of the cylinder below. Thus, in the figure, the entrance on the
right hand of the sliding-valve is represented as being open, and
the steam follows in the direction of the arrows into thecyliiuw.
where its expansive force will move the piston P in the direc-
tion of the arrow. The steam or air on the other side of the
piston passes out in the opposite direction, and is conveyed by a
tube passing through C C into the open air.
The motion of the piston in the direction of the arrow causes
the lever N to close the sliding-vah e on the right, and open a
communication for the steam on the opposite side of the piston
P, where it drives the piston back towards the arrow, at the
same time affording a passage for the steam on the right of the
piston to pass into the open air.
Motion being thus given to the piston, it is communicated, by
means of the rod R and the beam G, to the cranks K K, which,
being connected with the axle of the wheel, causes it to turu,
and thus move the machine.
Thus constructed, and placed on a railroad, the locomotive
Bteam-engine is advantageously used as a substitute for horse
power, for drawing heavy loads.
The apparatus of safety-valves, and other appliances for the
management of the power produced by the machine, are the
same in principle, though differing in form, with those used in
other steam-engines; for a particular description of which, the
student is referred to practical treatises upon the subject.
770. THE STATIONARY STEAM-ENGINE.-
best form of This engine is generally a high-pressure or
the ma * J team ~ n ~ non-condensing engine, used to propel ma-
chinery in work-shops and factories. As it is
designed for a labor-saving machine, it is desirable to com-
bine simplicity arid economy with safety and durability in
its construction ; and that form of this engine is to be pre-
ferred which in the greatest degree unites these qualities.
Describe the Sf a- 771. The figure on page 207 represents
tionary Steam- Tufts' stationary steam-engine, with sections ol
the interior. Like the double-acting condens-
ing engine of Mr. Watt, desci ibed in Fig. 109, it is furnished
with a governor, by which the supply of steam is regulated
and, like the locomotive, Fig. 112, the cylinder, with its piston,
has a horizontal position. The steam is admitted into the valve-
box through an aperture at E, in the section, and from thence
passes Into the cylinder through a sliding-valve, alternately to
each side of the piston P, as is represented by the direction of
the arrows, the sliding-valve being moved by the rod V, commu-
nicating with an " eccentric " apparatus attached to the axis of
the fly-wheel. The direction of the current of steam to the
valve-box is represented by the arrow at I, and its passage out-
ward from the cylinder, after it has moved the piston, is seen at
0. In this engine there is no working-beam, as in Watt's
engine, Fig. 109, but the motion is communicated from the pis-
ton-rod to a crank connected wilh the fly-wheel, which, turning
the wheel, will move all machinery connected either with the
axle or the circumference of that wheel.
Fig. 115 represents the Locomotive Steam-engine in one of
its most perfect forms, as used on railways at the present day.
772. OPTICS. Optics is the science which
What is Optics? c ,. - f f , . . .
treats of light, of colors, and of vision.
How are all sub- 773. The science of Optics divides all sub
stances consid- stances into the following classes : namely
luminous, transparent, and translucent; re-
flecting, refracting, and opaque.
What are lumi- ^^- Luminous bodies are those which
nous bodies f 8 ^ ne ty their own light such as the sun,
the stars, a burning lamp, or a fire.
There are in addition to the above quite a number of truly
luminous bodies, but of slight importance so far as their light is
concerned, because of its faintness. Such are the fire-fly, glow-
worm, decaying moist wood, and to these may be added certaic
mineral substances, such as the diamond, \\ hich are luminous for
a time after exposure to bright sunlight
What are trans- 775. Transparent substances are those
sub- which allow light to pass through them
freely, so that objects can be distinctly seen
through them; as glass, water, air, &c.*
776. Translucent bodies are those which
lucenlTodllsT' P ermifc a portion of light to pass through
them, but render the object behind them in-
distinct ; as horn, oiled paper, colored glass, &c.
What are re- ^7. Reflecting substances are those which
fleeting sub- do not permit light to pass through them;
but throw it off in a direction more or less
oblique, according as it falls on the reflecting surface ; as
polished steel, looking-glasses, polished metal, &c.
778. Refracting substances are those which
What are re- , ,. , ,, . .
fracting sub- turn tne "&"* " fr m lts course m its passage
stances ? through them ; and opaque substances are
those which permit nc light to pass through them, as met-
als, wood, &c.
What is light? 779. It is not known what light is. Sir
What are the j saac Newton supposed it to consist of
two tncories re- * A .
spelling the na- exceedingly small particles, moving from
tur t oj ught? luminous bodies; others think that it con-
sists of the undulations of an elastic medium, which fills
all space, f These undulations (as is supposed) produce the
* No substance that exists on our earth is perfectly transparent, and light
must, therefore, necessarily be impaired in its passage through all transpa*
rent media, and the diminution it suffers will vary as the medium is more
or less transparent, and as the passage it makes is of greater or less langth.
The exact ratio in which light is diminished has not yet been determined ;
it is, however, an established fact, that even those bodies which approach
most nearly to perfect transparency become opaque when their thickness ia
+ These two theories of light are called respectively the corpuscular and
the undulatnry theory. By the former the reflection of light is supposed to
lake place in the same manner as the reflection of solid elastic bodies, as
has been explained under the head of Mechanics [see No. 1(55, pnge 49]
By the Litter the propagation of light takes place from every luminous
ouiut.by meacs of the undulatnry movements of the ether. On this hypotb-
212 NATURAL PHILOSOPHY.
sensafron of light to the eje, in the same manner as th
vibrations of the air produce the sensation of sound to the
ear. The opinions of philosophers at the present day art
inclining to the undulatory theory. (See par. 1476.)
What is a ray 780. A ray of light is a single line of
of hght / \ig\\t proceeding from a luminous body.
781. Rays of light are said to diverge
When are rays , , ,
said to diverge? wnen the J separate more Fig. ne.
widely as they proceed
from a luminous body.
Fig. 116 represents the
^rp a ig. ra y g Q jjgjjj. diverging as they proceed from the
luminous body, from F to D.
782. It will be seen by this figure that, as light is projected In
every direction, its intensity must decrease with the distance, and
this decrease is determined by a fixed law. The light received upon
any surface decreases as the square of the distance increases.
Thus, if a portion of light fall on a surface at the distance of two
feet from any luaiinary, a surface twice that distance will receive
only one-fourth as much light; at three times that distance, one-
ninth ; at four times the distance, one-sixteenth, c. Hence a per-
son can see to read at a short distance from a single lamp much
better than at twice the same distance with two lamps, &c.
When are rays 783. Rays of light are said to converge
of Hght said to , . , , , mi
converge? when they approach each other. The point
esis, the waves of light follow the general laws of the reflection of all
elastic fluids , and, accordingly, every wave from every point, when it im-
pinges on any resisting object so as to be reflected, forms a new wave in its
course back, having its centre as much on the other side of the obstacle as
the centre of the original wave was on this side. In the case of light the
centre of the original wave is, obviously, the luminous point. There is a
remarkable similarity, therefore, between the reflection of light, pud echo
or the reflection of sound. It has been shown, under the head of Acc-usMc^,
that when two waves meet under certain circumstances, the elevation oi
one wave exactly filling up the depression of another wave, produces who, 1
is? called the acoustic paradox, namely, two sounds producing silt-nee. It wiT
readily be seen that the same undulatory movements in Optics will produce
the same analogous effect ; or, in other words, that two ray* of lizht n>,i^
produce darkness ; and this may, with equal propriety, be termed the optical
paradox. T3ut a clear understa tiding of the principles involved in what
is called respectively the hydrostatic, pneumatic, acoust*'' nd optical para
dox, shows that there is no paradox at all, but that each ia the necessarj
result of oertai- fixed and determinate laws
:tt which converging rays meet is called F; K n?.
Fig. 117 represents con-
Erplain fig. verging rays of light> of
which the point F is the focus.
What is abeam 784. A beam of Fig ' 118 -
of light ; light consists of many
rays running in parallel lines.
Explain Fig. Fig> 118 repr esents a beam of light.
785. A pencil of rays is a collection of
What ts a pen- , . . . r r , T .
ci/ of rays? diverging or converging rays. [&* F.#,
116 and 117.]
786. Light proceeding from a luminous
In what dtrec- . , . . , . .
tion, and with body is projected forward m straight lines in
what rapidity, every possible direction. It moves with a
does light mo vet . ,. , 1-1, i 111
rapidity but little short of two hundred thou
sand miles in a second of time.
787. Every point of a luminous body ia
From what part , . , , . , , .
of a luminous a Centre 5 trom which light radiates in every
body does ligkt direction. Rays of light proceeding from
different bodies cross each other witliO'U
interfering. The rays of light which issue from terrestrial
bodies continually diverge, until they meet with a refract-
ing substance , out the rays of the sun diverge so little, on
account of the immense distance of that luminary, that they
are considered parallel.
What is a 788. A shadow is the darkness produced
shadow ? ky the intervention of an opaque body, which
prevents the rays of light from reaching an object behind
the opaque body.
Why are shad- 789 ' Sliaclows are of different degrees of
' oic of different darkness, because the light from other luml-
214 NATURAL PHILOSOPHY.
degrees oj dark- nous bodies reaches the spot where the
shadow is formed. Thus, if a shadow be
formed when two candles are burning in a room, that
shadow will be both deeper and darker if one of the can
dies be extinguished. The darkness of a shadow is propor-
tioned to the intensity of the light, when the shadow is
produced by the interruption of the rays froui a single
What produces 79 - As the de g ree of H g ht and darkness
the darkest can be estimated only by comparison, the
strongest light will appear to produce the
deepest shadow. Hence, a total eclipse of the sun occa-
sions a more sensible darkness than midnight, because it is
immediately contrasted with the strong light of day. Hence
also, by causing the shadow of a single object to be thrown
on a surface. as, for instance, the wall, from two or mor*
lights, we can tell which is the brightest light, because it
will cause the darkest shadow.
791. When a luminous body is larger than
What is the , , , , , f ,,
thape of the an opaque body, the shadow of the opaque
shadow of an body will gradually diminish in size till it
( ' ue terminates in a point. The form of the
shadow of a spherical body will be that of a cone.
Fig. 119. A repre-
Kxplain Fig. gents th(J ^ ^ B
the moon. The sun
being much larger than the moon,
causes it to cast a converging shadow,
which terminates at E.
792. When the luminous body is smaller than the
opaque body, the shadow of the opaque body will gradually
increase in size with the distance, without limit.
IK Fig. 120 the shadow * 12 -
of the- object A increases
ui size at the different dis-
tances B, C, D, E; or, in
other words, it constantly
793. When several luminous bodies shine upon the same
object, each one will produce a shadow.
What is it the Fig. 121 represents a ball A, illuminated by
object of Fig. the three can-
dles B, C, .and
D. The light B produces the
shadow 3, the light C the shadow
c, and the light D the shadow d ;
but, as the light from each of th^
candles shines upon all the shad-
ows except its own, the shadows
will be faint.
What becomes of
the light which
falls on an
When is light
said to be re-
794. When raya of light fall upon an
opaque body, part o.^ them are absorbed, and
part are reflected.
Light is said to be reflected when it is
thrown off from the body on which it falls ;
and it is reflected in the largest quantities
from the most highly polished surfaces. Thus, although
most substances reflect it in a degree, polished metals, look-
ing-glasses, or mirrors, &c., reflect it "in so perfect a man-
ner as to convey to our eyes, when situated in a proper
position to receive them, perfect images of whatever objects
shine on them, either by their own or by borrowed lio;ht.
795. That part of the science of Optics
which relates to reflected light is called
What is Catop-
Wnat is the fun- T96 > The laws of r^cted light are the
damenfal law of same as those of reflected motion. Thus,
atop no, when light falls perpendicularly on an
opaque body, it is reflected back in the same line towards
the point whence it proceeded. If it fall obliquely, it will
ta reflected obliquely in the opposite direction ; and in all
cases the angle of incidence will be equal to the angle of
reflection. This is the fundamental law of Catoptrics, or
797. The angles of incidence and reflection have already beer
described under the head of Mechanics [see explanation of
/<%. 10, No. 162] ; but, as all the phenomena of reflected light
depend upon the law stated above, and a clear idea of these
angles is necessary in order to understand the law, it is deemed
expedient to repeat in this connection the explanation already
An incident ray is a ray proceeding to or^ falling on any sur-
face ; and a reflected ray is the ray which proceeds frorr any
Fig. 122 is designed to show
the angles of incidence and of
reflection. In this figure, M
A M is a mirror, or reflecting surface. P is
a line perpendicular to the surface. I A rep-
resents an incident ray, falling on the mirror
in such a manner as to form, with the perpen-
dicular P, the angle I A P. This is called
the angle of incidence. The line R A is to
be drawn on the othor side of P A in such a manner as to have
the same inclination with P A as I A has : that is, the angle
K A P is equal to I A P. The line R A will then show the
course of the reflected ray ; and the angle RAP will be
fche angle of reflection.
From whatever surface a ray of light is reflected, whether it
be a plain surface, a convex surface, or a concave surface, this
law invariably prevails ; so that, if we notice the inclination of
any incident raj, and the situation of the perpendicular to the
surface on which it falls, we can always determine in what man-
ner or to what point it will be reflected. This law explains thg
reason why, when we are standing on one side of a mirror, we
can see the reflection of objects on the opposite side of the room,
but not those on the same side on which we are standing. It also
explains the reason why a person can see his whole figure in a
mirror not more than half of his height. It also accounts for
all the apparent peculiarities of the reflection of the different
kinds of mirrors.
How are lu- 798. Opaque bodies are seen only by re-
minous and fl ec t e d light. Luminous bodies are seen by
opaque bodies ' .
respectively the rays of light which they send directly to
seen ? Qur
What effect 799. All bodies absorb a portion of the light
ontlwinten- which they receive ; therefore the intensity of
sity of light? light is diminished every time that it is reflected.
What does 800. Every portion of a reflecting surface
^a reflecting reflects an entire image of the luminous body
surface reflect ? shining upon it.
T , r , , Whon the sun or the moon shines upon a
H- liy do we
not see many sheet of water, every portion of the surface reflects
images of the an entire image of the luminary; but, as the image
same thing n .
reflected by a can " e seen on v "J reflected rays, and as the
reflecting sur- angle of reflection is always equal to the angle of
incidence, the image from any point can be ~,een
only in the reflected ray prolonged.
Why do objects 801. Objects seen by moonlight appear faiutc;
tppear fainter than when seen by daylight, because the light by
' which they are seen has been twice reflected ; for,
the moon is not a luminous body, but its light is caused by thu
Hun shining upon it. This light, reflected from the moon u.nd
fulling upon any object, is again reflected by tba* object. If
218 NATURAL PHILOSOUiY.
Buffers, therefore, two reflections ; and since a portion is absorbed
by each surface that reflects it, the light must be proportion*-
illy fainter. In traversing the atmosphere, also, the rays Loth
of the sun and moon, suffer diminution ; for, although pure air
is a transparent medium, which transmits the rays of light
freely, it is generally surcharged with vapors and exhalations,
by which some portion of light is absorbed.
802. All objects are seen by means of the
When is an . J J
object invisi- rays of light emanating or reflected from them ;
ble - and therefore, when no light falls upon an
opaque body, it is invisible.
This is the reason why none but luminous bodies can be
seen in the dark. For the same reason, objects in the shade or
in a darkened room appear indistinct, while those which are
exposed to a strong light can be clearly seen. We see the
things around us, when the sun does not shine directly upon them,
solely by means of reflected light. Everything on which it
shines directly reflects a portion of its rays in all possible direc-
tions, and it is by means of this reflected light that we are
enabled to see the objects around us in the day-time which are
not in the direct rays of the sun. It may here also be remarked
that it is entirely owing to the reflection of the atmosphere that
the heavens appear bright in the day-time. If the atmosphere
had no reflective power, only that part would be luminous in
which the sun is placed ; and, on turning our back to the sun, the
whole heavens would appear as dark as in the night ; we should
have no twilight, but a sudden transition from the brightest
sunsb?ne to darkness immediately upon the setting of the sun.
803. When rays of light, proceeding from
liht enter an 7 object, enter a small aperture, they cross
a small aper- one another, and form an inverted image of the
object. This is a necessary consequence of the
law that light always moves in straight lines.
K.rj>/am ^04. Fig. 123 represents the rays from an object
ttV. 123 a c, entering an aperture. The ray from a passes
down through the aperture to d, and the ray from c
up to f>, and thus these rays, crossing at the aper- Fi Jv!3 -
ture, form an inverted image on the wall. The
room in which this experiment is made should be
darkened, and no light permitted to enter, except-
ing through the aperture. It then becomes a
805. These words signify a darkened chamber. In the future de
Bcription which will be given of the eye, it will be setn that the
camera obscura is constructed on the same principle as the eye. If a
convex lens be placed in the aperture, an inverted picture, not only
of a single object, but of the entire landscape, will be found on the
wall. A portable camera obscura is made by admitting the light
into a box of any gize, through a convex lens, which throws the
image upon an inclined mirror, from whence it is reflected upwards
to a plate of ground glass. In this manner a beautiful but dimin-
ished image of the landscape, or. of any group of objects, is present-
ed on the plate in an erect position.
What is the 806. The angle of vision is the angle formed
angle of a t the eye by two lines drawn from opposite
vision? f J
parts ot an object.
What is the 807. The angle C, in Fig. 124, repiesents the
"figures 124 an o^ e ^ v i s i n - The line A C, proceeding from
and 125 ? one extremity of the object, meets the line B C
from the opposite extrem- Fig. 124.
ity, and forms an angle G
at the eye ; this is the
angle of vision.
808. Fig. 125 represents
the different angles made
by the same object at dif-
ferent distances. From an inspection of
the figure, it is evident that the nearer
the object is to the eye, the wider must c
be the opening of the lines to admit the
extremities of the object, and, consequent- E B
ly, the larger the angle under which it is seen ; and, on the con
trary, that objects at a distance will form small angles of vision
Thug, in this figure, the three crosses F G, D E, and A B. are
all of the same size ; but A B, being the most distant, subtesdr
the smallest angle A C B, while D E and F G, being nearer to
the eye, situated at C, form respectively the larger angles DOE
809. The apparent size of an object depends upon
On what docs , . ., , ,. . .
the apparent tne size * tne an g le * vision. But we are accus-
size of an ob- tomed to correct, by experience, the fallacy of ap-
yect depend ? p earances j ari & t therefore, since we know that real
objects do not vary in size, but that the angles under which we
see them do vary with the distance, we are not deceived by the
variations in the appearance of objects.
Thus, a house at a distance appears absolutely smaller than the
window through which we look at it ; otherwise we could not see
it through the window ; but our knowledge of the real size of the
house prevents our alluding to its apparent magnitude. In Fig. 124
in will be seen that the several crosses, A B, D E, F G, and II I,
although very different in size, on account of their different distances,
subtend the same angle A C B ; they, therefore, all appear to the
eye to be of the same size, while, in Fig. 125, the three objects A B,
D E, and F G, although of the same absolute size, are seen at a dif-
ferent angle of vision, and they, therefore, will seem of different
sizes, appearing larger as they approach the eye.
It is to a correct observance of the angle of vision that the art of
perspective drawing is indebted for its accuracy.
When is an 810. When an object, at any distance, does
of not subtend an angle of more than two seconds
its distance ? O f a degree, it is invisible.
At the distance of four miles a man of common stature
will thus become invisible, because his height at that distance
will not subtend an angle of two seconds of a degree. The size
of the apparent diameter of the heavenly bodies is generally
stated by the angle which they subtend.
Wh 811. When the velocity of a moving body
tion imper- does not exceed twenty degrees in an hour, its
teptible / motion is imperceptible to the eye.
It is for this reason that the motion of the heavenly
todies is invisible, notwithstanding their immense velocity.
812. The real velocity of a body in motion round a point de-
pends on the spuee comprehended in a degree. The more dis-
OPTICS. C ^i
tun j the moving body from the centre, or, in other words, the
larger the circle which it has to describe, the larger will be the
813. In Fig. 126, if the man at A, and the ** 126 -
man at B, both start together, it is manifest
that A must move more rapidly than B, to
arrive at C at the same time that B reaches