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. 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.