Engineering Wonders of the World
Volume I

RAILWAY BRAKES. 249 valve,” V2, which, when open, allows air to pass from the valve chamber through a hole in the body of the slide valve to ports y and x. It should be mentioned that at the top of the cylinder is a groove, G, cut in the walls, through which, when the piston is at the top of its stroke, air can pass the piston into the valve chamber, and so reach the reservoir ; also, that the piston rod can move some distance up or down without affecting the slide valve. In Fig. 8 the piston and valve are seen in their “ charging reservoir ” positions. Air from the main pipe is passing the piston by groove G, and the slide valve has been pushed full up so that a and b come opposite ports a1 and ö1, and open the brake cylinder to the outer atmosphere. As soon as the reservoir Fig. 7.—VACUUM BRAKE CYLINDER, EXTERIOR VIEW. On the engine is a steam-driven, double- acting air-pump, which compresses air into a reservoir situated below the engine or tender, and maintains a pressure of about 80 to 90 lbs. to the square inch in it, as well as in the train pipe and in reservoirs installed under the coaches. To reach one of the latter reservoirs the air must pass through a triple valve. This valve puts a coach reser- voir in connection with the train pipe when the pressure in the first is less than in the last, but, when the pipe pressure purposely or accidentally falls below that in the reser- voir, admits the reservoir air to the brake cylinder, until the pressure equilibrium has been restored. The force with which the brake is applied increases with the reduction of pressure in the train pipe. Fig. 8 is a diagrammatic representation of a triple valve. At the bottom is a cylinder, in which moves a piston. A piston rod, P.R, embraces a slide valve, VW The Triple (shown in solid black), pierced , . . with several ports, a, b, x, explained. 1 ’ ’ ’ and y. Ports a and b are in communication with each other. Projecting from the piston rod is a small “ graduating is fully charged the piston sinks a little, past groove G, and draws with it the valve V2. If the pressure on the train pipe be now