Under the tender and also under each car is a cylinder and piston. The compressed air is conducted to this cylinder in front of the piston when the brakes are to be applied. As the piston-rod is connected by a bell crank to the brake levers when the piston is forced out by the pressure of the air, the brakes are at once applied to the wheels. As the reservoir under the foot-board is connected by the pipes which have been described with the cylinders under each car and the tender, by simply opening communication between the reservoir and the pipes, the air at once rushes from the reservoir back through the whole length of the train, and so rapid is its motion and quick its action that only a second or two intervenes between the opening of communication and the application of the brakes. To relieve or “let off” the brakes it is only necessary to close the reservoir cock and open communication from the air-pipes to the external atmosphere, when the compressed air in the brake cylinders will escape, and the springs ordinarily used on car brakes will cause the pistons to resume their former positions. For the purpose of opening the connection from the reservoir to the brake cylinder, and closing this connection and opening one from the latter to the external air, a single three-way cock is commonly used. This is arranged at such a point as to be under the control of the engineer, so that he can at pleasure turn on the compressed air with any degree of force, instantaneously, or slowly, or with a varying power, or by another turn of the cock let it off as freely, still keeping it under the same complete control. [Illustration: Fig. 224. Scale 1¹⁄₂ inch = 1 foot.] QUESTION 444. _How does the vacuum brake act and how is it constructed?_