This fact must be borne in mind, for it is on this principle that the automatic action of the brakes depends. If a train parts, or if the air leaks out of the train-pipe, the brakes go on. This automatic principle is a vital one in most safety appliances, and it is secured in the case of the air-brake by one of the most ingenious little devices that man ever contrived, that is, the triple valve, which is placed in the piping system between the brake-cylinder and the car-reservoir. This triple valve has passages to the brake-cylinder, to the car-reservoir, to the train-pipe, and to the atmosphere. Which of these passages are open and which are closed depends upon the position of a piston inside of the triple valve, and the position of that piston is determined by the difference in air-pressure on either side of it. Thus, when the pressure in the train-pipe is greater than that in the car-reservoir, the triple valve piston is forced over, say to the left, a communication is opened from the train-pipe to the car-reservoir, and the air pressure in the latter is restored from the main reservoir on the locomotive. At the same time a passage is opened from the brake-cylinder to the atmosphere, the compressed air escapes, the brake-piston is driven back by a spring, and the brakes are released. If the pressure in the train-pipe is reduced, the triple-valve piston is driven to the right (we will assume) by the pressure from the car-reservoir, the passage to the atmosphere is closed, air flows freely from the car-reservoir to the brake-cylinder, and the brakes are applied. The function of the engineer's valve is to control these operations. Naturally the runner's left hand rests on this instrument, which is fixed to the back head of the boiler. To apply the brakes he turns the handle to such a position as to allow air to escape from the train-pipe; to release, he turns it to allow air to pass from the main or locomotive reservoir into the train-pipe, and thence into the car-reservoir. It is hardly necessary to say that the operation of the brake, which has been described for one car, is practically simultaneous throughout the train. The brakes on the driving-wheels of the engine are also automatically applied at the same time as those of the cars and the tender.