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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..3559c74 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #53778 (https://www.gutenberg.org/ebooks/53778) 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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margin-left:0; text-indent:0; } -p.pq { font-size:85%; margin-left:0; text-indent:1em; margin-bottom:0; } -p.pa { margin-left:0; text-indent:1em; margin-top:0; } -</style> -</head> -<body> - - -<pre> - -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 - - - - - - -</pre> - -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="Martindale’s First Lessons in Natural Philosophy" width="495" height="790" /> -</div> -<div class="box"> -<h1><span class="smaller">FIRST LESSONS</span> -<br /><span class="smallest">IN</span> -<br /><span class="sc">Natural Philosophy</span> -<br /><span class="smaller">FOR BEGINNERS.</span></h1> -<p class="tbcenter"><span class="smaller">BY</span> -<br />JOSEPH C. MARTINDALE, M.D.,</p> -<p class="center"><span class="smaller">LATE PRINCIPAL OF MADISON GRAMMAR SCHOOL, PHILADELPHIA; AUTHOR OF A HISTORY OF THE UNITED STATES FOR SCHOOLS, ETC.</span></p> -<div class="img"> -<img src="images/img001.jpg" alt="Paddlewheel steamboat." width="500" height="223" /> -</div> -<p class="tbcenter"><span class="small">PHILADELPHIA:</span> -<br />ELDREDGE & BROTHER, -<br /><span class="small">No. 17 North Seventh Street.</span> -<br />1881.</p> -</div> -<p class="center small">Entered, according to Act of Congress, in the year 1872, by -<br />ELDREDGE & BROTHER, -<br />in the Office of the Librarian of Congress at Washington.</p> -<p class="center small">J. FAGAN A SON, -<br />ELECTROTYPERS, PHILAD’A.</p> -<div class="img"> -<img src="images/img002.jpg" alt="" width="354" height="282" /> -<p class="pcap">KEYSTONE PRESS OF W<sup>M.</sup> J. DORNAN</p> -</div> -<div class="pb" id="Page_v">v</div> -<h2><span class="sc">Preface.</span></h2> -<div class="img"> -<img src="images/img003.jpg" alt="Decoration" width="150" height="16" /> -</div> -<p>In preparing this little work, our desire has been to afford -both pleasure and profit to those who may read or study its -pages.</p> -<p>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.</p> -<p>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.</p> -<p>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 -<span class="pb" id="Page_vi">vi</span> -boys and girls for answers to the many perplexing questions -which are so often presented to their inquiring minds.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p><span class="lr">J. C. M.</span></p> -<div class="img"> -<img src="images/img004.jpg" alt="Decoration" width="200" height="62" /> -</div> -<div class="pb" id="Page_vii">vii</div> -<div class="img"> -<img src="images/img005.jpg" alt="" width="200" height="81" /> -<p class="pcap">CONTENTS</p> -</div> -<dl class="toc"> -<dt class="small">PAGE</dt> -<dt><a href="#c1">CHAPTER I.—Astronomy.</a> 9</dt> -<dd><a href="#c2"><span class="sc">The Earth and other Planets</span></a> 9</dd> -<dd><a href="#c3"><span class="sc">The Moon</span></a> 14</dd> -<dd><a href="#c4"><span class="sc">Eclipses</span></a> 18</dd> -<dt><a href="#c5">CHAPTER II.—Light.</a> 22</dt> -<dd><a href="#c6"><span class="sc">Color</span></a> 22</dd> -<dd><a href="#c7"><span class="sc">Refraction of Light</span></a> 32</dd> -<dd><a href="#c8"><span class="sc">Reflection of Light</span></a> 34</dd> -<dd><a href="#c9"><span class="sc">The Eye</span></a> 38</dd> -<dt><a href="#c10">CHAPTER III.—Heat.</a> 42</dt> -<dd><a href="#c11"><span class="sc">Sources of Heat</span></a> 42</dd> -<dd><a href="#c12"><span class="sc">Expansion by Heat</span></a> 46</dd> -<dd><a href="#c13"><span class="sc">Conduction of Heat</span></a> 54</dd> -<dd><a href="#c14"><span class="sc">Convection of Heat</span></a> 59</dd> -<dd><a href="#c15"><span class="sc">Liquefaction</span></a> 61</dd> -<dd><a href="#c16"><span class="sc">Latent Heat</span></a> 63</dd> -<dd><a href="#c17"><span class="sc">Ebullition</span></a> 65</dd> -<dd><a href="#c18"><span class="sc">Vaporization</span></a> 66</dd> -<dd><a href="#c19"><span class="sc">Evaporation</span></a> 69</dd> -<dd><a href="#c20"><span class="sc">Radiation</span></a> 71</dd> -<dd><a href="#c21"><span class="sc">Reflection</span></a> 73</dd> -<dd><a href="#c22"><span class="sc">Absorption</span></a> 74</dd> -<dt><a href="#c23">CHAPTER IV.—The Air.</a> 77</dt> -<dd><a href="#c24"><span class="sc">Winds</span></a> 96</dd> -<dt><a href="#c25">CHAPTER V.—Water.</a> 99</dt> -<dd><a href="#c26"><span class="sc">Springs and Streams</span></a> 104</dd> -<dd><a href="#c27"><span class="sc">Fogs and Clouds</span></a> 106</dd> -<dd><a href="#c28"><span class="sc">Dew</span></a> 109</dd> -<dd><a href="#c29"><span class="sc">Frost</span></a> 112</dd> -<dd><a href="#c30"><span class="sc">Rain, Snow, and Hail</span></a> 113</dd> -<dd><a href="#c31"><span class="sc">Ice</span></a> 116</dd> -<dt><a href="#c32">CHAPTER VI.—Sound.</a> 118</dt> -<dd><a href="#c33"><span class="sc">Conduction of Sound</span></a> 122</dd> -<dd><a href="#c34"><span class="sc">Trumpets and Speaking-tubes</span></a> 125</dd> -<dd><a href="#c35"><span class="sc">Velocity of Sound</span></a> 127</dd> -<dd><a href="#c36"><span class="sc">Reflection of Sound</span></a> 128</dd> -<dt><a href="#c37">CHAPTER VII.—Matter.</a> 131</dt> -<dt><a href="#c38">CHAPTER VIII.—Attraction.</a> 137</dt> -<dd><a href="#c39"><span class="sc">Cohesion and Adhesion</span></a> 137</dd> -<dd><a href="#c40"><span class="sc">Capillary Attraction</span></a> 140</dd> -<dd><a href="#c41"><span class="sc">Gravitation</span></a> 141</dd> -<dd><a href="#c42"><span class="sc">Centre of Gravity</span></a> 145</dd> -<dd><a href="#c43"><span class="sc">Specific Gravity</span></a> 150</dd> -<dt><a href="#c44">CHAPTER IX.—Motion.</a> 154</dt> -<dt><a href="#c45">CHAPTER X.—Mechanical Powers.</a> 164</dt> -<dd><a href="#c46"><span class="sc">Lever</span></a> 164</dd> -<dd><a href="#c47"><span class="sc">Wheel and Axle</span></a> 174</dd> -<dd><a href="#c48"><span class="sc">Pulley</span></a> 176</dd> -<dd><a href="#c49"><span class="sc">Inclined Plane</span></a> 177</dd> -<dd><a href="#c50"><span class="sc">Wedge</span></a> 180</dd> -<dd><a href="#c51"><span class="sc">Screw</span></a> 181</dd> -<dt><a href="#c52">CHAPTER XI.—Electricity.</a> 183</dt> -<dd><a href="#c53"><span class="sc">Magnetism</span></a> 187</dd> -<dd><a href="#c54"><span class="sc">Electro-Magnetism</span></a> 189</dd> -</dl> -<div class="pb" id="Page_9">9</div> -<h1 title="">FIRST LESSONS -<br /><span class="smaller">IN</span> -<br /><span class="large"><span class="sc">Natural Philosophy.</span></span></h1> -<h2 id="c1">CHAPTER I. -<br /><span class="sc">Astronomy.</span></h2> -<h3 id="c2"><span class="sc">The Earth and Other Planets.</span></h3> -<div class="img" id="fig1"> -<img src="images/img006.jpg" alt="" width="400" height="242" /> -<p class="pcap">Fig. 1.</p> -</div> -<div class="img"> -<img src="images/img007.jpg" alt="Illustrated Capital W" width="167" height="200" /> -</div> -<p class="pq">What is the shape of the Earth on which we -live?</p> -<p class="pa">It is round like a ball.</p> -<p class="pq">What other names are given to the Earth?</p> -<p class="pa">It is called the World or Globe.</p> -<p class="pq">What else is it sometimes called?</p> -<p class="pa">It is called a Planet.</p> -<div class="pb" id="Page_10">10</div> -<p class="pq">Are there any other planets?</p> -<p class="pa">Yes; some of the stars are called planets.</p> -<p class="pq">Is our earth or world a star?</p> -<p class="pa">It is a star.</p> -<p class="pq">Why do the other stars seem to us different from the earth?</p> -<p class="pa">Because they are so far away from us.</p> -<p class="pq">What gives light to the planets?</p> -<p class="pa">The Sun gives light to the planets.</p> -<p class="pq">Of what is the sun the centre?</p> -<p class="pa">The sun is the centre of the solar system.</p> -<p class="pq">Do the planets remain in one place?</p> -<p class="pa">The planets do not remain in one place, but are -always moving.</p> -<p class="pq">How do the planets move?</p> -<p class="pa">They move around the sun.</p> -<p class="pq">Are some planets nearer the sun than others?</p> -<p class="pa">Yes; some planets are much nearer the sun than -others.</p> -<p class="pq">Which planet is nearest the sun?</p> -<p class="pa">The planet called Mercury.</p> -<p class="pq">Which is next to Mercury?</p> -<p class="pa">The planet called Venus.</p> -<p class="pq">And which is third in distance from the sun?</p> -<p class="pa">Our own planet, the Earth.</p> -<p class="pq">Are there any planets still farther from the sun?</p> -<p class="pa">Yes; Mars, Jupiter, Saturn, Uranus, and Neptune, -and many smaller planets.</p> -<div class="pb" id="Page_11">11</div> -<p class="pq">How does each planet move?</p> -<p class="pa">It moves in its own track or orbit around the sun, -as is represented in <a href="#fig2">Fig. 2</a>.</p> -<div class="img" id="fig2"> -<img src="images/img008.jpg" alt="" width="350" height="333" /> -<p class="pcap">Fig. 2.</p> -</div> -<p class="pq">Does the earth always move in the same orbit around the -sun?</p> -<p class="pa">Yes; the earth moves in the same orbit around the -sun, year after year.</p> -<p class="pq">How long does it take the earth to move around the sun?</p> -<p class="pa">365 days, 5 hours and 49 minutes, or one year.</p> -<p class="pq">What makes a year to us?</p> -<p class="pa">A year to us is the time required by the earth to -move around the sun.</p> -<p class="pq">If the earth is in a certain place at noon, on New Year’s day, -when will it be in that place again?</p> -<p class="pa">5 hours and 49 minutes after noon on New Year’s -day of the next year.</p> -<p class="pq">What, then, does the earth do every year?</p> -<p class="pa">The earth moves around the sun and comes back -to the place whence it started, once every year.</p> -<div class="pb" id="Page_12">12</div> -<p class="pq">Has the earth any other motion than the one around the sun?</p> -<p class="pa">It has; it turns over and over.</p> -<p class="pq">Does the earth stay in one place as it turns over and over?</p> -<p class="pa">It does not; it moves on in its track around the sun.</p> -<p class="pq">How many times does it turn over while going around the sun?</p> -<p class="pa">Three hundred and sixty-five times.</p> -<p class="pq">How many times, then, does it turn over in one year?</p> -<p class="pa">The earth turns over three hundred and sixty-five -times in a year.</p> -<p class="pq">What is the time which it takes to turn over once called?</p> -<p class="pa">It is called a day.</p> -<p class="pq">How many days in a year?</p> -<p class="pa">There are three hundred and sixty-five days in a -year.</p> -<p class="pq">What two motions, then, has the earth?</p> -<p class="pa">It turns over and over, and at the same time it -moves around the sun.</p> -<p class="pq">What change is produced on the earth by its moving around -the sun?</p> -<p class="pa">The change of Seasons.</p> -<p class="pq">Will you name the seasons?</p> -<p class="pa">Winter, Spring, Summer, and Autumn.</p> -<p class="pq">Do they follow each other in regular order?</p> -<p class="pa">They do; Spring follows Winter, Summer follows -Spring, Autumn follows Summer, Winter follows -Autumn, and Spring follows Winter again.</p> -<p class="pq">How often have we these seasons?</p> -<p class="pa">As often as the earth moves around the sun, which -is once a year.</p> -<div class="pb" id="Page_13">13</div> -<p class="pq">What change is produced on the earth by its turning over?</p> -<p class="pa">The change of day and night.</p> -<p class="pq">Where is it day?</p> -<p class="pa">It is day on that part of the earth which is turned -towards the sun.</p> -<p class="pq">Where is it night?</p> -<p class="pa">It is night on that part of the earth which is turned -away from the sun.</p> -<p class="pq">How often do we have the change from day to night?</p> -<p class="pa">As often as the earth turns over, which is once in -twenty-four hours.</p> -<p class="pq">What is a day?</p> -<p class="pa">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.</p> -<p class="pq">What does a day include?</p> -<p class="pa">It includes the night-time as well as the day-time, -of the twenty-four hours.</p> -<p class="pq">When it is day-time on one part of the earth, what is it on the -opposite part?</p> -<p class="pa">It is night-time on the opposite part.</p> -<p class="pq">When it is morning to us, what is it to people living on the -opposite side of the earth?</p> -<p class="pa">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.</p> -<p class="pq">On how much of the earth is it always day?</p> -<p class="pa">It is day on one-half of the earth, while on the -other half it is night.</p> -<div class="pb" id="Page_14">14</div> -<p class="pq">The sun seems to move around the earth; does it really do so?</p> -<p class="pa">No; it does not move around the earth.</p> -<p class="pq">Does the <i>earth</i> move?</p> -<p class="pa">Yes; it turns over once every day.</p> -<p class="pq">Why does it not seem to us to be moving?</p> -<p class="pa">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.</p> -<p class="pq">In what direction does the earth turn on its axis?</p> -<p class="pa">The earth turns from west to east.</p> -<p class="pq">Where do the sun, moon, and stars rise?</p> -<p class="pa">The sun, moon, and stars rise in the east.</p> -<p class="pq">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.</p> -<p class="pq">When is it sunrise to us?</p> -<p class="pa">When the part of the earth on which we are, first -comes into the sun’s rays.</p> -<p class="pq">When is it sunset to us?</p> -<p class="pa">When the part of the earth on which we are, moves -out of the sun’s rays.</p> -<h3 id="c3">The Moon.</h3> -<p class="pq">We have been told that the earth moves around the sun; does -any planet move around the earth?</p> -<p class="pa">Yes; one planet moves around the earth.</p> -<div class="pb" id="Page_15">15</div> -<p class="pq">What is that planet called?</p> -<p class="pa">It is called the Moon.</p> -<div class="img" id="fig3"> -<img src="images/img009.jpg" alt="" width="438" height="350" /> -<p class="pcap">Fig. 3.</p> -</div> -<p class="pq">How often does the moon move around the earth?</p> -<p class="pa">Once in 27 d. 7 h. 43 min.</p> -<p class="pq">How long is the time from new moon to new moon again?</p> -<p class="pa">It is 29 d. 12 h. 44 min.</p> -<p class="pq">How often do we have new moon?</p> -<p class="pa">Once every 29 d. 12 h. 44 min.</p> -<p class="pq">What is the period from one new moon to another called?</p> -<p class="pa">It is called a Lunar month.</p> -<p class="pq">Are the lunar months all of the same length?</p> -<p class="pa">Yes; they are all of the same length.</p> -<p class="pq">Is there any other kind of month?</p> -<p class="pa">Yes; the Calendar month, as it stands in almanacs.</p> -<p class="pq">How many calendar months in a year?</p> -<p class="pa">There are twelve calendar months in a year.</p> -<div class="pb" id="Page_16">16</div> -<p class="pq">Are the calendar months all of the same length?</p> -<p class="pa">No; some are thirty-one days long, some thirty days -long, and one only twenty-eight days long.</p> -<p class="pq">When can we see the moon and stars best?</p> -<p class="pa">We can see them best when it is night.</p> -<p class="pq">From what do the moon and the other planets get their light?</p> -<p class="pa">They get their light from the sun.</p> -<p class="pq">What do they do with this light?</p> -<p class="pa">They reflect it.</p> -<p class="pq">How do we see them?</p> -<p class="pa">We see them by the light which they reflect.</p> -<p class="pq">Why cannot we see the moon and stars in the day-time?</p> -<p class="pa">Because the bright light of the sun hides their -light.</p> -<p class="pq">Are there stars in every direction around the earth?</p> -<p class="pa">There are stars in every direction around the earth.</p> -<p class="pq">How do we know this?</p> -<p class="pa">Because stars may be seen from every part of the -earth, when it is night.</p> -<p class="pq">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.</p> -<p class="pq">What planet moves around the earth?</p> -<p class="pa">The moon moves around the earth.</p> -<p class="pq">Upon what part of the moon does the sun shine?</p> -<p class="pa">It shines upon the part towards the sun.</p> -<p class="pq">What does the moon do with the light which it receives from -the sun?</p> -<p class="pa">The moon reflects the light received from the sun.</p> -<div class="pb" id="Page_17">17</div> -<p class="pq">What is this reflected light called?</p> -<p class="pa">It is called moonlight when reflected from the -moon.</p> -<p class="pq">Is that part of the moon on which the sun shines always -towards the earth?</p> -<p class="pa">It is not always towards the earth.</p> -<p class="pq">When all of the moon on which the sun shines is towards the -earth, how does the moon appear?</p> -<p class="pa">The moon appears to be round, and the whole of it -seems to shine.</p> -<p class="pq">What is the moon then called?</p> -<p class="pa">It is called the Full Moon.</p> -<p class="pq">When no part of the moon on which the sun shines is towards -the earth, how does the moon appear?</p> -<p class="pa">We do not see the moon at all, and we say there is -no moon.</p> -<p class="pq">When a small part of the moon on which the sun shines is -first turned towards the earth, what is it called?</p> -<p class="pa">It is called the New Moon.</p> -<p class="pq">Does the lighted part of the moon grow?</p> -<p class="pa">It seems to grow larger and larger, until the whole -of the moon is lighted.</p> -<p class="pq">What is it then called?</p> -<p class="pa">It is then called the Full Moon.</p> -<p class="pq">What then takes place?</p> -<p class="pa">It seems to grow smaller and smaller, until the -whole of the lighted part is gone.</p> -<p class="pq">Just before the last part is gone, what is it called?</p> -<p class="pa">It is called the Old Moon.</p> -<div class="pb" id="Page_18">18</div> -<p class="pq">How long is it from one full moon to another full moon?</p> -<p class="pa">It is a lunar month.</p> -<p class="pq">How many full moons are there in a year?</p> -<p class="pa">There are thirteen full moons in a year.</p> -<p class="pq">Is the sun much larger than the moon?</p> -<p class="pa">The sun is many thousand times larger than the moon.</p> -<p class="pq">Why does the moon appear nearly as large as the sun?</p> -<p class="pa">Because it is so much nearer to us.</p> -<p class="pq">How far is the moon from the earth?</p> -<p class="pa">The moon is nearly 240,000 miles from the earth.</p> -<p class="pq">How far is the sun from the earth?</p> -<p class="pa">The sun is nearly 92,000,000 miles from the earth.</p> -<p class="pq">How long does it take the light of the sun to reach the earth?</p> -<p class="pa">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.</p> -<h3 id="c4">Eclipses.</h3> -<p class="pq">How do we know that the earth is round, like a ball?</p> -<p class="pa">We know that the earth is round like a ball, because -its shadow is similar to the shadow of a ball.</p> -<div class="pb" id="Page_19">19</div> -<p class="pq">How else do we know that the earth is round?</p> -<p class="pa">We also know that the earth is round, because we -can circumnavigate or sail around it.</p> -<p class="pq">Where can we see the shadow of the earth?</p> -<p class="pa">We can see the shadow on the moon, when the moon -is eclipsed.</p> -<p class="pq">What is the shadow on the moon like?</p> -<p class="pa">It is round, like the shadow of a ball.</p> -<p class="pq">When the Earth comes between the sun and the moon, where -does the shadow of the earth fall?</p> -<p class="pa">The shadow of the earth falls on the moon, as is -represented in <a href="#fig4">Fig. 4</a>.</p> -<div class="img" id="fig4"> -<img src="images/img010.jpg" alt="" width="500" height="115" /> -<p class="pcap">Fig. 4.</p> -</div> -<p class="pq">What is this shadow on the moon called?</p> -<p class="pa">It is called an Eclipse of the moon.</p> -<p class="pq">How do we know that the moon is round?</p> -<p class="pa">We know that the moon is round, because its shadow -is similar to the shadow of the earth.</p> -<p class="pq">When is the moon eclipsed?</p> -<p class="pa">When it comes into the earth’s shadow.</p> -<p class="pq">When the moon comes fully into the earth’s shadow, what is -it called?</p> -<p class="pa">It is called a total eclipse of the moon.</p> -<div class="pb" id="Page_20">20</div> -<p class="pq">When it comes partly into the earths shadow, what is it -called?</p> -<p class="pa">It is called a partial eclipse of the moon.</p> -<p class="pq">When the moon comes between the sun and the earth, where -does the shadow of the moon fall?</p> -<p class="pa">The shadow of the moon falls on the earth, as is -represented in <a href="#fig5">Fig. 5</a>.</p> -<div class="img" id="fig5"> -<img src="images/img011.jpg" alt="" width="400" height="121" /> -<p class="pcap">Fig. 5.</p> -</div> -<p class="pq">What does this produce?</p> -<p class="pa">It produces an eclipse of the sun.</p> -<p class="pq">When the moon is between the sun and the earth, can we see -the sun?</p> -<p class="pa">No; we cannot see the sun, because the moon prevents -the rays of the sun from coming to us.</p> -<p class="pq">When the moon prevents the sun’s rays from falling on us, -what is it called?</p> -<p class="pa">It is called an eclipse of the sun.</p> -<p class="pq">When the moon prevents <i>all</i> the sun’s rays from falling on us, -what is it called?</p> -<p class="pa">It is called a total eclipse of the sun.</p> -<p class="pq">When the moon prevents only a part of the sun’s rays from -falling on us, what is it called?</p> -<p class="pa">It is called a partial eclipse of the sun.</p> -<p class="pq">When will the <i>sun</i> be eclipsed?</p> -<p class="pa">Whenever the moon comes between the sun and the -earth.</p> -<div class="pb" id="Page_21">21</div> -<p class="pq">When will the <i>moon</i> be eclipsed?</p> -<p class="pa">Whenever the earth comes between the sun and the -moon.</p> -<p class="pq">Who made the sun, the moon, and the stars?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img012.jpg" alt="Paddlewheel steamboat." width="500" height="213" /> -</div> -<div class="pb" id="Page_22">22</div> -<div class="img"> -<img src="images/img013.jpg" alt="Decoration." width="500" height="179" /> -</div> -<h2 id="c5">CHAPTER II. -<br /><span class="sc">Light.</span></h2> -<h3 id="c6"><span class="sc">Color.</span></h3> -<div class="img"> -<img src="images/img014.jpg" alt="Illustrated Capital I" width="150" height="200" /> -</div> -<p class="pq">In what way is the sun useful to us?</p> -<p class="pa">It gives heat, which makes us warm, -and it gives light, so that we can see.</p> -<p class="pq">Is it useful in any other way?</p> -<p class="pa">It gives light and warmth to all animals as -well as to all plants.</p> -<p class="pq">What make the flowers and leaves so beautiful?</p> -<p class="pa">The light and heat from the sun make them beautiful.</p> -<p class="pq">What do the light and heat from the sun ripen?</p> -<p class="pa">They ripen the apples, cherries, and other fruits; -also, the wheat, corn, and other grains, and make -them fit for food.</p> -<p class="pq">When it is night-time or dark, can we distinguish objects?</p> -<p class="pa">No; because all things are then without color, and -they cannot be seen.</p> -<p class="pq">As it grows light in the morning, what takes place?</p> -<p class="pa">At first, we see the objects around us faintly; but -as it grows lighter, we see them more distinctly.</p> -<div class="pb" id="Page_23">23</div> -<p class="pq">Do they all show the same color in the light?</p> -<p class="pa">They do not; they show different colors.</p> -<p class="pq">What enables us to see the color of different things?</p> -<p class="pa">Light enables us to see the color of everything.</p> -<p class="pq">What kinds of light have we?</p> -<p class="pa">We have the light of the sun, moon, and stars; also, -the light from the burning of any substance.</p> -<p class="pq">Does the light show many colors?</p> -<p class="pa">It does show many colors or shades of color.</p> -<p class="pq">How may some colors be made?</p> -<p class="pa">They may be made by mixing other colors together.</p> -<p class="pq">How can we make a purple color?</p> -<p class="pa">By mixing a red and a blue color together.</p> -<p class="pq">How can a green color be made?</p> -<p class="pa">By mixing a blue and a yellow color together.</p> -<p class="pq">How can an orange color be made?</p> -<p class="pa">By mixing a red and a yellow color together.</p> -<p class="pq">How does the light come from the sun?</p> -<p class="pa">It comes in rays or straight lines.</p> -<p class="pq">What may be done with a ray of light?</p> -<p class="pa">It may be separated into different colors.</p> -<p class="pq">Into how many colors may a ray of light be separated?</p> -<p class="pa">Into <i>seven</i> colors, like the colors of the rainbow.</p> -<p class="pq">What names are given to these colors?</p> -<p class="pa">Red, orange, yellow, green, blue, indigo, violet.</p> -<p class="pq">How may these colors be separated?</p> -<p class="pa">By a prism, or three-sided piece of glass.</p> -<p class="pq">How may this be done?</p> -<p class="pa">If a sunbeam, shining through a hole in the window-shutter -<span class="pb" id="Page_24">24</span> -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.</p> -<div class="img" id="fig6"> -<img src="images/img015.jpg" alt="" width="500" height="163" /> -<p class="pcap">Fig. 6.</p> -</div> -<p class="pq">In <a href="#fig6">Fig. 6</a>, 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.</p> -<p class="pq">In what order will these colors appear?</p> -<p class="pa">They will always appear one above the other, in the -order named.</p> -<p class="pq">How may this order be remembered?</p> -<p class="pa">By the word Roy-g-biv, made from their initial -letters, beginning at the bottom.</p> -<p class="pq">From what are the colors of the rainbow made?</p> -<p class="pa">From the rays of light coming from the sun.</p> -<p class="pq">When is a rainbow seen?</p> -<p class="pa">A rainbow is seen when the sun shines through the -drops of water as they fall through the air.</p> -<p class="pq">How do the drops of water help to make a rainbow?</p> -<p class="pa">The drops of water act like little prisms, and separate -the rays of light passing through them into the -colors of the rainbow.</p> -<div class="pb" id="Page_25">25</div> -<p class="pq">What two things, then, are needed to make a rainbow?</p> -<p class="pa">A shower of water, and a bright sunshine.</p> -<p class="pq">In what part of the sky does the rainbow appear?</p> -<p class="pa">In the part opposite to that in which the sun is, at -the time of the shower.</p> -<p class="pq">How does the rainbow appear?</p> -<p class="pa">It appears like a great arch, spanning the sky, and -its splendid bright colors are very beautiful.</p> -<p class="pq">What may we remember in reference to the rainbow?</p> -<p class="pa">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.</p> -<p class="pq">Where else may we see a variety of colors?</p> -<p class="pa">In the soap-bubble.</p> -<p class="pq">Why are these colors constantly changing?</p> -<p class="pa">Because the walls of the bubble are constantly -growing thinner, and different colors are, therefore, -reflected from them.</p> -<p class="pq">Why do the walls of the soap-bubble grow thinner?</p> -<p class="pa">Because the water runs to the bottom of the bubble -until its top becomes so thin as to burst.</p> -<p class="pq">Can we tell the color of any object by touching it?</p> -<p class="pa">No; we must see it in order to tell its color.</p> -<p class="pq">How do we know this?</p> -<p class="pa">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.</p> -<p class="pq">What is necessary in order to see the colors?</p> -<p class="pa">Light is necessary; because all colors come from -the rays of light.</p> -<div class="pb" id="Page_26">26</div> -<p class="pq">Can we see the different colors in the ray itself?</p> -<p class="pa">We cannot; because God has so blended them together -that they cannot be seen by us.</p> -<p class="pq">When most of the rays of light pass through a substance, what -is said of it?</p> -<p class="pa">The substance is said to be <i>transparent</i>.</p> -<p class="pq">Name some transparent substances.</p> -<p class="pa">Glass, ice, diamonds, air, and clear water.</p> -<p class="pq">When only a few rays of light pass through a substance, what -is said of it?</p> -<p class="pa">The substance is said to be <i>translucent</i>.</p> -<p class="pq">Name some translucent substances.</p> -<p class="pa">Flint, isinglass, scraped-horn, and china-ware.</p> -<p class="pq">When no rays of light pass through a substance, what is said -of it?</p> -<p class="pa">The substance is said to be <i>opaque</i>.</p> -<p class="pq">Name some opaque substances.</p> -<p class="pa">Wood, iron, coal, and granite.</p> -<p class="pq">Can we see through a glass window?</p> -<p class="pa">Yes; because the glass is transparent, and the light -passes through it.</p> -<p class="pq">Can we see through a looking-glass?</p> -<p class="pa">No; because the back of the looking-glass is covered -with quicksilver, which prevents the rays of light -from passing through it.</p> -<p class="pq">What becomes of the light falling on the looking-glass?</p> -<p class="pa">It is thrown back from the glass.</p> -<p class="pq">What is this called?</p> -<p class="pa">It is called <i>reflection</i>.</p> -<div class="pb" id="Page_27">27</div> -<p class="pq">What objects are good reflectors of light?</p> -<p class="pa">Those having smooth and polished surfaces; such as -tin, silver, gold, and quicksilver.</p> -<p class="pq">What objects are poor reflectors of light?</p> -<p class="pa">Those having dull, uneven surfaces; such as iron, -wood, cloth, leather, and calico.</p> -<p class="pq">Of what is every ray of sunlight composed?</p> -<p class="pa">It is composed of the seven colors of the rainbow.</p> -<p class="pq">What does a looking-glass do with these colors?</p> -<p class="pa">A looking-glass throws them all back from its surface.</p> -<p class="pq">What does a glass window do with these colors?</p> -<p class="pa">A glass window permits all these colors to pass -through it.</p> -<p class="pq">What do opaque substances do with these colors?</p> -<p class="pa">They throw back or reflect some of them.</p> -<p class="pq">Do they all reflect the same color?</p> -<p class="pa">No; some reflect one color, and some another color.</p> -<p class="pq">What color does the grass reflect?</p> -<p class="pa">It reflects the green color; therefore, the grass is -green.</p> -<p class="pq">What does the grass do with the other colors?</p> -<p class="pa">It absorbs or hides them in itself.</p> -<p class="pq">How do we know the color of anything?</p> -<p class="pa">We know it by the color which it reflects.</p> -<p class="pq">Why is the rose red?</p> -<p class="pa">Because it reflects the red color, and absorbs or -hides the other colors in itself.</p> -<p class="pq">When anything <i>absorbs</i> all the colors of a ray of light, what -color is it?</p> -<p class="pa">It is black.</p> -<div class="pb" id="Page_28">28</div> -<p class="pq">When it <i>reflects</i> all the colors of light, what color is it?</p> -<p class="pa">It is white.</p> -<p class="pq">Why are the letters on this page black?</p> -<p class="pa">Because they absorb all the colors of light, and reflect -none.</p> -<p class="pq">Why is the paper white?</p> -<p class="pa">Because it reflects all the colors of light, and absorbs -none.</p> -<p class="pq">Why are all things black in the dark?</p> -<p class="pa">Because there are no rays of light falling on them; -hence, there is no color for them to reflect.</p> -<p class="pq">Does an object reflect its color in every direction?</p> -<p class="pa">It does reflect its color in every direction.</p> -<p class="pq">How do we know this?</p> -<p class="pa">Because thousands of persons, in different directions -from the same object, can see it at the same time.</p> -<p class="pq">How are the rays of color reflected from an object?</p> -<p class="pa">They are reflected in straight lines.</p> -<p class="pq">Where must a person be so as to see an object?</p> -<p class="pa">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.</p> -<p class="pq">What color does a red ribbon reflect?</p> -<p class="pa">It reflects a red color.</p> -<p class="pq">Why are some red ribbons of a brighter red than others?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_29">29</div> -<p class="pq">Upon what does the brightness or dullness of a color depend?</p> -<p class="pa">It depends upon the number of rays of that color -reflected.</p> -<p class="pq">What variety does this produce?</p> -<p class="pa">It produces a great variety in the shades and tints -of color.</p> -<p class="pq">Where may many of these shades and tints be seen?</p> -<p class="pa">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.</p> -<p class="pq">What may these things teach us?</p> -<p class="pa">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.</p> -<p class="pq">Why cannot we see the light of a lamp or candle distinctly, -in the day-time?</p> -<p class="pa">Because the strong rays from the sun hide the more -feeble rays from the lamp.</p> -<p class="pq">Why cannot we see the stars in the day-time?</p> -<p class="pa">Because the bright light from the sun hides their -more feeble light.</p> -<p class="pq">When can we see the glow-worms or fire-flies best?</p> -<p class="pa">At night, when it is dark; because, at other times, -the light of the sun hides their feeble light.</p> -<p class="pq">Where may we see them at night?</p> -<p class="pa">Flying about in all directions, looking like bits of -fire moving in the air.</p> -<div class="pb" id="Page_30">30</div> -<p class="pq">Does a looking-glass reflect all the colors of light falling -upon it?</p> -<p class="pa">It does; hence, the ray of light is not changed.</p> -<p class="pq">When the object is of a blue color, why is blue only reflected?</p> -<p class="pa">Because it is the only color falling upon the glass, -and there is no other color to reflect.</p> -<p class="pq">Is the color of any object changed by falling upon a looking-glass?</p> -<p class="pa">No; the color of every object is reflected from a -looking-glass without change.</p> -<p class="pq">Do other objects generally reflect the colors falling upon them?</p> -<p class="pa">They do not; they absorb or hide the color of all -other objects, and reflect only their own color.</p> -<p class="pq">If every object was a good reflector of color, like a looking-glass, -what would result?</p> -<p class="pa">Then every object would reflect the color of every -other object around it, and there would be an endless -confusion of colors.</p> -<p class="pq">What objects reflect the most light?</p> -<p class="pa">Those of a white or light color.</p> -<p class="pq">What objects reflect the least light?</p> -<p class="pa">Those of a darker color; and the nearer the color -approaches to black, the less light it reflects.</p> -<p class="pq">Why is it darker when the sky is covered by dark-colored, -than by light-colored, clouds?</p> -<p class="pa">Because the dark-colored clouds reflect less light, -than the light-colored clouds do.</p> -<p class="pq">What advantage do we derive from the green color of the -grass and the leaves?</p> -<p class="pa">They reflect a soft, pleasant light, which does not -dazzle or hurt our eyes.</p> -<div class="pb" id="Page_31">31</div> -<p class="pq">How would it be if those objects were white?</p> -<p class="pa">They would dazzle us by the amount of light reflected.</p> -<p class="pq">If we wish to make a room light, of what color should the -walls be?</p> -<p class="pa">They should be white; and when papered, it should -be with light-colored paper.</p> -<p class="pq">Why does dark-colored paper darken a room?</p> -<p class="pa">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.</p> -<p class="pq">Does the light from a burning body differ from sun-light?</p> -<p class="pa">Yes; it differs in intensity, in color, and in many -other ways.</p> -<p class="pq">What effect does the lamp-light have upon the color of some -substances?</p> -<p class="pa">It changes their color.</p> -<p class="pq">Where may we find an example of this?</p> -<p class="pa">In trying to match the colors of thread or silk at -night.</p> -<p class="pq">Why is the color different by lamp-light from what it is by -daylight?</p> -<p class="pa">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.</p> -<p class="pq">Are all persons able to distinguish one color from another?</p> -<p class="pa">No; some are color-blind. To such persons all -colors appear to be alike.</p> -<div class="pb" id="Page_32">32</div> -<p class="pq">What curious instances of color-blindness may be mentioned?</p> -<p class="pa">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.</p> -<h3 id="c7">Refraction of Light.</h3> -<p class="pq">How do the rays of light from the sun come to us?</p> -<p class="pa">They come to us in straight lines.</p> -<p class="pq">When the rays of light pass from one substance to another -more dense, what takes place?</p> -<p class="pa">They are bent or turned from a straight course.</p> -<p class="pq">What is said of these bent rays?</p> -<p class="pa">They are said to be <i>refracted</i>.</p> -<p class="pq">What is meant by refraction?</p> -<p class="pa">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.</p> -<p class="pq">Where may we see an example of refraction?</p> -<p class="pa">In the rays of light from a rod which -has one end in the water.</p> -<div class="img" id="fig7"> -<img src="images/img016.jpg" alt="" width="200" height="245" /> -<p class="pcap">Fig. 7.</p> -</div> -<p class="pq">Why does the rod appear to be bent?</p> -<p class="pa">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 -<span class="pb" id="Page_33">33</span> -surface, comes in straight lines to the eye; hence, the -rod appears to be bent at the surface, as is represented -in <a href="#fig7">Fig. 7</a>.</p> -<p class="pq">How does a pole appear when one end is placed in the water?</p> -<p class="pa">It appears to be bent where it meets the surface of -the water.</p> -<p class="pq">How does the part of the rod under water appear?</p> -<p class="pa">The part under the water appears to be raised up, -as may be seen in <a href="#fig7">Fig. 7</a>.</p> -<p class="pq">How does the bottom of a pond or stream of water appear?</p> -<p class="pa">It appears to be raised up, and the water does not -seem so deep as it really is.</p> -<p class="pq">Why is this the case?</p> -<p class="pa">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.</p> -<p class="pq">How much does the bottom of a pond or stream of water seem -to be raised up?</p> -<p class="pa">About one-third of the depth of the water.</p> -<p class="pq">If the water is really six feet deep, how deep does it appear?</p> -<p class="pa">It appears to be only four feet deep.</p> -<p class="pq">What has frequently happened from not knowing this fact?</p> -<p class="pa">People have been drowned from getting into water -deeper than it appeared to be.</p> -<p class="pq">Do the fish and other things floating in the water appear to -be raised up?</p> -<p class="pa">Yes; like the bottom of the stream, they also appear -to be only two-thirds as far from the surface as -they really are.</p> -<div class="pb" id="Page_34">34</div> -<p class="pq">If we wish to spear the fish, what must we do?</p> -<p class="pa">We must aim below the place where they seem to -be, or we must strike perpendicularly at them.</p> -<h3 id="c8">Reflection of Light.</h3> -<p class="pq">When a ball is thrown against a wall, what is the path through -which it goes called?</p> -<p class="pa">The line of incidence.</p> -<p class="pq">When the ball bounds back, what is its path called?</p> -<p class="pa">The line of reflection.</p> -<p class="pq">What is the angle at which a ball strikes the wall called?</p> -<p class="pa">The angle of incidence.</p> -<p class="pq">What is the angle at which the ball rebounds called?</p> -<p class="pa">The angle of reflection.</p> -<p class="pq">When a ray of light falls upon a looking-glass, what is the -path through which it goes called?</p> -<p class="pa">The line of incidence.</p> -<p class="pq">When the ray is reflected, what is its path called?</p> -<p class="pa">The line of reflection.</p> -<p class="pq">The <i>angle of incidence</i> is the angle between -a perpendicular and the line of -incidence; and the <i>angle of reflection</i> is -the angle between the perpendicular and -the line of reflection. The two angles are -always equal to each other, as is represented in <a href="#fig8">Fig. 8</a>.</p> -<div class="img" id="fig8"> -<img src="images/img017.jpg" alt="" width="200" height="92" /> -<p class="pcap">Fig. 8.</p> -</div> -<p class="pq">When a ray of light strikes a looking-glass obliquely or slantingly, -what is done with it?</p> -<p class="pa">The ray is reflected as obliquely from the glass as -it strikes or falls upon it.</p> -<div class="pb" id="Page_35">35</div> -<p class="pq">Why is the reflection of the sun in a pond of water seen near -the edge at noon?</p> -<p class="pa">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.</p> -<p class="pq">Where is the reflection seen in the morning or in the evening?</p> -<p class="pa">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.</p> -<p class="pq">When an opaque object is placed between a candle and a wall, -why will there be a shadow?</p> -<p class="pa">Because the opaque object prevents the rays of light -from falling on the wall.</p> -<p class="pq">Why will the shadow be larger, the nearer the object is to the -candle?</p> -<p class="pa">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 <a href="#fig9">Fig. 9</a>.</p> -<div class="img" id="fig9"> -<img src="images/img018.jpg" alt="" width="500" height="208" /> -<p class="pcap">Fig. 9.</p> -</div> -<p class="pq">In <a href="#fig9">Fig. 9</a>, 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.</p> -<div class="pb" id="Page_36">36</div> -<p class="pq">Why does an object seem to be smaller, the farther it is -from us?</p> -<p class="pa">Because the angle at which the light from it strikes -the eye, is less when at a distance, than when near to -the eye.</p> -<div class="img" id="fig10"> -<img src="images/img019.jpg" alt="" width="500" height="146" /> -<p class="pcap">Fig. 10.</p> -</div> -<p class="pa">In <a href="#fig10">Fig. 10</a>, 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.</p> -<p class="pq">Why does an object grow more dim the farther it is from us?</p> -<p class="pa">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.</p> -<p class="pq">What is the use of the telescope?</p> -<p class="pa">It is used to view distant objects.</p> -<p class="pq">Why can we see a distant object more distinctly with a telescope, -than without one?</p> -<p class="pa">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.</p> -<p class="pq">What class of objects do we view with a telescope?</p> -<p class="pa">We view the heavenly bodies; the sun, moon, and -stars.</p> -<div class="pb" id="Page_37">37</div> -<p class="pq">What is a spy-glass?</p> -<p class="pa">It is a glass mostly used to view distant objects on -the earth.</p> -<p class="pq">Is the spy-glass like the telescope?</p> -<p class="pa">The spy-glass is much like the telescope, only it is -smaller.</p> -<p class="pq">When looking through a spy-glass, how do objects appear?</p> -<p class="pa">They seem to be brought nearer to us, and they -appear larger.</p> -<p class="pq">By whom are spy-glasses much used?</p> -<p class="pa">Spy-glasses are much used by persons on board of -vessels, while at sea.</p> -<p class="pq">What instruments have we for magnifying small objects?</p> -<p class="pa">We have magnifying glasses and microscopes.</p> -<p class="pq">What are magnifying glasses?</p> -<p class="pa">They are single glasses or lenses, used for magnifying -small objects.</p> -<p class="pq">By whom are magnifying glasses used?</p> -<p class="pa">They are used by jewellers, watchmakers, engravers, -and others who examine small objects.</p> -<p class="pq">What are microscopes?</p> -<p class="pa">They are instruments used for examining very small -objects.</p> -<p class="pq">How does a drop of rain-water or vinegar appear under the -microscope?</p> -<p class="pa">It appears to be full of strange-looking creatures, -which are always in motion.</p> -<p class="pq">How do the smallest insects appear?</p> -<p class="pa">They appear to be as perfectly formed as the larger -ones which are seen with the naked eye.</p> -<div class="pb" id="Page_38">38</div> -<p class="pq">What may be seen with a microscope?</p> -<p class="pa">Thousands of things too small to be seen by us -unless they are magnified.</p> -<p class="pq">What kind of spectacles do near-sighted people use?</p> -<p class="pa">Near-sighted people use glasses, thickest at the edge -and thinnest at the centre.</p> -<p class="pq">What kind of spectacles do we use as we grow old?</p> -<p class="pa">As we grow old we use glasses, thinnest at the edge -and thickest at the centre.</p> -<p class="pq">Of what use are spectacles?</p> -<p class="pa">When the vision is defective, spectacles enable us to -see objects more clearly.</p> -<p class="pq">How should spectacles be made?</p> -<p class="pa">They should be so made as to render objects distinct, -but neither to magnify nor diminish them.</p> -<p class="pq">As people grow old, how do their eyes change?</p> -<p class="pa">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.</p> -<h3 id="c9">The Eye.</h3> -<p class="pq">What is that part of the eye called, which is blue, gray, or -brown, in different persons?</p> -<p class="pa">It is called the <i>iris</i>.</p> -<p class="pq">What is the pupil of the eye?</p> -<p class="pa">The pupil is the black spot or -hole in the centre of the iris.</p> -<div class="img" id="fig11"> -<img src="images/img020.jpg" alt="" width="200" height="107" /> -<p class="pcap">Fig. 11.</p> -</div> -<p class="pq">What is the use of the pupil?</p> -<p class="pa">The pupil is the window of the eye, and the rays of -light which enter the eye must pass through it.</p> -<div class="pb" id="Page_39">39</div> -<p class="pq">What power has the iris over the pupil?</p> -<p class="pa">The iris has the power of making the pupil larger -or smaller, according to the amount of light which it -receives.</p> -<p class="pq">How does a bright light affect the iris?</p> -<p class="pa">It causes the iris to contract, so that the pupil becomes -smaller.</p> -<p class="pq">How does a faint light affect the iris?</p> -<p class="pa">It causes the iris to dilate, so that the pupil becomes -larger.</p> -<p class="pq">Of what advantage is the iris to the eye?</p> -<p class="pa">The iris acts like a sentinel to protect the eye -from any sudden light.</p> -<p class="pq">How does a sudden light affect the eye?</p> -<p class="pa">A sudden light causes pain to the eye.</p> -<p class="pq">Of what shape is the pupil of the eye?</p> -<p class="pa">In man, the pupil of the eye is circular.</p> -<p class="pq">What is the size of the pupil of the eye?</p> -<p class="pa">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.</p> -<p class="pq">How else are our eyes protected from any sudden light?</p> -<p class="pa">By means of our eyelids, which may be closed, and -thus shut out all the light.</p> -<p class="pq">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?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_40">40</div> -<p class="pq">Why does the pupil become larger in the twilight?</p> -<p class="pa">So that more rays of light may enter the eye, -whereby objects may be more distinctly seen.</p> -<p class="pq">Why can an owl or a bat see at night?</p> -<p class="pa">Because the pupil of the eye in them is quite large, -and admits much light.</p> -<p class="pq">Why do they stay in dark places through the day?</p> -<p class="pa">Because the bright light of day hurts their eyes.</p> -<p class="pq">May the image of an object be retained in the eye after the -eyelids are closed?</p> -<p class="pa">It may for a very short time, as any one can prove -by first looking at some bright object, and then closing -the eyelids.</p> -<p class="pq">Why does a burning coal moved rapidly around, seem like a -circle of fire?</p> -<p class="pa">Because the light from it is retained a short time by -the eye, thus seeming to form a complete circle.</p> -<p class="pq">Why cannot we count the posts in a fence, when riding rapidly -in a car?</p> -<p class="pa">Because the light from each post falls in such quick -succession upon the eye, that it cannot distinguish one -post from another.</p> -<p class="pq">Can objects be seen distinctly when placed near the eye?</p> -<p class="pa">No; when objects are within six inches of the eye -they cannot be seen distinctly.</p> -<p class="pq">Can objects be seen when at a great distance from the eye?</p> -<p class="pa">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 -<span class="pb" id="Page_41">41</span> -of the stars which we behold in the heavens, are still -farther away than the sun.</p> -<p class="pq">Of what use is the eye to us?</p> -<p class="pa">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.</p> -<p class="pq">What may we remember about the light?</p> -<p class="pa">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!</p> -<div class="img"> -<img src="images/img021.jpg" alt="Locomotive." width="500" height="157" /> -</div> -<div class="pb" id="Page_42">42</div> -<div class="img"> -<img src="images/img022.jpg" alt="Decoration." width="500" height="180" /> -</div> -<h2 id="c10">CHAPTER III. -<br /><span class="sc">Heat.</span></h2> -<h3 id="c11"><span class="sc">Sources of Heat.</span></h3> -<div class="img"> -<img src="images/img023.jpg" alt="Illustrated Capital W" width="342" height="350" /> -</div> -<p class="pq">What is heat?</p> -<p class="pa">Heat is that agent which causes -the feeling of warmth.</p> -<p class="pq">Can heat be seen?</p> -<p class="pa">No; heat cannot be seen, it can only be -felt.</p> -<p class="pq">When we touch a substance hotter than we are, what takes -place?</p> -<p class="pa">A part of the heat from that substance comes to us -and causes a feeling of warmth.</p> -<p class="pq">When we touch a substance colder than we are, what takes -place?</p> -<p class="pa">A part of the heat from us goes to that substance, -and we experience a feeling of cold.</p> -<p class="pq">Does the amount of heat in any substance vary?</p> -<p class="pa">Yes; water may be heated until it is scalding hot, or -the heat may be taken from it until it is frozen into ice.</p> -<p class="pq">What causes a substance to become cold?</p> -<p class="pa">Taking away heat from a substance causes it to become -cold.</p> -<div class="pb" id="Page_43">43</div> -<p class="pq">Then what is cold?</p> -<p class="pa">Cold is only the absence or want of heat.</p> -<p class="pq">When we pass from a very hot room to one moderately warm, -how do we feel?</p> -<p class="pa">We experience a feeling of cold.</p> -<p class="pq">When we pass from a very cold room to one moderately warm, -how do we feel?</p> -<p class="pa">We have a feeling of warmth, although the room -is no warmer than in the first instance.</p> -<p class="pq">How else can we prove that we judge of heat and cold by our -feelings?</p> -<p class="pa">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.</p> -<p class="pq">Is there anything without heat?</p> -<p class="pa">No; the coldest substance known still contains some -heat.</p> -<p class="pq">How many kinds of heat are there?</p> -<p class="pa">There are two kinds of heat.</p> -<p class="pq">What are the two kinds of heat?</p> -<p class="pa">Heat accompanied by light, as the heat from the sun, -or a lamp; and heat without light, as the heat from -boiling water.</p> -<p class="pq">From what source does heat mostly come?</p> -<p class="pa">Heat mostly comes from the sun, which is also the -source of light.</p> -<div class="pb" id="Page_44">44</div> -<p class="pq">What is said of the heat and light from the sun?</p> -<p class="pa">Heat and light from the sun come together in the -sunbeam.</p> -<p class="pq">From what other source is heat obtained?</p> -<p class="pa">Heat is obtained from the burning of any substance.</p> -<p class="pq">What else does the burning of any substance produce?</p> -<p class="pa">It often produces light.</p> -<p class="pq">Can the light and heat from the sun easily be separated?</p> -<p class="pa">They cannot easily be separated.</p> -<p class="pq">What does a glass window do with the light and heat of the -sunbeam?</p> -<p class="pa">It permits both the light and the heat to pass into -the room, so that we can see the one and feel the other.</p> -<p class="pq">Are the light and heat from a fire united, as they are in the -sunbeam?</p> -<p class="pa">They are not; but they seem to be separated from -each other.</p> -<p class="pq">What does a glass window do with the light and heat from a -fire?</p> -<p class="pa">It permits the light to pass through; but it stops -the heat.</p> -<p class="pq">How may this be proved?</p> -<p class="pa">When a pane of glass is held between the face and -a fire, it will protect the face from the heat.</p> -<p class="pq">Is there any heat without light?</p> -<p class="pa">Yes; many substances contain heat, but do not emit -light.</p> -<p class="pq">Does light change the amount of heat in a substance?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_45">45</div> -<p class="pq">Is there any other source of heat?</p> -<p class="pa">Yes; heat is produced by rubbing or striking substances -together.</p> -<p class="pq">What will result from rubbing two pieces of wood together?</p> -<p class="pa">They may be rubbed until they take fire.</p> -<p class="pq">How do Indians kindle fires?</p> -<p class="pa">By rubbing two pieces of wood together until they -take fire.</p> -<p class="pq">How is heat produced by the brakes on railroad cars?</p> -<p class="pa">It is produced by the car-wheels rubbing against -the brakes.</p> -<p class="pq">What may be seen when a horse strikes his shoes against a -stone?</p> -<p class="pa">Small sparks of fire, which contain heat, may be -seen.</p> -<p class="pq">If a piece of iron be hammered, will its heat be increased?</p> -<p class="pa">Its heat will be increased.</p> -<p class="pq">How can this be proved?</p> -<p class="pa">A person can, by hammering a piece of cold iron, -make it red-hot.</p> -<p class="pq">Could we live without heat?</p> -<p class="pa">No; our bodies must be kept warm, and this can -be done only by heat.</p> -<p class="pq">Is this true of other living things?</p> -<p class="pa">Yes; all the animals and all the plants would die -if heat were taken away.</p> -<p class="pq">How should we feel in regard to these things?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_46">46</div> -<h3 id="c12">Expansion by Heat.</h3> -<p class="pq">How does heat generally affect substances?</p> -<p class="pa">Heat <i>expands</i> them or makes them larger.</p> -<p class="pq">Does heat expand all metals?</p> -<p class="pa">Yes; all metals are expanded by heat.</p> -<p class="pq">When a rod of iron is heated, is it longer or shorter than when -cold?</p> -<p class="pa">It is longer than when it is cold.</p> -<p class="pq">How do we know this?</p> -<p class="pa">We know it by measuring the rod when it is cold, -and again when it is heated.</p> -<p class="pq">Will the rod be larger, as well as longer?</p> -<p class="pa">The rod will be larger.</p> -<p class="pq">How can we prove this?</p> -<p class="pa">The rod, when red-hot, will not go through as small -a hole as when it is cold.</p> -<p class="pq">How do heat and cold affect most substances?</p> -<p class="pa">Heat expands most substances, and cold contracts -them.</p> -<p class="pq">Do all metals expand alike by heat?</p> -<p class="pa">No; some metals expand much more than others.</p> -<p class="pq">How does heat expand substances?</p> -<p class="pa">Heat pushes the parts or atoms of an object farther -from each other, and thus its size is increased.</p> -<p class="pq">What does a blacksmith do with a tire before putting it on a -wheel?</p> -<p class="pa">He heats the tire red-hot.</p> -<p class="pq">Why does he heat the tire red-hot?</p> -<p class="pa">So that it will be increased in size, and will go on -the wheel more easily.</p> -<div class="pb" id="Page_47">47</div> -<p class="pq">What takes place as the tire cools?</p> -<p class="pa">It becomes smaller, and thus binds the parts of the -wheel tightly together.</p> -<p class="pq">When a fire is made in a room, why does the furniture often -make a snapping noise?</p> -<p class="pa">Because heat expands the wood, and the particles -make a snapping noise upon separating.</p> -<p class="pq">Why does a stove make a crackling noise as it grows hot?</p> -<p class="pa">The particles of the iron make the noise in -expanding.</p> -<p class="pq">If a piece of glass be held in the hand, will the glass become -warm?</p> -<p class="pa">The glass will become as warm as the hand.</p> -<p class="pq">What part of the glass will be warmed first?</p> -<p class="pa">The part touching the hand will be warmed first.</p> -<p class="pq">What part of the glass will be warmed last?</p> -<p class="pa">The part farthest from the hand will be warmed -last.</p> -<p class="pq">Which part of the glass will be expanded first?</p> -<p class="pa">The part touching the hand, because it is first -warmed.</p> -<p class="pq">Which part will be expanded last?</p> -<p class="pa">The part farthest from the hand, because it is the -last part warmed.</p> -<p class="pq">If any other warm substance touch the glass, will the glass be -heated in the same manner?</p> -<p class="pa">The glass will be heated in the same manner.</p> -<p class="pq">What often results from this unequal heating?</p> -<p class="pa">The glass is broken.</p> -<div class="pb" id="Page_48">48</div> -<p class="pq">Why are glass plates or tumblers broken when placed in hot -water?</p> -<p class="pa">They are broken because the part next to the hot -water is expanded more than that farther away.</p> -<p class="pq">Why will a cloth dipped in hot water and wrapped around the -neck of a bottle loosen its stopper?</p> -<p class="pa">Because the heat from the cloth expands the neck -of the bottle before it does the stopper, so that the -stopper is loosened.</p> -<p class="pq">Why do stove-plates often break?</p> -<p class="pa">Because they are fitted together in the stove so that -they have not room to expand without breaking.</p> -<p class="pq">What effect has cold upon substances?</p> -<p class="pa">Cold causes them to contract or grow smaller.</p> -<p class="pq">Things <i>expand</i> unequally; do they also <i>contract</i> unequally?</p> -<p class="pa">They do contract unequally, and in this way may -be broken.</p> -<p class="pq">Why are lamp chimneys often broken while the lamp is -burning?</p> -<p class="pa">Because the heat of the flame causes them to -expand unequally.</p> -<p class="pq">In what other way are they broken?</p> -<p class="pa">By a draught of cold air, or a drop of cold water, -touching them, thus causing them to contract unequally.</p> -<p class="pq">Why are they sometimes broken when the lamp is not burning?</p> -<p class="pa">Because a current of cold air from a window or a -door, blows upon them.</p> -<p class="pq">Does quicksilver expand when heated?</p> -<p class="pa">Like other metals, quicksilver expands as it grows -warmer, and contracts as it grows cooler.</p> -<div class="pb" id="Page_49">49</div> -<p class="pq">Is quicksilver in the form of a solid or a fluid?</p> -<p class="pa">It is a fluid as we usually see it.</p> -<p class="pq">For what is quicksilver used?</p> -<p class="pa">It is used for filling the tubes of thermometers.</p> -<p class="pq">What is a Thermometer?</p> -<p class="pa">It is an instrument used for measuring the degree or -quantity of heat in any substance.</p> -<p class="pq">How does heat affect the quicksilver?</p> -<p class="pa">Heat expands the quicksilver, and causes it to rise -in the tube.</p> -<p class="pq">How does cold affect the quicksilver?</p> -<p class="pa">Cold contracts the quicksilver, and causes it to fall -in the tube.</p> -<p class="pq">Does heat expand liquids more than solids?</p> -<p class="pa">Heat does expand liquids more than solids.</p> -<p class="pq">Why are liquids expanded more than solids?</p> -<p class="pa">Because the particles of which they are formed are -more easily separated or pushed apart by the heat.</p> -<p class="pq">If we continue to apply heat to a liquid, what becomes of it?</p> -<p class="pa">The liquid is changed into a gas or vapor.</p> -<p class="pq">What common example have we of vapor?</p> -<p class="pa">Water, when heated, is turned into steam.</p> -<p class="pq">Mention some gas.</p> -<p class="pa">The <i>air</i> we breathe is a gas.</p> -<p class="pq">How does heat affect the air?</p> -<p class="pa">Heat warms the air and causes it to expand.</p> -<p class="pq">Do solids all expand equally when heated?</p> -<p class="pa">No; some solids expand more than others; thus, -zinc expands more than iron, and iron more than glass.</p> -<div class="pb" id="Page_50">50</div> -<p class="pq">Do liquids all expand equally when heated?</p> -<p class="pa">No; liquids, although more sensitive to heat, do not -expand as equally as solids.</p> -<p class="pq">Do gases expand equally when heated?</p> -<p class="pa">Yes; gases expand uniformly; thus, 491 cubic inches -of any gas, if heated one degree, will become 492 cubic -inches.</p> -<p class="pq">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?</p> -<p class="pa">The lead will weigh more than the water.</p> -<p class="pq">Which is the heavier substance, lead or water?</p> -<p class="pa">Lead is heavier than water.</p> -<p class="pq">Why do we say lead is heavier than water?</p> -<p class="pa">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.</p> -<p class="pq">Why is cork lighter than water?</p> -<p class="pa">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.</p> -<p class="pq">When lead is put in water, why does it sink to the bottom?</p> -<p class="pa">Because the lead is heavier than an equal bulk of -water.</p> -<p class="pq">When cork is put in water, why does it rise to the surface?</p> -<p class="pa">Because the cork is lighter than an equal bulk of -water.</p> -<p class="pq">Does heat increase the weight of any substance?</p> -<p class="pa">Heat does not increase the weight of any substance.</p> -<div class="pb" id="Page_51">51</div> -<p class="pq">How do we know this?</p> -<p class="pa">Because a piece of iron when cold will weigh as -much as when heated red-hot.</p> -<p class="pq">When a liquid is heated, does it become lighter or heavier?</p> -<p class="pa">It becomes lighter, bulk for bulk, when heated.</p> -<p class="pq">Why does a liquid become lighter when heated?</p> -<p class="pa">Because the same liquid is expanded and takes up -more room, although it has no more weight.</p> -<p class="pq">Why will a gallon of cold water weigh more than a gallon of -hot water?</p> -<p class="pa">Because a gallon of cold water, when heated, will -make more than a gallon of hot water.</p> -<p class="pq">When do we get the most molasses, by buying it in hot or in -cold weather?</p> -<p class="pa">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.</p> -<p class="pq">Does heat expand the air and make it lighter?</p> -<p class="pa">Heat does expand the air and make it lighter.</p> -<p class="pq">What does the air do when heated?</p> -<p class="pa">It ascends or rises up, because it is lighter, and the -cold or heavier air falls to take its place.</p> -<p class="pq">What part of a room, in which there is fire, is the warmest?</p> -<p class="pa">The part next to the ceiling is the warmest, because -the heated air always ascends.</p> -<p class="pq">Why does a soap-bubble ascend?</p> -<p class="pa">Because it is filled with heated air from the lungs, -and is, therefore, lighter than the air around it.</p> -<p class="pq">Why do balloons ascend?</p> -<p class="pa">Because they are filled with a gas lighter than the -air around them.</p> -<div class="pb" id="Page_52">52</div> -<p class="pq">Why does a chestnut split open when roasting?</p> -<p class="pa">Because the air in it is expanded by the heat, so -that it bursts the shell.</p> -<p class="pq">Why does the chestnut not split open when a hole is made in -the shell?</p> -<p class="pa">Because the air can then escape as it expands, without -bursting the shell.</p> -<p class="pq">What becomes of the air in an apple, when the apple is -roasted?</p> -<p class="pa">The air, upon being heated, bursts through the peel -of the apple, carrying the juice with it.</p> -<p class="pq">Why does an apple become soft when roasted?</p> -<p class="pa">Because the air in the cells of the apple expands, -and breaks those cells, so that the apple becomes soft.</p> -<p class="pq">What part of the apple first becomes soft?</p> -<p class="pa">The outside, because it is first heated.</p> -<p class="pq">Why do all fruits and vegetables become soft when cooked?</p> -<p class="pa">Because the heat expands the air in them, and thus -breaks up their cells, so that they become soft.</p> -<p class="pq">Why does wood make a snapping noise when burning?</p> -<p class="pa">Because the air in the cells of the wood bursts them -apart, thus making the noise.</p> -<p class="pq">Why are sparks of fire thrown out?</p> -<p class="pa">Because the heated air bursts the cells with such -force as to throw off small pieces of the burning -wood.</p> -<p class="pq">What kinds of wood snap the most?</p> -<p class="pa">The coarse-grained kinds, like chestnut; while the -finer-grained kinds, like walnut, seldom snap much.</p> -<div class="pb" id="Page_53">53</div> -<p class="pq">Why does dry wood snap more than green wood?</p> -<p class="pa">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.</p> -<p class="pq">Why does smoke rise in a chimney?</p> -<p class="pa">Because the air in the chimney is heated by the -fire, and as the air rises, it carries the smoke with it.</p> -<p class="pq">Why does a chimney smoke when the fire is first kindled?</p> -<p class="pa">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.</p> -<p class="pq">How are houses heated with hot air?</p> -<p class="pa">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.</p> -<p class="pq">Is there any substance that does not contract by cold?</p> -<p class="pa">Yes; water, when near freezing, does not contract -by cold.</p> -<p class="pq">What takes place with the water?</p> -<p class="pa">When it is reduced nearly to the freezing point, it -begins to expand, and when it freezes, it expands still -more.</p> -<p class="pq">How much does water expand by freezing?</p> -<p class="pa">It expands about one-seventh of its bulk. That -is, <i>seven</i> gallons of <i>water</i> will make <i>eight</i> gallons of -<i>ice</i>.</p> -<p class="pq">Does freezing water expand with much force?</p> -<p class="pa">Yes; it expands with great force.</p> -<div class="pb" id="Page_54">54</div> -<p class="pq">Why are vessels containing water often broken during a cold -night?</p> -<p class="pa">As the water in them freezes, it expands and breaks -the vessels.</p> -<p class="pq">Why do lead pipes and iron pipes often burst during the -winter?</p> -<p class="pa">Because the water in them is permitted to freeze, and -as there is not room for it to expand, the pipes are burst.</p> -<p class="pq">Is ice heavier or lighter than water?</p> -<p class="pa">Ice is lighter than the same bulk of water.</p> -<p class="pq">How much lighter is ice than water?</p> -<p class="pa">Eight gallons of ice weigh no more than seven -gallons of water.</p> -<p class="pq">Does the ice protect the water beneath it from freezing?</p> -<p class="pa">Ice does protect the water from freezing, to a very -great extent.</p> -<p class="pq">If ice were heavier than water, what would result?</p> -<p class="pa">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.</p> -<h3 id="c13">Conduction of Heat.</h3> -<p class="pq">Why does iron feel cold to the touch on a cold morning?</p> -<p class="pa">Because heat goes from the hand to the iron, thus -giving the sensation of cold.</p> -<p class="pq">What is the passage of heat from the hand to the iron called?</p> -<p class="pa">It is called <i>conduction</i>.</p> -<div class="pb" id="Page_55">55</div> -<p class="pq">When one end of a rod of iron is placed in a fire, does the -whole rod become heated?</p> -<p class="pa">The whole rod does become heated.</p> -<p class="pq">How does this take place?</p> -<p class="pa">The heat of the fire passes from one particle to -another throughout the rod.</p> -<p class="pq">What is this called?</p> -<p class="pa">It is called <i>conduction</i>.</p> -<p class="pq">What then is conduction of heat?</p> -<p class="pa">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.</p> -<p class="pq">What bodies are good conductors of heat?</p> -<p class="pa">Such bodies as gold, platinum, copper, silver, iron, -zinc, tin, and lead, in the order named, are the best -conductors of heat.</p> -<p class="pq">What bodies are poor conductors of heat?</p> -<p class="pa">Such bodies as glass, wood, charcoal, wool, hair, -and fur.</p> -<p class="pq">Fur is the poorest conductor of heat known.</p> -<p class="pq">Why does a piece of wood blazing at one end not feel hot at -the other end?</p> -<p class="pa">Because wood is a poor conductor, and the heat -goes slowly through it.</p> -<p class="pq">Why does a piece of iron feel cold in winter?</p> -<p class="pa">Because the iron is a good conductor, and carries -the heat away from our hands very rapidly.</p> -<p class="pq">Why does fur feel warmer than iron in the winter?</p> -<p class="pa">Because the fur is a poor conductor, and carries the -heat away from our hands very slowly.</p> -<div class="pb" id="Page_56">56</div> -<p class="pq">Why does a wooden pump-handle seem less cold than one of -iron?</p> -<p class="pa">Because wood is not so good a conductor as iron; -hence, it does not carry the heat of our hands away -so rapidly.</p> -<p class="pq">Why does carpet seem warmer than the bare floor?</p> -<p class="pa">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.</p> -<p class="pq">Why does a stone pavement make our feet cold in winter?</p> -<p class="pa">Because the stone is a good conductor, and rapidly -carries the heat away from our feet.</p> -<p class="pq">Why does heated iron feel hot to us?</p> -<p class="pa">Because the iron is a good conductor, and gives off -its heat rapidly to our bodies.</p> -<p class="pq">Why does a piece of cloth, when heated, not feel hot?</p> -<p class="pa">Because the cloth is a poor conductor, and gives -off its heat very slowly.</p> -<p class="pq">Why will a block of wood, when heated, remain hot longer -than a heated brick?</p> -<p class="pa">Because wood is a poorer conductor than brick, -and does not give off its heat so rapidly.</p> -<p class="pq">Why is a block of wood better than iron or brick, to keep our -feet warm while on a journey?</p> -<p class="pa">Because it does not give off its heat so rapidly, and -therefore remains warm longer than iron or brick.</p> -<p class="pq">Why should the wood or brick be wrapped in cloth?</p> -<p class="pa">Because the cloth, being a poor conductor, helps to -keep the heat from passing off so rapidly.</p> -<div class="pb" id="Page_57">57</div> -<p class="pq">Why does iron feel colder than water when both are at the -same temperature?</p> -<p class="pa">Because iron is a better conductor than water, and -carries away the heat from our hands more rapidly.</p> -<p class="pq">Are liquids good conductors of heat?</p> -<p class="pa">No; liquids are poor conductors of heat.</p> -<p class="pq">Is air a good conductor of heat?</p> -<p class="pa">No; air is a poor conductor of heat.</p> -<p class="pq">Are gases as good conductors as liquids?</p> -<p class="pa">No; gases are poorer conductors than liquids.</p> -<p class="pq">Why is water a better conductor than air?</p> -<p class="pa">Because the particles of water are closer together -than the particles of air.</p> -<p class="pq">Why is iron a better conductor than water?</p> -<p class="pa">Because the particles of iron are closer together -than the particles of water.</p> -<p class="pq">How do we know that water is a poor conductor of heat?</p> -<p class="pa">Because water may be made to boil at its surface -without melting ice a short distance below the surface.</p> -<p class="pq">How do we know that air is a poor conductor of heat?</p> -<p class="pa">Because the air at the ceiling of a room may be -made very warm without melting ice near the floor.</p> -<p class="pq">Why does a linen shirt feel cool?</p> -<p class="pa">Because linen is a good conductor, and there is but -little air among its fibres.</p> -<p class="pq">Why is a cotton shirt warmer than one made of linen?</p> -<p class="pa">Because the cotton contains more air among -its fibres, and air is a poor conductor of heat.</p> -<div class="pb" id="Page_58">58</div> -<p class="pq">Why is a woollen shirt warmer than one made of cotton?</p> -<p class="pa">Because wool contains more air among its fibres, -and is, therefore, a poorer conductor of heat.</p> -<p class="pq">Why is fur warmer than wool?</p> -<p class="pa">Because there is more air among the hairs of the -fur, and it is, therefore, a poorer conductor of heat.</p> -<p class="pq">What fur is the warmest?</p> -<p class="pa">Fur with very fine hairs, like that of the rabbit, -because it contains the most air.</p> -<p class="pq">Why is fur warmer when the hair is next to our bodies?</p> -<p class="pa">Because the fur and the air in it both help to keep -the heat of our bodies from passing off rapidly.</p> -<p class="pq">Is the earth a good conductor of heat?</p> -<p class="pa">No; the earth is a poor conductor of heat.</p> -<p class="pq">How do we know that the earth is a poor conductor of heat?</p> -<p class="pa">Because the heat of summer warms the earth only -a few inches below the surface.</p> -<p class="pq">How is it with the earth in winter?</p> -<p class="pa">In this latitude, the frost of winter reaches only a -few inches below the surface of the earth.</p> -<p class="pq">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.</p> -<p class="pq">Why is spring-water generally cool, even in summer?</p> -<p class="pa">Because the springs are mostly so far below the -surface of the earth, that they are not warmed by the -sun’s rays.</p> -<p class="pq">Why is snow a warm covering for the earth?</p> -<p class="pa">Because the air among the particles of snow prevents -the heat of the earth from passing off rapidly.</p> -<div class="pb" id="Page_59">59</div> -<p class="pq">Why does wrapping straw around a pump in winter, prevent -the water in it from freezing?</p> -<p class="pa">Because the stalks of straw are hollow, and the air -in the stalks makes them poor conductors of heat.</p> -<p class="pq">Will straw, wrapped around shrubbery, prevent it from freezing?</p> -<p class="pa">It will; because it keeps the heat of the shrubbery -from passing off into the air.</p> -<p class="pq">Why are rooms warmer from having double windows?</p> -<p class="pa">Because the air which is confined between the -double windows, being a poor conductor, keeps the -warmth of the room from escaping.</p> -<p class="pq">Why is the space between the double walls of ice-coolers filled -with charcoal?</p> -<p class="pa">Because charcoal is a poor conductor, and keeps -the heat of the air from the ice.</p> -<p class="pq">Why do ice-houses have double walls, with saw-dust or straw -between them?</p> -<p class="pa">Because the saw-dust and the straw are both poor -conductors, and keep the heat of the air from the ice.</p> -<p class="pq">Why does ice melt more slowly when wrapped in flannel?</p> -<p class="pa">Because the flannel is a poor conductor, and keeps -the heat of the air from the ice.</p> -<p class="pq">Why are woollen holders used about the stove?</p> -<p class="pa">Because wool is a poor conductor, and thus keeps -the heat of the iron from burning the hand.</p> -<h3 id="c14">Convection of Heat.</h3> -<p class="pq">When air near the floor of a room is heated, what becomes -of it?</p> -<p class="pa">It ascends to the ceiling of the room.</p> -<div class="pb" id="Page_60">60</div> -<p class="pq">What is done with the cold air at the ceiling?</p> -<p class="pa">It descends to the floor, to take the place of the -warm air.</p> -<p class="pq">When the air ascends to the ceiling, does it carry the heat -with it?</p> -<p class="pa">Yes; it carries the heat with it.</p> -<p class="pq">What is this mode of carrying heat called?</p> -<p class="pa">It is called <i>Convection</i>.</p> -<p class="pq">What is needed so as to heat a substance by convection?</p> -<p class="pa">Its particles must be able to move about freely.</p> -<p class="pq">Why is the air easily heated by convection?</p> -<p class="pa">Because the particles of the air move about freely.</p> -<p class="pq">Why is a liquid easily heated by convection?</p> -<p class="pa">Because its particles move about freely.</p> -<p class="pq">Why cannot a solid be heated by convection?</p> -<p class="pa">Because its particles cannot move.</p> -<p class="pq">When water near the bottom of a vessel is heated, what becomes -of it?</p> -<p class="pa">It rises to the surface.</p> -<p class="pq">What becomes of the cold water at the surface?</p> -<p class="pa">It falls to the bottom of the vessel, to take the place -of the warm water.</p> -<p class="pq">How long will this circulation continue?</p> -<p class="pa">It will continue so long as heat is applied to the -bottom of the vessel.</p> -<p class="pq">When heat is applied to the surface of the water, what is the -result?</p> -<p class="pa">The water near the surface only is heated.</p> -<div class="pb" id="Page_61">61</div> -<p class="pq">Why does it not heat all the water in the vessel?</p> -<p class="pa">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.</p> -<p class="pq">Where must heat be applied to warm a liquid or a gas?</p> -<p class="pa">It must be applied to the bottom of the vessel containing -the liquid or the gas.</p> -<p class="pq">Where must a fire be, so as to warm a room?</p> -<p class="pa">The fire must be near the floor of the room.</p> -<p class="pq">Why does a hot substance cool quickly in the air?</p> -<p class="pa">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.</p> -<p class="pq">How is water cooled by convection?</p> -<p class="pa">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.</p> -<p class="pq">Why does stirring hot coffee cool it quickly?</p> -<p class="pa">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.</p> -<p class="pq">Why does blowing hot coffee cool it quickly?</p> -<p class="pa">Because the hot air over the coffee is removed by -blowing, and the cold air is brought more rapidly in -contact with the coffee.</p> -<h3 id="c15">Liquefaction.</h3> -<p class="pq">Is ice a solid or a liquid?</p> -<p class="pa">Ice is a solid.</p> -<div class="pb" id="Page_62">62</div> -<p class="pq">When heat is applied to ice, what becomes of the ice?</p> -<p class="pa">The ice is changed into water.</p> -<p class="pq">Is water a liquid?</p> -<p class="pa">Water is a liquid.</p> -<p class="pq">How then can a <i>solid</i> be changed to a <i>liquid</i>?</p> -<p class="pa">By applying <i>heat</i> to a solid, it may be changed to -a liquid.</p> -<p class="pq">What is meant by liquefaction?</p> -<p class="pa"><i>Liquefaction</i> is the changing of a solid to a liquid, -by heat.</p> -<p class="pq">How may tallow be melted?</p> -<p class="pa">By heating the tallow, it may be melted.</p> -<p class="pq">Will the tallow become hard again when cold?</p> -<p class="pa">It will be hard again when cold.</p> -<p class="pq">Can lead and other metals be melted?</p> -<p class="pa">They can be melted or changed to a liquid state by -heat.</p> -<p class="pq">Can all solids be changed to liquids?</p> -<p class="pa">No; there are many solids, such as wood, coal, -paper, and leather, that cannot be changed to liquids.</p> -<p class="pq">How can a liquid be changed to a solid?</p> -<p class="pa">By taking heat away from the liquid, it may be -changed to a solid.</p> -<p class="pq">Why is a solid melted by heat?</p> -<p class="pa">Because heat forces the particles of the solid apart, -until it is changed to a liquid.</p> -<div class="img"> -<img src="images/img024.jpg" alt="Decoration." width="200" height="63" /> -</div> -<div class="pb" id="Page_63">63</div> -<h3 id="c16">Latent Heat.</h3> -<p class="pq">Why does warm iron feel warm to the hand?</p> -<p class="pa">Because it gives off some of its heat to the hand.</p> -<p class="pq">Is the hand sensible of the heat coming from the iron?</p> -<p class="pa">The hand is sensible of the heat.</p> -<p class="pq">What then may this heat be called?</p> -<p class="pa">It may be called <i>sensible</i> heat, because it can be felt.</p> -<p class="pq">How do we measure heat?</p> -<p class="pa">We measure heat by degrees.</p> -<p class="pq">How many degrees of heat in ice when at the melting point?</p> -<p class="pa">Thirty-two degrees of heat, as measured by the -thermometer.</p> -<p class="pq">How can we change ice into water?</p> -<p class="pa">By applying heat to the ice it will be changed into -water.</p> -<p class="pq">How many degrees of heat are necessary to change the ice -into water?</p> -<p class="pa">One hundred and forty degrees of heat are necessary.</p> -<p class="pq">How many degrees of heat are in the water when the ice is -all melted?</p> -<p class="pa">Only thirty-two degrees of heat, by the thermometer.</p> -<p class="pq">Does the water feel any warmer than the ice to the hand?</p> -<p class="pa">No; the water does not feel any warmer than the ice.</p> -<p class="pq">What has become of the one hundred and forty degrees of heat?</p> -<p class="pa">They have been hidden in the water.</p> -<p class="pq">What is this hidden heat called?</p> -<p class="pa">It is called <i>latent</i> heat.</p> -<p class="pq">What is latent heat?</p> -<p class="pa">It is heat that is not sensible to the touch, or to the -thermometer.</p> -<div class="pb" id="Page_64">64</div> -<p class="pq">When water is made to boil, what change takes place?</p> -<p class="pa">The water is changed into steam.</p> -<p class="pq">How many degrees of heat are necessary to change water into -steam?</p> -<p class="pa">More than nine hundred degrees of heat are necessary.</p> -<p class="pq">How many degrees of latent heat in steam?</p> -<p class="pa">More than nine hundred degrees of latent heat.</p> -<p class="pq">When steam is changed back again to water, what becomes -of the latent heat?</p> -<p class="pa">It passes off to the air and the surrounding objects, -and makes them warmer.</p> -<p class="pq">When water is changed into ice, what becomes of the latent -heat?</p> -<p class="pa">It passes off to the surrounding objects and makes -them warmer.</p> -<p class="pq">Is freezing a warming process?</p> -<p class="pa">Yes; because the water gives off its latent heat -while freezing; hence, the surrounding objects are -made warmer.</p> -<p class="pq">Is thawing a cooling process?</p> -<p class="pa">Yes; because the ice absorbs so much heat while -thawing that it cools the surrounding objects.</p> -<p class="pq">Where does the ice obtain its heat, while thawing?</p> -<p class="pa">The ice obtains its heat from the surrounding objects.</p> -<p class="pq">Are these objects made cooler by the melting of the ice?</p> -<p class="pa">They are made cooler by the melting of the ice.</p> -<p class="pq">What effect does salt have upon ice?</p> -<p class="pa">Salt makes the ice melt more rapidly.</p> -<p class="pq">When salt and ice are placed around a vessel of cream, what -takes place?</p> -<p class="pa">The salt causes the ice to melt quickly, and the ice -<span class="pb" id="Page_65">65</span> -in melting takes so much heat from the cream as to -freeze it, thus making it ice-cream.</p> -<h3 id="c17">Ebullition.</h3> -<p class="pq">What is meant by ebullition?</p> -<p class="pa"><i>Ebullition</i> means boiling.</p> -<p class="pq">When is a liquid in a state of ebullition?</p> -<p class="pa">When it has been heated so as to boil.</p> -<p class="pq">Can solids be made to boil?</p> -<p class="pa">No; liquids only can be made to boil.</p> -<p class="pq">What change takes place when a liquid boils?</p> -<p class="pa">The liquid is changed into a vapor or gas.</p> -<p class="pq">What becomes of water when it is boiled?</p> -<p class="pa">Water when boiled passes off into steam.</p> -<p class="pq">What is necessary to make water boil?</p> -<p class="pa">Heat is necessary to boil water or any other liquid.</p> -<p class="pq">How much heat is contained in boiling water?</p> -<p class="pa">Two hundred and twelve degrees of heat.</p> -<p class="pq">When water passes into steam, how much is its bulk increased?</p> -<p class="pa">About seventeen hundred times.</p> -<p class="pq">How much steam will a pint of water make?</p> -<p class="pa">A pint of water will make about seventeen hundred -pints of steam.</p> -<p class="pq">What do bubbles of boiling water contain?</p> -<p class="pa">They contain steam.</p> -<p class="pq">Why are these bubbles formed?</p> -<p class="pa">Because the water at the bottom of the vessel is -heated and changed into steam, which expands and -forms the bubbles.</p> -<div class="pb" id="Page_66">66</div> -<p class="pq">Why do these bubbles rise to the surface?</p> -<p class="pa">Because steam is lighter than water.</p> -<p class="pq">When does water simmer?</p> -<p class="pa">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.”</p> -<p class="pq">When does water boil?</p> -<p class="pa">Water boils when it becomes heated so that the bubbles -rise to the surface of the water before breaking.</p> -<p class="pq">Why will a vessel full of water run over when heated?</p> -<p class="pa">Because the water is expanded by heat, and some -of it must run over.</p> -<p class="pq">Why will the water boil over when the vessel is not full?</p> -<p class="pa">Because the steam, in trying to escape, throws some -of the water over the edge of the vessel.</p> -<h3 id="c18">Vaporization.</h3> -<p class="pq">When water is boiled, what change does it undergo?</p> -<p class="pa">It is changed into a vapor, called steam.</p> -<p class="pq">What is this process called?</p> -<p class="pa">It is called <i>Vaporization</i>.</p> -<p class="pq">What is meant by vaporization?</p> -<p class="pa">Vaporization is the changing of a liquid into a -vapor, by boiling.</p> -<p class="pq">What is necessary to change a liquid into a vapor?</p> -<p class="pa">Heat is necessary to change a liquid into a vapor.</p> -<div class="pb" id="Page_67">67</div> -<p class="pq">At what temperature is water changed into steam?</p> -<p class="pa">At the temperature of two hundred and twelve -degrees.</p> -<p class="pq">Are all liquids changed to vapor at the same temperature?</p> -<p class="pa">No; they vary very much in the amount of heat -required.</p> -<p class="pq">Ether boils at 96 degrees, alcohol at 176 degrees, and quicksilver -at a temperature of 662 degrees.</p> -<p class="pq">When water is changed into steam, what becomes of the steam?</p> -<p class="pa">The steam passes off into the air.</p> -<p class="pq">What does the air do with the steam?</p> -<p class="pa">The air condenses or changes it into very small particles -of water, so that it is seen like a fog or a cloud.</p> -<p class="pq">Can steam be seen?</p> -<p class="pa">No; steam is like the air in this respect, and cannot -be seen.</p> -<p class="pq">How can we prove this?</p> -<p class="pa">When water is boiled in a glass vessel, no steam is -visible, although it is there.</p> -<p class="pq">Cannot we see the steam coming from the spout of a tea-kettle?</p> -<p class="pa">No; we do not see anything for half an inch or more -from the spout.</p> -<p class="pq">Why does it become visible farther away?</p> -<p class="pa">Because the air condenses it, and then we see it in -its cloud-like state.</p> -<p class="pq">How much steam will one gallon of water make?</p> -<p class="pa">One gallon of water will make seventeen hundred -gallons of steam.</p> -<div class="pb" id="Page_68">68</div> -<p class="pq">When this steam is confined in a vessel, what does it try to -do?</p> -<p class="pa">It tries to expand or spread out in every direction.</p> -<p class="pq">Does it expand with much force?</p> -<p class="pa">Yes; it expands with so much force as frequently -to burst the vessel containing it.</p> -<p class="pq">Why is the lid of the tea-kettle lifted up when the water -boils?</p> -<p class="pa">Because the steam expands and raises the lid, so -that it can escape.</p> -<p class="pq">If the lid were fastened down, what would result?</p> -<p class="pa">The steam, in trying to escape, would burst the tea-kettle.</p> -<p class="pq">Why does water run out of the spout of the tea-kettle?</p> -<p class="pa">Because the steam in the tea-kettle presses on the -water with force enough to cause it to run out.</p> -<p class="pq">Why does steam burst vessels in which it is confined?</p> -<p class="pa">Because the vessels are not strong enough to keep -the steam from expanding.</p> -<p class="pq">How is the boiler of a steam-engine kept from bursting?</p> -<p class="pa">By means of a safety-valve.</p> -<p class="pq">What is a safety-valve?</p> -<p class="pa">It is a valve or opening through which steam may -escape.</p> -<p class="pq">What prevents all of the steam from escaping through the -valve?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_69">69</div> -<h3 id="c19">Evaporation.</h3> -<p class="pq">Does a wet blanket, when hung in the air, remain wet?</p> -<p class="pa">No; it soon becomes dry.</p> -<p class="pq">What becomes of the water in the blanket?</p> -<p class="pa">The water passes off into the air.</p> -<p class="pq">What is this process called?</p> -<p class="pa">It is called <i>Evaporation</i>.</p> -<p class="pq">What is meant by evaporation?</p> -<p class="pa">It is the gradual change of a liquid into a vapor.</p> -<p class="pq">When water is left in a shallow vessel, what becomes of it?</p> -<p class="pa">The water evaporates or passes off into the air.</p> -<p class="pq">Do liquids generally evaporate?</p> -<p class="pa">Yes; when exposed to the air, nearly all liquids -evaporate.</p> -<p class="pq">Does water change into steam when it evaporates?</p> -<p class="pa">Water does not change into steam when it evaporates.</p> -<p class="pq">How, then, does water evaporate?</p> -<p class="pa">It passes off into the air in particles or drops so -very small as not to be visible.</p> -<p class="pq">Is evaporation always going on around us?</p> -<p class="pa">Yes; wherever there is water exposed to the air, -evaporation is taking place.</p> -<p class="pq">Where does the most evaporation take place?</p> -<p class="pa">From the surface of the rivers, the lakes, and the -ocean.</p> -<p class="pq">What becomes of the water when evaporated?</p> -<p class="pa">It is formed into fogs and clouds.</p> -<div class="pb" id="Page_70">70</div> -<p class="pq">In what kind of air does evaporation go on most rapidly?</p> -<p class="pa">In air that is warm and dry.</p> -<p class="pq">Does evaporation take place in cool air?</p> -<p class="pa">It does; because clothing will dry during the coldest -weather of winter.</p> -<p class="pq">Why does water evaporate more in warm air than in cold air?</p> -<p class="pa">Because the warm air can hold more of the fine -particles of water, and, therefore, can absorb more.</p> -<p class="pq">Why does water evaporate more slowly in damp weather than -in dry weather?</p> -<p class="pa">Because the damp air contains so much moisture -that but little more can pass off into it.</p> -<p class="pq">What example have we of this?</p> -<p class="pa">Wet clothes dry much more slowly when the air is -damp than when it is dry.</p> -<p class="pq">Why do clothes dry more rapidly in windy weather?</p> -<p class="pa">Because the damp air in contact with them is blown -away, and the dry air is constantly taking its place.</p> -<p class="pq">Is heat necessary in evaporation?</p> -<p class="pa">Heat is necessary to produce evaporation.</p> -<p class="pq">Why do our hands feel cold when wet?</p> -<p class="pa">Because the water on our hands evaporates, and -thus heat is taken from them.</p> -<p class="pq">Why is the air cooler after a rain?</p> -<p class="pa">Because evaporation then takes place from every -wet object, and this absorbs heat from the air.</p> -<p class="pq">Why does watering streets and roads cool the air?</p> -<p class="pa">Because the water evaporates and absorbs heat -from the air, making it cooler.</p> -<div class="pb" id="Page_71">71</div> -<p class="pq">Why do we feel cold when our clothes are wet?</p> -<p class="pa">Because the moisture in them evaporates and absorbs -heat from our bodies.</p> -<p class="pq">Why is wet land always cold?</p> -<p class="pa">Because so much heat is absorbed from the land in -the evaporation which is constantly taking place.</p> -<p class="pq">What becomes of the dew that collects on plants and other -objects at night?</p> -<p class="pa">It is evaporated.</p> -<p class="pq">How does the sun hasten the evaporation of the dew?</p> -<p class="pa">By supplying the heat needed to change the dew -into a vapor, the sun hastens its evaporation.</p> -<p class="pq">Why do bread and biscuit become hard when kept a few days?</p> -<p class="pa">Because the moisture in them passes off into the -air, and thus they become dry and hard.</p> -<p class="pq">How can they be kept moist and soft for a longer time?</p> -<p class="pa">By keeping them in a covered vessel, so that evaporation -will take place more slowly, they may be kept -moist for a longer time.</p> -<h3 id="c20">Radiation.</h3> -<p class="pq">When we stand near the fire, why do we feel warm?</p> -<p class="pa">Because the fire gives off heat which makes us feel -warm.</p> -<p class="pq">How does the fire give off heat?</p> -<p class="pa">It gives off heat in straight lines, and in every -direction.</p> -<p class="pq">What is this giving off of heat called?</p> -<p class="pa">It is called <i>Radiation</i> of heat.</p> -<div class="pb" id="Page_72">72</div> -<p class="pq">Does the sun radiate heat?</p> -<p class="pa">The sun does radiate or give off both heat and light.</p> -<p class="pq">How do heat and light from the sun come to us?</p> -<p class="pa">They come together in the sunbeam.</p> -<p class="pq">Does heat travel as rapidly as light?</p> -<p class="pa">Yes; heat and light travel together at the rate of -200,000 miles in a second of time.</p> -<p class="pq">Do all substances radiate heat equally well?</p> -<p class="pa">No; some substances are much better radiators than -others.</p> -<p class="pq">What kind of objects are the best radiators?</p> -<p class="pa">Those with rough and dark surfaces.</p> -<p class="pq">What kind of objects are the poorest radiators?</p> -<p class="pa">Those with smooth and bright surfaces.</p> -<p class="pq">Why does water keep hot a long time in a bright tin vessel?</p> -<p class="pa">Because the tin is a poor radiator, and prevents the -heat of the water from passing off into the air.</p> -<p class="pq">Why does water soon cool in a dull earthen vessel?</p> -<p class="pa">Because the earthen vessel is a good radiator, and -gives off the heat of the water rapidly.</p> -<p class="pq">If the bright tin vessel be painted, will the water keep hot so -long?</p> -<p class="pa">No; the tin becomes a good radiator when painted, -and the water is soon cooled.</p> -<p class="pq">Why does a stove cool so quickly?</p> -<p class="pa">Because the iron is a good radiator, and gives off -its heat rapidly.</p> -<p class="pq">Do the rays from the sun give off much heat while passing -through the air?</p> -<p class="pa">The rays from the sun give off about one-fourth of -their heat while passing through the air.</p> -<div class="pb" id="Page_73">73</div> -<p class="pq">What becomes of the remaining heat from the sun?</p> -<p class="pa">It is absorbed or reflected at the earth’s surface.</p> -<p class="pq">What does the earth do with the heat?</p> -<p class="pa">The earth radiates the heat, and thus warms the air.</p> -<h3 id="c21">Reflection.</h3> -<p class="pq">When a piece of bright tin is held near the fire, does the tin -become hot?</p> -<p class="pa">No; the tin is scarcely warmed.</p> -<p class="pq">What does the tin do with the rays of heat from the fire?</p> -<p class="pa">The tin throws them back from its surface.</p> -<p class="pq">What is this called?</p> -<p class="pa">It is called <i>Reflection</i> of heat.</p> -<p class="pq">Do all substances reflect heat equally well?</p> -<p class="pa">No; some reflect much more heat than others.</p> -<p class="pq">What kind of objects reflect heat the best?</p> -<p class="pa">Those with bright and smooth surfaces.</p> -<p class="pq">What kind of objects reflect heat the least?</p> -<p class="pa">Those with rough and dark surfaces.</p> -<p class="pq">Does bright tin reflect heat better than glass?</p> -<p class="pa">Yes; bright tin reflects about eight times as much -heat as glass does.</p> -<p class="pq">Why does it take so long to heat water in a new tin cup?</p> -<p class="pa">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.</p> -<p class="pq">Why does a piece of tin or other bright metal, placed under a -stove, protect the carpet from burning?</p> -<p class="pa">Because the tin being a good reflector, remains cool, -and cannot burn the carpet under it.</p> -<div class="pb" id="Page_74">74</div> -<p class="pq">Why will not a piece of iron protect the carpet from burning?</p> -<p class="pa">Because the iron, being a poor reflector, soon -becomes heated, so as to burn the carpet under it.</p> -<h3 id="c22">Absorption.</h3> -<p class="pq">When a piece of iron is held near a fire, does the iron become -hot?</p> -<p class="pa">Yes; the iron soon becomes hot.</p> -<p class="pq">How is the iron heated?</p> -<p class="pa">The iron absorbs or takes in the heat from the fire.</p> -<p class="pq">What is this power to take in the heat called?</p> -<p class="pa">It is called <i>Absorption</i> of heat.</p> -<p class="pq">What is this process like?</p> -<p class="pa">It is somewhat like a sponge, that absorbs or sucks -up moisture.</p> -<p class="pq">Do all substances absorb heat equally well?</p> -<p class="pa">No; some substances absorb more heat than others.</p> -<p class="pq">What kind of objects absorb heat the best?</p> -<p class="pa">Those with rough surfaces absorb heat the best.</p> -<p class="pq">What kind of objects absorb heat the least?</p> -<p class="pa">Those with bright and smooth surfaces.</p> -<p class="pq">What colored objects absorb heat the best?</p> -<p class="pa">Objects of a dark or black color.</p> -<p class="pq">Do objects that absorb heat well, radiate well?</p> -<p class="pa">Yes; good absorbers are good radiators.</p> -<p class="pq">Why is this?</p> -<p class="pa">Because every substance must absorb heat before it -can have heat to give off.</p> -<p class="pq">Do objects that absorb heat well, reflect it well?</p> -<p class="pa">No; objects that absorb heat well, cannot reflect it -well.</p> -<div class="pb" id="Page_75">75</div> -<p class="pq">Why is this?</p> -<p class="pa">Because a ray of heat cannot be reflected from an -object, and be absorbed by that object, both at the -same time.</p> -<p class="pq">Why is water rapidly heated in a tea-kettle?</p> -<p class="pa">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.</p> -<p class="pq">Why does black cloth, when spread on the snow, cause the -snow to melt more rapidly?</p> -<p class="pa">Because the cloth absorbs heat from the rays of the -sun, and, therefore, melts the snow beneath it.</p> -<p class="pq">Why does not white muslin, when spread on the snow, hasten -its melting?</p> -<p class="pa">Because the white muslin reflects the heat from the -rays of the sun, so that but little of it enters the -snow.</p> -<p class="pq">Does the snow receive much heat from the sun?</p> -<p class="pa">Snow is a good reflector, and throws back most of -the heat from the sun.</p> -<p class="pq">What causes the snow to melt?</p> -<p class="pa">The heat it receives from touching the earth, causes -the most of it to melt.</p> -<p class="pq">Why does snow melt first on dark earth?</p> -<p class="pa">Because dark earth is a good radiator, and gives -off much heat to the snow.</p> -<p class="pq">Why does snow remain longest on light-colored earth?</p> -<p class="pa">Because the light-colored earth is a poor radiator, -and gives off but little heat to the snow.</p> -<div class="pb" id="Page_76">76</div> -<p class="pq">Why does the air feel cold in winter time?</p> -<p class="pa">Because it is so much cooler than our bodies that -it absorbs heat from them rapidly, and thus makes -them feel cold.</p> -<p class="pq">Why does the air feel warm in summer time?</p> -<p class="pa">Because it is then nearly as warm as our bodies, -and, therefore, absorbs heat from them slowly.</p> -<p class="pq">Why are greasy shoes warmer than those polished with blacking?</p> -<p class="pa">Because the greasy shoes absorb heat from the sun, -but the polished shoes throw off the heat of the sun -by reflection.</p> -<p class="pq">What may we remember about heat?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img025.jpg" alt="An outdoor walk." width="200" height="181" /> -</div> -<div class="pb" id="Page_77">77</div> -<div class="img"> -<img src="images/img026.jpg" alt="Decoration." width="500" height="186" /> -</div> -<h2 id="c23">CHAPTER IV. -<br /><span class="sc">The Air.</span></h2> -<div class="img"> -<img src="images/img027.jpg" alt="Illustrated Capital W" width="168" height="200" /> -</div> -<p class="pq">Where do we find air?</p> -<p class="pa">We find air everywhere around the -earth.</p> -<p class="pq">Does the air fill every place?</p> -<p class="pa">The air fills every place not filled by -some other substance.</p> -<p class="pq">When we say a bottle is empty, what do we mean?</p> -<p class="pa">We mean that it does not contain anything but air.</p> -<p class="pq">What do all vessels contain when empty?</p> -<p class="pa">All vessels contain air when seemingly empty.</p> -<p class="pq">Do all liquids contain air?</p> -<p class="pa">Yes; all liquids contain air.</p> -<p class="pq">Where is the air in a liquid?</p> -<p class="pa">The air is among the particles which compose the -liquid.</p> -<p class="pq">Is there air in water?</p> -<p class="pa">Yes; there is air in water.</p> -<p class="pq">Of what use is the air in the water?</p> -<p class="pa">It is used by the fish and other creatures that live -in the water.</p> -<div class="pb" id="Page_78">78</div> -<p class="pq">How do the fish and other creatures use it?</p> -<p class="pa">They breathe it into their lungs, and it supports -life in them, the same as the air which we breathe -supports life in us.</p> -<p class="pq">Do solids contain air?</p> -<p class="pa">Yes; solids contain more or less air.</p> -<p class="pq">Where is the air in fur and in wool?</p> -<p class="pa">It is among the hairs of the fur and the wool.</p> -<p class="pq">Where is the air in cotton and linen goods?</p> -<p class="pa">It is among the fibres of the cotton and the linen.</p> -<p class="pq">Where is the air in fruit?</p> -<p class="pa">It is among the particles composing the fruit.</p> -<p class="pq">Where is the air in wood?</p> -<p class="pa">It is in the pores or small spaces in the wood.</p> -<p class="pq">Is there air in the earth?</p> -<p class="pa">Yes; there is air among the particles of the earth.</p> -<p class="pq">How deep in the earth does the air extend?</p> -<p class="pa">We cannot tell; but both air and water have been -found several hundred feet below the earth’s surface.</p> -<p class="pq">Of what use is the air in the earth?</p> -<p class="pa">It is breathed by the worms and other creatures -that live in the earth.</p> -<p class="pq">When part of the air is taken from a room, what becomes of -the air left in the room?</p> -<p class="pa">It expands so as to fill the whole room.</p> -<p class="pq">Are the particles of air so close together after it expands as -they were before?</p> -<p class="pa">They are not so close together.</p> -<p class="pq">When air is thus expanded, what is said of it?</p> -<p class="pa">It is said to be <i>rare</i>.</p> -<div class="pb" id="Page_79">79</div> -<p class="pq">When pressure is put upon air in a vessel, what becomes of -the air?</p> -<p class="pa">It is forced or pressed into a smaller space, and, -therefore, occupies less room.</p> -<p class="pq">When pressure is put upon air, are its particles closer together?</p> -<p class="pa">They are closer together.</p> -<p class="pq">When air is thus compressed, what is said of it?</p> -<p class="pa">It is said to be <i>dense</i>.</p> -<p class="pq">Can air be much condensed by pressure?</p> -<p class="pa">It has been condensed so much that 100 cubic feet -of air was reduced to the bulk of only one cubic foot.</p> -<p class="pq">What makes the air so dense as it is at the surface of the earth?</p> -<p class="pa">The pressure of the air above the surface.</p> -<p class="pq">Does the air then become less dense the higher we ascend?</p> -<p class="pa">Yes; because there is less air above to press upon it.</p> -<p class="pq">Does the air become more dense the deeper we go in the earth?</p> -<p class="pa">It does; because there is more air above to press -upon it.</p> -<p class="pq">How can we understand this?</p> -<p class="pa">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.</p> -<p class="pq">How high does the air extend above the earth?</p> -<p class="pa">It is supposed to extend to the height of <i>fifty</i> miles.</p> -<p class="pq">Do these fifty miles of air press upon the earth?</p> -<p class="pa">They do press upon the earth.</p> -<p class="pq">With how much force does the air press on the earth?</p> -<p class="pa">With a force of <i>fifteen</i> pounds on every square inch -of surface.</p> -<div class="pb" id="Page_80">80</div> -<p class="pq">Then has the air weight?</p> -<p class="pa">The air has weight.</p> -<p class="pq">Why do we not feel the weight of the air?</p> -<p class="pa">Because it presses with equal force, in every direction; -up, down, and on every side.</p> -<p class="pq">How can we show that the air presses upwards?</p> -<p class="pa">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 <a href="#fig12">Fig. 12</a>.</p> -<div class="img" id="fig12"> -<img src="images/img028.jpg" alt="" width="250" height="247" /> -<p class="pcap">Fig. 12.</p> -</div> -<p class="pq">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?</p> -<p class="pa">The pressure of the air on the water in the bucket, -keeps the water in the tumbler.</p> -<p class="pq">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?</p> -<p class="pa">It will, the same as it keeps it in the tumbler.</p> -<p class="pq">How high will the air keep the water in the pipe?</p> -<p class="pa">It will keep the water to the height of <i>thirty-four</i> feet.</p> -<p class="pq">Why does the air keep the water up thirty-four feet?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_81">81</div> -<p class="pq">Does the weight of the air exactly balance the weight of the -column of water?</p> -<p class="pa">It does; they balance each other.</p> -<p class="pq">Is water heavier than air?</p> -<p class="pa">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.</p> -<p class="pq">Is quicksilver heavier than water?</p> -<p class="pa">Yes; quicksilver is about thirteen times heavier -than water.</p> -<p class="pq">How high a column of quicksilver can the air keep up?</p> -<p class="pa">The air can keep up a column of quicksilver only -thirty inches high.</p> -<p class="pq">Why is this?</p> -<p class="pa">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.</p> -<p class="pq">How can this be shown?</p> -<p class="pa">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 <a href="#fig13">Fig. 13</a>.</p> -<div class="img" id="fig13"> -<img src="images/img029.jpg" alt="" width="200" height="330" /> -<p class="pcap">Fig. 13.</p> -</div> -<p class="pq">If the cork be removed from the glass -tube, what will become of the quicksilver?</p> -<p class="pa">It will immediately sink into the -basin below.</p> -<div class="pb" id="Page_82">82</div> -<p class="pq">Why will the quicksilver sink into the basin?</p> -<p class="pa">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.</p> -<p class="pq">Does the weight of the air vary at different times?</p> -<p class="pa">Yes; the air does vary in weight at different times.</p> -<p class="pq">How can we tell when the air changes in weight?</p> -<p class="pa">By means of a <i>Barometer</i>.</p> -<p class="pq">What is a barometer?</p> -<p class="pa">It is an instrument having a tube filled with quicksilver.</p> -<p class="pq">When the air is heavier, how does it affect the quicksilver?</p> -<p class="pa">The quicksilver rises in the tube.</p> -<p class="pq">When the air is lighter, how does it affect the quicksilver?</p> -<p class="pa">The quicksilver sinks in the tube.</p> -<p class="pq">What is the use of the barometer?</p> -<p class="pa">It shows the changes about to take place in the -weather.</p> -<p class="pq">When the quicksilver rises, what kind of weather is indicated?</p> -<p class="pa">It indicates fair weather.</p> -<p class="pq">When the quicksilver sinks, what kind of weather is indicated?</p> -<p class="pa">It indicates cloudy or stormy weather.</p> -<p class="pq">Where is the barometer most useful?</p> -<p class="pa">It is most useful at sea, to warn the sailors of -coming storms.</p> -<p class="pq">What use is it to farmers?</p> -<p class="pa">They can use it as a guide when planting and -gathering their crops, and thus be better prepared for -stormy weather.</p> -<div class="pb" id="Page_83">83</div> -<p class="pq">For what else may the barometer be used?</p> -<p class="pa">It may be used to tell the height or depth of any -place.</p> -<p class="pq">Does the air become lighter as we ascend?</p> -<p class="pa">The air does become lighter as we ascend.</p> -<p class="pq">How does this affect the barometer?</p> -<p class="pa">As the air becomes lighter, the quicksilver in the -barometer sinks lower.</p> -<p class="pq">At what height does the quicksilver stand at the level of the sea?</p> -<p class="pa">It stands at the height of thirty inches.</p> -<p class="pq">When we ascend a mountain 10,000 feet high, how much does -the quicksilver sink?</p> -<p class="pa">It sinks ten inches, and, therefore, stands at the -height of only twenty inches.</p> -<p class="pq">Does the same thing take place when we ascend in a balloon?</p> -<p class="pa">Yes; the higher the balloon ascends, the lower the -quicksilver in the barometer sinks.</p> -<p class="pq">When we descend into the mines dug in the earth, does the -air become heavier?</p> -<p class="pa">It does; and the quicksilver in the barometer rises.</p> -<p class="pq">At what temperature does water boil?</p> -<p class="pa">Water boils at a temperature of <i>two hundred and -twelve</i> degrees.</p> -<p class="pq">When the weight of the air is taken off, will the water boil -at a lower temperature?</p> -<p class="pa">Water will then boil at a lower temperature.</p> -<p class="pq">Where will water boil at the lowest temperature, on a mountain, -or in a valley?</p> -<p class="pa">It will boil at the lowest temperature on a mountain.</p> -<div class="pb" id="Page_84">84</div> -<p class="pq">At how much lower temperature will water boil, as we ascend?</p> -<p class="pa">Water will boil at one degree less for every five -hundred feet we ascend.</p> -<p class="pq">How high is the city of Quito, in South America?</p> -<p class="pa">Quito is nine thousand feet above the level of the sea.</p> -<p class="pq">At what temperature does water boil there?</p> -<p class="pa">Water boils, in Quito, at a temperature of one hundred -and ninety-four degrees.</p> -<p class="pq">Will an egg boil hard at Quito?</p> -<p class="pa">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.</p> -<p class="pq">Do persons, while ascending mountains, often have difficulty -in cooking their food?</p> -<p class="pa">They do, on account of water boiling there at a -temperature below two hundred and twelve degrees.</p> -<p class="pq">What is a <i>vacuum</i>?</p> -<p class="pa">A <i>vacuum</i> is a place that does not contain anything, -not even air.</p> -<p class="pq">When a vessel contains water and we pour the water out, does -that produce a vacuum?</p> -<p class="pa">No; because air passes in and fills the vessel.</p> -<p class="pq">How then can a vacuum be produced?</p> -<p class="pa">A vacuum in any place, can be produced only by -drawing the air out of it.</p> -<p class="pq">When a vacuum is produced, what does the surrounding air -try to do?</p> -<p class="pa">It tries to get in so as to fill the vacuum.</p> -<p class="pq">With what force does the air try to fill a vacuum?</p> -<p class="pa">With a force of fifteen pounds on every square inch -of surface.</p> -<div class="pb" id="Page_85">85</div> -<p class="pq">When a vacuum is produced in contact with a liquid, what -does the liquid do?</p> -<p class="pa">The liquid tries to pass in to fill the vacuum.</p> -<p class="pq">Can there be a vacuum in contact with a gas or a liquid?</p> -<p class="pa">No; because the gas or the liquid will instantly fill -the vacant space.</p> -<p class="pq">Why does water make a gurgling noise when poured out of a -bottle?</p> -<p class="pa">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.</p> -<p class="pq">Why does water make a gurgling noise when running out of a -barrel?</p> -<p class="pa">Because the air passes into the barrel at the opening -where the water passes out, and thus the noise is made.</p> -<p class="pq">How can the gurgling noise be prevented?</p> -<p class="pa">By making an opening at the top of the barrel, so -as to admit the air.</p> -<p class="pq">Does water flow freely from a vessel that has but one opening?</p> -<p class="pa">No; because part of this opening is needed to -admit air into the vessel.</p> -<p class="pq">Why does the water flow freely, when there is an opening at -the top as well as at the bottom of the vessel?</p> -<p class="pa">Because air is admitted at the top, and there is -then nothing to obstruct the flow of water from the -opening at the bottom.</p> -<p class="pq">What causes fruit, vegetables, and meat to spoil?</p> -<p class="pa">The air causes them to spoil.</p> -<p class="pq">How do we know this?</p> -<p class="pa">Because when kept from the air they remain good -for a long time.</p> -<div class="pb" id="Page_86">86</div> -<p class="pq">What is this process of keeping fruit called?</p> -<p class="pa">It is called canning.</p> -<p class="pq">How is fruit canned?</p> -<p class="pa">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.</p> -<p class="pq">What becomes of the steam in the jar or can?</p> -<p class="pa">As the fruit cools, the steam is condensed to water, -and a vacuum is produced, so that the fruit is kept -from the air.</p> -<p class="pq">What is the plaything which boys call a sucker?</p> -<p class="pa">A <i>sucker</i> is a round piece of leather, with a string -fastened to its centre.</p> -<p class="pq">How is the sucker used?</p> -<p class="pa">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 -<a href="#fig14">Fig. 14</a>.</p> -<div class="img" id="fig14"> -<img src="images/img030.jpg" alt="" width="200" height="305" /> -<p class="pcap">Fig. 14.</p> -</div> -<p class="pq">What keeps the leather against the stone?</p> -<p class="pa">The pressure of the air upon the -leather keeps it against the stone.</p> -<p class="pq">With how much force does the air press the leather against -the stone?</p> -<p class="pa">With a force of fifteen pounds on every square -inch.</p> -<div class="pb" id="Page_87">87</div> -<p class="pq">When there is air between the stone and the leather, can the -stone be lifted with the sucker?</p> -<p class="pa">It cannot; because the air under the leather will -press it up as much as the air above will press it -down.</p> -<p class="pq">How does a fly walk upon the ceiling?</p> -<p class="pa">The fly’s feet are much like the sucker, and are -kept against the ceiling by the pressure of the air.</p> -<p class="pq">How do snails cling to objects?</p> -<p class="pa">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.</p> -<p class="pq">What is a <i>siphon</i>?</p> -<p class="pa">A <i>siphon</i> is a bent tube, having one branch longer -than the other, as is represented in <a href="#fig15">Fig. 15</a>.</p> -<div class="img" id="fig15"> -<img src="images/img031.jpg" alt="" width="250" height="295" /> -<p class="pcap">Fig. 15.</p> -</div> -<p class="pq">For what is the siphon used?</p> -<p class="pa">The siphon is used to draw liquids from one vessel -into another.</p> -<div class="pb" id="Page_88">88</div> -<p class="pq">How can water be drawn from a vessel by means of a -siphon?</p> -<p class="pa">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.</p> -<p class="pq">Where must the open end of the short branch be kept?</p> -<p class="pa">It must be kept beneath the surface of the water, -so as to prevent air from entering the tube.</p> -<p class="pq">Where must the open end of the long branch be kept?</p> -<p class="pa">It must be kept lower than the open end of the -short branch.</p> -<p class="pq">While the water is running out of the long branch, what is -produced at the bend of the tube?</p> -<p class="pa">The tendency is to produce a vacuum at the bend -of the tube.</p> -<p class="pq">What is the result of this?</p> -<p class="pa">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.</p> -<p class="pq">How do boys often suck cider from a barrel?</p> -<p class="pa">By means of a straw.</p> -<p class="pq">How is this done?</p> -<p class="pa">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.</p> -<p class="pq">What makes the cider rise to fill the vacuum in the straw?</p> -<p class="pa">The pressure of the air on the cider in the barrel.</p> -<div class="pb" id="Page_89">89</div> -<p class="pq">To what height will water rise in a tube to fill a vacuum?</p> -<p class="pa">The water will rise to the height of thirty-four feet.</p> -<p class="pq">To what useful purpose is this applied?</p> -<p class="pa">It is applied to making <i>pumps</i>.</p> -<p class="pq">Of what material are pumps made?</p> -<p class="pa">Either of wood, or of metal, such as lead or iron.</p> -<p class="pq">What is necessary in regard to the tube or pump-tree?</p> -<p class="pa">It must always be air-tight.</p> -<p class="pq">Why must the tube or pump-tree be air-tight?</p> -<p class="pa">So that a vacuum may be produced in it.</p> -<p class="pq">How is this accomplished?</p> -<p class="pa">By means of two valves in the pump-tree.</p> -<p class="pq">What is a valve like?</p> -<p class="pa">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.</p> -<p class="pq">Where is one valve placed?</p> -<p class="pa">One valve is placed in the lower part of the pump-tube.</p> -<p class="pq">How near the water must this valve be?</p> -<p class="pa">It must be within thirty-four feet of the water.</p> -<p class="pq">Why must the lower valve be within thirty-four feet of the -water in the well?</p> -<p class="pa">Because the air can force water up a tube only to -the height of thirty-four feet.</p> -<p class="pq">Does this valve move about in the pump-tube?</p> -<p class="pa">No; it remains fixed in one place.</p> -<p class="pq">Is this valve air-tight?</p> -<p class="pa">Yes; the lower valve or bucket in a pump must -always be air-tight.</p> -<div class="pb" id="Page_90">90</div> -<p class="pq">Where is the upper valve?</p> -<p class="pa">It is fastened on the end of a rod, which is attached -to the pump-handle.</p> -<p class="pq">Does this valve remain fixed in one place?</p> -<p class="pa">No; it moves up and down when the pump-handle -is moved.</p> -<p class="pq">Is the upper valve air-tight?</p> -<p class="pa">No; the upper valve is not air-tight.</p> -<div class="img" id="fig16"> -<img src="images/img032.jpg" alt="" width="223" height="374" /> -<p class="pcap">Fig. 16.</p> -</div> -<div class="img" id="fig17"> -<img src="images/img032a.jpg" alt="" width="228" height="374" /> -<p class="pcap">Fig. 17.</p> -</div> -<div class="img" id="fig18"> -<img src="images/img032b.jpg" alt="" width="218" height="375" /> -<p class="pcap">Fig. 18.</p> -</div> -<p class="pq">Explain the working of a pump.</p> -<p class="pa">When the pump-handle is lowered, the water, above -<span class="pb" id="Page_91">91</span> -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.</p> -<p class="pq">What does <a href="#fig16">Fig. 16</a> represent?</p> -<p class="pa">It represents a pump, when it is not working.</p> -<p class="pq">How are the valves?</p> -<p class="pa">The valves are both closed.</p> -<p class="pq">What keeps the water above the lower valve?</p> -<p class="pa">The lower valve is air-tight, and, therefore, water-tight, -so that it keeps the water above it.</p> -<p class="pq">What keeps the water in the tube below the lower valve?</p> -<p class="pa">The pressure of the air on the water in the well, -forces it up the tube to the lower valve.</p> -<p class="pq">What does <a href="#fig17">Fig. 17</a> represent?</p> -<p class="pa">It represents a pump, when the handle is being -raised.</p> -<p class="pq">How is the lower valve?</p> -<p class="pa">The lower valve is closed.</p> -<p class="pq">Why is the lower valve closed?</p> -<p class="pa">So as to prevent the water from being forced back -again into the well.</p> -<p class="pq">How is the upper valve?</p> -<p class="pa">The upper valve is open.</p> -<div class="pb" id="Page_92">92</div> -<p class="pq">Why is the upper valve open?</p> -<p class="pa">So that the water may pass up through the valve, -while the valve is descending.</p> -<p class="pq">What does <a href="#fig18">Fig. 18</a> represent?</p> -<p class="pa">It represents a pump when the handle is being -lowered.</p> -<p class="pq">How is the upper valve?</p> -<p class="pa">The upper valve is closed.</p> -<p class="pq">Why is the upper valve closed?</p> -<p class="pa">So as to lift up the water which is above it.</p> -<p class="pq">What becomes of the water which is lifted up?</p> -<p class="pa">It runs out of the spout of the pump.</p> -<p class="pq">As the upper valve is raised up, what is produced under it?</p> -<p class="pa">A vacuum is produced under it.</p> -<p class="pq">How is this vacuum filled?</p> -<p class="pa">The water below the lower valve is forced up, so as -to fill the vacuum.</p> -<p class="pq">How is the lower valve?</p> -<p class="pa">The lower valve is open.</p> -<p class="pq">Why is the lower valve open?</p> -<p class="pa">So that the water may pass through and fill the -vacuum above the valve.</p> -<p class="pq">Where are pumps mostly used?</p> -<p class="pa">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.</p> -<p class="pq">When does smoke ascend through the air?</p> -<p class="pa">When smoke is lighter than the air, it ascends -through it.</p> -<div class="pb" id="Page_93">93</div> -<p class="pq">When does smoke descend through the air?</p> -<p class="pa">When smoke is heavier than the air, it descends -through it.</p> -<p class="pq">Why does a cork rise to the surface of the water?</p> -<p class="pa">Because the cork is lighter than the same bulk of -water; hence, it ascends to the surface.</p> -<p class="pq">Why does heated air ascend?</p> -<p class="pa">Because it is lighter than the cold air.</p> -<p class="pq">If a feather be put in the heated air, will it be carried up with -the air?</p> -<p class="pa">The feather will be carried up with the air.</p> -<p class="pq">If warm air be confined in a bag, will it rise up and carry the -bag with it?</p> -<p class="pa">It will rise up and carry the bag with it.</p> -<p class="pq">What might this bag be called?</p> -<p class="pa">It might be called a <i>balloon</i>.</p> -<p class="pq">What is a balloon?</p> -<p class="pa">It is a bag filled with gas, and used for sailing in -the air.</p> -<div class="img" id="fig19"> -<img src="images/img033.jpg" alt="" width="400" height="311" /> -<p class="pcap">Fig. 19.</p> -</div> -<div class="pb" id="Page_94">94</div> -<p class="pq">What is placed over this bag?</p> -<p class="pa">A network of ropes is placed over it, and they are -so arranged as to extend beneath the bag, as is represented -in <a href="#fig19">Fig. 19</a>.</p> -<p class="pq">What is fastened to these ropes?</p> -<p class="pa">A car or basket is fastened to these ropes.</p> -<p class="pq">What is the car or basket for?</p> -<p class="pa">It is to carry up those who wish to ascend with the -balloon.</p> -<p class="pq">How large are balloons usually made?</p> -<p class="pa">They are from twenty to thirty feet in diameter.</p> -<p class="pq">With what are balloons filled?</p> -<p class="pa">Balloons are filled with a gas lighter than air.</p> -<p class="pq">What gas is often used for filling balloons?</p> -<p class="pa">The coal gas, such as is used to light our cities at -night.</p> -<p class="pq">What lighter kind of gas is sometimes used?</p> -<p class="pa">Hydrogen gas is sometimes used.</p> -<p class="pq">Is hydrogen gas much lighter than the air?</p> -<p class="pa">It is so much lighter than the air, that fifteen cubic -feet of hydrogen gas weigh no more than one cubic -foot of air.</p> -<p class="pq">How much less will a balloon thirty feet in diameter weigh, -when filled with hydrogen gas, than when filled with air?</p> -<p class="pa">It will weigh about ten thousand pounds less.</p> -<p class="pq">How much less, when filled with coal gas than when filled -with air?</p> -<p class="pa">About two thousand pounds less.</p> -<div class="pb" id="Page_95">95</div> -<p class="pq">How much force then is necessary to hold the balloon filled -with coal gas to the earth?</p> -<p class="pa">A force of two thousand pounds, because the balloon -is two thousand pounds lighter than the same bulk of -air.</p> -<p class="pq">If a weight of only one thousand pounds were fastened to the -balloon, what would result?</p> -<p class="pa">The balloon would rise in the air and carry the -weight of one thousand pounds with it.</p> -<p class="pq">How high will the balloon ascend?</p> -<p class="pa">It will ascend until the air displaced is of the same -weight as the balloon.</p> -<p class="pq">What will then become of the balloon?</p> -<p class="pa">It will be carried about in the currents of air.</p> -<p class="pq">With what is the car attached to the balloon loaded?</p> -<p class="pa">It is usually loaded with bags of sand, which serve -as a weight to keep it from rising too high.</p> -<p class="pq">When those in the car wish to go higher, what do they do?</p> -<p class="pa">They throw the sand out, so as to lighten the car; -then it will rise higher.</p> -<p class="pq">When they wish to descend, what do they do?</p> -<p class="pa">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.</p> -<p class="pq">How is the valve at the top of the balloon opened and closed?</p> -<p class="pa">It is opened and closed by means of small ropes, -reaching from the valve to the car beneath.</p> -<p class="pq">Can the balloon be made to go in any one direction?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_96">96</div> -<h3 id="c24">Winds.</h3> -<p class="pq">What is wind?</p> -<p class="pa"><i>Wind</i> is air moving from one place to another.</p> -<p class="pq">Does the air move in every direction?</p> -<p class="pa">It does move in every direction, upward, downward, -and to every side.</p> -<p class="pq">When the wind blows from the north towards the south, what -is it called?</p> -<p class="pa">It is called a north wind.</p> -<p class="pq">What is it called, when it blows from other directions?</p> -<p class="pa">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.</p> -<p class="pq">What causes the air to move from place to place?</p> -<p class="pa">Heat causes the air to move from place to place.</p> -<p class="pq">When air is heated, what change takes place?</p> -<p class="pa">The heated air ascends, and colder air moves in to -fill its place; thus causing the wind to blow.</p> -<p class="pq">What illustration have we of this?</p> -<p class="pa">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.</p> -<p class="pq">Why is this?</p> -<p class="pa">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.</p> -<p class="pq">Why does the wind blow towards a fire in the open air?</p> -<p class="pa">Because the air near the fire becomes heated, and -<span class="pb" id="Page_97">97</span> -ascends rapidly, so that the air rushes in from every -direction to fill its place.</p> -<p class="pq">What does the earth do with the heat it receives from the sun?</p> -<p class="pa">The earth gives off its heat to the air nearest to it.</p> -<p class="pq">What becomes of this heated air?</p> -<p class="pa">This heated air ascends, and colder air comes to -take its place; thus producing winds.</p> -<p class="pq">Are all parts of the earth heated alike by the sun?</p> -<p class="pa">No; they are not all heated alike.</p> -<p class="pq">How does this unequal heating affect the air?</p> -<p class="pa">It causes currents in the air all over the world.</p> -<p class="pq">Does water give off heat as rapidly as the earth does?</p> -<p class="pa">No; water is a poorer radiator of heat than the -earth is.</p> -<p class="pq">On which will the air be the cooler when the sun shines, on -the earth, or on the water?</p> -<p class="pa">It will be cooler on the water than on the earth.</p> -<p class="pq">Why does a sea-breeze feel cool in the daytime?</p> -<p class="pa">Because the air over the water, in the daytime, is -less heated than the air over the land; hence, the sea-breeze -feels cool.</p> -<p class="pq">Why does the earth become colder than the water at night?</p> -<p class="pa">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.</p> -<p class="pq">Why does a land-breeze feel cool at night?</p> -<p class="pa">Because the air over the land at night is less heated -than the air over the water; hence, the land-breeze -feels cool.</p> -<div class="pb" id="Page_98">98</div> -<p class="pq">How rapidly does the wind move?</p> -<p class="pa">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.</p> -<p class="pq">Of what use is the wind?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img034.jpg" alt="Paddlewheel steamboat." width="500" height="228" /> -</div> -<div class="pb" id="Page_99">99</div> -<div class="img"> -<img src="images/img035.jpg" alt="Decoration." width="500" height="173" /> -</div> -<h2 id="c25">CHAPTER V. -<br /><span class="sc">Water.</span></h2> -<div class="img"> -<img src="images/img036.jpg" alt="Illustrated Capital W" width="172" height="200" /> -</div> -<p class="pq">Where do we find water?</p> -<p class="pa">We find <i>water</i> almost everywhere; -above the earth, on the earth, and beneath -the earth’s surface.</p> -<p class="pq">Where do we find water above the earth?</p> -<p class="pa">We find it in the air, in the form of -vapor.</p> -<p class="pq">Where do we find water on the earth?</p> -<p class="pa">We find it in springs, in streams of water, and in -lakes and oceans.</p> -<p class="pq">Where else is water on the earth?</p> -<p class="pa">It is in the sap of plants, the juice of fruits, and -the blood of animals.</p> -<p class="pq">Where do we find water beneath the earth’s surface?</p> -<p class="pa">We find it in streams running in every direction.</p> -<p class="pq">Has water any influence on climate?</p> -<p class="pa">Yes; water causes many changes in climate.</p> -<p class="pq">What changes does water produce in climate?</p> -<p class="pa">It makes spring and summer cooler, and autumn -and winter warmer.</p> -<div class="pb" id="Page_100">100</div> -<p class="pq">How does water make the Spring of the year cooler?</p> -<p class="pa">The ice and snow while melting, take up a large -amount of heat, which passes into the water as latent -heat.</p> -<p class="pq">Where does the melting ice and snow obtain this heat?</p> -<p class="pa">They obtain it from the air, and from objects near -the surface of the earth.</p> -<p class="pq">What advantage is derived from this?</p> -<p class="pa">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.</p> -<p class="pq">How does water make Summer cooler?</p> -<p class="pa">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.</p> -<p class="pq">What effect does this heat produce upon the water?</p> -<p class="pa">The temperature of the water is gradually increased -by it during the hot weather of summer.</p> -<p class="pq">How is this useful?</p> -<p class="pa">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.</p> -<p class="pq">Does water store away a large amount of heat?</p> -<p class="pa">It does; and thus the summer is made much cooler.</p> -<p class="pq">How does water make Autumn warmer?</p> -<p class="pa">In the autumn the heat in the water is given off, -so that the air is kept warmer than it would otherwise -be.</p> -<div class="pb" id="Page_101">101</div> -<p class="pq">How does this affect autumn weather?</p> -<p class="pa">It makes that season of the year mild and pleasant.</p> -<p class="pq">What advantage is derived from this?</p> -<p class="pa">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.</p> -<p class="pq">How does water make Winter warmer?</p> -<p class="pa">By being changed into ice and snow.</p> -<p class="pq">When water is changed into ice or snow, what takes place?</p> -<p class="pa">The latent heat in the water is given off, and thus -the winter air is made warmer.</p> -<p class="pq">Can water be compressed into a smaller bulk?</p> -<p class="pa">Yes; it can be compressed, but not so much as air.</p> -<p class="pq">What is said of water that is compressed?</p> -<p class="pa">Its particles are pressed closer together, and it is -said to be more dense.</p> -<p class="pq">Why is the water at the bottom of a pond more dense than -that at the surface?</p> -<p class="pa">Because the water at the bottom of the pond has to -bear the weight of all the water above it.</p> -<p class="pq">Is the weight of the water very great?</p> -<p class="pa">The weight of the water is very great.</p> -<p class="pq">What is its weight at the depth of one hundred feet?</p> -<p class="pa">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.</p> -<p class="pq">What is its weight in the ocean at the depth of one mile?</p> -<p class="pa">At the depth of one mile it presses in every direction, -<span class="pb" id="Page_102">102</span> -with a force of two thousand three hundred -pounds on each square inch of surface.</p> -<p class="pq">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?</p> -<p class="pa">From the opening near the bottom of the vessel.</p> -<p class="pq">Why is this?</p> -<p class="pa">Because there is more weight of water to press it -out from the opening near the bottom.</p> -<p class="pq">When water is poured into a vessel, what becomes of it?</p> -<p class="pa">It spreads out so that its surface is level.</p> -<p class="pq">When different vessels are connected together, -what does the water do?</p> -<p class="pa">It falls to the same level in all of -them.</p> -<p class="pq">How high will the tea stand in the spout -of a tea-pot?</p> -<p class="pa">Just as high as the tea is in the -tea-pot.</p> -<div class="img" id="fig20"> -<img src="images/img037.jpg" alt="" width="200" height="195" /> -<p class="pcap">Fig. 20.</p> -</div> -<p class="pq">Will the tea in the spout balance that in the tea-pot?</p> -<p class="pa">The tea in the spout will balance that in the tea-pot, -as is shown in <a href="#fig20">Fig. 20</a>.</p> -<p class="pq">How else can it be shown that water will rise to the same level -in different vessels?</p> -<p class="pa">By a reference to <a href="#fig21">Fig. 21</a>, this may be seen.</p> -<div class="img" id="fig21"> -<img src="images/img038.jpg" alt="" width="500" height="218" /> -<p class="pcap">Fig. 21.</p> -</div> -<div class="pb" id="Page_103">103</div> -<p class="pq">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.</p> -<p class="pq">Where is this principle used to benefit mankind?</p> -<p class="pa">It is used in supplying the inhabitants of cities -with water.</p> -<p class="pq">What must be built in order to supply a city with water?</p> -<p class="pa">A reservoir must be built.</p> -<p class="pq">What is a reservoir?</p> -<p class="pa">It is a large basin made so as to hold water.</p> -<p class="pq">How high is the water in the basin?</p> -<p class="pa">It is usually as high as the tops of the houses in -the city.</p> -<p class="pq">How is the water raised into the basin?</p> -<p class="pa">It is raised into the basin by means of pumps.</p> -<p class="pq">How is the water conveyed from the basin to the houses in -the city?</p> -<p class="pa">By means of pipes laid under the surface of the -earth, and extending to all parts of the city.</p> -<p class="pq">How high will the water rise in these pipes?</p> -<p class="pa">Nearly as high as it is in the basin.</p> -<p class="pq">If an opening be made in a pipe, how high will the water be -forced up?</p> -<p class="pa">Nearly as high as the water in the basin.</p> -<p class="pq">What is the water flowing from such an opening called?</p> -<p class="pa">It is called a <i>fountain</i>.</p> -<p class="pq">What makes the water shoot up in a fountain?</p> -<p class="pa">The pressure of the water in the basin forces it to -shoot up in the fountain.</p> -<div class="pb" id="Page_104">104</div> -<p class="pq">To what does water always tend?</p> -<p class="pa">Water always tends to fall to a level.</p> -<p class="pq">What instrument is constructed on this principle?</p> -<p class="pa">The <i>water-level</i>.</p> -<p class="pq">What is a water-level?</p> -<p class="pa">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 <a href="#fig22">Fig. 22</a>.</p> -<div class="img" id="fig22"> -<img src="images/img039.jpg" alt="" width="300" height="48" /> -<p class="pcap">Fig. 22.</p> -</div> -<p class="pq">How can we tell when the instrument is level?</p> -<p class="pa">By the bubble of air, which will always be at the -centre of the tube when it is level.</p> -<p class="pq">What other liquid may be used to fill the tube?</p> -<p class="pa">Alcohol may be used to fill the tube, and it is then -called a <i>spirit-level</i>.</p> -<p class="pq">Of what use is the spirit-level?</p> -<p class="pa">It is used for levelling the walls and wood-work of -houses, bridges, and other structures.</p> -<h3 id="c26">Springs and Streams.</h3> -<p class="pq">When we dig into the earth, can we always find water?</p> -<p class="pa">Yes; we can always find water.</p> -<p class="pq">What reason may be given for believing this?</p> -<p class="pa">Because thousands of wells have been dug in different -parts of the country, and water is obtained -from them.</p> -<div class="pb" id="Page_105">105</div> -<p class="pq">Are these wells all of the same depth?</p> -<p class="pa">No; they vary in depth from four or five feet to -eighty or ninety feet.</p> -<p class="pq">What makes the water <i>hard</i> in some wells?</p> -<p class="pa">When the stream of water passes through iron ore, -before reaching the well, it absorbs some of the iron, -which makes it hard.</p> -<p class="pq">How else may it become hard?</p> -<p class="pa">By passing through other mineral substances, such -as limestone and sulphur, it will become hard.</p> -<p class="pq">Why is the water in some wells much warmer than in others?</p> -<p class="pa">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.</p> -<p class="pq">When these streams flow out at the surface of the earth, what -are they called?</p> -<p class="pa">They are called <i>springs</i>.</p> -<p class="pq">Where are springs found?</p> -<p class="pa">They are mostly found on hill-sides, or in valleys; -but they are sometimes found on the top of the hills.</p> -<p class="pq">Why is the water in some springs warmer than in others?</p> -<p class="pa">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.</p> -<p class="pq">What becomes of the water in the springs?</p> -<p class="pa">It runs away down the valleys, forming <i>brooks</i> and -<i>creeks</i>.</p> -<div class="pb" id="Page_106">106</div> -<p class="pq">When these brooks and creeks flow into other brooks and -creeks, what do they form?</p> -<p class="pa">They form <i>lakes</i> and <i>rivers</i>.</p> -<p class="pq">Into what do the rivers flow?</p> -<p class="pa">The rivers flow into the ocean.</p> -<p class="pq">How are waves produced?</p> -<p class="pa">Waves are produced by the winds blowing against -the surface of the water.</p> -<p class="pq">What does a light wind produce?</p> -<p class="pa">A light wind produces only ripples on the surface -of the water.</p> -<p class="pq">What does a stronger wind produce?</p> -<p class="pa">A stronger wind produces waves in the water.</p> -<p class="pq">Why is the water of the ocean and the larger lakes seldom -at rest?</p> -<p class="pa">Because the wind touching the water seldom ceases -to blow.</p> -<p class="pq">To what depth does the wind disturb the water of the ocean?</p> -<p class="pa">Wind seldom disturbs the ocean to a greater depth -than thirty feet below its surface.</p> -<h3 id="c27">Fogs and Clouds.</h3> -<p class="pq">What is always taking place from the surface of the water in -the rivers, lakes, and oceans?</p> -<p class="pa">The water is always passing off in the form of vapor.</p> -<p class="pq">Where does this vapor go?</p> -<p class="pa">It goes into the air so that the air becomes filled -with moisture.</p> -<p class="pq">What kind of air will hold the most vapor?</p> -<p class="pa">Warm air will hold the most vapor.</p> -<div class="pb" id="Page_107">107</div> -<p class="pq">When warm air becomes colder, what is done with the vapor?</p> -<p class="pa">The vapor is condensed into small particles of water.</p> -<p class="pq">When this takes place near the earth, what does it produce?</p> -<p class="pa">It produces a <i>fog</i>.</p> -<p class="pq">Where do fogs mostly occur?</p> -<p class="pa">Fogs mostly occur over low grounds and along the -course of streams.</p> -<p class="pq">When the vapor is condensed higher up in the air, what does -it produce?</p> -<p class="pa">It produces a <i>cloud</i>.</p> -<p class="pq">What is a cloud?</p> -<p class="pa">A cloud is only a fog, higher up in the air.</p> -<p class="pq">Do fogs ever ascend in the air?</p> -<p class="pa">Yes; they often ascend, and thus produce clouds.</p> -<p class="pq">What causes the vapor of the air to be condensed into clouds?</p> -<p class="pa">Cold causes it to be condensed into clouds.</p> -<p class="pq">Does the air grow colder as we ascend?</p> -<p class="pa">Yes; the higher we ascend, the colder we find the -atmosphere.</p> -<p class="pq">What becomes of the vapor in the air, as it ascends from the -earth?</p> -<p class="pa">The vapor is condensed into clouds in the upper -regions of the air.</p> -<p class="pq">Why do we mostly see clouds at the top of a mountain?</p> -<p class="pa">Because the vapor in the air is condensed into -clouds by the cold at the top of the mountain.</p> -<p class="pq">When is a cloud formed in the air?</p> -<p class="pa">When a warm, damp wind meets with a cold wind, -its vapor is condensed and a cloud is formed.</p> -<div class="pb" id="Page_108">108</div> -<p class="pq">Are clouds often formed in this way?</p> -<p class="pa">Yes; clouds are often quickly formed in this way.</p> -<p class="pq">When the cloud passes into a warmer current of air, what -becomes of it?</p> -<p class="pa">It is changed back into vapor, so that it cannot be -seen.</p> -<p class="pq">How then can a cloud in the sky disappear very quickly?</p> -<p class="pa">By passing into a warmer current of air the cloud -is changed into a vapor, and may thus quickly disappear.</p> -<p class="pq">Are clouds of different heights?</p> -<p class="pa">Yes; some clouds touch the earth, while other -clouds are high up in the air.</p> -<p class="pq">Why are clouds higher on a fine day?</p> -<p class="pa">Because there is not so much moisture in the air, -and, therefore, it rises higher before its vapor is condensed -into clouds.</p> -<p class="pq">Why are clouds lower on a rainy day?</p> -<p class="pa">Because there is so much moisture in the air, that -it is condensed into clouds near the earth.</p> -<p class="pq">Are the clouds high during a thunder-storm?</p> -<p class="pa">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.</p> -<p class="pq">Is there any difference in the thickness of the clouds?</p> -<p class="pa">Yes; some are only a few inches thick, while others -are a mile or more in thickness.</p> -<p class="pq">When the sun is about to rise, which of the seven colors of -light is the first to appear?</p> -<p class="pa">The <i>red</i> color appears first.</p> -<div class="pb" id="Page_109">109</div> -<p class="pq">When the sun sets, which of the seven colors is the last to -fade away?</p> -<p class="pa">The <i>red</i> color is the last to fade away.</p> -<p class="pq">Why are the clouds red just before sunrise?</p> -<p class="pa">Because they are tinged by the red ray of light, -before the other rays appear.</p> -<p class="pq">Why are the clouds red after sunset?</p> -<p class="pa">Because they are tinged by the red ray of light, -after the other rays have faded away.</p> -<h3 id="c28">Dew.</h3> -<p class="pq">What is dew?</p> -<p class="pa">Dew is the moisture that collects on plants and -other objects, during the night.</p> -<p class="pq">What causes dew upon any object?</p> -<p class="pa">The air in contact with that object has its vapor -condensed, so that the water is deposited on it in the -form of dew.</p> -<p class="pq">Does the same quantity of dew collect upon all objects?</p> -<p class="pa">No; it collects much more upon some objects than -upon others.</p> -<p class="pq">Upon what objects does it collect the most?</p> -<p class="pa">Upon those which radiate or give off heat the best.</p> -<p class="pq">Why do objects that are good radiators, collect the most dew?</p> -<p class="pa">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.</p> -<p class="pq">What objects are the best radiators of heat?</p> -<p class="pa">Grass, wood, the leaves of plants, and, in fact, all -things of a vegetable nature.</p> -<div class="pb" id="Page_110">110</div> -<p class="pq">What objects then collect the most dew?</p> -<p class="pa">Plants collect the most dew.</p> -<p class="pq">Of what use is the dew to plants?</p> -<p class="pa">It supplies them with moisture, and answers in the -place of rain.</p> -<p class="pq">When do we have the heaviest dew?</p> -<p class="pa">When the air is still and the sky is free from clouds.</p> -<p class="pq">Why does wind prevent the deposit of dew?</p> -<p class="pa">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.</p> -<p class="pq">Why are dews heaviest when the air is still?</p> -<p class="pa">Because the air remains in contact with the grass -and other objects, until it is cool enough to deposit the -dew.</p> -<p class="pq">Why is there but little dew on a cloudy night?</p> -<p class="pa">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.</p> -<p class="pq">Why is there a heavy dew on a clear night?</p> -<p class="pa">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.</p> -<p class="pq">How may dew be prevented from forming on any object?</p> -<p class="pa">By placing a cover over that object.</p> -<p class="pq">How does a cover prevent the formation of dew?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_111">111</div> -<p class="pq">Why is there but little dew under trees?</p> -<p class="pa">Because trees prevent the free radiation of heat -from the objects beneath them; hence, but little dew -is formed under trees.</p> -<p class="pq">Why do stone walls <i>sweat</i>?</p> -<p class="pa">Because the walls cool the air in contact with them, -so that its vapor is condensed into water, and deposited -on the walls.</p> -<p class="pq">Why does the sweating of stones indicate rain?</p> -<p class="pa">Because it shows that the air contains a large -amount of moisture.</p> -<p class="pq">When is a vessel of water said to sweat?</p> -<p class="pa">When drops of water collect on the outside of the -vessel, it is said to sweat.</p> -<p class="pq">Why does a pitcher of cold water sweat?</p> -<p class="pa">Because the vapor in the air in contact with the -cold pitcher is condensed into moisture, and deposited -on the pitcher.</p> -<p class="pq">Why does the pitcher sweat most before a rain?</p> -<p class="pa">Because the air then contains a large quantity of -moisture.</p> -<p class="pq">Why will not the pitcher sweat in fine weather?</p> -<p class="pa">Because the air contains only a small quantity of -moisture in fine weather.</p> -<p class="pq">Why does moisture sometimes collect on a glass window in a -warm room?</p> -<p class="pa">Because the glass cools the air, and causes its moisture -to collect in small drops on the glass.</p> -<p class="pq">What makes the glass cold enough to condense the vapor in -the air?</p> -<p class="pa">The cold air, outside of the room, in contact with -the glass.</p> -<div class="pb" id="Page_112">112</div> -<p class="pq">Where does the moisture in the air of the warm room come -from?</p> -<p class="pa">It comes from the breath of persons, and from the -steam of the tea-kettle.</p> -<p class="pq">When the breath is blown on a cold object, like a piece of -glass or metal, what is the result?</p> -<p class="pa">The moisture in the breath is condensed by the -cold object, and is collected in small drops of water -on its surface.</p> -<h3 id="c29">Frost.</h3> -<p class="pq">When the temperature of the air is reduced below thirty-two -degrees, what becomes of the moisture collected on plants and -other things?</p> -<p class="pa">The moisture is frozen.</p> -<p class="pq">What is this frozen moisture called?</p> -<p class="pa">It is called <i>frost</i>.</p> -<p class="pq">What then is frost?</p> -<p class="pa">Frost is the frozen moisture on the walls and -window panes, and the frozen dew on plants and -other objects.</p> -<p class="pq">On what objects do we find the most frost?</p> -<p class="pa">The most frost is found on objects having the most -dew.</p> -<p class="pq">What is a heavy dew when frozen called?</p> -<p class="pa">It is called a hoar-frost.</p> -<p class="pq">When do we have a hoar-frost?</p> -<p class="pa">We have heavy frosts when the air is full of moisture.</p> -<p class="pq">Why does a heavy frost indicate rain or snow?</p> -<p class="pa">Because it shows that the air contains much moisture.</p> -<div class="pb" id="Page_113">113</div> -<p class="pq">Are heavy frosts usually followed by rain or snow?</p> -<p class="pa">Yes; a heavy frost, on two successive mornings, is -mostly followed by rain or snow within a few hours.</p> -<h3 id="c30">Rain, Snow, and Hail.</h3> -<p class="pq">What is a <i>mist</i>?</p> -<p class="pa">A mist is very small drops of water falling through -the air.</p> -<p class="pq">When these drops are larger, what is it called?</p> -<p class="pa">It is called <i>rain</i>.</p> -<p class="pq">What causes rain?</p> -<p class="pa">Rain is caused by the sudden cooling of the air -above the earth.</p> -<p class="pq">How does cooling the air cause rain?</p> -<p class="pa">The cold condenses the vapor in the air, so that its -moisture is forced out in drops, which fall to the earth.</p> -<p class="pq">What becomes of the rain falling upon the earth?</p> -<p class="pa">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.</p> -<p class="pq">What becomes of the remaining part?</p> -<p class="pa">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.</p> -<p class="pq">How does rain purify the air?</p> -<p class="pa">By washing away many things that would, if left, -decay and make the air impure and unhealthy.</p> -<p class="pq">When the air is cooled below thirty-two degrees, what becomes -of the moisture in it?</p> -<p class="pa">The moisture in it is frozen.</p> -<div class="pb" id="Page_114">114</div> -<p class="pq">What is this frozen moisture in the air called?</p> -<p class="pa">It is called <i>snow</i>.</p> -<p class="pq">What are the particles of snow called?</p> -<p class="pa">They are called snow-flakes.</p> -<p class="pq">Are these snow-flakes all alike?</p> -<p class="pa">No; they vary very much in size and shape.</p> -<p class="pq">What does <a href="#fig23">Fig. 23</a> represent?</p> -<p class="pa">It represents a few of the various and beautiful -forms of the snow-flake or snow-crystals.</p> -<div class="img" id="fig23"> -<img src="images/img040.jpg" alt="" width="500" height="370" /> -<p class="pcap">Fig. 23.</p> -</div> -<p class="pq">At what temperature of the air does snow usually fall?</p> -<p class="pa">At a temperature of about thirty-two degrees.</p> -<p class="pq">Is there any reason for the remark that it is “<i>too cold to -snow</i>”?</p> -<p class="pa">Yes; for it seldom snows when the temperature of -the air is much below thirty-two degrees.</p> -<p class="pq">Of what use is snow?</p> -<p class="pa">The snow is a warm covering for the earth.</p> -<p class="pq">Why is snow a warm covering for the earth?</p> -<p class="pa">Because the air in the snow makes it a poor conductor -<span class="pb" id="Page_115">115</span> -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.</p> -<p class="pq">What is hail?</p> -<p class="pa">Drops of rain, when frozen, are called <i>hail</i>.</p> -<p class="pq">How is hail formed?</p> -<p class="pa">The rain-drops are frozen as they fall through the -air; hence, the hail-stones of winter are no larger -than the rain-drops.</p> -<p class="pq">When have we <i>dew</i>?</p> -<p class="pa">When the air in contact with the earth or the -plants is cooled, its vapor is condensed into dew.</p> -<p class="pq">When have we <i>frost</i>?</p> -<p class="pa">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.</p> -<p class="pq">When have we <i>fog</i>?</p> -<p class="pa">When the air near the earth is slightly cooled, its -vapor is condensed into fog.</p> -<p class="pq">When have we <i>clouds</i>?</p> -<p class="pa">When the air higher above the earth is slightly -cooled, its vapor is condensed into clouds.</p> -<p class="pq">When have we <i>mist</i>?</p> -<p class="pa">When the air is a little more cooled, its vapor is -condensed and mist falls.</p> -<p class="pq">When have we <i>rain</i>?</p> -<p class="pa">When the air is still more cooled, its vapor is condensed -and rain falls.</p> -<div class="pb" id="Page_116">116</div> -<p class="pq">When have we <i>snow</i>?</p> -<p class="pa">When the air is cooled below thirty-two degrees, its -vapor is condensed and frozen into snow.</p> -<p class="pq">When have we <i>hail</i>?</p> -<p class="pa">When the rain-drops, while falling, pass through a -current of air cold enough to freeze them, hail is produced.</p> -<h3 id="c31">Ice.</h3> -<p class="pq">What is ice?</p> -<p class="pa"><i>Ice</i> is frozen water.</p> -<p class="pq">When does water freeze?</p> -<p class="pa">Water freezes when its temperature is reduced to -thirty-two degrees.</p> -<p class="pq">Is water increased in bulk by freezing?</p> -<p class="pa">Water is increased in bulk by freezing.</p> -<p class="pq">Why do water-pipes often burst in cold weather?</p> -<p class="pa">The water in them freezes, and as it increases in -bulk, the pipes burst.</p> -<p class="pq">Why are some springs never frozen?</p> -<p class="pa">Because water, much above the freezing-point, is -always flowing into them from the earth, and thus -they are kept too warm to freeze.</p> -<p class="pq">Why are some other springs frozen?</p> -<p class="pa">Because the water flowing into them comes from -near the surface of the earth, and is soon made cold -enough to freeze.</p> -<p class="pq">Does salt water freeze at the same temperature as fresh water?</p> -<p class="pa">No; fresh water freezes at thirty-two degrees, but -<span class="pb" id="Page_117">117</span> -salt water requires a lower temperature depending -upon the amount of salt it contains.</p> -<p class="pq">How is this useful?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img041.jpg" alt="Horse." width="200" height="166" /> -</div> -<div class="pb" id="Page_118">118</div> -<div class="img"> -<img src="images/img042.jpg" alt="Decoration." width="500" height="180" /> -</div> -<h2 id="c32">CHAPTER VI. -<br /><span class="sc">Sound.</span></h2> -<div class="img"> -<img src="images/img043.jpg" alt="Illustrated Capital W" width="193" height="200" /> -</div> -<p class="pq">When the hands are brought quickly together, -what is produced?</p> -<p class="pa">They produce a shaking or vibration in -the air.</p> -<p class="pq">What is this vibration in the air called?</p> -<p class="pa">It is called a wave in the air.</p> -<p class="pq">What is this wave in the air like?</p> -<p class="pa">It is like a wave in the water.</p> -<p class="pq">When this wave in the air reaches the ear, what does it -produce?</p> -<p class="pa">It produces a sensation which is called <i>sound</i>.</p> -<p class="pq">When a stone is dropped into the water, what does it produce?</p> -<p class="pa">It produces waves in the water.</p> -<p class="pq">How do these waves move?</p> -<p class="pa">They move in every direction from the stone.</p> -<p class="pq">Do they get smaller the farther they go?</p> -<p class="pa">They do get smaller and smaller, until at last they -disappear.</p> -<p class="pq">Do the waves of sound in the air act in the same way as the -waves in the water?</p> -<p class="pa">They do act in the same way.</p> -<div class="pb" id="Page_119">119</div> -<p class="pq">What then becomes of the waves of sound?</p> -<p class="pa">They get smaller and smaller the farther they go, -until at last there is no wave left.</p> -<p class="pq">When the wave in the air is large, what kind of sound is made?</p> -<p class="pa">It makes a loud sound.</p> -<p class="pq">As the wave gets smaller, does the sound grow fainter?</p> -<p class="pa">Yes; the smaller the wave the fainter the sound, -until at last both the wave and the sound die away.</p> -<p class="pq">Where must we be to hear a loud sound?</p> -<p class="pa">We must be near the place in which the sound is -made.</p> -<p class="pq">What kind of waves will make a faint sound?</p> -<p class="pa">Small waves in the air make a faint sound.</p> -<p class="pq">Will these waves extend as far away as larger ones?</p> -<p class="pa">No; as all waves of sound decrease in size as they -recede, the smallest ones will soonest disappear.</p> -<p class="pq">Why will a loud sound be heard farther than a fainter sound?</p> -<p class="pa">Because the waves in the air from the loud sound -will extend farther away than those from the fainter -one.</p> -<p class="pq">How are sounds produced?</p> -<p class="pa">Sounds are produced by striking solid bodies together.</p> -<p class="pq">How else may sound be produced?</p> -<p class="pa">By striking a solid body against a liquid, as a stick -struck against the water.</p> -<p class="pq">In what other way may sound be made?</p> -<p class="pa">Sound may be made by causing a substance to -move rapidly through the air.</p> -<div class="pb" id="Page_120">120</div> -<p class="pq">When a piece of wood attached to a string is swung rapidly -around, what kind of a noise is made?</p> -<p class="pa">It makes a humming noise.</p> -<p class="pq">When lightning passes through the air, what noise is made?</p> -<p class="pa">A noise which we call thunder.</p> -<p class="pq">How can a murmur be produced?</p> -<p class="pa">When sounds, too faint to be heard separately, are -united, they produce a murmur.</p> -<p class="pq">Where may we hear a murmur?</p> -<p class="pa">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.</p> -<p class="pq">When a sound is heard but once, what is it called?</p> -<p class="pa">It is called a noise.</p> -<p class="pq">When sounds are made at irregular intervals, what are they -called?</p> -<p class="pa">They are called noises.</p> -<p class="pq">When sounds are repeated at regular intervals, what do they -become?</p> -<p class="pa">They become musical sounds.</p> -<p class="pq">Upon what do the high and low sounds in music depend?</p> -<p class="pa">They depend upon the rapidity with which the waves -are produced in the air.</p> -<p class="pq">When the waves are repeated rapidly, what sound is made?</p> -<p class="pa">They make a high or acute sound.</p> -<p class="pq">When they are repeated more slowly, what sound is made?</p> -<p class="pa">They make a low or grave sound.</p> -<p class="pq">How small a number of vibrations or waves in the air can be -heard?</p> -<p class="pa"><i>Sixteen</i> vibrations in a second of time, will produce -the gravest sound that can be heard.</p> -<div class="pb" id="Page_121">121</div> -<p class="pq">How large a number of vibrations in the air can be heard?</p> -<p class="pa">Twenty-four thousand vibrations in a second of -time, make the most acute sound that can be heard.</p> -<p class="pq">How is the human voice made?</p> -<p class="pa">It is made by means of the vocal chords at the upper -part of the larynx.</p> -<p class="pq">What is the larynx?</p> -<p class="pa">The upper part of the windpipe is called the -larynx.</p> -<p class="pq">How do these chords produce the voice?</p> -<p class="pa">When the air is forced out of the lungs, it causes -these chords to vibrate, so as to produce the voice.</p> -<p class="pq">Are these chords the same in all persons?</p> -<p class="pa">No; they are larger and coarser in some persons -than in others.</p> -<p class="pq">In whom are they larger and coarser?</p> -<p class="pa">They are usually larger and coarser in men, and -they are smaller and finer in women.</p> -<p class="pq">What kind of a voice is made by the large and coarse chords?</p> -<p class="pa">They make a rough, coarse voice.</p> -<p class="pq">What kind of voice is made by the finer chords?</p> -<p class="pa">They make a finer and more delicate voice.</p> -<p class="pq">Do these chords vibrate so rapidly in man as in woman?</p> -<p class="pa">No; they do not vibrate so rapidly in man as in -woman.</p> -<p class="pq">What kind of voice do rapid vibrations make?</p> -<p class="pa">They make an acute voice; hence, woman can, -when singing, raise her voice to a very high pitch.</p> -<p class="pq">Where is the sensation of sound produced?</p> -<p class="pa">The sensation of sound is produced on the ear.</p> -<div class="pb" id="Page_122">122</div> -<p class="pq">Do people vary in their ability to hear?</p> -<p class="pa">Yes; they vary very much in their ability to hear.</p> -<p class="pq">Is every person able to distinguish musical sounds?</p> -<p class="pa">No; some persons cannot tell one musical note from -another, and some cannot even tell one bird from -another by its notes.</p> -<p class="pq">Is the hearing of all animals alike?</p> -<p class="pa">No; some can hear more acutely than others; a -lion has more acute hearing than a man.</p> -<p class="pq">What other animals have acute hearing?</p> -<p class="pa">All such animals as hares, rabbits, and deer.</p> -<p class="pq">Of what use is the acute hearing to these animals?</p> -<p class="pa">It enables them quickly to hear the approach of -their enemies, so that they may run away in time to -escape the danger.</p> -<h3 id="c33">Conduction of Sound.</h3> -<p class="pq">How does sound come to our ears?</p> -<p class="pa">Sound is carried to our ears by the air.</p> -<p class="pq">Is the air a conductor of sound?</p> -<p class="pa">Yes; the <i>air</i> is a <i>conductor</i> of sound.</p> -<p class="pq">Is there any sound where there is no air?</p> -<p class="pa">No; there is not any sound where there is no air.</p> -<p class="pq">How do we know this?</p> -<p class="pa">When a bell is rung in a vacuum, no sound is -heard.</p> -<p class="pq">Can sounds be distinctly heard when the air is rare?</p> -<p class="pa">No; the rarer the air the more poorly it conducts -sound.</p> -<p class="pq">What proof have we of this?</p> -<p class="pa">On the top of a high mountain the human voice is -<span class="pb" id="Page_123">123</span> -heard only at a distance of a few yards; and a pistol -when exploded sounds as faintly as the breaking of a -stick.</p> -<p class="pq">Can sounds be heard better when the air is dense?</p> -<p class="pa">Yes; the denser the air, the better it conducts -sound.</p> -<p class="pq">What proof have we of this?</p> -<p class="pa">The human voice sounds so distinctly in the dense -air of deep mines, that conversation may be carried -on in a whisper.</p> -<p class="pq">Why are sounds heard better at night than in the day?</p> -<p class="pa">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.</p> -<p class="pq">Why is moist or damp air a better conductor of sound than -dry air?</p> -<p class="pa">Because the particles of water in the damp air -increase its power of conduction.</p> -<p class="pq">Why is sound heard farther when over the water?</p> -<p class="pa">Because the damp air over the water is a better -conductor, and because it contains fewer objects to -obstruct the waves of sound.</p> -<p class="pq">How far has the human voice been heard over the water?</p> -<p class="pa">The words “all’s well” have been heard across the -Strait of Gibraltar, a distance of <i>ten</i> miles.</p> -<p class="pq">How far has the human voice been heard over the land?</p> -<p class="pa">It has been heard at the distance of <i>four</i> miles.</p> -<p class="pq">How does the wind affect sound?</p> -<p class="pa">The wind carries the sound the way in which it is -<span class="pb" id="Page_124">124</span> -blowing, so that a noise may be heard much farther -with the wind than against it.</p> -<p class="pq">Why does water conduct sound faster than air?</p> -<p class="pa">Because the particles of water are closer together -than the particles of air.</p> -<p class="pq">How can we prove that water is a conductor of sound?</p> -<p class="pa">When a bell is rung under water, the sound may -be heard by any person whose head is beneath the -surface of the water.</p> -<p class="pq">Does a solid substance conduct sound more rapidly than air?</p> -<p class="pa">It does, much more rapidly.</p> -<p class="pq">How can we prove this?</p> -<p class="pa">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.</p> -<p class="pq">What use is made of this fact?</p> -<p class="pa">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.</p> -<p class="pq">Do all solid substances produce the same amount of sound -when struck?</p> -<p class="pa">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.</p> -<p class="pq">What are hard bodies, producing much sound, called?</p> -<p class="pa">They are called <i>sonorous</i> bodies.</p> -<p class="pq">Is a bell a very sonorous body?</p> -<p class="pa">Yes; a bell is one of the most sonorous bodies that -we have.</p> -<div class="pb" id="Page_125">125</div> -<p class="pq">Why does a bell ring when struck?</p> -<p class="pa">Because the stroke causes the particles of the bell -to vibrate.</p> -<p class="pq">When we touch a bell while ringing, why does the sound stop?</p> -<p class="pa">Because we stop its vibrations when we touch it.</p> -<p class="pq">Why does a crack in a bell check its ringing?</p> -<p class="pa">Because the crack stops the vibrations in the metal.</p> -<p class="pq">What other substances ring when struck?</p> -<p class="pa">All vessels made of glass, earth, stone, iron, etc., -ring when struck.</p> -<p class="pq">How may we detect a crack in any such vessel?</p> -<p class="pa">By striking the vessel; if it is cracked there will -be no ringing sound.</p> -<h3 id="c34">Trumpets and Speaking-Tubes.</h3> -<p class="pq">How do the waves of sound move?</p> -<p class="pa">The waves of sound move in every direction from -the place where the noise is made.</p> -<p class="pq">Why do we put both hands around the mouth when we call -to a person at a distance from us?</p> -<p class="pa">We do so to keep the waves of sound more together.</p> -<p class="pq">Does it make the voice louder?</p> -<p class="pa">It does make the voice louder.</p> -<p class="pq">What instrument has been made so as to operate in the same -way?</p> -<p class="pa">The <i>speaking-trumpet</i>, as is shown in <a href="#fig24">Fig. 24</a>.</p> -<div class="img" id="fig24"> -<img src="images/img044.jpg" alt="" width="400" height="147" /> -<p class="pcap">Fig. 24.</p> -</div> -<div class="pb" id="Page_126">126</div> -<p class="pq">Of what use are speaking-trumpets?</p> -<p class="pa">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.</p> -<p class="pq">Why do we hold the hand behind the ear when we wish to -hear more distinctly?</p> -<p class="pa">Because the hand catches more of the -waves of sound, and conducts them to -the ear.</p> -<p class="pq">What instrument has been made to serve the -same purpose?</p> -<p class="pa">The <i>ear-trumpet</i>, as is shown in <a href="#fig25">Fig. 25</a>.</p> -<div class="img" id="fig25"> -<img src="images/img045.jpg" alt="" width="200" height="192" /> -<p class="pcap">Fig. 25.</p> -</div> -<p class="pq">How does the ear-trumpet aid the hearing?</p> -<p class="pa">The large end of the trumpet collects more of the -waves of sound than the ear can; hence, more waves -reach the ear.</p> -<p class="pq">Why do such animals as the horse, the rabbit, and the deer, -hear better than we do?</p> -<p class="pa">Because their large ears act like ear-trumpets to -collect the waves of sound, and thus increase their -faculty of hearing.</p> -<p class="pq">How can these animals still farther increase their sense of -hearing?</p> -<p class="pa">By being able to turn their ears, just as we do the -ear-trumpet, in the direction whence the sound comes.</p> -<p class="pq">What are speaking-tubes?</p> -<p class="pa">They are metal pipes extending from one part of a -building to another part.</p> -<p class="pq">Of what use are speaking-tubes?</p> -<p class="pa">They are used to convey the voice to the more distant -parts of a building.</p> -<div class="pb" id="Page_127">127</div> -<p class="pq">Do they require loud talking?</p> -<p class="pa">No; even a whisper has been heard through a metal -pipe over three thousand feet long.</p> -<h3 id="c35">Velocity of Sound.</h3> -<p class="pq">How fast do the waves of sound move through the air?</p> -<p class="pa">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.</p> -<p class="pq">What effect does warm air have on sound?</p> -<p class="pa">Sound travels more rapidly in warm air.</p> -<p class="pq">How much does heat increase the velocity of sound?</p> -<p class="pa">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 <i>one mile</i> in <i>five seconds</i>.</p> -<p class="pq">Does light travel more rapidly than sound?</p> -<p class="pa">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.</p> -<p class="pq">What familiar examples of this difference may be given?</p> -<p class="pa">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.</p> -<p class="pq">How can we tell the distance of the wood-chopper from us?</p> -<p class="pa">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 -<span class="pb" id="Page_128">128</span> -this number, it will give us the distance in feet; because -sound travels eleven hundred and twenty feet -in a second.</p> -<p class="pq">How can we tell the distance of lightning from us?</p> -<p class="pa">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.</p> -<p class="pq">How rapidly does sound travel through water?</p> -<p class="pa">Sound travels about <i>four</i> times as rapidly through -<i>water</i> as it does through air.</p> -<p class="pq">How rapidly does sound travel through a solid?</p> -<p class="pa">Sound travels <i>ten</i> times as rapidly through <i>wood</i>, -and <i>sixteen</i> times as rapidly through <i>glass</i>, as it does -through air.</p> -<p class="pq">Does a loud sound travel more rapidly than a faint one?</p> -<p class="pa">No; it does not.</p> -<p class="pq">How do we know this?</p> -<p class="pa">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.</p> -<h3 id="c36">Reflection of Sound.</h3> -<p class="pq">What becomes of the waves of sound when they strike a solid -substance?</p> -<p class="pa">They are thrown back, as light from a looking-glass, -or a ball from a stone wall.</p> -<p class="pq">What is this called?</p> -<p class="pa">It is called <i>reflection</i> of sound.</p> -<div class="pb" id="Page_129">129</div> -<p class="pq">In what direction is sound reflected?</p> -<p class="pa">Sound, like light, is reflected so that the angles of -incidence and reflection are always equal.</p> -<p class="pq">When the reflected sound comes back to the ear, what does it -make?</p> -<p class="pa">It makes an <i>echo</i>.</p> -<p class="pq">When the sound comes back several times, what does it make?</p> -<p class="pa">It makes several echoes.</p> -<p class="pq">Where are echoes always to be heard?</p> -<p class="pa">Echoes may be heard in the deep caves of the earth; -because the walls of the caves reflect sounds striking -against them.</p> -<p class="pq">What curious echo may be mentioned?</p> -<p class="pa">There is an echo in Fairfax County, Virginia, -which sends back twenty notes played on a flute.</p> -<p class="pq">Does every reflection of sound produce an echo?</p> -<p class="pa">No; it does not.</p> -<p class="pq">Why is there no echo from the walls of a small room?</p> -<p class="pa">Because the walls are too close together to make an -echo.</p> -<p class="pq">How far away must the reflection of a sound be made, so as to -produce a perfect echo?</p> -<p class="pa">Sound must be made not less than one hundred and -twelve feet distant to produce a perfect echo.</p> -<p class="pq">How many syllables can be heard from an echo one hundred -and twelve feet distant?</p> -<p class="pa">Only one syllable can be heard.</p> -<p class="pq">How far must the echo be to hear two syllables?</p> -<p class="pa">It must be twice the one hundred and twelve feet, -or two hundred and twenty-four feet, to hear two syllables; -<span class="pb" id="Page_130">130</span> -and three times that distance to hear three -syllables, and so on.</p> -<p class="pq">Why can a person when speaking, be heard better in a room -than in the open air?</p> -<p class="pa">Because the walls of the building reflect his voice to -the hearers, so that more waves of sound reach their -ears.</p> -<p class="pq">Why are noises heard so distinctly in an unfurnished dwelling?</p> -<p class="pa">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.</p> -<p class="pq">What may be said of the harmony of sounds?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img046.jpg" alt="Horse-drawn trolley car." width="400" height="148" /> -</div> -<div class="pb" id="Page_131">131</div> -<div class="img"> -<img src="images/img047.jpg" alt="Decoration." width="500" height="176" /> -</div> -<h2 id="c37">CHAPTER VII. -<br /><span class="sc">Matter.</span></h2> -<div class="img"> -<img src="images/img048.jpg" alt="Illustrated Capital I" width="130" height="199" /> -</div> -<p class="pq">In what form is all matter found?</p> -<p class="pa">In the form of a solid, a liquid, or a gas.</p> -<p class="pq">What is true of all solids?</p> -<p class="pa">All solids must have length, breadth, and -thickness.</p> -<p class="pq">Do solids vary much in length, breadth, and thickness?</p> -<p class="pa">Yes; they vary so much that we can scarcely find -two things exactly alike in shape and size.</p> -<p class="pq">Can the shape of any substance be changed?</p> -<p class="pa">Yes; its shape may be changed in many ways.</p> -<p class="pq">What takes place in india-rubber when it is stretched?</p> -<p class="pa">Its particles are drawn farther apart, and its shape -is altered.</p> -<p class="pq">When the force is removed, what does the india-rubber do?</p> -<p class="pa">It springs back into its former shape.</p> -<p class="pq">What is this property of the india-rubber called?</p> -<p class="pa">It is called <i>Elasticity</i>.</p> -<p class="pq">Can air be pressed into a smaller bulk?</p> -<p class="pa">It can, by means of a weight.</p> -<div class="pb" id="Page_132">132</div> -<p class="pq">When the weight is removed, what does the air do?</p> -<p class="pa">The air goes back to its former bulk.</p> -<p class="pq">Is air very elastic?</p> -<p class="pa">Yes; air is one of the most elastic substances known.</p> -<p class="pq">Why does a ball rebound when thrown against the floor?</p> -<p class="pa">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.</p> -<p class="pq">Why does the ball rebound from the floor?</p> -<p class="pa">Because the floor is firm, and will not move from -the ball.</p> -<p class="pq">Does a ball rebound when it strikes a soft substance?</p> -<p class="pa">No; it does not rebound when it strikes a soft -substance, like a heap of wool or feathers.</p> -<p class="pq">Are solids ever elastic?</p> -<p class="pa">Yes; many solids are elastic.</p> -<p class="pq">Name some articles useful from their elastic properties.</p> -<p class="pa">Watch-springs, wagon-springs, steel pens, and spring -seats are very useful.</p> -<p class="pq">Why does a wooden bow throw an arrow from it?</p> -<p class="pa">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.</p> -<p class="pq">Can metals be bent?</p> -<p class="pa">Yes; nearly all metals can be bent.</p> -<p class="pq">What is this property of being bent called?</p> -<p class="pa">It is called <i>Flexibility</i>.</p> -<p class="pq">Do substances usually spring back after being bent?</p> -<p class="pa">No; they usually remain in the bent form.</p> -<div class="pb" id="Page_133">133</div> -<p class="pq">Can glass be bent?</p> -<p class="pa">Glass can be bent only a very little without being -broken.</p> -<p class="pq">What is this property of being easily broken called?</p> -<p class="pa">It is called <i>Brittleness</i>.</p> -<p class="pq">What may be said of brittle substances?</p> -<p class="pa">Brittle substances are generally hard, while flexible -substances are much softer.</p> -<p class="pq">What metal may be made either flexible or brittle?</p> -<p class="pa">Steel, when heated and cooled slowly, becomes flexible, -but when cooled quickly, it becomes brittle.</p> -<p class="pq">For what is brittle steel used?</p> -<p class="pa">It is used for making knives, razors, axes, and other -cutting instruments.</p> -<p class="pq">What may be done with metals that are flexible?</p> -<p class="pa">They may be hammered into thin plates.</p> -<p class="pq">What is this property in metals called?</p> -<p class="pa">It is called <i>Malleability</i>.</p> -<p class="pq">Which is the most malleable of all metals?</p> -<p class="pa">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.</p> -<p class="pq">Name some other metals that are malleable.</p> -<p class="pa">Silver, copper, tin, zinc, iron, and lead.</p> -<p class="pq">Can the particles of a piece of iron easily be separated?</p> -<p class="pa">No; its particles cannot easily be separated.</p> -<p class="pq">What is this property of iron called?</p> -<p class="pa">It is called <i>Tenacity</i>.</p> -<div class="pb" id="Page_134">134</div> -<p class="pq">Can the particles of a brittle substance, like glass, easily be -separated?</p> -<p class="pa">Yes; its particles can easily be separated.</p> -<p class="pq">Is there much tenacity in brittle substances?</p> -<p class="pa">No; there is very little tenacity in brittle substances.</p> -<p class="pq">Is there any tenacity in air?</p> -<p class="pa">No; because its particles are always trying to -separate from each other.</p> -<p class="pq">Is there any tenacity in water?</p> -<p class="pa">Only a little, just enough to hold its particles together -in the form of drops.</p> -<p class="pq">Upon what does the strength of substances depend?</p> -<p class="pa">Their strength depends upon their tenacity.</p> -<p class="pq">Do the different kinds of wood vary much in strength?</p> -<p class="pa">Yes; hickory and oak are much stronger than pine -or cedar.</p> -<p class="pq">When is a knowledge of the strength of wood necessary?</p> -<p class="pa">It is necessary to know the strength of wood used -in the construction of stores, bridges, houses, etc.</p> -<p class="pq">Which is the most tenacious of metals?</p> -<p class="pa">Iron is the most tenacious of metals.</p> -<p class="pq">How is this principle in iron made useful?</p> -<p class="pa">It is useful in the construction of suspension bridges, -and in very many other ways.</p> -<p class="pq">Can iron be drawn into wire?</p> -<p class="pa">Yes; iron can be drawn into wire.</p> -<p class="pq">What is this property of being drawn into wire called?</p> -<p class="pa">It is called <i>Ductility</i>.</p> -<div class="pb" id="Page_135">135</div> -<p class="pq">Which are the most ductile of metals?</p> -<p class="pa">Platinum, silver, iron, copper, and gold, are the -most ductile, in the order named.</p> -<p class="pq">When is glass ductile?</p> -<p class="pa">When glass is melted it becomes ductile, and may -be drawn into very fine threads.</p> -<p class="pq">If we take half the air out of a room, what will the remaining -half do?</p> -<p class="pa">Its particles will separate until they fill the whole -room.</p> -<p class="pq">What is said of air when its particles are thus separated?</p> -<p class="pa">The air is in a state of <i>rarity</i>.</p> -<p class="pq">What is said of it when the particles are pressed closer together?</p> -<p class="pa">The air is then said to be in a state of <i>density</i>.</p> -<p class="pq">Are the particles of water closer together than those of air?</p> -<p class="pa">Yes; therefore water is denser than air.</p> -<p class="pq">Upon what does the weight of a substance depend?</p> -<p class="pa">Its weight depends upon its density.</p> -<p class="pq">How much heavier is steel than water?</p> -<p class="pa">Steel is seven times as heavy as water, and is, therefore, -seven times as dense.</p> -<p class="pq">How much denser is mercury than water?</p> -<p class="pa">Mercury is thirteen times as dense as water, and is, -therefore, thirteen times as heavy.</p> -<p class="pq">Why can a bird move through the air?</p> -<p class="pa">Because the particles of air can be separated by -the bird.</p> -<div class="pb" id="Page_136">136</div> -<p class="pq">Why can a fish swim through the water?</p> -<p class="pa">Because the particles of water can be separated by -the fish.</p> -<p class="pq">Why can a needle be passed through cloth?</p> -<p class="pa">Because the fibres of the cloth can be separated by -the needle.</p> -<p class="pq">Why can a nail be driven through a board?</p> -<p class="pa">Because the fibres of wood can be separated by the -nail.</p> -<p class="pq">What name given to this property in these substances?</p> -<p class="pa">This property is called <i>Penetrability</i>.</p> -<p class="pq">What is necessary in order to penetrate a substance?</p> -<p class="pa">We must use a substance harder than the one -which we wish to penetrate.</p> -<p class="pq">Why will iron penetrate wood?</p> -<p class="pa">Because iron is harder than wood.</p> -<p class="pq">Why cannot our bodies penetrate the wood?</p> -<p class="pa">Because our bodies are softer than the wood.</p> -<p class="pq">Why can our bodies move more easily through air than -through water?</p> -<p class="pa">Because air is more easily penetrated than water.</p> -<p class="pq">Why can a nail be driven into a pine board more easily than -into an oak board?</p> -<p class="pa">Because the fibres of pine are more easily separated -than the fibres of oak; hence, pine wood is -more penetrable than oak wood.</p> -<div class="img"> -<img src="images/img049.jpg" alt="Decoration." width="150" height="48" /> -</div> -<div class="pb" id="Page_137">137</div> -<div class="img"> -<img src="images/img050.jpg" alt="Decoration." width="500" height="181" /> -</div> -<h2 id="c38">CHAPTER VIII. -<br /><span class="sc">Attraction.</span></h2> -<h3 id="c39"><span class="sc">Cohesion and Adhesion.</span></h3> -<div class="img"> -<img src="images/img051.jpg" alt="Illustrated Capital W" width="175" height="200" /> -</div> -<p class="pq">Why can a cup be lifted by taking hold of the -handle?</p> -<p class="pa">Because the particles of the cup do not -separate from each other.</p> -<p class="pq">What is the power which holds these particles -together called?</p> -<p class="pa">It is called <i>Cohesion</i>.</p> -<p class="pq">In what bodies is cohesion the strongest?</p> -<p class="pa">Cohesion is strongest in solid bodies.</p> -<p class="pq">Is the cohesion alike in all solids?</p> -<p class="pa">No; the cohesion in iron is much greater than the -cohesion in wood.</p> -<p class="pq">Upon what does the strength of a substance depend?</p> -<p class="pa">The strength of a substance depends upon the -amount of cohesion among its particles.</p> -<p class="pq">What effect does heat have upon the cohesion of bodies?</p> -<p class="pa">Heat tends to destroy the cohesion of bodies by -separating their particles.</p> -<div class="pb" id="Page_138">138</div> -<p class="pq">How does heat affect ice?</p> -<p class="pa">Heat destroys the cohesion between the particles -of ice, and thus it is changed into water.</p> -<p class="pq">How does heat affect water?</p> -<p class="pa">When still more heat is applied, the cohesion in the -water is destroyed, and thus it is changed into steam.</p> -<p class="pq">Is there any cohesion in steam?</p> -<p class="pa">No; there is not any cohesion in steam.</p> -<p class="pq">Is there any cohesion in air?</p> -<p class="pa">No; because the particles of air are always trying -to separate from each other.</p> -<p class="pq">Is there any cohesion in liquids?</p> -<p class="pa">Yes; there is some cohesion in liquids, but it is -only strong enough to hold them together in drops.</p> -<p class="pq">Does the cohesion in liquids vary?</p> -<p class="pa">Yes; the cohesion in mercury is stronger than it is -in water; therefore, a drop of mercury is larger than -a drop of water.</p> -<p class="pq">How is it in chloroform?</p> -<p class="pa">The cohesion in chloroform is weaker than it is in -water; therefore, a drop of chloroform is smaller than -a drop of water.</p> -<p class="pq">How is shot made?</p> -<p class="pa">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.</p> -<p class="pq">How high must a shot-tower be?</p> -<p class="pa">A shot-tower must be not less than two hundred -feet high, so that the lead may harden while falling.</p> -<div class="pb" id="Page_139">139</div> -<p class="pq">Why does putty stick to a pane of glass?</p> -<p class="pa">Because there is an attraction between the putty -and the glass.</p> -<p class="pq">What is this attraction called?</p> -<p class="pa">It is called <i>Adhesion</i>.</p> -<p class="pq">Upon what does adhesion depend?</p> -<p class="pa">Adhesion depends upon the attraction which substances -have for each other.</p> -<p class="pq">Has water any attraction for oil?</p> -<p class="pa">No; therefore, there is no adhesion between oil and -water.</p> -<p class="pq">Has glass any attraction for water?</p> -<p class="pa">Yes; therefore, the water collects on the glass in -drops.</p> -<p class="pq">What other examples of adhesion may be given?</p> -<p class="pa">Paint adheres to wood, dust to furniture, and tin -and mercury to the back of a looking-glass.</p> -<p class="pq">Will you name a few of the most adhesive substances?</p> -<p class="pa">Glue, mortar, paste, and putty.</p> -<p class="pq">Of what use is glue?</p> -<p class="pa">Glue fastens pieces of wood together.</p> -<p class="pq">Of what use is mortar?</p> -<p class="pa">Mortar fastens together the bricks and stones used -in the construction of houses and other buildings.</p> -<p class="pq">Of what use is paste?</p> -<p class="pa">Paste fastens pieces of paper together, and it fastens -paper to the walls and ceilings of rooms.</p> -<p class="pq">Of what use is putty?</p> -<p class="pa">Putty fastens the glass to the wood-work of our -windows.</p> -<div class="pb" id="Page_140">140</div> -<h3 id="c40">Capillary Attraction.</h3> -<p class="pq">Why does a sponge suck up water?</p> -<p class="pa">Because the particles of the sponge have an attraction -for the water.</p> -<p class="pq">What is this attraction called?</p> -<p class="pa">It is called <i>Capillary</i> attraction.</p> -<p class="pq">In what bodies do we find the most capillary attraction?</p> -<p class="pa">In those that are of a porous or spongy nature.</p> -<p class="pq">Why does a lump of sugar or salt, when partly dipped into -water, soon become entirely wet?</p> -<p class="pa">Because sugar and salt are porous substances, and -the water rises in them by capillary attraction.</p> -<p class="pq">Why does a heap of dry sand soon become damp when one -edge touches the water?</p> -<p class="pa">Because the water is drawn through the sand by -capillary attraction.</p> -<p class="pq">Why does water poured into the saucer of a flower-pot soon -wet the earth in the flower-pot?</p> -<p class="pa">Because the earth is porous, and the water is drawn -up through it by capillary attraction.</p> -<p class="pq">How does the oil in a lamp rise so as to be burned?</p> -<p class="pa">The oil is drawn up through the wick by capillary -attraction.</p> -<p class="pq">Where may examples of capillary attraction be noticed?</p> -<p class="pa">In blotting-paper when it takes up the ink, and in -bread when it soaks up the juice of meats.</p> -<p class="pq">What other examples may be given?</p> -<p class="pa">When one end of a towel is placed in a basin of -water, and the other end is left hanging outside the -<span class="pb" id="Page_141">141</span> -basin, the water will pass through the towel by capillary -attraction, until the basin is entirely dry.</p> -<p class="pq">Why is water in a glass tumbler a little -elevated at the edges?</p> -<p class="pa">Because the glass has a slight attraction -for the water near it, and -thus lifts it up a little.</p> -<p class="pq">Why will water rise in a small glass tube -higher than in the surrounding vessel?</p> -<p class="pa">Because the glass has an attraction -for the water, and elevates it at the -edges, as is shown in <a href="#fig26">Fig. 26</a>.</p> -<div class="img" id="fig26"> -<img src="images/img052.jpg" alt="" width="151" height="301" /> -<p class="pcap">Fig. 26.</p> -</div> -<p class="pq">Why do salt and other substances dissolve -in water?</p> -<p class="pa">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.</p> -<h3 id="c41">Gravitation.</h3> -<p class="pq">When a ball is thrown into the air, why does it fall back to -the earth?</p> -<p class="pa">Because the earth attracts the ball.</p> -<p class="pq">Why do rain and snow fall to the earth?</p> -<p class="pa">Because the earth attracts them.</p> -<p class="pq">What is this kind of attraction called?</p> -<p class="pa">It is called attraction of <i>Gravitation</i>.</p> -<p class="pq">Does the attraction of gravitation exist in all things?</p> -<p class="pa">Yes; every substance attracts every other substance.</p> -<div class="pb" id="Page_142">142</div> -<p class="pq">Upon what does the attraction of gravitation in a body depend?</p> -<p class="pa">It depends upon the size and weight of that body.</p> -<p class="pq">Do larger bodies attract with more force than smaller ones?</p> -<p class="pa">Yes; larger bodies do attract more than smaller ones.</p> -<p class="pq">Do heavy bodies attract with more force than light ones?</p> -<p class="pa">Yes; heavy bodies do attract more than light ones, -although they may be of the same size.</p> -<p class="pq">Where are all substances drawn by the attraction of gravitation?</p> -<p class="pa">They are all drawn towards the centre of the earth.</p> -<p class="pq">Why are substances drawn towards the centre of the earth?</p> -<p class="pa">Because the earth is larger than any substance on -or near its surface.</p> -<p class="pq">In what direction do all objects fall through the air?</p> -<p class="pa">They all fall towards the centre of the earth.</p> -<p class="pq">Which way then do we call “down”?</p> -<p class="pa"><i>Down</i> is always towards the centre of the earth.</p> -<p class="pq">Which way do we call “up”?</p> -<p class="pa"><i>Up</i> is always away from the centre of the earth.</p> -<p class="pq">Where must all plumb lines or perpendicular lines point?</p> -<p class="pa">They must all point with one end towards the centre -of the earth.</p> -<p class="pq">When the earth turns over, can any substance fall away from -its surface?</p> -<p class="pa">No; all things are drawn to the earth with so much -force that they cannot fall away.</p> -<p class="pq">Upon what does the weight of a body depend?</p> -<p class="pa">The weight of a body depends upon the force with -which it is drawn towards the earth.</p> -<div class="pb" id="Page_143">143</div> -<p class="pq">What kind of substances are attracted with the most force?</p> -<p class="pa">Those that are the most dense, like the metals; -hence, they are the heaviest.</p> -<p class="pq">What causes a liquid to form into a drop at the mouth of a -bottle?</p> -<p class="pa">The attraction of cohesion causes the drop to be -formed.</p> -<p class="pq">What causes the drop to fall to the earth?</p> -<p class="pa">The attraction of gravitation causes the drop to fall -to the earth.</p> -<p class="pq">When water is poured from a tumbler, why -does it run down the side of the tumbler?</p> -<p class="pa">Because the attraction of cohesion -draws the water to the side of the tumbler, -as is shown in <a href="#fig27">Fig. 27</a>.</p> -<div class="img" id="fig27"> -<img src="images/img053.jpg" alt="" width="200" height="232" /> -<p class="pcap">Fig. 27.</p> -</div> -<p class="pq">Why does a spout prevent the water from running -down the side of the vessel?</p> -<p class="pa">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 <a href="#fig28">Fig. 28</a>.</p> -<div class="img" id="fig28"> -<img src="images/img054.jpg" alt="" width="300" height="231" /> -<p class="pcap">Fig. 28.</p> -</div> -<p class="pq">Does the air obstruct bodies when -falling through it?</p> -<p class="pa">Yes; air obstructs all bodies -when falling through it; but it obstructs light bodies -more than heavy ones.</p> -<p class="pq">Do all bodies fall with equal rapidity in a vacuum?</p> -<p class="pa">Yes; all bodies fall with equal rapidity in a vacuum.</p> -<p class="pq">What bodies fall through the air most rapidly?</p> -<p class="pa">Those that are the heaviest or most dense; because -they are the smallest in bulk.</p> -<div class="pb" id="Page_144">144</div> -<p class="pq">Do heavy bodies increase in velocity as they descend?</p> -<p class="pa">They do increase rapidly in velocity as they -descend.</p> -<p class="pq">How far does a heavy body like lead fall in one second?</p> -<p class="pa">It falls sixteen feet in one second.</p> -<p class="pq">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.</p> -<p class="pa">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 <a href="#fig29">Fig. 29</a>.</p> -<div class="img" id="fig29"> -<img src="images/img055.jpg" alt="" width="147" height="399" /> -<p class="pcap">Fig. 29.</p> -</div> -<p class="pq">How may the distance a body falls in a given -time be found?</p> -<p class="pa">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.</p> -<p class="pq">What familiar example have we of the force of falling bodies?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_145">145</div> -<h3 id="c42">Centre of Gravity.</h3> -<p class="pq">What is the centre of gravity in a body?</p> -<p class="pa">The <i>centre of gravity</i> in a body, is the point on -which the body may be balanced, -as is shown in <a href="#fig30">Fig. 30</a>.</p> -<div class="img" id="fig30"> -<img src="images/img056.jpg" alt="" width="250" height="160" /> -<p class="pcap">Fig. 30.</p> -</div> -<p class="pq">Where is the centre of gravity in a -circle?</p> -<p class="pa">It is the centre of the circle.</p> -<p class="pq">Where is the centre of gravity in a -ball?</p> -<p class="pa">It is the centre of the ball.</p> -<p class="pq">What influence does the earth have on the centre of gravity?</p> -<p class="pa">The earth draws the centre of gravity in a body as -near to itself as possible.</p> -<p class="pq">Why will an egg, when placed on either end, fall over on its -side?</p> -<p class="pa">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 <a href="#fig31">Fig. 31</a> and <a href="#fig32">Fig. 32</a>.</p> -<div class="img" id="fig31"> -<img src="images/img057.jpg" alt="" width="152" height="200" /> -<p class="pcap">Fig. 31.</p> -</div> -<div class="img" id="fig32"> -<img src="images/img058.jpg" alt="" width="200" height="143" /> -<p class="pcap">Fig. 32.</p> -</div> -<p class="pq">Why will a ball remain in any position when placed on the -earth?</p> -<p class="pa">Because its centre of gravity is always the same -distance from the earth, whatever may be its position.</p> -<div class="pb" id="Page_146">146</div> -<p class="pq">When will a body change its position if left unsupported?</p> -<p class="pa">A body will change its position whenever by so -doing its centre of gravity will be lowered.</p> -<p class="pq">When may a body be readily moved about?</p> -<p class="pa">When its centre of gravity always remains at the -same distance from the earth while being moved.</p> -<p class="pq">Why is a round body, like a ball, easily rolled about the floor?</p> -<p class="pa">Because its centre of gravity remains at the same -distance from the floor.</p> -<p class="pq">When is the position of a body difficult to change?</p> -<p class="pa">Whenever the centre of gravity in it has to be -lifted up in order to make the change, as is shown in -<a href="#fig35">Fig. 35</a>.</p> -<div class="img" id="fig33"> -<img src="images/img059.jpg" alt="" width="250" height="212" /> -<p class="pcap">Fig. 33.</p> -</div> -<div class="img" id="fig34"> -<img src="images/img060.jpg" alt="" width="300" height="190" /> -<p class="pcap">Fig. 34.</p> -</div> -<div class="img" id="fig35"> -<img src="images/img061.jpg" alt="" width="200" height="172" /> -<p class="pcap">Fig. 35.</p> -</div> -<p class="pq">The centre of gravity, in <a href="#fig33">Fig. 33</a>, may be lowered; hence, it -will change its position if unsupported. The -centre of gravity, in <a href="#fig34">Fig. 34</a>, can neither be -lowered nor lifted up; hence, it may be -readily rolled about. The centre of gravity, -in <a href="#fig35">Fig. 35</a>, must be lifted up; hence, it remains -firm in its place.</p> -<p class="pq">When does an object stand firm?</p> -<p class="pa">It stands firm when it has a broad -base, and its centre of gravity is near the earth.</p> -<div class="pb" id="Page_147">147</div> -<p class="pq">What effect is produced by elevating the centre of gravity?</p> -<p class="pa">The higher the centre of gravity in any body is -raised above the earth, the greater is its tendency -to fall.</p> -<p class="pq">Why is a load of hay more easily overturned than a load of -iron, when upon the same wagon?</p> -<p class="pa">Because the centre of gravity in the load of hay is -much higher than it is in the load of iron.</p> -<p class="pq">Why do passengers on top of a stage cause it to overturn more -easily?</p> -<p class="pa">Because their weight on the top raises the centre of -gravity in the stage, and it is more readily overturned.</p> -<p class="pq">Why is a person more likely to fall when standing up than -when sitting down?</p> -<p class="pa">Because the centre of gravity is higher in a person -when standing than when sitting.</p> -<p class="pq">Why is a boy more likely to fall when raised on stilts?</p> -<p class="pa">Because the centre of gravity in the boy is farther -from the earth when he is raised on stilts.</p> -<p class="pq">When will a load of hay be overturned?</p> -<p class="pa">It will be overturned whenever -the line from the centre of -gravity to the earth falls outside -of the wheels, as is shown -in <a href="#fig36">Fig. 36</a>.</p> -<div class="img" id="fig36"> -<img src="images/img062.jpg" alt="" width="400" height="425" /> -<p class="pcap">Fig. 36.</p> -</div> -<p class="pq">Why should lamps have large -bottoms?</p> -<p class="pa">Lamps should have large -bottoms so that the line of -gravity may not fall outside of the bottom.</p> -<div class="pb" id="Page_148">148</div> -<p class="pq">Where must the line of gravity in a body always fall?</p> -<p class="pa">It must always fall within the base of that body, or -the body will be overturned.</p> -<p class="pq">How must a leaning tower be built so as not to fall?</p> -<p class="pa">It must be built so that the line of gravity will fall -within its base.</p> -<p class="pq">Why do we lean back when carrying a load in front of us?</p> -<p class="pa">We lean back so that the line of gravity may be -kept directly over our feet.</p> -<p class="pq">Why does a person standing up in a wagon fall when the wagon -is suddenly moved?</p> -<p class="pa">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.</p> -<p class="pq">Why does a person standing up in a wagon fall when the -wagon is suddenly stopped?</p> -<p class="pa">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.</p> -<p class="pq">Why do animals with <i>four</i> feet stand firmer than those with -only <i>two</i> feet?</p> -<p class="pa">Because the four feet afford a larger base to stand -upon than the two feet do, and, therefore, support the -centre of gravity better.</p> -<p class="pq">Why can a horse or a cow remain standing for many hours -without seeming to be weary?</p> -<p class="pa">Because their four feet afford a good support to the -centre of gravity.</p> -<div class="pb" id="Page_149">149</div> -<p class="pq">Why does a person who is standing soon become weary?</p> -<p class="pa">Because it requires a constant effort to keep the -line of gravity over the feet, and this soon produces -weariness.</p> -<p class="pq">Why can a person sit longer than he can stand?</p> -<p class="pa">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.</p> -<p class="pq">Why can a person lie down longer than he can sit or stand?</p> -<p class="pa">Because the centre of gravity is nearest the earth -when he is lying down, and it requires no effort to remain -in that position.</p> -<p class="pq">How do we measure time?</p> -<p class="pa">We measure time by means of a <i>pendulum</i>, such as -is used in a common clock.</p> -<p class="pq">How does the pendulum move?</p> -<p class="pa">The pendulum moves regularly -to and fro, from the line -of gravity.</p> -<p class="pq">Does the pendulum vary any in -its vibrations, or the time of moving -to and fro?</p> -<p class="pa">The pendulum does not -vary any in its vibrations.</p> -<div class="img" id="fig37"> -<img src="images/img063.jpg" alt="" width="200" height="297" /> -<p class="pcap">Fig. 37.</p> -</div> -<p class="pq">Why does the pendulum keep -moving?</p> -<p class="pa">When the pendulum is at -A, in <a href="#fig37">Fig. 37</a>, 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 -<span class="pb" id="Page_150">150</span> -motion it gains while falling to C carries it back to -A again.</p> -<p class="pq">What helps to keep the pendulum in motion?</p> -<p class="pa">The works inside of the clock help to keep the pendulum -in motion.</p> -<p class="pq">How long must the pendulum be, so as to tick once in every -second?</p> -<p class="pa">The pendulum must be thirty-nine inches long.</p> -<p class="pq">How does it tick when the pendulum is <i>shortened</i>?</p> -<p class="pa">It ticks more <i>rapidly</i>.</p> -<p class="pq">How does it tick when the pendulum is <i>lengthened</i>?</p> -<p class="pa">It ticks more <i>slowly</i>.</p> -<p class="pq">How then does <i>cold</i> affect a clock?</p> -<p class="pa">Cold makes the clock go faster by shortening the -pendulum.</p> -<p class="pq">How does <i>heat</i> affect a clock?</p> -<p class="pa">Heat makes the clock go more slowly by lengthening -the pendulum.</p> -<p class="pq">How may the pendulum always be kept of the same length?</p> -<p class="pa">By having the weight at the bottom of the pendulum, -so that it may be raised or lowered by means of -a screw.</p> -<h3 id="c43">Specific Gravity.</h3> -<p class="pq">Upon what does the weight of a body, when in the air, depend?</p> -<p class="pa">The weight of a body depends upon the force with -which it is drawn towards the earth.</p> -<p class="pq">What is this attractive force called?</p> -<p class="pa">It is called gravitation or gravity.</p> -<div class="pb" id="Page_151">151</div> -<p class="pq">What is gravity?</p> -<p class="pa"><i>Gravity</i> is the weight of any body.</p> -<p class="pq">What is specific gravity?</p> -<p class="pa"><i>Specific Gravity</i> is the weight of any body, compared -with an equal bulk of water.</p> -<p class="pq">Do substances weigh as much in water as they do in air?</p> -<p class="pa">No; they do not weigh so much.</p> -<p class="pq">When we drop lead in a vessel full of water, what becomes -of the water?</p> -<p class="pa">The water is forced over the sides of the vessel.</p> -<p class="pq">How much water is thus forced over the sides of the vessel?</p> -<p class="pa">A quantity equal in bulk to the lead dropped into -the vessel.</p> -<p class="pq">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?</p> -<p class="pa">It will weigh one pound less.</p> -<p class="pq">Then how much less is the weight of a body in water than it -is in air?</p> -<p class="pa">Its weight in the water is as much less as the weight -of the water which it displaces.</p> -<p class="pq">When a body has greater specific gravity than water, what will -it do?</p> -<p class="pa">It will sink in the water.</p> -<p class="pq">When it has less specific gravity than water, what will it do?</p> -<p class="pa">It will float on the surface of the water.</p> -<p class="pq">Will all parts of the body be above the surface of the water?</p> -<p class="pa">No; a part of it will sink into the water.</p> -<p class="pq">How much of it will sink into the water?</p> -<p class="pa">It will sink so as to displace water enough to balance -its own weight.</p> -<div class="pb" id="Page_152">152</div> -<p class="pq">Why does a boat float on the water?</p> -<p class="pa">Because it weighs less than a bulk of water of the -same size as the boat.</p> -<p class="pq">When a cargo is placed on the boat, why does it still float?</p> -<p class="pa">Because it still weighs less than a bulk of water of -the same size as the boat.</p> -<p class="pq">How do we make use of this principle?</p> -<p class="pa">We make use of it in the construction of vessels, -so as to carry on trade with other parts of the world.</p> -<p class="pq">Why can ducks and geese float on the water?</p> -<p class="pa">Because their light, downy feathers make their -specific gravity less than that of the water; hence, -they must float on its surface.</p> -<p class="pq">What may be said of the specific gravity of the human body?</p> -<p class="pa">When the lungs are filled with air, the specific -gravity of the human body is rather less than that -of water.</p> -<p class="pq">Can a person float in the water?</p> -<p class="pa">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.</p> -<p class="pq">What is the specific gravity of fish?</p> -<p class="pa">The specific gravity of fish is so nearly equal to -that of water, that they can move about anywhere -beneath its surface.</p> -<p class="pq">How do fish rise and sink in the water?</p> -<p class="pa">Fish rise and sink in the water by means of an air-bladder -in their bodies.</p> -<div class="pb" id="Page_153">153</div> -<p class="pq">How does the air-bladder help the fish to rise or sink?</p> -<p class="pa">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.</p> -<p class="pq">Why does cream rise to the surface of milk?</p> -<p class="pa">Because the specific gravity of the cream is less -than that of the milk.</p> -<p class="pq">Why does oil float on water?</p> -<p class="pa">Because the specific gravity of the oil is less than -that of the water.</p> -<p class="pq">Why can a fat person float more easily in the water than a -lean person?</p> -<p class="pa">Because the fat reduces the specific gravity of the -body, so that it floats more easily.</p> -<p class="pq">Why can a person float more easily in salt water than in fresh -water?</p> -<p class="pa">Because salt increases the weight of the water, and, -therefore, it buoys up with more force.</p> -<p class="pq">What familiar example have we of this?</p> -<p class="pa">An egg will sink when placed in fresh water; but -if salt be dissolved in the water, the egg will float -near the surface.</p> -<div class="img"> -<img src="images/img064.jpg" alt="Decoration." width="200" height="64" /> -</div> -<div class="pb" id="Page_154">154</div> -<div class="img"> -<img src="images/img065.jpg" alt="Decoration." width="500" height="180" /> -</div> -<h2 id="c44">CHAPTER IX. -<br /><span class="sc">Motion.</span></h2> -<div class="img"> -<img src="images/img066.jpg" alt="Illustrated Capital W" width="195" height="200" /> -</div> -<p class="pq">When is a body said to be in motion?</p> -<p class="pa">A body is in <i>motion</i> when it is moving -from one place to another.</p> -<p class="pq">When is a body said to be at rest?</p> -<p class="pa">A body is at rest when it is not moving -from one place to another.</p> -<p class="pq">What is necessary to give motion to a body?</p> -<p class="pa">Force of some kind is necessary to give motion to -a body.</p> -<p class="pq">How long will the motion given to a body continue?</p> -<p class="pa">The motion will continue until it is checked.</p> -<p class="pq">What is necessary to check the motion of a body?</p> -<p class="pa">Force of some kind is necessary to check its motion.</p> -<p class="pq">What force is always acting so as to bring bodies to a state of -rest?</p> -<p class="pa">The force of gravity, which is constantly drawing -all bodies to the earth.</p> -<p class="pq">What else may check the motion of a body?</p> -<p class="pa">The air, the water, and solids of all kinds may -check the motion of a body.</p> -<div class="pb" id="Page_155">155</div> -<p class="pq">In what direction will a body move when its course is not -obstructed?</p> -<p class="pa">It will move in a straight line.</p> -<p class="pq">Why does a ball struck by a bat make a curve to the earth?</p> -<p class="pa">Because the force of gravity gradually overcomes -the force given by the bat, so that the ball moves in -a curve.</p> -<p class="pq">Why does a bullet make a curve to the earth?</p> -<p class="pa">Because the force of gravity gradually overcomes -the force given by the powder, so that the bullet -moves in a curve to the earth.</p> -<p class="pq">When a person aims directly at a mark some distance from -him, where will the bullet strike?</p> -<p class="pa">The bullet will strike below the mark.</p> -<p class="pq">Why will the bullet strike below the mark?</p> -<p class="pa">Because the force of gravity draws it towards the -earth, while it is going from the gun to the mark.</p> -<p class="pq">When a ball attached to a string is whirled around, as in <a href="#fig38">Fig. 38</a>, -what is the tendency of the ball?</p> -<p class="pa">The tendency of the ball is to fly off in a straight -line.</p> -<div class="img" id="fig38"> -<img src="images/img067.jpg" alt="" width="500" height="255" /> -<p class="pcap">Fig. 38.</p> -</div> -<p class="pq">What is the force tending to make the ball fly off called?</p> -<p class="pa">It is called <i>centrifugal</i> force.</p> -<div class="pb" id="Page_156">156</div> -<p class="pq">What keeps the ball from flying off?</p> -<p class="pa">The string keeps the ball from flying off.</p> -<p class="pq">What is the force exerted by the string called?</p> -<p class="pa">It is called <i>centripetal</i> force.</p> -<p class="pq">What then is centrifugal force?</p> -<p class="pa">It is the force tending to fly off from the centre.</p> -<p class="pq">What is centripetal force?</p> -<p class="pa">It is the force tending to draw towards the centre.</p> -<p class="pq">Why does mud fly off from the wheel of a wagon when in -motion?</p> -<p class="pa">Because the motion of the wheel gives centrifugal -force to the mud, and causes it to fly off.</p> -<p class="pq">What does the rim of a wheel when in motion, tend to do?</p> -<p class="pa">The rim tends to fly off from the centre of the -wheel.</p> -<p class="pq">How is it prevented from flying off?</p> -<p class="pa">It is prevented by making the wheel very strong, so -that all parts of it are held firmly together.</p> -<p class="pq">What does this centrifugal force sometimes cause?</p> -<p class="pa">The centrifugal force is so great that it sometimes -causes grind-stones and mill-stones, when revolving -rapidly, to burst into pieces.</p> -<p class="pq">What becomes of these stones when they burst?</p> -<p class="pa">When they burst, their pieces fly off in different -directions, doing more or less injury to whatever obstructs -their course.</p> -<p class="pq">When a horse turns a corner, why does the rider lean in the -direction in which the horse is turning?</p> -<p class="pa">He leans so as to overcome the centrifugal force, -and is thus prevented from falling off the horse.</p> -<div class="pb" id="Page_157">157</div> -<p class="pq">How are cars prevented from running off the track, while -turning a curve?</p> -<p class="pa">The outer rail is made higher than the inner rail, -so as to overcome the tendency of the cars to run off -the track.</p> -<p class="pq">What grand example have we of the action of these two forces?</p> -<p class="pa">The earth and other planets that move around the -sun, are kept in their places by the action of these -two forces.</p> -<p class="pq">When a ball strikes a solid body and bounds back, what is -that motion called?</p> -<p class="pa">It is called <i>reflected</i> motion.</p> -<p class="pq">At what angle does the ball leave the solid body?</p> -<p class="pa">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 -<a href="#fig39">Fig. 39</a>.</p> -<div class="img" id="fig39"> -<img src="images/img068.jpg" alt="" width="300" height="156" /> -<p class="pcap">Fig. 39.</p> -</div> -<p class="pq">How may a stone be skipped on a smooth pond?</p> -<p class="pa">A stone may be skipped, by throwing it so obliquely -that the water will cause it to bound off from its -surface.</p> -<p class="pq">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?</p> -<p class="pa">The boat will go diagonally across the river.</p> -<p class="pq">Why will the boat go diagonally across the river?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_158">158</div> -<p class="pq">How do boatmen overcome this difficulty?</p> -<p class="pa">They overcome this by rowing the boat towards a -point higher up the stream.</p> -<p class="pq">Does it take longer to cross a stream when the current is strong?</p> -<p class="pa">Yes; because the boat is carried farther down the -stream by the current, and it must, therefore, go a -greater distance in crossing.</p> -<p class="pq">Why is a person apt to fall when he jumps from a wagon that -is moving?</p> -<p class="pa">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.</p> -<p class="pq">How can he prevent falling?</p> -<p class="pa">He can prevent falling by jumping in the same -direction as that in which the wagon is moving.</p> -<p class="pq">When a stone is dropped from the top of a mast of a vessel, -where does it fall?</p> -<p class="pa">The stone falls at the bottom of the mast.</p> -<p class="pq">Why does the stone fall at the bottom of the mast?</p> -<p class="pa">Because it partakes of the motion of the vessel, and -is carried forward while falling.</p> -<p class="pq">When a ball is thrown perpendicularly into the air by a -person on a boat, where does the ball fall?</p> -<p class="pa">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.</p> -<p class="pq">Why does the ball fall back into his hand?</p> -<p class="pa">Because it partakes of the motion of the boat, and -moves as far as the boat moves.</p> -<div class="pb" id="Page_159">159</div> -<p class="pq">When a stone thrown from a car is aimed at a post, where -will it strike?</p> -<p class="pa">The stone will strike ahead of the post.</p> -<p class="pq">How far ahead of the post will the stone strike?</p> -<p class="pa">It will strike as far ahead as the car moves while -the stone is going to the post.</p> -<p class="pq">Where must we aim so as to hit a bird that is flying?</p> -<p class="pa">We must aim ahead of the bird.</p> -<p class="pq">Why must we aim ahead of the bird?</p> -<p class="pa">Because the bird will move a short distance through -the air while the shot is reaching it.</p> -<p class="pq">When a body in motion strikes one at rest, is the force felt -alike by both bodies?</p> -<p class="pa">The force is felt alike by both bodies; thus, when one -head is struck against another, both are equally hurt.</p> -<p class="pq">Do bodies, when moving rapidly, strike with more force than -when moving slowly?</p> -<p class="pa">Yes; the force with which they strike depends upon -their velocity.</p> -<p class="pq">What examples of this force may be given?</p> -<p class="pa">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.</p> -<p class="pq">Why do large bodies start slowly?</p> -<p class="pa">Because it takes some time for the force to reach -all parts of them.</p> -<p class="pq">Why is it difficult for horses to start a loaded wagon?</p> -<p class="pa">Because they must pull some time before the force -reaches all parts of the wagon, causing it to move.</p> -<div class="pb" id="Page_160">160</div> -<p class="pq">Why is machinery slow at starting?</p> -<p class="pa">Because it requires time for the force applied to -reach all parts of the machinery.</p> -<p class="pq">When two solid bodies, moving slowly, strike against each -other, what takes place?</p> -<p class="pa">They are pushed off from each other in opposite -directions.</p> -<p class="pq">Which body is pushed off the more rapidly?</p> -<p class="pa">The one having the less weight, is pushed off the -more rapidly.</p> -<p class="pq">When the oars of a boat push against the water, what takes -place?</p> -<p class="pa">The boat and the water move in opposite directions.</p> -<p class="pq">When the wings of a bird strike the air, what takes place?</p> -<p class="pa">The bird and the air move in opposite directions.</p> -<p class="pq">How can a duck or a goose swim in the water?</p> -<p class="pa">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.</p> -<p class="pq">Can all birds swim in the water?</p> -<p class="pa">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.</p> -<p class="pq">What is produced by the rubbing of bodies together?</p> -<p class="pa"><i>Friction</i> is produced by rubbing bodies together.</p> -<p class="pq">What does friction tend to do?</p> -<p class="pa">Friction tends to check the motion of bodies.</p> -<p class="pq">How do we lessen the friction in machinery?</p> -<p class="pa">We lessen the friction in machinery by oiling the -parts where they rub together.</p> -<div class="pb" id="Page_161">161</div> -<p class="pq">How are waves produced in the water?</p> -<p class="pa">Waves are produced by the friction of the air upon -the water.</p> -<p class="pq">What causes the water to flow in creeks and rivers?</p> -<p class="pa">The attraction of gravitation, which causes water -always to fall to its lowest level.</p> -<p class="pq">What retards the current of water in streams?</p> -<p class="pa">The friction, caused by the water rubbing against -the banks and bottoms of the streams, retards their -current.</p> -<p class="pq">In what part of a stream is the current the strongest?</p> -<p class="pa">It is strongest in the middle of a stream, because -there is less friction there.</p> -<p class="pq">What retards the flow of water through a pipe?</p> -<p class="pa">The friction of the water against the sides of the -pipe, retards its flow very much.</p> -<p class="pq">Why do sudden turns in a pipe check the flow of water -through it?</p> -<p class="pa">Because every turn in the pipe increases the friction -made by the water.</p> -<p class="pq">Why does a wagon-wheel turn around when the wagon moves?</p> -<p class="pa">Because the friction between the wheel and the -earth, prevents the wheel from sliding along.</p> -<p class="pq">Why can a steam-engine draw a train of heavy cars?</p> -<p class="pa">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.</p> -<p class="pq">Why is sand sometimes put on the rails?</p> -<p class="pa">Sand is put on the rails to increase the friction between -<span class="pb" id="Page_162">162</span> -the wheels and the rails, so that the engine may -start a heavier train.</p> -<p class="pq">Why do we use sleighs when there is snow on the ground?</p> -<p class="pa">Because sleighs slip easily on the snow.</p> -<p class="pq">Why do sleighs slip easily on the snow?</p> -<p class="pa">Because there is but little friction between the -sleigh and the snow.</p> -<p class="pq">Why do wagons run more heavily when the ground is soft?</p> -<p class="pa">Because their wheels then sink farther into the -earth, and thus the friction is increased.</p> -<p class="pq">Why do wagons run more heavily when the ground is covered -with snow?</p> -<p class="pa">Because their wheels sink into the snow, and thus -the friction is increased.</p> -<p class="pq">Why will a person slip down upon the ice more easily than -upon the earth?</p> -<p class="pa">Because there is less friction between his feet and -the ice than there is between his feet and the earth.</p> -<p class="pq">Why do we put ashes on our walks when they are covered -with ice?</p> -<p class="pa">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.</p> -<p class="pq">Why is a lock or a brake used on a wagon?</p> -<p class="pa">It is used to check the motion of the wagon.</p> -<p class="pq">How does a lock check the motion of the wagon?</p> -<p class="pa">The lock is pressed against the wheels, so that -enough friction is produced to check the motion of -the wagon.</p> -<div class="pb" id="Page_163">163</div> -<p class="pq">Why does it require more force to draw a rusty plough -through the ground, than to draw one that is bright and -smooth?</p> -<p class="pa">Because the rust increases the friction between the -plough and the ground, and this makes more force -necessary in order to move it.</p> -<p class="pq">Why cannot we hold a live eel in our hand, as easily as we -can a live fish?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img069.jpg" alt="Explorer." width="400" height="358" /> -</div> -<div class="pb" id="Page_164">164</div> -<div class="img"> -<img src="images/img070.jpg" alt="Decoration." width="500" height="177" /> -</div> -<h2 id="c45">CHAPTER X. -<br /><span class="sc">Mechanical Powers.</span></h2> -<h3 id="c46"><span class="sc">Lever.</span></h3> -<div class="img"> -<img src="images/img071.jpg" alt="Illustrated Capital W" width="199" height="200" /> -</div> -<p class="pq">What is a lever?</p> -<p class="pa">A <i>Lever</i> is a rod or bar of some kind -used for raising weights.</p> -<p class="pq">Of what are levers made?</p> -<p class="pa">Levers are made of any solid substance, -such as wood and iron.</p> -<p class="pq">When a spoon is balanced on a cup, as in <a href="#fig40">Fig. 40</a>, what may -the spoon be called?</p> -<p class="pa">The spoon may then be called -a lever.</p> -<p class="pq">Upon what does the spoon rest?</p> -<p class="pa">It rests upon the edge of the -cup.</p> -<div class="img" id="fig40"> -<img src="images/img072.jpg" alt="" width="300" height="144" /> -<p class="pcap">Fig. 40.</p> -</div> -<p class="pq">What is the cup on which the spoon rests called?</p> -<p class="pa">It is called a <i>Fulcrum</i>.</p> -<p class="pq">What is a fulcrum?</p> -<p class="pa">A fulcrum is any solid body against which a lever -rests.</p> -<p class="pq">When the spoon is called a lever, what are the parts on each -side of the fulcrum called?</p> -<p class="pa">They are called the <i>arms</i> of the lever.</p> -<div class="pb" id="Page_165">165</div> -<p class="pq">What is necessary, so that a lever may balance?</p> -<p class="pa">The arms of the lever must be of equal weight, or -they will not balance.</p> -<p class="pq">If the bowl of the spoon be filled with water, what will result?</p> -<p class="pa">The bowl will sink, and the handle of the spoon -will rise.</p> -<p class="pq">What will cause the bowl of the spoon to sink?</p> -<p class="pa">The weight of the water in it will cause it to sink.</p> -<p class="pq">How can the bowl of the spoon be prevented from sinking?</p> -<p class="pa">By applying a force to the handle of the spoon, -equal to the weight of the water put in the bowl.</p> -<p class="pq">What is the force applied to the handle called?</p> -<p class="pa">It is called the <i>Power</i>.</p> -<p class="pq">What three things are always necessary when the lever is -used?</p> -<p class="pa">The power, the <i>fulcrum</i>, and the <i>weight</i>, are always -necessary when the lever is used.</p> -<p class="pq">Which of these three things are movable?</p> -<p class="pa">The power and the weight are movable.</p> -<p class="pq">Which one does not move?</p> -<p class="pa">The fulcrum does not move.</p> -<p class="pq">How many kinds of levers are there?</p> -<p class="pa">There are three kinds of levers.</p> -<p class="pq">What is the first kind of lever?</p> -<p class="pa">It is where the fulcrum is between the power and -the weight, as is shown in <a href="#fig41">Fig. 41</a>.</p> -<div class="img" id="fig41"> -<img src="images/img073.jpg" alt="" width="500" height="146" /> -<p class="pcap">Fig. 41.</p> -</div> -<div class="pb" id="Page_166">166</div> -<p class="pq">When the fulcrum is under the middle of the lever, how can -the lever be kept balanced?</p> -<p class="pa">The lever will remain balanced, so long as the -power and the weight used are equal to each other.</p> -<p class="pq">When the lever is moved, so that the fulcrum is nearer the -weight, which arm of the lever is the heavier?</p> -<p class="pa">The arm towards the power, because it is the longer -arm.</p> -<p class="pq">Will the power applied to the long arm of the lever be as -heavy as the weight?</p> -<p class="pa">No; the nearer the fulcrum is to the weight, the -less will be the power needed to balance that weight.</p> -<p class="pq">How will it be when the fulcrum is nearer to the power than -it is to the weight?</p> -<p class="pa">Then the power must be heavier than the weight.</p> -<p class="pq">When the long arm of the lever is twice the length of the -short arm, what will be the difference in the weights?</p> -<p class="pa">One pound applied to the long arm will then balance -two pounds applied to the short arm.</p> -<p class="pq">Upon what does the power of a lever depend?</p> -<p class="pa">It depends upon how much nearer the fulcrum is -placed to one end of the lever, than to the other end.</p> -<p class="pq">Of what use is the lever?</p> -<p class="pa">It is very useful in raising heavy bodies.</p> -<div class="img" id="fig42"> -<img src="images/img074.jpg" alt="" width="500" height="236" /> -<p class="pcap">Fig. 42.</p> -</div> -<div class="pb" id="Page_167">167</div> -<p class="pq">What is the weight to be lifted in <a href="#fig42">Fig. 42</a>?</p> -<p class="pa">The stone is the weight to be lifted.</p> -<p class="pq">Where is the fulcrum placed?</p> -<p class="pa">The fulcrum is placed near the stone.</p> -<p class="pq">Where is the power applied?</p> -<p class="pa">The power is applied by the hand to the long arm -of the lever.</p> -<p class="pq">How may a see-saw be made?</p> -<p class="pa">A see-saw may be made by placing a plank or a -board across a rail in a fence, so that it will balance.</p> -<p class="pq">Is the see-saw, as shown in <a href="#fig43">Fig. 43</a>, an example of the lever?</p> -<p class="pa">Yes; the see-saw is an example of the lever.</p> -<div class="img" id="fig43"> -<img src="images/img075.jpg" alt="" width="500" height="371" /> -<p class="pcap">Fig. 43.</p> -</div> -<p class="pq">What are the parts of the plank on each side of the fence-rail -called?</p> -<p class="pa">They are called the arms of the lever.</p> -<p class="pq">What is the fulcrum in the see-saw?</p> -<p class="pa">The fence-rail on which it rests, is the fulcrum.</p> -<p class="pq">What is the power in the see-saw?</p> -<p class="pa">The power is the boy on one end of the lever.</p> -<div class="pb" id="Page_168">168</div> -<p class="pq">What is the weight in the see-saw?</p> -<p class="pa">The weight is the boy on the other end of the lever.</p> -<p class="pq">How do children play at see-saw?</p> -<p class="pa">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.</p> -<p class="pq">Where must the fulcrum be when the two boys are of the same -weight?</p> -<p class="pa">The fulcrum must be under the middle of the lever.</p> -<p class="pq">Where must the fulcrum be when one boy is heavier than the -other?</p> -<p class="pa">The fulcrum must then be nearest to the heavier -boy, so that they may balance.</p> -<p class="pq">Does it require much force to play at see-saw?</p> -<p class="pa">No; when the see-saw is balanced, a very little -force will cause it to move up and down.</p> -<p class="pq">Of what use is a pair of scales?</p> -<p class="pa">A pair of scales is very useful in weighing different -substances.</p> -<div class="img" id="fig44"> -<img src="images/img076.jpg" alt="" width="400" height="270" /> -<p class="pcap">Fig. 44.</p> -</div> -<div class="pb" id="Page_169">169</div> -<p class="pq">Are the scales, as shown in <a href="#fig44">Fig. 44</a>, an example of the lever?</p> -<p class="pa">The scales are an example of the lever.</p> -<p class="pq">Where is the fulcrum of the scales?</p> -<p class="pa">The fulcrum is the point on which the lever rests.</p> -<p class="pq">What is the weight?</p> -<p class="pa">The weight is the lead put in one scale.</p> -<p class="pq">What is the power?</p> -<p class="pa">The power is the substance put in the other scale, -so as to balance the weight.</p> -<p class="pq">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?</p> -<p class="pa">Exactly one pound of sugar must be put in to -balance the lead.</p> -<p class="pq">For what is a balance mostly used?</p> -<p class="pa">A balance is mostly used for weighing heavy bodies.</p> -<p class="pq">Is the balance a lever?</p> -<p class="pa">Yes; the balance is a lever.</p> -<p class="pq">Which is the fulcrum in the balance?</p> -<p class="pa">The fulcrum is the hook that holds the balance up.</p> -<div class="img" id="fig45"> -<img src="images/img077.jpg" alt="" width="500" height="317" /> -<p class="pcap">Fig. 45.</p> -</div> -<div class="pb" id="Page_170">170</div> -<p class="pq">What is the weight?</p> -<p class="pa">The weight is the large body hanging from the short -arm of the lever.</p> -<p class="pq">What is the power?</p> -<p class="pa">The power is the smaller body hanging from the -long arm of the lever.</p> -<p class="pq">In <a href="#fig45">Fig. 45</a>, 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?</p> -<p class="pa">The large body must weigh ten pounds.</p> -<p class="pq">When the small body weighs ten pounds, how much must the -large body weigh, so as to balance it?</p> -<p class="pa">The large body must weigh ten times as much, or -one hundred pounds.</p> -<p class="pq">If the smaller body be moved half-way to the fulcrum, how -much weight will then balance it?</p> -<p class="pa">It will then take only -half the weight, or fifty -pounds to balance it.</p> -<p class="pq">What other familiar example -of this kind of lever may be -mentioned?</p> -<p class="pa">The pump-handle is a lever -of this kind.</p> -<div class="img" id="fig46"> -<img src="images/img078.jpg" alt="" width="250" height="388" /> -<p class="pcap">Fig. 46.</p> -</div> -<p class="pq">What is the power?</p> -<p class="pa">The power is the force applied -to the pump-handle.</p> -<p class="pq">What is the fulcrum?</p> -<p class="pa">The fulcrum is the screw -that fastens the handle to the -pump.</p> -<div class="pb" id="Page_171">171</div> -<p class="pq">What is the weight?</p> -<p class="pa">The weight is the water raised by the bucket.</p> -<p class="pq">Are scissors an example of the lever?</p> -<p class="pa">Yes; scissors are a double lever.</p> -<p class="pq">What is the fulcrum in the scissors?</p> -<p class="pa">The fulcrum is the rivet which holds the two levers -together.</p> -<p class="pq">What is the power?</p> -<p class="pa">The power is the force applied by the fingers.</p> -<p class="pq">What is the weight?</p> -<p class="pa">The weight is the paper, or other substance, to be -cut by the scissors.</p> -<p class="pq">Can scissors be made very strong?</p> -<p class="pa">Yes; some scissors are made so strong that they are -used for cutting iron, tin, and zinc.</p> -<p class="pq">What other useful instruments are made upon the same principle -as the scissors?</p> -<p class="pa">Such instruments as nippers, pincers, tweezers, and -blacksmith’s tongs.</p> -<p class="pq">What is the second kind of lever?</p> -<p class="pa">It is where the weight is between the power and the -fulcrum, as is shown in <a href="#fig47">Fig. 47</a>.</p> -<div class="img" id="fig47"> -<img src="images/img079.jpg" alt="" width="500" height="171" /> -<p class="pcap">Fig. 47.</p> -</div> -<p class="pq">Of what use is this kind of lever?</p> -<p class="pa">It is very useful in lifting heavy weights a short -distance from the earth.</p> -<div class="pb" id="Page_172">172</div> -<p class="pq">What example is there of this kind of lever?</p> -<p class="pa">The door on its hinges is a lever of this kind.</p> -<p class="pq">What is the fulcrum?</p> -<p class="pa">The hinges are the fulcrum.</p> -<p class="pq">What is the weight?</p> -<p class="pa">The door is the weight.</p> -<p class="pq">What is the power?</p> -<p class="pa">The force that opens or shuts the door is the power.</p> -<p class="pq">What other examples of this kind of lever have we?</p> -<p class="pa">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.</p> -<p class="pq">What is the third kind of lever?</p> -<p class="pa">It is where the power is between the fulcrum and -the weight, as is shown in <a href="#fig48">Fig. 48</a>.</p> -<div class="img" id="fig48"> -<img src="images/img080.jpg" alt="" width="500" height="142" /> -<p class="pcap">Fig. 48.</p> -</div> -<p class="pq">What examples of this kind of lever have we in common -use?</p> -<p class="pa">The shovel, the spade, and the pitch-fork, are levers -of this kind.</p> -<p class="pq">What is the fulcrum?</p> -<p class="pa">One hand is the fulcrum.</p> -<p class="pq">What is the power?</p> -<p class="pa">The other hand is the power.</p> -<p class="pq">What is the weight?</p> -<p class="pa">The earth raised on the shovel is the weight.</p> -<div class="pb" id="Page_173">173</div> -<p class="pq">What other familiar example have we of this kind of lever?</p> -<p class="pa">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 <a href="#fig49">Fig. 49</a>, representing a lad just drawing a -fish out of the water.</p> -<div class="img" id="fig49"> -<img src="images/img081.jpg" alt="" width="500" height="346" /> -<p class="pcap">Fig. 49.</p> -</div> -<p class="pq">There are several other examples of this kind of lever; will -you name some of them?</p> -<p class="pa">Yes; brooms, hoes, rakes, and axes are all levers -of this kind.</p> -<p class="pq">What is the first kind of lever mentioned?</p> -<p class="pa">It is where the fulcrum is between the power and -the weight, as in <a href="#fig50">Fig. 50</a>.</p> -<div class="img" id="fig50"> -<img src="images/img082.jpg" alt="" width="500" height="142" /> -<p class="pcap">Fig. 50.</p> -</div> -<div class="pb" id="Page_174">174</div> -<p class="pq">What is the second kind of lever mentioned?</p> -<p class="pa">It is where the weight is between the power and the -fulcrum, as in <a href="#fig51">Fig. 51</a>.</p> -<div class="img" id="fig51"> -<img src="images/img083.jpg" alt="" width="500" height="180" /> -<p class="pcap">Fig. 51.</p> -</div> -<p class="pq">What is the third kind of lever mentioned?</p> -<p class="pa">It is where the power is between the fulcrum and -the weight, as in <a href="#fig52">Fig. 52</a>.</p> -<div class="img" id="fig52"> -<img src="images/img084.jpg" alt="" width="500" height="197" /> -<p class="pcap">Fig. 52.</p> -</div> -<h3 id="c47">Wheel and Axle.</h3> -<p class="pq">What other way have we for raising weights?</p> -<p class="pa">We have the <i>wheel and axle</i>, as is shown in <a href="#fig53">Fig. 53</a>.</p> -<div class="img" id="fig53"> -<img src="images/img085.jpg" alt="" width="500" height="504" /> -<p class="pcap">Fig. 53.</p> -</div> -<div class="pb" id="Page_175">175</div> -<p class="pq">What is the wheel and axle?</p> -<p class="pa">The wheel and axle is only another form of the -lever.</p> -<p class="pq">Where is the power applied?</p> -<p class="pa">The power is applied to the rope which passes -around the wheel.</p> -<p class="pq">Where is the weight?</p> -<p class="pa">The weight is the bucket attached to the rope which -passes around the axle.</p> -<p class="pq">Where is the fulcrum?</p> -<p class="pa">The fulcrum is the frame on which the ends of the -axle rest.</p> -<p class="pq">Is the power applied to the wheel equal to the weight to be -raised?</p> -<p class="pa">No; the power is always less than the weight to be -raised.</p> -<p class="pq">Upon what do the power and the weight depend?</p> -<p class="pa">They depend upon the size of the wheel and the -axle.</p> -<p class="pq">When the diameter of the wheel is ten times the diameter of -the axle, how much will one pound -raise?</p> -<p class="pa">One pound applied to the -wheel, will then raise ten pounds -on the axle.</p> -<p class="pq">What is a windlass?</p> -<p class="pa">A <i>windlass</i> is another form of -the wheel and axle, as is shown -in <a href="#fig54">Fig. 54</a>.</p> -<div class="img" id="fig54"> -<img src="images/img086.jpg" alt="" width="405" height="367" /> -<p class="pcap">Fig. 54.</p> -</div> -<p class="pq">What is used in a windlass in place of the wheel?</p> -<p class="pa">A crank or handle is used in place of the wheel.</p> -<div class="pb" id="Page_176">176</div> -<p class="pq">Does the crank take the place of the wheel?</p> -<p class="pa">Yes; when the crank is turned, it forms a circle -the same as the outside of the wheel.</p> -<p class="pq">To what use is the windlass often applied?</p> -<p class="pa">The windlass is often used to draw water from -wells.</p> -<h3 id="c48">Pulley.</h3> -<p class="pq">What is a pulley?</p> -<p class="pa">A <i>pulley</i> is a wheel with a groove or hollow cut in -its edge.</p> -<p class="pq">What is the groove for?</p> -<p class="pa">The groove is cut so that a rope may move over the -wheel without slipping off.</p> -<div class="img" id="fig55"> -<img src="images/img087.jpg" alt="" width="191" height="300" /> -<p class="pcap">Fig. 55.</p> -</div> -<p class="pq">In <a href="#fig55">Fig. 55</a>, where is the power applied?</p> -<p class="pa">The power is applied to one end of the -rope by the hand.</p> -<p class="pq">Where is the weight?</p> -<p class="pa">The weight is attached to the other -end of the rope.</p> -<p class="pq">When the power and the weight are equal, what -do they do?</p> -<p class="pa">They balance each other, and neither of them will -move.</p> -<p class="pq">When the power is greater than the weight, what takes -place?</p> -<p class="pa">The weight is then lifted up.</p> -<p class="pq">Of what use is the single pulley?</p> -<p class="pa">The single pulley is used for hoisting flags to the -top of high poles, for raising sails to the topmasts -<span class="pb" id="Page_177">177</span> -of vessels, and for raising weights to the upper stories -of mills and factories.</p> -<p class="pq">How many pulleys are needed, when a horse is to raise the -weight?</p> -<p class="pa">When a horse is to raise the weight, two pulleys -are needed, as may be seen in <a href="#fig56">Fig. 56</a>.</p> -<div class="img" id="fig56"> -<img src="images/img088.jpg" alt="" width="350" height="404" /> -<p class="pcap">Fig. 56.</p> -</div> -<p class="pq">To what use do farmers apply the pulley?</p> -<p class="pa">They arrange the pulleys so as to lift the hay into -their barns, by means of a horse; thus saving themselves -much hard labor.</p> -<h3 id="c49">Inclined Plane.</h3> -<p class="pq">When one end of a plank is raised up, while the other end -remains on the floor, is its surface level?</p> -<p class="pa">No; its surface is not level.</p> -<p class="pq">What may the surface of the plank be called?</p> -<p class="pa">It may be called an inclined surface, or an <i>inclined -plane</i>.</p> -<div class="pb" id="Page_178">178</div> -<p class="pq">What is an inclined plane?</p> -<p class="pa">An inclined plane is a surface that inclines upwards -or downwards.</p> -<p class="pq">Of what use is the inclined plane?</p> -<p class="pa">It aids very much in raising weights.</p> -<p class="pq">Are all planes inclined alike?</p> -<p class="pa">No; some planes are much steeper than others.</p> -<div class="img" id="fig57"> -<img src="images/img089.jpg" alt="" width="500" height="270" /> -<p class="pcap">Fig. 57.</p> -</div> -<p class="pq">In <a href="#fig57">Fig. 57</a>, where is the power?</p> -<p class="pa">The power is at P.</p> -<p class="pq">Where is the weight?</p> -<p class="pa">The weight is at W.</p> -<p class="pq">Upon what does the power and the weight depend?</p> -<p class="pa">They depend upon the height and the length of the -inclined plane.</p> -<p class="pq">When an inclined plane is ten feet long and only one foot -high, how much will one pound at P balance?</p> -<p class="pa">One pound at P will balance ten pounds at W.</p> -<p class="pq">When an inclined plane ten feet long is two feet high, how -much will one pound at P balance?</p> -<p class="pa">One pound at P will then balance only five pounds -at W; because the plane is five times as long as it is -high.</p> -<div class="pb" id="Page_179">179</div> -<p class="pq">Where else may the power be exerted?</p> -<p class="pa">The power may be exerted behind the weight, as in -<a href="#fig58">Fig. 58</a>.</p> -<div class="img" id="fig58"> -<img src="images/img090.jpg" alt="" width="500" height="281" /> -<p class="pcap">Fig. 58.</p> -</div> -<p class="pq">Will any more power be needed behind the weight than -before it?</p> -<p class="pa">No; the power required is the same in both cases.</p> -<p class="pq">Would it require as much power to roll barrels into a wagon -as to lift them in?</p> -<p class="pa">No; they can be rolled into a wagon much more -easily than lifted in.</p> -<p class="pq">Can they be taken out of the wagon any more easily on an -inclined plane than without one?</p> -<p class="pa">Yes; they can be taken out more easily on an -inclined plane.</p> -<p class="pq">Why are drays, used in cities, made in the form of the inclined -plane?</p> -<p class="pa">So that barrels and hogsheads may the more easily -be rolled on them.</p> -<p class="pq">Are our stairs in the form of the inclined plane?</p> -<p class="pa">Yes; they are inclined planes with steps cut in them -to make the ascent easy.</p> -<div class="pb" id="Page_180">180</div> -<p class="pq">Why are house-roofs made in the form of the inclined plane?</p> -<p class="pa">So that the water falling on them may run off -readily.</p> -<p class="pq">What familiar example of the inclined plane may be mentioned?</p> -<p class="pa">The hills, all over the country, are so many inclined -planes.</p> -<p class="pq">Of what use are the hills?</p> -<p class="pa">The hills carry off the waste water from the earth, -so that it is kept dry enough for plants to grow.</p> -<p class="pq">What other example of the inclined plane may be mentioned?</p> -<p class="pa">The beds of the creeks and rivers are so many -inclined planes down which their waters flow.</p> -<h3 id="c50">Wedge.</h3> -<p class="pq">What is a wedge?</p> -<p class="pa">A <i>wedge</i> is a double inclined plane, as is represented -in <a href="#fig59">Fig. 59</a>.</p> -<div class="img" id="fig59"> -<img src="images/img091.jpg" alt="" width="500" height="122" /> -<p class="pcap">Fig. 59.</p> -</div> -<p class="pq">Of what use is the wedge?</p> -<p class="pa">The wedge is used in splitting logs of wood and -blocks of stone.</p> -<p class="pq">For what other purposes is the wedge used?</p> -<p class="pa">The wedge is used to fasten handles in hammers, -hatchets, axes, and hoes.</p> -<p class="pq">How is power applied to the wedge?</p> -<p class="pa">Power is applied to the wedge by means of sudden -blows from a hammer.</p> -<div class="pb" id="Page_181">181</div> -<p class="pq">What prevents the wedge from flying back after the blow?</p> -<p class="pa">The friction between the wedge and the other body, -keeps it from flying back.</p> -<p class="pq">What instruments are made in the form of a wedge?</p> -<p class="pa">Knife-blades, hatchets, axes, razors, and cutting -instruments of nearly all kinds, are in the form of the -wedge.</p> -<h3 id="c51">Screw.</h3> -<p class="pq">What is a screw?</p> -<p class="pa">A <i>screw</i> is another form of the inclined -plane.</p> -<p class="pq">Where is the inclined plane on a screw?</p> -<p class="pa">It begins at the bottom of the screw, and -winds around and around the screw until it -reaches the top, as is shown in <a href="#fig60">Fig. 60</a>.</p> -<div class="img" id="fig60"> -<img src="images/img092.jpg" alt="" width="100" height="207" /> -<p class="pcap">Fig. 60.</p> -</div> -<p class="pq">What is this inclined plane called?</p> -<p class="pa">It is called the thread of the screw.</p> -<p class="pq">How do the threads affect the screw?</p> -<p class="pa">When the threads are far apart, the inclined plane -is steep; but it becomes less steep the nearer the -threads are together.</p> -<p class="pq">When will the screw exert the most power?</p> -<p class="pa">The screw will exert the most power, when its -threads are nearest together.</p> -<p class="pq">Of what use are screws, such as is shown in <a href="#fig60">Fig. 60</a>?</p> -<p class="pa">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.</p> -<div class="pb" id="Page_182">182</div> -<p class="pq">For what other purposes is the screw used?</p> -<p class="pa">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.</p> -<div class="img" id="fig61"> -<img src="images/img093.jpg" alt="" width="440" height="396" /> -<p class="pcap">Fig. 61.</p> -</div> -<p class="pq">In what does a screw like that in -<a href="#fig61">Fig. 61</a> work?</p> -<p class="pa">It works in a nut which has -threads cut on the inside, so -that the threads of the screw -move in them.</p> -<p class="pq">Where is the power applied?</p> -<p class="pa">The power is applied to the -lever.</p> -<p class="pq">Can heavy bodies be raised by means of this screw?</p> -<p class="pa">Yes; loaded cars, and even heavy buildings, may be -raised by means of a screw of this kind.</p> -<div class="img"> -<img src="images/img094.jpg" alt="Sewing machine." width="500" height="496" /> -</div> -<div class="pb" id="Page_183">183</div> -<div class="img"> -<img src="images/img095.jpg" alt="Decoration." width="500" height="176" /> -</div> -<h2 id="c52">CHAPTER XI. -<br /><span class="sc">Electricity.</span></h2> -<div class="img"> -<img src="images/img096.jpg" alt="Illustrated Capital W" width="169" height="200" /> -</div> -<p class="pq">When a cat’s back is rubbed in the dark, what -does it give off?</p> -<p class="pa">It gives off small sparks.</p> -<p class="pq">What are these small sparks called?</p> -<p class="pa">They are called electric sparks, or <i>electricity</i>.</p> -<p class="pq">If a horse be rubbed in the winter when it is dark, what will -be given off from him?</p> -<p class="pa">Electricity will be given off from the horse.</p> -<p class="pq">When sealing-wax is rubbed with a silk handkerchief, or a -piece of flannel, what is produced?</p> -<p class="pa">Electricity is produced in the sealing-wax.</p> -<p class="pq">What will this electricity in the sealing-wax do?</p> -<p class="pa">It will attract bits of paper, straw, feathers, and -hairs, when it is held near them.</p> -<p class="pq">What other substance will act the same as the sealing-wax?</p> -<p class="pa">A dry glass tube will act in the same manner when -rubbed.</p> -<p class="pq">Where is electricity found?</p> -<p class="pa">Electricity is found in every body, and in every -thing.</p> -<div class="pb" id="Page_184">184</div> -<p class="pq">How is electricity developed?</p> -<p class="pa">Electricity is developed by motion, by friction, and -by almost every change that takes place.</p> -<p class="pq">Is electricity found in the air?</p> -<p class="pa">Yes; the air is almost constantly electrified.</p> -<p class="pq">With what is electricity often accompanied?</p> -<p class="pa">Electricity is often accompanied by heat and light.</p> -<p class="pq">How many kinds of electricity are there?</p> -<p class="pa">There are two kinds of electricity.</p> -<p class="pq">What are they called?</p> -<p class="pa">They are called <i>positive</i> and <i>negative</i> electricity.</p> -<p class="pq">What do these two kinds of electricity always do?</p> -<p class="pa">They always attract each other.</p> -<p class="pq">When one body contains positive and the other body negative -electricity, what will the two bodies do?</p> -<p class="pa">They will rush together.</p> -<p class="pq">When both bodies are positive or both are negative, what will -they do?</p> -<p class="pa">They will repel each other.</p> -<p class="pq">When a body permits electricity to pass freely through it, what -is it called?</p> -<p class="pa">It is called a <i>good conductor</i> of electricity.</p> -<p class="pq">What substances are good conductors of electricity?</p> -<p class="pa">Metals, charcoal, water, animals, vegetables, flame, -and smoke.</p> -<p class="pq">When a body does not permit the electricity to pass through -it, what is it called?</p> -<p class="pa">It is called a <i>non-conductor</i> of electricity.</p> -<p class="pq">What substances are non-conductors of electricity?</p> -<p class="pa">Gutta-percha, sulphur, glass, silk, wool, hair, feathers, -cotton, and paper.</p> -<div class="pb" id="Page_185">185</div> -<p class="pq">What is lightning?</p> -<p class="pa">Lightning is electricity passing from one place to -another.</p> -<p class="pq">When does lightning pass from one cloud to another cloud?</p> -<p class="pa">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.</p> -<p class="pq">When does lightning pass from the earth to a cloud?</p> -<p class="pa">Whenever the earth contains more electricity than -the cloud does.</p> -<p class="pq">When does lightning pass from a cloud to the earth?</p> -<p class="pa">Whenever the cloud contains more electricity than -the earth does.</p> -<p class="pq">How near are the clouds when the lightning comes to the -earth?</p> -<p class="pa">The clouds are seldom half a mile away when the -lightning comes from them to the earth.</p> -<p class="pq">What noise is made by the lightning?</p> -<p class="pa">The lightning passes so quickly through the air, -that it makes a noise called thunder.</p> -<p class="pq">Do we always hear the thunder when there is lightning?</p> -<p class="pa">No; the lightning is sometimes so far away that the -thunder cannot be heard.</p> -<p class="pq">What bodies attract the lightning to the earth?</p> -<p class="pa">Tall trees, steeples, high houses, and mountains.</p> -<p class="pq">Does the lightning do any harm?</p> -<p class="pa">Yes; it does much harm by destroying trees, houses, -animals, and, indeed, almost everything in its course.</p> -<div class="pb" id="Page_186">186</div> -<p class="pq">What does lightning sometimes do with buildings?</p> -<p class="pa">It sets the buildings on fire so that they are burned.</p> -<p class="pq">How are buildings protected from injury by lightning?</p> -<p class="pa">They are protected by means of the lightning-rod, -which was invented by Dr. Franklin.</p> -<p class="pq">Of what is the lightning-rod made?</p> -<p class="pa">The lightning-rod is made of some metal, usually of -iron or copper, which is a good conductor of electricity.</p> -<p class="pq">What does the lightning-rod do when the building is struck?</p> -<p class="pa">The lightning-rod conducts the electricity to the -earth, so that it does not injure the building.</p> -<p class="pq">Must the lightning-rod be higher than the building?</p> -<p class="pa">Yes; because the lightning is apt to strike the -highest object.</p> -<p class="pq">Of what should the point of the rod be made?</p> -<p class="pa">The point should be made of silver or platinum, so -that it will always remain bright.</p> -<p class="pq">Should the rod extend deep into the ground?</p> -<p class="pa">The rod should extend four or five feet beneath the -surface of the earth.</p> -<p class="pq">Why should ashes or charcoal be put around the bottom of the -rod?</p> -<p class="pa">Because they are good conductors and will help to -conduct the lightning into the earth.</p> -<p class="pq">How large should the lightning-rod be?</p> -<p class="pa">If made of iron, the lightning-rod should be about -three-fourths of an inch in diameter.</p> -<p class="pq">Why is glass put around the rod where it is joined to the -building?</p> -<p class="pa">The glass is a non-conductor, and prevents the -lightning from leaving the rod.</p> -<div class="pb" id="Page_187">187</div> -<p class="pq">Why is it not safe to stand near a fire during a thunder-storm?</p> -<p class="pa">Because the fire is a good conductor, and sometimes -draws the electricity into the room.</p> -<p class="pq">What part of the room is the safest place?</p> -<p class="pa">The middle of a room is thought to be the safest -place.</p> -<p class="pq">Is a feather-bed a safe place?</p> -<p class="pa">Yes; because the feathers are a non-conductor, and -will not attract the lightning.</p> -<p class="pq">Is dry air a non-conductor of electricity?</p> -<p class="pa">Yes; dry air is a non-conductor of electricity.</p> -<p class="pq">When are thunder-storms most frequent?</p> -<p class="pa">Thunder-storms are more frequent in summer than -in winter, and more so in the afternoon than in the -morning.</p> -<p class="pq">By what are thunder-storms usually attended?</p> -<p class="pa">They are usually attended by some change in the -direction of the wind.</p> -<p class="pq">Where do thunder-storms generally prevail?</p> -<p class="pa">They generally prevail in the lower regions of the -air, near the earth.</p> -<p class="pq">What is the rapidity of lightning?</p> -<p class="pa">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.</p> -<h3 id="c53">Magnetism.</h3> -<p class="pq">Of what does magnetism treat?</p> -<p class="pa"><i>Magnetism</i> treats of the property of magnets.</p> -<div class="pb" id="Page_188">188</div> -<p class="pq">What is a magnet?</p> -<p class="pa">A <i>magnet</i> is a piece of iron ore having the power of -attracting iron and steel.</p> -<p class="pq">How many kinds of magnets are there?</p> -<p class="pa">There are two kinds of magnets: natural magnets, -and artificial magnets.</p> -<p class="pq">What other name is given to a natural magnet?</p> -<p class="pa">It is called a loadstone.</p> -<p class="pq">Where are natural magnets or loadstones found?</p> -<p class="pa">They are dug out of the earth, and are found most -abundantly in Norway and Sweden.</p> -<p class="pq">What is an artificial magnet?</p> -<p class="pa">An artificial magnet is one made by magnetizing a -bar of steel.</p> -<p class="pq">How may a bar of steel be magnetized?</p> -<p class="pa">A bar of steel, when rubbed on a natural magnet, -becomes magnetized.</p> -<p class="pq">When the bar of steel is bent in the shape of a horse-shoe, -what is it called?</p> -<p class="pa">It is then called a horse-shoe magnet, from its -shape.</p> -<p class="pq">Where does a magnetic bar always point, when it is made to -move easily on a pivot?</p> -<p class="pa">One end of it always points to the north, and the -other end to the south.</p> -<p class="pq">What are the ends of the magnet called?</p> -<p class="pa">The north end is called the positive, and the south -end is called the negative.</p> -<p class="pq">What is there curious about the magnet?</p> -<p class="pa">The positive end of one magnet always attracts the -<span class="pb" id="Page_189">189</span> -negative end of every other magnet; but if the two -positive or the two negative ends are brought together, -they will repel each other.</p> -<p class="pq">Why are artificial magnets made of steel?</p> -<p class="pa">Because steel will remain magnetized, but iron soon -loses the magnetic properties.</p> -<p class="pq">What is a compass?</p> -<p class="pa">A <i>compass</i> is an instrument containing a magnetic -needle, so balanced as to move easily on a pivot.</p> -<p class="pq">In what direction does this needle always point?</p> -<p class="pa">This needle always points toward the north.</p> -<p class="pq">By whom is the compass used?</p> -<p class="pa">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.</p> -<p class="pq">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.</p> -<h3 id="c54">Electro-Magnetism.</h3> -<p class="pq">Are magnetism and electricity much alike?</p> -<p class="pa">Yes; they are so nearly alike that many persons -think they are identical.</p> -<p class="pq">What is an electro-magnet?</p> -<p class="pa">An <i>electro-magnet</i> is a magnet obtained by means of -electricity.</p> -<p class="pq">Of what is the magnet made?</p> -<p class="pa">The magnet is made of soft iron, around which are -<span class="pb" id="Page_190">190</span> -wound a great many coils of wire, and these are covered -with silk.</p> -<p class="pq">How is the soft iron made into a magnet?</p> -<p class="pa">The iron is magnetized by the passage of electricity -through the wire.</p> -<p class="pq">Of what use is this magnet?</p> -<p class="pa">It is used in the electro-magnetic telegraph.</p> -<p class="pq">When and where was the first line of telegraph built?</p> -<p class="pa">The first line of telegraph was built in 1844, from -Washington to Baltimore, by Professor Morse.</p> -<p class="pq">What is the principle of the telegraph?</p> -<p class="pa">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.</p> -<p class="pq">Does electricity move rapidly?</p> -<p class="pa">Yes; its velocity is such as to move around the -earth in a quarter of a second.</p> -<p class="pq">What is the electrical battery called?</p> -<p class="pa">The electrical battery is called a <i>key</i>.</p> -<p class="pq">What is the electro-magnet called?</p> -<p class="pa">The electro-magnet is called a <i>receiver</i>.</p> -<p class="pq">For what are the key and receiver used?</p> -<p class="pa">The key is used for sending messages, and the receiver -for receiving them.</p> -<div class="pb" id="Page_191">191</div> -<p class="pq">What must each station have?</p> -<p class="pa">Each station must have both a key and a receiver, -so as to send as well as receive messages.</p> -<p class="pq">What is the Atlantic cable?</p> -<p class="pa">The Atlantic cable is a telegraphic wire reaching -across the Atlantic Ocean, from America to Europe.</p> -<p class="pq">When was the first Atlantic cable laid?</p> -<p class="pa">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.</p> -<p class="pq">Of what use is the Atlantic cable?</p> -<p class="pa">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.</p> -<div class="img"> -<img src="images/img097.jpg" alt="Fruit bowl." width="500" height="499" /> -</div> -<div class="box"> -<h3 id="c55"><span class="smaller">THE</span> -<br /><i>Elements of Natural Philosophy</i></h3> -<p class="center">By E. J. 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