190. The Bicycle Pump. The bicycle pump is the best known of all compression pumps. Here, as in other pumps of its type, the valves open inward rather than outward. When the piston is lowered, compressed air is driven through the rubber tubing, pushes open an inward-opening valve in the tire, and thus enters the tire. When the piston is raised, the lower valve closes, the upper valve is opened by atmospheric pressure, and air from outside enters the cylinder; the next stroke of the piston drives a fresh supply of air into the tire, which thus in time becomes inflated. In most cheap bicycle pumps, the piston valve is replaced by a soft piece of leather so attached to the piston that it allows air to slip around it and into the cylinder, but prevents its escape from the cylinder (Fig. 144).
FIG. 144.—The bicycle foot pump.
191. How a Man works under Water. Place one end of a piece of glass tube in a vessel of water and notice that the water rises in the tube (Fig. 145). Blow into the tube and see whether you can force the water wholly or partially down the tube. If the tube is connected to a small compression pump, sufficient air can be sent into the tube to cause the water to sink and to keep the tube permanently clear of water. This is, in brief, the principle employed for work under water. A compression pump forces air through a tube into the chamber in which men are to work (Fig. 146). The air thus furnished from above supplies the workmen with oxygen, and by its pressure prevents water from entering the chamber. When the task has been completed, the chamber is raised and later lowered to a new position.
FIG. 145.—Water does not enter the tube as long as we blow into it.
Figure 147 shows men at work on a bridge foundation. Workmen, tools, and supplies are lowered in baskets through a central tube BC provided with an air chamber L, having air-tight gates at A and A'. The gate A is opened and workmen enter the air chamber. The gate A is then closed and the gate A' is opened slowly to give the men time to get accustomed to the high pressure in B, and then the men are lowered to the bottom. Excavated earth is removed in a similar manner. Air is supplied through a tube DD. Such an arrangement for work under water is called a caisson. It is held in position by a mass of concrete EE.
FIG. 146—The principle of work under water.