Each car is provided with its own cylinder which furnishes the necessary air for the operation of its brakes. There is, of course, a main cylinder on the locomotive in which air is pumped at high pressure by a steam-operated pump. When the pressure of the main reservoir falls below a predetermined amount, the air pump starts operating automatically and continues until the requisite pressure is restored. Air from the reservoir is fed to the train pipe at a certain pressure and feeds the local reservoirs on the cars of the train. At each car it passes through a very ingenious triple valve, which consists of a cylinder with a double piston, one operated by pressure in the train pipe, and the other by pressure in the local cylinder. The piston operated by the pressure of the train pipe is larger than the other and consequently the valve piston is normally pressed back, uncovering a small port through which air from the train pipe feeds into the local reservoir. In this way the air in each local reservoir is maintained at the same pressure as that of the train pipe. Whenever air is let out of the train pipe, the piston is pushed out by the excess pressure on the local reservoir side and uncovers another port which permits air to flow from the local reservoir to the brake cylinder operating the brakes. The advantage of this system is that each car is always supplied with sufficient power to act on the brakes immediately without having to draw its supply of air from the main reservoir. In case the train breaks apart, the air in the train pipe escapes and the brakes are set automatically. There have been a number of improvements in air brakes aimed to make the operation of the brakes more uniform and to insure immediate action, even on a very long train. Modern express trains weighing 920 tons and traveling at a velocity of 60 miles per hour, can be brought to a standstill in 860 feet, or practically their own length. Automatic arrangements are provided to insure the application of the brakes at graduated rate, except, of course, when the emergency brakes are applied. As the speed of the train slows down the pressure is gradually relieved, otherwise the train would stop with a severe jolt. The rate of applying the brakes so as to provide a smooth retardation depends in large measure upon the weight of the train. On the New York subways, it is interesting to note, every passenger who boards a train has his weight recorded by the brake mechanism, and allowance is automatically made for the inertia that his mass adds to the train when it is in motion. The weight operates through a system of levers to control the amount of pressure that is applied to the brakes so that when the engineer operates the brake lever, he does not have to consider whether the cars are light or jammed full of passengers; the brake mechanism itself takes care of this. PROPELLING CARS WITH AIR