with the pump of Fig. 44. Why will pouring in a little water at the top - that is, "priming"- often assist greatly in starting such a pump? Pumps used

in very deep wells are constructed with long plunger rods to bring the piston P to within the necessary distance of the water for suc

cessful operation.

55. The force pump. Fig. 45 illustrates the construction of the force pump, a device at least two thousand years old. The force pump is 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.

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

W

-b

FIG. 44. The lift pump

air that is directly responsible for the flow in the discharge tube; and as this reaction is continuous, the flow is continuous.

Air

56. 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 (see Fig. 46 (1)) has an opening in the lower end. It is filled in part with water and in part 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

H

dib

FIG. 45. The force pump

small bottle or test tube (Fig. 46 (2)) may be used instead ; it works equally well and brings out the principle even better. The modern submarine (see opposite

page 25) is essentially nothing but a huge Cartesian diver propelled, above water, by oil or steam engines and, when submerged, by electric motors driven by storage batteries. The volume of air in its chambers is changed by forcing water in or out, and it dives by a combined use of the propeller and the horizontal rudders.

(1)

(2)

FIG. 46. The Cartesian diver

57. The balloon. A reference to the proof of Archimedes' principle (see § 29, p. 24) 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 there

The R-34 photographed just as her gondola touched the ground at Mineola, Long Island, July 6, 1919, after the first trans-
atlantic 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 the 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 feet; height, 90 feet; diameter, 79 feet; 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 cubic feet; each engine, 12 cylinders;
propellers geared to engine speed; frame made of duralumin (= 95 per cent Al); catwalk inside envelope, 600 feet; total
weight, 59 gross tons, of which 16 tons was gasoline (= 4900 gallons); could rise to a height of 14,000 feet

1, helmet (spun copper); 2, air-escape valve; 3, hinged front window; 4, spitcock; 5, breastplate with collar straps;
6, breastplate wing nuts; 7, air-inlet control valve; 8, air hose; 9, telephone cable; 10, belt with lead weights; 11, double-
edged knife in case; 12, rubber gloves; 13, lead-soled shoes; 14, suit (rubberized canvas); 15, diver's air pump; 16, helmet
and outfit chest; 17, wrench for tightening wing nuts; 18, cuff expanders; 19, telephone box; 20, tender's telephone set;
21, telephone gooseneck connection; 22, air gooseneck connection; 23, safety helmet lock; 24, diver's combination trans-
mitter and receiver set into helmet. (Courtesy of Schrader's Sons)

fore rise if its own weight is less than the weight of the air which it displaces. When these weights become equal, the body floats.

A balloon is a large silk bag impregnated with rubber and usually filled either with hydrogen or with common illuminating gas. The former gas weighs about .09 kg. per cubic meter, and common illuminating gas weighs about .75 kg. per cubic meter. It will be remembered that ordinary air weighs about 1.20 kg. 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). From the weights given above it is easy to calculate the lifting power of any balloon whose volume is known.

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 partly 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 cm. of mercury should be only about one-fourth inflated when it is at the surface.

FIG. 47. The parachute

The parachute (Fig. 47) 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.

58. Helium balloons. One of the striking results of the World War was the development of the helium balloon. Helium is a noninflammable gas twice as dense as hydrogen and having a lifting power .92 as great. It is so rare an element that before the war not over 100 cu. ft. had been collected by anyone. Its pre-war price was $1700 per cubic foot. It is now being produced at a cost of ten cents a cubic foot from the gas wells of Texas and Oklahoma. The production of a balloon gas that assures safety from fire opens up a new era for the dirigible balloon (see opposite page 48).