Education Library FIRST LESSONS IN PHYSICS. FOR USE IN THE UPPER GRADES OF OUR COMMON SCHOOLS. In Nature, all ia Motion, Life, and Labor." Lesson xrii- BY C. L. HOTZE, Teacher of Physic* in the Central High School, Cleveland, O. ST. LOUIS: HENDRICKS, CHITTENDEN & CO. 1872. Entered according to Act of Congress, in the year 1871, by HENDRICKS & CHITTENDEN, In che Office of the Librarian of Congress, at Washington qc PREFACE. The conviction that an elementary knowledge of some important instruments, machines and physical phenomena can and should be given in our Common Schools, has in- duced the author to prepare the present little volume. Its object is the presentation of a number of phenomena, laws, and applications of the same, specially adapted to the per- ceptive capacities of the pupils of the upper grades. Inasmuch as the demand of a large amount of time might delay the introduction of physical science into the Common School, the book has been so prepared as to secure good results in the minimum of time ever given any study in our schools, viz. : one lesson a week. Each of the thirty-nine lessons commences with a fact familiar to the child, or an easy ', little experiment, which serves as the basrs for the development of a natural law. After this law, comes the application man makes of it such as the barometer, thermometer, pump and hydrostatic press. Costly apparatus is unnecessary. A pencil , a marble , a piece of board, of India-rubber, of wire ; glass tubes, and M289988 IV PREFACE. other objects of trifling expense, are sufficient, for our pur- poses even preferable. The steam engine and other com- plicated machines should be examined when in actual use at the workshop or other places, by the class in company with the teacher, but not until after the preparatory lesson in the school-room. Like all instruction, instruction in physics should proceed in concentric circles from the near to the remote. The present volume may be considered as the first and smallest of those circles. Its usefulness in the highest grade of our Common Schools has been shown by practical experience ; the author has written it, however, with a view of intro- ducing it into the second, and even into the third. At the end of every lesson, articles in books and popular magazines are pointed out, where the pupil may find inter- esting reading matter ; and where, while thus improving his leisure time, he may collect material for composition exer- cises in school. Explanations regarding the practical working and experi- ments will be given in the Teachers Guide, to be published simultaneously with the present volume. C. L. HOTZE. CLBVKLAND, O., April 8, 1871 Preface to the Third Edition. The hearty welcome given to the first edition of this work undoubt- edly had its reason in the long-felt want of a text-book suitable for the thousands of girls and boys whose school education ends in the com- mon school. Among the many things there learned, there are few things which they remember to greater advantage than the phenomena and daily applications of the laws of gravitation, the pressure of air, the lever, the pump, the steam-engine, and the telegraph. These realities train the observing powers, instill a love for knowledge, form a preven- tive against habits of superficial reasoning, and thus tend to diminish explosions, conflagrations, and other calamities, many of which are caused by persons ignorant of the powers of nature. The merchant, the laborer, or the manufacturer will do his work the better for having had his senses trained in observing nature's operations, and his mind disciplined by scientific thought. It may safely be stated that this view is held by most educators in this country, and that the time is fast approaching when physical science will no longer be a stranger in our common schools. And yet there are a few followers of the cramming-system, who would deny the right of nature to a share in the education of the young; who would not teach about the things themselves, but merely their names nd forms. These persons consider objective instruction in the lower grades of schools as simply a transient concession to ephemeral de- mands, although, during the last two centuries, such men as Cowley, Milton, Locke, Rousseau, Pestalozzi, Whewell, and Macaulay have advocated it. In the upper grades they refuse it admission altogether, notwithstanding its introduction there is urgently pressed by the scientific men of all countries, by the entire periodical press, and the most prom- inent educators of the world. These few opponents to progress in education are joined by a still smaller class of persons who are not adverse to the introduction of VI PREFACE. physical science into the schools, but who fear, lest the appropriation of time one lesson a week ! might diminish the habitual number of arith- metical examples, geographical names, and grammatical rules, and there- by vitiate the results of the annual examinations. So some people en- tertain a groundless prejudice against the acquisition of a foreign language, on the plea that the child's English might suffer. Huxley, in his "Answers to Certain Questions by the Schools Inquiry Commis- sion," says : " Physics lie at the foundation of all science ; and if nothing else were taught, it -would be a great gain to have the youth of this country soundly instructed in the laws of the elementary forces gravi- tation, heat, light, andso forth." An English Journal, " Nature," says : " The notion, that when a child has learned to read, write, and cipher, he is educated, must be eradi'zted. These are at best but means, and are only the instruments by which education is conducted." An editorial in the "Scientific American" (January 14, 1871), ends with the following significant words : " As object teaching is a mere handmaid of science is of use only to give scientific habits of 'thought, and to convey a know- ledge of scientific facts, and is worthless without science, the pnblic should see that its introduction into our schools be carried on under the advice of scientific experts, who shall direct what is best to be taught, and advise with the adepts in teaching how such knowledge may best be imparted. As a journal having the interests of science and education at heart, desiring to see science soundly popularized, and the masses made acquainted with its technical value, we make this suggestion, and further- more ask: Is there any man of- scientific attainments in the present Board of Education ? Is there any scientific authority upon its general staff? " Physical science was introduced into the B and C grammar classes of this city last September ; the pupils have now been using First Lessons in Physics for several months, and none of their other studies have been curtailed, yet the average of the monthly examinations does not suffer on that account, and, in the opinion of our teachers, it never will. A peculiar feature connected with the use of this book one which we trust will not be brought forward as an objection is, that the children ask a great many questions more or less to the point ; and that they find no rest until they have received a satisfactory answer, either from the teacher's experiments or their own. The fact is truly surprising, that PREFACE. VH the pupils of the C grade (sixth school-year) passed a very fair examina- tion a few days ago, on questions at the end of the book which were not found too easy for the C grade of the High- school (the tenth school- year). This shows what earnestness may accomplish ; and we have but begun It may be well to state that the modern technical sense of a word sometimes conflicts with its preconceived English meaning, or use; and as a book of this kind demands language both youthful and technical, the author may be excused for having given a slightly different dress to not a few of the laws. He has omitted several of the so-called "pro- perties " of matter which are very puzzling to the young; and, for the sake of simplicity, has treated the somewhat magic "impenetrability" of air as elasticity of air. The independent terms, Force, Motion, and Heat, are better understood by young pupils than Expansive Force, Moving Force, and so forth. The text in fine print, as well as pages 83, 84 and 120, must be omitted in a lesson of less than aitf-hour's length. The development of the steam-engine will find favor from those appre- ciating the historical element in the schooL While the lessons in Optics may claim special clearness in treatment, those in Chemical Electricity, being very difficult for young learners, will need forbearance. A two- fluid element was chosen, because it may be seen in actual use at the telegraph office. The questions in fine print serve for reviews and ex- aminations, but not as equivalents for experiments. Even a brief perusal of the volume will show the author's intention not to cram the pupil with meaningless facts, to be forgotten as rapidly as they are learned. As no special scientific qualification has been re- quired of the teacher who, to-morrow, may be called upon to impart scientific instruction to her class, a text-book in the hand of the pupil seems for the present a necessity. I earnestly hope that my feeble con- triburion to so great a cause may not be judged by its shortcomings alone, and that the day may soon come when physical science shall form a regular branch of study in the common school. C. L. H. CLEVELAND, O., December I, 1871. CONTENTS. OF ATTRACTION. PAOH LESS. 1. Gravity . 9 " 3. Gravity, Specific Floating and linking.... 14 " 3. Magnetic Attraction. .--,... 17 " 4. Electric Attraction , 20 " 5. Lightning. Lightning Rods , 26 " 6. Cohesion 29 " 7. Adhesion. Capillary Attraction 83 " 8.-Eeview 36 -OF PRESSURE. LESS. 9. Elasticity... . 39 " 10. Elasticity of Air 42 * 11. Pressure of Air ... 46 " 12. Barometer. .. ....- 48 " 13. Review. .. ,..-... . 51 " 14. Inertia 53 MOTION, OF MASSES. LESS. 15. Inclined Plane. ... 56 " 16. Lever. ... ........ . 59 " 17. Pendulum 63 " 18. Communicating Vessels. Hydraf'tc Press. 67 " 19. Breathing. The Bellows 71 " 20. Common Pump 74 " 21V Forcing Pump. Fire-Engine 77 " 22. Review 8J MOLECULAR. LESS. 23. Sound . . 86 " 24. Evaporation, Fog, Clouds, Rain, Sr ->, Hail, Dew, Frost 88 " 25. Heat. Conduction of Heat 92 " 26. Draught 96 " 27. Expansion by Heat. Thermometr . 99 " 28. Thermometer Compared with Baro3T*ter. .. 102 *' 29. Atmospheric Steam-Engine 105 " 30. Steam-Engine Ill " 31. Review 118 " 32. Light. Its Sources. Direction i 121 " 33. Radiant and Specular Reflection 124 34. Visible Direction. Refraction. }. Prisms. Lenses 36. Colors 37. Chemical Electricity . . . 127 131 135 140 " 38. Electro-Magnetic Telegraph 143. " 39. Review 150 QUESTIONS 153 APPENDIX 171 GLASS AMD CORK WORKING I LESSON I. GRAVITY. i. EXPERIMENT. A stone in our hand does not fall, because the hand supports it. But if the hand is withdrawn, the stone falls, and con- tinues to fall, until prevented from falling far- 10 FIRST LESSONS IN PHYSICS. ther by another obstacle, such as the floor or the ground. Familiar Facts. Chalk, pencils, paper, pens, and India-rubber, often fall from the desk upon the floor. A stone thrown into a pond sinks to the bottom ; a sign-board blown off by the storm falls upon the side-walk; rain, snow, and hailstones, descend to us from the clouds ; and large bodies of water, when precipitated from high rocks, form waterfalls. A cat may fall from a house-top ; a careless child tumbles down stairs ; coals fall through the grate ; meal falls through the sieve, and soot through the air. Branches of fruit-trees, hanging full with fruit, break off and fall to the ground ; the lily, whose stem is broken, droops its head ; the mighty oak in the Western forests, groaning under the blows of the settler's ax, falls with a crash to the ground. Heavy rods are at- tached to maps and curtains, to draw them down. Clocks are provided with weights, which move slowly in a downward direction; the heavy anchors of vessels plunge into the depths of the ocean. Having noticed these facts, you naturally in- quire, " Why is it that all bodies near the earth have a tendency to approach the earth ? " As every State and every town has its laws, so Nature has her laws, which all bodies must obey. All the facts given above may be comprised under GRAVITY. 11 the law: All bodies fall, if unsupported; they are attracted to the earth. The force which attracts them is called the Force of Gravity. 1 2. EXPERIMENT. This stone is not supported by my hand (Fig. 1). It is merely suspended. What pre- vents it from falling ? The string. When you draw the stone a little to one side, it moves back again; it wants to stay in one place. And, observe, that the string is kept straight. The string indicates the direction in which the stone would fall, if it were left free to do so. This direction is vertical. Who does not know the plumb-line used by carpenters and bricklayers ? The direction in which a body falls, if moved by the force of grav- ity atone, is vertical. no. I. That a body, instead of approaching the earth, may sometimes do the opposite, that is, ascend into the air, is due to the influence of other forces. Thus, when a boy leaps a few feet high, he succeeds in over- coming gravity ; however, he does so only for a few moments at a time. Birds and winged insects can overcome gravity longer by means of an action peculiar to them, which we call flying. An ordinary fly makes as many as five hundred beats with its wings during a second. But as soon as the influence of other forces ceases, the body must obey the law of gravity. The powerful eagle excels in swiftness the fastest loco- motive ; yet, when pierced by a deadly shot, he drops like a stone to the hunter's feet. 12 FIRST LESSONS IN PHYSICS. 3. EXPERIMENT. Place a large book upon the hand; the hand will be pressed downward. If a small book be taken, the downward pressure is much less. The small book has not as much weight as the large one. Familiar Facts. A large stone presses itself into the ground. The weight of a heavy wagon makes deep ruts in a road. When ladies buy silk robes, they lift the article on their hands. Do you know why ? All bodies press on their support ; this pressure is called their weight. 4. EXPERIMENT. A rod balanced on the edge of the hand has equal weight on each side of the support. The direction of the rod is level, or horizontal. Now, let a crayon be suspended from each end. The rod will still be horizontal, because both crayons have like weight ; they are attracted to the earth with the same force on either side. If a number of crayons be suspended from one end of the rod, and a standard of weight, such as i, or 1 lb., from the other, we have a crude form of the scale, or balance. A balance is an instrument for weighing. The pieces of iron, brass, or lead, used as standards, are the weights. Instead of the edge of the hand, a metal pivot is used. At each end of the beam a pan is sus- pended. When a person buys a pound of sugar, why does he see that the beam of the balance is horizontal ? Did it ever enter your mind that, when buying a pound of sugar, you actually bought a quantity of GKRAVITY. 13 sugar whose force of gravity amounted to a pound? That is, you bought a mass of sugar which is attracted by the earth to the amount of a pound. It matters not to gravity of what kind a substance is. A pound of coffee is as heavy as a pound of lead ; a pound of feathers, as a pound of iron. Application. The common balance clock weights hour glasses. Bead p. 224 and fig. on Weight of the Earth in " Things not Gener- ally Known," by David A. Wells. New York: Appleton & Co. Bead chapter on "Weight of the Earth" in Bernstein's Popular Treatise on Natural Science. New York : Chr. Schmidt, 39 Centre st. Gravity, as we have seen, is the force which attracts all bodies to the earth. This force is only a portion of the universal force of attraction between all bodies on the earth as well as in the universe (planets and fixed stars). A pound- weight has very nearly the same weight all over the earth; but if taken to the moon it would have less weight; it would weigh only about }/ of a pound there. On the sun, which contains 355,000 times as much matter as our earth, the pound would have the weight of about 28 pounds. Owing to that universal force, the planets revolve around the sun. The force with which the sun and moon attract our earth causes the huge tide-waves on the ocean ; while the earth's attraction for the moon causes this planet to revolve around the earth about once every four weeks. 14 FIRST LESSONS IN PHYSIOS. LESSON II. SPECIFIC GRAVITY FLOATING AND SINKING OF SOLIDS. 5. EXPERIMENT. Take two ink-wells of the same size. Fill the one with water, the other with oil, and place them on the pans of a balance. The one containing the water will be found to be de- pressed ; evidently the water has more weight than the same bulk of oil. In common words we say, water is heavier than oil ; but we ought to say, that water has greater specific weight than oil ; that is, a bulk of water has more weight than the same bulk of oil ; or, water is denser than oil. For is not a pound of water as heavy as a pound of oil? Specific Gravity is the weight of a substance compared with the weight of a like bulk of some other substance taken as a standard. 6. EXPERIMENT. Now first pour the oil into a tumbler, and then the water. The latter being the heavier, it settles in the bottom, the oil rising above it. Thus oil floats on water, because it has not as much weight as the same bulk of water. Familiar Facts. Smoke rises high into the air; balloons ascend into the clouds. Each is lighter than a like bulk of surrounding air. SPECIFIC GRAVITY. 15 Fluids of different specific gravity place them- selves in the order of their specific gravity the heaviest below, the lightest above. 7. EXPERIMENT. Drop a stone into a tumbler filled with water; it sinks. A piece of cork would float. Upon one pan of a balance place a tumbler filled to the brim with water ; upon the other place as many weights as are necessary to establish equilibrium. Remove the tumbler and drop a stone into it. The stone will sink and some water will run over. The space now occu- pied by the stone was before occupied by water, and th.at quantity of water was borne by the water in the tumbler. Now, if the stone had no greater weight than a like bulk of water , it would likewise be borne by the water. That it has, can easily be shown by placing the tumbler with the stone in it on the balance again ; the tumbler will have more weight than it had before. 8. EXPERIMENT. An empty flask, closed with a cork, floats on water. Look how little water it displaces. It evidently has less weight than a like bulk of water. It would float even if it con- tained a few pebbles, while a bottle filled with water sinks. A body floats, if it has less weight than an equal bulk of water ; it sinks, if it has more. 16 FIRST LESSONS IN PHYSICS. Familiar Facts. As the flask, so do vessels float, though they be heavily laden. The body of a man has scarcely more weight than a like bulk of water, and will float on water, provided the chest remains filled with air. QUESTIONS. I. Why is it difficult for bathers to walk in water chin- deep? 2. In drawing water from a well, why has the bucket more weight as it emerges from the water ? 3. Why may heavy stones be lifted in water, while on dry land they can scarcely be moved? Persons who can not swim, often lose their lives on falling into the water, because, when they first sink the water closes their mouth and nose, preventing them from inhaling air. Frightened by this, they lose their presence of mind, and, instead of holding their breath, they exhale the air from their lungs. Thus they diminish their volume, and are, of course, more apt to sink. Then they foolishly extend their arms into the air; the head then naturally sinks, and, unless rescued, they are drowned. The danger would have been very slight if these persons, on falling into the water, had first held their breath, spread out their limbs, and then quietly folded their arms over the crown of the head. For, by throwing the head slightly backward, a person is enabled to keep his mouth and nose above water, and thus may save his life. If the waves run high, he must, by all means, hold his breath as long as he is sub- merged; then no water can enter his mouth. Application. By means of specific gravity the purity of liquids and the value of substances, such as gold-quartz, can be ascertained. Bead Influence of Oil on Water, p. 256, in "Things Not Generally Known," by David A. Wells. New York : Appleton & Co. MAGNETIC ATTRACTION. 17 LESSON III. MAGNETIC ATTRACTION. 9. EXPERIMENT. Suspend an iron nail by a string. The direction of the string will be verti- cal (Lesson 2). But if we bring a magnet near the nail, the string will incline toward the magnet ; the more so, the nearer the magnet is brought to the nail. On approaching it still nearer it will attach itself to the magnet, and, if detached, con- trary to gravity, will not fall. This is owing to Magnetic Attraction. Reverse the last experiment. Suspend the mag- net at one of its ends, and lay the nail on the table. Holding the nail with one hand so as to keep it steady, the magnet will be seen to move toward the nail and adhere to it in spite of gravity. This shows that Magnets and unmag- netic iron attract each other. 10. EXPERIMENT. If iron filings be placed on a piece of paper or glass, they will likewise be attracted by the magnet. The latter need not be in contact with them ; it may be placed under the paper, or even under the table. Magnetic attrac- tion, like attraction of gravity, operates also through intervening bodies. 18 FIRST LESSONS IN PHYSICS. Let the magnet be placed lengthwise in the iron filings and turned round several times. On with- drawing it we find that it is covered at the ends with long threads of the filings, while toward the middle they become shorter, and in the center of the magnet the attraction is so slight that no fil- ings adhere. From this we see that the power of a magnet resides chiefly at its ends. 11. EXPERIMENT. The ends of a magnet are called its polls. Attach the magnet at its center to a string, and suspend it from the hand. The magnet will vibrate until it finally takes a certain position, which it keeps. If disturbed, it will again vibrate, and after many vibrations, resume the same position. It will do so anywhere, in the room or out-doors. Upon examining the direction, we find that it is north and south. That end of the magnet which points north is called its north pole, that which points couth, its south pole. A freely suspended magnet points north with one end; south, with the other. 12. EXPERIMENT. Bring the north pole of a magnet near the north pole of a magnet freely suspended; it will be repelled. The same is MAGNETIC ATTRACTION. 19 seen, if the south poles are brought together. The magnets will not come to rest before the north pole of the one has found the south pole of the other. Like poles repel each other; unlike poles attract each other. Application The most important application of this property of the magnet is the Magnetic Needle, or Compass, used by surveyors and mari- ners. A needle may easily be rendered magnetic by means of a magnet. Lay a needle upon the table and hold its point with the left hand. Taking the magnet with the right, place it with its north pole upon the center of the needle. Then pass it slowly along the right-hand part of the needle, rubbing the needle in the direction from the center to the eye. When arrived at the eye, the magnet must be raised from the needle and passed through the air back to the center, there to recom- mence the same operation with the same pole. This process must be repeated about thirty times. After that, the magnet is reversed, taken into the left hand, and, while the right now holds the needle, placed upon the center of the needle. By rubbing the magnet from the middle of the needle to the left end, returning through the air, and repeating this the same number of times as the first process, the needle becomes a perfect magnet. It will attract iron, and be attracted by the same ; it will point north and south, if suspended at the middle and if left to move freely. Magnets have usually the form of a horse-shoe, so that the poles are brought near together ; this more than doubles their supporting capacity. Bead "Magnetism," in Faraday's "Six Lectures on the Various Forces of Matter." New York : Harper & Brothers. Bead "Terrestrial Magnetism," in Harper's Monthly, Vol. I, p. 651. 20 FIRST LESSONS IN PHYSICS. LESSON IV. ELECTRIC ATTRACTION. The ancient Greeks gave amber the name of Electron ; they knew that if amber was rubbed it would attract small, light bodies. This attractive power is called Electricity. 13. EXPERIMENT. Eub apiece of sealing wax, a bar of sulphur, or a lamp-chimney, with a piece of flannel, and bring it near light bodies, such as tiny bits of paper, wafers, or small feathers. They will adhere to the sealing wax, sulphur or glass, which have become electric, and have now the power of attracting light bodies. 14. EXPERIMENT. Heat a piece of writing paper over a stove or lamp. Place it upon a table, rub it several times with a piece of India- rubber, and then bring it quickly near some light bodies ; it will attract them. From this we see that Friction produces Electricity, and that elec- tric bodies attract light bodies. 15. EXPERIMENT. If, in a very warm room, where there are but few persons, and where the atmosphere is perfectly dry, we bring the knuckle near electrified sulphur, glass or paper, we may see a spark pass from the substance to the ELECTRIC ATTRACTION. 21 hand. 1 At the same time, we also hear a crack- ling noise, feel a slight stinging in the hand, and smell a peculiar odor near the electrified object. Familiar Facts. The fur of a cat sparkles when rubbed with the hand in cold weather. The sparks are seen best in the dark. If the electric paper be held against one's face, a peculiar sensa- tion is felt, as though the face were being covered with a cobweb. The reason of this is, that the fine hair on the face is attracted by the paper and caused to move. Sparks a foot long are often seen when there is strong friction between the rubber bands and the wheels of a machine. But what has become of the electricity uiat passed from the sulphur, or glass, to the knuckle while emitting a spark ? If it had remained there, the knuckle would certainly attract light bodies ; but this is not the case. Neither the knuckle nor the hand shows any sign of electricity. It spread I. As it often depends upon uncontrollable circumstances whether a spark can be obtained by such simple means, the following contrivance has been suggested: "Take a glass tube of j-inch bore and a little over a foot long. Then take an iron wire, coil it spirally, and insert it into the tube the windings should be J^-inch distant from each other, and must rest firmly against the inner surface of the tube. One end of the wire is to protrude from the tube, and a tin ball to be soldered on to the protruding end. The other end of the spiral wire should not extend farther than the middle of the tube, in order that about six inches of the tube may be used as a handle. On rubbing the tube, a spark may be obtained from the tin ball." 22 FIKST LESSONS IN PHYSICS. all over the body and over the earth, and thus ix was sensibly lost. If we bring a key near elec- trified sulphur or glass, or a tin ball (see foot note p. 21), a spark will likewise be seen passing over to the key ; but the electricity which the key receives does not stay there; it passes into the hand, and thence through the body to the ground. This shows that metals and the human ~body are good conductors of electricity. If in place of the hand and the key, we take sealing wax, silk or glass, no spark will be seen, and they wil) remain electric after the contact. These objects do not conduct electricity. Hence sealing wax, silk and glass are non-conductors of electricity. The difference between conductors and non-con- ductors of electricity is this : A conductor re- ceives, and loses, electricity immediately on all the parts of its surface. A non-conductor receives, and loses, electricity only at the point of contact. 16. EXPERIMENT. Suspend a pith ball, 1 attached I. " Pith balls may be obtained best yi winter from young elder-trees of one year's growth. The stem is split open with a sharp knife, the pith is cut into small pieces, each of which is rolled between the hands into a ball. To suspend the balls, pierce each with a needle carrying a silk or linen thread, make a knot on the opposite side, and then draw the knot tight a little ways into the ball. The linen thread should be very fine. If silk thread is used, care must be taken that it contain no metal- lic color, as, for example, Prussic Blue, and that no cotton thread be inside, as cotton is a good conductor. The thread to which the little ball is attached is taken from three to five inches long ; one with a ball at each end should, of course, have double the length. They may be ELECTRIC ATTRACTION. 23 to a silk thread, from the hand or some other support (Fig. 3). On presenting it to an electrified bar of sealing wax, it will be seen that the ball is attracted by the seal- ing wax, that it comes in con- tact with the same, and that, after it has become electric itself, it is repelled. If we then slowly follow it with the sealing wax, it is repelled still farther. The repulsion between the two bodies con- tinues, until the aqueous vapor in the room, or gome other good conductor, or the contact of our hand, deprives the ball of its electricity. 17. EXPERIMENT. In a similar manner sus- pend two pith balls attached to a silk thread. On presenting electrified sealing wax, they be- come electric themselves by contact with it, and then repel each other. They hang no longer verti- cally ; the attracting and repelling force of elec- tricity may overcome gravity in the same way in which magnetic attraction overcomes gravity. 18. EXPERIMENT Repeat the 16th Experiment with a single pith ball ; after it becomes electric, suspended from a strong wire bent at right angle, which may be inserted in the cork of a bottle, so as to give it firm support." 24- FIRST LESSONS IN PHYSICS. present it to an electrified glass rod or tube. 1 The ball will be immediately attracted by the elec- tricity of the glass. 19. EXPERIMENT. Repeat the 17th Experi- ment, and after the two balls are separated by repulsion, present electrified glass to one of them. The glass will attract this ball and impart its electricity to it ; after which the ball will be re- pelled from the glass and at once fly to the other ball. When the two balls had the same kind of elec- tricity, they repelled each other ; now that they possess different electricities the one glass elec- tricity, the other sealing-wax electricity they attract each other. 20. EXPERIMENT. Again suspend two pith balls. Bring electrified sealing wax near the one, electrified glass near the other. The balls will at first be attracted and then repelled, when they will fly toward each other and stay together. This is easily understood if we remember that one ball had glass electricity, and the other seal- I. "Glass differs greatly with respect to electrical purposes. Some varieties are good conductors of electricity, because they contain metal. Hard glass, and common green bottle glass, if not colored with metal, are non-conductors, and, therefore, well adapted for that purpose. All kinds of glass, however, are hygroscopic, that is, they draw moisture from the atmosphere. For this reason thick glass rods are preferable to glass tubes. Before being used, both, tubes and rods, should be slightly heated, and should be rubbed with a warm cloth." ELECTRIC ATTRACTION. 25 ing wax, or, as it is called, resinous electricity. From all this it appears that there are two kinds of electricity Vitreous or Glass Electricity r , and Resinous Electricity. The former is also called positive electricity, the latter negative electricity. Like electricities repel each other ; unlike elec- tricities attract each other. (For a similar phe- nomenon see the preceding lesson.) Historical. The sparks obtained by the rubbing of furs, and light- ning, with its companion, thunder, must have been observed by the earliest people upon the earth. Although the Greeks, about 600 years before the Christian era, recorded the attracting property of amber, it was not before the beginning of the lyth century, that a book was published by Dr. Gilbert, an Englishman, who mentions many other substances, such as glass and sulphur, as having the same property. This author stated correctly that magnetism attracted as well as repelled, but, curiously enough, he added that electricity only attracted. In 1670, the first electric machine was constructed by Otto Guericke, burgomaster of Magdeburg, the inventor of the air-pump. He also discovered the property of electric repulsion. He excited electricity by means of sulphur (brimstone) exposed to friction. The distinction between conductors and non-conductors of electricity was discovered by Mr. Stephen Grey. He wished to electrify a cord suspended by linen threads, but was unsuccessful because the electricity, when entering the cord, at once passed over to the threads. The threads thus were found to be conductors of electricity. Upon the sug- gestion of a friend he tried silken threads, and as silk is a non-con- ductor, the experiment then met with the desired result. Du Fay distinguished between vitreous and resinous electricity. A number of other scientists afterward improved the electric machine, and by continuous research added largely to the progress of the science. But they were eclipsed by Dr. Franklin who astonished the world by draw- ing electricity from the clouds. 26 FIBST LESSONS IN PHYSIOS. LESSON Y. LIGHTNING. LIGHTNING KODS. A lamp-chimney yields only a small spark; but the glass disk in an electrical machine, such as is used in High Schools and Colleges, produces a long, zigzag spark, resembling a flash of light- ning, It had long been supposed that lightning was an electric phenomenon, but it was not until 1752 that, through the genius of our countryman, Benjamin Franklin, all doubts were removed. Having long been thinking over the subject, he one day saw a boy fly a kite, and the idea at once struck him that he must make one himself and send it into the clouds. Accordingly he stretched a silk handkerchief upon two sticks, in the form of a cross, on the top of which he fastened a pointed iron wire. This he connected with the hempen string holding the kite, and upon the approach of a thunder-storm he went out, accom- panied only by his little son. The hempen string was attached below to a key, and the key was insulated by silk string which Franklin held in his hand. The clouds were passing rapidly, but without any apparent effect upon the kite ; and the two observers, standing below and watching it with great anxiety, were about to abandon the LIGHTNING LIGHTNING KODS. 27 undertaking, when suddenly the fibres of the string bristled up, and a crackling noise was heard. Franklin now presented his knuckle to the key, and received an electric spark, which was soon followed by an abundance of sparks as the string became wet with the falling rain. Franklin's experiment, together with many experiments by scientific men in Europe, demon- strated beyond a doubt, that all rain clouds are electric. Familiar Facts. When two such clouds approach each other, their electricities try to unite. In doing so, one of them leaps over the space between them. This passage of electricity through the air produces a great electric spark which we call Lightning. Familiar Facts. Lightning mostly passes from one cloud to another. But it may also pass from the clouds to the earth, and from the earth upward to the clouds. It rarely happens thafc lightning strikes that is, strikes objects on the earth. Tall objects made of good conducting material are most liable to be struck tall objects, because they are nearer to the clouds ; good con- ductors, because electricity can get to the ground soonest through them. High houses, tall steeples, trees or chimneys, therefore, offer a good passage to electricity. In its onward course lightning 28 FIRST LESSONS IN PHYSICS. always prefers the best conductors ; thus it passes along the spouting of houses, along water-pipes, stove-pipes and iron pillars. It melts metallic objects ; it splits trees into fragments, and 'kills living beings by destroying the activity of their nerves. The safest place during a thunder-storm is that part of a room not too near the fire-place, stove, chandelier, gas-pipe or bell-rope. Why is it un- safe to seek shelter under tall trees, or in the en- trance of a house with rain pouring down over it? Knowing that lightning always follows the best conductors, Franklin devised a means by which he might direct its course, and invented the Lightning Rod. It consists of a metallic rod, with pointed upper ends, which protrudes several feet above the roof, in order that on the approach of a dense cloud the metallic point, and no part of the building, should be struck. The rod con- ducts the electricity into the ground, where it can do no harm. As lightning is an electric spark, so is on a large scale thunder the crackling noise which accompanies the electric spark. Bead " Thunder and Lightning," in "Illustrated Library of Won- ders." New York : Scribner & Co. Bead "Lightning and its Effects,'" page 291, in Wells' "Things Not Generally Known." Bead " Thunderstorm," in " The Earth and its Wonders." Cincin- nati : Hitchcock & Walden. COHESION. 29 LESSON VI. COHESION. Familiar Facts. In order to cut meat, to whittle a stick, to sharpen pencils, to split logs, to saw wood or to plane boards, we find it neces- sary to use instruments, such as a knife, an ax or a saw. We see that the parts of a solid body are not easily separated; evidently they are very close together. They are held together by a force which we call Cohesion. We know that it is difficult to break a piece of iron, because iron has a strong cohesive force ; yet a blow with a poker sometimes may break the door of an iron stove. Rolled or hammered iron is much stronger than cast-iron, because, by the process of rolling or hammering, its particles have been brought nearer together, and hence they cohere more firmly. The strength of our tools and building-material depends upon this cohesive force. 1 We can break wood more easily than iron, because it has less cohesive force. Easier yet to break, or separate, I. If iron be made to pass through fine openings, iron wire is ob- tained. (What is this property of iron called ?) Iron wire of the thick- ness of a match may support a weight of forty tons. A cable of wires, each wire having one-third of that thickness, may support a weight of ninety tons. Suspension bridges. 30 FIRST LESSONS IN PHYSICS. is water, oil, or air. Place the hand in water, now try to place it in wood. This is impossible, for the particles of a solid body cohere more closely than those of a liquid. How easily we can pour water from a pitcher into a tumbler, and oil from a can into a lamp ! And that our light- winged songsters can divide the air so swiftly, is owing to the fact that air has even less cohesion than water. We can walk, run, ride or jump in air. To do this in water is more difficult ; in molasses, it would be next to impossible. 1 To break a body, its force of cohesion must be overcome. Familiar Facts. When a little child breaks his slate, he tries to put the parts together again, but he quickly perceives that they will not re- main together; he must get a new slate. The particles on the surface of the edges can not be brought so near to each other as they were be- fore ; that is, they cohere no longer. A broken walking-cane, although the broken parts are glued together again, has lost its former strength. I. When we overcome the force of cohesion of a body, we do so by displacing its parts ; we do not in reality penetrate the body. Thus, in driving a nail into a board, the nail merely displaces parts of the board. A body can not occupy the space of another body unless that other body be first removed; that is, no two bodies can occupy the same space at the same time. If an inverted tumbler be placed in water, the water can not fill it, because the air in the tumbler has no means of escape. See Lesson X. COHESION. 31 But it is different with a liquid ; two parts of water can readily be made to form one mass by pouring them together. The greater or less resistance which the body offers when being broken, determines the degree of its cohesive force. A solid body has more co- hesion among its parts than a liquid. Gaseous bodies have no cohesion at all. Examples : Ice, water, steam. The great enemy of cohesion is Heat. Familiar Facts. Although solids and liquids cohere, they contain a great number of holes, which are called Pores. They may be of differ- ent size in the same body, and they may be visible or not. The pores of our skin are so minute that they can not be detected without a magnifying glass. Every square inch of our skin contains about 1,000 pores. Our health depends largely upon their activity. 1 Solid and liquid bodies are porous. Application. (a.) Of Cohesion : Beams and Pillars. Wire; Thread; Rope, &c., &c. (b.) Of Porosity : The Sponge ; Blotting-Paper. I. In the year 1661 the Academy of Florence proved that pores exist even in gold. A thin globe of gold was filled with water, and the orifice carefully closed. A violent pressure was then brought to bear upon it, and the result was, that the water was forced through the pores- of the gold, and stood like dew upon the outer surface of the globe. 32 FIRST LESSONS IN PHYSICS. i LESSON VII. ADHESION. CAPILLAEY ATTRACTION. 21. EXPERIMENT. Cut two leaden bullets with a pen-knife so as to form two bright surfaces, and let the two faces be pressed against each other until they are in the closest contact ; they will be found to adhere firmly to each other. Familiar Facts. The same takes place, if a piece of India-rubber be cut and the two surfaces be pressed together. Dealers in glass-ware know that when mirrors have been placed together with their surfaces, they are often broken in the at- tempt to separate them. Between solid bodies, adhesion takes place if ike surfaces are highly polished; that is, if they are so smooth that the parts of one surface closely approach those of the other. If not highly polished, the surfaces will not adhere as two bricks laid together. Nor will adhesion take place, if thin paper is placed between the two polished surfaces. As a general thing, bodies which we wish to adhere to one another, are not very smooth. Owing to the unevenness on their surface, many of the parts of one surface are prevented from coming in close contact with those of the other ; ADHESION. CAPILLARY ATTRACTION. 33 in this case there can be no adhesion. What may be done, then, in order to make two rough surfaces adhere? Simply put a liquid body be- tween the two, to fill out the unevenness. 22. EXPERIMENT. Put two moistened glass plates together, and it will require some effort to separate them. The same may be found if two boards are placed together with water between them. 1 Why does the hand become wet when immersed in water ? Why does it remain dry when drawn out from mercury ? Because, in the first case, the adhesive force between the water and hand is stronger than the cohesive force of the water ; in the other case, the cohesive force of the mercury is stronger than the adhesive force between it and the hand. Thus, when the hand is placed in water, a struggle takes place, as it were, be* tween the adhesive and cohesive force of the water. The hand comes out victorious, for on withdrawing, it carries off a portion of the water. I . Between paper we put mucilage ; between bricks, mortar ; between the pieces of a broken dish, cement. Adhesion takes place between the surface of these bodies and the liquid; cohesion between the parts of the liquid. It is thus that the two surfaces of glass, paper, brick and porce- lain are made to adhere to each other. 3 34 FIRST LESSORS IN PHYSICS. Adhesion is the attraction between the surfaces of bodies in contact with each other. Application. All gilding, painting, whitewash- ing, cementing, varnishing, gluing, writing, solder- ing, coating of looking-glasses, plating, &c., &c. Soot adheres to the chimney ; dust to the ceiling; chalk, and fresh paint, to one's dress. 23. EXPERIMENT. Immerse a clean glass plate partly in water, some of the water will be seen to rise on both sides of the plate. Evidently the adhe- sive force between the glass and the water is greater than the cohesive force of the water. Were it not so, the water would not rise, another glass plate near the first and not parallel to it (Pig. 4). Water will rise between them, and the form of its surface will be concave. The nearer the glass plates are brought to each other tlie higher will the water rise between them. This is natural, for the quantity of water between them is in this case very small ; and the cohesive force of the water, therefore, easily overcome by the adhesive force. If a glass tube be immersed (see FIG. 4. Now immerse ADHESION CAPILLARY ATTRACTION. 85 Pig. 5) the water will rise still higher, because here is a small quantity of water, surrounded on all sides by glass, and the force of adhesion is, therefore, comparatively of greater effect. 1 Capillary tubes are tubes so small that nothing thicker than a horse-hair m can pass through them. When such a tube comes in contact with a liquid FI0 - 6 - whose cohesive force it overcomes, the liquid is compelled to rise in it. The finer the bore of the tube the higher will the liquid rise in it. In a tube 1-100 of an inch in diameter water will rise over five inches. Capillary attraction is the result of adhesion operating between solid and liquid bodies. Application. Sponge, blotting paper. Eggs and meat may be kept fresh in sand or pulverized charcoal, these two substances containing capil- lary tubes which absorb any moisture that would otherwise affect the eggs or the meat. Lamp- wicks likewise contain capillary tubes ; these sup- port combustion, although there may be but little oil Grease spots in the floor may be removed by laying earth upon them. Our clothes become wet from the rain. In short, everything about us is filled with fine capillary tubes. i. But the reverse of all these phenomena takes place that is, water is always depressed about glass surfaces, if these are greased. Grease has no attraction for water ; the water, consequently, is left free to obey its cohesive force, and falls below the level of the liquid surrounding the tube. 36 FIRST LESSONS IN PHYSIOS. LESSON VIII. REVIEW. LESSON i. All bodies are attracted to the earth. The force which attracts them is called the Force of Gravity. 2. All bodies fall if unsupported ; if supported. they press upon their support ; if suspended. they pull in the direction in which they would fall if left free to do so. 3. The direction in which a body fatts, if it ir moved by gravity alone, is vertical. 4. The pressure of bodies upon their support is called Weight. 6. A pound is a weight, indicating a certain amount of that pressure taken as a standard. LESSON ii. 6. The specific gravity of a substance is it weight, compared with the weight of a like bulk of some other substance taken as a standard. When we say, mercury has a spe- cific gravity of 13.5, we mean that any bulk of mercury has 13.5 times as much weight as a like bulk of water. RKVIKW. 37 7. Fluids of different specific gravities, when brought together, place themselves in the order of their specific gravities, the heaviest below. 8. A body which is lighter than a quantity of water of equal bulk, floats on water; one which is heavier, sinks. LKSSON in. 9. The attraction between magnets and iron is called Magnetic Attraction. The attraction which the earth has for magnets, causes the magnetic needle, or any magnet freely sus- pended, to point north and south. LBSSON iv. 10. Tlve attraction of electrified bodies is called Electric Attraction. LBSSON vi. 11. The parts of a body are kept together by their mutual attraction. The attraction be- tween the parts of the same body is called Cohesion. 18. In order to separate a body, its cohesion must first be overcome. If it is difficult to break, we call the body tenacious ; if dim cult to penetrate, we call it hard. 38 FIKST LESSONS IN PHYSIOS. LESSON vii. 13 The attraction between the surfaces of bodies in contact, is called Adhesion. 14. The adhesion between solids and liquids is often called Capillary Attraction. 15. Gravity, Magnetism, Electricity, Cohesion, and Adhesion, are forces of attraction The last two are called Molecular Forces, because they bind molecules 1 together. 16. The first three Gravity, Magnetism and Elec- tricity act through great distances ; adhe- sion and cohesion only at an insensible dis- tance. 17. Instead of magnetic and electric attraction. we may witness magnetic and electric Repul- sion, while gravity, cohesion and adhesion, however, exert only attraction. Questions. What natural force is applied in the balance the compass the lightning rod suspension bridges blotting paper ? I. "A molecule is the smallest particle of matter into which a body can be divided without losing its identity." Thus, the smallest particles of bread or of salt, which are still bread or salt, respectively, are eules of bread or of salt 1LASTICITY 39 LESSON IX. ELASTICITY. Familiar Facts. More than a thousand years ago, long before powder was invented, our ances- tors used the cross-bow for the purpose of fighting the enemy as well as for the pleasures of the chase, At present, the cross-bow is used only by certain savage tribes, and as a plaything by our children. If you draw the string of the cross-bow, and then let it go again, the arrow placed before the string Hies off with astonishing rapidity. u How is it,'* may we ask, " that a string can obtain such great force ?" If we double a piece of India rubber be- tween two fingers, it straightens again when the pressure is removed. After pressing a steel pen gently against our thumb nail to try its writing qualities, it immediately returns to its former shape. Steel blades and whalebones likewise resume their former shape after having been bent. Steel, ivory, and India-rubber, possess this prop- I. Bodies such as lead, cotton, clay, show very little elasticity. For- merly, it was believed that they had none ; hence they were called inelas- tic; but even these bodies are not without elasticity ; and it may safely' be asserted that there are no inelastic bodies. Indeed, were it not that all bodies are more or less elastic, it would be difficult for us to live. Were not the ground, the floor, the walls of our houses, the tables and chairs elastic, every contact with them would hurt us. Were not our pa;>er and pens elastic, how long would it not take us to commit our thoughts to paper ! Were not wood elastic, every stroke of wind would blow branches of trees down upon us. 40 FIRST LESSONS IN PHYSIOS. erty in a high degree. Such substances are called elastic. Stone, lead, glass and many other bodies possess it to a small extent. Yet glass, when drawn out in tine threads, is so elastic that tissues have been woven from them. 23. EXPERIMENT. Take an ivory ball ; press it with your hand upon a slab of marble that has been blackened over a lamp. The ball will show a black spot about as large as a pin's head. Now lay the slab on the floor, stand on the table, and let the ball drop upon the slab from a consider- able height. The ball will then have a black spot very much larger than before. Although of a hard substance, the ball is flattened to that extent when it strikes the slab, and in resuming its for- mer shape, it rebounds. Familiar Facts.- An India-rubber ball is flat- tened still more, and therefore rebounds farther. A soap bubble, striking against the wall, some- times rebounds. Air, too, is elastic. This may be seen by striking upon a bladder inflated with air. When powder is ignited, gases are developed whose elastic force is so great that it overcomes everything before it. The parts of an elastic body return to their former position, when the external force which displaces them ceases to act. . Bodies, such as glass or sugar, that break if the displacing force is beyond the limits of their elas- ELASTICITY. 41 tieity, are called brittle Bodies, such as metals, whose parts instead of breaking may assume a different position, are either malleable or duc- tile. 1 ' The malleability of iron may be seen in sheet iron, and in the plates of gun- boats ; its ductility, in the telegraph wire. Questions. When are bodies said to be dense ? rare ? soft ? hard ? brittle ? malleable ? ductile ? Application of Elasticity. 1. To produce mo- tion : Watch-springs ; springs in watch-cases, boxes, and carriage-lanterns ; the ballista of the ancients ; the cross-bow ; locks, and triggers. 2. To counteract concussion: Wagon-springs; packing glass ware in hay or straw ; springs in mattresses, sofas, chairs and etui-cases. 3. To cause close contact or pressure: Springs in pocket- inkstands: printers' cylinders; some kinds of pen-holders. 4. For weighing : Spring-bal- ances i. Gold is -very- malleable. Gold leaf is hammered out so thin that it takes 300,000 sheets, placed one upon another, to make the thickness of an inch. Platinum is very ductile. 3,000 feet of platinum wire of a certain thickness were found to weigh only about one grain. A single *ilk-vrorm thread possesses a thickness equal to that of 140 such fine threads of platinum. Now, as a foot contains 144 lines, and as the tenth part of a line is readily visible to the naked eye, it follows that a single grain of platinum can be drawn out into 4,320,000 parts, each of which is distinctly visible. 42 FIRST LESSONS IN PHYSICS. LESSON X . ELASTICITY OF AIR. 24. EXPERIMENT. If we immerse an inverted tumbler perpendicularly in water, only a very little of the water will enter the tumbler, and, of course, the air in the tumbler is compressed. If the ves- sel is pressed down still farther, a little more water enters it, but it will never be entirely tilled with water, because it contains air. A cork previously placed in the tumbler, will show the position of the water-level in- side. (See Fig. 6.) Air maintains its v place like every other body, and presses upon bodies. Its pressure is distinctly felt, and if you withdraw the hand which presses the tumbler down, the tumbler will instantly rise. The air in the glass was compressed, and tended to expand again, be- cause air, like other bodies, is elastic. 2r>. EXPERIMENT. If a glass funnel be im- mersed instead of a tumbler, and if inverted with the mouth downward, the upper end being closed with the thumb, the air in the funnel is compressed. As the thumb is removed, however, water rushes into the funnel, forcing out some of the air. ELASTICITY OF AIR. 43 26. EXPERIMENT. Cement a funnel into the neck of a bottle and pour water into it. Only a small quantity of water will enter, unless the fun- nel is placed in the bottle loosely, so that there is a passage for the air. For, as the water is poured into the funnel, it forces the air in the tube of the funnel into the bottle. The air in the bottle being thus greatly compressed, its elastic force resists the downward pressure of the water. 27. EXPERIMENT. Another beautiful illustra- tion of the expansive force of air may be obtained by the " Hero's Fountain." Take a cork which fits into a bottle, and perforate it with a round file. The hole should be made so as to admit with difficulty a glass "nbe, which is now pushed through the cork. The tube should have a very fine opening above. This being done, fill the bottle about half with water and close it with the cork. Then PIQ ' *' drive the glass tube farther down, until it nearly reaches the bottom of the bottle. The bottle now contains air in its upper part and water in its lower. On blowing more air into the tube, the air will ascend through the water (Lesson II) and col- lect in the space above. In so doing, the air over the water is compressed, and in trying to expand, it forces the water upward through the tube. The inventor of this little apparatus was Hero, 44 FIRST LK880NS IN PHYSICS. a philosopher, who died in Alexandria, before Christ. Familiar Facts. The amusing toy, whose harmless missile darts off with such rapidity, the pop-gun, becomes a wonderful object, when we consider the powerful force it serves to illustrate. A piston moves air- tight in the tube of the pop-gun. Let it be at one end of the tube ; then insert, air-tight, a stopper into the other end of the tube and commence pushing down the rod ; the air inside is now compressed, it has the ten- dency to expand again ; but its force is not great enough, as yet, to drive out the stopper. If the rod is pushed in farther, the air is compressed still more, and the stopper is expelled with a loud report Another source of amusement is the blow-pipe. It consists of a long, smooth wooden tube, into which is fitted a sharp nail, around whose head shreds of cotton are tied. This nail is inserted, and by blowing into the tube at the same end, a great quantity of air is forced in, compressing the air inside ; this causes the nail to move forward. On blowing more strongly, the air is compressed more, and its expansive force, therefore, greatly increased. The nail is then expelled from the tube, and its speed will be in proportion to the force with which you have blown into the tube. The air in a diving-bell is so compressed by the water's trying to enter, that divers often experience great difficulty in breathing. Air is an elastic body; the more we compress it, the greater is its expansive force. A useful application of this property of air is the air chamber, used in connection with pumps. (Comp. Less. 77, p. 78.) The Diving-bell may also be considered an application of this force, because it is the expansive force of the compressed air which prevents the water from entering the bell. (Comp. 24 exp.) PRESSURE OF AIR. LESSON XI. PRESSURE OF AIR. 28. EXPERIMENT. A tumbler filled with water to the brim, with a piece of paper placed over it, is inverted. (See Pig. 8.) The hand on the paper, after pressing the latter firmly against the tumbler, is removed, bat the water does not flow out. How can this be accounted for? Notice that the tumbler contains no air; it is entirely filled with water. The air evidently presses upward against the paper. It is this upward pressure of the air, which supports the water in the tumbler. Were it not for the paper, the air would force its way into the water, by rushing up along a part of the inner side of the tumbler, leaving the water to fall down on the opposite part. 29. EXPERIMENT. Immerse a tumbler, hori- zontally, into a bowl of water, and press it down gradually. It will fill with water, and afterward be entirely below the surface of the liquid. Now turn it to the vertical position, and without, how- ever, raising its mouth above the surface, lift it as high as possible. The whole tumbler is still filled with water, and will remain filled, although 46 FIRST LBS8ON8 IN PHYSICS. the water in two communicating vessels ought to have the same height (Lesson XVIII). The tum- bler contains no air, while a large amount of air is over the remaining water, pressing downward upon the water. It is this downward pressure of air which supports the column of water in the tumbler. 30. EXPERIMENT. Let a vessel be filled with water; then take a narrow glass tube, open at both ends, and im- merse it perpendicularly in the vessel. The tube will partly fill with water; if taken out, the water will flow through the tube and fall, because attracted to the earth. Place the tube again in the water, but so that no air remains in it, and take it out again, keeping the upper opening closed with th