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50. The intermittent siphon. Fig. 40 represents an intermittent siphon. If the vessel is at first empty, to what level must it be filled before the water will flow out at o? To what level will the water then fall before the flow will cease?

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phere. A simple form of such a pump is shown in Fig. 41. When the piston is raised, the air from the receiver R expands into the cylinder B through the valve A. When the piston descends, it compresses this air and thus closes the valve A and opens the exhaust valve C. Thus, with each double stroke a certain fraction of the air in the receiver is transferred from R through the cylinder to the outside.

In many pumps the valve C is in the piston itself.


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FIG. 42. Automobile compression pump

52. The compression pump. A compression pump is used for compressing a gas into a container. If the pump shown in Fig. 41 be detached from the receiver plate and the vessel to receive the gas be attached at C, we have a compression pump. Fig. 42 shows a common form of compression pump used for

inflating automobile tires. Cup valves are shown at c and c'. They are leather disks a little larger than the barrel of the pump, attached to a loosely fitting metal piston.

When the pistons are forced down, the valve e spreads tightly against the wall, forcing the air past the valves c' and v. On the upstroke the valve c' spreads and forces the compressed air in the small barrel past v, while at the same time air passes by c, again filling the two barrels. v prevents any air from reëntering the small barrel from the hose h. The greater compressing power of the two-barreled pump is due to the fact that e' on the upstroke compresses air that has already been compressed by c on the downstroke.

Compressed air finds so many applications in such machines as air drills (used in mining), air brakes, air motors, etc. that the compression pump must be looked upon as of much greater importance industrially than the exhaust pump.



FIG. 43. The lift pump

53. The lift pump. The common water pump, shown in Fig. 43, has been in use at least since the time of Aristotle (fourth century B. C.). It will be seen from the figure that it is nothing more nor less than a simplified form of air pump. In fact, in the earlier strokes we are simply exhausting air from the pipe below the valve b. Water could never be obtained at S, even with a perfect pump, if the valve b were not within 34 feet of the surface of the water in W. Why? On account of mechanical imperfections this limit is usually about 28 feet instead of 34. Let the student analyze, stroke by stroke, the operation of pumping water from a well with the pump of Fig. 43. Why will pouring in a little water at the top, that is, "priming," often assist greatly in starting such a pump?

54. The force pump. Fig. 44 illustrates the construction of the force pump, a device commonly used when it is desired to deliver water at a point higher than the position at which it is convenient to place the pump

itself. Let the student analyze the action of the pump from a study of the diagram.

In order to make the flow of water in the pipe HS continue during the upstroke, an air chamber is always inserted between the valve a and the discharge point. As the water is forced violently into this chamber by the downward motion of the piston it compresses the confined air. It is, then, the reaction of this compressed



FIG. 44. The force pump

air which is immediately responsible for the flow in the discharge tube; and as this reaction is continuous, the flow is also continuous.



55. The Cartesian diver. Descartes (1596-1650), the great French philosopher, invented an odd device which illustrates at the same time the principle of the transmission of pressure by liquids, the principle of Archimedes, and the compressibility of gases. A hollow glass image in human shape (Fig. 45, (1)) has an opening in the lower end. It is filled partly with water and partly with air, so that it will just float. By pressing on the rubber diaphragm at the top of the vessel it may be made to sink or rise at will. Explain. If the diver is not available, a small bottle or test tube (Fig. 45, (2)) may be used instead; it works equally well and brings out the principle even better.

FIG. 45. The Cartesian diver

The modern submarine (see opposite page 23) is essentially nothing but a huge Cartesian diver which is propelled above water by oil or steam engines, and when submerged, by electric motors driven by storage batteries. The volume of the air in its chambers is changed by forcing water in or out, and it dives by a combined use of the propeller and horizontal rudders.

56. The balloon. A reference to the proof of Archimedes' principle (§ 29, p. 21) will show that it must apply as well to gases as to liquids. Hence any body immersed in air is buoyed up by a force which is equal to the weight of the displaced air. The body

will therefore rise if its own weight is less than the weight of the air which it displaces.

A balloon is a large silk bag (see opposite page 45) impregnated with rubber and filled either with hydrogen or with common illuminating gas. The former gas weighs about .09 kilogram per cubic meter, and common illuminating gas weighs about .75 kilogram per cubic meter. It will be remembered that ordinary air weighs about 1.20 kilograms per cubic meter. It will be seen, therefore, that the lifting force of hydrogen per cubic meter namely, 1.20 .09 1.11, is more than twice the lifting force of illuminating gas, 1.20 - .75 = .45.


FIG. 46. The parachute

Ordinarily a balloon is not completely filled at the start; for if it were, since the outside pressure is continually diminishing as it ascends, the pressure of the inside gas would subject the bag to enormous strain and would surely burst it before it reached any considerable altitude. But if it is but partially inflated at the start, it can increase in volume as it ascends by simply inflating to a greater extent. Thus, a balloon which ascends until the pressure is but 7 centimeters of mercury should be only about one fourth inflated when it is at the surface.

The parachute (Fig. 46) is a huge, umbrella-like affair with which the aëronaut may descend in safety to the earth. After opening, it descends very slowly on account of the enormous surface exposed to the air. A hole in the top allows air to escape slowly, and thus keeps the parachute upright.

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MONSTER BRITISH DIRIGIBLE AIRSHIP R-34 ARRIVING IN AMERICA The R-34 photographed just as her gondola touched the ground at Mineola, Long Island, July 6, 1919, after the first transatlantic flight of a lighter-than-air machine. This was the longest air flight in history, covering 3200 nautical miles, from Scotland to New York, in exactly 4 days, or 108 hours. On account of severe weather conditions and route taken the actual distance covered was 6300 nautical miles. She returned to Scotland in 75 hours. The characteristics of this historic airship were length, 672 ft.; height, 90 ft.; diameter, 79 ft.; 5 engines, 250 to 275 H. P. each (normal r.p.m. = 1600); total H.P., 1250 to 1375; 19 gas bags of goldbeater's skin (calf's intestine); capacity, 2,000,000 cu. ft.; each engine, 12 cylinders; propellers geared to engine speed; frame made of duralumin (= 95% Al); catwalk inside envelope, 600 ft.; total weight, 59 gross tons, of which 16 tons was gasoline (= 4900 gal.); could rise to a height of 14,000 ft.

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