Page:Motors and motor-driving (1902).djvu/296

264 to-and-fro motion of the piston is transformed into rotary motion.

The Slide Valve. The admission of the steam at alternate ends of the cylinder, and its outlet after it has forced the piston down or up, is controlled by the slide valve, which is worked by the crank-shaft of the engine. Figs. 17 to 20 show the slide valve and piston in different positions, and figs. 21 and 22 give details of the steam ports, exhaust port, and slide valve. Steam ports, or openings, are made at the top and bottom of the cylinder, fig. 17, and these are shown in plan at fig. 21. Between them is an opening, the exhaust port and the slide valve , which from fig. 22 it will be seen

is a hollow box, alternately covers and uncovers the steam ports for admitting steam to the piston. While it is doing that for one end of the cylinder it is connecting the other steam port with the exhaust port, so that the steam, after it has done its work, can escape freely.

We will assume that the engine is running, and at fig. 17 the piston in the cylinder  has reached the top of its stroke. At this moment the top steam port is closed by the edge of the slide valve, but as the piston commences to descend on its return stroke, the slide valve has also moved downward, so that the steam is admitted to the top of the piston and forces it downward, as shown in fig. 18.