velocity of sound, than you would imagine. If the wind sets from us, it
bears most of the aerial waves away, and renders the sound fainter; but
it is not very considerably longer in reaching the ear, than if the wind
blew towards us. This uniform velocity of sound, enables us to determine
the distance of the object, from which it proceeds; as that of a vessel
at sea, firing a cannon, or that of a thunder cloud. If we do not hear
the thunder, till half a minute after we see the lightning, we conclude
the cloud to be at the distance of six miles and a half.
_Emily._ Pray, how is the sound of an echo produced?
_Mrs. B._ When the aerial vibrations meet with an obstacle, having a hard
and regular surface, such as a wall, or rock, they are reflected back to
the ear, and produce the same sound a second time; but the sound will
then appear to proceed, from the object by which it is reflected. If the
vibrations fall perpendicularly on the obstacle, they are reflected back
in the same line; if obliquely, the sound returns obliquely, in the
opposite direction, the angle of reflection being equal to the angle of
incidence.
_Caroline._ Oh, then, Emily, I now understand why the echo of my voice
behind our house is heard so much plainer by you than it is by me, when
we stand at the opposite ends of the gravel walk. My voice, or rather, I
should say, the vibrations of air it occasions, fall obliquely on the
wall of the house, and are reflected by it, to the opposite end of the
gravel walk.
_Emily._ Very true; and we have observed, that when we stand in the
middle of the walk, opposite the house, the echo returns to the person
who spoke.
_Mrs. B._ Speaking-trumpets, are constructed on the principle, that
sound is reflected. The voice, instead of being diffused in the open
air, is confined within the trumpet; and the vibrations which would
otherwise spread laterally, fall against the sides of the instrument,
and are reflected from the different points of incidence, so as to
combine with those vibrations which proceed straight forwards. The
vibrations are thus forced onwards, in the direction of the trumpet, so
as greatly to increase the sound, to a person situated in that
direction. Figure 7, plate 14, will give you a clearer idea, of the
speaking-trumpet; in this, lines are drawn to represent the manner, in
which we may imagine the sound to be reflected. There is a point in
front of the trumpet, F, which is denominated its focus, because the
sound is there more intense, than at any other spot. The trumpet used by
deaf persons, acts on the same principle; although it does not equally
increase the sound.
_Emily._ Are the trumpets used as musical instruments, also constructed
on this principle?
_Mrs. B._ So far as their form tends to increase the sound, they are;
but, as a musical instrument, the trumpet becomes itself the sonorous
body, which is made to vibrate by blowing into it, and communicates its
vibrations to the air.
I will attempt to give you, in a few words, some notion of the nature of
musical sounds, which, as you are fond of music, must be interesting to
you.
If a sonorous body be struck in such a manner, that its vibrations, are
all performed in regular times, the vibrations of the air, will
correspond with them; and striking in the same regular manner on the
drum of the ear, will produce the same uniform sensation, on the
auditory nerve, and excite the same uniform idea, in the mind; or, in
other words, we shall hear one musical tone.
But if the vibrations of the sonorous body, are irregular, there will
necessarily follow a confusion of aerial vibrations; for a second
vibration may commence, before the first is finished, meet it half way
on its return, interrupt it in its course, and produce harsh jarring
sounds, which are called _discords_.
_Emily._ But each set of these irregular vibrations, if repeated alone,
and at equal intervals, would, I suppose, produce a musical tone? It is
only their irregular interference, which occasions discord.
_Mrs. B._ Certainly. The quicker a sonorous body vibrates, the more
acute, or sharp, is the sound produced; and the slower the vibrations,
the more grave will be the note.
_Caroline._ But if I strike any one note of the piano-forte, repeatedly,
whether quickly or slowly, it always gives the same tone.
_Mrs. B._ Because the vibrations of the same string, at the same degree
of tension, are always of a similar duration. The quickness, or slowness
of the vibrations, relate to the single tones, not to the various sounds
which they may compose, by succeeding each other. Striking the note in
quick succession, produces a more frequent repetition of the tone, but
does not increase the velocity of the vibrations of the string.
The duration of the vibrations of strings, or wires, depends upon their
length, their thickness, or weight, and their degree of tension: thus,
you find, the low bass notes are produced by long, thick, loose strings;
and the high treble notes by short, small, and tight strings.
_Caroline._ Then, the different length, and size, of the strings of
musical instruments, serve to vary the duration of the vibrations, and
consequently, the acuteness or gravity of the notes?
_Mrs. B._ Yes. Among the variety of tones, there are some which, sounded
together, please the ear, producing what we call harmony, or concord.
This arises from the agreement of the vibrations of the two sonorous
bodies; so that some of the vibrations of each, strike upon the ear at
the same time. Thus, if the vibrations of two strings are performed in
equal times, the same tone is produced by both, and they are said to be
in unison.
_Emily._ Now, then, I understand why, when I tune my harp, in unison
with the piano-forte, I draw the strings tighter, if it is too low, or
loosen them, if it is too high a pitch: it is in order to bring them to
vibrate, in equal times, with the strings of the piano-forte.
_Mrs. B._ But concord, you know, is not confined to unison; for two
different tones, harmonize in a variety of cases. When the vibrations of
one string (or other sonorous body) vibrate in double the time of
another, the second vibration of the latter, will strike upon the ear,
at the same instant, as the first vibration of the former; and this is
the concord of an octave.
If the vibrations of two strings are as two to three, the second
vibration of the first, corresponds with the third vibration of the
latter, producing the harmony called, a fifth.
_Caroline._ So, then, when I strike the key-note with its fifth, I hear
every second vibration of one, and every third of the other, at the same
time?
_Mrs. B._ Yes; and the key-note, struck with the fourth, is likewise a
concord, because the vibrations, are as three to four. The vibrations of
a major third, with the key-note, are as four to five; and those of a
minor third, as five to six.
There are other tones, which, though they cannot be struck together
without producing discord, if struck successively, give us that
succession of pleasing sounds, which is called melody. Harmony, you
perceive, arises from the combined effect of two, or more concordant
sounds, while melody, is the result of certain simple sounds, which
succeed each other. Upon these general principles, the science of music
is founded; but, I am not sufficiently acquainted with it, to enter into
it any further.
We shall now, therefore, take leave of the subject of sound; and, at our
next interview, enter upon that of optics, in which we shall consider
the nature of light, vision, and colours.
Questions
1. (Pg. 146) What is wind, and how is it generally produced?
2. (Pg. 146) How do the winds blow, around the place where the air
becomes rarefied?
3. (Pg. 146) What effect is likely to be produced where the winds meet?
4. (Pg. 147) In what part of the globe is the air most rarefied, and
what is the consequence?
5. (Pg. 147) How do these winds change their direction as they approach
the equator?
6. (Pg. 147) How are the trade-winds produced, and how far do they
extend?
7. (Pg. 147) How is the equilibrium in the air restored?
8. (Pg. 148) How can contrary currents of air be shown in a room?
9. (Pg. 148) What causes this?
10. (Pg. 148) What is meant by a periodical wind?
11. (Pg. 148) What occasions the land and sea breezes, and where do they
prevail?
12. (Pg. 149) What are monsoons?
13. (Pg. 149) How do they change, and what is the cause?
14. (Pg. 149) What is meant by their breaking up, and what effect is in
general produced?
15. (Pg. 149) Why is the wind most variable in high latitudes?
16. (Pg. 150) Why is the wind apt to lessen about sunset?
17. (Pg. 150) What effect must the sun and moon produce upon the
atmosphere, from their attraction?
18. (Pg. 150) Why do not the aerial tides affect the barometer?
19. (Pg. 151) How is sound produced?
20. (Pg. 151) Does sound exist in the sonorous body, if not, what is it?
21. (Pg. 151) By what experiment might we prove that air is the
principal vehicle of sound?
22. (Pg. 152) What other bodies convey sound, and how can it be shown
that they do so?
23. (Pg. 152) What is meant by a sonorous body?
24. (Pg. 152) To what do they owe this property?
25. (Pg. 152) How is this explained by fig. 6, plate 14?
26. (Pg. 152) How is it illustrated by a stone thrown into water, and
how far does this illustration apply?
27. (Pg. 153) How are the vibrations propagated?
28. (Pg. 153) How can we prove that sound, does not travel as rapidly as
light?
29. (Pg. 153) At what rate is sound said to travel?
30. (Pg. 153) Is the velocity much influenced by the direction of the
wind?
31. (Pg. 153) How will sound enable us to judge of the distance of
objects?
32. (Pg. 154) How are echoes produced?
33. (Pg. 154) What is the operation and effect of the speaking-trumpet
(fig. 7, plate 14)?
34. (Pg. 155) How is a musical tone produced?
35. (Pg. 155) What occasions discords?
36. (Pg. 155) Upon what does the acuteness or gravity of a sound depend?
37. (Pg. 155) Does the force, with which a string is struck, affect the
rapidity of its vibrations?
38. (Pg. 155) How are the strings made to produce the high and low
notes?
39. (Pg. 155) What is meant by harmony, or concord, and how is it
produced?
40. (Pg. 156) When are strings said to be in unison?
41. (Pg. 156) How are octaves produced?
42. (Pg. 156) How are fifths produced?
43. (Pg. 156) How major and minor thirds?
44. (Pg. 156) What is meant by melody, and in what particular does it
differ from harmony?
[Illustration: PLATE XV.]
CONVERSATION XIV.
ON OPTICS.
OF LUMINOUS, TRANSPARENT, AND OPAQUE BODIES. OF THE RADIATION OF LIGHT.
OF SHADOWS. OF THE REFLECTION OF LIGHT. OPAQUE BODIES SEEN ONLY BY
REFLECTED LIGHT. VISION EXPLAINED. CAMERA OBSCURA. IMAGE OF OBJECTS ON
THE RETINA.
CAROLINE.
I long to begin our lesson to-day, Mrs. B., for I expect that it will be
very entertaining.
_Mrs. B._ _Optics is that branch of philosophy, which treats of the
nature and properties of light._ It is certainly one of the most
interesting branches of Natural Philosophy, but not one of the easiest
to understand; I must, therefore, beg that you will give me your
undivided attention.
I shall first inquire, whether you comprehend the meaning of a _luminous
body_, an _opaque body_, and a _transparent body_.
_Caroline._ A luminous body is one that shines; an opaque....
_Mrs. B._ Do not proceed to the second, until we have agreed upon the
definition of the first. All bodies that shine, are not luminous; for a
luminous body is one that shines by its own light; as the sun, the fire,
a candle, &c.
_Emily._ Polished metal then, when it shines with so much brilliancy, is
not a luminous body?
_Mrs. B._ No, for it would be dark, if it did not receive light from a
luminous body; it belongs, therefore, to the class of dark, as well as
of opaque bodies, which comprehends all such as are neither luminous,
nor will admit the light to pass through them.
_Emily._ And transparent bodies, are those which admit the light to pass
through them, such as glass and water.
_Mrs. B._ You are right. Transparent, or pellucid bodies, are frequently
called mediums, because they allow the rays of light to pass through
them; and the rays which pass through, are said to be transmitted by
them.
Light, when emanated from the sun, or any other luminous body, is
projected forward in straight lines, in every possible direction; so
that the luminous body, is not only the general centre, from whence all
the rays proceed; but every point of it, may be considered as a centre,
which radiates light in every direction. (Fig. 1, plate 15.)
_Emily._ But do not the rays which are projected in different
directions, and cross each other, interfere, and impede each other's
course?
_Mrs. B._ Not at all. The particles of light, are so extremely minute,
that they are never known to interfere with each other. A ray of light,
is a single line of light, projected from a luminous body; and a pencil
of rays, is a collection of rays, proceeding from any one point of a
luminous body, as fig. 2.
_Caroline._ Is light then a substance composed of particles, like other
bodies?
_Mrs. B._ That is a disputed point, upon which I cannot pretend to
decide. In some respects, light is obedient to the laws which govern
bodies; in others, it appears to be independent of them: thus, though
its course is guided by the laws of motion, it does not seem to be
influenced by those of gravity. It has never been discovered to have
weight, though a variety of interesting experiments have been made with
a view of ascertaining that point; but we are so ignorant of the
intimate nature of light, that an attempt to investigate it, would lead
us into a labyrinth of perplexity, if not of error; we shall, therefore,
confine our attention to those properties of light, which are well
ascertained.
Let us return to the examination of the effects of the radiation of
light, from a luminous body. Since the rays of light are projected in
straight lines, when they meet with an opaque body through which they
are unable to pass, they are stopped short in their course; for they
cannot move in a curve line round the body.
_Caroline._ No, certainly; for it would require some other force besides
that of projection, to produce motion in a curve line.
_Mrs. B._ The interruption of the rays of light, by the opaque body,
produces, therefore, darkness on the opposite side of it: and if this
darkness fall upon a wall, a sheet of paper, or any object whatever, it
forms a shadow.
_Emily._ A shadow, then, is nothing more than darkness produced by the
intervention of an opaque body, which prevents the rays of light from
reaching an object behind it.
_Caroline._ Why then are shadows of different degrees of darkness; for
I should have supposed, from your definition of a shadow, that it would
have been perfectly black?
_Mrs. B._ It frequently happens that a shadow is produced by an opaque
body, interrupting the course of the rays from one luminous body, while
light from another, reaches the space where the shadow is formed; in
which case, the shadow is proportionally fainter. This happens when the
opaque body is lighted by two candles: if you extinguish one of them,
the shadow will be both deeper, and more distinct.
_Caroline._ But yet it will not be perfectly dark.
_Mrs. B._ Because it is still slightly illuminated by light reflected
from the walls of the room, and other surrounding objects.
You must observe, also, that when a shadow is produced by the
interruption of rays from a single luminous body, the darkness is
proportioned to the intensity of the light.
_Emily._ I should have supposed the contrary; for as the light reflected
from surrounding objects on the shadow, must be in proportion to the
intensity of the light, the stronger the light, the more the shadow will
be illumined.
_Mrs. B._ Your remark is perfectly just; but as we have no means of
estimating the degrees of light, and of darkness, but by comparison, the
strongest light will appear to produce the deepest shadow. Hence a total
eclipse of the sun, occasions a more sensible darkness than midnight, as
it is immediately contrasted with the strong light of noonday.
_Caroline._ The reappearance of the sun, after an eclipse, must, by the
same contrast, appear remarkably brilliant.
_Mrs. B._ Certainly. There are several things to be observed, in regard
to the form, and extent, of shadows. If the luminous body A (fig. 3.) is
larger than the opaque body B, the shadow will gradually diminish in
size, till it terminates in a point.
_Caroline._ This is the case with the shadows of the earth, and the
moon; as the sun, which illumines them, is larger than either of those
bodies. And why is it not the case with the shadows of terrestrial
objects? Their shadows, far from diminishing, are always larger than the
object, and increase with the distance from it.
_Mrs. B._ In estimating the effect of shadows, we must consider the
dimensions of the luminous body; when the luminous body is less, than
the opaque body, the shadow will increase with the distance. This will
be best exemplified, by observing the shadow of an object lighted by a
candle.
_Emily._ I have often noticed, that the shadow of my figure, against the
wall, grows larger, as it is more distant from me, which is owing, no
doubt, to the candle that shines on me, being much smaller than myself.
_Mrs. B._ Yes. The shadow of a figure as A, (fig. 4.) varies in size,
according to the distance of the several surfaces B C D E, on which it
is described.
_Caroline._ I have observed, that two candles, produce two shadows from
the same object; whilst it would appear, from what you said, that they
should rather produce only half a shadow, that is to say, a very faint
one.
_Mrs. B._ The number of lights (in different directions) while it
decreases the intensity of the shadows, increases their number, which
always corresponds with that of the lights; for each light, makes the
opaque body cast a different shadow, as illustrated by fig. 5. which
represents a ball A, lighted by three candles, B, C, D; and you observe
the light B, produces the shadow _b_, the light C, the shadow _c_, and
the light D, the shadow _d_; but neither of these shadows will be very
dark, because the light of one candle only, is intercepted by the ball;
and the spot is still illuminated by the other two.
_Emily._ I think we now understand the nature of shadows very well; but
pray, what becomes of the rays of light, which opaque bodies arrest in
their course, and the interruption of which, is the occasion of shadows?
_Mrs. B._ Your question leads to a very important property of light,
_Reflection_. When rays of light encounter an opaque body, they cannot
pass through it, and part of them are absorbed by it, and part are
reflected, and rebound; just as an elastic ball rebounds, when struck
against a wall.
By reflection, we mean that the light is turned back again, through the
same medium which it had traversed in its first course.
_Emily._ And is light, in its reflection, governed by the same laws, as
solid, elastic bodies?
_Mrs. B._ Exactly. If a ray of light fall perpendicularly on an opaque
body, it is reflected back in the same line, towards the point whence it
proceeded. If it fall obliquely, it is reflected obliquely, but in the
opposite direction; the ray which falls upon the reflecting surface, is
called the incident ray, and that which leaves it, the reflected ray;
the angle of incidence, is always equal to the angle of reflection. You
recollect that law in mechanics?
_Emily._ Oh yes, perfectly.
_Mrs. B._ If you will shut the shutters, we will admit a ray of the
sun's light, through a very small aperture, and I can show you how it is
reflected. I now hold this mirror, so that the ray shall fall
perpendicularly upon it.
_Caroline._ I see the ray which falls upon the mirror, but not that
which is reflected by it.
_Mrs. B._ Because it is turned directly back again; and the ray of
incidence, and that of reflection, are confounded together, both being
in the same line, though in opposite directions.
_Emily._ The ray then, which appears to us single, is really double, and
is composed of the incident ray, proceeding to the mirror, and of the
reflected ray, returning from the mirror.
_Mrs. B._ Exactly so. We will now separate them, by holding the mirror
M, (fig. 6,) in such a manner, that the incident ray, A B, shall fall
obliquely upon it--you see the reflected ray, B C, is marching off in
another direction. If we draw a line from the point of incidence B,
perpendicularly, to the mirror, it will divide the angle of incidence,
from the angle of reflection, and you will see that they are equal.
_Emily._ Exactly; and now, that you hold the mirror, so that the ray
falls more obliquely upon it, it is also reflected more obliquely,
preserving the equality of the angles of incidence, and of reflection.
_Mrs. B._ It is by reflected rays only, that we see opaque objects.
Luminous bodies, send rays of light immediately to our eyes, but the
rays which they send to other bodies, are invisible to us, and are seen,
only when they are reflected by those bodies, to our eyes.
_Emily._ But have we not just seen the ray of light, in its passage from
the sun to the mirror, and its reflections? yet, in neither case, were
those rays in a direction to enter our eyes.
_Mrs. B._ What you saw, was the light reflected to your eyes, by small
particles of dust floating in the air, and on which the ray shone, in
its passage to, and from, the mirror.
_Caroline._ Yet I see the sun, shining on that house yonder, as clearly
as possible.
_Mrs. B._ Indeed you cannot see a single ray, which passes from the sun
to the house; you see, by the aid of those rays, which enter your eyes;
therefore, it is the rays which are reflected by the house, to you, and
not those which proceed directly from the sun, to the house, that render
the building visible to you.
_Caroline._ Why then does one side of the house appear to be in
sunshine, and the other in shade? for, if I cannot see the sun shine
upon it, the whole of the house should appear in the shade.
_Mrs. B._ That side of the house, which the sun shines upon, receives,
and reflects more light, and therefore, appears more luminous and vivid,
than the side which is in shadow; for the latter is illumined only, by
rays reflected upon it by other objects; these rays are, therefore,
twice reflected before they reach your sight; and as light is more, or
less, absorbed by the bodies it strikes upon, every time a ray is
reflected, its intensity is diminished.
_Caroline._ Still I cannot reconcile to myself, the idea that we do not
see the sun's rays shining on objects, but only those which such objects
reflect to us.
_Mrs. B._ I do not, however, despair of convincing you of it. Look at
that large sheet of water; can you tell why the sun appears to shine on
one part of it only?
_Caroline._ No, indeed; for the whole of it is equally exposed to the
sun. This partial brilliancy of water, has often excited my wonder; but
it has struck me more particularly by moonlight. I have frequently
observed a vivid streak of moonshine on the sea, while the rest of the
water remained in deep obscurity, and yet there was no apparent obstacle
to prevent the moon from shining equally on every part of the water.
_Mrs. B._ By moonlight the effect is more remarkable, on account of the
deep obscurity of the other parts of the water; while by the sun's
light, the effect is too strong for the eye to be able to observe it so
distinctly.
_Caroline._ But, if the sun really shines on every part of that sheet of
water, why does not every part of it, reflect rays to my eyes?
_Mrs. B._ The reflected rays, are not attracted out of their natural
course, by your eyes. The direction of a reflected ray, you know,
depends on that of the incident ray; the sun's rays, therefore, which
fall with various degrees of obliquity upon the water, are reflected in
directions equally various; some of these will meet your eyes, and you
will see them, but those which fall elsewhere, are invisible to you.
_Caroline._ The streak of sunshine, then, which we now see upon the
water, is composed of those rays which by their reflection, happen to
fall upon my eyes?
_Mrs. B._ Precisely.
_Emily._ But is that side of the house yonder, which appears to be in
shadow, really illuminated by the sun, and its rays reflected another
way?