GIFT OF 
 Harry East Miller 
 
FIRST 
 
 LESSONS IN PHYSICS. 
 
 FOB USE 
 
 IN THE UPPER GRADES OF OUR COMMON 
 SCHOOLS-.: 
 
 ' In Nature all is Motion , Life , and Labor . ' ' Lesson xxii . 
 
 BY 
 
 C . L . H O T Z E , 
 
 Teacher of Natural Sciences in the Cleveland High School, 
 Author of " First Lessons in Physiology '," etc. 
 
 ST. LOUIS: 
 THE CENTRAL PUBLISHING COMP'Y. 
 
C. 3, 
 
 *', 
 -75 
 
 in the year 1871, by 
 
 In ihe Office of the Librarian of Congress, at Washington 
 
 Entered according to Act of Congress, in the year 1875, by 
 
 THE CENTRAL PUBLISHING COMP'Y, 
 In the Office of the Librarian of Congress, at Washington. 
 
 _, 
 
 
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 basis 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 
 
 M81995 
 
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. 
 
 CLEVELAND, O., Aprils, 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 
 and 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 tattght, 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 ot 
 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. VII 
 
 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 1 20, must be omitted in a lesson of less than an 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. ^ 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- 
 tribution 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 
 
 CLEVELAND, O., December I, 1871. 
 
PEEFACE TO EEY1SED EDITION, 
 
 In the present Edition, Lessons I, VI, IX, XIII, XX, will 
 be found materially altered. The Lesson on the Barometer is 
 entirely new; and page 174 on the Thermometer has been 
 added. 
 
 Cleveland, O., May i, 1875. 
 
 C. KLEINR, 274 Eighth Avenue, New York, will furnish sets of inex- 
 pensive Apparatus, for " Hotze's First Lessons in Physics," or any 
 apparatus required for the demonstration of Physical Science. 
 
CONTENTS. 
 
 FORCE, 
 
 OF ATTRACTION. 
 
 PAGE 
 
 LESS. 1. -Gravity 11 
 
 " 2. Gravity, Specific Floating and Sinking 14 
 
 " 3. Magnetic Attraction 17 
 
 " 4. Electric Attraction 20 
 
 " 5. Lightning. Lightning Rods 26 
 
 " 6. Cohesion 29 
 
 ' ' 7. Adhesion. Capillary Attraction 32 
 
 " 8. Review 36 
 
 OF PRESSURE. 
 
 LESS. 9. Elasticity 39 
 
 " 10. Elasticity of Air 42 
 
 " 11. Pressure of Air 45 
 
 " 12. Barometer 48 
 
 " 13 Review 52 
 
 " 14. Inertia 54 
 
 MOTION, 
 
 OF MASSES. 
 
 LESS. 15. Inclined Plane 56 
 
 " 16. Lever 59 
 
 " 17. -Pendulum ". 63 
 
 18. Communicating Vessels. Hydraulic Press.. 67 
 
 19. Breathing. The Bellows 71 
 
 20. Common Pump 74 
 
 21. Forcing Pump. Fire-Engine 77 
 
 22. Review 82 
 
 MOLECULAR. 
 
 LESS. 23. Sound 85 
 
 " 24. Evaporation, Fog, Clouds, Rain, Snow, Hail, 
 
 Dew, Frost 88 
 
 25. Heat. Conduction of Heat 92 
 
 26. Draught 96 
 
 27. Expansion by Heat. Thermometer 99 
 
 28. Thermometer Compared with Barometer 102 
 
 29. Atmospheric Engine 105 
 
 30. Steam-Engine Ill 
 
 31. Review 11& 
 
 32. Light. Its Sources. Direction 121 
 
 33. Radiant and Specular Reflection 121 
 
 34. Visible Direction. Refraction 127 
 
 35 Prisms. Lenses 131 
 
 36. Colors 135 
 
 87 . Chemical Electricity HO 
 
 38. Electro-Magnetic Telegraph 143 
 
 39.-Review 150 
 
 QUESTIONS * 
 
 APPENDIX * ' * 
 
 INDKX. . 17 > 
 
I, Barometer; 2, Microscope; 3, Heron's Fountain; 4, Pump; 5, Steam 
 Engine ; 6, Magnet ; 7, Telescope ; 8, Barometer ; 9, Galvanic 
 Battery; 10, Hour-Glass; n, Clock; 12, Thermom- 
 eter ; 13, Spirit Lamp ; 14, Magnetic Needle ; 
 15, Mirror; 16, Balance; 17, Weights ; 
 l8, Pump. 
 
LESSON I. 
 GRAVITY: ,% 
 
 1. EXPERIMENT. A stone in the hand does not 
 fall, because the hand supports it. But if we drop 
 the stone, it falls, and will continue to fall, until an 
 obstacle, such as the floor or the ground, prevents 
 it from falling farther. 
 
 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. Heavy rods are 
 attached to maps and curtains, to draw them down. 
 In clock-works moved by weights, the weights move 
 in a downward direction. 
 
 Having noticed these facts, you naturally inquire : 
 " Why is it that all bodies near the earth have a 
 tendency to fall toward it ? " 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 the law : All bodies 
 fall, if unsupported ; they are attracted to the 
 earth. The force which attracts them is called the 
 Force of Gravity. 
 
12 
 
 FIRST LESSONS IN PHYSICS. 
 
 2. EXPERIMENT. This stone is 
 
 not supported by the hand (Fig. 1), 
 
 but it is suspended. What prevents 
 
 it'f*o falling.^. The string. When 
 
 " 
 
 "ftoje a little to one 
 side s ia(L,tb.en leX it gty it will swing 
 ^afei^Siicl* fttrthj iafi'd .fihally come to 
 rest. In this po'sitioii the string in- 
 dicates the direction in which an 
 object would fall, if it were left free 
 to fall. This direction is vertical. 
 It gives rise to the plumb-line used 
 by carpenters and bricklayers. 
 
 The direction in which bodies fall 
 when tliey are moved ~by the force of 
 gravity alone, is vertical. 
 
 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 ; and we conclude that the small book 
 has less weight than 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 ; you will 
 now understand why this is done. 
 
 The pressure which bodies exert when supported, 
 or the pull which they exert wlien suspended, is 
 called weight. 
 
GRAVITY. 13 
 
 4. EXPERIMENT. A yard-stick balanced on the 
 
 edge of the hand has equal weight on each side ot 
 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, provided 
 both crayons have like weight. If a number of 
 crayons be suspended from one end of the rod, and 
 a weight from the other, we have a crude form of 
 the scale, or balance. 
 
 Allow the weight of fig. 1, suspended by a string, 
 to dip in water. The surface of the water is hori- 
 zontal ; a vertical line makes a right angle with 
 any horizontal. If a line makes a right angle with 
 another, both lines are perpendicular to each other. 
 
 A balance is an instrument for weighing. Pieces 
 of iron, brass, or lead, are used as standards ; they 
 are called the weights. There are different kinds 
 of balances ; one consists of a delicately poised rod 
 with a pan suspended from each end for holding the 
 weights ; another, also of a delicately poised rod, 
 but with the pans directly over its ends. Sub- 
 stances placed upon the former exert downward 
 pull; on the latter, downward pressure. Then, 
 there is the steel-yard and the spring-balance. 
 
 Gravity attracts all substances alike. Thus a 
 mass of feathers weighing a pound contains as 
 much matter as a mass of lead weighing a pound ; 
 a pound of water is as heavy as a pound of iron. 
 
 The force of gravity is put to use in balances 
 (the spring-balance excepted), the plumb-line, clock- 
 weights, hour-glasses. 
 
14 FIKST LESSONS IN PHYSICS. 
 
 LESSON II. 
 
 SPECIFIC GRAVITY FLOATING AND SINKING 
 OF SOLIDS. 
 
 5. EXPERIMENT. Take two ink-wells of like 
 size and weight ; fill one with water, the other with 
 oil, and place them on the pans of a balance. The 
 pan with the water will be found to be depressed ; 
 evidently the water has more weight than the same 
 bulk of oil. Commonly we say that water is heavier 
 than oil ; but we ought to say, that water has 
 greater specific weight than oil; that is, a given 
 bulk of water has more weight than the same balk 
 of oil ; or, water is denser than oil, because any vol- 
 ume of water has a greater mass than the same vol- 
 ume of common 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 to the bottom, the oil rising above 
 it. Thus oil floats on water, because it has less 
 weight than 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 GKAVITY. 
 
 Fluids of different specific gravities place them- 
 selves in tlie order of their specific gravities, 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 occupied by the stone 
 was occupied by as much water as ran over ; this 
 water was held up by the water in the tumbler. 
 Now, if the stone had no greater weight than a like 
 bulk of water, it would likewise be held up by the 
 water. It can easily be shown that it weighs more, 
 if we place the tumbler containing the stone 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, and displaces but very little 
 water. It evidently has less weight than a like 
 bulk of water. It might float even if it contained a 
 few small objects, or if half filled with water; but 
 
 . if entirely filled, it sinks. 
 
 A body floats on water, if it has less weight than 
 an equal bulk of water ; it sinks, if it has more. 
 
16 FIKST LESSONS IN PHYSICS. 
 
 Familiar Facts. As the flask just mentioned 
 is enabled to float, so is a ship ; but if it springs a 
 leak and fills with water, it will go down. The 
 human body has nearly the same weight as a like 
 bulk of water, and will float provided the lungs re- 
 main filled with air. 
 
 Persons who fall into the water and cannot swim, 
 often lose their lives 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, 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 naturally sinks, and, unless 
 rescued, they are drowned. The danger is greatly 
 diminished if these persons, on falling into the 
 water, quietly take in a full breath of air and hold 
 it , then throw the head back so that only the mouth 
 and nose remain above water. The object of this 
 is to enable the expanded chest to buoy up the 
 heavier portions of the body, so as to keep at least 
 the nostrils open for breathing. 
 
 Applications made of Specific Gravity. By 
 means of specific gravity the purity of liquids, and 
 the value of many other substances, may be ascer- 
 tained. 
 
MAGNETIC ATTRACTION. 17 
 
 LESSON III. 
 
 MAGNETIC ATTRACTION. 
 
 9. EXPERIMENT. Suspend an iron nail by a 
 
 string. The direction of the string will be vertical 
 (Lesson 1). But if we bring a magnet near the nail, 
 the nail will swing toward the magnet ; the more 
 so, the nearer the magnet is brought to the nail. 
 When near enough, the nail will attach itself to the 
 magnet, and, if separated from the string, will not 
 fall. This is owing to Magnetic Attraction. 
 
 Reverse the last experiment. Suspend the mag- 
 net by a string, and lay the nail on the table. 
 Holding the suspended magnet over the nail, 
 steadily bring it near the latter, the nail will 
 eventually rise and adhere to the magnet. 
 
 The preceding shows that Magnets and unmag- 
 netic iron attract each other. 
 
 10. EXPERIMENT. If iron filings be placed on a 
 
 piece of paper or glass, &nd a magnet be held be- 
 neath, the iron will also be attracted by the magnet, 
 and the little particles will arrange themselves in 
 curves about the end of the magnet. Magnetic 
 attraction, like attraction of gravity, is independent 
 of intervening bodies and spaces. 
 2 
 
18 FIRST LESSONS IN PHYSICS. 
 
 Let the magnet be placed horizontally 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 centre of the 
 magnet there is no attraction whatever. 
 
 From this we see that the attractive power of a 
 magnet resides chiefly at the ends of the magnet. 
 
 11. EXPERIMENT. The ends of a magnet are 
 
 called its poles. Tie a string 
 to the centre of a bar mag- 
 net, 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 
 FIG. 2. many vibrations, resume 
 
 the same position. It will do so whether in the 
 room or out of doors. On examining its direction, 
 we find it pointing north and south. That end 
 of the magnet which points north is called the north 
 pole of the magnet, that pointing south, its south 
 pole. 
 
 A freely suspended bar toagnet points north with 
 one end ; south, with the other. 
 
 12. EXPERIMENT. Bring the north pole of a 
 
 magnet near the north pole of a freely suspended 
 magnet, this north pole will be repelled. Now, if 
 the two south poles are brought together, they also 
 
MAGNETIC ATTRACTION. 19 
 
 repel each other ; but when the north pole of the 
 one is near the south pole of the other, there will be 
 mutual attraction between these unlike poles. 
 
 Like poles repel, unlike poles attract each other. 
 
 Application. The most important application of 
 magnetic attraction is the Compass, or Magnetic 
 Needle, used by mariners and surveyors. 
 
 A steel needle is easily rendered magnetic by means of a magnet. 
 Lay a needle upon the table and hold its point with the left hand ; with 
 the other hold the magnet vertically with one pole in the centre of the 
 needle. Then pass it slowly along the right-hand part of the needle, rub- 
 bing the needle in the direction from the centre to the eye. When arrived 
 at the eye, the magnet must be raised from the needle and passed through 
 the air back to the centre, there to recommence the same operation with 
 the same pole. This process may 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 centre 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 in the first process, 
 the needle becomes a true magnet. It will attract iron, and be attracted 
 by the same ; it will point north and south, if suspended at the middle and 
 left to move freely. 
 
 Magnets often have 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. 
 
 Read "Terrestrial Magnetism," in Harper's Monthly, Vol. I, p. 651. 
 
20 FIRST LESSONS IN PHYSICS. 
 
 LESSON IV. 
 
 ELECTRIC ATTRACTION. 
 
 13. EXPERIMENT. Rub a piece 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 ; 
 these 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. 
 
 Both experiments teach us that Friction produces 
 Electricity, and that electric bodies may exert at- 
 traction. 
 
 The ancient Greeks knew that if amber is rubbed 
 it would attract light bodies ; and as the Greek 
 name for amber is Electron, the name of this at- 
 tractive force is Electricity. 
 
 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 
 may notice a peculiar odor. 
 
 Familiar Facts. The fur of a cat when rubbed 
 with the hand, a gutta-percha comb passed through 
 the hair, or the strong friction of India-rubber 
 bands against rapidly moving axles or wheels, 
 will produce electric currencs more or less per- 
 ceptible according to circumstances. If an electri- 
 fied body be held close to the face, a peculiar sen- 
 sation is felt, as though the face were being 
 covered with a cobweb ; this is on account of the 
 attraction between the electric object and the fine 
 hair on the face, which causes the hair to move. 
 
 But what has become of the electricity that 
 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 is now electric. The electricity 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 }4-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 FIRST LESSONS IN PHYSICS. 
 
 all over the body and over the earth, and thus it 
 was sensibly lost. If we bring a key near elec- 
 trified sulphur or glass, or a tin ball (see foot note 
 p. 21), the electric current will pass 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 conductors of 
 electricity. If in place of the hand or the key, 
 we take sealing wax, silk, or glass, or India-rubber, 
 these objects will remain electric after the contact. 
 In other words, they do not conduct electricity. 
 Hence sealing wax, silk and glass are non-con- 
 ductors of electricity. If any part of a non-con- 
 ductor of electricity is electrified, the electricity 
 confines itself to the part that has been electrified ; 
 but if any part of a conductor is electrified, the 
 electricity spreads itself instantly over the whole 
 surface. 
 
 16. EXPERIMENT. Suspend a pith ball, 1 attached 
 
 I. " Pith balls may be obtained best in 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 
 (Pig. 3). On presenting it to 
 an electrified bar of sealing 
 wax, it will Tbe 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 
 some other good conductor, or the contact of the 
 hand, deprives the ball of its electricity. 
 
 17. EXPERIMENT. In a similar manner sus- 
 pend two pith balls each from a silk thread. On 
 presenting electrified sealing wax, they become 
 electric themselves by contact with it, and then 
 repel each other. They hang no longer vertically ; 
 the attracting and repelling force of electricity 
 may overcome gravity. We know from Lesson 
 III, that magnetic force may overcome 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 to it 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 or resinous elec- 
 tricity they attract each other. 
 
 20. EXPERIMENT. Bring electrified sealing 
 wax, or gutta-percha, near one of the two balls, 
 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, 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 iyth 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 fliat 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 FIRST LESSONS IN PHYSICS. 
 
 LESS01STY. 
 
 LIGHTNING. LIGHTNING EODS. 
 
 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 handker- 
 chief 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, accompanied only by his little son. The 
 hempen string was attached below to a key, and 
 the key was insulated by a silk string which Franklin 
 held in his hand. The clouds were passing rapidly, 
 but without any apparent effect on the kite ; and 
 the two observers, standing below and watching it 
 with great anxiety, were about to abandon the 
 undertaking, when suddenly the fibres of the string 
 bristled up, and a crackling noise was heard. 
 Franklin now presented his knuckle to the key, 
 
LIGHTNING LIGHTNING RODS. 27 
 
 and received an electric spark, which, was soon fol- 
 lowed by an abundance of sparks as the string 
 became wet with the falling rain. 
 
 Franklin's experiment, together with many experi- 
 ments by scientific men in Europe, demonstrated 
 beyond a doubt, that clouds are electric. 
 
 Suppose that a cloud happens to contain positive 
 electricity, -it will then call forth negative electricity, 
 either from some neighboring cloud, or from the 
 atmosphere, or from the earth. And if its electricity 
 were negative, it would call forth positive electricity. 
 When two clouds, or a cloud and the earth, are suf- 
 ficiently near to each other, their electricities unite. 
 When uniting, 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. Electric currents may pass 
 
 from one cloud to another, from a cloud to the earth, 
 or also from the earth upward to the clouds. It rarely 
 happens that lightning strikes that is, strikes ob- 
 jects on the earth. Tall objects made of good con- 
 ducting material are most liable to be struck, 
 inasmuch as they more than any other class of 
 bodies attract the electricity of the atmosphere or 
 clouds. High houses, tall steeples, trees or chim- 
 neys, therefore, oflvr a good passage to electricity. 
 In its onward course, lightning always prefers the 
 best conductors ; thus it passes along the spouting 
 
28 FIRST LESSONS IN PHYSICS. 
 
 of houses, along water-pipes, stove-pipes and iron 
 pillars. It heats metallic objects ; it splits trees 
 into fragments, and kills living beings by destroy- 
 ing the activity of their nerves. Lightning some- 
 times melts small metallic objects. 
 
 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 unsafe 
 to seek shelter under tall trees, or in the entrance 
 of a house with rain pouring down over it ? Know- 
 ing that electric currents follow the best conductors, 
 Franklin invented the Lightning Rod, as a means 
 to direct them into the~ ground. It consists of a 
 copper rod with a metallic point, which protrudes 
 several feet above the roof, in order that on the 
 approach of a current the metallic point, and no 
 part of the building, shall be struck. The rod con- 
 ducts the electricity into the earth where it can do 
 no harm. But the connection between the point and 
 the earth must be perfect that is, the rod must 
 not be put in direct contact with the wall, else the 
 electric current may pass over to the building and 
 produce great harm. 
 
 The purpose of lightning rods, then, is, 1st, to 
 attract electric currents ; 2d, to conduct them safely 
 into the ground. 
 
 Head " Thunderstorm, " in 'The Earth and its Wonders." Cincin- 
 nati : Hitchcock & Walden. 
 
COHESION. 
 
 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 stve. 
 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 cohesive force. The supporting ca- 
 pacity of wire ropes, iron pillars, or steel rods is 
 immense. A steel bar one inch square will support 
 a weight of forty tons or more. We can break 
 wood more easily than iron, because it has less 
 cohesive force ; hence the particles are more easily 
 separated. The particles of water, oil, or air, are 
 
30 FIRST LESSONS IN PHYSICS. 
 
 separated more easily still. Place the hand in 
 water, now try to place it in wood. This is im- 
 possible, for the particles of a solid body are 
 not so easily separated as those of a liquid. 
 We can pour water from a pitcher into a tumbler, 
 and oil from a can into a lamp. The cohesion of 
 liquids is well exhibited in the formation of water- 
 drops as seen when water is gently poured from a 
 vial. 
 
 To break a solid and to separate the particles of 
 a liquid, we must overcome the force of cohesion of 
 each. 
 
 We can overcome the force of cohesion of a 
 body only by displacing its parts ; we do not in 
 reality penetrate the body. Thus, in driving a 
 nail into a board, the nail crowds parts of the board 
 aside. One body can not occupy the space of 
 another unless the other body be first removed ; 
 that is, no two bodies can occupy the same space at 
 the same time. 
 
 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 remain 
 together. The reason of it is, the particles on the 
 surface of the edges can not be brought so near to 
 each other as they were before ; that is, they cohere 
 no longer. A broken walking cane, although the 
 broken parts are glued together again, has lost 
 much of its former strength. 
 
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. 
 
 Water in the form of ice has more cohesion than 
 in the liquid state ; in the form of steam, no cohe- 
 sion whatever. Solids have more cohesion than 
 liquids ; gases have none. Because by heating a 
 solid we may liquefy it, and by continuing the heat 
 we may convert it into gas, heat is said to destroy 
 cohesion. 
 
 Familiar Facts. Although solids and liquids 
 
 cohere, they contain a great number of little holes, 
 called Pores. These may be of different size in the 
 same body, and may be visible or not. The pores 
 of our sfcin are so minute that they can not be 
 detected without a magnifying glass. Every square 
 inch of the 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 Pil- 
 lars ; 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 PHYSIOS. 
 
 LESSON VII. 
 
 ADHESION. CAPILLARY 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 the 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 between 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. In both cases, however, the pressure of air 
 against the exterior surfaces greatly helps to hold 
 them together. 
 
 Why do postage stamps adhere to envelopes ? 
 Because there is cohesion among the particles of 
 the mucilage, and adhesion between the mucilage 
 and each paper surface. 
 
 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, between the parti- 
 cles of water next to it and those farther away. 
 The adhesive force of the former being stronger 
 than their cohesive force, they cling to the hand. 
 
 8 
 
34 
 
 FIEST 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, or fresh paint, to one's clothing. 
 
 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 FIG . 4 . 
 
 not so, the water would not rise. Now immerse 
 another glass plate near the first and not parallel 
 to it (Fig. 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 
 the 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 
 
ADHESION CAPILLARY ATTRACTION. 35 
 
 Fig. 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 (from the Latin, capil- 
 lus, a hair), are open tubes having a 
 very narrow bore. When such a tube 
 comes in contact with a liquid which is FIG 6 . 
 capable of moistening it, the liquid is compelled to 
 rise in it. The finer the bore of the tube, the higher 
 will the liquid ascend. In a tube T J^ 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 char- 
 coal, these two substances containing .capillary 
 tubes which absorb any moisture that would other- 
 wise affect the eggs or the meat. Lampwicks also 
 contain capillary tubes ; they suck up the oil in the 
 lamp. Sugar also has capillary tubes. Grease 
 spots on the floor may be removed by laying earth 
 upon them. Our clothes become wet from the rain. 
 In short, almost 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. 
 In this case we have capillary depression. 
 
36 FIEST LESSONS IN PHYSICS. 
 
 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 falls, if it if 
 
 moved by gravity alone, is vertical. 
 
 4. T Tie pressure of bodies upon their support, or 
 
 their pull when suspended, is called Weight. 
 
 5. 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 its 
 
 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 13.6, we mean that any bulk 
 of mercury has 13.6 times as much weight as 
 a like bulk of water. 
 
EBVIEW. 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. 
 
 LESSON in. 
 
 9. The attraction between magnets and iron is 
 
 called Magnetic Attraction. The magnetic 
 attraction of the earth causes the magnetic 
 needle, or any magnet freely suspended, to 
 point north and south. 
 
 LESSON iv. 
 
 10. The attraction of electrified bodies is called 
 Electric Attraction. 
 
 LESSON 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. 
 
 12. In order to separate tlie parts of a body, we 
 must overcome its cohesion. Solids have 
 more cohesion than liquids. Gaseous bodies 
 have no cohesion whatever. 
 
38 FIRST 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 mole- 
 cules of bread or of salt. 
 
ELASTICITY. 39 
 
 LESSON IX. 
 
 ELASTICITY. 
 
 23. EXPEKIMENT. 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 upon the floor, stand on the table, and let 
 the ball drop upon the slab from a considerable 
 height The ball will then have a black spot 
 much larger than before. Although of a hard sub- 
 stance, the ball is flattened to that extent when it 
 strikes the slab, and in resuming its former shape, 
 it rebounds. 
 
 If the string of a cross-bow, be drawn, and it is 
 then let go, the arrow placed before it flies off 
 with astonishing rapidity. " How is it," may we 
 ask, "that a string can obtain such great force?" 
 If a piece of india-rubber be doubled between the 
 fingers, when the pressure is removed, it will be- 
 come straight again. After pressing a steel pen 
 gently against the thumb nail to try its writing 
 qualities, it recovers its former shape. 
 
 Springs, ivory, or india-rubber, and many other bodies, 
 have the property of recovering their former figure when 
 the force which distorts them ceases to act, provided the 
 force be not too great. 
 
40 FIKST LESSONS IN PHYSICS. 
 
 This property is called Elasticity ; and all such 
 bodies show readily that they are elastic. All 
 bodies are more or less elastic; some substances, 
 however, such as lead or clay, do not readily ex- 
 hibit their elasticity, and were formerly called 
 inelastic. 
 
 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 coun- 
 teract 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- balances. 
 
 Hardness Softness. An ivory ball is elastic 
 and hard ; an India-rubber ball is elastic and soft. 
 If we take a glass rod and try to bend it as much 
 as a steel rod can be bent, it will break. The steel 
 rod is hard and elastic, while the glass rod is hard 
 but much less elastic. Moist clay is soft but has 
 very little elasticity. 
 
 All this shows that no intimate connection exists 
 between elasticity, and hardness or softness. A body 
 is hard, when a great force is required to scratch it, 
 or to displace its particles ; a body is soft, when 
 its particles can be displaced by a slight force. 
 
ELASTICITY. 41 
 
 Brittleness. To break a steel rod requires a great 
 force ; while, on the other hand, a pane of glass can 
 be broken by a slight blow. Every substance can 
 be broken, but the degree of force required is not 
 the same for each. Glass or chalk is brittle, that 
 is, easily broken. The property of being easily 
 fractured, is called Brittleness. 
 
 Ductility. Instead of breaking the steel rod, let 
 us endeavor to stretch it ; we find that only by a' 
 very great force can it be drawn out into a long 
 wire. A narrow glass tube can be drawn out very 
 thin by the heat of a flame. Bodies which can be 
 drawn out into a thread or wire are ductile ; this 
 property is called ductility. 
 
 Malleability. Many substances, such as iron, 
 copper, brass, gold, or silver, besides being ductile, 
 can -be hammered into sheets ; hence they are 
 called malleable. Some substances, as lead for 
 example, are malleable without being ductile. Gold, 
 the most malleable metal, has been hammered out 
 into leaves each onl sWoTJTj of an inch thick. 
 
 From this lesson we may learn that when the 
 force which displaces the parts of a body is within 
 certain limits, the body may exhibit elasticity ; if 
 beyond certain limits, it exhibits the properties of 
 brittleness, ductility or malleability. 
 
42 FIKST 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 filled 
 
 with water, because it contains 
 
 air. A cork previously placed 
 
 in the tumbler, will show the 
 
 position of the water-level in- FIG 6< 
 
 side. (See Fig. 6.) Air maintains its place like 
 
 every other body ; hence it shuts out the water 
 
 almost completely. But if you withdraw the hand 
 
 which presses the tumbler down, the tumbler will 
 
 instantly rise. The air in the glass was compressed, 
 
 and recovered its previous space, because air is 
 
 elastic. 
 
 25. EXEPERIMENT. 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, because, in this case, the air can 
 pass out. 
 
ELASTICITY OF AIR. 43 
 
 26. EXPERIMENT. Cement a funnel into the 
 neck of a small bottle, and pour water into it. 
 Only a little water will enter at first ; but, sub- 
 sequently, not a drop will get in, as there is no 
 escape 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 has 
 now no outlet, and, consequently, no water can 
 enter. 
 
 27. EXPERIMENT. A very good illustration of 
 
 the expansive force of air may be obtained by a 
 " 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 tube, 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 
 drive the glass tube farther down, until it 
 nearly reaches the bottom of the bottle. The bottle 
 now contains air in its npper 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. But now the air over the 
 water is compressed, and its expansive power 
 forces the water upward through the tube. The 
 inventor of this little apparatus was Hero, a phil- 
 osopher, who died in Alexandria, over two thous- 
 and years ago. 
 
44 FIRST LESSONS IN PHYSICS. 
 
 Air is an elastic T)ody ; 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. 21, p. 78.) The Diving-bell may also be con- 
 sidered an application of this force. 
 
 ' Another application is the pop-gun. 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 com- 
 mence pushing down the rod ; the air inside is now 
 compressed, it has the tendency 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 
 finally expelled with a loud report. 
 
 Another application is the blow-gun. 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. 
 
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 
 Fig. 8.) The hand on the paper, 
 after pressing the latter firmly 
 against the tumbler, is removed, 
 but the water does not flow out. 
 How can this be accounted for? 
 Notice that the tumbler contains FIG. e. 
 
 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 paper*(and the paper supports the water), or else 
 the air would force its way into the water, by rush- 
 ing up along a part of the inner side of the tum- 
 bler, leaving the water to fall down on the opposite 
 part. 
 
 29. EXPERIMENT. Immerse a tumbler, hori- 
 zontally, in 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 bottom upward, and without, however, 
 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 the water 
 
46 
 
 FIRST LESSONS IN PHYSICS. 
 
 in two communicating vessels ought to have the 
 same height (Lesson XVIII). The tumbler contains 
 no air, while a large amount of air is over the re- 
 maining 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 ves- 
 sel. The tube will be partly filled with 
 water; if taken out, the water will 
 flow through the tube and fall, be- 
 cause attracted to the earth. Place 
 the tube again in the vessel with 
 water, but on slowly removing it, be 
 sure that you keep the upper open- 
 ing closed with the thumb.* No water 
 will flow from the tube, because air 
 presses against the lower opening 
 and thus supports the column of 
 water in the tube. On removing the 
 thumb the water will flow out, be- 
 cause, in that case, the air presses as 
 strongly above as it does below ; the 
 water, consequently, obeys the force 
 of gravity and falls. Now fill the 
 glass tube again ; hold it obliquely, 
 until it is in a horizontal position : 
 FIG 9 - the water will still remain in the tube. 
 
PRESSURE OF AIR. 47 
 
 This shows that air presses not only upward ', 
 downward, and laterally, but in all directions. 
 
 In the preceding two experiments, a column of 
 water was sustained by the downward pressure of 
 air. Air, water, and all other fluids, exert pressure 
 in all directions, while solid bodies exert downward 
 pressure only. But downward pressure means 
 weight ; hence fluids and solids (that is, all bodies,) 
 have weight, while fluids have,besides weight, pres- 
 sure in all directions. 
 
 familiar facts. From an open faucet in a full 
 barrel with its bung-hole closed, the liquid does not 
 flow, because the air presses against the opening in 
 the faucet. To draw vinegar, or any other liquid 
 from a barrel, plunge a long tube into the liquid ; 
 close the upper end with the thumb and withdraw 
 the tube. The liquid in the tube will not flow out 
 (why not ?) as long as the thumb closes the upper 
 end. Oil-cans must be opened on the top in order 
 to obtain a ready flow. 
 
 Application. The Barometer (see next lesson). 
 Pneumatic Railway. 
 
48 FIRST LESSONS IN PHYSICS. 
 
 LESSON XII. 
 
 THE BAROMETER. 
 
 In the two preceding experiments the downward 
 pressure of air supported a column of water in a 
 tumbler, or in a glass tube. Nothing was said, 
 however, concerning the height of the liquid col- 
 umn. But we know with certainty, that if a pump 
 could be made with perfect valves it might raise 
 a column of water, nearly 34 feet high. That is to 
 say, the pressure of the atmosphere might then sup- 
 port a column of water about 34 feet high. 
 
 But what if, by the same pump, we had to lift a 
 fluid which has greater specific gravity than water, 
 say liquid tar, or mercury ? Would the atmosphere 
 be competent to support, e. g., a column of mercury 
 34 feet high ? The answer is : No. Such a column 
 weighs more than a column of water of the same 
 thickness ; hence its height must be less. The 
 truth of this Can be proved as follows: On each 
 of the scale-pans of a balance place a tall vessel 
 34 feet high, but only one square inch across. Both 
 vessels are to have equal weights ; hence the beam 
 of the balance will remain horizontal. Now, fill 
 one of them with pure water to the height of 34 feet, 
 and the other with mercury until it balances the 
 water; the mercury will then be found to be nearly 
 
THE BAKOMETEK. 49 
 
 30 inches high. We have now two columns of 
 equal weight and thickness, but of different heights. 
 The specific gravity of the mercury (Lesson II) is 
 13.6, and the height of the column of mercury = 34 
 feet divided by 13.6 = 30 inches. 
 
 But it was stated above, that the atmosphere 
 could support a column of water no higher than 
 about 34 feet, consequently, the atmosphere must 
 be competent to support a column of mercury of 
 egual weight , that is, a column of mercury about 30 
 inches high. 
 
 In order to test this reasoning, take a glass tube 
 about 36 inches long, closed at one end ; fill it with 
 mercury, close the open end of the tube with the 
 finger, and invert the tube into a small cup or 
 basin containing the same fluid. On withdrawing 
 the finger, it will be found that the mercury remains 
 suspended in the tube to the height of about 30 
 inches (see frontispiece, first figure to the left). The 
 space over the top of the mercury is entirely empty ; 
 it is a vacuum. We have here the weight of a col- 
 umn of mercury completely balanced by a column 
 of air of the same weight, but of indefinite height. 
 Any change in the weight of the air will be instantly 
 followed by a corresponding change in the weight 
 of the column of mercury. 
 
 From the preceding it appears, 1st, that a column 
 of water 34 feet high, or one of mercury 30 inches 
 high, nearly, can be supported by the pressure of a 
 column of air ; 3d, that the weight and thickness of 
 
50 FIRST LESSONS IN PHYSICS. 
 
 both columns are the same, while the height of the 
 column of air has not yet "been determined ; 3d, that 
 any change in the pressure of the atmosphere must 
 produce a corresponding change in the height of 
 the column of mercury. 
 
 The Barometer is an instrument for ascertaining 
 the pressure of the air. Its ordinary form is that 
 of an inverted tube (see frontispiece, centre figure), 
 with its open or lower end either bent upward a few 
 inches, or immersed in a small cistern filled with 
 mercury. This end may be in a wooden or metallic 
 case containing a fine opening for the air to pene- 
 trate to the mercury. The height of the fluid in a 
 barometer is the same, whether the instrument be 
 in the open air or within the house. For air presses 
 in all directions (Lesson XI) ; any difference in 
 pressure, therefore, would immediately be equal- 
 ized. 
 
 Uses of the Barometer. The atmosphere is an 
 ocean surrounding the earth. Its pressure, there- 
 fore, must be greatest at its greatest depth, viz. : at 
 the level of the sea. It will be gradually diminished 
 as we ascend in a balloon, because, in this case, we 
 leave air below us which does not press upon the 
 mercury in the balloon. The mercury of the 
 barometer actually falls in proportion to the eleva- 
 tion to which it is taken. Hence the barometer is 
 used for measuring the heights of mountains. 
 
 If the records of the barometer at different places, 
 but at identical times, be compared, probabilities 
 
THE BAROMETER. 51 
 
 may be obtained in regard to the weather which we 
 are about to have. Thus, if a surplus of air exists 
 at A and B, the air in these localities has more 
 weight, and the barometer will stand high ; at once 
 a quantity of air will be conveyed to some distant 
 place C, where the quantity of air is deficient, that 
 is, where the barometer stands low. If the records 
 show that at C the air was previously moist and 
 warm, while at A and B it was cold, the probabili- 
 ties for C are clouds, or rain. Hence the barometer 
 is used to some extent for ascertaining the weather 
 that is about to follow. 
 
 Amount of Pressure of Air. In the experiment 
 indicated on page 48, a weight of 15 pounds may, 
 at the level of the sea, be substituted for either col- 
 umn without seriously disturbing the balance. Thus 
 a column of mercury one inch square and 30 inches 
 high , or one of water one inch square and 34 feet 
 high, weighs 15 pounds. Consequently, the column 
 of air which can support either that is, one which 
 is one inch square but reaches to the top of the 
 atmosphere, also weighs 15 pounds. 
 
52 FIRST LESSONS IN PHYSIOS. 
 
 LESSON XIII. 
 
 REVIEW. 
 
 LESSON ix. 
 
 1. Springs, ivory, India-rubber, and many other 
 
 bodies, have the property of recovering their 
 former figure when the force which distorts 
 them ceases to act, provided the force be not 
 too great. This property is called Elasticity. 
 
 2. All bodies are more or less elastic. 
 
 3. Hardness is that property by which a body can 
 
 not be scratched, nor its particles displaced, ex- 
 cept by the application of great force. 
 
 4. Softness is the property by which the parts of a 
 
 body can be scratched or displaced, if a slight 
 force is applied. 
 
 5. Brittleness is the property by which a body is 
 
 easily fractured. 
 
 6. Ductility is' the property by which a body can 
 
 be drawn out into wire. 
 
 7. Malleability is the property which enables us to 
 
 hammer a body into thin sheets. 
 
 8. Elasticity is displayed only when the force dis- 
 
 placing the parts of a body is within certain 
 limits. 
 
 9. When the force displacing $ie parts of a body is 
 
 beyond certain limits, the., body may exhibit 
 brittleness, ductility, or malleability. 
 
KEVIEW. 53 
 
 LESSON x. 
 
 10. Air is an elastic body ; and the more we com- 
 press it, the greater is its expansive force. 
 
 LESSON xi. 
 
 11. Air, like water and other fluids, exerts pressure 
 in all directions. 
 
 LESSON xii. 
 
 12. The downward pressure of air can support a 
 column of any other fluid, and the height of 
 this column depends upon the specific gravity 
 of the fluid. 
 
 13. The downward pressure of the air is nearly 
 fifteen pounds to the square inch of surface. 
 
 14. A column of air extending from the earth to the 
 top of the atmosphere will support a column of 
 mercury about 30 inches high. 
 
 15. An entirely empty space is called a vacuum. 
 
 16. The barometer is an instrument for determining 
 the pressure of the air. 
 
 17. The uses of the barometer are, first, to measure 
 the heights of mountains ; second, to assist in 
 ascertaining the weather about to come. 
 
INERTIA. 
 
 LESSON XIY. 
 
 INERTIA. 
 
 31. EXPERIMENT. Place a piece of chalk upon 
 a book, and move the book quickly sideways. 
 The chalk drops to the floor without participating 
 in the motion of the book, because the book is 
 
 withdrawn from under it. 
 
 i 
 Familiar Facts. A coin laid upon a card on 
 
 the mouth of a bottle, drops into the bottle if the 
 card is snapped off quickly It falls, because its 
 support, the card, has been removed from under 
 it. Persons in a horse-car are thrown backward 
 if it starts suddenly. These, and numerous other 
 facts, attest that a body at rest remains at rest 
 until it is set in motion by some force. 
 
 32. EXPERIMENT. The motion given to the 
 book, and to the card, was so sudden that there 
 was not sufficient time for it to be communicated 
 to the chalk and coin. Hence it was that these two 
 bodies did not participate in the motion, but drop- 
 ped simply because they were left unsupported 
 (Lesson I). Now move the book and the card 
 slowly; the two objects upon them will partici- 
 pate in the motion, and not fall. This shows that 
 to set a body in motion, time is necessary. 
 
INERTIA. 55 
 
 Familiar Facts. A person running rapidly, or 
 a railway train in motion, cannot stop suddenly. 
 Place a piece of chalk on a book, and move the 
 book until it strikes against the wall ; the chalk 
 will continue to move after the book has ceased 
 moving. So do persons in a car which stops sud- 
 denly. A bell continues to ring for a time after it 
 has been struck. 
 
 A 'boat moves on a little after the action of the 
 oars has ceased. So coffee, when stirred, will re- 
 volve in the cup, although the spoon has been 
 removed. In winter, young persons when coasting 
 may be frequently seen flying down hill on their 
 sleds, and sometimes pass up an opposite hill a 
 short distance. A rabbit cannot run as fast as a 
 hound ; but if pursued by the hound, it may, by 
 suddenly changing its course to the right or left, 
 gain considerable advantage over the hound, which, 
 not being prepared for the change, must first over- 
 come his inertia before he can turn. From this we 
 see that a body once in motion, remains in motion 
 until stopped by some force or resistance. To stop 
 the motion of a body, time is necessary. 
 
 Inertia is the tendency which all bodies have to 
 persist in their state, whether this state be of rest 
 or of motion. 
 
 Application. Fly-wheels. The switching-off 
 of railway trains without a locomotive. 
 
56 FIKST LESSONS IN PHYSICS. 
 
 LESSON XV. 
 
 THE INCLINED PLANE. 
 
 33. EXPERIMENT. A ball lying on a book 
 
 upon a table will not fall as long as the book lies 
 in a horizontal position. But let the book be 
 raised on one side, and the ball rolls down. It 
 rolls down the faster the higher the book is tilted. 
 The ball presses with its whole weight upon the 
 book ; but when the book is raised a little on one 
 side the ball presses less, and begins to fall. The 
 higher we raise the book the less will the ball 
 press upon the book, and the more rapidly will it 
 descend. The surface of the book, when raised 
 on one side, is an inclined plane. 
 
 'Familiar facts. A wagon descending a steep 
 hill need not be drawn by the horses ; it is checked 
 rather, in order to prevent it from rolling down 
 too rapidly. When ascending a hill, the horses 
 have a more difficult task than on a level road. 
 It is tiresome for us to ascend steep stairs. In 
 loading wagons the skid is used. It saves much 
 labor, if it is not placed too steep. If tlie wagon 
 is very high, the skid must be quite long. The 
 steeper an inclined plane, the greater the velocity 
 of a body descending on it; and the greater the 
 force required to ascend it. 
 
THE INCLINED PLANE. 
 
 57 
 
 34. EXPERIMENT. Let an inclined plane be 
 formed by a board. (See Fig. 10.) Now it makes 
 a great difference whether the ball is rolled down 
 from the middle, or from 
 the top of the inclined 
 board. Try both. The 
 result will be 
 
 FIG. 10. 
 
 that the 
 
 ball, when rolling down from the top, acquires a 
 greater velocity. A body increases in velocity as 
 the space increases through which it descends. 
 
 35. EXPERIMENT. Before the lower end of 
 a grooved board place a ball. Then let another 
 ball be rolled down the inclined plane, so that it 
 strikes the first ball. Mark the place to which 
 the latter moves, and put it in its former position 
 again. Repeat the experiment, having the upper 
 
 end of the board raised a little higher ; that is, 
 having the inclined plane a little steeper (Fig. 11). 
 The ball rolling down will then cause the first ball 
 to move farther, perhaps to a, having struck it 
 with greater force. This is owing to the greater 
 steepness of the plane. We have seen that the 
 velocity of a body increases with the inclination 
 of the plane. The last experiment shows that 
 
58 FIRST LESSONS IN PHYSICS. 
 
 the greater the velocity of a body the greater its 
 striking force. 
 
 Familiar Facts, A bullet thrown with the 
 hand inflicts less harm than one fired from a gun. 
 A ~boy running slowly against a tree scarcely feels 
 the shock ; but by running against it rapidly, he 
 might be seriously injured. We throw a mar- 
 ble in the air and catch it again without being 
 hurt, but we should experience pain, if the mar- 
 ble were thrown up very high. Hailstones may 
 strike with force sufficient to break glass, and 
 to destroy standing grain. A boy jumps easily 
 from a fence, but would scarcely dare to jump 
 from the top of a house. 
 
 The descent of bodies on the inclined plane 
 shows that they are not supported by it with 
 their whole weight; if they were, they would not 
 descend. To say they are not wholly supported 
 means : An inclined plane overcomes a portion 
 of the weight of bodies upon it. Hence its 
 
 Application 1. To overcome weight: A road 
 winding up a hill. A skid ; an obliquely-placed 
 plank ; wedges and axes. 
 
 2. To exert strong pressure : Wedges used in 
 oil-wells, sugar-mills, &c., to press out the juice. 
 
 3. To overcome cohesion. Our knives, axes, 
 hatchets, scissors, needles, nails, swords, bay- 
 onets, saws, files, chisels, planes, plows, &c., &c. 
 
THE LEVER. 59 
 
 LESSON XVI. 
 
 THE LEVER. 
 
 36. EXPERIMENT. Balance a rod on the edge 
 of a slate supported "between two heavy books. 
 The rod is in a state of equilibrium, because on 
 each side of the point of support there is an 
 equal amount of matter. Now, place the rod in 
 such a manner that on one side of the support it 
 shall be twice as long as on the other. The longer 
 arm will descend, because it contains more mat- 
 ter. Let us repeat this slowly. Observe, that 
 in lifting the end of the long arm with the hand, 
 it moves through a greater space than is passed 
 through by the end of the short arm. The lengths 
 of the two arms of the lever are in the ratio of 1 
 to 2 ; and the space passed through by the end 
 of the long arm is twice as great as that passed 
 through by the other. Notice, also, that the ends 
 describe these unequal spaces in the same length 
 of time ; therefore, the end of the long arm of a 
 lever has greater velocity than the end of the 
 short arm. 
 
 But it was stated before (Less. XV), that, owing 
 to their great velocity, hailstones, although small 
 bodies, could acquire great power. So will any 
 small weight or object, if it be given great veloc- 
 ity. Apply this to the present case : 
 
60 FIRST LESSONS IN PHYSICS. 
 
 37. EXPERIMENT. Place a heavy weight on 
 
 the short arm of a lever. The greater the length 
 of the other arm, the smaller may be the weight 
 upon it requisite to lift the large weight on the 
 short arm. The weight or pressure to be applied 
 to the long arm for that purpose is called the 
 Power. Thus the small weight, with the great 
 velocity of the long arm, counterbalances the 
 large weight with the small velocity of the short 
 arm. A stiff bar made to turn on one point is 
 a lever. 
 
 The greater the length of one arm of a lever, 
 the less power needs be applied to that arm to 
 lift the load on the other arm. 
 
 Question. What power is needed in a lever to 
 counterbalance the load on the short arm ? The 
 amount of power depends, evidently, upon the 
 length of the long arm. If, as in the above case, 
 it has twice the length of the short arm, the power 
 needed to lift and counterbalance the load is one- 
 half the weight of the load. Thus, if a burden 
 of 100 pounds is to be lifted by means of a lever 
 whose long arm has twice the length of the short 
 arm, a power of 50 pounds is required ; if four 
 times, a power of one-fourth, or 25 pounds. To find 
 the power necessary to lift a load by means of a 
 lever, divide the product of the load into its dis- 
 tance from the point of support by the distance 
 from the point of support to the place where the 
 
THE LEVER. 61 
 
 power is to be applied. The quotient=the power 
 required. 
 
 The important points in a lever: 1. The point 
 of support, or Fulcrum. 2. The load (or weight) 
 to be lifted. 3. The power applied. 
 
 In the lever illustrated above, as well as in the 
 applications given below, the order of these three 
 points is; Load Fulcrum Power. Levers ar- 
 ranged in this order are called Levers of the First 
 Class. 
 
 Application. The Steelyard; Crowbars j Pump- 
 handles ; children teetering ; scissors and shears. 
 
 If, in order to lift a load, a laborer supports his 
 crowbar on a stone upon the ground, and enters 
 the short arm of the lever thus formed under the 
 weight , his lever is one of the first class ; why ? 
 But if he does not use the stone ; if he simply 
 rests his crowbar with one end on the ground, so 
 that the load comes to lie between him and the 
 fulcrum (the ground), then the order of his lever 
 is : Fulcrum Load Power ; and this constitutes 
 a Lever of the Second Class. 
 
 Application. The nut-cracker; where its limbs 
 are riveted together, is the Fulcruni ; the nut re- 
 presents the load (in this case the load, or resist- 
 ance, is to be crushed, not lifted); the power is 
 where the hands are applied. We have here two 
 levers combined. The chopping -Tinife is a lever 
 
62 FIRST LESSONS IN PHYSICS. 
 
 of the same class. Where the knife is fastened 
 is the Fulcrum. The object to be cut is the Load ; 
 the Power is at the handle (in this case, too, the 
 resistance is not a load to be lifted, but cohesion 
 to be overcome). Lemon-squeezers ; Cork-squeez- 
 ers ; the Wheel-barrow ; l the oar of a boat. 2 
 
 The great progress of our age does not lie so 
 much in the introduction of new forces of nature, 
 of which there are but a few, but in the ingenious 
 application of those few forces, and in their skill- 
 ful combination into machines. One of the offices 
 of machines is to communicate the effect of a force 
 to bodies which otherwise could not be acted upon 
 by that force. Thus, without the locomotive, the 
 expansive force of steam could not communicate 
 its effect upon a train of cars. 
 
 The Lever is the simplest of all machines ; and 
 probably, also, the most ancient. By means of a 
 very long arm, it becomes a most powerful instru- 
 ment. It is told of Archimedes, a Syracusan 
 philosopher (about 250 years before Christ), that 
 he offered to move the earth itself, if the king 
 would give him a place to stand on. 
 
 Read on Levers, and The Art of Walking, in "Things Not Generally 
 Known." 
 
 1. The Fulcrum is where the wheel rests on the ground. 
 
 2. The Fulcrum is where the oar rests in the water ; the Load is the 
 boat. 
 
THE PENDULUM. 63 
 
 LESSON XVII. 
 
 THE PENDULUM. 
 
 38. EXPEEIMENT. (a.) The string "by which a 
 stone is suspended has a vertical direction (Less. 
 I). If the stone is drawn a little to one side, the 
 direction of the string will be slanting. On letting 
 the stone go now, it will begin to move. Since all 
 bodies are drawn to the earth (Less. I), it will 
 approach the earth as near as possible. When 
 nearest the earth, it has again the vertical direc- 
 tion. But the weight does not stop there; its 
 inertia (Less. XIV) carries it onward ; being held 
 by the string it does not fall to the ground ; it 
 ascends, until gravity finally stops it. Gravity 
 not only stops it, but also pulls it down again. 
 Noticing its downward course more closely, we 
 see that it descends with increasing velocity. 
 Inertia causes it again to pass by the lowest point 
 of its path ; it ascends on the other side, stops an 
 instant of time, and is then forced back again by 
 gravity. Thus it swings back and forth for a 
 certain time. Each swinging in one direction is 
 called a vibration. The vibrations grow shorter, 
 and observation shows us that, finally, they cease 
 al together. 
 
 If, while the pendulum is vibrating, we beat 
 time, it will be found that the same length of time 
 is necessary for the shorter vibrations toward the 
 
64 FIRST LESSONS IN PHYSICS. 
 
 close of the experiment as for' the earlier, longer 
 ones. Thus if the pendulum at first made sixty 
 vibrations a minute, it will continue to make the 
 same number during the same time, although it 
 afterward passes through shorter arcs. 
 
 The vibrations of the same pendulum will talce 
 place in the same lengtli of time, unless these vibra- 
 tions pass through large spaces or arcs. 
 
 39. EXPERIMENT. (&.) Cut off three-fourths of 
 
 the string. The pendulum is now shorter than it 
 was before ; it has only one-fourth the former 
 length. After it is set vibrating, count the num- 
 ber of its vibrations in ten seconds ; the number 
 will be greater than that of the former pendulum.* 
 The reason of this is easily seen, if we suspend the 
 shorter and longer pendulum, both, from the same 
 horizontal height. The former descends on a shorter 
 incline than the latter, and, therefore, takes less 
 time to descend. This shows that a short pendu- 
 lum vibrates more rapidly than a long one} 
 
 I A pendulum which is four times as long as another will need twice 
 as much time to perform one vibration ; that is, it will vibrate half as fast 
 as the other. Let one pendulum be nine times as long as the other, it will 
 need three times as much time (it will vibrate one-third as fast) ; or, it will 
 vibrate once while the other vibrates three times. Hence the times of 
 vibrations of pendulums are to each other as the square roots of their 
 lengths. Thus, if one pendulum has a length of 4, and another the length 
 of 36, the former will vibrate faster than the latter ; the square roots being 
 2 and 6, the latter will require three times as much time as the other to 
 perform one vibration ; that is, if it vibrates once every three seconds, the 
 former will vibrate once every one second; or, the longer pendulum will 
 vibrate once in the same length of time that the shorter one vibrates three 
 times. 
 
THE PENDULUM. 
 
 65 
 
 The principal application of the pendulum 
 
 is its use for the 
 regulation of 
 clocks. A clock is 
 required to keep 
 good time. From 
 the preceding, it is 
 evident that a pen- 
 dulum, by its vi- 
 brations, whether 
 it moves fast or 
 
 slowly, gives us 
 equal portions of 
 time. But a pen- 
 dulum will cease 
 vibrating after a 
 short time ; what, 
 then, must be done 
 to meet this diffi- 
 culty? Everybody 
 FIG - 12 - knows that the 
 
 pendulum stops when the clock has " run down ; " 
 that is, when the weight has reached its lowest 
 point. It is plain that the downward tendency 
 of the weight is quite sufficient to meet that 
 difficulty ; for while the pendulum alone would 
 very soon cease vibrating, the descent of the 
 weight lasts at least twenty-four hours. (What is 
 meant by winding up a clock ?) But the weight, 
 after it begins to descend, increases in speed (Les- 
 son XV), and as the cord from which it is sus- 
 5 
 
66 
 
 FIKST LESSONS IN PHYSICS. 
 
 pended, passes round an axle which causes the 
 hands to move, the accelerated velocity would 
 cause the hands to move faster and faster. To 
 obviate this, the axle is connected with a wheel 
 of saw-shaped teeth (Fig. 12), which revolves with 
 it, and above which swings a curved hook, A. A., 
 called an escapement, whose two teeth work al- 
 ternately in the saw-shaped teeth of the wheel. 
 At every vibration of the pendulum, one of these 
 two teeth stops the revolution of the wheel, and 
 thus interrupts the descent of the weight. Now, 
 since the pendu- 
 lum vibrates in 
 equal portions of 
 time, the weight 
 descends through 
 equal spaces in 
 equal times. And 
 since the weight 
 descends through 
 equal s p a c e s in 
 equal times, it 
 turns the axle, the 
 work, and the 
 hands with uni- 
 form velocity. 
 Hence clocks are 
 moved by the de- 
 scent of the weights, and regulated by the vibra- 
 tions of the pendulum. (See Fig. 13.) 
 
COMMUNICATING VESSELS. 67 
 
 LESSON XVIII. 
 
 COMMUNICATING VESSELS HYDRAULIC PRESS. 
 
 40. EXPERIMENT. Fit a piece of thin board 
 
 into a tumbler, as a vertical partition divid- 
 ing the inside of the tumbler into two spaces. 
 The board should not touch the bottom of the 
 glass, but be a little above it. Now pour water 
 into the tumbler, and there will be two horizontal 
 surfaces of water, each having the same height. 
 Remove the board, and in place of it immerse a 
 wide glass tube. The two surfaces of water will 
 again be of the same height, nearly. 
 
 Familiar Facts. The same may be seen in 
 two glass tubes of unequal 
 width (Pig. 14) which are ce- 
 mented into a base made of 
 tin, and connected with each 
 other by means of a tin tube 
 (a). Also in a teapot. The tea 
 PIG. u. rises as high in the spout as in 
 
 the body of the pot, and if the body were higher 
 
 than^the spout the tea would flow from the spout. 
 
 Hence, in pouring out tea, we lift the pot and 
 
 lower the spout. 
 
 41. EXPERIMENT. Take a tube made of glass 
 or tin and bend it so that one limb be very short, 
 perhaps, only one-twentieth as long as the other, 
 
68 FIRST LESSONS IN PHYSIOS. 
 
 and let the opening of the short limb be drawn 
 out fine (Fig. 15). Then pour water into the 
 long tube, holding the short one closed with 
 the finger. On removing the finger, water 
 will jet forth. Thus we have a fountain on 
 a small scale. If the short tube were tall 
 enough, the water would rise until it stood 
 at a level with the water in the other tube. 
 The tube being short, however, the 
 water rises in a jet, but not to that level, 
 because there is friction, and because the 
 returning drops depress the rising jet. 
 
 Familiar Facts. Cisterns, offices, dwel- 
 ling-houses and factories are supplied with 
 water from large elevated reservoirs. 
 
 Vessels connected with each other, so that a 
 liquid 'can pass freely from one into the other, 
 are called Communicating Vessels. 
 
 Why may water pipes under ground be said to 
 be communicating tubes ? 
 
 42. EXPERIMENT. Take a cylindrical tin ves- 
 sel (about five inches high), with a 
 neck, B, perfectly cylindrical (Fig. 16), 
 into which a cork can be fitted tightly, 
 and with small holes in the sides of 
 the vessel as well as in the upper 
 (tapering) part. These openings are 
 carefully closed with beeswax, the 
 vessel filled with water to the very 
 
HYDEAULIO PEESS. 69 
 
 edge of B, and the cork set on the neck. If the 
 cork is then driven in by a sudden "blow with the 
 hand, the water jets forth from all the openings 
 simultaneously. 
 
 One end of a glass tube is cemented into a small, 
 hollow tin ball provided with about 
 a dozen fine openings. (See wood, 
 cut.) The other end is freely in- 
 serted in a rubber ball, previously filled with water. 
 When the ball is pressed with the hand, the same 
 phenomenon as above is witnessed. (To fill the 
 ball, squeeze it together under water, and then let 
 it go.) 
 
 The force of a pressure brought to bear upon a 
 small portion of a liquid, is transmitted equally 
 to all parts of the liquid. 
 
 Suppose, now, that the bottom of the tin vessel had 
 merely been telescoped in the vessel. The pressure 
 given to the water in B would evidently have 
 forced the bottom out ; and the bottom would then 
 have exerted a pressure upon any resisting object. 
 
 Application. Advantage has been taken of 
 this in a machine called the "Hydraulic Press," 
 invented in 1796 (Fig. 17). By means of a lever 
 (of the second kind) a pressure is exerted upon 
 the water in the narrow tube, A. This pressure is 
 communicated to the water in the wide tube, (7, 
 
70 
 
 FIRST LESSONS IN PHYSICS. 
 
 forcing the movable cylinder, J5, to ascend. Bales 
 of cotton, or any other object to be compressed, 
 lying on the plate, and prevented from rising 
 by the fixed plate, P, are thus compressed with 
 
 PIG. 17. 
 
 enormous force. For if the surface of the water 
 in the cylinder be 100 times that of the water in 
 the narrow tube, and if the pressure applied to 
 the liquid in the tube amount only to 50 pounds, 
 the pressure exerted upon the bale of cotton will 
 amount to 5,000 pounds But since the power 
 applied by the hand may be increased tenfold 
 with the advantage gained by a longer lever, the 
 amount of pressure may easily be raised to 
 50,000 pounds It can be farther increased by 
 steam-pressure so that the force of pressure may 
 amount to over a million pounds. 
 
BREATHING. THE BELLOWS. 71 
 
 LESSON" XIX. 
 
 BREATHING. THE BELLOWS. 
 
 43. EXPERIMENT. If a glass tube be placed 
 with one end in water, we can cause the water to 
 rise in the tube by sucking it up with the mouth. 
 This is the reason for it : We draw the air which 
 is in the tube, into the mouth ; a vacuum (Lesson 
 XIII) is thus created, and the pressure of the ex- 
 ternal air upon the water forces water into the 
 tube. 
 
 Familiar Facts. Instead of water we may 
 draw up air alone ; this is done in breathing. We 
 enlarge our lungs and the cavity in our chest 
 (Lesson II, p. 16) ; by this, the air in the chest is 
 rarefied, and the external air, by the pressure of 
 the layers of air above it, forced to rush into the 
 chest. This process is called Inspiration. During 
 the process. of Expiration we contract the chest, 
 and the air must rush out. If we immerse a pail 
 in a pond, and fill it with water, the moment the 
 pail is drawn out again, the water rushes in and 
 occupies the space where the pail was before. In 
 the same way 
 
 Air rushes into a vacuum, or into any space 
 containing rarefied air. 
 
72 FIKST LESSONS IN PHYSICS. 
 
 2-. very useful application of the pressure of 
 air is the Bellows, an instrument for blowing fire. 
 It consists of a space enclosed by two boards 
 opposite each other, which are united around the 
 edges by a wide strip of leather. In front, this 
 spaco opens in a narrow tube. In one of the 
 boards is a hole closed by a valve. A valve is a 
 sort of lid or cover, which admits a fluid into a 
 space, but prevents its return. When the bel- 
 lows is drawn out, the air inside is rarefied. The 
 external air now seeks to rush in, but it finds no 
 other way than through the valve ; this it opens 
 and instantly fills the extended bellows. When 
 the bellows is drawn in, the air inside is com- 
 pressed, and its expansive, or elastic, force (Les- 
 son X) being greatly increased, it presses against 
 the inner sides of the bellows, and, in doing so, 
 closes the valve. There being no other egress, 
 the air passes through the tube in front into the 
 fire. 
 
 On the same principle yon may explain Drink- 
 ing and Smoking. 
 
REVIEW. 73 
 
 REVIEW. 
 
 LESSON xv. 
 
 1. The steeper an inclined plane, the greater the 
 
 velocity of bodies descending on it ; and the 
 greater the force required to ascend it. 
 
 2. A body increases in velocity as the space in- 
 
 creases through which it descends. 
 
 3. The greater the velocity of a body, the greater 
 
 its striking force. 
 
 LESSON xvi. 
 
 4. A lever is an inflexible bar made to turn about 
 
 a fixed point. 
 
 5 When moving, the end of the long arm (where 
 the power is applied) has greater velocity 
 than the end of the short arm where the load 
 is attached. 
 
 6. The greater the length of the long arm of the 
 
 lever, the greater becomes its velocity; and, 
 consequently, the less power needs be ap- 
 plied to lift the load. 
 
 7. To find the power required to lift a load by 
 
 means of a lever, divide the product of the 
 load into its distance from the point of sup- 
 port by the distance between the point of 
 support and the place where the power is to 
 be applied. The quotient is the Power. 
 
74 
 
 FIRST LESSONS IN PHYSICS. 
 
 LESSON XX. 
 
 COMMON PUMP. 
 
 44. EXPERIMENT. Instead of immersing the 
 end of a glass tube, as we did in the preceding 
 lesson, let us dip a syringe into water. On draw* 
 ing up the piston, by means of the piston-rod, the 
 liquid is seen to rise in the 
 syringe. This is explained 
 by the law given in the pre- 
 ceding lesson, for the piston 
 being air-tight, it leaves, as 
 it rises, a vacuum below it, 
 which is eagerly filled by 
 the water. But what causes 
 the water to rise ? The answer 
 is : The pressure of air on the 
 surrounding water. 
 
 Application. Our pumps. 
 They act on the same princi- 
 ple. When we look at a 
 pump (Fig. 18), the first thing 
 that strikes our eye is the 
 cylinder, or barrel, C, the 
 spout, 8, and the lever, or 
 handle, H. The lower part 
 of the barrel, P, is called the 
 
 PIG. 18. 
 
COMMON PUMP. 75 
 
 suction-pipe ; it is submersed in the water. Inside 
 of the barrel works a piston 0, which fits air-tight, 
 and can be moved up and down by means of the 
 piston rod to which it is attached. It is pierced 
 with a hole, and the hole is covered by a valve, t), 
 which opens upward. The piston-rod is connected 
 at the top with the lever H. 
 
 When the handle of the pump is raised, and has 
 arrived at its highest point, the piston is at its 
 lowest, directly over the valve A. 
 
 Let us see now what happens when the handle is 
 being forced down, as in Fig. 18. First, observe 
 that the piston rises ; next, that the valve A opens. 
 But what causes this valve to open ? The answer 
 is : When the piston rises, the air previously con- 
 fined between valve A and the level of the water 
 in the suction-pipe now occupies a larger space ; 
 hence it is rarefied, and has less pressure than the 
 air over the outside water F F. The result of these 
 unequal pressures is that the level of F F is lowered 
 by the pressure of the atmosphere over it, and that 
 the water in the suction-pipe, P, must rise until the 
 air inside of the tube has reached the same pressure, 
 nearly ; that is, the same degree of density as the 
 outside air. At this instant, the valve A will close- 
 that is, it will fall of its own weight. It is evident 
 that the valve, V, remains closed whenever the piston 
 rises. 
 
 When the piston is lowered, the valve v is forced 
 open. Why ? Because the air below the piston is 
 
76 FIRST LESSONS IN PHYSIOS. 
 
 compressed, and, remembering p. 44, we know that 
 it is the expansion of the compressed air which 
 opens the valve v. When the piston is at its lowest 
 this valve falls by its own weight. On raising the 
 piston-rod the second time, more air is withdrawn 
 from the suction-pipe; water commences rushing 
 up, and enters through valve A. On lowering the 
 piston again, it descends into the water, and from 
 this moment all the air below the piston is expelled. 
 Some water is now above the piston, and the lower 
 valve again falls of its own weight. Henceforth, 
 whenever the piston descends, a large quantity of 
 water passes through the piston valve v; whenever 
 it rises, that quantity of water remains on the top 
 of the piston-valve. Afterward, at every rise of the 
 piston, the water above it flows out through the 
 spout. 
 
 The great principle of the pump is the fact, that 
 the pressure of the air upon a body of water, causes 
 the water to rush up into a vacuum that lias been 
 formed in a tube communicating with that body of 
 water. 
 
 The column of water in the suction-pipe, between 
 the level of F F and valve 7, is supported by the 
 pressure of the air on the water in the cistern. 
 
 Bead " Theory of Pump " p. 267, in " Things not Generally Known." 
 
 7" 
 
FORCING PUMP. FIRE ENGINE. 
 
 77 
 
 LESSON XXI. 
 
 FORCING PUMP. FIRE ENGINE. 
 
 A common pump cannot draw water to a ver- 
 tical height of 34 feet, because its vacuum cannot 
 be made perfect. 
 
 In order to elevate it to a greater height, the Forc- 
 
 ing Pump is used (Fig. 19). It is constructed on 
 
78 FIEST LESSONS IN PHYSICS. 
 
 the same principle as the Common Pump ; it dif- 
 fers from the latter in the following three points : 
 
 1. The piston of the Forcing Pump is not pierced. 
 
 2. In place of the spout there is a tube at the 
 lower part of the barrel, which leads to the place 
 wnere the water is to be carried. 3. That tube 
 contains a valve, a 2 , which opens outwardly. 
 
 When the piston P is raised this valve is closed; 
 thus the air below the piston becomes rarefied, and 
 water is drawn through the lower valve u 1 , the 
 same as in the common pump. When the piston 
 descends, the lower valve is closed on account of 
 its own weight ; the water above the valve v 1 is 
 then forced through valve v z into the tube, from 
 which it can not flow back. (Why not ?) 
 
 The Fire-Engine 
 
 Consists of a Heron 's Fountain (Lesson X) and 
 of two Forcing Pumps to pump water into it. 
 Both pumps stand in a large box filled with 
 water. Two iron levers (called brakes) L and L\ 
 work the iron piston-rods P and P 1 . A wide 
 cylinder, N^ stands between the two pumps. It 
 contains air, and a metallic tube which nearly 
 reaches to the bottom and is open at the top. 
 This cylinder acts like a Heron's Fountain, but in 
 the Fire-Engine, and in other pumps, it is called 
 an Air- Chamber. The tubes of the Forcing Pumps 
 enter the air-chamber ; each has a valve opening 
 outwardly into the air-chamber. 
 
FORCING PUMP. FIRE-ENGINE. 
 
 79 
 
 FIG. 
 
 When A, one of the pistons, rises, the valve of 
 tube B is closed by the pressure, which the air 
 over the water in the air-chamber exerts. Water, 
 at the same time, enters from the box through 
 the lower valve C into the barrel of the pump. 
 Why? When the piston E descends, the lower 
 valve D closes of its own weight, and water is 
 forced into the air-chamber through the valve 
 of the tube F. After continued pumping, the 
 water in the air-chamber has risen so high that 
 it has concentrated and compressed the air into 
 a much smaller space. But from Lesson X we 
 see that the more we compress air, the greater its 
 expansive force. Hence it is evident that the jet 
 
80 FIRST LESSONS IN PHYSICS. 
 
 of water sent forth from the metallic tube is sent 
 forth by the expansive force of the compressed 
 air in the air-chamber. There being two pumps 
 and an air-chamber the jet is continuous. 
 
 Give the difference between the Common Pump 
 and the Forcing Pump. 
 
 Also, between a Hero's Fountain and the Air- 
 Chamber of a Fire-Engine. 
 
 The Common Pump and Barometer Compared. 
 
 Four points in common : 
 
 1. Both have a tube or cylinder. 
 
 2. Both have a vacuum. 
 
 3. In both, the liquids rise in consequence of 
 the pressure of air. 
 
 4. In both, the liquids can not rise higher than 
 the capacity of that pressure permits. 
 
 Seven points of difference : 
 
 1. The Barometer has a glass tube ; pumps 
 usually have iron tubes. 
 
 2. The barometer-tube is closed above the vacu- 
 um ; while in pumps there is a valve above the 
 vacuum. 
 
 3. The vacuum in the pump can never be made 
 as perfect as that in the barometer. 
 
COMMON PUMP AND BAROMETER. 81 
 
 4. In the pump, the vacuum must first be pro- 
 duced ; in the barometer, the vacuum, once estab- 
 lished, remains. 
 
 5. Tho liquid in the barometer is always mer- 
 cury; in the pump it may be water, oil, vinegar, 
 &c., &c. 
 
 6. In the barometer neither spout nor lever is 
 required. 
 
 7. No graduated scale is attached to the pump. 
 
 8. The liquid column in the barometer usually 
 stands no higher than 30 inches. The liquid col- 
 umn in the pump stands higher than that of the 
 barometer (Comp. Lesson XII). Thus when the mer 
 cury in a barometer reads 29'5 inches, this num- 
 ber is an index of the pressure of the air at the 
 time ; and in a common pump with perfect valves, 
 the water could then be drawn up 29*5 x 13*6 = 
 401-2 inch. = 33-4 feet. 
 
 The distance between the level of the water in 
 the cistern and the lower valve must be propor- 
 tionate to the capacity of the pump. Suppose, 
 for instance, it were forty feet in the imagi 
 nary pump previously mentioned, then the water 
 would not come up to the lower valve, because the 
 atmospheric pressure cannot lift a column of 
 water higher than its natural limit, explained in 
 Lesson XII. 
 6 
 
82 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXII. 
 
 REVIEW. 
 
 LESSON xvir. 
 
 1. The vibrations of the same pendulum will take 
 
 place in the same length of time, unless these 
 vibrations pass through large spaces or arcs. 
 
 2. A short pendulum vibrates more quickly 
 
 makes a greater number of vibrations in the 
 same length of time than a longer one. 
 3 In Clocks the motory force is the force of 
 Gravity ; in Watches (and in clocks without 
 weights), the motory force is the force of Elas- 
 ticity. 
 
 4. In Clocks the motion is regulated by the Pen- 
 
 dulum ; in Watches by the Balance- wheel. 
 LESSON xvin. 
 
 5. Vessels connected with each other, so that a 
 
 liquid can pass freely from one into the other, 
 are called Communicating Vessels. 
 
 6. The force of pressure upon a small portion of 
 
 any liquid is transmitted equally and undi- 
 minished to all the parts of the liquid in all 
 directions. 
 
 LESSON xix 
 
 7. Air rushes into a vacuum, or into any spa~e 
 
 containing rarefied air. 
 
REVIEW. 83 
 
 LESSON xx. 
 
 8. If there is a Fluid between a vacuum and 
 
 the air, the pressure of air will force the 
 Fluid into the Vacuum. Thus water or mer- 
 cury rushes into a vacuum formed over a part 
 of its surface, because the pressure of air 
 upon the remaining portion of its surface 
 forces both to do so. (Pumps and Barometer.) 
 
 9. A stone on a support, a weight suspended by 
 
 a cord, are at rest. They may remain at rest 
 during thousands of years. The force of 
 gravity in them is also at rest. But as soon 
 as the support is withdrawn, or the cord 
 lengthened but the hundreth part of an inch, 
 they begin to move. Then the force of gravity 
 in them may be said to do work. This work 
 is called Motion. 
 
 10. An elastic spring may be compressed, and 
 may remain so for thousands of years. Dur- 
 ing this time the force of elasticity in it does 
 no work. But withdraw the pressure, and 
 the spring commences moving. Its motion is 
 t?ie work done by the Force of Elasticity. 
 
 11. Motion is a manifestation of the work done 
 by a Force, and is always accompanied by a 
 cliange of place. 
 
 12. a. A body on an incline (Fig. 10) will not fall ; 
 
 b. A pendulum (Fig. 13) will not vibrate ; 
 
 c. The long arm of a lever will not move ; 
 
84 FIRST LESSONS IN PHYSIOS. 
 
 d. The water in the wide tube of a communi- 
 cating vessel (if by means of a stop -cock 
 shut- off from the narrow tube) will not flow 
 into the narrow tube (Fig. 14); 
 
 e. The air outside the bellows will not enter ; 
 /. The air over the cistern of a pump will not 
 
 force the water up (Figs. 18 and 19) , 
 So long as the force of gravity (in e and / 
 the force of elasticity) does no work. But 
 from the moment that the rope at the top of 
 the incline, to which the body is fastened, is 
 cut; from the moment that the pendulum- 
 weight be drawn to one side ; that the long 
 arm of the lever be provided with additional 
 weight; that the stop- cock in the communi- 
 cating-tube be opened ; that the bellows be ex- 
 tended ; that the piston of the pump be moved ; 
 from that moment Work is done and Mo- 
 tion produced. 
 
 IB. The effect of the Force of Gravity is Pull. It 
 pulls all bodies to the earth. The effect of 
 the Force of Elasticity is Push (pressure). 
 These effects disappear when work is being 
 done by the forces ; the forces are then con- 
 verted into Motion. 
 
 14, The motion of masses is produced by the work 
 which their forces perform. The motions of 
 the human body are work which its forces 
 perform. When its forces cease to labor, 
 death takes place. In Nature all is Motion, 
 Life and Labor. 
 
SOUND. 85 
 
 LESSON XXIII. 
 
 SOUND. 
 
 Familiar Facts. The passage of an electric 
 spark through the air is followed by a crackling 
 noise, as the passage of lightning through air is 
 followed by thunder. The blow of a whip in the 
 air is also accompanied by a crackling noise ; and 
 a pencil, when it falls from the table, produces an 
 audible sound. So does a stone thrown into the 
 water, a book dropped upon the floor, or the hand 
 rapping at the door. Now, if the whip had not 
 moved through the air, nor the pencil upon the 
 table, nor the stone into the water, nor the book on 
 the floor, these sounds would evidently not have 
 been produced. 
 
 All sound is transmitted to the ear through tho 
 air ; no sound is heard in a vacuum. 
 
 45. EXPERIMENT. Insert the hlade of a knife 
 
 between the horizontal joints on the side of a desk 
 or table ; take the free end of the handle, press it 
 downward as far as convenient, and then let go : a 
 noise will be heard, and the knife will be seen to 
 move up and down very fast until it comes to rest. 
 This is a swinging, or a vibratory motion, similar to 
 that of the pendulum of a clock. 
 
 46. EXPERIMENT. Let a few drops c f water 
 fall into a tumbler filled with water. At first the 
 
86 FIRST LESSONS IN PHYSICS. 
 
 water is depressed, but quickly rises again. This 
 vibratory motion is communicated to the remaining 
 water. The water shows it in the circular elevations 
 (rings) round the point of contact. Thus the mo- 
 tion of the knife, as well as that of the water, is a 
 vibratory motion. 
 
 Familiar Facts. A vibratory motion may be 
 heard and felt, when a door is slammed or a gun 
 fired off. A bell is first set to vibrate, then it com- 
 municates its own vibrations to the air around it 
 
 FIG 21. 
 
 and the air in turn transmits its vibrations to the 
 ear. On water, the vibrations are rings ; in air, hol- 
 low spheres of compressed air, alternating with 
 iollow spheres of rarefied air. No sound is heard 
 if the vibrations are too faint, or if the organ of 
 hearing is defective. 
 
 Familiar Facts. That the sounding-board 
 of a piano vibrates while the instrument is being 
 played, may be seen if a pin or other small body 
 be placed on it. Blowing into a pipe sets the air 
 
SOUND. 87 
 
 vibrating. In windy weather the church-bells of 
 a city may be heard farther off than usual, at a 
 place which lies in the direction of the wind; 
 while at a place nearer by, but in an opposite di- 
 rection, they may not be heard at all. 
 
 Sound is caused by the vibratory motion of a 
 sounding body. 
 
 If a cannon is fired off at a distance of about 
 1100 feet, the flash is seen instantaneously, but 
 the report will be heard a second later. At twice 
 that distance, the report will be heard two sec- 
 onds later. From the time which elapses between 
 the flash of a gun on a vessel in distress, and 
 the hearing of the report on the shore, the dis- 
 tance of the vessel may be found. Thus, if ten 
 seconds have elapsed, the vessel is about 11,000 
 feet, a little over two miles, distant. The distance 
 of a thunderstorm may be ascertained in a like 
 manner, by counting the seconds that elapse be- 
 tween the lightning and the thunder following it. 
 
 Sound moves at the rate of about 1100 feet a 
 second. 
 
 Question. 1 What causes the noise when a 
 piece of paper is torn ? 2. What, when a piece 
 of wood is broken? 3. What, when a whip is 
 cracked ? 
 
 Bead "Wonders of Acoustics," in Illustrated Library of Wonders. 
 Bead "The Ear," in "Human Body" Illust. Library of Wonders. 
 "Bead "Sound and Echoes," p. 268, in Things not Generally Known. 
 
88 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXIV. 
 
 EVAPORATION FOG CLOUDS RAIN SNOW 
 HAIL DEW FROST. 
 
 Water is one of the most necessary elements in 
 human life. By the Hindoos and other pagan 
 nations it was revered as a Deity ; and the masses 
 of bleached bones lying around the few wells in 
 the desert show that during great heat the want of 
 water may be fatal to the traveling caravans. 
 
 Familiar Facts. Moisture on a slate or on a 
 piece of paper will disappear very soon. Water in 
 a tumbler, exposed to the air, constantly dimin- 
 ishes, until finally none is left. The water in 
 streets, cisterns, ponds, and brooks gradually dis- 
 appears. When water thus passes off into the air, 
 we say that it evaporates. Evaporation takes place 
 only at the surface of liquids. 
 
 By evaporation, water is changed into water 
 vapor (or aqueous vapor). 
 
 Familiar Facts. In summer our breath is in- 
 visible ; not so in winter, because it condenses 
 immediately after leaving the mouth. In warm 
 weather the vapors rising from rivers, swamps and 
 lakes are invisible. There may be a great quantity 
 of vapor in the atmosphere, and yet the vapor not 
 
FOG CLOUDS BAIN. 89 
 
 be seen. When the air near the earth is cool, the 
 vapor becomes visible, and then we call it Mist or 
 Fog. Aqueous vapor coming in contact with cool 
 air, forms Fog. 
 
 The vapor may not be perceived below, but be- 
 come visible higher up in the atmosphere. This 
 takes place especially when the warm, moist winds 
 (south or southwest winds) come in contact with 
 colder (north or northeast) winds. The vapor then 
 forms clouds. 
 
 Fog is clouds near the earth. Clouds are fog in the 
 upper regions of the air. 
 
 Familiar Facts. A piece of chalk, a piece of 
 earth, a lump of coal, drop quickly ; but dust, soot, 
 and finely powdered chalk, descend very slowly. 
 So the minute particles of which clouds and fog are 
 composed may float in the air for a length of time, 
 because, in this state, their downward passage is 
 resisted by the air. Remember that soap-bubbles 
 may do the same. But when aqueous vapor comes 
 in contact with cold air, its minute particles unite 
 form drops, and descend as rain. On their passage 
 through the air, these drops, small at first, increase 
 in size, because they meet with more aqueous vapor 
 in the air, which condenses upon them. The higher 
 up the clouds, the greater the rain-drops. (Why ?) 
 Rain is condensed aqueous vapor. In winter, the 
 
90 FIKST LESSONS IN PHYSICS. 
 
 aqueous vapor in the atmosphere, when it con- 
 denses, freezes and forms minute crystals. These 
 increase in size on their passage through the air, 
 Because more of the frozen vapor settles upon them, 
 and reach us as snow- flakes. Snow is frozen 
 aqueous vapor. 
 
 On stormy summer-days, stones of ice sometimes 
 fall from dense clouds, having an opaque kernel 
 and a transparent rind. They may be disastrous 
 to green-houses and to the crops. They are called 
 Hail-stones. But it is not known why, in summer, 
 such cold can be produced as to form solid masses 
 of ice in the atmosphere. 
 
 Familiar Facts. Inhabited rooms contain much 
 aqueous vapor. A part of it is exhaled from our 
 lungs. If, in summer, a tumbler is filled with cold 
 water, it becomes cold ; the aqueous vapor in the 
 air arounl it cools off, condenses, ,and forms 
 drops of water all over the glass. If, in winter, a 
 cold tumbler is brought into a warm room, the 
 vapor around the glass condenses, and forms, like- 
 wise, moisture on the glass. Axes, iron safes and 
 soda fountains are vulgarly said to " sweat." Moist- 
 ure is deposited when a person breathes against a 
 cold window-pane. The aqueous vapor of heated 
 apartments condenses on cold window-panes and 
 may run down as water. 
 
 Aqueous vapor is condensed into water when in con- 
 tact with oodies sufficiently cold. 
 
DEW FROST. 91 
 
 Familiar facts. The glistening dew-drops 
 
 which you have so often admired in the early 
 morning-sun, originate in the same manner. In 
 clear weather, the objects on the ground cool off 
 during the night ; and at the same time the aque- 
 ous vapor in the air about them is condensed. 
 Grass and leaves, in general all pointed objects, 
 cool more quickly, hence they have the most dew. 
 If the sky is cloudy, the clouds act like a screen ; 
 they throw the heat back to the earth. Then the 
 objects do not become sufficiently cold and no dew 
 is formed. Sometimes there is no dew, and yet 
 the sky is serene ; this is owing to winds, which 
 bring warmer air to the objects so that they can 
 not cool off sufficiently. As rain is aqueous va- 
 por condensed in the air, so Dew is aqueous vapor 
 condensed on solid bodies. If, during the night, 
 objects cool off to a greater extent, the dew which 
 is formed, freezes. Then we call it Frost. 
 Frost is frozen dew. 
 
 Bead "Lakes, Springs, Rain, Dew, Ice," in "The Earth and its 
 Wonders." 
 
 Bead "Atmosphere, Ocean, Rivers, Waterfalls, in "The Sublime in 
 Nature." Illustrated Library of Wonders. 
 
 Bead "Dew and Water-vapor," in "The Phenomena and Laws of 
 Heat." Illustrated Library of Wonders. 
 
92 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXV. 
 
 HEAT. CONDUCTION OF HEAT. 
 
 47. EXPERIMENT. Strike a piece of flint and 
 
 steel together ; sparks will fly off. 
 
 Familiar Facts. On a stone pavement, at 
 dusk, sparks may be seen when we are walking, 
 or when a horse is galloping. In these cases, 
 iron (the nails) has forcibly struck against stone. 
 The sparks which we see, are minute particles of 
 iron, or steel, which have been heated to redness 
 by friction, or percussion. 
 
 48. EXPERIMENT. Rub a key, or a copper coin, 
 on the floor. It will soon become heated. 
 
 49. EXPERIMENT. Try to ignite a match by 
 rubbing one gently on a piece of smooth glass. 
 It will not burn, because there is insufficient fric- 
 tion; it merely glides over the smooth surface. 
 But if rubbed against a rough surface, such as 
 the floor or a brick, the match presses against the 
 projecting parts of the rough surface and, owing 
 to the friction thus produced, it becomes heated 
 and ignites. 
 
 Familiar Facts. "Wagon wheels have so much 
 friction at their axles, that unless properly 
 greased, they may be set on fire. He that lets him- 
 self down by a rope has his hands blistered. On 
 a cold day, we sometimes rub our hands together. 
 
HEAT. CONDUCTION OF HEAT. 93 
 
 Saws and augurs, after being used, feel hot; a 
 piece of India-rubber, warm. This shows that 
 Friction produces heat. It shows, also, that Mo- 
 tion may be converted into heat ; for friction is 
 motion arrested. 
 
 Familiar Facts. "By holding our hands near 
 to a heated stove they become warm. Heat of 
 the stove passes first to those parts of the hands 
 nearest the stove, then it gradually passes to 
 the parts next ; and so on, until all the parts of 
 the hand are heated. 
 
 50. EXPERIMENT. Hold a short wire in the 
 flame of a burning lamp. It will be felt, that 
 even the part of the wire which is not in the 
 flame, is heated ; and that the heat increases so 
 that we must soon drop the wire. It is plain 
 that the heat of the flame was imparted first to 
 one end of the wire, and that it was communi- 
 cated successively to the remaining parts of the 
 wire. This shows that Heat may be communi- 
 cated by passing successively from any part of a 
 body to the remaining parts. This communica- 
 tion is called Conduction of Heat. 
 
 51. EXPERIMENT. Hold a taper, a straw, or a 
 thread in the flame. It may burn quite near your 
 fingers without hurting them. 
 
 52. EXPERIMENT. Take up the wire again 
 
 (50 Exp.), but wrap a strip of paper, or cloth, 
 around the end in the hand. If held in the 
 
94 FIEST LESSONS IN PHYSICS. 
 
 flame again, there is scarcely any heat felt. Tea- 
 pots and soldering-irons have usually wooden 
 handles. Why ? 
 
 Metals are good conductors of heat. Paper, 
 wood, cotton, wool, fur, feathers, ashes, snow, ice, 
 straw, and air, are bad conductors of Jieat. 
 
 53 EXPERIMENT. Place a wire and a piece of 
 wood upon a heated stove, and let them remain 
 there for a while. Both obtain the same temper- 
 ature ; yet, if touched with the hand, the wire 
 seems to be the warmer. This is owing to the fact 
 that, being a good conductor, it instantly imparts 
 all its heat to the hand. If you touch a cold iron 
 bar, it instantly takes heat from the hand, and, 
 therefore, seems cold. 
 
 Questions. 1. Why may ice be kept as well 
 in a feather bed as in an ice-chest? 
 
 2. Why do mittens keep the hands warmer than 
 gloves with fingers ? 
 
 3. Why does snow melt more readily on a 
 plank than on a rock ? 
 
 4. Why are steam-chests and steam-cylinders 
 often covered with wood ? 
 
 5. Why are the walls of safes often filled with 
 fine ashes ? 
 
 6. Why do wide garments keep us warmer than 
 tight ones ? 
 
 7. Why are frame houses warmer than stone 
 houses ? 
 
CONDUCTION OF HEAT. 95 
 
 Application of Conducting Substances. 
 
 /. Good Conductors. They conduct heat very rapidly, and, therefore,, 
 they are applied in order to diffuse heat quickly. Thus, to boil water 
 and roast meat, iron vessels are used. Iron stoves are heated in very 
 little time. 
 
 //. Bad Conductors. They conduct heat very slowly, but they also 
 part with it slowly ; for this reason we apply them to retain heat. 
 They serve to prevent a warm body from cooling off, and a cold body 
 from becoming heated. 
 
 Familiar Facts. If we wish to warm a tumbler on a heated stove,, 
 a piece of paper should be placed between the gla?s and stove ; other- 
 wise the glass may crack. In winter, pieces of heated wood are laid in 
 sleighs to keep the feet warm. Boards are placed on pavements, and 
 horsemen like to have wooden stirrups, because wood does not withdraw 
 the warmth from the foot. 
 
 Cotton quilts, woolen garments, blankets and furs keep the body 
 warm in winter ; they neither allow the warm air surrounding the body- 
 to pass off, nor do they permit the cold external air to enter. In cold 
 countries animals have very thick fur; some in our latitude have thicker 
 fur in winter than in summer. Northern birds have thick feathers. 
 Feather beds are in favor with persons fond of sleeping very warm. 
 Blast-furnaces are sometimes provided with double walls, and the space 
 between is filled with ashes. A cover of snow retains the heat of the 
 earth; thus it protects the winter grain from the cold. The Esquimaux , 
 build themselves huts of snow and ice. Tender trees, vines and pumps- 
 are covered with straw in winter to protect them against the cold. Ice- 
 houses are thatched with straw, and their walls filled with saw-dust, to- 
 prevent heat from entering. 
 
 Bead "ffeat,"by J. Abbott. Harper & Brother. 
 Bead " Sources of Heat," in The Phenomena and Laws of Heat. 
 Read " Good and Bad Conductors" in The Phen. and Laws of Heat. 
 Read " Woolen Clothing >" 'p. 296, in Things not Generally Known. 
 
96 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXVI. 
 
 DKAUGHT. 
 
 54. EXPERIMENT. Shreds of cotton, or small 
 strips of paper, held over a heated stove or regis- 
 ter, or over a lamp flame, will move upward, and, 
 if let go, they will ascend. The air above the 
 source of heat is heated. From the fact that boil- 
 ing water runs over, and from a great many other 
 facts (Less. XXVII), we know that heat expands 
 "bodies, and that heated air is expanded, and thus 
 takes up more space than before, and, therefore, 
 has less specific gravity (Less. II) than it had 
 when cold. Now, as air rises in bubbles through 
 water, so does heated air ascend in currents 
 through the colder air. 
 
 55. EXPERIMENT. Insert one end of a rod 
 
 upright in a ' cork, and stand the whole on a 
 heated stove or register. Suspend from the top a 
 band of paper, cut in the shape of a spiral , the 
 upward current of hot air will cause it to revolve. 
 
 56. EXPERIMENT. Bring a thermometer near 
 the floor of a room ; then, near the ceiling. It will 
 be seen that near the ceiling the air is warmer 
 than below. Heated air rises. 
 
 Why do balloons, smoke and steam rise ? (See 
 Lesson II.) 
 
DRAUGHT. 97 
 
 57. EXPERIMENT. If a window in a heated 
 room be opened above and below, the flame of a 
 burning candle, held in the opening above, will 
 be blown from the room ; if held in the opening 
 below, into the room. 
 
 Familiar Facts. The same may be observed 
 with cotton shreds in place of the flame. This 
 shows that the colder air from out- doors rushes 
 into the room from below, while the heated air of 
 the room flows out above. The colder air is con- 
 fined to the lower parts of a room, because it 
 has greater specific gravity than heated air. 
 Wherever a fire is burning, a current of air, or 
 draught, is produced. A draught is also noticed 
 when passing from the sun into the shade, for 
 where the sun shines, warmer air ascends, and is 
 replaced by the colder air from below. Chimneys 
 serve to increase the draught, because they en- 
 close a tall column of heated air, which has less 
 specific gravity than the outer, colder air. The 
 latter presses in with increased force proportion- 
 ate to the height of the chimney. If a handker- 
 chief be tied around the small openings under 
 the burner of a lighted lamp, the flame will be 
 extinguished. The same happens, also, if the top 
 of the chimney is covered with a piece of glass ; 
 in this case the draught is stopped because the 
 heated air can not pass out, and consequently no 
 fresh air come in. 
 7 
 
98 FIRST LESSONS IN PHYSICS. 
 
 Heated air rises ; colder air flows in to take its 
 place. 
 
 Familiar Facts. Near heated ground, the 
 
 air ascends and is replaced by colder air. This 
 causes our atmosphere to be in constant motion. 
 The currents thus produced are called Winds. 
 
 Application. Chimneys (in lamps, stores, fac- 
 tories, &c., &c.) Ventilation of rooms and halls. 
 
 Bead "Draught and Ventilation," p. 269, in Things not Generally 
 Known. 
 
 Bead " Winds and Currents" p. 279, in Things not Generally Known. 
 
 Read "Does the Sun Influence a Fire" p. 267, in Things not Gen- 
 erally Known. 
 
 REVIEW. 
 
 LESSON xxiv. 
 
 1. Heat changes liquids into Vapors. Vapor of 
 
 water is called Aqueous Vapor. The process 
 is called Evaporation. 
 
 2. Aqueous vapor coming in contact with cool 
 
 air, forms Fog. Fog is clouds near the earth. 
 Clouds are fog in the higher regions of air. 
 
 3. Aqueous vapor, in contact with cool air, forms 
 
 Fog ; in contact with cold air, Rain; with 
 cold, solid bodies, Dew ; with intensely cold 
 air, Snow. Frost is frozen dew. 
 
EXPANSION BY HEAT. 99 
 
 LESSON XXVII. 
 
 EXPANSION BY HEAT. THERMOMETER. 
 
 58. EXPERIMENT. Heat a fine glass tube, 
 
 closed at one end, and partly filled with water ; the 
 water will be seen to rise as it becomes heated. 
 Warm water takes up a larger space than cold. 
 
 Familiar Facts. A cold tumbler placed on a 
 heated stove will crack at the bottom. As it gets 
 hotter below than above, it suddenly expands below 
 more than it does above, and so the tumbler must 
 break. How may it be prevented from cracking ? 
 (Lesson XXY, p. 95.) A bladder, filled with air 
 and tied up at the end, expands if near a hot stove 
 or register. The air inside becomes heated, and 
 heated air takes up a greater space than cold air. 
 A flask with a ground glass- stopper is sometimes 
 difficult to open ; if it be gently heated around the 
 neck the stopper may be taken out without dif- 
 ficulty. The rails on a railroad track are laid so 
 that their ends shall be at a slight distance from 
 each other ; in summer their ends are very nearly 
 together; in winter they are farther apart. Tires 
 are heated, nearly red-hot before they are placed on 
 carriage-wheels, for they are then wider, and, on 
 cooling, fit tight to the wheels. 
 
 Heat expands all bodies. 
 
t 
 100 FIRST LESSONS IN PHYSICS. 
 
 Temperature. A substance is said to cool when 
 it parts with sensible heat, that is, with such heat 
 as may be felt. In the previous instance the tires 
 lost most of their sensible heat. The amount of 
 sensible heat which a body has, is its temperature. 
 
 Let us now consider the Thermometer. The 
 silvery substance in it is one of the few elements 
 having the liquid state at ordinary temperature ; at 
 an intense degree of cold such as Arctic explorers 
 experience it freezes into a solid mass. Its name 
 is Mercury, or Quicksilver. If the mercury is heated, 
 it expands ; it rises in the tube, simply because it 
 has no other place to which to go. On cooling, 
 it contracts, and falls. It may be heated by the at- 
 mosphere, that is, by the sun ; or by hot water ; by 
 steam; by heated oil, or by the natural warmth 
 of the hand when placed upon it. On examining 
 the thermometer, you notice that it consists of a 
 glass tube with a bulb below. Both tube and bulb 
 are closed. The bulb and a portion of the tube are 
 filled with mercury. Above the mercury is a vac- 
 uum. The vacuum is obtained by heating the 
 mercury to a very high degree ; its vapors then fill 
 the tube, the open end of which is now fused ; this 
 closes the tube. The whole is now exposed to cold ; 
 this condenses the mercury-vapors into liquid mer- 
 
THERMOMETER. ,, ,101 
 
 cury, leaving a vacuum behind. The frame is 
 not an essential part of the thermometer. A 
 little above the bulb is a point, marked Freez- 
 ing Point Everywhere on the earth, ice melts at 
 the same degree of temperature. So, after the 
 tube is sealed and cooled off, it is placed in melt- 
 ing ice. Immediately the mercury sinks, because 
 the cold contracts it. It occupies now a mucb 
 smaller space, and when it has settled, its low- 
 est point is carefully marked, either on the 
 frame or by etching it on the glass tube. This 
 point is called the " Freezing Point." It has also 
 been found that all over the earth, water, in low 
 countries, boils at the same temperature. So the 
 thermometer is now held upright in the hottest 
 steam issuing from boiling water. Heat expands 
 all bodies ; hence the mercury expands and is 
 seen to rise in the tube. The point to which it 
 ascends is carefully marked ; it is the "Boiling 
 Point." The space between the two points has 
 been divided into degrees. By means of these 
 degrees, we are enabled to indicate the tempera- 
 ture which a body has acquired. 
 
 Read " Expansion Thermometer," in "The Phenomena and Laws 
 of Heat. - ' 
 
FIRST LESSONS IN PHYSICS. 
 
 LESSON XXVIII. 
 
 THERMOMETER COMPARED WITH BAROMETER. 
 
 59. EXPERIMENT. If the palm of the hand, 
 
 after being rubbed a little so as to be perfectly 
 dry, is held to the thermometer-bulb, the mercury 
 will rise to a point which marks the Blood-heat 
 of the human body. It happens to be indicated 
 on our thermometers by the number 99. This is 
 owing to the fact, that in our country, and also in 
 England, the space between the freezing and the 
 boiling points is measured by very small degrees, 
 of which there are 180 between those two points. 
 Fahrenheit, a philosophical instrument maker, 
 divided that space into 180 degrees. He com- 
 menced counting, however, not at the Freezing- 
 point, but at a point below, which is the zero 
 point of his scale ; this brings the freezing point to 
 32; the boiling-point is marked 212. In some 
 European countries the Freezing-point is marked 
 ; the Boiling-point 80. That is, the space be- 
 tween the two points is divided into only 80 
 degrees. Each degree of this kind is much 
 larger (how many times as large ?) than one of 
 the former kind, the Fahrenheit. From the 
 name of the French philosopher who arranged 
 
THERMOMETER BAROMETER. 103 
 
 this scale, its degrees are called degrees Reau- 
 mur. Thus 80 R. is equivalent to 180 F. Far 
 more convenient than either of the two preceding 
 scales is the one of Celsius. He divided the space 
 between the freezing and the boiling points into 
 100 degrees. The use of this division is gradually 
 spreading. According to it, 100 a=l8QF= 80^. 
 
 The minus sign distinguishes numbers below 0. 
 
 Centigrade. Fahrenheit. 
 
 Boiling-point 100 - 2 12 
 
 50 122 
 
 25 77 
 
 Freezing-point O p 32 
 
 17J Q 
 
 50 58 
 
 The healthiest temperature for any room is 
 about 65^. Our rooms should not be heated 
 beyond that in winter. Thermometers should be 
 placed at equal distance from stove, or lireplace, 
 and the windows, so as to show the mean tem- 
 perature of the air. 
 
 Questions. If in New York the mercury stands 
 at 80 above zero, how would the same tem- 
 perature be indicated in Paris (according to O. 
 degrees)? How in Berlin (according to O. de- 
 grees) ? By what numbers would the blood-heat 
 point be indicated according to those scales ? By 
 what number is the point of healthiest tempera- 
 ture indicated in O. degrees ? (See p. 174.) 
 
104 FIRST LESSONS IN PHYSICS. 
 
 Thermometer and Barometer Compared. 
 
 Four points in common : 
 
 1. Both instruments consist of a glass tube. 
 
 2. Both have mercury in their tube. 
 
 3. Both have a vacuum. 
 
 4. Both have a graduated scale. 
 Four points of difference : 
 
 1. The thermometer -tube is closed above and 
 
 below ; 
 
 the barometer tube is closed above but open 
 below, so that the pressure of air may reach 
 the mercury within it. 
 
 2. In the thermometer-tube, the mercury rises 
 
 and falls on account of the effects of heat 
 and cold ; 
 
 in the barometer- tube, the mercury rises and 
 falls on account of the increase or decrease 
 of air-pressure. 
 
 3. The thermometer has a scale of degrees 
 
 whose size is arbitrary and may be differ- 
 ent in different thermometers ; 
 the barometer has a scale of inches, and frac- 
 tions of inches ; its scale is of less extent, 
 and only at the upper part of the tube. 
 
 4. Mercurial Thermometers may have any 
 
 length; 
 mercurial Barometers have uniform length. 
 
ATMOSPHERIC ENGINE. 105 
 
 LESSON XXIX. 
 
 THE ATMOSPHEEIO ENGINE. 
 
 1. If we look at a sewing-machine while it is 
 in motion, our attention is immediately called to 
 a long, upright rod, made to move up and down 
 by the stroke of the foot. The rod being fastened 
 to a wheel, it is evidently its up and down motion 
 that causes the motion of the wheel and with it, 
 that of the machine. You need but fasten a rod 
 to the edge of a toy-wheel, and you may demon- 
 strate the same. Motion in a straight line rec- 
 tilinear motion is thus converted into circular 
 motion. 
 
 2. This was known thousands of years ago; 
 but, strange to say, the principle upon which the 
 steam-engine is founded, was not thought of until 
 about 1690, A. D. At that time, Professor Papin, 
 an exiled Frenchman living in Germany, pub- 
 lished a little work, in which he says : " There is 
 a property peculiar to water, owing to which a 
 small quantity of that liquid, if heated and con- 
 verted into steam, acquires a force of elasticity 
 which much resembles that of air. When cooled 
 down, it returns to the liquid state, and loses its 
 elasticity. I am, therefore, inclined to believe 
 
106 FIRST LESSONS IN PHYSICS. 
 
 that machines may be constructed which are 
 moved by the application of heat to water." 
 
 3. These words laid the foundation for the 
 greatest change which human society ever ex- 
 perienced. The machine that effected this change 
 has benefited humanity more than all the gold 
 mines in the world. The steam-engine not only 
 reveals to us the hidden treasures of the earth ; 
 " it can engrave a seal ; crush masses of obdurate 
 metal like wax before it ; draw out, without break- 
 ing, a thread as fine as a gossamer, and lift a ship 
 of war like a bauble in the air. It can embroider 
 muslin and forge anchors ; cut steel into ribands 
 and impel loaded vessels against the fury of the 
 winds and waves." And when it flies with the 
 rapidity of a bird, over land and water, hurling 
 dense 'masses of steam and smoke into the air, 
 does it not look like some gigantic monster that 
 contains the strength and the power of thousands 
 of men ? Well may we admire the genius of man 
 that can turn one of Nature's simplest forces to 
 such wonderful account. 
 
 4. The simplicity of Papin's statement is demon- 
 strated by his own application. Knowing that 
 steam was elastic. like air (Lesson X), he immedi- 
 ately proceeded to the construction of an appar- 
 atus which, although its practical usefulness was 
 impeded by its slowness, was the first steam' 
 engine ever built. 
 
THE ATMOSPHERIC ENGINE. 107 
 
 60, EXPERIMENT. Papin's apparatus may be 
 
 illustrated by a test-tube (one of tin is preferable 
 inasmuch as glass breaks easily,) as shown 
 in Fig. 22. A small disk of wood, with a 
 packing of thread around it to make it fit 
 tight, is made into a piston, P, moving in a 
 tube nearly air-tight, and attached to a rod. 
 The tube is then filled with water about 
 an inch high, which is made to boil over a 
 flame after the piston is carefully placed 
 in the tube. The generation of steam causes 
 the piston to rise. E is an outlet for 
 surplus steam, and made in the upper part 
 of the tube. Between the water, when boiling, 
 and the piston there is no air ; the space is filled 
 with steam. On immersing the tube in cold water, 
 the rod descends again, because the steam below 
 .the piston is condensed by the cold, and because 
 a vacuum is thus formed between the piston and 
 the surface of the heated water. What is it that 
 forced the piston down ? The answer is : " At- 
 mospheric Pressure." (Lesson XI.) 
 
 5. In place of the small tube of this experiment, 
 Papin used a large iron cylinder, with proper 
 piston and piston-rod. We can readily imagine 
 how, by throwing, at regular intervals, a stream 
 of cold water on the cylinder, he produced an 
 up-and-down motion of the rod ; and how the ma- 
 chine must needs have been slow too slow to be 
 
108 FIRST LESSONS IN PHYSICS. 
 
 practically applied. A steam-engine built upon 
 the principle of Papin's that is, one not worked 
 by the expansive force of steam, but merely by 
 atmospheric pressure is not a steam-engine. It 
 is an "Atmospheric Engine." 
 
 6. Captain Savery, an Englishman, constructed 
 at about the same time, an apparatus in which 
 the steam served the purpose of raising water. 
 The steam was generated in a separate boiler, 
 and thence led into a chamber where it was con- 
 densed by cold water flowing over the chamber. 
 The apparatus, however, was very imperfect, and 
 used only for pumping water. Still, his was 
 the merit of having constructed the first Atmos- 
 pheric Engine that ever received practical ap< 
 'plication. 
 
 7. Thomas Newcomen, a hardware man, and 
 John Cowley, a glazier, both Englishmen, by their 
 brilliant invention, completely eclipsed Savery's 
 engine. They improved upon Papin's plan in 
 this, that they generated the steam in a boiler 
 not in the cylinder and that they condensed 
 it, not by cooling the boiler from without, but 
 by forcing a jet of cold water into the steam. 
 The machine was put to immediate use in the 
 coal-mines of England; and it is sometimes used 
 even at present, in places where a great mass of 
 water is to be pumped out. Its construction is 
 very simple. 
 
THE ATMOSPHERIC ENGINE. 
 
 109 
 
 8. It consists of the boiler, A (Fig. 23), where the 
 steam is generated, and the cylinder, Z?, which is 
 
 PIG, 23. 
 
 connected with the boiler by means of the pipe, 
 F. When steam has entered the cylinder, and 
 the piston O is raised, the stop-cock, a, is closed. 
 This shuts off the connection between the boiler 
 and the cylinder. The stop-cock, &, is then 
 opened, and a jet of cold water from the small 
 reservoir, O, is thrown into the cylinder. This 
 
110 FIEST LESSONS IN PHYSIOS. 
 
 condenses the steam in the cylinder ; a vacuum 
 is formed below the piston, and atmospheric 
 pressure forces the piston down. The water 
 from the condensed steam flows off through the 
 pipe, d, into a reservoir with water. (At the 
 end of d is a valve opening outward.) By means 
 of an iron chain, the piston-rod, H, is attached to 
 a working-beam, which swings on the pivot, Z>, 
 and which is connected at the other end with the 
 rod E. This rod is raised when the piston de- 
 scends. When the stop-cock, a, is opened again, 
 the steam rushes again into the cylinder ; but as 
 the force of pressure of the steam scarcely ex- 
 ceeds that of the air over the piston, the piston 
 would not rise, were it not for the heavy weight 
 attached to the rod, E. This weight falls when- 
 ever steam is let in under the piston, O; and in 
 falling, forces one arm of the working-beam 
 down, causing, at the same time, the piston at 
 the other arm to rise. The rod P is the piston- 
 rod of a pump, and is fastened to the weight. 
 
 In the Atmospheric ^Engine the piston is raised 
 by Gravity, and lowered by Atmospheric Pressure. 
 
 State the principal points of Papin's engine; 
 of Savery's ; and Newcomen's. 
 
 Read "H. Potter," in "Inventions and Discoveries," by Tetuple. 
 London: Groombridge. 
 
STEAM-ENGINE. Ill 
 
 LESSON XXX. 
 
 THE STEAM-ENGINE. 
 
 1. Half a century had passed away. New- 
 comen's engine had been introduced into most of 
 the coal-mines of England, when, in the winter of 
 1763, a young mechanic, James Watt, in Glasgow, 
 was employed by the University of that city to 
 repair one of Newcomen's engines. The task 
 which this man of uncommon mind was about to 
 undertake, marks a new era in the history of 
 steam-power, an era that finally resulted in the 
 perfection of a machine which is an element of 
 modern civilization. On trying the engine after 
 he had repaired it, young Watt perceived that it 
 was very imperfect. The principal defect con- 
 sisted in this, that the machine used a great deal 
 more steam than was needed for the motion of 
 the piston. For when the stream of cold water 
 was thrown into the cylinder, the steam was con- 
 densed ; but at the same time, the cylinder was 
 cooled down to such an extent, that when fresh 
 steam was admitted again, a great quantity of it 
 was wasted in reheating the cylinder ; and thus 
 there was a loss of money in direct proportion to 
 the amount of fuel necessary for producing the 
 quantity of steam equivalent to the quantity 
 
112 FIRST LESSONS IN PHYSICS. 
 
 wasted. On calculating the loss, it was found 
 that I of all the fuel used was wasted ; that is, 
 employed in reheating the cylinder. The question 
 with Watt now was, How can the cylinder, in- 
 stead of being cooled, be kept permanently hot ? 
 In other words, How can the steam be condensed 
 without at -the same time cooling the cylinder ? 
 
 2. Watt's genius solved the problem by an in- 
 vention of surprising simplicity. He condensed 
 the steam in a separate chamber, the condenser. 
 It stood in a chest filled with water, and was con- 
 nected with the cylinder by means of a pipe. 
 Thus the steam could be condensed without cool- 
 ing the cylinder, by simply leading it off. The 
 immediate result was the saving of I of thejfuel. 
 
 3. But Watt did not stop here. He noticed that 
 the air entering the heated cylinder as the piston 
 went down, also cooled the cylinder. This caused 
 a waste of steam, as the cylinder, in order not to 
 condense the fresh steam entering, had first to be 
 reheated to 212. To remedy this, he dispensed 
 with the air entirely, in providing the cylinder 
 with a cover pierced in the center so as to admit 
 the piston-rod air-tight. The air (atmospheric 
 pressure) could now no longer act upon the piston ; 
 how then was the piston to descend? It was 
 made to descend by allowing steam from the 
 boiler to enter above the piston, through a pipe 
 connecting the boiler with the upper part of the 
 
STEAM-ENGINE. 113 
 
 cylinder ; and to pass out again through a pipe 
 connecting the cylinder with the condenser. 
 Thus while there was a vacuum established in 
 the lower part of the piston, steam was admitted 
 into the upper part ; the upper part then being 
 made a vacuum by leading the steam off into the 
 condenser, fresh steam was admitted into the 
 lower part and forced the piston up. By this im- 
 provement, the steam not only served as a ready 
 means for obtaining a vacuum, as in Newcomen's 
 engine, but its expansive force was also made use 
 of, and from that time Watt's engine was no 
 longer an atmospheric, but a steam engine. 
 
 4. The atmospheric engine was "Single Act- 
 ing ; " it did work only while the piston descend- 
 ed ; the rise of the piston, as we remember from 
 the preceding lesson, was effected by gravity. 
 The power obtained by this machine was so small 
 that it could not overcome the resistance of a 
 wheel, and, therefore, it was used mainly for 
 pumping water out of coal-mines. 
 
 5. It will now be readily understood, that by 
 admitting the steam alternately above and below 
 the piston, Watt made the steam-engine "Double 
 Acting," and this was, perhaps, the most impor- 
 tant of all his improvements. For now, rotary 
 motion could be produced, without which no lo- 
 comotive or steamboat could ever have been 
 thought of. 
 
114 FIEST LESSONS IN PHYSICS. 
 
 Watt died in 1819, honored and admired by all 
 who knew him. Within a short time after his 
 death, five large statues were erected to his 
 memory. 
 
 6. In all the engines constructed by Watt, the 
 power of the steam was low ; it amounted scarcely 
 to more than li atmospheres (1J as much as the 
 pressure of our atmosphere; that is 1J times 15 
 pounds to the square inch of surface). The alter- 
 nate condensation of steam on either side of the 
 piston was, therefore, the only means of obtaining 
 the up-and-down motion of the piston ; for the 
 feeble expansive force of the steam was totally 
 insufficient to overcome the counter-pressure of 
 the atmosphere. But by employing steam of 
 greater expansive force that is, steam capable 
 of exerting a greater pressure, it was found that 
 the condenser could be dispensed with, as the 
 pressure of this steam was sufficiently great to 
 move the piston against the resistance of the 
 atmosphere. Engines usually having a steam-pres- 
 sure of from 3 to 15 atmospheres (45 to 225 pounds 
 of pressure to the square inch), and which, as a 
 rule, have no condenser, but send their exhaust 
 steam directly into the atmosphere, are called 
 Higli Pressure Engines; while those generally 
 working with a lower pressure, and always with a 
 condenser, are called Low Pressure Engines. 
 
STEAM-ENGINE. 
 
 115 
 
 7. The admission of steam into the cylinder 
 
 is now accomplished by means of a sliding-valve. 
 
 Steam- Chest. 
 
 CylindSr. 
 
 It is enclosed in a square box, called the steam- 
 chest (See Fig. 24), which is attached to one side 
 of the cylinder. When the steam from the boiler 
 reaches the steam-chest through the opening, 0, 
 it fills the chest at once, and, as the sliding-valve 
 keeps the opening, 6, closed, it presses through 
 the opening a into the cylinder. There it fills the 
 upper part and forces the piston down. This it 
 does because a vacuum has been formed on the 
 other side of the piston, or, as is the case in High 
 Pressure Engines (see Fig. above), by the immense 
 expansive force of the steam. At the same time, 
 however, the sliding-valve (which rises when the 
 piston-rod descends, and descends when the pis- 
 
116 
 
 FIRST LESSON'S IN PHYSICS. 
 
 ton-rod rises,) has moved upward, and shuts off 
 the steam from a (see Fig. 25) ; the steam must 
 
 Steam-Chest. 
 
 Cjhnder 
 
 now enter through the opening b and force the 
 piston up. Meanwhile the old steam above the 
 piston passes through a and e into a tube lead- 
 ing to the condenser. In a steam-eiigine which 
 has no condenser, as, for example, the loco- 
 motive, the old steam passes through e into the 
 air. After the piston has arrived above, the 
 process is renewed, owing to the sliding-valve 
 having a motion opposite to that of the piston ; 
 thus steam is admitted alternately above and be- 
 low the piston which, as mechanics say, moves in 
 a vacuum, or rather, in a space filled with steam. 
 
 In figs. 24 and 25, the exhaust e is too narrow. It should be consider- 
 ably wider than either a or b. 
 
STEAM-ENGINE. 117 
 
 8. When we look at a locomotive rushing past 
 us at full speed, we notice a horizontal iron rod 
 moving back and forth. The rod connects two 
 large wheels, and runs at one end in a wide brass 
 cylinder. Next to this cylinder is the steam-chest, 
 a small square box. It is in the cylinder that 
 the motory power is imparted to the engine. In 
 addition to these things, we see a great many 
 wheels, pipes and rods ; but they mostly serve 
 minor purposes. The main parts of the locomo- 
 tive are the steam -chest, cylinder, piston, piston- 
 rod, the large wheels, and the boiler. 
 
 9. The steam, by means of the sliding- valve, 
 causes the back-and-forth motion of the piston in 
 the cylinder ; by this, it causes the back-and-forth 
 motion of the piston-rod ; and by this, the. revolu- 
 tion of the large wheels. The wheels roll on 
 the track ; they cause the locomotive to move on- 
 ward, and the locomotive pulls the cars attached 
 to it. 
 
 Read "James Watt," in "Pursuit of Knowledge," Vol. II. New 
 York: Harper & Bros. 
 
 Read "The Locomotive Engine," by C. Colburn. H. C. Baird, Phila. 
 
 Read "The Steam- Engine, " \>y David Read. Hurd & Houghton, 
 New York. 
 
 Read "The Railway and its Cradle" "The Youth of James 
 Watt" in "Inventions and Discoveries." Groombridge & Sons, 
 London. 
 
118 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXXI. 
 
 REVIEW. 
 
 LESSON xxni. 
 
 1. The motion of a body produces vibrations in 
 
 the air which, ir they impress the ear, give us 
 the sensation of sound. Sound, therefore, is 
 merely the effect of a vibrating motion upon 
 the ear. 
 LESSON xxv. 
 
 2. Heat may be communicated by passing suc- 
 
 cessively from one part of a body to the other 
 parts. This mode of communication is called 
 Conduction of Heat. 
 
 LESSON xxvi. 
 
 3. Heated air rises, because it has less specific 
 
 gravity than cold air. This fact causes 
 Draught and Winds. 
 LESSON xxvn. 
 
 4. All bodies are expanded by heat; and con- 
 
 tracted by cold. 
 
 5. What we call " Heat," is merely a vibrating 
 
 motion among the minute invisible parts 
 (molecules) of a heated body. We can not 
 see that vibrating motion, but we can feel it. 
 
 6. What we call "Sound," is merely a vibrating 
 
 motion of masses. We can neither see nor 
 feel that vibrating motion, but we can hear it. 
 
KEVIEW. 119 
 
 7. As sound is the effect of vibratory motion upon 
 
 the ear, so heat is the effect of vibratory mo- 
 tion upon the nerves. 
 LESSON xxix. 
 
 8. In the old Atmospheric Engine the piston is 
 . raised by Gravity ; and forced down by At- 
 mospheric Pressure. 
 
 LESSON xxx. 
 
 9. Low Pressure-engines generally work with 
 
 steam of about 1 atmospheres. The steam- 
 pressure in High Pressure-engines is often as 
 high as 15 atmospheres. 
 
 10. In the locomotive, steam causes (by means of 
 
 a sliding-valve) the back-and-forth motion of 
 the piston in the cylinder ; and by this mo- 
 tion, the back-and-forth motion of the piston- 
 rod ; and by this, the revolution of the large 
 wheels. The wheels roll on the track ; this causes 
 the locomotive to move onward and draw the cars 
 attached to it. 
 
 11. On dropping a stone to the floor, the floor and 
 
 the air over the floor, commence vibrating. 
 This shows that Force (Force of Gravity in 
 this case) may be converted into Motion. 
 
 12. The motion of a train of cars heats the axles 
 
 and wheels of the cars. This shows that 
 Motion is convertible into Heat. 
 
120 FIRST LESSONS IN PHYSIOS. 
 
 13. Heat expands all bodies (Less. XXVII); and 
 
 as expansion (the work done by heat) is mo- 
 tion, we may say that Heat is also converti- 
 ble into Motion. (Thermometer.) 
 
 14. Heat expands water into steam. Steam ex- 
 
 pands still farther. The particles of steam, 
 therefore, are in continual motion. The ef- 
 fect of this motion is the Expansive Force of 
 Steam. This shows that Motion is convertible 
 into Force. (Compare Less XXII, Review.) 
 
 15. The Expansive Force disappears as soon as 
 
 the steam has moved the piston of the engine. 
 The motion of the piston is the work done by 
 the steam. Thus, in this case, Force is con- 
 verted into Motion. (Compare No. 14, above, 
 and Less. XXII, No. 13.) 
 
 16. Force of Pressure is convertible into Motion 
 
 of Masses. (Wind Barometer Pumps.) 
 17 From all the preceding, we see that 
 
 a. Force is convertible into Motion. (The 
 
 pump.) 
 
 b. Motion is convertible into Heat. (Friction.) 
 
 c. Heat is convertible into Motion. (Thermom- 
 
 eter.) 
 
 d. Motion is convertible into Force. (Expan- 
 
 sive Force of Steam.) 
 
LIGHT ITS SOURCES DIRECTION. 121 
 
 LESSON XXXII. 
 
 LIGHT ITS SOURCES DIRECTION. 
 
 Familiar Facts. In the daytime, whether 
 
 the sun is visible or not, we can see objects 
 around us. But we can not see objects at 
 night, for then it is dark ; the sun is on' the other 
 side of the earth. The light of the stars, or flashes 
 of lightning, may somewhat relieve the darkness 
 of night; glow-worms may feebly illuminate our 
 immediate vicinity. If we rub a match in the 
 dark against the hand, the phosphorus will shine 
 on the hand. This property is called Phosphor- 
 escence. Glimmers of light are also noticeable in 
 decaying animal and vegetable substances. Two 
 pieces of sugar, after being rubbed together, also 
 emit light. Candles, oil and gas, at times also 
 torch-lights, are our usual means of illumination. 
 But our greatest luminary is the sun. 
 
 1. The Sun and the Fixed Stars, Electricity^ 
 Phosphorescence and Burning Substances are 
 Sources of Light. The sun, stars, lightning, 
 phosphorus, glow-worm and flame are Self-lu- 
 minous Bodies. 
 
 Familiar Facts. The moon sends light to us ;, 
 so do other planets. But this light is not her own j 
 
122 FIRST LESSONS IN PHYSICS. 
 
 she receives it from the sun, the same as the other 
 planets do. She is invisible when her non-illu- 
 mined portion faces us. When a room is dark a 
 Ibook upon the table can not be seen ; neither can 
 the table, nor the desks, nor the streets, nor any 
 thing else. None of these objects is self -lumi- 
 nous ; that is, in order to be seen, these objects 
 need light from some luminous body. 
 
 2. Neither the planets, nor most of the objects 
 surrounding us, are self-luminous bodies. 
 
 Familiar Facts. If we close our eyes we can 
 
 not see. Nor can persons who were born blind, 
 or have become blind from accident or disease. 
 In order to see objects behind us, we must turn 
 around ; to see things above us, we must turn our 
 eyes upward. 
 
 3. Bodies not self-luminous are visible only 
 when they receive light from some luminous 
 body; and then only, if a part of that light 
 forms an impression on our eye. 
 
 Pencils, crayons, glass, water, ice, trees, houses, and all other ob- 
 jects are seen by us, because when light falls upon objects, a portion of 
 the light is diffused from their surface in all directions, and because a 
 small portion of that diffused light enters our eye and forms an im- 
 pression on the retina. From our room we see objects out-doors very 
 clearly; but when looking from without, objects in the room are not 
 seen so well. The amount of light diffused in a room is much smaller 
 than that diffused out-doors. It is light in daytime, although it may be 
 very cloudy. The clouds receive all the light from the sun, and diffuse 
 a portion of it. 
 
LIGHT ITS SOUEOES DIRECTION. 123 
 
 61. EXPERIMENT. Place a large paste-board 
 
 (with a small hole in it) a few inches from the 
 blackboard. Light a candle and place it in front 
 of the hole in the pasteboard. A bright spot will 
 be seen on the blackboard. It is a spot illumined 
 by the rays of the light that pass from the" flame 
 through the opening. The direction from the flame 
 through the hole to the illumined spot is that of a 
 straight line. Let the flame be moved about, the 
 spot will move also. 
 
 Familiar Facts. Through the cracks in the 
 
 shutter of a darkened room, rays of light are ob- 
 served to enter in straight lines. The hunter levels 
 his gun at a squirrel in the direction in which the 
 rays of light diffused from the squirrel enter his 
 eye. Opera-glasses and telescopes have straight 
 tubes. 
 
 4. Light emanates from self-luminous bodies 
 in all directions, and travels in straight lines. 
 
 Bead " Sun, Moon and Stars " in "The Wonders of the Heavens" 
 
 Illustrated Library of Wonders. 
 
 Bead "Light and Color," in "The Earth and its Wonders." 
 Bead "The Eye," in "The Human Body "Illustrated Library of 
 
 Wonders. 
 
124 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXXIII. 
 
 RADIANT AND SPECULAR REFLECTION. 
 
 During the daytime, sunlight is diffused in the 
 atmosphere as well as in the air of our rooms, 
 whether the sun is visible or not. Some of this 
 light in the air falls upon the walls and upon the 
 objects in the room ; and the walls, as well as the 
 objects, reflect (throw back) that light in all di- 
 rections. They reflect it thus: Every point of 
 their surface radiates the light in all directions ; 
 hence any point of this surface may be seen by a 
 person in the room, whatever part of the room he 
 may be in, provided that a portion of that reflected 
 light strikes his eye. 
 
 Familiar Facts. Here is a pencil. What ena- 
 bles us to see it? It is not a self-luminous body ; 
 but there is diffused light in the room, and as the 
 pencil has a more or less rough surface, every 
 point on that surface receives some of this dif- 
 fused light, and in turn reflects some of it. It 
 does so by radiating the liglit in all directions. 
 Of this radiated light, a portion enters our eye, 
 and we say " we see the pencil," and may then 
 describe it. 
 
 We see a looking-glass, owing to the light which 
 is reflected from it by radiation. True, its surface 
 

 LIGHT, CONTINUED. 125 
 
 is smoother than that of the pencil, or of most 
 objects; yet even in a looking-glass there are very 
 many uneven places, from every point of which 
 light is reflected by radiation. Were it not for 
 that, we would not see the glass at all. The sur- 
 face of a perfect mirror would be invisible. 
 
 All bodies reflect light by radiation. We call 
 this Radiant Reflection of Light. 
 
 62. EXPERIMENT. If an India-rubber ball be 
 
 thrown upon the floor in the direction in which a 
 ray of light would pass through a crack on the 
 floor of a darkened room, the ball will rebound, 
 and may be made to strike the wall opposite the 
 crack. Let the place where it strikes the wall 
 be marked. Now lay a looking-glass upon the 
 bright spot on the floor of the darkened room 
 (the spot is caused by the rays of light entering 
 through the crack), and it will be seen that the 
 rays, like the India-rubber ball, rebound to where 
 the ball struck the wall, nearly. The rays are 
 reflected by the looking-glass. Any other highly 
 polished surface would have caused the same re- 
 flection. 
 
 All this shows that there are objects which not 
 only reflect light by Radiant Reflection in all 
 directions, but which, in addition, reflect an 
 extra amount of light in certain definite direc- 
 tions. We call this Specular (mirror-like) Re- 
 flection of Light. 
 
126 FIEST LESSONS IN PHYSIOS. 
 
 Familiar Facts. Burnished metal plates, pol- 
 ished wood, the surface of water or mercury, the 
 coating of mercury in looking-glasses, and even 
 common glass plates when viewed in a very ob- 
 lique position, exert both, "Radiant" and "Specu- 
 lar" reflection of light. Objects with polished 
 surface reflect light radiantly and specularly. 
 
 Light reflected Radiantly compared with Light 
 reflected Specularly. 
 
 Four points in common : 
 
 1. Both have emanated first from a self-lumin- 
 
 ous body. 
 
 2. Both have been thrown back from the sur- 
 
 face of bodies not self-luminous. 
 
 3. Both have their rays travel in straight lines. 
 
 4. Both may enter the eye. 
 Three points of difference : 
 
 1. Radiantly reflected light proceeds from the 
 
 surface of all bodies ; 
 
 Specularly reflected light only from the sur- 
 face of highly polished objects. 
 
 2. Radiantly reflected light is thrown back from 
 
 a surface in all directions ; 
 Specularly reflected light is thrown back 
 from a surface only in certain directions. 
 
 3. Radiantly reflected light enables us to see 
 
 objects ; 
 
 Specularly reflected light enables us to see 
 images of objects. 
 
LIGHT, CONTINUED. 127 
 
 LESSON XXXIV. 
 
 VISIBLE DIRECTION. REFRACTION. 
 
 Familiar Facts. -1. When a person is hit 
 with a stone he does not seek the person who 
 threw the stone, in the direction in which the stone 
 flies, but in the direction from which it comes. 
 Thus, he whose forehead has been struck by a 
 stone in a downward direction, looks upward for 
 the perpetrator ; he supposes him to be in the di- 
 rection from which the stone came. If struck by 
 a stone in an upward direction, he will look down- 
 ward to find the evil-doer. 
 
 It is the same with a ray of light. 
 
 2. A boy looking at a steeple, receives its top- 
 most ray of light in downward direction. Imagine 
 
 Image Inverted. 
 
 this ray to be extended after passing through the 
 opening in his eye (Fig. 26). 
 
128 FIRST LESSONS IN PHYSIOS. 
 
 Since light travels in straight lines (Lesson 
 XXXII), the ray strikes the lower part of his eye 
 in a downward direction. The lowest ray coming 
 from the foot of the steeple would, if extended, 
 traverse the upper part of his eye in upward di- 
 rection. But if, turning round and then bending 
 his head down to his knees, he looks at the 
 steeple from between his knees (Fig 27), the 
 
 Eye Inverted. 
 
 FIG. 27. Image Upright. 
 
 topmost ray will, if extended, pass downward 
 through the upper part of his eye ; and the lowest 
 ray, upward through the lower part of his eye. In 
 this case he would see the steeple inverted, were it 
 not for the fact that the eye sees an object, or any 
 part of an object, in the direction from which the 
 rays of the object come. All bodies appear to be 
 situated in the direction from which their rays 
 enter the eye. 
 
 63. EXPERIMENT. Immerse a pencil in water 
 perpendicularly; it looks as straight as before. 
 But when immersed obliquely, it appears bent, or 
 broken. Oars, when partly immersed, present 
 
LIGHT, CONTINUED. 129 
 
 the same appearance. When the pencil is out of 
 the water, we see it by means of the light diffused 
 from it (Lesson XXXIII). Consequently, when 
 the pencil is partly immersed, we see the portion 
 above the liquid for the same reason. The light 
 diffused from the immersed portion, however, 
 must first travel through the water, and then 
 through the air. Now, since the immersed por- 
 tion seems to be bent, it follows that the rays dif- 
 fused from it are bent; that is, they travel in 
 straight lines through the liquid, but on entering 
 the air, they are made to deviate from their 
 straight course. But the eye is in the habit of 
 following the direction of the rays, and must see 
 the pencil bent simply because the rays coming 
 from it are bent. 
 
 Let a b be the pencil partly immersed ; the part 
 immersed, a c, appears to be at d c, because the 
 ray coming from a, which ought to 
 pass out in the direction of a &, is 
 made to deviate from its course 
 when leaving the water a c, and 
 enters the eye in the direct on of d 
 e. The eye, believing the point a to 
 FIG. 28. be in the direction from which its 
 ray comes, sees the point a actually as being at d. 
 The same takes place with the other rays entering 
 the eye ; hence the whole part a c is seen as being 
 at do. 
 9 
 
130 FIEST LESSONS IN PHYSIOS. 
 
 64. EXPERIMENT. A coin placed on the bottom 
 of a filled tumbler, is seen in its 
 true direction if viewed perpendicu- 
 larly; but if viewed obliquely, it 
 will be seen in a more elevated 
 place. 
 
 FIG. 29. 
 
 Familiar Facts. Owing to refraction of light, 
 
 the bottom of clear waters appears to be more 
 elevated than it really is ; that is, water often ap- 
 pears less deep than it in reality is. This must be 
 taken into account by persons bathing, so that 
 they may not go beyond their depth. 
 
 Hays of light, on passing obliquely through 
 substances of different densities (such as air 
 and water, or glass and water), demote from 
 their straight course; they are bent. This de- 
 viation is called Refraction of Light. 
 
PEISMS LENSES. 
 
 131 
 
 LESSON XXXY. 
 
 PEISMS. LENSES. 
 
 65. EXPEEIMENT. On a blackboard make a 
 
 mark in the shape of 
 an arrow, and look 
 at it through a glass 
 prism, which should 
 be held so that only 
 one edge of it is di- 
 rected upward (Fig. 
 30). The arrow will 
 then be seen as being above its true place. The 
 reason is this : Rays of light, a e we take but 
 two for the sake of simplicity diffused from the 
 arrow, strike the surface, b c, obliquely, and are, 
 therefore, refracted to / (Less. XXXIV ). On pass- 
 ing from the glass prism at /, they are again re- 
 fracted, and enter the eye which is stationed at g. 
 But the eye follows the direction of the refracted 
 
 rays (Less. XXXIY); 
 consequently it sees 
 the arrow as being 
 at Ti. 
 
 66. EXPEEIMENT. 
 
 Now look at the 
 
 k\ ,/ v arrow on the black- 
 
 FIO.SI. board through the 
 
132 
 
 FIRST LESSONS IN PHYSICS. 
 
 prism inverted (Fig. 31); that is, placed so as to 
 present two edges upward. The arrow will then 
 be seen below its true place. The reason of this 
 is the same as before. The rays a e are refracted 
 to/; consequently the arrow is seen as being at#. 
 If now we place two prisms together, as in 
 Fig. 32, rays diffused from the arrow and entering 
 
 PIG. 33. 
 
 the glass surface, a I c, will, in like manner, be 
 refracted twice, and meet each other in several 
 
PEISMS LENSES. 
 
 133 
 
 points behind the prism. In order that the eye 
 of a person situated at /, might receive all these 
 refrn^ted rays, and thus be able to see the whole 
 arrow through the prisms, it would be necessary 
 to have these rays blend into a common point. 
 To do this, we must have the surface, a ~b c, curved 
 (Fig. 33) ; that is, we must have a curved glass in 
 place of the prisms. Such a curved glass is a 
 convex Lens, commonly called a Burning-glass. 
 For it not only brings rays of light to a common 
 point, the Focus, but at the same time it blends 
 rays of heat into a focus. In consequence of this, 
 a match ignites, and a hole is burnt in a piece of 
 paper, if either of these objects be held in the 
 focus. 
 
 FIG 84. 
 
 The arrow, as viewed through a lens, is seen 
 larger (Fig. 34) ; that is, it is magnified, because 
 
134 FIKST LESSONS IN PHYSIOS. 
 
 taking the two extreme rays for the sake of illus- 
 tration the rays, d e and f g, refracted to the 
 eye, are seen as coming from the points Ji L 
 (Why? Lesson XXXIY, p, 130.) The common 
 Burning-glass, therefore, is also called Magnify- 
 ing -Glass. 
 
 An object in front of a convex lens is seen mag- 
 nified ~by an eye placed behind tlie lens. 
 
 Application. The lenses in spectacles ; opera- 
 glasses and telescopes ; all magnifying glasses. 
 
 Read "Magnifying and Burning Glasses," in "Pursuit of KnowK 
 edge," Vol. II. Harper & Bros. 
 
 Bead "Lenses," in "The Wonders of Optics" Illustrated Library of 
 Wonders. 
 
 Head "How to View Pictures," p. 248, in "Spectacles," p. 250 m 
 "Things not Generally Known." 
 
OOLOES. 135 
 
 LESSON XXXVI. 
 
 C O LO E. 
 
 66. EXPEEIMENT. If a large pasteboard with 
 a small hole be placed facing the sun ; or if a 
 room be darkened and only a few rays of light 
 admitted through a crack in the shutter, these 
 rays will pass to the floor and there form a spot 
 of white light. But if a prism 1 be held before 
 the crack or before the hole in the pasteboard, 
 the rays of light will be refracted (Less. XXXIV), 
 and spread out in the form of a long band. In- 
 stead of being white, this band will be colored. 
 The colors of the rainbow may be distinguished 
 in it ; viz. : Violet, indigo, blue, green, yellow, 
 orange and red. The colored band is called the 
 Solar Spectrum ; and this spreading out of light 
 
 I. A prism which is to show the refraction of light, may be of solid 
 glass, or, if such a one can not be had, it may be constructed in the fol- 
 lowing manner : Procure two strips of common 
 glass, having the shape of a rectangle, each of the 
 same size, about 5 inches long by i^ inches wide. 
 FIG. 35 One of the long edges of each is heated over an 
 
 alcohol flame; both edges are then cemented together with sealing-wax, 
 allowing a distance of l% inches between the two remaining long edges. 
 The ends of the vessel thus formed are closed by triangular pieces of 
 thin board, measuring \y z inches on each side, and which are likewise 
 cemented to the glass. Water is then put in, and when used, the prism 
 is held so as to have the long cemented edge below. 
 
136 FIRST LESSONS IN PHYSICS. 
 
 is called Dispersion. If the spectrum be made to 
 fall upon a mirror, it will be reflected in straight 
 lines like ordinary light. 
 
 67. EXPERIMENT. To convince ourselves that 
 ordinary sunlight contains the seven colors of 
 the rainbow, let the spectrum produced by one 
 
 prism fall upon a second 
 prism of the same size 
 as the first, but placed 
 PIQ>86 - as shown in the figure 
 
 annexed. The rays of light, dispersed by the first 
 prism, will be collected by the second, and will 
 then produce white light again. 
 
 68. EXPERIMENT. The same may be shown if 
 a top, painted with the seven colors of the rain- 
 bow, is set spinning rapidly. The impressions 
 made in the eye by these different colors are 
 mixed together, and thus produce a mixture of 
 the colors which is nearly white. 
 
 All this shows that white sunlight is composed 
 of the seven colors of the rainbow. 
 
 69. EXPERIMENT. Between the crack, or the 
 hole in the pasteboard, and the prism insert a 
 piece of red glass. The spectrum will then be 
 almost entirely red, and the other colors be found 
 wanting. Insert a piece of green glass in place of 
 the red ; the spectrum will be almost exclusively 
 green ; with blue glass it will be nearly blue, &c. 
 It is manifest, that white light falls upon each 
 
COLOES. 137 
 
 piece of colored glass ; and that only one color at 
 a time falls upon the prism. Thus when the red 
 glass is inserted, only red light falls upon the 
 prism, and consequently there can be but a spec- 
 trum of red light. The question now is, what 
 becomes of the remaining light or colors which 
 fall on the red glass ? Evidently the red glass 
 absorbs all the light except the red, and this it 
 throws out. The same takes place with each of 
 the other colored glasses ; the green absorbs all 
 the light it receives except the green light ; this it 
 throws out. The blue absorbs all the light it re- 
 ceives save the blue, which it throws out, &c. 
 White glass, however, transmits nearly all the 
 light, and absorbs very little or scarcely any. 
 
 70. EXPERIMENT. If a sheet of red colored 
 paper be held facing the sun, and a sheet of 
 white paper before it, so as to form an oblique 
 angle with it, the portion of the white paper 
 which is near the red, will appear red. In this 
 case red rays are diffused from the red body and 
 fall upon the white. But the red paper receives 
 white light from the sun, hence it must have ab- 
 sorbed all of the white light save the red ; this it 
 throws out. 
 
 Familiar Facts. Objects near a blue curtain 
 often have a bluish hue. The curtain receives 
 white light, and absorbs it all except the blue, 
 which it reflects. Objects near the foliage of 
 
138 FIRST LESSONS IN PHYSICS. 
 
 trees and bushes often have a greenish hue, be- 
 cause green leaves absorb all the light that falls 
 upon them save the green ; this they diffuse in all 
 directions, and thus send green light to the ob- 
 jects near by. 
 
 A body is colored when it reflects only a por- 
 tion of wMte light. A body is white when it re- 
 flects all the white light; and a body is black 
 when it reflects (almost) no light, that is, when it 
 absorbs all the light. 
 
 Questions. What causes a piece of red cloth 
 to appear red ? It sends only red rays to the eye, 
 the other rays it absorbs. What causes a sheet 
 of white paper to appear white ? It absorbs no 
 light, but rejects nearly all of it. Some of this 
 rejected or reflected light enters the eye and thus 
 produces the sensation of white in us. 
 
 What causes a black coat to be black ? It ab- 
 sorbs nearly all the light, consequently it sends 
 scarcely any to the eye. The eye receives just 
 enough light from it to become aware of its pres- 
 ence, but not sufficient to perceive any color. 
 
 Why is every thing black in a dark night? 
 Because, when there is no light, objects receive 
 none, and, therefore, they can not send any to the 
 eye. But if no light enters the eye, we see 
 nothing (Lesson XXXIII). 
 
 Color is not a quality inherent in bodies. 
 
COLORS. 139 
 
 Application. The application of colors is so 
 manifold, that it is impossible to mention each. 
 They serve to enliven the scenery around us , to 
 improve our own appearance , to indicate joy or 
 mourning. We imitate the thousand delicate 
 hues and tinges of the colors in nature in our 
 paintings, artificial flowers, and in many different 
 contrivances. Colors also serve as signals to be 
 seen from afar, hence their use in lighthouses, 
 on railroads (colored lights), and with the mili- 
 tary (flags), &c., &c. 
 
 Bead " Color- Blindness," p. 242, and "Principles of Harmony and 
 Contrasts in Color," p. 244, in "Things Not Generally Known." 
 
 Bead " Color," in "The Earth and its Wonders." 
 
140 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXXVII. 
 
 CHEMICAL ELECTRICITY. 
 
 71. EXPERIMENT. Take a plain glass tumbler, 
 
 and place in it a porous cup of earthenware (un- 
 glazed) in a manner such, that between the cup 
 and the tumbler there is a finger's width of space 
 left. Next have a small sheet of zinc cut as high 
 as the cup. Then bend it into a cylinder wide 
 enough to encircle the porous cup freely. This 
 cylinder is open above and below, with a slit 
 through its whole height. On the top, and oppo- 
 site the slit, about a square inch of zinc is left 
 higher than the rest. To this piece, one end of a 
 copper wire about a foot long is soldered. The 
 zinc cylinder is put into the space between the 
 tumbler and the cup ; the space is then filled with 
 diluted sulphuric acid (a table-spoon full of the 
 acid mixed with ten times the quantity of water). 
 The cup is filled with strong nitric acid. In the 
 acid place a plate of carbon, to the top of which 
 the end of another copper wire is secured. If no 
 carbon plate can be had, a narrow strip of pla- 
 tinum may be used, and another wire soldered on 
 it. If that, too, can not be obtained, fill the cup 
 with crushed coke. 1 Thus prepared, the cup is 
 
 I. The filling with coke must be done in the following manner : Coke 
 is pulverized in a mortar, then a small quantity is first put in the cup 
 
CHEMICAL ELECTEICITY. 141 
 
 placed inside the zinc cylinder ; the diluted acid, 
 of course, surrounding it. Such an apparatus is 
 called a cell or element ; if two or more cells are 
 connected with each other, the apparatus is called 
 a battery. 
 
 The free end of the wires must be scraped clean 
 with a file or knife. If, then, they are brought 
 quite near to each other, a small, bright spark 
 is produced. If the tongue is held between the 
 two ends, a thrilling sensation is felt. 
 
 In the first place, the diluted acid acts upon 
 the zinc; this action may be seen by the minute 
 bubbles rising from the zinc; it may also be 
 heard. They are bubbles of a gas called hydro- 
 gen. In the second place, we have carbon, or 
 platinum, in contact with nitric acid; the action 
 which takes place here is invisible. Thirdly, the 
 two liquids penetrate the porous cup, and, there- 
 fore, meet with each oilier. This action is in- 
 visible also. 
 
 The mutual contact of two different metals 
 (or of zinc and carbon), each placed in a certain 
 liquid, produces Chemical Electricity. 
 
 This electricity is also called " Galvanic Elec- 
 
 and a little nitric acid mixed with it, so that the powder may be soaked 
 with acid. This is repeated several times, until the cup is nearly filled 
 with saturated coke. On top of the coke a lump of coke is placed, 
 around which a copper wire is wound several times, so that about a foot 
 length of wire remains free. That the coke lump may stand firmly, 
 surround the lower part of it by coke powder. 
 
142 FIRST LESSONS IN PHYSICS. 
 
 tricity," because it was discovered by Galvani, 
 an Italian physician, toward the end of the last 
 century. 
 
 The electric spark is seen only when the free 
 ends of the wires are brought together. The zinc 
 is in contact with the diluted acid; electricity 
 passes from the zinc to the acids, and thence to 
 the carbon, and from the carbon, electricity, that 
 is the invisible electric current, passes along the 
 copper wire, returns to the zinc, then to the diluted 
 acid again, and so forth, in the same manner as 
 above, forming an uninterrupted current of Elec- 
 tricity. If the metals (wires) are not very near 
 to each other, no spark is seen, and the current is 
 interrupted. If the end of one of the wires be 
 attached to a pair of scissors, the spark will be 
 seen at the point of the scissors on bringing it 
 very near to the other wire. 
 
 Chemical Electricity is also produced by two different metals im- 
 mersed in only one liquid. Thus, a simple battery can be made by 
 taking a piece of sheet-zinc, and one of sheet-lead, e ach about one-fourth 
 the size of this page, and separated from each other by a cloth or blot- 
 ting paper a little larger than the metals. A few feet of thin copper- 
 wire conveniently fastened to each metal near the edge, and the lead 
 may be placed in a dish, then the cloth or pnper, and the zinc on top. 
 The whole is now covered with a solution of blue vitriol in water. 
 Another simple battery can be fitted up from a strip of zinc and a piece 
 of carbon, immersed in a solution of bi-chromate of potash, mixed with 
 a small quantity of sulphuric acid. Either cell will suffice to work an 
 ordinary electro-magnet. 
 
THE ELECTRO-MAGNETIC TELEGRAPH. 143 
 
 LESSON XXXVIII. * 
 
 THE ELECTRO-MAGNETIC TELEGRAPH. 
 
 Next to the steam-engine the telegraph forms 
 the wonder of our age. Its eminent usefulness 
 and, more yet, the incredible rapidity with which 
 it communicates messages from one place to an 
 other, is something so new, so extraordinary, that 
 we are tempted to believe there is nothing which 
 the human mind is not capable of achieving. 
 
 The fire- signals of the ancients were no longer 
 sufficient for the increasing demands of civiliza- 
 tion. Toward the end of the last century, so-called 
 " optical" telegraphs, consisting of high poles 
 erected upon high buildings or hills were used in 
 France. By means of moveable arms attached to 
 them, signs could be made which in clear weather 
 were visible at great distances. But when, in 1820, 
 it had been discovered by Oerstedt, a Danish pro- 
 fessor, that the electric current running along a 
 wire, exerted a certain influence upon iron, it was 
 at once proposed to apply that influence to the 
 telegraph. 
 
 The first electric wire by which messages were 
 sent, was put up by Steinheil, between his place 
 of residence in Munich and the astronomical 
 observatory near that city. England soon fol- 
 
144 FIRST LESSONS IN PHYSICS. 
 
 lowed the example; so did America. As is 
 always the case with new inventions, a great 
 many improvements were made in rapid succes- 
 sion. It was an American, Morse, who, by a very 
 simple but ingenious improvement, brought the 
 telegraph to its present degree of perfection. 
 
 The principle of Morse's telegraph may be 
 illustrated easily by the following experiment : 
 
 72. EXPERIMENT. A cylindrical rod of soft 
 iron is bent into the shape of a horse-shoe. The 
 rod may be J inch in diameter and 10 inches long. 
 Its two ends must be filed smooth ; the whole is 
 then covered with clay and placed in a coal fire. 
 There it is left for a time and allowed to coo] 
 gradually, when the fire has gone out. After this 
 the clay is removed, and the two ends filed smooth 
 again. Then take a coil of copper wire of about 
 of an inch in diameter, heat it red-hot and cool 
 it in water. The wire must then be 
 completely insulated, by either wrap- 
 ping it in silk or in paper ; or, as 
 the insulation may be very thin, by 
 varnishing it. The trouble of heat- 
 ing and insulating is avoided by 
 buying insulated wire. It is then 
 FIG. ST. wrapped round the iron in close 
 windings. (See Fig. 37.) When beginning to wrap, 
 leave about two feet of wire free, wind then closely 
 near to the bend ; leave the bend uncovered, and 
 
ELECTRO -MAGNETIC TELEGRAPH. 145 
 
 stretch the wire across to the other arm. Then 
 proceed downward to the other end, and leave the 
 last two feet of the wire again free. Both ends of 
 the wire are to be scraped clean, and afterward 
 connected with the wires of the galvanic element, 
 so that the wire starting from the carbon (or 
 platinum) be connected with one of the wires of 
 the horse- shoe ; and the wire of the zinc cylinder, 
 with the other wire of the same. 1 
 
 If now a piece of soft iron, smooth on one 
 side, or a nail, be held at a short distance from 
 the ends of the bent rod, it will be attracted by 
 them, and adhere. Tlie electricity flowing around 
 the iron rod, has rendered the rod magnetic ; its 
 ends are now magnetic poles. (See Lesson III.) 
 
 The galvanic current now travels from the car- 
 bon along the wire, passes through the place 
 where the wires are fastened together, and enters 
 the wire leading to the horse-shoe. Then it runs 
 through all the windings of the two coils, and, in 
 doing so, constantly flows around the iron rod. 
 Leaving the iron rod at the other end, it passes 
 along the copper wire, enters the (zinc) wire where 
 the two wires are connected with each other, and, 
 finally, arrives again at the zinc, whence it starts 
 again to make the same travel anew. 
 
 i . The connection may be effected either by holding the two respective 
 wire-ends firmly together with both hands, or by twisting them closely 
 together. 
 
 10 
 
146 FIRST LESSONS IN PHYSICS. 
 
 Disconnect one of the wires, either by withdraw- 
 ing one hand, or by untwisting the wire ends ; if 
 the iron rod is of the right kind, the piece of soft 
 iron attached will drop instantly. If held up 
 against the poles again, it will not be attracted 
 so long as the wires remain disconnected. The 
 iron rod shows no trace of magnetism. Evidently 
 it was magnetic only as long as the electric cur- 
 rent flowed around it. 1 
 
 Iron becomes magnetic when an electric current 
 passes around it ; but at the instant that the cur- 
 rent is interrupted it ceases to be magnetic. 
 
 Such an iron rod may have the shape of a horse- 
 shoe, or of two -spools. It is an Electro-Magnet. 
 The piece of soft iron applied to its poles is called 
 the Keeper. 
 
 Principles of the Electric Telegraph. 
 
 I. According to Lesson III, magnets have the 
 power of attracting iron ; by means of alternately 
 closing and breaking the electric current, the 
 electro-magnet renders a piece of soft iron alter- 
 nately magnetic and unmagnetic. 
 
 II. The length of the wires connecting the gal- 
 vanic battery with the electro-magnet is imma- 
 
 I In most cases some electricity is left after the current has been in- 
 terrupted. It lasts, however, but a short time. 
 
ELECTRO-MAGNETIC TELEGRAPH. 
 
 147 
 
 terial ; it may be thousands of miles. Thus a 
 battery may be in the city of New York, while 
 the electro-magnet with which it is connected is 
 set np in St, Louis, a distance of 1200 miles, nearly. 
 III. A person stationed at the battery, may, 
 by disconnecting and connecting the wires, break 
 and close the current at Ms pleasure. 
 
 The three principles can be demonstrated 
 by a simple apparatus shown in Fig. 38. Two up- 
 right pieces of board, M N^ are fastened to a table 
 so as to admit the 
 wooden piece, #, 
 between them. The 
 horse- shoe rod, J., 
 is made an electro- 
 magnet whenever 
 the wires are pro- 
 p e r 1 y connected 
 with the galvanic 
 element. A piece 
 of soft iron, d e, on 
 which thin paper 
 has been pasted, is 
 attached to a one 
 armed lever, b c> 
 whose fulcrum is at D. When the electric cur- 
 
148 FIRST LESSONS IN PHYSICS. 
 
 rent passes through A, the poles of the Electro- 
 magnet attract the Keeper d e ; but on breaking 
 the current by disconnecting one of the wires, the 
 Keeper will drop. To prevent its dropping too 
 far, there is a wooden support, ^, which does not 
 allow the Keeper to separate from the poles of the 
 Electro -magnet more than perhaps 1-10 of an inch. 
 A piece of wire previously wound around a lead- 
 pencil, serves to draw the lever promptly down- 
 ward. The paper pasted on the Keeper immedi- 
 ately disconnects the latter from the poles of the 
 magnet when the current is broken. Lastly, a 
 wooden point, /, writes the message upon an end- 
 less band of paper, which is unwound from a 
 cylinder above it. This cylinder is not repre- 
 sented in the drawing. 
 
 When the keeper is attracted by the magnet, 
 the point f makes a mark or indentation, on the 
 paper. But when the current is interrupted, the 
 Keeper drops, and the point drops at the same 
 time ; consequently no mark is then made. To 
 
 represent the letter a, for example, a sign : , 
 
 is impressed upon the paper ; the operator at the 
 delivery station closes the current for an instant 
 only, this produces the small line ; then he 
 breaks it, but immediately afterward closes it 
 again, and keeps it closed three times as long as 
 
 before. This produces the other line, , and 
 
 now the letter a is on the paper of the operator 
 
ELECTRO-MAGNETIO TELEGRAPH. 149 
 
 In the receiving station. To write the word table, 
 the following signs are necessary : 
 
 t a l> I e 
 
 Experienced operators are able to write down 
 the messages merely from the clicking of the lever. 
 
 Magnet and Electro-Magnet Compared. 
 
 Five points in common : 
 
 1. Both attract iron. 
 
 2. Each has usually the form of a horse- shoe. 
 
 3. Each has two poles. 
 
 4. In both the power resides chiefly at the ends. 
 
 5. Both are eminently useful to man : the mag- 
 
 netic-needle as a guide upon the ocean ; 
 the 'electro-magnet as a carrier of messages. 
 Two points of difference : 
 
 1. A magnet has no wire coil (helix) around it; 
 
 an electro-magnet has. 
 
 2. A magnet always attracts iron ; 
 
 an electro-magnet, only when an electric 
 current passes around it. 
 
 Read " The Old Telegraphs, 1 ' p. 69" The Laying of the Atlantic 
 Cable," p. 193, in "Inventions and Discoveries," by Temple. Groom- 
 bridge. London. 
 
150 FIRST LESSONS IN PHYSICS. 
 
 LESSON XXXIX. 
 
 EE VI E W . 
 
 LESSON xxxn. 
 
 1. The Sun, the Fixed Stars, Electricity, Phos- 
 
 phorescence, Luminous Animals, and Burning 
 Substances, are Sources of Light. 
 
 2. Neither the plants nor most of the objects 
 
 around us, are self-luminous bodies. 
 
 3. Bodies not self-luminous are visible only when 
 
 they receive light from some luminous body, 
 and when a portion of that light forms an im- 
 pression upon our eye. 
 
 4. Light emanates from a self-luminous body in 
 
 all directions, and travels in straight' lines. 
 LESSON xxxin. 
 
 5. All bodies reflect light radiantly. 
 
 6. Objects with polished surface reflect light, both, 
 
 radiantly and specularly. 
 
 7. All bodies appear to be in the direction whence 
 
 their rays enter the eye. 
 
 8. Rays of light, on passing obliquely through 
 
 substances of different density, such as glass, 
 water, or air, deviate from their straight 
 . course ; they are refracted. 
 LESSON xxxv. 
 
 9. An object before a convex lens, appears mag- 
 
 nified to the eye situated behind the lens. 
 
REVIEW. 151 
 
 LESSON XXXVI. 
 
 10. White sunlight is composed of the colors of 
 the rainbow. 
 
 11. A body is colored when it diffuses only a por- 
 tion of the white light it receives ; a body is 
 white when it diffuses all the white light it 
 receives ; a body is black when it absorbs all 
 the white light it receives. 
 
 12. Color is not a quality inherent in bodies. 
 
 LESSON XXXVII. 
 
 13. The mutual contact of two different metals 
 
 (or of zinc and carbon), each placed in a 
 certain liquid, produces chemical electricity. 
 
 14. Chemical electricity travels in a circuit from 
 its source and back again. 
 
 LESSON xxxvni. 
 
 15. Soft iron is magnetic, when an electric current 
 passes around it. 
 
 When the current is interrupted, it ceases to 
 be magnetic. 
 
 16. The principles of the Electric Telegraph are : 
 
 1. A piece of soft iron may be rendered alter- 
 
 nately magnetic and unmagnetic by means 
 of an electro-magnet. 
 
 2. The electric current travels over any length 
 
 of wire. 
 
 3. A person stationed at the electric battery, 
 
 may close and break the current at his 
 pleasure. 
 
QUESTIONS. 
 
 (Questions preceded by a = are of a more difficult character.) 
 LESSON 1. GRAVITY. 
 
 PAGE 1 1. 
 
 I. Why does a stone in our hand 
 not fall ? 
 
 2 Why does it fall when drop'd ? 
 
 3 Why does a pencil roll down 
 
 from the desk ? 
 
 4 Whither does a stone thrown 
 
 into a pond fall ? and why ? 
 
 5 Whither does a sign-board 
 
 blown off by the storm ? and 
 why ? 
 
 6 Whence does rain, snow and 
 
 hailstones come ? and why ? 
 
 7 When does water form water- 
 
 falls ? 
 
 8. Why do coals fall through the 
 
 grate ? 
 
 9. Why does soot, through the 
 
 air? 
 
 10. To what purpose are heavy 
 
 rods attached to maps and 
 curtains ? 
 
 11. To what purpose are clocks 
 
 provided with weights ? 
 ' 12. Why is it that all bodies near 
 the earth have a tendency to 
 approach the earth? 
 
 13. Give the law of gravity. 
 
 PAGE 12. 
 
 14. Why is a string, with a weight 
 
 attached, drawn straight? 
 
 15. What prevent^ the weight 
 
 from falling ? " 
 
 16. What does the string indicate? 
 
 17. Define vertical. 
 
 1 8. What is a plumb-line ? 
 
 19. Give the law of Direction of 
 
 Force of Gravity. 
 
 20. Why does a large stone press 
 
 itself partly into the ground? 
 
 PAGE 12. 
 
 21. Why do heavy wagons make 
 
 ruts? 
 
 22. In what manner do ladies 
 
 judge of silk robes ? 
 
 23. Define weight. 
 
 24. What is a balance ? 
 
 25. What are the weights ? 
 PAGE 13. 
 
 26. How does it come that a pound 
 
 of coffee has as much weight 
 as a pound of lead ? 
 
 27. a pound of feathers as 
 
 much as a pound of iron ? 
 
 28. Of what force of Nature is the 
 
 Balance an application of? 
 
 29. Clock weights ? 
 
 30. Hour-glasses? 
 
 31. Why is a large drop of mer- 
 
 cury lying upon the table 
 never entirely round ? 
 
 32. Why do wagons, unless- 
 
 checked, roll down hill with 
 great rapidity ? 
 
 33. Why do light bodies, such as- 
 
 feathers, bits of paper, &c., 
 fall to the ground more slow- 
 ly than heavy bodies, such as 
 stones and the like ? 
 
 34. Where must a rod be support- 
 
 ed to be evenly balanced ? 
 
 35. How can the weight of a body 
 
 be found by means of a bal- 
 anced rod ? 
 
 36. Has a body the same weight 
 
 on different heavenly bodies? 
 
 37. Wha^ will a pound of tea 
 
 weigh on the moon ? 
 
 38. What, on the sun ? 
 
 39. What, in the center of the 
 
 earth ? 
 
 40. What, half way between cen- 
 
 ter and surface ? 
 
154 
 
 FIRST LESSONS IN PHYSICS. 
 
 41. Would it weigh more, or less, 
 if at a considerable distance 
 above the surface ? 
 
 42. What causes the tide-waves ? 
 
 43. What, the revolution of the 
 
 moon around the earth ? 
 
 LESSON II. SPECIFIC GRAVITY. 
 
 FLOATING AND SINKING SOLIDS. 
 
 PAGE 14. 
 
 44. What is meant by the state- 
 
 ment "Water is heavier than 
 oil ?" 
 
 45. How should the statement be? 
 
 46. Prove that a pound of water 
 
 is as heavy as (better: has 
 the same weight as) a pound 
 of oil. 
 
 47. Why does a pint of nercury 
 
 weigh more than a pint of 
 
 water ? 
 .48. Why has a solid rubber-ball 
 
 more weight than a hollow 
 
 one ? 
 49. Have all solids the same 
 
 weight ? 
 
 50. Have all liquids ? 
 
 51. Define Specific Gravity. 
 
 52. What makes oil float on water? 
 
 (Answer: The fact that oil, 
 &c., &c.) 
 
 53. How does it come that smoke 
 
 rises, while soot falls? 
 (Quest. 9.) 
 
 54. Why does oil rise thro' water? 
 
 55. Why do balloons rise through 
 
 the air ? 
 PAGE 15. 
 
 56. Give law about Fluids of dif- 
 
 ferent specific gravity. 
 
 57. Why does a piece of wood 
 
 float, while a st-ne sinks, 
 when thrown into w iter ? 
 
 58. Prove that liquids have 
 
 weight. 
 
 59. Will the weight of a pai 1 of 
 
 water be increased when a 
 
 fish is thrown in ? 
 60. Why does an empty flask float 
 
 on water ? 
 61. Why does it not also in air? 
 
 PAGE 15. 
 
 62. Why does a bottle filled with 
 
 water sink in water ? 
 
 63. Why does it float on mercury? 
 
 64. Under what circumstances 
 
 does a body float ? sink ? 
 
 65. Why do iron-clads float ? 
 
 66. When will the body of man 
 
 float? 
 
 67. Why is it difficult for bathers 
 
 to walk in water chin deep? 
 
 68. In drawing water from a well, 
 
 why has the bucket more 
 weight as it emerges from 
 the water ? 
 
 69. Why may heavy stones be 
 
 lifted in water, while on dry 
 land they can scarcely be 
 moved ? 
 
 70. What should persons who can 
 
 not swim, do on falling in 
 the water ? 
 
 71. Why does ice float on water ? 
 
 72. Why does a full tumbler run 
 
 over when a stone is thrown 
 in, and not when a piece of 
 sponge? 
 
 73. Why does wood saturated with 
 
 water, sink ? 
 
 74. ^ 'hy do some bodies, floating 
 
 on water, sink in it more 
 than others; thus oak wood 
 more than pine wood ? 
 
 75. Why can persons float on 
 
 water by means of life-pre- 
 servers or bladders filled 
 with air ? 
 
QUESTIONS. 
 
 155 
 
 76. Why do we often see a sedi- 
 
 ment on the bottom of ves 
 Sels containing liquids, after 
 they have leen standing for a 
 time? 
 
 77. Why do drowned persons, 
 
 after having lain under water 
 for a time, rise to the sur- 
 face ? 
 
 78. Why do ships sink deeper in 
 
 river water than in the ocean? 
 
 79. Why does a hen's egg float on 
 
 water strongly salted, while 
 it sinks in fresh water ? 
 
 80. Why does water in a vesse' 
 
 rise higher on dropping intc 
 it a pound of iron than i; 
 does when a pound ot lead 
 is dropped in ? 
 
 81. Why must a dog sometimes 
 
 drop a heavy stone (after hav- 
 ing fetched it from the bot- 
 tom of a water) when he 
 reaches the surface? 
 
 82. What enables fish to move up 
 
 and down in the water at 
 pleasure ? 
 
 LESSON III. MAGNETIC ATTRACTION. 
 
 PAGE 17. 
 
 83. Under what circumstances 
 
 will a plumb-line change 
 from the vertical direction ? 
 
 84. Will it also change if its weight 
 
 is a stone ? 
 
 85. Mention a force which may 
 
 overcome gravity. 
 
 86. Show that magnets and un- 
 
 magnetic iron attract each 
 other. 
 
 87. Give a property common to 
 
 both, magnetic attraction an J 
 gravity-attraction. 
 
 PAGE 1 8. 
 
 88. Where does the power of a 
 
 magnet chiefly reside ? 
 
 89. What is the difference between 
 
 a magnet and a piece of un- 
 magnetic iron ? 
 
 90. What is the name of the ends 
 
 of the magnet ? 
 
 91. Where do these ends point ? 
 
 92. In what position must the 
 
 magnet be in that case ? 
 
 PAGE 1 8. 
 
 93. State the law of direction of a 
 
 magnet. 
 
 94. What action is seen in two 
 
 magnets whose like ends 
 are brought together ? 
 PAGE 19. 
 95., Give law for it. 
 
 96. Whence the application of 
 
 magnets ? 
 
 97. Is a magnetic needle liable to 
 
 deviate more on a wooden 
 vessel than on an iron ? 
 
 98. How may a magnet be made ? 
 
 99. Why have magnets usually 
 
 that form ? 
 
 100. Describe a magnet. 
 
 101. Does the earth act like a mag-' 
 
 net? 
 Give reasons for your answer. 
 
 102. What reason have the French 
 for calling the north pole of 
 a magnet its "South Pole," 
 and the south pole its "North 
 Pole?" 
 
 LESSON IV. ELECTRIC ATTRACTION. 
 
 PAGE 20. 
 
 103. Whence the term "Electri- 
 
 city ?" 
 
 104. What power may sealing-wax, 
 
 sulphur and glass acquire; 
 and on what condition? 
 
 PAGE 20. 
 
 105. Same, regarding paper. 
 
 1 06. State the source of electricity. 
 
 107. What peculiar property do 
 
 electric bodies manifest . 
 
156 
 
 FIKST LESSONS IN PHYSICS. 
 
 PAGE 21. 
 
 108. What phenomena may accom- 
 
 pany electrified bodies ? 
 
 109. Why the peculiar sensation 
 
 felt on holding electrified 
 objects against one's face? 
 
 PAGE 22. 
 
 110. W r hat becomes of electricity 
 
 after it has left the sulphur, 
 or the glass ? 
 
 in. Mention two good conductors 
 of electricity. 
 
 112. Three non-conductors. 
 
 113. Give difference between good 
 
 conductors and non-conduc- 
 ors. 
 
 Can electricity be produced 
 upon both classes of bodies? 
 
 PAGE 23. 4 
 
 114. What phenomena take place 
 
 when electrified sealing-wax 
 is presented to a suspended 
 pith ball ? 
 
 115. When, only, do they take 
 
 place ? 
 
 1 1 6. Did you notice anything simi- 
 
 lar in magnets ? 
 
 117. In gravity ? 
 
 1 1 8. What phenomena, when elec- 
 
 trified sealing-wax is pre- 
 sented to two pith balls ? 
 
 1 19. What force is o.vercome in that 
 
 case? 
 
 1 20. Was that same force ever 
 
 overcome before ? (Com p. 
 question 85.) 
 
 PAGE 23. 
 
 121. What phenomena, if first seal- 
 
 ing-wax and then glass is 
 presented to the single pith 
 ball? 
 
 122. How do you explain your 
 
 answer ? 
 PAGE 24. 
 
 123. What phenomena if first seal- 
 
 ing-wax and then glass is 
 presented to one of the two 
 pith balls ? 
 
 124. What phenomena if to one of 
 
 the two pith balls you pre- 
 sent sealing-wax, and at the 
 same time, glass to the other? 
 
 PAGE 25. 
 
 125. How many kinds of electri- 
 
 city? Name them. 
 
 126. Give law of electricity. 
 
 127. Explain principle and action 
 
 of Lightning Rods. 
 
 128. On rubbing glass on flannel, 
 
 do you produce only one 
 kind of electricity, or both 
 kinds ? 
 
 129. Why does an electrified bar 
 
 of sealing-wax gradually lose 
 its electricity ? 
 
 130. Why do small pith balls upon 
 
 a table jump up and down, 
 if a sheet of electrified paper 
 be held over them. 
 
 LESSON V. LIGHTNING. LIGHTNING-RODS. 
 
 PAGE 26. 
 
 131. What was Franklin's merit 
 
 regarding the explanation of 
 lightning ? 
 
 132. Give an account of Franklin's 
 
 experiment. 
 
 Why the pointed iron 
 
 wire on top of his kite ? 
 
 133. Could he have taken a silk 
 
 string instead of a hempen 
 one ? 
 
 134. What was the purpose of the 
 
 the key ? 
 
 PAGE 27. 
 
 135. What made the fibres of the 
 
 string bristle up ? 
 
 136. What does Franklin's experi- 
 
 ment demonstrate ? 
 
 137. What is the cause of light- 
 
 ning? 
 
 138. Give three paths which light- 
 
 ning may follow ? 
 
 139. What objects are most liable 
 
 to be strucl; I aid why ? 
 
QUESTIONS. 
 
 157 
 
 PAGE 27. 
 
 140. Why should you not stand 
 
 under a tall tree during a 
 thunderstorm ? 
 PAGE 28. 
 
 141. Which is the safest place in a 
 . room during a thunder- 
 
 stoi m ? 
 
 PAGE 28. 
 
 142. Give an account of the light- 
 
 ning-rod. 
 
 143. On what conditions may a 
 
 lightning-rod be called good? 
 
 144. What becomes of the light- 
 
 ning after passing down 
 ' along the rod ? 
 
 LESSON VI. COHESION. 
 
 PAGE 29. 
 
 145. Why is it that meat must be 
 
 cut, while bread may easily 
 be broken ? 
 
 146. Why is water easily divided, 
 
 while ice is not ? 
 
 147. What is the name of the force 
 
 which causes the parts of a 
 solid to remain together ? 
 
 148. Why is rolled iron stronger 
 
 than common iron ? 
 
 149. To what purpose does a 
 
 knowledge of the cohesive 
 force serve ? 
 
 150. Could birds fly in water ? 
 PAGE 30. 
 
 151. Why would it be difficult for 
 
 us to walk through molasses? 
 
 152. What must be done to break 
 
 a body ? 
 
 153. Why has a walking-cane lost 
 
 its strength if after being 
 broken, the parts are glued 
 together ? 
 
 PAGE 3 1. 
 
 154. What is the great enemy of 
 
 cohesion ? 
 155- Why does oil form larger 
 
 drops than water ? 
 
 156. What are pores ? 
 
 157. Why is a dry sponge smaller 
 
 than a wet one ? 
 
 158. What makes blotting-paper 
 
 remove fresh ink ? 
 
 159. Why do doors, window-frames 
 
 and drawers often swell in 
 damp weather ? 
 
 1 60. How is :t that mercury can be 
 
 pressed through a leather 
 bag? 
 
 161. What causes wooden tubs to 
 
 leak in summer ? 
 
 162. What may be done to prevent 
 
 this ? 
 
 163. How do solids, liquids and 
 
 gases differ as to cohesion. 
 
 164. Give 2 app'c of cohe. force. 
 
 LESSON VII. ADHESION CAP. ATTRACTION. 
 
 PAGE 32. 
 
 165. How can two leaden bullets 
 
 be made to adhere ? 
 
 1 66. Why do not two bricks ad- 
 
 here in the same manner ? 
 
 167. When, only, does adhesion 
 
 take place ? 
 LV.GE33. 
 
 1 68. How may two rough surfaces 
 
 be made to adhere ? 
 
 169. Why does the hand become 
 
 wet when immersed in 
 water? (Given in text.) 
 
 170. vVhy does it remain dry when 
 
 drawn out of mercury ? 
 
 PAGE 33 
 
 171. What two forces are in strug- 
 
 gle with each other when the 
 hand is placed in water ? 
 
 172. Define adhesion. 
 
 173. Why are two smoothly pol- 
 
 ished plates separated with 
 great difficulty, if laid to- 
 gether and firmly pressed ? 
 
 174. Why does fresh paint adhere 
 
 to one's dress ? 
 PAGE 35. 
 
 175. What is a capillary tube 
 
 176. Define capillary attraction. 
 
158 
 
 FIRST LESSONS IN PHYSICS. 
 
 PAGE 35. 
 
 177. What causes the sponge to 
 
 absorb water? (Comp. 157.) 
 
 178. Why may eggs and meat be 
 kept fresh in sand ? 
 
 179. Explain the action of oil in 
 
 lamp- wicks. 
 
 1 80. How, and why, may grease 
 
 spots be removed from the 
 floor? 
 
 181. Why do two papers pasted 
 
 together adhere firmly ? 
 
 182. Why may the hand be drawn 
 
 out of the water dry, if, be- 
 fore immersed it was cover'd 
 with Lycopodium powder? 
 
 183. Why is a greased glass not 
 
 moistened when immersed 
 in water ? 
 
 184. Why does a small drop of 
 
 water on a board remain tho' 
 the board be inverted? 
 
 185. Why does a drop of mercury 
 
 fall when the board is in- 
 verted ? 
 
 186. Why does a small drop of 
 
 mercury on a tin plate re- 
 main when the plate is in- 
 verted ? 
 
 187. Why do figures drawn with 
 
 the finger upon a window- 
 pane, become visible if we 
 breathe on them ? 
 
 LESSON IX. ELASTICITY. 
 
 PAGE . 
 
 1 88. Why is the spot which an 
 
 ivory ball receives upon 
 falling on a blackened sur- 
 face, larger if the ball has 
 fallen from a considerable 
 height than if it has merely 
 been pressed with the hand 
 upon that surface? 
 
 189. What makes an arrow, shot 
 
 from a cross-bow, fly a 
 great distance ? 
 What makes steel, ivory 
 and india-rubber resume 
 
 190. 
 
 PAGE 40. 
 
 their former position after 
 being bent ? 
 
 191. Define elasticity ? (El. is the 
 
 property of bodies to re- 
 cover their former fig., etc. 
 
 192. When are bodies hard soft ? 
 PAGE 41. 
 
 193. Mention four applications of 
 
 the elasticity of bodies. 
 
 194. Define btittleness ductility. 
 
 195. Define malleability. 
 
 196. Give examples of brittle, 
 
 malleable and ductile bod's. 
 
 LESSSON X. ELASTICITY OF AIR. 
 
 PAGE 42. 
 
 197. Show that air, like every other 
 body, maintains its place. 
 
 PAGE 43. 
 
 198- Why does not water enter a 
 bottle in the neck of which 
 a funnel is cemented ? 
 
 199. Describe the action of the 
 
 pop-gun. (Page 44.) 
 
 200. What is its principle ? 
 
 201. Principle of the blow-gun? 
 
 202. Principle of the Diving-bell ? 
 
 PAGE 43 
 
 203. W hat causes the air inside a 
 
 Heron's Fountain to be 
 compressed ? 
 
 204. Desci ibe action of Heron's F. 
 PAGE 44. 
 
 205. Give the law on elast'y of ah. 
 
 206. What is an air-chamber ? 
 
 207. Describe its action. 
 
 208. Why do fire- wheels turn ? 
 
 209. Why do sky-rockets ascend ? 
 
 210. Why do cannons recoil when 
 
 fired off? 
 
QUESTIONS. 
 
 159 
 
 LESSON XL PRESSURE OF AIR. 
 
 PAGE 46. 
 
 211. Why does the water not flow 
 
 from a filled inverted tum- 
 bler, with a piece of paper 
 pressing against it ? 
 
 212. Show that air presses down- 
 
 ward. 
 PAGE 47. 
 
 213. Show that air presses in all 
 
 directions. 
 
 214. Why does not vinegar flow 
 
 from a barrel whose bung- 
 hole is closed ? 
 
 215. Explain the action of the 
 "Thief." 
 
 2 1 6. Why do we not feel the 
 
 pressure of air exerted upon 
 us? 
 
 217. Why are travelers more easily 
 
 fatigued on high lands than 
 on low lands ? 
 
 2 1 8. What makes us feel tired dur- 
 
 ing excessive heat, or before 
 a thunderstorm ? 
 
 219. Wh^ is it that, when a bottle 
 
 filled with air in the low 
 land is taken up on high 
 land, the air will escape with 
 violence when the bottle is 
 opened ? 
 
 LESSON XIL BAROMETER. 
 
 PAGE 48. 
 
 220. What can be the height of a 
 
 col. of water supported by 
 the atmosphere? 
 
 221. What is the height of a col- 
 
 nmn of mercury ? 
 
 222. Comp. the weight, thickness 
 
 and height of these liquid 
 columns with the corresp'g 
 column of air. 
 PAGE 49. 
 
 223. Describe barometer, (p. 50. ) 
 
 224. Why dues it read the same 
 
 whether in or out of doors? 
 22$. What are the chief uses of 
 
 the barometer? 
 PAGE 51. 
 
 226. Lxplain its use in measuring 
 
 heights. 
 
 227. Its use as an index of the 
 
 weather. 
 
 228. What is the real object of 
 
 the barometer ? 
 
 229. What causes the mercury in 
 
 the barometer to fall ? 
 
 230. What causes it to rise ? 
 
 231. Suppose we wished to em- 
 
 PAGE 51., 
 
 ploy water instead of mer- 
 cury, how high would the 
 barometer tube have to be 
 made ? 
 
 232. What is a vacuum ? 
 
 233. What is the amount of the 
 
 pressure of air ? 
 
 234. How can this be proved? 
 
 235. To what extent may wind in- 
 
 fluence the barometer? (Re- 
 member that wind is air in 
 motion. ) 
 
 236. Why does the mercury in the 
 
 barometer fall when carried 
 up on the mountains ? 
 
 237. Does atmospheric pressure 
 
 increase or decrease, as we 
 go away from the earth ? 
 
 238. Supposing the moon to have 
 
 a terrestrial atmosphere, 
 how high would the mer- 
 curial column stand there ? 
 
 239. How high on the sun ? 
 
 240. At the centre of the earth ? 
 
160 
 
 FIRST LESSONS IN PHYSICS. 
 
 LESSON XIV INERTIA. 
 
 PAGE 54, 
 
 241. Snow that a body at rest re- 
 
 mains at rest until set in 
 motion by some force. 
 
 242. Show that for a body !b be 
 
 set in motion, time is neces- 
 sary. 
 PAGE 55. 
 
 243. Show that a body once in mo- 
 
 tion remains in motion until 
 stopped. 
 
 Show that for the motion of a 
 body to stop, time is neces- 
 sary. 
 
 244. Define inertia. 
 
 245. Why is a fly-wheel an applica- 
 
 tion of inertia. 
 
 246. Why may the loose handle of 
 
 a hammer be fastened again 
 by knocking the end of the 
 handle against a hard object? 
 
 247. Why may a stopped-up pipe 
 
 be cleaned again by forcibly 
 knocking against one of its 
 ends ? 
 
 248. Why must good bridges have 
 
 a great mass ? 
 
 249. Why may a' candle be shot 
 
 from a great distance thro' 
 a board ? 
 
 250. Why do cannon or rifle balls 
 
 make a circular hole if fired 
 at a window-pane ?. 
 
 LESSON XV. INCLINED PLANE. / 
 
 PAGE 58. 
 
 258 Why does a bullet thrown 
 with the hand inflict less 
 harm than one fired from a 
 
 PAGE 56. 
 
 251. What is an Inclined Plane ? 
 
 252. Why does a ball on it fall ? 
 
 253. Give three familiar instances 
 
 of an inclined plane. 
 PAGE 57. 
 
 254. Show that the steeper an in- 
 
 clined plane is, the greater 
 is the velocity of a body fall- 
 ing on it. 
 
 255. Show that in that case the 
 
 force required to ascend it is 
 greater. 
 
 256. Show that a body increases in 
 
 velocity as th> space in- 
 creases through which it 
 falls. 
 
 257. Show that the greater the ve- 
 
 locity of a body, the greater 
 its striking force. 
 
 gun ? 
 
 259. Why may hailstones destroy 
 
 standing grain ? 
 
 260. What does the falling of bodies 
 
 on an inclined plane show ? 
 
 261. Whence the practical applica- 
 
 tion of the inclined plane ? 
 
 262. What is the principle of the 
 
 wedge ? 
 
 263. of the ax ? 
 
 264. of the skid ? 
 
 265. Why are roads leading up 
 
 steep mountains made in 
 windings ? 
 
 266. What is meant by the length 
 
 of an inclined plane ? 
 
 267. What, the height? 
 
 LESSON XVI. LEVER. \ 
 
 PAGE 59. PAGE 59. 
 
 268. When is a balanced rod in a 271. What is to be noticed in lift- 
 
 state of equilibrium ? ing the end of the longer 
 
 269. Why then ? * rm w r ith the hand ? 
 
 272. What, if the lengths of the 
 
 270. Why will the longer arm of a two arms have the ratio cf 
 
 rod fall ? I to 2 ? 
 
QUESTIONS. 
 
 161 
 
 PAGE 59. 
 
 2 73- What characterizes the end 
 of the long arm of a lever ? 
 PAGE 60. 
 
 274. What have hailstones in com- 
 
 mon with a small weight at 
 the end of the long arm ? 
 
 275. Define the lever. 
 
 276. Give a general law about it. 
 
 277. What does the amount of 
 
 power needed to lift a load 
 by means of a lever, depend 
 upon? 
 
 278. How may it be found ? 
 
 PAGE 61. 
 
 279. Give the three important 
 
 points in a lever. 
 
 PAGE 61. 
 
 280. What is a lever of the first 
 
 class ? 
 
 281. Give three examples, and ex- 
 
 plain. 
 
 282. What is a lever of the second 
 
 class ? 
 
 283. Give three examples, and ex- 
 
 plain. 
 
 284. To which class of levers does 
 
 the oar belong ? and why ? 
 
 285. The wheel -barrow ? 
 
 286. How may a heavy stone be 
 
 lifted ? 
 
 287. What is the stone then called? 
 
 288. Why are levers used only for 
 
 moving loads through short 
 distances ? 
 
 of a 
 
 LESSON XVII. THE PENDULUM. 
 
 PAGE 66. 
 
 297. What is the office of the 
 
 crutch ? 
 
 298. Explain how it comes that 
 
 weight in a clock causes the 
 hands to move with uniform 
 velocity. 
 
 299. What is the motory force of 
 
 clocks ? 
 
 300. What the regulating force ? 
 
 PAGE 63. 
 
 189. What is a vibration? 
 
 290. Explain the vibration 
 
 pendulum. 
 
 291. How many forces act upon it, 
 
 and what are they ? 
 
 PAGE 64. 
 
 291. Show that the vibration of the 
 same pendulum, whether 
 quite short or not, takes 
 place in the same length of 
 time. 
 
 293. Show that a short pendulum 
 
 vibrates more quickly than 
 a long one. 
 PAGE 65. 
 
 294. What is the principle applica- 
 
 tion of the pendulum f 
 
 295. Explain its action. 
 
 296. What is meant by winding up 
 
 a clock ? 
 
 Would a pendulum placed 
 high up above the earth's 
 surface, vibrate more quickly 
 or more slowly than on 
 earth? 
 
 How on the moon ? 
 
 303. How on the sun? 
 
 304. Midway between the earth's 
 
 surface and center? 
 
 301- 
 
 302. 
 
 305. At the center of the earth ? 
 
 LESSON XVm.--COM]Vfl!fNlCATING VESSELS-HYDRAULIC 
 PRESS. 
 
 PAGE 67. PAGE 68. 
 
 306. Show that the surface of quiet 309. Explain fountains. 
 
 310. What causes fountain-jets to be 
 
 shorter than they ought to be? 
 
 311. How does it come that our 
 
 water-pipes can lead water 
 to the upper part of houses, 
 
 water is always level. 
 
 307. How does water stand in a 
 
 tea-pot ? 
 PAGE 68. 
 
 308. Show that your statement 
 
 must be true. 
 II 
 
 contrary to gravity ? 
 
162 
 
 FIRST LESSONS IN PHYSICS. 
 
 PAGE 68. 
 
 312. Define Communicating Ves- 
 
 sels. 
 
 313. Why may water-pipes under 
 
 ground be said to be com- 
 municating tubes? (Text.) 
 
 314. Give law about pressur 
 
 liquids. 
 
 PAGE 70. 
 
 315. Demonstrate it. 
 
 316. Give name and date of its ap- 
 
 plication. 
 
 ~, PAGE 70. 
 
 of 317. Explain the action of the hy 
 draulic press. 
 
 LESSON XIX. BREATHING BELLOWS. 
 
 PAGE 71. 
 
 318. Why can we, with a tube, suck 
 
 up water with the mouth ? 
 
 319. Explain the process of Inspi- 
 ration. 
 
 320. That of Expiration. 
 
 321. What is meant by breathing? 
 
 322. What is a vacuum ? 
 
 323. What takes place when air has 
 
 access to a vacuum? 
 PAGE 72. 
 
 324. Explain the action of the bel- 
 
 lows. 
 
 325. What is a valve ? 
 
 PAGE 72. 
 
 326. Compare the action of the bel- 
 
 lows with the action oi 
 breathing. 
 
 327. Explain the act of smoking. 
 
 328. That of drinking. 
 
 329. Could we breathe in a vacu'm? 
 Give reasons for your answer. 
 
 331 
 
 Would the bellows work in a 
 
 vacuum ? 
 Give reasons. 
 Would the bellows work in 
 
 water ? 
 
 LESSON XX. COMMON PUMP. 
 
 PAGE 74. 
 
 332. Explain the action of the sy- 
 ringe. 
 
 333- What causes the water to rise 
 in it ? 
 
 334. Would it rise if water and 
 
 syringe, both, were in a 
 vacuum ? 
 
 335. What are the principal parts 
 
 of a common pump ? 
 PAGE 75. 
 
 336. Where is the piston when the 
 
 handle is drawn out farthest? 
 PAGE 76. 
 
 337. When is the piston at its 
 
 highest ? 
 
 338. In that case, what is below 
 
 the piston ? 
 
 339' When the piston commences 
 rising, which of the two 
 valves is opened ? 
 
 340. Why ? 
 
 341. When the piston is at its high- 
 
 est, which valve is closed ? 
 
 PAGE 76. 
 
 342. What is meant by rarified air? 
 
 343. Why does the water in the 
 
 suction-pipe rise ? 
 
 344. What is the position of the 
 
 valves when the piston is 
 being lowered? 
 
 345. What do you pump out first ? 
 340. What causes the water to flow 
 
 out through the spout ? 
 
 347. What causes the lower valve 
 
 to close ? 
 
 348. What, the higher? 
 
 349. Give the principle of the com- 
 
 mon pump. 
 
 350. What is a pump ? 
 
 351. To what purpose is the lower 
 
 valve ? 
 
 352. The upper ? 
 
 353. Is there any similarity between 
 
 the common pump and the 
 bellows ? 
 
 354. Explain your statement. 
 
QUESTIONS. 
 
 163 
 
 355' 
 
 Since the one serves to pump 
 out water, and the other to 
 pump out air, why has the 
 latter but one valve ? 
 
 356. Comparing the common pump 
 with the barometer, give 
 four points which they have 
 in common. 
 
 357- Eight points of difference. 
 
 LESSON XXL FORCING PUMP FIRE-ENGINE. 
 
 PAGE 77. 
 
 358. How high (theoretically speak- 
 
 ing) may water be lifted with 
 a common pump? 
 
 359. Give reason. 
 
 360. To elevate water to a greater 
 
 height, what must be used? 
 PAGE 78. 
 
 361. Give three points in which it 
 
 differs from the common 
 pump. 
 
 362. What are the principal parts 
 
 of the forcing pump? 
 
 363. Where is the piston when the 
 
 handle is at its lowest? 
 When at its highest ? 
 
 364. When the piston is at its high- 
 
 est what is the position of 
 the valves ? 
 
 365. When does the lower valve 
 
 close ? and why ? 
 
 366. When is the upper valve 
 
 opened? 
 
 367. Why does it close ? and when? 
 
 368. Are both valves ever open at 
 
 the same time? 
 Closed at the same time ? 
 
 369. W r hy not ? 
 
 370. When the piston rises, why 
 
 does which valve open? 
 
 PAGE 78. 
 
 371. When the piston is at its low- 
 
 est which valve is open ? 
 
 372. Why does not the water flow 
 
 back from the tube ? 
 
 373. Explain the action of the forc- 
 
 ing pump. 
 
 374. Why is it that, by means of 
 
 this pump, water may be 
 raised higher than by the 
 other ? 
 PAGE 79. 
 
 375. Give the parts of the Fire- 
 
 Engine. 
 
 376. Why does it not have com- 
 
 mon pumps ? 
 
 377. Explain its action. 
 PAGE 8O. 
 
 378. What causes the flow? 
 
 379. What makes the flow continu- 
 
 ous? 
 
 380. Give the difference between a 
 
 Heron's fountain and an air- 
 chamber ? 
 
 381. W 7 hich of the two would work 
 
 better in a vacuum ? 
 
 382. How long will either of the 
 
 two "run"? 
 
 LESSON XXIIL SOUND. 
 
 PAGES 85 AND 119. 
 
 383. What causes sound ? 
 
 384. What is sound? 
 
 385. What makes us hear the crack 
 
 of a whip ? 
 
 386. Would we hear it if there were 
 
 no air ? 
 
 387. Show that sound is the effect 
 
 of a vibratoty motion. 
 PAGES 86 AND 120. 
 
 388. What are sound-waves? 
 
 PAGES 86 AND 120. 
 
 389. Do we hear all sound-waves ? 
 
 390. Give velocity of sound. 
 
 391. What causes the noise when 
 
 paper is torn? (Text.) 
 
 392. When wood is broken ? 
 
 393. When a whip is cracked ? 
 
 394. Why do we not hear the alarm 
 
 of a clock in an exhausted 
 receiver (in a vacuum) ? 
 
164 
 
 FIRST LESSONS IN PHYSICS. 
 
 395. Why is music heard mOre dis- 
 
 tinctly when near than at a 
 great distance ? 
 
 396. Why do some bodies give a 
 
 louder sourd than others? 
 (Because they have a different 
 degree of elasticity.) 
 
 397. Why r, the ax of a wood-chop- 
 
 per, at a distance, seen to 
 fall before the blow is heard? 
 
 398. Why may distant cannon- 
 
 thunder, be heard better by 
 
 putting the ear on the 
 ground ? 
 
 399. Why do deaf persons not hear? 
 
 400. Why are the bells of a neigh- 
 
 boring place heard ringing 
 at times, and not at other 
 times ? 
 
 401. Why is it so quiet on the 
 
 mountains ? 
 
 402. Why need we not speak so 
 
 loud on a calm lake as on 
 land? 
 
 LESSON XXIV. EVAPORATION, FOG, CLOUDS, RAIN, 
 SNOW, HAIL, &c., &c. 
 
 PAGE 88. 
 
 403. Define evaporate. 
 
 404. When does evaporation take 
 
 place ? 
 
 405. What change does it effect? 
 
 406. Why is the breath visible in 
 
 winter? 
 
 407. Is all aqueous vapor visible? 
 PAGE 89. 
 
 408. When is it visible ? 
 
 409. What name has it then ? 
 
 410. Under what circumstances 
 
 does it become visible higher 
 up in the atmosphere? 
 
 411. What name has it then? 
 
 412. Why do clouds stay in the 
 
 air? 
 
 413. Why do soap-bubbles? 
 
 414. Why is it that the higher up 
 
 the clouds, the greater the 
 rain-drops ? 
 
 415. What is rain? 
 PAGE 90. 
 
 416. What is snow? 
 
 417. What is strange about hail? 
 
 418. Whence the drops on the out- 
 
 side of a tumbler with cold 
 water, in summer ? 
 
 419. Why do iron safes "sweat "? 
 
 420. Whence the moisture on a 
 
 window-pane when a person 
 breathes against it? 
 
 PAGE 90. 
 
 421. When is aqueous vapor con* 
 
 densed ? 
 
 422. What is meant by condense? 
 PAGE 91. 
 
 423. What is dew ? 
 
 424. Why is there no dew in cloudy 
 
 nights ? 
 
 425. Why none sometimes, al- 
 
 though the sky is serene? 
 
 426. What is frost? 
 
 427. Why do we not have frost in 
 
 summer ? 
 
 428. Why does it rain in moun- 
 
 tainous countries more than 
 on low land? 
 
 429. Has the direction of the water- 
 
 sheds anything to do with 
 the quantity of rain? 
 
 430. When does it ram more, in 
 
 daytime or at night ? 
 
 431. Give your reasons. 
 
 432. Why does it not rain every 
 
 cold night? 
 
 433. Is snow useful? Why? 
 
 434. Upon what does the solid 
 
 state of water, its liquid 
 state and its gaseous state 
 depend ? 
 
 435. Will it do to compare the at- 
 
 mosphere to a boiler ? 
 
 436. Give your reasons. 
 
QUESTION'S. 
 
 165 
 
 LESSON XXV. HEAT CONDUCTION OF HEAT. 
 
 PAGE 92. 
 
 437. Whence the sparks which we 
 
 see when flint and steel are 
 struck together? 
 
 438. When a horse is galloping? 
 
 439. What effect is produced by 
 
 rubbing a copper coin on 
 the floor? 
 
 440. W T hy does not a match ignite 
 
 by being rubbed against 
 glass ? 
 
 441. Why does it ignite on a brick ? 
 
 442. Why has he his hands blis- 
 
 tered who lets himself down 
 along a rope ? 
 
 443. Why does a saw feel warm 
 
 after use? 
 PAGE 93. 
 /I/14- How is heat produced? 
 
 445. What may motion be con- 
 
 verted into? (Were you to 
 ask " Into what may motion 
 be converted?" the pupil 
 would be inclined to answer 
 merely a word or two. ) 
 
 446. Why do the hands get warm 
 
 on holding them to a heated 
 stove ? 
 
 447. What sensation is felt on keep- 
 
 ing the end of a wire in a 
 flame? Why? 
 
 448. What is conduction of heat ? 
 
 449. Why is it that that wire (Ques- 
 
 tion 447) may be held longer, 
 if the end in the hand is en- 
 veloped in paper ? 
 PAGE 94. 
 
 450. Why have teapots and solder- 
 
 ing - irons usually wooden 
 handles ? (Text. ) 
 
 451. What class of bodies are good 
 
 conductors of heat? 
 
 452. What is a good conductor of 
 
 heat ? 
 
 453. What is a non-conductor (or 
 
 bad conductor) of heat? 
 
 454. Mention six non-conductors 
 
 of heat? 
 
 PAGE 94. 
 
 455. When a wire and a piece of 
 
 paper that have been lying 
 on a heated stove, are touch- 
 ed, the wire feels the warm- 
 er. Why? 
 
 456. Why do iron stoves heat well ? 
 
 457- Why may ice be kept as well 
 in a feather bed as in an ice- 
 chest ? 
 
 458. Why do mittens keep the 
 
 hands warmer than gloves 
 with fingers ? 
 
 459. Why does snow melt more 
 
 readily on a plank than on 
 a rock ? 
 
 460. Why are steam -chests and 
 
 steam-cylinders often cover- 
 ed with wood ? 
 
 461. Why are the walls of safes 
 
 often filled with fine ashes ? 
 
 462. Why do wide garments keep 
 
 us warmer than tight ones ? 
 
 463. Why are frame houses warm- 
 
 er than stone ones ? 
 PAGE 95. 
 
 464. Whence the use of good con- 
 
 ductors of heat ? 
 
 465. Why are metallic vessels used 
 
 for boiling water and other 
 liquids ? 
 
 466. Whence the use of bad con- 
 
 ductors of heat? 
 
 467. Give their effect upon warm 
 
 and cold bodies ? 
 
 468. How should a tumbler be 
 
 heated? Why? 
 
 469. Why is less heat given out by 
 
 a stove when its inner sur- 
 face is covered with soot ? 
 
 470. What advantage in a long 
 
 stove pipe ? 
 
 471. Why is a glowing coal rapidly 
 
 extinguished if placed on 
 iron? 
 
 472. Why do double windows keep 
 
 the room warm ? 
 
166 
 
 FIRST LESSONS IN PHYSICS 
 
 473. Why does cold wind chill us 477. Give reason for your state- 
 
 all through ? ment ? 
 
 474. Why does fruit ripen quicker 4;8> m does drawi the cur . 
 
 against a dark waif than t / ns down ^ a room 
 
 when isolated ?^ warmer? 
 
 475- What advantage in air being , TT1 , 
 
 a bad conductor? 479- Why does snow protect the 
 
 476. Does fanning us make the air ground from freezing ? 
 
 around us cool? 
 
 LESSON XXVI. DRAUGHT. 
 
 PAGE 96. 
 
 480. Why will paper strips held 
 
 over a heated stove, move 
 upward ? 
 
 481. Why will they rise if let go? 
 
 482. What is the universal effect of 
 
 heat upon bodies ? 
 
 483. Why does heated air rise? 
 
 484. Explain the revolving of a 
 
 spiral paper owing to heat. 
 
 485. Prove that the air is warmer 
 
 near the ceiling. 
 
 486. Why do balloons, smoke and 
 
 steam rise? (Text.) 
 PAGE 97. 
 
 487. When is a flame, held in the 
 
 upper opening of a room, 
 blown from the room ? 
 
 488. How about a flame held in the 
 
 lower opening ? 
 
 489. Give reasons for your state- 
 
 ments. 
 
 PAGE 97. 
 
 490. What is draught ? 
 
 491. What is the cause of draught? 
 
 492. Why is a lamp extinguished 
 
 if the draught is stopped 
 below ? 
 
 493. Why, if its chimney is closed 
 
 above ? 
 
 494. Compare this with Experiment 
 
 26, p. 43. 
 
 495. Show that heated air rises, and 
 
 that colder flows toward the 
 source of heat. 
 596. What is the cause of winds ? 
 
 497. How long does wind last? 
 
 498. What is ventilation ? 
 
 499. Is it sufficient for the ventila- 
 
 tion of a room to simply 
 admit fresh air ? 
 
 500. Prove your statement by a 
 
 previous experiment. 
 
 LESSON XXVII. EXPANSION BY HEAT-THERMOMETER. 
 
 PAGE 99. 
 
 501. Why does boiling water often 
 
 run over ? 
 
 502. Why does a cold tumbler crack 
 
 if placed on a heated surface? 
 
 503. How may the cracking be 
 
 prevented ? 
 
 504. What is the effect of heat upon 
 
 all bodies ? 
 
 505. Why are rails placed on the 
 
 track with space between? 
 
 506. How are tires placed on 
 
 wheels? 
 
 507. Why does pop-corn pop ? 
 
 PAGE 100. 
 
 508. When is a substance said to 
 
 cool? JDefine temperature. 
 
 509. Give the parts of the ther- 
 
 mometer. 
 
 510. Why the vacuum? 
 
 511. Why could not the glass tube 
 
 be open above ? 
 
 512. Can you heat the vacuum? 
 
 and what will be the effect 
 upon -the mercury ? 
 
 513. Why does the mercury ex- 
 
 pand from heat ? 
 
 514. Why does the mercury rise? 
 Why does it fall ? 
 
QUESTIONS. 
 
 167 
 
 PAGE 101 
 
 515. How are thermometers made ? 
 
 516. How are the freezing and 
 
 boiling points obtained? 
 
 517. What advantage in dividing 
 
 the space between those two 
 points into degrees? 
 
 PAGE 100. 
 
 518. Why are the plates of metallic 
 
 roofing not nailed together ? 
 
 519. When, and why, will hot water 
 
 crack a cold tumbler ? 
 
 520. What advantage in thermom- 
 
 eters? 
 
 LESSON XXVIII. THERMOMETER COMPARED WITH 
 BAROMETER. 
 
 522 
 523 
 
 PAGE 102. 
 
 521. How is the blood-heat point of 
 the thermometer obtained? 
 
 How is it marked? 
 
 How did Fahrenheit divide 
 the space between the freez- 
 ing and boiling points? 
 
 524. Where did he not commence? 
 
 525. Where did he commence? 
 PAGE 103. 
 
 526. How did Reaumur divide that 
 
 space ? 
 
 527. How, Celsius? 
 
 528. W T hat are the equivalents of 
 
 80 C? 
 
 529. What of 50 C? 
 
 530. What of 77 F? 
 
 531. What of 32 F? 
 
 532. What of 17 7-9 C? 
 
 533. What of 40 C ? 
 
 PAGE 103. 
 
 534. What is the healthiest tem- 
 perature of a room ? 
 
 535- Where should thermometers 
 be placed? 
 
 536. If in New York the mercury 
 
 stands 85, how would it 
 stand in Paris (according to 
 C. )? (Text.) 
 
 537. Ho win Berlin (C. )? (Text.) 
 
 538. According to those scales, 
 
 what numbers would indi- 
 cate the blood-heat point? 
 (Text.) 
 
 539. Indicate the point of healthiest 
 
 temperature in Centigrade 
 degrees. (Text.) 
 
 PAGE 104. 
 
 540. Give four points which the 
 
 thermometer and barometer 
 have in common. 
 
 541. Give four points they differ in. 
 
 LESSON XXIX. ATMOSPHERIC ENGINE. 
 
 PAGE 105. 
 
 542. How is a sewing-machine 
 
 made to work? 
 
 543. What is rectilinear motion ? 
 
 544. Circular motion? 
 
 545. Give two instances of each ? 
 
 546. Who was Papin, and why is 
 
 he celebrated? 
 
 PAGE 1 06. 
 
 547. Mention five diff. kinds of work 
 
 done by the steam-engine. 
 PAGE 107. 
 
 548. Describe Papin's apparatus. 
 
 549. Describe the experiment with 
 
 the same. 
 
 550. What causes the piston to 
 
 rise ? 
 
 PAGE 105. 
 
 551. What to sink? 
 
 PAGE 1 08. 
 
 552. What is meant by an atmos- 
 
 pheric steam-engine ? 
 
 553. Describe Savery's apparatus. 
 
 554. Compare it with Papin's. 
 
 555. Give Newcomen's improve- 
 
 ments on Papin's apparatus. 
 
 PAGE III. 
 
 556. Explain Newcomen's atmos- 
 
 pheric steam-engine. 
 
 557. What causes the piston in it 
 
 to rise? Explain. 
 
 558. What causes it to sink ? Ex- 
 
 plain. 
 
168 
 
 FIRST LEGSONS IN PHYSICS. 
 
 PAGE III. 
 
 559. State the principal points of 
 
 Papin's engine. (Text.) 
 
 560. Of Savery's. (Text.) 
 
 561. Of Newcomen's. (Text.) 
 
 PAGE III. 
 
 562. Compare Savery's apparatus 
 
 with Newcomen's engine. 
 
 563. Compare Fapin's with New- 
 
 comen's. 
 
 LESSON XXX. STEAM-ENGINE. 
 
 PAGE 112. 
 
 564. Who was Watt? 
 
 565. Whence his familiarity with 
 
 the defects of Newcomen's 
 engine ? 
 
 566. What was its principal defect ? 
 
 567. What was the cause of this 
 
 defect? 
 PAGE 113. 
 
 568. How great a loss was caused 
 
 thereby ? 
 
 569. What question arose? 
 
 570. What was Watt's first im 
 
 provement ? 
 
 571. Explain. 
 
 572. What did Watt's next im- 
 
 provement consist in ? 
 
 573. What defect did it overcome? 
 
 574. What caused now the piston 
 
 to rise and sink ? 
 
 PAGE 114. 
 
 575. Did henceforth the steam 
 
 merely serve to produce a 
 vacuum ? 
 
 576. Why was Newcomen's ma- 
 
 chine a single-acting engine? 
 
 577. Why was it used only for 
 
 pumping water ? 
 
 578. What constitutes a double- 
 
 acting steam-engine ? 
 
 PAGE 115. 
 
 579. When did Watt die? 
 
 580. What is meant by steam of low 
 
 pressure ? of high pressure ? 
 
 581. What are high and low press- 
 
 ure engines ? 
 
 582. What is the use of the sliding 
 
 valve ? 
 PAPE 1 1 6. 
 
 583. Explain action of high-press- 
 
 ure engine. 
 
 LESSON XXXII. LIGHT ITS SOURCES DIRECTION. 
 
 PAGE 122. 
 
 584. What are our sources of light? 
 
 585. Mention s i x self-luminous 
 
 bodies. 
 
 586. What is a self-luminous body? 
 
 (One which makes its own 
 light.) 
 
 PAGE 123. 
 
 587. Are the planets self-luminous? 
 Are they luminous ? 
 
 588. What, then, is the difference 
 
 between luminous and self- 
 luminous ? 
 
 PAGE 123 
 
 589. When, only, are bodies not 
 
 self-luminous, visible ? 
 
 590. Show that light emanates from 
 
 self-luminous bodies in alt 
 directions. 
 
 PAGE 124. 
 
 591. Show that it travel? in straight 
 
 lines. 
 Why have opera-glasses 
 
 592. 
 
 straight tubes ? 
 
 LESSON XXXII I. -RADIANT AND SPECULAR REFLECTION. 
 
 PAGE 125. 
 
 593. What makes our rooms light 
 
 in the daytime ? 
 
 594. How do all objects reflect 
 
 light ? 
 
 595. What enables us to see a pencil? 
 
 596. A looking-glass ? 
 
 PAGE 126. 
 
 597. What is radiant reflection of 
 
 light ? 
 
 598. Show that there are objects 
 
 which reflect light also in 
 certain directions. 
 
QUESTIONS. 
 
 PAGE 126. 
 
 599. What is this kind of reflection 
 
 called? 
 
 PAGE 127. 
 
 600. What class of objects reflect 
 
 light, both, radiantly and 
 specularly ? 
 
 PAGE 127. 
 
 60 1. Compare light reflected radi- 
 antly with light reflected 
 specularly by 
 
 (a.) Giving four points in com- 
 mon. 
 
 (.) Three points of difference. 
 
 LESSON XXXIV. VISIBLE DIRECTION REFRACTION. 
 
 PAGE 128. 
 
 602. Whither does a person hit 
 
 with a stone, look? 
 
 603. Show that a boy looking at a 
 
 steeple does somet'ng simiPr. 
 PAGE 129. 
 
 604. On page 128 the two images 
 
 have opposite directions ; 
 why does the person, never- 
 theless, see the object in 
 only one direction ? 
 
 605. Give general statement of the 
 
 case. (All bodies appear to 
 be situated &c., &c.) 
 
 PAGE 129. 
 
 606. Why do oars, when immersed 
 
 obliquely, appear bent ? 
 PAGE 130. 
 
 607. Why does the eye see the oar 
 
 bent? 
 
 608. When is a coin on the bot- 
 
 tom of a filled tumbler not 
 seen in its true place and 
 direction ? 
 PAGE 131. 
 
 609. Why do clear waters appear 
 
 more shallow than they are? 
 
 610. What is refraction of light? 
 
 LESSON XXXV. PRISMS LENSES. 
 
 PAGE 132. 
 
 611. Show the passage of rays (of 
 
 an arrow) through a prism 
 with edge upward 
 PAGE 133. 
 
 612. With the edge downward. 
 
 613. What is a prism ? 
 
 614. Show the path of rays of an 
 
 arrow through" two prisms 
 with their bases adjacent (as 
 on page 133). 
 
 615. Why the use of a curved glass 
 
 in place of the prisms ? 
 PAGE 134. 
 
 6 1 6. What two names are given to 
 
 this? 
 
 PAGE 134. 
 
 617. What is the Focus ? 
 
 618. Why the term Burning-glass ? 
 
 619. What effect has such a lens 
 
 upon objects ? 
 
 620. Is your answer true in every 
 
 sense of the word ? 
 PAGE 135. 
 
 621. Give the general statement 
 
 true of such a lens. 
 
 622. Does a telescope really mag- 
 
 nify distant objects ? 
 
 623. Upon what, then, is its use 
 
 based ? 
 
 LESSON XXXVI. COLOR. 
 
 PAGE 136. 
 
 624. What is dispersion of light ? 
 
 6*5. How can it be shown ? 
 
 ^6. What effect has it upon white 
 
 light ? 
 627. Give the principal colors of 
 
 the rainbow. 
 
 PAGE 136. 
 
 628. What is the whole series of 
 
 colors called ? 
 PAGE 137.- - 
 
 629. How can you prove that ordi- 
 
 nary sunlight contains these 
 colors ? 
 
170 
 
 FIRST LESSONS IN PHYSICS. 
 
 PAGE 137. 
 
 '630. Is color a substance ? 
 
 PAGE 138. 
 
 631. Why does white glass look 
 
 white ? 
 632. Why does blue glass look 
 
 blue? 
 
 633. Why do objects near a blue 
 
 curtain have a blueish tinge? 
 PAGE 139. 
 
 634. When is a body said to be 
 
 colored ? 
 ^635. When white ? 
 
 PAGE 139. 
 
 636. When black? 
 
 637. What causes a piece of red 
 
 cloth to appear red ? (Text.) 
 
 638. What, a sheet of paper to ap- 
 
 pear white? (Text.) 
 
 639. A black coat to appear black? 
 
 (Text.) 
 
 640. Why is everything black on 
 
 a dark night? (Text.) 
 
 641. Is color a quality inherent in 
 
 bodies ? 
 
 642. Is it a property of a body ? 
 
 LESSON XXXVIL CHEMICAL ELECTRICITY. 
 
 PAGE 141. 
 
 '643. What constitutes a galvanic 
 element, or cell? 
 
 644. What is a galvanic battery ? 
 
 645. How is a coke-cell prepared ? 
 646. Why must the cup used be 
 
 un glazed ? 
 (The two liquids pass each 
 
 other through its pores.) 
 647. Why must the wire ends be 
 
 entirely clean ! 
 (The electric current does not 
 
 leap over. ) 
 
 PAGE 141. 
 
 648. Whence the thrilling sensation 
 
 if the current passes through 
 the tongue ? 
 PAGE 142. 
 
 649. Explain the action of such a 
 
 galvanic cell. 
 
 650. How is chemical or galvanic 
 
 electricity produced ? 
 
 651. Whence the rwa\t galvanic? 
 
 652. Explain the uninterrupted cur- 
 
 rent of electricity 
 
 653. Is the length of the wires of 
 
 importance ? 
 
 LESSON XXXVIII. THE ELECTRO-MAGNETIC TELE- 
 GRAPH. 
 
 TAGE 144. 
 
 654. What is next to the steam- 
 engine the wonder of our 
 age, and why ? 
 
 '655. Who discovered the effect of 
 the galvanic current upon 
 iron? 
 
 656. Who put up the first telegr'ph? 
 PAGE 145. 
 
 657. Who perfected the telegraph? 
 
 658. How may the principle of 
 
 Morse's telegraph be illus- 
 trated ? 
 PAGE 146. 
 
 659. What renders the horse-shoe 
 
 rod magnetic? 
 
 >66o. Describe the path of the elec- 
 tric current of the cell, when 
 passing around the rod. 
 
 PAGE 147. 
 
 661. What effect has the interrup- 
 
 tion of the current upon the 
 rod? 
 
 662. What is an Electro- Magnet 
 
 663. What is the keeper ? 
 
 PAGE 148. 
 
 664. What are the principles of the 
 
 re the principl 
 ic telegraph ? 
 
 electric 
 
 665. Describe the apparatus by 
 
 which they may be demon- 
 strated. 
 
 666. Compare the Magnet with the 
 
 Electro-Magnet, giving 
 (a.) Five points in common. 
 (.) Two points of difference. 
 
APPEN DIX. 
 
 I. REMARKS. 
 
 LESSON III. To preserve their magnetism, the poles of magnets 
 should constantly be kept in contact with iron. 
 
 LESSON IV. Bar-sulphur, or a solid glass rod, is often preferred to a 
 lamp-chimney. For negative electricity, use "hard" rubber (gutta percha). 
 
 LESSON V. May be used as a reading lesson, or as one in which the 
 unfinished part of any previous lesson can be brought up. 
 
 LESSON VII. Time is gained if the bullets are flattened with a 
 hammer ; then scrape one of the flattened surfaces of each with a knife ; 
 press the two surfaces together by a few strokes of a hammer. 
 
 The glass plates, and tubes, must be free from grease. 
 
 The pores of a body may be compared to the open ends of capillary 
 tubes ; the capillary tubes may be compared to glass tubes of very fine 
 bore. 
 
 LESSON IX. Instead of an ivory ball, which is expensive, a large 
 marble gives the same result. The round spot may be shown also on 
 the slab. 
 
 LESSON X. A piece of India-rubber tubing around the tube of the 
 funnel will do as well as sealing-wax, or any other cement. Any kind 
 of tube, about ^ inch in diameter and about 3 feet long, may serve as a 
 blow-gun. 
 
 LESSON XL A tumbler with a brim curved outward is best. 
 
 LESSON XVI. A square wooden beam about 20 inches long for a 
 lever, with the sharp edge of a ruler, paper knife or knife-blade as a 
 fulcrum. The fulcrum must be firmly fixed. 
 
 LESSON XVIII. The bees-wax must be put on very thin, or else 
 the water cannot force its way through. Draw Fig. 1 7 on the board. 
 The small triangular valve, when lowered, is just large enough to close 
 the right hand side of the short horizontal tube. 
 
 LESSONS XX AND XXI. Draw the Fig. on the board. 
 
 LESSON XXV. Copper being a better conductor than iron, copper 
 wire is preferable. 
 
 LESSONS XXIX AND XXX. Draw the Fig. on the board. 
 
 LESSON XXXV EXPERIMENT 65. Draw an arrow on the board, 
 place the prism in proper position, and sufficiently elevated for each 
 scholar to see the arrow through the prism. As this requires but a few 
 seconds, it may be found convenient to let the class slowly file past the 
 prism. 
 
172 FIRST LESSONS IN PHYSICS. 
 
 II. GLASS AND CORK WORKING. 
 
 The following is taken nearly literally from Vernon Harcourt's excel- 
 lent work, "Exercises in Practical Chemistry, Clarendon Press, 
 Oxford:" . 
 
 1. To CUT A GLASS TUBE. Take glass tubing about f-inch external 
 diameter ("hard glass " is preferable). For a Hero's fountain (Lesson 
 X, Experiment 27, and also frontispiece), it should reach from within half 
 an inch of the bottom of the flask to about eight inches above the cork. 
 As mercantile tubing is much longer, a piece of that length should be 
 cut off. To do this, lay the tube on a table, hold it between the thumb 
 and forefinger of the left hand, placed close to where it is to be cut. 
 Take a triangular file, press your left thumb and forefinger firmly against 
 the tube, put the edge of the file upon the tube so as to touch and lean 
 against the thumb, which will thus prevent the file from slipping over 
 the glass, and make a notch on the glass by a few short, energetic 
 strokes in a forward direction. While in the act of cutting, do not bear 
 down too heavily with the left hand; rather have your left thumb yield 
 a little as the file passes forward, so that the tube may turn a little in 
 the direction of the advancing file. To guard against injury, in case the 
 tube should yield, put on a glove. Now take up the tube, holding it so 
 that the thumb-nails are opposite to each other, with the notch between 
 them, and that you tightly press the tube (where the notch is) with 
 thumb-nail and forefinger of each hand, and with a resolute grasp break 
 the tube (moving your hands in a direction from you) as you would 
 break a stick. The edges of the new end will be sharp and rugged; to 
 prevent their tearing the cork, pass the file lightly over them; then hold 
 them a few seconds in the tip of an alcohol (or gas) flame. 
 
 For a Hero's fountain like the one in the frontispiece a more con- 
 venient form than that in Lesson X you should have a bent glass tube. 
 
 2. To BEND A GLASS TUBE. If the external diameter of the tube 
 does not exceed half an inch, a common gas flame is very suitable; but 
 if the gas is not at hand, a spirit lamp with a large flame may be used. 
 Light the gas, or spirit lamp ; then holding the piece of tube by its ex- 
 tremities, bring it a little above the flame, turning it constantly around 
 and moving it laterally so as to heat about two inches of it equally on 
 both sides. After a few seconds lower it gradually into the flame, still 
 constantly turning it round. 
 
 If the gas burner be used, the glass will become covered with soot 
 when immersed in the flame ; but this is of no consequence, as the heat 
 of such a burner is never high enough to incorporate the carbon with 
 the glass. When the heated portion becomes soft and yielding, which 
 will take place even before it has acquired a visible red heat, withdraw 
 it from the flame, and gently bend it to a right angle, avoiding the use 
 of much force. When the proper bend is completed, lay the tube on a 
 bit of glass in such a position that the heated portion does not come into 
 contact with any cold surface, and leave it to cool slowly. 
 
 3. To MAKE A GLASS JET. Take the straight tube, previously ob- 
 tained; heat the tube two inches from one of its ends by holding it to 
 
GLASS AND CORK WORKING 173 
 
 the extent oi half an inch in the upper part of a flame. The thumb and 
 forefinger of each hand should hold the glass about an inch from the 
 heated part. The heated part will soon become soft and a little narrower. 
 Then withdraw it from the flame, and draw the heated part out by pul- 
 ling the two ends of the glass apart. But pull very gently or else the 
 tube will be drawn out too thin; the jet should have about ^-inch 
 external diameter. Gently place the whole on the table before you and 
 allow it to cool ; then make a fine notch at the middle of the drawn-out 
 part, and break the tube there. The long part is the jet for a Hero's 
 fountain. If the aperture is too wide, hold it for a second or two in the 
 flame. 
 
 4. To PERFORATE A CORK. It now remains to fit these tubes the 
 bent tube and the jet to the bottle by means of a cork having two holes. 
 Take a good, sound cork, about an inch in diameter, squeeze it until it 
 becomes soft and elastic (a pair of pliers or nut-crackers will serve the 
 purpose of a regular cork-squeezer), then take it up between the second 
 finger and the thumb of the left hand, and place the sharpened end of 
 the smallest cork-borer against it, one end of the cork midway between 
 the center and the edge. Urge the cork-borer into the cork with a twist- 
 ing motion, as if you were using a cork screw. Some care will be re- 
 quired to make the hole straight through the cork, so that it may be 
 truly central. Of the proper direction the eye will be the best judge. 
 And when the cork-borer has penetrated some little way, it will be advis- 
 able to turn the cork a quarter round in order that it may be seen 
 whether the axis of the cork-borer and of the cork are still in the same 
 straight line. If not, a slight pressure on the cork-borer in one direc- 
 tion or the other will set it straight. When the borer has penetrated 
 quite through the cork, it may be withdrawn with a twitching motion, 
 and will bring with it a cylindrical plug of cork, leaving a hole, the sides 
 of which should be smoothed with a round file. In the same manner 
 make the other hole midway between the center and the circumference. 
 Take a cork-borer rather smaller than the tubing which you have; see 
 that the holes do not run into each other, or pierce the side of the cork. 
 The holes should next be smoothed and slightly enlarged by a rat-tail 
 file, until the end of one of the tubes will just enter them when some 
 little pressure is used. (If much pressure is used, the tube is not un- 
 likely to break, and the splinters of glass may cause injury. The hole 
 should never be so much smaller than the tube as to make it necessary 
 to use much force in passing the latter through it. It is a good plan, 
 also, to wrap the tube in a cloth or handkerchief while it is being inserted 
 in the cork. ) Now pass the longer of the two tubes through the cork, 
 with moderate pressure and a twisting motion, until it projects so far as 
 to reach, when the cork is fitted into its place, nearly to the bottom of 
 the bottle. When this is done pass the other tube through the other 
 hole in the cork, until it projects one or two inches on the other side. 
 
174 FIKST LESSONS IN PHYSICS. 
 
 III. PROBLEMS ON THE THERMOMETER. 
 F. C. 
 
 212. 
 
 32- 
 
 100 Boiling Ft. Tne questions on page 103 need a few 
 
 explanations. Only the F and C scales 
 require problems, they being the most 
 important. To illustrate the problems be- 
 low, let the two vertical lines annexed 
 Freez'g Pt. represent the two thermometers. 
 
 Problem, i. The mercury stands at 86 F, how will it stand accord- 
 ing to degrees C ? 
 
 Solution : 86 F = 86 32 54 F above freezing-point. Now, 
 since 180 F above freezing-point = 100 C, we have 9 F - 5 C, 
 or i F = 5 C; and 54 = 54 X f = 3 C = answer. 
 
 Rule I. To convert any number of degrees F above freezing-point 
 into degrees C, first subtract 32 from that number, then multiply the re- 
 mainder by 5-9. 
 
 Problem 2. Mercury at 25 C, how must this be read in degrees F ? 
 
 Solution : 100 C = equals 180 F above freezing-point, or 5 C 
 = 9? F ; hence i C = | F, and 25 C = 25 X f 45 F above 
 freezing-point. Now, 45 F above freezing-point means 45 above 32 9 
 F, so we must add 45 to 32, which is 77 F, answer. 
 
 Rule II. To convert any number of degrees , above the freezing-point 
 into degrees F, multiply the number by |, and then add 32. 
 
 Problem. 3. Thermometer at 23 F, how is this read in degrees C? 
 
 Solution : Consulting the two thermometers represented above, we 
 find that 23 F means 32 23 = 9 F below the freezing point. The 
 problem now is : Resolve 9 F below freezing-point into its equivalent 
 degrees C. Hence we have 9 X | ^ below freezing-point = 5 
 C, answer. 
 
 Problem. 4. Thermometer at 13 F, required its standing in de- 
 grees C. 
 
 Solution: By inspection we find that the point of the F scale is 32 
 below the freezing-point, and that 13 is 13 below the point, that is, 
 32 -f 13 = 45 F. the actual number of degrees F. below the freezing- 
 point. Now, 45^ F below freezing-point X = 2 5 Q c below freez- 
 ing-point, or 25^ C, answer. 
 
 Rule III. To convert any given number of degrees F below freezing- 
 point into C, ascertain the difference between 32 F and the number 
 given, and multiply this difference by |, the product will be C. 
 
 Problem. 5. Thermometer at 30 C ; give the same in degrees F. 
 
 Solution: 30 C means below the freezing point ; hence 30 C X 
 9 = 54 F below freezing-point. By inspection it will be found that 
 we must subtract 54 Q from 32 ; or, 32 54 = 22 Q F, answer. 
 
 Rule IV. To convert degrees C below the freezing-point into degrees 
 F, multiply the given number of C^ by J ; subtract the product from 
 32, and the remainder = answer. If the subtrahend is greater than 
 the minuend, as in the problem above, the difference between the product 
 and 32* is negative, that is, below 0? F, and consequently marked F 
 
INDEX. 
 
 PAGE. 
 
 Academy of Florence 31 
 
 Adhesion 32 
 
 Attraction, Capillary 35 
 
 Attraction, Electric 2C 
 
 Attraction, Magnetic 17 
 
 Balance 13 
 
 Barometer 49 
 
 Barometer comp. with Pump. 80 
 Barometer compared -with 
 
 Thermometer 104 
 
 Bellows 72 
 
 Blotting-paper 38 
 
 Blow-pipe 43 
 
 Breathing 71 
 
 Burning-glass 134 
 
 Cell, Galvanic 141 
 
 Clock Weights 13 
 
 Clocks....:.. 65 
 
 Clouds 89 
 
 Cohesion 29 
 
 Color 136 
 
 Compass 38 
 
 Communicating Vessels 87 
 
 Condenser 113 
 
 Conductors of Electricity 22 
 
 Conductors of Heat 94 
 
 Contraction by Cold 100 
 
 Conversion of Force, Motion, 121 
 
 Contents, Table of 7 
 
 Current, Electric 142 
 
 Dew gi 
 
 Direction, Visible 128 
 
 Diving-bell 44 
 
 Draught ....... 96 
 
 Drowning ^ 16 
 
 Ductile... 
 
 41 
 
 Ductility 29 
 
 PAGE. 
 
 Elasticity . 39. 
 
 Elasticity, Application of 41 
 
 Elasticity of Air 42 
 
 Electricity, Pos. and N eg 25 
 
 Electricity, Chemical 140 
 
 Electric Attraction 20 
 
 Electric Repulsion 23 
 
 Electro- Magnet 150- 
 
 Element, Galvanic 141 
 
 Evaporation 88 
 
 Exhalation 71 
 
 Expansion by Heat 99 
 
 Fire-Engine 7& 
 
 Fly-Wheels 55 
 
 Fog 89 
 
 Force, into Motion 84 
 
 Franklin's Experiment 26 
 
 Frost 91 
 
 Fulcrum 6l 
 
 Glass for Electric Purposes. . . 24. 
 
 Glass for Prism 136- 
 
 Gravity, Direction of 1 1 
 
 Gravity, Force of 9' 
 
 Gravity, Specific 14 
 
 Hail 90 
 
 Heat. 9 2 
 
 Heat, Conduction of. 93 
 
 Heron's Fountain 44 
 
 High Pressure 1*5 
 
 Horizontal *3 
 
 Hour-glass *3 
 
 Hydraulic Press 67 
 
 Impenetrability 3* 
 
 Inclined Plane 5 & 
 
 Inertia 53 
 
 Inhalation 7 1 
 
INDEX. 
 
 PAGE. 
 
 Lenses 133 
 
 Level 12 
 
 Lever 59 
 
 Light, Direction 124 
 
 Light, Sources 122 
 
 Light, Radiant and Specular 
 
 Reflection 125 
 
 Light, Radiant and Specular 
 
 Reflection Compared 127 
 
 Lightning 26 
 
 Lightning- Rod 27 38 
 
 Locomotive 117 i *8 
 
 Low Pressure 115 
 
 Magnetic Attraction 17 
 
 Magnet 18 
 
 Magnet compared with Elec- 
 tro-Magnet 150 
 
 Malleable 41 
 
 Metals, Conductors of Heat.. 94 
 
 Morse's Telegraph 145 
 
 Needle, How rend. Magnetic. 19 
 
 Newcomen's Engine 108 
 
 NuJt-Cracker ^ 6" 
 
 Papin's Apparatus 105 
 
 Pendulum 63 
 
 Persons Drowning 16 
 
 Pith-balls, How made 22 
 
 Plumb-line 1 1 
 
 Poles of Magnets 19 
 
 Pop-gun 43 
 
 Pores . 31 
 
 Pressure of Air 46 50 
 
 Pressure, Downward 12 
 
 Prisms 132, 136, 137 
 
 PAGE. 
 
 Pump, Common 74 
 
 Pump, Forcing 77 
 
 Pull 84 
 
 Push.. 8/L 
 
 Radiation of Ligfrt 
 
 Rain 
 
 Reflection of Light 
 
 Refraction of Light 
 
 Refraction of Light, Law . . . 
 Repulsion, Electric 
 
 Self-luminous 
 
 Sliding-valve 
 
 Snow 
 
 Sound 
 
 Spark, Electric 
 
 Steam-Engine, Atmospheric. 
 Steam-Engine, Newcomen's. 
 
 Steam-Engine, Papin's 
 
 Steam-Engine, Savery's 
 
 Steam-Engine, Watt's , 
 
 125 
 89 
 
 125 
 129 
 
 131 
 
 23 
 
 123 
 
 "5 
 9 
 85 
 
 21 
 
 I0 5 
 108 
 
 112 
 
 Telegraph '. 144 
 
 Telegraph, Principle of, 147 
 
 Telegraph, Prin. Demonst'd. 148 
 
 Thermometer ..100, 102 
 
 Thermometer compared with 
 
 Barometer 104 
 
 Thermometer Problems 174 
 
 Vacuum - 49 
 
 Vertical 1 1 
 
 Visible Direction 128 
 
 Watt, James...* 112 
 
 Weight 12 
 
 Winds, Cause of 98 
 
 Work done by forces 83 
 
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 University of California 
 
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 M81995 
 
 THE UNIVERSITY OF CALIFORNIA LIBRARY