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?
Sixteenvibrations 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.
How does sound come to our ears?
Sound is carried to our ears by the air.
Is the air a conductor of sound?
Yes; theairis aconductorof 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 isheard only at a distance of a few yards; and a pistol when exploded sounds as faintly as the breaking of a stick.
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 oftenmiles.
How far has the human voice been heard over the land?
It has been heard at the distance offourmiles.
How does the wind affect sound?
The wind carries the sound the way in which it isblowing, so that a noise may be heard much farther with the wind than against it.
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 calledsonorousbodies.
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.
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?
Thespeaking-trumpet, as is shown inFig. 24.
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?
Theear-trumpet, as is shown inFig. 25.
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.
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 nearlyone mileinfive 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 bythis number, it will give us the distance in feet; because sound travels eleven hundred and twenty feet in a second.
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 aboutfourtimes as rapidly throughwateras it does through air.
How rapidly does sound travel through a solid?
Sound travelstentimes as rapidly throughwood, andsixteentimes as rapidly throughglass, 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.
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 calledreflectionof 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 anecho.
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;and three times that distance to hear three syllables, and so on.
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.
Illustrated Capital I
In what form is all matter found?
In the form of a solid, a liquid, or a gas.
What is true of all solids?
All solids must have length, breadth, and thickness.
Do solids vary much in length, breadth, and thickness?
Yes; they vary so much that we can scarcely find two things exactly alike in shape and size.
Can the shape of any substance be changed?
Yes; its shape may be changed in many ways.
What takes place in india-rubber when it is stretched?
Its particles are drawn farther apart, and its shape is altered.
When the force is removed, what does the india-rubber do?
It springs back into its former shape.
What is this property of the india-rubber called?
It is calledElasticity.
Can air be pressed into a smaller bulk?
It can, by means of a weight.
When the weight is removed, what does the air do?
The air goes back to its former bulk.
Is air very elastic?
Yes; air is one of the most elastic substances known.
Why does a ball rebound when thrown against the floor?
Because the substance of the ball is pressed out of shape when it strikes the floor, and, in its effort to spring back to its former shape, it is forced to rebound.
Why does the ball rebound from the floor?
Because the floor is firm, and will not move from the ball.
Does a ball rebound when it strikes a soft substance?
No; it does not rebound when it strikes a soft substance, like a heap of wool or feathers.
Are solids ever elastic?
Yes; many solids are elastic.
Name some articles useful from their elastic properties.
Watch-springs, wagon-springs, steel pens, and spring seats are very useful.
Why does a wooden bow throw an arrow from it?
When the force that bends the bow is removed, it springs back to its former shape, and in so doing throws the arrow from it.
Can metals be bent?
Yes; nearly all metals can be bent.
What is this property of being bent called?
It is calledFlexibility.
Do substances usually spring back after being bent?
No; they usually remain in the bent form.
Can glass be bent?
Glass can be bent only a very little without being broken.
What is this property of being easily broken called?
It is calledBrittleness.
What may be said of brittle substances?
Brittle substances are generally hard, while flexible substances are much softer.
What metal may be made either flexible or brittle?
Steel, when heated and cooled slowly, becomes flexible, but when cooled quickly, it becomes brittle.
For what is brittle steel used?
It is used for making knives, razors, axes, and other cutting instruments.
What may be done with metals that are flexible?
They may be hammered into thin plates.
What is this property in metals called?
It is calledMalleability.
Which is the most malleable of all metals?
Gold is the most malleable; and it can be made into leaves so thin that three hundred and sixty thousand of them, when placed together, make a bulk only one inch in thickness.
Name some other metals that are malleable.
Silver, copper, tin, zinc, iron, and lead.
Can the particles of a piece of iron easily be separated?
No; its particles cannot easily be separated.
What is this property of iron called?
It is calledTenacity.
Can the particles of a brittle substance, like glass, easily be separated?
Yes; its particles can easily be separated.
Is there much tenacity in brittle substances?
No; there is very little tenacity in brittle substances.
Is there any tenacity in air?
No; because its particles are always trying to separate from each other.
Is there any tenacity in water?
Only a little, just enough to hold its particles together in the form of drops.
Upon what does the strength of substances depend?
Their strength depends upon their tenacity.
Do the different kinds of wood vary much in strength?
Yes; hickory and oak are much stronger than pine or cedar.
When is a knowledge of the strength of wood necessary?
It is necessary to know the strength of wood used in the construction of stores, bridges, houses, etc.
Which is the most tenacious of metals?
Iron is the most tenacious of metals.
How is this principle in iron made useful?
It is useful in the construction of suspension bridges, and in very many other ways.
Can iron be drawn into wire?
Yes; iron can be drawn into wire.
What is this property of being drawn into wire called?
It is calledDuctility.
Which are the most ductile of metals?
Platinum, silver, iron, copper, and gold, are the most ductile, in the order named.
When is glass ductile?
When glass is melted it becomes ductile, and may be drawn into very fine threads.
If we take half the air out of a room, what will the remaining half do?
Its particles will separate until they fill the whole room.
What is said of air when its particles are thus separated?
The air is in a state ofrarity.
What is said of it when the particles are pressed closer together?
The air is then said to be in a state ofdensity.
Are the particles of water closer together than those of air?
Yes; therefore water is denser than air.
Upon what does the weight of a substance depend?
Its weight depends upon its density.
How much heavier is steel than water?
Steel is seven times as heavy as water, and is, therefore, seven times as dense.
How much denser is mercury than water?
Mercury is thirteen times as dense as water, and is, therefore, thirteen times as heavy.
Why can a bird move through the air?
Because the particles of air can be separated by the bird.
Why can a fish swim through the water?
Because the particles of water can be separated by the fish.
Why can a needle be passed through cloth?
Because the fibres of the cloth can be separated by the needle.
Why can a nail be driven through a board?
Because the fibres of wood can be separated by the nail.
What name given to this property in these substances?
This property is calledPenetrability.
What is necessary in order to penetrate a substance?
We must use a substance harder than the one which we wish to penetrate.
Why will iron penetrate wood?
Because iron is harder than wood.
Why cannot our bodies penetrate the wood?
Because our bodies are softer than the wood.
Why can our bodies move more easily through air than through water?
Because air is more easily penetrated than water.
Why can a nail be driven into a pine board more easily than into an oak board?
Because the fibres of pine are more easily separated than the fibres of oak; hence, pine wood is more penetrable than oak wood.
Decoration.
Decoration.
Illustrated Capital W
Why can a cup be lifted by taking hold of the handle?
Because the particles of the cup do not separate from each other.
What is the power which holds these particles together called?
It is calledCohesion.
In what bodies is cohesion the strongest?
Cohesion is strongest in solid bodies.
Is the cohesion alike in all solids?
No; the cohesion in iron is much greater than the cohesion in wood.
Upon what does the strength of a substance depend?
The strength of a substance depends upon the amount of cohesion among its particles.
What effect does heat have upon the cohesion of bodies?
Heat tends to destroy the cohesion of bodies by separating their particles.
How does heat affect ice?
Heat destroys the cohesion between the particles of ice, and thus it is changed into water.
How does heat affect water?
When still more heat is applied, the cohesion in the water is destroyed, and thus it is changed into steam.
Is there any cohesion in steam?
No; there is not any cohesion in steam.
Is there any cohesion in air?
No; because the particles of air are always trying to separate from each other.
Is there any cohesion in liquids?
Yes; there is some cohesion in liquids, but it is only strong enough to hold them together in drops.
Does the cohesion in liquids vary?
Yes; the cohesion in mercury is stronger than it is in water; therefore, a drop of mercury is larger than a drop of water.
How is it in chloroform?
The cohesion in chloroform is weaker than it is in water; therefore, a drop of chloroform is smaller than a drop of water.
How is shot made?
When lead is melted and poured through a sieve at the top of a high tower, the attraction of cohesion forms it into round drops or globules; these cool, while falling, thus forming the grains of shot.
How high must a shot-tower be?
A shot-tower must be not less than two hundred feet high, so that the lead may harden while falling.
Why does putty stick to a pane of glass?
Because there is an attraction between the putty and the glass.
What is this attraction called?
It is calledAdhesion.
Upon what does adhesion depend?
Adhesion depends upon the attraction which substances have for each other.
Has water any attraction for oil?
No; therefore, there is no adhesion between oil and water.
Has glass any attraction for water?
Yes; therefore, the water collects on the glass in drops.
What other examples of adhesion may be given?
Paint adheres to wood, dust to furniture, and tin and mercury to the back of a looking-glass.
Will you name a few of the most adhesive substances?
Glue, mortar, paste, and putty.
Of what use is glue?
Glue fastens pieces of wood together.
Of what use is mortar?
Mortar fastens together the bricks and stones used in the construction of houses and other buildings.
Of what use is paste?
Paste fastens pieces of paper together, and it fastens paper to the walls and ceilings of rooms.
Of what use is putty?
Putty fastens the glass to the wood-work of our windows.
Why does a sponge suck up water?
Because the particles of the sponge have an attraction for the water.
What is this attraction called?
It is calledCapillaryattraction.
In what bodies do we find the most capillary attraction?
In those that are of a porous or spongy nature.
Why does a lump of sugar or salt, when partly dipped into water, soon become entirely wet?
Because sugar and salt are porous substances, and the water rises in them by capillary attraction.
Why does a heap of dry sand soon become damp when one edge touches the water?
Because the water is drawn through the sand by capillary attraction.
Why does water poured into the saucer of a flower-pot soon wet the earth in the flower-pot?
Because the earth is porous, and the water is drawn up through it by capillary attraction.
How does the oil in a lamp rise so as to be burned?
The oil is drawn up through the wick by capillary attraction.
Where may examples of capillary attraction be noticed?
In blotting-paper when it takes up the ink, and in bread when it soaks up the juice of meats.
What other examples may be given?
When one end of a towel is placed in a basin of water, and the other end is left hanging outside thebasin, the water will pass through the towel by capillary attraction, until the basin is entirely dry.
Why is water in a glass tumbler a little elevated at the edges?
Because the glass has a slight attraction for the water near it, and thus lifts it up a little.
Why will water rise in a small glass tube higher than in the surrounding vessel?
Because the glass has an attraction for the water, and elevates it at the edges, as is shown inFig. 26.
Fig. 26.
Fig. 26.
Why do salt and other substances dissolve in water?
Because water overcomes the force of cohesion in the salt, and thus separates its particles. The capillary attraction between the salt and the water is greater than the cohesion between the particles of salt; hence, the salt is dissolved by the water.
When a ball is thrown into the air, why does it fall back to the earth?
Because the earth attracts the ball.
Why do rain and snow fall to the earth?
Because the earth attracts them.
What is this kind of attraction called?
It is called attraction ofGravitation.
Does the attraction of gravitation exist in all things?
Yes; every substance attracts every other substance.