of the United States army rose without injury to a height of about 8 miles (42,470 feet), his success being due to the artificial inhalation of oxygen. The French aviator Callizo, at Villaconblay, France, on October 10, 1924, ascended in an airplane to a height of 40,820 feet (7 miles). The temperature recorded was about 100° F. below freezing.

By sending up self-registering thermometers and barometers in balloons which burst at great altitudes (the instruments being protected by parachutes from the dangers of rapid fall), the atmosphere has been explored to a height of 35,080 meters (21.8 miles), this being the height attained on December 7, 1911, by a little balloon which was sent up at Pavia, Italy. These extreme heights are calculated from the indications of the self-registering barometers.

At a height of 35 miles the density of the atmosphere is estimated to be but 30boo as great as at sea level. By calcu

lating how far below the horizon the sun must be when the last traces of color disappear from the sky, we find that at a height as great as 45 miles there must be air enough to reflect some light. How far beyond this an extremely rarefied atmosphere may extend, no one knows. It has been estimated at all the way from 100 to 500 miles. These estimates are based on observations of the height at which meteors first become visible, on the height of the aurora borealis, and on the darkening of the surface of the moon just before it is eclipsed by the shadow of the solid earth.

FIG. 40

SUMMARY. Liquids, being practically incompressible, exert pressure proportional to depth below the free surface.

Gases, being highly compressible, do not follow the depthpressure law.

Boyle's law. The pressure exerted by a given mass of gas at constant temperature is inversely proportional to its volume or directly proportional to its density.

QUESTIONS AND PROBLEMS

1. Blow as hard as possible into the tube of the bottle shown in Fig. 40. Then withdraw the mouth and explain all the effects observed.

2. What sort of change in volume do the bubbles of air which escape from a diver's suit experience as they ascend to the surface?

3. With the aid of the experiment in which the rubber dam was burst under the exhausted receiver of an air pump, explain why high mountain climbing often causes pain and bleeding in the ears and nose.

4. Pressure tests for boilers or steel tanks of any kind are always made by filling them with water rather than with air. Why?

5. The deepest sounding in the ocean is about 6 mi. Find the pressure in tons per square inch at this depth. (Specific gravity of ocean water = 1.026.) Will a pebble thrown overboard reach the bottom? Explain.

6. Why must the cap be removed from a kerosene can in order to secure a proper flow from the spout?

7. If 100 cu. ft. of hydrogen gas at normal pressure are forced into a steel tank having a capacity of 5 cu. ft., what is the absolute gas pressure in pounds per square inch? What is the gauge pressure?

8. A gas cylinder 5 ft. long and 1 sq. ft. in cross section contains gas at a gauge pressure of 210 lb. per square inch. How many cubic feet would this gas occupy at normal atmospheric pressure?

9. There is a pressure of 80 cm. of mercury on 1000 cc. of gas. What pressure must be applied to reduce the volume to 600 cc. if the temperature is kept constant?

10. An automobile tire having a capacity of 1500 cu. in. is inflated to a gauge pressure of 35 pounds per square inch. What is the density of the air within the tire? To what volume would the air expand if there should be a "blow-out"?

11. A glass tube 25 in. long and closed at the upper end is attached to the sounding lead of a ship. On drawing the lead from the bottom of the ocean, the sea water is found to have wet 24 in. of the inner surface of the tube. How many atmospheres of pressure acted upon the air inclosed in the tube? How many were due to the water? How many fathoms (a fathom = 6 ft.) deep was the ocean (the density of ocean water being 64 lb. per cubic foot)?

12. Under ordinary conditions a gram of air occupies about 800 cc. Find what volume a gram will occupy at the top of Mont Blanc (altitude 15,781 ft.), where the barometer indicates that the pressure is only about one half what it is at sea level.

13. The mean density of the air at sea level is about .0012. g. per cubic centimeter. What is its density at the top of Mont Blanc? What fractional part of the earth's atmosphere has one left beneath him when he ascends to the top of this mountain?

PNEUMATIC APPLIANCES

50. The siphon. Let a rubber or glass tube be filled with water and then placed in the position shown in Fig. 41. Water will be found to flow through the tube from vessel A into vessel B. If B is then raised until the water in it is at a higher level than that in A, the direction of flow will be reversed. This instrument, which is called the siphon, is very useful for removing liquids from

vessels which cannot be overturned, or for drawing off the upper layers of a liquid without disturbing the lower layers. Many commercial applications of it are found in various siphon flushing systems.

The explanation of the action of the siphon is readily seen from Fig. 41. Since the tube acb is full of water, water must evidently flow through it if the force which pushes it one way is greater than that which pushes it the other way. Now the upward pressure at a is equal to the atmospheric pressure minus the downward pressure of the water column ad, and the upward pressure at

b is the atmospheric pressure minus the downward pressure of the water column be. Hence the pressure at a exceeds the pressure at b by the pressure of the water column fb. The

A

B

FIG. 41. The siphon

FIG. 42. The intermittent siphon

siphon will evidently cease to operate when the water is at the same level in the two vessels, since then fb = 0, and the forces acting at the two ends of the tube are therefore equal and opposite. It will also cease to act when the bend c is more than 34 feet above the surface of the water in A, atmospheric pressure being unable to raise water to a height greater than this in either tube.

Would a siphon flow in a vacuum?

51. The intermittent siphon. Fig. 42 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?

52. The air pump. The air pump was invented in 1650 by Otto von Guericke, who deserves the greater credit since he was apparently altogether without knowledge of the discoveries which Galileo, Torricelli, and Pascal had made a few

years earlier in regard to the character of the earth's atmosphere. A simple form of such a pump is shown in Fig. 43. 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, opening, in turn, the exhaust valve C. Thus, with each double stroke a certain fraction of the air contained in the receiver is transferred from R through the cylinder to the outside. In many pumps the valve C is located in the piston itself. 53. The compression pump.

A compression pump is used for compressing a gas into a container. If the pump shown in Fig. 43 is detached from the receiver plate, and the vessel to receive the gas is attached at C, we have a compression pump.

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. (See opposite page 40 for tunnel construction.)

54. The lift pump. The common water pump, shown in Fig. 44, 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 a did not work 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