G. P. (George Payn) Quackenbos.

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

. (page 22 of 42)
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the piston, and the steam above is drawn off into the condenser and convert-
ed into water as before. While this action is going on, the cold-water pump

valves work. Describe the condensing apparatus. 570. How is the engine worked?



THE STEAM-ENGINE.



225



is constantly supplying the cistern in which the condenser is immersed ; while
the air-pump is drawing off the hot water from the condenser to the upper
reservoir, whence it is conveyed by the hot-water pump to the cistern that
supplies the boiler. An up-and-down motion is thus communicated to the
piston, and by it to the working-beam, which causes the fly to revolve, and
moves the machinery with which it is connected.

571. The Governor. The Governor, an ingenious piece
of mechanism, by which the throttle-valve in the steam-
pipe is opened and closed, and the supply of steam regu-
lated as the machinery requires, is worthy of further de-
scription.

The governor and its Fig. 223.

connection with the throt-
tle-valve are represented in
Fig. 223. It consists of two
heavy balls of iron, E, E,
suspended by metallic arms
from the point e. At e they
cross, forming a joint, and
are continued to/,/, where
they are attached by pivots
to other bars,/ h,f h. These
bars are joined to one end
of a lever, the other end of
which, H, is connected at
\V with the handle of the
valve Z. The spindle D D, to which the balls are attached, turns with the
fly-wheel. When the fly-wheel revolves very rapidly, the balls E E, under
the influence of the centrifugal force, fly out from the spindle, and with the
aid of the bars/A/^, pull down the end of the lever g. The other end, H,
is of course raised, and with it the handle of the valve Z, which is thus made
to close the mouth of the steam-pipe A and cut off the supply of steam. On
the other hand, when the motion of the fly diminishes, the centrifugal force
of the balls E E also diminishes, and they fall towards the spindle. The near-
er end of the lever g is thus raised, while the end H is depressed. The valve
Z is by this means opened, and admits a full supply of steam. The governor
thus acts almost with human intelligence, now admitting, and now cutting
off the steam, just as is required.

572. The Boiler. The boiler is made of thick wrought-
iron or copper plates, riveted as strongly as possible, so as
to resist the expansive force of the steam generated within.

How are the cisterns supplied ? 571. What is the Governor ? Describe the gov-
ernor, and ita connection with the throttle-valve. Bhow the workings of the gov
10*




THE GOVERNOR.



226 PYRONOMICS.

The fire is applied in an apartment beneath or within the
boiler called the Furnace.

Boilers are made of different shapes, but are generally
cylindrical, because this form is one of the strongest. Watt
made his concave on the bottom, in order to bring a greater
extent of surface in contact with the flame.

573. The Safety Valve. The pressure on the boiler, in
consequence of the expansive force of steam, is immense.
If it is allowed to become too great, the boiler bursts, often
with fatal effects. To prevent such catastrophes, a Safety
Valve is fixed in the upper part of the boiler, which is forced
open and allows some of the steam to escape whenever the
pressure exceeds a certain amount. A lever, with a weight
which slides to and fro on its arm, is attached to the valve ;
and the engineer, by placing the weight at different dis-
tances, can determine the amount of pressure which the
boiler shall sustain before the valve will open.

574. KINDS OF ENGINES. Engines are divided into two
kinds, Low Pressure and High Pressure.

In the Low Pressure Engine, one form of which has been
described above, the steam is carried off and condensed ;
while in the High Pressure Engine it is allowed to escape
into a chimney, and thence into the open air. The latter,
having no condensing apparatus, is much the simpler in its
construction. It is noisy when in operation, in consequence
of the puffing sound made by the steam as it escapes.

575. As regards their use, engines may be divided into
three classes ; Stationary Engines, employed in manufactur-
ing, Marine Engines, for propelling boats, and Locomotive
Engines, for drawing wheeled carriages.

576. THE LOCOMOTIVE ENGINE. The Locomotive is a
high pressure engine. The principle on which it works may
be understood from Fig. 224.



crnor. 5T2. Of what is the boiler made ? Where is the fire applied ? What is the
Usual shape of boilers ? What shape did Watt make his, and why ? 573. What is the
Use of the Safety Valve ? How is it worked ? 574. How are engines divided ? What
constitutes the difference between Low Pressure and High Pressure Engines ? Which
are the simpler? "Which are the more noisy, and why? 575. As regards their use,



THE LOCOMOTIVE ENGINE.

Fig. 224



227




The cylinder A in this engine is horizontal instead of vertical, and the pis-
ton works horizontally. B, the piston-rod, is connected by a crank, D, with
the axle E E of the wheels, F, F. The piston, moving alternately in and out
of the cylinder, with the aid of the crank causes the axle and wheels to re-
volve ; and the wheels, by their friction on the rails, move forward the en-
gine and whatever may be attached to it. The heavy line represents the
position of the parts when the piston is at the remote extremity of the cylin-
der ; the dotted line shows their position, when the piston has reached the
other end. Steam is first introduced on one side of the piston, and then on
the other, being allowed to escape as soon as it has done its work, that is,
driven the piston to the opposite extremity. The rest of the machinery con-
sists of arrangements for boiling the water, for regulating the admission of
steam into the cylinder and its discharge, for providing draught for the fire,
and for giving the driver the means of starting and stopping the engine, and
reversing the direction of its motion.

577. History. Watt seems to have been the first to
conceive the idea of propelling wheeled carriages by steam ;
but he was so engaged in perfecting the stationary engine
that he did not attempt to carry out his idea. "William
Murdoch, in 1784, first constructed a locomotive. Though
little more than a toy, it worked successfully, and travelled
so fast that on one occasion its inventor in vain tried to
keep pace with it.

Eighteen years passed before any use was made of Mur-
doch's invention ; at the end of that time, in 1802, Richard
Trevithick publicly exhibited a locomotive engine, so con-



into what three classes may engines be divided ? 576. With Fig. 224, show the prin-
ciple on which the locomotive engine works. What does the rest of the machinery
consist of? 577. Who first conceived the idea of the locomotive engine ? Who first
carried out the idea? What is said of Murdoch's engine? Who exhibited an im-



228 EXAMPLES FOB PKACTICE.

structed that it could be used for transporting cars. Im-
portant modifications and improvements have since been
made, for many of which the world is indebted to George
Stephenson, who shares with Trevithick the honor of this
great invention.

EXAMPLES FOE PRACTICE.

1. (See 510.) A joint of meat stands 2 feet from a fire, a fowl 4 feet; how-

does the heat which strikes the former compare with that received by
the latter?

2. How does the heat which my finger receives from the blaze of a candle,

when held an inch from it, compare with what it receives when held a
foot from it ?

3. If we were but one-fifth of our present distance from the sun, how many

times as much heat would we receive from it ?

4. The planet Neptune is about SO times as far from the sun as the earth is ;

how does its solar heat compare with ours ?

5. To receive a certain amount of heat from a fire, an object is placed 3 feet

from it ; to receive only one-fourth as much heat, how far from the fire
must it be placed ?

6. (See 526.) A quantity of water at the freezing-point measures 22 gallons ;

how much will it measure when its temperature has increased to the
boiling-point ?

7. I have a vessel which holds 46 gallons ; how much water at a temperature

of 32 must I put in it, to exactly fill the vessel when it boils ?

8. What will be the increase in measure of 18 gallons of alcohol, when raised

from 32 to 212 ? What will be the increase in weight ?

9. (See 554.) Under a pressure of one atmosphere, how many cubic inches

of steam will be generated from 2 cubic inches of water? From 10 cubic
inches of water ?

10. If 3,400 cubic feet of steam (under a pressure of one atmosphere) be con-
densed, how much water will it make ?

11. (See 555.) Under a pressure of two atmospheres, about how many cubic
inches of steam will two inches of water generate ? How many, under
a pressure of three atmospheres ?

12. About how many cubic inches of steam will be required, to raise 10 tons
10 feet high? If the steam were condensed, how many cubic inches of
water would it make ?

proved locomotive in 1S02 ? "Who subsequently made important improvements In
the locomotive?



OPTICS. 229




CHAPTER XIV.

OPTICS.

578. OPTICS is the science that treats of light and vision.

Mature of ILigfat.

579. Light is an agent, by the action of which upon the
eye we are enabled to see.

Light is imponderable ; for it moves with great velocity, and if it had
any weight, though it were ever so little, its striking force would be felt by
every object with which it comes in contact. Yet it does not affect even the
most sensitive balance.

580. "With respect to the nature of light, two theories
have been advanced, the Corpuscular and the Undulatory.

581. Corpuscular Theory. The Corpuscular Theory teaches that light
consists of extremely minute particles of matter, thrown off from luminous
bodies, which strike the eye and produce the sensation of light, just as par-
ticles thrown off by an odoriferous substance affect the organ of smell. This
theory, held as long ago as the days of Pythagoras, was received by New-
ton ; but, failing to account for many of the facts more recently discovered
in connection with light, it has now but few supporters.

582. Undulatory Theory. According to the Undulatory Theory, light is
produced by the undulations of an exceedingly subtile imponderable medi-
um, known as Ether, with which space is filled ; just as sound is produced by
the vibrations of air. A luminous object millions of miles away causes the
ether in contact with it to move in minute waves, like the surface of a pond
rippled by throwing in a stone. These undulations are transmitted with in-
conceivable rapidity, till they reach the eye, strike the sensitive membrane
that lines it, and produce the phenomena of vision. This theory, advanced
by Descartes [dd-karf], but first definitely laid down by Huygens, explains
most of the phenomena of optics, and is now generally received.

578. What Is Optics ? 579. What is Light ? How is it proved that light is impon-
derable? 580. What two theories have been advanced with respect to tihe nature ol
light ? 581. State the chief points of the Corpuscular Theory. By whom was it held ?
582. According to the Undulatory Theory, how is light produced ? By whom was tho
Undulatory Theory advanced ? Which of these theories is now generally received 1




230 OPTICS.

583. Rays. Rays are single lines of light, the smallest
distinct parts into which light can be resolved.

rig. 225. rig. 226. Fig. 227. R ays of light from the

same body either move in
parallel lines, as in Fig.
225 ; or diverge, that is, sep-
arate from each other, as in
Fig. 226 ; or converge, that
is, come together at a point called the Focus, as in Fig. 227.

A Beam of light is a collection of parallel rays.

A Pencil of light is a collection of rays not parallel.

A Diverging Pencil is a collection of diverging rays.

A Converging Pencil is a collection of converging rays.

Division of Bodies.

584. SELF-LUMINOUS AND NON-LUMINOUS BODIES. As
regards the production of light, bodies are divided into two
classes, Self-luminous and Non-luminous.

Self-luminous bodies are those which are seen by the
light that they themselves produce ; as, the sun, the stars,
a lighted candle.

Non-luminous bodies are those that produce no light of
their own, but are seen only by that of other bodies. The
moon is non-luminous, its light being borrowed from the
sun. The furniture in a dark room is non-luminous, being
invisible until the light of the sun, a lamp, or some other
luminous body, is admitted.

Many non-luminous bodies, when exposed to a heat of 977 F., become
incandescent, and grow brighter and brighter with every increase of temper-
ature beyond that point, till they reach a white heat. This is a striking proof
of the connection between light and heat.

585. TEANSPAKENT, TRANSLUCENT, AND OPAQUE BODIES.



583. What are Eays ? How may rays move ? What is a Beam of light ? What is a Pen-
cil of light ? What is a Diverging Pencil ? What is a Converging Pencil ? 584. As
regards the production of light, how are bodies divided ? What are Self-luminous
bodies? What are Non-luminous bodies? Give examples. What striking proof
have wo of the connection between light and heat ? 585. As regards the transmission



TBANSPAKENT AND OPAQUE BODIES. 231

As regards the transmission of light, bodies are divided
into three classes ; Transparent, Translucent, and Opaque.

Transparent bodies are such as allow light to pass freely
through them ; air, water, glass, are transparent.

Translucent bodies are such as allow light to pass through
them, but not freely ; ground glass, thin horn, paper, are
translucent.

Opaque bodies are such as do not allow light to pass
through them ; wood, stone, the metals, are opaque.

Transparent and opaque are relative terms. No substance transmits
light without intercepting some by the way. It is computed that the sun's
rays lose nearly one-fourth of their brilliancy by passing through the earth's
atmosphere ; and that, if this atmosphere extended fifteen times as far from
the surface as it now does, we should receive no light at all from the sun,
but should be plunged in perpetual night. On the other hand, an opaque
substance, if made very thin, may become transparent. Gold leaf, for in-
stance, held in the sun's rays, transmits a dull greenish light.

586. MEDIA. By a Medium (plural, media) is meant
any substance through which a body or agent moves in
passing from one point to another. Air is the medium in
which birds fly ; water, the medium in which fish swim ;
ether, the medium in which the planets move. In connec-
tion with light, any substance through which it passes is a
medium ; as air, water, glass, &c.

587. A Uniform Medium is one that is of the same
composition and density throughout.

Sources of ILIgSat.

588. The principal sources of light are nearly the same
as those of heat ; viz., the Sun and Stars, Chemical Action,
Mechanical Action, Electricity, and Phosphorescence.

Most of our artificial light is produced by chemical action, as exhibited in
the process of combustion (see 479). To this is due the light of lamps, can-

of light, how are bodies divided ? What arc Transparent bodies ? What are Trans-
lucent bodies ? What are Opaque bodies ? What is said of the terms transparent
and opaque f How much of their brilliancy do the sun's rays lose in passing through
the atmosphere ? What would be the consequence if the atmosphere extended fif-
teen times as far as at present ? How may an opaque substance be made transparent?
586. What is a Medium? Give examples. 58T. What is a Uniform Medium?
5S8. Name the principal sources of light. How is most of our artificial light pro-



232 OPTICS.

dies, gas, fires, &c. The mechanical action involved in percussion is also a
source of light. Sparks are produced when flint and steel are struck vio-
lently together. Lightning and the sparks given off from the electrical ma-
chine are examples of light produced by electricity. Phosphorescent light is
unaccompanied with heat. It is seen in decayed wood, fire-flies, glow-worms,
and certain marine animals. Vast tracts of ocean are sometimes rendered
luminous by myriads of phosphorescent creatures.

589. THE SUN AND STARS, SOURCES OF LIGHT. The sun
has already been mentioned ( 474) as the great natural
source of heat and light to the earth. Notwithstanding
the loss of some of its brightness in consequence of passing
through our atmosphere, its light is more intense than any
other with which we are acquainted. The most dazzling
artificial lights look like black specks, when held up be-
tween the eye and the sun, so much more brilliant is the
latter. It would require the concentrated brightness of
5,563 wax candles at the distance of a foot, to equal the
light which we receive from the sun at a distance of
95,000,000 miles.

The fixed stars are the suns of other systems. Like our
sun, they are self-luminous, and therefore sources of light,
though unimportant to us as such by reason of their great
distance. The light we get from Sirius, one of the bright-
est of the fixed stars, is only one twenty-thousand-millionth
of what we receive from the sun. When the sun shines,
the stars are invisible, their light being lost in his superior
brightness.

The light of some of the stars is so faint, that it is entirely absorbed by
the atmosphere before it reaches the eye of an observer at the level of the sea.
This is the reason why more stars are visible from the top of a mountain than
from its base.

590. The moon and planets are non-luminous, receiving from the sun the

duccd ? Give an example of light produced by mechanical action. Of light pro-
duced by electricity. "What is the peculiarity of phosphorescent light ? In what is
it seen ? 589. What is the great natural source of light to the earth ? How does the
sun's light compare with other lights with which we are acquainted ? Prove this.
To how many wax candles is the light received from the sun equal ? What are tha
fixed stars ? What renders them unimportant to us, as sources of light ? How does
the light of Sirius compare with that of the sun? Why are the stars invisible in tha
day-time ? Why can more stars be seen from the top of a mountain than from ita
base ? 590. "What heavenly bodies are non-luminous ? What follows with respect to






PROPAGATION OF LIGHT. 233



light with which they shine. This light, reflected to the earth, is much inferior
in brightness to that received directly from the sun. The latter body, for
example, gives us 800,000 times as much light as the moon.

Propagation of ILigSal.

591. DIRECTION. Light radiates from every point of a
luminous surface in every direction.

The flame of a candle can be seen by thousands of persons at once, be-
cause a ray from the flame meets the eye of each. Within the immense space
belonging to the solar system, there is no point at which an observer can be
placed without seeing the sun, provided no opaque body intervenes. From
the sun, therefore, and from every luminous body, an infinite number of rays
proceed.

592. In a uniform medium, light is propagated in
straight lines.

Look through a straight tube at the sun, and you see it ; not so, if you
look through a bent or curved tube. Place a book between your eye and a
gas-burner ; the latter is not visible, because, to reach your eye, the light from
it would have to deviate from a straight line. Darken a room, and admit a
sunbeam through a small hole in a shutter. Its* path, marked out by the
floating dust that it illuminates, is seen to be a straight line.

593. The rays proceeding in straight lines from different particles of a
luminous body cross at every point within the sphere of its illumination, but
without at all interfering with each other ; just as different forces may act
on an object, and each produce the same effect as if it acted alone. A dozen
candles will shine through a hole in the wall of a dark room, and each with
the same intensity and direction as if no other rays than its own traversed
the narrow passage.

594. VELOCITY. Light travels with the enormous ve-
locity of 192,000 miles in a second. While you count one,
it goes eight times round the earth ; it would take the swift-
est bird three weeks to fly once around it. Light traverses
the space between the sun and the earth in about 8 min-
utes ; a cannon-ball would be seventeen years in going the
same distance.



their light ? How does the moon's light compare with the sun's ? 591. What is the
law for the direction of radiated light? Show the truth of this law in the case of a
candle and the sun. 592. In a uniform medium, how is light propagated ? Prove
this by some familiar experiments. 593. What is said of the rays proceeding in
straight lines from different particles of a luminous body ? Illustrate this with can-
dles shining through a hole. 594. What is the velocity of light ? How does it com-
pare with that of the swiftest bird ? With that of a cannon-ball ? By whom was the



234 OPTICS.

The velocity of light was discovered accidentally, by Roemer, an eminent
Danish astronomer, when engaged in a series of observations on one of the
moons of the planet Jupiter. This moon, in a certain part of its path, be-
comes invisible to an observer on the earth, in consequence of getting be-
hind its planet. Knowing that the revolutions of the moon must be per-
formed in the same time, Roemer supposed that the intervals between these
invisible periods would of course be uniform. To his surprise, he found that
they differed a little every time ; increasing for six months (at the expiration
of which, the eclipse was sixteen minutes later than at first), and then de- 1 ,
creasing at the same rate for a similar period, till at the end of a year he
found the interval precisely the same as at first. The conclusion was inevi-
table. The discrepancy was caused by the difference in the earth's distance.
If the first observation was made when the earth was at that point of her
orbit which was nearest to Jupiter, six months afterwards she would be at
the most distant point; and the light from Jupiter's moon, to reach the ob-
server's eye, would have to travel the whole distance across the orbit (about
190,000,000 miles) farther than before. Here was the key to a grand discov-
ery. If light was sixteen minutes, or 960 seconds, in travelling 190,000,000
miles, it was easy to find how far it travelled in one second.

595. INTENSITY AT DIFFERENT DISTANCES. The intensi-
ty of light diminishes according to the square of the dis-
tance from the luminous body that produces it.

Let several objects be placed respectively 1 foot, 2 feet, 3 feet, &c., from a
luminous body ; they will then receive different degrees of light proportioned
to each other as 1, 1 / tt y 9 , &c. A planet twice as far from the sun as the
earth is, would receive from it only / 4 as much light ; one three times as far,
l / 9 as much ; one ten times as far, Vioo as much.

rj<r 228 59 G. This is illustrated with Fig. 228. A

square card placed at A, a distance of 1 foot
from the candle, receives from a given point in
the flame a certain amount of light. This same
light, if not intercepted at A, goes on to B at a
distance of 2 feet ; it there illuminates four
squares of the same size as the card, and has,
therefore, but one-fourth of its former intensity.
If allowed to proceed to C, 3 feet, it illuminates nine such squares, and has
but one-ninth of its original intensity, &c.

Slaaclow.

597. Light falling on an opaque body is intercepted,

velocity of light discovered ? State the facts and reasoning by which Eoenier arrived
at this discovery. 595. What is the law relating to the intensity of light at different
distances? Give examples. 596. Illustrate this law with Fig. 228. 597. What fs




SHADOWS. 235

The darkness thus produced behind the opaque body is
called its Shadow.

598. Shadows are not all equally dark. They may be more or less illu-
mined by reflected light or by rays from some luminous body that are not
intercepted. Thus, if there are two lighted candles in different parts of a
room, the shadow cast by either is less dark than if it were burning alone.
Again, the brighter the light that produces a shadow, the darker it appears
by contrast. Hence, to compare the intensity of different lights, observe the
shadows respectively cast at equal distances ; the one that throws the dark-
est shadow is the brightest light.

599. When the luminous body is larger than the opaque
body it shines on, the latter throws a shadow smaller than



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 22 of 42)