G. P. (George Payn) Quackenbos.

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

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end of a long stick of timber, the scratch of a pin at the other end can be
distinctly heard, owing to the conducting power of the wood. An approaching
locomotive can be heard at a great distance by placing one's ear on the rails.
The American Indians knew by experience the facility with which solids
transmit sounds, and were in the habit of applying their ears to the earth
when they suspected the approach of an enemy, or wanted a more distinct
impression of any sound that attracted their attention.

723. The denser air is, the more readily it transmits sounds. On the tops
of high mountains, where, as we have already learned, the atmosphere is
rare, the human voice can be heard only a few rods off, and the report of a
musket sounds no louder than the snapping of a whip at the level of the sea.
On the other hand, the air in a diving-bell let down to the bottom of the sea,
which is condensed by the upward pressure of the water, transmits sound so
freely that those who descend can hardly speak to each other above their
breath ; conversation in an ordinary tone would pain the ear. Frosty air is
a much better conductor of sound than warm air. In the polar regions, con-
versation has been carried on by two persons a mile apart.

Still air of uniform density transmits sounds more freely than air which
is agitated by variable currents or contains strata of different density. This
is one reason why sounds are more distinctly heard by night than by clay.
Falling rain or snow interferes with the vibrations, and tends to make sounds
less distinct ; so, likewise, do contrary winds.

724. If the air were perfectly still and of uniform densi-
ty, sound transmitted through it would decrease in loud-
ness as the square of the distance from the vibrating body

we ordinarily hear, transmitted ? "What else may transmit sound-waves in the same
way ? 722. How do solid, liquid, and aeriform bodies compare, as conductors of
Bound ? Give a proof of the conducting power of water. State some facts illustrating
the facility with which solids conduct sound. 723. How do rare and dense air com-
pare, as conductors of sound ? Give examples. How does cold air compare with warm
in conducting power ? Under what circumstances does air transmit sound most free-
]y ? What is the effect of falling rain or snow ? 724. If the air were perfectly still



VELOCITY OF SOUND. 277

increased. The report of a cannon, for instance, would
seem only one-fourth as loud at a distance of 200 feet as at
a distance of 100 feet.

725. VELOCITY OF SOUND. Under ordinary circum-
stances, sound is transmitted through air with a velocity
0/1,120 feet in a second, which is at the rate of a mile in
about 4 seconds.

All sounds, whether loud or faint, high or low, are
transmitted by a given medium with equal rapidity. Were
it not so, there would be no such thing as harmony in mu-
sical performances, for the notes of the different instruments
would reach the ear at different intervals.

Sound, it will be observed, travels much more slowly than light. The
latter moves 192,000 miles while the former is going only 1,120 feet. The
difference in their velocities is perceptible even at short distances. If we
look at a man splitting wood a few rods off, we see the axe descend on the
log some time before we hear the noise of the blow. So, the report of a can-
non is not heard till after the flash is seen, the interval being long or short
according to its distance.

726. When the sound is accompanied with a flash, knowing the relative
velocity of sound "and light, we can calculate very nearly the distance from
which it comes. We have only to notice the number of seconds that elapse
after the flash is seen before the sound is heard, and multiplying this by
1,120, we get the distance in feet. The time which it takes the light to trav-
erse the given distance and reach the eye, is so small that it does not enter
into the calculation. For example, if a clap of thunder is heard 3 seconds
after the accompanying flash is seen, the cloud from which they proceed is 3
times 1,120 (or 3,360) feet distant. The sooner the report follows the flash,
the nearer the cloud.

727. Water transmits sound 4i times as rapidly as air ;
iron, 10 times; and different kinds of wood, from 11 to 17
tunes.

Place the ear at one end of a very long stick of timber, and let some one
strike the other end with a hammer. The wood conducts the sound to the
ear so much more quickly than the air that the blow is heard twice. So,

and of uniform density, what would be the law for tho loudness of a sound heard at
different distances ? Give an example. 725. What is the velocity of sound ? How
Is the velocity of sound affected by its loudness and pitch ? What proof have we of
this? How does the velocity of sound compare with that of light? Give some fa-
miliar instances showing their difference of velocity. 726. When the sound is accom-
panied with a flash, how may we calculate the distance from which it comes ? Give
an example. 727. With what velocity does water transmit sound, aa compared with



278 ACOUSTICS.

when a bell at the end of a long iron tube is struck, two sounds are heard at
the opposite extremity, the first conducted by the iron, the second by the
air within it.

728. DISTANCE TO WHICH SOUND is TRANSMITTED. So
many changes are constantly taking place in the atmos-
phere, in its temperature, moisture, density, and the veloc-
ity and direction of its currents, that no universal law can
be laid down as to the distance, at which sound is audible.
The human voice, when raised to its highest pitch and loud-
est tones, may be heard at the distance of an eighth of a
mile ; the report of a musket, at 5 miles.

Through the water, or in the atmosphere directly over it, sounds are trans-
mitted to a great distance. The ringing of a bell under water has been heard
across the whole breadth of Lake Geneva, not less than nine miles. The
" all's well " of the sentinel at Gibraltar has been distinguished twelve miles
off, and naval engagements have been heard at a distance of 200 miles. An
eruption of the volcano of St. Yincent has been heard at Demerara, 340 miles
off, the greatest distance on record to which sound has been transmitted by
the atmosphere.

729. ACOUSTIC TUBES. It is their dispersion in the sur-
rounding air that makes sounds finally inaudible. Hence,
when they are confined within tubes, they are carried to a
much greater distance. The slightest whisper has been
heard through an iron pipe 3,120 /eet (more than half a
mile) in length.

This fact has been turned to account in several ways. The voice is con-
veyed by speaking-tubes from one part of a building to another, frequently
to a considerable distance and by a circuitous route. The Stethoscope, an
instrument for examining the lungs and other internal organs, is an applica-
tion of the same principle. It is a hollow cylinder of wood with a funnel-
shaped extremity, which is placed on the organ to be examined while the ear
is applied to the other end. The sounds produced by the vital action within
are thus conveyed to the ear, and enable the experienced examiner to judge
whether the organ is in a healthy state.



air ? Iron ? Wood ? What experiments prove that solids conduct sound more rap-
idly than air ? 728. What makes it impossible to lay down a universal law as to the
distance at which sound is audible ? How far may the human voice be heard? The
report of a musket ? What instances are mentioned showing the great distance to
which sound is transmitted by water ? What is the greatest distance on record to
which sound has been transmitted by the atmosphere ? 729, What makes sounds
finally inaudible ? How may this difficulty be in a measure removed? How far has
a faint whisper been heard through a tube ? How has this principle been turned to



THE SPEAKKTG-TKUMFET. 279

730. The Speaking-trumpet. Even if the tube is short,
the more intense pulsation excited in a column of confined
air makes a given sound audible at a much greater distance
than if it is at once diffused in the atmosphere. This is
proved by the Speaking-trumpet, an instrument used by
seamen and others who wish to give additional power to
their voices. The narrowness of the tube prevents the easy
flow of the air which the voice sets in vibration. The or-
gans of articulation, therefore, operate on it with concen-
trated force, as they do on condensed air ; and, conse-
quently, when the vibrations escape from the tube, they
are propelled to a greater distance. A loud voice with a
speaking-trumpet 20 feet long, can be heard at a distance
of three miles. No one can use the speaking-trumpet long
without being exhausted, which shows that an unusual
effort has to be made with the voice.

731. INTERFERENCE OF SOUND. Two sets of vibrations
of equal intensity, meeting in such a way that the depres-
sions of one correspond with the elevations of the other,
interfere, or neutralize each other, and an interval of silence
is the result.

Cause a tuning-fork to vibrate and hold it over a cylindrical glass vessel.
Vibrations will soon be communicated to the glass, and a musical note will
be heard. Place a similar glass vessel at right angles to the first and oppo-
site the tuning-fork, and the note previously heard will cease. Withdraw it,
and the note is again heard. The vibrations of the first vessel produce the
sound, but are neutralized by those of the second.

732. REFLECTION OF SOUND. Vibrations striking a
plane surface are reflected from it (like light and heat) in
such a way as to make the angle of reflection equal to the
angle of incidence.

733. Echoes. When a sound is heard a second time by
reflection, after a certain interval, an Echo is said to be
produced. A sound is sometimes repeated more than once,

account ? What instrument is constructed on this principle ? Describe the Stetho-
scope, and its operation. 730. By whom is the Speaking-trumpet used ? Explain the
principle on which it operates. How far has a loud voice heen hoard with a speaking-
trumpet? 731. What is meant by the Interference of sound, and how is it caused?
Give'an example. 732. What is the law for the reflection of sound ? 733. What is an



280 ACOUSTICS.

according to the number of reflecting surfaces on which it
strikes. An echo near Milan repeats a single syllable thirty
times.

To be distinctly heard, the echo must not reach the ear till one-ninth of a
second after the original sound has ceased. Otherwise they will run together
and form one continuous sound. Hence, the reflecting surface must be a
certain distance from where the original sound is produced. The farther it
is off, the longer the reflected sounds will be in reaching the observer's ear,
and the more syllables will be repeated. At Woodstock, England, there is
an echo which repeats from 17 to 20 syllables ; in this case the reflecting sur-
face is distant about 2,300 feet. In mountainous regions echoes are quite
common. There are several remarkable ones among the Alps ; and the
mountaineers contrive to sing one of their national songs in such time that
the echo forms an agreeable accompaniment.

In ordinary rooms no echo is perceived, because the distance of the walla
is so small that th reflected sound is mingled with the original one ; but in
large halls, unless the principles of Acoustics are regarded, an unpleasant
echo follows the speaker's words and makes them confused and indistinct.

734. Ear-trumpets. Ear-trumpets, used by deaf per-
sons, concentrate and reflect to the interior membrane of
the ear, vibrations that strike it, and thus render audible
sounds that could not otherwise be heard. The principle
on which they operate will be understood from Fig. 263.

p jo . 2(53 The sounds enter the large end, and are united by

successive reflections at the small end, which is applied
to the ear. The outer part of the ear is itself of such a
shape as to collect the sound-waves that strike it and re-
flect them to the membrane within. To enable them to
hear more distinctly, we often see people putting up their
hands behind their ears, so as to form a concave reflcct-

TIIE EAR-TKUMPET. , . , ,

ing surtace ; in which case, the hand acts somewhat on
the principle of the ear-trumpet. Instinct teaches animals to prick up their
ears when they want to catch a sound more clearly.

Shells of a certain shape reflect from their inner surface the vibrations
that strike it from the external air, and hence the peculiar sound that is
heard when they are applied to the ear.

Echo ? In what case may a sound be repeated more than once ? How often does an
echo near Milan repeat a syllable ? What is essential to the distinctness of an echo ?
On what does the number of syllables repeated depend ? Give an account of the
echo at Woodstock, England. Where are echoes quite common ? What is said of
those in the Alps ? Why is there no echo in ordinary rooms ? 734. How is it that
Ear-trumpets render audible sounds that could not otherwise be heard ? What is
said of the outer part of the ear ? How is the hand made to act on the principle of
a speaking-trumpet ? Why do animals prick up their ears ? Explain the roaring of




WHISPERING GALLERIES. 281

735. Whispering Galleries. Sound reflected from curved
surfaces follows the same law as light and heat. Let two
large concave brass mirrors be placed opposite to each
other, as shown in Fig. 213 ; the ticking of a watch, or the
faintest whisper in the focus of one, is distinctly heard,
after two reflections, at the focus of the other, though in-
audible at any other point. Two persons with their backs
to each other can thus carry on a conversation, while those
between them are not aware that anything is being said.

An apartment in which such a reflection is produced by
the walls is called a Whispering Gallery. An oval form is
the best for such a gallery, because there are two points
within, to either of which all the vibrations^ produced at
the other are reflected at the same instant from every point
of the surrounding walls. The dome of St. Paul's Church,
London, and that of the Capitol at Washington, are exam-
ples of fine whispering galleries.

One of the most remarkable structures of this kind in ancient times was
" the ear of Dionysius", a dungeon so called from the tyrant of Syracuse, by
whom it was constructed. The walls and roof were so arranged that every
sound from within was reflected and conveyed to a neighboring apartment,
where the tyrant could ensconce himself and hear even the whispers of his
unsuspecting victims.

736. MUSICAL SOUNDS. Musical Sounds are produced
by regular vibrations, uniform in their duration and in-
tensity.

737. Loudness, Pitch, and Quality. In connection
with musical sounds, three things must be considered ; their
Loudness, their Pitch, and their Quality.

The Loudness of a musical sound depends on the extent
of the vibrations producing it. The greater the vibrations,
the louder is the sound.

The Pitch of a musical sound depends on the rapidity



shells. 735. What law does sound reflected from curved surfaces follow ? Illustrate
this law in the case of sounds reflected from two concave mirrors. What is a Whis-
pering Gallery? What is the best form for such a gallery, and why? What build-
ings contain whispering galleries? Give an account of "the ear of Dionysius 1 '.
736. How are Musical Sounds produced ? 737. What three things must be considered
In connection with musical sounds ? On what does the Loxidness of a musical sound



282 ACOUSTICS.

of the vibrations producing it. The more rapid the vibra-
tions, the higher is the pitch.

The slowest vibrations that produce audible musical sounds follow each
other at the rate of 8 in a second, and a very low note is the result. As the
vibrations become more rapid the pitch rises, till they recur at the rate of
24,000 in a second, when a very high note is produced. Beyond this the vi-
brations last so short a time that they no longer affect an ordinary ear, and
no musical sound is heard. t

The Quality of a musical sound depends on the nature
of the vibrating body. The human voice, the piano, and
the flute, may all produce a note of precisely the same
loudness and pitch, and yet we readily distinguish them
apart. The difference lies in their Quality.

738. All musical sounds are produced by the regular
vibrations either of solids or confined air. This gives rise
to a division of musical instruments into two classes :
Stringed Instruments, like the violin ; and Wind Instru-
ments, like the flute.

739. STRINGED INSTRUMENTS. The strings used in mu-
sical instruments are made of metal or cat-gut. They are
fastened at each end, and are set in vibration with the fin-
ger, as in the case of the harp, or by the stroke of a ham-
mer, as in the piano, or by drawing across them an instru-
ment made for the purpose, like the bow of a violin.

740. To produce notes of different pitch, two strings
must vibrate with different degrees of rapidity. That they
may do so, one must be longer than the other, or thicker,
or stretched more tightly.

The longer a string is, with a given thickness and tension, the more
slowly it vibrates and the graver its tone. The thicker a string is, with
a given length and tension, the more slowly it vibrates and the graver its
tone. The more tightly a string is stretched, with a given length and thick-
ness, the more rapidly it vibrates and the more acute its tone.

depend ? On what, its Pitch ? How rapidly do the vibrations that produce the low-
est audible musical sounds follow each other ? How rapidly, those that produce the
highest notes ? On what does the Quality of a musical sound depend ? Give an ex-
ample of difference in quality. 738. By what are all musical sounds produced ? How
are musical instruments, then, divided ? 739. Of what are the strings used in mu-
sical instruments made ? HOAV are they set in vibration ? 740. How are two strings
made to produce notes of different pitch ? State the three laws relating to the length,



WIND INSTRUMENTS. 283

Stringed instruments are tuned, that is, brought to their proper pitch,
by turning pegs to which the strings are attached. Changes in the condition
of the atmosphere affect the length and consequently the tone of the strings.

741. The music of the .(Eolian Harp is produced by the action of currents
of air on strings which are stretched between two small uprights two or three
feet apart. The most pleasing combinations of sounds sometimes proceed
from this simple instrument, commencing with a strain, soft and low, as if
wafted to the ear from a distance, then swelling as if it were coming nearer,
while other notes break forth, mingling with the first with indescribable
sweetness.

742. In the case of the drum, musical sounds are produced by the vibra-
tions of a tense membrane acting on the same principle as strings.

743. WIND INSTRUMENTS. In wind instruments, such as
the flute, the trumpet, &c., musical sounds are produced
by the vibrations of air confined within tubes. In tubes
of equal diameter, the pitch of the note differs according
to the length of the vibrating column ; the shorter the col-
umn, the higher or sharper the note.

There are two ways of producing notes of different pitch
with the same instrument : 1. By joining tubes of dif-
ferent length and diameter, as in the organ. 2. By having
but one tube and providing apertures in it at different in-
tervals, by uncovering which the air is allowed to escape,
and the internal vibrations are stopped at any desired point.
This is the arrangement in the flute.

A wind and a stringed instrument produce notes of the same pitch when
the column of air contained within the former vibrates with the same rapid-
ity as the string which produces the note of the latter.

744. The tubes of wind instruments may be open at both ends, or closed
at both ends, or open at one end and closed at the other. In the last case,
the note produced is twice as low as in either of the other cases, the length
of the tubes being the same.

745. Musical notes are produced with wind instruments by blowing into
one end, by causing a current of air to enter an aperture, or by making

thickness, and tension of strings. How are stringed instruments tuned? What
causes them to get out of tune ? 741. How are the sounds of the ^Eolian Harp pro-
duced ? Describe the music of this instrument. 742. How are musical sounds pro-
duced in the case of the drum ? 743. How are musical sounds produced in wind
instruments ? On what does the pitch of the note depend ? How many ways aro
there of producing notes of different pitch with the same wind instrument? Mention
them. When do a wind and a stringed instrument produce notes of the same pitch ?
"44. What is said respecting the openings of the tubes of wind instruments ?
"45. What three modes of producing musical notes with wind instruments are men-



284 ACOUSTICS.

such a current act on thin plates of metal or wood properly arranged
within.

746. A jet of hydrogen gas, ignited and made to pass through a glass tube
about an inch in diameter, produces sweet musical sounds, which may be
made soft or loud at pleasure by raising or lowering the tube. These sounds
are caused by vibrations excited in the confined air by the burning hydrogen.

747. The Organ. The grandest and most complicated
of wind instruments is the organ. It combines the tones
of almost every other wind instrument, in such a way that
they may be used singly or together at the pleasure of the
performer. An organ in Switzerland has tones so closely
resembling those of the human voice, that visitors who hear
it imagine they are listening to a full choir of singers. The
great organ at Haarlem, in Holland, which is the most cel-
ebrated one in the world, has no less than 5,000 pipes, as
the tubes of the organ are technically called.

The water-organ, or hydraulicon, was known more than two hundred
years before the Christian era. Its invention is attributed to Ctesibius, the
barber of Alexandria, already mentioned as the inventor of the lifting-pump.
Wind-organs appear to have been little known until the eighth century after
Christ, though perhaps invented some time before. We read that an instru-
ment of this kind was sent to King Pepin, of France, in the year 757, by the
Greek Emperor, Constantino.

748. THE GAMUT. Notes are said to be in unison when
the vibrations that produce them are performed in equal
times.

Two notes, one of which is produced by twice as many
vibrations as the other, are called Octaves. In passing
from a note to its octave, there are several intermediate
sounds, produced by intermediate numbers of vibrations,
each of which the ear recognizes as a distinct note. These
notes are distinguished by different names, as shown be-
low. Assuming the number of vibrations producing the
first to be 1, the relative number of vibrations producing



tioncd ? 746. How may musical notes bo produced with a jet of hydrogen gas ?
747. What is the grandest of wind instruments ? What are combined in the organ ?
What is said of an organ in Switzerland ? How many pipes has the great Haarlem
organ? How long ago was the water-organ known? By whom was it invented?
When do wind-organs appear to have first become known ? 748. When are notes
eaid to bo in unison t What is meant by Octaves ? Between a note and its octave,



THE GAMUT. 285

the other notes will be expressed by the fractions respec-
tively placed below them, the number of the eighth note
being, as already stated, double that of its octave.
Names of the notes, CDEFGABC

or, do re mi fa sol la si do

Pronounced, do ra, me fait, sole laJi se do

No. of vibrations, 1 f f J/- 2

These eight notes constitute the Gamut, or Diatonic Scale. The notes
of the next higher octave bear the same relations to each other^ but are pro-
duced by vibrations performed in half the time, and therefore twice as nu-
merous in each case. The notes of the next lower octave again bear the same
relations to each other, but their vibrations take twice the time, and are there-
fore only half as numerous. In other words, a given note of any octave is
produced by vibrations twice as rapid as the same note of the next octave
below, and only half as rapid as the same note of the next octave above.

749. HABMCOSTY. Some notes, reaching the ear simul-
taneously, produce an agreeable impression in consequence
of their vibrations' frequently coinciding, and constitute
what is called concord. Other notes, whose vibrations
rarely coincide, impress the ear unpleasantly and produce



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 27 of 42)