Richard Green Parker.

A school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of online

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Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 13 of 38)
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constructed vast aqueducts across valleys, at great expense, to con-
vey water over them. The moderns effect the same object by meana
of wooden, metallic, or stone pipes.

How arefoun- 503. Fountains are formed by water carried
tains formed? through natural or artificial ducts from a reser-
voir. The water will spout from the ducts to nearly the height
of the surface of the reservoir. (See par. 1456.)

504. In Fig. 76 a fountain is represented at i,
issuing from the reservoir, the height of which is
represented by a c. The jet at i will rise nearly
as high as c.

505. A simple method of making an artificial
fountain may be understood by Fig. 77. A
glass siphon a b c is immersed in a vessel of
water, and the air being exhausted from the
siphon, a jet will be produced at <z, proportioned
to the fineness of the bore and the length of the

[N. B. The force of this kind of artificial jet is in
great measure dependent on a pneumatic principle.]

Explain the
fountain by
Fig. 76.


506. HERO'S FOUNTAIN. The hydraulic instrument called
Hero's Fountain is an a'pparatus for projecting water by means
of the pressure of confined air.

Fig. 78 represents Hero's Fountain. It consists of two ves
rig. 78 se ^ s bth air-tight, and communicating by a

pipe, which, being inserted into the top of the
lower vessel, reaches nearly to the top of the
upper vessel, which is in two parts, the upper
part being filled with water, which descends in
a pipe, seen on the right in the figure to the
lower vessel, and, as it fills the lower vessel
condenses the air, forcing it up through the left-
hand pipe, and causing it to press on the sur-
face of the water in the lower part of the upper
vessel. The water in the upper vessel is thus
forced through the central pipe in a jet, to' a
height nearly as great as the length of the pipe on the right.
The supply of water is furnished in the upper part of the upper
vessel, which may always be kept full by any external supply.

a'mecMM -Water becomes a mechanical agent of
agent? great power by means of its weight, its

momentum and its fluidity. (See par. 1450.)

It is used as the moving power of presses, to raise portions of
itself, and to propel or turn wheels of different constructions, which,
being connected with machinery of various kinds, form mills and
other engines capable of exerting great force.

What is ^8. PNEUMATICS. Pneumatics treats of

Pneumatics? the mechanical properties and effects of air
and similar fluids, called elastic fluids and gases, or aeri-
form fluids. (See par. 1460.)

What is meant 509. Aeriform fluids are those which have the
Tyu an aeriform . ,.. ,, . ., . ,

fluid? form of air. Many of them are invisible,* or

* Gases are all invisible, except when colored, which happens only in a
few instances.


nearly so, and all of them perform very important operations in
the mateiial world. But, notwithstanding that they are in
most instances imperceptible to our sight, they are really
material, and possess all the essential properties of matter.
They possess, also, in an eminent degree, all the properties
which have been ascribed to liquids in general, besides others
by which th 3y are distinguished from liquids.

,-rr, . . J7 510. Elastic fluids are divided into two classes.

What is the

difference be- namely, permanent gases and vapors. The gases

tween a perma- cannot be converted into the liquid state by any
nent gas and , . . ., ,.,

a vapor ? known process ot art ;*but the vapors are readily

reduced to the liquid form either by pressure or
diminution of temperature. There is, however, no essential dif-
ference between the mechanical properties of both classes of fluids.

Wh ub' t **^" ^" s ^ e a * r w ^ cn we breathe, and. which
are embraced surrounds us, is the most familiar of all this class
in the science O f bodies, it is generally selected as the subject
of Pneumatics. But it must be premised that
the same laws, properties and effects, which belong to air, belong
in common, also, to all aeriform fluids or gaseous bodies.

512. There are two principal properties of air,"
What are the namely, gravity and elasticity. These are called
wo principal th^ principal properties of this class of bodies.
Because they are the means by which their pres-

gaseous bodies? ence and mechanical agency are especially ex-

What degree 513. Although the aeriform fluids all have

oj cohesive at- ^eight, t h e y appear to possess no cohesive at-
J rr

traction home


514. The great degree of elasticity possessed by all aeriform
fluids, renders them susceptible of compression and expansion to ail
almos^ unlimited extent. The repulsion of their particles canse
them to expand, while within certain limits they are easi'y com-

* Carbonic acid gas forms an exception to th4s remark. Water also is
the union of oxygen and hydrogen gas.



pressed. This materially affects the state of density and rarity
under which they are at times exhibited.*

,, , 515. It may here be stated, that all the laws

pertain to aeri- and properties of liquids (which have been de~

form bodies in scribed under the heads of Hydrostatics and

en Hydraulics) belong also to aeriform fluids.

The chemical properties of both liquids and fluids belong pecu-
fiarly to the science of Chemistry, and are not, therefore, considered
in this volume.

What is the ^' ^e air which we breathe is an elastic

air which we fluid, surrounding the earth, and extending
forty or fifty miles above its surface, and con-
stantly decreasing upwards in density.

. 517. It has already been stated that the air

air in its most near tne surface of the earth bears the weight of
condensed that which is above it. Being comprespod, there-
/orm, and for ^ ^ ^ we ^ nt of tnat aboye j t} it mugt exigt

in a condensed form near the surface of the
earth, while in the upper regions of the atmosphere, where
there is no pressure, it is highly rarefied. This condensation,
or pressure, is very similar to that of water at great depths' m
the sea.f

518. As the air diminishes in density upwards, it follows
that it must be more rare upon a hill than on a plain. In very
elevated situations it is so rare that it is scarcely fit for respir-
ation or breathing, and the expansion which takes place in the
more dense air contained within the body is often painful. It

* The terms '* rarefaction " and " condensation," and " rarefied " and " con
densed," must be clearly understood in this connexion. They are applied
respectively to tho expansion and compression of a body.

f The air is necessary to animal and vegetable life, and to combustion.
It is a very heterogeneous mixture, being filled with vapors of all kinds.
It consists, however, of two principal ingredients, cal'el oxygen ani

occasions distension, and sometimes causes the bursting, of the
smaller blood-vessels in the nose and ears. Besides, in such
situations we are more exposed both to heat and cold ; for,
though the atmosphere is itself transparent, its lower regions
abound with vapors and exhalations from the earth, which float
in it, and act in some degree as a covering, which preserves us
equally from the intensity of the sun's rays and from the
severity of the cold.

519. Besides the two principal properties, gravity * and elasticity,
the operations of which produce most of the phenomena of Pneu-
matics, it will be recollected that as air,. although an invisible is
yet a material substance, possessing all the common properties of
matter, it possesses also the common property of impenetrability
This will be illustrated by experiments.


Where is the **^. -^e pressure of the atmosphere caused
pressure of the by its weight is exerted on all substances, inter-

nally and externally, and it is a necessary conse-
J J ' J

What pressure

does a man of quence of its fluidity. The body of a man of
common stat- common stature has a surface of about 2000
from the square inches, whence the pressure, at 15 pounds

weight of the per square inch, will be 30,000 pounds. The
**"' reason why this immense weight is not felt is,

that the air within the body and its pores counterbalances the
weight of the external air. When the external pressure is arti-
ficially removed from any part, it is immediately felt by the
reaction of the internal air.

TPTT . -, , 521. Heat acts upon the minute particles

What effect

has heat upon of bodies and forces them asunder, in opposition

air and other to tne attraction of cohesion and of gravity ; it
elastic fluids ?

therefore exerts its power against both the attrac-

tion of gravitation and the attraction of cohesion. But as the
attraction of cohesion does not exist in aeriform fluids, the
expansive power of heat upon them has nothing to contend with

* It has been computed that the weight of the whole atmosphere is eqnal
to that of a globe of lead sixty miles in diameter, or to five thousand billions
of tous.


but gravity. Any increase of temperature, therefore, expands
an elastic fluid prodigiously, and a diminution of heat con-
denses it.

, tri , . JL 522. A column of air, having a base an inch

*\Vhat is the
weight of a squarej and reaching to the top of the atmo-

column of air sphere, weighs about fifteen pounds. This press-

ch? ure ' like the P ressure of li( l uids > is exertcd
equally in all directions.

What is meant 523. The elasticity of air and other aeriform

by tlie elasticity fl^g j s t h at property by which they are in-

of air and ' . . * , . '

other aeriform creased or diminished in extension, according as

fluids ? they are compressed.

What effect ' 524. This property exists in a much greater.
has an increase degree in air and other similar fluids than in any

tionof n r ensure other substance - In fact > Jt has no known limit
upon an aeri- for, when the pressure is removed from any per-
form body ? tion of air, it immediately expands to such a
degree that the smallest quantity will diffuse itself over an
indefinitely large space. And, on the contrary, when the press-
ure is increased, it will be compressed into indefinitely smal

What is Ma- 525. The elasticity or pressure of air and
riotte's Law ? a }j g ase s is in direct proportion to their dens-
ity ; or, what is the same thing, inversely proportional to
the space which the fluid occupies. This law, which was
discovered by MariottP, is called " Mariotte's Law"
This law may perhaps be better expressed in the following
language ; namely, the density of an . elastic fluid is i-ti
direct proportion to the pressure which it sustains.
IIow does 526. Air becomes a mechanical agent by

m T ecMM a means of its wei gH its elasticity, its inertia,
agent ? and its fluidity.

With what 527. The 'fluidity of air invests it, as it invests

power does %\\ o ther fluids, with the power of tranmittinf

TN K UMATICS . 1 4 '{*,

fluidity invest pressure. But it has already been shown, under
a fluid? the head of Hydrostatics, that fluidity is a neces-

sary consequence of the independent gravitation of the particle?
of a fluid. It may, therefore, be included among the effects of

528. The inertia of air is exhibited in the. resistance which it
opposes to motion, which has already been noticed under the head
}f Mechanics.* This is clearly seen in its effects upon foiling
bodies, as will be exemplified in the experiments with the air-pump

What is a 529. A Vacuum is a space from which aii

Vacuum ? an( j ever v O ther substance have been removed

530. The Torricellian vacuum was discovered
Vihat is the ,-,.. , i * i .1 * n t

most perfect D y Torncelh, and was obtained m the following

vacuum that manner : A tube, closed at one end, and about
thirty-two inches long, was filled with mercury ;

the open end was then covered with the finger, so
as to prevent the escape of the mercury, and the tube inverted
and plunged into a vessel of mercury. The finger was then
removed, and the mercury permitted to run out of the tube. It
was found, however, that the mercury still remained in the tube
to the height of about thirty inches, leaving a vacuum at the
top of about two inches. This vacuum, called from the dis-
joverer the Torricellian vacuum, is the most perfect that has
been discovered.!

* The fly, as it is called, in the mechanism of a clock by which the hours
are strucK, is an instance of the application of the inertia of the air in

t Torricelli was a pupil of the celebrated Galileo. The Grand Duke of
Tuscany having had a deep well dug, the workmen found that the water
would rise no higher than thirty-two feet. Galileo was applied to for an
explanation of the reason without success. Torricelli conceived the idea of
substituting mercury for water, arguing that if it was the pressure of the
atmosphere that would raise the water in the pump to the height of thirty-
two feet, that it would sustain a column of mercury only one-fourteenth as
hifh, or thirty inches only, on account of its greater specific gravity. He
therefore determined to test it by experiment. He accordingly filled a
linall glass tube, about four feet long, with mercury, and, stopping the
open end with his finger, he inverted it into a basin of mercury. On
removing his finger, the mercury immediately descended in the tube, and
rtood at the height of about thirty inches ; thus demonstrating the fact
that it was the pressure of the air on the surface of the mercury in the one
>ase, and of the water in the other, that sustain* ! the column of mercury
\n tlic tube, and of the water in the puuip.


531. As this is one of the most important discoveries of the
science of Pneumatics, it is thought to be deserving of a labored
explanation. The whole phenomenon is the result of the equilibrium
of fluids. The atmosphere, pressing by its weight (fifteen pounds
on every square inch) on the surface of the mercury in the vessel,
counterpoised the column of mercury in the tube when it was about
thirty inches high, showing thereby that a column of the atmo
sphere is equal in weight to a column of mercury of the same base,
having a height of thirty inches. Any increase or diminution in
the density of the air produces a corresponding alteration in its
weight, and, consequently, in its ability to sustain a longer or a
shorter column of mercury. Had water been used instead of mer-
cury, it would have required a height of about thirty-three feet to
counterpoise the weight of the atmospheric column. Other fluids
may be used, but the perpendicular height of the column of any
fluid, to counterpoise the weight of the atmosphere, must be as
much greater than that of mercury as the specific gravity of mercury
exceeds that of the fluid employed.

532. This discovery of Tomcelli led to the construction of the
Darometer,* for it was reasoned that if it was the weight of the
atmosphere which sustained the column of mercury, that on ascend- '
>ng any eminence the column of mercury would descend in pro-
portion to the elevation.

What is a Ba- 533. The Barometer is an instrument to
rometer? measure the weight of the atmosphere, and
thereby to indicate the variations of the weather, f

534. Fig. 83 represents a barometer. It ^79.
&?P la congists O f a long glass tu k ej . ^0^ thirty-
three inches in length, closed at the upper
end and filled with mercury. The tube is then in-
ver /ed in a cup or leather bag of mercury, on which
the pressure of the atmosphere is exerted. As the
tube is closed at the top, it is evident that the mercury
cannot descend in the tube without producing a vacuum.
The pressure of the atmosphere (which is capable of
supporting a column of mercury of about thirty inches
in height) prevents the descent of the mercury ; and

* Among those to whom the world is indebted for the invention of the
barometer and its applications in science,rnay be mentioned the names of
Descartes, Pascal, Morienue, and Boyle. The original idea is due to Torri-
telli's experiment.

t The word barometer is from the Greek, and signifies "a measure oftk<
weight" that is, of the atmosphere.



Fig. 80.

the instrument, thus constructed, becomes an implement for
ascertaining the weight of the atmosphere. As the air varies
in weight or pressure, it must, of course, influence the mercury
in the tube, which will rise or fall in exact proportion with the
pressure. When the air is thin and light, the pressure is less,
and the mercury will descend ; and, when the air is dense and
heavy the mercury will rise.* 1 At the side of the tube there
is a scale, marked inched and tenths of an inch, to note the rise
and fall of the mercury.

535. The barometer, as thus constructed, only required the
addition of an index and a weather-glass, as seen
in Fig. 80, tc give a fair and true announcement
of the state and weight of the atmosphere. The
instruments are now manufactured in several dif-
ferent forms. The different forms of the barometer
in general use are the common Mercurial Barom-
eter, the Diagonal, and the Wheel Barometer, all
of which are constructed with a column of mer-
cury. The Aneroid or Portable Barometer is a
new instrument, in which confined air is substi-
tuted for mercury. This is a convenient form of
the instrument for portable purposes. But the
principle is the same in all, and repeated observa-
tions during the ascent of the loftiest mountains
in Europe and America have confirmed the truth
of barometrical announcements ; for, by its indi-
cations, the respective heights of the acclivities in
high regions can now be ascertained by means of
this instrument better than by any other course,
with this advantage, too, that no proportionate
height need be known to ascertain the altitude.!

* The elasticity of the air causes an increase or diminution of its bulk,
according as it is affected by heat and cold; and this increase and diminu-
tion of bulk materially affect its specific gravity. The height of a column
of mercury that can be sustained by a column of the atmosphere must,
therefore, be affected by the state of the atmosphere.

t From the explanation whieh has now been given of the barometer, it


On hat ^^' ^ e P ressure f *^ e atmosphere on th<

principle is mercury, in the bag or cup of a barometer, being

the barometer exerted on the principle of the equilibrium of

fluids, must vary according to the situation in

which the barometer is placed. For this reason, it will be the
greatest in valleys and low situations, and least on the top of
high mountains. Hence the barometer is often used to ascer-
tain the height of mountains and other places above the level of
the sea.

Wlien is the ^37. The air is the heaviest in dry weather,

atmosphere and consequently the mercury will then rise

highest. In wet weather the dampness renders

will readily be seen that a column of any other fluid will answer as well as
mercury, provided the tube be extended in an inverse proportion to the
specific gravity of the fluid. But mercury is the most convenient, because
it requires the shortest tube.

In navigation the barometer has become an important element of
guidance, and a most interesting incident is recounted by Captain Basil
liall, indicative of its value in the open sea. While cruising off the coast
of South America, in the Medusa frigate, one day, when within the tropics
the commander of a brig in company was dining with him. After dinner
Vthe conversation turned on the natural phenomena of the region, when
.'Captain Hali's attention was accidentally directed to the barometer in the
*tate-room where they were seated, and, to his surprise, he observed it to
' evince violent and frequent alteration. His experience told him to expect
bad weather, and he mentioned it to his friend. His companion, however,
only laughed, for the day was splendid in the extreme, the sun was shining
with its utmost brilliance, and not a cloud specked the deep-blue sky
above. But Captain Hall was too uneasy to be satisfied with bare appear-
ances. He hurried his friend to his ship, and gave immediate directions
for shortening the top hamper of the frigate as speedily as possible. His
lieutenants and the men looked at hiir, in mute surprise, and one or two of
the former ventured to suggest the inucility of the proceeding. The cap-
tain, however, persevered. The sails were furled, the top-masts were
struck; -in short, everything thi?,t could oppose the wind was made as
snug as possible. His friend, on the contrary, stood in under every sail.

The wisdom of Captain Hall's proceedings was, however, speedily evi-
dent ; just, indeed, as he was beginning to doubt the accuracy of his in-
strument. For hardly had the necessary preparations been made, and
while his eye was ranging over the vessel to see if his instructions had been
obeyed, a dark hazy hue was seen to rise in the horizon, a leaden tint
rapidly overspread the sullen waves, and one of the most tremendous hur-
ricanes burst upon the vessels that ever seaman encountered on his ocean
home. The sails of the brig were immediately torn to ribbons, her masts
went by the board, and she was left a complete wreck on the tempestuous
surf which raged around her, while the frigate was driven wildly along at a
furious rate, and had to scud under bare poles across the wide Pacific, full
three thousand miles, before it could be said that she was in safety from the


the air less salubrious, and it appears, therefore, more heavy
then, although it is, in fact, much lighter.
A. what time ^38. The greatest depression of the barometer
of the day is occurs daily at about four o'clock, both in the morn-

'and lowest * n & an( ^ * n *k a ft em0011 5 an ^ ^8 highest elevation
state of the at about ten o'clock, morning and night. In sun*
barometer ? mer these extreme points are reached an hour or
two earlier in the morning, and as much later in the afternoon,

589. Rules have been proposed by which the changes of th
weather may be predicted by means of the barometer. Heno
the graduated edge of the instrument is marked with the words
"ram," "fair," "changeable." "frost," .&c. These expressions
are predicated on the assumption that the changes of the weathsr
may correctly be predicted by the absolute height of the mercury.-
But on this little reliance can be placed. The best authorities agree
that it is rather the change in the height on which the predications
must be made.

540. As the barometer is much used at the present day, it hag
been thought expedient to subjoin a few general and special rules,
from different authorities, by which some knowledge of the uses of
the instrument may be acquired.

541. General Rules by which Changes of the Weather may be prognost*

cated by means of the Barometer.*

(1.) Generally the rising of the mercury indicates the approach of fair

(2.) In sultry weather the fall of the mercury indicates coming thunder
In winter the rise of the mercury indicate? frost In frost, its fall indicates
thaw, and its rise indicates snow.

(3.) Whatever change of weather suddenly follows a change in the
barometer, may be expected to last but a short time. Thus, if fair weather
follow immediately the rise of the mercury, there will be very little of it,,
and, in the same way, if foul weather follow the fall of the mercury, it will
last but a short time.

'4.) If fair weather continue for several days, during which the mercury
continually falls, along succession of foul weather will probably ensue; and
again, if foul weather continue for several days, while the mercury con-
tinually rises, a long succession of fair weather will probably succeed.

(5.) A fluctuating and unsettled state in the mercurial column indicate*
changeable weather. Lardner, page 75, Pneumatics.

542. Special Rules ly which we may know the. Changes of the Weather ly

means of the Barometer^

(1.) The barometer is highest of all during a long frost, and it generally
rises with a north-west wind.

* These rules, says Dr. Lardner, from whose work they are extracted,
may to some extent be relied upon, but they are subject to some uncer-

t These rules are from a different authority. ^


(2 ) The barometer is lowest of all during a thaw which follows a long
frost, and it generally falls with a south or east wind.

Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 13 of 38)