However, air plays another and highly important part in transportation and here it is employed not to deliver energy stored in it, but to serve as a cushion. Without the soft, flexible grip of the air-filled rubber tubes with which automobile wheels are shod, high-speed motoring would be practically impossible. There is nothing that can compare with air for absorbing shocks and unevennesses in the road. Whenever an obstruction is encountered, not only is the shock absorbed by elasticity of the air, but the impact is immediately distributed uniformly over the whole tire, so that the strain on the tire is not localized and the life of the tire is correspondingly increased. There have been many attempts to introduce substitutes for pneumatic tires, such as combinations of metallic springs and straps, but these have failed, chiefly for the reason that they cannot distribute the shock as the pneumatic tire does. Consequently they are not only liable to damage, but they do not absorb the obstructions as readily as the pneumatic tire does and the vehicle which they carry is subjected to heavy stresses and strains.
Air as a cushion is used to prevent the rebound of the springs of an automobile and is also widely employed in machinery to absorb the momentum of moving parts or to slow the action of a spring. In a door check, for instance, a powerful spring is provided which would slam the door shut were it not for the cushioning action of air. A plunger is connected to the door and slides in a cylinder on the door frame. The air compressed by the plunger can escape only very slowly through a small port.
AIR CUSHIONS FOR ELEVATORS
Air cushions are used in elevators to prevent too rapid a fall of the car in case of accident. The lower part of the elevator shaft is completely inclosed and fitted with steel doors to form an air pocket. The car fits the shaft closely enough to compress the air under it when it is moving downward. At normal speeds, this air escapes quite readily around the sides of the car and through cracks in the doors, but when the speed is excessive the air cannot escape fast enough and sufficient pressure is built up to retard the car so that it will strike the bottom with a moderate impact not at all dangerous to the passengers.
In the Woolworth Building, New York, the highest elevator shafts are 680 feet high and the air pockets are 137 feet deep—i. e., they reach up to the tenth story. If the car broke away from its supports at the top of the shaft it would be traveling at the rate of 132 miles an hour when it struck the air pocket, but before it reached the bottom the air would bring it practically to a stop. Imagine a heavy automobile traveling at 132 miles per hour and brought down to a standstill in half a city block! The passengers would be hurled out of their seats, but in an elevator the passengers are standing and can brace themselves against the pressure produced by their own momentum. The velocity acquired in falling 543 feet to the air pocket must be overcome in 137 feet or ¼ of the distance. This means that the weight of each passenger is multiplied by four. A man who weighed 150 pounds would find that his pressure on the floor of the car had mounted to 600 pounds. One of the inventors of the pneumatic cushion for elevators was killed when testing out a car, not because of the impact of the car, but because he was foolish enough to sit in a chair. The chair gave way under his suddenly acquired weight and the poor man was fatally stabbed by one of the splinters.
Air jets are used for a variety of purposes ranging from tamping railway tracks to painting buildings. When a track is depressed under the tamping action of trains passing over it the ties must be raised and new ballast introduced under them so as to bring them up to level. In order to save hand labor and insure the perfect filling of all cavities a pneumatic tamper is sometimes used. The ballast, consisting of sand or gravel up to ¾ inch in diameter, is fed out of a hopper and meets a blast of air that hurls it into place like shot from an air rifle. This furnishes a very solid and compact road bed.
It is in much the same way that weather-stained stone walls are cleansed by means of a sand blast. The sand particles projected by the jet of air act like myriads of tiny bullets which chip away the face of the stone.
LAYING CEMENT WITH AN AIRGUN
It is after this same fashion that the cement gun is used to project cement against wire reenforcement to form walls of buildings. Carl E. Akeley was led to the invention of the cement gun by his efforts to find an expeditious and economical method of mounting specimens of large animals for the Field Museum in Chicago. He constructed a pneumatic device for spraying cement and water upon a canvas-covered framework, thus building up a body upon which the skin of the animal could be mounted. This machine was improved and tried out successfully, on a large scale, in constructing buildings.
The machine as now constructed consists of a hopper into which a proper mix of sand and cement is introduced. Compressed air blows this mixture out of a nozzle. Here it meets a jet of water also propelled by compressed air. The water, sand, and cement combine and strike the wall or surface to be coated with such an impact as to make a compact fine-grained coating known to the trade as “gunite.”