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The Westinghouse Brake 129 the pipe to escape until the pressure within the pipe became the same as that of the outer atmosphere —namely, 14 pounds per square inch. As the com- pressed air escaped, the pistons of the brake cylinders moved inwards, so that the brake-blocks were drawn clear, leaving the wheels free. While the foregoing system is delightfully simple, that of the automatic quick-acting brake is far more so, although the means whereby the desired ends are achieved appear to be somewhat elaborate, if not intricate. The valve mechanism, placed between the brake pipe, brake cylinder, and the auxiliary reservoir, fulfils the following function. When compressed air is admitted into the train pipe, the valve mechanism acts in such a manner that the compressed air is allowed to pass into the auxiliary reservoir. Thus the two are open to one another, so that the air pressure in the train pipe and the reservoir is identical. At the same time the valve mechanism opens a second passage from the cylinder, in which the piston con- nected to the brake-block system moves to and fro, to the outer air, so that the air within the brake cylinder is not under pressure. This is the position when the brakes are “ off,” as when the train is travelling. To apply the brakes the driver moves a lever, which causes a certain proportion of the compressed air within the train pipe to escape into the outer atmosphere. This action causes the valve mechan- ism to close the connection between the train pipe and the auxiliary reservoir carrying air under pressure, so that the pressure of air within the auxiliary re- servoir is now higher than that obtaining in the J