Copyright
Richard Green Parker.

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

. (page 16 of 38)
Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 16 of 38)
Font size
QR-code for this ebook


more or less perturbation, according to the situation of the country,
the position of mountains, valleys, and a variety of other causes
Hence every climate must be liable to variable winds. The (fuality
of winds is affected by the countries over which they pass ? uud



172



NATURAL PHILOSOPHY.



ciiey fire sometimes rendered Destilential by the heat of deserts or
the imtrid exhalations of marshes /and lakes. Thus, from the
deserts of Africa, Arabia and the neighboring countries, a hot wind
blows, called Samiel, or Simoon, which sometimes produces instant
death. A similar wind blows from the desert of Sahara, upon the
western coast of Africa, called the Harmattan, producing a dryness
and heat which is almost insupportable, scorching like the blasts
of a furnace.

622. WHIRLWINDS AND WATERSPOUTS. The
How is wind
sometimes af- direction ot winds is sometimes influenced by the

form of lofty and precipitous mountains, which,
resisting their direct course, causes them to
descend with a spiral and whirling motion, and
with great force.

623. A similar effect is produced by two winds meeting at an
angle, and then turning upon a centre. If a cloud happen to be
between these two winds thus encountering each other, it will be
condensed and rapidly turned round, and all light substances will
be carried up into the air by the whirling motion thus produced.

What is sup- 624. The whirlwind, occurring at sea, occa



fected by the
face of a
country ?



to *e the



cause oj vater-
tpouts ? spout



iong ^ singular phenomenon of the water



Fig. 3U6.




ACOUSTICS. 173

What doe* ^25. Fie. 105 represents the several forma in

Fig. 105 rep- . .

resent * which water-spouti are sometimes seen.

626. From a dense cloud a cone descends in the form of a trumpet,
with the small end downwards. At the same time, the surface of
the sea under it is agitated and whirled round, the waters are con-
verted into vapor, and ascend with a spiral motion, till they unite
with the cone proceeding froin the cloud. Frequently, however,
they disperse before the junction is effected. Both columns diminish
towards their point of contact, where they are sometimes not more
than three or four feet in diameter. In the centre of the water-
spout there is generally a vacant space, in which none of the small
particles of water ascend. In this, as well as around the outer
edges of the water-spout, lurge drops of rain fall Water-spouts
sometimes disperse suddenly, and sometimes continue to move
rapidly over the surface of the sea, continuing sometimes in sight
for the space of a quarter of an hour When the wate^-spout
breaks, the water usually descends in the form of heavy rain. It is
proper here to observe that by some authorities the phenomena of
water-spouts are considered as due to electrical causes.

627. A notion has prevailed that water-spouts are dangerous to
shipping. It is true that small vessels incur a risk of being overset
if they carry much sail, because sudden gusts of wind, from all
points of the compass, are very common in the vicinity of water-
spouts ; but large vessels, under but a small spread of canvas,
encounter, as is said, but little danger.

628. Pneumatics forms a branch of physical science which has
been entirely created by modern discoveries. Galileo first demon-
strated that air possesses weight. His pupil, Torricelli, invented
the barometer; and Pascal, by observing the difference of the alti-
tudes of the mercurial column at the top and the foot of the Puy de
Dome, proved that the suspension of the mercury is caused by the
pressure -of the atmosphere. Otto Guericke, a citizen of Magde-
burg, invented the air-pump about the year 1654 ; and Boyle und
Manotte soon after detected, by its means, the principal mechanical
properties of atmospheric air. Analogous properties have been
proved to belong to all the other aeriform fluids. The problem of
determining the velocity of their vibrations was solved by Newton
and Euler, but more completely by Lagrange. The theoretical prin-
ciples relative to the pressure and motion of elastic fluids, from
which the practical formulae are deduced, were established by
Daniel Bernoulli in his Hydrodynamica (1738), but have bee&
rendered more general by Navier.

What is 629. ACOUSTICS. Acoustics is the -science

Acoustics? which treats of the nature and laws of sound
It includes the theory of musical concord or harmony.



'74 NATURAL PHILOSOPHY.

\Vhat is 630. Sound is the sensation produced in the
*>mnd? or g ans O f hearing by the vibrations or undulations
transmitted through the air around.*

631. If a bell be rung under an exhausted receiver, no sound can
be heard from it ; but when the air is admitted to surround the bell,
the vibrations immediately produce sound.

032. Again, if the experiments be made by enclosing the bell in
a small receiver, full of air, and placing that under another receiver,
from which the air can be withdrawn, though the bell, when struck,
must then produce sound, as usual, yet it will not be heard if the
outer receiver be well exhausted, and care be taken to prevent the
vibrations from being communicated through any solid part of the
apparatus, because there is no medium through which the vibrations
of the bell in the smaller receiver can be communicated to the ear.f

Why is a sound ^^' Bounds are louder when the air sur-
louder in cold rounding the sonorous body is dense than when
weather? ft j s m a rarefied state, and in general the

intensity of sound increases with the density of the medium
by which it is propagated.

634. For this reason the sound of a bell is louder in cold than
in warm weather ; and sound of any kind is transmitted to a
greater distance in cold, clear weather, than in a warm, sultry
day. On the top of mountains, where the air is rare, the human
voice can be heard only at the distance of a few rods ; and the
iring of a gun produces a sound scarcely louder than the crack-
ing of a whip.

What are So- 635. Sonorous bodies are those which pro-
norous bodies ? ^ uce c i earj distinct, regular, and durable
sounds, such as a bell, a drum, wind instruments, musical
strings and glasses. These vibrations can be communicated
to a distance not only through the air, but also through
liquids and soM bodies.

* " The sensation of sound is produced by the wave of air impinging on
the membrane of the ear-drum, exactly as the momentum of a wave of the
sea would strike the shore." [ Lardner.]

t In performing these experiments, the bell must be placed in such a man-
aer thac whatever supports it will rest on a soft cushion of wool, so as to
prevent the vibrations from being communicated to the plate of the air
putup, or any other of tUe solid parts of the apparatus.



ACOUSTICS. 175

To what do 636. Bodies owe their sonorous property

bodies owe their , ,,. , .., -r, ,, ,..

sonorous prop- io thelr elasticity. But, although it is un-

erties ? doubtedly the case that all sonorous bodies are

elastic, it is not to be inferred that all elastic bodies are
sonorous.

637. The vibrations of a sonorous body give a tremulous or un-
dulatory motion to the air or the medium by which it is surrounded,
similar to the motion communicated to smooth water when a stono
is thrown into it.

What are the 638. Sound ' l & communicated more rapidly
best conductors, and with greater power through solid bodies
than through the air, or fluids. It is conducted
by water about four times quicker than by air, and by solids
about twice as rapidly as by water.

639. If a person lay his head on a long piece of timber, he can
hear the scratch of a pen at the other end, wnile it could not be
heard through the air.

640. If the ear be placed against a long, dry brick wall, and a
person strike it once with a hammer, the sound will be heard tivice,
because the wall will convey it with greater rapidity than the air,
though each will brin^ it to the ear.

641. It is on the principle of the greater power of solid bodies ti
communicate sound that the instrument called the Stethoscope * is
constructed.

What is the 642. The Stethoscope is a perforated cylin-

Stethoscope, fe r O f li^ht, fine-drained wood, with a funnel-

and on what ' . . . ;

principle is it shaped extremity, which is applied externally to

constructed? fa cav ities of the body, to distinguish the
sounds within.

T YJ t ,7 643. By means of the stethoscope the phy-

of the stetho- sioian is enabled to form an opinion of the healthy
scope ? action of the lungs, and other organs to which the

ear cannot be directly applied.

* The word Stethoscope is derived from two Greek words, art &oS, the
breast, and <rxo7i:st, to examine, and is given to this instrument because 't
is applied to the breast of a person for the purpose of ascertaining the oon-
dition of the lungs and other internal organs. Dr. Webster suggests that
the term Pkonophorus, or Sound-conductor, would be a preferable name lor
fclie instrument.



176 NATUBAL PHILOSOPHY

With what rapidity 644. Sound passing through the air
does sound move? moveg at tne rate O f ;Q20 f eet j n a secorK J

of time ; and this rule applies to all kinds of sound, whether
loud or soft.*

What kind of 645. The softest whisper, tnerefore, flies as fast
sounds move as the loudest thunder ; and the force and direction
of the wind, although they affect the continuance
of a sound, have but slight effect on its velocity.

646. Were it not for this uniform velocity of all kinds of sound,
the music of a choir, or of an orchestra, at a short distance, woulc*
be but a strange confusion of discordant sounds ; for the different
instruments or voices, having different degrees of loudness, could not
simultaneously reach the ear.

647. The air is a better conductor of sound when it is humid than
when it is dry. A bell can be more distinctly hoard just before a
rain ; and sound is heard better in the night than in the day, because
the air is generally more damp in the night.

648. The distance to which sound may be heard depends upon
various circumstances, on which no definite calculations can be pre-
dicated. Volcanoes, among the Andes, in South America, have been
heard at the distance of three hundred miles ; naval engagements
have been heard two hundred; and even the watchword "All '5
toe//," pronounced by the unassisted human voice, has been heard
from Old to New Gibraltar, a distance of twelve miles. It is said
that, the cannon fired at the battle of Waterloo were heard at Dover.

649. A clear and frosty atmosphere is favorable to the trans-
mission of Sound, especially where the surface over which it passes is
smooth and level. Conversation in the polar regions has been carried
on between persons more than a mile apart. The cannon in naval
engagements in the English Channel have been heard in the centre
of England.

650. A blow struck under the water of the Lake of Geneva was
heard across the whole breadth of the lake, a distance of nine miles.
The earth itself is a good conductor of sound* The trampling of
horses can be heard at a great distance by putting the ear to the
ground, and the approach of railroad-cars can be ascertained when
very far off by applying the ear to the rail.

* The velocity of sound has sometimes been estimated as much as eleven
nundred and forty -two feet in a second. The state of the air must, h< wever,
be taken into consideration. The higher the temperature, the greater tha
velocity; and it has been ascertained that within certain limits the velocity
is increased about one foot for every degree that the thermometer rises. Ex-
periments made with a cannon at midnight by Arago, Gay Lassac, and
others, when the thermometer stood at 61, gave 1118.39 feet per second as
the velocity of sound. The rate stated in No. 644 will not therefore be far
from the truth. The experiments which gave a result >i leven hundred and
forty-two feet in a second were probably made when the weather was e*-
trmufcl warm



ACOUSTICS. 177

To what prao 651. This uniform velocity of sound enab es us

velocit l of h " to ascertain with some Degree of accuracy, the
sound applied? distance of an object from which it proceeds.

If, for instance, the flash of a gun at sea is seen a half of a minute
before the report is heard, the vessel must be at the distance of about
six miles.

652. In the same manner the distance of a thunder-cloud may
be estimated by counting the seconds thi.t intervene between the
flash of the lightning and the roaring of the thunder, and multiplying
them by 1120.

. 7 653, THE ACOUSTIC PARADOX. Sound, as has
i is me a ] reac i y 5 een s t a ted, is propagated by the undulations
Acoustic Vara-



of ^ ^ Now? ag these undu]ation8 or waves are

precisely analogous to the case of two series of waves
formed upon the surface of a liquid, there is a point where the
elevation of a wave, produced by one cause, will coincide with the
depression of another wave produced by another cause, and the con-
sequence will be neither elevation nor depression of the liquid.



Explain the ^' ^ ien ' therefore, two sounds are produced
acoustic para- in different places, there is a point between them
dox- where the undulations will counteract each other,

r.nd the two sounds may produce silence.

655. A simple illustration of this fact may be made with a
tuning-fork. If this instrument be put into vibration and held up tc
the ear and rapidly turned, the sound, instead of being continuous,
will appear to be pulsative or interrupted ; but, if slowly caused to
revolve at a distance from the ear, a position of the forks will be
found at which the sound will be inaudible.

656. A similar experiment may be made with the tuning-fork
iield over a cylindrical glass vessel. Another glass vessel of similai
kind being placed with its mouth at right angles to the first, no
sound will be heard ; but, if either cylinder be removed, the sound
will be distinctly audible in the other. The silence produced in this
way is due to the interference of the undulations.

This seeming paradox, when thus explained, like the paradox
mentioned under the heads of Hydrostatics and Pneumatics, and
another to be mentioned under the head of Optics, will be found
to be perfectly consistent with the laws of sound.
What is 657. An echo is produced by the vibrations of the
an echo? a i r meeting a hard and regular surface, such as a wall,
a rock, a mountain, and being reflected back to the ear, thus
producing the same sound a second and sometimes a third and
fourth time.



178 NATUKAL PHILOSOPHY.

Why are there ^58. For this reason, it is evident tbat no echo
no echoes sea can be heard at sea, or on an extensive plain,
where there are no objects to reflect the sound.

Bywhatlawis 659. Sound, as well as light and heat, is re-
sound reflected ? fleeted in obedience to the same law that has
already been stated in Mechanics namely, the angles of inci-
dence and of reflection are always equal.

660. It is only necessary, therefore, to know how sound strikes
against a reflecting surface to know how it will be reflected. It is
related of Dionysius, the tyrant of Sicily, that he had a dungeon
(called the ear of Dionysius) in which the roof was so constructed
as to collect the words and even the whispers of the prisoners con-
fined therein, and direct them along a hidden conductor to the place
where he sat to listen, and thus he became acquainted with the
most secret expressions of his unhappy victims.

What is said 661. Speaking-trumpets do not depend for

of speaking-

trumpets? . their efficiency upon the reflection of sound.

662. The voice, instead of being diffused in the open air, is con-
fined within the trumpet, and the vibrations imparted by the lips
to the column of air within the trumpet produce better waves in
the open air than the lips alone would be able to do. Speaking-
trumpets are chiefly used by naval officers to aid the voice, so that
the word of command may be heard above the sound of winds and
waves.

How is a hear- 663 - Hearing-trumpets, or the trumpets used
ing-trumpet by deaf persons, are also constructed on the same
e ' principle ; but as the voice enters the large end
of the trumpet instead of the small one, it is not so much con-
fined, nor so much increased.*

664. The musical instrument called the trumpet acts also on the
same principle with the speaking-trumpet, so far as its form, tends
to increase the sound.

665. The smooth and polished surface of the interior parts of
certain kinds of shells, particularly if they be spiral or undulating,

* In this connection the author cannot refrain from giving publicity to
the value of a pair of acoustic instruments worn by one of the members of
his family. They consist of two small hearing-trumpets of a peculiar con-
struction, connected by a slender spring with an adjusting slide, which,
passing over the head, keeps both trumpets in their place. They are con-
cealed from observation by the head-dress ? and enable the wearer to join
in conversation of ordlc&ry tone, from which without them she is wholly
debarred.



ACOUSTICS. |7''

dt them to collect ar4 reflect the various sounds which are taking
place in the vicinity. Hence the Cyprias, the Nautilus, and som-o
other shells, when held near the ear, give a continued sound, whici
resembles the roar of the distant ocean.

On what prin- 666. Sound, like light, after it has been reflect
wAis erin *- e( ^ ^ rom severa ^ surfaces may be collected into one
galleries cot,- point, as a focus, where it will be more audibk
<tructed? tnaa j n a ny other part; and on this principle

whispering-galleries may be constructed.

667. The famous whispering-gallery in the dome of St. Paul's
church, in London, is constructed on this principle. Persons at
rery remote parts of the building can carry on a conversation in a
soft whisper, which will be distinctly audible to one another, while
others in the building cannot hear it ; and the ticking of a watch
may be heard from side to side.

668. There is a church in the town of Newburyport, in Massa
chusetts, which, as was accidentally discovered, has the same prop-
erty as a whispering-gallery. Persons in opposite corners of the
building, by facing the wall, may carry on a conversation in the
softest whisper, unnoticed by others in any other part of the build
ing. It is the building which contains in its cemetery the remain*
of the distinguished preacher, Wh icefield.

What is an 669. ACOUSTIC TUBES. Sounds may be coi:
AcousticTube ? veyed to a much greater distance through contin-
uous tubes than through the open air. The tubes used to con-
rey sounds are called Acoustic Tubes. They are much used iu
public houses, stores, counting-rooms, &c., to convey communi-
cations from one room to another.

670. The quality of sound is affected by the furniture of a room,
particularly the softer kinds, such as cur tains, carpets, &c.; because
having little elasticity, they present surfaces unfavorable to vibni
tions.

671. For this reason, music always sounds better in rooms with
bare walls, without carpets, and without curtains. For the sanv
reason, a crowded audience increases the difficulty of speaking.

672. As a general rule, it may be stated that plane and smooth
tur faces reflect sound without dispersing it; convex surfaces disperse it.
ind concave surfaces collect' it.



How is the ^^' ~^ e soun( ^ ^ tne aumaa voice is pro
round of the duced by the vibration of two delicate membrane?

human voice s i tua ted at the top of the windpipe, between whicb
produced? . f

the air from the lungs passes.

8



180 NATURAL PHILOSOPHY. *

674. The tones are varied from grave to acute, by penmg 01
contracting the passage ; and. they are regulated by the muscles
belonging to the throat, by the tongue, and by the cheeks. The
management of the voice depends much upon cultivation ; and
although many persons can both speak and sing with ease, and with
great power, without much attention to its culture, yet it is found
that they who cultivate their voices by use acquire a degree of flexi-
bility and ease in its management, which, in a great measure, guj
plies the deficiency of nature.*

675. Ventriloquism t is the arc of speaking ir

, l - ? such a manner as to cause the voice to appear

to proceed from a distance.

676. The art of ventriloquism was not unknown to the ancients ,
and it is supposed by some author^ that the famous responses of the
oracles at Delphi, at Ephesus, <KC., were delivered by persons who
possessed this faculty. There is no doubt that many apparently
wonderful pieces of deception, which, in the days of superstition
and ignorance, were considered as little short of miracles, were per-
formed by means of ventriloquism. Thus houses have been made

* Dr. Rush's very valuable work on " The Philosophy of the Human
Voice " contains much valuable matter in relation to the human voice
Dr. Barber's " Grammar of Elocution," and the " Rhetorical Reader," by
the author of this volume, contain useful instructions in a practical form.
To the work of Dr. Rush both of the latter works are largely indebted.

f The word Ventriloquism literally means, " speaking from the belly," and
It is so defined in Chambers' Dictionary of Arts and Sciences. The ven-
triloquist, by a singular management of the voice, seems to have it in his
power " to throw his voice " in any direction, so that the sound shall appear
to proceed from that spot. The words are pronounced by the organs usu-
ally employed for that purpose, but in such a manner as to give little or no
motion to the lips, the organs chiefly concerned being those of the throat
and tongue. The variety of sounds which the human voice is capablu of
thus producing is altogether beyond common belief, and, indeed, is truly
surprising. Adepts in this art will mimic the voices of all ages and condi-
tions of human life, from the smallest infant to the tremulous voice of tot-
tering age, and from the intoxicated foreign beggar to the high-bred, arti-
ficial tones of the fashionable lady. Some will also imitate the warbling
of the nightingale, the loud tones of the whip-poor-will, and the scream of
the peacock, with equal truth and facility. Nor are these arts confined to
professed imitators ; for in many villages boys may be found who are in
f.he habit of imitating the brawling and spitting of cats in such a manner
as to deceive almost every hearer.

The human voice is also capable of imitating almost every inanimate
sound. Thus, the turning and occasional creaking of a grindstone, with
the rush of the water, the sawing of wood, the trundling and creaking
of a wheeltirrow, the drawing of corks, and the gurgling of the flow-
ing liquor, the sound of air rushing through a crevice on a wintry night
oad a great variety of other noises of the same kind, are imitated by the
voice so exactly as to deceive any hearer who does not know whence tbej
proceed



ACOUSTICS. 181

t<> appear haunted, voices have been heard from tomls, and the iead
have been made to appear to speak, to the great dismay of the
neighborhood, by means of this wonderful art.

Ventriloquism is, without doubt, in great measure the gift of
nature ; but many persons can, with a little practice, utter sounds
and pronounce words without opening the lips or moving the mus-
cles of the face ; and this appears to be the great secret of the
art.

How is the ^' ^ USICAL SOUNDS, OR HARMONY. The
soundofamu- sound produced by a musical string is caused by
steal ^string its vibrations ; and the height or depth of the
tone depends upon the rapidity of these vibra-
tions. Long strings vibrate with less rapidity than short ones;
and for this reason the low tones in a musical instrument pro-
ceed* from the long strings, and the high tones from the short
ones. That character of sound depending upon rapidity of vi-
bration is called pitch.

678. Fig. 106. AB represents a musical string.
Explain T f f r, p
Fig. 106. " tt b Fi g- m

drawn __ ___ G - ......

up to G-, its elas- ^-'"'. - - - - ^ '"" - -IT"^^

ticity will not on- x^ '^*- - _ ~



ly carry it back *x**>~7 - - - - & _________ l

again, but will *"**-*.. F - - - - ~,-'''*

*~-~ iy ____ - ~

give it a momen-

tum which will carry it to H, from whence it will successively
return to T, F, C, D, &c., until the resistance of the air entirely
destroys its motion.

, 679. The pitch ^of the sound produced by

the pitch of the strings depends upon their length, thickness,

tone of a string we ight, and degree of tension. The pitch oi



Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 16 of 38)