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The United States army observation balloon, commonly called a kite balloon, has a length of 90 ft., a diameter of 29 ft., and a capacity of 37,000 cu. ft. It is allowed to rise to various heights from anchorage to the earth, and observations are communicated to earth by telephone. The close-up view shows an American major of the balloon service in the basket of a balloon near the front-line trenches in France, June, 1918. In case of very sudden attack by enemy airplanes the observer escapes by means of the parachute seen hanging from the side of the basket


57. 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 cu. ft. At the close of the war 147,000 cu. ft., extracted at a cost of ten cents a cubic foot from the gas wells of Texas and Oklahoma, were ready for shipment to France, and plans were under way for producing it at the rate of 50,000 cu. ft. per day. The production of a balloon gas that assures safety from fire opens up a new era for the dirigible balloon (see opposite page 44).



FIG. 47. The diving bell

58. The diving bell. The diving bell (Fig. 47) is a heavy, bell-shaped body with rigid walls, which sinks of its own weight. Formerly the workmen who went down in the bell had at their disposal only the amount of air confined within it, and

the water rose to a certain height within the bell on account of the compression of the air. But in modern practice the air is forced in from the surface through a connecting tube a (Fig. 48) by means of a force pump h. This arrangement, in addition to furnishing a continual supply of fresh air, makes it possible to force the water down to the level of the bottom of the bell. In practice a continual stream of bubbles is kept flowing out from the lower edge of the bell, as shown in Fig. 48, which illustrates subaqueous construction.

FIG. 48. Laying foundations of piers with the diving bell

The pressure of the air within the bell must, of course, be the pressure existing within the water at the depth of the level of the water inside the bell; that is, in Fig. 47 at the depth AC. Thus, at a depth of 34 feet the pressure is 2 atmospheres. Diving bells are used for putting in the foundations of bridge piers, doing subaqueous excavating, etc. The so-called caisson, much used in bridge building, is simply a huge stationary diving bell, which the workmen enter through compartments provided with air-tight doors. Air is pumped. into it precisely as in Fig. 48.

59. The diving suit. For most purposes except those of heavy engineering the diving suit (Fig. 49) has now replaced the diving bell. This suit is made of rubber and has a metal helmet. The diver is sometimes connected with the surface by a tube through which air is forced down to him. It passes out into the water through the valve V in his suit. But more commonly the diver is entirely independent of the surface, carrying air under a pressure of about 40 atmospheres in a tank on his back. This air is allowed to escape gradually through the suit and out into the water through the valve V as fast as the diver needs it. When he wishes to rise to the surface, he simply admits enough air to his suit to make him float.

In all cases the diver is subjected to the pressure existing at the depth at which the suit or bell communicates with the outside water. Divers seldom work at depths greater than 60 feet, and 80 feet is usually considered the limit of safety. But Chief Gunner's Mate Frank Crilley, investigating the sunken U. S. submarine F-4 at Honolulu in 1915, descended to a depth of 304 feet.


FIG. 49. The diving suit

The diver experiences pain in the ears and above the eyes when he is ascending or descending, but not when at rest. This is because it requires some time for the air to penetrate into the interior cavities of the body and establish equal pressure in both directions.

60. The gas meter. Gas from the city supply enters the meter through P (Fig. 50) and passes through the openings o and o, into the compartments B and B of the meter. Here its pressure forces in the diaphragms

d and d1. The gas already contained in A and A1 is therefore pushed out to the burners through the openings o' and of and the pipe P1. As soon as the diaphragm d has moved as far as it can to the right, a lever which is worked by the movement of d causes the slide valve u to move to the left, thus closing o and shutting off connection between P and B, but at the same time opening o' and allowing the gas from P to enter compartment A through o'. A quarter of a cycle later u1 moves to the right and connects A1 with P and B1 with P1. If u and u1 were set so as to work exactly together, there would be slight fluctuations in the gas pressure at P1. The movement of the diaphragms is recorded by a clockwork device, the dials of which indicate the number of cubic feet of gas which have passed through the meter.



FIG. 50. The gas meter


1. A water tank 8 ft. deep, standing some distance above the ground, closed everywhere except at the top, is to be emptied. The only means of emptying it is a flexible tube. (a) What is the most convenient way of using the tube, and how could it be set into operation? (b) How long must the tube be to empty the tank completely?

2. Kerosene has a specific gravity of .8. Over what height can it be siphoned at normal pressure?

3. Let a siphon of the form shown in Fig. 51 be made by filling a flask one third full of water, closing it with a cork through which pass two pieces of glass tubing, as in the figure, and then inverting so that the lower end of the straight tube is in a dish of water. If the bent arm is of considerable length, the fountain will play forcibly and continuously until the dish is emptied. Explain.

4. Diagram a lift pump on upstroke. What causes the water to rise in the suction pipe? What happens on downstroke?

5. Diagram a force pump with air dome on downstroke. What happens on upstroke?

FIG. 51

6. If the cylinder of an air pump is of the same size as the receiver, what fractional part of the air is removed by one complete stroke? What fractional part is left after 3 strokes? after 10 strokes?

7. If the cylinder of an air pump is one third the size of the receiver, what fractional part of the original air will be left after 5 strokes? What will be the reading of a barometer within the receiver, the outside pressure being 76?

8. Theoretically, can a vessel ever be completely exhausted by an air pump, even if mechanically perfect?

9. Explain by reference to atmospheric pressure why a balloon rises. 10. How many of the laws of liquids and gases do you find illustrated in the experiment of the Cartesian diver?

11. Pneumatic dispatch tubes are now used in many large stores for the transmission of small packages. An exhaust pump is attached to one end of the tube in which a tightly fitting carriage moves, and a compression pump to the other. If the air is half exhausted on one side of the carriage and has twice its normal density on the other, find the propelling force acting on the carriage when the area of its cross section is 50 sq. cm.

12. What determines how far a balloon will ascend? Under what conditions will it begin to descend? Explain these phenomena by the principle of Archimedes.

13. If a diving bell (Fig. 47) is sunk until the level of the water within it is 1033 cm. beneath the surface, to what fraction of its initial volume has the inclosed air been reduced? (1033 g. per sq. cm. = 1 atmosphere.)

14. If a diver's tank has a volume of 2 cu. ft. and contains air under a pressure of 40 atmospheres, to what volume will the air expand when it is released at a depth of 34 ft. under water?

15. A submarine weighs 1800 tons when its submerging tanks are empty, and in that condition 10 per cent by volume of the submarine is above water. What weight of water must be let into the tanks to just submerge the boat?




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FIG. 52. The dials of a gas meter

16. (a) The upper figure shows a reading of 84,600 cu. ft. of gas. The lower figure shows the reading of the meter a month later. What was the amount of the bill for the month at $.80 per 1000 cu. ft.? (b) Diagram the meter dials to represent 49,200 cu. ft.

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