G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

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the point D is lower than the

end C, and the water descends to D by the force of gravity. Another half-
revolution brings the point E lower than D, and again the water descends.
This is continued till the water is discharged at the upper end. As new wa-
ter is constantly scooped up, there will be a continuous flow as long as the
handle is turned. Archimedes' Screw operates only at short distances.

385. The Chain Pump. The Chain Pump is much
used for raising water. The principle it involves is also
applied in dredging-machines, for cleaning out the channels
of rivers.

This machine (see Fig. 173) consists of a continuous chain, to which cir-
cular plates, c, d, e,f, &c., are attached at equal distances. The plates are
of such a size as exactly to fit the cylinder G II, the lower end of which rests
in the water. The chain passes over the two wheels, I, J ; to the upper one
of which, I, a handle is attached. When the handle is turned, the chain is
set in motion. The plates, ascending through G II, carry up water before
tlftm, which has no opportunity of escaping till it reaches the opening K.



machines for raising water? Of what does Archimedes' Screw consist ? Describe its
mode of operation. At what distances docs Archimedes' screw operate ? 3 NX What
machine is mu^h used for raising water ? What other application is made of the
principle :!; involves? Describe the Chain. Pump, and its mode of operating.



THE HYDRAULIC HAM.



1G3



There it is discharged, as long as the Fig. 173.

handle is turned.

386. Tlie Hydraulic Earn.
The Hydraulic Ram was in-
vented in France, in 1796. It
raises water by successive im-
pulses, which have been com-
pared to the butting of a ram,
and hence its name. The re-
quisite power is gained by mo-
mentarily stopping a stream in
its course, and causing its mo-
mentum to act in an upward
direction.

Fig. 174 represents a simple form of
the Hydraulic Ram. To a stream or res-
ervoir at A, is adapted an inclined pipe,
B, through which the water that works
the ram is conveyed. Near the lower
end of the pipe B rises an air-chamber,
D, with which an upright pipe, F, is con-
nected. The passage connecting B with
the air-chamber is commanded by a valve
opening upward. At the extremity of
the pipe B is another valve, E, opening TI1E CIIAIN PUMP .

downward, and made just heavy enough
to fall when the water in B is at rest.

Fig. 174. The play of the valve E makes the machine self-

acting. Suppose the pipe B to be filled from the res-
ervoir ; the valve E opens by its weight, and allows
some of the water to escape. Soon, however, the
water acquires momen-
tum enough to raise the
valve and close the open-
ing. The stream is thus
suddenly stopped, and
the pipe would be in
danger of bursting from
THE HYDRAULIC RAM. the shock were it not for

the valve in the air-chamber D, which is at once forced upward, and allows





8?fi. When and where was the Hydraulic Ram invented 1 Why is it so called ?
IIo\v is the requisite power gained in the ram? Describe the hydraulic ram, and



164 H YDE AULICS.

some of the water to enter. The air in D is at first condensed by the pressure
of the water thus admitted ; but, immediately expanding by reason of its elas-
ticity, it drives the water into F, for the closing of the valve prevents it from
returning to B. By this time the water in B is again at rest, the valve E
opens, and the whole process is repeated.

By successive impulses the water may be raised in F to a great height. A
descent of four or five feet from the reservoir is sufficient. Care must be
taken to have the valve E just heavy enough to fall when B is at rest, and
not so heavy as to prevent it from readily rising as the momentum of the
stream increases. The pipe B must also be of such length that the water,
when arrested in its course, may not be thrown back on the reservoir.

387. Hydraulic Rams afford a cheap and convenient
means of raising water in small quantities to great heights,
wherever there is a spring or brook having a slight eleva-
tion. They are used for a variety of purposes, and partic-
ularly when a supply of water is needed for agricultural
operations.

EXAMPLES FOR PRACTICE.

|^ Friction is left out of account in tttese examples.

1. (See 356, rule in italics.} Two streams issue from different orifices in the

same vessel with velocities that are to each other as 1 to 6. How many
times farther from the surface is the one than the other?

2. The stream A runs from an orifice in a vessel three times as fast as the

stream B. How do their distances below the surface of the liquid com-
pare?

3. In a vat full of beer there are two orifices of equal size; one 9 inches be-

low the surface, and the other 25. How does the velocity of the latter
compare with that of the former?

4. There are three apertures in a reservoir of water, 1, 4, and 16 feet below

the surface. With what comparative velocity will their streams flow ?

5. A stream flows from an aperture in a vessel at the rate of 4 feet in a sec-

ond. I wish to have another stream from the same vessel with a velo-
city of 16 feet per second. How much farther below the surface than the
first must it be?

6. (See 359.) A vat full 'of ale, 3 feet high, has four apertures in it, 3, 12, 13,
* and 24 inches respectively from the top. Through which will the liquid

spout to the greatest horizontal distance? Which next? Which next?

7. (See 360.) How much water will be discharged every minute from an

orifice of 3 square inches, the stream flowing at the rate of 5 feet in a
second, and the vessel being kept replenished ?

its mode of operating. TTow creat a descent is required ? What precautions are
necessary ? 3S7. In what case may hydraulic rams be used with advantage ?



EXAMPLES FOR PRACTICE. 165

How much will be discharged every minute from another orifice in
the same vessel, equally large, but situated four times as far below the
surface of the liquid?

8- A stream Hows from a hole in the bottom of a vessel with a velocity of G
feet in a second. The hole has an area of 5 square inches, and the ves-
sel is emptied in 15 seconds. How much water does the vessel hold ?
9' \See 37ti.) A stream having a momentum equivalent to 100 units of work
is applied to an Undershot Wheel ; how many units of work will it per-
form? Ana. 25.

(See 377.) How many units of work will it perform, if applied to an. Over-
shot Wheel ?

(See 378.) How many, if applied to a Breast-wheel?
(See 379.) How many, if applied to a Turbine?



CHAPTER XII.



PNEUMATICS.

388. PNEUMATICS is the science that treats of air and the
other elastic fluids, their properties, and the machines in
which they are applied.

389. DIVISION OP ELASTIC FLUIDS. The elastic fluids
arc divided into two classes :

I. GASES, or such as retain their elastic form under ordi-
nary circumstances. Some of the gases, under a
high degree of pressure, assume a liquid form ; as,
carbonic acid and chlorine ; others, such as oxy-
gen and nitrogen, can not be converted into liquids
by any known process.

II. VAPORS, or elastic fluids produced by heat from
liquids and solids. When cooled down, they re-
sume the liquid or solid form. Steam, the vapor
of water, is an example.

390. All gases and vapors have the same properties.

3SS. What is Pneumatics ? 339. Into what two classes are elastic fluids divided?
What are gases ? What difference is there in the gases ? What are vapors ? 890. In



166



PNEUMATICS.



The principles of Pneumatics, therefore, relate to all alike,
though they are most frequently exhibited and applied in
the case of air, with which we have far more to do than
with any other elastic fluid.

Air.

391. Air is the elastic fluid that we breathe. It sur-
rounds the earth to a distance of about fifty miles from its
surface, and forms what is called the Atmosphere. It exists
in every substance, entering the minutest pores.

392. VACUUMS. Air may be removed from a vessel with
an instrument called the Air-pump. A Vacuum is then said
to be produced. Vacuums sometimes result from natural
causes ; but they last only for an instant, as the surround-
ing air at once rushes in to fill them. Hence the old phi-
losophers used to say, Nature abhors a vacuum.

393. PROPERTIES or AIR. Air can not be seen, but it

can be felt by moving the hand
rapidly through it. It is there-
fore material, and has all the
essential properties of matter.
394. Air is impenetrable.
395. TJie Divinff-lelLTbe impen-
etrability of air is shown by the Diving-
bell, represented in Fig. 175. A C is a
large iron vessel, shaped somewhat like
an inverted tumbler, and attached to a
chain, by which it is let down in the
water. As the vessel descends, the air
in it is condensed by the upward pres-
sure of the liquid, and water enters.
The lower it gets, the more the air
is compressed, and the greater the
amount of water admitted. The im-
penetrability of the air, however,
keeps the greater part of the bell



Fig. 175.




THE DIVING-BELL.



what are the principles of Pneumatics most frequently exhibited, and why?
891. What is Air? How far does it extend from the earth's surface? What does it
constitute ? 892. What is a Vacuum ? What did the old philosophers say, and why ?
393. What proves the air to be material ? 894. What apparatus shows the impenetra-
bility of air? 395. Describe the Diving-bell. Explain how descents are made with



PROPERTIES OF AIE.



167



clear of water, so that several persons may descend in it to the bottom of
the sea.

As fast as the air is vitiated by the breath, it is let off by a stop-cock,
while fresh air is supplied from above by a coadensing S3 r riuge, through the
pipe 13. Air may be thus forced down in sufficient quantities to expel the
water altogether from the bell, so that the divers can move about without
difficulty on the bottom of the sea. If air were not impenetrable, the bel
would be tilled with water, and the divers drowned.

When the diving-bell was invented, is not known. History makes no
mention of it before the sixteenth century. At that time, we are told, two
Greeks, in the presence of the emperor Charles V. and several thousand spec-
tators, let themselves down under water, at Toledo in Spain, in a large in-
verted kettle, and rose again without being wet. In 1665, a kind of bell was
employed off the Hebrides, for the purpose of recovering the treasure lost
in several ships belonging to the Invincible Armada. From that time to the
present, various improvements have been made in the diving-bell; and it is
now extensively used for clearing out harbors, laying the foundation of sub-
marine walls, and recovering articles lost by shipwreck.

396. Air is compressible.

This is proved with the diving-bell. If the air Fi? - 176 -
were not compressible, no water would enter the
bell as it descended.

397. Air is elastic.

This also may be shown with the diving-
bell. When, on its descent, water has entered,
on account of the air's being compressed, let the
bell be raised, and the air will resume its origi-
nal bulk, expelling the water.

Bottle Imps. The compressibility and elasticity of air may
be exhibited in an amusing way with the apparatus represent-
ed in Fig. 176. In a vessel nearly full of water are placed sev-
eral small balloons, or hollow figures of men, &c., made of col-
ored glass, and called Bottle Imps. Each figure has a little
hole in the bottom, and is of such specific gravity that it will
just float in water. A piece of thin india rubber is tied over
the mouth of the vessel, so as to cut off communication with
the external air. Now press on the india rubber cover. The BOTTLE r
water at once transmits the pressure to the air in the hollow figures. This
air is condensed, water enters, the specific gravity of the figures is increased,

it. What is the first mention made of the divinsr-bell in history? In 1665, for wlat
purpose was it used? For what is it now extensively used? 396. How does the
diving-bell prove air to be compressible? 307. How docs it prove air to be elastic?
What properties in air do the Bottle Imps illustrate ? Describe the botik- imps, and



168 PNEUMATICS.

and they descend. On removing the fingers from the cover, the air, by rea-
sou oi its elasticity, resumes its original bulk, and the figures rise. By thus
playing on the india rubber, the figures may be made to dance up and down.

398. Mar lotto's Law. The elastic fluids are the most
easily compressed of all substances. The greater the pres-
sure to which they are subjected, the less space they occupy,
and the greater their density. A body of air which under
a certain pressure occupies a cubic foot, under twice that
pressure will be condensed into half a cubic foot ; under
three times that pressure, into one-third of a cubic foot, &c.
This principle, variously stated, is called, from its discov-
erer, Mariotte's Law.

The more the elastic fluids are compressed, the greater
is, their resistance to the pressure. Hence, their elastic force
increases with their density.

399. The Air-gun. By subjecting a body of air to a great pressure, we
may increase its elastic force sufficiently to produce wonderful effects. The
Air-gun is an example. It consists of a strong metallic vessel, into which
air is forced till it is in a state of high condensation. The vessel is then at-
tached to a barrel like that of an ordinary gun, to the bottom of which a bul-
let is fitted. Pulling a trigger opens a valve, the condensed air rushes forth,
and drives the bullet out with great force.

One supply of condensed air is sufficient for several discharges, though
each is weaker than the preceding one. The labor required for condensing
the air prevents this instrument from being much used ; but as it makes less
noise, when discharged, than the ordinary gun, it is sometimes employed by
assassins.

400. Air has weight.

Weigh a flask full of air, and then weigh the same flask
with the air exhausted. The difference indicates the weight
of the air contained.

401. Experiments show the weight of 100 cubic inches of air to be about
30V2 grains. This makes it 828 times lighter than water. It has been com-
puted that the weight of the whole atmosphere surrounding the earth is equal
to that of a globe -of lead GO miles in diameter.



explain the principle on which they dance np and down. 393. "What subst.inros are
the most easily compressed ? What is Mariotte's Law ? To what is the elastic force
of gases and vapors proportioned ? 399. JIow may a body of air be made to produce
wonderful effects? What instrument proves this? Describe the Air-gun, and its
operation. Why is not the air-gun used more? By whom is it sometimes employed ?
400. Prove that air has weight. 401. What is the weight of 100 cubic inches of air?



ATMOSPHERIC PRESSURE.



169



Atmosplieric Presmre.

402. The particles of air, like those of the other elastic
fluids, mutually repel each other. The atmosphere would
therefore spread out into space, and become exceedingly
rare, if it were not for the attraction of the earth. This
prevents it from extending more than fifty miles from the
eurface, arid gives it weight.

403. Since air has weight, it exerts a pressure on all
terrestrial bodies. This is known as Atmospheric Pressure.
The pressure on any given body is equal to the weight of
the column of air resting upon it, and therefore rig. ITT.
varies according to its size.

404. EXPERIMENTS. The pressure of the
atmosphere is proved by experiments.

Experiment 1. Take a common syringe, represented in
Fig. 177, and let the piston, P, rest on the bottom of the bar-
rel. Insert the nozzle, 0, in a vessel of water, and raise the
piston. The water enters through 0, and follows the piston,
as shown in the Figure.

What causes the water to rise? The piston, being air-
tight, as it is drawn up, leaves a vacuum behind it ; and the
pressure of the atmosphere on the water in the vessel drives
it into the barrel through 0. If the piston does not fit the
barrel tightly enough to exclude the air above, no water
enters, because the pressure of the air from without is then
counterbalanced by that from within the barrel.

Ex,p. 2. Take a small tube, close one end with the
finger, fill it with water, and carefully invert it, as
shown in Fig. 178. The water is kept in the tube by
atmospheric pressure. Remove the finger, and the
downward pressure of the atmosphere, which was be-
fore cut off, will counterbalance the upward pressure,
and the water will fall by its own weight.

Exp. 3. Fill a wine-glass with water, and cover the
mouth with a piece of stiffpaper. Place the hand over
the paper, and invert the glass. On carefully removing

What is the weight of the whole atmosphere? 402. "What prevents the atmosphere
from spreading out into space ? 493. What is Atmospheric Pressure ? What causes-
atmospheric pressure? To what is the atmospheric pressure on any given body
equal ? 404. Describe the experiment with the syringe that proves the pressure of
the atmosphere. What will prevent the water from rising in the syringe ? Describe




170



PNEUMATICS.



the hand, the water will be found to remain in the glass, supported there by
atmospheric pressure.



Fig. 179.




THE BELLOWS.



Fig. ISO.



Ex.p. 4. When we raise the top board,
A, of a common bellows (see Fig. 17y), the
valve B in the lower board opeus. This is
because a vacuum is formed within the bel-
lows, and the atmospheric pressure forces
the valve up and drives in a portion of the
external air.

The same principle is involved in the act of breathing. The cells in the
lungs are expanded by muscular action, a vacuum is thus formed, and the
pressure of the atmosphere drives in the outer air through the nose or mouth.
In a few seconds the muscles contract, and the same air, laden with impuri-
ties received from the blood in the lungs, is expelled.

405. The Sucker, a play-thing used by
boys, shows the force of atmospheric pres-
sure. It consists of a circular piece of
leather with a string attached to the mid-
dle. The leather, being first wet so that it
may adapt itself to the surface, is pressed
firmly upon a flat stone. The string is then
gently pulled, so as to form a vacuum be*
tween the leather and the stone. On this,
the atmospheric pressure from above, not
being counterbalanced from beneath, acts
on the leather with such force that a stone
of great weight may be lifted without the sucker's becom-
ing detached. If a hole is made in the leather, air rushes
in, the pressure from above is counterbalanced, and the
stone falls by its own weight.

When flies walk on a ceiling, their feet act like suckers. Vacuums are
formed beneath them, and they are sustained by atmospheric pressure. It is
in the same way that the shell-fish called limpets fasten themselves to rocks.

406. Supported by the pressure of the atmosphere below, while it is cut
off from that above, a liquid will not flpw from the tap of a barrel unless a
small opening is made in the top. As soon as this is done, air is admitted,

the experiment with a small tube that proves the pressure of the atmosphere. How
may water be supported in a wine-jrlass by atmospheric pressure ? IIow is the pres-
sure of the atmosphere exhibited with a common bellows? How do we breathe ?
405. Explain the principle involved in the Sucker. How are flies able to walk on a
ceiling? 405. Why, when a barrel is tapped, must a hole be made in the top?




THE SUCKEE.



THE BAROMETER. 171

the upward pressure is counterbalanced, and the liquid flows continuously.
Ou the same principle, a small hole is made in the lid of a tea-pot.

407. THE BAROMETER. The pressure of the atmosphere
differs at different times and different places. To measure
it, an instrument called the Barometer is used.

The barometer was invented about the middle of the
seventeenth century. It was the result of a celebrated ex-
periment performed by Torricelli [to-re-chel'-le], the friend
and pupil of Galileo.

403. Torricellian Experiment. The Duke of Tuscany, having dug a well
of great depth, and tried to raise water from it with an ordinary pump, found
to his surprise that the water would not rise more than 32 feet. Galileo, to
whom the fact was referred, was unable to explain it ; but shortly before his
death he requested Torricelli to investigate the subject. Torricelli,
suspecting that the water was raised and supported by atmospheric
pressure, proceeded to test the truth of his opinion by experiment-
ing with a column of mercury. Mercury is nearly 14 times as heavy
as water; if, therefore, atmospheric pressure supported a column
of water 32 feet high, it would support a column of mercury only
about one-fourteenth of that height, or 28 inches. Accordingly, he
procured a tube 3 feet long, sealed at one end ; and having filled it
with mercury, and stopped the open end with his finger, he invert-
ed the tube in a vessel of mercury, as shown in Fig. 181. When he
removed his finger, the mercury fell, and finally settled, as he had
supposed it would, at a height of about 23 inches, leaving a vacuum
in the upper part of the tube. This is the famous Torricellian Vac-
uum.

Torricelli did not live to follow up his discovery ; but the French
philosopher, Pascal, succeeded him with a variety of ingenious ex-
periments. It occurred to Pascal that, if the columns of water and
mercury were supported by the pressure of the atmosphere, then
at great elevations, where this pressure would necessarily be less,
the height of the columns supported would also be less. He tried
the experiment on a mountain in Auvergne [o-viirn 1 ]. At the foot
of the mountain, the mercury stood at 28 inches ; at the top, it was
below 25 ; and at intervening distances it stood between the two.
This proved beyond doubt that the atmosphere exerted a pressure,
and that this pressure varied according to the distance above the
level of the sea. Perceiving how valuable such an instrument would be for

T. What is the Barometer? When was it invented? Of what was it the result?
40-. Jlelate the circumstances that first directed attention to the subject. Give an
account of Torricelll's experiment. What is meant by the Torricellian Vacuum ?
Who followed up Torrlcelli's discovery? Give an account of Pascal's experiment.



172



PNEUMATICS.



Fig. 182.



measuring heights, Pascal soon constructed a Barometer, consisting of a tube
and vessel of mercury so attached as to be conveniently carried.

409. Kinds of Barometers. There are several kinds
of barometers. The simplest consists of Torricelli's tube
and vessel of mercury, with a graduated scale attached to
the upper part. The mercury never rises above 31 inches,
and seldom falls below 27. The scale is therefore applied
only to that part of the tube which lies
between these limits.

The Wheel Barometer is exhibited
in Fig. 182.

Here the tube, instead of resting in a vessel of
mercury, is bent upward at its lower extremity.
A float, F, is supported by the mercury in the short
arm of the tube. To this float is attached a thread,
which passes over the pulley P, and is attached to
the ball W. When the mercury falls in the long
arm of the tube, it must rise in the short arm, and
with it rises the float F. The thread turns the
pulley P, and this moves the index I, which is so
arranged as to traverse the graduated scale S S.

410. TJie Barometer as a Weather-
guide. The barometer shows that the
pressure of the atmosphere at any
given place is different at different
times. This is because the air is con-
stantly varying in density, on account
of a greater or less intermixture of for-
eign substances. When the air is
densest, the mercury stands highest,
and we generally have clear weather ;
but, when the air is rarefied, the mer-
THE WHEEL BAROMETER. cui*y falls, and rain not unfrequently
follows. Hence, the barometer has been used for predict-




w



What did it prove ? 409. Of what does the simplest kind of barometer consist? To
what part of the tube is the scale confined, and why ? Describe the Wheel Barom-
etor, and its mode of operation. 410. What does the barometer show with respect to
the pressure of the atmosphere ? What occasions this difference ? When the air is
densest, what generally follows ? When it is rarefied, what follows ? In view of this,






THE BAROMETER. 173

ing changes of weather; and the words FAIR, CHANGE, RAIX,
are placed at different points on the scale, to indicate the
weather which may be expected when the mercury reaches
either of those levels.



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 16 of 42)