[Illustration:  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.

[Illustration:  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.

[Illustration:  FIG. 146—­The principle of work under water.]

[Illustration:  FIG. 147—­Showing how men can work under water.]

In many cases men work in diving suits rather than in caissons; these suits are made of rubber except for the head piece, which is of metal provided with transparent eyepieces.  Air is supplied through a flexible tube by a compression pump.  The diver sometimes carries on his back a tank of compressed air, from which the air escapes through a tube to the space between the body and the suit.  When the air has become foul, the diver opens a valve in his suit and allows it to pass into the water, at the same time admitting a fresh supply from the tank.  The valve opens outward from the body, and hence will allow of the exit of air but not of the entrance of water.  When the diver ceases work and desires to rise to the surface, he signals and is drawn up by a rope attached to the suit.

192.  Combination of Pumps.  In many cases the combined use of both exhaust and compression pumps is necessary to secure the desired result; as, for example, in pneumatic dispatch tubes.  These are employed in the transportation of letters and small packages from building to building or between parts of the same building.  A pump removes air from the part of the tube ahead of the package, and thus reduces the resistance, while a compression pump forces air into the tube behind the package and thus drives it forward with great speed.