59. The diving suit. For most under-water work except that of heavy engineering the diving suit (see opposite page 49) is used. This suit is made of rubber and has a metal helmet. The diver is usually connected with the surface by a tube through which air is forced down to him. It passes out into the water through a valve in his suit. But sometimes 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 a valve 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 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 United States submarine F-4 at Honolulu in 1915, descended to a depth of 304 feet.

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

B A

B

A1

60. The gas meter. Gas from the city supply enters the meter through P (Fig. 48) and passes through the openings o and 01 into the compartments B and B1 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 o'1 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

1

1

FIG. 48. The gas meter

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.

SUMMARY. The siphon flows because of unbalanced pressures within its unequal arms.

An air pump is a device for removing air from a vessel by taking advantage of the natural tendency of gases to expand.

A lift pump must have its piston work within the limits of the height to which atmospheric pressure can raise a column of water. A force pump can deliver water to any height.

Balloons rise or descend in accordance with Archimedes' law. Divers are subject to the laws of liquid pressure and to Archimedes' law.

QUESTIONS AND PROBLEMS*

1. Let a siphon of the form shown in Fig. 49 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.

2. A water tank 8 ft. deep, standing some distance above the ground and closed everywhere except at the top, is to be emptied. The only means of emptying it is a flexible tube. (1) What is the most convenient way of using the tube, and how could it be set in operation? (2) How long must the tube be, in order to empty the tank completely?

FIG. 49

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

4. Describe the construction and explain the operation of the piston air pump, employed for exhausting a receiver.

* Supplementary questions and problems for Chapter III are given in the Appendix.

5. 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 three strokes? after ten strokes?

6. 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?

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

8. Draw a diagram of a lift pump on the upstroke. What causes the water to rise in the pipe? What happens on the downstroke?

12. If a balloonist wishes to descend, he causes the gas bag FIG. 50. The dials of a gas meter to become smaller by letting out

some of the gas through a valve at the top. Why does this allow the balloon to drop to a lower level? How may he again ascend?

13. Explain by reference to the weight of a balloon and the upward and downward forces of the atmosphere upon it why it rises. 14. How many of the laws of liquids and gases do you find illustrated in the experiment of the Cartesian diver?

15. Hydrogen is lighter than air. Would pumping more of it into a balloon already expanded to its maximum capacity increase the lifting power of the balloon? Explain fully.

16. A liter of air weighs 1.29 g. and a liter of helium 0.18 g. at 0° C. If a small rubberized-silk balloon weighing 10 g. is filled with 40 1. of helium, what is its lifting power?

17. If air is forced into a caisson until the level of the water within it is 1033 cm. beneath the surface of the river, to what fraction of its initial volume has the inclosed air been reduced? (1033 g. per square centimeter 1 atmosphere.)

=

18. In Fig. 50 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 cents per 1000 cu. ft.? Draw a diagram of the meter dials to represent 49,200 cu. ft.

19. 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.

20. 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 in order just to submerge the boat?

CHAPTER IV

MOLECULAR MOTIONS

KINETIC THEORY OF GASES

61. Molecular constitution of matter. In order to account for some of the simplest facts in nature - for example, the fact that two substances often apparently occupy the same space at the same time, as when two gases are crowded together in the same vessel or when sugar is dissolved in water - it has been found that all substances are composed of very minute particles called molecules. Spaces exist between these molecules; so that when one gas enters a vessel which is already full of another gas, the molecules of the one scatter themselves about among the molecules of the other. Since molecules cannot be seen with the most powerful microscopes, it is evident that they must be very minute. The number of them contained in a cubic centimeter of air at normal temperature and normal pressure is twenty-seven billion billion (27 x 1018). It has been estimated that this is approximately the number of grains of sand required to make a beach 1 mile long, 1000 feet wide, and 3 feet deep. It would take as many as a thousand molecules laid side by side to make a speck long enough to be seen with the best microscopes.

62. Evidence for molecular motions in gases. Certain very simple observations lead us to the conclusion that the molecules of gases, even in a still room, must be in continual and rapid motion. Thus, if a little ammonia, or any gas of powerful odor, is introduced into a room, in a very short time it will have become perceptible in all parts of the room. This shows clearly that enough of the molecules of the gas to affect the olfactory nerves must have found their way across the room.