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.” Large surfaces can be painted much more readily and more evenly with compressed air than with a brush. A widely spreading air jet is used which draws the paint out of a receptacle, breaking it up into minute droplets that are projected as a mist against the surface, covering it with a uniform coating. AIR AS A TENSION SPRING So far we have been considering air as a compression spring. It may also be employed in a manner somewhat analogous to a tension spring. When air is exhausted from a receptacle we have a partial vacuum which will produce work, but in the opposite direction from that produced by compressed air. Of course there is no such thing as suction, and it is the pressure of the atmosphere which is employed to do the work. We are apt to overlook the fact that a vacuum cleaner depends upon atmospheric pressure to drag the dust and dirt out of a carpet. A vacuum cleaner would not work on the moon, where there is no atmospheric pressure. The apparatus consists of an air pump, usually driven by an electric motor, which exhausts, or partially exhausts, the air from a foot piece that is dragged along the carpet. As the air under atmospheric pressure rushes in through the carpet to take the place of the air that has been pumped out, it carries with it all loose particles of dirt. The dirt is filtered out of the air or a mechanical trap is provided to catch and retain the dirt while the air is allowed to escape. Large-scale vacuum cleaners are used for cleaning city streets. The dirt is loosened by revolving brushes and is then drawn up by atmospheric pressure into a tank. Another use of atmospheric pressure is to be found in the pneumatic tubes which convey mail from one post office to another. Air is exhausted from the tube and when a carrier is inserted which fits the tube closely the atmospheric pressure behind it forces the carrier along to its destination. The same system is used to carry money to and from the cashier’s office in department stores. In later developments of the pneumatic mail carrier compressed air was used back of the carrier, but the principle of operation was not altered. In either case propulsion is effected by the difference of air pressure on the opposite sides of the car. In 1869 Alfred E. Beach built an experimental subway line under Broadway, New York, near Warren Street. The line was only 200 feet long. A cylindrical car large enough to hold eighteen people, fitted the bore snugly and was propelled in one direction by compressing the air in the tunnel and in the other direction by exhausting the air.