  Illustrated Capital W When the hands are brought quickly together, what is produced? They produce a shaking or vibration in the air. What is this vibration in the air called? It is called a wave in the air. What is this wave in the air like? It is like a wave in the water. When this wave in the air reaches the ear, what does it produce? It produces a sensation which is called sound. When a stone is dropped into the water, what does it produce? It produces waves in the water. How do these waves move? They move in every direction from the stone. Do they get smaller the farther they go? They do get smaller and smaller, until at last they disappear. Do the waves of sound in the air act in the same way as the waves in the water? They do act in the same way. What then becomes of the waves of sound? They get smaller and smaller the farther they go, until at last there is no wave left. When the wave in the air is large, what kind of sound is made? It makes a loud sound. As the wave gets smaller, does the sound grow fainter? Yes; the smaller the wave the fainter the sound, until at last both the wave and the sound die away. Where must we be to hear a loud sound? We must be near the place in which the sound is made. What kind of waves will make a faint sound? Small waves in the air make a faint sound. Will these waves extend as far away as larger ones? No; as all waves of sound decrease in size as they recede, the smallest ones will soonest disappear. Why will a loud sound be heard farther than a fainter sound? Because the waves in the air from the loud sound will extend farther away than those from the fainter one. How are sounds produced? Sounds are produced by striking solid bodies together. How else may sound be produced? By striking a solid body against a liquid, as a stick struck against the water. In what other way may sound be made? Sound may be made by causing a substance to move rapidly through the air. When a piece of wood attached to a string is swung rapidly around, what kind of a noise is made? It makes a humming noise. When lightning passes through the air, what noise is made? A noise which we call thunder. How can a murmur be produced? When sounds, too faint to be heard separately, are united, they produce a murmur. Where may we hear a murmur? We may hear a murmur from the brook, as its waves flow over the pebbles, and from the wind, as it passes through the trees. When a sound is heard but once, what is it called? It is called a noise. When sounds are made at irregular intervals, what are they called? They are called noises. When sounds are repeated at regular intervals, what do they become? They become musical sounds. Upon what do the high and low sounds in music depend? They depend upon the rapidity with which the waves are produced in the air. When the waves are repeated rapidly, what sound is made? They make a high or acute sound. When they are repeated more slowly, what sound is made? They make a low or grave sound. How small a number of vibrations or waves in the air can be heard? Sixteen vibrations in a second of time, will produce the gravest sound that can be heard. How large a number of vibrations in the air can be heard? Twenty-four thousand vibrations in a second of time, make the most acute sound that can be heard. How is the human voice made? It is made by means of the vocal chords at the upper part of the larynx. What is the larynx? The upper part of the windpipe is called the larynx. How do these chords produce the voice? When the air is forced out of the lungs, it causes these chords to vibrate, so as to produce the voice. Are these chords the same in all persons? No; they are larger and coarser in some persons than in others. In whom are they larger and coarser? They are usually larger and coarser in men, and they are smaller and finer in women. What kind of a voice is made by the large and coarse chords? They make a rough, coarse voice. What kind of voice is made by the finer chords? They make a finer and more delicate voice. Do these chords vibrate so rapidly in man as in woman? No; they do not vibrate so rapidly in man as in woman. What kind of voice do rapid vibrations make? They make an acute voice; hence, woman can, when singing, raise her voice to a very high pitch. Where is the sensation of sound produced? The sensation of sound is produced on the ear. Do people vary in their ability to hear? Yes; they vary very much in their ability to hear. Is every person able to distinguish musical sounds? No; some persons cannot tell one musical note from another, and some cannot even tell one bird from another by its notes. Is the hearing of all animals alike? No; some can hear more acutely than others; a lion has more acute hearing than a man. What other animals have acute hearing? All such animals as hares, rabbits, and deer. Of what use is the acute hearing to these animals? It enables them quickly to hear the approach of their enemies, so that they may run away in time to escape the danger. Conduction of Sound.How does sound come to our ears? Sound is carried to our ears by the air. Is the air a conductor of sound? Yes; the air is a conductor of sound. Is there any sound where there is no air? No; there is not any sound where there is no air. How do we know this? When a bell is rung in a vacuum, no sound is heard. Can sounds be distinctly heard when the air is rare? No; the rarer the air the more poorly it conducts sound. What proof have we of this? On the top of a high mountain the human voice is Can sounds be heard better when the air is dense? Yes; the denser the air, the better it conducts sound. What proof have we of this? The human voice sounds so distinctly in the dense air of deep mines, that conversation may be carried on in a whisper. Why are sounds heard better at night than in the day? Because the air at night is condensed by cold, and becomes a better conductor of sound; also, because the nights are stiller, and fewer sounds take our attention. Why is moist or damp air a better conductor of sound than dry air? Because the particles of water in the damp air increase its power of conduction. Why is sound heard farther when over the water? Because the damp air over the water is a better conductor, and because it contains fewer objects to obstruct the waves of sound. How far has the human voice been heard over the water? The words “all’s well” have been heard across the Strait of Gibraltar, a distance of ten miles. How far has the human voice been heard over the land? It has been heard at the distance of four miles. How does the wind affect sound? The wind carries the sound the way in which it is Why does water conduct sound faster than air? Because the particles of water are closer together than the particles of air. How can we prove that water is a conductor of sound? When a bell is rung under water, the sound may be heard by any person whose head is beneath the surface of the water. Does a solid substance conduct sound more rapidly than air? It does, much more rapidly. How can we prove this? When the ear is placed at one end of a log of wood and the other end is struck with a hammer, two sounds will be heard; first, the one coming through the log, and, afterwards, the one coming through the air. What use is made of this fact? The tramping of horses or the rumbling of cars and wagons, when at a distance, may be heard by placing the ear near the ground. Do all solid substances produce the same amount of sound when struck? No; soft substances, such as lead and wood, produce but little sound; while hard substances, such as copper, bell-metal, and glass, produce much more sound. What are hard bodies, producing much sound, called? They are called sonorous bodies. Is a bell a very sonorous body? Yes; a bell is one of the most sonorous bodies that we have. Why does a bell ring when struck? Because the stroke causes the particles of the bell to vibrate. When we touch a bell while ringing, why does the sound stop? Because we stop its vibrations when we touch it. Why does a crack in a bell check its ringing? Because the crack stops the vibrations in the metal. What other substances ring when struck? All vessels made of glass, earth, stone, iron, etc., ring when struck. How may we detect a crack in any such vessel? By striking the vessel; if it is cracked there will be no ringing sound. Trumpets and Speaking-Tubes.How do the waves of sound move? The waves of sound move in every direction from the place where the noise is made. Why do we put both hands around the mouth when we call to a person at a distance from us? We do so to keep the waves of sound more together. Does it make the voice louder? It does make the voice louder. What instrument has been made so as to operate in the same way? The speaking-trumpet, as is shown in Fig. 24. Fig. 24. Of what use are speaking-trumpets? They are of much use to firemen when giving orders at a fire, and to those on board of vessels when giving orders to the sailors. Why do we hold the hand behind the ear when we wish to hear more distinctly? Because the hand catches more of the waves of sound, and conducts them to the ear. What instrument has been made to serve the same purpose? The ear-trumpet, as is shown in Fig. 25. Fig. 25. How does the ear-trumpet aid the hearing? The large end of the trumpet collects more of the waves of sound than the ear can; hence, more waves reach the ear. Why do such animals as the horse, the rabbit, and the deer, hear better than we do? Because their large ears act like ear-trumpets to collect the waves of sound, and thus increase their faculty of hearing. How can these animals still farther increase their sense of hearing? By being able to turn their ears, just as we do the ear-trumpet, in the direction whence the sound comes. What are speaking-tubes? They are metal pipes extending from one part of a building to another part. Of what use are speaking-tubes? They are used to convey the voice to the more distant parts of a building. Do they require loud talking? No; even a whisper has been heard through a metal pipe over three thousand feet long. Velocity of Sound.How fast do the waves of sound move through the air? They move at the rate of ten hundred and ninety feet in a second of time, when the temperature of the air is thirty-two degrees. What effect does warm air have on sound? Sound travels more rapidly in warm air. How much does heat increase the velocity of sound? The velocity of sound is increased one foot in a second of time, for every degree of heat added to the air; so that sound travels eleven hundred and twenty feet in a second, when the temperature is at sixty-two degrees, which is nearly one mile in five seconds. Does light travel more rapidly than sound? Yes; light comes from the moon to the earth, a distance of two hundred and forty thousand miles, while sound moves eleven hundred and twenty feet. What familiar examples of this difference may be given? A wood-chopper’s axe is seen to descend before we hear the stroke, and the smoke from a gun is seen before we hear the report. How can we tell the distance of the wood-chopper from us? If we count the number of seconds between the stroke of the axe and the time when the sound reaches our ear, and multiply eleven hundred and twenty by How can we tell the distance of lightning from us? If we count the number of seconds between the lightning and the thunder, and divide this number by five, it will give the distance in miles; because sound travels one mile in every five seconds. How rapidly does sound travel through water? Sound travels about four times as rapidly through water as it does through air. How rapidly does sound travel through a solid? Sound travels ten times as rapidly through wood, and sixteen times as rapidly through glass, as it does through air. Does a loud sound travel more rapidly than a faint one? No; it does not. How do we know this? Because the notes from a band of music come to us in the order in which they are played, whether they are loud or faint. Reflection of Sound.What becomes of the waves of sound when they strike a solid substance? They are thrown back, as light from a looking-glass, or a ball from a stone wall. What is this called? It is called reflection of sound. In what direction is sound reflected? Sound, like light, is reflected so that the angles of incidence and reflection are always equal. When the reflected sound comes back to the ear, what does it make? It makes an echo. When the sound comes back several times, what does it make? It makes several echoes. Where are echoes always to be heard? Echoes may be heard in the deep caves of the earth; because the walls of the caves reflect sounds striking against them. What curious echo may be mentioned? There is an echo in Fairfax County, Virginia, which sends back twenty notes played on a flute. Does every reflection of sound produce an echo? No; it does not. Why is there no echo from the walls of a small room? Because the walls are too close together to make an echo. How far away must the reflection of a sound be made, so as to produce a perfect echo? Sound must be made not less than one hundred and twelve feet distant to produce a perfect echo. How many syllables can be heard from an echo one hundred and twelve feet distant? Only one syllable can be heard. How far must the echo be to hear two syllables? It must be twice the one hundred and twelve feet, or two hundred and twenty-four feet, to hear two syllables; Why can a person when speaking, be heard better in a room than in the open air? Because the walls of the building reflect his voice to the hearers, so that more waves of sound reach their ears. Why are noises heard so distinctly in an unfurnished dwelling? Because there is then nothing but the walls to obstruct the waves of sound; but when the dwelling is furnished, each article of furniture helps to obstruct the sound and to make it less distinct. What may be said of the harmony of sounds? The babbling of the brook, the roaring of the cataract, the wailing of the wind, and the singing of the birds, everywhere show the Divine love of harmony; for where there is no human ear to listen, they still sing their heavenly anthems of praise to His ear alone. Horse-drawn trolley car. Decoration.
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