The _air cushion_ (Fig. 24) is a metal pipe or dome filled with air attached to a water pipe where sudden changes in pressure are to be controlled. At many faucets in a city water system such an air cushion is employed. It contains air; this, unlike water, is easily compressible and the confined air when the tap is suddenly closed receives and checks gradually the rush of water in the pipe. Even with an air cushion, the "pound" of the water in the pipe when a tap is suddenly closed is often heard. If air cushions were not provided, the "water hammer" would frequently crack or break the pipes. [Illustration: FIG. 24.--The short pipe above the faucet contains air forming an air cushion.] Important Topics 1. Pascal's law. 2. Hydraulic press. 3. Artesian wells. 4. Standpipes and air cushions. Exercises 1. Where have you _seen_ an air cushion? Describe it and its use. 2. Where have you _seen_ an hydraulic press? Why and how used? 3. Where have you _seen_ hydraulic elevators? What moves them? 4. Where do you know of liquids under pressure? Three examples. 5. What is the pressure in water at a depth of 1500 cm. Express in grams per square centimeter and in kilograms per square centimeter. 6. What head[C] of water is required to give a pressure of 200 g. per square centimeter? 2 kg. per square centimeter? [C] "Head" is a term used to express the vertical height of water in pipes. 7. What _pressure_ will be produced by a "head" of water of 20 meters? 8. If 1728 cu. in. of water are placed in a vertical tube 1 sq. in. in cross section to what height would the water rise? It would give how many feet of _head_? 9. What would the water in problem 8 weigh? What pressure would it produce at the bottom, in pounds per square inch? From this, compute how many feet of "head" of water will produce a pressure of 1 lb. per square inch. 10. Using the result in problem 9, what "head" of water will produce a pressure of 10 lbs. per square inch? 100 lbs. per square inch? 11. From the result in 9, 100 ft. of "head" of water will produce what pressure? 1000 ft. of "head?" 12. If the diameter of the pump piston in a hydraulic press is 2 cm. and that of the press piston 50 cm. what will be the force against the latter if the former is pushed down with a force of 40 kg.? (3) ARCHIMEDES' PRINCIPLE =44. A Body Supported by a Liquid.=--Among the applications of the force exerted by a liquid upon a surface, Archimedes' Principle is one of the most important. Most persons have noted that a body placed in water is partly or wholly supported by the force of the water upon it. A stone held by a cord and lowered into water is felt to have a part of its weight supported, while a piece of cork or wood is wholly supported and floats.