Martindale’s First Lessons in Natural Philosophy, by Joseph C. Martindale: a Project Gutenberg eBook

Entered, according to Act of Congress, in the year 1872, by
ELDREDGE & BROTHER,
in the Office of the Librarian of Congress at Washington.

Preface.

In preparing this little work, our desire has been to afford both pleasure and profit to those who may read or study its pages.

The most interesting, as well as the most useful things to us, are those with which we meet in every-day life; but we must have some knowledge of these things, ere we can contemplate them with much pleasure, or use them to our greatest profit.

Simple truths in natural science may be learned at an early age; for it requires no more than the ordinary intelligence possessed by boys and girls to understand them. Such facts are not only valuable in themselves, but they are also useful in disciplining the mind by teaching it to observe the many changes constantly going on around us, and to reason and reflect in regard to the same. Habits of observation and reflection thus formed are likely to go with us through life; and they may prove a blessing in a thousand ways; for they will not only add to our comfort and our enjoyment, but they will also produce many a pleasing thought.

We have endeavored to make a book that can be easily understood; but few scientific terms will be found in it, for its language is the language in familiar use; hence, it may serve to interest the fireside circle, and it may be referred to by intelligent vi boys and girls for answers to the many perplexing questions which are so often presented to their inquiring minds.

Many pupils have not the chance to attend even an Academy or a High School. It is to such that this book is especially adapted, for it is believed to contain as much matter as can be taught with satisfaction and profit in our public or our private schools; but to other pupils, it will prove a valuable introduction to larger and more comprehensive works.

Whenever a drawing would illustrate a fact, and make its application plainer, it has been made; but by far the best illustrations of hundreds of facts, explained in the text, will be found in the world around us, and the attention of both pupil and teacher should always be directed to this reliable and never-ending source.

In presenting these Familiar Lessons in Natural Philosophy to the youth of our country, it is with the earnest hope that it may make them better acquainted with the natural objects surrounding us; that it may lead them to observe the order, the harmony, and the beauty in nature everywhere; and that it may help to turn their thoughts more and more to Him, who is the Creator and Ruler of all things.

FIRST LESSONS
IN
Natural Philosophy.

CHAPTER I.
Astronomy.

The Earth and Other Planets.

If the earth is in a certain place at noon, on New Year’s day, when will it be in that place again?

The earth moves around the sun and comes back to the place whence it started, once every year.

They do; Spring follows Winter, Summer follows Spring, Autumn follows Summer, Winter follows Autumn, and Spring follows Winter again.

A day is the time from sunrise till sunrise again, from sunset till sunset again, from noon till noon again, or from midnight till midnight again.

When it is morning to us, what is it to people living on the opposite side of the earth?

When it is morning to us, it is evening to them; when it is day-time to us, it is night-time to them; and when it is noon to us, it is midnight to them.

Because we move along with it. If we are on a railroad car, the trees and houses along the road appear to move, and the car to stand still; but the car really moves, while the trees and houses stand still.

The teacher may, by means of a ball or small globe, readily make these things so plain that they can be understood even by small children.

The Moon.

We have been told that the earth moves around the sun; does any planet move around the earth?

No; some are thirty-one days long, some thirty days long, and one only twenty-eight days long.

Stars are in every direction around the earth, just as we may imagine the apples to be in every direction around a boy who has climbed into a tree full of that fruit.

When all of the moon on which the sun shines is towards the earth, how does the moon appear?

When no part of the moon on which the sun shines is towards the earth, how does the moon appear?

When a small part of the moon on which the sun shines is first turned towards the earth, what is it called?

It seems to grow smaller and smaller, until the whole of the lighted part is gone.

It takes about eight minutes; therefore, the light of the sun must travel about twelve millions of miles in a minute, or two hundred thousand miles in a second of time. At this rate, the light would come from the moon to the earth in a little more than one second of time. It would take no longer for it to come than it does for the pulse to beat once, or the clock to make one tick.

Eclipses.

We know that the earth is round like a ball, because its shadow is similar to the shadow of a ball.

We also know that the earth is round, because we can circumnavigate or sail around it.

When the Earth comes between the sun and the moon, where does the shadow of the earth fall?

We know that the moon is round, because its shadow is similar to the shadow of the earth.

When the moon comes between the sun and the earth, where does the shadow of the moon fall?

No; we cannot see the sun, because the moon prevents the rays of the sun from coming to us.

When the moon prevents all the sun’s rays from falling on us, what is it called?

When the moon prevents only a part of the sun’s rays from falling on us, what is it called?

God, the creator of all things, made the sun, the moon, and the stars. He placed them in the heavens, where they remain in obedience to His will. He made the bright rays from the sun to light up the day, and give beauty to the world; but the feebler rays of the moon and the stars are seen best at night, when the sun is hid.

CHAPTER II.
Light.

Color.

They ripen the apples, cherries, and other fruits; also, the wheat, corn, and other grains, and make them fit for food.

At first, we see the objects around us faintly; but as it grows lighter, we see them more distinctly.

We have the light of the sun, moon, and stars; also, the light from the burning of any substance.

If a sunbeam, shining through a hole in the window-shutter 24 of a dark room, should fall upon a prism, it will be broken up as it passes through the prism, and be shown on the opposite wall, in the seven rainbow colors.

In Fig. 6, D is a sunbeam passing through a shutter. When it passes through the prism, at E, it is separated into the seven rainbow colors, as is shown on the opposite wall.

By the word Roy-g-biv, made from their initial letters, beginning at the bottom.

A rainbow is seen when the sun shines through the drops of water as they fall through the air.

The drops of water act like little prisms, and separate the rays of light passing through them into the colors of the rainbow.

It appears like a great arch, spanning the sky, and its splendid bright colors are very beautiful.

We may remember that God set his “bow” in the cloud, as a token that the world should no more be destroyed by a flood of waters.

Because the walls of the bubble are constantly growing thinner, and different colors are, therefore, reflected from them.

Because the water runs to the bottom of the bubble until its top becomes so thin as to burst.

If there are two coats, one blue and the other black, we cannot tell which is the blue one or which is the black one, by feeling them.

We cannot; because God has so blended them together that they cannot be seen by us.

No; because the back of the looking-glass is covered with quicksilver, which prevents the rays of light from passing through it.

Those having smooth and polished surfaces; such as tin, silver, gold, and quicksilver.

Those having dull, uneven surfaces; such as iron, wood, cloth, leather, and calico.

Because it reflects the red color, and absorbs or hides the other colors in itself.

Because there are no rays of light falling on them; hence, there is no color for them to reflect.

Because thousands of persons, in different directions from the same object, can see it at the same time.

He must be in a position so that there is nothing to prevent the rays of light passing in a straight line from the object to his eyes.

When they reflect a great many red rays, they are of a bright red color; but when they reflect only a few red rays, they are of a dull red color.

In the pretty flowers that grow in the woods, fields, and gardens; in the beautiful tints of the leaves in autumn; and in the plumage of the birds that visit us every spring, and cheer us with their songs.

They may teach us the goodness of God, in giving us a world so full of beauty; and we should feel ever thankful to Him for the great pleasure that we have in seeing them.

At night, when it is dark; because, at other times, the light of the sun hides their feeble light.

Because it is the only color falling upon the glass, and there is no other color to reflect.

They do not; they absorb or hide the color of all other objects, and reflect only their own color.

If every object was a good reflector of color, like a looking-glass, what would result?

Then every object would reflect the color of every other object around it, and there would be an endless confusion of colors.

Those of a darker color; and the nearer the color approaches to black, the less light it reflects.

Why is it darker when the sky is covered by dark-colored, than by light-colored, clouds?

Because the dark-colored clouds reflect less light, than the light-colored clouds do.

Because it absorbs some of the rays of light which come in at the windows; hence, there is less light in the room. Dark-colored carpets, or dark furniture, darken the room for the same reason.

Because the yellow color of the lamp’s rays acts upon the color of the silk, forming a third color. Blue silk becomes green by lamp-light, and red silk becomes orange.

Dr. Mitchell tells of an officer who bought a blue coat and a red waistcoat, thinking them to be of the same color. He also tells of a tailor who patched a black waistcoat with a piece of crimson; and of another, who put a red collar on a blue coat. Dr. Dalton lost a piece of red sealing-wax in the grass, and he could not find it because it and the grass seemed of the same color to him.

Refraction of Light.

When the rays of light pass from one substance to another more dense, what takes place?

Bending a ray of light, as it passes from one substance to another of different density; as from air to water, or from water to air.

Because light coming from the part of the rod under the water, is refracted or bent at the surface of the water, while the light from the part above the 33 surface, comes in straight lines to the eye; hence, the rod appears to be bent at the surface, as is represented in Fig. 7.

It appears to be raised up, and the water does not seem so deep as it really is.

Because the light from the bottom of the pond is refracted or bent at the surface of the water; hence, the bottom appears to be raised up.

Yes; like the bottom of the stream, they also appear to be only two-thirds as far from the surface as they really are.

We must aim below the place where they seem to be, or we must strike perpendicularly at them.

Reflection of Light.

When a ball is thrown against a wall, what is the path through which it goes called?

When a ray of light falls upon a looking-glass, what is the path through which it goes called?

The angle of incidence is the angle between a perpendicular and the line of incidence; and the angle of reflection is the angle between the perpendicular and the line of reflection. The two angles are always equal to each other, as is represented in Fig. 8.

When a ray of light strikes a looking-glass obliquely or slantingly, what is done with it?

The ray is reflected as obliquely from the glass as it strikes or falls upon it.

Because the angle of reflection is equal to the angle of incidence; and the observer must be nearly over the place where the rays strike, so that, when reflected, they may meet his eye.

It is seen nearer the middle of the pond; because the sun’s rays then fell more obliquely on the pond, and are reflected more obliquely to the eye.

When an opaque object is placed between a candle and a wall, why will there be a shadow?

Because the rays of light diverge, in every direction from the candle, in straight lines, and the nearer the object, the more rays will it intercept, as is represented in Fig. 9.

In Fig. 9, let L represent the flame of a candle. When the object is placed at A, the shadow on the wall will extend from C to D; but when the object is moved nearer, as at B, the shadow will extend from E to F.

Because the angle at which the light from it strikes the eye, is less when at a distance, than when near to the eye.

In Fig. 10, the diameter of the near circle is from A to B; but the diameter of the more distant circle, although just as large, seems only to be from C to D.

Because the rays of light from it spread out in every direction; and fewer rays enter the eye when it is at a distance than when it is near by.

Why can we see a distant object more distinctly with a telescope, than without one?

Because the telescope enlarges the image of the object, and it also collects more light from that object than is collected by the unaided eye; thus, making the image distinctly visible.

They are used by jewellers, watchmakers, engravers, and others who examine small objects.

They appear to be as perfectly formed as the larger ones which are seen with the naked eye.

Near-sighted people use glasses, thickest at the edge and thinnest at the centre.

They should be so made as to render objects distinct, but neither to magnify nor diminish them.

Their eyes change so that they see objects at a distance better than those which are nearer, hence, in reading they hold a book farther from the eye.

The Eye.

What is that part of the eye called, which is blue, gray, or brown, in different persons?

The pupil is the window of the eye, and the rays of light which enter the eye must pass through it.

The iris has the power of making the pupil larger or smaller, according to the amount of light which it receives.

The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye.

When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards?

Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.

So that more rays of light may enter the eye, whereby objects may be more distinctly seen.

It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids.

Because the light from it is retained a short time by the eye, thus seeming to form a complete circle.

Because the light from each post falls in such quick succession upon the eye, that it cannot distinguish one post from another.

No; when objects are within six inches of the eye they cannot be seen distinctly.

Yes; objects may be seen even when millions of miles away from us; thus, the sun is seen although ninety-five millions of miles from the earth, and some 41 of the stars which we behold in the heavens, are still farther away than the sun.

The eye gives us ideas of the size, the shape, the color, the place, the distance, and the movements of things around us, so that we can use them the better to promote our comfort and our happiness.

And God said, “Let there be light;” and there was light. Thus the day was separated from the night, and the sun was made to send forth his silvery rays upon hill and valley, field and forest, causing the rarest buds and most beautiful blossoms to come forth from the lifeless earth, although no eye, save the All-Seeing One, was there to behold them!

CHAPTER III.
Heat.

Sources of Heat.

A part of the heat from that substance comes to us and causes a feeling of warmth.

A part of the heat from us goes to that substance, and we experience a feeling of cold.

Yes; water may be heated until it is scalding hot, or the heat may be taken from it until it is frozen into ice.

We have a feeling of warmth, although the room is no warmer than in the first instance.

If one hand be held in quite warm water, and the other in quite cold water, for a few moments, and then both hands be plunged into tepid water, the tepid water will feel cold to the hand that was in the warm water, and warm to the hand that was in the cold water.

Heat accompanied by light, as the heat from the sun, or a lamp; and heat without light, as the heat from boiling water.

It permits both the light and the heat to pass into the room, so that we can see the one and feel the other.

When a pane of glass is held between the face and a fire, it will protect the face from the heat.

No; boiling water is as hot in the dark as it is in the light; and ice is as cold in the daytime as it is at night.

We should feel thankful to an All-wise Creator, for having provided us with both heat and light from a never-failing source—the sun.

Expansion by Heat.

Heat pushes the parts or atoms of an object farther from each other, and thus its size is increased.

When a fire is made in a room, why does the furniture often make a snapping noise?

Because heat expands the wood, and the particles make a snapping noise upon separating.

If any other warm substance touch the glass, will the glass be heated in the same manner?

They are broken because the part next to the hot water is expanded more than that farther away.

Why will a cloth dipped in hot water and wrapped around the neck of a bottle loosen its stopper?

Because the heat from the cloth expands the neck of the bottle before it does the stopper, so that the stopper is loosened.

Because they are fitted together in the stove so that they have not room to expand without breaking.

By a draught of cold air, or a drop of cold water, touching them, thus causing them to contract unequally.

Like other metals, quicksilver expands as it grows warmer, and contracts as it grows cooler.

It is an instrument used for measuring the degree or quantity of heat in any substance.

Because the particles of which they are formed are more easily separated or pushed apart by the heat.

No; some solids expand more than others; thus, zinc expands more than iron, and iron more than glass.

No; liquids, although more sensitive to heat, do not expand as equally as solids.

Yes; gases expand uniformly; thus, 491 cubic inches of any gas, if heated one degree, will become 492 cubic inches.

If one cup be filled with lead, and another cup of the same size with water, which will weigh the more, the lead or the water?

Because a cup full of lead will weigh more than the same cup fall of water, or any bulk of lead will weigh more than an equal bulk of water.

Because any bulk of cork is lighter than an equal bulk of water; a cup filled with cork will weigh less than when it is filled with water.

Because the same liquid is expanded and takes up more room, although it has no more weight.

Because a gallon of cold water, when heated, will make more than a gallon of hot water.

We get the most by buying it in cold weather, because a gallon bought then will make more than a gallon when the weather becomes warmer.

It ascends or rises up, because it is lighter, and the cold or heavier air falls to take its place.

The part next to the ceiling is the warmest, because the heated air always ascends.

Because it is filled with heated air from the lungs, and is, therefore, lighter than the air around it.

The air, upon being heated, bursts through the peel of the apple, carrying the juice with it.

Because the air in the cells of the apple expands, and breaks those cells, so that the apple becomes soft.

Because the heat expands the air in them, and thus breaks up their cells, so that they become soft.

Because the air in the cells of the wood bursts them apart, thus making the noise.

Because the heated air bursts the cells with such force as to throw off small pieces of the burning wood.

The coarse-grained kinds, like chestnut; while the finer-grained kinds, like walnut, seldom snap much.

Because the sap in the pores of the green wood dries up as the wood becomes dry, and its place is filled with air, so that there is more air in dry wood than in green wood.

Because the air in the chimney is heated by the fire, and as the air rises, it carries the smoke with it.

Because the cold walls of the chimney cool the heated air so rapidly as to prevent its rising to the top, and the smoke then sinks back into the room.

By having the fires made in the cellar, so that the air when heated, may ascend through pipes to the different parts of the building.

When it is reduced nearly to the freezing point, it begins to expand, and when it freezes, it expands still more.

It expands about one-seventh of its bulk. That is, seven gallons of water will make eight gallons of ice.

Because the water in them is permitted to freeze, and as there is not room for it to expand, the pipes are burst.

The ice would sink as rapidly as formed, and our streams would, every winter, freeze to the bottom, thus destroying the fish and other creatures living in them. In this, as in many other ways, we may learn the lesson, that in the creation of the world, God wisely made all things to serve some useful purpose.

Conduction of Heat.

When one end of a rod of iron is placed in a fire, does the whole rod become heated?

It is the passage of heat from one body to another which it touches, or from one particle of a body to another particle of the same body.

Such bodies as gold, platinum, copper, silver, iron, zinc, tin, and lead, in the order named, are the best conductors of heat.

Because the iron is a good conductor, and carries the heat away from our hands very rapidly.

Because the fur is a poor conductor, and carries the heat away from our hands very slowly.

Because wood is not so good a conductor as iron; hence, it does not carry the heat of our hands away so rapidly.

Because the carpet is a poorer conductor than the wood of the floor, and does not carry away the heat of our bodies so rapidly.

Because the stone is a good conductor, and rapidly carries the heat away from our feet.

Because the iron is a good conductor, and gives off its heat rapidly to our bodies.

Because wood is a poorer conductor than brick, and does not give off its heat so rapidly.

Why is a block of wood better than iron or brick, to keep our feet warm while on a journey?

Because it does not give off its heat so rapidly, and therefore remains warm longer than iron or brick.

Because the cloth, being a poor conductor, helps to keep the heat from passing off so rapidly.

Because iron is a better conductor than water, and carries away the heat from our hands more rapidly.

Because water may be made to boil at its surface without melting ice a short distance below the surface.

Because the air at the ceiling of a room may be made very warm without melting ice near the floor.

Because linen is a good conductor, and there is but little air among its fibres.

Because the cotton contains more air among its fibres, and air is a poor conductor of heat.

Because wool contains more air among its fibres, and is, therefore, a poorer conductor of heat.

Because there is more air among the hairs of the fur, and it is, therefore, a poorer conductor of heat.

Fur with very fine hairs, like that of the rabbit, because it contains the most air.

Because the fur and the air in it both help to keep the heat of our bodies from passing off rapidly.

In this latitude, the frost of winter reaches only a few inches below the surface of the earth.

In all parts of the earth, except in the Frigid Zones, the heat of the summer takes away the frost and ice of the winter, and the earth is warmed so that plants may grow.

Because the springs are mostly so far below the surface of the earth, that they are not warmed by the sun’s rays.

Because the air among the particles of snow prevents the heat of the earth from passing off rapidly.

Why does wrapping straw around a pump in winter, prevent the water in it from freezing?

Because the stalks of straw are hollow, and the air in the stalks makes them poor conductors of heat.

It will; because it keeps the heat of the shrubbery from passing off into the air.

Because the air which is confined between the double windows, being a poor conductor, keeps the warmth of the room from escaping.

Because charcoal is a poor conductor, and keeps the heat of the air from the ice.

Because the saw-dust and the straw are both poor conductors, and keep the heat of the air from the ice.

Because the flannel is a poor conductor, and keeps the heat of the air from the ice.

Because wool is a poor conductor, and thus keeps the heat of the iron from burning the hand.

Convection of Heat.

Because the water when heated, becomes lighter, and stays at the surface, while the cold water is heavier and remains at the bottom of the vessel.

It must be applied to the bottom of the vessel containing the liquid or the gas.

Because the air touching a substance ascends as soon as heated, and cold air takes its place; as this process goes on rapidly, the substance is quickly cooled.

The water at the surface is cooled by giving off its heat to the air; it then falls to the bottom, and the warm water rises to take its place at the surface.

Because the hot coffee at the bottom of the vessel, is brought more rapidly in contact with the air, and, therefore, gives off its heat more rapidly.

Because the hot air over the coffee is removed by blowing, and the cold air is brought more rapidly in contact with the coffee.

Liquefaction.

No; there are many solids, such as wood, coal, paper, and leather, that cannot be changed to liquids.

Because heat forces the particles of the solid apart, until it is changed to a liquid.

Latent Heat.

Yes; because the water gives off its latent heat while freezing; hence, the surrounding objects are made warmer.

Yes; because the ice absorbs so much heat while thawing that it cools the surrounding objects.

The salt causes the ice to melt quickly, and the ice 65 in melting takes so much heat from the cream as to freeze it, thus making it ice-cream.

Ebullition.

Because the water at the bottom of the vessel is heated and changed into steam, which expands and forms the bubbles.

When water is heated, the steam at first forms in very small bubbles at the bottom of the vessel, and as these bubbles rise through the colder water, the steam in them is condensed to water, so that they break before reaching the surface, thus producing what is called “simmering.”

Water boils when it becomes heated so that the bubbles rise to the surface of the water before breaking.

Because the steam, in trying to escape, throws some of the water over the edge of the vessel.

Vaporization.

Ether boils at 96 degrees, alcohol at 176 degrees, and quicksilver at a temperature of 662 degrees.

The air condenses or changes it into very small particles of water, so that it is seen like a fog or a cloud.

When water is boiled in a glass vessel, no steam is visible, although it is there.

Yes; it expands with so much force as frequently to burst the vessel containing it.

Because the steam in the tea-kettle presses on the water with force enough to cause it to run out.

The valve is kept closed by a weight, which prevents the escape of the steam until a certain quantity of it is formed in the boiler; it then lifts up the valve, and a portion of it escapes, the same as it does with the lid of the tea-kettle.

Evaporation.

It passes off into the air in particles or drops so very small as not to be visible.

Because the warm air can hold more of the fine particles of water, and, therefore, can absorb more.

Because the damp air contains so much moisture that but little more can pass off into it.

Because the damp air in contact with them is blown away, and the dry air is constantly taking its place.

Because evaporation then takes place from every wet object, and this absorbs heat from the air.

Because so much heat is absorbed from the land in the evaporation which is constantly taking place.

By supplying the heat needed to change the dew into a vapor, the sun hastens its evaporation.

Because the moisture in them passes off into the air, and thus they become dry and hard.

By keeping them in a covered vessel, so that evaporation will take place more slowly, they may be kept moist for a longer time.

Radiation.

Yes; heat and light travel together at the rate of 200,000 miles in a second of time.

Because the tin is a poor radiator, and prevents the heat of the water from passing off into the air.

Because the earthen vessel is a good radiator, and gives off the heat of the water rapidly.

The rays from the sun give off about one-fourth of their heat while passing through the air.

Reflection.

Because the bright tin is a good reflector, and throws off the greater part of the rays of heat that come to it from the fire.

Why does a piece of tin or other bright metal, placed under a stove, protect the carpet from burning?

Because the tin being a good reflector, remains cool, and cannot burn the carpet under it.

Because the iron, being a poor reflector, soon becomes heated, so as to burn the carpet under it.

Absorption.

Because a ray of heat cannot be reflected from an object, and be absorbed by that object, both at the same time.

Because the rough, dark iron of the tea-kettle absorbs the heat from the fire, and gives it off rapidly to the water contained in the kettle. The iron is also a good conductor of heat; therefore, the heat from the fire passes rapidly through it to the water.

Why does black cloth, when spread on the snow, cause the snow to melt more rapidly?

Because the cloth absorbs heat from the rays of the sun, and, therefore, melts the snow beneath it.

Because the white muslin reflects the heat from the rays of the sun, so that but little of it enters the snow.

Because the light-colored earth is a poor radiator, and gives off but little heat to the snow.

Because it is so much cooler than our bodies that it absorbs heat from them rapidly, and thus makes them feel cold.

Because it is then nearly as warm as our bodies, and, therefore, absorbs heat from them slowly.

Because the greasy shoes absorb heat from the sun, but the polished shoes throw off the heat of the sun by reflection.

And God said, Let there be light; and there was light. But along with the light, came its companion—heat; for when the sunbeam first came forth at His command, the twin sisters, light and heat, together touched the earth, causing it to bloom with life and beauty. And the earth brought forth grass, the herb yielded its seed, and the tree its fruit: And God saw that it was all good.

CHAPTER IV.
The Air.

They breathe it into their lungs, and it supports life in them, the same as the air which we breathe supports life in us.

We cannot tell; but both air and water have been found several hundred feet below the earth’s surface.

When part of the air is taken from a room, what becomes of the air left in the room?

Are the particles of air so close together after it expands as they were before?

It is forced or pressed into a smaller space, and, therefore, occupies less room.

It has been condensed so much that 100 cubic feet of air was reduced to the bulk of only one cubic foot.

In a large heap of wool, the hairs of the wool at the bottom of the heap are pressed closely together, but they are farther and farther apart the nearer they are to the top of the heap.

Because it presses with equal force, in every direction; up, down, and on every side.

Fill a tumbler with water and put a piece of writing-paper over the top. Hold the hand on the paper and quickly invert the tumbler, or turn it bottom upwards; then remove the hand, and the pressure of the air against the paper will keep the water in the tumbler. This is represented in Fig. 12.

If a tumbler be filled with water in a bucket, and then raised bottom upwards until its edge is just below the surface of the water, it will remain full of water; why is this?

The pressure of the air on the water in the bucket, keeps the water in the tumbler.

If we use an iron pipe or a lead pipe, instead of a tumbler, will the pressure of the air keep the water in the pipe?

Because the air presses down with a force of fifteen pounds on every square inch of surface, and a column of water thirty-four feet high presses down with a force of fifteen pounds on every square inch of surface.

Water is heavier than air; because a column of water thirty-four feet high weighs as much as a column of air fifty miles high.

Because a column of quicksilver thirty inches high, weighs exactly the same as a column of air fifty miles high, or a column of water thirty-four feet high.

Take a glass tube about thirty-three inches long, cork up one end, and fill the tube with quicksilver; then put the open end of the tube beneath the surface of some quicksilver, in a basin, and the quicksilver in the tube will fall to the height of thirty inches, as is shown in Fig. 13.

If the cork be removed from the glass tube, what will become of the quicksilver?

Because the air will then press upon the quicksilver in the tube, the same as it does on that in the basin; hence, all the quicksilver will fall to the same level.

They can use it as a guide when planting and gathering their crops, and thus be better prepared for stormy weather.

It sinks ten inches, and, therefore, stands at the height of only twenty inches.

Yes; the higher the balloon ascends, the lower the quicksilver in the barometer sinks.

When the weight of the air is taken off, will the water boil at a lower temperature?

No; because the water passes off into steam at one hundred and ninety-four degrees, and it requires more than one hundred and ninety-four degrees of heat to boil an egg hard.

Do persons, while ascending mountains, often have difficulty in cooking their food?

They do, on account of water boiling there at a temperature below two hundred and twelve degrees.

When a vessel contains water and we pour the water out, does that produce a vacuum?

Because air is passing into the bottle at the same time that the water is coming out, and the air makes the noise in passing through the water.

Because the air passes into the barrel at the opening where the water passes out, and thus the noise is made.

Why does the water flow freely, when there is an opening at the top as well as at the bottom of the vessel?

Because air is admitted at the top, and there is then nothing to obstruct the flow of water from the opening at the bottom.

The fruit is put into a glass jar, or into a tin can, along with some water, and then heated until the jar or can is filled with steam and the air is all forced out; it is then sealed so as to be air-tight, and, in this condition, may be kept for many months.

As the fruit cools, the steam is condensed to water, and a vacuum is produced, so that the fruit is kept from the air.

The leather is made very soft and wet, so that when pressed against a stone or brick, all the air under the leather is forced out; and when the leather is lifted up, the stone or brick is lifted with it, as is represented by Fig. 14.

When there is air between the stone and the leather, can the stone be lifted with the sucker?

It cannot; because the air under the leather will press it up as much as the air above will press it down.

The fly’s feet are much like the sucker, and are kept against the ceiling by the pressure of the air.

They force out all the air between the object and their bodies, and are then held against the object, as the sucker is held against the stone.

A siphon is a bent tube, having one branch longer than the other, as is represented in Fig. 15.

Put the short branch in the water, then, by means of the mouth applied to the long branch, suck the air from it, and the water will begin immediately to flow from the long branch.

It must be kept beneath the surface of the water, so as to prevent air from entering the tube.

While the water is running out of the long branch, what is produced at the bend of the tube?

The water is continually forced up the short branch to fill this vacuum, so that it runs out of the long branch in a steady stream.

One end of the straw is placed in the cider, and the other end in the mouth; then the air is sucked out of the straw, and the cider is forced up to fill the vacuum. When the cider is sucked out, more cider is forced up, so that the straw is kept full.

A valve is like the lid on a tea-kettle, or the cover on an inkstand; and it is kept in its place by a hinge on one side.

Because the air can force water up a tube only to the height of thirty-four feet.

When the pump-handle is lowered, the water, above 91 the valve or bucket attached to the end of the rod, is raised so that it runs out at the spout; and the vacuum, thus produced by raising the water, is filled by the water in the well, which is forced up the pump-tube by the pressure of the air. When the pump is at rest, the lower valve, which is air-tight, prevents the water from sinking back into the well, so that the pump-tube is always full of water, whether we are pumping or not.

The lower valve is air-tight, and, therefore, water-tight, so that it keeps the water above it.

The pressure of the air on the water in the well, forces it up the tube to the lower valve.

Pumps are mostly used by people in the country, for the purpose of obtaining water from the wells which have been sunk in the earth.

Because the cork is lighter than the same bulk of water; hence, it ascends to the surface.

A network of ropes is placed over it, and they are so arranged as to extend beneath the bag, as is represented in Fig. 19.

It is so much lighter than the air, that fifteen cubic feet of hydrogen gas weigh no more than one cubic foot of air.

How much less will a balloon thirty feet in diameter weigh, when filled with hydrogen gas, than when filled with air?

How much force then is necessary to hold the balloon filled with coal gas to the earth?

A force of two thousand pounds, because the balloon is two thousand pounds lighter than the same bulk of air.

If a weight of only one thousand pounds were fastened to the balloon, what would result?

The balloon would rise in the air and carry the weight of one thousand pounds with it.

It is usually loaded with bags of sand, which serve as a weight to keep it from rising too high.

They open a valve at the top of the balloon, which permits some of the gas to escape; then it will sink towards the earth.

It is opened and closed by means of small ropes, reaching from the valve to the car beneath.

No; it is carried about by the currents of air, and those in the car cannot control its course; therefore, it has never been of much use to man.

Winds.

When it blows from the east, it is called an east wind; from the south, a south wind; and from the west, a west wind.

The heated air ascends, and colder air moves in to fill its place; thus causing the wind to blow.

When a candle is held in the open door of a heated room, its flame is blown outwards if placed at the top of the door, and inwards if placed at the bottom of the door; thus showing two currents of air.

The heated air escapes from the top of the room, so as to ascend higher, while the colder air comes in at the bottom, to fill its place.

Because the air near the fire becomes heated, and 97 ascends rapidly, so that the air rushes in from every direction to fill its place.

This heated air ascends, and colder air comes to take its place; thus producing winds.

On which will the air be the cooler when the sun shines, on the earth, or on the water?

Because the air over the water, in the daytime, is less heated than the air over the land; hence, the sea-breeze feels cool.

Because the earth gives off its heat more rapidly than the water, and as it receives no heat from the sun at night, it soon becomes colder than the water.

Because the air over the land at night is less heated than the air over the water; hence, the land-breeze feels cool.

A gentle breeze goes four or five miles in an hour; a high wind goes from twenty to fifty miles in an hour, and a hurricane goes from sixty to one hundred miles in an hour.

The wind carries away the impure air and injurious gases from cities and other places, so that they are the more healthy; it dries up the water on the surface of the earth, and thus promotes vegetation; it sweeps the clouds filled with moisture from the lakes and the oceans, over every part of the world, so that the water in them may fall in refreshing showers, to fill the springs, to purify the air, and in many other ways to make the earth a fitting place of abode for man. But these are only a few of the many blessings that an all-wise Creator has given us; and, for them all, we should ever remember to offer up to Him the homage of a grateful heart.

CHAPTER V.
Water.

We find water almost everywhere; above the earth, on the earth, and beneath the earth’s surface.

The ice and snow while melting, take up a large amount of heat, which passes into the water as latent heat.

The tender buds and blossoms are kept back in the spring until the ice and the snow are melted, and the earth becomes warmed, so that they are seldom injured by the cold.

By the streams, lakes, and oceans, which absorb heat from the sun; and, as the water is a poor radiator, but little of this heat is given off to the air.

The temperature of the water is gradually increased by it during the hot weather of summer.

It gives heat to the plants in the water, so that they may grow; and it rouses into active life, the fish and other breathing things, from the torpid state in which they are during winter.

In the autumn the heat in the water is given off, so that the air is kept warmer than it would otherwise be.

It gives the late fruits and seeds a chance to ripen; and it affords time for the sap of plants to return to the roots, and to become prepared for the frosts of winter.

The latent heat in the water is given off, and thus the winter air is made warmer.

Because the water at the bottom of the pond has to bear the weight of all the water above it.

At the depth of one hundred feet, it presses in every direction, with a force of forty-three pounds on every square inch of surface.

At the depth of one mile it presses in every direction, 102 with a force of two thousand three hundred pounds on each square inch of surface.

When one opening is made near the top, and another near the bottom of a vessel full of water, from which will the water flow with the most force?

Because there is more weight of water to press it out from the opening near the bottom.

How else can it be shown that water will rise to the same level in different vessels?

Let these vessels be made so as to connect with each other by the tube at the bottom; then, when water is poured into one of them, it will rise up in each of the others, so as to be the same height in all.

By means of pipes laid under the surface of the earth, and extending to all parts of the city.

It is a glass tube so nearly filled with water that it contains only a bubble of air. This tube is fastened in a piece of wood, as shown in Fig. 22.

By the bubble of air, which will always be at the centre of the tube when it is level.

It is used for levelling the walls and wood-work of houses, bridges, and other structures.

Springs and Streams.

Because thousands of wells have been dug in different parts of the country, and water is obtained from them.

When the stream of water passes through iron ore, before reaching the well, it absorbs some of the iron, which makes it hard.

By passing through other mineral substances, such as limestone and sulphur, it will become hard.

In wells where the water is warm, the streams flowing into them are near the surface of the earth, and are heated by the sun; but in the wells of cold water, the streams are much deeper in the earth.

They are mostly found on hill-sides, or in valleys; but they are sometimes found on the top of the hills.

In springs where the water is warm, the streams flowing into them are so near the surface of the earth as to be warmed by the sun, while in springs where the water is colder, the streams are much deeper in the earth.

When these brooks and creeks flow into other brooks and creeks, what do they form?

Wind seldom disturbs the ocean to a greater depth than thirty feet below its surface.

Fogs and Clouds.

What is always taking place from the surface of the water in the rivers, lakes, and oceans?

Because the vapor in the air is condensed into clouds by the cold at the top of the mountain.

When a warm, damp wind meets with a cold wind, its vapor is condensed and a cloud is formed.

By passing into a warmer current of air the cloud is changed into a vapor, and may thus quickly disappear.

Because there is not so much moisture in the air, and, therefore, it rises higher before its vapor is condensed into clouds.

Because there is so much moisture in the air, that it is condensed into clouds near the earth.

No; during a thunder-storm the clouds are seldom more than one-third of a mile high, and very often are much nearer the surface of the earth.

Yes; some are only a few inches thick, while others are a mile or more in thickness.

When the sun is about to rise, which of the seven colors of light is the first to appear?

Because they are tinged by the red ray of light, after the other rays have faded away.

Dew.

Dew is the moisture that collects on plants and other objects, during the night.

The air in contact with that object has its vapor condensed, so that the water is deposited on it in the form of dew.

Because they give off their heat rapidly after sunset, and before morning, become so cold that they condense the vapor in the air touching them, and cause the dew to be deposited on their surfaces.

Grass, wood, the leaves of plants, and, in fact, all things of a vegetable nature.

Because the air in contact with the grass and other objects, is removed so quickly that it does not get cool enough to deposit dew.

Because the air remains in contact with the grass and other objects, until it is cool enough to deposit the dew.

Because the clouds prevent radiation of heat from the earth; hence, the earth and the objects near it, remain too warm to cause any dew.

Because there is nothing then to prevent free radiation of heat from the earth; hence, it soon cools so as to cause a heavy dew.

It prevents the object that is covered from giving off its heat, so that it does not get cool enough to cause a deposit of dew.

Because trees prevent the free radiation of heat from the objects beneath them; hence, but little dew is formed under trees.

Because the walls cool the air in contact with them, so that its vapor is condensed into water, and deposited on the walls.

Because the vapor in the air in contact with the cold pitcher is condensed into moisture, and deposited on the pitcher.

Because the glass cools the air, and causes its moisture to collect in small drops on the glass.

When the breath is blown on a cold object, like a piece of glass or metal, what is the result?

The moisture in the breath is condensed by the cold object, and is collected in small drops of water on its surface.

Frost.

When the temperature of the air is reduced below thirty-two degrees, what becomes of the moisture collected on plants and other things?

Frost is the frozen moisture on the walls and window panes, and the frozen dew on plants and other objects.

Yes; a heavy frost, on two successive mornings, is mostly followed by rain or snow within a few hours.

Rain, Snow, and Hail.

The cold condenses the vapor in the air, so that its moisture is forced out in drops, which fall to the earth.

A part of it remains on the surface of the earth, and is either absorbed again by the air, or is carried away to the ocean by the creeks and rivers.

It sinks into the earth, to furnish moisture to the roots of plants, and to supply the streams beneath the earth’s surface with water.

By washing away many things that would, if left, decay and make the air impure and unhealthy.

When the air is cooled below thirty-two degrees, what becomes of the moisture in it?

It represents a few of the various and beautiful forms of the snow-flake or snow-crystals.

Yes; for it seldom snows when the temperature of the air is much below thirty-two degrees.

Because the air in the snow makes it a poor conductor 115 of heat, so that it prevents the warmth of the earth from passing off into the air. Snow is, therefore, a protection to the plants, and prevents their being frozen by the cold weather of winter.

The rain-drops are frozen as they fall through the air; hence, the hail-stones of winter are no larger than the rain-drops.

When the air in contact with the earth or the plants is cooled, its vapor is condensed into dew.

When the air in contact with the earth or the plants is cooled below thirty-two degrees, its vapor is condensed into dew, and the dew is frozen into frost.

When the air near the earth is slightly cooled, its vapor is condensed into fog.

When the air higher above the earth is slightly cooled, its vapor is condensed into clouds.

When the air is cooled below thirty-two degrees, its vapor is condensed and frozen into snow.

When the rain-drops, while falling, pass through a current of air cold enough to freeze them, hail is produced.

Ice.

Because water, much above the freezing-point, is always flowing into them from the earth, and thus they are kept too warm to freeze.

Because the water flowing into them comes from near the surface of the earth, and is soon made cold enough to freeze.

No; fresh water freezes at thirty-two degrees, but 117 salt water requires a lower temperature depending upon the amount of salt it contains.

The surface of the ocean is never frozen over except in very cold latitudes; hence, vessels may sail on it at all seasons of the year, and carry the products of one country to another; thus adding much to our comfort and our enjoyment.

CHAPTER VI.
Sound.

They get smaller and smaller the farther they go, until at last there is no wave left.

Yes; the smaller the wave the fainter the sound, until at last both the wave and the sound die away.

No; as all waves of sound decrease in size as they recede, the smallest ones will soonest disappear.

Because the waves in the air from the loud sound will extend farther away than those from the fainter one.

When a piece of wood attached to a string is swung rapidly around, what kind of a noise is made?

When sounds, too faint to be heard separately, are united, they produce 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.

Sixteen vibrations in a second of time, will produce the gravest sound that can be heard.

Twenty-four thousand vibrations in a second of time, make the most acute sound that can be heard.

When the air is forced out of the lungs, it causes these chords to vibrate, so as to produce the voice.

They are usually larger and coarser in men, and they are smaller and finer in women.

They make an acute voice; hence, woman can, when singing, raise her voice to a very high pitch.

No; some persons cannot tell one musical note from another, and some cannot even tell one bird from another by its notes.

No; some can hear more acutely than others; a lion has more acute hearing than a man.

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.

On the top of a high mountain the human voice is 123 heard only at a distance of a few yards; and a pistol when exploded sounds as faintly as the breaking of a stick.

The human voice sounds so distinctly in the dense air of deep mines, that conversation may be carried on in a whisper.

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.

Because the particles of water in the damp air increase its power of conduction.

Because the damp air over the water is a better conductor, and because it contains fewer objects to obstruct the waves of sound.

The words “all’s well” have been heard across the Strait of Gibraltar, a distance of ten miles.

The wind carries the sound the way in which it is 124 blowing, so that a noise may be heard much farther with the wind than against it.

When a bell is rung under water, the sound may be heard by any person whose head is beneath the surface of the water.

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.

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.

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.

Trumpets and Speaking-Tubes.

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?

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.

Because the hand catches more of the waves of sound, and conducts them to the ear.

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.

By being able to turn their ears, just as we do the ear-trumpet, in the direction whence the sound comes.

No; even a whisper has been heard through a metal pipe over three thousand feet long.

Velocity of Sound.

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.

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.

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.

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.

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 128 this number, it will give us the distance in feet; because sound travels eleven hundred and twenty feet in a second.

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.

Sound travels about four times as rapidly through water as it does through air.

Sound travels ten times as rapidly through wood, and sixteen times as rapidly through glass, as it does through air.

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.

They are thrown back, as light from a looking-glass, or a ball from a stone wall.

Sound, like light, is reflected so that the angles of incidence and reflection are always equal.

Echoes may be heard in the deep caves of the earth; because the walls of the caves reflect sounds striking against them.

There is an echo in Fairfax County, Virginia, which sends back twenty notes played on a flute.

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?

It must be twice the one hundred and twelve feet, or two hundred and twenty-four feet, to hear two syllables; 130 and three times that distance to hear three syllables, and so on.

Because the walls of the building reflect his voice to the hearers, so that more waves of sound reach their ears.

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.

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.

CHAPTER VII.
Matter.

Yes; they vary so much that we can scarcely find two things exactly alike in shape and size.

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.

No; it does not rebound when it strikes a soft substance, like a heap of wool or feathers.

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.

Brittle substances are generally hard, while flexible substances are much softer.

Steel, when heated and cooled slowly, becomes flexible, but when cooled quickly, it becomes brittle.

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.

It is necessary to know the strength of wood used in the construction of stores, bridges, houses, etc.

It is useful in the construction of suspension bridges, and in very many other ways.

Platinum, silver, iron, copper, and gold, are the most ductile, in the order named.

When glass is melted it becomes ductile, and may be drawn into very fine threads.

Steel is seven times as heavy as water, and is, therefore, seven times as dense.

Mercury is thirteen times as dense as water, and is, therefore, thirteen times as heavy.

Because the fibres of pine are more easily separated than the fibres of oak; hence, pine wood is more penetrable than oak wood.

CHAPTER VIII.
Attraction.

Cohesion and Adhesion.

The strength of a substance depends upon the amount of cohesion among its particles.

Heat destroys the cohesion between the particles of ice, and thus it is changed into water.

When still more heat is applied, the cohesion in the water is destroyed, and thus it is changed into steam.

Yes; there is some cohesion in liquids, but it is only strong enough to hold them together in drops.

Yes; the cohesion in mercury is stronger than it is in water; therefore, a drop of mercury is larger than a drop of water.

The cohesion in chloroform is weaker than it is in water; therefore, a drop of chloroform is smaller than a drop of water.

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.

A shot-tower must be not less than two hundred feet high, so that the lead may harden while falling.

Paint adheres to wood, dust to furniture, and tin and mercury to the back of a looking-glass.

Mortar fastens together the bricks and stones used in the construction of houses and other buildings.

Paste fastens pieces of paper together, and it fastens paper to the walls and ceilings of rooms.

Capillary Attraction.

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 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.

In blotting-paper when it takes up the ink, and in bread when it soaks up the juice of meats.

When one end of a towel is placed in a basin of water, and the other end is left hanging outside the 141 basin, the water will pass through the towel by capillary attraction, until the basin is entirely dry.

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 in Fig. 26.

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.

Gravitation.

Yes; heavy bodies do attract more than light ones, although they may be of the same size.

No; all things are drawn to the earth with so much force that they cannot fall away.

The weight of a body depends upon the force with which it is drawn towards the earth.

When water is poured from a tumbler, why does it run down the side of the tumbler?

Because the attraction of cohesion draws the water to the side of the tumbler, as is shown in Fig. 27.

Because the spout is curved so that its lowest point is where the water falls from it; hence, gravitation causes the water to fall directly to the earth, as is shown in Fig. 28.

Yes; air obstructs all bodies when falling through it; but it obstructs light bodies more than heavy ones.

Those that are the heaviest or most dense; because they are the smallest in bulk.

A falling body increases thirty-two feet in velocity for every second of time it is in falling. During the first second it falls sixteen feet. During the second second, it falls sixteen feet plus thirty-two feet, or forty-eight feet. During the third second, it falls forty-eight feet plus thirty-two feet, or eighty feet, etc.

A rising body decreases thirty-two feet in velocity for every second of time it is in rising. If it be three seconds in rising, its velocity will be eighty feet the first second, forty-eight feet the second second, and sixteen feet the third second, being just the reverse of the same body when falling, as is shown in Fig. 29.

It may be found by multiplying the number of seconds it is in falling, by the same number, and this product by sixteen feet. Thus, when it is three seconds in falling, multiply three by three, and this product by sixteen feet, which gives one hundred and forty-four feet, the distance it falls in three seconds.

We have an example in the large hail-stones which fall to the earth in summer-time, breaking the glass in our windows and doing much injury to the crops.

Centre of Gravity.

The centre of gravity in a body, is the point on which the body may be balanced, as is shown in Fig. 30.

Because the centre of gravity is nearer the earth when the egg is placed upon its side than when upon either end, as is shown in Fig. 31 and Fig. 32.

Because its centre of gravity is always the same distance from the earth, whatever may be its position.

A body will change its position whenever by so doing its centre of gravity will be lowered.

When its centre of gravity always remains at the same distance from the earth while being moved.

Whenever the centre of gravity in it has to be lifted up in order to make the change, as is shown in Fig. 35.

The centre of gravity, in Fig. 33, may be lowered; hence, it will change its position if unsupported. The centre of gravity, in Fig. 34, can neither be lowered nor lifted up; hence, it may be readily rolled about. The centre of gravity, in Fig. 35, must be lifted up; hence, it remains firm in its place.

It stands firm when it has a broad base, and its centre of gravity is near the earth.

The higher the centre of gravity in any body is raised above the earth, the greater is its tendency to fall.

Why is a load of hay more easily overturned than a load of iron, when upon the same wagon?

Because the centre of gravity in the load of hay is much higher than it is in the load of iron.

Because their weight on the top raises the centre of gravity in the stage, and it is more readily overturned.

Because the centre of gravity is higher in a person when standing than when sitting.

Because the centre of gravity in the boy is farther from the earth when he is raised on stilts.

It will be overturned whenever the line from the centre of gravity to the earth falls outside of the wheels, as is shown in Fig. 36.

Lamps should have large bottoms so that the line of gravity may not fall outside of the bottom.

It must always fall within the base of that body, or the body will be overturned.

Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported.

Why does a person standing up in a wagon fall when the wagon is suddenly stopped?

Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported.

Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better.

Why can a horse or a cow remain standing for many hours without seeming to be weary?

Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness.

Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position.

Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position.

When the pendulum is at A, in Fig. 37, gravity causes it to descend to C, and the motion it gains while falling to C carries it up to B; gravity then causes it to descend to C, and the 150 motion it gains while falling to C carries it back to A again.

By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw.

Specific Gravity.

The weight of a body depends upon the force with which it is drawn towards the earth.

Specific Gravity is the weight of any body, compared with an equal bulk of water.

When the water thus displaced weighs just one pound, how much less will the lead weigh while in the water than it did in the air?

Its weight in the water is as much less as the weight of the water which it displaces.

We make use of it in the construction of vessels, so as to carry on trade with other parts of the world.

Because their light, downy feathers make their specific gravity less than that of the water; hence, they must float on its surface.

When the lungs are filled with air, the specific gravity of the human body is rather less than that of water.

Yes; if a person, who cannot swim, will throw his head back so that his mouth and nose only remain above the water, he may float in this position for some time.

The specific gravity of fish is so nearly equal to that of water, that they can move about anywhere beneath its surface.

When the bladder is filled with air, the specific gravity of the fish becomes lighter, and it rises towards the surface; and when the air is forced out, the specific gravity becomes heavier, and the fish sinks towards the bottom.

Because the fat reduces the specific gravity of the body, so that it floats more easily.

Because salt increases the weight of the water, and, therefore, it buoys up with more force.

An egg will sink when placed in fresh water; but if salt be dissolved in the water, the egg will float near the surface.

CHAPTER IX.
Motion.

Because the force of gravity gradually overcomes the force given by the bat, so that the ball moves in a curve.

Because the force of gravity gradually overcomes the force given by the powder, so that the bullet moves in a curve to the earth.

When a person aims directly at a mark some distance from him, where will the bullet strike?

Because the force of gravity draws it towards the earth, while it is going from the gun to the mark.

When a ball attached to a string is whirled around, as in Fig. 38, what is the tendency of the ball?

Because the motion of the wheel gives centrifugal force to the mud, and causes it to fly off.

It is prevented by making the wheel very strong, so that all parts of it are held firmly together.

The centrifugal force is so great that it sometimes causes grind-stones and mill-stones, when revolving rapidly, to burst into pieces.

When they burst, their pieces fly off in different directions, doing more or less injury to whatever obstructs their course.

When a horse turns a corner, why does the rider lean in the direction in which the horse is turning?

He leans so as to overcome the centrifugal force, and is thus prevented from falling off the horse.

The outer rail is made higher than the inner rail, so as to overcome the tendency of the cars to run off the track.

The earth and other planets that move around the sun, are kept in their places by the action of these two forces.

It bounds off from the solid body, so that the angle of reflection is always equal to the angle of incidence, as is shown in Fig. 39.

A stone may be skipped, by throwing it so obliquely that the water will cause it to bound off from its surface.

When a boat moves across a river at the rate of four miles an hour, and the current runs four miles an hour, in what direction will the boat go?

Because the boat will be carried as far down the river as the river is wide. If the river be one mile wide, then the boat, while crossing, will be carried down it one mile.

Yes; because the boat is carried farther down the stream by the current, and it must, therefore, go a greater distance in crossing.

Because his body partakes of the motion of the wagon and moves on after his feet are stopped by the ground, so that he falls forwards.

He can prevent falling by jumping in the same direction as that in which the wagon is moving.

Because it partakes of the motion of the vessel, and is carried forward while falling.

When a ball is thrown perpendicularly into the air by a person on a boat, where does the ball fall?

The ball falls back into the person’s hand, although the boat may have carried him several feet while the ball was in the air.

Because it partakes of the motion of the boat, and moves as far as the boat moves.

It will strike as far ahead as the car moves while the stone is going to the post.

Because the bird will move a short distance through the air while the shot is reaching it.

When a body in motion strikes one at rest, is the force felt alike by both bodies?

The force is felt alike by both bodies; thus, when one head is struck against another, both are equally hurt.

A nail may be driven into a board by a hammer, when it falls rapidly upon the nail; a bullet may be shot through a plank; and even a tallow candle, when shot from a gun, may move with velocity enough to pass through a pine board.

Because they must pull some time before the force reaches all parts of the wagon, causing it to move.

Because it requires time for the force applied to reach all parts of the machinery.

When two solid bodies, moving slowly, strike against each other, what takes place?

The toes of a duck or a goose are joined together by a web which makes the foot broad, like a paddle; and when these broad feet are pressed against the water, the duck or the goose moves forward.

No; only those birds can swim whose toes are joined together by a web, thus giving them a broad foot to be pushed against the water.

We lessen the friction in machinery by oiling the parts where they rub together.

The attraction of gravitation, which causes water always to fall to its lowest level.

The friction, caused by the water rubbing against the banks and bottoms of the streams, retards their current.

It is strongest in the middle of a stream, because there is less friction there.

The friction of the water against the sides of the pipe, retards its flow very much.

Because the friction between the wheel and the earth, prevents the wheel from sliding along.

Because the friction, between the wheels and the rails, causes the wheels to turn around instead of sliding; and thus the engine moves forward drawing the cars with it.

Sand is put on the rails to increase the friction between 162 the wheels and the rails, so that the engine may start a heavier train.

Because their wheels then sink farther into the earth, and thus the friction is increased.

Because there is less friction between his feet and the ice than there is between his feet and the earth.

We put ashes on our walks to increase the friction between the ice and our feet, so that we may not slip down when walking on them.

The lock is pressed against the wheels, so that enough friction is produced to check the motion of the wagon.

Why does it require more force to draw a rusty plough through the ground, than to draw one that is bright and smooth?

Because the rust increases the friction between the plough and the ground, and this makes more force necessary in order to move it.

Because the skin of the eel is so smooth, that there is much less friction between it and our hand, than there is between the fish and our hand. Dry sand or dry ashes sprinkled on the eel will increase the friction so that it can be held.

CHAPTER X.
Mechanical Powers.

Lever.

When the spoon is called a lever, what are the parts on each side of the fulcrum called?

By applying a force to the handle of the spoon, equal to the weight of the water put in the bowl.

The power, the fulcrum, and the weight, are always necessary when the lever is used.

It is where the fulcrum is between the power and the weight, as is shown in Fig. 41.

When the fulcrum is under the middle of the lever, how can the lever be kept balanced?

The lever will remain balanced, so long as the power and the weight used are equal to each other.

When the lever is moved, so that the fulcrum is nearer the weight, which arm of the lever is the heavier?

No; the nearer the fulcrum is to the weight, the less will be the power needed to balance that weight.

How will it be when the fulcrum is nearer to the power than it is to the weight?

When the long arm of the lever is twice the length of the short arm, what will be the difference in the weights?

One pound applied to the long arm will then balance two pounds applied to the short arm.

It depends upon how much nearer the fulcrum is placed to one end of the lever, than to the other end.

A see-saw may be made by placing a plank or a board across a rail in a fence, so that it will balance.

By pushing against the earth with their feet, the arms of the lever are alternately forced to rise and fall in the air, thus making a see-saw.

No; when the see-saw is balanced, a very little force will cause it to move up and down.

When the lead put in one scale weighs exactly a pound, how much sugar must be put in the other scale to balance the lead?

In Fig. 45, the distance from the fulcrum to the power is ten times the distance from the fulcrum to the weight; now, if the small body weighs one pound, how much must the large body weigh, so as to balance it?

When the small body weighs ten pounds, how much must the large body weigh, so as to balance it?

If the smaller body be moved half-way to the fulcrum, how much weight will then balance it?

Yes; some scissors are made so strong that they are used for cutting iron, tin, and zinc.

It is where the weight is between the power and the fulcrum, as is shown in Fig. 47.

We have an example in the lid of the tea-kettle, and in the cover of the ink-stand, when they are fastened on by hinges.

It is where the power is between the fulcrum and the weight, as is shown in Fig. 48.

The fishing-rod is an example of this kind. One hand is the fulcrum, the other hand is the power, and the line, with the fish, is the weight. This is made plain by Fig. 49, representing a lad just drawing a fish out of the water.

There are several other examples of this kind of lever; will you name some of them?

Wheel and Axle.

When the diameter of the wheel is ten times the diameter of the axle, how much will one pound raise?

Yes; when the crank is turned, it forms a circle the same as the outside of the wheel.

Pulley.

The single pulley is used for hoisting flags to the top of high poles, for raising sails to the topmasts 177 of vessels, and for raising weights to the upper stories of mills and factories.

When a horse is to raise the weight, two pulleys are needed, as may be seen in Fig. 56.

They arrange the pulleys so as to lift the hay into their barns, by means of a horse; thus saving themselves much hard labor.

Inclined Plane.

When one end of a plank is raised up, while the other end remains on the floor, is its surface level?

When an inclined plane is ten feet long and only one foot high, how much will one pound at P balance?

When an inclined plane ten feet long is two feet high, how much will one pound at P balance?

One pound at P will then balance only five pounds at W; because the plane is five times as long as it is high.

Can they be taken out of the wagon any more easily on an inclined plane than without one?

The hills carry off the waste water from the earth, so that it is kept dry enough for plants to grow.

The beds of the creeks and rivers are so many inclined planes down which their waters flow.

Wedge.

Knife-blades, hatchets, axes, razors, and cutting instruments of nearly all kinds, are in the form of the wedge.

Screw.

It begins at the bottom of the screw, and winds around and around the screw until it reaches the top, as is shown in Fig. 60.

When the threads are far apart, the inclined plane is steep; but it becomes less steep the nearer the threads are together.

Screws are used to fasten hinges and bolts on doors and shutters, and to fasten pieces of wood together in the construction of houses, boats, wagons, and other things.

The screw is used where much power is needed, as in the vise; where pressure is desired, as in copying letters, and in coining money; and, also, for squeezing the juice from apples, grapes, and sugar-cane.

It works in a nut which has threads cut on the inside, so that the threads of the screw move in them.

Yes; loaded cars, and even heavy buildings, may be raised by means of a screw of this kind.

CHAPTER XI.
Electricity.

If a horse be rubbed in the winter when it is dark, what will be given off from him?

When sealing-wax is rubbed with a silk handkerchief, or a piece of flannel, what is produced?

It will attract bits of paper, straw, feathers, and hairs, when it is held near them.

Electricity is developed by motion, by friction, and by almost every change that takes place.

When one body contains positive and the other body negative electricity, what will the two bodies do?

When a body does not permit the electricity to pass through it, what is it called?

Whenever one cloud becomes filled with more electricity than another, the cloud having the most electricity gives off a portion to the cloud having the least electricity.

The clouds are seldom half a mile away when the lightning comes from them to the earth.

The lightning passes so quickly through the air, that it makes a noise called thunder.

Yes; it does much harm by destroying trees, houses, animals, and, indeed, almost everything in its course.

They are protected by means of the lightning-rod, which was invented by Dr. Franklin.

The lightning-rod is made of some metal, usually of iron or copper, which is a good conductor of electricity.

The lightning-rod conducts the electricity to the earth, so that it does not injure the building.

The point should be made of silver or platinum, so that it will always remain bright.

Because they are good conductors and will help to conduct the lightning into the earth.

If made of iron, the lightning-rod should be about three-fourths of an inch in diameter.

Because the fire is a good conductor, and sometimes draws the electricity into the room.

Yes; because the feathers are a non-conductor, and will not attract the lightning.

Thunder-storms are more frequent in summer than in winter, and more so in the afternoon than in the morning.

The rapidity of lightning is probably not less than 250,000 miles in a second, and the flash does not exceed the millionth part of a second in duration.

Magnetism.

A magnet is a piece of iron ore having the power of attracting iron and steel.

They are dug out of the earth, and are found most abundantly in Norway and Sweden.

Where does a magnetic bar always point, when it is made to move easily on a pivot?

The positive end of one magnet always attracts the 189 negative end of every other magnet; but if the two positive or the two negative ends are brought together, they will repel each other.

Because steel will remain magnetized, but iron soon loses the magnetic properties.

A compass is an instrument containing a magnetic needle, so balanced as to move easily on a pivot.

It is used by travellers to guide them on their journey; by surveyors to mark out the boundary lines between different properties; and by seamen to point out their way across the sea from one place to another.

Before the Mariner’s Compass was invented, seamen were afraid to venture out of the sight of land, for fear of being lost on the ocean; for they then had no means of telling where they were, or in what direction to guide their vessels, so as to reach the land again.

Electro-Magnetism.

The magnet is made of soft iron, around which are 190 wound a great many coils of wire, and these are covered with silk.

The first line of telegraph was built in 1844, from Washington to Baltimore, by Professor Morse.

At one end of the telegraph wire is an electrical battery, and at the other end is an electro-magnet. When a message is to be sent, the electricity is transmitted through the wire to the soft iron of the electro-magnet, which causes the iron to move, and this motion is conveyed to the machinery, which is needed to print the message in letters or signals, so as to be understood.

Each station must have both a key and a receiver, so as to send as well as receive messages.

The Atlantic cable is a telegraphic wire reaching across the Atlantic Ocean, from America to Europe.

The first Atlantic cable was successfully laid in 1866, between Newfoundland and Ireland. It is eighteen hundred and sixty-four miles long, and its success is chiefly due to Cyrus W. Field.

The Atlantic cable forms a telegraphic connection between Europe and America, so that news may be sent from one country to the other in a few moments of time; thus the people of the two continents, although so far apart, can communicate with each other many times in a single day.

THE
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The Project Gutenberg eBook of First Lessons in Natural Philosophy for Beginners

FIRST LESSONS IN Natural Philosophy FOR BEGINNERS.