another purpose, will also show how the slide valve is operated. Fig. 25 shows the position of the valve when the engine is running forward. Two eccentrics are keyed to the crank-shaft. These two