all automatically-braked cars and coaches. These are known as—(1) The vacuum automatic, using the pressure of the atmosphere on a piston from the other side of which air has been mechanically exhausted; and (2) the Westinghouse automatic, using compressed air. The action of these brakes will now be explained as simply as possible.
THE VACUUM AUTOMATIC BRAKE.
Under each carriage is a vacuum chamber (Fig. 85) riding on trunnions, E E, so that it may swing a little when the brakes are applied. Inside the chamber is a cylinder, the piston of which is rendered air-tight by a rubber ring rolling between it and the cylinder walls. The piston rod works through an air-tight stuffing-box in the bottom of the casing, and when it rises operates the brake rods. It is obvious that if air is exhausted from both sides of the piston at once, the piston