G. P. (George Payn) Quackenbos.

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

. (page 17 of 42)
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411. The only reliable indications, however, afforded by the barometer
are cltanges in the level of the mercury. No regard should be paid to the
particular point at which it stands at any given time ; we should merely ask,
is it rising or falling ? The following rules generally hold good :

1. After much dry weather, if the mercury falls steadily, rain will ensue,

though it may not begin for several days. The longer it is in com-
ing, the longer it will last.

2. After much wet weather, if the mercury, standing below its medium

height, rises steadily, fine weather will ensue, though it may not be-
gin for several days. The longer it is in coming, the longer it will last.

3. A sudden fall of the barometer, in spring or fall, indicates wind ; in

Y_ery hot weather, a thunder-storm ; in winter, a change of wind, and
rain or snow according to the temperature.

4. Sudden changes of the mercury indicate violent changes of the weather,

but not permanent ones.

5. A rise of mercury in autumn, after much wet and windy weather, indi-

cates the approach of cold.

412. At sea, the barometer may be relied on with tole-
rable certainty, and it is therefore exceedingly useful to
navigators. Violent and frequent changes in the mercury
almost invariably precede a sudden storm. Warned in
time, the prudent mariner furls his sails, and thus escapes
the fury of the hurricane which would have proved fatal to
his craft had it struck her unprepared.

Dr. Arnott gives the following account of his preservation at sea through
the warning of the barometer : " It was in a southern latitude ; the sun
had just set with placid appearance, closing a beautiful afternoon ; and the
usual mirth of the evening watch was proceeding, when the captain's order
came to prepare with all haste for a storm : the barometer had begun to fall
with appalling rapidity. As yet the oldest sailors had not perceived even a
threatening in the sky, and were surprised at the extent and hurry of the
preparation ; but the required preparations were not completed, when a more
awful hurricane burst upon them than the most experienced had ever braved.

to what use has the barometer been applied ? 411. What are the only reliable indi-
cations afforded by the barometer? What does a stead}' fall of mercury in the ba-
rometer after much dry weather indicate ? What does a rise of mercury after much
wet weather indicate? What does a sudden fall indicate at the different seasons?
What do sudden changes indicate ? What does a rise of mercury in autumn indicate ?
412. What is said of the barometer at sea ? Relate the circumstances of Dr. Arnott's



174



PNEUMATICS.



Nothing could withstand it ; the sails, already furled and closely bound to
the yards, were riveu away in tatters ; even the bare yards and masts were
in great part disabled, and at one time the whole rigging had nearly fallen
by the board. In that awful night, but for the little tube of mercury which
had given the warning, neither the strength of the noble ship nor the skill
and energies of the commander could have saved one man to tell the tale."

413. DENSITY OF THE AIR AT DIFFERENT LEVELS. The
lowest parts of the atmosphere are the densest, as they



Fig. 183.




1. Highest Peak of the Himalayas.

2. Highest Peak of the Alps.
Q. Highest Peak of the Andes.

4. Mount Mitchell, N. Carolina.



have the greatest quanti-
ty of air pressing on them
from above.

414. At the level of
the sea, the pressure of
the atmosphere on every
square inch of surface is
15 pounds. The body of
a man of ordinary size has
a surface of about 2,000
square inches, and is there-
fore subjected to the enor-
mous pressure of 30,000
pounds. We do not feel
this pressure, because it is
counterbalanced by that
of the air within our
bodies.

415. The higher we go
above the level of the sea,
the less is the pressure of
the atmosphere and the
rarer the air. At an ele-

i 3 vation of 18 miles, the
mercury would fall to 1
inch, that is, the air
above that point is- so rare,



preservation at sea by means of the barometer. 413. What parts of the atmosphere
are densest, and why ? 414. How great is the pressure of the atmosphere at the level
of the sea ? How great is the pressure oil the body of a man of ordinary size ? Why



DENSITY OF AIR AT DIFFERENT LEVELS. 1*75

that a column of it 30 miles high weighs no more than an
equal column of mercury 1 inch in height.

The shading in Fig. 183 shows the gradual increase in the density of the
air as the surface of the earth is approached. The figures in the left margin
represent the height of the atmosphere in miles ; those on the right, the cor-
responding height, in inches, at which the mercury stands in the barometer.
On the top of Mount Mitchell and Mount Washington, the most elevated peaks
in the United States east of the Mississippi, somewhat over a mile high, it
stands at 24 inches ; on the highest peaks of the Himalayas and Andes, which
are about five miles high, at no more than 12.

41G. The rarity of the air is painfully felt by those who
ascend to great heights on mountains. The pressure of the
external air being diminished, that which is in the body
expands, the delicate blood-vessels burst, the skin cracks,
and blood issues from the nose and ears. Among the Andes,
the Indians are subject to a malady called veta, which is
caused by the rarity of the air. The head aches violently,
its veins are swollen, the extremities grow cold, and breath-
ing becomes difficult.

Effect of Meat on Air.

417. Air is rarefied by heat.

Throw some burning paper into a wine-glass, and before the flame goes
out place your hand over the top. The glass will be found to adhere to your
hand. This is because the heat rarefies the air within, and thus expels most
of it before the top is covered. The pressure of the external air, not being
counterbalanced by any pressure from within, fastens the glass and hand
together.

413. Cupping-glasses are made to draw on this principle. Incisions hav-
ing been made in the skin, the sides of the glass are moistened with alcohol,
and flame is applied. While the alcohol is burning, the glass is inverted on
the skin. The pressure of the air in the body, no longer counterbalanced by
4he external pressure, causes a flow of blood into the cup.

419. Heated air, being lighter than that which surrounds



do we not feel this pressure ? 415. What is said of the air, as we ascend above the
sea-level ? How would the mercury stand at a height of IS miles? What does Fig.
1S3 show ? How does the mercury in the barometer stand on the top of Mount
Mitchell? On the tops of the Himalayas? 416. What sensations are experienced
by persons who ascend to great heights on mountains ? Describe the symptoms of
the reta. 417. What is (he effect of heat on air? How may tho rarefaction of air
by heat be shoAvn ? 418. Explain the operation of cupping-glasses. 419. Why doe




176 PNEUMATICS.

it, ascends till it reaches a region of the atmosphere as rare
as itself.

This is the reason why smoke rises. So, when a fire is kindled in a grate,
a draft is produced in the chimney. The air near the fire is rarefied and as-
cends. A vacuum is thus formed for the instant ; cold air rushes in to fill it ;
this in turn is heated and rises, and thus there is a constant passage of hot
air up through the chimney.

To show the ascent of hot air, take a circular
* ' piece of paper, as represented in Fig. 184, and,

commencing at any point of the outer edge, as
A, cut in the direction of the dotted line. Sup-
port it from beneath at B on a piece of wire, and
it will hang down, resembling in shape the
threads of a cork-screw. If the paper thus sus-
pended be held over a hot stove, it will be carried
rapidly round by the ascending currents of heat-
ed air.

420. BALLOONS. By observing the rise of smoke, Ste-
phen and Joseph Montgolfier \mon-g ol-fe-af], paper-manu-
facturers in France, were led in 1782 to the invention of
balloons. The following year, they exhibited their invention
to the public.

An immense bag of linen lined with paper was prepared, and brought di-
rectly over a fire of chopped straw. In a few minutes, the balloon was filled
with rarefied air and released from its fastenings. It rose about a mile, re-
mained suspended ten minutes, and reached the ground a mile and a half
from the place of its ascent. The same year, two persons ascended to a
height of 3,000 feet in the basket of a smoke balloon, and came down in safety.

On the 1st of January, 1784, a successful ascent was
made in a balloon inflated with hydrogen. This gas is now
generally used for the purpose, on v acepun\ of its superior
buoyancy. Even when badly prepared, it has but one-sixth
of the weight of air, and is three times as light as Montgol-
fier's mixture of heated air and smoke.

421. Balloons have not as yet been turned to any practical use, from the
fact that they are completely at the mercy of the wind, no way of steering
them having been devised. A theory has lately been put forth, however,
that at a certain height of the atmosphere currents are always setting from

heated air rise? Explain how the kindling of afire causes a draft in a chimney. How
may the ascent of hot air be shown ? 420. By whom and when were balloons invent-
ed? Describe the Montgolfiers' balloon, and its ascent. When was the first success-
ful ascent made in a balloon inflated with hydrogen ? Why is hydrogen now used for



NAVIGATION OF THE AIK. 177

west to east; if this be so, a<"rial voyages may be made with tolerable cer-
tainty, at least in one direction. The theory in question has been in part
confirmed by a balloon voyage (the most remarkable on record) made July 1,
1859. Four persons started from St. Louis, and in ID hours, 40 minutes, land-
ed in Jefferson Co., N. Y., near Lake Ontario, having travelled about 1,000
miles, at a rate exceeding that of the fastest railroad train.

422. Long before the invention of balloons, attempts were made to navi-
gate the air. At different periods not long after the Christian era, adventur-
ous men launched themselves from the tops of high buildings, and with
different sorts of apparatus which they had prepared moved a short distance
through the air. Mechanical contrivances resembling wings were more than
once resorted to ; but several who tried them met with serious accidents,
and it was at last proved that wings sufficiently large to support a man in
the air would be too heavy for him to move.

Tlie

423. The Air-pump is an instrument Fig. iss.
used for removing the air .from a vessel

called a Receiver. Receivers are made of
glass, and are usually of the shape repre-
sented in Fig. 185.

424. INVENTION OF THE AIR-PUMP.
The air-pump was invented 1654 A. D., by
Otto Guericke [gz'-re-ka], burgomaster of
Magdeburg, Germany.

Guericke's first attempt to obtain a vacuum was made
with a barrel full of water. Having closed it tightly, he
applied a pump to the lower part and commenced drawing
off the water. Could he have done this and kept the air
out, a vacuum would have been formed ; but he had not
proceeded far, when the air from without began to force
its way with a loud noise through the seams of the barrel.
To remedy the difficulty, Guericke substituted a metallic
globe for his barrel of water, and the experiment was then
successful.
>

inflating balloons ? 421. Why have not balloons been turner! to practical use ? What
remarkable voyage has lately boon made? 422. Give an account of the early at-
tempts to navipate the air. 423. What is the Air-pump ? Of what are receivers made?
424. By whom and when was the air-pump invented ? Give an account of Guericke's
first attempt to obtain a vacuum, llow did he finally succeed? Describe Gue-

8*




ITS



PNEUMATICS.




MAGDEBURG
HEMISPHERES.



. 187.



Great improvements have been made on the rude air-pump employed by
Guericke; yet, imperfect as his instrument was, it produced .results of deep
interest to the learned men of that day. His most famous experiment was
performed before the Emperor of Germany and his court. Two hollow me-
Fig. 1S6. tallic hemispheres of great size were prepared, fitting each

other so closely as to form an air-tight globe. From this
globe the air was removed with the pump, and a stop-cock
prevented any new air from entering. Fifteen horses
were then harnessed to each hemisphere ; but their united
strength was unable to effect a separation, so tightly were
the two parts held together by atmospheric pressure. On
turning the stop-cock and readmitting the air, they fell
asunder by their own weight.

425. This experiment is often repeated at the present
day, on a small scale. The Magdeburg hemispheres, as they
are called from Guericke's native city, are represented in
Fig. 186. They are fixed to the plate of an air-pump, in-
stead of a receiver ; and on exhausting the air they are
pressed together so tightly that two men can not pull them
apart.

426. SINGLE-BARRELLED AIR-PUMP. A single-barrelled

air-pump is repre-
sented in Fig. 187.
A is a receiver with
its edge carefully
ground, resting on a
plate near the centre
of the stand. In
this plate there is a
hole leading into a
pipe beneath, which
connects the receiv-
er with the barrel B.
The lower part
of the barrel is rep-
resented as cut away
in the figure, in or-
der to show the interior. A piston is tightly fitted to it,
containing a valve opening upward, and connected with a

ricke's famous experiment before the Emperor of Germany. 425. Describe the ex-
periment with tho Magdeburg hemispheres. 426. Describe the single-barrelled air-




THE SIXGLE-BARP.ELLED AIR-PUMP.



THE AIR-PUMP.



179



handle, by which it may be worked up and down. At the
base of the barrel there is another valve, also opening up-
ward.

427. Operation. The plate having been carefully dusted and rubbed with
a little oil, the receiver is placed on it, and the piston is drawn up. A vac-
uum is thus formed in the lower part of the cylinder, and the air in the re-
ceiver, by reason of its elasticity, pushes up the lower valve and enters the
barrel. The piston is now in turn driven down ; the pressure at once closes
the lower valve, while the resistance of the air in the barrel opens the valve
in the piston. Through the latter the air passes out, and by the time the
piston has reached the bottom, it has all escaped. The piston is then again
raised, and the whole operation is repeated, a barrel-full of air being drawn
out from the receiver as often as the piston ascends, and expelled from the
barrel as it descends. At last the air in the receiver becomes so rare that it
has not sufficient elasticity to open the valve at the base of the barrel. After
this the exhaustion can not be carried any further. A perfect vacuum, there-
fore, is not produced ; but the air is rarefied to such a degree that we speak
of it as such.

428. DOUBLE-BAR- rig.m

RELLED AlR-PUMP.

The double-barrelled
air-pump (see Fig.
188) acts on the same
principle as the above,
but exhausts the air
more quickly in con-
sequence of having
two barrels and pis-
tons. A section of
the instrument is rep-
sented in Fig. 189, from which
understood.

A and B are the barrels, in which the pistons, C, D, work
up and down. Each piston is connected with a rack, E, F,
the teeth of which work in the cog-wheel G, turned by
the handle M. When C is raised, D is lowered; and
when C is lowered, D is raised. H I is the passage which

p'lmp, as represented in Fig. 1ST. Describe the interior of the barrel. 427. How
does the single-barrelled air-pump operate ? 428. How does the double-barrelled air-
pump differ from tho single-barrelled? Describe the operation of the double-bar-




THE DOUBLE-BARRELLED AIR-PUMP.



its working will be



readily



180



PNEUMATICS.




connects the bar-
rels with the re-
ceiver J. K is a
stop-cock by which
the connection may
be cut off. L is a
tube resting at one
end in a small ves-
sel of mercury, and
at the other con-
nected with the re-
ceiver. This tube
is called a barome-
ter gauge. As the air in the receiver is rarefied, the external
atmospheric pressure on the mercury in the vessel causes
it to rise in the tube ; .the degree of rarefaction is there-
fore shown by the position of the mercury.

429. EXPERIMENTS WITH THE AIR-PUMP. With the air-
pump and different pieces of apparatus which accompany
it, may be performed a variety of experiments, illustrating
the properties of air.

430. The Hand-glass. The Hand-glass (Fig. 190)
is a receiver open at both ends. Set the large end
on the plate of the air-pump, and place the hand
flat upon the top. As soon as the pump is worked,
the pressure of the atmosphere is felt. When the
air is exhausted, the hand can hardly be removed
from the glass ; on readmitting the air through a
stop-cock, it is raised without difficulty. The ex-
pansion of the air in the palm of the hand is shown
by the redness of the flesh, and its puffing out while
over the exhausted glass.
431. TJie Apple-cutter. The Apple-cutter (Fig. 191) is a metallic cylinder
with a sharp upper edge. An apple that fits it closely having been placed
on its top, the air is exhausted. The pressure of the atmosphere forces the
apple down on the sharp edge ; the middle part is cut out and falls inside of
the vessel.



Fig. 190.




THE HAND-GLASS.



relied air-pump, with the aid of Fig. 1S3. What is the use of the barometer g;mo;e 5
430. What is the Hand-glass ? Describe the experiment with the hand-glass. Wha:
causes the redness of the hand ? 431. What is the Apple-cutter ? Describe the ex



EXPERIMENTS WITH THE AIR-PUMP.



181




Fig. 191. 432. Tlie Bladder-glass. Over the large

end of the hand-glass tie a wet bladder, as
shown in Fig. 192. When the bladder has
become dry, place the open end on the plate,
and exhaust the air from the glass. The
pressure of the atmosphere, unsupported
from within, soon bursts the bladder with a
loud noise. If a piece of thin india rubber
be substituted for the bladder, it will be
THK APPLE- drawn in and distended, till it covers near-
CUTTER. \j the whole inside of the glass.
433. The Lungs-glass. The Lungs-glass (Fig. 193) illus-
trates the elasticity of air. It is a small glass globe with
a metallic stopper. Through this stopper passes a tube,
to the lower part of which a bladder is tied. The whole is
placed under a receiver, and the air exhausted. The air
in the bladder, communicating through the tube with the
receiver, is gradually rarefied. The air around it in the
Fig 194. glass, having no communication

with the receiver, remains of the
same density. Owing to its pres-
sure, the bladder becomes shrivelled



y. 192.




Fig. 193.





THE LUNGS-GLASS.



VACUUM FOUNTAIN.



when the receiver is exhausted ;
but, on the readmission of the air, it resumes its former
dimensions. This movement, regularly repeated, re-
sembles the action of the lungs in breathing, and hence
the name given to the apparatus.

434. Vacuum Fountain. Fig. 194 represents o
tall glass receiver, terminating at the bottom in a me/
tallic cap, through which a tube passes. This tube is
furnished with a stop-cock, and a screw, by means of
which it may be fastened to the plate of an air-pump.
A jet communicating with the tube rises into the re-
ceiver. Screw this appai atus to the plate of the pump,
exhaust the air, and close the stop-cock. Then un-
screw the whole, place the lower end of the tube in a
vessel of water, and open the stop-cock. The pres-
sure of the atmosphere will force the water up through
the tube and jet into the vacuum, forming a beautiful
miniature fountain.

Another mode of producing a vacuum fountain is
with the apparatus shown in Fig. 195. It consists of



periment with the apple-cuttor. 432. How is the experiment with tho bladder-glass
performed ? 433. What does the Lungs : glass illustrate ? What does it consist of? De-
scribe the experiment Why is the lungs-glass so called ? 434. What does Fig. 194 rep-
resent ? How is the vacuum fountain produced ? Describe another mode of producing



182



PNEUMATICS.




Fig. 195. a glass vessel with an air-tight stopper, through which a
tube extends almost to the bottom. The vessel, nearly filled
with water, is placed under a tall receiver, and the air ex-
hausted. The elasticity of the air within the vessel, not be-
ing counterbalanced by any pressure from without, forces the
water through the tube in the form of a fountain.

435. Bottle Imps. The bottle imps, described in 397,
. may be made to dance up and down in a jar of water in an
exhausted receiver. These figures are hollow ai d contain
air. When the receiver is exhausted, the pressure on the surface of the
water being removed, the air in the figures expands and drives out some of
the water. This diminishes their specific gravity, and causes them to
Fig. 196. r i sc - When the air is readmitted, the pressure is restored,

the air in the figures is compressed, water enters, their
specific gravity is increased, and they sink.

436. The Mercury Shower. On an open-mouthed re-
ceiver, D, place the cup A, in the bottom of which is a plug
of oak wood, B, projecting downward about two inches.
Put some mercury in A, and set the saucer C beneath the
oaken plug. Exhaust the air from D, and the mercury
will soon be forced by atmospheric pressure through the
pores of the oak, and fall into the saucer in a silvery
shower.

437. TJie Weight-lifter. This, is an apparatus with
which the pressure of the atmosphere is made to lift a

heavy weight (see Fig. 197). A is a cylinder attached to a frame, firmly sup-
ported by three legs. On the bottom of the cylinder rests a closely fitting
piston, to which the platform F is attached. A tube, B C, connects the in-
terior of the cylinder with the plate E of the pump D. When the air is ex-
hausted from A, the pressure of the atmosphere raises the piston, together
with the platform and its contents, the whole length of the cylinder. Atmos-
pheric pressure being 15 pounds to the square inch, the number of pounds
that can be lifted by a given cylinder may be found by multiplying its area
expressed in inches by 15.

438. It has been proposed to transmit mails between distant points, by
atmospheric pressure, on the principle of the weight-lifter. A strong me-
tallic tube, perfectly smooth on the inside, would have to be laid between
the places, and a piston tightly fitted to it. Large air-pumps, worked by
steam, would be placed at both ends of the tube. The mail being attached
to the piston at one end of the line, the pumps at the other would be set in
motion. A partial vacuum would be produced, and atmospheric pressure
would drive the piston through the tube at a rate estimated at 500 miles an

a vacuum fountain. 435. How may bottle imps be made to dance up and down in a
jar of water? Explain the principle. 436. How is the mercury shower produced?
437. "What is the Weight-lifter? Describe it, and its mode of operating. How many
pounds will a given cylinder lift ? 438. T6" what has it boon proposed to apply this




EXPERIMENTS WITH THE AIK-PUMP.

Fig. 197.



183




hour. Such is the theory ; whether it can be practically applied, remains to
be proved.

439. Vacuum Sell. This apparatus is intended
to show that air is essential to the production of
sound. A bell is so fixed under a receiver that it
can be rung by pushing down a sliding-rod which
passes through the top. When rung before the re-
ceiver is exhausted, the bell is distinctly heard;
but, when the air is withdrawn, it is almost inaudi-
ble. If a perfect vacuum could be produced, it would
not be heard at all.

440. Freezing Apparatus. Water may be frozen
in a vacuum, with the apparatus shown in Fig. 199.

Having placed the liquid
in a shallow vessel over a
basin containing strong
sulphuric acid, set the
whole under a receiver
and exhaust the air. Un-
der the diminished pres-

prindple ? Give the theory of the process. 439. What is the apparatus known as the
vacuum bell intended to show ? Describe the experiment. 4iO. Describe tho freez-



Fig. 199.





1 84 PNEUMATICS.

sure, the water is rapidly converted into vapor, which is as rapidly absorbed
by the acid. The continued evaporation cools the water to such a degree
that it is finally covered with ice.

441. Miscellaneous Experiments. In a vacuum, boiling commences at a
much lower temperature than in the air. This is shown by placing some
hot water under a receiver and exhausting the air. The pressure of the at-



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 17 of 42)