# How do you use compressed air underwater

Working under water. Do you know how men are able to work under water, as in building foundations for bridges or dams or in building tunnels under rivers? They can do so only because compressed air is used to hold back the water while the work proceeds. Such work is carried. on inside steel walled cylinders called caissons. The air pressure inside the caisson has to be as much as the pressure under water at the bottom of the caisson in order to keep the water out.

The pressure of water 34 feet deep is 1 atmosphere, or 15 pounds per square inch. There is, of course, an atmospheric pressure of 15 pounds per square inch at the surface of the water. Hence 34 feet below the surface there is a pressure of 2 atmospheres, or 30 pounds per square inch. There is an additional 15 pounds per square inch for every 34 feet of added depth under water.

Men have worked in caissons more than 100 feet below the surface of a river. At that depth the pressure is more than 4 atmospheres, that is, about 60 pounds per square inch.

Men cannot work very long under such enormous pressures. Also, they cannot withstand too sudden changes in air pressure. There is air in the human body. It is in the blood, in the lungs, in the ears, and, to some extent, in other parts of the body. When the air pressure on the outside of the body is greatly increased, air is dissolved in the blood and other body liquids. If later the air pressure is suddenly reduced, the air in the liquids forms bubbles. These may cause a serious illness, called the “bends,” or even death. Also, sudden changes in pressure may injure the eardrums. Therefore, when caissons are needed for work under water, they are built in several compartments.’ Each compartment can be closed from the others. Different air pressures are maintained in the different compartments.

The workmen go from one compartment into the next. There they remain for a few minutes, until they get used to the new pressure. Then they pass to the third compartment. Thus the workmen become adjusted gradually to the different air pressures. In this way the air pressures gradually become equal inside and outside their bodies.

Lifting water with atmospheric pressure and compressed air. The water merely flows out of the spout of a lift pump. Hence a lift pump is of no use when it is necessary to throw water a long distance or high into the air, as in fighting a fire. For such purposes a force pump is needed.