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diff --git a/old/53778-0.txt b/old/53778-0.txt deleted file mode 100644 index 83b3e33..0000000 --- a/old/53778-0.txt +++ /dev/null @@ -1,9719 +0,0 @@ -The Project Gutenberg EBook of First Lessons in Natural Philosophy, by -Joseph C. Martindale - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - -Title: First Lessons in Natural Philosophy - For Beginners - -Author: Joseph C. Martindale - -Release Date: December 20, 2016 [EBook #53778] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK NATURAL PHILOSOPHY *** - - - - -Produced by Stephen Hutcheson, Kate Rooney & Ian Smith, -MFR and the Online Distributed Proofreading Team at -http://www.pgdp.net - - - - - - - - - - FIRST LESSONS - IN - Natural Philosophy - FOR BEGINNERS. - - - BY - JOSEPH C. MARTINDALE, M.D., - - LATE PRINCIPAL OF MADISON GRAMMAR SCHOOL, PHILADELPHIA; AUTHOR OF A - HISTORY OF THE UNITED STATES FOR SCHOOLS, ETC. - - [Illustration: Paddlewheel steamboat.] - - - PHILADELPHIA: - ELDREDGE & BROTHER, - No. 17 North Seventh Street. - 1881. - - Entered, according to Act of Congress, in the year 1872, by - ELDREDGE & BROTHER, - in the Office of the Librarian of Congress at Washington. - - J. FAGAN A SON, - ELECTROTYPERS, PHILAD’A. - - [Illustration: KEYSTONE PRESS OF W^M. J. DORNAN] - - - - - Preface. - - - [Illustration: Decoration] - -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 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. - -J. C. M. - - [Illustration: Decoration] - - [Illustration: CONTENTS] - - - PAGE - CHAPTER I.—Astronomy. 9 - The Earth and other Planets 9 - The Moon 14 - Eclipses 18 - CHAPTER II.—Light. 22 - Color 22 - Refraction of Light 32 - Reflection of Light 34 - The Eye 38 - CHAPTER III.—Heat. 42 - Sources of Heat 42 - Expansion by Heat 46 - Conduction of Heat 54 - Convection of Heat 59 - Liquefaction 61 - Latent Heat 63 - Ebullition 65 - Vaporization 66 - Evaporation 69 - Radiation 71 - Reflection 73 - Absorption 74 - CHAPTER IV.—The Air. 77 - Winds 96 - CHAPTER V.—Water. 99 - Springs and Streams 104 - Fogs and Clouds 106 - Dew 109 - Frost 112 - Rain, Snow, and Hail 113 - Ice 116 - CHAPTER VI.—Sound. 118 - Conduction of Sound 122 - Trumpets and Speaking-tubes 125 - Velocity of Sound 127 - Reflection of Sound 128 - CHAPTER VII.—Matter. 131 - CHAPTER VIII.—Attraction. 137 - Cohesion and Adhesion 137 - Capillary Attraction 140 - Gravitation 141 - Centre of Gravity 145 - Specific Gravity 150 - CHAPTER IX.—Motion. 154 - CHAPTER X.—Mechanical Powers. 164 - Lever 164 - Wheel and Axle 174 - Pulley 176 - Inclined Plane 177 - Wedge 180 - Screw 181 - CHAPTER XI.—Electricity. 183 - Magnetism 187 - Electro-Magnetism 189 - - - - - FIRST LESSONS - IN - Natural Philosophy. - - - - - CHAPTER I. - Astronomy. - - - The Earth and Other Planets. - - [Illustration: Fig. 1.] - - [Illustration: Illustrated Capital W] - - -What is the shape of the Earth on which we live? - -It is round like a ball. - - -What other names are given to the Earth? - -It is called the World or Globe. - - -What else is it sometimes called? - -It is called a Planet. - - -Are there any other planets? - -Yes; some of the stars are called planets. - - -Is our earth or world a star? - -It is a star. - - -Why do the other stars seem to us different from the earth? - -Because they are so far away from us. - - -What gives light to the planets? - -The Sun gives light to the planets. - - -Of what is the sun the centre? - -The sun is the centre of the solar system. - - -Do the planets remain in one place? - -The planets do not remain in one place, but are always moving. - - -How do the planets move? - -They move around the sun. - - -Are some planets nearer the sun than others? - -Yes; some planets are much nearer the sun than others. - - -Which planet is nearest the sun? - -The planet called Mercury. - - -Which is next to Mercury? - -The planet called Venus. - - -And which is third in distance from the sun? - -Our own planet, the Earth. - - -Are there any planets still farther from the sun? - -Yes; Mars, Jupiter, Saturn, Uranus, and Neptune, and many smaller -planets. - - -How does each planet move? - -It moves in its own track or orbit around the sun, as is represented in -Fig. 2. - - [Illustration: Fig. 2.] - - -Does the earth always move in the same orbit around the sun? - -Yes; the earth moves in the same orbit around the sun, year after year. - - -How long does it take the earth to move around the sun? - -365 days, 5 hours and 49 minutes, or one year. - - -What makes a year to us? - -A year to us is the time required by the earth to move around the sun. - - -If the earth is in a certain place at noon, on New Year’s day, when will -it be in that place again? - -5 hours and 49 minutes after noon on New Year’s day of the next year. - - -What, then, does the earth do every year? - -The earth moves around the sun and comes back to the place whence it -started, once every year. - - -Has the earth any other motion than the one around the sun? - -It has; it turns over and over. - - -Does the earth stay in one place as it turns over and over? - -It does not; it moves on in its track around the sun. - - -How many times does it turn over while going around the sun? - -Three hundred and sixty-five times. - - -How many times, then, does it turn over in one year? - -The earth turns over three hundred and sixty-five times in a year. - - -What is the time which it takes to turn over once called? - -It is called a day. - - -How many days in a year? - -There are three hundred and sixty-five days in a year. - - -What two motions, then, has the earth? - -It turns over and over, and at the same time it moves around the sun. - - -What change is produced on the earth by its moving around the sun? - -The change of Seasons. - - -Will you name the seasons? - -Winter, Spring, Summer, and Autumn. - - -Do they follow each other in regular order? - -They do; Spring follows Winter, Summer follows Spring, Autumn follows -Summer, Winter follows Autumn, and Spring follows Winter again. - - -How often have we these seasons? - -As often as the earth moves around the sun, which is once a year. - - -What change is produced on the earth by its turning over? - -The change of day and night. - - -Where is it day? - -It is day on that part of the earth which is turned towards the sun. - - -Where is it night? - -It is night on that part of the earth which is turned away from the sun. - - -How often do we have the change from day to night? - -As often as the earth turns over, which is once in twenty-four hours. - - -What is a day? - -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. - - -What does a day include? - -It includes the night-time as well as the day-time, of the twenty-four -hours. - - -When it is day-time on one part of the earth, what is it on the opposite -part? - -It is night-time on the opposite part. - - -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. - - -On how much of the earth is it always day? - -It is day on one-half of the earth, while on the other half it is night. - - -The sun seems to move around the earth; does it really do so? - -No; it does not move around the earth. - - -Does the _earth_ move? - -Yes; it turns over once every day. - - -Why does it not seem to us to be moving? - -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. - - -In what direction does the earth turn on its axis? - -The earth turns from west to east. - - -Where do the sun, moon, and stars rise? - -The sun, moon, and stars rise in the east. - - -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. - - -When is it sunrise to us? - -When the part of the earth on which we are, first comes into the sun’s -rays. - - -When is it sunset to us? - -When the part of the earth on which we are, moves out of the sun’s rays. - - - The Moon. - - -We have been told that the earth moves around the sun; does any planet -move around the earth? - -Yes; one planet moves around the earth. - - -What is that planet called? - -It is called the Moon. - - [Illustration: Fig. 3.] - - -How often does the moon move around the earth? - -Once in 27 d. 7 h. 43 min. - - -How long is the time from new moon to new moon again? - -It is 29 d. 12 h. 44 min. - - -How often do we have new moon? - -Once every 29 d. 12 h. 44 min. - - -What is the period from one new moon to another called? - -It is called a Lunar month. - - -Are the lunar months all of the same length? - -Yes; they are all of the same length. - - -Is there any other kind of month? - -Yes; the Calendar month, as it stands in almanacs. - - -How many calendar months in a year? - -There are twelve calendar months in a year. - - -Are the calendar months all of the same length? - -No; some are thirty-one days long, some thirty days long, and one only -twenty-eight days long. - - -When can we see the moon and stars best? - -We can see them best when it is night. - - -From what do the moon and the other planets get their light? - -They get their light from the sun. - - -What do they do with this light? - -They reflect it. - - -How do we see them? - -We see them by the light which they reflect. - - -Why cannot we see the moon and stars in the day-time? - -Because the bright light of the sun hides their light. - - -Are there stars in every direction around the earth? - -There are stars in every direction around the earth. - - -How do we know this? - -Because stars may be seen from every part of the earth, when it is -night. - - -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. - - -What planet moves around the earth? - -The moon moves around the earth. - - -Upon what part of the moon does the sun shine? - -It shines upon the part towards the sun. - - -What does the moon do with the light which it receives from the sun? - -The moon reflects the light received from the sun. - - -What is this reflected light called? - -It is called moonlight when reflected from the moon. - - -Is that part of the moon on which the sun shines always towards the -earth? - -It is not always towards the earth. - - -When all of the moon on which the sun shines is towards the earth, how -does the moon appear? - -The moon appears to be round, and the whole of it seems to shine. - - -What is the moon then called? - -It is called the Full Moon. - - -When no part of the moon on which the sun shines is towards the earth, -how does the moon appear? - -We do not see the moon at all, and we say there is no moon. - - -When a small part of the moon on which the sun shines is first turned -towards the earth, what is it called? - -It is called the New Moon. - - -Does the lighted part of the moon grow? - -It seems to grow larger and larger, until the whole of the moon is -lighted. - - -What is it then called? - -It is then called the Full Moon. - - -What then takes place? - -It seems to grow smaller and smaller, until the whole of the lighted -part is gone. - - -Just before the last part is gone, what is it called? - -It is called the Old Moon. - - -How long is it from one full moon to another full moon? - -It is a lunar month. - - -How many full moons are there in a year? - -There are thirteen full moons in a year. - - -Is the sun much larger than the moon? - -The sun is many thousand times larger than the moon. - - -Why does the moon appear nearly as large as the sun? - -Because it is so much nearer to us. - - -How far is the moon from the earth? - -The moon is nearly 240,000 miles from the earth. - - -How far is the sun from the earth? - -The sun is nearly 92,000,000 miles from the earth. - - -How long does it take the light of the sun to reach the earth? - -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. - - -How do we know that the earth is round, like a ball? - -We know that the earth is round like a ball, because its shadow is -similar to the shadow of a ball. - - -How else do we know that the earth is round? - -We also know that the earth is round, because we can circumnavigate or -sail around it. - - -Where can we see the shadow of the earth? - -We can see the shadow on the moon, when the moon is eclipsed. - - -What is the shadow on the moon like? - -It is round, like the shadow of a ball. - - -When the Earth comes between the sun and the moon, where does the shadow -of the earth fall? - -The shadow of the earth falls on the moon, as is represented in Fig. 4. - - [Illustration: Fig. 4.] - - -What is this shadow on the moon called? - -It is called an Eclipse of the moon. - - -How do we know that the moon is round? - -We know that the moon is round, because its shadow is similar to the -shadow of the earth. - - -When is the moon eclipsed? - -When it comes into the earth’s shadow. - - -When the moon comes fully into the earth’s shadow, what is it called? - -It is called a total eclipse of the moon. - - -When it comes partly into the earths shadow, what is it called? - -It is called a partial eclipse of the moon. - - -When the moon comes between the sun and the earth, where does the shadow -of the moon fall? - -The shadow of the moon falls on the earth, as is represented in Fig. 5. - - [Illustration: Fig. 5.] - - -What does this produce? - -It produces an eclipse of the sun. - - -When the moon is between the sun and the earth, can we see the sun? - -No; we cannot see the sun, because the moon prevents the rays of the sun -from coming to us. - - -When the moon prevents the sun’s rays from falling on us, what is it -called? - -It is called an eclipse of the sun. - - -When the moon prevents _all_ the sun’s rays from falling on us, what is -it called? - -It is called a total eclipse of the sun. - - -When the moon prevents only a part of the sun’s rays from falling on us, -what is it called? - -It is called a partial eclipse of the sun. - - -When will the _sun_ be eclipsed? - -Whenever the moon comes between the sun and the earth. - - -When will the _moon_ be eclipsed? - -Whenever the earth comes between the sun and the moon. - - -Who made the sun, the moon, and the stars? - -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. - - [Illustration: Paddlewheel steamboat.] - - [Illustration: Decoration.] - - - - - CHAPTER II. - Light. - - - Color. - - [Illustration: Illustrated Capital I] - - -In what way is the sun useful to us? - -It gives heat, which makes us warm, and it gives light, so that we can -see. - - -Is it useful in any other way? - -It gives light and warmth to all animals as well as to all plants. - - -What make the flowers and leaves so beautiful? - -The light and heat from the sun make them beautiful. - - -What do the light and heat from the sun ripen? - -They ripen the apples, cherries, and other fruits; also, the wheat, -corn, and other grains, and make them fit for food. - - -When it is night-time or dark, can we distinguish objects? - -No; because all things are then without color, and they cannot be seen. - - -As it grows light in the morning, what takes place? - -At first, we see the objects around us faintly; but as it grows lighter, -we see them more distinctly. - - -Do they all show the same color in the light? - -They do not; they show different colors. - - -What enables us to see the color of different things? - -Light enables us to see the color of everything. - - -What kinds of light have we? - -We have the light of the sun, moon, and stars; also, the light from the -burning of any substance. - - -Does the light show many colors? - -It does show many colors or shades of color. - - -How may some colors be made? - -They may be made by mixing other colors together. - - -How can we make a purple color? - -By mixing a red and a blue color together. - - -How can a green color be made? - -By mixing a blue and a yellow color together. - - -How can an orange color be made? - -By mixing a red and a yellow color together. - - -How does the light come from the sun? - -It comes in rays or straight lines. - - -What may be done with a ray of light? - -It may be separated into different colors. - - -Into how many colors may a ray of light be separated? - -Into _seven_ colors, like the colors of the rainbow. - - -What names are given to these colors? - -Red, orange, yellow, green, blue, indigo, violet. - - -How may these colors be separated? - -By a prism, or three-sided piece of glass. - - -How may this be done? - -If a sunbeam, shining through a hole in the window-shutter 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. - - [Illustration: Fig. 6.] - - -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. - - -In what order will these colors appear? - -They will always appear one above the other, in the order named. - - -How may this order be remembered? - -By the word Roy-g-biv, made from their initial letters, beginning at the -bottom. - - -From what are the colors of the rainbow made? - -From the rays of light coming from the sun. - - -When is a rainbow seen? - -A rainbow is seen when the sun shines through the drops of water as they -fall through the air. - - -How do the drops of water help to make a rainbow? - -The drops of water act like little prisms, and separate the rays of -light passing through them into the colors of the rainbow. - - -What two things, then, are needed to make a rainbow? - -A shower of water, and a bright sunshine. - - -In what part of the sky does the rainbow appear? - -In the part opposite to that in which the sun is, at the time of the -shower. - - -How does the rainbow appear? - -It appears like a great arch, spanning the sky, and its splendid bright -colors are very beautiful. - - -What may we remember in reference to the rainbow? - -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. - - -Where else may we see a variety of colors? - -In the soap-bubble. - - -Why are these colors constantly changing? - -Because the walls of the bubble are constantly growing thinner, and -different colors are, therefore, reflected from them. - - -Why do the walls of the soap-bubble grow thinner? - -Because the water runs to the bottom of the bubble until its top becomes -so thin as to burst. - - -Can we tell the color of any object by touching it? - -No; we must see it in order to tell its color. - - -How do we know this? - -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. - - -What is necessary in order to see the colors? - -Light is necessary; because all colors come from the rays of light. - - -Can we see the different colors in the ray itself? - -We cannot; because God has so blended them together that they cannot be -seen by us. - - -When most of the rays of light pass through a substance, what is said of -it? - -The substance is said to be _transparent_. - - -Name some transparent substances. - -Glass, ice, diamonds, air, and clear water. - - -When only a few rays of light pass through a substance, what is said of -it? - -The substance is said to be _translucent_. - - -Name some translucent substances. - -Flint, isinglass, scraped-horn, and china-ware. - - -When no rays of light pass through a substance, what is said of it? - -The substance is said to be _opaque_. - - -Name some opaque substances. - -Wood, iron, coal, and granite. - - -Can we see through a glass window? - -Yes; because the glass is transparent, and the light passes through it. - - -Can we see through a looking-glass? - -No; because the back of the looking-glass is covered with quicksilver, -which prevents the rays of light from passing through it. - - -What becomes of the light falling on the looking-glass? - -It is thrown back from the glass. - - -What is this called? - -It is called _reflection_. - - -What objects are good reflectors of light? - -Those having smooth and polished surfaces; such as tin, silver, gold, -and quicksilver. - - -What objects are poor reflectors of light? - -Those having dull, uneven surfaces; such as iron, wood, cloth, leather, -and calico. - - -Of what is every ray of sunlight composed? - -It is composed of the seven colors of the rainbow. - - -What does a looking-glass do with these colors? - -A looking-glass throws them all back from its surface. - - -What does a glass window do with these colors? - -A glass window permits all these colors to pass through it. - - -What do opaque substances do with these colors? - -They throw back or reflect some of them. - - -Do they all reflect the same color? - -No; some reflect one color, and some another color. - - -What color does the grass reflect? - -It reflects the green color; therefore, the grass is green. - - -What does the grass do with the other colors? - -It absorbs or hides them in itself. - - -How do we know the color of anything? - -We know it by the color which it reflects. - - -Why is the rose red? - -Because it reflects the red color, and absorbs or hides the other colors -in itself. - - -When anything _absorbs_ all the colors of a ray of light, what color is -it? - -It is black. - - -When it _reflects_ all the colors of light, what color is it? - -It is white. - - -Why are the letters on this page black? - -Because they absorb all the colors of light, and reflect none. - - -Why is the paper white? - -Because it reflects all the colors of light, and absorbs none. - - -Why are all things black in the dark? - -Because there are no rays of light falling on them; hence, there is no -color for them to reflect. - - -Does an object reflect its color in every direction? - -It does reflect its color in every direction. - - -How do we know this? - -Because thousands of persons, in different directions from the same -object, can see it at the same time. - - -How are the rays of color reflected from an object? - -They are reflected in straight lines. - - -Where must a person be so as to see an object? - -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. - - -What color does a red ribbon reflect? - -It reflects a red color. - - -Why are some red ribbons of a brighter red than others? - -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. - - -Upon what does the brightness or dullness of a color depend? - -It depends upon the number of rays of that color reflected. - - -What variety does this produce? - -It produces a great variety in the shades and tints of color. - - -Where may many of these shades and tints be seen? - -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. - - -What may these things teach us? - -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. - - -Why cannot we see the light of a lamp or candle distinctly, in the -day-time? - -Because the strong rays from the sun hide the more feeble rays from the -lamp. - - -Why cannot we see the stars in the day-time? - -Because the bright light from the sun hides their more feeble light. - - -When can we see the glow-worms or fire-flies best? - -At night, when it is dark; because, at other times, the light of the sun -hides their feeble light. - - -Where may we see them at night? - -Flying about in all directions, looking like bits of fire moving in the -air. - - -Does a looking-glass reflect all the colors of light falling upon it? - -It does; hence, the ray of light is not changed. - - -When the object is of a blue color, why is blue only reflected? - -Because it is the only color falling upon the glass, and there is no -other color to reflect. - - -Is the color of any object changed by falling upon a looking-glass? - -No; the color of every object is reflected from a looking-glass without -change. - - -Do other objects generally reflect the colors falling upon them? - -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. - - -What objects reflect the most light? - -Those of a white or light color. - - -What objects reflect the least light? - -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. - - -What advantage do we derive from the green color of the grass and the -leaves? - -They reflect a soft, pleasant light, which does not dazzle or hurt our -eyes. - - -How would it be if those objects were white? - -They would dazzle us by the amount of light reflected. - - -If we wish to make a room light, of what color should the walls be? - -They should be white; and when papered, it should be with light-colored -paper. - - -Why does dark-colored paper darken a room? - -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. - - -Does the light from a burning body differ from sun-light? - -Yes; it differs in intensity, in color, and in many other ways. - - -What effect does the lamp-light have upon the color of some substances? - -It changes their color. - - -Where may we find an example of this? - -In trying to match the colors of thread or silk at night. - - -Why is the color different by lamp-light from what it is by daylight? - -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. - - -Are all persons able to distinguish one color from another? - -No; some are color-blind. To such persons all colors appear to be alike. - - -What curious instances of color-blindness may be mentioned? - -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. - - -How do the rays of light from the sun come to us? - -They come to us in straight lines. - - -When the rays of light pass from one substance to another more dense, -what takes place? - -They are bent or turned from a straight course. - - -What is said of these bent rays? - -They are said to be _refracted_. - - -What is meant by refraction? - -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. - - -Where may we see an example of refraction? - -In the rays of light from a rod which has one end in the water. - - [Illustration: Fig. 7.] - - -Why does the rod appear to be bent? - -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 surface, comes in straight lines to the eye; hence, the -rod appears to be bent at the surface, as is represented in Fig. 7. - - -How does a pole appear when one end is placed in the water? - -It appears to be bent where it meets the surface of the water. - - -How does the part of the rod under water appear? - -The part under the water appears to be raised up, as may be seen in Fig. -7. - - -How does the bottom of a pond or stream of water appear? - -It appears to be raised up, and the water does not seem so deep as it -really is. - - -Why is this the case? - -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. - - -How much does the bottom of a pond or stream of water seem to be raised -up? - -About one-third of the depth of the water. - - -If the water is really six feet deep, how deep does it appear? - -It appears to be only four feet deep. - - -What has frequently happened from not knowing this fact? - -People have been drowned from getting into water deeper than it appeared -to be. - - -Do the fish and other things floating in the water appear 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. - - -If we wish to spear the fish, what must we do? - -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? - -The line of incidence. - - -When the ball bounds back, what is its path called? - -The line of reflection. - - -What is the angle at which a ball strikes the wall called? - -The angle of incidence. - - -What is the angle at which the ball rebounds called? - -The angle of reflection. - - -When a ray of light falls upon a looking-glass, what is the path through -which it goes called? - -The line of incidence. - - -When the ray is reflected, what is its path called? - -The line of reflection. - - -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. - - [Illustration: 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. - - -Why is the reflection of the sun in a pond of water seen near the edge -at noon? - -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. - - -Where is the reflection seen in the morning or in the evening? - -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 opaque object prevents the rays of light from falling on the -wall. - - -Why will the shadow be larger, the nearer the object is to the candle? - -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. - - [Illustration: 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. - - -Why does an object seem to be smaller, the farther it is from us? - -Because the angle at which the light from it strikes the eye, is less -when at a distance, than when near to the eye. - - [Illustration: Fig. 10.] - -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. - - -Why does an object grow more dim the farther it is from us? - -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. - - -What is the use of the telescope? - -It is used to view distant objects. - - -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. - - -What class of objects do we view with a telescope? - -We view the heavenly bodies; the sun, moon, and stars. - - -What is a spy-glass? - -It is a glass mostly used to view distant objects on the earth. - - -Is the spy-glass like the telescope? - -The spy-glass is much like the telescope, only it is smaller. - - -When looking through a spy-glass, how do objects appear? - -They seem to be brought nearer to us, and they appear larger. - - -By whom are spy-glasses much used? - -Spy-glasses are much used by persons on board of vessels, while at sea. - - -What instruments have we for magnifying small objects? - -We have magnifying glasses and microscopes. - - -What are magnifying glasses? - -They are single glasses or lenses, used for magnifying small objects. - - -By whom are magnifying glasses used? - -They are used by jewellers, watchmakers, engravers, and others who -examine small objects. - - -What are microscopes? - -They are instruments used for examining very small objects. - - -How does a drop of rain-water or vinegar appear under the microscope? - -It appears to be full of strange-looking creatures, which are always in -motion. - - -How do the smallest insects appear? - -They appear to be as perfectly formed as the larger ones which are seen -with the naked eye. - - -What may be seen with a microscope? - -Thousands of things too small to be seen by us unless they are -magnified. - - -What kind of spectacles do near-sighted people use? - -Near-sighted people use glasses, thickest at the edge and thinnest at -the centre. - - -What kind of spectacles do we use as we grow old? - -As we grow old we use glasses, thinnest at the edge and thickest at the -centre. - - -Of what use are spectacles? - -When the vision is defective, spectacles enable us to see objects more -clearly. - - -How should spectacles be made? - -They should be so made as to render objects distinct, but neither to -magnify nor diminish them. - - -As people grow old, how do their eyes change? - -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? - -It is called the _iris_. - - -What is the pupil of the eye? - -The pupil is the black spot or hole in the centre of the iris. - - [Illustration: Fig. 11.] - - -What is the use of the pupil? - -The pupil is the window of the eye, and the rays of light which enter -the eye must pass through it. - - -What power has the iris over the pupil? - -The iris has the power of making the pupil larger or smaller, according -to the amount of light which it receives. - - -How does a bright light affect the iris? - -It causes the iris to contract, so that the pupil becomes smaller. - - -How does a faint light affect the iris? - -It causes the iris to dilate, so that the pupil becomes larger. - - -Of what advantage is the iris to the eye? - -The iris acts like a sentinel to protect the eye from any sudden light. - - -How does a sudden light affect the eye? - -A sudden light causes pain to the eye. - - -Of what shape is the pupil of the eye? - -In man, the pupil of the eye is circular. - - -What is the size of the pupil of the eye? - -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. - - -How else are our eyes protected from any sudden light? - -By means of our eyelids, which may be closed, and thus shut out all the -light. - - -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. - - -Why does the pupil become larger in the twilight? - -So that more rays of light may enter the eye, whereby objects may be -more distinctly seen. - - -Why can an owl or a bat see at night? - -Because the pupil of the eye in them is quite large, and admits much -light. - - -Why do they stay in dark places through the day? - -Because the bright light of day hurts their eyes. - - -May the image of an object be retained in the eye after the eyelids are -closed? - -It may for a very short time, as any one can prove by first looking at -some bright object, and then closing the eyelids. - - -Why does a burning coal moved rapidly around, seem like a circle of -fire? - -Because the light from it is retained a short time by the eye, thus -seeming to form a complete circle. - - -Why cannot we count the posts in a fence, when riding rapidly in a car? - -Because the light from each post falls in such quick succession upon the -eye, that it cannot distinguish one post from another. - - -Can objects be seen distinctly when placed near the eye? - -No; when objects are within six inches of the eye they cannot be seen -distinctly. - - -Can objects be seen when at a great distance from the eye? - -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 of the stars which we behold in the heavens, are still farther -away than the sun. - - -Of what use is the eye to us? - -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. - - -What may we remember about the light? - -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! - - [Illustration: Locomotive.] - - [Illustration: Decoration.] - - - - - CHAPTER III. - Heat. - - - Sources of Heat. - - [Illustration: Illustrated Capital W] - - -What is heat? - -Heat is that agent which causes the feeling of warmth. - - -Can heat be seen? - -No; heat cannot be seen, it can only be felt. - - -When we touch a substance hotter than we are, what takes place? - -A part of the heat from that substance comes to us and causes a feeling -of warmth. - - -When we touch a substance colder than we are, what takes place? - -A part of the heat from us goes to that substance, and we experience a -feeling of cold. - - -Does the amount of heat in any substance vary? - -Yes; water may be heated until it is scalding hot, or the heat may be -taken from it until it is frozen into ice. - - -What causes a substance to become cold? - -Taking away heat from a substance causes it to become cold. - - -Then what is cold? - -Cold is only the absence or want of heat. - - -When we pass from a very hot room to one moderately warm, how do we -feel? - -We experience a feeling of cold. - - -When we pass from a very cold room to one moderately warm, how do we -feel? - -We have a feeling of warmth, although the room is no warmer than in the -first instance. - - -How else can we prove that we judge of heat and cold by our feelings? - -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. - - -Is there anything without heat? - -No; the coldest substance known still contains some heat. - - -How many kinds of heat are there? - -There are two kinds of heat. - - -What are the two kinds of heat? - -Heat accompanied by light, as the heat from the sun, or a lamp; and heat -without light, as the heat from boiling water. - - -From what source does heat mostly come? - -Heat mostly comes from the sun, which is also the source of light. - - -What is said of the heat and light from the sun? - -Heat and light from the sun come together in the sunbeam. - - -From what other source is heat obtained? - -Heat is obtained from the burning of any substance. - - -What else does the burning of any substance produce? - -It often produces light. - - -Can the light and heat from the sun easily be separated? - -They cannot easily be separated. - - -What does a glass window do with the light and heat of the sunbeam? - -It permits both the light and the heat to pass into the room, so that we -can see the one and feel the other. - - -Are the light and heat from a fire united, as they are in the sunbeam? - -They are not; but they seem to be separated from each other. - - -What does a glass window do with the light and heat from a fire? - -It permits the light to pass through; but it stops the heat. - - -How may this be proved? - -When a pane of glass is held between the face and a fire, it will -protect the face from the heat. - - -Is there any heat without light? - -Yes; many substances contain heat, but do not emit light. - - -Does light change the amount of heat in a substance? - -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. - - -Is there any other source of heat? - -Yes; heat is produced by rubbing or striking substances together. - - -What will result from rubbing two pieces of wood together? - -They may be rubbed until they take fire. - - -How do Indians kindle fires? - -By rubbing two pieces of wood together until they take fire. - - -How is heat produced by the brakes on railroad cars? - -It is produced by the car-wheels rubbing against the brakes. - - -What may be seen when a horse strikes his shoes against a stone? - -Small sparks of fire, which contain heat, may be seen. - - -If a piece of iron be hammered, will its heat be increased? - -Its heat will be increased. - - -How can this be proved? - -A person can, by hammering a piece of cold iron, make it red-hot. - - -Could we live without heat? - -No; our bodies must be kept warm, and this can be done only by heat. - - -Is this true of other living things? - -Yes; all the animals and all the plants would die if heat were taken -away. - - -How should we feel in regard to these things? - -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. - - -How does heat generally affect substances? - -Heat _expands_ them or makes them larger. - - -Does heat expand all metals? - -Yes; all metals are expanded by heat. - - -When a rod of iron is heated, is it longer or shorter than when cold? - -It is longer than when it is cold. - - -How do we know this? - -We know it by measuring the rod when it is cold, and again when it is -heated. - - -Will the rod be larger, as well as longer? - -The rod will be larger. - - -How can we prove this? - -The rod, when red-hot, will not go through as small a hole as when it is -cold. - - -How do heat and cold affect most substances? - -Heat expands most substances, and cold contracts them. - - -Do all metals expand alike by heat? - -No; some metals expand much more than others. - - -How does heat expand substances? - -Heat pushes the parts or atoms of an object farther from each other, and -thus its size is increased. - - -What does a blacksmith do with a tire before putting it on a wheel? - -He heats the tire red-hot. - - -Why does he heat the tire red-hot? - -So that it will be increased in size, and will go on the wheel more -easily. - - -What takes place as the tire cools? - -It becomes smaller, and thus binds the parts of the wheel tightly -together. - - -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. - - -Why does a stove make a crackling noise as it grows hot? - -The particles of the iron make the noise in expanding. - - -If a piece of glass be held in the hand, will the glass become warm? - -The glass will become as warm as the hand. - - -What part of the glass will be warmed first? - -The part touching the hand will be warmed first. - - -What part of the glass will be warmed last? - -The part farthest from the hand will be warmed last. - - -Which part of the glass will be expanded first? - -The part touching the hand, because it is first warmed. - - -Which part will be expanded last? - -The part farthest from the hand, because it is the last part warmed. - - -If any other warm substance touch the glass, will the glass be heated in -the same manner? - -The glass will be heated in the same manner. - - -What often results from this unequal heating? - -The glass is broken. - - -Why are glass plates or tumblers broken when placed in hot water? - -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. - - -Why do stove-plates often break? - -Because they are fitted together in the stove so that they have not room -to expand without breaking. - - -What effect has cold upon substances? - -Cold causes them to contract or grow smaller. - - -Things _expand_ unequally; do they also _contract_ unequally? - -They do contract unequally, and in this way may be broken. - - -Why are lamp chimneys often broken while the lamp is burning? - -Because the heat of the flame causes them to expand unequally. - - -In what other way are they broken? - -By a draught of cold air, or a drop of cold water, touching them, thus -causing them to contract unequally. - - -Why are they sometimes broken when the lamp is not burning? - -Because a current of cold air from a window or a door, blows upon them. - - -Does quicksilver expand when heated? - -Like other metals, quicksilver expands as it grows warmer, and contracts -as it grows cooler. - - -Is quicksilver in the form of a solid or a fluid? - -It is a fluid as we usually see it. - - -For what is quicksilver used? - -It is used for filling the tubes of thermometers. - - -What is a Thermometer? - -It is an instrument used for measuring the degree or quantity of heat in -any substance. - - -How does heat affect the quicksilver? - -Heat expands the quicksilver, and causes it to rise in the tube. - - -How does cold affect the quicksilver? - -Cold contracts the quicksilver, and causes it to fall in the tube. - - -Does heat expand liquids more than solids? - -Heat does expand liquids more than solids. - - -Why are liquids expanded more than solids? - -Because the particles of which they are formed are more easily separated -or pushed apart by the heat. - - -If we continue to apply heat to a liquid, what becomes of it? - -The liquid is changed into a gas or vapor. - - -What common example have we of vapor? - -Water, when heated, is turned into steam. - - -Mention some gas. - -The _air_ we breathe is a gas. - - -How does heat affect the air? - -Heat warms the air and causes it to expand. - - -Do solids all expand equally when heated? - -No; some solids expand more than others; thus, zinc expands more than -iron, and iron more than glass. - - -Do liquids all expand equally when heated? - -No; liquids, although more sensitive to heat, do not expand as equally -as solids. - - -Do gases expand equally when heated? - -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? - -The lead will weigh more than the water. - - -Which is the heavier substance, lead or water? - -Lead is heavier than water. - - -Why do we say lead is heavier than 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. - - -Why is cork lighter than 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. - - -When lead is put in water, why does it sink to the bottom? - -Because the lead is heavier than an equal bulk of water. - - -When cork is put in water, why does it rise to the surface? - -Because the cork is lighter than an equal bulk of water. - - -Does heat increase the weight of any substance? - -Heat does not increase the weight of any substance. - - -How do we know this? - -Because a piece of iron when cold will weigh as much as when heated -red-hot. - - -When a liquid is heated, does it become lighter or heavier? - -It becomes lighter, bulk for bulk, when heated. - - -Why does a liquid become lighter when heated? - -Because the same liquid is expanded and takes up more room, although it -has no more weight. - - -Why will a gallon of cold water weigh more than a gallon of hot water? - -Because a gallon of cold water, when heated, will make more than a -gallon of hot water. - - -When do we get the most molasses, by buying it in hot or in cold -weather? - -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. - - -Does heat expand the air and make it lighter? - -Heat does expand the air and make it lighter. - - -What does the air do when heated? - -It ascends or rises up, because it is lighter, and the cold or heavier -air falls to take its place. - - -What part of a room, in which there is fire, is the warmest? - -The part next to the ceiling is the warmest, because the heated air -always ascends. - - -Why does a soap-bubble ascend? - -Because it is filled with heated air from the lungs, and is, therefore, -lighter than the air around it. - - -Why do balloons ascend? - -Because they are filled with a gas lighter than the air around them. - - -Why does a chestnut split open when roasting? - -Because the air in it is expanded by the heat, so that it bursts the -shell. - - -Why does the chestnut not split open when a hole is made in the shell? - -Because the air can then escape as it expands, without bursting the -shell. - - -What becomes of the air in an apple, when the apple is roasted? - -The air, upon being heated, bursts through the peel of the apple, -carrying the juice with it. - - -Why does an apple become soft when roasted? - -Because the air in the cells of the apple expands, and breaks those -cells, so that the apple becomes soft. - - -What part of the apple first becomes soft? - -The outside, because it is first heated. - - -Why do all fruits and vegetables become soft when cooked? - -Because the heat expands the air in them, and thus breaks up their -cells, so that they become soft. - - -Why does wood make a snapping noise when burning? - -Because the air in the cells of the wood bursts them apart, thus making -the noise. - - -Why are sparks of fire thrown out? - -Because the heated air bursts the cells with such force as to throw off -small pieces of the burning wood. - - -What kinds of wood snap the most? - -The coarse-grained kinds, like chestnut; while the finer-grained kinds, -like walnut, seldom snap much. - - -Why does dry wood snap more than green wood? - -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. - - -Why does smoke rise in a chimney? - -Because the air in the chimney is heated by the fire, and as the air -rises, it carries the smoke with it. - - -Why does a chimney smoke when the fire is first kindled? - -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. - - -How are houses heated with hot air? - -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. - - -Is there any substance that does not contract by cold? - -Yes; water, when near freezing, does not contract by cold. - - -What takes place with the water? - -When it is reduced nearly to the freezing point, it begins to expand, -and when it freezes, it expands still more. - - -How much does water expand by freezing? - -It expands about one-seventh of its bulk. That is, _seven_ gallons of -_water_ will make _eight_ gallons of _ice_. - - -Does freezing water expand with much force? - -Yes; it expands with great force. - - -Why are vessels containing water often broken during a cold night? - -As the water in them freezes, it expands and breaks the vessels. - - -Why do lead pipes and iron pipes often burst during the winter? - -Because the water in them is permitted to freeze, and as there is not -room for it to expand, the pipes are burst. - - -Is ice heavier or lighter than water? - -Ice is lighter than the same bulk of water. - - -How much lighter is ice than water? - -Eight gallons of ice weigh no more than seven gallons of water. - - -Does the ice protect the water beneath it from freezing? - -Ice does protect the water from freezing, to a very great extent. - - -If ice were heavier than water, what would result? - -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. - - -Why does iron feel cold to the touch on a cold morning? - -Because heat goes from the hand to the iron, thus giving the sensation -of cold. - - -What is the passage of heat from the hand to the iron called? - -It is called _conduction_. - - -When one end of a rod of iron is placed in a fire, does the whole rod -become heated? - -The whole rod does become heated. - - -How does this take place? - -The heat of the fire passes from one particle to another throughout the -rod. - - -What is this called? - -It is called _conduction_. - - -What then is conduction of heat? - -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. - - -What bodies are good conductors of heat? - -Such bodies as gold, platinum, copper, silver, iron, zinc, tin, and -lead, in the order named, are the best conductors of heat. - - -What bodies are poor conductors of heat? - -Such bodies as glass, wood, charcoal, wool, hair, and fur. - - -Fur is the poorest conductor of heat known. - - -Why does a piece of wood blazing at one end not feel hot at the other -end? - -Because wood is a poor conductor, and the heat goes slowly through it. - - -Why does a piece of iron feel cold in winter? - -Because the iron is a good conductor, and carries the heat away from our -hands very rapidly. - - -Why does fur feel warmer than iron in the winter? - -Because the fur is a poor conductor, and carries the heat away from our -hands very slowly. - - -Why does a wooden pump-handle seem less cold than one of iron? - -Because wood is not so good a conductor as iron; hence, it does not -carry the heat of our hands away so rapidly. - - -Why does carpet seem warmer than the bare floor? - -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. - - -Why does a stone pavement make our feet cold in winter? - -Because the stone is a good conductor, and rapidly carries the heat away -from our feet. - - -Why does heated iron feel hot to us? - -Because the iron is a good conductor, and gives off its heat rapidly to -our bodies. - - -Why does a piece of cloth, when heated, not feel hot? - -Because the cloth is a poor conductor, and gives off its heat very -slowly. - - -Why will a block of wood, when heated, remain hot longer than a heated -brick? - -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. - - -Why should the wood or brick be wrapped in cloth? - -Because the cloth, being a poor conductor, helps to keep the heat from -passing off so rapidly. - - -Why does iron feel colder than water when both are at the same -temperature? - -Because iron is a better conductor than water, and carries away the heat -from our hands more rapidly. - - -Are liquids good conductors of heat? - -No; liquids are poor conductors of heat. - - -Is air a good conductor of heat? - -No; air is a poor conductor of heat. - - -Are gases as good conductors as liquids? - -No; gases are poorer conductors than liquids. - - -Why is water a better conductor than air? - -Because the particles of water are closer together than the particles of -air. - - -Why is iron a better conductor than water? - -Because the particles of iron are closer together than the particles of -water. - - -How do we know that water is a poor conductor of heat? - -Because water may be made to boil at its surface without melting ice a -short distance below the surface. - - -How do we know that air is a poor conductor of heat? - -Because the air at the ceiling of a room may be made very warm without -melting ice near the floor. - - -Why does a linen shirt feel cool? - -Because linen is a good conductor, and there is but little air among its -fibres. - - -Why is a cotton shirt warmer than one made of linen? - -Because the cotton contains more air among its fibres, and air is a poor -conductor of heat. - - -Why is a woollen shirt warmer than one made of cotton? - -Because wool contains more air among its fibres, and is, therefore, a -poorer conductor of heat. - - -Why is fur warmer than wool? - -Because there is more air among the hairs of the fur, and it is, -therefore, a poorer conductor of heat. - - -What fur is the warmest? - -Fur with very fine hairs, like that of the rabbit, because it contains -the most air. - - -Why is fur warmer when the hair is next to our bodies? - -Because the fur and the air in it both help to keep the heat of our -bodies from passing off rapidly. - - -Is the earth a good conductor of heat? - -No; the earth is a poor conductor of heat. - - -How do we know that the earth is a poor conductor of heat? - -Because the heat of summer warms the earth only a few inches below the -surface. - - -How is it with the earth in winter? - -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. - - -Why is spring-water generally cool, even in summer? - -Because the springs are mostly so far below the surface of the earth, -that they are not warmed by the sun’s rays. - - -Why is snow a warm covering for the earth? - -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. - - -Will straw, wrapped around shrubbery, prevent it from freezing? - -It will; because it keeps the heat of the shrubbery from passing off -into the air. - - -Why are rooms warmer from having double windows? - -Because the air which is confined between the double windows, being a -poor conductor, keeps the warmth of the room from escaping. - - -Why is the space between the double walls of ice-coolers filled with -charcoal? - -Because charcoal is a poor conductor, and keeps the heat of the air from -the ice. - - -Why do ice-houses have double walls, with saw-dust or straw between -them? - -Because the saw-dust and the straw are both poor conductors, and keep -the heat of the air from the ice. - - -Why does ice melt more slowly when wrapped in flannel? - -Because the flannel is a poor conductor, and keeps the heat of the air -from the ice. - - -Why are woollen holders used about the stove? - -Because wool is a poor conductor, and thus keeps the heat of the iron -from burning the hand. - - - Convection of Heat. - - -When air near the floor of a room is heated, what becomes of it? - -It ascends to the ceiling of the room. - - -What is done with the cold air at the ceiling? - -It descends to the floor, to take the place of the warm air. - - -When the air ascends to the ceiling, does it carry the heat with it? - -Yes; it carries the heat with it. - - -What is this mode of carrying heat called? - -It is called _Convection_. - - -What is needed so as to heat a substance by convection? - -Its particles must be able to move about freely. - - -Why is the air easily heated by convection? - -Because the particles of the air move about freely. - - -Why is a liquid easily heated by convection? - -Because its particles move about freely. - - -Why cannot a solid be heated by convection? - -Because its particles cannot move. - - -When water near the bottom of a vessel is heated, what becomes of it? - -It rises to the surface. - - -What becomes of the cold water at the surface? - -It falls to the bottom of the vessel, to take the place of the warm -water. - - -How long will this circulation continue? - -It will continue so long as heat is applied to the bottom of the vessel. - - -When heat is applied to the surface of the water, what is the result? - -The water near the surface only is heated. - - -Why does it not heat all the water in the vessel? - -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. - - -Where must heat be applied to warm a liquid or a gas? - -It must be applied to the bottom of the vessel containing the liquid or -the gas. - - -Where must a fire be, so as to warm a room? - -The fire must be near the floor of the room. - - -Why does a hot substance cool quickly in the air? - -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. - - -How is water cooled by convection? - -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. - - -Why does stirring hot coffee cool it quickly? - -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. - - -Why does blowing hot coffee cool it quickly? - -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. - - -Is ice a solid or a liquid? - -Ice is a solid. - - -When heat is applied to ice, what becomes of the ice? - -The ice is changed into water. - - -Is water a liquid? - -Water is a liquid. - - -How then can a _solid_ be changed to a _liquid_? - -By applying _heat_ to a solid, it may be changed to a liquid. - - -What is meant by liquefaction? - -_Liquefaction_ is the changing of a solid to a liquid, by heat. - - -How may tallow be melted? - -By heating the tallow, it may be melted. - - -Will the tallow become hard again when cold? - -It will be hard again when cold. - - -Can lead and other metals be melted? - -They can be melted or changed to a liquid state by heat. - - -Can all solids be changed to liquids? - -No; there are many solids, such as wood, coal, paper, and leather, that -cannot be changed to liquids. - - -How can a liquid be changed to a solid? - -By taking heat away from the liquid, it may be changed to a solid. - - -Why is a solid melted by heat? - -Because heat forces the particles of the solid apart, until it is -changed to a liquid. - - [Illustration: Decoration.] - - - Latent Heat. - - -Why does warm iron feel warm to the hand? - -Because it gives off some of its heat to the hand. - - -Is the hand sensible of the heat coming from the iron? - -The hand is sensible of the heat. - - -What then may this heat be called? - -It may be called _sensible_ heat, because it can be felt. - - -How do we measure heat? - -We measure heat by degrees. - - -How many degrees of heat in ice when at the melting point? - -Thirty-two degrees of heat, as measured by the thermometer. - - -How can we change ice into water? - -By applying heat to the ice it will be changed into water. - - -How many degrees of heat are necessary to change the ice into water? - -One hundred and forty degrees of heat are necessary. - - -How many degrees of heat are in the water when the ice is all melted? - -Only thirty-two degrees of heat, by the thermometer. - - -Does the water feel any warmer than the ice to the hand? - -No; the water does not feel any warmer than the ice. - - -What has become of the one hundred and forty degrees of heat? - -They have been hidden in the water. - - -What is this hidden heat called? - -It is called _latent_ heat. - - -What is latent heat? - -It is heat that is not sensible to the touch, or to the thermometer. - - -When water is made to boil, what change takes place? - -The water is changed into steam. - - -How many degrees of heat are necessary to change water into steam? - -More than nine hundred degrees of heat are necessary. - - -How many degrees of latent heat in steam? - -More than nine hundred degrees of latent heat. - - -When steam is changed back again to water, what becomes of the latent -heat? - -It passes off to the air and the surrounding objects, and makes them -warmer. - - -When water is changed into ice, what becomes of the latent heat? - -It passes off to the surrounding objects and makes them warmer. - - -Is freezing a warming process? - -Yes; because the water gives off its latent heat while freezing; hence, -the surrounding objects are made warmer. - - -Is thawing a cooling process? - -Yes; because the ice absorbs so much heat while thawing that it cools -the surrounding objects. - - -Where does the ice obtain its heat, while thawing? - -The ice obtains its heat from the surrounding objects. - - -Are these objects made cooler by the melting of the ice? - -They are made cooler by the melting of the ice. - - -What effect does salt have upon ice? - -Salt makes the ice melt more rapidly. - - -When salt and ice are placed around a vessel of cream, what takes place? - -The salt causes the ice to melt quickly, and the ice in melting takes so -much heat from the cream as to freeze it, thus making it ice-cream. - - - Ebullition. - - -What is meant by ebullition? - -_Ebullition_ means boiling. - - -When is a liquid in a state of ebullition? - -When it has been heated so as to boil. - - -Can solids be made to boil? - -No; liquids only can be made to boil. - - -What change takes place when a liquid boils? - -The liquid is changed into a vapor or gas. - - -What becomes of water when it is boiled? - -Water when boiled passes off into steam. - - -What is necessary to make water boil? - -Heat is necessary to boil water or any other liquid. - - -How much heat is contained in boiling water? - -Two hundred and twelve degrees of heat. - - -When water passes into steam, how much is its bulk increased? - -About seventeen hundred times. - - -How much steam will a pint of water make? - -A pint of water will make about seventeen hundred pints of steam. - - -What do bubbles of boiling water contain? - -They contain steam. - - -Why are these bubbles formed? - -Because the water at the bottom of the vessel is heated and changed into -steam, which expands and forms the bubbles. - - -Why do these bubbles rise to the surface? - -Because steam is lighter than water. - - -When does water simmer? - -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.” - - -When does water boil? - -Water boils when it becomes heated so that the bubbles rise to the -surface of the water before breaking. - - -Why will a vessel full of water run over when heated? - -Because the water is expanded by heat, and some of it must run over. - - -Why will the water boil over when the vessel is not full? - -Because the steam, in trying to escape, throws some of the water over -the edge of the vessel. - - - Vaporization. - - -When water is boiled, what change does it undergo? - -It is changed into a vapor, called steam. - - -What is this process called? - -It is called _Vaporization_. - - -What is meant by vaporization? - -Vaporization is the changing of a liquid into a vapor, by boiling. - - -What is necessary to change a liquid into a vapor? - -Heat is necessary to change a liquid into a vapor. - - -At what temperature is water changed into steam? - -At the temperature of two hundred and twelve degrees. - - -Are all liquids changed to vapor at the same temperature? - -No; they vary very much in the amount of heat required. - - -Ether boils at 96 degrees, alcohol at 176 degrees, and quicksilver at a -temperature of 662 degrees. - - -When water is changed into steam, what becomes of the steam? - -The steam passes off into the air. - - -What does the air do with the steam? - -The air condenses or changes it into very small particles of water, so -that it is seen like a fog or a cloud. - - -Can steam be seen? - -No; steam is like the air in this respect, and cannot be seen. - - -How can we prove this? - -When water is boiled in a glass vessel, no steam is visible, although it -is there. - - -Cannot we see the steam coming from the spout of a tea-kettle? - -No; we do not see anything for half an inch or more from the spout. - - -Why does it become visible farther away? - -Because the air condenses it, and then we see it in its cloud-like -state. - - -How much steam will one gallon of water make? - -One gallon of water will make seventeen hundred gallons of steam. - - -When this steam is confined in a vessel, what does it try to do? - -It tries to expand or spread out in every direction. - - -Does it expand with much force? - -Yes; it expands with so much force as frequently to burst the vessel -containing it. - - -Why is the lid of the tea-kettle lifted up when the water boils? - -Because the steam expands and raises the lid, so that it can escape. - - -If the lid were fastened down, what would result? - -The steam, in trying to escape, would burst the tea-kettle. - - -Why does water run out of the spout of the tea-kettle? - -Because the steam in the tea-kettle presses on the water with force -enough to cause it to run out. - - -Why does steam burst vessels in which it is confined? - -Because the vessels are not strong enough to keep the steam from -expanding. - - -How is the boiler of a steam-engine kept from bursting? - -By means of a safety-valve. - - -What is a safety-valve? - -It is a valve or opening through which steam may escape. - - -What prevents all of the steam from escaping through the valve? - -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. - - -Does a wet blanket, when hung in the air, remain wet? - -No; it soon becomes dry. - - -What becomes of the water in the blanket? - -The water passes off into the air. - - -What is this process called? - -It is called _Evaporation_. - - -What is meant by evaporation? - -It is the gradual change of a liquid into a vapor. - - -When water is left in a shallow vessel, what becomes of it? - -The water evaporates or passes off into the air. - - -Do liquids generally evaporate? - -Yes; when exposed to the air, nearly all liquids evaporate. - - -Does water change into steam when it evaporates? - -Water does not change into steam when it evaporates. - - -How, then, does water evaporate? - -It passes off into the air in particles or drops so very small as not to -be visible. - - -Is evaporation always going on around us? - -Yes; wherever there is water exposed to the air, evaporation is taking -place. - - -Where does the most evaporation take place? - -From the surface of the rivers, the lakes, and the ocean. - - -What becomes of the water when evaporated? - -It is formed into fogs and clouds. - - -In what kind of air does evaporation go on most rapidly? - -In air that is warm and dry. - - -Does evaporation take place in cool air? - -It does; because clothing will dry during the coldest weather of winter. - - -Why does water evaporate more in warm air than in cold air? - -Because the warm air can hold more of the fine particles of water, and, -therefore, can absorb more. - - -Why does water evaporate more slowly in damp weather than in dry -weather? - -Because the damp air contains so much moisture that but little more can -pass off into it. - - -What example have we of this? - -Wet clothes dry much more slowly when the air is damp than when it is -dry. - - -Why do clothes dry more rapidly in windy weather? - -Because the damp air in contact with them is blown away, and the dry air -is constantly taking its place. - - -Is heat necessary in evaporation? - -Heat is necessary to produce evaporation. - - -Why do our hands feel cold when wet? - -Because the water on our hands evaporates, and thus heat is taken from -them. - - -Why is the air cooler after a rain? - -Because evaporation then takes place from every wet object, and this -absorbs heat from the air. - - -Why does watering streets and roads cool the air? - -Because the water evaporates and absorbs heat from the air, making it -cooler. - - -Why do we feel cold when our clothes are wet? - -Because the moisture in them evaporates and absorbs heat from our -bodies. - - -Why is wet land always cold? - -Because so much heat is absorbed from the land in the evaporation which -is constantly taking place. - - -What becomes of the dew that collects on plants and other objects at -night? - -It is evaporated. - - -How does the sun hasten the evaporation of the dew? - -By supplying the heat needed to change the dew into a vapor, the sun -hastens its evaporation. - - -Why do bread and biscuit become hard when kept a few days? - -Because the moisture in them passes off into the air, and thus they -become dry and hard. - - -How can they be kept moist and soft for a longer time? - -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. - - -When we stand near the fire, why do we feel warm? - -Because the fire gives off heat which makes us feel warm. - - -How does the fire give off heat? - -It gives off heat in straight lines, and in every direction. - - -What is this giving off of heat called? - -It is called _Radiation_ of heat. - - -Does the sun radiate heat? - -The sun does radiate or give off both heat and light. - - -How do heat and light from the sun come to us? - -They come together in the sunbeam. - - -Does heat travel as rapidly as light? - -Yes; heat and light travel together at the rate of 200,000 miles in a -second of time. - - -Do all substances radiate heat equally well? - -No; some substances are much better radiators than others. - - -What kind of objects are the best radiators? - -Those with rough and dark surfaces. - - -What kind of objects are the poorest radiators? - -Those with smooth and bright surfaces. - - -Why does water keep hot a long time in a bright tin vessel? - -Because the tin is a poor radiator, and prevents the heat of the water -from passing off into the air. - - -Why does water soon cool in a dull earthen vessel? - -Because the earthen vessel is a good radiator, and gives off the heat of -the water rapidly. - - -If the bright tin vessel be painted, will the water keep hot so long? - -No; the tin becomes a good radiator when painted, and the water is soon -cooled. - - -Why does a stove cool so quickly? - -Because the iron is a good radiator, and gives off its heat rapidly. - - -Do the rays from the sun give off much heat while passing through the -air? - -The rays from the sun give off about one-fourth of their heat while -passing through the air. - - -What becomes of the remaining heat from the sun? - -It is absorbed or reflected at the earth’s surface. - - -What does the earth do with the heat? - -The earth radiates the heat, and thus warms the air. - - - Reflection. - - -When a piece of bright tin is held near the fire, does the tin become -hot? - -No; the tin is scarcely warmed. - - -What does the tin do with the rays of heat from the fire? - -The tin throws them back from its surface. - - -What is this called? - -It is called _Reflection_ of heat. - - -Do all substances reflect heat equally well? - -No; some reflect much more heat than others. - - -What kind of objects reflect heat the best? - -Those with bright and smooth surfaces. - - -What kind of objects reflect heat the least? - -Those with rough and dark surfaces. - - -Does bright tin reflect heat better than glass? - -Yes; bright tin reflects about eight times as much heat as glass does. - - -Why does it take so long to heat water in a new tin cup? - -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. - - -Why will not a piece of iron protect the carpet from burning? - -Because the iron, being a poor reflector, soon becomes heated, so as to -burn the carpet under it. - - - Absorption. - - -When a piece of iron is held near a fire, does the iron become hot? - -Yes; the iron soon becomes hot. - - -How is the iron heated? - -The iron absorbs or takes in the heat from the fire. - - -What is this power to take in the heat called? - -It is called _Absorption_ of heat. - - -What is this process like? - -It is somewhat like a sponge, that absorbs or sucks up moisture. - - -Do all substances absorb heat equally well? - -No; some substances absorb more heat than others. - - -What kind of objects absorb heat the best? - -Those with rough surfaces absorb heat the best. - - -What kind of objects absorb heat the least? - -Those with bright and smooth surfaces. - - -What colored objects absorb heat the best? - -Objects of a dark or black color. - - -Do objects that absorb heat well, radiate well? - -Yes; good absorbers are good radiators. - - -Why is this? - -Because every substance must absorb heat before it can have heat to give -off. - - -Do objects that absorb heat well, reflect it well? - -No; objects that absorb heat well, cannot reflect it well. - - -Why is this? - -Because a ray of heat cannot be reflected from an object, and be -absorbed by that object, both at the same time. - - -Why is water rapidly heated in a tea-kettle? - -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. - - -Why does not white muslin, when spread on the snow, hasten its melting? - -Because the white muslin reflects the heat from the rays of the sun, so -that but little of it enters the snow. - - -Does the snow receive much heat from the sun? - -Snow is a good reflector, and throws back most of the heat from the sun. - - -What causes the snow to melt? - -The heat it receives from touching the earth, causes the most of it to -melt. - - -Why does snow melt first on dark earth? - -Because dark earth is a good radiator, and gives off much heat to the -snow. - - -Why does snow remain longest on light-colored earth? - -Because the light-colored earth is a poor radiator, and gives off but -little heat to the snow. - - -Why does the air feel cold in winter time? - -Because it is so much cooler than our bodies that it absorbs heat from -them rapidly, and thus makes them feel cold. - - -Why does the air feel warm in summer time? - -Because it is then nearly as warm as our bodies, and, therefore, absorbs -heat from them slowly. - - -Why are greasy shoes warmer than those polished with blacking? - -Because the greasy shoes absorb heat from the sun, but the polished -shoes throw off the heat of the sun by reflection. - - -What may we remember about heat? - -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. - - [Illustration: An outdoor walk.] - - [Illustration: Decoration.] - - - - - CHAPTER IV. - The Air. - - - [Illustration: Illustrated Capital W] - - -Where do we find air? - -We find air everywhere around the earth. - - -Does the air fill every place? - -The air fills every place not filled by some other substance. - - -When we say a bottle is empty, what do we mean? - -We mean that it does not contain anything but air. - - -What do all vessels contain when empty? - -All vessels contain air when seemingly empty. - - -Do all liquids contain air? - -Yes; all liquids contain air. - - -Where is the air in a liquid? - -The air is among the particles which compose the liquid. - - -Is there air in water? - -Yes; there is air in water. - - -Of what use is the air in the water? - -It is used by the fish and other creatures that live in the water. - - -How do the fish and other creatures use it? - -They breathe it into their lungs, and it supports life in them, the same -as the air which we breathe supports life in us. - - -Do solids contain air? - -Yes; solids contain more or less air. - - -Where is the air in fur and in wool? - -It is among the hairs of the fur and the wool. - - -Where is the air in cotton and linen goods? - -It is among the fibres of the cotton and the linen. - - -Where is the air in fruit? - -It is among the particles composing the fruit. - - -Where is the air in wood? - -It is in the pores or small spaces in the wood. - - -Is there air in the earth? - -Yes; there is air among the particles of the earth. - - -How deep in the earth does the air extend? - -We cannot tell; but both air and water have been found several hundred -feet below the earth’s surface. - - -Of what use is the air in the earth? - -It is breathed by the worms and other creatures that live in the earth. - - -When part of the air is taken from a room, what becomes of the air left -in the room? - -It expands so as to fill the whole room. - - -Are the particles of air so close together after it expands as they were -before? - -They are not so close together. - - -When air is thus expanded, what is said of it? - -It is said to be _rare_. - - -When pressure is put upon air in a vessel, what becomes of the air? - -It is forced or pressed into a smaller space, and, therefore, occupies -less room. - - -When pressure is put upon air, are its particles closer together? - -They are closer together. - - -When air is thus compressed, what is said of it? - -It is said to be _dense_. - - -Can air be much condensed by pressure? - -It has been condensed so much that 100 cubic feet of air was reduced to -the bulk of only one cubic foot. - - -What makes the air so dense as it is at the surface of the earth? - -The pressure of the air above the surface. - - -Does the air then become less dense the higher we ascend? - -Yes; because there is less air above to press upon it. - - -Does the air become more dense the deeper we go in the earth? - -It does; because there is more air above to press upon it. - - -How can we understand this? - -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. - - -How high does the air extend above the earth? - -It is supposed to extend to the height of _fifty_ miles. - - -Do these fifty miles of air press upon the earth? - -They do press upon the earth. - - -With how much force does the air press on the earth? - -With a force of _fifteen_ pounds on every square inch of surface. - - -Then has the air weight? - -The air has weight. - - -Why do we not feel the weight of the air? - -Because it presses with equal force, in every direction; up, down, and -on every side. - - -How can we show that the air presses upwards? - -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. - - [Illustration: 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? - -It will, the same as it keeps it in the tumbler. - - -How high will the air keep the water in the pipe? - -It will keep the water to the height of _thirty-four_ feet. - - -Why does the air keep the water up thirty-four feet? - -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. - - -Does the weight of the air exactly balance the weight of the column of -water? - -It does; they balance each other. - - -Is water heavier than air? - -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. - - -Is quicksilver heavier than water? - -Yes; quicksilver is about thirteen times heavier than water. - - -How high a column of quicksilver can the air keep up? - -The air can keep up a column of quicksilver only thirty inches high. - - -Why is this? - -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. - - -How can this be shown? - -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. - - [Illustration: Fig. 13.] - - -If the cork be removed from the glass tube, what will become of the -quicksilver? - -It will immediately sink into the basin below. - - -Why will the quicksilver sink into the basin? - -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. - - -Does the weight of the air vary at different times? - -Yes; the air does vary in weight at different times. - - -How can we tell when the air changes in weight? - -By means of a _Barometer_. - - -What is a barometer? - -It is an instrument having a tube filled with quicksilver. - - -When the air is heavier, how does it affect the quicksilver? - -The quicksilver rises in the tube. - - -When the air is lighter, how does it affect the quicksilver? - -The quicksilver sinks in the tube. - - -What is the use of the barometer? - -It shows the changes about to take place in the weather. - - -When the quicksilver rises, what kind of weather is indicated? - -It indicates fair weather. - - -When the quicksilver sinks, what kind of weather is indicated? - -It indicates cloudy or stormy weather. - - -Where is the barometer most useful? - -It is most useful at sea, to warn the sailors of coming storms. - - -What use is it to farmers? - -They can use it as a guide when planting and gathering their crops, and -thus be better prepared for stormy weather. - - -For what else may the barometer be used? - -It may be used to tell the height or depth of any place. - - -Does the air become lighter as we ascend? - -The air does become lighter as we ascend. - - -How does this affect the barometer? - -As the air becomes lighter, the quicksilver in the barometer sinks -lower. - - -At what height does the quicksilver stand at the level of the sea? - -It stands at the height of thirty inches. - - -When we ascend a mountain 10,000 feet high, how much does the -quicksilver sink? - -It sinks ten inches, and, therefore, stands at the height of only twenty -inches. - - -Does the same thing take place when we ascend in a balloon? - -Yes; the higher the balloon ascends, the lower the quicksilver in the -barometer sinks. - - -When we descend into the mines dug in the earth, does the air become -heavier? - -It does; and the quicksilver in the barometer rises. - - -At what temperature does water boil? - -Water boils at a temperature of _two hundred and twelve_ degrees. - - -When the weight of the air is taken off, will the water boil at a lower -temperature? - -Water will then boil at a lower temperature. - - -Where will water boil at the lowest temperature, on a mountain, or in a -valley? - -It will boil at the lowest temperature on a mountain. - - -At how much lower temperature will water boil, as we ascend? - -Water will boil at one degree less for every five hundred feet we -ascend. - - -How high is the city of Quito, in South America? - -Quito is nine thousand feet above the level of the sea. - - -At what temperature does water boil there? - -Water boils, in Quito, at a temperature of one hundred and ninety-four -degrees. - - -Will an egg boil hard at Quito? - -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. - - -What is a _vacuum_? - -A _vacuum_ is a place that does not contain anything, not even air. - - -When a vessel contains water and we pour the water out, does that -produce a vacuum? - -No; because air passes in and fills the vessel. - - -How then can a vacuum be produced? - -A vacuum in any place, can be produced only by drawing the air out of -it. - - -When a vacuum is produced, what does the surrounding air try to do? - -It tries to get in so as to fill the vacuum. - - -With what force does the air try to fill a vacuum? - -With a force of fifteen pounds on every square inch of surface. - - -When a vacuum is produced in contact with a liquid, what does the liquid -do? - -The liquid tries to pass in to fill the vacuum. - - -Can there be a vacuum in contact with a gas or a liquid? - -No; because the gas or the liquid will instantly fill the vacant space. - - -Why does water make a gurgling noise when poured out of a bottle? - -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. - - -Why does water make a gurgling noise when running out of a barrel? - -Because the air passes into the barrel at the opening where the water -passes out, and thus the noise is made. - - -How can the gurgling noise be prevented? - -By making an opening at the top of the barrel, so as to admit the air. - - -Does water flow freely from a vessel that has but one opening? - -No; because part of this opening is needed to admit air into the vessel. - - -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. - - -What causes fruit, vegetables, and meat to spoil? - -The air causes them to spoil. - - -How do we know this? - -Because when kept from the air they remain good for a long time. - - -What is this process of keeping fruit called? - -It is called canning. - - -How is fruit canned? - -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. - - -What becomes of the steam in the jar or can? - -As the fruit cools, the steam is condensed to water, and a vacuum is -produced, so that the fruit is kept from the air. - - -What is the plaything which boys call a sucker? - -A _sucker_ is a round piece of leather, with a string fastened to its -centre. - - -How is the sucker used? - -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. - - [Illustration: Fig. 14.] - - -What keeps the leather against the stone? - -The pressure of the air upon the leather keeps it against the stone. - - -With how much force does the air press the leather against the stone? - -With a force of fifteen pounds on every square inch. - - -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. - - -How does a fly walk upon the ceiling? - -The fly’s feet are much like the sucker, and are kept against the -ceiling by the pressure of the air. - - -How do snails cling to objects? - -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. - - -What is a _siphon_? - -A _siphon_ is a bent tube, having one branch longer than the other, as -is represented in Fig. 15. - - [Illustration: Fig. 15.] - - -For what is the siphon used? - -The siphon is used to draw liquids from one vessel into another. - - -How can water be drawn from a vessel by means of a siphon? - -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. - - -Where must the open end of the short branch be kept? - -It must be kept beneath the surface of the water, so as to prevent air -from entering the tube. - - -Where must the open end of the long branch be kept? - -It must be kept lower than the open end of the short branch. - - -While the water is running out of the long branch, what is produced at -the bend of the tube? - -The tendency is to produce a vacuum at the bend of the tube. - - -What is the result of this? - -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. - - -How do boys often suck cider from a barrel? - -By means of a straw. - - -How is this done? - -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. - - -What makes the cider rise to fill the vacuum in the straw? - -The pressure of the air on the cider in the barrel. - - -To what height will water rise in a tube to fill a vacuum? - -The water will rise to the height of thirty-four feet. - - -To what useful purpose is this applied? - -It is applied to making _pumps_. - - -Of what material are pumps made? - -Either of wood, or of metal, such as lead or iron. - - -What is necessary in regard to the tube or pump-tree? - -It must always be air-tight. - - -Why must the tube or pump-tree be air-tight? - -So that a vacuum may be produced in it. - - -How is this accomplished? - -By means of two valves in the pump-tree. - - -What is a valve like? - -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. - - -Where is one valve placed? - -One valve is placed in the lower part of the pump-tube. - - -How near the water must this valve be? - -It must be within thirty-four feet of the water. - - -Why must the lower valve be within thirty-four feet of the water in the -well? - -Because the air can force water up a tube only to the height of -thirty-four feet. - - -Does this valve move about in the pump-tube? - -No; it remains fixed in one place. - - -Is this valve air-tight? - -Yes; the lower valve or bucket in a pump must always be air-tight. - - -Where is the upper valve? - -It is fastened on the end of a rod, which is attached to the -pump-handle. - - -Does this valve remain fixed in one place? - -No; it moves up and down when the pump-handle is moved. - - -Is the upper valve air-tight? - -No; the upper valve is not air-tight. - - [Illustration: Fig. 16.] - - [Illustration: Fig. 17.] - - [Illustration: Fig. 18.] - - -Explain the working of a pump. - -When the pump-handle is lowered, the water, above 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. - - -What does Fig. 16 represent? - -It represents a pump, when it is not working. - - -How are the valves? - -The valves are both closed. - - -What keeps the water above the lower valve? - -The lower valve is air-tight, and, therefore, water-tight, so that it -keeps the water above it. - - -What keeps the water in the tube below the lower valve? - -The pressure of the air on the water in the well, forces it up the tube -to the lower valve. - - -What does Fig. 17 represent? - -It represents a pump, when the handle is being raised. - - -How is the lower valve? - -The lower valve is closed. - - -Why is the lower valve closed? - -So as to prevent the water from being forced back again into the well. - - -How is the upper valve? - -The upper valve is open. - - -Why is the upper valve open? - -So that the water may pass up through the valve, while the valve is -descending. - - -What does Fig. 18 represent? - -It represents a pump when the handle is being lowered. - - -How is the upper valve? - -The upper valve is closed. - - -Why is the upper valve closed? - -So as to lift up the water which is above it. - - -What becomes of the water which is lifted up? - -It runs out of the spout of the pump. - - -As the upper valve is raised up, what is produced under it? - -A vacuum is produced under it. - - -How is this vacuum filled? - -The water below the lower valve is forced up, so as to fill the vacuum. - - -How is the lower valve? - -The lower valve is open. - - -Why is the lower valve open? - -So that the water may pass through and fill the vacuum above the valve. - - -Where are pumps mostly used? - -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. - - -When does smoke ascend through the air? - -When smoke is lighter than the air, it ascends through it. - - -When does smoke descend through the air? - -When smoke is heavier than the air, it descends through it. - - -Why does a cork rise to the surface of the water? - -Because the cork is lighter than the same bulk of water; hence, it -ascends to the surface. - - -Why does heated air ascend? - -Because it is lighter than the cold air. - - -If a feather be put in the heated air, will it be carried up with the -air? - -The feather will be carried up with the air. - - -If warm air be confined in a bag, will it rise up and carry the bag with -it? - -It will rise up and carry the bag with it. - - -What might this bag be called? - -It might be called a _balloon_. - - -What is a balloon? - -It is a bag filled with gas, and used for sailing in the air. - - [Illustration: Fig. 19.] - - -What is placed over this bag? - -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. - - -What is fastened to these ropes? - -A car or basket is fastened to these ropes. - - -What is the car or basket for? - -It is to carry up those who wish to ascend with the balloon. - - -How large are balloons usually made? - -They are from twenty to thirty feet in diameter. - - -With what are balloons filled? - -Balloons are filled with a gas lighter than air. - - -What gas is often used for filling balloons? - -The coal gas, such as is used to light our cities at night. - - -What lighter kind of gas is sometimes used? - -Hydrogen gas is sometimes used. - - -Is hydrogen gas much lighter than the air? - -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? - -It will weigh about ten thousand pounds less. - - -How much less, when filled with coal gas than when filled with air? - -About two thousand pounds less. - - -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. - - -How high will the balloon ascend? - -It will ascend until the air displaced is of the same weight as the -balloon. - - -What will then become of the balloon? - -It will be carried about in the currents of air. - - -With what is the car attached to the balloon loaded? - -It is usually loaded with bags of sand, which serve as a weight to keep -it from rising too high. - - -When those in the car wish to go higher, what do they do? - -They throw the sand out, so as to lighten the car; then it will rise -higher. - - -When they wish to descend, what do they do? - -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. - - -How is the valve at the top of the balloon opened and closed? - -It is opened and closed by means of small ropes, reaching from the valve -to the car beneath. - - -Can the balloon be made to go in any one direction? - -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. - - -What is wind? - -_Wind_ is air moving from one place to another. - - -Does the air move in every direction? - -It does move in every direction, upward, downward, and to every side. - - -When the wind blows from the north towards the south, what is it called? - -It is called a north wind. - - -What is it called, when it blows from other directions? - -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. - - -What causes the air to move from place to place? - -Heat causes the air to move from place to place. - - -When air is heated, what change takes place? - -The heated air ascends, and colder air moves in to fill its place; thus -causing the wind to blow. - - -What illustration have we of this? - -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. - - -Why is this? - -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. - - -Why does the wind blow towards a fire in the open air? - -Because the air near the fire becomes heated, and ascends rapidly, so -that the air rushes in from every direction to fill its place. - - -What does the earth do with the heat it receives from the sun? - -The earth gives off its heat to the air nearest to it. - - -What becomes of this heated air? - -This heated air ascends, and colder air comes to take its place; thus -producing winds. - - -Are all parts of the earth heated alike by the sun? - -No; they are not all heated alike. - - -How does this unequal heating affect the air? - -It causes currents in the air all over the world. - - -Does water give off heat as rapidly as the earth does? - -No; water is a poorer radiator of heat than the earth is. - - -On which will the air be the cooler when the sun shines, on the earth, -or on the water? - -It will be cooler on the water than on the earth. - - -Why does a sea-breeze feel cool in the daytime? - -Because the air over the water, in the daytime, is less heated than the -air over the land; hence, the sea-breeze feels cool. - - -Why does the earth become colder than the water at night? - -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. - - -Why does a land-breeze feel cool at night? - -Because the air over the land at night is less heated than the air over -the water; hence, the land-breeze feels cool. - - -How rapidly does the wind move? - -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. - - -Of what use is the wind? - -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. - - [Illustration: Paddlewheel steamboat.] - - [Illustration: Decoration.] - - - - - CHAPTER V. - Water. - - - [Illustration: Illustrated Capital W] - - -Where do we find water? - -We find _water_ almost everywhere; above the earth, on the earth, and -beneath the earth’s surface. - - -Where do we find water above the earth? - -We find it in the air, in the form of vapor. - - -Where do we find water on the earth? - -We find it in springs, in streams of water, and in lakes and oceans. - - -Where else is water on the earth? - -It is in the sap of plants, the juice of fruits, and the blood of -animals. - - -Where do we find water beneath the earth’s surface? - -We find it in streams running in every direction. - - -Has water any influence on climate? - -Yes; water causes many changes in climate. - - -What changes does water produce in climate? - -It makes spring and summer cooler, and autumn and winter warmer. - - -How does water make the Spring of the year cooler? - -The ice and snow while melting, take up a large amount of heat, which -passes into the water as latent heat. - - -Where does the melting ice and snow obtain this heat? - -They obtain it from the air, and from objects near the surface of the -earth. - - -What advantage is derived from this? - -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. - - -How does water make Summer cooler? - -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. - - -What effect does this heat produce upon the water? - -The temperature of the water is gradually increased by it during the hot -weather of summer. - - -How is this useful? - -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. - - -Does water store away a large amount of heat? - -It does; and thus the summer is made much cooler. - - -How does water make Autumn warmer? - -In the autumn the heat in the water is given off, so that the air is -kept warmer than it would otherwise be. - - -How does this affect autumn weather? - -It makes that season of the year mild and pleasant. - - -What advantage is derived from this? - -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. - - -How does water make Winter warmer? - -By being changed into ice and snow. - - -When water is changed into ice or snow, what takes place? - -The latent heat in the water is given off, and thus the winter air is -made warmer. - - -Can water be compressed into a smaller bulk? - -Yes; it can be compressed, but not so much as air. - - -What is said of water that is compressed? - -Its particles are pressed closer together, and it is said to be more -dense. - - -Why is the water at the bottom of a pond more dense than that at the -surface? - -Because the water at the bottom of the pond has to bear the weight of -all the water above it. - - -Is the weight of the water very great? - -The weight of the water is very great. - - -What is its weight at the depth of one hundred feet? - -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. - - -What is its weight in the ocean at the depth of one mile? - -At the depth of one mile it presses in every direction, 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? - -From the opening near the bottom of the vessel. - - -Why is this? - -Because there is more weight of water to press it out from the opening -near the bottom. - - -When water is poured into a vessel, what becomes of it? - -It spreads out so that its surface is level. - - -When different vessels are connected together, what does the water do? - -It falls to the same level in all of them. - - -How high will the tea stand in the spout of a tea-pot? - -Just as high as the tea is in the tea-pot. - - [Illustration: Fig. 20.] - - -Will the tea in the spout balance that in the tea-pot? - -The tea in the spout will balance that in the tea-pot, as is shown in -Fig. 20. - - -How else can it be shown that water will rise to the same level in -different vessels? - -By a reference to Fig. 21, this may be seen. - - [Illustration: Fig. 21.] - - -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. - - -Where is this principle used to benefit mankind? - -It is used in supplying the inhabitants of cities with water. - - -What must be built in order to supply a city with water? - -A reservoir must be built. - - -What is a reservoir? - -It is a large basin made so as to hold water. - - -How high is the water in the basin? - -It is usually as high as the tops of the houses in the city. - - -How is the water raised into the basin? - -It is raised into the basin by means of pumps. - - -How is the water conveyed from the basin to the houses in the city? - -By means of pipes laid under the surface of the earth, and extending to -all parts of the city. - - -How high will the water rise in these pipes? - -Nearly as high as it is in the basin. - - -If an opening be made in a pipe, how high will the water be forced up? - -Nearly as high as the water in the basin. - - -What is the water flowing from such an opening called? - -It is called a _fountain_. - - -What makes the water shoot up in a fountain? - -The pressure of the water in the basin forces it to shoot up in the -fountain. - - -To what does water always tend? - -Water always tends to fall to a level. - - -What instrument is constructed on this principle? - -The _water-level_. - - -What is a water-level? - -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. - - [Illustration: Fig. 22.] - - -How can we tell when the instrument is level? - -By the bubble of air, which will always be at the centre of the tube -when it is level. - - -What other liquid may be used to fill the tube? - -Alcohol may be used to fill the tube, and it is then called a -_spirit-level_. - - -Of what use is the spirit-level? - -It is used for levelling the walls and wood-work of houses, bridges, and -other structures. - - - Springs and Streams. - - -When we dig into the earth, can we always find water? - -Yes; we can always find water. - - -What reason may be given for believing this? - -Because thousands of wells have been dug in different parts of the -country, and water is obtained from them. - - -Are these wells all of the same depth? - -No; they vary in depth from four or five feet to eighty or ninety feet. - - -What makes the water _hard_ in some wells? - -When the stream of water passes through iron ore, before reaching the -well, it absorbs some of the iron, which makes it hard. - - -How else may it become hard? - -By passing through other mineral substances, such as limestone and -sulphur, it will become hard. - - -Why is the water in some wells much warmer than in others? - -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. - - -When these streams flow out at the surface of the earth, what are they -called? - -They are called _springs_. - - -Where are springs found? - -They are mostly found on hill-sides, or in valleys; but they are -sometimes found on the top of the hills. - - -Why is the water in some springs warmer than in others? - -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. - - -What becomes of the water in the springs? - -It runs away down the valleys, forming _brooks_ and _creeks_. - - -When these brooks and creeks flow into other brooks and creeks, what do -they form? - -They form _lakes_ and _rivers_. - - -Into what do the rivers flow? - -The rivers flow into the ocean. - - -How are waves produced? - -Waves are produced by the winds blowing against the surface of the -water. - - -What does a light wind produce? - -A light wind produces only ripples on the surface of the water. - - -What does a stronger wind produce? - -A stronger wind produces waves in the water. - - -Why is the water of the ocean and the larger lakes seldom at rest? - -Because the wind touching the water seldom ceases to blow. - - -To what depth does the wind disturb the water of the ocean? - -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? - -The water is always passing off in the form of vapor. - - -Where does this vapor go? - -It goes into the air so that the air becomes filled with moisture. - - -What kind of air will hold the most vapor? - -Warm air will hold the most vapor. - - -When warm air becomes colder, what is done with the vapor? - -The vapor is condensed into small particles of water. - - -When this takes place near the earth, what does it produce? - -It produces a _fog_. - - -Where do fogs mostly occur? - -Fogs mostly occur over low grounds and along the course of streams. - - -When the vapor is condensed higher up in the air, what does it produce? - -It produces a _cloud_. - - -What is a cloud? - -A cloud is only a fog, higher up in the air. - - -Do fogs ever ascend in the air? - -Yes; they often ascend, and thus produce clouds. - - -What causes the vapor of the air to be condensed into clouds? - -Cold causes it to be condensed into clouds. - - -Does the air grow colder as we ascend? - -Yes; the higher we ascend, the colder we find the atmosphere. - - -What becomes of the vapor in the air, as it ascends from the earth? - -The vapor is condensed into clouds in the upper regions of the air. - - -Why do we mostly see clouds at the top of a mountain? - -Because the vapor in the air is condensed into clouds by the cold at the -top of the mountain. - - -When is a cloud formed in the air? - -When a warm, damp wind meets with a cold wind, its vapor is condensed -and a cloud is formed. - - -Are clouds often formed in this way? - -Yes; clouds are often quickly formed in this way. - - -When the cloud passes into a warmer current of air, what becomes of it? - -It is changed back into vapor, so that it cannot be seen. - - -How then can a cloud in the sky disappear very quickly? - -By passing into a warmer current of air the cloud is changed into a -vapor, and may thus quickly disappear. - - -Are clouds of different heights? - -Yes; some clouds touch the earth, while other clouds are high up in the -air. - - -Why are clouds higher on a fine day? - -Because there is not so much moisture in the air, and, therefore, it -rises higher before its vapor is condensed into clouds. - - -Why are clouds lower on a rainy day? - -Because there is so much moisture in the air, that it is condensed into -clouds near the earth. - - -Are the clouds high during a thunder-storm? - -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. - - -Is there any difference in the thickness of the clouds? - -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? - -The _red_ color appears first. - - -When the sun sets, which of the seven colors is the last to fade away? - -The _red_ color is the last to fade away. - - -Why are the clouds red just before sunrise? - -Because they are tinged by the red ray of light, before the other rays -appear. - - -Why are the clouds red after sunset? - -Because they are tinged by the red ray of light, after the other rays -have faded away. - - - Dew. - - -What is dew? - -Dew is the moisture that collects on plants and other objects, during -the night. - - -What causes dew upon any object? - -The air in contact with that object has its vapor condensed, so that the -water is deposited on it in the form of dew. - - -Does the same quantity of dew collect upon all objects? - -No; it collects much more upon some objects than upon others. - - -Upon what objects does it collect the most? - -Upon those which radiate or give off heat the best. - - -Why do objects that are good radiators, collect the most 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. - - -What objects are the best radiators of heat? - -Grass, wood, the leaves of plants, and, in fact, all things of a -vegetable nature. - - -What objects then collect the most dew? - -Plants collect the most dew. - - -Of what use is the dew to plants? - -It supplies them with moisture, and answers in the place of rain. - - -When do we have the heaviest dew? - -When the air is still and the sky is free from clouds. - - -Why does wind prevent the deposit of dew? - -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. - - -Why are dews heaviest when the air is still? - -Because the air remains in contact with the grass and other objects, -until it is cool enough to deposit the dew. - - -Why is there but little dew on a cloudy night? - -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. - - -Why is there a heavy dew on a clear night? - -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. - - -How may dew be prevented from forming on any object? - -By placing a cover over that object. - - -How does a cover prevent the formation of 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. - - -Why is there but little dew under trees? - -Because trees prevent the free radiation of heat from the objects -beneath them; hence, but little dew is formed under trees. - - -Why do stone walls _sweat_? - -Because the walls cool the air in contact with them, so that its vapor -is condensed into water, and deposited on the walls. - - -Why does the sweating of stones indicate rain? - -Because it shows that the air contains a large amount of moisture. - - -When is a vessel of water said to sweat? - -When drops of water collect on the outside of the vessel, it is said to -sweat. - - -Why does a pitcher of cold water sweat? - -Because the vapor in the air in contact with the cold pitcher is -condensed into moisture, and deposited on the pitcher. - - -Why does the pitcher sweat most before a rain? - -Because the air then contains a large quantity of moisture. - - -Why will not the pitcher sweat in fine weather? - -Because the air contains only a small quantity of moisture in fine -weather. - - -Why does moisture sometimes collect on a glass window in a warm room? - -Because the glass cools the air, and causes its moisture to collect in -small drops on the glass. - - -What makes the glass cold enough to condense the vapor in the air? - -The cold air, outside of the room, in contact with the glass. - - -Where does the moisture in the air of the warm room come from? - -It comes from the breath of persons, and from the steam of the -tea-kettle. - - -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? - -The moisture is frozen. - - -What is this frozen moisture called? - -It is called _frost_. - - -What then is frost? - -Frost is the frozen moisture on the walls and window panes, and the -frozen dew on plants and other objects. - - -On what objects do we find the most frost? - -The most frost is found on objects having the most dew. - - -What is a heavy dew when frozen called? - -It is called a hoar-frost. - - -When do we have a hoar-frost? - -We have heavy frosts when the air is full of moisture. - - -Why does a heavy frost indicate rain or snow? - -Because it shows that the air contains much moisture. - - -Are heavy frosts usually followed by rain or snow? - -Yes; a heavy frost, on two successive mornings, is mostly followed by -rain or snow within a few hours. - - - Rain, Snow, and Hail. - - -What is a _mist_? - -A mist is very small drops of water falling through the air. - - -When these drops are larger, what is it called? - -It is called _rain_. - - -What causes rain? - -Rain is caused by the sudden cooling of the air above the earth. - - -How does cooling the air cause rain? - -The cold condenses the vapor in the air, so that its moisture is forced -out in drops, which fall to the earth. - - -What becomes of the rain falling upon 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. - - -What becomes of the remaining part? - -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. - - -How does rain purify the air? - -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? - -The moisture in it is frozen. - - -What is this frozen moisture in the air called? - -It is called _snow_. - - -What are the particles of snow called? - -They are called snow-flakes. - - -Are these snow-flakes all alike? - -No; they vary very much in size and shape. - - -What does Fig. 23 represent? - -It represents a few of the various and beautiful forms of the snow-flake -or snow-crystals. - - [Illustration: Fig. 23.] - - -At what temperature of the air does snow usually fall? - -At a temperature of about thirty-two degrees. - - -Is there any reason for the remark that it is “_too cold to snow_”? - -Yes; for it seldom snows when the temperature of the air is much below -thirty-two degrees. - - -Of what use is snow? - -The snow is a warm covering for the earth. - - -Why is snow a warm covering for the earth? - -Because the air in the snow makes it a poor conductor 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. - - -What is hail? - -Drops of rain, when frozen, are called _hail_. - - -How is hail formed? - -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 have we _dew_? - -When the air in contact with the earth or the plants is cooled, its -vapor is condensed into dew. - - -When have we _frost_? - -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 have we _fog_? - -When the air near the earth is slightly cooled, its vapor is condensed -into fog. - - -When have we _clouds_? - -When the air higher above the earth is slightly cooled, its vapor is -condensed into clouds. - - -When have we _mist_? - -When the air is a little more cooled, its vapor is condensed and mist -falls. - - -When have we _rain_? - -When the air is still more cooled, its vapor is condensed and rain -falls. - - -When have we _snow_? - -When the air is cooled below thirty-two degrees, its vapor is condensed -and frozen into snow. - - -When have we _hail_? - -When the rain-drops, while falling, pass through a current of air cold -enough to freeze them, hail is produced. - - - Ice. - - -What is ice? - -_Ice_ is frozen water. - - -When does water freeze? - -Water freezes when its temperature is reduced to thirty-two degrees. - - -Is water increased in bulk by freezing? - -Water is increased in bulk by freezing. - - -Why do water-pipes often burst in cold weather? - -The water in them freezes, and as it increases in bulk, the pipes burst. - - -Why are some springs never frozen? - -Because water, much above the freezing-point, is always flowing into -them from the earth, and thus they are kept too warm to freeze. - - -Why are some other springs frozen? - -Because the water flowing into them comes from near the surface of the -earth, and is soon made cold enough to freeze. - - -Does salt water freeze at the same temperature as fresh water? - -No; fresh water freezes at thirty-two degrees, but salt water requires a -lower temperature depending upon the amount of salt it contains. - - -How is this useful? - -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. - - [Illustration: Horse.] - - [Illustration: Decoration.] - - - - - CHAPTER VI. - Sound. - - - [Illustration: Illustrated Capital W] - - -When the hands are brought quickly together, what is produced? - -They produce a shaking or vibration in the air. - - -What is this vibration in the air called? - -It is called a wave in the air. - - -What is this wave in the air like? - -It is like a wave in the water. - - -When this wave in the air reaches the ear, what does it produce? - -It produces a sensation which is called _sound_. - - -When a stone is dropped into the water, what does it produce? - -It produces waves in the water. - - -How do these waves move? - -They move in every direction from the stone. - - -Do they get smaller the farther they go? - -They do get smaller and smaller, until at last they disappear. - - -Do the waves of sound in the air act in the same way as the waves in the -water? - -They do act in the same way. - - -What then becomes of the waves of sound? - -They get smaller and smaller the farther they go, until at last there is -no wave left. - - -When the wave in the air is large, what kind of sound is made? - -It makes a loud sound. - - -As the wave gets smaller, does the sound grow fainter? - -Yes; the smaller the wave the fainter the sound, until at last both the -wave and the sound die away. - - -Where must we be to hear a loud sound? - -We must be near the place in which the sound is made. - - -What kind of waves will make a faint sound? - -Small waves in the air make a faint sound. - - -Will these waves extend as far away as larger ones? - -No; as all waves of sound decrease in size as they recede, the smallest -ones will soonest disappear. - - -Why will a loud sound be heard farther than a fainter sound? - -Because the waves in the air from the loud sound will extend farther -away than those from the fainter one. - - -How are sounds produced? - -Sounds are produced by striking solid bodies together. - - -How else may sound be produced? - -By striking a solid body against a liquid, as a stick struck against the -water. - - -In what other way may sound be made? - -Sound may be made by causing a substance to move rapidly through the -air. - - -When a piece of wood attached to a string is swung rapidly around, what -kind of a noise is made? - -It makes a humming noise. - - -When lightning passes through the air, what noise is made? - -A noise which we call thunder. - - -How can a murmur be produced? - -When sounds, too faint to be heard separately, are united, they produce -a murmur. - - -Where may we hear a murmur? - -We may hear a murmur from the brook, as its waves flow over the pebbles, -and from the wind, as it passes through the trees. - - -When a sound is heard but once, what is it called? - -It is called a noise. - - -When sounds are made at irregular intervals, what are they called? - -They are called noises. - - -When sounds are repeated at regular intervals, what do they become? - -They become musical sounds. - - -Upon what do the high and low sounds in music depend? - -They depend upon the rapidity with which the waves are produced in the -air. - - -When the waves are repeated rapidly, what sound is made? - -They make a high or acute sound. - - -When they are repeated more slowly, what sound is made? - -They make a low or grave sound. - - -How small a number of vibrations or waves in the air can be heard? - -_Sixteen_ vibrations in a second of time, will produce the gravest sound -that can be heard. - - -How large a number of vibrations in the air can be heard? - -Twenty-four thousand vibrations in a second of time, make the most acute -sound that can be heard. - - -How is the human voice made? - -It is made by means of the vocal chords at the upper part of the larynx. - - -What is the larynx? - -The upper part of the windpipe is called the larynx. - - -How do these chords produce the voice? - -When the air is forced out of the lungs, it causes these chords to -vibrate, so as to produce the voice. - - -Are these chords the same in all persons? - -No; they are larger and coarser in some persons than in others. - - -In whom are they larger and coarser? - -They are usually larger and coarser in men, and they are smaller and -finer in women. - - -What kind of a voice is made by the large and coarse chords? - -They make a rough, coarse voice. - - -What kind of voice is made by the finer chords? - -They make a finer and more delicate voice. - - -Do these chords vibrate so rapidly in man as in woman? - -No; they do not vibrate so rapidly in man as in woman. - - -What kind of voice do rapid vibrations make? - -They make an acute voice; hence, woman can, when singing, raise her -voice to a very high pitch. - - -Where is the sensation of sound produced? - -The sensation of sound is produced on the ear. - - -Do people vary in their ability to hear? - -Yes; they vary very much in their ability to hear. - - -Is every person able to distinguish musical sounds? - -No; some persons cannot tell one musical note from another, and some -cannot even tell one bird from another by its notes. - - -Is the hearing of all animals alike? - -No; some can hear more acutely than others; a lion has more acute -hearing than a man. - - -What other animals have acute hearing? - -All such animals as hares, rabbits, and deer. - - -Of what use is the acute hearing to these animals? - -It enables them quickly to hear the approach of their enemies, so that -they may run away in time to escape the danger. - - - Conduction of Sound. - - -How does sound come to our ears? - -Sound is carried to our ears by the air. - - -Is the air a conductor of sound? - -Yes; the _air_ is a _conductor_ of sound. - - -Is there any sound where there is no air? - -No; there is not any sound where there is no air. - - -How do we know this? - -When a bell is rung in a vacuum, no sound is heard. - - -Can sounds be distinctly heard when the air is rare? - -No; the rarer the air the more poorly it conducts sound. - - -What proof have we of this? - -On the top of a high mountain the human voice is heard 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 of _ten_ miles. - - -How far has the human voice been heard over the land? - -It has been heard at the distance of _four_ miles. - - -How does the wind affect sound? - -The wind carries the sound the way in which it is blowing, 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 called _sonorous_ bodies. - - -Is a bell a very sonorous body? - -Yes; a bell is one of the most sonorous bodies that we have. - - -Why does a bell ring when struck? - -Because the stroke causes the particles of the bell to vibrate. - - -When we touch a bell while ringing, why does the sound stop? - -Because we stop its vibrations when we touch it. - - -Why does a crack in a bell check its ringing? - -Because the crack stops the vibrations in the metal. - - -What other substances ring when struck? - -All vessels made of glass, earth, stone, iron, etc., ring when struck. - - -How may we detect a crack in any such vessel? - -By striking the vessel; if it is cracked there will be no ringing sound. - - - Trumpets and Speaking-Tubes. - - -How do the waves of sound move? - -The waves of sound move in every direction from the place where the -noise is made. - - -Why do we put both hands around the mouth when we call to a person at a -distance from us? - -We do so to keep the waves of sound more together. - - -Does it make the voice louder? - -It does make the voice louder. - - -What instrument has been made so as to operate in the same way? - -The _speaking-trumpet_, as is shown in Fig. 24. - - [Illustration: Fig. 24.] - - -Of what use are speaking-trumpets? - -They are of much use to firemen when giving orders at a fire, and to -those on board of vessels when giving orders to the sailors. - - -Why do we hold the hand behind the ear when we wish to hear more -distinctly? - -Because the hand catches more of the waves of sound, and conducts them -to the ear. - - -What instrument has been made to serve the same purpose? - -The _ear-trumpet_, as is shown in Fig. 25. - - [Illustration: Fig. 25.] - - -How does the ear-trumpet aid the hearing? - -The large end of the trumpet collects more of the waves of sound than -the ear can; hence, more waves reach the ear. - - -Why do such animals as the horse, the rabbit, and the deer, hear better -than we do? - -Because their large ears act like ear-trumpets to collect the waves of -sound, and thus increase their faculty of hearing. - - -How can these animals still farther increase their sense of hearing? - -By being able to turn their ears, just as we do the ear-trumpet, in the -direction whence the sound comes. - - -What are speaking-tubes? - -They are metal pipes extending from one part of a building to another -part. - - -Of what use are speaking-tubes? - -They are used to convey the voice to the more distant parts of a -building. - - -Do they require loud talking? - -No; even a whisper has been heard through a metal pipe over three -thousand feet long. - - - Velocity of Sound. - - -How fast do the waves of sound move through the air? - -They move at the rate of ten hundred and ninety feet in a second of -time, when the temperature of the air is thirty-two degrees. - - -What effect does warm air have on sound? - -Sound travels more rapidly in warm air. - - -How much does heat increase the velocity of sound? - -The velocity of sound is increased one foot in a second of time, for -every degree of heat added to the air; so that sound travels eleven -hundred and twenty feet in a second, when the temperature is at -sixty-two degrees, which is nearly _one mile_ in _five seconds_. - - -Does light travel more rapidly than sound? - -Yes; light comes from the moon to the earth, a distance of two hundred -and forty thousand miles, while sound moves eleven hundred and twenty -feet. - - -What familiar examples of this difference may be given? - -A wood-chopper’s axe is seen to descend before we hear the stroke, and -the smoke from a gun is seen before we hear the report. - - -How can we tell the distance of the wood-chopper from us? - -If we count the number of seconds between the stroke of the axe and the -time when the sound reaches our ear, and multiply eleven hundred and -twenty by this 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 about _four_ times as rapidly through _water_ as it does -through air. - - -How rapidly does sound travel through a solid? - -Sound travels _ten_ times as rapidly through _wood_, and _sixteen_ times -as rapidly through _glass_, as it does through air. - - -Does a loud sound travel more rapidly than a faint one? - -No; it does not. - - -How do we know this? - -Because the notes from a band of music come to us in the order in which -they are played, whether they are loud or faint. - - - Reflection of Sound. - - -What becomes of the waves of sound when they strike a solid substance? - -They are thrown back, as light from a looking-glass, or a ball from a -stone wall. - - -What is this called? - -It is called _reflection_ of sound. - - -In what direction is sound reflected? - -Sound, like light, is reflected so that the angles of incidence and -reflection are always equal. - - -When the reflected sound comes back to the ear, what does it make? - -It makes an _echo_. - - -When the sound comes back several times, what does it make? - -It makes several echoes. - - -Where are echoes always to be heard? - -Echoes may be heard in the deep caves of the earth; because the walls of -the caves reflect sounds striking against them. - - -What curious echo may be mentioned? - -There is an echo in Fairfax County, Virginia, which sends back twenty -notes played on a flute. - - -Does every reflection of sound produce an echo? - -No; it does not. - - -Why is there no echo from the walls of a small room? - -Because the walls are too close together to make an echo. - - -How far away must the reflection of a sound be made, so as to produce a -perfect echo? - -Sound must be made not less than one hundred and twelve feet distant to -produce a perfect echo. - - -How many syllables can be heard from an echo one hundred and twelve feet -distant? - -Only one syllable can be heard. - - -How far must the echo be to hear two syllables? - -It must be twice the one hundred and twelve feet, or two hundred and -twenty-four feet, to hear two syllables; 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. - - [Illustration: Horse-drawn trolley car.] - - [Illustration: Decoration.] - - - - - CHAPTER VII. - Matter. - - - [Illustration: 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 called _Elasticity_. - - -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 called _Flexibility_. - - -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 called _Brittleness_. - - -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 called _Malleability_. - - -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 called _Tenacity_. - - -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 called _Ductility_. - - -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 of _rarity_. - - -What is said of it when the particles are pressed closer together? - -The air is then said to be in a state of _density_. - - -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 called _Penetrability_. - - -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. - - [Illustration: Decoration.] - - [Illustration: Decoration.] - - - - - CHAPTER VIII. - Attraction. - - - Cohesion and Adhesion. - - [Illustration: 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 called _Cohesion_. - - -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 called _Adhesion_. - - -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. - - - Capillary Attraction. - - -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 called _Capillary_ attraction. - - -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 the basin, 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 in Fig. 26. - - [Illustration: 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. - - - Gravitation. - - -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 of _Gravitation_. - - -Does the attraction of gravitation exist in all things? - -Yes; every substance attracts every other substance. - - -Upon what does the attraction of gravitation in a body depend? - -It depends upon the size and weight of that body. - - -Do larger bodies attract with more force than smaller ones? - -Yes; larger bodies do attract more than smaller ones. - - -Do heavy bodies attract with more force than light ones? - -Yes; heavy bodies do attract more than light ones, although they may be -of the same size. - - -Where are all substances drawn by the attraction of gravitation? - -They are all drawn towards the centre of the earth. - - -Why are substances drawn towards the centre of the earth? - -Because the earth is larger than any substance on or near its surface. - - -In what direction do all objects fall through the air? - -They all fall towards the centre of the earth. - - -Which way then do we call “down”? - -_Down_ is always towards the centre of the earth. - - -Which way do we call “up”? - -_Up_ is always away from the centre of the earth. - - -Where must all plumb lines or perpendicular lines point? - -They must all point with one end towards the centre of the earth. - - -When the earth turns over, can any substance fall away from its surface? - -No; all things are drawn to the earth with so much force that they -cannot fall away. - - -Upon what does the weight of a body depend? - -The weight of a body depends upon the force with which it is drawn -towards the earth. - - -What kind of substances are attracted with the most force? - -Those that are the most dense, like the metals; hence, they are the -heaviest. - - -What causes a liquid to form into a drop at the mouth of a bottle? - -The attraction of cohesion causes the drop to be formed. - - -What causes the drop to fall to the earth? - -The attraction of gravitation causes the drop to fall to 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. - - [Illustration: Fig. 27.] - - -Why does a spout prevent the water from running down the side of the -vessel? - -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. - - [Illustration: Fig. 28.] - - -Does the air obstruct bodies when falling through it? - -Yes; air obstructs all bodies when falling through it; but it obstructs -light bodies more than heavy ones. - - -Do all bodies fall with equal rapidity in a vacuum? - -Yes; all bodies fall with equal rapidity in a vacuum. - - -What bodies fall through the air most rapidly? - -Those that are the heaviest or most dense; because they are the smallest -in bulk. - - -Do heavy bodies increase in velocity as they descend? - -They do increase rapidly in velocity as they descend. - - -How far does a heavy body like lead fall in one second? - -It falls sixteen feet in one second. - - -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. - - [Illustration: Fig. 29.] - - -How may the distance a body falls in a given time be found? - -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. - - -What familiar example have we of the force of falling bodies? - -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. - - -What is the centre of gravity in a body? - -The _centre of gravity_ in a body, is the point on which the body may be -balanced, as is shown in Fig. 30. - - [Illustration: Fig. 30.] - - -Where is the centre of gravity in a circle? - -It is the centre of the circle. - - -Where is the centre of gravity in a ball? - -It is the centre of the ball. - - -What influence does the earth have on the centre of gravity? - -The earth draws the centre of gravity in a body as near to itself as -possible. - - -Why will an egg, when placed on either end, fall over on its side? - -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. - - [Illustration: Fig. 31.] - - [Illustration: Fig. 32.] - - -Why will a ball remain in any position when placed on the earth? - -Because its centre of gravity is always the same distance from the -earth, whatever may be its position. - - -When will a body change its position if left unsupported? - -A body will change its position whenever by so doing its centre of -gravity will be lowered. - - -When may a body be readily moved about? - -When its centre of gravity always remains at the same distance from the -earth while being moved. - - -Why is a round body, like a ball, easily rolled about the floor? - -Because its centre of gravity remains at the same distance from the -floor. - - -When is the position of a body difficult to change? - -Whenever the centre of gravity in it has to be lifted up in order to -make the change, as is shown in Fig. 35. - - [Illustration: Fig. 33.] - - [Illustration: Fig. 34.] - - [Illustration: 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. - - -When does an object stand firm? - -It stands firm when it has a broad base, and its centre of gravity is -near the earth. - - -What effect is produced by elevating the centre of gravity? - -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. - - -Why do passengers on top of a stage cause it to overturn more easily? - -Because their weight on the top raises the centre of gravity in the -stage, and it is more readily overturned. - - -Why is a person more likely to fall when standing up than when sitting -down? - -Because the centre of gravity is higher in a person when standing than -when sitting. - - -Why is a boy more likely to fall when raised on stilts? - -Because the centre of gravity in the boy is farther from the earth when -he is raised on stilts. - - -When will a load of hay be overturned? - -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. - - [Illustration: Fig. 36.] - - -Why should lamps have large bottoms? - -Lamps should have large bottoms so that the line of gravity may not fall -outside of the bottom. - - -Where must the line of gravity in a body always fall? - -It must always fall within the base of that body, or the body will be -overturned. - - -How must a leaning tower be built so as not to fall? - -It must be built so that the line of gravity will fall within its base. - - -Why do we lean back when carrying a load in front of us? - -We lean back so that the line of gravity may be kept directly over our -feet. - - -Why does a person standing up in a wagon fall when the wagon is suddenly -moved? - -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. - - -Why do animals with _four_ feet stand firmer than those with only _two_ -feet? - -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 their four feet afford a good support to the centre of gravity. - - -Why does a person who is standing soon become weary? - -Because it requires a constant effort to keep the line of gravity over -the feet, and this soon produces weariness. - - -Why can a person sit longer than he can stand? - -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. - - -Why can a person lie down longer than he can sit or stand? - -Because the centre of gravity is nearest the earth when he is lying -down, and it requires no effort to remain in that position. - - -How do we measure time? - -We measure time by means of a _pendulum_, such as is used in a common -clock. - - -How does the pendulum move? - -The pendulum moves regularly to and fro, from the line of gravity. - - -Does the pendulum vary any in its vibrations, or the time of moving to -and fro? - -The pendulum does not vary any in its vibrations. - - [Illustration: Fig. 37.] - - -Why does the pendulum keep moving? - -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 motion it gains while -falling to C carries it back to A again. - - -What helps to keep the pendulum in motion? - -The works inside of the clock help to keep the pendulum in motion. - - -How long must the pendulum be, so as to tick once in every second? - -The pendulum must be thirty-nine inches long. - - -How does it tick when the pendulum is _shortened_? - -It ticks more _rapidly_. - - -How does it tick when the pendulum is _lengthened_? - -It ticks more _slowly_. - - -How then does _cold_ affect a clock? - -Cold makes the clock go faster by shortening the pendulum. - - -How does _heat_ affect a clock? - -Heat makes the clock go more slowly by lengthening the pendulum. - - -How may the pendulum always be kept of the same length? - -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. - - -Upon what does the weight of a body, when in the air, depend? - -The weight of a body depends upon the force with which it is drawn -towards the earth. - - -What is this attractive force called? - -It is called gravitation or gravity. - - -What is gravity? - -_Gravity_ is the weight of any body. - - -What is specific gravity? - -_Specific Gravity_ is the weight of any body, compared with an equal -bulk of water. - - -Do substances weigh as much in water as they do in air? - -No; they do not weigh so much. - - -When we drop lead in a vessel full of water, what becomes of the water? - -The water is forced over the sides of the vessel. - - -How much water is thus forced over the sides of the vessel? - -A quantity equal in bulk to the lead dropped into the vessel. - - -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? - -It will weigh one pound less. - - -Then how much less is the weight of a body in water than it is in air? - -Its weight in the water is as much less as the weight of the water which -it displaces. - - -When a body has greater specific gravity than water, what will it do? - -It will sink in the water. - - -When it has less specific gravity than water, what will it do? - -It will float on the surface of the water. - - -Will all parts of the body be above the surface of the water? - -No; a part of it will sink into the water. - - -How much of it will sink into the water? - -It will sink so as to displace water enough to balance its own weight. - - -Why does a boat float on the water? - -Because it weighs less than a bulk of water of the same size as the -boat. - - -When a cargo is placed on the boat, why does it still float? - -Because it still weighs less than a bulk of water of the same size as -the boat. - - -How do we make use of this principle? - -We make use of it in the construction of vessels, so as to carry on -trade with other parts of the world. - - -Why can ducks and geese float on the water? - -Because their light, downy feathers make their specific gravity less -than that of the water; hence, they must float on its surface. - - -What may be said of the specific gravity of the human body? - -When the lungs are filled with air, the specific gravity of the human -body is rather less than that of water. - - -Can a person float in the 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. - - -What is the specific gravity of fish? - -The specific gravity of fish is so nearly equal to that of water, that -they can move about anywhere beneath its surface. - - -How do fish rise and sink in the water? - -Fish rise and sink in the water by means of an air-bladder in their -bodies. - - -How does the air-bladder help the fish to rise or sink? - -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. - - -Why does cream rise to the surface of milk? - -Because the specific gravity of the cream is less than that of the milk. - - -Why does oil float on water? - -Because the specific gravity of the oil is less than that of the water. - - -Why can a fat person float more easily in the water than a lean person? - -Because the fat reduces the specific gravity of the body, so that it -floats more easily. - - -Why can a person float more easily in salt water than in fresh water? - -Because salt increases the weight of the water, and, therefore, it buoys -up with more force. - - -What familiar example have we of this? - -An egg will sink when placed in fresh water; but if salt be dissolved in -the water, the egg will float near the surface. - - [Illustration: Decoration.] - - [Illustration: Decoration.] - - - - - CHAPTER IX. - Motion. - - - [Illustration: Illustrated Capital W] - - -When is a body said to be in motion? - -A body is in _motion_ when it is moving from one place to another. - - -When is a body said to be at rest? - -A body is at rest when it is not moving from one place to another. - - -What is necessary to give motion to a body? - -Force of some kind is necessary to give motion to a body. - - -How long will the motion given to a body continue? - -The motion will continue until it is checked. - - -What is necessary to check the motion of a body? - -Force of some kind is necessary to check its motion. - - -What force is always acting so as to bring bodies to a state of rest? - -The force of gravity, which is constantly drawing all bodies to the -earth. - - -What else may check the motion of a body? - -The air, the water, and solids of all kinds may check the motion of a -body. - - -In what direction will a body move when its course is not obstructed? - -It will move in a straight line. - - -Why does a ball struck by a bat make a curve to the earth? - -Because the force of gravity gradually overcomes the force given by the -bat, so that the ball moves in a curve. - - -Why does a bullet make a curve to the earth? - -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? - -The bullet will strike below the mark. - - -Why will the bullet strike below the mark? - -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? - -The tendency of the ball is to fly off in a straight line. - - [Illustration: Fig. 38.] - - -What is the force tending to make the ball fly off called? - -It is called _centrifugal_ force. - - -What keeps the ball from flying off? - -The string keeps the ball from flying off. - - -What is the force exerted by the string called? - -It is called _centripetal_ force. - - -What then is centrifugal force? - -It is the force tending to fly off from the centre. - - -What is centripetal force? - -It is the force tending to draw towards the centre. - - -Why does mud fly off from the wheel of a wagon when in motion? - -Because the motion of the wheel gives centrifugal force to the mud, and -causes it to fly off. - - -What does the rim of a wheel when in motion, tend to do? - -The rim tends to fly off from the centre of the wheel. - - -How is it prevented from flying off? - -It is prevented by making the wheel very strong, so that all parts of it -are held firmly together. - - -What does this centrifugal force sometimes cause? - -The centrifugal force is so great that it sometimes causes grind-stones -and mill-stones, when revolving rapidly, to burst into pieces. - - -What becomes of these stones when they burst? - -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. - - -How are cars prevented from running off the track, while turning a -curve? - -The outer rail is made higher than the inner rail, so as to overcome the -tendency of the cars to run off the track. - - -What grand example have we of the action of these two forces? - -The earth and other planets that move around the sun, are kept in their -places by the action of these two forces. - - -When a ball strikes a solid body and bounds back, what is that motion -called? - -It is called _reflected_ motion. - - -At what angle does the ball leave the solid body? - -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. - - [Illustration: Fig. 39.] - - -How may a stone be skipped on a smooth pond? - -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? - -The boat will go diagonally across the river. - - -Why will the boat go diagonally across the river? - -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. - - -How do boatmen overcome this difficulty? - -They overcome this by rowing the boat towards a point higher up the -stream. - - -Does it take longer to cross a stream when the current is strong? - -Yes; because the boat is carried farther down the stream by the current, -and it must, therefore, go a greater distance in crossing. - - -Why is a person apt to fall when he jumps from a wagon that is moving? - -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. - - -How can he prevent falling? - -He can prevent falling by jumping in the same direction as that in which -the wagon is moving. - - -When a stone is dropped from the top of a mast of a vessel, where does -it fall? - -The stone falls at the bottom of the mast. - - -Why does the stone fall at the bottom of the mast? - -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. - - -Why does the ball fall back into his hand? - -Because it partakes of the motion of the boat, and moves as far as the -boat moves. - - -When a stone thrown from a car is aimed at a post, where will it strike? - -The stone will strike ahead of the post. - - -How far ahead of the post will the stone strike? - -It will strike as far ahead as the car moves while the stone is going to -the post. - - -Where must we aim so as to hit a bird that is flying? - -We must aim ahead of the bird. - - -Why must we aim ahead of the bird? - -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. - - -Do bodies, when moving rapidly, strike with more force than when moving -slowly? - -Yes; the force with which they strike depends upon their velocity. - - -What examples of this force may be given? - -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. - - -Why do large bodies start slowly? - -Because it takes some time for the force to reach all parts of them. - - -Why is it difficult for horses to start a loaded wagon? - -Because they must pull some time before the force reaches all parts of -the wagon, causing it to move. - - -Why is machinery slow at starting? - -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? - -They are pushed off from each other in opposite directions. - - -Which body is pushed off the more rapidly? - -The one having the less weight, is pushed off the more rapidly. - - -When the oars of a boat push against the water, what takes place? - -The boat and the water move in opposite directions. - - -When the wings of a bird strike the air, what takes place? - -The bird and the air move in opposite directions. - - -How can a duck or a goose swim in the water? - -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. - - -Can all birds swim in the water? - -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. - - -What is produced by the rubbing of bodies together? - -_Friction_ is produced by rubbing bodies together. - - -What does friction tend to do? - -Friction tends to check the motion of bodies. - - -How do we lessen the friction in machinery? - -We lessen the friction in machinery by oiling the parts where they rub -together. - - -How are waves produced in the water? - -Waves are produced by the friction of the air upon the water. - - -What causes the water to flow in creeks and rivers? - -The attraction of gravitation, which causes water always to fall to its -lowest level. - - -What retards the current of water in streams? - -The friction, caused by the water rubbing against the banks and bottoms -of the streams, retards their current. - - -In what part of a stream is the current the strongest? - -It is strongest in the middle of a stream, because there is less -friction there. - - -What retards the flow of water through a pipe? - -The friction of the water against the sides of the pipe, retards its -flow very much. - - -Why do sudden turns in a pipe check the flow of water through it? - -Because every turn in the pipe increases the friction made by the water. - - -Why does a wagon-wheel turn around when the wagon moves? - -Because the friction between the wheel and the earth, prevents the wheel -from sliding along. - - -Why can a steam-engine draw a train of heavy cars? - -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. - - -Why is sand sometimes put on the rails? - -Sand is put on the rails to increase the friction between the wheels and -the rails, so that the engine may start a heavier train. - - -Why do we use sleighs when there is snow on the ground? - -Because sleighs slip easily on the snow. - - -Why do sleighs slip easily on the snow? - -Because there is but little friction between the sleigh and the snow. - - -Why do wagons run more heavily when the ground is soft? - -Because their wheels then sink farther into the earth, and thus the -friction is increased. - - -Why do wagons run more heavily when the ground is covered with snow? - -Because their wheels sink into the snow, and thus the friction is -increased. - - -Why will a person slip down upon the ice more easily than upon the -earth? - -Because there is less friction between his feet and the ice than there -is between his feet and the earth. - - -Why do we put ashes on our walks when they are covered with ice? - -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. - - -Why is a lock or a brake used on a wagon? - -It is used to check the motion of the wagon. - - -How does a lock check the motion of the wagon? - -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. - - -Why cannot we hold a live eel in our hand, as easily as we can a live -fish? - -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. - - [Illustration: Explorer.] - - [Illustration: Decoration.] - - - - - CHAPTER X. - Mechanical Powers. - - - Lever. - - [Illustration: Illustrated Capital W] - - -What is a lever? - -A _Lever_ is a rod or bar of some kind used for raising weights. - - -Of what are levers made? - -Levers are made of any solid substance, such as wood and iron. - - -When a spoon is balanced on a cup, as in Fig. 40, what may the spoon be -called? - -The spoon may then be called a lever. - - -Upon what does the spoon rest? - -It rests upon the edge of the cup. - - [Illustration: Fig. 40.] - - -What is the cup on which the spoon rests called? - -It is called a _Fulcrum_. - - -What is a fulcrum? - -A fulcrum is any solid body against which a lever rests. - - -When the spoon is called a lever, what are the parts on each side of the -fulcrum called? - -They are called the _arms_ of the lever. - - -What is necessary, so that a lever may balance? - -The arms of the lever must be of equal weight, or they will not balance. - - -If the bowl of the spoon be filled with water, what will result? - -The bowl will sink, and the handle of the spoon will rise. - - -What will cause the bowl of the spoon to sink? - -The weight of the water in it will cause it to sink. - - -How can the bowl of the spoon be prevented from sinking? - -By applying a force to the handle of the spoon, equal to the weight of -the water put in the bowl. - - -What is the force applied to the handle called? - -It is called the _Power_. - - -What three things are always necessary when the lever is used? - -The power, the _fulcrum_, and the _weight_, are always necessary when -the lever is used. - - -Which of these three things are movable? - -The power and the weight are movable. - - -Which one does not move? - -The fulcrum does not move. - - -How many kinds of levers are there? - -There are three kinds of levers. - - -What is the first kind of lever? - -It is where the fulcrum is between the power and the weight, as is shown -in Fig. 41. - - [Illustration: 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? - -The arm towards the power, because it is the longer arm. - - -Will the power applied to the long arm of the lever be as heavy as the -weight? - -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? - -Then the power must be heavier than 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. - - -Upon what does the power of a lever depend? - -It depends upon how much nearer the fulcrum is placed to one end of the -lever, than to the other end. - - -Of what use is the lever? - -It is very useful in raising heavy bodies. - - [Illustration: Fig. 42.] - - -What is the weight to be lifted in Fig. 42? - -The stone is the weight to be lifted. - - -Where is the fulcrum placed? - -The fulcrum is placed near the stone. - - -Where is the power applied? - -The power is applied by the hand to the long arm of the lever. - - -How may a see-saw be made? - -A see-saw may be made by placing a plank or a board across a rail in a -fence, so that it will balance. - - -Is the see-saw, as shown in Fig. 43, an example of the lever? - -Yes; the see-saw is an example of the lever. - - [Illustration: Fig. 43.] - - -What are the parts of the plank on each side of the fence-rail called? - -They are called the arms of the lever. - - -What is the fulcrum in the see-saw? - -The fence-rail on which it rests, is the fulcrum. - - -What is the power in the see-saw? - -The power is the boy on one end of the lever. - - -What is the weight in the see-saw? - -The weight is the boy on the other end of the lever. - - -How do children play at see-saw? - -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. - - -Where must the fulcrum be when the two boys are of the same weight? - -The fulcrum must be under the middle of the lever. - - -Where must the fulcrum be when one boy is heavier than the other? - -The fulcrum must then be nearest to the heavier boy, so that they may -balance. - - -Does it require much force to play at see-saw? - -No; when the see-saw is balanced, a very little force will cause it to -move up and down. - - -Of what use is a pair of scales? - -A pair of scales is very useful in weighing different substances. - - [Illustration: Fig. 44.] - - -Are the scales, as shown in Fig. 44, an example of the lever? - -The scales are an example of the lever. - - -Where is the fulcrum of the scales? - -The fulcrum is the point on which the lever rests. - - -What is the weight? - -The weight is the lead put in one scale. - - -What is the power? - -The power is the substance put in the other scale, so as to balance the -weight. - - -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? - -Exactly one pound of sugar must be put in to balance the lead. - - -For what is a balance mostly used? - -A balance is mostly used for weighing heavy bodies. - - -Is the balance a lever? - -Yes; the balance is a lever. - - -Which is the fulcrum in the balance? - -The fulcrum is the hook that holds the balance up. - - [Illustration: Fig. 45.] - - -What is the weight? - -The weight is the large body hanging from the short arm of the lever. - - -What is the power? - -The power is the smaller body hanging from the long arm of the lever. - - -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? - -The large body must weigh ten pounds. - - -When the small body weighs ten pounds, how much must the large body -weigh, so as to balance it? - -The large body must weigh ten times as much, or one hundred pounds. - - -If the smaller body be moved half-way to the fulcrum, how much weight -will then balance it? - -It will then take only half the weight, or fifty pounds to balance it. - - -What other familiar example of this kind of lever may be mentioned? - -The pump-handle is a lever of this kind. - - [Illustration: Fig. 46.] - - -What is the power? - -The power is the force applied to the pump-handle. - - -What is the fulcrum? - -The fulcrum is the screw that fastens the handle to the pump. - - -What is the weight? - -The weight is the water raised by the bucket. - - -Are scissors an example of the lever? - -Yes; scissors are a double lever. - - -What is the fulcrum in the scissors? - -The fulcrum is the rivet which holds the two levers together. - - -What is the power? - -The power is the force applied by the fingers. - - -What is the weight? - -The weight is the paper, or other substance, to be cut by the scissors. - - -Can scissors be made very strong? - -Yes; some scissors are made so strong that they are used for cutting -iron, tin, and zinc. - - -What other useful instruments are made upon the same principle as the -scissors? - -Such instruments as nippers, pincers, tweezers, and blacksmith’s tongs. - - -What is the second kind of lever? - -It is where the weight is between the power and the fulcrum, as is shown -in Fig. 47. - - [Illustration: Fig. 47.] - - -Of what use is this kind of lever? - -It is very useful in lifting heavy weights a short distance from the -earth. - - -What example is there of this kind of lever? - -The door on its hinges is a lever of this kind. - - -What is the fulcrum? - -The hinges are the fulcrum. - - -What is the weight? - -The door is the weight. - - -What is the power? - -The force that opens or shuts the door is the power. - - -What other examples of this kind of lever have we? - -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. - - -What is the third kind of lever? - -It is where the power is between the fulcrum and the weight, as is shown -in Fig. 48. - - [Illustration: Fig. 48.] - - -What examples of this kind of lever have we in common use? - -The shovel, the spade, and the pitch-fork, are levers of this kind. - - -What is the fulcrum? - -One hand is the fulcrum. - - -What is the power? - -The other hand is the power. - - -What is the weight? - -The earth raised on the shovel is the weight. - - -What other familiar example have we of this kind of lever? - -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. - - [Illustration: Fig. 49.] - - -There are several other examples of this kind of lever; will you name -some of them? - -Yes; brooms, hoes, rakes, and axes are all levers of this kind. - - -What is the first kind of lever mentioned? - -It is where the fulcrum is between the power and the weight, as in Fig. -50. - - [Illustration: Fig. 50.] - - -What is the second kind of lever mentioned? - -It is where the weight is between the power and the fulcrum, as in Fig. -51. - - [Illustration: Fig. 51.] - - -What is the third kind of lever mentioned? - -It is where the power is between the fulcrum and the weight, as in Fig. -52. - - [Illustration: Fig. 52.] - - - Wheel and Axle. - - -What other way have we for raising weights? - -We have the _wheel and axle_, as is shown in Fig. 53. - - [Illustration: Fig. 53.] - - -What is the wheel and axle? - -The wheel and axle is only another form of the lever. - - -Where is the power applied? - -The power is applied to the rope which passes around the wheel. - - -Where is the weight? - -The weight is the bucket attached to the rope which passes around the -axle. - - -Where is the fulcrum? - -The fulcrum is the frame on which the ends of the axle rest. - - -Is the power applied to the wheel equal to the weight to be raised? - -No; the power is always less than the weight to be raised. - - -Upon what do the power and the weight depend? - -They depend upon the size of the wheel and the axle. - - -When the diameter of the wheel is ten times the diameter of the axle, -how much will one pound raise? - -One pound applied to the wheel, will then raise ten pounds on the axle. - - -What is a windlass? - -A _windlass_ is another form of the wheel and axle, as is shown in Fig. -54. - - [Illustration: Fig. 54.] - - -What is used in a windlass in place of the wheel? - -A crank or handle is used in place of the wheel. - - -Does the crank take the place of the wheel? - -Yes; when the crank is turned, it forms a circle the same as the outside -of the wheel. - - -To what use is the windlass often applied? - -The windlass is often used to draw water from wells. - - - Pulley. - - -What is a pulley? - -A _pulley_ is a wheel with a groove or hollow cut in its edge. - - -What is the groove for? - -The groove is cut so that a rope may move over the wheel without -slipping off. - - [Illustration: Fig. 55.] - - -In Fig. 55, where is the power applied? - -The power is applied to one end of the rope by the hand. - - -Where is the weight? - -The weight is attached to the other end of the rope. - - -When the power and the weight are equal, what do they do? - -They balance each other, and neither of them will move. - - -When the power is greater than the weight, what takes place? - -The weight is then lifted up. - - -Of what use is the single pulley? - -The single pulley is used for hoisting flags to the top of high poles, -for raising sails to the topmasts of vessels, and for raising weights to -the upper stories of mills and factories. - - -How many pulleys are needed, when a horse is to raise the weight? - -When a horse is to raise the weight, two pulleys are needed, as may be -seen in Fig. 56. - - [Illustration: Fig. 56.] - - -To what use do farmers apply the pulley? - -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? - -No; its surface is not level. - - -What may the surface of the plank be called? - -It may be called an inclined surface, or an _inclined plane_. - - -What is an inclined plane? - -An inclined plane is a surface that inclines upwards or downwards. - - -Of what use is the inclined plane? - -It aids very much in raising weights. - - -Are all planes inclined alike? - -No; some planes are much steeper than others. - - [Illustration: Fig. 57.] - - -In Fig. 57, where is the power? - -The power is at P. - - -Where is the weight? - -The weight is at W. - - -Upon what does the power and the weight depend? - -They depend upon the height and the length of the inclined plane. - - -When an inclined plane is ten feet long and only one foot high, how much -will one pound at P balance? - -One pound at P will balance ten pounds at W. - - -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. - - -Where else may the power be exerted? - -The power may be exerted behind the weight, as in Fig. 58. - - [Illustration: Fig. 58.] - - -Will any more power be needed behind the weight than before it? - -No; the power required is the same in both cases. - - -Would it require as much power to roll barrels into a wagon as to lift -them in? - -No; they can be rolled into a wagon much more easily than lifted in. - - -Can they be taken out of the wagon any more easily on an inclined plane -than without one? - -Yes; they can be taken out more easily on an inclined plane. - - -Why are drays, used in cities, made in the form of the inclined plane? - -So that barrels and hogsheads may the more easily be rolled on them. - - -Are our stairs in the form of the inclined plane? - -Yes; they are inclined planes with steps cut in them to make the ascent -easy. - - -Why are house-roofs made in the form of the inclined plane? - -So that the water falling on them may run off readily. - - -What familiar example of the inclined plane may be mentioned? - -The hills, all over the country, are so many inclined planes. - - -Of what use are the hills? - -The hills carry off the waste water from the earth, so that it is kept -dry enough for plants to grow. - - -What other example of the inclined plane may be mentioned? - -The beds of the creeks and rivers are so many inclined planes down which -their waters flow. - - - Wedge. - - -What is a wedge? - -A _wedge_ is a double inclined plane, as is represented in Fig. 59. - - [Illustration: Fig. 59.] - - -Of what use is the wedge? - -The wedge is used in splitting logs of wood and blocks of stone. - - -For what other purposes is the wedge used? - -The wedge is used to fasten handles in hammers, hatchets, axes, and -hoes. - - -How is power applied to the wedge? - -Power is applied to the wedge by means of sudden blows from a hammer. - - -What prevents the wedge from flying back after the blow? - -The friction between the wedge and the other body, keeps it from flying -back. - - -What instruments are made in the form of a wedge? - -Knife-blades, hatchets, axes, razors, and cutting instruments of nearly -all kinds, are in the form of the wedge. - - - Screw. - - -What is a screw? - -A _screw_ is another form of the inclined plane. - - -Where is the inclined plane on a 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. - - [Illustration: Fig. 60.] - - -What is this inclined plane called? - -It is called the thread of the screw. - - -How do the threads affect the screw? - -When the threads are far apart, the inclined plane is steep; but it -becomes less steep the nearer the threads are together. - - -When will the screw exert the most power? - -The screw will exert the most power, when its threads are nearest -together. - - -Of what use are screws, such as is shown in Fig. 60? - -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. - - -For what other purposes is the screw used? - -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. - - [Illustration: Fig. 61.] - - -In what does a screw like that in Fig. 61 work? - -It works in a nut which has threads cut on the inside, so that the -threads of the screw move in them. - - -Where is the power applied? - -The power is applied to the lever. - - -Can heavy bodies be raised by means of this screw? - -Yes; loaded cars, and even heavy buildings, may be raised by means of a -screw of this kind. - - [Illustration: Sewing machine.] - - [Illustration: Decoration.] - - - - - CHAPTER XI. - Electricity. - - - [Illustration: Illustrated Capital W] - - -When a cat’s back is rubbed in the dark, what does it give off? - -It gives off small sparks. - - -What are these small sparks called? - -They are called electric sparks, or _electricity_. - - -If a horse be rubbed in the winter when it is dark, what will be given -off from him? - -Electricity will be given off from the horse. - - -When sealing-wax is rubbed with a silk handkerchief, or a piece of -flannel, what is produced? - -Electricity is produced in the sealing-wax. - - -What will this electricity in the sealing-wax do? - -It will attract bits of paper, straw, feathers, and hairs, when it is -held near them. - - -What other substance will act the same as the sealing-wax? - -A dry glass tube will act in the same manner when rubbed. - - -Where is electricity found? - -Electricity is found in every body, and in every thing. - - -How is electricity developed? - -Electricity is developed by motion, by friction, and by almost every -change that takes place. - - -Is electricity found in the air? - -Yes; the air is almost constantly electrified. - - -With what is electricity often accompanied? - -Electricity is often accompanied by heat and light. - - -How many kinds of electricity are there? - -There are two kinds of electricity. - - -What are they called? - -They are called _positive_ and _negative_ electricity. - - -What do these two kinds of electricity always do? - -They always attract each other. - - -When one body contains positive and the other body negative electricity, -what will the two bodies do? - -They will rush together. - - -When both bodies are positive or both are negative, what will they do? - -They will repel each other. - - -When a body permits electricity to pass freely through it, what is it -called? - -It is called a _good conductor_ of electricity. - - -What substances are good conductors of electricity? - -Metals, charcoal, water, animals, vegetables, flame, and smoke. - - -When a body does not permit the electricity to pass through it, what is -it called? - -It is called a _non-conductor_ of electricity. - - -What substances are non-conductors of electricity? - -Gutta-percha, sulphur, glass, silk, wool, hair, feathers, cotton, and -paper. - - -What is lightning? - -Lightning is electricity passing from one place to another. - - -When does lightning pass from one cloud to another cloud? - -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. - - -When does lightning pass from the earth to a cloud? - -Whenever the earth contains more electricity than the cloud does. - - -When does lightning pass from a cloud to the earth? - -Whenever the cloud contains more electricity than the earth does. - - -How near are the clouds when the lightning comes to the earth? - -The clouds are seldom half a mile away when the lightning comes from -them to the earth. - - -What noise is made by the lightning? - -The lightning passes so quickly through the air, that it makes a noise -called thunder. - - -Do we always hear the thunder when there is lightning? - -No; the lightning is sometimes so far away that the thunder cannot be -heard. - - -What bodies attract the lightning to the earth? - -Tall trees, steeples, high houses, and mountains. - - -Does the lightning do any harm? - -Yes; it does much harm by destroying trees, houses, animals, and, -indeed, almost everything in its course. - - -What does lightning sometimes do with buildings? - -It sets the buildings on fire so that they are burned. - - -How are buildings protected from injury by lightning? - -They are protected by means of the lightning-rod, which was invented by -Dr. Franklin. - - -Of what is the lightning-rod made? - -The lightning-rod is made of some metal, usually of iron or copper, -which is a good conductor of electricity. - - -What does the lightning-rod do when the building is struck? - -The lightning-rod conducts the electricity to the earth, so that it does -not injure the building. - - -Must the lightning-rod be higher than the building? - -Yes; because the lightning is apt to strike the highest object. - - -Of what should the point of the rod be made? - -The point should be made of silver or platinum, so that it will always -remain bright. - - -Should the rod extend deep into the ground? - -The rod should extend four or five feet beneath the surface of the -earth. - - -Why should ashes or charcoal be put around the bottom of the rod? - -Because they are good conductors and will help to conduct the lightning -into the earth. - - -How large should the lightning-rod be? - -If made of iron, the lightning-rod should be about three-fourths of an -inch in diameter. - - -Why is glass put around the rod where it is joined to the building? - -The glass is a non-conductor, and prevents the lightning from leaving -the rod. - - -Why is it not safe to stand near a fire during a thunder-storm? - -Because the fire is a good conductor, and sometimes draws the -electricity into the room. - - -What part of the room is the safest place? - -The middle of a room is thought to be the safest place. - - -Is a feather-bed a safe place? - -Yes; because the feathers are a non-conductor, and will not attract the -lightning. - - -Is dry air a non-conductor of electricity? - -Yes; dry air is a non-conductor of electricity. - - -When are thunder-storms most frequent? - -Thunder-storms are more frequent in summer than in winter, and more so -in the afternoon than in the morning. - - -By what are thunder-storms usually attended? - -They are usually attended by some change in the direction of the wind. - - -Where do thunder-storms generally prevail? - -They generally prevail in the lower regions of the air, near the earth. - - -What is the rapidity of lightning? - -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. - - -Of what does magnetism treat? - -_Magnetism_ treats of the property of magnets. - - -What is a magnet? - -A _magnet_ is a piece of iron ore having the power of attracting iron -and steel. - - -How many kinds of magnets are there? - -There are two kinds of magnets: natural magnets, and artificial magnets. - - -What other name is given to a natural magnet? - -It is called a loadstone. - - -Where are natural magnets or loadstones found? - -They are dug out of the earth, and are found most abundantly in Norway -and Sweden. - - -What is an artificial magnet? - -An artificial magnet is one made by magnetizing a bar of steel. - - -How may a bar of steel be magnetized? - -A bar of steel, when rubbed on a natural magnet, becomes magnetized. - - -When the bar of steel is bent in the shape of a horse-shoe, what is it -called? - -It is then called a horse-shoe magnet, from its shape. - - -Where does a magnetic bar always point, when it is made to move easily -on a pivot? - -One end of it always points to the north, and the other end to the -south. - - -What are the ends of the magnet called? - -The north end is called the positive, and the south end is called the -negative. - - -What is there curious about the magnet? - -The positive end of one magnet always attracts the negative end of every -other magnet; but if the two positive or the two negative ends are -brought together, they will repel each other. - - -Why are artificial magnets made of steel? - -Because steel will remain magnetized, but iron soon loses the magnetic -properties. - - -What is a compass? - -A _compass_ is an instrument containing a magnetic needle, so balanced -as to move easily on a pivot. - - -In what direction does this needle always point? - -This needle always points toward the north. - - -By whom is the compass used? - -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. - - -Are magnetism and electricity much alike? - -Yes; they are so nearly alike that many persons think they are -identical. - - -What is an electro-magnet? - -An _electro-magnet_ is a magnet obtained by means of electricity. - - -Of what is the magnet made? - -The magnet is made of soft iron, around which are wound a great many -coils of wire, and these are covered with silk. - - -How is the soft iron made into a magnet? - -The iron is magnetized by the passage of electricity through the wire. - - -Of what use is this magnet? - -It is used in the electro-magnetic telegraph. - - -When and where was the first line of telegraph built? - -The first line of telegraph was built in 1844, from Washington to -Baltimore, by Professor Morse. - - -What is the principle of the telegraph? - -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. - - -Does electricity move rapidly? - -Yes; its velocity is such as to move around the earth in a quarter of a -second. - - -What is the electrical battery called? - -The electrical battery is called a _key_. - - -What is the electro-magnet called? - -The electro-magnet is called a _receiver_. - - -For what are the key and receiver used? - -The key is used for sending messages, and the receiver for receiving -them. - - -What must each station have? - -Each station must have both a key and a receiver, so as to send as well -as receive messages. - - -What is the Atlantic cable? - -The Atlantic cable is a telegraphic wire reaching across the Atlantic -Ocean, from America to Europe. - - -When was the first Atlantic cable laid? - -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. - - -Of what use is the Atlantic cable? - -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. - - [Illustration: Fruit bowl.] - - - - - THE - _Elements of Natural Philosophy_ - - By E. J. HOUSTON, A.M., -_Professor of Physics and Physical Geography in the Central High School, - Philadelphia, Pa._ - -This book is intended for Graded Schools, Academies, Seminaries, Normal -Schools, etc. It gives the elements of the science in a concise form and -in logical sequence, so that the book forms a _system_ of Natural -Philosophy, and not a mere collection of disconnected facts. 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