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. CHAPTER XIX THE WATER PROBLEM OF A LARGE CITY 193. It is by no means unusual for the residents of a large city or town to receive through the newspapers a notification that the city water supply is running low and that economy should be exercised in its use. The problem of supplying a large city with an abundance of pure water is among the most difficult tasks which city officials have to perform, and is one little understood and appreciated by the average citizen.