All sorts of tools have been built in which a small air-driven motor furnishes the motive power. There are hand drills in which the tool is revolved by a set of pistons, shears for shearing sheep which operate somewhat on the principle of hair clippers, and pneumatic chisels used for chipping stone. Pneumatic motors are widely used in operating cranes and air hoists. They consist of a simple cylinder and piston, and are used for short, direct lifts of all sorts.

STOPPING TRAINS WITH AIR

Air has played a most important part in transportation. When the air brake was first introduced its purpose was to prevent collisions and provide greater safety of operation. It was not generally realized that efficient air brakes are not only a safety precaution, but a means of increasing schedule speeds. The more quickly a train can stop, the better speed it may make, particularly on a schedule that calls for frequent stops.

In the first air brake invented by George Westinghouse, the locomotive was provided with a reservoir in which air was stored and compressed by means of a steam-operated air pump. The cars were each provided with a cylinder and piston connected to the air reservoir through a valve conveniently located in the engineer’s cab. The piston of the air cylinder was connected with the air brakes so that whenever the engineer wished to stop his train he merely turned on the compressed air and all the brakes in the train were operated. This was a great saving over the previous system of providing brakemen to operate hand brakes.

Unfortunately, the problem of stopping a train was not as simple as all this. On a long train the cylinders near the engine were the first to receive pressure sufficient to operate the brakes, and as a result, the forward part of the train was retarded more than the rear part at the start of the brake operation, and the train came to a stop with a series of jolts. Another serious disadvantage was the fact that an enormous reservoir was required to furnish all the air necessary for a long train. But the most serious drawback was the fact that occasionally a train broke apart and then the engineer was powerless to control the brakes of the detached portion of the train. If the accident occurred on a down grade, there would result a collision between the cars running by their own momentum and gravity and those attached to the engine. This led to the present system of reversing the process which consists in keeping air pressure constantly in the train pipe and setting the brakes by relieving the pressure in the train pipe.

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

If air can be used to stop a train, why cannot air be used to propel it? This question occurred to many inventors and they answered it by building air-propelled cars and locomotives. Pneumatic cars were tried out on street railways, but they have had to give way to electric cars which do not need to carry their power around with them, but can draw it from a central power plant through a trolley wire. In only a few situations, such as in mines where electric sparking is feared, are air-propelled cars and locomotives still used to any considerable extent.