MECHANICS OF GASES
(1) WEIGHT AND PRESSURE OF THE AIR
=51. Weight of Air.=--It is said that savages are unaware of the
presence of _air_. They feel the _wind_ and hear and see it moving the
leaves and branches of the trees, but of air itself they have little
conception.
To ordinary observers, it seems to have no weight, and to offer little
resistance to bodies passing through it. That it has weight may be
readily shown as follows: (See Fig. 29.) If a hollow metal sphere, or a
glass flask, provided with tube and stopcock, be weighed when the
stopcock is open, and then after the air has been exhausted from it by
an air pump, a definite loss of weight is noticeable.
[Illustration: FIG. 29.--Proof that air has weight.]
If the volume of the sphere is known and it is well exhausted of air, a
fair approximation of the weight of air may be obtained. Under
"_standard conditions_," which means _at the freezing temperature_ and a
barometric pressure of 76 cm., a liter of air weighs 1.293 g. while 12
cu. ft. of air weigh approximately 1 lb.
=52. Pressure of Air.=--Since air has weight it may be supposed to exert
pressure like a liquid. That it does so may be shown in a variety of
ways.
If a plunger fitting tightly in a glass cylinder be drawn upward, while
the lower end of the tube is under water, the water will rise in the
tube (Fig. 30). The common explanation of this is that the water rises
because of "suction." The philosophers of the ancient Greeks explained
it by saying that "nature abhors a vacuum," and therefore the water
rises. Neither explanation is correct. It was found in 1640 that water
would not rise in a pump more than 32 ft. despite the fact that a vacuum
was maintained above the water. Galileo was applied to for an
explanation. He said, "evidently nature's horror of a vacuum does not
extend above 32 ft." Galileo began tests upon "the power of a vacuum"
but dying left his pupil Torricelli to continue the experiment.
Torricelli reasoned that if water would rise 32 ft., then mercury, which
is 13.6 times as dense as water, would rise about 1/13 as much. To test
this, he performed the following famous experiment.
[Illustration: FIG. 30.--Air pressure forces the liquid up the tube.]
=53. Torricelli's Experiment (1643).=--Take a glass tube about 3 ft.
long, sealed at one end, and fill it with mercury. Close the end with
the finger and invert, placing the end closed by the finger under
mercury in a dish (Fig. 31). Remove the finger and the mercury sinks
until the top of the mercury is about 30 in. above the level of the
mercury in the dish. Torricelli concluded that the rise of liquids in
exhausted tubes is due to the pressure of the atmosphere acting on the
surface of the mercury in the dish.
To test this, place the tube with its mercury upon the plate of an air
pump and place a tubulated bell jar over the apparatus so that the tube
projects through a tightly fitting stopper. (See Fig. 32.) If the air
pressure is the cause of the rise of mercury in the tube, on removing
the air from the bell jar the mercury should fall in the tube. This is
seen to happen as soon as the pump is started. It is difficult to remove
all the air from the receiver so the mercury rarely falls to the same
level in the tube as in the dish. A small tube containing mercury is
often attached to air pumps to indicate the degree of exhaustion. Such
tubes are called _manometers_.
[Illustration: FIG. 31.--Torricelli's experiment.]
[Illustration: FIG. 32.--The mercury drops as the air is removed.]
=54. The Amount of Atmospheric Pressure.=--Torricelli's experiment
enables us to compute readily the pressure of the atmosphere, since it
is the atmospheric pressure that balances the column of mercury in the
tube. By Pascal's Law, the pressure of the atmosphere on the surface of
the mercury in the dish is transmitted as an exactly equal pressure on
the mercury column in the tube at the same level as the mercury
outside.
This pressure, due to the air, must balance the weight of the column of
mercury in the tube. It therefore equals the weight of the column of
mercury of unit cross-section. The average height of the column of
mercury at sea-level is 76 cm. Since the weight of 1 cc. of mercury is
13.6 grams, the pressure inside the tube at the level of the surface of
the mercury in the dish is equal to 1 × 76 × 13.6 or 1033.6 g. per
square centimeter. Therefore the _atmospheric pressure_ on the surface
of the mercury in the dish is 1033.6 g. per square centimeter,
approximately _1 kg. per square centimeter or 15 lbs. per square inch_.
=55. Pascal's Experiment.=--Pascal tested in another way the action of
atmospheric pressure upon the column of mercury by requesting his
brother-in-law, Perrier, who lived near a mountain, to try the
experiment on its top. Perrier found that on ascending 1000 meters the
mercury fell 8 cm. in the tube. Travelers, surveyors, and aviators
frequently determine the altitude above sea-level by reading the
barometer, an ascent of 11 meters giving a fall of about 1 mm. in the
mercury column, or 0.1 in. for every 90 ft. of ascent.
[Illustration: FIG. 33.--A standard barometer.]
_56. The Barometer._--The modern barometer (Fig. 33), consists of a
Torricellian tube properly mounted. Reading a barometer consists in
accurately reading the height of the mercury column. This height varies
from 75 to 76.5 cm. or 29 to 30 in. in localities not far from the
sea-level. The atmospheric pressure varies because of disturbances in
the atmosphere. It is found that these disturbances of the atmosphere
pass across the country from west to east in a somewhat regular manner,
hence a series of readings of the barometer may give reliable
information of the movement of these disturbances and so assist in
forecasting the weather. The weather Bureau has observations taken at
the same moment at various stations over the country. These observations
form the basis for the daily forecast of the weather.
[Illustration: FIG. 34.--An aneroid barometer]
Another form of barometer in common use is the _Aneroid Barometer_
(Fig. 34). Its essential parts are a cylindrical air-tight box with
an elastic corrugated cover. Inside the box is a partial vacuum.
This makes the cover very sensitive to slight changes of pressure.
The motion of the top of the box is conveyed by a series of levers
to an indicating hand which moves over a dial. This barometer can
be made so sensitive as to indicate the change of air pressure from
a table top to the floor. It is much used by travelers, explorers,
surveying parties and aviators, since the mercurial barometer is
inconvenient to carry.
Important Topics
1. _Weight_ and _Pressure_ of air in English and metric units. How
shown. Evidences.
2. Work of Galileo, Torricelli, and Perrier.
3. Barometer: construction, action, mercurial, aneroid.
[Illustration: FIG. 35.--Air pressure keeps the water In the tumbler.]
[Illustration: FIG. 36.--Cross-section of a modern drinking fountain.]
Exercises
1. Do you think Archimedes' Principle applies to the air? Does Pascal's
Law? Why?
2. Find the downward pressure of the mercury in a barometer tube if the
cross-section is 1 sq. cm. and the height 75 cm. at the level of the
mercury surface in contact with the air. (The density of mercury is 13.6
grams per cc.)
3. What is the weight of the air in a room if it is 10 × 8 × 4 meters?
4. What weight of air is in a room 10 × 15 × 10 ft.?
5. When smoke rises in a straight line from chimneys, is it an
indication of a high or low barometric pressure? Why?
6. Why does a tumbler filled with water and inverted in a dish with its
rim under water remain full?
7. If the barometer tube is inclined the mercury remains at the same
horizontal level. How can this be explained?
8. When the mercurial barometer stands at 76 cm., how high would a water
barometer stand? Explain.
9. Explain why it is possible for one to suck soda water through a tube?
10. Fill a tumbler with water. Place a sheet of paper over the top and
invert. The paper clings to the tumbler and prevents the water from
escaping. Explain. (See Fig. 35.)
11. Why must a kerosene oil can have two openings in order to allow the
oil to flow freely?
12. Explain the action of the modern drinking fountain (Fig. 36).
(2) COMPRESSIBILITY AND EXPANSIBILITY OF THE AIR
=57. Effect of Pressure on Liquids and Gases.=--Both classes of fluids,
liquids and gases, have many characteristics in common. Both are
composed of molecules that move freely; hence both _flow_. At any point
within a fluid the _pressure is the same in all directions_. Archimedes'
Principle applies, therefore, to both liquids and gases.
We now come to an important _difference_ between liquids and gases.
_Liquids_ are _practically incompressible_. "So much so, that if water
is subjected to a pressure of 3000 kg. per sq. cm., its volume is
reduced only about one-tenth." Gases show a very different behavior from
liquids on being subjected to pressure. They may readily be compressed
to a small fraction of their volume as is noticed on inflating a
pneumatic tire. A gas has also the _ability to spring back_ to a larger
volume as soon as the pressure is released, as when a cork is driven
from a pop gun. Not only is compressed air able to expand, but air
under ordinary conditions will expand if it is released in a space where
the pressure is less.
Hollow bodies, animals and plants, are not crushed by atmospheric
pressure, because the air and gases contained within exert as much force
outward as the air exerts inward.
=58. Boyle's Law.=--The relation between the volume and pressure of a
gas was first investigated by Robert Boyle in the seventeenth century.
The experiment by which he first discovered the law or the relation
between the volume and the pressure of a gas is briefly described as
follows:
[Illustration: FIG. 37 _a_.
FIG. 37 _b_.
FIGS. 37 _a_ AND 37 _b_.--Boyle's law apparatus.]
A glass tube is bent in the form of the capital letter J, the short
arm being closed. A little mercury is poured in to cover the bend.
(See Fig. 37 _a_.) Since the mercury is at the _same level in both
arms_, the pressure in (_A_) is the same as in (_B_). Mercury is
now poured into (_A_) until it stands in the long tube at a height
above that in (B) which is equal to the height of the mercury
column of the barometer. (See Fig. 37 _b_.) The air in (_BC_) is
now under a pressure of two atmospheres (one atmosphere is due to
the mercury column). On measurement the air in (_BC_) will be found
to have just one-half of its original volume.
Thus doubling the pressure to which a gas is subjected reduces its
volume to one-half. Tripling the pressure, reduces the volume to
one-third and so on.
Careful experiments reveal the following law: _The volume of a given
mass of gas at constant temperature is inversely proportional to the
pressure to which it is subjected_.
This law is often expressed mathematically. _P/P´ = V´/V_, or _PV =
P´V´_. Since doubling the pressure reduces the volume one-half, it
doubles the density. Tripling the pressure triples the density. We
therefore have _P/P´ = D/D´_ or the density of a gas directly
proportional to its pressure.
[Illustration: FIG. 38.--Height and density of the air.]
=59. Height of the Atmosphere.=--From its properties of _compression_
and _expansion_, the air varies in density and pressure as one ascends
in it. At a height of 3 miles the pressure is reduced to about one-half.
This is an indication that one-half of the air is below this level.
Balloonists have gone to a height of 7 miles, Glaser and Coxwell in
England in 1862 and Berson in France in 1901. The atmosphere has been
explored to a height of 30,500 meters (18.95 miles) by sending up
self-registering barometers in small balloons which burst at great
altitudes. A parachute protects the instruments from breakage from too
rapid fall. This height of 30,500 meters was reached by a balloon sent
up by William R. Blair, at Huron, South Dakota, September 1, 1910.
At a height of 35 miles, the density is estimated at 1/30,000 of its
value at sea-level. (See Fig. 38.) It is believed that some rarefied air
exists for a considerable distance above this point, some estimates
placing the extent at 100 miles, and others from 200 to 500 miles.
Evidences of some air at such heights are shown by: (a) the height at
which meteors first appear, (b) the height of the Aurora Borealis, and
(c), the distance that the sun is below the horizon when the last traces
of color disappear from the sky in the evening.
Although the exact limits of the atmosphere are unknown, the weight of a
column of air 1 sq. cm. in cross-section, and extending _upward as high
as the atmosphere_, may be accurately computed. For this column of air
exactly balances the column of mercury in the tube of the barometer.
Below sea-level, the air increases rapidly in density and it is
estimated that at a depth of 35 miles, the density of the air would be a
thousand times that at the earth's surface, or more than that of water.
Important Topics
1. Evidence of compressibility of gases and incompressibility of
liquids.
2. Boyle's Law. Proof, applications.
3. Extent of the atmosphere--three evidences.
Exercises
1. Mention three illustrations of the compressibility and expansibility
of air that you know from your own experience.
2. Increasing the pressure increases the amount of a gas that will be
absorbed by a liquid? Explain this. Have you ever observed this fact?
Where?
3. If a toy balloon containing 2000 ccm. of gas at the earth's surface
where the barometer reading is 76 cm., rises to an elevation where the
barometer reads 54 cm., the balloon will tend to expand to what volume?
Explain. Will it attain this volume?
4. If a gas is compressed, it changes in temperature. How do you explain
this?
5. What change in temperature will occur when compressed air is allowed
to expand? Explain.
6. Air blowing up a mountain side has its pressure lessened as it
approaches the top. How will this affect the temperature? Why? What may
result from this change in temperature? Explain.
7. To what pressure must 500 ccm. of air be subjected to compress it to
300 ccm. the barometer reading at first being 75 cm. Explain.
[Illustration: FIG. 39.--The air pump.]
(3) PNEUMATIC APPLIANCES
=60. The Air Pump.=--The air pump is used to remove air or other gases
from a closed vessel. It was invented about 1650 by Otto Von Guericke,
burgomaster of Magdeburg, Germany. One form of air pump is shown in
Fig. 39. _C_ is a cylinder within which slides a tightly fitting piston.
_R_ is the vessel from which the air is to be exhausted. _r_ and _u_ are
valves opening upward. The action of the pump is as follows:
On pushing the piston down, the air in _C_ is compressed. This opens
valve _r_ allowing the confined air to escape above the piston. The
piston is then raised making the space in _C_ a partial vacuum. The
pressure in _R_ now being greater than in _C_, _u_ is pushed up and the
air from _R_ rushes into _C_, until the pressure is equalized. On
pushing down the piston again, valve _u_ closes and the process is
repeated until the pressure in _R_ is no longer able to raise the valve
_u_. Some air pumps are so constructed that the valves are opened and
closed automatically by the movement of the piston. With these pumps a
higher degree of rarefaction can be obtained.
Air is often partially exhausted from receivers or vessels by the use of
a filter pump or _aspirator_. A stream of water flowing through a
constriction causes a reduced pressure, draws in air and carries it
away, and thus produces a partial vacuum. See Fig. 40 for a section of
the device.
[Illustration: FIG. 40.--An aspirator.]
=61. The Condensing Pump.=--This is like the exhaust pump except that
its valves are reversed. It is used in compressing illuminating gases
into cylinders for use in lighting vehicles, stereopticons, Pintsch
lights, gas light buoys, etc., and also for compressing air to operate
air brakes, pneumatic hammers and drills, and for other uses.
The common condensing pump is the kind used for inflating tires. (See
Fig. 41.) In this, a loosely fitting metal piston is attached to a disc
of leather somewhat larger than the cylinder. This device is called a
_cup valve_. On raising the piston, air rushes in from the top past the
valve, but on pushing the piston down, the valve is pressed tightly
against the sides of the cylinder and prevents the escape of any air.
The compressed air pushes open a valve on the tire and enters it. This
valve closes as soon as the pressure is lessened from outside. It is
well to notice in all of these pumps that _two_ valves are used. One
holds the air already secured while the other opens for a new supply.
Both valves are never open at the same time.
[Illustration: FIG. 41.--Condensing pump used in inflating tires.]
=62. Water Pumps.--The Common Lift Pump.= This, the simplest pump for
raising water, consists of a cylinder _C_ (Fig. 42) connected by a pipe
_R_ to a supply of water as a cistern or well. A valve opening upward is
placed at the bottom of the cylinder over the entrance to the pipe. In
the cylinder is a tightly fitting piston connected by a rod to a lever
for ease in action. The piston contains a valve opening upward. In
operating this pump water is usually first poured into the cylinder to
"prime" it. This helps to close the valves and prevents air leaking past
them. When the piston is lowered the lower valve closes, the air in the
cylinder being compressed pushes the upper valve open and passes above
the piston. On raising the piston the upper valve closes. This forms a
partial vacuum in the cylinder.
The air pressing on the surface of the water below forces the water and
air that may be in the tube upward through the lower valve to fill this
partial vacuum.
When the cylinder becomes filled with water, this is lifted out on the
up-stroke, whence its name, "lift pump." Since the atmospheric pressure
at sea-level can only support a column of water about 34 ft. high, the
lower valve must be within this distance of the water surface. In actual
practice the limit is about 27 ft. In deeper wells, the cylinder and
valves are placed so that they are within 25 or 27 ft. of the surface of
the water in the well, a long piston rod reaching above the surface of
the ground and connected to a pump handle operates the piston. A
discharge pipe extends from the cylinder to the surface of the ground
above.
[Illustration: FIG. 42.--The common lift pump.]
[Illustration: FIG. 43.--A force pump with an air chamber (_A_).]
=63. The Force Pump.=--The force pump is used to deliver water under
pressure either for spraying or to an elevated reservoir. The piston is
solid, the second valve being placed at the entrance of the discharge
pipe. (See Fig. 43.) The action is the same as that of the lift pump,
with this exception; the piston in its down stroke forces the water out
through the discharge pipe, the velocity depending upon the pressure
exerted.
A force pump is usually provided with an air chamber which is connected
with the discharge pipe. On the down stroke of the piston, water is
forced into the air chamber. This compresses the air it contains. The
compressed air reacts and exerts pressure on the water forcing it out in
a steady stream.
Force pumps are used in deep wells, being placed at the bottom.
The pumps used in city water works, fire engines, and all steam pumps,
are force pumps. (See Fig. 44.)
[Illustration: FIG. 44.--A steam pump used on a fire engine.]
=64. The Siphon.=--The siphon is a tube used to convey a liquid from one
level over an elevation to a lower level by atmospheric pressure. It is
used to remove liquids from tanks or vessels that have no opening at the
bottom.
The siphon cannot be completely understood until one has mastered the
laws of the flow of liquids. The following is offered as an incomplete
explanation of its behavior. Consider the siphon to be full of water
and closed at _d_ (Fig. 45). Atmospheric pressure on _a_ will hold the
siphon full if _ab_ does not exceed 34 feet. If _d_ is opened the water
falls out with a speed equal to that acquired in falling from the level
of _a_ to that of _d_. This speed is acquired by all the water in the
siphon and results in a drop in pressure throughout it. The pressure at
_a_ inside the siphon becomes less than the pressure at the same level
outside as soon as the water starts flowing. The water in the vessel
then flows into the siphon and out at _d_. This flow continues as long
as there is a fall from the free surface of the water in the vessel to
the outlet at _d_.
[Illustration: FIG. 45.--Cross-section of a siphon.]
[Illustration: FIG. 46.--The Cartesian diver.]
=65. The Cartesian Diver.=--This is a device which illustrates at the
same time transmission of pressure by liquids, Archimedes' principle,
and compressibility of gases. It was invented by Des Cartes (1596-1650).
As ordinarily made, it is a hollow glass image with a small opening in
the foot. It contains air and water in such amounts that the average
density of image and contents is slightly less than that of water. It is
placed in a tall glass jar filled with water and covered with tightly
stretched rubber tissue. (See Fig. 46.) By pressing on the rubber cover
the diver may be made to sink, since the air and water transmit the
pressure on the cover which compresses the air inside the figure
admitting some water to it, thus making the diver more dense than
water. By varying the pressure it can be made to sink, rise, or remain
stationary at will.[D] A small vial can be used instead of the image.
[D] The position of a submarine in or under water is controlled in
a similar manner.
=66. Hydraulic Ram.=--The hydraulic ram (see Fig. 47) is an automatic
device that is much used for raising water from springs to houses
located on higher ground. Water flows through the pipe _A_ through the
opening at _B_. The pressure closes the valve at _B_. The increased
pressure in the pipe due to the closing of _B_ opens the valve _C_ and
some of the water flows into the air chamber _D_. This reduces the
pressure against the valve _B_ so that it drops and allows a little
water to escape. Just as this happens, valve _C_ closes. The pressure in
the pipe then closes _B_ and forces water past _C_. This action being
continually repeated, the air in _D_ becomes so compressed that it has
elastic force enough to raise the water in a steady stream to a height
of many feet.
[Illustration: FIG. 47.--Cross-section of a hydraulic ram.]
=67. The Balloon.=--Since air is a fluid, Archimedes' principle applies
to it as well as to liquids. Therefore any object in the air is lifted
up by a force equal to the weight of the air it displaces. The object
will rise, if it weighs less than this displaced air and will continue
to rise until both weights are equal.
_The Balloon_ (Fig. 48) rises because it weighs less than the air it
displaces, and therefore it is pushed up by the heavier air, the
"lifting power" being the difference between its weight and that of the
air displaced. The neck at the bottom is left open to allow for
expansion of the gas. When the aeronaut wishes to descend, he opens a
valve at the top allowing some of the gas to escape.
[Illustration: FIG. 48.--Winner of international championship race,
Paris, 1913.]
_Hydrogen_ is the lightest gas, weighing 0.09 kg. per cubic meter, and
so gives the greatest lifting power, but as it is expensive to make,
coal gas, density 0.75 kg. per cubic meter, is ordinarily employed.
Helium has recently been used to fill military balloons because it
cannot be set on fire.
_The Parachute_ (Fig. 49) is an umbrella-shaped device for use in
descending from a balloon. After falling a few seconds it opens, the
large surface exposed to the air causing it to descend slowly. The hole
in the top keeps the parachute upright by allowing the air to escape
through it, thus relieving the pressure.
[Illustration: FIG. 49.--A parachute.]
[Illustration: FIG. 50.--Cross-section of a Westinghouse air brake.]
=68. The Air Brake.=--Compressed air is used to do work in many
machines, such as pneumatic drills, hammers, and air brakes. The
Westinghouse air brake (Fig. 50) uses air at a pressure of about 70 lbs.
to the square inch. The essential parts as shown are a reservoir _R_,
the brake cylinder _C_ and a triple valve _V_, placed under each car
with an air pipe _P_, leading to the engine. This is connected to _R_ by
the triple valve _V_. When the pressure in _P_ is reduced by the
engineer or by accident, the triple valve operates so as to admit air
from _R_ into the cylinder _C_ pushing the piston _H_ to the left. _H_
is connected to the brakes by levers which press the brake shoes
strongly against the wheels. When the air pressure in _P_ is restored
the triple valve acts so as to permit the air in _C_ to escape while _R_
is filled again from _P_. The hissing sound heard when a train stops is
caused by air escaping from cylinder _C_. The spring in _C_ keeps the
brakes from the wheels except when the "air is on."
[Illustration: FIG. 51.--Cross-section of a gas meter showing its
construction and action.]
=69. The Gas Meter.=--The gas meter consists of a box divided into two
parts by a vertical partition (Fig. 51). Two bellows are attached to
this partition, one on each side. The valves that regulate the flow of
gas to and from the bellows and the chambers _A_ and _D_ are opened and
closed by levers connected with the bellows. These levers also operate
the hands upon the dials. When the inlet to the bellows _B_ is opened,
the outlet of _A_ is also opened. Gas entering _B_ opens the bellows and
forces the gas in _A_ out into the house-pipe _E_. When _B_ is full its
inlet valve closes and its outlet valve opens. The inlet of _A_ also
opens and its outlet closes. Gas now flows into _A_, compressing the
bellows and _B_, and forcing the gas from it into the house-pipe. At
each filling of the bellows _B_ there will be displaced from _A_ and
forced into the house-pipe as much gas as enters _B_. It is evident that
at each emptying of _B_ an equal amount of gas enters _A_. Thus we have
_A_ and _B_ alternately filling and emptying as long as the gas burner
is open. To have a continuous flow of gas in the house-pipes two pipes
and two chambers are necessary, one being filled while the other is
being emptied.
Fig. 52 represents the dials upon a gas meter showing a reading of
54,600 cu. ft.
[Illustration: FIG. 52.--Dials of a gas meter.]
=70. Centrifugal Pumps.= Fluids, such as water and air, are often put in
motion by devices called _centrifugal pumps_ (see Art. 78). These pumps
contain a revolving part, like a wheel without a rim, whose spokes are
replaced by thin blades. This revolving part resembles the paddle wheel
of some steam boats and is enclosed in a case or cover having one
opening at the rim and another opening on one side about the axle.
[Illustration: FIG. 53.--A vacuum sweeper. (_Courtesy of the Hoover
Suction Sweeper Co._)]
When the wheel is rapidly revolved, the fluid is driven out with
considerable force through the opening at the rim, while a partial
vacuum is produced at the axle causing a rapid flow into the device at
this point.
This is the principle of the action of the _vacuum cleaner_. Fig. 53 is
a section of a vacuum sweeper showing the revolving wheel and the
current of air passing into the wheel at the lower side and out of the
rim of the case at the rear.
_Centrifugal water pumps_ work on the same principle and furnish a
continuous flow of water, often large in volume and at considerable
pressure.
Important Topics
1. Air pump.
2. Condensing pump.
3. Lift and force pumps.
4. Siphon.
5. Cartesian diver.
6. Hydraulic ram.
7. Balloon.
8. Air brake.
9. Gas meter.
10. Vacuum cleaner.
Exercises
1. Explain why smoke settles to the ground before storms.
2. Why does the water rise in the suction pipe of a pump?
3. Why is it easier to float in water when the lungs are filled with air
than when they are not filled?
4. Why is it easier to swim in salt water than in fresh water?
5. How are submarines made to sink? to rise to the surface?
6. How can a fish rise or sink in water?
7. Explain why a life preserver made of cork will enable a person to
float.
8. Hold the open hand out flat with the fingers together. Place
underneath the fingers a piece of paper. Blow between the first and
second fingers against the paper. As long as you blow hard the paper
will not fall but will stick to the hand. Explain.
9. Why does pressing the bulb of an atomizer force out the liquid in a
fine spray?
10. Why is air that contains a large amount of water vapor lighter than
air that only contains a small amount?
11. How are heights above sea-level ascertained by a barometer?
12. Oil floats on water but sinks in alcohol. Explain.
13. In a balloon the lower end is often open to the air. Why does not
the gas escape and prevent the balloon from rising?
14. How long will a balloon continue to rise?
15. If the pressure against the 8-in. piston of an air brake is 70 lbs.
per square inch, how much force does the piston exert?
16. The capacity of a balloon is 40,000 cu. ft. The weight of the
balloon, car, etc., is 600 lbs.; specific gravity of the gas used is
0.46 that of the air. Find how much weight the balloon can carry.
17. The so-called Magdeburg hemispheres were invented by Otto von
Guericke of Magdeburg, Germany. When the hemispheres (see Fig. 54) are
placed in contact and the air exhausted it is found very difficult to
pull them apart. Explain.
18. Von Guericke's hemispheres had an inside diameter of 22 in. What
force would be required to pull them apart if all the air were exhausted
from them? (Find the atmospheric force on a circle, 22 in. in diameter.)
19. Von Guericke made a water barometer whose top extended through the
roof of his house. On the top of the water in the tube was placed a
wooden image. In fair weather the image appeared above the roof, but it
descended before a storm. Explain.
20. The balloon "Goodyear" (Fig. 48), which won the International
championship race at Paris in 1913, has a capacity of 80,000 cu. ft. The
gas bag weighs 653 lbs., the net 240 lbs. and the basket 92 lbs. How
large a load can it carry when filled with hydrogen specific gravity
0.069 (compared with air).
[Illustration: FIG. 54.--Magdeburg hemispheres.]
Review Outline: Liquids and Gases
Liquids: Force, pressure, and density. Floating and immersed bodies.
Laws: Liquid force, _F = A.h.d_, Pascal's, Archimedes. Illustrations and
Applications:
Specific gravity, _W_{a}/(W_{a} - W_{w})_, _(W_{a} - W_{l})/(W_{a}
- W_{w})_, Boyle's, _PV = P´V´_
Devices: Hydraulic press, air cushion, barometer--mercurial and aneroid.
Pumps, lift, force, vacuum, compression, centrifugal, balloon, siphon,
etc. Construction and action of each.