EDUCATION DEPT, 
 
 
7> 
 
POPULAR 
 
 PHYSICS 
 
 BY 
 
 J DORMAN STEELE, PH.D., F.G.S. 
 
 J II 
 
 AUTHOR OF FOURTEEN-WEEKS SERIES IN NATURAL SCIENCK 
 
 The works of God are fair for naught. 
 
 Unless our eyes, in seeing, 
 See hidden in the thing the thought 
 
 That animates its being. 
 
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 EDUCATION DEPT. 
 
 Copyright, 1888, by A. S. BARNES & Co. 
 Pop. Phys. 
 
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AUTHOR'S PREFACE. 
 
 work has grown up in the class-room. It 
 _!_ contains those definitions, illustrations, and ap- 
 plications which seemed at the time to interest and 
 instruct the author's pupils. "Whenever any explana- 
 tions fixed the attention of the learner, it was laid 
 aside for future use. Thus, by steady accretions, the 
 process has gone on until a book is the result. 
 
 As Physics is generally the first branch of Natural 
 Science pursued in schools, it is important that the 
 beginner should not be wearied by the abstractions 
 of the subject, and so lose interest in it at the very 
 start. The author has therefore endeavored to use 
 such simple language and practical illustrations as 
 will attract the learner, while he is at once led out 
 into real life. From the multitude of philosophical 
 principles, only those have been selected which are 
 essential to the information of every well-read person. 
 Withiri the limits of a small text-book, no subject 
 can be exhaustively treated. This is, however, of 
 less importance now, when every teacher feels that 
 he must of necessity be above and beyond any 
 school-work in the fullness of his information. The 
 object of an elementary work is not to advance the 
 peculiar ideas of any person, but simply to state the 
 currently-accepted facts and theories. The time-hon- 
 
 M187542 
 
Vl AUTHOR'S PKEFACE. 
 
 ored classifications recognized in all scientific works, 
 have been retained. In order to familiarize the 
 pupil with the metric system, now generally used 
 by scientific men, it is continually employed in the' 
 problems. The notes contain many illustrations and 
 additional suggestions, but their great value will ap- 
 pear in the descriptions of simple experiments which 
 are within the reach of any pupil. 
 
 New plates being required for this edition, the 
 author has taken the opportunity thoroughly to re- 
 vise the entire work. By carefully comparing the 
 criticisms of teachers, he has tried to obtain the 
 "parallax" of all its statements and methods, and 
 to eliminate, as far as possible, the errors growing 
 out of his "personal equation." Hearty thanks are 
 tendered to the many friends of the book who, by 
 their suggestions and criticisms, have so greatly 
 added to the value of this revision. To name them 
 all in this Preface would be impossible, and to dis- 
 criminate would be invidious. The author can not, 
 however, allow the opportunity to pass without ex- 
 pressing his profound sense of obligation. By untir- 
 ing study and the continued help of his friends, he 
 hopes thus, year by year, to make the series more 
 and more worthy the favor which his fellow-teachers 
 have so abundantly bestowed upon it. Happy indeed 
 will he be if he succeed in leading some young mind 
 to become a lover and an interpreter of Nature, and 
 thus come at last to see that Nature herself is but 
 a "thought of God." 
 
PUBLISHERS' PREFACE. 
 
 series of elementary text-books in science, 
 -L written by the late Dr. J. Dorman Steele, at- 
 tained an extraordinary degree of popularity, due to 
 the author's attractive style, his great skill in the 
 selection of material suited to the demands of the 
 schools for which the books were intended, his sym- 
 pathetic spirit toward both teachers and pupils, and 
 his earnest Christian character, which was exhibited 
 in all his writing. 
 
 Shortly before his death, finding his health too 
 feeble to permit of extra labor, the author requested 
 Dr. W. Le G. Stevens, Professor of Physics in the 
 Packer Collegiate Institute, Brooklyn, to revise the 
 text-book in Physics, as important advances in this 
 department of science had been made since the issue 
 of the edition of 1878. In performing this work, 
 Professor Stevens has endeavored to impose the least 
 possible modification upon the peculiar style of the 
 author. Nevertheless, every chapter has received 
 some alterations and slight enlargement. In a work 
 intended for higher classes, the reviser would natu- 
 rally make the treatment of every subject more thor- 
 ough ; but this would unfit the present book for the 
 
PUBL1J3HEKS' PREFACE. 
 
 schools to which it was originally adapted. It is 
 difficult, moreover, to combine a strictly popular style 
 with that precision which is demanded by advanced 
 students in exact science. But although the field in 
 a rudimentary text-book is limited, it is thought that 
 there are no important errors of statement in the 
 present hand-book. 
 
 In order to distinguish this revision from the 
 older editions, the name is changed to " Steele's Pop- 
 ular Physics." 
 
 NOTICE. 
 
 The publishers of this book will still issue the former edition, known as 
 "Steele's fourteen Weeks in Physics ," for classes already organized and for 
 teachers who may prefer to continue its use. Any book-seller can obtain the 
 former edition if requested to do so. 
 
SUGGESTIONS TO TEACHERS. 
 
 OTUDENTS are expected to obtain information from this 
 vO book, without the aid of questions, as they must always 
 do in their general reading. When the subject of a paragraph 
 is announced, the pupil should be prepared to tell all he knows 
 about it. He should never be allowed to answer a question, 
 except it be a short definition, in the language of the book. 
 The diagrams and illustrations, as far as possible, should be 
 drawn upon the blackboard and explained. Although pupils 
 may, at first, manifest an unwillingness to do this, yet in a 
 little time it will become an interesting feature of the recita- 
 tion. . In his own classes, the author has been accustomed to 
 place upon the blackboard the analysis of each chapter of the 
 book, and require the pupils to recite from that, without the 
 interposition of questions, except such as were necessary to 
 bring out the topic more clearly, or to throw a side light upon 
 it. Where the analysis" given in the book does not include all 
 the minor points of the lesson, the pupils can easily supply the 
 omission. The "Practical Questions" given at the close of 
 each general subject, have been found a profitable exercise in 
 awakening inquiry and stimulating thought. They may be 
 used at the pleasure of the instructor. The equations con- 
 tained in the text are designed to be employed in the solution 
 of the problems. 
 
 It should constantly be borne in mind that, as far as possible, 
 every question and principle should be submitted to Nature for 
 a direct answer by means of an experiment. Pupils should be 
 encouraged to try the simple illustrations necessary. The stu- 
 dent who brings in a bit of apparatus made by himself, does 
 better than if he were merely to memorize pages of text. 
 
X SUGGESTIONS TO TEACHERS. 
 
 The following works, to which the author acknowledges his 
 obligation for valuable material, will be useful to teacher as 
 well as pupil, in furnishing additional illustrations and in eluci- 
 dating difficult subjects, viz.: Tait's "Recent Advances on Phys- 
 ical Science"; Arnott's " Elements of Physics" (7th ed.); Stewart's 
 "Elementary Physics," also his "Conservation of Energy," and 
 "Treatise on Heat"; Atkinson's "Deschanel's Natural Philoso- 
 phy"; Lockyer's "Guillemin's Forces of Nature"; Herschel's 
 "Introduction to the Study of Physical Science"; Tomlinson's 
 "Introduction to the Study of Natural Philosophy"; Beale's 
 "How to Work with the Microscope"; Schellen's "Spectrum 
 Analysis"; Roscoe on "Spectrum Analysis"; Lockyer's "The 
 Spectroscope," and "Studies in Spectrum Analysis"; Airy's 
 "Geometrical Optics"; Nugent's "Optics"; "Chevreul on Col- 
 ors"; Thomson and Tait's "Natural Philosophy"; Maxwell's 
 "Electricity and Magnetism"; Silvanus Thompson's "Lessons 
 in Electricity and Magnetism"; Faraday's "Forces of Matter"; 
 Youmans' "Correlation of Physical Forces"; Maury's "Physical 
 Geography of the Sea"; Atkinson's " Ganot's Physics"; Silli- 
 man's "Physics"; Tyndall's Lectures on Light, Heat, Sound, Elec- 
 tricity, also his "Forms of Water"; Snell's " Olmsted's Philoso- 
 phy " (revised edition) ; Loomis' " Meteorology "; Miller's " Chem- 
 ical Physics"; Urbanitzky's " Electricity in the Service of Man"; 
 Cooke's "Religion and Chemistry"; Daniell's "Principles of 
 Physics"; Anthony and Brackett's "Text-book of Physics," and 
 also numerous works named in the " Reading References " at 
 the close of each general division. They may be procured of 
 the publishers of this book. The pupil should continually be 
 impressed with the thought that the text-book only introduces 
 him to a subject, which he should seek every opportunity to 
 pursue in larger works and in treatises on special topics. 
 
 The editor will be pleased to correspond with teachers con- 
 cerning the apparatus for the performance of the experiments, 
 or with reference to any of the "Practical Questions." 
 
TABLE OF CONTENTS. 
 
 PAGE 
 
 I. INTRODUCTION 1 
 
 I. GENERAL DEFINITIONS 3 
 
 II. GENERAL PROPERTIES OF MATTER ... 6 
 
 III. SPECIFIC PROPERTIES OF MATTER ... 10 
 
 II. MOTION AND FORCE ... . . .19 
 
 III. ATTRACTION . . . . .... 41 
 
 I. MOLECULAR FORCES . . * ... 43 
 II. GRAVITATION . . . , . . , . 55 
 
 IV. ELEMENTS OF MACHINES . . . . .79 
 
 V. PRESSURE OF LIQUIDS AND GASES . .99 
 
 I. HYDROSTATICS . *> 101 
 
 II. HYDRODYNAMICS . . . 121 
 
 III. PNEUMATICS 129 
 
 VI. SOUND 151 
 
 VII. LIGHT . . .189 
 
 VIII. HEAT 241 
 
Xll CONTENTS. 
 
 PACE 
 
 IX. MAGNETISM . . , . . . . . .279 
 
 X. ELECTRICITY . .* . . . . '. .291 
 
 XI. APPENDIX . . . . . . . . .355 
 
 1. QUESTIONS . . .... . . 357 
 
 2. INDEX . . . 375 
 
I. 
 
 INTRODUCTION. 
 
 " WE have no reason to believe that the sheep or the dog, or indeed 
 any of the lower animals, feel an interest in the laws by which natural 
 phenomena are regulated. A herd may be terrified by a thunder-storm ; 
 birds may go to roost, and cattle return to their stalls during a solar 
 eclipse; but neither birds nor cattle, so far as we know, ever think of 
 inquiring into the causes of these things. It is otherwise with man. The 
 presence of natural objects, the occurrence of natural events, the varied 
 appearances of the universe in which he dwells, penetrate beyond his 
 organs of sense, and appeal to an inner power of which the senses are the 
 mere instruments and excitants. No fact is to him either final or original. 
 He can not limit himself to the contemplation of it alone, but endeavors 
 to ascertain its position in a series to which the constitution of his mind 
 assures him it must belong. He regards all that he witnesses in the pres- 
 ent as the afflux and sequence of something that has gone before, and as 
 the source of a system of events which is to follow. The notion of spon- 
 taneity, by which in his ruder state he accounted for natural events, is 
 abandoned ; the idea that nature is an aggregate of independent parts also 
 disappears, as the connection and mutual dependence of physical powers 
 become more and more manifest ; until he is finally led to regard Nature 
 as an organic whole, as a body each of whose members sympathizes with 
 the rest, changing, it is true, from age to age, but without any real break 
 of continuity, or interruption of the fixed relations of cause and effect." 
 
 TYNDALL. 
 
ANALYSIS OF THE INTRODUCTION. 
 
 I. GENERAL DEFINI- 
 TIONS. 
 
 II. GENERAL PROPER- 
 TIES OF MATTER. 
 
 III. SPECIFIC PROPER- 
 TIES OF MATTER. 
 
 1. Of Matter, Body, and Substance 
 
 2. General and Specific Properties. 
 
 3. The Atomic Theory. 
 
 4. Physical and Chemical Changes. 
 
 5. Energy. 
 
 6. Physical and Chemical Forces. 
 
 7. Definition of Physics and Chem- 
 
 istry. 
 
 1. Extension and its Measurement. 
 
 2. Impenetrability. 
 
 3. Divisibility. 
 
 4. Porosity. 
 
 5. Indestructibility. 
 
 1. Ductility. 
 
 2. Malleability. 
 
 3. Tenacity. 
 
 4. Elasticity. 
 
 5. Hardness. 
 
 6. Brittleness. 
 
 (1.) Compression. 
 (2.) Expansion. 
 (3.) Torsion. 
 (4.) Flexure. 
 
INTRODUCTION. 
 
 I. GENERAL DEFINITIONS. 
 
 1. Matter. Whatever occupies space is called mat- 
 ter. A definite portion of matter is termed a ~body. 
 Examples: A lake, a dew-drop, a quart of oil, an an- 
 vil, a pendulum. A particular kind of matter is styled 
 a substance. Examples: Gold, wood, stone, oxygen. 
 
 2. General and Specific Properties, A general 
 property of matter is a quality that belongs to all sub- 
 stances. Example: Extension. A specific property 
 is one which distinguishes particular substances. Ex- 
 amples: The yellow color of gold, the brittleness of 
 glass, the sweetness of sugar. These properties are 
 so distinctive that we say, " yellow as gold," "brittle 
 as glass," "sweet as sugar." 
 
 3. The Atomic Theory supposes 
 
 (1.) That the smallest particle of matter we can 
 see is composed of still smaller particles or molecules 
 (tiny masses),* each possessing the specific properties 
 of the substance to which it belongs. 
 
 * A molecule is a group of atoms held together by chemical force, and 
 is the smallest particle of a substance which can exist by itself. Even in 
 a simple substance, i. ., one in which the atoms are all of one kind, it is 
 thought that they are generally clustered in molecules. (See "Chemistry," 
 p. 4.) In water, the molecules are the small masses which, when driven 
 
VI:: : A INTRODUCTION. 
 
 (2.) That each molecule consists of still smaller 
 portions, called atoms,* which are regarded as indi- 
 visible and unchangeable. Examples : A molecule of 
 water is made up of two atoms of hydrogen and 
 one of oxygen. A molecule of salt consists of one 
 atom of chlorine and one of sodium. The smallest 
 piece of salt contains many molecules. By dissolving 
 in water, we divide it into its separate molecules, and 
 the solution has a briny taste, because each one pos- 
 sesses the savor of salt. 
 
 4. Physical and Chemical Changes. A physical 
 change is one that does not affect the composition 
 of the molecule, and so does not alter the specific 
 properties of a substance. Examples : The falling of 
 a stone, the dissolving of sugar in water. A chemical 
 change is one that implies the re-arrangement of 
 the atoms into new molecules and so destroys the 
 specific properties of a substance. Examples: The 
 rusting of iron, the burning of coal. 
 
 5. Energy. -7- The power of producing change of 
 any kind is called energy. Heat, light, electricity, etc., 
 
 apart, form steam. In a gas, they move about with great velocity, colliding 
 with one another and with the sides of the containing vessel. The collisions 
 of the molecules with the walls of the vessel account for the pressure which 
 the gas exerts upon them. 
 
 * Animalcules furnish a striking illustration of the minuteness of 
 atoms. In the drop of stagnant water that clings to the point of a needle, 
 swarming legions swim as in an ocean, full of life, frisking, preying upon 
 one another, waging war, and re-enacting the scenes of the great world 
 about them. These tiny animals possess organs of digestion and assimila- 
 tion. Their food, coursing in channels more minute than we can conceive, 
 may be composed of solid as well as liquid matter; and finally, at the 
 lowest extreme of this descending series, we come to the atoms of which 
 the matter itself is composed. The most powerful of microscopes fails com- 
 pletely to reveal the separate molecule. 
 
GENERAL DEFINITIONS. 5 
 
 are different forms of energy. Each is capable of 
 producing change in matter. Examples: Heat melts 
 ice ; light blackens silver chloride ; a wire becomes hot 
 when an electric current passes through it. The same 
 energy may be manifested successively in different 
 forms. Examples: Heat is converted by the steam- 
 engine into motion ; this motion imparted to a dynamo 
 is converted into electricity, which may be further 
 transformed into light, heat and magnetism. 
 
 6. Physical and Chemical Forces. When energy 
 is manifested as an attraction or repulsion we speak of 
 it as a forcCj and we distinguish between physical and 
 chemical forces as the changes produced by them are 
 physical or chemical. Examples : The attraction of 
 an electrified body for small bits of paper is a physical 
 force, and so also the attraction of a magnet for one 
 end of a compass-needle, or the repulsion for the other 
 end. The attraction of iron for oxygen in the forma- 
 tion of iron-rust is a chemical force. 
 
 7. Physics and Chemistry. The former treats of 
 physical changes, the latter of chemical changes in 
 matter. The unit of the physicist is the molecule, of 
 the chemist the atom. 
 
 PRACTICAL QUESTIONS. 
 
 1. Name some specific property of coal; ink; chalk; grass; tobacco; 
 snow. 
 
 2. My knife-blade is magnetized, so that it will pick up a needle ; is 
 that a physical or a chemical change? 
 
 3. Is it treated in Physics or Chemistry? 
 
 4. Is the burning of coal a physical or a chemical change? 
 6. The production of steam? The formation of dew? 
 
INTRODUCTION. 
 
 6. The falling of a stone? The growth of a tree? 
 
 7. The flying of a kite? The chopping of wood? 
 
 8. The explosion of powder? The boiling of water? 
 
 9. The melting of iron? The drying of clothes? 
 
 10. The freezing of water? The dissolving of sugar? 
 
 11. The forging of a nail? The making of bread? 
 
 12. The sprouting of a seed? The decay of vegetables? 
 13* The condensation of steam? 
 
 II. GENERAL PROPERTIES OF MATTER. 
 
 THERE are two essential properties without which, 
 matter is inconceivable. These are extension and 
 impenetrability. 
 
 1. Extension is the property of occupying space. 
 The amount of space a body occupies is called its 
 volume. 
 
 Measurement of Extension. A body has three di- 
 mensions: length, breadth, and thickness. To meas- 
 ure these, some standard is required. The standard 
 of length popularly in use in England and the United 
 States is the yard. Its length is the distance between 
 two lines on a certain bar of bronze, kept in London 
 and measured at a certain temperature, 62F. (see 
 p. 248). There is only one yard in the world ; all 
 that we call yards are imperfect copies from it. The 
 yard is inconveniently divided into three feet, or 
 thirty-six inches. The standard of length used in 
 France, and by scientific men throughout the world, 
 is the meter.* Its length is nearly, but not exactly, 
 
 * The meter is divided into ten decimeters (dm.) ; each of theae into ten 
 centimeters (cm.) ; and each of these into ten millimeters (mm.). In Kg. 1 is 
 
GENERAL PROPERTIES OF MATTER. 
 
 There 
 
 PlQ. 1. 
 
 4 o-o 0*0 ootf f an entire meridian of the earth. 
 is only one meter in the world. It 
 is the length of a certain bar of 
 platinum, kept in Paris, and meas- 
 ured at the temperature of melting 
 ice. Most copies of the meter and 
 yard are accurate enough for the 
 purposes to which they are applied. 
 
 2. Impenetrability is the prop- 
 
 erty of so occupying space as to 
 exclude all other matter.* JSTo two 
 bodies can occupy the same space 
 at the same time. A book lies upon 
 the table before me ; no human 
 power is able to place another in 
 the same spot, until the first book 
 
 shown a line, AS, whose length is a decimeter, 
 divided into centimeters and millimeters. At the 
 side of it is another line, AC, slightly longer. It 
 is made up of four inches, divided into halves, 
 quarters, and eighths. The length of the meter 
 is about 39.37 inches, or nearly 1.1 yard. 
 
 For the measurement of surface, we use square 
 meters (sq. m.), square centimeters (sq. cm.), etc. 
 
 The unit adopted for the measurement of vol- 
 ume is the cubic decimeter. It is called a liter. 
 A vessel that contains just a liter of water will 
 hold a little more than a quart of the same liquid. 
 Since the liter has a length, breadth, and thickness 
 of one decimeter, it contains 1,000 cubic centi- 
 meters. 
 
 * In common language, we say a needle pene- 
 
 trates cloth, a nail enters wood, etc. ; but a moment's examination shows 
 that they merely push aside the fibers of the cloth or wood, and so press 
 them closer together. With care we can drop a quarter of a pound of 
 shingle-nails into a tumbler brimful of water, without causing it to over- 
 flow. The surface of the water, however, becomes convex. 
 
 Comparison of Metric 
 and English Measures 
 of Length. 
 
8 INTRODUCTION. 
 
 is removed. I attempt to fill a bottle through a 
 closely-fitting funnel ; but before the liquid can run 
 in, the air must gurgle out, or the water will trickle 
 down the outside of the bottle. 
 
 In addition to these two essential properties of 
 matter, there are others which have been found to 
 be general, such as divisibility, porosity, and inde- 
 structibility. 
 
 3. Divisibility is that property by which a body 
 may be separated into parts. The extent to which 
 the divisibility of matter may be carried is almost 
 incredible.* Example: A grain of strychnine will 
 flavor 1,750,000 grains of water; hence there will 
 be in each grain of the liquid only l 7g } 000 of a 
 grain of strychnine, yet this amount can be dis- 
 tinctly tasted. 
 
 4. Porosity is the property of having pores. By 
 this is meant not the sensible pores to which we 
 refer when in common language we speak of a por- 
 ous body, as bread, wood, unglazed pottery, a sponge, 
 etc., but the finer or physical pores. The latter are 
 as invisible to the eye as the atoms themselves, and 
 are caused by the fact that the molecules of which 
 a body is composed are not in actual contact, but 
 
 * Newton estimated that the film of a soap-bubble at the instant of 
 breaking is less than g-goo <y o of an inch thick. Pure water will acquire the 
 requisite viscidity for making bubbles by adding only ^ part of soap. It 
 is evident that there must be at least one molecule of soap in every cubic 
 jreJ-STO of an inch of the film, and that the molecule must be smaller than 
 one hundredth of a cubic ^-mjirres of an incb -' * * than *n5-s*tnnso of a 
 cubic inch. Now a molecule of soft-soap (if it is a pure potassium stearate, 
 "Chemistry," p. 207) contains 56 atoms, and this point must be reached 
 before we come to^the possible limit of divisibility. 
 
GENERAL PROPERTIES OF MATTER. 9 
 
 are separated by minute spaces.* Examples: To a 
 bowl-full of water it is easy to add a quantity of 
 fine salt without the liquid running over. Only care 
 must be taken to drop in the salt slowly, giving time 
 for it to dissolve and the bubbles of air to pass off. 
 When the water has dissolved -all the salt it can, we 
 can still add other soluble solids, f In testing large 
 cannon by hydrostatic pressure (p. 104), water is 
 forced into the gun until it oozes through the thick 
 metal and covers the outside of the gun like froth, 
 then gathers in drops and runs to the ground in 
 minute streams.J 
 
 It is in virtue of these physical pores that a body 
 changes in volume when warmed or cooled. The 
 molecules become farther apart or nearer as heat is 
 
 * These spaces are so small that they can not be discerned with the 
 most powerful microscope, yet it is thought that they may be very large 
 when compared with the size of the atoms themselves. If we imagine a 
 being small enough to live on one of the atoms near the center of a stone, 
 as we live on the earth, then we are to suppose that he might possibly see 
 the nearest atoms at great distances from him, as we see the moon and 
 stars, and might perchance have need of a fairy telescope to examine them, 
 as we investigate the heavenly bodies. It is impossible, however, for us to 
 have any definite knowledge on such a topic. 
 
 t In this case we suppose that the particles of salt are smaller than 
 those of water, and those of the different substances used are smaller than 
 those of salt. The particles of salt fill the spaces between the particles of 
 water, and the others occupy the still smaller spaces left between the par- 
 ticles of salt. We may better understand this if we suppose a bowl filled 
 with oranges. It will hold a quantity of peas, then of gravel, then of fine 
 sand, and lastly some water. 
 
 J In the course of some experiments performed during the eighteenth 
 century at the Florence Academy, Italy, hollow globes of silver were filled 
 with water and placed in a screw-press. The spheres being flattened, their 
 size was diminished, and the water oozed through the pores of the metal. 
 The philosophers of the day thought this to show that water is incom- 
 pressible. We now see that it proved only that silver has pores larger than 
 the molecules of water. 
 
10 INTRODUCTION. *;; . 
 
 applied or withdrawn. We can not conceive this to 
 be possible if they are in perfect contact. 
 
 5. Indestructibility is the property which renders 
 matter incapable of being destroyed. We can not 
 conceive of the annihilation of matter. We may 
 change its form, but we can not deprive it of exist- 
 ence. Example: We cut down a tree, saw it into 
 boards, and build a house. The house burns, and 
 only little heaps of ashes remain. Yet in the ashes, 
 * and in the smoke of the burning building, exist the 
 identical atoms, which have passed through these 
 various forms unchanged.* 
 
 III. SPECIFIC PROPERTIES OF MATTER. 
 
 AMONG the most important specific properties of 
 matter are ductility, malleability, tenacity, elasticity, 
 hardness, and brittleness. 
 
 1. Ductility. A ductile body is one which can be 
 drawn into wire. Fig. 2 represents a machine for 
 making wire. B is a steel drawing-plate pierced with 
 a series of gradually diminishing holes. A rod of 
 iron, A, is hammered at the end so as to pass 
 
 * Walter Raleigh, while smoking in the presence of Queen Elizabeth, 
 offered to bet her majesty that he could tell the weight of the smoke that 
 curled upward from his pipe. The wager was accepted. Raleigh quietly 
 finished, and then weighing the ashes, subtracted this amount from the 
 weight of the tobacco he had placed in the pipe, thus finding the weight 
 of the smoke. When we reach the subject of combustion in chemistry, we 
 shall be able to detect Raleigh's mistake. The smoke and the ashes really 
 weighed more than the original tobacco, since the oxygen of the air had 
 combined with the tobacco in burning. 
 
SPECIFIC PROPERTIES OF MATTER. 11 
 
 through the largest. It is then grasped by a pair of 
 pincers, (7, and, by turning the crank, _D, is drawn 
 through the plate, 
 diminished in di- 
 ameter and propor- 
 tionately increased 
 in length. The 
 tenacity of the 
 metal is greatly 
 
 Drawing Wire. 
 
 improved by the 
 
 process of drawing, so that a cable of fine wire is 
 stronger than a chain or bar of the same weight. 
 Gold, silver, and platinum are the most ductile 
 metals. A silver rod an inch thick, covered with 
 gold leaf, may be drawn to the fineness of a hair 
 and yet retain a perfect coating of gold, three ounces 
 of the latter metal making 100 miles of the gilt- 
 thread used in embroidery. Platinum wire has been 
 drawn so fine that, though it is nearly three times as 
 heavy as iron, a mile's length weighed only a grain. 
 
 2. Malleability. A malleable body is one which 
 can be hammered or rolled into sheets. Example: 
 Gold may be beaten until it is only ?60 V og - of an inch 
 thick. It would require 1,800 such leaves to equal 
 the thickness of common printing-paper.* ' Copper is 
 
 * An ingot of gold is passed many times between steel rollers, which 
 are so adjusted as to be continually brought nearer together. The metal is 
 thus reduced to a ribbon about ses of an inch thick. This is cut into 
 inch squares, 150 of which are piled up alternately with leaves of strong 
 paper four inches square. A workman with a hammer beats the pile 
 until the gold is spread to the size of the leaves. Each piece is next 
 quartered, and the 600 squares are placed between leaves of goldbeaters' 
 
12 
 
 INTRODUCTION. 
 
 Pro. 3. 
 
 so malleable, that a workman can hammer out a 
 kettle from a solid block. 
 
 3. Tenacity. A tenacious body is one which can 
 not easily be pulled apart. Iron possesses this quality 
 in a remarkable degree. Steel wire will sustain many 
 thousand times its own weight. 
 
 4. Elasticity is of four kinds, according as a body 
 tends to resume its original form when compressed, 
 extended, twisted, or bent. 
 
 (1.) ELASTICITY OF COMPRESSION. Many solids, as 
 iron, glass, and caoutchouc, are highly elastic. Ex- 
 ample : Spread a thin coat 
 of oil on a smooth marble 
 slab. If an ivory ball be 
 dropped upon it, the size 
 of the impression will vary 
 with the distance at which 
 the ball is held above the 
 table. This shows that the 
 ivory is flattened, some- 
 what like a soap-bubble 
 when it strikes a smooth 
 
 :r ,, : ,,, nm , CTMWa . Ml ^ T ,, m ,,,,, |;|l , l , l , surface and rebounds. 
 
 Elasticity of ivory. Liquids are compressed 
 
 with great difficulty, so that for a long time they 
 were considered incompressible. When the force is 
 
 skin and pounded. They are then taken out, spread by the breath, cut, 
 and the 2,400 squares pounded as before. They are finally trimmed and 
 placed between tissue-paper in little books, each of which contains twenty- 
 five gold leaves. 
 
SPECIFIC PROPERTIES OF MATTER. 13 
 
 removed, they regain their exact volume, and are 
 therefore perfectly elastic. 
 
 Gases are easily compressed, and are also perfectly 
 elastic. A pressure of 15 Ibs. to the square inch re- 
 duces the volume of water only -jnrfoir, whereas it 
 diminishes the volume of a perfect gas . A gas may 
 be kept compressed for years, but on being released 
 will instantly return to its original form. 
 
 (2.) ELASTICITY OF EXPANSION is possessed largely 
 by very many substances. Examples: India rubber, 
 when stretched, tends to fly back to its original di- 
 mensions. A drop of water hanging to the nozzle 
 of a bottle may be touched by a piece of glass and 
 drawn out to considerable length, but when let go 
 it will resume its spherical form. 
 
 (3.) ELASTICITY OF TOESION is the tendency of a 
 thread or wire which has been Pto 4 
 
 twisted, to untwist again. If a 
 weight be suspended by means of a 
 steel wire, twisted around and then 
 released, it constitutes a torsion pen- 
 dulum. (Fig. 4.) 
 
 (4.) ELASTICITY OF FLEXURE is the- 
 property ordinarily meant by the 
 term elastic. Many solids possess 
 this quality, within certain limits, 
 
 ,~ n , , The Torsion Pendulum. 
 
 to a high degree. Swords have been 
 made which could be bent into a circle without 
 breaking. "Watch-springs, bows, cushions, etc., are 
 useful because of their elasticity. Glass, though 
 brittle, is one of the most elastic substances known. 
 
14 INTRODUCTION, 
 
 5. Hardness. One body is harder than another 
 when it will scratch or indent it. This property does 
 not depend on density.* Examples: Gold is about 
 2-J times as dense as iron, yet it is much softer. 
 Mercury is a liquid, yet it is almost twice as dense 
 as steel. The diamond is the hardest known sub- 
 stance, yet it is not one third as heavy .as lead. 
 
 6. Brittleness. A brittle body is one that is easily 
 broken. This property is a frequent characteristic 
 of hard bodies. Example: Glass will scratch pure 
 iron, yet it is extremely brittle. 
 
 SUMMARY. 
 
 MATTER is that which occupies space. A separate portion is 
 called a body, and a particular kind a substance. A general 
 property of matter belongs to all substances, and a specific one 
 to particular kinds. Matter is composed of very minute atoms. 
 A group of atoms forms a molecule, in which reside the specific 
 properties of a substance. A physical change never affects the 
 molecule, but a chemical change breaks it up, and so makes 
 new combinations possible. Physics deals with physical forces 
 and changes ; Chemistry with chemical attraction, and chemical 
 changes. Extension and impenetrability are the essential prop- 
 erties of matter. Extension is the property of occupying space. 
 The amount of space a body fills is its volume. In virtue of 
 impenetrability two bodies can not occupy the same space at 
 the same time. The divisibility of matter is without perceptible 
 limit so far as we know. Porosity is the property in virtue of 
 which the molecules of a body are not in absolute contact. In- 
 destructibility prohibits the extinction of matter by man. Duc- 
 
 * A dense body has its molecules closely compacted. The word rare, the 
 opposite of dense, is applied to gases. Mass, or the quantity of matter a 
 body contains, should be distinguished from weight or size. 
 
HISTORICAL SKETCH. 15 
 
 tility, malleability, tenacity, elasticity, hardness, and brittleness 
 are the principal specific properties of matter. A ductile body 
 can be drawn into wire ; gold, silver, and platinum are the 
 most noted for this property. A malleable body can be ham- 
 mered into sheets ; gold possesses this quality in a remarkable 
 degree. A tenacious body resists pulling apart ; iron is the best 
 example. An elastic body permits a play of its particles, so 
 that they return to their original position when the disturbing 
 force is removed. A hard body can not easily be indented. A 
 brittle body is readily broken. 
 
 HISTORICAL SKETCH. 
 
 IN ancient times, any seeker after truth was termed a 
 philosopher (a lover of. wisdom), and philosophy included all in- 
 vestigations concerning both mind and matter. In the fourth 
 century B.C., Plato assumed 4;hat there are two principles, 
 matter and form, which by combining produce the five ele- 
 ments, earth, air, fire, water, and ether. Aristotle, his pupil, 
 established the first philosophical ideas concerning matter and 
 space. But the method of study generally pursued for 2,000 
 years was one of pure metaphysical speculation. Observation 
 had no place, but the philosophers made up a theory, and then 
 accommodated facts to it. They guessed about the real essence 
 of things, as to whether matter exists except when perceived 
 by the mind,* and how a change in matter can produce a 
 change in mind. In 1620, Bacon published his "Novum Or- 
 ganum," advocating the inductive method of studying nature. 
 He argued that the philosopher should seek to benefit mankind, 
 and that, instead of wasting his time in sterile and ingenious 
 theories about the world and matter, he should watch the 
 phenomena of life, gather facts, and then reasoning from 
 effects back to their causes, reach the general law. This work 
 
 * Dr. Johnson once remarked to a gentleman who had been defending 
 the theory that there is no external world, as he was going away, " Pray, 
 sir, don't leave us, for we may perhaps forget to think of you, and then 
 you will cease to exist. 1 ' 
 
16 INTBODUCTION. 
 
 is commonly said to have established the modern method of in- 
 vestigation. Ptolemy, Archimedes, Galileo, and other physicists, 
 however, had long before proved its value. 
 
 The Atomic Theory was propounded by Democritus, in the 
 fifth century B.C., and twenty-two centuries later elaborated by 
 Dalton, an English physicist. The grander generalization and 
 development of this law was advanced in 1811 by Avogadro, 
 an Italian, and afterward extended by the French philosopher, 
 Ampere. The latter asserted that " equal volumes of all sub- 
 stances, when in the gaseous form and under like conditions, 
 contain the same number of molecules." For half a century 
 this hypothesis was ignored. Its adoption within the past thirty 
 years produced fundamental changes in chemistry. Displacing 
 the old it gave birth to a new and consistent notation, and, raised 
 to the rank of a law, it served as a criterion by which the correct- 
 ness of its formulae might be tested. 
 
 The history of the establishment of a standard of measures 
 is a curious one. Anciently, length was referred to some por- 
 tion of the human body, as the foot ; the cubit (the length of 
 the fore-arm from the elbow to the end of the middle finger) ; 
 the finger's length or breadth ; the hand's breadth ; the span, 
 etc. In England, Henry I. (1120) ordered that the ell, the an- 
 cient yard, should be the exact length of his arm. Afterward 
 a standard yard-stick was kept at the Exchequer in London ; 
 but it was so inaccurate, that a commissioner, who examined it 
 in 1742, wrote: "A kitchen poker filed at both ends would 
 make as good a standard. It has been broken, and then re- 
 paired so clumsily that the joint is nearly as loose as a pair of 
 tongs." In 1760, Mr. Bird carefully prepared a copy of this for 
 the use of the government. It was not legally adopted until 
 1824, when it was ordered that if destroyed, it should be re- 
 stored by a comparison with the length of a pendulum vibrating 
 seconds at the latitude of London. At the great fire in London, 
 1834, the Parliament House was burned, and with it Bird's 
 yard-stick. Repeated attempts were then made to find the 
 length of the lost standard by means of the pendulum. This 
 was found impracticable, on account of errors in the original 
 directions. At last the British government adopted a standard 
 prepared from the most reliable copies of Bird's yard-stick. A 
 
HISTORICAL SKETCH. 17 
 
 copy of this was taken by Troughton, an instrument-maker of 
 London, for the use of our Coast Survey.* 
 
 The French had previously adopted for the length of the 
 legal foot that of the royal foot of Louis XIV., as perishable a 
 standard as Henry's arm. In 1790, the Prince de Talleyrand 
 proposed to the Constituent Assembly of France the foundation 
 of a system based on a single and universal standard, which 
 might be used by all civilized nations. The selection of this 
 was committed to five members of the Academy of Sciences, 
 MM. Borda, Lagrange, Laplace, Monge, and Condorcet, who 
 decided that the ten-millionth part of a quarter of the earth's 
 meridian should be taken as the standard of length, from which 
 the standards of surface, volume, capacity, and weight should 
 be derived. A trigonometric measurement was made of the 
 arc of a meridian extending through France from Dunkirk to 
 Barcelona, a work which occupied seven years. In 1799, an 
 international commission was assembled at Paris, with repre- 
 sentatives from most of the governments of Europe. They 
 deposited at the Palace of the Archives, in Paris, the standard 
 meter-bar of platinum, and the standard kilogram weight, made 
 of the same metal. In English denominations the length of the 
 meter is almost exactly 3.28 feet, or 39.37 inches ; the weight 
 of the kilogram almost exactly 2.2 pounds avoirdupois. 
 
 But after the establishment of the metric system it was 
 found that a slight mistake had been made in the measurement 
 of the arc of the meridian. The English, who had declined to 
 accept the French system, discovered also a difficulty in the 
 determination of the yard from the pendulum beating seconds. 
 Both the yard and the meter are therefore arbitrary and not 
 absolute standards. Copies of each have been made so care- 
 fully and distributed so widely that there is no probability of 
 any appreciable loss resulting from the accidental destruction 
 of the originals. The metric system is by far the simplest and 
 best in use, but it has not yet generally supplanted other sys- 
 
 * A bronze bar. which, has the standard length at 61.79 P., has been 
 presented by the English government to that of the United States. Ac- 
 cording to Act of Congress, sets of weights and measures have been dis- 
 tributed to the governors of the several States. Both the yard and the 
 meter are legal standards in the United States and Great Britain. 
 
18 INTRODUCTION. 
 
 terns that retain their popularity, not on account of merit, but 
 only because of human conservatism and the inconveniences 
 resulting from change. 
 
 Consult Cooke's "New Chemistry," chapter on Molecules, etc.; 
 Powell's "History of Natural Philosophy"; Buckley's "History 
 of Natural Science"; Whewell's "History of the Inductive 
 Sciences"; Roscoe's "John Dalton and his Atomic Theory," in 
 Manchester Science Lectures, '73-4; "Appleton's Cyclopedia," 
 Art. Molecules; Outer bridge's "Divisibility of Gold and Other 
 Metals," in Popular Science Monthly, Vol. XL, p. 74; Crookes' 
 " The Radiometer a fresh evidence of a Molecular Universe," 
 Popular Science Monthly, Vol. XIII. , p. 1; Tait's "Recent Ad- 
 vances in Physical Science," Chap. XII., The Structure of Mat- 
 ter ; Hoefer, " Histoire de la Physique et de la Chimie"; Draper's 
 "History of Intellectual Development of Europe"; Barnard on 
 "The Metric System." 
 
II. 
 
 MOTION AND FORCE. 
 
 BEST is nowhere. The winds that come and go, the ocean that un- 
 easily throbs along the shore, the earth that revolves about the sun, the 
 light that darts through space all tell of a universal law of Nature. The 
 solidest body hides within it inconceivable velocities. Even the molecules 
 of granite and iron have their orbits as do the stars, and move as cease- 
 lessly. 
 
 No energy is ever lost. It changes its form, but the eye of philosophy 
 detects it and enables us to drive it from its hiding-place undiminished. 
 It assumes Protean guises, but is every-where a unit. It may disappear 
 from the earth; still 
 
 "Somewhere yet that atom's force 
 Moves the light-poised universe." 
 
ANALYSIS OF MOTION AND FORCE. 
 
 1. DEFINITIONS. 
 
 2. COMMUNICATION OF MOTION. 
 
 3. RESISTANCES TO MOTION. 
 
 MOTION AND 
 FORCE. 
 
 4. FIRST LAW OF MOTION 
 
 (1.) Inertia. 
 
 (2.) Momentum. 
 
 5. SECOND LAW OF MOTION. 
 
 6. THIRD LAW OF MOTION. 
 
 7. COMPOSITION OF MOTIONS. 
 
 8. COMPOSITION OF FORCES. 
 
 9. TRIANGLE OF FORCES. 
 
 10. POLYGON OF FORCES. 
 
 11. RESOLUTION OF FORCES. 
 
 12. MOTION IN A CURVE. 
 
 13. CIRCULAR MOTION. 
 
 14. REFLECTED MOTION. 
 
 15. ENERGY. 
 
 16. KINETIC AND POTENTIAL ENERGY. 
 
 17. CONSERVATION OF ENERGY, 
 
MOTION AND FORCE. 
 
 1. Motion is change of place. All motion, as 
 well as rest, with which we are acquainted, is rela- 
 tive. Examples : When we ride in the cars, we judge 
 of our motion by the objects around us. A man on 
 a steamer may be in motion with regard to the 
 shore, but at rest with reference to the objects on 
 the deck of the vessel. Force is that which produces 
 or tends to produce or to destroy motion. Velocity is 
 the rate at which a body moves. When the rate is 
 constant, i. e., when the moving body passes over equal 
 spaces in equal times, the body moves with constant 
 velocity. It is expressed by the number of units of 
 space through which the body moves in a unit of time. 
 Example: Ten miles an hour or fifteen feet a sec- 
 ond. When the rate is variable, i. e., when the spaces 
 passed over by the body in equal times are unequal, 
 the body moves with variable velocity. Example: 
 The velocity of a train between two stations is vari- 
 able. In cases of this kind it is convenient to speak 
 of the average velocity of the train, which is the ratio 
 of the whole distance passed over to the time required 
 for such passage. The train will evidently arrive at 
 the second station in the same time, whether it move 
 uniformly with the average velocity or with its ac- 
 tually varying velocity. 
 
22 MOTION AND FORCE. 
 
 2. The Communication of Motion is not instan- 
 taneous. If I strike one end of a rail a mile long the 
 tremor communicated from particle to particle will 
 take a definite time to reach the other end. A stone 
 thrown against a pane of glass shatters it, but a 
 bullet fired through it will make only a round hole. 
 The bullet is gone before the motion has i^me to be 
 communicated to the surrounding particles. A frac- 
 tion of time is required for a ball to receive the force 
 of the exploding powder and get under full headway. 
 
 3. The Resistances to Motion are friction and the 
 resistance of air and water. (1.) Friction is the resis- 
 tance caused by the surface over which a body moves. 
 It is of great value in common life. Without it, nails, 
 screws, and strings would be useless ; engines could 
 not draw the cars ; we could hold nothing in our hands ; 
 and we should everywhere walk as on glassy ice. (2.) 
 The resistance which a body meets in passing through 
 air or water is caused largely by the particles displaced. 
 
 LAWS OF MOTION. There are three laws of mo- 
 tion, which were first distinctly formulated by Sir 
 Isaac Newton. 
 
 4. First Law of Motion. A body set in motion 
 will move forever in a straight line with constant 
 velocity, unless acted on by some external force. Ob- 
 viously, no experiment will directly prove this law. 
 Common experience, moreover, seems to contradict 
 it ; for everywhere on the surface of the earth bodies 
 in motion show a tendency to stop. A little reflection 
 will show, however, that these bodies are subject to 
 
LAWS OF MOTION. 23 
 
 the action of external forces tending to arrest the 
 motion. It is supposed that could these resistances 
 be removed a body once in motion would never stop. 
 Inertia. The law just stated is often called the 
 law of inertia. Matter has no inherent power of 
 producing change upon itself. If a body be already 
 in motion, force has to be expended in stopping it. 
 If it be at rest, force is required to start it in mo- 
 tion. In either case we " overcome its inertia." The 
 danger in jumping from a car in rapid motion lies 
 in the fact that the body has the speed of the train, 
 while the forward motion of the feet is checked by 
 contact with the ground. It is necessary to jump as 
 nearly as possible in the direction in which the train 
 is moving, and be ready to run the instant the feet 
 touch the ground. Those who do so can then grad- 
 ually overcome the inertia of the body, and after a 
 few yards can turn as they please. 
 
 Momentum. To measure any force we must know 
 what quantity of matter is moved, and what velocity 
 it receives in a unit of time. The quantity of matter 
 in a body is called its mass. It is not the same as 
 weight, but is proportional to this (see p. 57), so 
 that we speak of pounds of mass as* well as pounds 
 of weight. The product of mass by velocity is called 
 momentum* Thus if a mass of five pounds move 
 
 * A heavy body may be moving very slowly and yet have an immense 
 momentum. Examples ; An iceberg, with a scarcely perceptible motion, will 
 crush the strongest ship as if it were an egg-shell. Soldiers have thought 
 to stop a spent cannon-ball by putting a foot against it, but have found its 
 momentum sufficient to break a leg. 
 
 On the other hand, a light body moving with a high velocity may have 
 
24 MOTION AND FORCE. 
 
 with a velocity of twenty feet per second, it has one 
 hundred units of momentum. 
 
 5. Second Law of Motion. A force acting upon 
 a ~body in motion or at rest, produces the same effect 
 whether it acts alone or with other forces. Examples : 
 All bodies upon the earth are in constant motion 
 with it, yet we act with the same ease that we 
 should were the earth at rest.* We throw a stone 
 
 an enormous momentum. Examples : The air in a hurricane will tear up 
 trees by the roots and level buildings to the ground. Sand driven from a 
 tube by steam is used for drilling and in stone-cutting, engraving, etc. 
 
 "In a rude age, before the invention of means for overcoming friction, 
 the weight of bodies formed the chief obstacle to setting them in motion. 
 It was only after some progress had been made in the art of throwing 
 missiles, and in the use of wheel-carriages and floating vessels, that men's 
 minds became practically impressed with the idea of mass as distinguished 
 from weight. Accordingly, while almost all the metaphysicians who dis- 
 cussed the qualities of matter, assigned a prominent place to weight among 
 the primary qualities, few or none of them perceived that the sole unalter- 
 able property of matter is its mass. At the revival of science this property 
 was expressed by the phrase ' The inertia of matter ' ; but while the men 
 of science understood by this term the tendency of the body to persevere 
 in its state of motion (or rest), and considered it a measurable quantity, 
 those philosophers who were unacquainted with science understood inertia 
 in its literal sense as a quality mere want of activity or laziness I there- 
 fore recommend to the student that he should impress his mind with the 
 idea of mass by a few experiments, such as setting in motion a grindstone 
 or a well-balanced wheel, and then endeavoring to stop it, twirling a long 
 pole, etc., till he comes to associate a set of acts and sensations with the 
 scientific doctrines of dynamics, and he will never afterward be in any dan- 
 ger of loose ideas on these subjects." MAXWELL'S "Theory of Heat," p. 85 
 
 * A ball thrown up into the air with a force that would cause it to rise 
 fifty feet, will ascend to that height whatever horizontal wind may be blow- 
 ing. While riding on a car, we throw a stone at some object at rest. The 
 stone, having the motion of the train, strikes just as far ahead of the 
 object as it would have gone had it remained on the train. In order to 
 hit the mark, we should have aimed a little back of it. The circus-rider 
 wishes, while his horse is at full speed, to jump through a hoop suspended 
 before him. He simply springs directly upward. Going forward by the 
 momentum which he had acquired before he leaped from the horse, he 
 passes through the hoop and alights upon the saddle again. A person 
 
LAWS OF MOTION. 
 
 25 
 
 directly at an object and hit it, yet, within the sec- 
 ond, the mark has gone forward many feet.* If a 
 cannon-ball be thrown horizontally, it will fall as fast 
 and strike the earth as soon as if dropped to the 
 ground from the muzzle of the gun. In Fig. 5, D is 
 an arm driven by a wooden spring, E, and turning 
 
 Fio. 5. 
 
 Illustration of the Second Law of Motion. 
 
 on a hinge at C. At D is a hollow containing a 
 bullet, so placed that when the arm is sprung, the 
 ball will be thrown in the line FK. At F is a simi- 
 lar ball, supported by a thin slat, G, and so arranged 
 that the same blow which throws the ball, D, will 
 let the ball, F, fall in the line FH. The two balls 
 will strike the floor at the same instant. 
 
 6. Third Law of Motion. Action is equal to re- 
 action, and in the contrary direction. Examples: A 
 bird in flying beats the air downward, but the air 
 
 riding in a coach drops a cent to the floor. It apparently strikes where it 
 would if the coach were at rest. 
 
 * The earth moves in its orbit around the sun at the rate of about 
 eighteen miles ner second. (See "Fourteen Weeks in Astronomy," p. 106.) 
 
26 
 
 MOTION AND FORCE. 
 
 FIG. 6. 
 
 reacts and supports the bird. The powder in a gun 
 explodes with equal force in every direction, driving 
 the gun backward and the ball forward, with the 
 same momentum. Their velocities vary with their 
 weights ; the heavier the gun, the less will the recoil 
 be noticed. When we spring from a boat, unless we 
 are cautious, the reaction will drive it from the 
 shore. When we jump from the ground, we tend to 
 push the earth from us, while it reacts and pushes 
 us from it ; we separate from each other with equal 
 
 momentum, and our veloc- 
 ity is as much greater 
 than that of the earth as 
 we are lighter. We walk 
 therefore by reason of the 
 reaction of the ground on 
 which we tread. 
 
 The apparatus shown in 
 Fig. 6 consists of ivory 
 balls hung so as to vibrate readily.* If a ball be let 
 fall from one side, it will strike the second ball, 
 which will react with an equal force, and stop the 
 motion of the first, but transmit the motion to the 
 third ; this will act in the same manner, and so on 
 through the series, each acting and reacting until 
 the last ball is reached ; this will react and then 
 bound off, rising as high as the first ball fell (except 
 the loss caused by resistances to motion). If two 
 
 * The same experiments can be performed by means of glass marbles 
 or billiard balls placed in a groove. Better still, attach strings to glass 
 marbles by means of mucilage and bits of paper and suspend them from a 
 simple wooden frame. 
 
 Illustration of the Third Law of Motion. 
 
COMPOSITION OF FORCES. 27 
 
 balls be raised, two will fly off at the opposite end; 
 if two be let fall from one side and one from the 
 other, they will respond alternately. 
 
 7. Composition of Motions. Let a ball at A 
 (Fig. 7) be acted on by a force which would drive 
 it in a given time to B, and also at the same instant 
 by another which would drive it to D in the same 
 time; the ball will move in the direction AC. Ex- 
 amples : A person wishes to row a ^ ^ 
 boat across a swift current which A^ B 
 
 v^^ y 
 
 would carry him down stream. \^^^^ \ 
 
 He therefore steers toward a point \___^X 
 
 above that which he wishes to Com 8ition of Motions C 
 reach, and so goes directly across. 
 A bird, beating the air with both its wings, flies 
 
 in a direction different from that which would be 
 given by either one. 
 
 8. Composition of Forces. When a body is thus 
 
 acted on by two forces, 
 we draw lines represent- 
 ing their directions, and 
 mark off AD and AB, 
 whose lengths represent 
 their comparative mag- 
 nitudes. We next com- 
 plete the parallelogram 
 and draw the diagonal 
 AC, which denotes the 
 composition of TWO Forces. resultant of these forces, 
 
 and gives the direction in which the body will move. 
 
28 
 
 MOTION AND FORCE. 
 
 If more than two forces act, we find the resultant 
 of two, then of that resultant and a third force, and 
 so on. 
 
 9. Triangle of Forces. In Fig. 8 the resultant, 
 AC, could have been obtained more easily by draw- 
 ing AB to represent the magnitude and direction 
 of one force, and then similarly BO for the other 
 force. Connecting the initial point, A, of the first 
 line with the terminal point, C, of the second line, 
 we have AC for the magnitude and direction of the 
 resultant, which completes a triangle. 
 
 1C. Polygon of Forces. Let F t , F z , F z , F, and 
 
 F 5 (Fig. 9), represent five forces acting 
 on the same point at the same time. 
 To find their resultant, we draw (Fig. 
 10) OA, AB, BO, OD, and DE, equal 
 and parallel respectively to F^, F^, F 3 , 
 Ft, and F 6 . Then, joining the first point 
 with the last, we have 
 Five Forces acting OE to represent the 
 
 at the same Point. 
 
 Pie. 9. 
 
 Polygon of Forces. 
 
 tion of their combined resultant. 
 For OB is the resultant for OA and 
 AB; OC for OB and BC ; OD for 
 OC and CD ; and OE for OD and 
 DE. This method is applicable to 
 the representation of any number of forces. 
 
 11. Resolution of Forces consists in finding what 
 forces are equivalent to a given force under special 
 conditions. A triangle is drawn, having the given 
 
RESOLUTION OF FORCES. 
 
 29 
 
 force as one side. Example : There is a wind, blow- 
 ing nearly from the 
 west (Fig. 11) against 
 the sail, AS, of a vessel 
 going northward. We 
 may regard the wind- 
 force, WC, as the re- 
 sultant of two forces, 
 WD and DC. The 
 former, being parallel 
 to the sail, is not effect- 
 ive ; the latter is per- 
 pendicular to it, and 
 tends to drive the ves- 
 
 -i-iji Eesolution of Forces. Ship sailing northward. 
 
 sel nearly north-east. 
 
 Again, resolving D<7, we find this equivalent to two 
 
 forces, DE and EG. The 
 former pushes the vessel 
 sideways, but is largely 
 counteracted by the re- 
 sistance of the water 
 against the broad side ; 
 EC is in the direction 
 of the ship's course, and 
 propels it north. 
 
 By shifting the rig- 
 ging, one vessel may 
 sail into the harbor 
 while another is sailing 
 out, both driven by the 
 same wind. In Fig. 12, 
 
 FIG. 12. 
 
 Resolution of Forces. Ship sailing south- 
 ward. 
 
80 MOTION AND FORCE. 
 
 which represents a ship sailing southward, the letter- 
 ing and explanation is the same as for Fig. 11, if 
 we substitute "south" for "north."* If the ship 
 were required to go westward, it would tack alter- 
 nately NW and SW. In this way its resultant direc- 
 tion might be almost in the "teeth of the wind." 
 
 A canal-boat drawn by horses is acted upon by a 
 force which tends to bring it to the bank. This 
 force may be resolved into two, one pulling toward 
 the tow-path, and the other directly ahead. The 
 former is counteracted by the shape of the boat and 
 the action of the rudder ; the latter draws the boat 
 forward. 
 
 12. Motion in a Curve. To change the direction 
 of a moving body requires an external force (p. 22, 
 4). When this force is a continuous force and the 
 direction in which it acts differs continually from the 
 path in which the body moves, the body will describe 
 a curved line. Example: A body thrown obliquely 
 into the air describes a curve, because the force of 
 gravity continually changes the direction of the mov- 
 ing body. 
 
 13. Circular Motion is produced when a moving 
 body is drawn toward a center by a constant force. 
 
 * In a similar manner we may resolve the three forces which act upon 
 a kite viz., the pull of the string, the force of the wind, and its own 
 weight. In Fig. 11, let AB represent the face of the kite. We can resolve W C, 
 the force of the wind, into WD and DC. We next resolve DC into DE and 
 EC. We then have a force, EC, which overcomes the weight of the kite 
 and tends to lift it upward. The string pulls in the .direction CD, perpen- 
 dicularly to the face. The kite obeys neither one of these forces alone, but 
 both, and so ascends in a direction CA between the two. It is really drawn 
 up an inclined plane by the joint force of the wind and the string. 
 
CIRCULAR MOTION. 31 
 
 Thus, when a sling is whirled, the stone is pulled 
 toward the hand by the string, and as, according to 
 the third law of motion, every action has its equal 
 and opposite reaction, the hand is pulled toward the 
 stone. If the string break, the stone will continue 
 to move, according to the first law of motion, in a 
 straight line in the direction of a tangent to the 
 circle at that point. The tension of the string, act- 
 ing inward, is called the Centripetal (centrum, the 
 center, petere, to seek) force ; and the reaction of 
 the stone upon the string, acting outward, is termed 
 the Centrifugal (centrum, the center, fugere, to flee) 
 force.* 
 
 The following examples are among those usually 
 
 * It should be noticed that in circular motion there is but one true 
 force concerned. It acts, however, upon a body in motion. The so-called 
 centrifugal force has nothing to do with the production of the motion, 
 being merely the resistance which the body offers by its inertia to the 
 operation of the centripetal force, and ceases the instant that force is dis- 
 continued. It does not act at right angles to the centripetal force, as is 
 often stated, but in direct opposition. A body never flies off from the 
 center impelled by the centrifugal force, since that can never exceed the 
 centripetal (action = reaction), and moreover the path of such a body is in 
 the direction of a tangent, and not the radius of a circle. Thus, when 
 water is thrown off a grindstone in rapid rotation, the tendency of the 
 water to continue to move on in the direction of the straight line in which 
 it is going at each instant (in other words, the inertia of the water) over- 
 comes its adhesion to the stone, and it flies off in obedience to the first law 
 of motion. So, also, when a grindstone, driven at a high speed, breaks, 
 and the fragments are thrown with great velocity, we are not to suppose 
 that the centrifugal force impels them through the air. That force existed 
 only while the stone was entire. It was opposed to the force of cohesion, 
 and in the moment of its triumph ceased, and the fragments of the stone 
 fly off in virtue of the velocity they possess at that instant. Again, the 
 so-caUed centrifugal force is not a real force urging bodies upward at the 
 equator. The earth's surface is merely falling away from a tangent, and, a 
 part of the force of gravity is spent in overcoming the inertia of bodies. 
 The term centrifugal force has caused much confusion, and will perhaps 
 be discarded. 
 
32 
 
 MOTION AND FORCE. 
 
 given to illustrate the action of the center-fleeing 
 force : Water flies from a grindstone on account of 
 the centrifugal force produced in the rapid revolu- 
 tion, which overcomes the adhesion. In factories, 
 grindstones are sometimes revolved with such veloc- 
 ity that this force overcomes that of cohesion, and 
 the ponderous stones fly into fragments. A pail full 
 
 of water may be whirled 
 around so rapidly that 
 none will spill out, be- 
 cause the centrifugal 
 force overcomes that of 
 gravity. When a horse is 
 running around a small 
 circle, he bends inward 
 to overcome the centrif- 
 ugal force. 
 
 The heavenly bodies 
 present the grandest ex- 
 ample of circular motion. We may suppose the earth 
 to have been moving originally in the direction AE. 
 The attraction of the sun, however, drawing it in 
 the direction US, it passes along the line EE'. If 
 the centripetal force were suddenly to cease, the earth 
 would fly off into space along a tangent, as EA. The 
 rapid revolution of the earth on its axis tends to 
 throw off all bodies headlong. As this acts in oppo- 
 sition to gravity, it diminishes the weight of bodies 
 at the equator, where it is greatest, being there 
 equivalent to -^ of the force of gravity. It also 
 tends to drive the water on the earth from the poles 
 
 Circular Motion. 
 
REFLECTED MOTION; 
 
 33 
 
 FIG. 14. 
 
 toward the equator. Were the velocity of the earth's 
 rotation to diminish, the water 
 would flow back toward the poles, 
 and tend to restore the earth to 
 a spherical form.* This influence 
 is well illustrated by the appara- 
 tus shown in Fig. 14. The hoop 
 is made to slide upon its axis, 
 and if revolved rapidly it will 
 assume an oval form, bulging out 
 more and more as the velocity is 
 increased.f 
 
 14. Reflected Motion is produced by the reaction 
 of a surface against which an elastic body is cast. 
 If a perfectly elastic ball be thrown in the direction 
 
 * Since the earth's polar diameter is nearly twenty-seven miles shorter 
 than its equatorial diameter, we are not sure that this motion of its waters 
 would make it perfectly spherical. 
 
 t This apparatus is accompanied by objects to illustrate the principle 
 that all bodies tend to revolve about their shortest diameters. " Tie to the 
 middle of a lead-pencil a piece of string about three feet long. Suspend so 
 that the pencil will balance itself. Now twist the end of the string between 
 the thumb and the first finger of the right hand, steadying and holding 
 the string with the left hand. A circular motion will thus be communi- 
 cated to the pencil, and it will revolve around the point on which it is 
 suspended. Tie a piece of white string around the middle of the pencil, 
 or its center of gravity, simply to show the position of that point. Now 
 tie the first piece of string half-way between the end of the pencil and 
 the center of gravity, and communicate the circular motion described 
 above, and we shall observe that the pencil will still revolve around the 
 center of gravity, the point marked by the white string being at rest. It 
 can thus be shown that any thing, of whatever shape, will tend to revolve 
 on its shortest diameter. If the end links of a small steel chain (such as 
 is often attached to purses or parasols) be hooked together, the string tied 
 to a link, and the circular motion given, it will be observed that the chain 
 begins to take an elliptical form, which gradually approaches that of a 
 circle, until at last it becomes a circle, when it revolves horizontally. 
 This shows that even a ring revolves on its shortest axis." 
 
34 MOTION AND FOKCE. 
 
 OP against the surface AS, it will rebound in the 
 line PQ. The angle, i, between the direction OP and 
 the perpendicular, PR, drawn at the point of inci- 
 dence, is called the angle of incidence. The angle 
 
 of reflection, r, is 
 that between this 
 perpendicular, PR, 
 and the direction 
 PQ. If OP repre- 
 sent the magnitude 
 
 Reflected Motion. and directi(m of the 
 
 incident force, it may be resolved into OR and RP. 
 But the reaction, PR, is equal to the vertical portion, 
 RP, of the incident force, while the horizontal por- 
 tion is not checked. Hence PQ = OP, and the angle 
 of incidence is equal to the angle of reflection. 
 
 15. Energy has been defined as the power of produc- 
 ing change of any kind (p. 4, 5). To produce a change 
 resistance must be overcome, and " work is done when 
 resistance is overcome." Therefore energy may also be 
 defined as the power of doing work. It is in general 
 a power put into a body by means of work, and which 
 comes out of it when it does work. Examples: A wound- 
 up clock, a red-hot iron. The difference between energy 
 and momentum is easily illustrated. When a bullet is 
 fired from a rifle, the momenta of both are equal, but 
 the energy of the former, i. e., its power of doing work, 
 as piercing a board, is far greater. Energy is propor- 
 tional to the square of the velocity of the moving body. 
 Thus, a cannon-ball given double speed will penetrate 
 four times as far into a wall ; and a stone thrown up- 
 
ENEKG-Y. 35 
 
 ward at the rate of ninety- six feet per second will rise 
 nine times as far as with a velocity of thirty-two feet. 
 
 1G. Two Forms of Energy. Energy may be either 
 active or latent. When a rock is tumbling down a moun- 
 tain-side, it exhibits the force of gravity in full sway ; 
 but when the rock was lodged on the mountain-top, it 
 possessed the same energy, which could be developed 
 at any moment by loosening it from its place. These 
 two forms are known as energy of motion and energy 
 of position, or kinetic and potential energy.* 
 
 17. Conservation of Energy. The sum of all the 
 
 energy in the universe remains the same while its 
 transformations are infinite. One kind of energy is 
 changed into another ; from an available form to one 
 that is not controllable. A hammer falls by the force 
 of gravity. In coming to rest when stopped, it does 
 the work of crushing what it hits, and its motion as 
 a mass is converted into one of molecules, revealing 
 
 * The following may be taken as examples to show the difference 
 between kinetic and potential energy. We wind a watch, and by a few 
 moments of labor condense in the spring a potential energy, which is doled 
 out for twenty-four hours in the kinetic energy of the moving wheels and 
 hands. Lift a pendulum, and you thereby give the weight potential energy. 
 Let it fall, and the potential changes gradually to kinetic. At the center 
 of the arc the potential is gone and kinetic is possessed. Then the kinetic 
 changes again to potential, which increases till thend of the arc is reached 
 and the pendulum ceases to rise, when the energy is that of position, not 
 of motion. Potential energy is like what is concealed, lying in wait and 
 ready to burst forth on the instant. It is that of a loaded gun prepared 
 for the arm of the marksman. It is that of a river trembling on the brink 
 of a precipice, about to take the fearful leap. It is that of a weight wound 
 up and held against the tug of gravity. It is that of the engine on the 
 track with the steam hissing from every crevice. On the contrary, kinetic 
 energy is that in actual operation. The bullet is speeding to the mark; 
 the river is tumbling ; the weight is falling ; the engine is flying over the 
 rails. It is that of heat radiating from our fires, and electricity carrying our 
 messages over the continent. 
 
36 MOTION AND FORCE. 
 
 itself to our touch as heat. The sun is continually 
 sending forth radiant energy, which has been stored 
 up in it by the aggregation of matter during untold 
 ages. Its kinetic energy is thus becoming dissipated 
 into potential energy; but, even after it ceases to 
 glow, the grand total, including all that was once 
 kinetic, will remain unchanged. 
 
 PRACTICAL QUESTIONS. 
 
 1. A rifle-ball thrown against a board standing edgewise, will knock it 
 down ; the same bullet fired at the board will pass through it without dis- 
 turbing its position. Why is this? 
 
 2. Why can a boy skate safely over a piece of thin ice, when, if he 
 should pause, it would break under him directly? 
 
 3. Why can a cannon-ball be fired through a door standing ajar, with- 
 out moving it on its hinges? 
 
 4. Why can we drive on the head of a hammer by simply striking the 
 end of the handle? 
 
 5. Suppose you were on a train of cars moving at the rate of 30 miles 
 per hour; with what velocity would you be thrown forward if the train 
 were stopped instantly? 
 
 6. In what line does a stone fall from the mast-head of a vessel in 
 motion ? 
 
 7. If a ball be dropped from a high tower, it will strike the ground a 
 little east of a vertical line. Why is this? 
 
 8. It is stated that a suit was once brought by the driver of a light 
 wagon against the owner of a coach for damages caused by a collision. 
 The complaint was "the latter was driving so fast that when the two car- 
 riages struck, the driver of the former was thrown forward over the dash- 
 board." On trial he was nonsuited, because his own evidence showed him 
 to be the one who was driving at the unusual speed. Explain. 
 
 9. Suppose a train moving at the rate of 30 miles per hour: on the 
 rear platform is a spring gun aimed parallel to the track and in a direction 
 precisely opposite to the motion of the car. Let a ball be discharged with 
 the exact speed of the train; where would it fall? 
 
 10. Suppose a steamer in rapid motion, and on its deck a man jumping. 
 Can he jump farther by leaping the way the boat is moving than in the 
 opposite direction? 
 
 11. Could a party play ball on the deck of an ocean steam-ship when 
 
PRACTICAL QUESTIONS. 37 
 
 steaming along at the rate of 20 miles per hour, without making allowance 
 for the motion of the ship ? 
 
 12. Since action "is equal to reaction, why is it not so dangerous to 
 receive the ' kick " of a gun as the force of the bullet? 
 
 13. If you were to jump from a carriage in rapid motion, would you 
 leap directly toward the spot on which you wished to alight? 
 
 14. If you wished to shoot a bird in swift flight, would you aim di- 
 rectly at it? 
 
 15. At what parts of the earth is the centrifugal force least ? 
 
 16. What causes the mud to fly from the wheels of a carriage in rapid 
 motion ? 
 
 17. What proof have we that the earth was once a soft mass? 
 
 18. On a curve in a railroad, one track is always higher than the 
 other. Why is this? 
 
 19. What is the principle of the sling? 
 
 20. The mouth of the Mississippi River is about 2J miles farther from 
 the center of the earth than its source. In this sense it may be said to 
 "run up hill." What causes this apparent opposition to the attraction of 
 gravity ? 
 
 21. Is it action or reaction that breaks an egg, when I strike it against 
 the table? 
 
 22. Was the man philosophical who said that it "was not the falling 
 so far, but the stopping so quick, that hurt him " ? 
 
 23. If one person runs against another, which receives the greater 
 blow? 
 
 24. Would it vary the effect if the two persons were running in oppo- 
 site directions? In the same direction? 
 
 25. Why can you not fire a rifle-ball around a hill? 
 
 26. Why is it that a heavy rifle "kicks" less than a light shot-gun? 
 
 27. A man on the deck of a large vessel draws a small boat toward 
 him. Can you express the ratio of the ship's motion to that of the boat? 
 
 28. Suppose a string, fastened at one end, will just support a weight of 
 25 Ibs. at the other. Unfasten it, and let two persons pull upon it in oppo- 
 site directions. How much can each pull without breaking it? 
 
 29. Can a man standing on a platform-scale make himself lighter by 
 lifting up on himself ? 
 
 30. Why can not a man lift himself by pulling up on his boot-straps? 
 
 31. With what momentum would a steam-boat weighing 1,000 tons, and 
 moving with a velocity of 10 ft. per second, strike against a sunken rock? 
 
 32. With what momentum would a train of cars weighing 100 tons, 
 and running 10 miles per hour, strike against an obstacle? 
 
 33. What would be the comparative kinetic energy of two hammers, 
 one driven with a velocity of 20 ft. per second and the other 10 ft. ? 
 
 34. If a 100 horse-power engine can propel a steamer 5 miles per hour, 
 will one of 200 horse-power double its speed ? 
 
 35. Why are ships becalmed at sea often floated by strong currents 
 into dangerous localities without the knowledge of the crew? 
 
38 MOTION AND FORCE. 
 
 36. A man in a wagon holds a 50-lb. weight in his hand. Suddenly 
 the wagon falls over a precipice. Will he, while dropping, bear the strain 
 of the weight? 
 
 37. Why are we not sensible of the rapid motion of the earth? 
 
 38. A feather is dropped from a balloon which is immersed in and 
 swept along by a swift current of air. Will the feather be blown away or 
 will it appear to drop directly down? 
 
 39. Suppose a bomb-shell, flying through the air at the rate of 500 ft. 
 per second, explodes into two parts of equal weight, driving one half for- 
 ward in the same direction as before, but with double its former velocity. 
 What would become of the other half? 
 
 40. Which would have the greater penetrating power, a 10-lb. cannon- 
 ball with a velocity of 1,000 ft. per second, or a 100-lb. ball with a velocity 
 of 100 ft. per second? 
 
 41. There is a story told of a man who erected a huge pair of bellows 
 in the stern of his pleasure-boat, that he might always have a fair wind. 
 On trial, the plan failed. In which direction should he have turned the 
 beUows? 
 
 42. If a man and a boy were riding in a wagon, and, on coming to the 
 foot of a hill, the man should take up the boy in his arms, would that 
 help the horse? 
 
 43. If we whirl a pail of water swiftly around with our hands, why 
 will the water tend to leave the center of the pail? 
 
 44. Why will the foam collect at the hollow in the center? 
 
 45. If two cannon-balls, one weighing 8 Ibs. and the other 2 Ibs., be 
 fired with the same velocity, which will go the farther? 
 
 46. Resolve the force of the wind which turns a common windmill, 
 and show how one part acts to push the wheel against its support, and one 
 to turn it around. 
 
 47. When an animal is jumping or falling, can any exertion made in 
 mid-air change the motion of its cent^r of gravity? 
 
 48. If one is riding rapidly, in which direction will he be thrown when 
 the horse is suddenly stopped? 
 
 49. When standing in a boat, why, as its starts, are we thrown back- 
 ward? 
 
 50. When carrying a cup of tea, if we move or stop quickly, why is 
 the liquid liable to spill ? 
 
 51. Why, when closely pursued, can we escape by dodging? 
 
 52. Why is a carriage or sleigh, when sharply turning a corner, liable 
 to tip over? 
 
 53. Why, if you place a card on your finger and on top of it a cent, can 
 you snap the card from under the cent, which will then drop on your finger? 
 
 54. Why is a "running jump" longer than a "standing jump"? 
 
 55. Why, after the sails of a vessel are furled, does it still continue to 
 move ? and why, after the sails are spread, does it require some time to get 
 it tinder full headway ? 
 
 56. Why can a tallow candle be fired through a board ? 
 
SUMMAKY. 39 
 
 SUMMARY. 
 
 MATTER, so far as we know it, is in constant change. 
 Change of place is termed motion. Terrestrial motion is re- 
 stricted by friction, by the air, and by water. Friction is 
 caused by the roughness of the surface over which a body 
 moves. It may be decreased by the use of grease to fill up the 
 minute projections, or by changing the sliding into rolling fric- 
 tion. Air and water must be displaced by a moving body ; the 
 resistance they offer is measured by the kinetic energy expended 
 in overcoming it, and is hence proportional to the square of its 
 velocity. Motion takes place in accordance with three laws ; 
 viz. : A moving body left to itself tends to go forever in a 
 straight line with a constant velocity ; a force has the same effect 
 whether it acts alone or with other forces, and upon a body at 
 rest or in motion ; and action is equal and opposed to reaction. 
 By means of the principles of the composition and resolution of 
 forces, we can find the individual effect of a single force or the 
 combined effect of several forces. To change the path of a mov- 
 ing body requires an external force. When the direction of the 
 action of this force continually differs from the direction of the 
 motion of the body, the path of the latter is that of a curved line. 
 A particular case of this is circular motion. The path of a 
 bullet or rocket in the air exemplifies curvilinear motion; and 
 the movement of a stone whirled in a sling and of a planet re- 
 volving about the sun are illustrations of circular motion. When 
 a rubber ball bounds back from a surface against which it is 
 thrown, the angle of reflection equals the angle of incidence. 
 
 Energy is the power of overcoming resistance, i.e., of doing 
 work. This power, possessed by bodies, may be either latent 
 potential as in the case of a suspended weight, or actual- 
 kinetic as in the case of the weight when falling.* 
 
 * When energy refers to that possessed by visible masses it is called molar 
 energy ; when it refers to the energy possessed by molecules it is called molecu- 
 lar energy. A body is hot because of the kinetic energy of its molecules. 
 Heat is, therefore, an example of molecular energy. Light, sound, electricity 
 are other forms of molecular energy. The different forms of energy are 
 mutually convertible. The molar energy of a falling mass when stopped is 
 transferred to its molecules and becomes the molecular energy of heat. This 
 
40 MOTIOK AND FORCE. 
 
 HISTORIC AL SKETCH. 
 
 ARISTOTLE taught that all motion is naturally circular, and 
 this view was held by his school. He divided the phenomena 
 of motion into two classes the natural and the violent. As an 
 instance of the former, he gave the falling of a stone, which 
 constantly increases in velocity ; and of the latter, a stone 
 thrown vertically up, which being against nature, continually 
 goes slower. Newton, in his "Principia," published in 1687, 
 propounded the laws of motion as now received. Other philos- 
 ophers, notably Galileo, Hooke, and Huyghens, had anticipated 
 much of his reasoning, yet so slowly were his opinions accepted 
 that "at his death," says Voltaire, "he had not more than 
 twenty followers outside of England." 
 
 The law of the Conservation of Energy, Faraday, the great 
 English physicist, pronounced "the grandest ever presented for 
 the contemplation of the human mind." It has been established 
 within the present century ; yet we now know that former 
 scholars had inklings of the wonderful truth. It arose in con- 
 nection with discoveries on the subject of Heat, and its history 
 will be treated of hereafter. 
 
 Consult Stewart's "Conservation of Energy"; Youmans' 
 "Correlation of the Physical Forces"; Faraday's "Lectures on 
 the Physical Forces"; Everett's " Deschanel's Natural Philos- 
 ophy"; Tait's "Recent Advances in Physical Science"; Max- 
 well's "Matter and Motion *' ; "Appleton's Cyclopedia," Art. 
 Correlation of Forces; Tyndall's "Crystalline and Molecular 
 Forces," in Manchester Science Lectures, '73-4; Crane's "Ball 
 Paradox," in Popular Science Monthly, Vol. X., p. 725. 
 
 energy in the steam-engine is reconverted into the molar energy of the train. 
 Every change that takes place in the universe is a case either of transfer- 
 ence or transformation of energy. Energy, though convertible, is as inde- 
 structible as matter it is conserved. The principle of the Conservation of 
 Energy teaches that the quantity of energy in the universe is constant, 
 that it can neither be increased nor diminished. "We cannot account for 
 its origin; we only know its law of action, which we must finally refer to a 
 Supreme Being. 
 
III. 
 
 ATTRACTION. 
 
 " THE smallest dust which floats upon the wind 
 Bears this strong impress of the Eternal mind: 
 In mystery round it subtle forces roll, 
 And gravitation binds and guides the whole." 
 
 " Attraction, as gravitation, is the muscle and tendon of the universe, 
 by which its mass is held together and its huge limbs are wielded. As 
 cohesion and adhesion, it determines the multitude of physical features of 
 its different parts. As chemical or interatomic action, it is the final source 
 to which we trace all material changes." ABNOTT. 
 
ANALYSIS OF MOLECULAR FORCES. 
 
 ' ATTRACTIVE AND REPELLENT FORCES. 
 
 1. COHESION. * 
 
 2. ADHESION. H 
 
 II. ATTRACTION 
 
 OF 
 GRAVITATION. 
 
 1. Definition of Cohesion. 
 
 2. Three States of Matter. 
 
 3. Cohesion acts at Insensible Distances. 
 
 4. Surface Tension of Liquids. 
 
 5. Liquids Tend to Form Spheres. 
 
 6. Solids Tend to Form Crystals. 
 
 7. Annealing and Tempering. 
 
 8. Rupert's Drop. 
 
 1. Definition and Illustration of Adhe- 
 
 sion. 
 
 f (1.) Direct. 
 
 2. Capillarity. J (2.) Reversed. 
 
 [ (3.) Law of Capillarity. 
 
 3. Imbibition. 
 
 4. Solution. 
 
 5. Diffusion of Liquids. 
 
 6. Diffusion of Gases. 
 
 7. Osmose. 
 
 1. Law of Gravitation. 
 
 2. Illustrations of Gravity. 
 
 3. Mass and Weight. 
 
 4. The Earth's Center of Gravity. 
 
 5. The Center of Gravity of a Body. 
 
 6. Laws of Weight. 
 
 C (1.) In a Vacuwm. 
 
 (2.) In the Air, with At- 
 
 7. Falling I ^^ MacM ^ 
 
 Bodies. I (3) ExpeT i ment s with this 
 {_ Machine. 
 
 8. Equations of Bodies Falling Freely. 
 
 9. Measurement of Kinetic Energy. 
 
 10. Resistance of the Air to Falling 
 
 Bodies. 
 
 11. Equilibrium. 
 
 1 12. The Pendulum. 
 
ATTRACTION. 
 
 I. MOLECULAR FORCES. 
 
 Attractive and Repellant Forces. If we take a 
 piece of iron and attempt to pull it to pieces, we find 
 that there is a force which holds the molecules to- 
 gether and resists our efforts. If we try to compress 
 the metal, we find that there is a force which holds 
 the molecules apart and resists our efforts as before. 
 We thus see that there are two opposing forces ah 
 attraction and a repulsion which operate between the 
 molecules and resist change of distance between 
 them. 
 
 1. COHESION. 
 
 1. Cohesion is that force which holds together 
 molecules of the same kind. 
 
 2. Three States of Matter. Matter occurs in 
 three states solid, liquid, and gaseous. The mole- 
 cules of a solid, are held together by cohesion, and this 
 force resists increase of distance between them. This 
 resistance may, however, be overcome by the applica- 
 tion of heat-energy. The molecules of a solid are not 
 really fixed and at rest, but move swiftly to and fro 
 within narrow limits, without being able to leave these 
 limits. By the application of heat the path over which 
 
44 ATTRACTION. 
 
 the molecules move and the intensity of their motion 
 is increased, and the solid expands. At a certain stage 
 in this process of heating the distance between the mole- 
 cules becomes great enough to permit them to wander 
 slowly through the substance. The solid has now be- 
 come a liquid. Matter in this state has no independent 
 form, but assumes the shape of the containing vessel. 
 Continuing with the application of heat, the spaces be- 
 tween the molecules become finally so great that the 
 attraction between them is canceled, permitting them 
 to dart hither and thither with great velocity. This rep- 
 resents the gaseous state. In this state the molecules 
 require to be restrained by the walls of the containing 
 vessel. By pressure and cooling gases are liquefied ; 
 by further cooling liquids are rendered solid. 
 
 3. Cohesion Acts at Insensible Distances. Take 
 two bullets, and having flattened and cleaned one 
 side of each, press them together with a twisting 
 motion. They will cohere when the molecules are 
 crowded into apparent contact.* If two globules of 
 mercury be brought near each other, at the instant 
 they seem to touch they will suddenly coalesce. Two 
 freshly-cut surfaces of rubber, when warmed and 
 pressed together, will cohere as if they formed one 
 piece. The process of welding illustrates this princi- 
 ple. A wrought-iron tool being broken, we wish to 
 mend it. So we bring the iron to a white heat at the 
 
 * Surfaces may appear to the eye to be in contact when they are not 
 actually so. Newton found, during some experiments on light (p. 220), that 
 a convex lens or a watch-glass laid on a flat glass does not touch it, and 
 can not be made to do so, even by a force of many pounds. 
 
COHESION. 45 
 
 ends which we intend to unite. This partly over- 
 comes the attraction of cohesion, and the molecules 
 will move easily upon one another. Laying now one 
 of the two heated ends upon the other, we pound 
 them until the molecules are brought near enough 
 for cohesion to grasp them. 
 
 4. Surface Tension of Liquids. Within a liquid 
 each molecule is pressed by the weight of all those 
 above it, so that the slight cohesion between it and 
 its neighbors is masked. At the surface there is 
 no such liquid pressure. Cohesion causes the sur- 
 face film to be in a state of tension like a sheet of 
 stretched India rubber. A double film may be de- 
 tached by using soapy water and lifting out of it the 
 bowl of a pipe. The elasticity and toughness of the 
 film are shown by blowing it into a bubble, whose 
 surface is many times greater than that across the 
 bowl that held it. A candle flame may be blown out 
 by the bubble as it contracts toward its own center 
 and expels a breeze of air through the tube of the 
 pipe.* 
 
 5. Liquids Tend to Form Spheres. Mix alcohol 
 and water in such proportion that 'a drop of olive-oil 
 
 * There are many charming experiments that can be made with soap 
 films. (See Popular Science Monthly, Vol. IX., p. 575 ; Scientific American, May 
 15, 1886; Scientific American Supplement, Jan. 25, 1879.) A good recipe for 
 making soap solution is as follows : Procure some of the best white Castile, 
 or palm-oil soap. Scrape from it about four ounces of thin shavings, put 
 these into a quart bottle of purest rain-water, or distilled water, and shake 
 until the strongest clear solution of the soap is had. Then add a pint of 
 pure concentrated glycerine. A film from this mixture will last for hours, 
 and bubbles over a foot in diameter are easily made. 
 
ATTRACTION. 
 
 will sink into it without going to the bottom. The 
 action of gravity on it is just balanced by the buoy- 
 ancy of the liquid, so that cohesion can act without 
 much interference. Like a soap-bubble, the outside 
 film of the oil tends to contract upon its interior, 
 and a nearly perfect sphere remains suspended. The 
 contraction of the tough surface film is what pro- 
 duces the roundness of dew-drops, rain-drops, glob- 
 ules of mercury, and melted lead as it falls and cools 
 into round shot. 
 
 6. Solids Tend to Form Crystals. A liquid in 
 becoming solid shows a tendency to assume a regular 
 form bounded by plane surfaces termed a crystal. 
 
 FIG. 16. 
 
 Ice Flowers. 
 
 Crystals of different substances show in most cases 
 marked differences in form, which may serve for the 
 recognition of the substance.* When different sub- 
 
 * Epsom salt crystallizes in four-sided prisms, common salt in cubes, 
 and alum in octahedra. We can illustrate the formation of the last by 
 adding alum to hot water until no more will dissolve. Then suspend strings 
 across the dish and set it away to cool. Beautiful octahedral crystals will 
 
COHESION. 47 
 
 stances are contained in the same solution, they sep- 
 arate on crystallization, and each molecule goes to its 
 'own. The exquisite beauty of these crystalline forms 
 is seen in snow-flakes and the frost-work traced on a 
 cold morning upon the windows or the stone-flagging. 
 A beam of light passed through a block of ice reveals 
 these crystals as a mass of star-like flowers (Fig. 16).* 
 Melted iron rapidly cooled in a mold has not time 
 to arrange its crystals. If, however, the iron be after- 
 ward violently jarred, as when used for cannon, rail- 
 cars, etc., the molecules take on the crystalline form 
 and the metal becomes brittle. f 
 
 7. Annealing and Tempering. If a piece of 
 wrought-iron be heated and then plunged into water, 
 it becomes hard and brittle. If, on the contrary, it 
 be heated and cooled slowly, it is made tough and 
 flexible. Steel is tempered by heating red-hot, then 
 cooling quickly, and afterward re-heating and cooling 
 slowly. It becomes then one of the most elastic and 
 tough of known substances. 
 
 8. The Rupert's Drop is a tear of melted glass 
 dropped into water, and cooled quickly. The exte- 
 
 collect on the threads and sides of the vessel. The slower the process, the 
 larger the crystals. 
 
 * It is noticeable that, as the crystals melt, at the center of each liquid 
 flower Is a vacuum, showing that there is not enough water formed to fill 
 the space occupied by the crystal, and that the solid contracts as it passes 
 into a fluid (p. 271). This experiment is easily tried. The ice must be cut 
 parallel to the plane of its freezing and be not over half an inch thick. A 
 common oil lamp will furnish the light. 
 
 t On examining such a piece of iron, which can easily be procured at 
 a car or machine shop, we can see in a fresh fracture the smooth, shin- 
 ing faces of the crystals. 
 
48 ATTRACTION. 
 
 rior at once becomes rigid, while the interior is still 
 hot and expanded. When the whole mass is cool, the 
 interior is in a state of strain. If the tail 
 of the drop be nipped off, so that the ex- 
 terior shell is broken, the tension will cause 
 the mass to fly into powder with a sharp 
 explosion. All glassware, when first made, 
 is brittle, but it is annealed by being 
 
 Rupert's Drop. 
 
 drawn slowly through a long oven, highly 
 heated at one end, but quite cool at the other. Dur- 
 ing this passage, the molecules of glass have time 
 to arrange themselves in a stable position.* 
 
 PRACTICAL QUESTIONS. 
 
 1. Why can we not weld a piece of copper to one of iron? 
 
 2. Why is a bar of iron stronger than one of wood ? 
 
 3. Why may a piece of iron, when perfectly welded, be stronger than 
 before it was broken ? 
 
 4. Why do drops of different liquids vary in size? 
 
 5. When you drop medicine, why will the last few drops contained in 
 the bottle be of a larger size than the others? 
 
 6. Why are the drops larger if you drop them slowly? 
 
 7. Why, if you melt scraps of lead, will they form a solid mass when 
 cooled ? 
 
 8. In what liquids is the force of cohesion greatest? 
 
 9. Why does iron, by continued jarring, become brittle? 
 
 10. Why can glass be welded? 
 
 11. Name some substances that can not be welded. Why not? 
 
 12. What liquids would you select for showing surface tension? 
 
 * " The restoration of cohesion is beautifully seen in the gilding of china. 
 A figure is drawn upon the china with a mixture of oxide of gold and an 
 essential oil. The article is then heated, whereby the essential oil and the 
 oxygen of the gold are expelled, and a red-brown pattern remains. This 
 consists of pure gold in a finely-divided state, without luster. By rubbing 
 with a hard burnisher, the particles of gold cohere and reflect the rich 
 yellow color of the polished metal." 
 
ADHESION. 
 
 49 
 
 2. ADHESION. 
 
 1. Adhesion is the force which holds together 
 molecules of different kinds. Examples: Two pieces 
 of wood are fastened together with glue, two pieces 
 of china with cement, two bricks with mortar, two 
 sheets of paper with mucilage, and two pieces of tin 
 with solder. Syrup and coal-oil are purified by filter- 
 ing through animal charcoal. Bubbles can be blown 
 from soap-suds, because the soap by its adhesive force 
 holds together the particles of water. 
 
 2. Capillarity. If there is strong adhesion between 
 a liquid and a solid partly immersed in it, the liquid 
 rises above the general level and wets the solid, caus- 
 ing the surface film along the line of PI O . is. 
 contact to be concave upward. This is 
 
 shown in Fig. 18, which represents a 
 tube of glass dipped in water. On the 
 inner side of the tube the concavity is 
 more marked in proportion as the bore 
 FIG. 19. is less. The contraction of the 
 surface film (p. 45) balances 
 the weight of the liquid above Direct capillarity, 
 the level within the tube. This is called 
 capillarity (capillus, a hair), because best 
 shown in tubes with a bore as fine as a hair. 
 If the tube be thrust into a liquid, like 
 mercury, which does not wet it, the capil- 
 larity is reversed ; the liquid is convex up- 
 ward, and within the tube it is depressed. 
 A striking illustration of the effect of narrowing 
 
 Keversed 
 Capillarity. 
 
50 
 
 ATTRACTION. 
 
 FIG. 20. 
 
 The Capillary Curve. 
 
 the exposed surface of the film may be seen by 
 putting two clean glass plates edgewise into colored 
 water ' so that their lower part 
 shall be immersed, with a pair of 
 upright edges touching (Fig. 20.) 
 Just at the edge the liquid rises 
 to the top ; the height decreases 
 as the successive distances be- 
 tween the two plates increases, 
 and the liquid surface seen edge- 
 wise forms a curve called the 
 Hyperbola. 
 
 Law of Capillarity.* The height to which a liquid 
 rises in a wetted tube varies inversely as the diam- 
 eter. Example: In a tube whose inner diameter is 
 1 mm. (see p. 12), water rises at ordinary tempera- 
 ture (62F) to a height of 29 mm. (a little over an 
 inch) ; if the tube be only | mm. in diameter, the 
 height will be twice as great. Cold water rises higher 
 than warm water, or than alcohol or ether. 
 
 3. Imbibition. Many porous bodies (sensible pores, 
 p. 8), such as sugar, blotting paper, sand, a lamp 
 wick, a towel, absorb liquids at a rate which is in- 
 creased if the materials be warm.t This is due to the 
 
 * For further discussion of Surface Tension and Capillarity, consult 
 Maxwell's "Theory of Heat," p. 280; "American Journal of Science," 
 Dec.. 1882, p. 416; Pickering's "Physical Manipulation," Vol. I., p. 102; 
 Daniells' "Physics," p. 245; Deschanel's "Natural Philosophy," p. 130. 
 
 t In the same way, water is drawn to the surface of the ground to fur- 
 nish vegetation with the materials of growth. Even in the winter, when 
 the surface is frozen, the water still finds its way upward, and freezes into 
 ice, which in the spring produces mud, although there may have been little 
 rain or snow. 
 
ADHESION. 51 
 
 attraction between the solid and the liquid. Hopes 
 absorb water by imbibition, swell, and shrink often 
 to breaking.* 
 
 4. Solution. Sugar will dissolve in water, because 
 the adhesion between the two substances is stronger 
 than the cohesion of the sugar, f As heat weakens 
 cohesion, it hastens solution, so that a substance gen- 
 erally dissolves more rapidly in hot water than in 
 cold. In like manner, pulverizing a solid aids solu- 
 tion. Liquids also absorb gases by adhesion. Thus 
 water contains air, which renders it pleasant to the 
 -taste. As pressure and cold weaken the repellent 
 force, they favor the adhesion between the molecules 
 
 * It is 1586. The Egyptian obelisk, weighing a million pounds, is to be 
 raised in the square of St. Peter's, Borne. Pope Sixtus V. proclaims that no 
 one shall utter a word aloud until the engineer announces that all danger 
 is passed. As the majestic column ascends, all eyes watch it with wonder 
 and awe. Slowly it rises, inch by inch, foot by foot, until the task is almost 
 completed, when the strain becomes too great. The huge ropes yield and 
 slip. The workmen are dismayed, and fly wildly to escape the impending 
 mass of stone. Suddenly a voice breaks the silence. "Wet the ropes," rings 
 out clear-toned as a trumpet. The crowd look. There, on a high post, stand- 
 ing on tiptoe, his eyes glittering with the intensity of excitement, is one of 
 the eight hundred workmen, a sailor named Bresca di S. Bemo. His voice 
 and appearance startle every one; but his words inspire. He is obeyed. 
 The ropes swell and bite the stone. The column ascends again, and in due 
 time stands securely on its pedestal. The daring sailor is not only for- 
 given, but his descendants to this day enjoy the reward of providing the 
 palm-branches used on Palm Sunday at St. Peter's. 
 
 t This contest between adhesion and cohesion is seen when we let fall 
 on water a drop of oil. Adhesion tends to draw the oil to the liquid, so as 
 to mix thoroughly, and cohesion to prevent this. The extent to which the 
 drop will spread will depend on the relation of the two attractions, and vary 
 for every substance. Thus each oil has its own COHESION FIGTJBE, which en- 
 ables the chemist readily to detect differences and mixtures. Experiments: 
 Dissolve a little salt in a glass of water, and touch the surface of the liquid 
 with a pen full of ink. The characteristic figures will quickly appear. Dis- 
 solve in water a pinch of salt and a lump of loaf-sugar. Touch the surface 
 with lunar caustic. Tha figure of nitrate of silver will be seen. 
 
52 ATTRACTION. 
 
 of a gas and water. Soda-water receives its effer- 
 vescence and pungent taste from carbonic-acid gas, 
 which, being absorbed under great pressure, escapes 
 21 in sparkling bubbles when the pressure is 
 removed. 
 
 5. Diffusion of Liquids. Let a jar be 
 
 partly filled with water colored by blue 
 litmus. Then, by a funnel-tube, pour clear 
 water containing sulphuric acid to the 
 bottom, beneath the colored water. At 
 first, the two will be distinctly defined, but 
 Diffusion of in a few days they will mix, as will be 
 Liquids. seen "by the change of color from blue to 
 red. A drop of sulphuric acid may thus be distrib- 
 uted through a quart of water. Most liquids will min- 
 gle when brought in contact. If, however, 
 
 TFrn OO 
 
 there is no adhesion between their mole- 
 cules, they will not mix, and will separate 
 even after having been thoroughly shaken 
 together. For example, shake together mer- 
 cury, water, and sweet oil ; they will soon sep- 
 arate and settle in layers with the oil at the 
 top and the mercury at the bottom. Diffu- 
 sion is a slow process, so we generally help it Diffusion of 
 by shaking or stirring the mixture of liquids.* 
 
 6. Diffusion of Gases. Hydrogen gas is only -fa as 
 heavy as common air. Yet, if two bottles be ar- 
 ranged as in Fig. 22, the lower one filled with the 
 
 * A story is told of some negroes in the West Indies who supplied them- 
 selves with liquor by inverting the neck of a bottle of water in the bung- 
 
ADHESION. 
 
 53 
 
 FIG. 23. 
 
 heavy gas, and the upper with the lighter, the gases 
 will soon be uniformly mixed.* 
 
 7. Osmose. When two liquids are separated by a 
 thin substance, the interchange may be modified in 
 
 hole of a cask of rum. The water sank into the barrel, -while the rum rose 
 to take its place. Water and rum diffuse readily, "but rum is lighter and 
 requires time to diffuse to the bottom. 
 
 * This phenomenon is explained by the theory that the molecules of all 
 bodies are in rapid motion (p. 44, 2). In a gas the molecules are assumed to 
 move in straight lines, collide with each other, rebound, and so have their di- 
 rections continually changed. The paths over which they move are inconceiv- 
 ably minute, but the velocity with which they move is correspondingly great. 
 The particles of ammonia gas, for example, are flying to and fro at the rate 
 of twenty miles per minute. " Could we, by any means," says Prof. Cooke, 
 "turn in one direction the actual motion of the molecules of what we call 
 still air, it would become at once a wind blowing seventeen miles per min- 
 ute, and exert a destructive power compared with which the most violent 
 tornado is feeble." Invert a bottle over a 
 lighted candle, and the oxygen of the inclosed 
 air being soon consumed the flame goes out. 
 Instead of the bottle, use a foolscap-paper cone. 
 There will be an interchange of gases through 
 the pores of the paper and the light will 
 burn with moderate freedom. 
 
 Diffusion of Gases is still more strikingly 
 shown in the experiment of Pig. 23, devised 
 by Prof. Graham. Mt a porous cup used in 
 Grove's Battery (p. 320) with a cork and glass 
 tube. Fasten the tube so that it will dip be- 
 neath the colored water in the glass. Then 
 invert over the cup a jar of hydrogen. The 
 gas will pass through the sensible pores of 
 the earthenware and down the tube so rapidly, 
 as almost instantly to bubble up through the 
 water. Rose balloons lose their buoyancy, be- 
 cause the hydrogen escapes through the pores 
 of the rubber. If they were filled with air and 
 placed in a jar of hydrogen, that gas would 
 creep in so rapidly as to burst them. In per- 
 forming the experiment shown in Kg. 23, 
 coal-gas may be used. After the bubbling 
 
 ceases, on withdrawing the jar water rises in the tube. The thin gas 
 diffuses out into the denser air, producing a partial vacuum within. 
 
 Graham's Diffusion Tube. 
 
54 
 
 ATTRACTION. 
 
 FIG. 34. 
 
 a curious manner, according to the nature of the 
 liquid and the substance used. At the end of a glass 
 tube (Fig. 24) fasten a bladder filled with alcohol. 
 Insert into a jar of water, and 
 mark the height to which the 
 alcohol ascends in the tube. The 
 column will soon begin to rise 
 slowly. On examination, we shall 
 see that the alcohol is passing out 
 through the pores of the blad- 
 der and mixing with the water, 
 while the water is coming in 
 more rapidly. The bladder is 
 not porous in the sense of hav- 
 ing sensible pores. 
 
 Diffusion of fluids, through 
 the medium of a substance 
 which attracts them unequally, 
 is called osmose. It is applied 
 by the chemist in methods of 
 analysis where the separation of substances is based 
 on their unequal diffusibility. Crystals in solution 
 pass readily through animal membrane, like bladder, 
 while substances that do not crystallize, like gum, 
 gelatine, or white of egg, are stopped. 
 
 Osmose. 
 
 PRACTICAL QUESTIONS. 
 
 1. Why does cloth, shrink when wet ? 
 
 2. Why do sailors at a boat-race wet the sails ? 
 
 3. Why is writing-paper sized ? 
 
 4. Why does paint prevent wood from shrinking f 
 
ATTRACTION OF GRAVITATION. 55 
 
 5. What is the shape of the surface of a glass-full of water? Of mercury? 
 
 6. "Why can we not perfectly dry a towel by wringing ? 
 
 7. Why will not water run through a fine sieve when the wires are 
 greased? 
 
 8. Why will camphor dissolve in alcohol, and not in water? 
 
 9. Why will mercury rise in zinc tubes as water will in glass tubes? 
 
 10. Why will ink spilled on the edge of a book extend farther inside 
 than if spilled on the side of the leaves? 
 
 11. If you should happen to spill some ink on the edge of your book, 
 ought you to press the leaves together? 
 
 12. Why can you not mix water and oil? 
 
 13. What is the object of the spout on a pitcher? Ans. The water 
 would run down the side of the pitcher by the force of adhesion, but the 
 spout throws it into the hands of gravitation before adhesion can catch it. 
 
 14. Why will water wet your hand, while mercury will not? 
 
 15. Why is a pail or tub liable to fall to pieces if not filled with water 
 or kept in a damp place? 
 
 16. Name instances where the attraction of. adhesion is stronger than 
 that of cohesion. 
 
 17. Why does the water in Pig. 18 stand higher inside of the tube 
 than next the glass on the outside? 
 
 18. Why will clothes-lines tighten and sometimes break during a shower? 
 
 19. In casting large cannon, the gun is cooled by a stream of cold 
 water. Why ? 
 
 20. Why does paint adhere to wood? Chalk to the blackboard? 
 
 21. Why does a towel dry one's face after washing? 
 
 22. Why will a greased needle float on water? 
 
 23. Why is the point of a pen slit? 
 
 24. Why is a thin layer of glue stronger than a thick one? 
 
 II. ATTRACTION OF GRAVITATION. 
 
 WE have spoken of the attraction existing between 
 the molecules of bodies at minute distances. We now 
 notice an attraction which acts at all distances. 
 
 1. Law of Gravitation. Hold a stone in the hand, 
 and you feel a power constantly drawing it to the 
 ground. "We call this familiar phenomenon weight. 
 It is really the attraction of the earth pulling the 
 
56 
 
 ATTK ACTION. 
 
 PIG. 25. 
 
 stone back to itself an instance of a general law, 
 one operation of an ever-active force. For every par- 
 ticle of matter in the universe* attracts every other 
 particle with a force proportional to the product of 
 their masses, and increasing as the square of the dis- 
 tance decreases. 
 
 Gravitation is the general term applied to the 
 attraction that exists between all 
 bodies in the universe. Gravity is 
 the earth's attraction for terrestrial 
 bodies; it tends to draw them to- 
 ward the center of the earth. 
 
 2. Illustrations of Gravity. A 
 
 stone falls to the ground because the 
 earth attracts it; but in turn the 
 stone attracts the earth. Each moves 
 to meet the other, but the stone 
 passes through as much greater dis- 
 tance than the earth as its mass is 
 less. The mass of the earth is so 
 great that its motion is impercepti- 
 
 Deflection of a Plumb- ' 
 
 line by a mountain, ble. A plumb-line hanging near a 
 
 (Exaggerated.) . . 
 
 mountain is attracted from the ver- 
 tical. In Fig. 25, AB represents the ordinary posi- 
 
 * The force of gravitation acts on every particle of matter, and hence 
 it is not confined to our own world. By its action the heavenly bodies are 
 bound to one another, and thus kept in their orbits. It may help us to con- 
 ceive how the earth is supported, if we imagine the sun letting down a 
 huge cable, and every star in the heavens a tiny thread, to hold our globe 
 in its place, while it in turn sends back a cable to the sun and a thread to 
 every one of the stars. So we are bound to them and they to us. Thus the 
 worlds throughout space are linked together by these cords of mutual attrac- 
 tion, which, interweaving in every direction, make the universe a unit. 
 
 r 
 
ATTRACTION OF GRAVITATION. 57 
 
 tion of the line, while AC indicates the attractive 
 power (greatly exaggerated) of the mountain.* 
 
 3. Mass and Weight. The quantity of matter in 
 a body is its mass. The measure of the earth's at- 
 traction upon it is its weight. If m be the number 
 of units of mass in a body, and g be the number of 
 units of force expressing the earth's attraction, then 
 its weight, w, is equal to m multiplied by g; or, 
 
 w = mg, whence 
 
 w 
 
 m = . 
 9 
 
 4. The Earth's Center of Gravity is that point 
 within it where the attraction of all the particles 
 on any one side is equal to the attraction of all 
 those on the opposite side. As an attracting mass 
 the whole earth may be regarded as if it were con- 
 centered at this point. Its position is probably very 
 near the geometric center. When the earth's center 
 is mentioned we generally mean its center of gravity. 
 
 5. The Center of Gravity of a Body is that point 
 about which it may be balanced. A straight line 
 from this point to the earth's center of gravity is 
 called the line of direction. It is also called a verti- 
 cal, or plumb-line. Gravity tends to cause the body 
 to move along this line toward the center. f 
 
 * Maskelyne, in 1774, found the attraction of Mount Schehallien to de- 
 flect a plumb line 12". By comparing this force with that of the earth, the 
 specific gravity of the earth was estimated to be five times that of water. 
 Later investigations make it 5.67. 
 
 t Downward means toward the earth's center; upward means the oppo- 
 site. Any two bodies moving downward, one from America and the other 
 from Europe, if unresisted, would meet at the earth's center if their fall 
 were properly timed. 
 
58 ATTRACTION. 
 
 6. Laws of Weight. I. The weight of a body at 
 the center of the earth is nothing ; for since the oppo- 
 site attractions are mutually balanced there can be 
 no tendency to motion in any direction. 
 
 II. The weight of a ~body above the surface of the 
 earth decreases as the square of the distance from 
 the center of the earth increases* 
 
 III. The weight of a body varies on different por- 
 tions of the surface of the earth.\ It will be least at 
 the equator, because (1), on account of the bulging 
 form of our globe a body is there farther from the 
 earth's center; and (2), the centrifugal force is there 
 strongest. It will be greatest at the poles, because 
 (1), on account of the flattening of the earth a body 
 at a pole is there nearer to the earth's center; (2), 
 there is no centrifugal force at the poles. 
 
 7. Falling Bodies. I. Under the influence of the 
 constant force of gravity alone all bodies fall with 
 equal rapidity. 
 
 * A body at the surface of the earth (4,000 miles from the center) weighs 
 100 Ibs. What would be its weight 1,000 miles above the surface (5,000 miles 
 from the center)? SOLUTION: (5,000 mi.) 2 : (4,000 mi.) 2 :: 100 Ibs. :x = 64 
 
 Ibs, Or, its weight would decrease in the ratio of = if- Hence it 
 
 Would weigh f x 100 Ibs. = 64 Ibs. The weight of a body below the surface 
 of the earth is commonly said to decrease directly as the distance from the 
 center decreases. Thus, 1,000 miles below the surface, a body would lose 
 J its weight. In fact, however, the density of the earth increases so much 
 toward the center, that " for T T 5 of the distance the force of gravity actually 
 becomes stronger than on the surface." 
 
 t In these statements concerning weight, a spring-balance is supposed 
 to be employed. If it be graduated to indicate correctly at a medium lati- 
 tude, it would show too little at the equator, and too much at the poles. 
 In other words, a pound weighed with such a spring-balance at the equator 
 would contain a greater mass of matter than one weighed at the poles by 
 about T*5 nart. 
 
FALLING BODIES. 
 
 59 
 
 FIG. 26. 
 
 This is well illustrated by the " guinea and feather 
 experiment." Let a coin and a feather be placed in 
 a tube, and the air exhausted. 
 Quickly invert the tube, and the 
 two bodies will fall in nearly the 
 same time. Let in the air again, 
 and the feather will nutter down 
 long after the coin has reached 
 the bottom.* Hence we conclude 
 that in a vacuum all bodies de- 
 scend with equal velocity, and 
 that the resistance of the air and 
 the adhesion of the feather to 
 the tube are the causes of the 
 variation we see between the fall- 
 ing of light and of heavy bodies 
 in it. 
 
 II. AT WOOD'S MACHINE.! To 
 deduce the laws of falling bodies 
 we make use of Atwood's ma- 
 chine (Fig. 27). This consists of 
 a very light grooved wheel, w, 
 
 Guinea and Feather Ex- 
 periment. 
 
 * The same fact may be noticed in the case of a sheet of paper. When 
 spread out, it merely nutters to the ground ; but when rolled in a compact 
 mass, it falls quickly. In this case we have not increased the force of at- 
 traction, but we have diminished the resistance of the air. "It is difficult 
 for many pupils to understand how, under the influence of gravity alone, 
 all bodies fall with equal rapidity. An illustration, which is usually effect- 
 ive, is that of a number of bodies of the same kind, say bricks, which will 
 separately fall in the same space of time. The pupil will admit that, if all 
 of them are connected together, inasmuch as nothing is thereby added to 
 their weight, there is no reason why the mass of bricks should not fall in the 
 time of a Dingle one, notwithstanding it is a larger body." WM. H. TAYLOB. 
 
 t If the ieacher is not provided with Atwood's machine, or if the pupils 
 
ATTRACTION. 
 
 Fro. 27. 
 
 Atwood's Machine. 
 
 pivoted at the top of a firm 
 vertical pillar, on one side 
 of which is a graduated 
 strip, s, divided into inches 
 or centimeters. A silk 
 thread, passing over the 
 wheel, supports two equal 
 masses, m and m'. The 
 force of gravity on one 
 of these just balances its 
 force on the other. A 
 small cross-bar, n, placed 
 upon m, gives vertical mo- 
 tion, by the action of grav- 
 ity on it, to the whole 
 mass, which we may call 
 M } made up of (m -f- m' 
 + n). Since the momen- 
 tum of m, moving down- 
 ward, is balanced by that 
 of m', moving upward, the 
 rate of motion of M is as 
 much less than if it were 
 falling freely as the mass 
 of the cross-bar is less than 
 
 are unfamiliar with, algebra, it may 
 be found best to omit this discussion 
 of Falling Bodies. The subject re- 
 ceives special attention in the present 
 edition on account of the united re- 
 quest of many teachers. It has been 
 reduced within the narrowest limit 
 consistent with clearness. 
 
FALLING BODIES. 61 
 
 the whole mass, M. An allowance has to be made 
 for the mass of the wheel, which is put in circular 
 motion at the same time. By properly choosing the 
 mass of the cross-bar, the motion of M may be 
 made so slow that the distance through which it 
 passes in each succeeding second may easily be 
 measured on the graduated strip along which m falls. 
 A pendulum, o, marks the successive seconds, and is 
 so arranged as to release the support of m at the proper 
 moment, thus allowing M to move. Attached to the 
 graduated strip are a movable ring, r, and a movable 
 plate, p. If these be placed as shown in Fig. 27, the 
 cross-bar would be caught by the ring, while m pass- 
 ing through would continue to move until stopped 
 by the plate p. 
 
 EXPERIMENTS WITH AT WOOD'S MACHINE. Suppose 
 the mass of the cross-bar be so adjusted to that of m 
 and m' that when m is released by the pendulum it 
 will pass through 5 cm. (p. 4) during the first sec- 
 ond. Let the plate, _p, be removed, and the ring, r, 
 be placed 5 cm. below the starting-point of m. Then 
 allowing the system M to fall, the cross-bar will be 
 caught by the ring exactly at the end of one second 
 and stopped; the remainder of the system, however, 
 will continue its motion (p. 22, 4). m passing 
 through the ring will be seen to pass over 1 cm. in 
 each successive second until it reaches the foot of 
 the instrument. 10 cm. per second, then, is the ve- 
 locity produced by the force of gravity acting for 
 one second on the cross-bar. Let the ring, for a sec- 
 ond experiment, be placed 20 cm. below the starting- 
 
62 
 
 ATTRACTION. 
 
 point. 
 
 FIG. 28. 
 
 I 6 
 
 a 
 to 
 
 On allowing the system to fall, the cross-bar 
 will now be caught exactly at the 
 end of two seconds, m passing 
 through the ring will be seen to 
 move with a velocity of 2 cm. per 
 second. We have evidently doubled 
 the velocity by allowing the force of 
 gravity to act for two seconds on the 
 cross-bar. Similarly, by allowing 
 gravity to act for three seconds, 
 the velocity will be trebled. The 
 longer, therefore, the force is al- 
 lowed to act the greater will be the 
 velocity produced. The distances 
 passed over during the successive 
 seconds were 5, 15, 25 cm. (see Fig. 
 28). Under the action of a constant 
 force, therefore, a body will move 
 with a continually increasing ve- 
 locity. The gain in velocity per unit 
 of time is called the -acceleration, 
 and the motion itself is described as 
 accelerated motion. If the accelera- 
 tion for successive seconds, as in our 
 experiments, has the same numeri- 
 cal value, the motion is called uni- 
 formly accelerated motion, and this 
 is the motion of a falling body. 
 
 From the first experiment it is 
 seen that during the first second the 
 system M passed over 5 cm., but that the acceleration 
 
ATTRACTION. 
 
 63 
 
 produced in that time was double this, or 1 cm. per 
 second. A body, therefore, under the action of a con- 
 stant force and starting from a state of rest passes over 
 a distance during the first second equal to one half the 
 acceleration produced during that time. 
 
 On repeating the experiments with cross-bars of 
 greater mass, it will be found that the accelerations 
 produced increase with the mass of the cross-bar, 
 that is, with the force employed. A constant force 
 may, therefore, be measured by the acceleration it 
 imparts to unit mass (page 23). 
 
 EQUATIONS OF FALLING BODIES. These results are 
 combined in the accompanying table, where the ac- 
 celeration, 10 cm., is represented by/. 
 
 FIG. 29. 
 
 t 
 
 1 
 
 2 
 
 3 
 
 4 
 
 
 V 
 
 / 
 
 /x2 
 
 /X3 
 
 /x4 
 
 *=/* (1) 
 
 s 
 
 M 
 
 i/x3 
 
 |/X5 
 
 I/X7 
 
 S= i/(2*-l) (2) 
 
 8 
 
 if 
 
 i/x4 
 
 i/x9 
 
 i/xl6 
 
 S=|/* 2 (3) 
 
 The horizontal line marked t gives the time in 
 seconds; that marked v, the velocity at the end of 
 each second; that marked s, the space traversed 
 during each successive second ; and that marked S, 
 the whole space traversed from the beginning of fall 
 to the end of each second. Taking any one of the 
 vertical columns, such as the fourth, we see that 
 
 (1.) At the end of any given second, the velocity 
 
64 ATTRACTION. 
 
 is equal to the acceleration multiplied by the num- 
 ber of the second ; or, v ft. 
 
 (2.) During any particular second, the space trav- 
 ersed is equal to half the acceleration multiplied by 
 one less than double the number of the second ; or, 
 8 = tf(M- I). 
 
 (3.) The whole space traversed during a given 
 number of seconds is equal to half the acceleration 
 multiplied by the square of the time in seconds; or, 
 8 = 
 
 8. Bodies Falling Freely. If a body falls freely, 
 the acceleration is about 9.8 meters, or 32 feet.* If 
 we represent this by g in the equations just deduced, 
 we have 
 
 .............. 
 
 a = $g(2t - 1) .......... (2) 
 
 (3)f 
 
 When a body is thrown upward, gravity causes it 
 to lose 32 feet in velocity each second until it ceases 
 to ascend. The velocity with which it begins to 
 rise, and its time of rising, must be the same as the 
 velocity with which it ends its fall, and the time of 
 falling. The laws of falling bodies may hence be 
 applied. 
 
 * For the latitude of New York, g = 32.16 more nearly, or 980.2 cm. 
 This varies slightly with the latitude and elevation of a place; but for 
 ordinary problems it will be sufficient to assume the values given in the 
 text. 
 
 t An additional formula that is often useful may be obtained by elim- 
 inating t in combining equations (1) and (3). The result is v = V2gS. 
 Since 8 represents height (h), it is often expressed t? 3 = 2gh. (4). 
 
EQUILIBRIUM. 65 
 
 9. Measurement of Kinetic Energy. (See p. 84.) 
 To lift a body through any height energy must 
 be expended. The measure of this is the weight 
 (w), multipled by the height (h), to which it is lifted. 
 Using the initial letters of these words for symbols, 
 
 we have 
 
 K=wh. 
 
 But from the equation, at the bottom of p. 64, 
 
 v 2 
 we have h = ~-. And from p. 57, w = mg. Sub- 
 
 ^9 
 stituting these values and reducing, we have 
 
 1C. Resistance of the Air to Moving Bodies. A 
 
 body in falling or otherwise moving through the air 
 expends energy in overcoming the resistance of the 
 air. From the formula just deduced we see that 
 this is proportional to the mass of air moved and to 
 the square of the velocity. The flight of a cannon- 
 ball is never so great as it would be if shot through 
 a vacuum. Practically it is not easy to calculate 
 beforehand the amount of energy to be lost through 
 resistances. 
 
 11. Equilibrium. When a body is at rest the 
 forces which act on every molecule in it are said to 
 balance one another, or to be in equilibrium. The 
 most important of these forces is gravity. 
 
 (1.) THREE STATES OF EQUILIBRIUM. 1st. A body 
 is in stable equilibrium when the center of gravity 
 is below the point of support, or when any move- 
 ment tends to raise the center of gravity. In Fig. 30, 
 
66 ATTRACTION, 
 
 the cork and two knives together form a connected 
 body whose center of gravity is outside, just be- 
 neath the needle. By pushing either knife a few 
 oscillations are produced, but a posi- 
 
 FIG. 30. 
 
 tion of rest is soon recovered. Any 
 movement of the toy shown in Fig. 
 31 tends to raise the center of grav- 
 ity, and it returns quickly to a state 
 of rest. 
 
 2d. A body is said to be in un- 
 
 Stable Equilibrium. , -, . 7 .-, . , ,, 
 
 stable eqmhomum when the center 
 of gravity is above the point of support, or when 
 any movement tends to lower the center of gravity. 
 If we take the cork as arranged with 
 the knives in Fig. 30, and invert it, Fl0 ' 31< 
 
 we shall have difficulty in balancing 
 the needle ; and, if we 
 succeed, it will readily 
 topple off, as the least 
 motion tends to lower the 
 center of gravity. 
 
 3d. A body is said to 
 be in indifferent equilib- 
 rium when the center of 
 gravity is at the point of 
 
 Stable Equilibrium. 
 
 support, or when any 
 
 movement tends neither to elevate nor lower the 
 
 center of gravity. A ball of uniform density on a 
 
 level surface will rest in any position, because the 
 
 center of gravity moves in a line parallel to the 
 
 floor. 
 
EQUILIBRIUM. 
 
 67 
 
 (2.) GENERAL PRINCIPLES. (a.) The center of grav- 
 ity tends to seek the lowest point. 
 
 (u) A body will not tip over while the line of 
 direction falls within the base, but will as soon as it 
 falls without.* 
 
 (c.) In general, narrowness of base combined with 
 height of center of gravity, tends to instability ; f 
 breadth of base and lowness of center of gravity, 
 produce stability. 
 
 (3.) PHYSIOLOGICAL FACTS. Our feet and the space 
 between them form the base on which we stand. 
 By turning our toes outward, we increase its breadth. 
 
 Fro. 
 
 * The Leaning Tower of Pisa, in Italy, beautifully illustrates this prin- 
 ciple. It is about 188 feet high, and its top leans 15 feet, yet the line of 
 direction falls so far within the base that it 
 is perfectly stable, having stood for seven cent- 
 uries. The feeling experienced by a person 
 who for the first time looks down from the 
 lower side of the top of this apparently im- 
 pending structure is startling indeed. 
 
 t " This is shown by the difficulty in learn- 
 ing to walk upon stilts. The art of balancing 
 one's self may, however, be acquired by prac- 
 tice, as is seen in the Landes of south-western 
 France. During a portion of the year these 
 sandy plains are half covered with water, and 
 in the remainder are still very bad walking. 
 The natives accordingly double the length of 
 their legs by stilts. Mounted on these wooden 
 poles, which are put on and off as regularly 
 as the other parts of their dress, they appear 
 to strangers as a new and extraordinary race, 
 marching with steps of six feet in length, 
 and with the speed of a trotting-horse. While 
 watching their flocks, they support themselves 
 "by a third staff behind, and then with their Walking on Stats, 
 
 rough sheep-skin cloaks and caps, like thatched 
 
 roofs, seem to be little watch-towers, or singular lofty tripods, scattered 
 over the country." ABNOTT. 
 
68 
 
 ATTRACTION. 
 
 FIG. 83. 
 
 When we stand on one foot, we bend over so as to 
 bring the line of direction within this narrower base. 
 When we walk, we incline to the right and the left 
 alternately. When we walk up hill we lean forward, 
 and in going down hill we incline backward, in un- 
 conscious obedience to the laws of gravity. We bend 
 forward when we wish to rise from a chair, in order 
 to bring the center of gravity over our feet. In walk- 
 ing we lean forward, so as bring the center of gravity 
 as far in front as possible. Thus, walking is a pro- 
 cess of falling forward and then checking the fall. 
 When we run, we lean farther forward, and so fall 
 faster. ("Hygienic Physiology/' p. 37.) 
 
 12. The Pendulum consists of a weight so sus- 
 pended as to swing freely. Its move- 
 ments to and fro are termed vibra- 
 tions or oscillations. The path through 
 which it passes is called the arc. The 
 extent to which it goes in either direc- 
 tion from the lowest point is styled 
 its amplitude. Vibrations performed 
 in equal times are termed i-soch'ro- 
 nous (isos, equal; chronos, time). 
 
 (1.) THREE LAWS. I. In the same 
 pendulum, all vibrations of small am- 
 plitude are isochronous. If we let one 
 of the balls represented in Fig. 3 3 
 swing through a short arc, and then 
 Pendulums. through a longer one, on counting the 
 number of oscillations per minute, we shall find them 
 very uniform. 
 
 
THE PENDULUM. 
 
 69 
 
 Fw. 34. 
 
 II. The times of the vibrations of different pendu- 
 lums are proportional to the square roots of their re- 
 spective lengths. Example : 
 
 A pendulum -J the length of 
 another, will vibrate three 
 times as fast.* Conversely, 
 the lengths of different pen- 
 dulums are proportional to 
 the squares of their times of 
 vibration. 
 
 III. The time of the vibra- 
 tion of the same pendulum 
 will vary at different places, 
 since it decreases as the 
 square root of the number ex- 
 pressing the acceleration of 
 gravity increases. At the 
 equator a pendulum vibrates 
 most slowly. The length of 
 
 a Seconds-pendulum at New Pendulums of apparently the same 
 . . , . , length, but really different lengths. 
 
 York is about 39-^ inches. 
 
 (2.) CENTER OF OSCILLATION. The upper part of a 
 
 pendulum tends to move faster than the lower part, 
 
 f 
 
 * A pendulum which vibrates seconds must be four times as long as 
 one which vibrates half-seconds. The apparatus represented in Figs. 33 and 
 34 can be made by any carpenter or ingenious pupil, and will serve excel- 
 lently to illustrate the three laws of the pendulum. The law of the pendu- 
 lum may be conveniently expressed in symbols. If t be the time of a sin- 
 gle vibration in seconds, / the length of the pendulum, g the acceleration 
 of gravity, I and g being expressed in feet, or in meters, and if tr be the 
 ratio of the circumference to the diameter of a circle, then 
 
 This formula is convenient for use in solving problems. 
 
70 ATTRACTION. 
 
 and so hastens the speed. The lower part of a pen- 
 dulum tends to move slower than the upper part, 
 and so retards the speed. Between these extremes 
 is a point which is neither quickened nor impeded 
 by the rest, but moves in the same time that it 
 would if it were a particle swinging by an imaginary 
 line. This point is called the center of oscillation. 
 It lies a little below the center of gravity.* In Fig. 
 34 is shown an apparatus containing pendulums of 
 different shapes, but of the same length. If they are 
 started together, they will immediately diverge, no 
 two vibrating in the same time. As pendulums, they 
 are not of the same length. 
 
 (3.) THE CENTER OF OSCILLATION is FOUND BY 
 TRIAL, f Huyghens discovered that the point of sus- 
 pension and the center of oscillation are interchange- 
 able. If, therefore, a pendulum be inverted, and a 
 
 * This determines the real length of a pendulum, which is the distance 
 from the point of support to the center of oscillation. The imaginary pen- 
 dulum above described is known in Physics as the Simple Pendulum. 39.1 
 inches = 993.3 mm. 
 
 t " Take a flat board of any form and drive a piece of wire through it 
 near its edge, and allow it to hang in a vertical plane, holding the ends of 
 the wire by the finger and thumb. Take a small bullet, fasten it to the 
 end of a thread, and allow the thread to pass over the wire so that the 
 bullet hangs close to the board. Move the hancl by which you hold the 
 wire horizontally in the plane of the board, and observe whether the board 
 moves forward or backward with respect to the bullet. If it moves forward, 
 lengthen the string ; if backward, shorten it till the bullet and the board 
 move together. Now mark the point of the board opposite the center of 
 the bullet, and fasten the string to the wire. You will find that, if you hold 
 the wire by the ends and move it in any manner, however sudden and 
 irregular, in the plane of the board, the bullet will never quit the marked 
 spot on the board. Hence this spot is called the center of oscillation, be- 
 cause, when the board is oscillating about the wire when fixed, it oscillates 
 as if it consisted of a single particle placed at the spot. It is also called 
 the center of percussion, because, if the board is at rest and the wire is 
 
THE PENDULUM. 
 
 71 
 
 FIG. 35. 
 
 point found at which it will vibrate in the same time 
 as before, this is the former center of oscillation ; while 
 the old point of suspension becomes 
 the new center of oscillation.* 
 
 (4.) THE PENDULUM AS A TIME- 
 KEEPER. The friction at the point of 
 suspension, and the resistance of the 
 air, soon destroy the motion of the 
 pendulum. The clock is a machine 
 for keeping up the vibration of the 
 pendulum, and counting its beats. 
 In Fig. 35, R is the scape-wheel 
 driven by the force of the clock- 
 weight or spring, and mn the escape- 
 ment, moved by the forked arm, AB, 
 so that only one cog of the wheel 
 can pass at each double vibration of 
 the pendulum. Thus the oscillations 
 are counted by the cogs on the wheel, 
 while the friction and the resistance 
 of the air are overcome by the ac- 
 tion of the weight or spring, f As 
 " heat expands and cold contracts,". 
 
 suddenly moved horizontally, the board will at first 
 begin to rotate about the spot as a center." J. 
 CLEBK MAXWELL, on " Matter and Motion," p. 104. 
 
 * The center of oscillation is the same as the 
 center of percussion. The latter is the point where we 
 must strike a suspended body, if we wish it to re- 
 volve about its axis without any strain. If we do 
 not hit a ball on the bat's center of percussion, our Clock Pendulum, 
 hands " sting" with the jar. 
 
 t The action of a clock is clearly seen by procuring the works of an old 
 clock and watching the movements of the various parts. 
 
72 ATTRACTION. 
 
 a pendulum lengthens in summer and shortens in 
 winter. A clock, therefore, tends to lose time in 
 summer and gain in winter. To regulate a clock, 
 we raise or lower the pendulum-bob, L, by the nut v. 
 (5.) OTHER USES OF THE PENDULUM. (a.) Since the 
 time of vibration of a pendulum indicates the force 
 FIG gg of gravity, and the force 
 
 of gravity decreases as the 
 square of the distance from 
 the center of the earth in- 
 creases, we may thus find 
 the semi-diameter of the 
 earth at various places, and 
 ascertain the figure of our 
 globe. (&.) Knowing the 
 
 Foucault's Method. ., 
 
 force of gravity at any 
 
 point, the velocity of a falling body can be deter- 
 mined, (c.) The pendulum may be used as a stand- 
 ard of measures, (d.) Foucault devised a method 
 of showing the rotation of the earth on its axis, 
 founded upon the fact that the pendulum vibrates 
 constantly in one plane.* (e.) By observing the dif- 
 ference in the length of a seconds-pendulum at the 
 
 * A pendulum 220 feet in length was suspended from the dome of the 
 Pantheon in Paris. The lower end of the pendulum traced its vibrations 
 north and south upon a table beneath, sprinkled with fine sand. These 
 paths did not coincide, but at each return to the outside, the pendulum 
 marked a point to the right. At the poles of the earth the pendulum, 
 constantly vibrating in the same vertical plane, would perform a complete 
 revolution in twenty-four hours, making thus a kind of clock. At the equa- 
 tor it would not change east or west, as the plane of vibration would go 
 forward with the diurnal rotation of the earth. The shifting of the plane 
 would increase as the pendulum was carried north or south from the equa- 
 tor. 
 
PRACTICAL QUESTIONS. 78 
 
 top of a mountain and at the level of the^sea, the 
 density of the earth may be estimated. 
 
 PRACTICAL QUESTIONS. 
 
 1. When an apple falls to the ground, does the earth rise to meet it? 
 
 2. Will a body weigh more in a valley than on a mountain ? 
 
 3. Will a pound weight fall more slowly than a two-pound weight? 
 
 4. How deep is a well if it takes three seconds for a stone to fall to 
 the bottom? 
 
 5. Is the center of gravity always within a body as, for example, a 
 pair of tongs? 
 
 6. In a ball of equal density throughout, where is the center of grav- 
 ity? 
 
 7. Why does a ball roll down hill ? 
 
 8. Why is it easier to roll a round body than a square one? 
 
 9. Why is it easier to tip over a load of hay than one of stone? 
 
 10. Why is a pyramid such a stable structure? 
 
 11. When a hammer is thrown, on which end does it most often strike? 
 
 12. Why does a rope-walker carry a heavy balancing-pole ? 
 
 13. What would become of a ball if dropped into a hole bored through 
 the center of the earth? 
 
 14. Would a clock lose or gain time if carried to the top of a mount- 
 ain ? If carried to the North Pole ? 
 
 15. In the winter, would you raise or lower the pendulum-bob of your 
 clock? 
 
 16. Why is the pendulum-bob generally made flat? 
 
 17. What "beats-off" the time in a watch? 
 
 18. What should be the length of a pendulum to vibrate minutes at 
 the latitude of New York? Solution: (1 sec.) 2 : (60 sec.) 2 :: 39.1 in. : x = 2.2 
 + miles. 
 
 19. What should be the length of the above to vibrate half -seconds ? 
 Quarter-seconds? Hours? 
 
 20. What is the proportionate time of vibration of two pendulums, re- 
 spectively 16 and 64 inches long? 
 
 21. Why, when you are standing erect against a wall, and a piece of 
 money is placed between your feet, can you not stoop forward and pick it 
 up? 
 
 22. If a tower were 198 ft. high, with what velocity would a stone, 
 dropped from the summit, strike the ground ? (In these problems on falling 
 bodies we may disregard the resistance of the air.) 
 
 23. A body falls in 5 seconds ; with what velocity does it strike the 
 ground? 
 
74 ATTRACTION. 
 
 24. How far will a body fall in 10 seconds ? "With what velocity will it 
 strike the ground? 
 
 25. A body is thrown upward with a velocity of 192 ft. the first second : 
 to what height will it rise ? 
 
 26. A ball is shot upward with a velocity of 256 ft. ; to what height 
 will it rise? How long will it continue to ascend? 
 
 27. Why do not drops of water, falling from the clouds, strike with a 
 force equal to that calculated according to the laws of falling bodies? Be- 
 cause the mass of each drop is so small in proportion to its surface that the 
 resistance of the air soon balances the acceleration of gravity, so that they 
 fall with uniform velocity instead of accelerated velocity. 
 
 28. Are any two plumb-lines parallel ? 
 
 29. A stone let fall from a bridge strikes the water in 3 seconds. What 
 is the height ? 
 
 30. A stone falls from a church-steeple in 4 seconds. What is the 
 height of the steeple? 
 
 31. How far would a body fall in the first second at a distance of 
 12,000 miles above the earth's surface? 
 
 32. A body at the surface of the earth weighs 100 tons ; what would 
 be its weight 1,000 miles above ? 
 
 33. A boy wishing to find the height of a steeple, lets fly an arrow that 
 just reaches the top and then falls to the ground. It is in the air 6 sec- 
 onds. Required the height. 
 
 34. An object let fall from a balloon reaches the ground in 10 seconds. 
 Required the distance. 
 
 35. In what time will a pendulum 40 ft. long make a vibration? 
 
 36. Two bodies in space are 12 miles apart. Their masses are, respect- 
 ively, 100 and 200 Ibs. If they should fall together by their mutual attrac- 
 tion, what portion of the distance would be passed over by each body? 
 
 37. If a body weighs 2,000 Ibs. upon the surface of the earth, what 
 would it weigh 2,000 miles above? 500 miles above? 
 
 38. At what distance above the earth will a body fall, the first second, 
 21^ inches? 
 
 39. How far will a body fall in 8 seconds ? In the 8th second ? In 10 
 seconds? In the 30th second? 
 
 40. How long would it take for a pendulum one mile in length to make 
 a vibration? 
 
 41. What would be the time of vibration of a pendulum 64 meters 
 long? 
 
 42. A ball is dropped from a height of 64 ft. At the same moment a 
 second ball is thrown upward with sufficient velocity to reach the same 
 point. How far from the ground will the two balls pass each other? 
 
 43. Explain the following fact: A straight stick loaded with lead at 
 one end, can be more easily balanced vertically on the finger when the 
 loaded end is upward than when it is downward. 
 
 44. If a body weighing a pound on the earth were carried to the sun it 
 would weigh about 27 Ibs. How much, would it then attract the sun? 
 
SUMMARY. 75 
 
 45. Why does watery vapor float and rain fall? 
 
 46. If a body weighs 10 kilos, on the surface of the earth, what would 
 it weigh 1,000 kilometers above (the earth's radius being 6,366 km.) ? 
 
 47. A body is thrown vertically upward with a velocity of 100 meters ; 
 how long before it will return to its original position ? 
 
 48. Required the time needed for a body to fall a distance of 2,000 
 meters. 
 
 49. What would be the time of vibration of a pendulum 39.1 inches 
 long at the surface of the moon, where the acceleration of gravity is only 
 4.8 ft. ? 
 
 50. What would be the time of vibration for the same pendulum at 
 the surface of the sun, where the acceleration of gravity is 27 times what 
 it is at the earth's surface? 
 
 51. How many vibrations per minute would be made at the surface of 
 the moon by a pendulum 40 ft. long ? 
 
 52. A pendulum vibrates 200 times in 15 minutes. What is its length ? 
 
 53. For a certain clock in New York the pendulum was made 500 Ibs. 
 in weight. What was the object in making it so heavy? 
 
 54. Pendulums are often supported by knife-edges of steel resting on 
 plates of agate. Why? 
 
 55. The acceleration of gravity at the equator is 32.088 ft. ; at the pole, 
 32.253 ft. If a pendulum vibrates 3,600 times an hour at the equator, how 
 many times an hour will it vibrate at the pole ? 
 
 SUM MARY. 
 
 THERE are certain forces operating between molecules and act- 
 ing only at insensible distances, which are known as the Molecular 
 Forces. The one which ties together molecules of the same kind 
 is styled cohesion. The relation between this force and that of 
 heat chiefly determines whether a body is solid, liquid, or gase- 
 ous. Under the action of cohesion, liquids tend"*to form spheres ; 
 and many solids, crystals. The processes* of welding and tem- 
 pering, and the annealing of iron and glass, illustrate curious 
 modifications of the cohesive force. Molecules of different kinds 
 are held together by adhesion. Its action is seen in the use of 
 cement, paste, etc., in the solution of solids, in capillarity, diffu- 
 sion of gases, and osmose. 
 
 Gravitation, though weak,* compared with cohesion, acts 
 
 * As the attraction of gravitation acts so commonly upon great masses 
 of matter, we are apt to consider it a tremendous force. We, however, 
 readily detect its relative feebleness when we compare the weight of bodies 
 
76 ATTRACTION. 
 
 universally. Its force is directly as the product of the attract- 
 ing and attracted masses, and inversely as the square of their 
 distance apart. Gravity makes a stone fall to the ground. The 
 earth and a kilogram of iron in mid-air attract each other equally, 
 but the mass of the former is so much greater that they move 
 toward each other with unequal velocity, and the motion of the 
 earth is imperceptible. "Weight is the measure of the attraction 
 of the earth. At the center of the earth the weight of a body 
 would be nothing ; at the poles it would be greatest, and at the 
 equator least. Increase of distance above or far below the sur- 
 face of the earth will diminish weight. "Were the resistance of 
 the air removed, all bodies would fall with equal rapidity. The 
 laws of falling bodies may be studied with the aid of Atwood's 
 Machine. The first second a body falls 16 ft. (4.9 meters), and 
 gains a velocity of 32 ft. (9.8 meters). In general, the final ve- 
 locity of a falling body is 32 ft., multiplied by the number cor- 
 responding to the second, and the distance is 16 ft. multiplied 
 by tha square of the number expressing the seconds. The center 
 of gravity is the point about which the weights of all the par- 
 ticles composing a body will balance one another, i. e,, be in 
 equilibrium. There are three states of equilibrium stable, un- 
 stable, and indifferent according as the point of support in a 
 body is above, below, or at the center of gravity. As the center 
 of gravity tends to seek the lowest point, its position determines 
 the stability of a body. A body suspended so as to swing freely 
 is a pendulum. The time of a pendulum's vibration is independ- 
 ent of its material, proportional to the square root of its length 
 and variable according to the latitude. The pendulum is our 
 time-keeper and "useful in many scientific investigations. 
 
 "We are so accustomed to see all the objects around us pos- 
 sess weight, that we can hardly conceive of a body deprived of 
 a property which we are apt to consider as an essential attri- 
 bute of matter. Nothing is more natural, apparently, than the 
 falling of a stone to the ground. "Yet," says D'Alembert, "it 
 is not without reason that philosophers are astonished to see a 
 stone fall, and those who laugh at their astonishment would 
 
 with their tenacity. Example : Think how much easier it is to lift an iron 
 wire against gravity than to pull it to pieces against cohesion. 
 
HISTORICAL SKETCH. 77 
 
 soon share it themselves, if they would reflect on the subject/' 
 Gravity is constantly at work about us, at one moment produc- 
 ing equilibrium or rest, and at another, motion. When it seems 
 to be destroyed, it is only counterbalanced for a time, and re- 
 mains, apparently, as indestructible as matter itself. The sta- 
 bility and the incessant changes of nature are alike due to its 
 action. Not only do rivers flow, snows fall, tides rise, and 
 mountains stand in obedience to gravitation, but smoke ascends 
 and clouds float through the combined influence of heat and 
 weight. 
 
 HISTORICAL SKETCH. 
 
 THE latter part of the sixteenth century witnessed the estab- 
 lishment of the principles of falling bodies. Galileo, while sitting 
 in the cathedral at Pisa and watching the swinging of an im- 
 mense chandelier which hung from its lofty ceiling, noticed that 
 its vibrations were isochronous. This was the germ-thought of 
 the pendulum and the clock. Up to his time it had been taught 
 that a 4-lb. weight would fall twice as fast as a 2-lb. one. He 
 proved the fallacy of this view by dropping from the Leaning 
 Tower of Pisa balls of different metals gold, copper, and lead. 
 They all reached the ground at nearly the same moment. The 
 slight variation he correctly accounted for by the resistance of 
 the air, which was not the same for all. 
 
 Newton and his immediate predecessors knew the law of 
 terrestrial gravity as manifested in falling bodies. When quite 
 a young man, Newton entertained the idea that the attraction 
 which draws bodies downward at the earth's surface must exist 
 also between masses widely separated in sfcace, such as the earth 
 and the moon. To test this, he calculated how far the moon 
 bends from a straight line, i. e., falls toward the earth every 
 second. Knowing the distance a body falls in a second at the 
 surface of the earth, he endeavored to see how far it would fall 
 at the distance of the moon. For years he toiled over this prob- 
 lem, but an erroneous estimate of the earth's diameter then 
 accepted by physicists prevented his obtaining a correct result. 
 Finally, a more accurate measurement having been made, he 
 inserted this in his calculations. Finding the result was likely 
 
78 ATTRACTION. 
 
 to verify his conjecture, his hand faltered with the excitement, 
 and he was forced to ask a friend to complete the task. The 
 truth was reached at last, and the grand law of gravitation 
 discovered (1682). 
 
 The sun-dial was doubtless the earliest device for keeping 
 time. The clepsydra was afterward employed. This consisted 
 of a vessel containing water, which slowly escaped into a dish 
 below, in which was a float that by its height indicated the 
 lapse of time. King Alfred used candles of a uniform size, six 
 of which lasted a day. The first clock erected in England, about 
 1288, was considered of so much importance that a high official 
 was appointed to take charge of it. The clocks of the middle 
 ages were extremely elaborate. They indicated the motions of 
 the heavenly bodies ; birds came out and sang songs, cocks crowed, 
 and trumpeters blew their horns ; chimes of bells were sounded, 
 and processions of dignitaries and military officers, in fan- 
 tastic dress, marched in front of the dial and gravely announced 
 the time of day. Watches were made at Nuremberg in the 
 fifteenth century. They were styled Nuremberg eggs. Many 
 were as small as the watches of the present day, while others 
 were as large as a dessert-plate. They had no minute or second 
 hand, and required winding twice per day. 
 
 On Attraction, as well as on subsequent topics treated in this 
 book, consult Guillemin's "Forces of Nature;" Atkinson's " Ga- 
 not's Physics"; Arnott's "Elements of Physics"; Snell's " Olm- 
 stead's Natural Philosophy"; Stewart's "Elementary Physics"; 
 Silliman's "Physics"; Everett's "Text-book of Physics"; Young's 
 "Lectures on Natural Philosophy"; "Appleton's Cyclopedia," 
 articles on Clocks and "Watches, Weights and Measures, Gravi- 
 tation, Mechanics, etc.; Peck's " Ganot's Natural Philosophy"; 
 Miller's "Chemical Physics," Chap. III., on Molecular Force; 
 Weinhold's " Experimental Physics"; Pickering's " Elementary 
 Physical Manipulation"; "Fourteen Weeks in Astronomy," sec- 
 tions on Galileo and Newton, pp. 29-34. 
 
 The current numbers of " Harper's Magazine," "The Century 
 Magazine," "Scribner's Magazine," "Popular Science Monthly," 
 "Boston Journal of Chemistry," "Scientific American," " Knowl- 
 edge," and "Nature," contain the latest phases of science. 
 
ELEMENTS OF MACHINES. 
 
 NATURE is a reservoir of power. Tremendous forces are all about us, 
 but they are not adapted to our use. We need to remold the energy to 
 fit our wants. A water-fall can not grind corn nor the wind draw water. 
 Yet a machine will gather up these wasted forces, and turn a grist-mill or 
 work a pump. A kettle of boiling water has little of promise ; but hus- 
 band its energy in the steam-engine, and it will weave cloth, forge an 
 anchor, or bear our burdens along the iron track. 
 
 " The hero in the fairy tale had a servant who could eat granite rocks, 
 another who could hear the grass grow, and a third who could run a hun- 
 dred leagues in half an hour. So man in nature is surrounded by a gang 
 of friendly giants who can accept harder stints than these. There is no 
 porter like gravitation, who will bring down any weight you can not carry, 
 and if he wants aid, knows how to get it from his fellow-laborers. "Water 
 sets his irresistible shoulder to your mill, or to your ship, or transports vast 
 bowlders of rock, neatly packed in his iceberg, a thousand miles." 
 
ANALYSIS OF THE ELEMENTS OF MACHINES. 
 
 THE SIMPLE MACHINES. 
 
 THE LAW OF MECHANICS. 
 
 1. THE LEVER. 
 
 1. Definition. 
 
 2. Three Classes f 
 of Levers. 
 
 2. THE WHEEL 
 AXLE. 
 
 AND 
 
 3. THE INCLINED PLANE. 
 
 4. THE SCREW. 
 
 5. THE WEDGE. 
 
 Class ' 
 Second Class. 
 
 (3.) Third Class. 
 
 3. Law of Equilibrium. 
 
 4. Steelyard. 
 
 5. Compound Lever. 
 
 1. Definition and Illustration. 
 
 2. Law of Equilibrium. 
 
 3. Wheel-work. 
 
 1. Definition and Illustration, 
 
 2. Law of Equilibrium. 
 
 j 1. Definition and Illustration. 
 ( 2. Law of Equilibrium. 
 
 j 1. Definition and Illustration- 
 1 2. Law of Equilibrium. 
 
 6. THE PULLEY. 
 
 1. Definition and Illustration. 
 
 2. Fixed and Movable Pulleys. 
 
 3. Combinations of Pulleys. 
 
 4. Law of Equilibrium. 
 
 7. CUMULATIVE CONTRIVANCES. 
 
 8. PERPETUAL MOTION. 
 
ELEMENTS OF MACHINES. 
 
 The Simple Machines are the elements to which 
 all machinery can be reduced. The watch with its 
 complex system of wheel-work, and the engine with 
 its belts, cranks, and pistons, are only various modi- 
 fications of some of the six elementary forms the 
 lever, the wheel and axle, the inclined plane, the 
 screw, the wedge, and the pulley. These six may be 
 still further reduced to two the lever and the in- 
 clined plane. 
 
 They are often termed the Mechanical Powers, 
 but they do not produce work ; they are only the 
 means of applying it. Here again the doctrine of 
 the Conservation of Energy holds good. The work 
 done by the power is always equal to the resistance 
 overcome in the weight. 
 
 The Law of Mechanics is, the power multiplied 
 by the distance through which it moves, is equal to 
 the weight multiplied ~by the distance through which 
 it moves. Example: 1 Ib. of power moving through 
 10 feet= 10 Ibs. of weight moving through one foot, 
 or vice versa. In theory, the parts of a machine 
 have no weight, move with no friction, and meet no 
 resistance from the air. In practice, these influences 
 must be considered. 
 
82 
 
 ELEMENTS OF MACHINES. 
 
 1. The Lever is a bar turning on a pivot. The 
 force used is termed the power (P), the object to be 
 lifted the weight ("FT), the pivot on which the lever 
 turns the fulcrum (F), and the parts of the lever 
 each s'ide of the fulcrum the arms. 
 
 THREE CLASSES OF LEVERS. In the three kinds, the 
 fulcrum, weight, and power are each respectively be- 
 
 FIG. 37. 
 
 FIG. 38. 
 
 fia. 
 
 J7T 1 
 
 1 
 F 
 
 T 
 
 1 ' 
 
 P , 
 
 V 
 
 7 ' ' 
 
 A 
 
 J 
 
 First Class. 
 
 Second Class. 
 
 Third Class. 
 
 FIG. 40. 
 
 tween the other two, as may be seen by comparing 
 Figs. 37-39. 
 
 First Class. We wish to lift a heavy stone. Ac- 
 cordingly we put one end of a handspike under it, 
 and resting the bar on a block at I 1 , bear down at 
 P. A pump-handle is a lever of 
 the first class. The hand is the 
 P, the water lifted the W, and 
 the pivot the F. A pair of scis- 
 sors is a double lever of the same 
 class. The cloth to be cut is the 
 W, the hand the P, and the rivet the F. 
 
 Second Class. We may also raise the stone by 
 resting one end of the lever on the ground, which 
 acts as a fulcrum, and lifting up on the bar. An 
 oar is a lever of the second class. The hand is the 
 
 w 
 
 Lifting a Stone. 
 
THE LEVEE. 83 
 
 P, the boat the W, and the water the F. In this 
 case the F is not immovable. 
 
 Third Class. The treadle of a sewing-machine is 
 a lever of the third class. The front end resting on 
 the ground is the F, the foot is the P, and the force 
 is transmitted by a rod to the W, the arm above. 
 
 LAW OF EQUILIBRIUM. The product of P multiplied 
 by the perpendicular distance between its line of ac- 
 tion and F, is called the mo- n4r 
 ment of P. In the lever, P 
 balances TFwhen the moments 
 about the fulcrum are equal. 
 In Fig. 41, assume AB to be 
 the initial position of a lever, 
 which is then turned into the 
 position A'B' by application of 
 the power, P, which balances the weight W, its line 
 of action being A'P, while that of W is B'W. The 
 power moves through a distance equal to AA, while 
 the weight moves through a distance equal to BB'. 
 But these distances are proportional to A'F and B'F. 
 We may represent A'F by Pd, the distance of the 
 power's' line of action from F ; and B'F by Wd f , the 
 distance of the weight's line of action from F. Sub- 
 stituting these terms in the general expression of 
 the law, we have, 
 
 W v "WV7' 
 P x Pd =W x Wd' (1) P : W : : Wd> : Pd (2) P = pd (3) 
 
 In the first and second classes, as ordinarily used, 
 we gain power and lose time ; in the third class we 
 lose power and gain time. 
 
84 
 
 ELEMENTS OF MACHINES. 
 
 The BALANCE is a iever of the first class with 
 equal arms. The bar, AS (Fig. 42), has a pair of 
 scale pans suspended from its ends. At the middle 
 
 PIG. 42. 
 
 Tlie Balance. 
 
 an axis, n, made of steel and provided with a knife 
 edge, rests upon a hard surface, so that the friction 
 may be the least possible ; this is the fulcrum. 
 
 The STEELYARD (Fig. 43) is a lever of the first class. 
 The P is at E, the F at (7, and the TFat D. If the dis- 
 
THE LEVER. 
 
 85 
 
 FIG. 43. 
 
 tance from the pivot of the hook D to the pivot of 
 the hook C be one inch, and from the 
 pivot of the hook Q to the notch where 
 E hangs be 1 2 inches, then 1 Ib. at E will 
 balance 12 Ibs. at W. If the steelyard be 
 reversed (Fig. 44), then the distance of 
 the F from the W is 
 only 
 
 FIG. 44. 
 
 J- as great, and 
 1 Ib. at E will balance 
 481bs.at J D. Two sets 
 
 of notches on opposite sides of the bar cor- 
 respond to these different positions. 
 
 The COMPOUND LEVER consists of several 
 levers so connected that the short arm of the first 
 acts on the long arm of the second, and so 
 on to the last of the series. If the distance 
 of A (Fig. 45) from the F be four times 
 that of B, a P of 5 Ibs. at A will balance 
 a W of 20 Ibs. at B. 
 If the arms of the 
 second lever are of 
 the same comparative 
 
 length, a P of 20 Ibs. at C will balance 80 
 Ibs. at E. In the third lever, a P df 80 Ibs. 
 at D will balance 320 Ibs. at G. With this 
 
 system of three levers, 5 
 Ibs. at A will according- 
 ly balance 320 Ibs. at G. 
 To raise the W 1 ft., how- 
 
 Tbe Compound Lever. , , -^ 
 
 ever, the P must move 
 64 ft. Thus what is gained in power is lost in time. 
 
 Fio. 45. 
 
86 ELEMENTS OF MACHINES. 
 
 There is no creation of force by the use of the 
 levers; on the contrary, there is an appreciable loss 
 because of friction. 
 
 Hay scales are constructed upon the principle of 
 the compound lever. Considering the large mass on 
 
 the platform as the power, 
 its pressure is transmitted 
 at the points P and P' (Fig. 
 46) to a pair of levers of the 
 third class, whose fulcrums 
 are at F and F'. Pressure 
 is thus produced at P" on 
 Hay scales. another lever whose fulcrum 
 
 is at F". At the remote end of this in turn, press- 
 ure is transmitted by the upright bar to the end, P"', 
 of a lever of the first class whose fulcrum is at F'". 
 The weight, W, can be adjusted at will until a bal- 
 ance is secured. 
 
 2. The Wheel and Axle is a 
 kind of perpetual lever. As both 
 arms work continuously, we are not 
 obliged to prop up the W and re-ad- 
 just the lever. In the windlass used 
 for drawing water from a well, the 
 P is applied at the handle, the W is 
 the bucket, and the F is the axis of the windlass. 
 The long arm of the lever is the length of the handle, 
 and the short arm is the semi-diameter of the axle. 
 This is shown in a cross-section (Fig. 47) where the 
 center, 0, is the F, OA the long arm, and OB the short 
 arm. In Fig. 48, instead of turning a handle we take 
 
THE WHEEL AND AXLE. 
 
 87 
 
 hold of pins inserted in the rim of the wheel. Fig. 
 4 9 represents a capstan used FIG. 48. 
 
 on vessels for raising the an- 
 chor. The P is applied by 
 handspikes inserted in the 
 axle. Fig. 50 shows a form 
 of the capstan employed in 
 moving buildings, in which a 
 horse furnishes the power. 
 
 LAW OF EQUILIBRIUM. By 
 turning the handle or wheel 
 
 around once, the rope will be 
 wound around the axle and the 
 W be lifted that distance. Ap- 
 plying the law of mechanics, 
 P x the circumference of the 
 wheel = W x circumference of 
 the axle ; or, as circles are pro- 
 portional to their radii, 
 
 Wheel and Axle. 
 
 FIG. 49. 
 
 Capstan. 
 
 P : W : : radius of the axle : radius of the wheel. 
 
 (4) 
 
 FIG. 50. 
 
 Capstan. 
 
88 
 
 ELEMENTS OF MACHINES. 
 
 51 
 
 . 
 
 w 
 
 WHEEL-WORK consists of a series of wheels and 
 axles which act upon one another on the principle of 
 the compound lever. The projections on the circum- 
 ference of the wheel are termed 
 teeth, on the axle leaves, and the 
 axle itself is called a pinion. If 
 the radius of the wheel F be 12 
 . p inches, and that of each pinion 2 
 inches, then a P of 1 Ib. will ap- 
 ply a force of 6 Ibs. to the second 
 wheel E. If the radius of this be 12 inches, then 
 the second wheel will apply a P of 36 Ibs. to the 
 third wheel, which, acting on its axle, will balance 
 a W of 2 1 6 Ibs. The W will pass through only ^ 
 the distance of the P. We thus gain power and lose 
 speed. If we wish to reverse this we can apply the 
 P to the axle, and so gain speed. This is the plan 
 adopted in factories, where a water-wheel furnishes 
 abundant power, and spindles or other machines are 
 to be turned with great rapidity. 
 
 3. The Inclined Plane. If we wish to lift a 
 heavy cask into a wagon, we rest one end of a plank 
 on the wagon-box and the other on the ground. We 
 can then easily roll the cask up this inclined plane. 
 When roads are to be made over steep hills, they 
 are sometimes constructed around the hill, like the 
 thread of a screw, or in a winding manner as shown 
 in Fig. 52. There is a remarkable ascent of this kind 
 on Mount Royal, Montreal. 
 
 LAW OF EQUILIBRIUM. In Fig. 53 the P must de- 
 scend vertically a distance equal to the length of the 
 
THE INCLINED PLANE. 
 
 89 
 
 plane, AC, in order to move the W from A to C and" 
 thus elevate it through the vertical height BC. Ap- 
 
 Fio 
 
 Inclined Plane. 
 
 plying the law of mechanics, P x length of inclined 
 
 plane = W x height of inclined plane ; hence, 
 
 P : W : : height of inclined plane : length of inclined plane.* (5) 
 
 100 ft., then a horse 
 
 FIG. 53. 
 
 W 
 
 Inclined Plane. 
 
 If a road ascend 1 ft. in 
 drawing up a wagon has to 
 lift only yj^ of the load, besides 
 overcoming the friction. A 
 body sliding down a perfectly 
 smooth inclined plane acquires 
 the same velocity that it would in falling the same 
 
 * If we roll into a wagon a barrel of pork, weighing 200 Ibs., up a plane 
 12 ft. long and 3 ft. high, we have 12 ft. : 3 ft. : : 200 Ibs. : x = 50 Ibs. We 
 lift only 50 Ibs., or i of the barrel, but we move it through four times the 
 space that would be necessary if we could elevate it directly into the wagon. 
 We thus lose speed and gain power. The longer the inclined plane, the 
 heavier the load we can lift, but the more time it will take to do it. 
 
90- 
 
 ELEMENTS OF MACHINES. 
 
 FIG. 54. 
 
 height perpendicularly. A train descending a grade 
 of 1 ft. in 100 ft. tends to go down with a force 
 equal to ^ of its weight.* 
 
 4. The Screw consists of an inclined plane wound 
 around a cylinder, the former being called the thread, 
 and the latter the body. It works in a nut which 
 is fitted with reverse threads to move on the thread 
 of the screw. The nut may turn on the screw, or the 
 screw in the nut. The P may be applied to either, 
 
 by means of a wrench 
 or lever. The screw is 
 used in vises ; in raising 
 buildings ; in copying let- 
 ters, and in presses for 
 squeezing the juice from 
 apples, sugar-cane, etc. 
 
 LAW OF EQUILIBRIUM. 
 When the P is applied at 
 the end of a lever, at- 
 tached to the head of the 
 screw, it describes a circle 
 screw. of which the lever is the 
 
 radius. The distance through which the P passes, is 
 the circumference of this circle ; and the height to 
 which the W is elevated at each revolution of the 
 screw, is the distance between two of the threads. 
 
 * Near Lake Lucerne is a forest of firs on the top of Mount Pilatus, an 
 almost inaccessible Alpine summit. By means of a wooden trough, the 
 log is conducted into the water below, a distance of eight miles, in but 
 little more than as many minutes. The force with which it falls is so 
 prodigious, that if it jumps out of the trough it is dashed to pieces. 
 
THE WEDGE AND PULLEY. 
 
 91 
 
 Applying the law of mechanics, P x circumference 
 of circle = W x interval between the threads ; hence, 
 
 P : W : : interval \ : circumference (6) 
 
 The efficiency of the screw may be increased by 
 lengthening the lever, or by diminishing the distance 
 between the threads. 
 
 5. The Wedge consists generally 
 of two inclined planes placed back to 
 back. It is used for splitting wood 
 and stone and lifting vessels in the 
 dock. Leaning chimneys have been 
 righted by wedges driven in on the 
 lower side. Nails, needles, pins, knives, 
 axes, etc., are made on the principle of the wedge. 
 
 THE LAW OF EQUILIBRIUM is the same as that of 
 inclined plane viz.:* 
 
 P : W : : thickness of wedge : length of wedge. . . (7) 
 
 Wedge. 
 
 Pis. 56. 
 
 W 
 
 Fixed Pulley. 
 
 6. The Pulley consists of a wheel, 
 within the grooved edge of which runs a 
 cord. 
 
 A FIXED PULLEY (Fig. 56) merely 
 changes the direction of the force. There 
 is no gain of power or speed, as the hand 
 P must move down as much as the 
 weight W rises, and both with the same 
 
 * In practice, however, this by no means accounts for the prodigious 
 power of the wedge. Friction, in the other mechanical powers, diminishes 
 their efficiency ; in this it is essential, else the wedge would fly back and 
 the effect be lost. In the others, the P is applied as a steady pressure ; in 
 this it is a sudden blow, and depends upon the kinetic energy expended in 
 the stroke of the hammer. 
 
92 
 
 ELEMENTS OF MACHINES. 
 
 velocity. It is simply a lever of the first class with 
 FIG. sr. equal arms. By its use a man standing 
 on the ground will hoist a flag to the top 
 of a lofty pole, and thus avoid the p IG . 5 s. 
 trouble and danger of climbing up 
 with it. Two fixed pulleys, ar- 
 ranged as shown in Fig. 57, make 
 it -possible to elevate a heavy load 
 to the upper story of 
 a building by horse- 
 power. 
 
 Application of Fixed Pulleys. A MOVABLE PULLEY 
 
 is represented in Fig. 58. One half of the barrel is 
 sustained by the hook while the hand lifts the other. 
 As the P is one half the W, it must move through 
 twice the space ; in other words, by taking twice the 
 time we can lift twice as much. Thus power is 
 
 gained and time lost. 
 
 We may also explain 
 p the single movable pulley 
 
 by Fig. 59. A represents 
 
 the F, R the W acting in 
 
 the line OR, and B the 
 
 P acting in the line BP. 
 
 This is a lever of the sec- 
 
 ond class ; and as A = P 
 
 Systems of Pulleys. 
 
 FIG. 
 
 FIG. Cl. 
 
 COMBINATIONS OF PULLEYS. (1.) 
 In Fig. 60, we have the W sus- 
 tained by three cords, each of which 
 is stretched by a tension equal to the P; hence, 
 
THE PULLEY. 
 
 93 
 
 FIG. 62, 
 
 FIG. 63. 
 
 1 lb. of power will balance 3 Ibs. of weight. (2.) In 
 Fig. 61, the P will sustain a W of 4 Ibs. (3.) In 
 Fig. 62, the cord marked 1 1 has a tension equal 
 to P in each part ; the one 
 marked 2 2 has a tension 
 equal to 2P in each part, 
 and so on with the others. 
 The total tension acting on 
 W is 16; hence, W=16P. 
 In this system, D rises twice 
 as fast as (7, four times as 
 fast as .B, etc. Work must 
 stop when D reaches E, which 
 gives little sweep to A for 
 lifting W. (4.) Fig. 63 rep- 
 
 w 
 
 System of 
 Pulleys. 
 
 Tackle Block. 
 
 resents the ordinary " tackle-block " used 
 by mechanics. 
 
 LAW OF EQUILIBRIUM. When a continuous rope is 
 used, let n represent the number of separate parts 
 of the cord which sustain the movable block. We 
 then have 
 
 j> = . ... ... . : . . . (8) 
 
 When the number of movable arid of fixed pulleys 
 is equal, in general, W P x twice the number of 
 movable pulleys. 
 
 7. Cumulative Contrivances. A hammer, club, 
 pile-driver, sling, fly-wheel, etc., are instruments for 
 accumulating energy to be used at the proper mo- 
 ment. Thus we may press a hammer on the head 
 of a nail with all our strength to no purpose ; but 
 
94 ELEMENTS OF MACHINES. 
 
 swing the hammer the length of the arm, and the 
 blow will bury the nail to the head. The strength 
 of our muscles and the attraction of gravity during 
 the fall both gather energy to be exerted at the in- 
 stant of contact. A fly-wheel by its momentum 
 equalizes an irregular force, or produces a sudden 
 effect.* 
 
 8 Perpetual Motion. It is impossible to make a 
 machine capable of perpetual motion. No combina- 
 tion can produce energy ; it can only direct that 
 which is applied. In all machinery there is friction ; 
 this must ultimately exhaust the power and bring 
 the motion to rest. The only question is, how long 
 a time will be required for the leakage to drain the 
 reservoir. Every year brings to light new seekers 
 after perpetual motion. The mere fact that a man 
 devotes himself to the solution of this impossible 
 problem is now generally regarded as a proof that 
 either his mental balance has been disturbed, or his 
 knowledge of the laws of nature is too meager to 
 entitle him to consideration. 
 
 PRACTICAL QUESTIONS. 
 
 1. Describe the rudder of a boat as a lever. A door. A door-latch. A 
 lemon-squeezer. A pitchfork. A spade. A shovel. A sheep-shears. A 
 poker. A pair of tongs. A balance. A pair of pincers. A wheelbarrow. 
 A man pushing open a gate with his hand near the hinge. A sledge-ham- 
 
 * We see the former illustrated in a sewing-machine, and the latter 
 in a punch operated by a treadle. In the one case, the irregular action of 
 the foot is turned into a uniform motion, and in the other it is concen- 
 trated in a heavy blow that will pierce a thick piece of metal. 
 
PRACTICAL QUESTIONS. 95 
 
 mer, when the arm is swung from the shoulder. A nut-cracker. The arm 
 (see "Physiology,' p. 48). 
 
 2. Show the change that occurs from the second to tl^e third class of 
 lever, when you take hold of a ladder at one end and raise it against a 
 building. 
 
 3. Why is a pinch from the tongs near the hinge more severe than one 
 near the end? 
 
 4. Two persons are carrying a weight of 250 Ibs., hanging between them 
 from a pole 10 ft. in length. Where should it be suspended so that one 
 will lift only 50 Ibs. ? 
 
 5. In a lever of the first class, 6 ft. long, where should the F be placed 
 so that a P of 1 Ib. will balance a W of 23 Ibs. ? 
 
 6. What P would be required to lift a barrel of pork with a windlass 
 whose axle is 1 ft. in diameter, and handle 3 ft. long? 
 
 7. What sized axle, with a wheel 6 ft. in diameter, would be required 
 to balance a W of one ton by a P of 100 Ibs. ? 
 
 8. What number of movable pulleys would be required to lift a W of 
 200 Ibs. by a P of 25 Ibs. ? 
 
 9.. How many pounds could be lifted with a system of 4 movable pul- 
 leys by a P of 100 Ibs. ? 
 
 10. What W could be lifted with a single horse-power * acting on a sys- 
 tem of pulleys shown in Fig. 62? 
 
 11. What distance should there be between the threads of a screw to 
 enable a P of 25 Ibs. acting on a handle 3 ft. long, to lift a ton ? 
 
 12. How high would a P of 12 Ibs., moving 16 ft. along an inclined 
 plane, lift a W of 96 Ibs. ? 
 
 13. I wish to roll a barrel of flour into a wagon, the box of which is 
 4 ft. from the ground. I can lift but 24 Ibs. How long a plank must 
 I get? 
 
 14. What W can be lifted with a P of 100 Ibs. acting on a screw hav- 
 ing threads 1 in. apart, and a handle 4 ft. long ? 
 
 15. What is the object of the balls often cast on the ends of the han- 
 dle of the screw used in presses for copying letters? 
 
 16. In a steelyard 2 ft. long, the distance from the weight-hook to the 
 fulcrum-hook is 2 in. ; how heavy a body can be weighed with a pound 
 weight ? 
 
 17. Describe the change from the, first to the third class of lever, in 
 the different ways of using a pitchfork or a spade. 
 
 18. Why are not blacksmiths' tongs and fire-tongs constructed on the 
 same principle? 
 
 19. In a lever of the third class, what W will a P of 50 Ibs. balance, if 
 one arm be 12 ft. and the other 3 ft. long? 
 
 * A horse-power is a force which is equivalent to 550 foot-pounds, i. e., can 
 raise against gravity 550 Ibs. one foot in one second, or 33,000 Ibs. one foot 
 in one minute. 
 
96 ELEMENTS OF MACHINES. 
 
 20. In a lever of the second class, what W will a P of 50 Ibs. balance, 
 with a lever 12 ft. long, and the W 3 ft. from the F? 
 
 21. In a lever of the first class, what W will a P of 50 Ibs. balance, 
 with a lever 12^. long, and the F 3 ft. from the W? 
 
 22. In a wheel and axle, the P = 40 Ibs., the W = 360 Ibs., and the 
 diameter of the axle = 8 in. Required the circumference of the wheel. 
 
 23. Suppose, in a wheel and axle, the P = 20 Ibs., the W = 240 Ibs., and 
 the diameter of the wheel = 4 ft. Required the circumference of the axle. 
 
 24. Required, in a wheel and axle, the diameter of the wheel, the 
 diameter of the axle being 10 in., the P 100 Ibs., and the W 1 ton. 
 
 25. Why is the rim of a fly-wheel made so heavy? 
 
 26. Describe the hammer, when used in drawing a nail, as a bent lever^ 
 i. ., one in which the bar is not straight. 
 
 27. Describe the four levers shown in Kg. 46, when both the load of 
 hay and the weight are considered, respectively, as the W and the P. 
 
 SUM MARY. 
 
 ALL machines can be resolved into a few elementary forms. 
 Of these there are six, viz., the lever, the wheel and axle, the 
 inclined plane, the screw, the wedge, and the pulley. Though 
 called the mechanical powers, they are only instruments by 
 which we can avail ourselves of the forces of nature. Molar 
 energy or the motion of masses, as of air, water, steam, 
 etc., is thus utilized, while the strength of a horse does the 
 work of many men. A force of small intensity made to act 
 through a considerable distance becomes one of great intensity 
 acting through a small distance, and vice versa. By the use of 
 the mechanical powers, the application of energy is made more 
 convenient, but always some energy is absorbed in moving the 
 machine and overcoming friction, and hence prevented from 
 doing useful work. No machine can be a source of power, but, 
 on the contrary, it thus involves a loss of power. The law of 
 machines is, that the power multiplied by the distance through 
 which it moves is equal to the weight multiplied by the distance 
 through which it moves, plus the internal work involved in the 
 motion of the machine. This law is equivalent to a statement 
 that perpetual motion is impossible ; for no known terrestrial 
 source of energy is exhaustless. 
 
HISTOKICAL SKETCH. 97 
 
 The lever is a bar resting at some point on a prop as a 
 center of motion. The crowbar, claw-hammer for drawing nails, 
 pincers, windlass, and steelyard are examples of various classes 
 of levers. The compound lever consists of several levers, so 
 connected, that the short arm of one acts on the long arm of 
 the next, as in the hay scales. In the bent lever, the power 
 and weight act in lines that are not necessarily parallel, but 
 still tend to produce rotation of the lever about its fulcrum, if 
 the product of the power by the perpendicular distance from its 
 line of action to the fulcrum be not equal to the weight multi- 
 plied by the distance from its line of action to the fulcrum. 
 These two products are called the moments about the fulcrum. 
 If the two moments are equal and opposite, the result is equi- 
 librium. 
 
 To the lever may be reduced the wheel and axle, and the 
 pulley. To the inclined plane may be reduced the wedge and 
 the screw. The awl, vise, carpenter's plane, corkscrew, and 
 stairs are common modifications of the inclined plane. The 
 blade of a pocket-knife is a familiar example of the wedge, 
 which itself is only a movable inclined plane. In the applica- 
 tion of these last mechanical powers, friction becomes a most 
 important and useful element ; and it interferes so much with 
 the operation of the simple machine alone, which should be de- 
 void of friction in order to make exact calculation possible, that 
 it is usually impossible to calculate the ratio between the power 
 applied and the work accomplished through the medium of a 
 wedge. 
 
 HISTORICAL SKETCH. 
 
 SIMPLE machines for moving large bodies are as old as his- 
 tory. The Babylonians knew the use of the lever, the pulley, 
 and the roller. The Romans were acquainted with the lever, 
 the wheel and axle, and the pulley (simple and compound). The 
 Egyptians, it is thought, raised the immense stones used in 
 building the Pyramids, by inclined planes made of earth which 
 was afterward removed. Archimedes, in the third century B.C., 
 
98 ELEMENTS OF MACHINES. 
 
 discovered the law of equilibrium in the lever.* His investiga- 
 tions, however, were too far in advance of his time to be fully 
 understood, and the teachings of Aristotle were long after ac- 
 cepted by scientific men. The law of mechanics, or of Virtual 
 Velocities, as it is called, was discovered by Galileo. 
 
 * It is often said that Archimedes, in allusion to the tremendous power 
 of the lever, asserted that, Give him a fulcrum and he could move the 
 world. Had he been allowed such a chance, " the fulcrum being nine 
 thousand leagues from the center of the earth, with. power of 200 Ibs. 
 the geometer would have required a lever 12 quadrillions of miles long and 
 the power would have needed to move at the rate of a. cannon-ball to lift 
 the earth one inch in 27 trillions of years." 
 
V. 
 
 PRESSURE OF LIQUIDS AND 
 GASES. 
 
 "THE waves that moan along the shore, 
 
 The winds that sigh in blowing, 
 Are sent to teach a mystic lore 
 
 Which men are wise in knowing.* 1 
 
ANALYSIS. 
 
 2. LIQUIDS INFLUENCED 
 
 BY GRA.VITY. 
 
 (4.) Specific 
 Gravity. 
 
 1. RULES CONCERNING 
 A JET. 
 
 1. LIQUIDS INFLUENCED / (1.) Law of Transmission. 
 
 BY EXTERN ALPBES- -j (2.) Water as a Mechanical Power. 
 SURE ONLY. ( (3.) Hydrostatic Press. 
 
 (1.) Four Laws of Equilibrium. 
 (2.) Rules for Computing Pressure. 
 (3.) Water Level. 
 
 (a.) Definition and Illustra- 
 tion. 
 
 (b.) Buoyant Force of Liq- 
 uids. 
 
 (c.) To Find Specific Grav- 
 ity of a Solid. 
 
 (d.) To Find Specific Grav- 
 ity of a Liquid. 
 (e.) To Find Weight of Given 
 
 Volume, 
 (f.) To Find Volume of Given 
 
 Weight. 
 (g.) To Find Volume of a 
 
 Body. 
 
 (h.) Floating Bodies. 
 DEFINITION AND GENERAL PRINCIPLES. 
 
 (1.) The Velocity that of a Palling Body. 
 (2.) To Pind the Velocity. 
 (3.) To Pind the Quantity. 
 
 2. EFFECT OF TUBES. 
 
 3. PLOW OF WATER IN RIVERS. 
 
 f (1.) Overshot. 
 
 4. WATER-WHEELS. 4 < 2 '\ Undershot. 
 
 (3.) Breast. 
 I (4.) Turbine. 
 DEFINITION AND GENERAL PRINCIPLES. 
 
 1. AIR-PUMP. 
 
 2. CONDENSER. 
 
 / (1.) Weight. 
 
 3. PROPERTIES OF AIR. < (2.) Elasticity. 
 
 ( (3.) Expansibility. 
 
 (1.) The Proof. 
 
 (2.) Upward Pressure. 
 
 (3.) Buoyant Porce. 
 
 (4.) Amount of Pressure. 
 
 (5.) Pressure Varies. 
 
 (6.) Mariotte's (or Boyle's) Law. 
 
 (7.) Barometer. 
 
 (1.) Lifting. 
 
 (2.) Porcing. 
 ( (3.) Pire-engine. 
 
 6. SIPHON. 
 
 7. PNEUMATIC INSSTAND. 
 
 8. HYDRAULIC RAM. 
 9. . ATOMIZER. 
 
 10. HEIGHT OF THE ATMOSPHERE. 
 
 4. PRESSURE OF THE 
 AIR. 
 
 5. PUMPS. 
 
PRESSURE OF LIQUIDS AND GASES. 
 
 1. HYDROSTATICS. 
 
 HYDROSTATICS treats of liquids at rest. Its princi- 
 ples apply to all liquids; but water, on account of 
 its abundance, is taken as the type. 
 
 1. Liquids Influenced by External Pressure only. 
 
 (1.) PASCAL'S LAW.* Liquids transmit 
 pressure equally in all directions, and this 
 acts at right angles upon the surface 
 pressed. 
 
 As the particles of a liquid move freely 
 among themselves, there is no loss by 
 friction, and any force will be transmitted 
 equally upward, downward, and side wise. 
 Thus, if a bottle be filled with water and 
 a pressure of 1 Ib. be applied upon the 
 cork, it will be communicated from par- 
 ticle to particle throughout the water. If 
 the area of the cork be 1 sq. in., the pressure upon 
 
 Illustration of 
 Pascal's Law. 
 
 * This law is named after the celebrated geometrician, Blaise Pascal, 
 who first enunciated it in 1663. At first thought it may seem impossible 
 for a pressure of 1 Ib. to produce a pressure of 100 Ibs. ; but it should be 
 remembered that the general law of mechanics applies to liquids as well as 
 solids. If a force of 1 Ib. on i sq. in. should came motion by pressing 
 through the medium of a liquid on 100 sq. in., the velocity of the body 
 moved will be only ii 5 th of that of the body applying the pressure. 
 
102 L^PKE1SS;UR;E:QF LIQUIDS AND GASES. 
 
 PIG. 65. 
 
 Tube with Cylinder of Lead. 
 
 FIG. 66. 
 
 Bent Tube Full of Water. 
 
 any sq. in. of the glass at n, a, b, or c, will be 1 Ib. 
 If the inside surface of the bottle be 100 sq. in., a 
 pressure of 1 Ib. upon the cork will produce a force 
 of 100 Ibs., tending to burst the bottle. 
 
 Illustrations. The 
 P transmission of press- 
 ure by liquids under 
 soms circumstances, is 
 more perfect than by solids. Let a straight tube, 
 AB, be filled with a cylinder of lead, and a piston 
 be fitted to the end of 
 the tube. If a force be 
 applied at P, it will be 
 transmitted to with- 
 out sensible loss. If in- 
 stead, we use a bent 
 tube, the force will be transmitted in the lines of the 
 
 arrows, and will act on 
 Pbut slightly. If, how- 
 ever, we fill the tube 
 with water, the force 
 will be transmitted 
 without diminution.* 
 Take a glass bulb and 
 stem full of water, as in 
 Fig. 67.f 1^ you are 
 pounding with a Glass Buib. careful to let the stem 
 
 slip loosely through your fingers as the bulb strikes, 
 
 * With cords, pulleys, levers, etc., we lose often more than one half of 
 the force by friction; but this "liquid rope" transmits it with no appre- 
 ciable loss. 
 
 t The process of filling such bulbs is shown on p. 248. They are cheaply 
 
 FIG. 67. 
 
HYDROSTATICS. 
 
 103 
 
 FIG. 68. 
 
 Rupert's Drop in Vial. 
 
 you may pound it upon a smooth surface. The force 
 
 of the blow is instantly transmitted from the thin 
 
 glass to the water, which is almost incompressible, 
 
 and this makes the bulb 
 
 nearly as solid as a ball of 
 
 glass. If a Rupert's drop 
 
 be held in a closed vial of 
 
 water so as not to touch 
 
 the glass (Fig. 68), and 
 
 the tapering end be broken, 
 
 the water will transmit 
 
 the concussion and shatter 
 
 the vial. 
 
 (2.) WATER AS A MECHANICAL POWER. Take two 
 
 cylinders, P and p (Fig. 69), fitted with pistons and 
 
 filled with water. Let the area of p be 1 sq. in. and 
 
 that of P be 100 sq. in. 
 Then a downward press- 
 ure of 1 Ib. on each sq. in. 
 of the small piston will 
 produce an upward press- 
 ure of 1 Ib. on each sq. in. 
 of the large piston. H&nce 
 a P of 1 IV. moving at a 
 
 rate of 1 in. per second will lift a W of 100 Ibs. at 
 
 a rate of yfoth of an inch per second.* 
 
 purchased of apparatus dealers, and explain not only this point, but also the 
 method of filling thermometers. 
 
 * Pascal announced the discovery of this principle in the following 
 words : " If a vessel full of water closed on all sides has two openings, the 
 one a hundred times as large as the other, and if each be supplied with a 
 piston which fits exactly, a man pushing the small piston will exert a force 
 which will balance that of a hundred men pushing the large one." 
 
 FTG. 69. 
 
 Principle of the Hydrostatic Press. 
 
104 PRESSURE OF LIQUIDS AND GASES. 
 
 (3.) THE HYDROSTATIC PRESS (Fig. 70) utilizes the 
 principle just explained. As the workman depresses 
 the lever 0, he forces down the piston a upon the 
 water in the cylinder A. The pressure is transmitted 
 through the bent tube of water d under the piston 
 
 PIG. 70 
 
 The Hydrostatic Press. 
 
 (7, which lifts up the platform K, and compresses 
 the bales. If the area of a be 1 in. and that of C 
 100 in., a force of 100 Ibs. will balance 10,000 Ibs. 
 The handle is a lever of the second class. If the dis- 
 tance of the hand from the pivot be ten times that 
 of the piston, a P of 100 Ibs. will produce a force 
 
HYDROSTATICS. 
 
 105 
 
 of 1,000 Ibs. at a. This will become 100,000 Ibs. at 
 (7.* According to the principle of mechanics, PxPd 
 = Wx Wdj the platform will ascend T -oVo of the dis- 
 tance the hand descends. This machine is used for 
 baling hay and cotton, for launching vessels, and for 
 testing the -strength of ropes, chains, etc. 
 
 2. Liquids Influenced by Gravity. The lower 
 part of a vessel of water must bear the weight of 
 the upper part. Thus each particle of water at rest 
 is pressed downward by the weight of the minute 
 column it sustains. It must, in turn, press in every 
 direction with the same force, else it would be driven 
 out of its place and the liquid would no longer be at 
 rest. Indeed, when a liquid is disturbed it comes to 
 rest i. e., there is an equilibrium established only 
 when this equality of pressure is produced. The fol- 
 lowing laws obtain : 
 
 FIG. 71. 
 
 Transmission of Pressure in all Directions. - 
 
 (1.) FOUR LAWS or EQUILIBRIUM. I. At any point 
 within a liquid at rest the pressure is the same in 
 
 * The presses employed for raising the immense tubes of the Britannia 
 Bridge across the Menai Strait, were each capable of lifting 2,672 tons, and 
 of "throwing water in a vacuum to a height of nearly six miles, or over 
 the top of the highest mountain." 
 
106 PRESSURE OF LIQUIDS AND GASES. 
 
 PIG. 72. 
 
 all directions. If the series of glass tubes shown in 
 Fig. 71 be placed in a pail of water, the liquid will 
 be forced up 1 by the upward pressure of the water, 
 2 by the downward pressure, 3 by the lateral press- 
 ure, and 4 by the three combined in different por- 
 tions of the tube. The water 
 will rise in them all to the same 
 height i. e., to the level of the 
 water in the pail. 
 
 II. The pressure increases 
 with the depth. Into a tall jar 
 full of water put a bent tube 
 open at both ends, as shown 
 in Fig. 72, a little mercury 
 having been previously poured 
 in so as to fill the bend. The 
 pressure of the water forces 
 down the mercury in the short 
 arm, that in the long arm being 
 exposed only to the pressure of 
 the air. Suppose the difference 
 of level to be an inch when the 
 tube is lowest. Then a column 
 of mercury an inch long will 
 just balance the weight of a 
 column of water of the same 
 thickness and nearly equal in 
 
 ^^ ^ ^ height Qf the ^ 
 
 Let the tube now be raised until the surface of the 
 mercury in the short arm is only a fourth of its 
 previous distance from the surface. The difference 
 
 lucre** of Pressure withDepth. 
 
HYDROSTATICS. 
 
 107 
 
 of level in the two arms is now found to be only a 
 fourth of an inch. 
 
 A cubic foot of water weighs about 62.5 Ibs. 
 (1,000 oz.) ; the same volume of sea-water weighs 
 64.4 Ibs.; hence the pressure is proportionally greater 
 in sea-water. At the greatest depth ever measured 
 in the sea, a little over five miles, the pressure is 
 about six tons on every square inch. An empty 
 
 PIG. 73. 
 
 glass bottle securely stoppered may /be crushed before 
 sinking a hundred yards. 
 
 III. The pressure does not depend on the shape 
 or size of the vessel, but on the area and depth. In 
 the apparatus shown in Fig. 73, a disk is held up by 
 a string against the bottom of an open tube, to 
 which may be screwed vessels of different shapes 
 and sizes, such as A, B, or C. The string is attached 
 to the beam of a balance. In the scale pan is put 
 
108 PRESSUKE OF LIQUIDS AND GASES. 
 
 Fie, 74. 
 
 such a weight, Jif, as to balance the pressure of the 
 liquid against the disk when the vessel is full up to 
 a certain point, n. The addition of any more liquid 
 causes the disk to sink and thus spill the liquid, 
 whether the large vessel, A, or the smaller ones, B or 
 (7, be used. Against the same area at the bottom 
 the same pressure is obtained even if A is three 
 times B in volume, provided the 
 depth be kept the same. If the 
 depth of water be increased, M 
 has to be increased in the same 
 ratio. Thus a pound of water 
 in B may be made to exert a 
 greater pressure at the bottom 
 than 2 Ibs. of water in A. 
 
 The Hydrostatic Bellows is an 
 application of the principles just 
 discussed, and is like the Hydro- 
 static Press on a small scale. It 
 consists of two boards connected 
 by a band of leather and pro- 
 vided with a supply tube for 
 water. Suppose the area of C 
 (Fig. 74) to be 500 sq. in., and 
 the area across the small tube 
 A to be a single sq. in. Let the 
 stiffness of the leather, along 
 with an added weight on (7, be 
 equivalent to 100 Ibs. Then 
 only one fifth of a pound of water in A is needed to 
 balance the 100 Ibs. If at A we use a larger tube 
 
 Hydrostatic Bellows. 
 
HYDROSTATICS. 109 
 
 across which the area is 10 sq. in., then 2 Ibs. of 
 water in it are required to maintain equilibrium. 
 The surface of the water in A will be about 6 in. 
 above (7, whether the large or small tube is used. 
 If a cubic inch more be poured into the small tube, 
 the same quantity will be forced into the bellows, so 
 that C rises ^-th of an inch. If a larger weight is 
 put on (7, a higher column in A is required, but still 
 C can be raised by any addition to A, however small. 
 
 A strong cask fitted with a FIG. 75. 
 
 small pipe 30 or 40 feet long, 
 if filled with water will be burst 
 asunder.* The pressure is as 
 great as if the tube were of the 
 same diameter as the cask. In 
 a coffee-pot the small quantity 
 of liquid in the spout balances 
 the large quantity in the vessel. 
 If it were not so, it would rise 
 in the spout and run out. 
 
 IV. Water seeks its level. In 
 the apparatus shown in Fig. 76 
 the water rises to the same 
 
 height in the various tubes which communicate with 
 one another, because so long as the surfaces are 
 not at the same level a particle below any surface 
 
 * Suppose the pipe to have an area of ts sq. in., and to hold \ Ib. of 
 water. The pressure on each > of an inch of the interior of the cask 
 would be J Ib., or 2,880 Ibs. on each sq. ft. a pressure no common barrel 
 could sustain. The principle that a small quantity of water will thus bal- 
 ance another quantity, however large, or will lift any weight, however great, 
 is frequently termed the "Hydrostatic Paradox." It is only an instance of 
 a general law, and is no more paradoxical than the action of the lever. 
 
110 PRESSURE OF LIQUIDS AND GASES. 
 
 must be unequally pressed from opposite sides, and 
 
 must move until equi- 
 librium is attained. In 
 Fig. 7 7 tank is situ- 
 ated on a hill, whence 
 the water is conducted 
 underground through 
 a pipe to the fountain. 
 In theory the jet will 
 rise to the level of the reser- 
 voir, but 'in practice it falls 
 short, owing chiefly to the 
 friction in the pipe, the 
 weight of the falling drops, 
 
 Water in Communicating . . ., 
 
 vessels. and the resistance of the air. 
 
 FIG. 77. 
 
 Construction of a Fountain. 
 
HYDROSTATICS. 
 
 Ill 
 
 Artesian Wells.* Let A B and C D represent 
 curved strata of clay impervious to water, and K K 
 a layer of gravel. The rain falling on the hills filters 
 down to C D, and collects in this basin. If a well 
 be bored at H, as soon as it reaches the gravel the 
 
 Artesian Well. 
 
 water will rush upward, under the tremendous lateral 
 pressure, to the surface of the ground, often spout- 
 ing high in air.f 
 
 Wells. Of the rain which falls on the land, a part 
 runs directly to the streams and part soaks into the 
 soil. The latter portion may filter down to an im- 
 
 * They are so Jiamed because they have long been used in the province 
 of Artois (Latin, Artesium), Prance. They were, however, early employed 
 by the Chinese for the purpose of procuring gas and salt water. 
 
 t The famous well at Grenelle, Paris, is at the bottom of a basin which 
 extends miles from the city. It is about 1,800 feet deep, and furnishes 656 
 gallons of water per minute. The two wells of Chicago are about 700 feet 
 deep, and discharge daily about 432,000 gallons. Being situated on the 
 level prairie, the force with which the water comes to the surface indicates 
 that it may be supplied perhaps from Bock River, 100 miles distant. There 
 are also valuable artesian wells at Louisville, Kentucky, and at Charleston, 
 South Carolina. When the water comes from a great depth, it is generally 
 warm. 
 
112 PRESSURE OF LIQUIDS AND GASES. 
 
 permeable' layer of rock or clay, and then run along 
 till it oozes out at some lower point as a spring ; or, 
 if it can not escape, it will collect in the ground. If 
 a well be sunk into this subterranean reservoir, the 
 water will rise in it to the level of the source.* 
 
 (2.) RULES FOR COMPUTING PRESSURE. I. To find 
 the pressure on the bottom of a vessel. Multiply the 
 area of the base in square feet by the vertical height 
 in feet, and that product by the weight of a cubic 
 foot of the liquid. 
 
 FIG 79. 
 
 The Curvature of a Water Level. 
 
 II. To find the pressure on the side of a vessel. 
 Multiply the area of the side in square feet by half 
 
 * "From a forgetfulness of this principle the company which dug the 
 Thames and Medway Canal, England, incurred heavy damages. Having 
 planned the canal to "be filled at high tide, the salt water spread immedi- 
 ately into all the wells of the surrounding region. Had the canal been dug 
 a few feet lower, the evil would have been avoided." ABNOTT. 
 
HYDROSTATICS. 113 
 
 of the vertical height in feet,* and that product by 
 the weight of a cubic foot of the liquid. The press- 
 ure on the bottom of a cubical vessel of water is the 
 weight of the water ; on each side, one half ; and on 
 the four sides, twice the weight ; therefore, on the 
 five sides the pressure is three times the weight of 
 the water. 
 
 (3.) WATER-LEVEL. The surface of standing water 
 is said to be level i. e., horizontal to a plumb-line. 
 This is true for small sheets of water, but for larger 
 bodies an allowance must be made for the spherical 
 form of the earth (Fig. 79). The curvature is 8 
 inches for the first mile, and increases as the square 
 of the distance, f 
 
 The spirit-level is an instrument used by builders 
 for leveling. It consists of a slightly-curved glass 
 tube nearly full ^ ^ 
 
 of alcohol, so that 
 it holds only a 
 bubble of air. 
 When the level 
 is horizontal, the The 8 P irit - level - 
 
 bubble remains at the center of the upper side of 
 the tube. 
 
 (4.) SPECIFIC GRAVITY, or relative weight, is the 
 ratio of the weight of a substance to that of the 
 
 * This clause of the rule holds only when the center of gravity of the 
 side is at half the vertical height. In general, the depth of its center of 
 gravity below the surface should be used as the multiplier. 
 
 t For two miles it is 8 inchesx2 3 =32 inches. If one's eye were at the 
 level of still water, he could barely see the top of an object 67 feet high at 
 a distance of 10 miles in a perfectly clear atmosphere. 
 
114 PKESSUKE OF LIQUIDS AND GASES. 
 
 same volume cf another substance taken as a stand- 
 ard. Water is taken as the standard* for solids and 
 PIQ gi liquids, and air for gases. A 
 
 cubic inch of sulphur weighs 
 twice as much as a cubic inch 
 of water ; hence its specific 
 gravity =2. A cubic inch of car- 
 bonic-acid gas weighs 1.52 times 
 as much as the same volume of 
 air; hence its specific gravity = 
 1.52.f 
 
 Buoyant Force of Liquids. 
 The cube a b c d is immersed 
 
 in water. The lateral pressure at a is equal to 
 that at &, because both sides are at the same 
 depth ; hence the body has no tendency toward 
 either side of the jar. The upward pressure at c is 
 greater than the downward pressure at d, because 
 its depth is greater ; hence the cube has a tendency 
 to rise. This upward pressure is called the buoyant 
 force of the water. It is equal to the weight of the 
 
 * "The water must be at 39.2 F., its greatest density. In all exact 
 measurements, especially of standards, it is necessary to know the tempera- 
 ture. For the scale that is a foot long to-day may be more or less than a 
 foot long to-morrow; the measure that holds a pint to-day may hold more 
 or less than a pint to-morrow. Nay, more, these measures may not be the 
 same in two consecutive moments. When a carpenter takes up his rule 
 and applies it to some object, the size of which he wishes to determine, it 
 becomes in that instant longer than it was before ; when a druggist grasps 
 his measuring glass in his hand to dispense some of his preparations, the 
 glass increases in size. A person, enters a cool room, and at once it becomes 
 more capacious, for its walls, ceiling, and floor, because of the heat he im- 
 parts, immediately expand." DRAPER. 
 
 t The term density is often used in the same sense as specific gravity, 
 especially in relation to gases. 
 
HYDROSTATICS. 
 
 115 
 
 FIG. 82. 
 
 liquid displaced. For the downward pressure at d is 
 the weight of a column of water whose area is that 
 of the top of the cube, and whose vertical height 
 is n d, and the upward pressure at c is equal to 
 the weight of a column of the same size whose 
 vertical height is n c. The difference between the 
 two, or the buoyant force, is the weight of a volume 
 of water equal to the size of the cube. 
 
 The same principle is shown in the " cylinder-and- 
 bucket experiment." The cylinder a exactly fits in 
 the bucket &. When the glass vessel in which the 
 cylinder hangs is empty, the apparatus is balanced 
 by weights placed in 
 the scale-pan. Next, 
 water is poured into 
 the glass vessel. Its 
 buoyant force raises 
 the cylinder and de- 
 presses the opposite 
 scale-pan. Then water 
 is dropped into the 
 bucket; when it is 
 exactly full, the scales 
 will balance again. 
 This proves 
 that a body in 
 water is buoyed 
 up by a for^e 
 equal to the 
 weight of the wa r it displaces." This is called Ar- 
 chimedes' law. 
 
 Cylinder-and-bucket Experiment. 
 
116 PRESSURE OF LIQUIDS AND GASES. 
 
 FIG. 83. 
 
 To find the specific gravity of a heavy solid. Weigh 
 the body in air, and in water ; the difference is the 
 weight of its volume of water ; divide its weight in 
 air by its loss of weight in water; the quotient is 
 the specific gravity. Thus, sulphur loses one half its 
 weight when immersed in water; hence it is twice 
 as heavy as water, and its specific gravity =2.* 
 
 To find the specific gravity of a liquid ~by the 
 specific-gravity flask. This is a bottle 
 which holds exactly 1,000 grains of 
 water. If it will hold 1,840 grains of 
 sulphuric acid, the specific gravity of 
 the acid is 1.84. 
 
 To find the specific gravity of a 
 liquid ~by a hydrometer. This instru- 
 ment consists of a glass tube, closed at 
 one end and having at the other a bulb 
 containing mercury. A graduated scale 
 is marked upon the tube. The alcohol- 
 ometer, used in testing alcohol, is so 
 balanced as to sink in pure water to 
 the zero point. As alcohol is lighter than water, the 
 instrument will descend for every addition of spirits. 
 
 * In careful measurements an allowance is made for the weight of the 
 air displaced by the body, so that its weight in a vacuum becomes known. 
 Strictly, it is the weight in a vacuum that has to be compared with the 
 loss of weight in water. If the body will not sink in water, attach it to a 
 heavy body. 1. Weigh the lighter body in air (A). 2. Weigh the heavy 
 body in water (E). 3. Weigh both together in water (C). Now C is less 
 than B because the light body buoys up the heavy one ; I, e., its weight A 
 is more than balanced, and is replaced by an upward or lifting force=l? 
 <7. Therefore the loss of the light body in wa,teT=A+BC.'. spec. grav.= 
 
 A 
 A+B-C' 
 
 Hydrometer. 
 
HYDROSTATICS. 117 
 
 The degrees of the scale indicate the percentage of 
 alcohol. Similar instruments are used for determin- 
 ing the strength of milk, acids, etc. 
 
 To find the weight of a given volume of any sub- 
 stance. Multiply the weight of one cubic foot of water 
 by the specific gravity of the substance, and that 
 product by the number of cubic feet. Example : What 
 is the weight of three cubic feet of cork? Solution: 
 1,000 oz. x. 240*^240 oz. ; 240 oz. x 3 = 720 oz. 
 
 To find the volume of a given weight of any sub- 
 stance. Multiply the weight of a cubic foot of water 
 by the specific gravity of the substance, and divide 
 the given weight by that product. The quotient is 
 the required volume in cubic feet. Example: What 
 is the volume of 20,000 oz. of lead? Solution: 1,000 
 oz.x 11. 36 = 11, 360; 20,000-r-ll, 360 = 1. 76+cu. ft. 
 
 To find the volume of a body. Weigh it in water. 
 The loss of weight is the weight of the displaced 
 water. Then, as a cubic foot of water weighs 1,000 
 oz., we can easily find the volume of water displaced. 
 Example : A body loses 1 oz. on being weighed 
 in water. The displaced water weighs 10 oz. and 
 is riir of 'a cubic foot ; this is the exact volume of 
 the body. 
 
 * TABLE or SPECIFIC Q-RAVITT. (See "Chemistry," p. 288.) 
 
 Iridium 21.80 
 
 Platinum 21.53 
 
 Gold 19.34 
 
 Mercury ... 13.59 
 
 Lead 11.36 
 
 Silver 10.50 
 
 Copper 8.90 
 
 Cast-iron 7.21 
 
 Zinc 7.15 
 
 Diamond. .. .about 3.50 
 
 Flint Glass 2.76 
 
 Chalk 2.65 
 
 Sulphur 2.00 
 
 Ice 93 
 
 Potassium 86 
 
 Quicklime 80 
 
 Pine Wood 66 
 
 Cork 24 
 
 Sulphuric Acid 1.84 
 
 Water from Dead Sea. 1.24 
 
 Milk 1.03 
 
 Sea-water 1.03 
 
 Absolute Alcohol .79 
 
 N 
 
118 PRESSURE OF LIQUIDS AND GASES. 
 
 Floating Bodies. A body will float in water when 
 its weight is no.t greater than that of an equal vol- 
 ume of the liquid, and its weight 
 always equals that of the fluid 
 displaced. An egg dropped into 
 a glass jar half full of water 
 (Fig. 84) sinks directly to the 
 bottom. If, by means of a funnel 
 with a long tube, we pour brine 
 beneath the water, the egg will 
 rise. We may vary the experiment by 
 not dropping in the egg until we have 
 half filled the jar with the brine. The 
 egg will then fall to the center, and there 
 float. Almost any solid, if dissolved in 
 water, fills the pores of the water with- 
 out adding much to its volume. This E gg m water, 
 increases its density and buoyant power. A person 
 can therefore swim more easily in salt than in 
 fresh water.* An iron ship will not only float 
 itself, but also carry a heavy cargo, because it 
 displaces a great volume of water. A body floating 
 in water has its center of gravity at the lowest 
 point, when it is in stable equilibrium, f Fishes 
 have air-bladders, by which they can rise or sink 
 
 * Bayard Taylor says that he could float on the surface of the Dead 
 Sea, with a log of wood for a pillow, as comfortably as if lying on a spring 
 mattress. Another traveler remarks, that on plunging in he was thrown 
 out again like a cork ; and that on emerging and drying himself, the crys- 
 tals of salt which covered his body made him resemble an "animated stick 
 of rock-candy." 
 
 t Herschel tells an amusing story of a man who attempted to walk on 
 water by means of large cork boots. Scarcely, however, had he ventured 
 
HYDROSTATICS. 119 
 
 at pleasure.* By compressing the air-bladder, the 
 fish diminishes the volume of its own body. The 
 buoyant effect of the water is correspondingly de- 
 creased and the fish descends. By relaxing the com- 
 pression on the bladder, the air in it expands and 
 the fish rises. 
 
 PRACTICAL QUESTIONS. 
 
 1. Why can housekeepers test the strength of lye by trying whether or 
 not an egg will float on it? 
 
 2. How much water will it take to make a gallon of strong brine? 
 
 3. Why ought a fat man to swim more easily than a lean one ? 
 
 4. Why does the firing of a cannon sometimes bring to the surface the 
 body of a drowned person? Ans. Because by the concussion it shakes the 
 body loose from the mud or any object with which it is entangled. 
 
 5. Why does the body of a drowned person generally come to the sur 
 face of the water after a time ? Ans. Because the gases which are generated 
 by decomposition in the body make* its specific gravity less. 
 
 6. If we let bubbles of air pass up through a glass of water, why will 
 they become larger as they ascend? 
 
 7. What is the pressure on a canal lock-gate 14 feet high and 10 feet 
 wide, when the lock is full of water? 
 
 8. Will a pail of water weigh any more with a live fish in it than 
 without? 
 
 9. If the water filtering down through a rock should collect in a 
 crevice an inch square and 250 feet high, opening at the bottom into a 
 closed fissure having 20 square feet of surface, what would be the total 
 pressure tending to burst the rock? 
 
 10. Why can stones in water be moved so much more easily than on 
 land? 
 
 11. Why is it so difficult to wade in water when there is any current? 
 
 out ere the law of gravitation seized him, and all that could be seen was a 
 pair of heels, whose movements manifested a great state of uneasiness in 
 the human appendage below. 
 
 * It was formerly thought that a fish in water has no weight. It Is 
 said that Charles II . of England once asked the philosophers of his time to 
 explain this phenomenon. They offered many wise conjectures, but no one 
 thought of trying the experiment. At last a simple-minded man balanced 
 a vessel of water, and on adding a fish, found it weighed just as much as 
 if placed on a dry scale-pan. 
 
120 PRESSURE OF LIQUIDS AND GASES. 
 
 12. "Why is a mill-dam or canal embankment small at the top and large 
 at the bottom? 
 
 13. In digging canals, ought the engineer to take into consideration the 
 curvature of the earth? 
 
 14. Why does the bubble of air in a spirit-level move as the instrument 
 is turned? 
 
 15. Can a swimmer tread on pieces of glass at the bottom of the water 
 with less danger than on land ? 
 
 16. "Will a vessel displace more water in a fresh river than in the 
 ocean ? 
 
 17. "Will iron sink in mercury? 
 
 18. The water in the reservoir in New York is about 80 feet above the 
 fountain in the City Hall Park. "What is the pressure upon a single inch of 
 the pipe at the latter point? 
 
 19. "Why does cream rise on milk? 
 
 20. There is a story told of a Chinese boy who accidentally dropped 
 his ball into a deep hole where he could not reach it. He filled the hole 
 with water, but the ball would not quite float. He finally thought of a 
 successful expedient. Can you guess it? 
 
 21. "Which has the greater buoyant force, water or oil? 
 
 22. "What is the weight of four cubic feet of cork? 
 
 23. How many ounces of iron will a cubic foot of cork float in water? 
 
 24. What is the specific gravity of a body whose weight in air is 30 
 grs. and in water 20 grs. ? How much is it heavier than water? 
 
 25. Which is heavier, a gallon of fresh or one of salt water? 
 
 26. The weights of a piece of syenite-rock in air and water were 3941.8 
 grs. and 2607.5 grs. Find its specific gravity. 
 
 27. A specimen of green sapphire from Siam weighed in air 21.45 grs. 
 and in water 16.33 grs. ; required its specific gravity. 
 
 28. A specimen of granite weighs in air 534.8 grs., and in water 334.6 
 grs. ; what is its specific gravity ? 
 
 29. What is the volume of a ton of iron ? A ton of gold ? A ton of 
 copper? 
 
 30. What is the weight of a cube of gold 4 feet on each side ? 
 
 31. A cistern is 12 feet long, 6 feet wide, and 10 feet deep; when full 
 of water, what is the pressure on each side ? 
 
 32. Why does a dead fish always float on its back? 
 
 33. A given volume of water weighs 62.5 grs., and the same volume of 
 muriatic acid 75 grs. What is the specific gravity of the acid? 
 
 34. A vessel holds 10 Ibs. of water; how much mercury would it 
 contain? 
 
 35. A stone weighs 70 Ibs. in air and 50 in water ; what is its volume ? 
 
 36. A hollow ball of iron weighs 10 Ibs. ; what must be its volume to 
 float in water? 
 
 37. Suppose that Hiero's crown was an alloy of silver and gold, and 
 weighed 22 oz. in air and 20* oz. in water. What was the proportion of 
 each metal? 
 
HYDRODYNAMICS. 121 
 
 38. Why will oil, which floats on water, sink in alcohol? 
 
 39. A specific gravity bottle holds 100 gms. of water and 180 gms. of 
 sulphuric acid. Required the density of the acid. 
 
 40. What is the density of a body which weighs 58 gms. in air and 46 
 gms. in water? 
 
 41. What is the density of a body which weighs 63 gms. in air and 35 
 gms. in a liquid of a density of .85? 
 
 II. HYDRODYNAMICS. 
 
 HYDRODYNAMICS treats of liquids in motion. In 
 this, as in Hydrostatics, water is taken as the type. 
 In theory, its principles are those of falling bodies, 
 but in practice they can not be relied upon except 
 when verified by experiment. The discrepancy 
 arises from changes of temperature which vary the 
 fluidity of the liquid, from friction, the shape of the 
 orifice, etc. 
 
 1. Rules Concerning a Jet. (1.) THE VELOCITY 
 
 OF A JET IS THE SAME AS THAT OF A BODY FALLING 
 FROM THE SUEFACE OF THE WATER. We Can S66 that 
 
 this must be so, if we recall two principles : First, 
 " a jet will rise to the level of its source ; " and 
 second, "to elevate a body to any height, it must 
 have the same velocity that it would acquire in fall- 
 ing that distance." It follows that the velocity of a 
 jet depends on the height of the liquid above the 
 orifice. 
 
 (2.) TO FIND THE VELOCITY OF A JET OF WATER, 
 
 use the 4th equation of falling bodies, v* = 2gh, in 
 which h is the distance of the orifice below the sur- 
 face of the water. Example: The depth of water 
 
122 PRESSURE OF LIQUIDS AND GASES. 
 
 above the orifice is 49 feet; required the velocity 
 
 Substituting, v=^/2 x 32 x-49 = 56 feet. 
 
 (3.) TO FIND THE QUANTITY OF WATER DISCHARGED 
 
 IN A GIVEN TIME, multiply the area of the orifice by 
 the velocity of the water, and that product by the 
 number of seconds. Example : What quantity of 
 water will be discharged in 5 seconds from an orifice 
 having an area of sq. foot at an average depth of 
 49 feet ? At that depth, v = ^2 x 32 x 49 = 56 feet 
 per second ; multiplying by |, we have 2 8 cubic feet 
 discharged in one second and 140 -cubic feet in five 
 seconds.* In practice, much less than this can be 
 realized. 
 
 2. Effect of Tubes. If we examine a jet of 
 water, we see its size is decreased just outside the 
 orifice to about two thirds that at the opening. This 
 neck is called the vena contracta, and is caused by 
 the water producing cross currents as it flows from 
 different directions toward the orifice. If a tube of 
 a length twice or thrice the diameter of the opening 
 be inserted, the water will adhere to the sides so 
 that there will be no contraction, and the flow be 
 increased to about 80 per cent, of the theoretical 
 amount. If the tube be conical, and inserted with 
 the large end inward, the discharge may be aug- 
 mented to 95 per cent. ; and if the outer end be 
 
 * If , at a foot below the surface, an opening will furnish 1 gallon per 
 minute, to double that quantity the opening must be 4 feet below the top. 
 Again, if a certain power will force through a nozzle of a fire-engine a 
 given quantity of water in a minute, to double the quantity the power 
 must be quadrupled. 
 
HYDRODYNAMICS. 123 
 
 flaring, it may reach 98 per cent. Long tubes 
 or short angles, by friction, diminish the flow of 
 water. 
 
 3. Flow of Water in Rivers, A fall of three 
 inches per mile is sufficient to give motion to water, 
 and produce a velocity of as many miles per hour. 
 The Ganges descends but 800 feet in 1,800 miles. 
 Its waters require a month to move down this long 
 inclined plane.* A fall of three feet per mile will 
 make a mountain torrent. The current moves more 
 swiftly at the center than near the shores or bottom 
 of a channel, since there is less friction. 
 
 4. Water-wheels are machines for using the 
 force of falling water. By bands FIG 85 
 
 or cog-wheels the motion of the 
 wheel is conducted from the axle 
 into the milLf 
 
 The OVERSHOT WHEEL has on its 
 circumference a series of buckets 
 which receive the water flowing 
 from a sluice, C. These hold the 
 water as they descend on one side, 
 and empty it as they come up ori the other. Over- 
 shot wheels are valuable where a great fall can be 
 secured, since they require but little water. If W de- 
 notes the weight of the water and h the distance it 
 
 * "The fall of 800 feet would theoretically give a velocity of more 
 than 150 miles per hour. This is reduced by friction to about three miles." 
 
 t The principle is that of a lever with the P acting on the short arm. 
 In this way the movement of the slow creaking axle reappears in the 
 swiftly buzzing saw or flying spindle. 
 
124 PKESSURE OF LIQUIDS AND OASES. 
 
 falls, then the total work = Wh. Of this amount, 75 
 per cent, can be made available under good conditions. 
 
 FIG. 86. 
 
 FIG. 87. 
 
 Undershot Wheel. 
 
 Breast wheel. 
 
 FIG. 88. 
 
 .The UNDERSHOT WHEEL has projecting boards, or 
 floats, which receive the force of the current. It is 
 
 of use where there 
 is little fall and a 
 large quantity of 
 water. It utilizes 
 not more than 25 
 per cent, of the 
 energy of the wa- 
 ter. 
 
 The BREAST- 
 WHEEL (Fig. 87) is 
 a medium between 
 the two kinds al- 
 ready named. 
 
 The TURBINE 
 WHEEL is placed 
 horizontally and 
 immersed in the 
 water, In Fig. 88, G is the dam and DA the spout 
 
 Turbine Wheel. 
 
HYDRODYNAMI OS. 
 
 125 
 
 FIG. 89. 
 
 by which the water is furnished. E is a scroll-like 
 casing encircling the wheel, and open at the center 
 above and below. The axis of the wheel is the ver- 
 tical cylinder B, from which radi- 
 ate plane-floats against which the 
 water strikes. This form utilizes 
 as high as 90 per cent, of the 
 energy. F is a band-wheel which 
 conducts the power to the ma- 
 chinery. 
 
 The principle of the unbal- 
 anced pressure of a column of 
 water may also be employed. It 
 is illustrated in Barker's Mill or 
 Reaction-wheel.* This consists 
 of an upright cylinder with hori- 
 zontal arms, on the opposite sides 
 of which are small apertures. It 
 rests in a socket, so as to 
 revolve freely. Water is 
 supplied from a tank 
 above. If the openings in 
 the arms are closed, when 
 the cylinder is filled with 
 water the pressure is 
 
 equal in all directions and the machine is at rest. 
 If now we open an aperture, the pressure is relieved 
 
 * Bevolving fire-works and the whirligig, used for watering lawns and 
 as an ornament in fountains, are constructed on the same principle. An 
 ingenious pupil can easily construct a Reaction-wheel of straws or quills, 
 pouring the water into the upright tube by means of a pitcher, or admitting 
 it slowly through a siphon from a pail of water placed on a table above. 
 
 Barker's Mill. 
 
126 PKESSUKE OF LIQUIDS AND GASES. 
 
 on that side, and the arm flies back on account of 
 the unbalanced pressure of the column of . water 
 above. 
 
 5. Waves are produced by the friction of the 
 wind against the surface of the water. The wind 
 raises the particles of water and gravity draws them 
 back again. They thus vibrate up and down, but in 
 deep water the liquid mass does not advance. The 
 forward movement of the wave is an illusion. The 
 form of the wave progresses like the apparent motion 
 of the thread of the screw which we turn in our 
 hand, or the undulations of a rope or carpet which 
 is shaken, or the stalks of grain which bend in bil- 
 lows as the wind sweeps over them. 
 
 The corresponding parts of different waves are 
 said to be like phases. Thus, in Fig. 90, A and E, 
 
 FIG. 90. 
 
 B and F r C and G are like phases. The distance 
 between two like phases, or between the crests of 
 two succeeding waves, is called a wave-length. Thus 
 the distance AE, or BF, or CO- is a wave-length. 
 Opposite phases are those parts which are vibrating 
 in opposite directions, as E and (7, or B and D. The 
 successive particles of water move each in an ellipse, 
 and in regular succession, so that when a particle at 
 
HYDRODYNAMICS. 
 
 127 
 
 E is moving forward, one at C is moving backward, 
 one at B upward, and one at D downward. This is 
 easily observed at sea.* 
 
 COMPOSITION OF WAVE MOTION. A tide-wave may 
 be setting steadily toward the west ; waves from 
 
 FIG. 91. 
 
 Interference of Waves. 
 
 distant storms may be moving upon this ; and, above 
 the whole, ripples from the breeze then blowing may 
 diversify the surface. These different systems of 
 
 * Near the shore the osciltetions become shorter; the lower particles 
 being checked in their elliptic motion by the friction on the sandy beach, 
 the front becomes well-nigh vertical, and the tipper part curls over and 
 falls beyond. The size of " mountain billows " has been exaggerated. Along 
 the coast in a gorge they may reach 90 feet, but in the open sea the highest 
 wave, from the deepest "trough" to the very topmost "crest," rarely 
 ures over 30 feet. 
 
128 PKESSURE OF LIQUIDS AND GASES. 
 
 waves will compound into a resultant system in ac- 
 cordance with the following general principles : If any 
 two systems coincide with like phases, the crest of one 
 meeting the crest of the other, and the furrow of one 
 meeting the furrow of ^the other, the resulting wave 
 will have a height equal to the sum of the two. If any 
 two systems coincide with opposite phases, the hollow 
 of one striking the crest of another, the height will be 
 the difference of the two. Thus, if in two systems hav- 
 ing the same wave-length and height, one is exactly 
 half a length behind the other, they will destroy each 
 other. This is termed the interference of waves.* 
 
 The manner in which different waves move 
 among and upon one another, is seen by dropping a 
 handful of stones in water and watching the waves 
 as they circle out from the various centers in ever- 
 widening curves. In Fig. 91 is shown the beautiful 
 appearance these waves present when reflected from 
 the sides of a vessel. 
 
 * " In the port of Batsha the tidal-wave comes up by two distinct chan- 
 nels so unequal in length that their time of arrival varies by six hours. 
 Consequently when the crest of high water reaches the harbor by one 
 channel, it meets the low water returning by the other, and when these 
 opposite phases are equal, they neutralize each other, so that at particular 
 seasons there is no tide in the port, and at other times there is but one 
 tide per day, and that equal to the difference between the ordinary morn- 
 ing and evening tide." Lloyd's Wave Theory. 
 
 Another striking example of interference of tide- waves is seen in the 
 immediate neighborhood of New York. The tide-wave from the ocean, 
 coming from the south-east, divides, a part^passing up New York Bay, and 
 another part sweeping around and turning westward through Long Island 
 Sound. The meeting-place of these two branches is at Hell Gate, the narrow- 
 est ship-channel between Long Island and New York. If a wall were built 
 across Hell Gate, the water on one side would sometimes be five feet above 
 that on the other. In the absence of such a wall, the current surges with 
 great rapidity under the Brooklyn Bridge, alternately in opposite directions. 
 
PNEUMATICS. 129 
 
 PRACTICAL QUESTIONS. 
 
 1. Two faucets, one 8 feet and the other 4 feet below the surface of the 
 water in a cistern, are kept Open for a minute. How many times as much 
 water can be drawn from the first as the second? 
 
 2. How much water will be discharged per second from a short pipe 
 having a diameter of 4 inches and a depth of 48 feet below the surface of 
 the water? 
 
 3. When we pour molasses from a jug, why is the stream so much 
 larger near the nozzle than at some distance from it? 
 
 4. Ought a faucet to extend into a barrel beyond the staves? 
 
 5. What would be the effect if both openings in one of the arms of 
 Barker's Mill were on the same side? 
 
 III. PNEUMATICS. 
 
 PNEUMATICS treats of the general properties and 
 the pressure of gases. Since the molecules move 
 among themselves more freely even than those of 
 liquids, the conclusions which we have reached with 
 regard to transmission of pressure, buoyancy, and 
 specific gravity apply also to gases. Since air is the 
 most abundant gas, it is taken 
 
 FIG. 92. 
 
 as the type of the class, just 
 as water is of liquids. 
 
 1. The Air-pump is shown 
 in its essential features in 
 Fig. 92. A is a glass receiver 
 standing on an oiled pump- B \ 
 plate. The tube D connecting The 
 
 the receiver with the cylinder, 
 
 is closed by the valve E, opening upward. There is 
 a second valve, P, in the piston, also opening up- 
 ward. Suppose the piston is at the bottom and both 
 
130 PRESSURE OF LIQUIDS AND GASES. 
 
 valves shut. Let it now be raised, and a vacuum 
 will be produced in the cylinder ; the expansive force 
 of the atmosphere in the receiver will open the valve 
 E and drive the air through to fill this empty space. 
 When the piston descends, the valve E will close, 
 while the valve P will open, and the air will pass 
 up above the piston. On elevating the piston a sec- 
 ond time, this air is removed from the cylinder, 
 while the air from the receiver passes through as 
 before. At each stroke a portion of the atmosphere 
 is drawn off; but the expansive force becomes less 
 and less, until finally it is insufficient to lift the 
 valves. For this reason a perfect vacuum can not be 
 obtained. 
 
 2. The Condenser, in construction, is the same 
 as the air-pump, except that the valve opens inward 
 instead of outward. Instead of exhausting, it forces 
 more air into a vessel.* 
 
 3. Properties of Air. (1.) WEIGHT. Exhaust the 
 air from a flask which holds 100 cubic inches, and 
 then balance it. On turning the stop-cock, the air 
 will rush in with a whizzing noise and the flask 
 
 * The practical applications of this pump are numerous. The soda 
 manufacturer uses it to condense carbonic acid in soda-water reservoirs. 
 The engineer employs it in laying the foundations of bridges. Large tubes 
 or caissons are lowered to the bed of the stream, and air being forced in, 
 drives out the water. The workm'en are let into the caissons by a sort of 
 trap, and work in this condensed atmosphere. Pneumatic dispatch-tubes 
 contain a kind of train holding the mail, and back of this a piston fitting 
 the tube Air is forced in behind the piston or exhausted before it, and so 
 the train is driven through the tube at a high speed. In the Westinghouse 
 air-brake, condensed air is forced along a tube running underneath the 
 cars, and by its elastic force drives the brakes against the wheel. 
 
PNEUMATICS. 
 
 131 
 
 FIG. 93. 
 
 Weighing Air. 
 
 Pio. 94. 
 
 will descend (Fig. 93). It will require 31 grains or 
 more to restore the equipoise. 
 
 (2.) ELASTICITY is shown in a pop- 
 gun. We compress the atmosphere in 
 the barrel until the elastic force drives 
 out the stopper with a loud report. As 
 we crowd down the piston we feel the 
 elasticity of the air yielding to our 
 strength, like a bent spring. The bottle- 
 imps, or Cartesian divers, illustrate the 
 same property. Fig. 94 represents a 
 simple form of this apparatus. The 
 cover of a fruit-jar is fitted with a 
 tube, which is inserted in a syringe-bulb. The jar 
 is filled with water and the diver 
 placed within. This is a hollow image 
 of glass, having a small opening at the 
 end of the curved tail. If we squeeze 
 the bulb, the air will be forced into the 
 jar and the water will transmit the 
 pressure to the air in the image. This 
 being compressed, more water will 
 enter, and the diver, thus becoming 
 heavier, will descend. On relaxing the 
 grasp of the hand on the bulb, the air 
 will return into it, the air in the image 
 will expand, by its elastic force driving 
 out the water, and the diver, thus 
 lightened of his ballast, will ascend. 
 The nearer the image is to the bottom, the less 
 force will be required to move it. With a little care 
 
 Cartesian Diver. 
 
132 PRESSURE OF LIQUIDS AND GASES. 
 
 FIG. 95. 
 
 it can be made to respond to the slightest pressure, 
 and* will rise and fail as if instinct with life.* 
 
 (3.) EXPANSIBILITY. Let a well- 
 dried bladder be partly filled with air 
 and tightly closed. Place it under 
 the receiver and exhaust the air. 
 The air in the bladde'r expanding 
 will burst it into shreds. 
 
 Take two bottles partly filled with 
 colored water. Let a bent tube be 
 
 Expansibility of Air. mgerted tightly ^ A and loogely ^ 
 
 B. Place this apparatus under the receiver and 
 exhaust the air. The expansive 
 force of the air in A will drive the 
 water over into B. On readmitting 
 the air into the receiver, the press- 
 ure will return the water into A. 
 It may thus be driven from bottle 
 to bottle at pleasure, f 
 
 mere? 8 fountain acts on the same principle, as may 
 
 FIG. 
 
 Transfer of Liquid under 
 Receiver. 
 
 * This experiment shows also the buoyant force of liquids, their trans- 
 mission of pressure in every direction, and the increase of the pressure in 
 proportion to the depth. The elasticity of the air, as well as the principles 
 explained by the Cartesian diver, Fig. 94, may be illustrated in the follow- 
 ing simple manner: Fill with water a wide-mouth 8-oz. bottle, and also a 
 tiny vial, such as is used by homeopathists. Invert the vial and a few 
 drops of water will run out. Now put it inverted into the bottle, and if it 
 does not sink just below the surface and there float, take it out and add or 
 remove a little water, as may be needed. When this result is reached, cork 
 the bottle so that the cork touches the water. Any pressure on the cork 
 will then be transmitted to the air in the vial, as in the image in 
 Fig 94. 
 
 t Prick a hole in the small end of an egg, and place the egg with the 
 big end up in a wine-glass. On exhausting the receiver, the bubble of air 
 in the upper part of the egg will drive the contents down into the glass, 
 and on admitting the air they will be forced back again. 
 
PNEUMATICS. 
 
 133 
 
 Fie. 97. 
 
 FIG. 98. 
 
 be seen by an examination of Fig. 9 7. Having removed 
 the jet-tube, the upper globe is partly filled with 
 water. The tube being then replaced, 
 water is poured into the basin on top. 
 The liquid runs down the pipe at the 
 right, into the lower globe. The air in 
 that globe is driven up the tube at the 
 left into the upper 
 globe, and by its elas- 
 ticity forces the water 
 there out through the 
 jet-tube, forming a tiny 
 fountain. 
 
 4. Pressure of the 
 Air. (1.) THE PROOF. 
 If we cover a hand-glass 
 with one hand, as in 
 Fig. 98, on exhausting the air we shall find the 
 pressure painful.* Tie 
 over one end of the 
 glass a piece of wet 
 bladder. When dry, ex- 
 haust the air, and the 
 membrane will burst 
 
 With a Sharp report, f Magdeburg Hemispheres. 
 
 * The exhaustion of the air does not prodtice the pressure on the hand ; 
 it simply reveals it. The average pressure on each person is 16 tons. It is 
 equal, however, on all parts of the body, and is counteracted by the air 
 within. Hence we never notice it. Persons who go up high mountains or 
 go down in diving-bells feel the change in the pressure. 
 
 t To show the crushing force of the atmosphere, take a tin cylinder 15 
 inches long and 4 inches in diameter. Fit one end with a stop-cock for the 
 exit of the steam. Put in a little water and boil. When the air is entirely 
 
 Hand-glass. 
 
 Hiero's Fountain. 
 
 FIG. 99. 
 
134 PRESSURE OF LIQUIDS AND GASES. 
 
 Fio. 100. 
 
 The Magdeburg Hemispheres are named from the 
 city in which Ghiericke, their inventor, resided. They 
 consist of two small brass hemispheres, which fit 
 closely together, but may be separated at pleasure. 
 If, however, the air be exhausted from within, several 
 
 persons will be required to pull them 
 
 apart.* In whatever "position the 
 
 hemispheres are held, the pressure 
 
 is the same. 
 (2.) UPWARD 
 
 PRESSURE. Fill 
 
 a tumbler with 
 
 water, and then 
 
 lay a sheet of 
 
 paper over the 
 
 top. Quickly invert the glass, 
 and the water will be supported 
 by the upward pressure of the 
 air. Within the glass cylinder, 
 Fig. 101, is a piston working 
 air-tight. Connect the nozzle 
 above with the air-pump by 
 means of a rubber tube and exhaust the air. 
 weight will leap up as if caught by a spring. 
 
 Water held up by Press- 
 ure of Air. 
 
 The Weight-lifter. 
 
 The 
 
 driven out, turn the stop-cock. Pour cold water over the outside to condense 
 the steam, when the cylinder will collapse as if struck by a heavy blow. 
 
 * In the museum at Berlin the hemispheres used by Q-uericke in his 
 experiments are preserved. They are of copper, and 22 inches interior 
 diameter, with a flange an inch wide, making the entire diameter 2 feet. 
 Accompanying is a Latin book by the burgomaster describing numerous 
 pneumatic experiments which he had performed, and containing a wood- 
 cut representing three spans of horses on each side trying to separate the 
 hemispheres. 
 
PNEUMATICS. 
 
 135 
 
 (3.) BUOYANT FOKCE OF THE AIR. The law of 
 Archimedes (p. 114) holds true in gases. A hollow 
 sphere of glass or copper, Fig. 102, is balanced in 
 the air by a solid lead weight, but on being placed 
 
 FIG. 102. 
 
 FIG. 103. 
 
 Buoyancy of Air. 
 
 under the receiver it steadily 
 falls while the air is becom- 
 ing exhausted. This shows 
 that its weight was partly 
 sustained by the buoyant 
 force of the air. 
 
 (4.) AT SEA-LEVEL THE PRESS- 
 URE OF THE AIR SUSTAINS A COL- 
 UMN OF MERCURY 30 INCHES 
 HIGH, OR OF WATER NEARLY 34 FEET HIGH, AND IS NEARLY 
 
 15 LBS. PER SQUARE INCH. Take a strong glass tube 
 about three feet in length, and tie over one end a 
 piece of wet bladder. When dry, fill the tube with 
 mercury, and invert it in a cup of the same liquid. 
 The mercury will sink to a height of about 30 
 
 Torricelli's Experiment. 
 
136 PRESSURE OF LIQUIDS AND GASES: 
 
 inches. If the area across the tube be one square 
 inch, the metal will weigh about 14.7 Ibs. The 
 weight of the column of mercury is equal to the 
 downward pressure on each square inch of the 
 surface of the mercury in the cup. Hence we 
 conclude that the pressure of the atmosphere is 
 14.7 Ibs. per square inch, and will balance a column 
 of mercury 30 inches high. As water is 13 times 
 lighter than mercury, the same pressure would 
 balance a column of that liquid 13 times higher, 
 or 33} feet.* 
 
 (5.) PRESSURE OF THE AIR VARIES.! Changes of 
 temperature, moisture, etc., continually vary the 
 density of the air, and change the height of the 
 column of liquid it can support. The pressure also 
 increases with the depth. Hence, in a valley the 
 column of mercury stands higher than on a mountain. 
 The pressure of the atmosphere is 29.92 inches only 
 at the level of the sea, and at the temperature of 
 melting ice at latitude 45. The variation due to 
 latitude is very slight; that due to temperature is 
 greater, and that due to elevation is greatest. Ob- 
 servations on the barometer at any given station 
 
 * On account of the unwieldly length of the tube required to exhibit 
 the column of water, it is not easy to verify this. It may, however, be 
 prettily illustrated. Pour on the mercury in the cup (Fig. 103) a little 
 water colored with red ink. Then raise the end of the tube above the sur- 
 face of the metal, but not above that of the water; this will rise in the 
 tube, the mercury passing down in beautifully-beaded globules. The mer- 
 curial column is only 30 inches high, while the water will fill the tube. 
 Finish the experiment by puncturing the bladder with a pin, when the 
 water will instantly fall to the cup below. 
 
 t We live at the bottom of an aerial ocean whose depth is greater than 
 that of the deepest sea. Its invisible currents surge round us on every side. 
 
PNEUMATICS. 
 
 137 
 
 are generally "reduced" to what they would be 
 under the standard conditions just mentioned. 
 
 (6.) MARIOTTE'S LAW.* 
 Fig. 104 represents a 
 long, bent glass tube with 
 the end of the short arm 
 closed. Pour mercury into 
 the long arm until it rises 
 to the point marked zero.f 
 It stands at the same 
 height in both arms, and 
 there is an equilibrium. 
 The air presses on the 
 mercury in the long arm 
 with a force equal to a 
 column of mercury 30 
 inches high, and the elas- 
 tic force of the air con- 
 fined in the short arm is 
 equal to the same amount. 
 Now pour additional mer- 
 cury into the long arm 
 until it stands at 30 
 inches above that in the Marietta Tube, 
 
 short arm (Fig. 105), and the pressure is doubled. In 
 the short arm, the air is condensed to one half its 
 
 * This law was independently discovered by the Englishman, Boyle, 
 and the Frenchman, Mariotte, during the latter part of the seventeenth 
 century. It is often called Boyle's Law. 
 
 t By cautiously inclining the apparatus, when a little air will escape, 
 and adding more mercury if needed, the liquid can be made to stand at 
 zero in both arms. 
 
138 PRESSURE OF LIQUIDS AND GASES. 
 
 FIG. 106. former dimensions, and the elastic force 
 is also doubled.* We therefore conclude 
 that the elasticity of a gas increases, and 
 the volume diminishes in proportion to 
 the pressure upon it. 
 
 (7.) The BAROMETER is an instrument 
 for measuring the pressure of the air. It 
 consists essentially of the tube and cup 
 of mercury in Fig. 103. A scale is at- 
 tached for convenience of reference. The 
 barometer is used (a) to indicate the 
 weather, and (&) to measure the height of 
 mountains. 
 
 It does not directly foretell the weather. 
 It simply shows the varying pressure of 
 the air, from which we must draw our 
 conclusions. A continued rise of the mer- 
 cury indicates fair weather, and a con- 
 tinued fall, foul weather, f Since the press- 
 
 * The force with which the flying molecules of air 
 beat against the walls of any confining vessel will increase 
 with the diminution of the space through which they can 
 pass. If we give them only half the distance to fly 
 through, they will strike twice as often and exert twice 
 the pressure. 
 
 t Mercury is used for filling the barometer because of 
 its weight and low freezing-point. It is said that the first 
 barometer was filled with water. The inventor, Otto von 
 Guericke, erected a tall tube reaching from a cistern in the 
 cellar up through the roof of his house. A wooden image 
 was placed within the tube, floating upon the water. On 
 fine days, this noVel weather-prophet would rise above the 
 roof-top and peep out upon the queer old gables of that 
 ancient city, while in foul weather he would retire to the 
 protection of the garret. The accuracy of these movements 
 The Barometer, attracted the attention of the neighbors. Finally, becoming 
 
PNEUMATICS. 
 
 139 
 
 ure diminishes above the level of the sea, the ob- 
 server ascertains the fall of the mercury in the 
 barometer, and the temperature by the thermometer ; 
 and then, by reference to tables, determines the 
 height. 
 
 PIG. 107. 
 
 FIG. 108. 
 
 FIG. 109. 
 
 The Lifting-pump. 
 
 5. Pumps. (1.) The LIFTING-PUMP contains two 
 valves opening upward one, a, at the top of the 
 suction-pipe, B ; the other, c, in the piston. Suppose 
 the handle to be raised, the piston being at the bot- 
 tom of the cylinder and both valves closed. Now 
 
 suspicious of Otto's piety, they accused him of being in league with the 
 devil. So the offending philosopher relieved this wicked wooden man from 
 longer dancing attendance upon the weather, and the staid old city was 
 once more at peace. 
 
140 PRESSURE OF LIQUIDS AND GASES. 
 
 Fio. 110. 
 
 depress the pump-handle and thereby elevate the 
 piston. This will produce a partial vacuum in the 
 suction-pipe. The pressure of the air on the surface 
 of the water below will force the water up the pipe, 
 open the valve, and partly fill the 
 chamber. Let the pump-handle be 
 elevated again, and the piston de- 
 pressed. The valve a will then close, 
 the valve c will open, and the water 
 will rise above the piston (Fig. 108). 
 When the pump-handle is lowered 
 the second time and the piston ele- 
 vated, the water is lifted up to the 
 spout, whence it flows out ; while at 
 the same time the lower valve opens 
 and the water is forced up from be- 
 low by the pressure of the air (Fig. 
 109).* 
 
 (2.) The FORCE-PUMP has no valve 
 in the piston. The water rises above 
 the lower valve as in the lifting-pump. 
 When the piston descends, -the press- 
 ure opens the valve in the pipe D, and forces the 
 water up. This pipe may be made of any length, 
 and thus the water driven to any height. 
 
 (3.) The FIRE-ENGINE consists of two force-pumps 
 with an air-chamber. The water is driven by the 
 
 * If the valves and piston were fitted air-tight, the water could be 
 raised 34 feet (more exactly 13 times the height of the barometric column) 
 to the lower valve, but owing to various imperfections it commonly reaches 
 about 28 feet. For a similar reason we sometimes find a dozen strokes 
 necessary to "bring water." 
 
 Force-pump. 
 
PNEUMATICS. 
 
 141 
 
 FIG. 111. 
 
 Fire-engine. 
 
 pistons ra, n, alter- 
 nately, into the 
 chamber R, whence 
 the air, by its ex- 
 pansive force, throws 
 it out in a continu- 
 ous stream through 
 the hose-pipe at- 
 tached at Z (Fig. 1 1 1 ). 
 
 6. The 
 
 Siphon is a 
 U-shaped 
 tube, hav- 
 ing one arm 
 longer than 
 
 FIG. 112. 
 
 siphon. 
 
142 PRESSURE OF LIQUIDS AND GASES. 
 
 the other. Insert the short arm in the water, and 
 then applying the mouth to the long arm, exhaust 
 the air. The water will flow from the long arm 
 until the end of the short arm is uncovered. 
 
 THEORY OF THE SIPHON. The pressure of the air 
 at 5 holds up the column of water a &, and the up- 
 ward pressure is the weight of the air less the weight 
 of the column of water a Z>. The upward pressure 
 at d is the weight of the air minus the weight of 
 the column of water c d. Now c d is less than a &, 
 and the water in the tube is driven toward the 
 longer arm by a force equal to the difference in the 
 
 weight of the two arms.* 
 
 FIG. 113. 
 
 7. The Pneumatic Inkstand 
 
 can be filled only when tipped 
 so that the nozzle is at the 
 top. The pressure of the air 
 will retain the ink when the 
 stand is placed upright. When 
 
 Pneumatic Inkstand. nil i-iii j? 
 
 used below o, a bubble of air 
 passes in, forcing the ink into the nozzle. 
 
 8. The Hydraulic Ram is a machine for raising 
 water where there is a slight fall. The water enters 
 
 * The siphon is used more conveniently if two tubes of glass or metal 
 are connected with a flexible tube of India rubber. An instructive experi- 
 ment can then be made if we allow the water to run from one tumbler 
 into another until just before the flow ceases; then quickly elevate the 
 glass containing the long arm, carefully keeping both ends of the siphon 
 under the water, when the flow will set back to the first tumbler. Thus 
 we may alternate until we see that the water flows to the lower level, and 
 ceases whenever it reaches the same level in both glasses. It will add to 
 the beauty of this as weU as of many other experiments, to color the water 
 in one tumbler with a few scales of magenta, or with red ink. 
 
PNEUMATICS. 
 
 143 
 
 through the pipe A, fills the reservoir B, and lifts 
 the valve D. As that closes, the shock raises the 
 
 PIG. 115. 
 
 Hydraulic Ram. 
 
 valve E and drives the water into the air-chamber 
 G. D falls again as soon as an equilibrium is re- 
 stored. A second shock follows, and more water is 
 thrown into G. When the 
 air in G- is sufficiently con- 
 densed, its elastic force 
 drives the water through 
 the pipe H. 
 
 9. The Atomizer is used 
 to turn a liquid into spray. 
 The blast of air driven from 
 the rubber bulb as it passes 
 over the end of the upright 
 tube, sweeps along the 
 neighboring molecules of air 
 and produces a partial vacuum in the tube.* The 
 
 Atomizer. 
 
 * In locomotives, this principle of the adhesion of gases to gases is ap- 
 plied to produce a draft. The waste steam is thrown into the smoke-pipe, 
 
14.4 PRESSURE OF LIQUIDS AND GASES. 
 
 pressure of the air in the bottle drives the liquid up 
 the tube, and at the mouth the blast of air carries 
 it off in fine drops. 
 
 The action of a current of air in dragging along 
 with it the adjacent still atmosphere and so tending 
 to produce a vacuum, is shown by the apparatus 
 represented in Fig. 116. A globe, a, is connected 
 
 FIG. 116. 
 
 Tube for showing Adhesion of Air. 
 
 with a horizontal tube, c, containing colored water. 
 Close the opening d with the finger, and with the 
 mouth at & draw the air out of the globe. A slight 
 rarefaction will cause the liquid, by the pressure of 
 the air at the opening /, to be forced into a. Now, 
 
 and this current sweeps off the smoke from the fire, while the pressure of 
 the atmosphere outside forces the air through the furnace and increases 
 the combustion. A familiar illustration may be devised by taking two 
 disks of card-board, the lower one fitted with a quill, and the upper one 
 merely kept from sliding off by a pin thrust through it and extending into 
 the quill. The more forcibly air is driven through the quill against the 
 upper disk, the more firmly it will be held to its place. See article " Ball 
 Paradox," in "Popular Science Monthly," April, 1877. Faraday used to 
 illustrate the principle thus: Hold the hand out flat with the fingers 
 extended and pressed together. Place underneath a piece of paper two 
 inches square. Blow through the opening between the index and the 
 middle finger, and so long as the current is passing the paper will not 
 fall. 
 
PKACTICAL QUESTIONS. 145 
 
 if, instead of drawing the air out at &, a jet of air 
 be forced through the tube and out at d, the same 
 effect will be produced. 
 
 1C. Height of the Atmosphere. Three opposing 
 forces act upon the air, viz.: gravity, which binds it 
 to the earth, and the centrifugal and repellent forces, 
 which tend to hurl it into space. There must be a 
 point where these balance. At the height of 3.4 
 miles the mercury in the barometer stands at 15 
 inches, indicating that half the atmosphere is within 
 about 3^ miles of the earth's surface. Beyond a 
 height of 40 miles the quantity of air is too small 
 to be perceptible in any way.* If it were every-where 
 as dense as it is at sea-level, the upper limit of our 
 atmosphere would be about five miles high. 
 
 PRACTICAL QUESTIONS. 
 
 [In these questions, assume the standard conditions mentioned on p. 1S6."] 
 
 1. Why must we make two openings in a barrel of cider when we 
 tap it? 
 
 2. What is the weight of 10 cubic feet of air? 
 
 3. What is the pressure of the air on 1 square rod of land? 
 
 4. What is the pressure on a pair of Magdeburg hemispheres 4 inches 
 in diameter? 
 
 5. How high a column of water can the air sustain when the barometric 
 column stands at 28 inches? 
 
 6. If we should add a pressure of two atmospheres (30 Ibs. to the 
 square inch), what would be the volume of 100 cubic inches of common 
 air? 
 
 * In mountain climbing, or ascending to a great height in a balloon, 
 the voyager is apt to suffer on account of the decrease in density of the 
 air. In 1862, Mr. G-laisher ascended nearly 7 miles, and there fainted. 
 His assistant was barely able to open the valve and cause the balloon to 
 descend. 
 
146 PRESSURE OF LIQUIDS AND GASES. 
 
 7. If, while the water is running through the siphon, we quickly lift 
 the long arm, what is the effect on the water in the siphon ? If we lift the 
 entire siphon? 
 
 8. When the mercury stands at 29$ inches in the barometer, how high 
 above the surface of the water can we place the lower pump- valve? 
 
 9. Can we raise water to a higher level by means of a siphon? 
 
 10. If the air in the chamber of a fire-engine be condensed to T V its 
 former bulk, what will be the pressure due to the expansive force of the 
 air on every square inch of the air-chamber? 
 
 11. What causes the bubbles to rise to the surface when we put a lump 
 of loaf-sugar in hot tea? 
 
 12. When will a baUoon stop rising? What weight can it lift? 
 
 13. The rise and fall of the barometric column shows that the air is 
 lighter in foul and heavier in fair weather. Why is this? Ans. Vapor of 
 water is only half as heavy as dry air. When there is a large quantity 
 present in the atmosphere, displacing its own volume of air, the weight of 
 the atmosphere will be correspondingly diminished. 
 
 14. When smoke ascends in a straight line from chimneys, is it a proof 
 of the rarity or the density of the air? 
 
 15. Explain the action of the common leather-sucker. 
 
 16. Did you ever see a bottle really empty? 
 
 17. Why is it so tiresome to walk in miry clay ? Ans. Because the up- 
 ward pressure of the air is removed from our feet. 
 
 18. How does the variation in the pressure of the air affect those who 
 ascend lofty mountains? Who descend in diving-bells? 
 
 19. Explain the theory of "sucking cider" through a straw. 
 
 20. Would it make any difference in the action of the siphon if the 
 limbs were of unequal diameter? 
 
 21. What would be the effect of making a small hole in the top of a 
 diving-bell while in use? 
 
 22. The pressure of the atmosphere being 1.03 kg. per sq. cm., what is 
 the amount on 10 sq. meters? 
 
 SUMMARY. 
 
 Hydrostatics treats of the laws of equilibrium in liquids. 
 Pressure is transmitted by liquids equally in every direction. 
 Water thus becomes a "mechanical power," as in the "Hy- 
 draulic Press." Liquids acted on by their weight only, at the 
 same depth, press downward, upward, and sidewise with equal 
 force. This pressure is independent of the size of the vessel, 
 but increases with the depth. Wells, springs, aqueducts, fount- 
 ains, and the water-supply of cities illustrate the tendency of 
 
SUMMARY. 147 
 
 water to seek its level. The ancients understood this law, but 
 had no suitable material for making the immense pipes needed ; 
 just so the art of printing awaited the invention of paper. Spe- 
 cific gravity, or the relative weights of the same volume of dif- 
 ferent substances, is found by comparing them with the weight 
 of the same volume of water. This is easily done, since, according 
 to the law of Archimedes, a body immersed in water is buoyed 
 up by a force equal to the weight of the water displaced ; i. e., 
 it loses in weight an amount equal to that of the same volume of 
 
 weight in vacuum 
 
 water. Hence spec. grav.= - : 
 
 weight in vacuum weight in water 
 
 A floating body displaces only its own weight of liquid. This ex- 
 plains the buoyancy which supports a ship, why a floating log 
 is partly out of water, and many similar phenomena. 
 
 Hydrodynamics treats of moving liquids. The laws of falling 
 bodies in theory apply ; so that a descending jet of water 
 will acquire the same velocity that a stone would in falling to 
 the ground from the surface of the water; and an ascending 
 jet would need to have the same velocity in order to reach that 
 height. The quantity of water discharged through any orifice 
 equals the area of the opening multiplied by the velocity of the 
 stream. The chief resistance to the motion of a liquid is the 
 friction of the air and against the sides of the pipe, and, in the 
 case of rivers, against the banks and bottom of the channel. 
 The force, of falling water is utilized in the arts by means of 
 water-whee]s. There are four kinds overshot, undershot, 
 breast, and turbine. The principles of wave-motion, so essen- 
 tial to the understanding of sound, light, etc., are most easily 
 studied in connection with water. A stone let fall into a quiet 
 pool sets in motion a series of concentric* waves, whose particles 
 move in ellipses, while the movement passes to the outermost 
 edge of the water, and is then transmitted to the ground be- 
 yond. The velocity of the particles is much less than that of 
 the wave itself. A handful of stones acts in the same way, but 
 sets in motion many series of waves. Hence arise the phenom- 
 ena of interference. 
 
 Pneumatics treats of the properties and the laws, of equilib- 
 rium of gases. The air being composed of matter, has all the 
 properties we associate with matter, as weight, indestructibility, 
 
148 PRESSURE OF LIQUIDS AND GASES. 
 
 extension, compressibility, etc. The elasticity of the air, accord- 
 ing to Mariotte's law, is inversely proportional to its volume, and 
 this is inversely proportional to the pressure upon the air ; both 
 heat and pressure increasing the elasticity of a gas. The air, like 
 other fluids, transmits the weight of its own particles, as well 
 as any outside pressure, equally in every direction ; hence the 
 upward pressure or buoyant force of the atmosphere. A balloon 
 rises because it is buoyed up by a force equal to the weight of 
 the air it displaces. It floats in the air for the same reason 
 that a ship floats on the ocean. When smoke falls it is heavier 
 than the surrounding atmosphere. When it rises, it is carried up 
 by adhesion of warm air, which is lighter than that surrounding 
 the current. The air-pump is used for exhausting the air from, 
 and the condenser for condensing the air into, a receiver. A 
 vacuum in which there remains only i 00 ^ OTrg of the atmosphere 
 can be obtained by means of Sprengel's air-pump, which acts on 
 the principle of the adhesion of the air to a column of falling 
 mercury. The average pressure of the air being 15 Ibs. to the 
 square inch, equals that of a column of water 34 feet, and of 
 mercury 30 inches or 760 millimeters high. This amount varies 
 incessantly through atmospheric changes caused by alterations 
 in the wind, heat of the sun, etc. The barometer measures the 
 pressure of the atmosphere, and is used to determine the height 
 of mountains and the changes of the weather. The action of 
 the siphon, the pneumatic inkstand, and of the different kinds 
 of pumps, is based upon the pressure of the air. 
 
 HISTORICAL SKETCH. 
 
 Hydrostatics is comparatively a modern science. The Ro- 
 mans had a knowledge of the fact that " liquids rise to the 
 level of their source," but they had no means of making iron 
 pipes strong enough to resist the pressure.* They were there- 
 
 * The ancient engineers sometimes availed themselves of this principle. 
 Not far from Rachel's Tomb, Jerusalem, are the remains of a conduit once 
 used for supplying the city with water. The valley was crossed by means 
 of an inverted siphon. The pipe was about two miles long and fifteen 
 
HISTORICAL SKETCH. 149 
 
 fore forced to carry water into the Imperial City by means of 
 enormous aqueducts, one of which was 63 miles long, and was 
 supported by arches 100 feet high. The ancient Egyptians and 
 Chaldeans were probably the first to investigate the most obvi- 
 ous laws of liquids from the necessity of irrigating their land. 
 Archimedes, in the third century B.C., invented a kind of pump 
 called Archimedes' Screw, demonstrated the principle of equilib- 
 rium, known now as "Archimedes' Law" (p. 114), and found 
 out the method of obtaining the specific gravity of bodies. The 
 discovery of the last is historical. Hiero of Syracuse suspected 
 that a gold crown had been fraudulently alloyed with silver. 
 He accordingly asked Archimedes to find out the fact without 
 injuring the workmanship of the crown. One day going into a 
 bath-tub full of water, the thought struck the philosopher that 
 as much water must run over the side as was equal to the 
 volume of his body. Electrified by the idea, he sprang out and 
 ran through the streets, shouting : " Eureka ! " (I have found it !) 
 The ancients never dreamed of associating the air with gross 
 matter. To them it was the spirit, the life, the breath. No- 
 ticing how the atmosphere rushes in to fill any vacant space, 
 the followers of Aristotle explained it by saying, "Nature 
 abhors a vacuum." This principle answered the purpose of 
 philosophers for 2,000 years. In 1640, some workmen were 
 employed by the Duke of Tuscany to dig a deep well near 
 Florence. They found to their surprise that the water would 
 not rise in the pump as high as the lower valve. More disgusted 
 with nature than nature was with the vacuum in their pump, 
 they applied to Q-alileo. The aged philosopher answered half 
 in jest, we hope, certainly he was half in earnest "Nature does 
 not abhor a vacuum beyond 34 feet." His pupil, Torricelli, how- 
 ever, discovered the secret. He reasoned that there is a force 
 which holds up the water, and as mercury is 13 times as 
 heavy as water, it would sustain a column of that liquid only 
 34 feet -f- 13 = 30 inches high. Trying the experiment shown in 
 Fig. 103, he verified the conclusion that the weight of the air is 
 the unknown force. But the opinion was not generally received. 
 
 inches in diameter. It consisted of perforated blocks of stone, ground 
 smooth at the joints, and fastened with a hard cement. 
 
150 PRESSURE OF LIQUIDS AND GASES. 
 
 Pascal next reasoned that if the weight of the air is really the 
 force, then at the summit of a high mountain it is weakened, 
 and the column would be lower. He accordingly carried his 
 apparatus to the top of a tower, and finding a slight fall in the 
 mercury, he asked his brother-in-law, Perrier, who lived near 
 Puy de D6me, a mountain in Southern France, to test the con- 
 clusion. On trial, it was found that the mercury fell 3 inches. 
 "A result," wrote Perrier, "which ravished us with admiration 
 and astonishment." Thus was discovered the germ of our mod- 
 ern barometer, and the dogma of the philosophers soon gave 
 place to the law of gravitation and our present views concern- 
 ing the atmosphere. 
 
 Consult Pepper's " Cyclopedic Science"; Bert's "Atmos- 
 pheric Pressure and Life," in "Popular Science Monthly," Vol. 
 XI., p. 316; "Appleton's Cyclopedia," Articles on Hydrome- 
 chanics, Atmosphere, Pneumatics, etc.; Delaunay, " Me"canique 
 JRationelle " ; Boutan et D' Almeida, "Cours de Physique"; 
 Miiller, "Lehrbuch der Physik und Meteorologie." 
 
 On the theory of Wave-motion, and the subjects of Sound 
 and Light, which are now to follow, consult Lockyer's " Studies 
 in Spectrum Analysis"; Lloyd's "Wave Theory"; Taylor's 
 "Science of Music"; Blaserna's "Theory of Sound in Relation 
 to Music"; Tyndall's "Sound" and "Light"; Lockyer's " Water- 
 waves and Sound-waves" in "Popular Science Monthly," Vol. 
 XIII., p. 166; Shaw's "How Sound and Words are Produced," 
 in "Popular Science Monthly," Vol. XIII., p. 43; Mayer on 
 "Sound"; Schellen's "Spectrum Analysis"; Airy's "Optics"; 
 Lockyer's "Spectroscope"; Chevreul's "Colors"; Spottiswoode's 
 "Polarization of Light"; Lommel's "Nature of Light"; Helm- 
 holtz's "Popular Lectures on Scientific Subjects"; "Appleton's 
 Cyclopedia," Articles on Sound, Light, Spectrum, Spectrum Anal- 
 ysis, Spectacles, Heat, etc. ; Stokes' "Absorption and Colors," and 
 Forbes' "Radiation," in "Science Lectures at South Kensington," 
 Vol. I.; Mayer and Barnard's "Light"; Draper's "Popular Ex- 
 position of some Scientific Experiments," in "Harper's Maga- 
 zine" for 1877; Core's "Modern Discoveries in Sound," in 
 Manchester Science Lectures, '77-8 ; Dolbear's " Art of Project- 
 ing"; Draper's "Scientific Memoirs"; -Steele's "Physiology," 
 Section on Sight, pp. 187-196. 
 
VI. 
 
 ON SOUND. 
 
 SCIENCE ought to teach us to see the invisible as well as the visible in 
 nature : to picture to our mind's eye those operations that entirely elude 
 the eye of the body ; to look at the very atoms of matter, in motion and in 
 rest, and to follow them forth into the world of the senses." TYNDALL. 
 
ANALYSIS OF SOUND. 
 
 1. PRODUCTION OF SOUND. 
 
 2. TRANSMISSION OF SOUND. 
 
 3. REFRACTION OF SOUND. 
 
 4. REFLECTION OF SOUND. 
 
 5. MUSICAL SOUNDS. 
 
 6. INTERFERENCE OF SOUND. 
 
 7. VIBRATION OF CORDS. 
 
 (1.) Through Air. 
 (2.) In a Vacuum. 
 (3.) In Liquids. 
 (4.) In Solids. 
 
 (5.) Production of Motion by Sound. 
 (6.) Co-vibration through Air as a Me- 
 dium. 
 
 (7.) Velocity of Transmission. 
 . (8.) Loudness of Sound. 
 
 !(1.) Law of Reflection. 
 (2.) Echoes. 
 (3.) Decrease by Reflection. 
 (4.) Acoustic Clouds. 
 
 f (1.) Difference between Noise and Music. 
 
 (2.) Pitch. 
 
 (3.) The Siren. 
 
 (4.) Wave-lengths. 
 
 (5.) Tones in Unison. 
 
 (1.) The Sonometer. 
 
 (2.) Laws of Vibration. 
 
 (3.) Nodes. 
 
 (4.) Acoustic Figures. 
 
 (5.) Harmonics. 
 
 (6.) Nodes of a Bell. 
 
 (7.) Nodes of a Sounding-board. 
 
 L (8.) Musical Scale. 
 
 8. VIBRATION OF COLUMNS OF AIR. 
 
 9. "Wnn> INSTRUMENTS. 
 
 10. CO-VIBRATION. 
 
 11. THE PHONOGRAPH. 
 
 12. THE BAR. 
 
 (1.) Sensitive Flames. 
 (2.) Singing Flames. 
 
 (1.) Range of the Ear. 
 
 (2.) Ability to Analyze Sound 
 
ACOUSTICS, OR THE SCIENCE OF 
 SOUND.* 
 
 1. Production of Sound. By lightly tapping a 
 glass fruit-dish, we can throw the sides into motion 
 visible to the eye. Fill a goblet half-full of water, 
 and rub a wet finger lightly around the upper edge 
 of the glass. The sides will vibrate, and cause tiny 
 waves to ripple the surface of the water. Hold a 
 card close to the prongs of a vibrating 
 tuning-fork, and you 
 can hear the repeat- 
 ed taps. 
 Place 
 
 the cheek near them, *"**** * tatata its vibrations - 
 and you will feel the little puffs of wind. Insert the 
 handle between your teeth, and you will experience 
 the indescribable thrill of the swinging metal. The 
 
 * The term sound is used in two senses the subjective (which has refer- 
 ence to our mind) and the objective (which refers to the objects around us). 
 (1.) Sound is the sensation produced upon the organ of hearing by vibra- 
 tions in matter. In this use of the word there can be no sound where 
 there is no ear to catch the vibrations. An oak falls in the forest, and if 
 there is no ear to hear it there is no noise, and the old tree drops quietly 
 to its resting-place. Niagara's flood poured over its rocky precipice for 
 ages, but fell silently to the ground. There were the vibrations of earth 
 and air, but there was no ear to receive them and translate them into 
 sound. When the first foot trod the primeval solitude, and the ear felt the 
 pulsations from the torrent, then the roaring cataract found a voice and 
 broke its lasting silence. (2.) Sound consists of those vibrations of matter 
 
154 
 
 ACOUSTICS. 
 
 tuning-fork may be made to draw the outline of its 
 vibrations upon a smoked glass. Fasten upon one 
 prong a sharp point, and drawing the fork along, a 
 sinuous line will show the width (amplitude) of the 
 vibrations. 
 
 2. Transmission of Sound. (1.) THROUGH AIR. 
 In order that any medium shall transmit sound, it 
 must be elastic. Most known bodies possess some 
 elasticity, and hence sound may be transmitted 
 through gases, liquids, and solids. The prong of a 
 tuning-fork advances, condensing the elastic air in 
 front of it. This transmits the compression to the 
 air next forward, while the fork swings backward, 
 leaving a rarefaction next to the compression. This 
 
 in turn advances, on account of the elasticity 
 of the air, and is followed by a compression 
 due to the second forward swing of the fork. 
 This process is repeated, until the fork comes to rest, 
 and the sound ceases. Each vibration produces a 
 sowid^wave of air, which contains one condensation 
 and one rarefaction. In water, we measure a wave- 
 capable of producing a sensation upon the organ of hearing. In this use 
 of the word there can be a sound in the absence of the ear. An object 
 falls and the vibrations are produced, though there may be no organ of 
 hearing to receive an impression from them. This is the sense in which 
 the term sound is used in Physics. 
 
TRANSMISSION OF SOUND. 
 
 165 
 
 length from crest to crest ; in air, from condensation 
 to condensation. The condensation of the sound- 
 wave corresponds to the crest, and the rarefaction of 
 the sound-wave to the hollow of the water-wave. In 
 Fig. 118, the dark spaces a, 5, c, d represent the con- 
 densations, and a', 5', c' the rarefactions; the wave- 
 lengths are the distances a&, &c, cd. 
 
 If we fire a gwi, the gases which are produced 
 expand suddenly and force the air outward in every 
 
 Fio. 119. 
 
 alii 
 
 Propagation of Sound. 
 
 direction. This hollow shell of condensed air imparts 
 its motion to the next one, while it springs back by 
 its elasticity and becomes rarefied. The second shell 
 rushes forward with the motion received, then 
 bounds back and becomes rarefied. Thus each shell 
 of air takes up the motion and imparts it to the 
 next. The wave, consisting of a condensation and a 
 rarefaction, proceeds onward. It is, however, as in 
 water-waves, a movement of the form only, while 
 the particles vibrate but a short distance to and fro. 
 
156 
 
 ACOUSTICS. 
 
 FIG. 130. 
 
 The molecules in water-waves oscillate vertically; 
 
 those in sound-waves horizontally, or parallel to the 
 
 line of motion.* 
 
 If a bell be rung, the adja- 
 cent air is set in motion ; 
 thence, by a series of conden- 
 sations and rarefactions, the 
 vibrations are conveyed to the 
 ear.f 
 
 (2.) IN A VACUUM. The bell 
 B (Fig. 120) may be set in 
 motion by the sliding-rod r. 
 The apparatus is suspended 
 by silk cords, that no vibration 
 may be conducted through the 
 pump. If the air be exhausted, 
 the sound will become so faint 
 that it can not be heard, except 
 when the ear is placed close to 
 the receiver.]: 
 
 In very elevated regions 
 
 sounds are diminished in loudness, and it is difficult 
 
 Bell in Vacuum. 
 
 * A continuous blast of air produces no sound. The rush of the grand 
 aerial rivers above us we never hear. They flow on in the upper regions 
 ceaselessly but silently. Let, however, the great billows strike a tree and 
 wrench it from the ground, and we can hear the secondary, shorter waves 
 which set out from the struggling limbs and the tossing leaves. 
 
 t " It is marvelous," says Youmans, " how slight an impulse throws a 
 vast amount of air into motion. We can easily hear the song of a bird 
 500 feet above us. For its melody to reach us it must have filled with 
 wave-pulsations a sphere of air 1,000 feet in diameter, or set in motion 
 eighteen tons of the atmosphere." 
 
 $ There would be perfect silence in a perfect vacuum. ITo sound is 
 transmitted to the earth from the regions of space. The movements of the 
 heavenly bodies are noiseless. 
 
TKAKSM1SS1ON OF SOUND. 157 
 
 to carry on a conversation. The reverse takes place 
 in deep mines and diving-bells. 
 
 (3.) IN LIQUIDS. Let two persons immerse them- 
 selves in water at a distance of twenty or thirty 
 yards from one another. If one of them strikes two 
 pebbles together, or rings a bell, the other will hear 
 the sound with the utmost clearness. 
 
 (4.) IN SOLIDS. The "Lovers' Telephone" consists 
 of a pair of little cups, the bottom of each being 
 made of an elastic substance, like stretched bladder, 
 and connected with that of the other by a string. 
 By stretching the string elastic force is developed, 
 and on talking into one of the cups the sound is 
 readily heard at the other. On relaxing the string 
 and thus diminishing the elasticity, sound ceases to 
 be conveyed perceptibly by it. By putting the ear 
 against the ground, one may hear the tread of foot- 
 steps that are inaudible through the air alone.* 
 
 (5.) PRODUCTION OF MOTION BY SOUND. If a tuning- 
 fork be excited and its stem be pressed firmly against 
 a table, the sound will become much louder. The 
 solid fork communicates its vibration to the table, 
 which in turn gives its vibration to a much larger 
 body of air than that in contact with the fork alone. 
 Tuning-forks are generally mounted upon resonance 
 boxes, the whole body of air within, as well as the 
 box itself, thus co-vibrating with the fork. 
 
 * Wheatstone invented a beautiful experiment to show the transmission 
 of sound through wood. Upon the top of a music-box, he rested the end of a 
 wooden rod reaching to the room above, and insulated from the ceiling by 
 India rubber. A violin being placed on the top of the rod, the sounds from 
 the box below filled the upper room, appearing to emanate from the violin. 
 
158 
 
 ACOUSTICS. 
 
 (6.) CO-VIBRATION THROUGH Am AS A MEDIUM. The 
 air between any source of sound and the ear is like 
 an elastic spring between a pair of tuning-forks of 
 the same size and material. When the prong of the 
 first fork swings from a to & (Fig. 121), a condensa- 
 tion is propagated through the spring and makes 
 
 yxx 
 
 FIG. 121. 
 
 Tuning-forks Connected by a Delicate Wire. 
 
 the prong of the second fork swing slightly from x 
 toward y. A rarefaction follows, making it swing 
 from x toward z. The succession of these properly- 
 timed impulses causes an accumulation of energy to 
 be imparted through the air to the second fork, 
 which soon gives forth an audible sound. The elas- 
 tic bodies composing the ear in like manner accept 
 vibrations from outside, and their motion is perceived 
 as sound. 
 
 (7.) THE VELOCITY OF SOUND depends on the ratio 
 of the elasticity to the density of the medium 
 through which it passes. The higher the elasticity, 
 
TRANSMISSION OF SOUND. 159 
 
 the, more promptly and rapidly the motion is trans- 
 mittted, since the elastic force acts like a bent 
 spring between the molecules; and the greater the 
 density, the more molecules to be set in motion, and 
 hence the slower the transmission. 
 
 Sound travels through air (at 32 F.) 1,090 feet 
 per second. A rise in temperature diminishes the 
 density of the air, and thus increases the velocity of 
 sound. A difference of 1 F. makes a variation of a 
 little more than one foot. Sound also moves faster 
 in damp than in dry air. 
 
 Sound travels through water about ^,700 feet per 
 second. Water being denser than air should on this 
 account conduct sound more slowly ; but its high 
 elasticity (p. 10), measured by the amount of force re- 
 quired to compress it, more than quadruples the rate. 
 
 Sound travels through solids faster than through 
 air. This may be illustrated by placing the ear close 
 to the horizontal bar at one end of an iron fence, 
 while a person strikes a sharp blow at the other end. 
 Two sounds will reach the ear one through the 
 metal, and afterward another through the air. The 
 velocity varies with the nature of the solid. In the 
 metals it is from four to sixteen times that in air. 
 
 Different sounds travel with sensibly the same 
 velocity* A band may be playing at a distance, yet 
 
 * It has been said that the "heaviest thunder travels no faster than 
 the softest whisper." Mallet, however, found that in blasting with a 
 charge of 2,000 Ibs., the velocity was 967 feet per second, while with 12,000 
 Ibs. it was increased to 1,210 feet. Parry in his Arctic travels states that, 
 on a certain occasion, the sound of the sunset-gun reached his ears before 
 the officer's word of command to fire, proving that the report of the can- 
 non traveled sensibly faster than the sound of the voice. 
 
160 ACOUSTICS. 
 
 the harmony of the different instruments is pre- 
 served. The soft and the loud, the high and the low 
 notes reach the ear at the same time. 
 
 Velocity of sound used to find distance. Light 
 travels instantaneously so far as all distances on the 
 earth are concerned. Sound moves more slowly. 
 We see a chopper strike with his ax, and a moment 
 elapses before we hear the blow. If one second in- 
 tervenes the distance is about 1,090 feet. By means 
 of the second-hand of a watch or the beating of our 
 pulse, we can count the seconds that elapse between 
 a flash of lightning and the peal of thunder which 
 follows. Multiplying the velocity of sound by the 
 number of seconds, we obtain the distance of the 
 tfiunder-bolt. 
 
 (8.) THE LOUDNESS OF SOUND depends chiefly on 
 the amplitude of the vibration, if the air be quiet. 
 The energy of the vibration is proportional to the 
 square of the amplitude, i. e., the arc through which 
 the molecule swings to either side of its position of 
 rest. But loudness is a sensation, and no accurate 
 measurement of sensations has yet been made. The 
 loudness of sound depends also on the density of the 
 air. On the top of a mountain, because of the rare 
 atmosphere, there are fewer molecules to be set in 
 motion, hence the effect on the ear is less intense. 
 
 Mechanically considered, the intensity of sound* 
 
 * The same proportion obtains in Gravitation, Sound, Light, and Heat. 
 We have seen how the motion of the common Pendulum is due to the force 
 of Gravity, and reveals the Laws of Falling Bodies. Now we find that the 
 Pendulum, and even the principles of Beflected Motion and Momentum, 
 are linked with the phenomena of Sound, As we progress further, we shall 
 
BEFRACTION OF SOUND. 161 
 
 diminishes as the square of the distance increases. 
 The sound-wave expands in the form of a sphere. 
 The larger the sphere, the greater the number of air 
 particles to be set in motion, and the feebler their 
 vibration. The surfaces of spheres are proportional 
 to the squares of their radii ; the radii of sound- 
 spheres are their distances from the center of dis- 
 turbance. Hence the force with which the molecules 
 will strike the ear decreases as the square of our 
 distance from the sounding body increases. 
 
 Speaking-tubes conduct sound to distant rooms 
 because they prevent the waves from expanding and 
 losing their intensity.* The ear-trumpet collects 
 waves of sound and reflects them into the ear. The 
 speaking-trumpet is based on the same principle as 
 the speaking-tube. The sound of the voice is strength- 
 ened also by the co-vibration of the walls of the 
 trumpet. 
 
 3. Refraction of Sound When a sound-wave 
 goes obliquely from one medium to another, it is 
 bent out of its course. Like light, it may be passed 
 through a lens and brought to a focus. In Fig. 122 
 is shown a bag of thin India rubber or collodion, filled 
 with carbonic acid gas so as to assume the form of 
 a lens. A watch is placed at one focus of this and 
 the ear at the other. The ticks of the watch can be 
 heard, while outside the focus they are inaudible. 
 
 find how Nature is thus interwoven every-where with proofs of a common 
 plan and a common Author. 
 
 * Biot held a conversation through a Paris water-pipe 3,120 feet long. 
 He says that " it was so easy to be heard, that the only way not to be 
 heard was not to speak at all." 
 
162 
 
 ACOUSTICS. 
 
 4. Reflection of Sound. When a sound-wave 
 strikes against the surface of another medium, a 
 portion goes on while the rest is reflected. 
 
 FIG. 122. 
 
 Refraction of Sound. 
 
 (1.) THE LAW is that of Motion ; the angle of 
 incidence is equal to that of reflection.* If the re- 
 flecting surface be very near, the reflected sound will 
 join the direct one and strengthen it. This accounts 
 for the well-known fact that a speaker can be heard 
 
 * Domes and curved walls reflect sound as mirrors do light. Thus, in 
 the gallery under the dome of St. Paul's Cathedral, London, persons stand- 
 ing close to the wall can whisper to each other and be heard at a great 
 distance. Two persons, placed with their backs to each other, at the foci 
 of an oval room, or "Whispering G-allery," can carry on a conversation 
 that will be inaudible to spectators standing between them. The covered 
 recesses on the opposite sides of a street, or the arches of a stone bridge, 
 oftentimes reflect sound so as to enable persons seated at the foci to con- 
 verse in whispers while loud noises are being made in the open space 
 between these semi-domes. 
 
REFLECTION OF SOUND. 
 
 163 
 
 more easily in a room than in the open air, and that 
 a smooth wall back of the stand re-enforces the voice. 
 The old-fashioned "sounding-boards" were by 
 
 no 
 
 FIG. 123. 
 
 Reflection of Sound. 
 
 means inefficient, however singular may have been 
 their appearance. 
 
 By revolving a disk of card-board from which a 
 pair of sectors have been cut out, and blowing 
 against it with a trumpet or whistle, a person sta- 
 tioned at the proper angle will notice a beating 
 
164 ACOUSTICS. 
 
 sound due to successive reflection and transmission 
 of the waves. 
 
 (2.) ECHOES are produced where the reflecting 
 surface is so distant that we can distinguish the 
 reflected from the direct sound. If the sound be 
 short and quick, this requires at least fifty or sixty 
 feet ; but if it be an articulate one, as in ordinary 
 speech, more than a hundred feet are necessary. It 
 is possible to pronounce and hear distinctly about 
 five syllables in a second; 1,120 ft. (the velocity at 
 a medium temperature) -=- 5 = 224 ft.* If the wave 
 travel 224 feet in going and returning, the ad- 
 vancing and returning sounds will not blend, and 
 
 * When several parallel surfaces are properly situated, the echo may be 
 repeated backward and forward in a surprising manner. In Princeton, 
 Ind., there is an echo between two buildings that will return the word 
 "Knickerbocker" twenty times. So many persons visited the place tlu.t 
 the city council forbade the use of the echo after 9 o'clock at night. At 
 Woodstock, England, an echo returns seventeen syllables by day and 
 twenty by night. The reflecting surface is distant about 2,300 feet, and a 
 sharp ha! will come back a ringing ha, ha, ha! The echo is often softened, 
 as in the Alpine regions, where it warbles a beautiful accompaniment to 
 the shepherd's horn. The celebrated echo of the Metelli at "Rome is said 
 to have been capable of distinctly repeating the first line of the ^Eneid 
 eight times. In Fairfax County, Va., is an echo which will return twenty 
 notes played on a flute. The tick of a watch may be heard from one end 
 of the Church of St. Albans to the other. At Carisbrook Castle, Isle of 
 Wight, is a well 210 feet deep and twelve feet wide, lined with smooth 
 masonry. When a pin is dropped into the well it is distinctly heard to 
 strike the water. In certain parts of the Colosseum at London the tearing of 
 paper sounds like the patter of hail, while a single exclamation comes back 
 a peal of laughter. The dome of the Baptistery of the Cathedral at Pisa 
 has a wonderful echo. During some experiments there, the author found 
 every noise, even the rattle of benches on the pavement below, to be reflected 
 back as if from an immense distance and to return mellowed and softened 
 into music. An interesting illustration of the reflection of sound is found 
 at the so-called Echo River, of the Mammoth Cave, Ky. Sounding in succes- 
 sion the notes G, E, C, at the middle of the tunnel, the boatman receives the 
 echoes, all mingled into a full chord, for eight or ten seconds afterward. 
 
MUSICAL SOUNDS. 165 
 
 the ear will be able to detect an interval between 
 them. A person speaking in a loud voice squarely 
 in front of a large smooth wall 112 feet distant, 
 can distinguish the echo of the last syllable he 
 utters; at 224 feet, the ]ast two syllables, etc. 
 
 (3.) DECREASE OF SOUND BY REFLECTION. If we 
 strike the bell, represented in Fig. 120, before a 
 vacuum is produced, we shall find a marked differ- 
 ence between its sound under the glass receiver and 
 in the open air. Floors are deadened with tan-bark 
 or mortar, since as the sound-wave passes from par- 
 ticle to particle of the unhomogeneous mass, it be- 
 comes weakened by partial reflection. The air at night 
 is more homogeneous, and hence sounds are heard 
 farther and more clearly than in the day-time. 
 
 (4.) ACOUSTIC CLOUDS are masses of moist air of 
 varying density, which act upon sounds as common 
 clouds do upon light, wasting it by repeated reflec- 
 tions. They may exist in the clearest weather. To 
 their presence is to be attributed the variation often 
 noticed in the distance at which well-known sounds, 
 as the ringing of church bells, blowing of engine- 
 whistles, etc., are heard at different times.* 
 
 5. Musical Sounds. (1.) THE DIFFERENCE BE- 
 TWEEN NOISE AND MUSIC is that between irregular and 
 
 * The extinction of sound by such agencies is often almost incredible. 
 Thus two observers looking across the valley of the Chickahominy at the 
 battle of Q-aines' Mill failed to hear a sound of the conflict, though they 
 could clearly see the lines of soldiers, the batteries, and the flash of the 
 guns. These phenomena are ascribed by many to. an elevation or a depres- 
 sion of the wave-front so that the sound passes .above the observer or is 
 stopped before it reaches Mm. See " Stewart's Physics," p. 141. 
 
166 ACOUSTICS. 
 
 regular vibrations. Whatever the cause which sets 
 the air in motion, if the vibrations are uniform and 
 rapid enough, the sound is musical. If the ticks of 
 a watch could be made with sufficient rapidity, they 
 would lose their individuality and blend into a mu- 
 sical tone. If the puffs of a locomotive could reach 
 fifty or sixty a second, its approach would be her- 
 alded by a tremendous organ-peal.* 
 
 (2.) PITCH depends on the rapidity of the vibra- 
 tions. Thus, if we hold a card against the cogs of 
 a rapidly-revolving wheel, we shall obtain a clear 
 tone ; and the faster the wheel turns, the shriller 
 the tone, i. e., the higher the pitch. 
 
 (3.) THE NUMBER OF VIBRATIONS PER SECOND IS 
 
 determined by an instrument called the siren. It 
 consists of a cylindrical box (Figs. 124 and 125), the 
 top of which is pierced with a series of holes. Over 
 this is a plate with a corresponding series, fixed to a 
 vertical rod, which is pivoted on the lower plate so 
 as to revolve easily. It is provided with an endless 
 screw (Fig. 125), which operates some clock-work. 
 
 * The pavement of London is largely composed of granite blocks, four 
 inches in width. A cab- wheel jolting over this at the rate of eight miles 
 per hour produces a succession of 35 sounds per second. These link them- 
 selves into a soft, deep musical tone, that will bear comparison with notes 
 derived from more sentimental sources, even though it may seem con- 
 fused to a hearer in its midst. This tendency of Nature to music is 
 something wonderful. "Even friction," says Tyndall, "is rhythmic." A 
 bullet flying through the air sings softly as a bird. The limbs and leaves 
 of trees murmur as they sway in the breeze. Palling water, singing birds, 
 sighing winds, every- where attest that the same Divine love of the beautiful 
 which causes the rivers to wind through the landscape, the trees to bend in 
 a graceful curve the line of beauty and the rarest flowers to bud and 
 blossom where no eye save His may see them, delights also in the anthem 
 of praise which Nature sings for His ear alone. 
 
MUSICAL SOUNDS. 
 
 167 
 
 On the dial (Fig. 124), we can see the number of 
 turns made by the upper disk. The holes in the two 
 disks are oppositely inclined, so that when a current 
 of air is forced in from below it passes up through 
 the openings in the lower disk, and striking against 
 
 FIG. 124. 
 
 FIG. 125 
 
 The Siren. 
 
 the sides of those in the upper disk, causes it to re- 
 volve. As that turns, it alternately opens and closes 
 the orifices in the lower disk, and thus converts the 
 steady stream of air into uniform puffs. At first 
 they succeed each other so slowly that they may be 
 counted. But, as the motion increases, they link 
 themselves together, and pass into a full, melodious 
 note. As the velocity augments, the pitch rises, 
 
168 ACOUSTICS. 
 
 until the music becomes painfully shrill. Diminish 
 the speed, and the pitch falls. 
 
 To find, therefore, the number of vibrations in a 
 given sound, force the air through the siren until the 
 required pitch is reached. See on the dial, at the end 
 of a minute, the number of revolutions of the disk. 
 Suppose the number of holes in a disk to be 10, and 
 the tone produced to be in unison with that of a (7 3 
 tuning-fork. The number of revolutions indicated on 
 the dial at the end of a minute is found to be 1,536. 
 There were 10 puffs, or 10 waves of sound, for each 
 revolution. 1,536 x 10 = 15,360. Dividing this by 
 60, we have 256, the number per second. Increasing 
 now the blast until the tone produced is in unison 
 with a (7 4 tuning-fork, the octave above the first, the 
 number of vibrations per second is found to be 512. 
 Hence the octave of a tone is caused by double the 
 number of vibrations. 
 
 (4.) To FIND THE LENGTH OF THE WAVE. Suppose 
 the air in the last experiment was of such a temper- 
 ature that the foremost sound-wave traveled 1,120 
 feet in a second. In that space there were 256 
 sound-waves. Dividing 1,120 by 256, we have 4| ft. 
 as the length of each. We thus find the wave-length 
 by dividing the velocity by the number of vibrations 
 per second. As the pitch is elevated by rapidity of 
 vibration, we perceive that the low tones in music 
 are produced by the long waves and the high tones 
 by the short ones.* 
 
 * The aerial waves are seemingly shortened when the source of sound 
 is approaching, whether by its own motion or the hearer's, and lengthened 
 
INTERFEKENCE OF SOUND-WAVES. 169 
 
 (5.) TONES IN UNISON. If the string of a violin, 
 the cord of a guitar, the parchment of a drum, and 
 the pipe of an organ, produce the same tone, it is be- 
 cause they are executing the same number of vibra- 
 tions per second. If a voice and a piano perform the 
 same music, the steel strings of the piano and the 
 vocal cords of the singer vibrate together and send 
 out sound-waves of the same length. 
 
 6. Interference of Sound-waves. Just as two 
 water-waves by meeting in opposite phases may de- 
 stroy one another, so by a proper adjustment two 
 sound-waves may be made to interfere, and, if ex- 
 actly equal and opposite, to produce silence. Fig. 
 126 represents a piece of apparatus intended to show 
 this. Let a tone, such as <7 3 , be sounded in the 
 mouthpiece at a. The waves divide at the end of 
 the first India-rubber tube and reunite on entering the 
 second, before entering the ear at 5. One branch o 
 the channel is made of two tubes, One of which slides 
 over the other so that the branch may be lengthened 
 at will. If it be pulled up so high that the waves 
 passing through it shall traverse a half wave-length 
 more in distance than those in th6 fixed branch, oppo- 
 site phases will meet where they reunite, and the list- 
 
 is receding. In the former case, the tone of the sound is more acute ; 
 in the latter, graver. This is strikingly illustrated when a swift train rushes 
 past a station, the whistle blowing. While the cars are approaching, a per- 
 son hears a note somewhat sharper ; after it has passed, one somewhat flatter 
 than the true note. Still more obvious is the change when two trains pass 
 each other. A person unfamiliar with the arrangement would suppose a 
 different bell was rung. In one case more and in the other fewer waves 
 reach the ears in a second, 
 
170 
 
 ACOUSTICS. 
 
 ener notices great weakening of the sound. By 
 proper handling, the sound received may be made to 
 become alternately strong and weak without any 
 change in the sound given.* 
 
 FIG. 126. 
 
 Tube for Interference of Sound. 
 
 If we strike a tuning-fork and turn it slowly 
 around before the ear, we shall find four points 
 where the interference of the sound-waves causes 
 
 * We can not produce complete extinction because, 1. The sound is con- 
 ducted not only by the inclosed air, but by the solid tube also ; 2. There 
 is loss by friction in the longer branch ; 3. There is loss by leakage between 
 the tubes that slide against each other. 
 
VIBRATIONS OF COEDS. 171 
 
 great weakening. The two prongs swing alternately 
 toward and from each other. When a condensation 
 is produced between the prongs, a rarefaction is pro- 
 duced on their outer sides. Certain lines can be 
 found where these interfere. 
 
 If two forks are nearly but not quite in unison, 
 the waves from them are unequal in length. They 
 alternately conjoin and oppose each other, producing 
 "beats." These are often noticed in the sound from 
 a large bell, the opposite sides of which are not quite 
 equally elastic. A pair of mistuned organ-pipes pro- 
 duces a similar effect, and the discord of an inferior 
 piano, or indeed all discord, is due to beats. 
 
 7. Vibrations of Cords. Let ab be a stretched 
 cord made to vibrate. The motion from e to d and 
 
 PIG. 127. 
 
 d 
 
 Vibrating Cord. 
 
 back again is termed a vibration; that from e to d, 
 a half-vibration. The distance, cd, from the middle 
 to either of the extreme positions is the amplitude. 
 (1.) THE SONOMETER is an instrument used to in 
 vestigate the laws of vibration of stretched cords. 
 It consists of two cords stretched by weights, P, 
 across fixed bridges, A and B. The movable bridge, 
 Z>, serves to lengthen or shorten the vibrating part 
 of either cord. Beneath is a resonance box, to which 
 the vibrations are conducted by the bridges. This is 
 the body whose sound is chiefly heard. 
 
172 ACOUSTICS. 
 
 (2.) THREE LAWS. I. The number of vibrations 
 per second increases as the length of the cord de- 
 creases. By plucking the cord with the finger, or 
 drawing ^a violin bow across it, make it vibrate, giving 
 the note of the entire string. Place the bridge D at 
 the center of the cord, and the sound will be the oc- 
 tave above the former. Thus, by taking one half the 
 length of the cord we double the number of vibra- 
 tions. Examples: If an entire cord make 20 vibra- 
 tions per second, one half will make 40, and one 
 
 FIG. 128. 
 
 third, 60. The violin or guitar player elevates the 
 pitch of a string by moving his finger, thus shorten- 
 ing the vibrating portion. In the piano, harp, etc., 
 the long and the short strings produce the low and 
 the high notes respectively. 
 
 II. The number of vibrations per second increases 
 as the square root of the tension. The cord when 
 stretched by 1 Ib. gives a certain tone. To double 
 the number of vibrations and obtain the octave re- 
 quires 4 Ibs. Stringed instruments are provided with 
 
VIBRATIONS OF CORDS. 173 
 
 keys, by which the tension of the cord and the cor- 
 responding pitch may be increased or diminished. 
 
 III. The number of vibrations per second decreases 
 as the square root of the weight of the cord increases. 
 If two strings of the same material be equally 
 stretched, and one have four times the weight of 
 the other, it will vibrate only half as often. In the 
 violin the bass notes are produced by the thick 
 strings. In the piano fine wire is coiled around the 
 heavy strings to increase their weight. 
 
 FIQ. 129. 
 
 Production of Two Segments. 
 
 (3.) NODES. In the experiments just described, the 
 cord is shortened by means of a firm, movable bridge. 
 If, instead, we rest a feather lightly on the string, 
 and draw the bow over one half, the cord will 
 vibrate in two portions and give the octave as be- 
 fore. Remove the feather, and it will continue to 
 vibrate in two parts and to yield the same tone. 
 We can show that the second half vibrates by plac- 
 ing across that portion a little paper rider. On draw- 
 ing the bow it will be thrown off. Hold the feather 
 
174 
 
 ACOUSTICS. 
 
 so as to separate one third of the string and cause 
 it to vibrate ; the remainder of the cord will vibrate 
 in two segments. When the feather is removed, the 
 
 FIG. 131. 
 
 Production of Three Segments. 
 
 entire cord will vibrate in three different parts of 
 equal length, separated by stationary points called 
 nodes. This may be shown by the riders ; the one 
 at the node remains, while the others are thrown off. 
 (4.) ACOUSTIC FIGURES. Sprinkle fine sand on a 
 metal plate. Place 
 the finger-nail on 
 one edge to stop 
 the vibration at 
 that point, as the 
 feather did in the 
 last experiment, 
 and draw the bow 
 lightly across the 
 
 opposite edge. The sand will be tossed 
 away from the vibrating parts of the 
 plate and will collect along the nodal vibration of a Plate. 
 
VIBRATIONS OF COKDS. 
 
 175 
 
 FIG. 132. 
 
 lines, which divide the large square. It is wonder- 
 ful to see how the sand will seemingly start into life 
 and dance into line at the touch of the bow. Fig. 
 132 shows some of 
 the beautiful patterns 
 obtained by Chladni. 
 (5.) HAEMONICS.* 
 Whenever a cord vi- 
 brates, it separates 
 into segments at the 
 same time. Thus we 
 have the full or fun- 
 damental note of the 
 entire string, and su- 
 perposed upon it the 
 higher notes pro- 
 duced by the vibrat- 
 ing parts. These are called overtones or harmonics. 
 The mingling of the two classes of vibrations deter- 
 mines the quality of the sound, and enables us to 
 distinguish the music of different instruments. 
 
 (6.) NODES OF A BELL. Let the heavy circle in 
 Fig. 133 represent the circumference of a bell when 
 at rest. Let the hammer strike at a, Z>, c, or d. At 
 
 Chladni's Figures. 
 
 * Press gently but firmly down the notes C, Q-, and C, in the octave 
 above middle C, on the piano-forte. Without releasing these keys, give to 
 C below middle C a quick, hard blow. The damper will fall, and the sound 
 will stop abruptly. At the same instant a low, soft chord will be heard. 
 This comes from the three strings whose dampers are raised, leaving them 
 free to sound in sympathy with the overtones of the lower C, which sounds 
 are identical with their own. When a goblet or wine-glass is tapped with 
 a knife-blade, we can distinguish three sounds, the fundamental and two 
 harmonics. 
 
176 ACOUSTICS. 
 
 one moment, as the bell vibrates, it forms an oval 
 with ab, at the next with cd, for its longest diam- 
 eter. When it strikes its deepest note, the bell vi- 
 brates in four segments, with 
 n, n, n, n } as the nodal points, 
 whence nodal lines run up 
 from the edge to the crown 
 of the bell. It tends, however, 
 to divide into a greater num- 
 ber of segments, especially if 
 it is very thin, and to produce 
 harmonics. The overtones 
 
 ' Vibration of a Bell. wMch accompany the dee p 
 
 tones of the bell are frequently very striking, even 
 in a common call-bell, and often make it hard to de- 
 termine at once what is its fundamental. Usually 
 they die away sooner than the fundamental. 
 
 (7.) NODES OF A SOUNDING-BOARD. The case of a 
 violin or guitar is composed of thin wooden plates 
 which divide into vibrating segments, separated by 
 nodal lines according to the pitch of the note 
 played. The inclosed air vibrating in unison with 
 these, re-enforces the sound and gives it fullness and 
 richness. 
 
 (8.) MUSICAL SCALE. The lowest tone that can be 
 distinctly perceived as musical by most ears is pro- 
 duced by 32 vibrations per second. This is called 
 C . The octave above this is C^ 64 vibrations; the 
 double octave, C 2 , 128 vibrations, etc. If a string be 
 stretched so as to give C 8 , the tones of the common 
 musical scale between this and C 3 are obtained from 
 
VIBRATION OF COLUMNS OF AIR. 177 
 
 the parts of the string indicated by the following 
 fractions : 
 
 C 2 D a E a F 8 G 2 A 2 B 3 3 
 1 "***!*** 
 
 As the number of vibrations varies inversely as 
 the length of the cord, we have only to invert these 
 fractions to obtain the relative number of vibrations 
 per second; thus,* 
 
 C 2 D 3 E 8 F 8 G 3 A 2 B 2 C 8 
 
 1 t I t I I 2 
 
 128 144 160 170 192 214 240 256 
 
 8. Vibration of Columns of Air. If a tuning-fork 
 be excited and its prongs be held before the open 
 end of a tube of proper length, the sound will be- 
 come much louder. If the pitch of the fork is C 4 , 
 512 vibrations, the length of such a tube, open at 
 both ends, is about 18 inches; if open at only one 
 end, 6|- inches. A hollow globe of proper size, with 
 an opening on one side, will respond in like manner. 
 Such bodies are called resonators. 
 
 9. Wind Instruments produce sounds by the vi- 
 bration of the columns of air which they inclose. 
 An organ-pipe is merely a tube-resonator. The sound- 
 
 * In this table, "C 3 = 256 vibrations" represents the middle C of a 
 piano-forte. This number is purely arbitrary. The so-called " concert- 
 pitch '' varies in different countries. The Stuttgart Congress of 1834 fixed 
 the standard tuning-fork middle A at 440 vibrations per second; while 
 the Paris Conservatory (1859) gave to middle A 437.5. The pitch agreed upon 
 as an international standard by the Vienna Conference in 1885 for A= 435 
 vibrations. This was adopted by the Piano Manufacturers' Association of 
 New York and vicinity in 1891. The ratio of the different numbers is identi- 
 cal, whatever the pitch. 
 
178 
 
 ACOUSTICS. 
 
 waves in organ-pipes are set in motion by either 
 fixed mouth-pieces or vibrating reeds. The air is 
 forced from the bellows into the tube P, through the 
 vent i, and striking against the thin edge a, produces 
 a flutter. The column of air above, thrown into vi- 
 
 Fio. 134. 
 
 Organ-pipes. 
 
 bration, re-enforces the sound and gives a full mu- 
 sical tone. The length of the pipe, if open, should be 
 J wave length corresponding to the pitch to which 
 it responds; if closed, wave length. If a tuning- 
 fork which produces this pitch be held at & while vi- 
 brating, the sound will at once become much stronger. 
 
CO-VIBKATION. 179 
 
 The air co-vibrates, whatever may be the source of 
 sound, if only the pitch be properly adjusted. 
 
 1C. Co-vibration. We have already seen (p. 158) 
 how one tuning-fork may co-vibrate with another 
 through the medium of the air. Vibrations thus 
 produced are often called sympathetic, and bodies 
 which thus strengthen sound are said to be resonant. 
 Produce a musical tone with the voice near a piano, 
 and a certain wire will seem to select that sound and 
 respond to it. Change the pitch, and the first string 
 will cease, while another replies. If a hundred tun- 
 ing-forks of different tones are sounding at the foot 
 of an organ-pipe, 'it will strengthen the sound of the 
 one to which it can reply, and answer that alone. 
 Helmholtz has applied this principle to the construc- 
 tion of the resonance globe, an instrument which 
 will respond to a particular harmonic in a compound 
 tone, and strengthen it so as to make it audible. 
 
 (1.) SENSITIVE FLAMES. Flames are sensitive to 
 sound. At an instrumental concert the gas-lights 
 vibrate with certain pulsations of the music. This 
 is noticeable when the pressure of gas is so great 
 that the flame is just on the verge of flaring, and 
 the vibration of the sound-wave is sufficient to " push 
 it over the precipice." * 
 
 (2.) SINGING FLAMES. If we lower a glass tube 
 over a small gas-jet, we soon reach a point where 
 
 * Prof. Barrett, of Dublin, describes a peculiar jet -which is so sensitive 
 that it trembles and cowers at a hiss, like a human being, beats time to 
 the ticking of a watch, and is violently agitated by the rumpling of a silk 
 
180 
 
 ACOUSTICS. 
 
 FIG. 135. 
 
 the flame leaps spontaneously into song. At first 
 the sound seems remote, but gradually approaches 
 until it bursts into an almost full song. The length 
 of the tube and the size of the jet determine the 
 pitch of the note.* The flame, owing to the friction 
 
 at the mouth of the 
 pipe, is thrown into 
 vibration. The air 
 vibrates in unison 
 with the jet, and, 
 like that in the or- 
 gan-pipe, selects the 
 tone corresponding 
 to the length of the 
 tube. 
 
 11. The Phono- 
 graph is an instru- 
 ment for recording 
 and reproducing 
 the vibrations of 
 sound. Its essen- 
 tial features are as 
 follows : 
 
 (1.) A metallic 
 cylinder which can 
 be rotated on a 
 
 Singing Flame. 
 
 screw as axis, so as to secure motion that is side- 
 ward as well as rotary. 
 
 (2.) A hollow cylinder of wax which fits over the 
 
 * The jets are easily made by drawing out glass tubing to a fine point 
 over a spirit-lamp. 
 
THE PHONOGRAPH. 
 
 181 
 
 metallic cylinder, and may be removed after receiv- 
 ing impressions from a source of sound. 
 
 (3.) A mouth-piece into which the speaker vo- 
 calizes. At the bottom of this is an elastic disk, 
 which is set into vibration by the voice. 
 
 (4.) A lever which is actuated by the disk. At 
 one end of it is a specially prepared needle, which 
 makes indentations upon the rotating cylinder of 
 
 wax. 
 
 Pia. 136. 
 
 The Phonograph 
 
 After the line of indentations has been made on 
 the wax, the cylinder is brought back to its first 
 position. On turning it, the needle, pressing on the 
 serrated surface, receives vibratory motion like that 
 which had been given it by the voice. This is re- 
 ceived by the disk, and the instrument thus talks 
 out what had been talked into it. 
 
182 ACOUSTICS. 
 
 There are usually two mouth-pieces, interchange- 
 able in position, one of which is used in speaking to 
 the phonograph, and the other in giving out what 
 this has to say. Each is specially adapted to the 
 work it has to do. The metallic cylinder is rotated 
 by means of an electric motor. The arm which car- 
 ries the mouth-pieces is provided with a turning 
 tool for smoothing the wax before this receives the 
 record from the voice. 
 
 In Fig. 136, the phonograph is shown ready to 
 talk. A conical speaking-trumpet is fixed upon the 
 mouth-piece, so that the sound may be strengthened 
 by co-vibration. The wax cylinder can be kept any 
 length of time, and be made to speak out its message 
 repeatedly. The phonograph reproduces so accurately 
 the sounds it has received, that even the peculiarities 
 which result from the special quality of the speaker's 
 Voice can be recognized. 
 
 12. The Human Ear is also an instrument for 
 receiving sound vibrations, which affect the auditory 
 nerve and produce sensation thus at the base of the 
 brain. ("Hygienic Physiology," p. 216.) 
 
 (1.) RANGE OF THE EAR. JSTo definite limit can 
 be assigned to the range through which musical 
 sounds are perceptible. The highest limit has been 
 roughly estimated to be about 38,000, and the low- 
 est, 16, vibrations per second. When the number of 
 impressions on the ear in each second is less than 
 15 or 16, we become able to perceive them sepa- 
 rately. To be musical they must come fast enough 
 to appear to coalesce. From 16 to 33,000 is about 
 
THE EAR. 183 
 
 eleven octaves. The capacity to hear the higher 
 tones varies in different persons. A sound audible 
 to one may be silence to another. Some ears can not 
 distinguish the squeak of a bat or the chirp of a 
 cricket, while others are acutely sensitive to these 
 shrill sounds. Indeed, the auditory nerve seems gen- 
 erally more alive to the short, quick vibrations than 
 to the long, slow ones. The whirr of a locust is 
 much more noticeable than the sighing of the wind 
 through the trees.* 
 
 (2.) THE ABILITY OF THE EAR TO DETECT AND AN- 
 ALYZE SOUND is wonderful beyond comprehension. 
 Sound-waves chase one another up and down through 
 the air, superposed in entangled pulsations, yet a cyl- 
 inder not larger than a quill conveys them to the 
 ear, and each string of that wonderful harp selects 
 its appropriate sound, and repeats the music to the 
 soul. Though a thousand instruments be played at 
 once, there is no confusion, but each is heard, and 
 all blend in harmony, f 
 
 * To this, however, there are remarkable exceptions. The author knows 
 a lady who is insensible to the higher tones of the voice, but acutely sensi- 
 tive to the lower ones. Thus, on one occasion, being in a distant room, she 
 did not notice the ringing of the bell announcing dinner, but heard the 
 noise the bell made when returned to its place on the shelf. v 
 
 t "Is not the ear the most perfect sense? A needle-woman will distin- 
 guish by the sound whether it is silk or cotton that is torn. Blind people 
 recognize the age of persons by their voices. An architect, comparing the 
 length of two lines separated from each other, if he estimate within ^, we 
 deem very accurate ; but a musician would not be considered very precise 
 who estimated within a quarter of a note (128 -4- 30 = 4 nearly). In a large 
 orchestra, the leader will distinguish each note of each instrument. We 
 recognize an old-time friend by the sound of his voice, when the other 
 senses utterly fail to recall him. The musician carries in his ear the idea 
 of the musical key and every tone in the scale, though he is constantly 
 hearing a multitude of sounds," 
 
184 ACOUSTICS. 
 
 ' PRACTICAL QUESTIONS. 
 
 1. "Why can not the rear of a long column of soldiers keep time to the 
 music in front? 
 
 2. Three minutes elapse between the flash and the report of a thunder- 
 bolt; how far distant is it? 
 
 3. Five seconds expire between the flash and the report of a gun ; what 
 is its distance? 
 
 4. Suppose a speaking-tube should connect two villages ten miles apart ; 
 how long would it take the sound to travel ? 
 
 5. The report of a pistol-shot was returned to the ear from the face of 
 a cliff in four seconds ; what was the distance ? 
 
 6. What is the cause of the difference between the voice of man and 
 woman? A base and a tenor voice? 
 
 7. What is the number of vibrations per second necessary to produce 
 the fifth tone of the scale of C 3 ? 
 
 8. What is the length of each sound-wave in that tone when the tem- 
 perature is at zero? 
 
 9. What is the number of vibrations in the fourth tone above C a ? 
 
 30. If a meteor were to explode at a height of 60 miles, would it be 
 possible for its sound to be heard at sea-level? 
 
 11. A stone is let fall into a well, and in four seconds is heard to strike 
 the bottom; how deep is the well? 
 
 12. What time would be required for a sound to travel five miles in 
 the still water of a lake? 
 
 13. Does sound travel faster at the foot than at the top of a mountain ? 
 
 14. Why is an echo weaker than the original sound? 
 
 15. Why is it so fatiguing to talk through a speaking-trumpet? 
 
 16. Why will the report of a cannon fired in a valley be heard on the 
 top of a neighboring mountain, better than one fired on the top of a mount- 
 ain will be heard in the valley? 
 
 17. Why do our footsteps in unfurnished dwellings sound so startlingly 
 distinct ? 
 
 18. Why do the echoes of an empty church disappear when the audi- 
 ence assemble? 
 
 19. What is the object of the sounding-board of a piano? 
 
 20. During some experiments, Tyndall found that a certain sound 
 would pass through twelve folds of a dry silk handkerchief, but would be 
 stopped by a single fold of a wet one. Explain. 
 
 21. What is the cause of the musical murmur often heard near tele- 
 graph lines? 
 
 22. Why will a variation in the quantity of water in a goblet, when 
 this is made to sound, cause a difference in the tone produced by its 
 vibration ? 
 
 23. At what rate (in meters) will sound move through air at sea-level, 
 the temperature being 20 s C.? 
 
SUMMARY. 185 
 
 SUMMARY. 
 
 SOUND is produced by vibrations. These are transmitted in 
 waves through the air (60 F.) at sea-level at the rate of 1,120 
 ft. per second ; through water four times, and through iron fif- 
 teen times as fast. In general, the velocity depends on the re- 
 lation between the density and the elasticity of the medium ; 
 and the intensity is proportional to the square of the amplitude 
 of the vibrations. Sound, like light, may be reflected and re- 
 fracted to a focus. Echoes* are produced by the reflection of 
 sound from smooth surfaces, not less than 112 ft. (about 33 
 meters) distant. Rapidly-repeated vibrations make a continuous 
 sound ; regular and rapid vibrations produce music ; irregular 
 ones cause a noise. 
 
 The pitch of a sound depends on the rapidity of the vibra- 
 tions. The number of waves, and their consequent length in a 
 given sound, is found by means of the siren. Unison is pro- 
 duced by identical wave-motions. Any number of sound-waves 
 may traverse the air, as any number of water-waves may the 
 surface of the sea, without losing their individuality. The mo- 
 tion of each molecule of air is the algebraic sum of the several 
 motions it receives. Two systems of waves may therefore de- 
 stroy or strengthen each other, according as they meet in oppo- 
 
 * Several acoustic phenomena have become of historical interest. (1.) 
 Near Syracuse, Sicily, is a cave known as the Ear of Dionysius. A whisper 
 at the farther end of the cavern is easily heard by a person at the entrance, 
 though the distance is 200 ft. Tradition says that the Tyrant of Syracuse 
 used this as a dungeon, and was thus enabled to listen to the conversation 
 of his unfortunate prisoners. (2.) On the banks of the Nile, near Thebes, 
 is a statue 47 ft. high, and extending 7 ft. below the ground. It is called 
 the Vocal Memnon. Ancient writers tell us that about sunrise each morn- 
 ing, there issued from this gigantic monolith a musical sound resembling 
 the breaking of a harp-string. It is now believed that this was produced 
 by friction due to unequal expansion of different parts under the morning 
 sun. (3.) Near Mount Sinai, in Arabia, remarkable sounds are produced by 
 the sand falling down a declivity. The sand, which is very white, fine, 
 and dry, lies at such an angle as to be easily set in motion by any cause, 
 such as scraping away a little at the foot of the slope. The sand then rolls 
 down with a sluggish motion, causing at first a low moan, that gradually 
 swells to a roar like thunder, and finally dies away as the motion 
 
186 ACOUSTICS. 
 
 site or in similar phases. Interference is the mutual weakening 
 of two systems of waves which meet in opposite phases. Beats 
 are the effect produced by two musical sounds of nearly the 
 same pitch, which alternately interfere and coalesce. The vibra- 
 tions of a cord produce a musical sound, which is re-enforced by 
 a sounding-board. The rate of vibration and consequent pitch 
 depends on the length, the tension, and the weight of the cord. 
 A sounding body vibrates not only as a whole, but also in seg- 
 ments. Its vibration as a whole produces the fundamental tone, 
 and the additional vibration in segments gives rise to the over- 
 tones. These together form either a complete or an interrupted 
 harmonic series. The quality of the compound sound depends 
 on the number, orders, relative intensities, and mode of com- 
 bination of the overtones into which it can be resolved. The 
 various notes in the musical scale are determined by fixed por- 
 tions of the length of the cord. The music of a wind instru- 
 ment is produced by vibrating columns of air. Resonance is a 
 sympathetic vibration caused by one sonorous body acting on 
 another, through a conducting medium, as seen in the resonance 
 globe, etc. The voice is a reed instrument, with its vibrating 
 cords and resonant cavity. The ear collects the sound-waves. 
 It consists of the outer ear, the drum, and the labyrinth. The 
 auditory nerve transmits to the brain the motions produced in 
 the ear by sound-waves. 
 
 HISTORICAL SKETCH. 
 
 THE ancients knew that without air we should be plunged 
 in eternal silence. " What is the sound of the voice," cried 
 Seneca, "but the concussion of the air by the shock of the 
 tongue? What sound could be heard except by the elasticity of 
 the aerial fluid ? The noise of horns, trumpets, hydraulic organs, 
 is not that explained by the elastic force of the air?" Pythag- 
 oras, who lived in the sixth century before Christ, conceived 
 that the celestial spheres are separated from each other by in- 
 tervals corresponding with the relative lengths of strings arranged 
 to produce harmonious tones. In his musical investigations he 
 
HISTOEICAL SKETCH. 187 
 
 used a monochord, the original of the sonometer now employed 
 by physicists, and wished that instrument to be engraved on his 
 tomb. Pythagoras held that the musical intervals depend on 
 mathematics ; while his great rival, Aristoxenes, claimed that 
 they should be tested by the ear alone. The theories of these 
 two philosophers long divided the attention of the scientific 
 world. The former considered the subject from the stand-point 
 of Physics, the latter from that of Physiology. 
 
 Many centuries elapsed before any marked advance was 
 made. Galileo called attention to the sonorous waves travers- 
 ing the surface of a glass of water, when the glass is made to 
 vibrate. He gave an accurate explanation of the phenomena 
 of resonance, and referred to the fact that every pendulum has 
 a fixed oscillation period of its own ; that a succession of 
 properly timed small impulses may throw a heavy pendulum 
 into vibration, and that this may communicate vibration to a 
 second pendulum of the same vibration period. Galileo also 
 described the first experiment involving the direct determination 
 of a vibration ratio for a known musical interval. He related 
 that he was one day engaged in scraping a brass plate with an 
 iron chisel, in order to remove some spots from it, and noticed 
 that the passage of the chisel across the plate was sometimes 
 accompanied by a shrill whistling sound. On looking closely at 
 the plate, he found that the chisel had left on its surface a long 
 row of indentations parallel to each other and separated by 
 exactly equal intervals. This occurred only when a sound was 
 heard. It was found that a rapid passage of it gave rise to a 
 more acute sound, a slower passage to a graver sound, and that 
 in the former case the indentations were closer together. After 
 many trials, two sets of markings were obtained, which cor- 
 responded to a pair of tones making an exact fifth with each 
 other. The indentations were 30 and 45, respectively, to a 
 given length. Galileo's inference from this was exactly what 
 we now accept as true. 
 
 The present century has witnessed a more complete demon- 
 stration of the laws of the vibrations of cords and the general 
 principles of sound. In 1822, Arago, Gay-Lussac, and others 
 decided the velocity of sound to be 337 meters at 10 C. Savart 
 invented a toothed wheel by which he determined the number 
 
188 ACOUSTICS. 
 
 of vibrations in a given sound ; Latour invented the siren, which 
 gave still more accurate results ; Colladon and Sturm, by a 
 series of experiments at Lake Geneva, found the velocity of 
 sound in water ; Helmholtz made known the laws of harmonics ; 
 Lissajous, by means of a mirror attached to the vibrating body, 
 threw the vibrations on a screen in a series of curves, and so 
 rendered them visible ; while Tyndall has investigated the causes 
 modifying the propagation of sound, as acoustic clouds, fogs, 
 etc., and popularized the whole subject of acoustics. 
 
VII. 
 
 ON LIGHT. 
 
 THE sunbeam comes to the earth as simply motion of ether-waves, yet 
 it is the grand source of beauty and power. Its heat, light, and chemical 
 energy work every-where the wonder of life and motion. In the growing 
 plant, the burning coal, the flying bird, the glaring lightning, the blooming 
 flower, the rushing engine, the roaring cataract, the pattering rain we see 
 only varied manifestations of this one protean energy which we receive 
 from the sun. 
 
ANALYSIS OF LIGHT. 
 
 1. PRODUCTION AND PROPA- 
 GATION OF LlGHr. 
 
 2. REFLECTION OF LIGHT. 
 
 3. REFRACTION OF LIGHT. 
 
 4. DECOMPOSITION OF 
 LIGHT. 
 
 6. OPTICAL INSTRUMENTS. 
 
 3. Lenses. 
 
 1. Definitions. 
 
 2. Visual Angle. 
 
 3. Laws of Light. 
 
 4. Velocity of Light. 
 
 5. Theory of Light. 
 
 1. Definition and Law. 
 
 2. Action of Rough and Polished Surfaces. 
 
 f(l.) Plane. 
 (2.) Concave. 
 (3.) Convex. 
 
 1. Definition, and Illustrations. 
 
 2. Laws of Refraction, and Illustrations. 
 
 j (1.) Convex. 
 1 (2.) Concave. 
 
 4. Spherical Aberration. 
 
 5. Total Reflection. 
 
 6. Mirage. 
 
 r 1. The Prismatic Spectrum. 
 
 2. Solar Energy. 
 
 3. Properties of the Spectrum. 
 
 4. Interruptions in the Spectrum. 
 
 5. The Spectroscope. 
 
 6. Three Kinds of Spectra. 
 
 7. Color. 
 
 8. Complementary Colors. 
 
 9. The Rainbow. 
 
 10. Chromatic Aberration. 
 
 r (1.) Definition. 
 (2.) By Double Refraction. 
 (3.) By Reflection. 
 (4.) The Polariscope. 
 
 11. Polarization. 
 
 1. Microscope. 
 
 2. Telescope. 
 
 3. Opera-glass. 
 
 4. Projecting Lantern. 
 
 5. Camera. 
 
 6. The Eye. 
 
 7. The Stereoscope. 
 
OPTICS, OR THE SCIENCE OF 
 LIGHT. 
 
 I. PRODUCTION AND TRANSMISSION OF LIGHT. 
 
 1. Definitions. A luminous body is one that emits 
 light. A medium is any substance through which 
 light passes. A transparent* body is one that ob- 
 structs light so little that we can see objects through 
 it. A translucent body is one that lets some light 
 pass, but not enough to render objects visible through 
 it. An opaque body is one that does not transmit 
 light. A ray of light is a single line of light. A 
 pencil or beam of light is a collection of rays, which 
 may be parallel, diverging, or converging ; it may be 
 traced in a dark room into which a sunbeam is ad- 
 mitted by the floating particles of dust which reflect 
 the light to the eye. 
 
 2. The Visual Angle is the angle formed at the 
 eye by rays coming from the extremities of an ob- 
 
 * The terms transparent and opaque are relative. No substance is per- 
 fectly transparent, or entirely opaque. Glass obstructs some light. Accord- 
 ing to Miller, 7 ft. of the clearest water will arrest one half the light which 
 falls upon it. While Young asserts that the beam of the setting sun, pass- 
 ing through 200 miles of air, loses of its fo'rce. On the other hand, gold, 
 beaten into leaf, becomes translucent, transmitting green light ; and scraped 
 horn is semi-transparent. 
 
192 OPTICS. 
 
 ject. The angle AOB. is the angle of vision sub- 
 tended by the object AB. The size of this angle 
 varies with the distance of the body. AB and A'B' 
 are of the same length, and yet the angle A OB' is 
 
 FIG. 137. 
 
 Variation of Visual Angle with Distance. 
 
 smaller than AOB, and hence A'B' will seem shorter 
 than AB. The distance and the apparent size of 
 objects are intimately connected, since by experience 
 we have learned to associate them. Knowing the 
 distance of an object, we immediately estimate its 
 size from the visual angle.* 
 
 3. Laws of Light. 1. Light passes off from a 
 luminous body equally in every direction. 2. Light 
 travels through a uniform medium in straight lines. 
 3. The intensity of light decreases as the square of 
 the distance increases. 
 
 4. The Velocity of Light has been determined in 
 various ways. The following was the first method: 
 The planet Jupiter has five moons. As these revolve 
 around the planet, they are eclipsed at regular inter- 
 vals. In Fig. 138, let J represent Jupiter, e one of 
 the moons, S the sun, and T and t different positions 
 of the earth in its orbit around the sun. When the 
 
 * We can vary the apparent size of any body at which we are looking by 
 increasing or diminishing this angle a principle that will be found of great 
 importance in the formation of images by mirrors and lenses. 
 
PRODUCTION AND TRANSMISSION OF LIGHT. 193 
 
 earth is at T, the eclipse occurs 16 min. and 36 sec. 
 earlier than at t. That interval of time is required 
 
 FIG. 138. 
 
 ilillllllllH 
 
 The Sun, Earth, and Jupiter. 
 
 for the light to travel across the earth's orbit, giving 
 a velocity of about 186,000 miles per second.* 
 
 5. Undulatory Theory of Light. To account for 
 the phenomena presented by light a substance is as- 
 sumed to exist which pervades all space. To this sub- 
 stance the name luminiferous ether, or, more shortly, 
 ether, has been given. In it the heavenly bodies are 
 immersed, and with it the pores of all substances are 
 filled, so that matter may be spoken of as being " ether- 
 soaked." The best attainable vacuum still contains 
 this substance. A luminous body sets in motion waves 
 of ether, which go off in every direction. They move 
 at the rate of 186,000 miles per second, and, break- 
 ing upon the eye, give the impression of light. In 
 the wave-motion of light, the vibrations are transverse 
 (crosswise) to the direction of propagation, f 
 
 * This rate is so great that for all distances on the earth it is instanta- 
 neous. A sunbeam would girt the globe quicker than we can wink, if its 
 path could be appropriately curved. 
 
 t Thus, if we suppose a star directly overhead and a ray of light com- 
 ing down to us, we should conceive that some of the particles which com- 
 
194 OPTICS. 
 
 II. REFLECTION OF LIGHT. 
 
 1. Definition. Light falling on a surface is divided 
 into two portions. One enters the body; the other 
 is reflected* according to the familiar law of Motion 
 and of Sound: The angle of incidence is equal to 
 that of reflection. 
 
 2. Action of Rough and Polished Surfaces. 
 
 When the surface is rough, the numerous little ele- 
 vations scatter the reflected rays in every direction, 
 forming diffused light. Such a body can be seen 
 from any point. When the surface is polished, the 
 rays are uniformly reflected in particular directions, 
 and may bring to us the images of other objects. 
 We thus see non-luminous objects by irregularly-re- 
 flected (diffused) light, and images of objects by reg- 
 ularly-reflected light, f 
 
 3. Mirrors. All highly-reflecting surfaces are mir- 
 rors. These are of three kinds -plane, concave, and 
 convex. The first has a flat surface ; the second, one 
 
 pose the waves are vibrating E. and W., others N. and S., and others to- 
 ward all other possible points of the compass in succession. 
 
 * The amount of light reflected varies with the angle at which light 
 falls. Thus, if we look at the images of objects in still water, we notice 
 that those near us are not so distinct as those on the opposite bank. The 
 rays from the latter striking the water more obliquely, are more perfectly 
 reflected to the eye. Fill any dark-colored pail with water tinted with 
 bluing or red ink. The color will be quite invisible to a spectator at a little 
 distance. Now insert in the water a plate. This will reflect the transmitted 
 light and reveal the hue of the water. 
 
 t The most perfectly polished substance, however, diffuses some light- 
 enough to enable us to trace its surface ; were it not so, we should not be 
 aware of its existence. The deception of a large plate-glass mirror is often 
 nearly complete ; but dust or vapor, increasing the irregular reflection, will 
 bring its surface to view. 
 
REFLECTION OF LIGHT. 195 
 
 like the inner surface of a hollow globe ; the third, 
 like part of its outer surface. The general principle 
 of mirrors is that the image is seen in the direction 
 of the reflected ray as it enters the eye. 
 
 (1.) PLANE MIRRORS. Rays of light retain their 
 relative direction after reflection from a plane sur- 
 face.* While standing before a plane mirror, one 
 sees his image erect and of the same size as him- 
 self. It is, however, reversed right and left. 
 
 Why the image is as far behind the mirror as the 
 object is in front. Let AB 
 
 FIG. 139. 
 
 be an arrow held in front 
 
 of the mirror MN. Rays 
 
 of light from the point A 
 
 striking upon the mirror 
 
 at Cj are reflected, and 
 
 enter the eye as if they 
 
 came from a. Rays from 
 
 B seem to come from 6. 
 
 Since the image is seen in the direction of the 
 
 reflected rays, it appears at a&, a point which can 
 
 easily be proved to be as far behind MN as the 
 
 arrow is in front of it. Such an image is called a 
 
 * The perpendiculars are not given in the figures of the book, as the 
 pupil at recitation should draw all the cuts on the blackboard, erect the perpendiculars, 
 and demonstrate the location of the reflected ray. It will aid in drawing the per- 
 pendicular to a convex or concave surface, to remember that it is a radius 
 of the sphere of which the mirror forms a part. A book held in various 
 positions before a looking-glass illustrates the action of plane mirrors. A 
 beam of light admitted into a dark room and reflected from a mirror will 
 show that the angles of incidence and reflection are in the same plane. 
 Many of the grotesque effects of concave and convex mirrors may be seen 
 on the inner and outer surfaces of a bright spoon, call-bell, or metal cup 
 (see "Mayer & Barnard's Light" for inexpensive experiments). 
 
196 OPTICS. 
 
 virtual one, as it has no real existence apart from 
 the observer's eye. 
 
 Why we can see several images of an- object in a 
 mirror. Metallic mirrors form only a single image. 
 FIG 140 ^ however, we look obliquely at 
 
 the image of a candle in a looking- 
 glass, we shall see several images, 
 the first feeble, the next bright, and 
 the others diminishing in intensity. 
 The ray from A is in part reflected 
 to the eye from the glass at &, and 
 gives rise to the image a ; the re- 
 mainder passes on and is reflected from the metallic 
 surface at c, and coming to the eye forms a second 
 image a'. The ray cd, when leaving the glass at 
 d, loses a part, which is reflected back to form a 
 third image. This ray in turn is divided to form 
 a fourth, and so on. 
 
 If two mirrors are arranged as in Fig. 141, three 
 images of a candle may be seen. (Let the pupil 
 trace the formation of each by the diagram of Fig. 
 142.) To vary the experiment, hold the mirrors to- 
 gether like the covers of a book placed on end, and 
 put the candle between them on the table, opening 
 and shutting the mirror-cover so as to vary the 
 angle ; or hold the mirrors parallel to each other 
 with the light between them. When the mirrors are 
 inclined at 90, three images are formed ; at 60, 
 five images; and at 45, seven images. As the 
 angle increases, the number diminishes. The images 
 are upon the circumference of a circle whose center 
 
REFLECTION OF LIGHT. 
 
 197 
 
 is on a line in which the reflecting surfaces would in- 
 tersect if produced. Where the mirrors are parallel the 
 
 FIG. 141. 
 
 PIG. 142. 
 
 Multiple Reflection. 
 
 images are in a straight line. They become dimmer 
 
 as they recede, light being 
 
 lost at each reflection. The 
 
 Kaleidoscope contains three 
 
 mirrors set at an angle of 
 
 60. Small bits of colored 
 
 glass at one end reflect to 
 
 the eye at the other multi- ^ 
 
 pie images which change in 7: / 
 
 varying patterns as the tube '*' x 
 
 is revolved. 
 
 Images seen in water are symmetrical, but in- 
 verted. The reason of this can be understood by 
 holding an object in front of a horizontal looking- 
 glass and noticing the angle at which the rays must 
 strike the surface in order to be reflected to the eye. 
 
198 
 
 OPTICS. 
 
 Fro. 148. 
 
 When the moon is high in the heavens, we see the 
 image in the water at only one spot, while the rest 
 
 of the surface ap- 
 pears dark. The 
 light falls upon all 
 parts, but each 
 ray is reflected 
 from only one 
 point at the proper 
 angle to reach the 
 eye. Each ob- 
 server sees the 
 image at a differ- 
 ent place. When 
 the surface of the 
 water is ruffled, a 
 tremulous line of 
 light is reflected 
 from the side of 
 each tiny wave 
 
 that is turned toward us. As every little billow 
 rises, it flashes a gleam of light to our eyes, and 
 then sinking, comes up beyond, to reflect another 
 ray. 
 
 (2.) A CONCAVE MIRROR tends to collect the rays 
 of light to a focus. In Fig. 144, O is the center of 
 curvature, i.e., the center of the hollow sphere of 
 which the mirror is a part ; V is the vertex, or mid- 
 dle of the mirror; F is the principal focus; it is 
 half-way between C and V. Any ray which passes 
 through G is an axis ; it is called the principal axis 
 
REFLECTION OF LIGHT. 199 
 
 if it pass also through V, otherwise it is a secondary 
 axis. All axial rays are reflected back upon their 
 own paths. All 
 
 rays parallel to FlG> 144< 
 
 the principal axis 
 cross at the prin- 
 cipal focus after 
 reflection, and 
 
 conversely all ^ > 
 
 rays which pass 
 
 through the prin- Parallel Rays Reflected to the Focus. 
 
 cipal focus will 
 
 be reflected parallel to the principal axis.* An image 
 is real if the rays after reflection cross before reach- 
 ing the eye ; it will appear to be at the crossing 
 point. Otherwise, the image is virtual. 
 
 Images formed ~by Concave Mirrors. In a dark 
 room place a candle (PQ, Fig. 145) in front of. a 
 concave mirror at some distance beyond its center 
 of curvature. A small inverted image of it will ap- 
 pear to be suspended in mid-air near its focus. It is 
 easy to determine the position of this image. From 
 P draw an axial ray through (7; it will be reflected 
 back on its own path, hence the image of P must be 
 
 * These statements are approximately true only for mirrors of slight 
 curvature, where the angle M CW, or angular aperture, does not exceed 8 or 
 10=. When greater, the rays reflected near the edge of the mirror meet the 
 principal axis VC, nearer the mirror than F. This is called the aberration 
 of the mirror. The reflected rays will then cross at points in a curved 
 surface called a caustic. A section of such a curve can be seen when the 
 light of a candle is reflected from the inside of a cup partly full of milk. 
 All of these phenomena can be proved mathematically to be necessary con- 
 sequences of the one law, that the angles of incidence and reflection are 
 equal. 
 
200 OPTICS. 
 
 on this line. From P draw also a ray, Pa, parallel 
 to the principal axis ; after reflection it will pass 
 through the focus, F, and cross the secondary axis 
 at P', which is hence the position of the image of 
 P. In like manner we may determine Q'. If a piece 
 of thin white paper or roughened glass be put at 
 P'Q'j the light will seem to come from it since the 
 rays cross here. 
 
 Bring the candle closer to the mirror. The image 
 will grow larger and move from F toward C. When 
 the candle reaches C, the image will fall upon it and 
 
 Fie. 145. 
 
 Inverted Real Image of a Candle. 
 
 just cover it. When it reaches P'Q', the image will 
 have receded to PQ, and in every case the ratio of 
 the lengths of candle and image will be the same as 
 the ratio of their distances from C. P and P' are 
 called conjugate points; for they are so related that 
 an object placed at one of them will be imaged at 
 the other. 
 
 If the candle be moved toward F, the image in- 
 creases in size and recedes from C. At J^the image 
 vanishes. Passing F the image again appears, but now 
 as if it were behind the mirror, larger than the object 
 
REFLECTION OF LIGHT. 
 
 201 
 
 FIG. 146. 
 
 and erect. Let the student trace the rays, as shown 
 in Fig. 146, and satisfy himself that they can never 
 cross after reflec- 
 tion. The image is 
 hence virtual; it 
 can not be caught 
 on a screen ; but 
 its apparent length 
 is as much greater 
 
 Virtual Image in a Concave Mirror. 
 
 than that of the 
 candle as its appar- 
 ent distance from O is greater. Moreover, it appears 
 erect, and not inverted like the real image of the 
 more distant candle. 
 
 (3.) CONVEX MIRRORS. Let the position of a can- 
 dle be varied in front of a convex mirror. It will be 
 found that the image is always virtual, erect, and 
 
 FIG. 147. 
 
 
 Virtual Image in a Convex Mirror. 
 
 smaller than the candle. Parallel rays are made to 
 diverge after reflection, as if they had come from a 
 
202 
 
 OPTICS. 
 
 point within the sphere, half-way between its surface 
 and center. The image of P is at the crossing point 
 of the axial ray from P and the backward prolonga- 
 tion of the ray from P which was parallel before re- 
 flection. The student can easily trace the rays and 
 determine the position of the image. 
 
 III. REFRACTION OF LIGHT. 
 
 1. Definition. When a ray of light passes ob- 
 liquely from one medium to another of different 
 density, it is refracted or bent out of its course. 
 Examples: A spoon in clear tea appears bent. An 
 oar dipping in still water seems broken at the point 
 where it enters the water.* Put a cent in a bowl. 
 Standing where you can not see the coin, let another 
 
 * In Pig. 148 is shown a stick sunk till the end is at the bottom of 
 
 the water. Bays of light from 
 this end are bent as they 
 emerge from the liquid and 
 reach the eye as if they had 
 come from a point consider- 
 ably higher. The entire bot- 
 tom, therefore, seems lifted up. 
 Hence, water is always deeper 
 than it appears. Look oblique- 
 ly into a pail of water, then 
 place your finger on the out- 
 side where the bottom seems 
 to be ; you will be surprised to 
 find the real bottom is several 
 inches below. Fill a glass dish 
 with water, and, darkening the 
 windows, let a sunbeam fall 
 
 upon the surface. The ray will bend as it enters. Dust scattered through 
 
 the air will make the beam distinct. 
 
 Apparent breaking of a Stick in Water. 
 
REFRACTION OF LIGHT. 
 
 203 
 
 FIG. 149. 
 
 person pour water into the vessel, when the coin will 
 be lifted into view. To understand the apparent 
 change of position, remember that the 
 object is seen in the direction of the re- 
 fracted ray as it enters the eye. Let L, 
 Fig. 149, be a body beneath the water. 
 A ray, LA, coming to the surface, is 
 bent away from the vertical, LK, and 
 strikes the eye as if it came from L. 
 The object will therefore apparently be elevated 
 above its true place. 
 
 2. Laws of Refraction. From any point, A, Fig. 
 150, let a beam of light, AS, pass through air and 
 meet a denser transparent medium at B, such as 
 
 water or glass. At this 
 point let a line, DE, be 
 drawn perpendicular to 
 the surface. Then some 
 of the light will be re- 
 flected at B, the angle 
 of reflection DBF being 
 equal to the angle of in- 
 cidence, DBA. A little 
 of it will be absorbed 
 and changed into heat. 
 The rest will be trans- 
 mitted, but its direction 
 changed to BC. This apparent breaking of the ray 
 is called refraction, and the angle EEC, which is less 
 than DBA, is the angle of refraction. If the source 
 of light were at C, its direction on emerging at B 
 
 Reflection and Refraction. 
 
204 
 
 OPTICS. 
 
 would be BA. The angle of refraction now is DBA 
 and the angle of incidence is EBC. Hence, 
 
 I. In passing into a denser medium, the ray is 
 bent toward the perpendicular. 
 
 II. In passing into a rarer medium, the ray is 
 bent from the perpendicular.* 
 
 ILLUSTRATIONS. Path of rays through a window- 
 glass. When a ray enters a win- 
 dow-glass, it is refracted toward 
 the perpendicular (1st law), and, 
 on leaving, is refracted equally 
 from the perpendicular (2d law). 
 The general direction of objects 
 is therefore unchanged. A poor 
 quality of glass produces distor- 
 tion by its unequal density and 
 uneven surface. 
 Path of rays through a prism. A ray of light, on 
 
 entering and on 
 
 leaving a glass *^ 
 
 prism, is refracted. 
 
 The inclination of 
 
 the sides causes 
 
 the ray to be bent 
 
 twice in the same 
 
 direction. The 
 
 candle, L, will 
 
 therefore appear to be in the direction of r. 
 
 Passage of a Ray through 
 Window-glass. 
 
 Passage of a Ray through a Prism, 
 
 * Both the incident and the refracted ray lie in the same plane as the 
 normal (perpendicular). The ratio hetween the nines of the angles of inci- 
 dence and refraction is termed the index of refraction. It varies with the 
 media. Example; From air to water it is | and from air to glass f. 
 
REFRACTION OF LIGHT. 
 
 205 
 
 3. Lenses. A lens is a transparent body, with at 
 least one curved surface. There are two general 
 classes of lenses, concave and convex* (See Mg. 153.) 
 
 FIG. 153. 
 
 Magnifying Lenses. 
 
 Diminishing Lenses. 
 
 (1.) THE BI-CONVEX LENS has two convex surfaces. 
 Its action on light is like that of a concave mirror. 
 A ray, X, striking perpendicularly, is not refracted. 
 
 FIG. 154. 
 
 Bi-convex Lens. 
 
 The parallel rays, M, L, etc., are refracted both on 
 entering and on leaving the lens, and are converged 
 at Fj the principal focus. \ If a luminous point be 
 placed at F, its rays will emerge parallel. 
 
 * Forms of lenses: M, double-convex; N, plano-convex; 0, meniscus 
 (crescent) ; P, double-concave ; Q, plano-concave ; E, concavo-convex. The 
 first three are styled magnifiers, and the second, diminishers. 
 
 t The convex lens is sometimes termed a burning-glass, from the fact that, 
 like a concave mirror, it collects and brings to a focus the rays of the sun ; 
 and combustible substances placed at this focus may be burned. Even 
 glass globes of water, such as are used for gold-fishes or in the windows of 
 drug stores, may fire adjacent objects. 
 
206 OPTICS. 
 
 Construction of Images. There is a point, called 
 the optical center of the lens, through which the 
 passing -ray does not change its general direction 
 after emergence. These are called axial rays. The 
 principal axis passes not only through the optical 
 center (C, Fig. 155), but also through the principal 
 focus, F. All rays parallel to it are so refracted as 
 to pass through F. Let a candle, PQ, be placed in 
 front of a bi-convex lens,* at a distance of ten or 
 
 p' 
 
 Formation of an Image with a Bi-convex Lens. 
 
 twelve feet. An axial ray, PC, continues its path 
 unchanged. A parallel ray, Pa, will after refraction 
 pass through F. Where this cuts the axial ray at P', 
 the image of P is found. In like manner Q' is found 
 as the point conjugate to Q. The image, P'Q', is 
 real, and may be caught on a screen. ,It is inverted, 
 and as much smaller than the object as its distance 
 from the optical center is less. 
 
 As the candle is made to approach the lens on 
 one side, the image recedes on' the other. When 
 
 * An ordinary magnifying hand-glass, such as is often used in looking 
 at photographs, or even a spectacle-lens used by an aged person in reading, 
 will be sufficient for these experiments. 
 
REFRACTION OF LIGHT. 
 
 207 
 
 Fie. 156. 
 
 brought nearly to F, the image on the other side 
 grows very large and distant. When it arrives with- 
 in the focal dis- 
 tance, FC, the 
 image suddenly 
 
 appears on the / - jc^l 
 
 same side with 
 the object, erect, 
 and as much 
 larger as its ap- 
 parent distance 
 from O is greater. This image is virtual. The 
 student can determine this by tracing the rays in 
 Fig. 156. 
 
 (2.) THE BI-CONCAVE LENS has two concave sur- 
 faces. Its action on light is like that of a convex 
 mirror. Thus, diverging rays from L (Fig. 157) are 
 
 FIG. 157. 
 
 Virtual Image with Convex Lens. 
 
 Bi-concave Lens. 
 
 rendered more diverging, and, to an eye which re- 
 ceives the rays MN, the candle would seem to be at 
 Z, where the image is seen.* 
 
 * Unscrew the eye-piece from an opera-glass; it serves well for experi- 
 ments with concave lenses. Unscrew the glass at the other end ; it serve* 
 for those with convex lenses. 
 
208 
 
 OPTICS. 
 
 The image formed ~by a concave lens, like that of 
 a convex mirror, is virtual, erect, and diminished in 
 
 FIG. 158. 
 
 
 Formation of an Image with a Concave Lens. 
 
 size (Fig. 158). Let the student determine this by 
 tracing the rays in Fig. 158, in which the arrow PQ 
 is the object and P'Q' the image.* 
 
 4. Spherical Aberration of Lenses. Parallel rays 
 falling on a lens whose surface is like that of a 
 sphere are not all refracted to a single focus. Those 
 which pass through marginal parts of the lens are 
 collected to a focus nearer than that to which the 
 central rays are collected. With a single lens, there- 
 fore, it is not easy to secure perfect distinctness of 
 image. 
 
 5. Total Reflection. In passing from a dense 
 into a rare medium, the angle of refraction is greater 
 than the angle of incidence. It can not exceed 90, 
 for then the ray would cease to emerge. The angle 
 
 * Remember that parallel rays, Pa and $>, become divergent after re- 
 fraction, as if they had come from a focus, F, on the same side of the lens. 
 The images of P and Q must hence be found on the backward prolongations 
 of these emergent rays. Axial rays are drawn from P and Q to C. 
 
REFRACTION OF LIGHT. 
 
 209 
 
 Flo. 159. 
 
 of incidence for which the emergent ray would make 
 an angle of 90 with the per- 
 pendicular is called the critical 
 angle. The light is then totally 
 reflected in the dense medium 
 as if its surface were the most 
 perfect of mirrors. When we 
 look obliquely into a pond, we 
 can not see the bottom, because 
 the rays of light from below 
 are reflected downward at the 
 surface of the wate^r. Hold a 
 glass of water above the level 
 of the eye, and the upper part 
 will gleam like burnished silver.* 
 Thus the internal surface of a 
 transparent body becomes a 
 
 mirror Total Internal Reflection. 
 
 6. Mirage. Over the heated deserts of Arabia 
 and Africa the traveler sometimes sees a shimmer- 
 ing expanse, as if a quiet lake were in the distance, 
 in which the scattered trees are mirrored upside 
 down. The layer of air close to the uniformly heated 
 sand is less dense than the cooler air above. A ray 
 coming obliquely downward from a tree-top may be 
 so bent from its first direction by passing through 
 
 * Place a bright spoon in the glass and notice its image reflected from 
 the surface of the water. Turn the spoon about in the glass and, changing 
 the angle of observation, notice the effect. The real handle may apparently 
 be attached to the image in the water. The spoon will soon be covered 
 with bubbles of air shining, like pearls, from total reflection. This shows 
 also the presence of air in water and the adhesion of gases to solids. 
 
210 
 
 OPTICS. 
 
 these different media as to be sent obliquely upward 
 to the eye. The low warm layer of air acts like a 
 totally reflecting mirror, and inverted images are 
 dimly seen amid the bright light along the horizon. 
 
 FIG. 160. 
 
 In Fig. 160, rays of light from a clump of trees are 
 refracted more and more until finally they are bent 
 upward from a layer at a, and enter the eye of the 
 Arab as if they came from the surface of a quiet 
 lake. 
 
 IV. COMPOSITION OF LIGHT. 
 
 1. The Prismatic Spectrum. When a sunbeam 
 is received through a narrow slit and transmitted 
 through a prism, properly placed, the ray is not only 
 bent from its course, but is also spread out into a 
 band of rainbow colors the solar spectrum. This 
 includes a multitude of tints grading imperceptibly 
 
COMPOSITION OF LIGHT. 
 
 211 
 
 from one to another. The most prominent are violet, 
 indigo, blue, green, yellow, orange, red* If we re- 
 ceive the spectrum on a concave mirror or pass it 
 through a convex lens appropriately adjusted in 
 position, these colors may be recombined so as to 
 form a white band. We therefore conclude that 
 
 FIG. 161. 
 
 The Prismatic Spectrum. 
 
 white light is made up of these many tints. Be- 
 cause each has its own separate index of refraction 
 (see p. 204, foot-note) when passing through the 
 same prism, this refracts them unequally. The de- 
 viation of the violet is the greatest, and that of the 
 red is the least, for the visible rays of the spectrum. 
 
 * Notice that the initial letters spell the mnemonic word, 
 
212 OPTICS. 
 
 2. Solar Energy. What we receive from the Sun 
 is called SOLAE ENERGY. It reaches us in tiny waves, 
 the longest of which are so minute that 8,000 of 
 them in succession would be required to cover an 
 inch. The shortest that have been measured are 
 about a tenth as long, or -50 ibr of an inch. The 
 longer ones are manifested largely as heat ; some of 
 the intermediate ones as light, and the shortest as 
 chemical energy. All these waves come mixed to- 
 gether in the sunbeam. The prism changes their 
 direction, and arranges them in the order of their 
 wave-lengths. 
 
 3. Properties of the Spectrum. Only a small pr^rt 
 of the solar spectrum is perceptible to the eye. Be- 
 yond the violet all is dark, but by employing a pho- 
 tographic plate this invisible part of the spectrum 
 may be photographed. By means of Langley's bo- 
 lometer (page 347), an instrument sensitive to varia- 
 tions of temperature, we may explore the region 
 beyond the red, and show that the spectrum continues 
 also in this direction. Solar energy appears thus to 
 have been separated by the prism into parts having 
 different properties Part producing the sensation of 
 light, another part lying beyond the red manifesting 
 itself mainly as heat, and still another part beyond 
 the violet distinguished by its chemical effects. This 
 difference in properties is, however, only apparent. 
 There is physically no distinction between the rays 
 occupying different parts of the spectrum except a 
 difference of wave-length. A ray belonging to any 
 part of the visible spectrum shows all three properties. 
 
COMPOSITION OF LIGHT. 213 
 
 It affects the eye with, the sensation of color, the 
 photographic plate in producing chemical change, 
 and the bolometer in heating the filament of platinum. 
 
 4. Interruptions in the Spectrum. When the 
 spectrum of the sun is carefully examined, it is found 
 that there are numerous breaks in both the visible 
 and invisible parts. Numerous black lines, parallel 
 to the slit that transmits the light, may be detected 
 in the visible part. The more prominent of these 
 have been named. Thus, the A, B, and O lines are 
 in the red ; the D line in the yellow ; the E line in the 
 green ; the F line in the blue ; the Q- and H lines in 
 the violet. The interruptions in the invisible portion 
 are far broader, becoming bands rather than narrow 
 lines. 
 
 5. The Spectroscope is an instrument used for the 
 production and examination of spectra. It consists of 
 one or mere prisms for producing the spectrum, and a 
 telescope for examining it. From the source of light, 
 Q-j Fig. 162, the rays pass through an adjustable slit, 
 and are made parallel by the lens in the tube, _B, before 
 passing through the prism, P. The spectrum is seen 
 through the telescope A. The tube, (7, has at one 
 end a scale on glass through which passes the light 
 from a candle or coal-gas jet at F. This is reflected 
 from the surface of the prism into the telescope A t 
 where an image of the scale is seen alongside of the 
 spectrum. Each part of the spectrum can thus be 
 distinguished by its own scale number. Instead 
 of a single prism, often a train of prisms is used, 
 
214 
 
 OPTICS. 
 
 thus widening the spectrum and diminishing its 
 brightness.* 
 
 FIG. 162. 
 
 
 The Spectroscope. 
 
 6. Three Kinds of Spectra. If in the spectro- 
 scope we examine the light of a glowing thin gas 
 or vapor, its spectrum is seen to consist of one or 
 more bright lines only. Thus burning sodium gives 
 
 * On the uses of the spectroscope, examine " New Astronomy," p. 258, 
 and " Popular Chemistry," p. 147. In the former, opposite p. 258, is a colored 
 illustration of the spectra. The dark lines which cross the solar spectrum 
 are known as Fraunhofer''s lines, being so named in honor of the physicist who 
 first carefully studied and mapped them. The spectroscope affords an un- 
 rivaled mode of analysis. No chemical test is so delicate. Strike together 
 two books near the light at the slit of the spectroscope, and the dust blown 
 into the flame will contain enough sodium (the basis of common salt) to 
 cause the yellow D lines its test to flash out distinctly. A very effective 
 spectroscope may be contrived thus : Cut a slit not over ^ inch wide and 
 2 inches long in a piece of tin-foil, and gum it on a pane of glass. Hold 
 this before a flame and look at it through a prism. 
 
COMPOSITION OF LIGHT. 215 
 
 a pair of brilliant yellow lines close together ; zinc 
 vapor, a number of lines among which the blue are 
 very prominent ; and strontium, a number among 
 which the red are conspicuous. Each element, if 
 made gaseous, can be thus recognized by its spec- 
 trum. 
 
 If the light from a glowing solid be examined, its 
 spectrum is found to be continuous, giving all the 
 colors without interruption. White-hot lime, or the 
 particles of carbon in a common candle-flame, furnish 
 a continuous spectrum. 
 
 The bright lines given by a- glowing gas may be 
 made to broaden into bands, and these finally to be- 
 come joined into a nearly continuous spectrum by 
 subjecting the gas to very great pressure, and thus 
 making it very dense. There is no sharp distinction 
 between line spectra and continuous spectra. 
 
 The interrupted spectrum is that given by the 
 sun and stars. It may be produced to a limited ex- 
 tent by interposing a glowing vapor, like that of 
 sodium, between the spectroscope and a white-hot 
 solid, like lime. It is believed that the body of the 
 sun and of each of the stars is made up of very 
 dense glowing matter, which is surrounded by less 
 hot vapors. These absorb some of the light from 
 within, and thus produce the interruptions observed 
 in the spectra of the sun and stars. Moreover, it has 
 been proved that each gas or vapor absorbs the same 
 waves as those given out by itself in glowing. By 
 comparing the bright lines of a known gas or vapor 
 with the dark lines in the sun or star spectrum, it 
 
216 OPTICS. 
 
 becomes possible to determine whether this vapor 
 exists in the atmosphere of the sun or star. 
 
 7. Color. If a piece of pure red paper is put 
 against the successive parts of the spectrum on a 
 screen, it will look red only when in the red part, 
 but dark gray or black in the other parts. It reflects 
 red light and absorbs the other tints. Color is anal- 
 ogous to pitch, violet corresponding to the high and 
 red to the low sounds in music. Intensity of color, 
 as of sound, depends on the amplitude of the vibra- 
 tions. When a body absorbs all the colors of the 
 spectrum except blue, but reflects that to the eye, 
 we call it a blue body ; when it absorbs all but green, 
 we call it a green body.* Red glass has the power 
 of absorbing all except the red rays, which it trans- 
 mits. When a substance reflects all the colors to the 
 eye, it seems to us white. If it absorbs all the colors, 
 it is black. Thus color is not an inherent property 
 of objects.f In darkness all things are colorless. 
 
 8. Complementary Colors. Two colors, which by 
 their mixture produce white light, are termed com- 
 
 * Some eyes are blind to certain colors, as some ears are deaf to certain 
 sounds. " Color-blindness " generally exists as to red. Such a person can 
 not by the color distinguish ripe cherries from green ones. Doubtless rail- 
 way accidents have occurred through this inability to apprehend signals. 
 Dr. Mitchell mentions a naval officer who chose a blue coat and red waist- 
 coat, believing them of the same color ; a tailor who mended a black silk 
 waistcoat with a piece of crimson ; and another who put a red collar on a 
 blue coat. Dalton could see in the solar spectrum only two colors, blue and 
 yellow, and having once dropped a piece of red sealing-wax in the grass, he 
 could not distinguish it. 
 
 t Moisten a swab with alcohol saturated with common salt. On ignit- 
 ing this in a dark room, every object will take on a curious ghastly yellow 
 hue from the burning sodium. The gay colors of flowers will instantly be 
 quenched. 
 
COMPOSITION OF LIGHT. 
 
 217 
 
 FIG. 163. 
 
 Complementary Colors. 
 
 plementary to each other. Thus, if we sift the red 
 rays out of a beam of light and bring the remainder 
 to a focus, a bluish-green image 
 will be formed.* In Fig. 163 
 the colors opposite each other 
 are complementary. Place a red 
 and a blue ribbon side by side. 
 The former will take on a yel- 
 lowish and the latter a greenish 
 tint. Lay a piece of tissue pa- 
 per upon black letters printed 
 on brightly colored paper. The 
 dark letters will appear of a color complementary to 
 that of the background.! 
 
 9. The Rainbow is formed by the refraction and 
 reflection of the sunbeam in drops of falling water. 
 The white light is thus decomposed into its simple 
 colors. The inner arch is termed the primary bow ; 
 the outer or fainter arch, the secondary. 
 
 PRIMARY Bow. A ray of light, S", enters, and is 
 bent downward at the top of a falling drop, passes 
 to the opposite side, is there reflected, then passing 
 out of the lower side, is bent upward. By the refrac- 
 
 * Certain substances are able to split a ray of light into two colors, 
 and are said to be dichroic. Gold-leaf reflects the yellow, transmits the 
 green, and absorbs the rest. 
 
 t A color is heightened when placed near its complement. A red apple 
 is the brighter for the contrast of the green leaf. Observe a white cloud 
 through a bit of red glass with one eye and through green glass with the 
 other eye. After some moments, transfer both eyes to the red glass, open- 
 ing and closing them alternately. The strengthening of the red color in the 
 eye, fatigued by its complementary green, is very striking. In examining 
 ribbons of the same color, the eye becomes wearied and unable to detect 
 the shade, because of the mingling of the complementary hue. 
 
218 OPTICS. 
 
 tion the ray of white light is decomposed, so that 
 when it emerges it is spread out fan-like, as in the 
 solar spectrum. Suppose that the eye of a spectator 
 is in a proper position to receive the red ray, he can 
 not receive any other color from the same drop, be- 
 cause the red is bent upward the least, and all the 
 others will pass directly over his head. He sees the 
 violet in a drop below. Intermediate drops furnish 
 the other colors of the spectrum. 
 
 FIG. 164. 
 
 The Rainbow. 
 
 SECOISTDARY Bow. A ray of light, S, strikes the 
 bottom of a drop, v, is refracted upward, passes to 
 the opposite side, where it is twice reflected, and 
 thence passes out at the upper side of the drop. 
 The violet ray being most refracted, is bent down to 
 the eye of the spectator. Another drop, r, refracting 
 another ray of light, is in the right position to send 
 the red ray to the eye. 
 
 WHY THE Bow is CIRCULAR. When the red ray of 
 
COMPOSITION OF LIGHT. 219 
 
 the primary bow leaves the drop, it forms an angle 
 with the sun's ray, S"r, of about 42, and the violet 
 forms with it one of 40. These angles are constant. 
 Let ab be a straight line drawn from the sun through 
 the observer's eye. If produced, it would pass through 
 the center of the circle of which the rainbow is an 
 arc. This line is termed the visual axis. It is paral- 
 lel to the rays of the sun ; and when it is also parallel 
 to the horizon, the rainbow is a semicircle. Suppose 
 the line EV in the primary bow to be revolved 
 around Eb, keeping the angle ~bE V unchanged ; the 
 point V would describe an arc of a circle on the 
 sky, and every drop over which it passed would be 
 at the proper angle to send a violet ray to the eye 
 at E. Imagine the same with the drop r. We can 
 thus see (a) the bow must be circular ; (&) when the 
 sun is high in the heavens, the whole bow sinks 
 below the horizon; (c) the lower the sun the larger 
 is the visible circumference ; and (d) on lofty mount- 
 ains a perfect circle may sometimes be seen.* 
 
 1C. Chromatic Aberration of Lenses. Since in 
 passing through any medium, such as a lens, the 
 violet rays are bent farther away from their first 
 direction than the red rays, they will be brought to 
 a focus nearer than that of the red rays. An image 
 on a screen produced with a single lens is therefore 
 fringed with a reddish or a bluish fringe according 
 
 * Halos, coronas, sun-dogs, circles about the moon, and the tinting at 
 sunrise and sunset, are produced by the refraction and reflection of the 
 sun's rays by the clouds. The phenomenon known as the " sun's drawing 
 water," consists of the long shadows of broken clouds. Twilight and kin- 
 dred topics are treated in Astronomy. 
 
220 OPTICS. 
 
 to the position of the screen. By combining two 
 lenses properly, one made of crown glass and the 
 other of flint glass, it is possible to correct much of 
 this coloring, which is called chromatic aberration, 
 and at the same time much of the spherical aberra- 
 tion.* 
 
 11. Interference of Light (Newton's Rings). Let 
 the convex side of a plano-convex lens be pressed 
 down upon a plane of glass. The two surfaces will 
 FIG 165 apparently touch at the center. 
 
 If different circles be described 
 Newton's around this point, at all parts of 
 
 Rings * each circle the surfaces will be 
 
 the same distance apart, and the larger the circle the 
 greater the distance. Now let a beam of red light fall 
 upon the flat surface. A black spot is seen at the cen- 
 ter ; around this a circle of red light, then a dark ring, 
 then another circle of red light, and so alternating 
 to the circumference. The distances between the 
 surfaces of the glass, where the successive dark rings 
 appear, are proportional to the numbers 0, 2, 4 . . . . , 
 and the bright circles to 1, 3, 5 ..... This fact 
 suggests the cause. There are two sets of waves, 
 one reflected from the upper surface of the plane 
 glass, and the other from the lower surface of the 
 
 * The crown-glass lens must be bi-convex and the flint-glass lens plano- 
 concave or meniscus. Flint glass gives a spectrum nearly twice as long as 
 crown glass. The two lenses oppose each other in their action on light. 
 They may be so adjusted that each tends almost completely to reverse the 
 spectrum that the other would produce, and yet the excess of deviation 
 produced by the crown glass may still be enough to bring the rays of this 
 nearly white light to a focus. 
 
COMPOSITION OF LIGHT. 221 
 
 convex glass. These alternately interfere, producing 
 darkness, and combine, making an intenser color.* 
 To determine the length of a wave of red light, we 
 have only to measure the distance between the two 
 glasses at the first ring. 
 
 When beams of light of the various colors are 
 used corresponding circles are obtained, having dif- 
 ferent diameters ; red light gives the largest, and 
 violet the smallest. We hence conclude that red 
 waves are the longest, and violet the shortest. The 
 minuteness of these waves passes comprehension. 
 About 40,000 red waves, or 60,000 violet ones, are 
 comprised within a single inch. Knowing the veloc- 
 ity of light, we can calculate how many of these 
 tiny waves reach our eyes each second. When we 
 look at a violet object, 757 million million of ether- 
 waves break on the retina every moment! 
 
 12. Polarization of Light. (1.) DEFINITION. If we 
 could look at the end of a ray of light coming to- 
 
 * The play of colors in mother-of-pearl is due to the interference of 
 light in its thin overlapping plates. In a similar manner the plumage of 
 certain birds reflects changeable hues. A metallic surface ruled with fine 
 parallel lines not more than 5-^5 of an inch apart, gleams with brilliant 
 colors. Thin cracks in plates of glass or quartz, mica when two layers are 
 slightly separated, even the scum floating in stagnant water, breaks up the 
 white light of the sunbeam and reflects the varying tints of the rainbow. 
 The rich coloring of a soap-bubble is caused by the interference of the rays 
 reflected from the upper and lower surfaces of the bubble. DIFFRACTION is 
 interference produced by a beam of light passing along the edge of an 
 opaque body or through a small opening, or reflected by a surface ruled 
 with fine lines. Examples ; Place the blades of two knives closely together 
 and hold them up to the sky ; waving lines of interference will shade the 
 open space. Look at the sky through the meshes of a veil, or at a lamp- 
 light through a bird-feather or a fine slit in a card, and delicate colors like 
 those of the prism will appear. 
 
222 
 
 OPTICS. 
 
 FIG. 166. 
 
 FIG. 167. 
 
 FIG. 168. 
 
 ward us, as we can at the end of a rod, we should 
 see the molecules of ether vibrating across the direc- 
 tion of the ray in all possible planes, as 
 shown in Fig. 166. There are certain 
 conditions under which reflected or re- 
 fracted light may be made to vibrate in 
 but a single plane. It is then called 
 polarized light. 
 
 The crystal tourmaline has this power upon trans- 
 mitted light. If two thin plates of this be cut parallel 
 to the axis of the crystal and light be passed perpen- 
 dicularly through them, when one is placed parallel 
 to the other, as in Fig. 167, some of it will be ab- 
 sorbed, but what passes through 
 vibrates only in a plane the 
 same as that of the axis. This 
 is proved by crossing them, as 
 in Fig. 168; at once the light 
 is quenched. What passed Tourmalines 
 through the first plate had been Paralleh 
 polarized, and was stopped by the second plate when 
 crossed. If they be placed with axes oblique to each 
 other, part of the polarized light is transmitted and 
 part quenched. 
 
 (2.) DOUBLE REFRACTION. In tourmaline and many 
 other crystals the ether is unequally elastic in two 
 directions at right angles to each other. The light 
 is hence divided into two parts which pass through 
 with unequal velocities. If transmitted across the 
 axis of the crystal, these parts are separated so that 
 two beams become perceptible. Iceland spar shows 
 
COMPOSITION OF LIGHT. 223 
 
 this remarkably well. An object viewed through it 
 appears double. If the crystal be placed over a dot 
 and turned around, two dots will be seen; one ap- 
 pears a little nearer than the other and revolves 
 around it, or a word will appear double if viewed in 
 like manner. (Fig. 169.) FIG. 109. 
 
 If now a plate of tour- 
 maline be put between 
 the eye and the rotating 
 crystal of spar, the dots 
 will alternately disappear. 
 This shows that the two 
 beams were polarized at Double Re fraction - 
 
 right angles to each other. One of them is called 
 the ordinary and the other the extraordinary ray. 
 Tourmaline is a doubly refracting crystal in which 
 the ordinary ray is absorbed unless the plate be ex- 
 ceedingly thin. 
 
 (3.) POLARIZATION BY REFLECTION. When light falls 
 upon a surface of glass at such an angle that the 
 reflected and refracted beams are at right angles to 
 each other, each of these is polarized, just as in pass- 
 ing through a doubly-refracting crystal. This special 
 polarizing angle of incidence for glass is about 56. 
 Many other substances polarize the light reflected at 
 the proper angle from them.* 
 
 (4.) THE POLARISCOPE. The best polarizer is a 
 crystal of Iceland spar specially arranged so as to 
 
 * If a tourmaline is rotated before the eye while looking obliquely at 
 the surface of a varnished table, or leather-seated chair, the reflected light 
 will be found to be polarized. 
 
224 OPTICS. 
 
 transmit the extraordinary ray and quench the ordi- 
 nary ray. It is called a Nicol's prism. Whatever is 
 used for examining the light after it has been polar- 
 ized is called an analyzer. The Mcol's prism makes 
 the best analyzer also. An instrument that includes 
 both polarizer and analyzer is called a polariscope. 
 A glass plate fixed at the proper angle makes an 
 excellent polarizer, and a small Nicol's prism, or piece 
 of tourmaline, for analyzer is enough for many beau- 
 tiful experiments. Exquisite displays of complement- 
 ary colors, due to interference of polarized beams in 
 transmission, nfay be seen by examining thin pieces 
 of crystallized gypsum, mica, horn, strained glass, 
 etc., between polarizer and analyzer.* Polarized light 
 affords a delicate means of examining the molecular 
 structure of a body. 
 
 * A simple polariscope is shown in Fig. 170. Upon a wooden frame a 
 plate of glass, P, blackened on the under side, is fixed so that light falling 
 on it at the polarizing angle shall be reflected through the tube W. This 
 contains a small Nicol's prism, n, for analyzer, and a lens, /, through which 
 an object, , may be examined with polarized light. The student who 
 
 FIG. 170. 
 
 Polariscope. 
 
 makes one will find it a source of fascination and continued delight. He 
 will find useful information very clearly expressed in the " Scientific Amer- 
 ican Supplement" for Nov. 20, 1886, p. 9072; also, a full and admirable 
 explanation of these beautiful experiments in "Light," by Lewis Wright, 
 published by Macmillan & Co., London. 
 
OPTICAL INSTRUMENTS. 
 
 225 
 
 V. OPTICAL INSTRUMENTS. 
 
 1. Microscopes (to see small things) are of two 
 kinds, simple and compound. The former consists of 
 one or more convex lenses through which the object 
 
 FIG. 171. 
 
 The Microscope. 
 
 is seen directly; the latter contains a simple magni- 
 fier for viewing the image of an object produced by 
 a second lens. Fig. 171 represents a compound mi- 
 
226 OPTICS. 
 
 croscope. At M is a mirror which reflects the rays 
 of light through the object a. The object-lens (ob- 
 jective), o, forms, in the tube above, a magnified, in- 
 verted image of the object. The eye-lens, (ocular), 
 magnifies this image. The magnifying power of the 
 instrument is nearly equal to the product of that of 
 the two lenses. If a microscope increases the appar- 
 ent diameter of an object 100 times, it is said to 
 have a power of 100 diameters, the surface being 
 magnified 100 2 = 10,000 times. The eye-piece may 
 be only a single lens, and is really a simple micro- 
 scope. The object-lens often consists of several lenses, 
 and each one of a combination of convex crown glass 
 and concave flint glass (p. 219) to prevent aberration. 
 
 2. Telescopes (to see afar off) are of two kinds, 
 reflecting and refracting. The former contains a 
 large metallic mirror (speculum) which reflects the 
 rays of light to a focus. The observer stands at the 
 side and examines the image with an eye-piece.* 
 
 FIG. 172. 
 
 Formation of Image in the Telescope. 
 
 The Refracting Telescope contains an object-lens, 
 0, which forms an inverted image, ab. This is viewed 
 through the eye-piece, 0, which produces a magnified 
 
 * The largest reflecting telescope is that of Lord Eosse (see Frontispiece 
 to " Astronomy "). Its speculum is 6 ft. in diameter and gathers about 
 120,000 times as much light as would ordinarily enter the eye. 
 
OPTICAL INSTRUMENTS. 
 
 227 
 
 image, cd, of the first image, ab. The image cd is 
 as much larger than ab as the focal distance of the 
 object-glass exceeds that of the eye-glass. The larger 
 
 FIG. 173. 
 
 Cambridge Equatorial. 
 
 the object-lens the more light is collected with which 
 to view the image. The magnifying power is due 
 to the eye-piece.* The apparent inversion of the ob- 
 
 * The use of the telescope depends upon (1st) its light-collecting and 
 (2d) its magnifying power. Thus Herschel, illustrating the former point, 
 says that once he told the time of night from a clock on a steeple invisible 
 on account of the darkness. It is noticeable that while in the compound 
 microscope the image is as much larger than the object as the image is 
 farther than the object from the object-glass, in the telescope the image is 
 as much smaller than the object as it is nearer than the object to the ob- 
 ject-glass ; while in both cases the image is examined with a magnifier. If 
 
228 
 
 OPTICS. 
 
 ject is of no importance for astronomical purposes. 
 In terrestrial observations additional lenses are used 
 to erect the image. 
 
 3. The Opera-glass contains an object-glass, 0, 
 and an eye-piece, o. The latter is a double-concave 
 lens ; this increases the visual angle by diverging 
 
 FIG. 174. 
 
 Formation of Image in Opera glass. 
 
 the rays of light, which would otherwise come to a 
 focus beyond the eye -piece. An erect and magni- 
 fied image is seen at ab. 
 
 4. The Projecting Lantern consists of a system 
 of lenses attached to a dark box, within which is 
 a powerful source of illumination such as the elec- 
 tric light or lime light. Sometimes an oil-lamp is 
 used. From the white-hot source the light is con- 
 verged by the condensing lens, C, Fig. 175, so as to 
 send through the projecting lens, P, as much of it as 
 possible. A picture on glass is placed in front of the 
 
 a power of 1,000 "be used in looking at the sun, we shall evidently see the 
 sun as if it were only 93,000 miles away, or less than one half the distance 
 of the moon. The same power used upon the moon would bring that body 
 apparently to within 240 miles of us. 
 
 The National Observatory telescope at Washington has an object-glass 
 26 inches in diameter, and of excellent denning power. The Lick telescope, 
 erected in 1887 upon Mt. Hamilton, in California, has an object-glass just 
 one yard in diameter. Its light-collecting power is estimated to be about 
 30,000 times that of the unaided eye. 
 
OPTICAL INSTRUMENTS. 
 
 229 
 
 condenser, and is thus strongly illuminated. An 
 image of it, greatly enlarged, is formed by the pro- 
 
 FIG. 175. 
 
 Projecting Lantern for the Lime Light. 
 
 jecting lens and focalized on a distant white screen 
 in a dark room. Dissolving views are produced by 
 
 Projecting Lantern for the Oil Light. 
 
 using two lanterns together. While one view is on 
 the screen, another is projected upon it. The light 
 
230 
 
 OPTICS. 
 
 PIG. 177. 
 
 is then cut off from the first lantern so as to leave 
 only the second view. Fig. 176 is an outline of one 
 form of oil-lantern. The reflector, M, helps to illu- 
 minate the transparent picture, ab, in front of the 
 condenser. 
 
 5. The Camera, used by photographers, contains 
 a double-convex lens, L, which throws an inverted 
 
 image of the ob- 
 ject upon a re- 
 movable ground- 
 glass screen, S. 
 When the focus 
 has been obtain- 
 ed, the screen is 
 removed and a 
 slide, containing 
 a sensitive film, 
 is inserted in its 
 place. (" Chemis- 
 try," p 171.) 
 
 Photographer's Camera. 
 
 6. The Eye is 
 
 a unique optical 
 instrument resembling a camera. The outer mem- 
 brane is termed the sclerotic coat, S (Fig. 178). It 
 is tough, white, opaque, and firm. A little portion in 
 front, called the cornea, c, is more convex and per- 
 fectly transparent. The middle or choroid coat, (7, is 
 soft and delicate, like velvet. It lines the inner part 
 of the eye and is covered with a black pigment. Over 
 it the optic nerve, which enters at the rear, expands 
 
OPTICAL INSTKUMENTS. 231 
 
 in a net-work of delicate fibers termed the retina, 
 the seat of vision. Back of the cornea is a colored 
 curtain, hi, the iris (rainbow), in which is a round 
 hole called the pupil. The crystalline lens, o, is a 
 double-convex lens, composed of concentric layers 
 somewhat like an onion, weighing about four grains 
 and transparent as glass. Between the cornea and 
 the crystalline lens is a limpid fluid termed the 
 aqueous humor ; while the vitreous humor, a trans- 
 parent, jelly-like liquid, fills the space back of the 
 crystalline lens. 
 
 Pia. 178. 
 
 Vertical Section of the Eye. 
 
 Let AB represent an object in front of the eye. 
 Rays of light are first refracted by the cornea and 
 aqueous humor, next by the crystalline lens, and last 
 by the vitreous humor, forming on the retina an 
 image, ab* which is real, inverted, and smaller than 
 the object. To render vision distinct, the rays must 
 
 * The diameter of the eye is less than an inch ; yet, as we look over an 
 extended landscape, every feature, with all its variety of shade and color, 
 is repeated in miniature on the retina. Millions upon millions of ether 
 waves, converging from every direction, break on that tiny beach, while we, 
 oblivious to the marvelous nature of the act, think only of the beauty of 
 the revelation. Yet in it the physicist sees a new illustration of the sim- 
 plicity and perfection of the laws and methods of the Divine Workman, 
 and a continued reminder of His forethought and skill. 
 
232 OPTICS. 
 
 be accurately focused on the retina. If we gaze 
 steadily at an object near by, and at the same time 
 regard a distant object in the same direction, we find 
 our vision of this blurred. If now we gaze at the 
 more distant object, our vision of the nearer one be- 
 comes blurred. The eye thus has the power of adapt- 
 ing itself to the varying distances of objects. This 
 is done by a change in the convexity of the front 
 surface of the crystalline lens under the action of 
 the ciliary muscle which surrounds it at its edge. 
 When clear vision can not be had of distant objects, 
 the person is near-sighted. When the ciliary muscle 
 is strained to produce clear vision of objects less 
 than ten or twelve inches distant, the person, if 
 young, is over-sighted. In the first case, the distance 
 of the retina from the crystalline lens is too great 
 to permit of distinct f ocalization ; in the second case, 
 this distance is too small. The remedy for near- 
 sightedness is to wear concave glasses, selected by a 
 competent oculist. Rays from distant objects are 
 thus made to diverge before entering the eye, as if 
 they had come from very near objects. For over- 
 sightedness, the remedy is properly selected convex 
 glasses.* As old age approaches, the crystalline lens 
 
 * There are other defects for which the aid of the oculist should be 
 sought. If glasses are needed, they should never be selected except after 
 examination by a thoroughly competent person. If not properly adapted, 
 they may do much more harm than good. No eye is optically perfect, and 
 but few are free from defects that may be detected on examination. That 
 glasses are more used now than during the previous generations is due not 
 so much to increase of habits injurious to vision as to the better knowledge 
 of the eye and the better opportunities for every person to find out his own 
 defects. 
 
OPTICAL INSTRUMENTS. 233 
 
 becomes less elastic so that tbe eye loses the power 
 of accommodation to near objects. Convex glasses 
 become necessary for reading, while the vision of 
 distant objects may remain perfect. 
 
 The retina retains an impression for a brief time 
 after the object has been removed, usually a fraction 
 of a second, which varies according to the bright- 
 ness.* This explains why a lighted coal, rapidly 
 moved in the dark, appears as a line of light. Many 
 of the most brilliant effects from fire-works depend 
 on this property of the retina. The Zoetrope is an 
 instrument by which a succession of pictures of the 
 same object in different phases of motion are made 
 to pass rapidly before the eye. The persistence of 
 the successive sensations causes an apparent blending, 
 so that the illusion is that of an object actually in 
 motion. 
 
 7. Binocular Vision. In looking with both eyes 
 at an object that is not very distant, we obtain a 
 much better idea of its position and form, or its 
 " depth in space," than when a single eye is employed. 
 The two retinal images differ slightly because the 
 two eyes are different in direction from the object, 
 and hence to a slight extent we see around the ob- 
 ject on two sides. The illusion of depth in space is 
 well brought out by means of Fig. 179. The tunnel, 
 A, appears as if viewed by the left eye alone ; B, as if 
 
 * When one is riding slowly on the cars and looking at the landscape 
 between the upright fence-boards, he catches only glimpses of the view ; 
 but when mooing rapidly, these snatches will combine to form a perfect landscape, 
 which has, however, a grayish tint, owing to the decreased amount of light 
 reflected to the eye. 
 
234 OPTICS. 
 
 by the right eye alone. Bring the page close up to 
 the face, so that one picture is immediately in front 
 of each eye. The two images at once seem combined 
 into a single blurred image. Now withdraw the page 
 a few inches;* the haziness gives place to distinct- 
 
 FIG. 179. 
 
 A B 
 
 A Stereograph which may be Viewed without a Stereoscope. 
 
 ness and the tunnel appears startlingly deep, as if it 
 were a hole through the book. While still gazing 
 into its depths two more tunnels may be indirectly 
 seen, one on each side of it ; but the illusion of depth 
 in them is far less clear. Each is seen by but a sin- 
 gle eye, while the middle one is a binocular perception. 
 The Stereoscope is an instrument intended to aid 
 in attaining binocular vision of a pair of properly 
 prepared pictures, which together compose the stereo- 
 graph. With a little practice like that just described, 
 any one may become independent of the stereoscope. 
 
 * In performing this experiment, it is very important to avoid crossing 
 the eyes. Perfect relaxation of the muscles of the eyeballs will make it 
 very easy. Imagine yourself to be looking thr&ugk the page at the opening 
 of a distant tunnel, and keep the muscles relaxed. 
 
 For further discussion of the Stereoscope, consult an article on this 
 subject in the "Popular Science Monthly" for May and June, 1882. 
 
PRACTICAL QUESTIONS. 235 
 
 PRACTICAL QUESTIONS. 
 
 1. Why is the secondary bow fainter than the primary? "Why are the 
 colors reversed? 
 
 2. Why can we not see around the corner of a house, or through a bent 
 tube? 
 
 3. What color would a painter use if he wished to represent an open- 
 ing into a dark cellar? 
 
 4. Is white a color? Is black? 
 
 5. By holding an object nearer a light, will it increase or diminish the 
 size of the shadow? 
 
 6. What must be the size of a glass in order to reflect a full-length 
 image of a person ? Ans. Half the person's height. 
 
 7. Where should we look for a rainbow in the morning? 
 
 8. Can two spectators see the same bow? 
 
 9. Why, when the drops of water are falling through the air, does the 
 rainbow appear stationary? 
 
 10. Why can a cat see in the night better than a human being? 
 
 11. Why can not an owl see distinctly in daylight? 
 
 12. Why are we blinded when we pass quickly from a dark into a 
 lighted room? 
 
 13. If the light of the sun upon a distant planet is T o of that which 
 we receive, how does its distance from the sun compare with ours? 
 
 14. If, when I sit six feet from a candle, I receive a certain amount of 
 light, how much shall I diminish it if I move back six feet farther? 
 
 15. Why do drops of rain, in falling, appear like liquid threads? 
 
 16. Why does a towel turn darker when wet? 
 
 17. Does color exist in the object, or in the mind of the observer? 
 
 18. Why is lather opaque, while air and a solution of soap are each 
 transparent ? 
 
 19. Why does it whiten molasses candy to " pull it " ? 
 
 20. Why does plastering become lighter in color as it dries? 
 
 21. Why does the photographer use a lamp with a chimney of red 
 glass in the " dark room " ? 
 
 22. Is the common division of colors into '"cold" and "warm" veri- 
 fied in philosophy? 
 
 23. Why is the image on the camera, Mg. 177, inverted? 
 
 24. Wliy is the second image seen in a mirror, Pig. 140, brighter than 
 thtf first? 
 
 25. Why does a blow on the head make one "see stars"? Ans. The 
 blow excites the optic nerve, and so produces the sensation of light. 
 
 26. What is the principle of the kaleidoscope ? (If you can not discover 
 this, consult Deschanel's " Natural Philosophy," pp. 886-891.) 
 
 27. Which can be seen at the greater distance gray or yellow? 
 
 28. When a star is near the horizon, does it seem higher or lower than 
 its true place? 
 
236 OPTICS. 
 
 29. Why can we not see a rainbow at midday? 
 
 30. What conclusion do we draw from the fact that moonlight shows 
 the same dark lines in the spectrum as sunlight? 
 
 31. Why does the bottom of a boat seen under clear water appear 
 natter than it really is? 
 
 32. Of what shape does a round body appear in water? 
 
 33. Why is rough glass translucent while smooth glass is transparent ? 
 
 34. Why can a carpenter, by looking along the edge of a board, tell 
 whether it is straight ? 
 
 35. Why can we not see out of the window after we have lighted the 
 lamp in the evening? 
 
 36. Why does a ground-glass globe soften the light? 
 
 37. Why can we not see through ground-glass or painted windows? 
 
 38. Why does the moon's surface appear flat? 
 
 39. Why can we see farther with a telescope than with the naked eye? 
 
 40. Why is not snow transparent, like ice? 
 
 41. Are there rays in the sunbeam which we can not perceive with the 
 eye? 
 
 42. Why, when we press the finger on one eyeball, do we see objects 
 double ? 
 
 43. Why does a distant light, in the night, seem like a star? 
 
 44. Why does a bright light, in the night, seem so much nearer than 
 it is? 
 
 45. What color predominates in artificial lights? Ans. Yellow. 
 
 46. Why are we not sensible of darkness when we wink? 
 
 47. Under what condition do the eyes of a portrait seem to follow a 
 spectator to all parts of a room? 
 
 48. Why do the two parallel tracks of a railroad appear to approach in 
 the distance? 
 
 49. Why does a fog apparently magnify objects? 
 
 50. If you sit where you can not see another person's image, why can 
 not that person see yours? 
 
 51. Why can we see the multiple images in a mirror better if we look 
 Into it very obliquely? 
 
 52. Why is an image seen in water inverted? 
 
 53. Why is the sun's light fainter at sunset than at midday? 
 
 54. Why can we not see the fence-posts when we are riding rapidly? 
 
 55. Ought a red flower to be placed in a bouquet close to an orange 
 one? A pink or blue with a violet one? 
 
 56. Why are the clouds white while the clear sky is blue? 
 
 57. Why does skim-mflk look blue and new milk white? 
 
 58. Why is not the image of the sun in water at midday so bright 
 as near sunset? 
 
 59. Why is the rainbow always opposite the sun? 
 
 60. Hold a card with its edge close in front of your eye and look at a 
 distant candle flame in a dark room. You will probably perceive either a 
 reddish or a bluish fringe on one side. Explain. 
 
SUMMARY. 237 
 
 SUMMARY. 
 
 Light comes from the sun and other self-luminous bodies. It 
 is transmitted by means of vibrations in ether, in accordance 
 with the laws of wave-motion. It is radiated equally in all 
 directions, travels in straight lines, decreases as the square of the 
 distance increases, and is propagated 186,000 miles per second. 
 Light falling upon a body may be absorbed, transmitted, or re- 
 flected. If the surface be rough, the irregularly-reflected light 
 enables us to see the body ; if it be smooth and highly polished, 
 the rays are reflected so as to form an image of the original 
 object. Surfaces producing such images are termed mirrors 
 plane, concave, or convex. The image is seen in the direction 
 from which the reflected ray enters the eye, and, in a plane 
 mirror, as far behind the mirror as the object is in front. Mul- 
 tiple images are produced by repeated reflections, as in the 
 kaleidoscope. A concave mirror, as generally used, collects the 
 rays, and serves to produce either a magnified erect virtual 
 image or a magnified or diminished inverted real image of an 
 object. A convex mirror scatters the rays, and diminishes the 
 apparent size of an object. 
 
 When a ray enters or leaves a transparent body obliquely, it is 
 refracted ; if passing into a rarer medium, it is bent away from the 
 perpendicular erected at the point of incidence ; if into a denser 
 medium, it is bent toward this perpendicular. A lens is a 
 transparent body with one or more curved surfaces, which are 
 usually spherical, so as to refract the light either to a focus, or 
 as if it had come from a focus. There are two classes convex 
 and concave. The former lens, as generally used, tends, like a 
 concave mirror, to collect the rays of light ; the latter, like a 
 convex mirror, causes the rays of light to diverge. Mirage is 
 an optical delusion caused by refraction of light in passing 
 through air composed of strata of unequal density. Owing 
 to the varying refrangibility of the different waves of the 
 sunbeam, a prism can disperse them into a colored band called 
 the solar spectrum. The spectrum shows white light to consist 
 of many tints, and that the solar energy may produce lumi- 
 nous, heating, or chemical effects according to the nature of the 
 
238 OPTICS. 
 
 body receiving it. By means of the spectroscope we can ex- 
 amine the spectrum of a flame, and find whether its light is 
 due to the incandescence of a gas or to the glowing of solid 
 particles disseminated through it. Each substance 'in the gaseous 
 state gives a spectrum with its peculiar lines of color. A gas ab- 
 sorbs the same rays that it is capable of emitting ; if. therefore, a 
 burning gas or vapor is interposed between the eye and a glow- 
 ing solid, the spectrum of the solid is interrupted by dark lines 
 due to absorption by the vapor. A delicate mode of analysis is 
 thus furnished, whereby the elements even of the distant stars 
 can be detected. The rainbow is formed by the refraction 
 and reflection of the sunbeam in rain-drops. Light, when 
 reflected by. or transmitted through bodies, is so modified, 
 chiefly by absorption, as to produce the varied phenomena of 
 color. Each color has its own wave-length, which is less than 
 -55~Gtt inch. Different systems of light-waves, as of sound- 
 waves, may be combined. But if any two coincide with simi- 
 lar phases they will strengthen each other ; and if with opposite 
 phases, weaken each other. Interference of light, as thus pro- 
 duced, causes the play of colors in the soap-bubble, mother-of- 
 pearl, etc. Polarized light is that in which the molecular vibra- 
 tions are made in the same plane. Many of the most beauti- 
 ful color effects may be produced by polarization. 
 
 The principal optical instruments, including the eye, are 
 adapted to produce and examine the image formed by a lens. 
 In the projecting lantern and solar microscope, the image is 
 thrown on a screen in a dark room. In the refracting telescope 
 and the microscope, the image is formed in a tube by a lens at 
 one end and looked at from behind by a lens at the other end. 
 In the ey , which is a small camera-obscura, the image is formed 
 on the retina, whence the sensation is carried by the optic nerve 
 to the brain. The retinal sensation continues for a short time 
 after the impression is made. Advantage is taken of this fact 
 in the use of the zoetrope, by which a succession of images is 
 made to appear in motion. Vision with two eyes is superior 
 to that with a single eye, because we are thus enabled to form 
 better ideas of depth in space, and hence of the distance and 
 form of a body. The stereoscope is an instrument for studying 
 the peculiarities of binocular vision. 
 
HISTORICAL SKETCH. 239 
 
 HISTORICAL SKETCH. 
 
 THE ancients knew that light is propagated in straight lines. 
 They discovered the laws of reflection, and one of the ancient 
 fables is that Archimedes set fire to the Roman ships off Syra- 
 cuse by means of concave mirrors. Euclid and Plato, however, 
 thought that the ray of light proceeds from the eye to the ob- 
 ject, an error that was long uncorrected. One thousand years did 
 not bring much advancement in this department of knowledge. 
 The Arabian philosopher, Alhazen, who lived in the eleventh 
 century, discovered the apparent displacement of a body seen 
 in water. The law of intensity of light was established by 
 Kepler, and the first researches on the comparison of intensity 
 from different sources were made by Maurolycus, Huygens, 
 and Francis Marie. About 1608, the telescope was invented by 
 the Dutch.* Jansen, Metius, and Lippersheim each claimed the 
 honor, and the legend is that the discovery grew out of some 
 children at play, accidentally arranging two watch-glasses so as 
 apparently to magnify an object. In fact, however, the action 
 of the convex lens was already known, the compound microscope 
 had been invented by Jansen twenty years previously, and the 
 simple microscope was known to the ancient Chaldeans. In 
 1621, Snell discovered the law of refraction. By its aid Des- 
 cartes explained the rainbow. Half a century of waiting, and 
 Newton published his investigations in the decomposition of 
 light. He, however, believed in what is known as the "corpus- 
 cular theory." This holds that light consists of minute particles 
 of matter radiated in straight lines from a- luminous object, the 
 ray being endued with alternate "fits" of easy reflection and 
 easy transmission. In 1676, Roemer, by observing Jupiter's 
 moons (p. 192), found out the velocity of light, which up to 
 that time had been considered instantaneous. In 1665, Gri- 
 
 * "In 1609, the government of Venice made a considerable present to 
 Signor Galileo, of Florence, Professor of Mathematics at Padua, and in- 
 creased his annual stipend by 100 crowns, because, with diligent study, he 
 found out a rule and measure by which it is possible to see places 30 miles 
 distant as if they were near, and, on the other hand, near objects to appear 
 much larger than they are before our eyes." From an old paper in the Library 
 of Heidelberg University. 
 
240 OPTICS. 
 
 maldi discovered the existence of fringes of light and shade 
 when a beam is received through a narrow slit. Huygens soon 
 afterward advanced the undulatory theory, which was originated 
 independently about the same time by Hooke. This involved 
 them in vigorous disputes with Newton, without the definite 
 establishment of their theory. In 1802, Thomas Young revived 
 the undulatory theory, accounting by it for all the phenomena 
 of interference then known. In 1817, Fresnel extended the 
 researches of Young, and Newton's corpuscular theory began to 
 fall into discredit. The elementary phenomena of polarization 
 were discovered by Malus in 1808, and this subject was after- 
 ward studied with great thoroughness by Fresnel, Arago, Biot, 
 and Brewster. 
 
VIII. 
 
 ON HEAT. 
 
 " THE combustion of a single pound of coal, supposing it to take place 
 in a minute, is equivalent to the work of three hundred horses ; and the 
 force set free in the burning of 300 Ibs. of coal is equivalent to the work 
 of an able-bodied man for a life-time." 
 
ANALYSIS OF HEAT. 
 
 PRODUCTION 
 HEAT. 
 
 OF 
 
 II. 
 
 PHYSICAL EFFECTS 
 OF HEAT. 
 
 III. 
 
 COMMUNICATION OF 
 HEAT. 
 
 THE STEAM -EN- 
 GINE. 
 
 . V, METEOROLOGY. 
 
 1. Definitions. 
 
 2. Relation between the Forms of 
 
 Radiant Energy. 
 
 3. Theory of Heat. 
 
 4. Sources of Heat. 
 
 5. Mechanical Equivalent of Heat, 
 
 1. Expansion. 
 
 2. Temperature. 
 
 3. The Heat Unit. 
 
 4. Liquefaction. 
 
 5. Vaporization. 
 
 6. Evaporation. 
 
 7. Spheroidal State. 
 
 8. Specific Heat. 
 
 1. Conduction. 
 
 2. Convection. 
 ] 3. Radiation. 
 
 I 4. Absorption and Reflection. 
 
 {1. General Principle. 
 2. The Q-overnor. 
 3. The High-pressure Engine. 
 
 1. General Principles. 
 
 2. Dew. 
 
 3. Fogs. 
 
 4. Clouds. 
 
 5. Rain. 
 
 6. Winds. 
 
 7. Ocean Currents, 
 
 ^ 8. Adaptations of Water. 
 
HEAT. 
 
 I. PRODUCTION OF HEAT. 
 
 1. Definitions. Radiant Energy is the name of 
 what we receive from the sun, stars, and other 
 heated bodies. It may be manifested as light, as 
 temperature, as chemism, or in all of these ways at 
 the same time. 
 
 2. Relation between the Forms of Radiant En- 
 ergy. Thrust a cold iron into the fire. It is at first 
 dark, but soon becomes luminous, like the glowing 
 coals. Raise the temperature of a platinum wire. 
 We quickly feel the radiation of obscure heat-rays. 
 As the metal begins to glow, our eyes detect a red 
 color, then orange combined with it, and so on 
 through the spectrum. At last all the colors are 
 emitted, and the metal is dazzling white. Like light, 
 heat may be reflected, refracted, 'and polarized. It 
 radiates in straight lines in every direction, and de- 
 creases in intensity as the square of the distance 
 increases. It moves with the same velocity as light. 
 
 It is believed that each of the forms of radiant 
 energy is merely the manifestation of wave-motion 
 at a special rate.* The longer and slower waves of 
 
 * According to Tyndall, 95 per cent, of the rays from a candle are 
 
244 HEAT. 
 
 ether falling upon the nerves of touch produce the sen- 
 sation of heat. The more rapid affect the optic nerve 
 and produce the sensation of light. The shortest 
 are especially active in producing chemical changes. 
 
 3. Theory of Heat. Heat is motion. The inole- 
 
 invisible or heat-rays. These may be brought to a focus and bodies fired 
 in the darkness. Each of the five classes of nerves seems to be adapted to 
 transmit vibrations of its own kind, while it is insensible to the others. 
 Thus, if the rate of oscillation be less than that of red, or more than that 
 of violet, the optic nerve is uninfluenced by the waves. We can not see 
 with our fingers, taste with our ears, or hear with our nose. Yet these are 
 organs of sensation and sensitive to their peculiar impressions." Suppose, 
 by a wild stretch of imagination, some mechanism that will make a rod 
 turn round one of its ends, quite slowly at first, but then faster and faster, 
 till it will revolve any number of times in a second ; which is, of course, 
 perfectly imaginable, though you could not find such a rod or put together 
 such a mechanism. Let the whirling go on in a dark room, and suppose a 
 man there knowing nothing of the rod ; how will he be affected by it ? So 
 long as it turns but a few times in the second, he will not be affected at 
 all unless he is near enough to receive a blow on the skin. But as soon as 
 it begins to spin from sixteen to twenty times a second, a deep growling 
 note will break in upon him through his ear ; and as the rate then grows 
 swifter, the tone will go on becoming less and less grave, and soon more 
 and more acute, till it will reach a pitch of shrillness hardly to be borne, 
 when the speed has to be counted by tens of thousands. At length, about 
 the stage of forty thousand revolutions a second, more or less, -the shrill- 
 ness will pass into stillness ; silence will again reign as at first, nor any 
 more be broken. The rod might now plunge on in mad fury for a long 
 time without making any difference to the man ; but let it suddenly come 
 to whirl some million times a second, and then through intervening space 
 faint rays of heat will begin to steal toward him, setting up a feeling of 
 warmth in his skin ; which again will grow more and more intense, as 
 now through tens and hundreds and thousands of millions the rate of 
 revolution is supposed to rise. Why not billions? The heat at first will 
 be only so much the greater. But, lo 1 about the stage of four hundred 
 billions there is more a dim red light becomes visible in the gloom; and 
 now, while the rate still mounts up, the heat in its turn dies away, till it 
 vanishes as the sound vanished ; but the red light will have passed for the 
 eye into a yellow, a green, a blue, and, last of all, a violet. And to the 
 violet, the revolutions being now about eight hundred billions a second, 
 there will succeed darkness night, as in the beginning. This darkness 
 too, like the stillness, will never more be broken. !Let the rod whirl on as 
 it may, its doings can not come within the ken of that man's senses." 
 
PRODUCTION OF HEAT. 245 
 
 cules of a solid are in constant vibration. When we 
 increase the rapidity of this oscillation, we heat the 
 body ; when we decrease it, we cool the body. The 
 vacant spaces between the molecules are filled with 
 ether. As the air moving among the limbs of a 
 tree sets its boughs in motion, and in turn may be 
 kept in motion by the waving of branches, so the 
 ether puts the molecules in vibration, or is thrown 
 into vibration by them. Example : Insert one end of 
 a poker in the fire. The particles immersed in the 
 flame are made to vibrate intensely ; the swinging 
 molecules strike their neighbors, and so on, continu- 
 ally, until the oscillation reaches the other end. If 
 we handle the poker, the motion is imparted to the 
 delicate nerves of touch ; they carry it to the brain, 
 and pain is felt. In popular language, " the iron is 
 hot," and we are burned. If, without touching it, 
 we hold our hand near the poker, the ether-waves 
 set in motion by the vibrating molecules of iron 
 strike against the hand, and produce a less intense 
 sensation of heat. In the former case, the fierce 
 motion is imparted directly ; in the latter, the ether 
 acts as a carrier to bring it to us. 
 
 4. The Sources of Heat are the sun, the stars, 
 and mechanical and chemical energy. 
 
 (1.) The molecules of the sun and stars are in 
 rapid vibration. These set in motion waves of ether, 
 which are propagated across the intervening space, 
 and meeting the earth, give up their motion to it. 
 (2.) Friction and percussion produce heat, the mo- 
 tion of a mass being changed into motion among 
 
246 HEAT. 
 
 molecules.* (3.) Chemical action is seen in fire. The 
 oxygen of the air has an affinity for the carbon and 
 hydrogen of the fuel. They combine, and chemical 
 energy is transformed into that of sensible heat. 
 
 5. Mechanical Equivalent of Heat (Joule's Law). 
 In these various changes of mechanical motion 
 into motion of molecules no energy is destroyed, 
 though some of it may be so transformed as to be- 
 come incapable of being made to do useful work. 
 If the energy transformed by the fall of a black- 
 smith's hammer on his anvil could be gathered up, 
 it would be sufficient to lift the hammer to the point 
 from which it fell. A pound-weight falling vertically 
 772 feet, will generate enough heat to raise the tem- 
 perature of 1 pound of water through 1 F.; con- 
 versely, this amount of heat is the equivalent of the 
 energy required to lift 1 pound mechanically to a 
 height of 772 feet. This important truth was first 
 demonstrated by Mr. Joule, of Manchester, England, 
 and we express it by saying 772 foot-pounds is the 
 mechanical equivalent of heat. Expressed in metric 
 measures, it is 424 kilogram-meters for 1 C. 
 
 * A horse hits his shoes against a stone and " strikes fire " ; little par- 
 ticles of the metal being torn off are heated by the shock, and some of the 
 energy is manifested also as light. A train of cars is stopped by the press- 
 ure of the brakes. In a dark night, we see the sparks flying from the 
 wheels, the motion of the train being converted into heat. A blacksmith 
 pounds a piece of iron until it glows. His strokes set the particles of metal 
 vibrating rapidly enough to send ether-waves of such swiftness as to affect 
 the eye of the observer. As a cannon-shot strikes an iron target, a shower 
 of sparks is scattered around. Were the earth instantly stopped, enough 
 heat would be produced to " raise a lead ball the size of our globe to 384,000 
 C." If it were to fall to the sun its impact would produce a thousand times 
 more heat than its burning. 
 
PHYSICAL EFFECTS OF HEAT. 247 
 
 II. PHYSICAL EFFECTS OF HEAT. 
 
 1. Expansion. If the molecules of a body have 
 an increase of energy imparted to them they swing, 
 like pendulums, through wider arcs. Each tends to 
 push against its neighbor, and the mass as a whole 
 grows larger. Hence the general law, "Heat ex- 
 pands and cold contracts," cold being merely a rela- 
 tive term implying the withdrawal of energy. The 
 ratio of the increase of volume to the original vol- 
 ume for a change of 1 in temperature is called the 
 Coefficient of Expansion. Generally this is greatest 
 for gases, less for liquids, and least for solids, each 
 particular substance having its own co- efficient. 
 The force of expansion is for many substances 
 -irresistible. A rise in temperature of 80 F. will 
 lengthen a bar of wrought-iron, 10 feet long, about 
 ^ of an inch ; and if its cross-section is one square 
 inch it will push in expanding with a force of about 
 2 5 tons. When the metal cools it will contract with 
 the same force.* 
 
 A familiar application of expansion is in the pen- 
 
 * A carriage-tire is put on when hot, in order that, when cooled, it may 
 bind the wheel together. Rivets used in fastening the plates of steam-boilers 
 are inserted red-hot." The ponderous iron tubes of the Britannia Bridge 
 writhe and twist, like a huge serpent, under the varying influence of the 
 solar heat. A span of the tube is depressed only a quarter of an inch by 
 the heaviest train of cars, while the sun lifts it 2 inches." The same may 
 be noticed on the great Brooklyn Bridge, more than a mile long, where an 
 allowance of nearly a yard has to be made for expansion with the change 
 of seasons. The Bunker-hill monument nods as it follows the sun in its 
 daily course. Tumblers of thick glass break on the sudden application of 
 heat, because the surface dilates before the heat has time to be conducted 
 to the interior. 
 
248 
 
 HEAT. 
 
 PICK 180. 
 
 dulum of a clock, which lengthens in summer and 
 shortens in winter. A clock, therefore, tends to lose 
 time in summer and gain in winter. To 
 regulate it we raise or lower the pendu- 
 lum bob. 
 
 The gridiron pendulum consists of 
 brass and steel rods, so connected that 
 the brass, h, Jc, will lengthen upward, 
 and the steel, a, &, c, d, downward, and 
 thus the center of oscillation remain 
 unchanged. The mercurial pendulum 
 contains a cup of mercury which ex- 
 pands upward, while the pendulum-rod 
 expands downward. 
 
 2. Temperature. When one body is 
 in a condition to communicate heat to 
 another, the first is said to have a 
 higher temperature than the second, or 
 to be warmer. We measure temperature 
 usually by noting its effect in producing 
 expansion. Within narrow limits we 
 may form a rough estimate of it by 
 the sensation of touch, but this is 
 
 very unreliable. 
 
 The thermometer is an instrument for measuring 
 
 temperature, usually by the expansion of mercury.* 
 
 * Take a glass tube terminating in a bulb, and heat the bulb to expel 
 the air. Then plunge the stem in colored water. As the bulb cools, the 
 water will rise and partly fill it. Heat the bulb again until the steam 
 pours out of the stem. On inserting it a second time, the water will fill 
 the bulb. Tn the manufacture of thermometers, it is customary to have a 
 cup blown at the upper end of the stem. This is filled with mercury, and 
 
 Gridiron Pendu- 
 lum. 
 
PHYSICAL EFFECTS OF HEAT. 
 
 249 
 
 FIG. 181. 
 
 To graduate it, according to Fahrenheit's scale 
 each thermometer is put in melting ice, and the 
 point to which the mercury sinks is 
 marked 32, Freezing-point.* It is then 
 placed in a steam-bath, and the point to 
 which the mercury rises (when the baro- 
 metric column stands at 30 inches) is 
 marked 212, Boiling-point. The space 
 between these two points is divided into 
 180 equal parts. In the Centigrade scale 
 (O.) the freezing-point is 0, and the boil- 
 ing-point 100. In Reaumur's scale (J2.), 
 the boiling-point is 80.f The thermom- 
 eter does not measure the quantity of 
 heat, but only its intensity. 
 
 F. c. R. 
 
 Thermometers. 
 
 3. The Heat Unit. For measuring 
 quantity of heat, the unit commonly em- 
 ployed in England and America is that 
 quantity which is required to raise the temperature 
 of one pound (avoirdupois) of water through one 
 degree (Fahrenheit) above the freezing-point. 
 
 the air, when expanded, bubbles out through it, while the metal trickles 
 down as the bulb cools. The mercury is thn highly heated, when the 
 tube is melted off and sealed at the end of the column of mercury. The 
 metal contracts on cooling, and leaves a vacuum above. 
 
 * The inventor placed zero 32 below the temperature of freezing 
 water, because he thought that to be absolute cold a point now. estimated 
 to be about 492 below the freezing-point on his scale. 
 
 t The following formulae will be of use in comparing the readings of 
 the different scales : 
 
 R.= | C. = | (F. - 32). . t , ,...,... . (1.) 
 
 C. = R. = | (F. - 32) ....... (2.) 
 
 F. = f C. + 32 = | R. + 32 (3.) 
 
 1 O. = 1.8 F. . , . . (4.) 
 
250 HEAT. 
 
 4. Liquefaction or Fusion. When heat is commu- 
 nicated to a solid body a point is finally reached 
 when the vibratory swing of its molecules is so great 
 that they are driven apart, each toward the limit of 
 the sphere of attraction of its neighbor, so that all 
 rigidity is lost.* The molecules then move freely 
 among themselves. The energy that is applied raises 
 the temperature of the body up to a fixed point called 
 its melting or fusing point, when liquefaction begins. 
 Additional energy then does the work of driving the 
 molecules apart without further rise of temperature, 
 until fusion is complete ; after which the liquid rises 
 still further in temperature. Energy that does thus 
 the work of changing the state of a body without at 
 the same time changing its temperature is often 
 called latent heat.\ If a pound of ice at 32 F. be 
 heated, it requires 142 heat units to melt it, and 
 180 more to raise its temperature then up to the 
 boiling-point. 
 
 Freezing. The converse of fusion is freezing. 
 Ice melts at 32 F., and in doing so. it absorbs en- 
 ergy. Water freezes at 32 F., and in doing so it 
 gives out the energy which had been keeping its 
 molecules apart. Thawing is thus a cooling process 
 and freezing is a warming process. Freezing mixt- 
 ures depend on this principle. In freezing ice- 
 cream, salt and pounded ice are put around the 
 
 * This is true only of bodies which are not broken into their chemical 
 constituents before the melting-point is reached. A large variety of sub- 
 stances, such as wood* bone, flesh, etc., become chemically changed instead 
 of melting. 
 
 t The term latent heat is gradually going out of use. 
 
PHYSICAL EFFECTS OF HEAT. 251 
 
 vessel that contains the cream. The strong attrac- 
 tion between salt and water causes the ice to melt 
 rapidly, and the solid salt becomes liquid by solu- 
 tion. This rapid thawing involves much absorption 
 of energy, which comes from the nearest objects 
 whose temperature is higher than that of the solu- 
 tion. The cream thus loses energy, its temperature 
 becoming reduced down to the freezing-point.* 
 
 5. Vaporization. When heat is applied to a 
 liquid the temperature rises until the boiling-point 
 is reached, when it stops and the liquid is changed 
 to vapor at that constant temperature. The vapor 
 is nearly free from solids dissolved in the liquid. 
 Example: Pure or distilled water is obtained by 
 heating water in a boiler, A, whence the steam 
 passes through the pipe, (7, and the worm within the 
 condenser, S, where it is condensed and drops into 
 the vessel, D. The pipe is coiled in a spiral form 
 within the condenser, and is hence termed the 
 worm. The condenser is kept full of cold water 
 from the tub at the left. By carefully regulating 
 the temperature, one liquid may be separated from 
 another by "fractional" distillation, advantage being 
 taken of the fact that each liquid has its own boil- 
 
 * That freezing is a warming process may be conclusively shown as fol- 
 lows : Gently melt some sodium sulphate (a cheap salt that may be ob- 
 tained from any apothecary) in a flask by heating it over a lamp flame. 
 Put it aside to cool slowly in a perfectly quiet place. After cooling it re- 
 mains liquid, but ready to freeze as soon as motion among its molecules is 
 started. Disturb it by putting a thermometer bulb into the liquid. At 
 once crystals are seen shooting out, and the mass is soon frozen hard. The 
 mercury in the thermometer meanwhile rises, and the warming may be 
 felt with the hand. 
 
252 
 
 HEAT. 
 
 ing-point, higher or lower than that of the liquid 
 with which it is mixed. 
 
 Boiling-point. When we heat water, the bubbles 
 which pass off first are the air dissolved in the 
 liquid ; next bubbles of steam form on the bottom 
 and sides of the vessel, and, rising a little distance, 
 
 FIG. 182. 
 
 A Still. 
 
 are condensed by the cold water. Collapsing, they 
 produce the sound known as "simmering." As the 
 temperature of the water rises, they ascend higher, 
 until they burst at the surface, and pass off into the 
 air. The violent agitation of the water thus pro- 
 duced is termed boiling.* Some substances vaporize 
 
 * The temperature of water can not be raised above the boiling-point, 
 unless the steam be confined. The extra energy is applied in expanding 
 the water into stea'm. This occupies 1,700 times the space, and is of the 
 same temperature as the water from which it is made. Nearly 1,000 units 
 
PHYSICAL EFFECTS OF HEAT. 253 
 
 at ordinary temperatures ; others only at the high- 
 est ; while the gases of the air are but the vapor of 
 substances which boil at exceedingly low tempera- 
 tures. The distinction between gases and vapors in 
 ordinary language is only relative. 
 
 The boiling-point of water depends on three cir- 
 cumstances: (1.) Purity of the water. A solid sub- 
 stance dissolved in water ordinarily elevates the 
 boiling-point. Thus salt water boils at a higher 
 temperature than pure water. The air dissolved in 
 water tends by its elastic force to separate the mole- 
 cules. If this be removed, the boiling-point may be 
 elevated to 275 F., when the water will be con- 
 verted into steam with explosive violence. 
 
 (2.) Nature of the vessel. Water will boil at a 
 lower temperature in iron than in glass. When the 
 surface of the glass is chemically clean, the boiling- 
 point is still higher. This seems to depend in some 
 degree on the strength of the adhesion between the 
 water and the containing vessel. 
 
 (3.) Pressure upon the surface raises the boiling- 
 point.* Water, therefore, boils at a lower tempera- 
 ture on a mountain than in a valley. The tempera- 
 ture of boiling water at Quito" is 194 F., and on 
 
 of heat for each pound of water are expended in this process, but are made 
 sensible again as temperature when the steam is condensed. Steam is in- 
 visible. This we can verify by examining it where it issues from the 
 spout of the tea-kettle. It soon condenses, however, into minute globules, 
 which become visible in white clouds. 
 
 * Pressure opposes the repellent heat-force, and so renders it easier for 
 cohesion to hold the particles together. In the interior of the earth there 
 may be masses of matter heated red or white hot and yet solid, more 
 rigid even than glass, in consequence of their melting-point being raised 
 so high by the tremendous pressure that they can not liquefy. TAIT. 
 
254 
 
 HEAT. 
 
 FIG. 183. 
 
 Mont Blanc, 183 F. The variation is so uniform 
 that the height of a place can thus be a'scertained; 
 
 an ascent of 596 feet pro- 
 ducing a difference of 1 F. 
 The influence of pressure 
 is well illustrated by the fol- 
 lowing experiment : Half fill 
 a strong glass flask with 
 water, and boil this until 
 all the air is expelled from 
 both the water and the 
 space above it. Now quickly 
 apply a tight stopper and 
 invert. The pressure of the 
 steam will stop ebullition. 
 A few drops of cold water 
 will condense the steam, and 
 boiling will recommence. 
 This will soon be checked, 
 but can be restored as before. The process may be 
 repeated until the water cools to the ordinary tem- 
 perature of the air, and even then the liquid inside 
 may be made to boil by rubbing the outside of the 
 flask with ice. The cushion of air which commonly 
 breaks the fall of water is removed, and if the cork 
 be air-tight, the water, when cold, will strike against 
 the flask with a sharp, metallic sound. 
 
 6. Evaporation is a slow formation of vapor, 
 which takes place at ordinary temperatures. Water 
 evaporates even at the freezing-point. Clothes dry 
 in the open air in the coldest weather. The wind 
 
 Boiling Water by Condensing its 
 Vapor. 
 
PHYSICAL EFFECTS OF HEAT. 255 
 
 quickens the process, because it drives away the 
 moist air near the clothes and supplies dry air. 
 Evaporation is also hastened by an increase of sur- 
 face and a gentle heat. 
 
 Vacuum pans are employed in condensing milk 
 and in the manufacture of sugar. They are so ar- 
 ranged that the air above the liquid in the vessel 
 may be exhausted, and then the evaporation takes 
 place rapidly, and at so low a temperature that 
 burning is avoided. 
 
 The cooling effect of -evaporation is due to the ab- 
 sorption of energy required to drive the molecules 
 apart beyond their spheres of mutual attraction. 
 Water may be frozen under the receiver of an air- 
 pump by placing a small watch-glass containing it 
 over a pan of strong sulphuric acid, which absorbs 
 the vapor as fast as it is formed in the vacuum. 
 The cooling due to rapid evaporation of a part is 
 sufficient to freeze the rest. By strong pressure and 
 cooling, carbonic acid is easily liquefied. Allowing a 
 jet of this liquid to escape, the evaporation of a part 
 of it causes the rest to freeze into a snowy powder 
 which may be pressed into a ball.* Nitrogen, oxy- 
 gen, and air, which is a mixture chiefly of these 
 
 * Mercury in contact with it is quickly solidified. On throwing the 
 frozen metal into a little water, the mercury instantly liquefies, but the 
 water turns to ice, the solid thus becoming a liquid and the liquid a solid 
 by the exchange of heat. A cold knife cuts through the mass of frozen 
 mercury as a hot knife would ordinarily through butter. The author, on 
 one occasion, saw Tyndall, during a course of lectures at the Eoyal Insti- 
 tution at London, when freezing a ladle of mercury in a red-hot crucible, 
 add some ether to hasten the evaporation. The liquid caught fire, but the 
 metal was drawn out from the glowing crucible, through the midst of the 
 flame, frozen into a solid mass. 
 
256 HEAT. 
 
 two gases, have been liquefied. Liquid air boils at 
 
 337 F. in a vacuum. Nitrogen has been obtained 
 in "snow-like crystals of remarkable size," and by 
 reducing the pressure on these a temperature of 
 
 373 F. was attained, the lowest recorded up to 
 the present date (1887). 
 
 7. Spheroidal State. If a few drops of water be 
 put in a hot, bright spoon, they will gather in a 
 globule, which will dart to and fro over the surface. 
 It rests on a cushion of steam, while the currents of 
 air drive it about. If the spoon cool, the water will 
 lose its spheroidal form, and coming into contact with 
 the metal, burst into steam with a slight explosion.* 
 
 8. Specific Heat. More energy is required to 
 raise the temperature of a pound of water through 
 one degree than for any other substance except the 
 gas hydrogen. The number of heat units (p. 249, 3) 
 required to raise one pound of a given substance one 
 degree F. is called its specific heat; thus for mer- 
 cury it is about -gV ; for iron, \ ; for air, nearly 
 ; for hydrogen, 3^. On this account the ocean 
 changes its temperature far less quickly than the 
 land, and sea-side cities are subject to less extremes 
 of temperature than those on the middle of a conti- 
 nent. On the elevated plateau region around the 
 Great Salt Lake the temperature during the year 
 varies from 115 F. to - 30 F. 
 
 * Drops of water spilled on a hot stove illustrate* the principle. By 
 moistening the finger, we can touch a hot flat-iron with impunity. The 
 water assumes this state, and thus protects the flesh from injury.- 
 Furnace-men can dip their moistened hands into molten iron. 
 
COMMUNICATION OF HEAT. 257 
 
 III. COMMUNICATION OF HEAT. 
 
 Heat tends to become diffused equally among 
 neighboring bodies.* There are three modes of dis- 
 tribution. 
 
 1. Conduction is the process of heating by the 
 passage of heat from molecule to molecule. Examr 
 pie : Hold one end of a poker in the fire, and the 
 other end soon becomes hot enough to burn the 
 hand. Of the ordinary metals, silver and copper 
 are the best conductors.! Wood is a poor con- 
 ductor, especially " across the grain." 
 
 Gases are the poorest conductors ; hence porous 
 bodies, as wool, fur, snow, charcoal, etc., which con- 
 tain large quantities of air, are excellent non-con- 
 ductors. Refrigerators and ice-houses have double 
 walls, filled between with charcoal, sawdust, or other 
 non-conducting substances. Air is so poor a con- 
 ductor that persons have gone into ovens that were 
 hot enough to cook meat, which they carried in and 
 laid on the metal shelves; yet, so long as they did 
 not themselves touch any good conductor, they ex- 
 perienced little inconvenience. 
 
 Liquids are also poor conductors. Example: 
 
 * If we touch an object colder than we are, it abstracts heat from us, 
 and we say " it feels cold " ; if a warmer body, it imparts heat to us, and 
 we say "it feels warm." Adjacent objects have, however, the same tem- 
 perature, though flannel sheets feel warm, and linen cold. These effects 
 depend upon the relative conducting power of different substances. Iron 
 feels colder than feathers because it robs us faster of our heat. 
 
 t Place a silver, a German-silver, and an iron spoon in a dish of hot 
 water. ^Notice how much sooner the handle of the silver spoon is heated 
 than the others. 
 
258 
 
 HEAT. 
 
 FIG. 184. 
 
 Hold the upper end of a test-tube of water in the 
 flame of a lamp. The water nearest the blaze will 
 boil without the heat being felt 
 by the hand. 
 
 2. Convection is the process 
 of heating ~by circulation. (1.) 
 CONVECTION OF LIQUIDS. Place a 
 little sawdust in a flask of water, 
 and apply heat. We shall soon 
 find that an ascending and a 
 descending current are estab- 
 lished. The water near the 
 lamp becoming heated, expands 
 and rises. The cold water above 
 
 Heating by Convection. ginks to take ^ placa 
 
 (2.) OF GASES. By testing with a lighted candle, 
 we shall find at the bottom of a door opening into 
 cold air, a current setting inward, and at the top, 
 one setting outward. The cold air in a room flows 
 to the stove along the floor, is heated, and then rises 
 to the ceiling. Heating by hot-air furnaces depends 
 upon the principle that warm air rises. 
 
 3. Radiation. Two bodies not in contact may yet 
 exchange heat across the intervening space. This 
 transfer is effected by means of the ether of space, and 
 is called radiation. A hot stove throws the ether sur- 
 rounding it into vibrations, which are transmitted ra- 
 dially outward. On being absorbed (stopped) by bodies 
 radiant energy is again converted into heat.* The 
 
 *The Radiometer is an instrument that, for a time, was supposed to 
 exhibit the actual mechanical force of the sunbeam. It consists of a tiny 
 
COMMUNICATION OF HEAT. 
 
 259 
 
 power of radiating heat and of absorbing radiant en- 
 ergy differs for different substances. In general, good 
 radiators are also good absorbers, and vice versa. The 
 space between the earth and the sun is not warmed by 
 the sunbeam, because the ether of space is incapable of 
 absorbing radiant energy. Dry air is so poor an ab- 
 sorber that meat can be cooked by radiation, while the 
 surrounding air remains at the freezing-point. Aque- 
 ous vapor is a comparatively good absorber, especially 
 for the longer waves. For this reason the moisture in 
 the atmosphere stops much of the energy of the sun- 
 beam passing through it, and converts it into heat. 
 The greater part of the solar energy absorbed by the 
 earth is again radiated forth into space, but in longer 
 waves. To these glass is especially Fra * 185 - 
 
 opaque.* Hence its use in the con- 
 struction of green-houses. At lofty 
 
 vane delicately pivoted in a glass globe from 
 which, the air is exhausted as fully as possible, 
 when the globe is hermetically sealed. The four 
 arms of the vane carry each a thin light disk of 
 mica or aluminium, covered with lamp-black on 
 one side and uncovered on the other. When day- 
 light falls upon it the little vane revolves rapidly. 
 The motion ceases as soon as the light is cut off. 
 When different gases are admitted into the globe, 
 the rate of rotation varies. It is now believed 
 that the unequal heating of the black and white 
 surfaces of the disks causes unequal reaction 
 of the molecules of air left in the vacuum. 
 Lamp-black being the best absorber and radiator, 
 receives the more forcible bombardment of flying 
 air molecules. 
 
 * In the course of Prof. Langley's experiments 
 upon Mount Whitney, water was boiled by expos- 
 ing it in a copper vessel covered by a pane of win- 
 dow-glass, to the direct rays of the sun. This 
 
 The liiidiometer. 
 
260 HEAT. 
 
 elevations the dry air allows the heat received by 
 the soil during the day to escape so rapidly that a 
 frost occurs before morning, though the heat of the 
 afternoon may be torrid. 
 
 4. Absorption and Reflection. A good reflector 
 is a poor absorber and also a poor radiator. Snow 
 is a good reflector, but a poor absorber or radiator. 
 Light colors often absorb solar heat less and re- 
 flect more than dark colors.* White is generally 
 considered the best reflector, and black the best 
 absorber and radiator. But the nature of the ma- 
 terial is of more importance than its tint. If on a 
 bright summer day three thermometers are exposed 
 to the sun, one held up in mid-air, another resting 
 on a bed of black silk, and the third on a bed of 
 white sand, it will be found in a short time that the 
 temperatures indicated will be very different. The 
 thermometer on the sand will have its bulb more 
 warmed than that on the bed of black silk ; and both 
 of these will be warmer than the one in mid-air. 
 
 shows that many of the heat-rays of the sunbeam are stricken down by the 
 air before reaching low levels, but may be utilized at high elevations. So, 
 Were the atmosphere removed the earth would receive far more heat and yet 
 be much colder than now, because there would be no beds of water vapor to 
 check the radiation back into space. See " American Journal of Science," 
 March, 1883. 
 
 * Experiments show that with artificial heat the molecular condition 
 of the surface varies radiation as well as reflection. In fact, white lead is 
 as good a radiator as lamp-black. On one side of a sheet of paper paste let- 
 ters of gold-leaf. Spread over the opposite side a thin coating of scarlet 
 iodide of mercury a salt which turns yellow on the application of heat. 
 Turn the scarlet side down. Hold over the paper a red-hot iron. The gold- 
 leaf will reflect the heat, but the paper spaces between the letters will 
 absorb it, and on turning the paper over, the gilt letters will be found 
 traced in scarlet on a yellow background. 
 
THE STEAM-ENGINE. 
 
 261 
 
 FIG. 186. 
 
 IV. THE STEAM-ENGINE. 
 
 WHEN steam rises from water at a temperature 
 of 212, it has an elastic force of nearly 15 Ibs. per 
 square inch. If the steam be 
 confined and the temperature 
 raised, the elastic force will be 
 rapidly increased. 
 
 1. The Steam-engine is a 
 machine for using the elastic 
 force of steam as a motive 
 power. There are two classes, 
 high-^pressure and low-pressure. 
 In the former, the steam, after 
 it has done its work, is forced 
 
 OUt into the air ; in the latter, Steam-chest and Cylinder of 
 
 it is condensed in a separate 
 
 chamber by a spray of cold water. As the steam 
 is condensed in the low-pressure engine, a vacuum 
 is formed behind the piston ; while the piston of the 
 high-pressure engine acts against the pressure of the 
 air. The elastic force of the steam must be 15 Ibs. 
 per square inch greater in the latter case. The 
 figure represents the piston and connecting pipes of 
 an engine. The steam from the boiler passes 
 through the pipe, a, into the steam-chest, &, as indi- 
 cated by the arrow. The sliding-valve worked by 
 the rod h lets the steam into the cylinder, alter- 
 nately above and below the piston, which is thus 
 made to play up and down by the expansive force. 
 This valve is so arranged that at the moment fresh 
 
262 HEAT. 
 
 steam, is let in on one side of the piston, the spent 
 steam on the other side is released into the outer 
 air, or into the condensing chamber. 
 
 2. The Governor is an apparatus for regulating 
 the supply of steam. AB is the axis around which 
 
 iw t ^ 16 keavy balls E and D revolve. 
 
 They are so connected by hinge- 
 joints that the ring at B may be 
 pulled down or lifted by them, 
 while that at C is fixed. When 
 trie machine is going too fast, the 
 balls fly out and thus pull down 
 the rod, K, which is in connection 
 with a valve that controls the pipe 
 
 The Governor. 
 
 supplying steam. A portion of the 
 steam is thus obstructed, and the revolution of the 
 balls becomes slower. This in turn makes them 
 descend, and in so doing they lift the rod, K. The 
 valve is thereby opened and more steam supplied, 
 whenever the speed of revolution becomes too small. 
 
 3. A High-pressure Engine is shown in Fig. 188. 
 A represents the cylinder ; B, the steam-chest at its 
 side, connected with it on the interior by the slid- 
 ing-valve already shown ; C, the throttle-valve in 
 the pipe through which steam is admitted from the 
 boiler ; Z>, the governor ; E, the band-wheel by 
 which the governor is driven ; F, the pump ; G, the 
 crank ; 7, the conductor attached to a, the cross- 
 head ; H, the eccentric rod (h in Fig. 186) which 
 works the sliding-valve in the steam-chest ; K, the 
 
264 HEAT. 
 
 governor-valve ; S, the shaft by which the power is 
 conveyed to the machinery. The cross-head, a, 
 slides to and fro in a groove, and is fastened to the 
 rod which works the piston in the cylinder A. The 
 expansive force of the steam is thus communicated 
 to a, thence to J, by which the crank is turned. 
 The heavy fly-wheel renders the motion uniform 
 (p. 23). 
 
 V. METEOROLOGY. 
 
 1. General Principles. (1.) The air always con- 
 tains moisture. The amount it can receive depends 
 upon the temperature ; warm air absorbing more, 
 and cold air less. At 100 F., a cubic foot of air 
 can hold nearly 2 grains of invisible water vapor ; 
 a reduction of 70 will cause nine tenths of that 
 quantity to be condensed into visible droplets. 
 When the air at any temperature contains all the 
 vapor it can hold in an invisible state, it is said to 
 be saturated ; any fall of temperature will then con- 
 dense a part of the vapor. 
 
 (2.) When air expands against pressure (i.e., doing 
 work in the expansion), its energy, being thus ex- 
 pended, ceases to be manifested as temperature. 
 The warm air from the earth ascending into the 
 upper regions, is thus rarefied and cooled. Its vapor 
 is then condensed into clouds, and often falls as 
 rain. Owing to this expansion of the atmosphere 
 and the greater radiation of heat in the dry air of 
 
METEOROLOGY. 265 
 
 the upper regions, there is a gradual diminution of 
 the temperature as the altitude increases, the mean 
 rate in the north temperate zone being about 1 for 
 300 feet. 
 
 2, Dew.* The grass at night, becoming cooled 
 by radiation, condenses the vapor of the adjacent air 
 upon its surface. Dew will gather most freely upon 
 the best radiators, as they will the soonest become 
 cool. Thus grass, leaves, etc., receive the largest 
 deposits. It will not form on windy nights, nor 
 when there are clouds in the sky to reflect the heat 
 radiated from the ground. In tropical regions the 
 nocturnal radiation on clear nights is often so great 
 as to render the formation of ice possible. In Bengal, 
 water is exposed for this purpose in shallow earthen 
 dishes resting on rice straw In parts of Chili, Arabia, 
 etc., by its abundance, dew feebly supplies the place 
 of rain. When the temperature of plants falls below 
 32, the vapor is frozen upon them directly, and is 
 called white, or hoar-frost. 
 
 3. Fogs are formed when the temperature of the 
 air falls below the dew-point, i. e., the temperature 
 at which dew is deposited for a given degree of hu- 
 midity. They are characteristic of low lands, rivers, 
 etc., where the air is saturated with moisture. 
 
 * Dew was anciently thought to possess wonderful properties. Baths 
 in this precious liquid were said greatly to conduce to beauty. It was col- 
 lected for this purpose, and for the use of the alchemists in their weird 
 experiments, by spreading fleeces of wool upon the ground. Laurens, a 
 philosopher of the middle ages, claimed that dew is ethereal, so that if we 
 should fill a lark's egg with it and lay it out in the sun, immediately on 
 the rising of that luminary, the egg would fly off into the air 1 
 
266 
 
 HEAT. 
 
 4. Clouds differ from fogs only in their elevation 
 in the atmosphere. They are produced chiefly by 
 the cooling due to expansion as currents of warm 
 moist air rise high above the surface of the ground. 
 In tropical regions they float only at great heights ; 
 in arctic regions, near the groimd. 
 
 FIG. 189. 
 
 Different kinds of Clouds one bird indicates the Nimbus, two birds the Stratus, 
 three birds the Cumulus, and four birds the Cirrus cloud. 
 
 The stratus cloud is composed of broad, widely- 
 extended cloud-belts, sometimes spread over the 
 whole sky. It is the lowest cloud, and often rests 
 on the earth, where it forms a fog. It is the night- 
 cloud. 
 
 The cumulus cloud is made up of large cloud- 
 masses looking like snow-capped mountains piled up 
 along the horizon. It forms the summits of pillars 
 
METEOROLOGY. 267 
 
 of vapor, which, streaming up from the earth, are 
 condensed in the upper air. It is the day-cloud. 
 When of small size and seen near midday, it is a 
 sign of fair weather. 
 
 The cirrus (curl) cloud consists of light, fleecy 
 clouds floating high in air. It is composed of 
 little needles of ice or flakes of snow. 
 
 The cirro-cumulus is formed by small rounded 
 portions of cirrus cloud, having a clear "sky between. 
 Sailors call this a " mackerel sky." It accompanies 
 warm, dry weather. 
 
 The cirro-stratus is produced when the cirrus 
 cloud spreads into long, slender strata. It forebodes 
 rain or snow. 
 
 The cumulo-stratus is due to increase in thick- 
 ness of the cumulus clouds, becoming denser and 
 darker below, while the upper parts flatten out and 
 thus appear like the stratus clouds. They often pre- 
 cede thunder-storms. 
 
 The nimbus cloud is that from which rain falls. 
 It may be produced by the thickening of any of the 
 forms just described. 
 
 5. Rain is the product of rapid condensation of 
 vapor in the upper regions. At a low temperature 
 the vapor is frozen directly into snow. This may 
 melt before it reaches the earth, and fall as rain or 
 sleet. A sudden draught of cold air into a heated 
 ball-room has produced a miniature snow-storm. The 
 wonderful variety and beauty of snow-crystals are 
 illustrated in the figure. 
 
 Rain always warms the air. Vapor can not con- 
 
268 HEAT. 
 
 dense without giving out as heat to the surrounding 
 medium the energy it absorbed in assuming the va- 
 porous state.* It has been estimated that the heat given 
 to the west coast of Ireland by rain-fall is equivalent 
 
 FIG. 190. 
 
 Snow-crystals. 
 
 to half of that derived from the sun. At Cherra- 
 poonjee, in India, the annual rain-fall is four times 
 as great as on the coast of Ireland. 
 
 6. Winds are produced by variations in the tem- 
 
 * "A gallon of water weighs ten pounds, and if spread out so as to 
 form a layer an inch thick, it would cover about two square feet of space. 
 To cover a square mile an inch in depth, 60,000 tons of rain are required, 
 or 12,000,000 gallons. In the condensation of the vapor needed to produce 
 a single gallon, heat enough is given out to melt 75 pounds of ice, or to 
 make 45 pounds of cast-iron white-hot. An inch of rain-fall on each square 
 mile hence implies an evolution of heat sufficient to melt a layer of ice 
 spread over the ground 8 inches thick, or to liquefy a globe of iron 130 
 feet in diameter, or a rod of it a foot in thickness and 260 miles in 
 length." 
 
METEOROLOGY. 269 
 
 perature of the air. The atmosphere at some point 
 is heated and expanded; it rises and colder air flows 
 in to supply its place. This produces currents. The 
 land and sea breezes of tropical islands are caused 
 by the unequal specific heat of land and water. Dur- 
 ing the day the land becomes more highly heated 
 than the water, and hence toward noon a sea-breeze 
 sets in from the ocean, and is strongest in the after- 
 noon. At night the land cools faster than the wa- 
 ter, and so a land-breeze sets out from the land, and 
 is strongest after midnight. 
 
 Trade-winds are so named because by their regu- 
 larity they favor commerce. A vessel on the Atlantic 
 Ocean will sometimes, without shifting a sail, set 
 steadily before this wind from Cape Verde to the 
 American coast. The air about the equator is highly 
 heated, and, rising to the upper regions, flows off 
 north and south. The cold air near the poles sets 
 toward the equator to fill its place. If the earth 
 were at rest this would make an upper current 
 flowing from the equator, and a lower current flow- 
 ing toward it. As the earth is rotating on its axis 
 from west to east, the under current starting from 
 the poles is constantly coming to a part moving 
 faster than itself. It therefore lags behind. When 
 it reaches the north equatorial regions, it lags so 
 much that it becomes a current from the north-east, 
 and in the south equatorial regions a current from 
 the south-east. 
 
 7. Ocean Currents are produced in a similar 
 manner. The water heated by the vertical sun of 
 
270 HEAT. 
 
 the tropics rises and flows toward the poles. The 
 Gulf Stream carries the heat of the Caribbean Sea 
 across the Northern Atlantic to the shores of Scot- 
 land and Norway. This great stream of warm water, 
 flowing steadily through the cold water of the ocean, 
 rescues England from the snows of Labrador. Were 
 it not for the barrier of a chain of mountains con- 
 necting North and South America, Great Britain 
 would be condemned to arctic glaciers. 
 
 8. Adaptations of Water. The great specific 
 heat of water exercises a marked influence on 
 climate. It tends to prevent sudden changes of 
 weather. In the summer it absorbs vast quantities 
 of heat, which it gives off in the fall, and thus mod- 
 erates the approach of winter. In the spring the 
 melting ice and snow drink in the warmth of the 
 sunbeam. Since so much heat is required to melt 
 the ice and snow, they dissolve very slowly, and thus 
 ward off the disastrous floods which would follow, 
 if they passed quickly into the liquid state. 
 
 Water contains air, which is necessary for the 
 support of animal life. This air not only makes it 
 available as a home for fish and other creatures that 
 inhabit the water, but also makes the change from 
 water to steam more gradual. Much of it is driven 
 off when the water is heated. When water has been 
 carefully deprived of the air usually held in solution, 
 it is liable to violent commotion at any moment 
 when it is heated above 212 F. With such water 
 every stove-boiler would need a thermometer. A 
 tea-kettle would require as careful watching as a 
 
METEOROLOGY. 271 
 
 steam-engine, and our kitchens would witness fre- 
 quent and perhaps disastrous explosions. 
 
 Water, like other liquids, expands when heated and 
 contracts when cooled for any temperatures above 
 39.2 F. The density of water is greatest at this tem- 
 perature. Cooled below 39.2 F. water expands till 
 it becomes solid at 32 F. One volume at 39.2 F. 
 becomes 1.00013 volumes at 32 F.* This expan- 
 sion in cooling is probably due to the regrouping of the 
 molecules of the water preparatory to crystallization, 
 the crystalline structure requiring more space than 
 the liquid form. As soon as a few crystals are 
 definitely formed, each serves as a nucleus around 
 which others gather, and the process becomes then 
 far more rapid. Certain metals, such as bismuth 
 and iron, act like water in this respect, and are 
 hence well fitted for making sharp castings, filling 
 every crevice of the mold as they expand in crys- 
 tallizing, f This crystallization of water is of great 
 importance in connection with the freezing of our 
 lakes and rivers. Were it not crystalline when 
 
 * Since ice when it melts contracts, pressure aids in liquefaction and 
 so lowers the melting-point. In descending over the rough surface of a 
 mountain slope, glacier ice is subjected to alternate pressure and extension. 
 The pressure melts it and makes the mass slide farther down. Passing over 
 some ledge it snaps, producing great fissures. When the walls of these 
 come into contact they freeze together again, but only to be re-melted. 
 
 t Fit a small flask with a cork, through which passes an upright glass 
 tube. Fill with colored water. Apply heat to the flask until the liquid 
 runs over the top of the tube. This shows the expansion by heat. Now 
 apply a freezing mixture to the flask, and at first the liquid in the tube 
 falls, but soon begins to rise. When it runs over as before, apply heat 
 and it shrinks back again. Thus cold will expand and heat contract it. 
 When water is at its maximum density (about 39) expansion sets in 
 alike, whether you heat or cool it. 
 
272 HEAT. 
 
 frozen, the water at the surface during severe 
 weather, radiating its heat and becoming chilled, 
 would contract and fall to the bottom, while the 
 warm water below would rise to the top. This 
 process would continue until the freezing-point was 
 reached, when the whole mass would solidify into 
 ice. Our lakes and rivers would freeze solid every 
 winter. This would be fatal to all animal life in it, 
 at least of the higher orders, such as fish. In the 
 spring the ice would not, as now, buoyant and light, 
 float and melt in the direct sunbeam, but, lying at 
 the bottom, would be protected by the non-conduct- 
 ing water above. The longest summer would not be 
 sufficient to thaw the deeper bodies of water. As it 
 is, the ice is formed at the surface, and there it floats, 
 protecting the water beneath from further reduction 
 of temperature.* 
 
 Water, in freezing, has a tendency to free itself 
 from salts and other substances dissolved in it. 
 Thus, melting ice furnishes a means of obtaining 
 fresh water in Arctic regions. If a barrel of vinegar 
 freeze, we shall find much of the acid collected in a 
 mass about the center of the ice. 
 
 * Water distills from the ocean and land as vapor, at one time cooling 
 and refreshing the air, at another moderating its wintry rigor. It con- 
 denses into clouds, which shield the earth from the direct rays of the sun, 
 and protect against excessive radiation. It falls as rain, cleansing the air 
 and quickening vegetation with renewed life. It descends as snow, and, 
 like a coverlet, wraps the grass and tender buds in its protecting em- 
 brace. It bubbles up in springs, invigorating us with cooling, healing 
 draughts in the sickly heat of summer. It purifies our system, dissolves 
 our food, and keeps our joints supple. It flows to the ocean, fertilizing 
 the soil, and floating the products of industry and toil to the markets of 
 the world. (See " Chemistry," p. 56-63.) 
 
PRACTICAL QUESTIONS. 273 
 
 PRACTICAL QUESTIONS. 
 
 1. "Why will one's hand, on a frosty morning, freeze to a metallic 
 door-knob sooner than to one of porcelain ? 
 
 2. Why does a piece of bread toasting curl up on the side exposed to 
 the fire ? 
 
 3. Why do double windows protect better than single ones from the 
 cold? 
 
 4. "Why do furnace-men wear flannel shirts in summer to keep cool, 
 and in winter to keep warm? 
 
 5. Why do we blow our hands to make them warm, and our soup to 
 make it cool? 
 
 6. Why does snow protect the grass in winter? 
 
 7. Why does water "boil away" more rapidly on some days than on 
 others? 
 
 8. What causes the crackling sound of a stove when a fire is lighted? 
 
 9. Why is the tone of a piano higher in a cold room than in a warm 
 one? 
 
 10. Ought an inkstand to have a large or a small mouth? 
 
 11. Why is there a space left between the ends of the rails on a rail- 
 road track? 
 
 12. Why is a person liable to. take cold when his clothes are damp? 
 
 13. What is the theory of corn-popping? 
 
 14. Could vacuum-pans be employed in cooking? 
 
 15. Why does the air feel so chilly in the spring, when snow and ice 
 are melting? 
 
 16. Why, in freezing ice-cream, do we put the ice in a wooden vessel 
 and the cream in a tin one? 
 
 17. Why does the temperature generally moderate when snow falls? 
 
 18. What causes the singing of a tea-kettle? Aw. The escaping steam 
 is thrown into vibration by friction against the spout. 
 
 19. Why does sprinkling a floor with water cool the air? 
 
 20. How low a degree of temperature can be marked by a mercurial 
 thermom eter ? 
 
 21. If the temperature is 70 R, what is it C.? 
 
 22. Will dew form on an iron bridge? On a plank walk? 
 
 23. Why will not corn pop when very dry? 
 
 24. When the interior of the earth is so hot, why do we get the coldest 
 water from a deep well? 
 
 25. Ought the bottom of a tea-kettle to be polished? 
 
 26. Which boils the sooner, milk or water? 
 
 27. Is it economy to keep our stoves highly polished? 
 
 28. If a thermometer be held in a running stream, will it indicate the 
 same temperature that it would in a pailful of the same water? 
 
 29. Which makes the better holder when one wishes to protect his 
 hands from a hot dish, woolen or cotton? 
 
274 HEAT. 
 
 30. Which will give out the more heat, a plain stove or one with orna- 
 mental designs? 
 
 31. Does dew fall? 
 
 32. What causes the "sweating" of a pitcher? 
 
 33. Why is evaporation hastened in a vacuum? 
 
 34. Does stirring the ground around plants aid in the deposition of 
 dew? 
 
 35. Why does the snow at the foot of a tree melt sooner than that in 
 tho open field? 
 
 36. Why is the opening in a chimney made to decrease in size from 
 bottom to top? 
 
 37. Will tea keep hot longer in a bright or a dull tea-pot? 
 
 38. What causes the snapping of wood when laid on the fire ? Am. The 
 expansion of the air in the cells of the wood. 
 
 39. Why is one's breath visible on a cold day? 
 
 40. What gives the blue color to air? Am. The particles floating in it 
 reflect the blue light of the sunbeam. 
 
 41. How does the heat at two feet from the fire compare with that at 
 four feet? 
 
 42. Why does the frost remain later in the morning upon some objects 
 than upon others? 
 
 43. Is it economy to use green wood? 
 
 44. Why does not green wood snap? 
 
 45. Why will a piece of metal dropped into a glass or porcelain dish of 
 boiling water increase the ebullition? 
 
 46. Which can be ignited the more quickly with a burning-glass, lamp- 
 black or white paper? 
 
 47. Why does the air feel colder on a windy day? 
 
 48. Could a burning-lens be made of ice? 
 
 49. Why is an iceberg frequently enveloped by a fog? 
 
 50. Would dew gather more freely on a rusty stove than on a bright 
 kettle ? 
 
 51. Why is a clear night colder than a cloudy one during the same 
 season ? 
 
 52. Why is no dew formed on cloudy nights ? 
 
 53. Why will "fanning" cool the face? 
 
 54. How are safes made fire-proof? 
 
 55. Why can you heat water more quickly in a tin than a china cup? 
 
 56. Why will a woolen blanket keep ice from melting? 
 
 57. Does dew form under trees? 
 
 58. What is the principle of heating by steam? 
 
 59. What is the cause of "cloud-capped" mountains? 
 
 60. Show how the glass in a hot-house acts as a trap to catch the sun- 
 beam. 
 
 61. Does the heat of the sun come in through our windows? 
 
 62. Does the heat of our stoves pass out in the same way? 
 
 03. The top of a mountain is nearer the sun, why is it not warmer? 
 
SUMMARY. 275 
 
 64. What is hoar-frost? Ans. Frozen dew. 
 
 65. "Why will a slight covering protect plants from frost? Ans. Because 
 it prevents radiation. 
 
 66. Why is there no frost on cloudy nights? Ans. The clouds act like 
 a blanket, to prevent radiation and keep the earth warm. 
 
 67. Can we find frost on the windows and on the stone flagging the 
 same morning? 
 
 68. Why will not snow "pack " into halls except in mild weather? 
 
 69. Why is the sheet of zinc under a stove so apt to become puckered? 
 
 70. Why does a mist gather in the receiver of the .^-pump as the air 
 becomes rarefied? 
 
 71. Why are the tops of high mountains in the tropics covered with 
 perpetual snow? 
 
 SUMMARY. 
 
 Heat is produced, by longer and less refrangible waves and 
 slower vibrations of ether than those which cause light. Solar 
 energy may be radiated, reflected, refracted, absorbed, and po- 
 larized, whether manifested as light or heat. If we elevate 
 the temperature of a body sufficiently, such as a piece of plati- 
 num, we can cause it to emit rays of both heat and light. A 
 body which allows the radiant heat to pass through it easily 
 is styled diathermanous ; rock-salt is such a body, being to 
 heat-rays what glass is to light-rays. The sun is the principal 
 source of heat. But heat can be obtained by chemical and 
 mechanical means. In burning coal we secure it by the former 
 method. Mechanical energy may be changed directly into heat, 
 as in striking fire with flint and steel, and in hammering a 
 bullet on an anvil until it is hot. According to Joule's law, 
 772 feet fall of a given weight corresponds to 1 of rise of tem- 
 perature in the same weight of water. 
 
 Among the physical effects of heat are a change of temper- 
 ature, expansion, liquefaction, vaporization, and evaporation. 
 The heat-force increases the kinetic energy of the molecules, 
 thus elevating the temperature ; and the increased vibration of 
 the molecules causes an expansion of the body. The latter is 
 so uniform in certain substances, such as mercury, that it is 
 used to indicate changes of temperature, as in the thermometer. 
 The expansion of the metals by heat is turned to account in 
 
276 HEAT, 
 
 many art processes. The walls of a gallery in the Conservatoire 
 des Arts et Metiers in Paris, had begun to bulge. To remedy 
 this, iron rods were passed across the building and screwed into 
 plates on the outside of the walls. By heating the bars, they 
 were expanded, when they were screwed up tightly. Being 
 then allowed to cool, they contracted, thus drawing the walls 
 back toward a perpendicular. The same has been done for 
 weakened walls in many other' places. 
 
 Heat is the great antagonist of cohesion. The liquid and gas- 
 eous states of bodies depend on its relative presence or absence 
 (absolute cold is as yet only a theoretical condition, all bodies 
 with which we are familiar being relatively warm). When the 
 heat-force nearly balances that of cohesion, the body breaks 
 down into a liquid, and when the repellent fairly triumphs, the 
 particles fly off as a gas. Immediately before and after each 
 of these marked changes, viz., of a solid to a liquid and of a 
 liquid to a gas, the thermometer indicates a constant temper- 
 ature. Thus water from melting ice affects the thermometer 
 just as the ice does, and steam is no hotter than the boiling 
 water. The heat which, in these processes, becomes hidden 
 from the thermometer is called latent, though we now know 
 that, having been occupied in doing internal work, it has merely 
 become potential, and can be readily turned again into kinetic 
 energy. The so-called latent heat of water is only the potential 
 heat-energy of the separated molecules, which will reappear the 
 instant the molecules collapse and come once more within the 
 grasp of cohesion. On this principle is based the method of 
 heating by steam. Evaporation is a slow change to vapor that 
 takes place at all temperatures, but may be greatly increased 
 by a diminution of pressure, as in a vacuum. It is a cooling 
 process, and is practically applied to the manufacture of ice. 
 
 By the subtraction of heat, i. e., by cold, and by the addi- 
 tion of pressure, which antagonizes the repellent heat-force, 
 gases may be liquefied and even congealed, the transparent car- 
 bonic-acid gas thus becoming a snowy solid. What were for- 
 merly called the "permanent gases" (oxygen, hydrogen, etc.), 
 have been liquefied by means of the cold produced by their 
 rarefaction when they were suddenly released from a pressure 
 of two or three hundred atmospheres. 
 
HISTORICAL SKETCH. 277 
 
 Heat is conducted from molecule to molecule of a body, 
 radiated in straight lines through air (or space), and circulated 
 by the transference of heated masses through a change of spe- 
 cific gravity due to expansion. The first method is character- 
 istic of solids, and the third of liquids and gases. The elastic 
 force of steam increases when it is confined and a higher tem- 
 perature is reached. The steam-engine utilizes this principle. 
 There are two forms of this machine, the high-pressure and the 
 low-pressure, according as the waste steam is ejected into the 
 air or condensed in a separate chamber. The phenomena of 
 dew, rain, etc., depend upon the fact that a change from a 
 higher to a lower temperature causes the air to deposit its 
 moisture. 
 
 HISTORICAL SKETCH. 
 
 DEMOCRITUS, the originator of the Atomic Theory, held that 
 heat consists of minute spherical particles radiated rapidly 
 enough to penetrate every substance. Until very recently, heat 
 and light were thus reckoned among the Imponderables, i. e., 
 matter which has no weight. Aristotle considered heat more a 
 condition than a substance. Bacon, in his "Novum Organum," 
 wrote : "Heat is a motion of expansion." Locke, half a century 
 later, said: "Heat is a very brisk agitation of the insensible 
 parts of an object, which produces in us the sensation from 
 whence we denominate the object hot, so that what in our 
 sensation is heat, in the object is motion." 
 
 The material view, however, held its ground. At the begin- 
 ning of the 18th century, Stahl elaborated a theory that a 
 buoyant substance called phlogiston is the principle of heat, and 
 that when a body burns, its phlogiston escapes as fire. In 
 1760, Dr. Black investigated and made known the principles of 
 what he termed latent heat, i. e., heat which becomes hidden 
 when ice is turned into water or water into steam. Priestley 
 discovered, in 1774, and Lavoisier afterward developed, the 
 modern view of combustion. But the latter philosopher then 
 advanced the theory that heat (caloric) is an actual substance, 
 which passes freely from one body to another and combines at 
 
278 HEAT. 
 
 pleasure. Toward the close of the 18th century, Benjamin 
 Thompson, better known as Count Rumford, a native of Wo- 
 burn, Mass., but in the employ of the Elector of Bavaria, 
 proved the convertibility of force. "He first took the subject," 
 as Professor Youmans well remarks, "out of the domain of 
 metaphysics, where it had been speculated upon since the time 
 of Aristotle, and placed it on the true basis of physical experi- 
 ment." 
 
 Soon the scientific world seemed to be ripe for this dis- 
 covery, and it appears to have sprung up spontaneously in 
 men's thoughts every-where. Mayer, a physician of Germany, 
 and Grove, of England, proved the mutual relation of the 
 forces, the latter first using the term " Correlation of Forces," 
 since changed to Conservation of Energy. Joule discovered the 
 law of the "Mechanical Equivalent of Heat," about 1843. In 
 his famous experiments, he used pound-weights made to fall 
 through a measured distance. Cords were attached to them, so 
 that, as they fell, they turned a paddle-wheel placed in a box of 
 water. Other liquids were used instead of the water. The rise 
 of temperature in the liquids was carefully marked. The loss 
 by friction in the apparatus was estimated, and so, at last, the 
 dynamical theory of heat was fully demonstrated. Names of 
 philosophers well known to us, such as Henry, Helmholtz, 
 Faraday, Thomson, Maxwell, Le Conte, Youmans, Stewart, and 
 Tyndall, are associated with the final establishment of this 
 theory. 
 
 Consult, on this interesting subject, Tait's " Recent Advances 
 in Physical Science"; Stewart's "Treatise on Heat"; Tyndall's 
 "Heat a Mode of Motion"; MaxweU's "Theory of Heat"; 
 Thurston's " History of the Growth of the Steam-engine " ; 
 Buckley's "Short History of Natural Science''; Smiles' "Lives 
 of Boulton and Watt" ; Youmans' "Correlation of the Physical 
 Forces"; "Read and the Steam-engine"; "American Cyclope- 
 dia," Art. "Steam-engine"; "Popular Science Monthly," Vol. 
 XII., p. 616, Art. "Liquefaction of Gases"; Scott's "Meteor- 
 ology," and Thomson's "Cruise of the Challenger." 
 
IX. 
 
 MAGNETISM. 
 
 ' THAT power which, like a potent spirit, guides 
 The sea-wide wanderers over distant tides, 
 Inspiring confidence where'er they roam, 
 By indicating still the pathway home; 
 Through Nature, quickened by the solar beam, 
 Invests each atom with a force supreme, 
 Directs the cavern'd crystal in its birth, 
 And frames the mightiest mountains of the earth 
 Each leaf and flower by its strong law restrains 
 And binds the monarch Man within its mystic chains." 
 
 HUNT. 
 
ANALYSIS OF MAGNETISM. 
 
 MAGNETISM. . 
 
 1. MAGNETS. 
 
 2. THE MAGNETIC MERIDIAN. 
 
 3. LAWS OF MAGNETISM. 
 
 4. INDUCTION. 
 
 5. How TO MAKE A MAGNET. 
 
 6. THE COMPASS. 
 
 7. LINES OF FORCE. 
 
 8. POLARITY OF THE NEEDLE. 
 
 9. TERRESTRIAL MAGNETISM. 
 
MAGNETISM. 
 
 1. Magnets.* A natural magnet is an ore 6f iron 
 (Fe 3 4 , "Popular Chemistry," p. 156), generally known 
 
 FIG. 191. 
 
 Magnet Dipped in Iron Pilings 
 
 as lodestone (Saxon, laedan, to lead), which has the 
 power of attracting iron.f The artificial magnet is a 
 
 * The term is derived from the fact that an ore of iron possessing this 
 property was first found at Magnesia, in Asia Minor. 
 
 t A few other elements, such as nickel and cobalt, are attracted 
 
282 
 
 MAGNETISM. 
 
 steel bar that has acquired properties like those of 
 lodestone. If it be straight, it is called a bar mag- 
 net; if U-shaped, a horseshoe magnet. A piece of 
 soft iron called the armature is placed so as to con- 
 nect the two ends of the horseshoe. 
 
 If we insert a magnet in iron filings, they will 
 cling chiefly to its ends termed the poles. The 
 magnetic force will be exerted even through any in- 
 tervening body that is not itself magnetic. 
 
 FIG. 192. 
 
 Influence of one Magnet on another. 
 
 2. The Magnetic Meridian. If a slender bar 
 magnet be suspended so as to swing freely in a 
 horizontal plane, it will come to rest in a definite 
 position ; one pole pointing north, the other south. A 
 vertical plane passing through the two poles in this 
 
 slightly by the magnet. They are all called magnetic bodies, but for ordi- 
 nary purposes iron may be regarded as the only one of importance. 
 
INDUCTION. 283 
 
 position is called the magnetic meridian. The pole 
 pointing north is called the north or (-h) pole, the 
 other the south or ( ) pole. 
 
 3. Laws of Magnetism. If we hold a magnet 
 near a magnetic needle, we shall find that the south 
 pole of one attracts the north pole, and repels the 
 south pole of the other.* This proves the law 
 
 " Like poles repel, and 
 
 FIG. 193. 
 
 unlike poles attract. 
 
 Two opposite poles 
 
 Magnetic induction. ^^ placed near together 
 
 attract each other 
 
 strongly, but this force, like gravitation, dimin- 
 ishes as the square of the distance increases. 
 
 4. Induction is the process of developing 
 magnetism by bringing a magnetic body and a mag- 
 net near together. If a piece of soft iron be brought 
 near a magnet, it immediately assumes the magnetic 
 state, but loses it on being removed. In steel the 
 
 * Experiments. 1. Rub the point of a sewing-needle across the north 
 pole of a magnet. Bring the point near the south pole of the magnetic 
 needle. The needle will be repelled, showing that the point of the sewing- 
 needle has become a south pole. 2. Suspend a key from the north pole of 
 a magnet. Bring the south pole of an equal magnet close to the upper end 
 of the key. The key will instantly fall. 3. Suspend a long iron wire from 
 the north pole of a magnet. Bring the north pole of the second magnet 
 near the lower end of the wire. The wire is repelled, because its lower 
 extremity possesses north polarity. 4. Immerse the unlike poles of two 
 magnets in iron filings. Bring the two poles near each other. The filings 
 will move toward one another. But if the poles of the magnets are like, 
 the filings will fall off the magnets. 5. To ascertain whether a metallic 
 substance contains iron : Bring the substance near one of the extremities 
 of a magnetic needle. If the position of the needle be affected, then the 
 substance almost certainly contains Iron. A piece of copper will not affect 
 the magnetic needle. 
 
284 MAGNETISM. 
 
 change is induced and lost much more slowly. The 
 end of the bar next to the south pole of the magnet 
 becomes the north pole of the new magnet, and vice 
 versa. When opposite states are thus developed in 
 the opposite ends of a body, it is said to be polarized. 
 Whenever an object is attracted by a magnet, it is 
 supposed first to be made a magnet (polarized) by 
 induction, and then the attraction consists in that of 
 unlike poles for each other. Thus we may suspend 
 from a magnet a chain of rings held together by 
 magnetic attraction.* Each link is a magnet with 
 its north and south poles. Each particle of the tuft 
 of filings in Fig. 191 is a distinct magnet. By in- 
 ducing magnetism, a magnet does not lose force. It 
 rather gains by the reciprocal influence of the new 
 magnet. An armature acts in this manner to 
 
 strengthen a magnet. If 
 
 we break a ma net ' the 
 smallest fragment 
 
 9 IP i * i ii< n mi B B u x^g ilai I* 
 
 Polarization of an Iron Bar. 
 
 have a north and a south 
 
 pole. This is explained by supposing that every 
 molecule contains two opposite kinds of energy which 
 neutralize each other. When the bar is magnetized 
 these are separated, but do not leave the molecule. 
 This is hence polarized, the halves assuming opposite 
 magnetic states, as shown in Fig. 194. The light half 
 of each little circle represents the positive, and the 
 dark the negative side. All the molecules exert their 
 negative force in one direction, and their positive in 
 
 * Repeat this experiment with keys, or nails of different sizes, or bits 
 of wire of varying length. 
 
LINES OF FORCE. 285 
 
 the other. The forces thus neutralize each other at 
 the center, but manifest themselves at the ends of 
 the magnet. Hence it is impossible PIG 195 
 
 to produce a magnet with only one 
 pole. Each pole necessitates the pres- 
 ence of the other. 
 
 5. How to Make a Magnet. 
 Place the inducing magnet, as shown ^"H 
 
 in Fig. 195, on the unmagiietized bar Makiug a Magnet 
 
 (which any blacksmith can make from 
 
 a bar of steel), and draw it from right to left several 
 
 times, always carrying it back through the air to the 
 
 starting-point.* 
 
 6. The Compass is a magnetic needle used by 
 mariners, surveyors, etc. It is delicately poised over 
 a card on which the "points of the compass" are 
 marked. At most places the needle does not point 
 directly N. and S. The " line of no variation " in the 
 United States passes near Wilmington, N. C., Char- 
 lottesville, Va., and Pittsburg, Pa, East of this the 
 declination of the needle from true north is toward 
 the west, and west of it the declination is toward 
 the east. 
 
 7. Lines of Force. The region in the neighbor- 
 hood of a magnet pole within which it can have a 
 perceptible effect upon magnetic bodies is called its 
 
 * A needle may be magnetized by laying it across the poles of a horse- 
 shoe magnet. After remaining a few moments, the end in contact with the 
 north pole of the magnet will become a south pole and the other a north 
 pole. If it be suspended from the middle by a thread it will point north 
 and south. A knife- blade may be magnetized by rubbing it several times, 
 in the same direction, across one of the poles of the magnet. 
 
286 
 
 MAGNETISM, 
 FIG. 196. 
 
 The Compass. 
 
 field. The directions which these bodies tend to 
 assume are called lines of force. Over a bar magnet 
 lay a sheet of paper or a plate of glass, and sprinkle 
 fine iron filings over this. On gently tapping the 
 
 PIG. 197. 
 
POLABITT OF THE NEEDLE. 
 
 287 
 
 FIG. 198. 
 
 Pio. 
 
 plate they become arranged in curved lines, many of 
 which seem to radiate from the poles. These are 
 the indicators of the lines of force, the position of 
 each iron filing being determined by its direction 
 and distance from the two poles of the magnet. 
 
 Between the two opposite poles of a horseshoe 
 magnet, or of two separate magnets brought near 
 together, the lines of force are straight. 
 
 8. Polarity of the Needle. WHY THE NEEDLE 
 POINTS NORTH AND SOUTH. The earth is a great mag- 
 net, whose opposite poles produce lines of force that 
 permeate its body and the space around. A needle 
 when magnetized tends to assume the direction of 
 the line of force that 
 passes through it. The 
 position of the terres- 
 trial magnetic poles is 
 not constant, and hence 
 the needle changes its 
 direction accordingly. 
 
 Suppose a magnet g 
 NS passing through the 
 center of a small globe. 
 The needle sn will hang parallel to it (Fig. 198), its 
 positive pole being attracted by the negative pole of 
 the magnet, and vice versa. If the globe be revolved 
 (Fig. 199), the positive pole of the needle will turn 
 dip, as it is termed downward. If the globe be re- 
 volved in the other direction, the negative pole of the 
 needle will dip in the same manner.* 
 
 * Similar phenomena are noticed in the compass. At the magnetic 
 
 Magnet in Globe. 
 
288 MAGNETISM. 
 
 A DIPPING-NEEDLE is poised as shown in Fig. 200. 
 At the magnetic equator it hangs horizontally, but 
 FIG 200 when carried north its positive end 
 
 dips downward more and more until 
 it points vertically downward at a 
 point in Boothia Peninsula, north of 
 Hudson Bay, called the pole of ver- 
 ticity, or often simply the magnetic 
 pole. This is the negative pole of the 
 The Dipping-needle, earth. The position of its positive 
 pole has been calculated to be beneath the Antarctic 
 Ocean.* 
 
 9. Magnetism of the Earth. Magnetic bodies such 
 as iron fences, lightning-rods, iron standards of chairs, 
 etc., are found to be fully magnetic when examined 
 with a magnetic needle. This is due to the inductive 
 action of the earth's magnetism. In the northern 
 hemisphere the upper end of such bodies is south- 
 magnetic, the lower end north-magnetic. In the 
 southern hemisphere the polarity is the reverse of this. 
 The cause of the earth's magnetism and of the 
 variations in it is not yet known. 
 
 equator it is horizontal, but dips whenever taken north or south. An un- 
 magnetized needle, if poised, in our latitude, on being magnetized, settles 
 down, as if the north end were the heavier. This is remedied by making 
 the north end of the needle lighter, or by attaching a little weight upon 
 the south end. The reverse is true in the southern hemisphere. 
 
 * The declination and dip of the magnetic needle have daily and yearly 
 variation, and also very slow changes requiring centuries to complete. In 
 1686, at New York, the declination was 9 west; in 1750, 6 20' west; In 
 1790, 4 15' west; in 1847, 6 30' west; in 1885, 8 west. The line of no 
 variation was becoming slowly shifted eastward from 1686 to 1790, then 
 became stationary, and has since been moving westward. The intensity of 
 terrestrial magnetism at any given place has also daily variations, growing 
 stronger by day and weaker by night. 
 
HISTORICAL SKETCH. 289 
 
 SUMMARY. 
 
 NATURAL magnets are found in certain regions, but in 
 practice, magnets made of steel are generally used. These 
 bars may be magnetized either by contact with other magnets 
 or by placing them within the magnetic field of a coil conduct- 
 ing an electric current (see p. 332). The existence of magnetism 
 is manifested by polarity. Like poles repel, unlike attract each 
 other. The intensity of the force varies inversely as the square 
 of the distance. A magnet induces magnetism in any neighbor- 
 ing magnetic body. This is not prevented by intervening bodies 
 which are not themselves magnetic. If free to move, small 
 bodies thus influenced by induction tend to place themselves in 
 certain directions, called lines of force, around the inducing 
 magnet. The declination, the dip, and the intensity are the 
 magnetic elements of a place. Each of these is subject to daily 
 variations, and additionally to slow changes requiring many 
 years for a cycle. The cause of the earth's magnetism is un- 
 known. Sudden variations in it accompany the outbreak of 
 spots on the sun, and magnetic storms are usually attended by 
 the appearance of the aurora. 
 
 HISTORICAL SKETCH. 
 
 MAGNETS probably became first known to European nations 
 through the discovery of natural magnets by the Greeks in the 
 Thessalian district of Magnesia. From this the name was 
 taken. The tendency of a magnetized needle to point in a defi- 
 nite direction was early noticed, and it is thought that the 
 compass was invented by the Chinese. The first mention of the 
 use of the magnetic needle in Europe occurs in 1190. The 
 needle was floated on a cork, and in this way it served as a 
 guide to the Chinese travelers. By the end of the fifteenth cent- 
 ury the compass was known to most European sailors, and its use 
 was specially frequent among the Spanish and Portuguese. The 
 declination of the needle was known to the Chinese in the be- 
 ginning of the twelfth century. Columbus discovered it inde- 
 
290 MAGNETISM. 
 
 pendently in 1482, just ten years before his discovery of Amer- 
 ica. The first known work for the use of seamen was written 
 during the reign of Queen Elizabeth. It was entitled "A Dis- 
 course on the Variation of the Cumpas or Magneticall Needle," 
 and is dedicated to "the travaillers, sea-men, and mariners of 
 England." The dip was discovered accidentally in 1576 by Robert 
 Norman, an English instrument-maker. He found the dip at 
 London to be 71 50'. Dr. Gilbert, the physician of Queen Eliza- 
 beth, published his great work, "De Magnete," about 1600. In 
 this he announces his belief that the earth is a great magnet, 
 controlling the direction of the needle. The variation in intensity 
 of the earth's magnetic force has become known chiefly during 
 the present century. 
 
ELECTRICITY. 
 
 " MAXWELL has demonstrated that luminous vibrations can be nothing 
 else than periodic variations of electro- magnetic forces. Hertz, in proving 
 by experiments that electro-magnetic oscillations are propagated like light, 
 has given an experimental basis to the theory of Maxwell. This gave birth 
 to the idea that the luminiferous ether and the seat of electric and magnetic 
 forces are one and the same thing. This being established I can now . . . 
 reply to the question you put to me : What is electricity ? It is not only the 
 formidable agent which now and then shatters and tears the atmosphere, 
 terrifying you with the crash of its thunder, but it is also the life-giving 
 agent which sends from heaven to earth, with light and heat, the magic of 
 colors and the breath of life. It is that which makes your heart beat to the 
 palpitations of the outside world ; it is that which has the power to transmit 
 to your soul the enchantment of a look and the grace of a smile." PBOF. 
 FERRARIS. 
 
ANALYSIS OF ELECTRICITY. 
 
 I. FBICTIONAL ELEOTBICITY. 
 
 H. VOLTAIC ELEOTBICITY. 
 
 IH. TBANSFOBMATIONS or 
 ELECTBIO ENEBGY. 
 
 1. Development of Electricity. 
 
 2. The Electroscope. 
 
 3. Electrical Attraction and Repulsion, 
 
 4. Theory of Electricity. 
 
 5. Electrical Potential. 
 
 6. Electrical Conduction. 
 
 7. Electrical Density. 
 
 8. Electrical Induction. 
 
 9. The Plate Machine. 
 
 10. Theory of Attraction. 
 
 11. Free and Bound Charges. 
 
 12. Inductive Capacity. 
 
 13. Electrical Condensation. 
 
 14. The Leyden Jar. 
 
 15. The Voss Machine. 
 
 16. Lightning. 
 
 17. Effects of Fractional Electricity. 
 
 1. Simple Voltaic Cell. 
 
 2. Action in the Voltaic Cell. 
 
 3. The Electric Current. 
 
 4. The Volt. 
 
 5. Electrical Resistance. 
 
 6. The Ohm. 
 
 7. The Ampere. 
 
 8. The Battery. 
 
 9. Polarization within the Battery. 
 
 10. Special Forms of Battery. 
 
 11. Effects of Voltaic Electricity. 
 
 1. Effect of a Voltaic Current on a Mag- 
 
 netic Needle. 
 
 2. Magnetic Coils. 
 
 3. The Tangent Galvanometer. 
 
 4. The Electro-magnet. 
 
 5. The Electro-magnetic Telegraph. 
 
 6. Ocean Cables. 
 
 7. Current Induction. 
 
 8. The Induction Coil. 
 
 9. Magneto-induction. 
 
 10. The Telephone. 
 
 11. The Microphone. 
 
 12. The Magneto-electric Machine. 
 
 13. The Dynamo-electric Machine. 
 
 14. The Electric Light. 
 
 15. Thermo-electricity. 
 
 16. Animal Electricity. 
 
ELECTRICITY. 
 
 a piece of sealing-wax is rubbed with flannel 
 it temporarily acquires the new property of attracting, 
 when brought near them, other light bodies, such as 
 shreds of paper, gold leaf, lint, etc. If .the hand is ap- 
 proached sufficiently near the excited sealing-wax, small 
 sparks may not infrequently be seen in the dark to leap 
 across the space between the hand and the sealing- 
 wax.* The excited sealing-wax is evidently in a dif- 
 ferent state from what it was before it was rubbed 
 with the flannel. The supposed agent producing this 
 state is called electricity, and the sealing-wax in this 
 excited condition is said to be electrified. Other sub- 
 stances may be used instead of the sealing-wax. A 
 piece of vulcanite, glass, resin, shellac, or amber will do 
 as well. The substance also with which friction is pro- 
 duced may be varied. Instead of flannel, fur, silk, or 
 chamois-skin covered with tin-amalgam may be used, 
 and it is found that with some rubbers better results are 
 .obtained than with others. Under proper conditions 
 most bodies may be rendered electric by friction. 
 
 * In cold, frosty weather, a person, by shuffling about in his stocking- 
 feet upon the carpet, can develop so much electricity in his body that he 
 can ignite a jet of gas by simply applying his finger to it. Blasts in mines 
 intended to be fired by electricity have thus been prematurely discharged 
 by the workmen touching the wires. To prevent this disastrous effect, at 
 tne Sutro Tunnel, Nevada City, the workmen who are handling exploders 
 wet their boots, stand on an iron plate to conduct off the electricity of the 
 body, and wear rubber gloves. 
 
294 
 
 ELECTRICITY. 
 
 I. FRICTIONAL ELECTRICITY. 
 
 1. Electricity developed either directly or indirectly 
 by friction is called frictional electricity. For its detec- 
 tion it is convenient to employ instruments specially 
 adapted for this purpose, called electroscopes. 
 
 2. The Electroscope. A pith ball suspended by a 
 silk thread, as shown in Fig. 201, constitutes a simple 
 and very effective electroscope. A straw 3 or 4 inches 
 
 Electroscopes. 
 
 in length, and suspended by a silk fibre at the middle 
 so as to hang horizontally, may be substituted for the 
 pith ball. A lath balanced on an egg placed in a wine- 
 glass may also serve as an electroscope. 
 
 3. Electrical Attraction and Repulsion. If a warm 
 dry glass, such as a lamp-chimney, be rubbed with a 
 silk handkerchief, a crackling sound will be heard. 
 
FRICTIONAL ELECTRICITY. 295 
 
 If the tube be held near the face, a sensation like 
 that of touching cobwebs will be felt. Make with the 
 electrified tube the following experiments.* Present 
 
 * The following simple experiments are instructive : 1. A rubber comb 
 passed a few times through the hair will furnish enough electricity to turn 
 the lath entirely around, and empty egg-shells, paper hoops, etc., will follow 
 the comb over the table in the liveliest way. 2. Take a thin sheet of gutta- 
 percha, about a foot square ; lay it upon the table, and rub it briskly a few 
 times with an old fur cuff ; the gutta-percha will become powerfully elec- 
 trified. 3. Lift the gutta-percha by one corner, and some force will be re- 
 quired to separate it from the table. 4. Hold the electrified gutta-percha in 
 the left hand ; bring the fingers of the right near the paper ; it will be at- 
 tracted to the hand, and sparks will pass to the fingers with a snapping 
 sound. 5. Hold some feathers, suspended by a silk thread, near the excited 
 gutta-percha, and the feathers will be attracted. 6. Hold the excited paper, 
 or the excited sheet of gutta-percha, over the head of a person with dry 
 hair; the hair will be attracted by the gutta-percha, and each particular 
 hair will stand on end. 7. Hold the excited gutta-percha near the wall ; 
 the gutta-percha will fly to it, and remain some minutes without falling. 
 8. Place a sheet of gutta-percha on a tea-tray ; rub the gutta-percha briskly 
 with a fur cuff ; place the tea-tray with the excited sheet of gutta-percha 
 on a dry tumbler ; lift off the gutta-percha from the tea-tray ; bring the 
 knuckle of your hand near the tray, and you will receive a spark. Replace 
 the gutta-percha on the tray and apply your knuckle, and you will receive 
 another spark. This may be repeated a dozen times, 9. Take a sheet of 
 foolscap paper and a board about the same size. Heat both till they are 
 thoroughly dry. While hot, lay the paper on the board and rub the former 
 briskly with a piece of rubber. The paper and board will cling together. 
 Tear the paper loose and try experiments 4, 5, 6, and 7. Return the paper 
 and rub as before. Cut the paper so as to form a tassel. Then lift, and the 
 strips of the tassel will repel one another. 10. Take a piece of common 
 brown paper, about the size of an octavo book, hold it before the fire till 
 quite dry and hot, then draw it briskly under the arm several times, so as 
 to rub it on both sides at once by the coat. The paper will be found so 
 powerfully electrical, that if placed against a wainscoted or papered wall of 
 a room, it will remain there for some minutes without falling. 11. While 
 the paper still clings to the wall hold against it a light, fleecy feather, and it 
 will be attracted to the paper in the same way the paper is to the wall. 
 12. If the paper be warmed, drawn under the arm as before, and then 
 hung up by a thread attached to one corner, it will sustain several feathers 
 on each side ; should these fall off from different sides at the same time, 
 they will cling together very strongly; and if after a minute they are all 
 shaken off, they will fly to one another in a singular manner. 13. Warm 
 and excite the paper as before, and then lay on it a ball of elder-pith, 
 
296 ELECTRICITY. 
 
 it to the pith ball of an electroscrope. This will be 
 attracted till it touches, and then fly off. The end 
 of the suspended straw will likewise be first attracted, 
 but then repelled just after it is touched. Grasp the 
 pith ball or straw for a moment. It will no longer 
 be repelled. Rub a stick of sealing-wax with a woolen 
 cloth or some fur. The behavior of the pith ball or 
 straw toward it will be the same as toward the glass. 
 But bring the rubbed sealing-wax near to the pith 
 ball or straw that is repelled by the rubbed glass ; 
 there will be attraction instead of repulsion. If the 
 excited glass be held on one side of a ball and the 
 
 about the size of a pea ; the ball will immediately roll across the paper, 
 and if a needle be pointed toward it, it will again roll to another part, and 
 so on for a considerable time. 14. Support a pane of glass, well dried and 
 warmed, upon two books, one at each end, and place some bran underneath ; 
 then rub the upper side of the glass with a silk handkerchief, or a piece of 
 flannel, and the bran will dance up and down like the images in Fig. 208. 
 15. Place a common tea-tray on a dry, clean tumbler. Then take a sheet 
 of foolscap writing-paper (as in No. 9) and dry it carefully until all its 
 hygrometric moisture is expelled. Holding one end of the sheet on a table 
 with the finger and thumb, rub the paper with a large piece of India rubber 
 a dozen times vigorously from left to right, beginning at the top. Now take 
 up the sheet by two of the corners and bring it over the tray, and it will 
 fall like a stone. This, as well as the apparatus in No. 8, forms a simple 
 Electroplvorus, fit to perform many experiments ordinarily performed with 
 that instrument. If the tip of a finger be held close to the bottom of the 
 tray, a sensible shock will be felt. Next, lay a needle on the tray with its 
 point projecting outward, remove the paper, and, in the dark, a star sign 
 of the negative electricity will be seen ; return the paper, and the positive 
 brush will appear. Lay a dry, hot board, as in No. 9, on top of four tum- 
 blers. If a boy stand on the board he will be insulated, and on his holding 
 the tray vertically, the paper will not fall. Sparks may then be drawn 
 from his body, and his hair will be electrified. 16. Warm a lamp-chimney, 
 rub it with a hot flannel, and then bring a downy feather near it. On the 
 first moment of contact, the feather will adhere to the glass, but soon after 
 will fly rapidly away, and you may drive it about the room by holding the 
 glass between it and the surrounding objects ; should it, however, come in 
 contact with any thing not under the influence of electricity, it will in- 
 stantly fly back to the glass. 
 
FRICTIONAL ELECTRICITY. 297 
 
 excited wax on the other, it will fly between the two, 
 touching each in succession alternately. From this 
 we conclude that (1), there are two kinds of mani- 
 festation of frictional electricity ; and (2), like kinds 
 are manifested ~by repulsion, and unlike ~by attrac- 
 tion. The electricity from the glass is termed posi- 
 tive [ + ], and that from the wax, negative [ ].* 
 
 4. Theory of Electricity. It is thought that posi- 
 tive and negative electricity exist in every body, in 
 a state of total or partial equilibrium. When this is 
 disturbed, as by friction, electrical separation follows, 
 and each kind becomes manifested, just as in the 
 polarization of a magnet, if the proper conditions are 
 observed. Electricity is not a fluid, as was long 
 taught. It may be a condition of strain among the 
 molecules of a body, capable of being communicated 
 like a fluid. We know only its laws, and not its 
 nature. 
 
 5. Electric Potential. A body electrically excited 
 by friction or otherwise is said to be charged. The 
 charge may be either positive or negative, strong or 
 weak. If two bodies equally and oppositely charged 
 are put into contact, the charge of each is neutral- 
 ized by that of the other. A body strongly charged 
 positively is said to be at high potential; if nega- 
 
 * In the following list, each substance becomes positively electrified 
 when rubbed with the body following it ; but negatively, with the one pre- 
 ceding it. GANOT. 
 
 1. Cat's fur. 5. Cotton. 9. Shellac. 13. Caoutchouc. 
 
 2. Flannel. 6. Silk. 10. Resin. 14. Gutta-percha. 
 
 3. Ivory. 7. The hand. 11. The metals. 15. Gun-cotton. 
 
 4. Glass. 8. Wood. 12. Sulphur. 
 
298 ELECTRICITY. 
 
 tively, at low potential; when discharged, at zero 
 potential The surface of the earth is electrically 
 at zero. 
 
 6. Conductors and Insulators. A body which 
 allows electricity to pass through it freely is termed 
 a conductor; one which does not, is called a ~bad 
 conductor, or insulator. Copper is one of the best 
 conductors, and hence it is used in many electrical 
 experiments. Glass is one of the best insulators. 
 A body is said to be insulated when it is supported 
 by some bad conductor, which is generally glass or 
 vulcanite. A body can be highly charged only when 
 insulated. In damp air electricity is quickly dissi- 
 pated. This is due to the deposit, on the glass insu- 
 lators, of a thin film of moisture, which conducts 
 away the electricity. For success in electrical experi- 
 ments, therefore, it is important to keep the air dry 
 and warm, since dry air is one of the best of insu- 
 lators.* 
 
 7. Distribution of Electricity on Bodies. A charge 
 communicated to one part of an insulator is not 
 spread over its whole surface ; but when a good con- 
 ductor is charged at any point the spread is instan- 
 taneous. It spreads, however, only on the surface, and 
 
 * The following list contains some of the most common conductors and 
 insulators : 
 
 Conductors. Insulators. 
 
 Metals. Vegetables. Dry Air. GMass. 
 
 Charcoal. Animals. Shellac. Silk. 
 
 Flame. Linen. Amber. Dry Paper. 
 
 Minerals. Cotton. Sulphur. Caoutchouc. 
 
 Acids. Dry Wood. Wax. 
 Water. Ice. 
 
FKICTIONAL ELECTRICITY. 
 
 299 
 
 not through the interior. A pith ball, if made to 
 touch the outside of an electrified metal cup or hol- 
 
 Fra. 202. 
 
 Variation in Electric Density. 
 
 low ball, is strongly repelled ; but on the interior 
 there is no such effect.* If the ball is spherical, the 
 
 FIG. 203. 
 
 * Faraday once made a hollow cube of wood, measuring 12 ft. each way 
 and covered with tin-foil. Insulating this, he 
 charged it with a powerful machine until 
 sparks darted off from every corner on the 
 outside. Going within this little room with 
 his most delicate electroscopes, he could not 
 detect the least effect upon them. He made 
 a conical bag of linen, and fastened its open 
 end to an insulated ring. Pulling it out with 
 a silken cord, he electrified it. The charge 
 was manifest on the outside, zero on the 
 inside. Beversing the pull so as to turn it 
 inside out, the new exterior was found to be 
 charged. A half -minute previously it had 
 been a neutral interior. The student should 
 try this interesting experiment, using the 
 most delicate electroscope that he can make. Faraday's Conical Bag. 
 
300 
 
 ELECTRICITY. 
 
 Fio. 204. 
 
 amount of electricity at all points of its surface is 
 the same ; or, we may say that the electric density 
 is uniform over its surface. On a cylinder the elec- 
 tric density is greatest at the ends. If one end is 
 blunt and the other sharp, the density at the sharp 
 end becomes so great that the neighboring air mole- 
 cules are quickly electrified by contact and instantly 
 repelled. Others in turn are successively repelled, 
 and the body is soon discharged. Electricity thus 
 escapes rapidly from jutting points.* 
 
 8. Electrical Induction. Let an insulated con- 
 ductor, Fig. 204, be brought near another conductor 
 
 that has been strongly 
 Sk charged positively, and 
 let a series of pairs of 
 pith balls be suspend- 
 ed from the first. The 
 motion of the balls 
 shows that the ends 
 of the insulated con- 
 ductor are electrically 
 excited, while the mid- 
 dle is neutral. The end 
 Electrical induction. nearest the. charged 
 
 conductor is excited negatively and the remote end 
 positively. If the charged conductor be removed, all 
 of the pith balls collapse. Place several insulated 
 
 * The electric whirl, mounted on the prime conductor of an electrical 
 machine, illustrates this action. As each molecule of air is repelled from 
 a point, it reacts with equal force against the point. This is sufficient to 
 set the light wire-wheel in rapid rotation. 
 
FRICTIONAL ELECTRICITY. 301 
 
 conductors, as shown in Fig. 205, the balls being 
 strongly charged, that at the right positively, and 
 that at the left negatively. Each intermediate con- 
 ductor becomes excited, as indicated, and becomes 
 
 FIG. 205. 
 
 Electrical Induction. 
 
 neutral when the balls are discharged. It has been 
 polarized by induction, like a magnetic body when 
 brought into the field of a magnet pole.* 
 
 9. The Plate Electrical Machine consists of (1) 
 a circular glass plate which can be turned by means 
 of a crank ; (2) a pair of leather or cloth rubbers 
 pressed against the plate and covered with electrical 
 amalgam or tin disulphide ; f (3) a metallic comb or 
 fork with sharp points which nearly touch the plate ; 
 (4) a prime conductor, consisting of a rounded brass 
 cylinder, insulated by resting on a glass standard, 
 and connected at one end with the comb. Frequently 
 
 * The experiment in Fig. 204 can be nicely performed by means of an 
 egg placed flatwise on the top of a dry wine-glass and the glass tube rep- 
 resented in Pig. 201. Several eggs and glasses will show the principle of 
 Fig. 205. See TyndalFs " Lessons in Electricity," p. 39. 
 
 t Electrical amalgam is a mixture of two parts each of tin and zinc, and 
 four parts of mercury. By experience it has been found that, when this ia 
 rubbed on glass, electrical separation is most easily effected. Tin disulphide 
 is often called "mosaic gold," because of its metallic yellow color. It is 
 used in bronzing. 
 
302 
 
 ELECTRICITY. 
 
 the lower half of the plate is made to revolve between 
 a pair of silken flaps (Fig. 206). A chain is usually 
 attached to the knob in connection with the rubber, 
 and connects this with the ground through the me- 
 dium of a gas-pipe or other conductor. 
 
 On turning the crank, the friction of the plate 
 against the rubbers produces electrical separation ; 
 
 FIG. 206. 
 
 The Plate Electrical Machine. 
 
 the rubbers becoming charged negatively, the plate 
 positively. The negative charge is conducted off to 
 the earth by the chain, which thus restores the rub- 
 bers to zero potential. The positive charge on the 
 plate, when this is brought opposite the comb, polar- 
 izes the prime conductor and comb by induction, 
 Positive electricity becomes manifested on the re- 
 mote conductor, and negative electricity at the comb 
 
FKICTIONAL ELECTRICITY. 
 
 303 
 
 is communicated at once by the sharp points to 
 the air, whose molecules are repelled into contact 
 with the plate, thus neutralizing its positive charge. 
 The prime conductor is hence left charged to high 
 potential. 
 
 The action of the plate machine is thus an appli- 
 cation of both friction and induction. 
 
 1C. The Theory of Attraction is likewise an ap- 
 plication of induction. In Fig. 201, where a glass 
 rod at high potential is brought near a pith ball, 
 this is polarized by induction, the nearer half becom- 
 ing negative, and the remote half positive. The 
 charge on the rod attracts the negative half and re- 
 pels the positive half. But since the negative half 
 is nearer, the attraction exceeds the repulsion, and 
 the pith ball moves toward the rod. On touching 
 this the negative charge is wholly neutralized, and 
 only repulsion can be effective. Every case of elec- 
 trical attraction is thus a case of induction. 
 
 The electric chime consists of three bells, two 
 of which, c and 5, are hung 
 by brass chains, while the 
 middle one is insulated above 
 by a silk cord, and connected 
 below with the earth by a 
 chain. The balls between 
 them are also insulated. The 
 outer bells becoming charged 
 with positive electricity from 
 the prime conductor of an Biectnc chimes, 
 
 electrical machine, polarize the balls by induction 
 
 FIG. 207. 
 
304 
 
 ELECTRICITY. 
 
 FIG. 208. 
 
 through the intervening air. The balls being then 
 attracted to the bells, are charged and immediately 
 
 repelled. Swinging away, they 
 strike against the middle bell, 
 discharging their electricity, 
 and are forthwith attracted 
 again. Flying to and fro, they 
 ring out a merry song. 
 
 The dancing image consists 
 of a pith-ball figure placed be- 
 tween two metallic plates, the 
 upper one hanging from the 
 
 Dancing Images. prime conductorj and the lower 
 
 one connected with the earth. The dance is con- 
 ducted by alternate attraction and repulsion.* 
 
 11. Free and Bound Electricity. The gold-leaf 
 electroscope is more sensitive than one of pith balls. 
 Within a dry glass jar a pair of strips of gold-leaf 
 are suspended from a metal rod terminating at the 
 top in a knob or plate. If a rod, excited for example 
 negatively, be brought near the knob, then by induc- 
 tion this becomes charged positively while both 
 leaves are charged negatively, and hence repel each 
 other. By placing the finger on the knob and with- 
 drawing it while the rod is still near, the leaves col- 
 lapse. Their negative charge has been conducted off 
 to the earth. But on withdrawing now the rod, they 
 diverge again and remain apart. The positive charge 
 
 * A slow motion should be given to the electrical wheel, and a pin 
 thrust into the heel of the image will add much to the stamp of the tiny 
 feet. 
 
FKICTIONAL ELECTRICITY. 
 
 305 
 
 on the knob was "bound" there by the presence of 
 the negatively excited rod and* could not be con- 
 ducted away, like 
 the negative charge 
 on the leaves. On 
 removing the rod 
 after the finger has 
 been taken away the 
 positive charge be- 
 comes "free " ; it is 
 distributed over 
 knob and leaves, 
 and these now repel 
 each other with a 
 positive charge. So 
 long as a charge is 
 " bound," i. e., so long 
 as the knob and 
 leaves under the in- 
 
 fluence of the excited rod are at zero potential, it fails 
 to manifest itself. 
 
 12. Inductive Capacity. A body through which 
 induction occurs is called a dielectric. The power with 
 which inductive influence acts across a dielectric is 
 called its inductive capacity, and this is different for 
 different dielectrics. Induction takes place better 
 through a plate of glass than through a layer of equal 
 thickness of shellac or air. In other words, glass has a 
 higher inductive capacity than shellac or air, and is, 
 therefore, the best dielectric of the three. 
 
 13. Electrical Condensation. By putting a good 
 
306 ELECTRICITY. 
 
 dielectric between two conducting surfaces, one of 
 which is connected with the prime conductor of an 
 electrical machine and the other with the earth, elec- 
 tricity may be strongly " condensed" on these sur- 
 faces. In Fig. 210, let the strip on the left represent 
 a conductor, of tin-foil, positively charged from the 
 machine, and that on the right a similar conductor 
 connected with the earth, the intervening space being 
 occupied by a plate of glass. This dielectric becomes 
 polarized, the surface on the right attaining a nega- 
 tive charge which is bound there, while the corre- 
 sponding positive electricity on the same side is neu- 
 tralized by connection with the earth. 
 i 2, The negative charge in turn reacts 
 
 N^^^^N through the dielectric, binding a pos- 
 itive charge on the left, whose energy 
 thus becomes potential. The con- 
 ductor can then receive a new charge 
 OOC^ from the machine, and the process 
 is repeated until the greatest charge 
 is accumulated that the condenser can carry. Its 
 molecules are then in a condition of great strain. 
 
 14. The Leyden Jar consists of a glass jar, serv- 
 ing as dielectric, coated inside and outside, not quite 
 to the top, with tin-foil. It is fitted with a cover of 
 baked wood through which passes a metal rod with 
 a knob at the top, and below a metal chain extend- 
 ing down to the inner coating. The jar is charged 
 by bringing the knob near the prime conductor of 
 the machine, while the outer coating communicates 
 with the earth. The inner coating becomes charged 
 
FRICTIONAL ELECTRICITY. 
 
 307 
 
 FIG. 211. 
 
 The Leyden Jar. 
 
 first from the machine, a succession of sparks being 
 received until the two coatings acquire a large charge 
 of bound electricity, positive with- 
 in and negative without. To dis- 
 charge it * one end of a conductor 
 with an insulated handle is put 
 on the outer coating, while the 
 other is brought near the knob 
 above. A sharp snap and a brill- 
 iant flash through the air an- 
 nounce that equilibrium is re- 
 stored. Minute particles detached 
 from the solid conductors are 
 made momentarily white - hot, 
 giving brilliancy to the spark, f 
 
 The tin-foil on a Leyden jar serves only as a con- 
 ductor, and not as an accumulator, of the charge. 
 The jar may be made with movable coatings. After 
 it is charged these may be removed. Putting the 
 same jar then into another set of coatings, it may 
 be discharged in the usual manner. 
 
 * It is said that Cuneus, a pupil at Leyden, discovered the principle of 
 the Leyden jar in the following curious way: While experimenting, he 
 held a bottle of water to the prime conductor of his electrical machine. 
 Holding the bottle with one hand, he happened to touch the water with 
 the other, when he received a shock so unexpected, and so unlike any thing 
 he had ever felt before, that he was filled with astonishment. It was two 
 days before he recovered from his fright. A few days afterward, in a 
 letter to a friend, the physicist innocently remarked, that he would not 
 take another shock for the whole kingdom of France. 
 
 t The incredibly small quantity of the metal volatized in this way is a 
 striking proof of the divisibility of matter. During some experiments at 
 the Philadelphia mint a gold pole lost in weight by a strong spark one 
 millionth of a grain ; and rsso^oo of a grain of nickel signed its name in the 
 spectroscope brilliantly. See " Popular Science Monthly," May, 1877. 
 
308 
 
 ELECTRICITY. 
 
 15. The Toepler-Holtz Electrical Machine. Many 
 improvements have been made on the plate electrical 
 machine. One of the best is the Toepler-Holtz m achine.* 
 This consists of a fixed glass plate in front of which re- 
 
 FIG. 212. 
 
 The Toepler-Holtz Machine. 
 
 volves a smaller one provided with six metallic buttons 
 (Fig. 212, &). On the rear of the fixed plate are two 
 sheets of varnished paper, a and a/, called the armature. 
 On each a strip of tinfoil is cemented, from which a 
 metallic arm extends around to the front, ending in 
 a rubber of brass filaments, r. Under this each but- 
 
 * The " Wimshurst " electrical machine is the most recent improvement 
 of electrical machines depending on induction. In these machines electricity 
 is generated by glass discs faced with metallic sectors, which are rotated in 
 opposite directions. These machines are certain in their action, and furnish 
 large quantities of electricity at a high potential. 
 
FRICTIONAL ELECTRICITY. 309 
 
 ton passes. A pair of combs, c and c', connect with 
 adjustable discharging rods, p and n. Another pair 
 of combs and brushes are attached to the brass rod, 
 dd' ; this extends across in front of the plate, which 
 revolves in the direction shown by the arrows. 
 
 If there be the least possible difference of poten- 
 tial between the two armatures, such as is naturally 
 due to accidental conditions on their surfaces, it may 
 be greatly increased by revolving the plate. Suppose 
 the left armature, a', to be faintly charged positively, 
 while the right armature, a, is neutral ; then a' in- 
 duces a slight negative bound charge on the button 
 in front, which in revolving passes under d'. Passing 
 from d' to r, the button comes opposite a neutral 
 armature. Its negative bound charge at once be- 
 comes free and is conducted through the rubber r to 
 the armature behind, charging it negatively. This 
 at once acts inductively on the button, causing it to 
 acquire a positive bound charge with which it passes 
 d. This charge is freed at r' and conducted to the 
 armature a/, strengthening its positive charge. This 
 process continues, both armatures becoming soon 
 strongly and oppositely charged. The comb, c', by 
 induction from a, is polarized. It discharges nega- 
 tive electricity, while the rod, p, acquires a strong 
 positive charge. In like manner c discharges positive 
 electricity and n acquires a strong negative charge. 
 A succession of sparks soon passes between p and n, 
 the strength of which is greatly increased by con- 
 densation in the Leyden jars, with which the dis- 
 charging rods are connected. 
 
310 ELECTKICITY. 
 
 16. Lightning is only the discharge of a Leyden 
 jar on a grand scale. If two clouds with opposite 
 charges of electricity come near together, the inter- 
 vening air reaches its limit of polarization, and a 
 flash occurs like that between the discharging-rods 
 of an electrical machine.* The air is never quite uni- 
 form in conducting power at all places, and the im- 
 mense spark, moving along the line of least resist- 
 ance, describes a zigzag course. It suddenly heats 
 the air, which expands and instantly collapses. The 
 concussion produces a series of air-waves from suc- 
 cessive parts of the spark. These constitute thunder, 
 which continues to roll because the sound is reflected 
 many times from clouds, and from masses of air 
 which differ among themselves in density. Often 
 the charged cloud approaches the ground rather 
 than another cloud. Discharge takes place, and ex- 
 posed objects, such as tall houses or trees, are de- 
 stroyed if included in the lightning's path. 
 
 LIGHTNING-KODS were invented by Franklin, f They 
 are based on the principle that electricity always 
 
 * The air is constantly electrified. In clear weather it is in a positive 
 state, but in foul weather it changes rapidly from positive to negative, and 
 vice versa. Dr. Livingstone tells us that in South Africa the hot wind 
 which blows over the desert is so highly electrified, that a bunch of ostrich 
 feathers held for a few seconds against it becomes as strongly charged as if 
 attached to an electrical machine, and will clasp the hand with a sharp, 
 crackling sound. 
 
 t Franklin's plan was opposed by many men of his day, who declared 
 it was as impious to ward off Heaven's lightning, "as for a child to ward 
 off the chastening rod of its father." There was much discussion as to 
 whether the conductors should be pointed or not. Wilson persuaded 
 George III. that the points were a republican device to injure His Majesty, 
 as they would certainly "invite" the lightning, and so the points on the 
 lightning-rods upon Buckingham Palace were changed for balls. 
 
FRICTIONAL ELECTRICITY. 311 
 
 seeks the best conductor. The rod should be pointed 
 at the top with some metal which will not easily 
 corrode. If constructed in several parts, they should 
 be securely jointed. The lower end should extend 
 into water, or else deep into the damp ground, be- 
 yond a possibility of any drought rendering the 
 earth about it a non-conductor, and be packed about 
 with ashes or charcoal. If the rod is of iron, it 
 needs to be much larger than one of copper, which 
 is a better conductor. Every elevated portion of the 
 building should be protected by a separate rod. 
 Chimneys need especial care, because of the ascend- 
 ing column of vapor and smoke. Water conductors, 
 tin roofs, etc., should be connected with the damp 
 ground or the lightning-rod, that they may aid in 
 conveying off the electricity.* 
 
 DURATION OF THE FLASH. The duration of the 
 flash from a Ley den jar has been found to vary from 
 two thousandths to forty billionths of a second. 
 When the plate of the Toepler-Holtz machine is re- 
 volving at the highest speed, each button can be mo- 
 mentarily seen, as if it were still, when illuminated by 
 the spark. The trees swept by the. tempest, or a train 
 of cars in rapid motion, when seen by a flash of 
 
 * The value of a lightning-rod consists, most of all, in its power of 
 quietly restoring the equilibrium between the earth and the clouds. By 
 erecting lightning-rods, we thus lessen the liability of a sudden discharge. 
 Every drop of rain, and every snow-flake, falls charged with electric energy, 
 and thus quietly disarms the clouds of their terror. The balls of electric 
 light, called by sailors " St. Elmo's fire," which sometimes cling to the masts 
 and shrouds of vessels and the flames said to play about the points of 
 bayonets, indicate the quiet escape of electricity from the earth toward the 
 clouds. 
 
312 ELECTRICITY. 
 
 lightning, seem motionless ; while a cannon-ball, in 
 swift flight, appears poised in mid-air. 
 
 17. Effects of Frictional Electricity. (1.) PHYS- 
 ICAL. Discharges from a large battery of Leyden 
 jars will melt metal rods, perforate glass, split wood, 
 magnetize steel bars, etc. Let a person stand upon 
 an insulated stool and become charged from the 
 prime conductor. His hair, through repulsion, will 
 stand erect in a ludicrous manner. On presenting 
 his hand to a little ether contained in a warm spoon, 
 a spark leaping from his extended finger will ignite 
 
 FIG. 213. 
 
 Rod of Steel ready for Magnetization. 
 
 it. If he hold in his hand an icicle, the spark will 
 readily dart from it to the liquid.* A card held be- 
 tween the knob of a Leyden jar and that of the dis- 
 charger, will be punctured by the spark. A piece of 
 steel may be magnetized by the discharge from a 
 Leyden jar. Wind a covered copper wire around a 
 steel bar, as in Fig. 213, or inclose a needle in a 
 small glass tube, around which the wire may be 
 wound. On passing the spark through the wire, the 
 needle will attract iron filings. When strips of tin- 
 foil are pasted on glass, and figures of various pat- 
 terns cut from them, the electric spark leaping from 
 
 * This experiment can be more surely performed by using disulphide 
 of carbon. The insulating stool may be merely a board laid on four dry 
 flint-glass bottles or goblets, and the electricity be developed by rubbing a 
 glass tube. 
 
VOLTAIC ELECTKICITY. 
 
 313 
 
 FIG. 214. 
 
 
 one to the other presents a beautiful appearance. If 
 a battery be discharged through a small wire the 
 electricity will be changed to heat, 
 and the wire, if sufficiently small, 
 will be fused into globules or dissi- 
 pated in smoke. 
 
 (2.) CHEMICAL EFFECTS. The 
 "electric gun" is filled with a mixt- 
 ure of oxygen and hydrogen gases. 
 A spark causes them to combine 
 with a loud explosion and form 
 water. The sulphurous smell which 
 accompanies the working of an electrical machine, 
 and is noticed in places struck by lightning, is 
 owing to the production of ozone, a peculiar form of 
 the oxygen of the air. (See "Popular Chemistry," 
 p. 23.) 
 
 (3.) PHYSIOLOGICAL EFFECTS. A slight charge from 
 a Leyden jar produces a contraction of the muscles 
 and a spasmodic sensation in the wrist. A stronger 
 one becomes painful and even dangerous. 
 
 Illuminated Pane. 
 
 II. VOLTAIC ELECTRICITY.* 
 
 1. Simple Voltaic Circuit. If a strip of zinc, 
 coated over with mercury, be put into a mixture of 
 sulphuric acid and water, no perceptible chemical 
 action will be noticed. But if a strip of copper or 
 
 * This name is given in honor of the Italian physicist who made the 
 first discoveries in this branch of electricity. 
 
314 ELECTRICITY. 
 
 platinum be immersed at the same time, and the 
 upper ends of the two pieces of metal be touched 
 together or connected by wires, many little bubbles 
 of gas will be seen on the second strip, forming and 
 rising to the surface. When the experiment is per- 
 formed in the dark, an almost in- 
 
 FIG 215. 
 
 finitesimal spark is perceived at 
 the moment the wires are joined.* 
 Two metal plates joined in this 
 way form a voltaic pair. The ex- 
 posed end of the copper or platinum 
 plate is called the positive pole, and 
 that of the zinc the negative pole 
 of the pair, f Joining the wires. 
 
 A Voltaic Pair. 
 
 or otherwise connecting the poles, 
 is termed closing the circuit; and separating them, 
 breaking the circuit. A cup prepared for such an ex- 
 periment is called a voltaic cell. 
 
 2. Action in the Voltaic Cell. Zinc is far more 
 easily acted upon by sulphuric acid than copper is. 
 Each molecule of the acid is composed of two atoms 
 of hydrogen, one of sulphur, and four of oxygen. J 
 This maybe expressed by the symbol, H 2 S0 4 . When 
 the acid, mixed with water, acts on zinc (Zn), its hy- 
 drogen is set free, and a new substance, called zinc 
 
 * We can easily form a simple galvanic circuit by placing a silver coin 
 between our teeth and upper lip, and a piece of zinc under our tongue. 
 On pressing the edges of the two metals together, a peculiar taste will be 
 perceived. 
 
 t These names may easily be remembered if we associate the p's with 
 copper and positive, and the n's with zinc and negative. 
 
 t For the properties of these elements, refer to "Popular Chemistry," 
 pp. 11, 38, and 103. 
 
VOLTAIC ELECTKICITY. 315 
 
 sulphate, is produced. Its symbol is Zn S0 4 . It is at 
 once dissolved in the water, leaving a fresh surface 
 of metal to be attacked by the acid. It is thought 
 that each molecule of liquid between the copper and 
 zinc becomes polarized, then decomposed, giving up 
 its H 2 to its neighbor on the side toward the copper, 
 and its S0 4 to its neighbor on the side toward the 
 zinc. The H 3 liberated, in contact with the copper, 
 gathers in bubbles of gas ; the S0 4 , in contact with 
 the zinc, unites with this metal to form zinc-sulphate, 
 which dissolves in the liquid of the cell. If the ex- 
 posed ends of the two plates be examined with a suf- 
 ficiently delicate electroscope, while they are still 
 separate, it is found that the copper is electrically at 
 higher potential than the zinc. When they are con- 
 nected, neutralization instantly takes place, but the 
 action of the acid on the zinc renews the difference of 
 potential between the copper and the zinc, so that the 
 process continues as long as this action continues.* 
 
 3. The Electric Current. The term "current" is 
 applied to this continuous neutralization and renewal 
 of difference of potential in the closed voltaic circuit. 
 The current is said to " flow " through the conduct- 
 ing wire from the copper at high potential toward the 
 zinc at low potential, just as water flows from an 
 
 * With, what inconceivable rapidity must these successive changes take 
 place in an iron wire to transmit the electric energy, as in actual experi- 
 ments, from Valentia, Ireland, across the bed of the Atlantic and the 
 American continent to San Francisco and return, a distance of 14,000 
 miles, in two minutes ! In fact, it far surpassed the velocity of the earth's 
 rotation, by which we measure time and leaving Valentia at 7:21 A. M., 
 Feb. 1, it reached San Francisco at 11:20 p. M., Jan. 31. 
 
816 ELECTRICITY. 
 
 elevated reservoir through a pipe toward a lower reser- 
 voir. There is no actual transfer of matter, no cur- 
 rent of fluid ; but only by analogy we may call it a 
 current of energy transmitted through the entire 
 thickness and length of the conducting wire. By 
 analogy also the current is said to pass through the 
 cell from zinc to copper, thus completing its circuit. 
 
 4. The Volt. We measure the difference of tem- 
 perature between two bodies in degrees on the ther- 
 mometer scale, or the difference of level between the 
 surfaces of two reservoirs in feet or meters. These 
 are the accepted units of measurement. In like 
 manner, for measuring the difference of potential 
 between two bodies, a unit called the volt* has been 
 selected. In the simple voltaic cell already described, 
 when freshly set in action, the difference of poten- 
 tial between its poles is about one volt. The force 
 due to difference of potential is called electro-motive 
 force, and is always measured in volts, f 
 
 * Named in honor of Alessandro Volta, an Italian physicist, who was 
 born in 1745. In 1793, he communicated to the Royal Society of London 
 an account of his important experiments on which the modern science of 
 electricity has been largely built. 
 
 . t The difference of potential between the discharging rods of a Voss 
 electrical machine when giving long sparks is often several hundreds of 
 volts. When passed through the body, such momentary currents are pain- 
 ful. The potential of the air during a thunder-storm quickly changes 
 through thousands of volts. The voltaic cell furnishes a current that is 
 exceedingly steady in comparison with the stream of sparks from an elec- 
 trical machine, but of only small electro-motive force. Frictional electricity 
 is sudden, noisy, convulsive; voltaic is gentle, silent, yet powerful. The 
 one is like a quick, violent blow, as of a swiftly moving bullet ; the other like 
 the steady uniform pressure produced by a large mass slowly advancing 
 against resistance. Lightning leaps across miles of air ; the voltaic current 
 will pass through a conductor from England to California rather than 
 spark across half an inch of air. The most powerful frictional machine 
 
VOLTAIC ELECTRICITY. 317 
 
 5. Electrical Resistance. Every conductor op- 
 poses resistance to the electric current. The amount 
 of resistance offered depends upon the nature of the 
 conductor, its length and cross-section. Metals offer 
 much less resistance than liquids, and these again less 
 than substances classed as insulators (7, p. 298). For 
 the same material the resistance increases with its 
 length and decreases with increase of its cross-section. 
 
 6. The Ohm. To measure resistance, a unit called 
 the ohm* has been selected. A piece of common 
 copper wire, as thick as the band shown in Fig. 216, 
 and fifty yards long, opposes a resist- FIG. 210. 
 ance of about one ohm. Coils whose 
 
 resistance in ohms is known are much used in elec- 
 trical measurement. 
 
 7. The Ampere. To measure the effective cur- 
 rent strength obtained from a voltaic cell, a unit 
 called an ampere \ has been selected. It is the 
 
 would be insufficient for telegraphing ; while signals have been sent across 
 the ocean with a tiny battery composed of " a gun-cap and a strip of zinc, 
 excited by a drop of water the bulk of a tear." " Faraday immersed a 
 voltaic pair, composed of a wire of platinum and one of zinc, in a solution 
 of four ounces of water and one drop of oil of vitriol. In three seconds this 
 produced as great a deviation of the galvonometer needle as was obtained 
 by 30 turns of the powerful plate-glass machine. If this had been concen- 
 trated in one millionth of a second, the duration of an electric spark, it 
 would have been sufficient to kill a cat ; yet it would require 800,000 such 
 discharges to decompose a grain of water." 
 
 * Named in honor of Dr. Q-. S. Ohm, a German physicist, who deter- 
 mined the relation existing between current strength, electro-motive force, 
 and resistance. 
 
 t Named for Andre Marie Ampere, a French physicist, born in 1775, 
 whose splendid work in electricity was such as to give him the highest 
 rank along with Volta. 
 
 The definitions of electrical units given in the text are not exact 
 enough to furnish more than the most elementary ideas. The student will 
 
318 ELECTRICITY. 
 
 amount of current obtained when one volt of electro- 
 motive force acts against one ohm of resistance. 
 
 With these units electricity can be measured with 
 as much exactness as we measure quantities of grain 
 or water. 
 
 8. A Battery consists of two or more voltaic cells 
 so connected as to secure a stronger current than 
 can be obtained from a single cell. According to 
 Ohm's Law, the current strength (C) in amperes is 
 equal to the electro-motive force (E) in volts divided 
 by the resistance in ohms. The resistance is partly 
 in the external conductor (R) and partly in the 
 liquid of the battery (r). The law is expressed in a 
 formula, thus, 
 
 c _ E 
 
 ~ R + r 
 
 With a large number of cells a battery, therefore, 
 can be arranged either to overcome a large external 
 resistance, or, when this is small, to furnish a strong 
 current. 
 
 9. Polarization within the Battery. As soon as 
 the action of a battery is well begun, the electro- 
 motive force becomes rapidly diminished because 
 hydrogen tends to collect upon the plate in connec- 
 tion with the positive pole. The bubbles interfere 
 with further action and start a counter electro- 
 motive force which neutralizes much of that in 
 operation. Many different devices have been em- 
 find them more accurately defined in any text-book devoted specially to 
 Electricity, such as that of Thompson or Urbanitzky. 
 
VOLTAIC ELECTRICITY. 
 
 319 
 
 FIG. 217. 
 
 ployed to diminish this evil, and each gives rise to a 
 special kind of battery. Only a few need be de- 
 scribed. 
 
 1O. The Potassium Bichromate Battery. Instead 
 of copper, a pair of plates of car^ 
 bon are immersed, with a plate of 
 zinc between them, arranged so 
 as to slide into the liquid or out 
 of it at will. A solution of po- 
 tassium bichromate in sulphuric 
 acid and water is used. The sul- 
 phuric acid acts on the zinc, and 
 the hydrogen is prevented from 
 forming in bubbles by being com- 
 bined at once with some of the 
 oxygen which the chromic acid 
 yields. 
 
 DANIELL'S BATTERY. In this 
 
 battery there are two fluids separated by a cup of 
 porous earthenware, which does not prevent the pas- 
 sage of 'the current. In the outer 
 vessel of glass there is a strong solu- 
 tion of copper sulphate (CuS0 4 ) in 
 which a split copper cylinder is im- 
 mersed. Within this is placed the 
 porous cup, containing a rod of zinc 
 coated with mercury and a mixture 
 of sulphuric acid with water. Zinc 
 sulphate is produced, and the liber- 
 ated hydrogen decomposes some of the copper sul- 
 phate, taking its S0 4 and causing a deposit of metal- 
 
 Potassium Bichromate Cell. 
 
 FIG. 218. 
 
 Daniell Cell. 
 
320 
 
 E L E C T R L C 1 T Y . 
 
 PIG. 219. 
 
 lie copper. Polarization is thus prevented, and this 
 is one of the most constant batteries known. 
 
 GROVE'S BATTEEY. In this the outer cup contains 
 the zinc and dilute sulphuric acid. With- 
 in the porous cup a strip of platinum dips 
 into strong nitric acid. The hydrogen de- 
 composes some of the nitric acid, taking 
 oxygen from it to produce water and lib- 
 erating red fumes of nitrogen tetroxide, 
 which are unpleasant and hurtful. This 
 battery gives very high electro-motive force. 
 BUNSEN'S BATTERY. In this rods of carbon are 
 substituted for the strips of platinum used in the 
 Grove battery. Sometimes potassium bichromate 
 
 Fro. 220 
 
 Grove Cell. 
 
 Bunsen Battery. 
 
 solution is substituted for nitric acid in order to 
 avoid the production of nitrous fumes. Fig. 220 
 shows a Bunsen battery arranged in series. 
 
 11. Effects of Voltaic Electricity. (1.) PHYSICAL. 
 If a current of electricity is passed through a wire 
 too small to conduct it readily, it is converted into 
 
VOLTAIC ELECTRICITY. 
 
 321 
 
 FIG. 221. 
 
 heat. The poorer the conducting power of the wire, 
 and hence the greater the resistance, the more 
 marked the effect. With ten or twelve Grove's cups 
 several inches of fine steel wire may be fused ; and 
 with a powerful battery, several yards of platinum 
 wire may be made to glow with very brilliant effect, 
 giving a steady light.* 
 
 In closing or breaking the circuit, we produce a 
 spark, the size of which depends on the electro- 
 motive force and cur- 
 rent strength of the 
 battery. With several 
 cells, beautiful scintil- 
 lating sparks are ob- 
 tained by fastening 
 one pole to a file and 
 rubbing the other up- 
 on it. When charcoal 
 or gas -carbon elec- 
 trodes are used with 
 a powerful battery, on 
 slightly separating the 
 points, the intervening 
 space is spanned by an arch of the most dazzling 
 light (Fig. 221). The flame, reaching out from the 
 positive pole like a tongue, vibrates around the neg- 
 ative pole, licking now ^ on this side and now on that. 
 The heat is intense. Platinum melts in it like wax 
 
 * Torpedoes and blasts are fired on this principle. Two copper wires 
 leading from the battery to the spot are separated in the powder by a short 
 piece of small steel wire. When the circuit is completed, the fine wire be- 
 comes red-hot and explodes the charge, 
 
 The Arc Light. 
 
322 ELECTRICITY. 
 
 in the flame of a candle,* the metals burn with their 
 characteristic colors ; and lime, quartz, etc., are fused. 
 The effect is not produced by burning the charcoal 
 points, since in a vacuum it is equally brilliant. 
 
 (2.) CHEMICAL EFFECTS. Electrolysis (to loosen by 
 electricity) is the process of the decomposition of 
 compound bodies by the voltaic current. If plati- 
 num electrodes be held a little distance apart in a 
 cup of water mixed with sulphuric acid, tiny bubbles 
 will immediately begin to rise to the surface. When 
 the gases are collected, they are found to be oxygen 
 and hydrogen, in the proportion of two volumes of 
 the latter to one of the former, f In the electrolysis of 
 
 * To show the varying conducting power of the different metals, fasten 
 together alternate lengths of silver and platinum wire and pass the current 
 through them. The latter will glow, while the former, conveying the elec- 
 tricity more perfectly, will scarcely manifest its presence. 
 
 There are two forms of the electric light now used the arc (shown in 
 Pig. 221), where the current passes between two carbon points; and the 
 incandescent, where the current heats to a dazzling white a carbon strip 
 placed in the circuit. The former is employed in lighting streets, railroad 
 stations, and large halls ; the latter is generally used in dwellings, etc., as it 
 gives a softer light, and is much more steady. Edison's Lamp consists of a 
 tiny carbon loop placed in a glass globe from which the air has been so 
 completely exhausted as to leave only i ti oo ooo of an atmosphere. When 
 exposed to the air, the voltaic arc rapidly wastes the carbon points. Elec- 
 tric lamps have therefore been devised that, by a self-acting apparatus, 
 keep the points at a proper distance from each other. 
 
 t If the copper poles be inserted, bubbles will pass off from the nega- 
 tive, but none from the positive pole, since the oxygen combines with the 
 copper wire. That gas has no effect on platinum. The burning of an atom 
 of zinc in the battery develops enough electricity to set free an atom of 
 oxygen at the positive pole. It is interesting to notice that in the battery 
 there is zinc burning, i. e., combining with oxygen, but without light or 
 heat ; in the electric light the real force of the combustion is revealed. We 
 may thus transfer the light and heat to a great distance from the place 
 where they take their origin. The transmission of energy thus to a distance 
 is better effected through electricity than through any other agency. Much 
 ingenuity has been expended on machines for this purpose. 
 
VOLTAIC ELECTRICITY. 
 
 323 
 
 compounds, their elements are found to be in differ- 
 ent electrical conditions. Hydrogen and most of the 
 metals go to the negative pole, and are electro-posi- 
 tive. Oxygen, chlorine, sulphur, etc., go to the posi- 
 tive pole, and are therefore electro-negative. 
 
 On disconnecting the electrodes of the voltameter 
 (Fig. 2^22) from the battery and joining them with a 
 conductor, a current passes through the conductor 
 
 FIG. 222. 
 
 H 
 
 from the electrode covered with oxygen to that cov- 
 ered with hydrogen. A voltameter thus charged con- 
 stitutes a secondary cell in which the energy of a 
 current may be stored up and again given out.* 
 
 Electrotyping is the process of depositing metals 
 from their solutions by electricity. It is use'd in 
 
 * Faure's accumulator consists of two lead plates coated with red lead, 
 rolled together with flannel between them, and immersed in dilute sulphuric 
 acid. A current of electricity passed through such a cell changes a part of 
 the red lead on the positive plate into peroxide of lead ; and a part of the red 
 lead on the negative plate into spongy metallic lead. A battery of these cells 
 when freshly charged will retain its energy and produce a sustained current 
 when desired. 
 
324 
 
 ELECTRICITY. 
 
 copying medals, wood-cuts, types, etc. An impres- 
 sion of the object is taken with gutta-percha or 
 wax. The surface to be copied is brushed with 
 black-lead to render it a conductor. The mold is 
 then suspended in a solution of copper sulphate, 
 from the negative pole of the battery, and a plate 
 
 PIG. 223 
 
 Electrotyping. 
 
 of copper is hung opposite on the positive pole. 
 The electric current decomposes the copper sul- 
 phate ; the metal goes to the negative pole and is 
 deposited upon the mold, while the acid, passing to 
 the positive pole, dissolves the copper, and preserves 
 the strength of the solution.* 
 
 * While the plate is hanging in the solution there is no noise heard or 
 bubbling seen. The most delicate sense fails to detect any movement. Yet 
 the mysterious electric force is continually drawing particles of ruddy, solid 
 copper out of the blue liquid, and, noiselessly as the fall of snow-flakes, drop- 
 ping them on the mold ; producing a metal purer than any chemist can 
 manufacture, spreading it with a uniformity no artist can attain, and 
 copying every line with a fidelity that knows no mistake. 
 
VOLTAIC ELECTRICITY. 325 
 
 Electro-elating is the process of coating with sil- 
 ver or gold by electricity. The metal is readily de- 
 posited on German silver, brass, copper, or nickel 
 silver (a mixture of copper, zinc, and nickel). The 
 objects to be plated are thoroughly cleansed, and 
 then hung from the negative pole in a solution of 
 silver, while a plate of silver is suspended on the 
 positive pole. In five minutes a " blush " of the 
 metal will be deposited, which conceals the other 
 metal and is susceptible of polish.* 
 
 (3.) PHYSIOLOGICAL EFFECTS. With a single cell no 
 special sensation is experienced when the two poles 
 are held in the hands. With a large battery a sud- 
 
 A * Place in a large test-tube a silver coin with a little nitric acid. If 
 the fumes of the decomposed acid do not soon rise, warm the liquid. 
 When the silver is dissolved, fill the tube nearly full of soft water. Next 
 drop hydrochloric acid into the liquid until the white precipitate (silver 
 chloride) ceases to fall. When the chloride has settled, pour off the colored 
 water which floats on top. Pill the tube again with soft water ; shake it 
 thoroughly ; let it settle, and then pour off as before. Continue this proc- 
 ess until the liquid loses all color. Finally, fill with water and heat mod- 
 erately, adding potassium cyanide (the pupil will remember that this sub- 
 stance is exceedingly poisonous) in small bits as it dissolves, until the 
 chloride is nearly taken up. The liquid is then ready for electro-plating. 
 Thoroughly cleanse a brass key, hang it from the negative pole of a small 
 battery, and suspend a silver coin from the positive pole. Place these in 
 the silver solution, very near and facing each other. When well whitened 
 by the deposit of silver, remove the key and polish it with chalk. In the 
 arts the polishing is performed by rubbing with "burnishers." These are 
 made of polished steel, and fit the surfaces of the various articles upon 
 which they are to be used. It is said that an ounce of silver can be spread 
 over two acres of surface. A well-plated spoon receives about as much sil- 
 ver as there is In a ten-cent piece. The only method of deciding accurately 
 the amount deposited is by weighing the article before and after it is 
 plated. A vessel may be "gold-lined" by filling it with a solution of gold, 
 suspending in it a slip of gold from the positive pole of the battery, and 
 then attaching the negative pole to the vessel. The current passing 
 through the liquid causes it to bubble like soda-water, and in a few mo- 
 ments deposits a thin film of gold over the entire surface. 
 
326 
 
 ELECTRICITY. 
 
 den twinge is felt, and the shock becomes painful 
 and even dangerous, especially if the palms are 
 moistened with salt or acid water to increase the 
 conduction. Rabbits which had been suffocated for 
 half an hour, have been restored by an application 
 of a strong voltaic current. 
 
 III. TRANSFORMATIONS OF ELECTRIC ENERGY. 
 
 1. Effect of a Voltaic Current on a Magnetic 
 Needle. If a wire conducting an electric current be 
 placed over a poised magnetic needle, this tends to 
 place itself at right angles to the wire. 
 
 FIG. 224. 
 
 Effect of Current on Needle. 
 
 Assuming the direction of the current to be 
 northward, the north pole of the needle will be 
 turned toward the left. The same effect will be 
 produced if the current pass southward under the 
 
TRANSFORMATIONS OF ENERGY. 
 
 327 
 
 needle, or vertically downward on the north side of 
 it, or vertically upward on the south side of it. 
 By reversing these conditions, the north pole of the 
 needle will be turned toward the right. The play of 
 the needle becomes thus a test of the presence and 
 direction of an electric current.* The delicacy of 
 
 FlG. 225. 
 
 \ 
 
 Pivoted Hoop of Conducting Wire. 
 
 this test is greatly increased if the wire, properly 
 insulated, be coiled into a ring with many turns, at 
 the center of which the needle is pivoted or sus- 
 pended. 
 
 2. A Wire bearing a Current acts like a Mag- 
 net. Let a copper-wire hoop be pivoted, as shown 
 in Fig. 225, so that its plane is in a north and south 
 
 * Ampere gave a very convenient rule for determining the direction 
 of the current from the motion of the magnetic needle. Imagine the cur- 
 rent to be like a stream of water, with a little swimmer in it, facing the 
 needle and swimming along with the current. The north pole of the needle 
 will always turn toward his left. The pupil should try the experiment and 
 test it in all possible ways. 
 
328 
 
 ELECTRICITY. 
 
 FIG. 226. 
 
 direction. On passing an electric current through the 
 hoop it slowly turns until its plane assumes an east 
 and west position and its axis points north and south. 
 Evidently the current-circle behaves like a magnet ; 
 
 one side of it being north, 
 the other south-magnetic. 
 A more effective arrange- 
 ment consists of a wire 
 wound spirally as shown in 
 Fig. 226. A wire so wound 
 is called a solenoid. On pass- 
 ing a current through the 
 
 solenoid it promptly places itself with its axis in the 
 magnetic meridian. With a pair of such solenoids (see 
 Fig. 227) the experiments relating to the attraction and 
 
 FIG. 227. 
 
 Pivoted Helix of Wire. 
 
 Two Helices acting like Magnets. 
 
 repulsion of poles of magnets may be repeated. The 
 polarity of the solenoid depends upon the direction in 
 which the current passes through it, and may be deter- 
 mined by the following rule : If on looking along the 
 axis of the solenoid the current runs through it in a 
 
TRANSFORMATIONS OF ENERGY. 329 
 
 direction contrary to the direction of the hand of a 
 watch (viewed face up), the observer is looking at the 
 north end of the solenoid. 
 
 FIG. 228. 
 
 The Tangent Galvanometer. 
 
 3. The Tangent Galvanometer. Any instrument 
 designed to measure the length of an electric cur- 
 rent is called a galvanometer. Of the many varieties, 
 the tangent galvanometer is the most important. It 
 consists of one or more coils of insulated wire wound 
 upon a wooden hoop, at the center of which a small 
 
330 
 
 ELECTRICITY. 
 
 PlO. 229. 
 
 magnetic needle is pivoted or suspended (Fig. 228). 
 The plane of the coils is made to coincide with that 
 of the magnetic meridian. The earth's magnetism 
 tends to keep the needle in this plane. 
 The magnetic effect of a current 
 tends to make it assume a position 
 across this plane. Obeying both forces 
 it assumes an oblique position, so as 
 to make a measurable angle with the 
 meridian. The strength of current is 
 proportional to the tangent of this 
 angle. 
 
 4. The Electro-magnet. If a 
 
 current be passed through a coil held 
 vertically (Fig. 229), a rod of soft 
 iron placed below will be drawn up 
 into the coil, springing up as if en- 
 dowed with life at the moment the 
 A Magnetic Mahomet's current begins. It drops as soon as 
 
 Coffin- 
 
 the circuit is broken.* Let a pair of 
 such coils be fixed around the arms of a U-shaped 
 rod of soft iron. This becomes a strong horseshoe 
 magnet, whose strength comes and goes as the cur- 
 rent is made or broken. It is therefore called an 
 electro-magnet. Such magnets have been made 
 strong enough to sustain a weight of several tons 
 attached to the armature below. 
 
 5. The Electro-magnetic Telegraph depends on 
 
 * Thus is realized in science the fabulous story of Mahomet's coffin, 
 which is said to have been suspended in mid-air. 
 
TRANSFORMATIONS OF ENERGY. 
 
 331 
 
 FIG. 230. 
 
 the principle of closing and breaking the circuit at 
 one station, and thereby making and unmaking an 
 electro-magnet at the station 
 with which communication is 
 held. A single wire connects 
 the two stations and is joined 
 at each station to a key, a reg- 
 ister (sounder), and a battery. 
 One pole of each battery is con- 
 nected with the ground. When 
 a current is sent along the wire 
 the circuit is completed through 
 the earth. The key is used for 
 sending messages ; the register for receiving them. 
 
 FIG. 231. 
 
 The Electro-magnet. 
 
 The Telegraph Key. 
 
 The key is shown in Fig. 231. E and F are 
 screws which fasten the instrument to the table, 
 and also hold the two ends of the wire. F is insu- 
 
332 ELECTRICITY. 
 
 lated by a ring of vulcanite where it passes through 
 the table and the metal plate B. H is a lever with 
 a finger-button G, a spring /, to keep it lifted, and a 
 screw Dj to regulate the distance it can move. At 
 A is a break between two platinum points, which 
 form the real ends of the wires. When Gr is de- 
 pressed, the circuit is complete, and when lifted, it 
 is broken. C is a circuit-closer that is used when 
 the key is not in operation ; the arm being pushed 
 under A touches the platinum wire, and so com- 
 pletes the circuit. It is pushed out whenever the 
 operator manipulates @. Then, by moving G-, he 
 can "close" or "open" the circuit at pleasure. He 
 thus sends a message. 
 
 The Register. 
 
 The register contains an electro-magnet, E (Fig. 
 232). When the circuit is complete, the current, 
 passing through the coils of wire at E, attracts the 
 armature m. This elevates n, the other end of the 
 lever mn, "and forces the rounded point x firmly 
 against the soft paper a. As soon as the circuit is 
 broken, E ceases to be a magnet, and the spring R 
 lifts the armature, drawing the point from the paper. 
 
TRANSFORMATIONS OF ENERGY. 
 
 833 
 
 Clock-work attached to the rollers at z moves the 
 paper along uniformly beneath the point x. When 
 the circuit is completed and broken again instantly, 
 there is a short dot made on the paper. This is 
 called e ; two dots, i ; three dots, s ; four dots, h. If 
 the current is closed for a longer time, the mark be- 
 comes a dash, t ; two dashes, m ; a dot and a dash, a. 
 
 TABLE OF MOKSE'S SIGNS. 
 
 a 
 
 j 
 
 s 
 
 b 
 
 k 
 
 t 
 
 c . . . 
 
 1 
 
 u 
 
 d 
 
 m 
 
 v ... 
 
 e 
 
 n 
 
 w 
 
 f 
 
 o ' . - 
 
 x . " 
 
 
 
 y. 
 
 
 
 
 h .... 
 
 q 
 
 z - - 
 
 i - 
 
 r ' 
 
 & . ... 
 
 A skillful operator becomes so accustomed to the 
 sound that the clicking of the armature is perfectly 
 intelligible. He uses, therefore, simply a "sounder" 
 i.e., a register without the paper and clock-work at- 
 tachment. Indeed, the register has now gone almost 
 entirely out of use, and every operator is required 
 to read by sound. 
 
 RELAY. When the stations are more than fifty or 
 sixty miles apart, the current becomes generally too 
 weak to work the register. By substituting the relay 
 for it in the line circuit, the force of a local battery 
 may be employed to work the sounder or register. 
 
334 
 
 ELECTKICITY. 
 
 In Fig. 233, which represents a relay, D is connected 
 with the line wire, and C with the ground wire ; A 
 is connected with the positive pole of the local bat- 
 tery, and B with the register or sounder, and thence 
 with the negative pole of this battery. The main 
 current passes in at D, traverses the fine wire of 
 the electro-magnet, K, and thence passes out at C to 
 the ground. The armature E, playing to and fro as 
 
 FIG. 233. 
 
 The Relay. 
 
 the current from the distant station passes through 
 or is cut off, moves the lever F. This works on an 
 axis at the lower end and is drawn back by the 
 spring H, which is regulated by the thumb-screw I. 
 As E is attracted, the circuit at G is closed ; the 
 current from A traverses a wire underneath, up F, 
 and down L, and back through another wire under- 
 neath to B-, thence to the electro - magnet of the 
 sounder, which therefore attracts its armature. 
 
 The operator who sends the message simply com- 
 pletes and breaks the circuit with the key ; the ar- 
 mature of the relay, at the station where the mes- 
 
TRANSFORMATIONS OF ENERGY. 335 
 
 sage is received, vibrates in unison with these move- 
 ments ; the register or sounder repeats them with 
 greater force ; and, the second operator interprets 
 their meaning.* 
 
 6. Ocean Cables. The Atlantic and Indian Oceans 
 have been spanned with cables of insulated wire for 
 the transmission of telegraphic messages. The cable 
 must be well insulated and p^ 334. 
 
 very strong. In the middle is 
 a bundle of copper wires, 
 (Fig. 234) ; this is buried with- 
 in a sheathing of gutta-percha, 
 (7; around this is a group of 
 cords of tarred hemp, H, to 
 
 protect the gutta-percha ; and, to give still further 
 protection and strength, fifteen or twenty iron wires 
 are twisted around the whole, so as to make it a 
 rope about an inch thick. 
 
 In signaling over a long cable, allowance has to 
 be made for the great resistance of the long wire, 
 and the slowness with which the circuit has to be 
 operated. Instead of using a relay or sounder, it is 
 necessary to use a delicate galvanometer as a re- 
 ceiver. A beam of light is reflected from a little 
 
 * The simple telegraph, instrument is but one of a multitude of appli- 
 cations that have been made of the electro-magnet. By various ingenious 
 devices it has become possible to send two, or even four, messages with 
 reasonable rapidity over the same line at the same time. By one system, 
 devised by Mr. Delany, as many as seventy-two circuits have been operated 
 with a single instrument at the rate of two or three words per minute. 
 The message is often printed at the moment it is received. Prom the Stock 
 Exchange in New York hundreds of printed reports are thus sent at the 
 same time to offices in various parts of the city. 
 
SB6 
 
 ELECTRICITY. 
 
 mirror attached to the magnetic needle, and the 
 swinging of the bright spot on a screen is interpreted 
 as an alphabet. Or, a fine glass siphon tube is at- 
 tached to a movable galvanometer coil which swings 
 between the poles of an electro-magnet. Its short 
 arm dips into a vessel of ink which is insulated and 
 can be electrified. The long arm has its end over a 
 strip of paper moved by clock-work. The electrifica- 
 
 FIG. 235. 
 
 K 
 Siphon-recorder Alphabet. 
 
 tion of the ink causes it to issue in fine drops over 
 the moving paper, and a sinuous line is recorded. 
 This " Siphon-recorder " alphabet is partly shown in 
 Fig. 235. 
 
 7. Electro-Magnetic Induction. An electric cur- 
 rent or a magnet produces in the space about it what 
 is called a field of force. The greater the strength of 
 the current or magnet, the greater is the force in the 
 field. The farther we proceed from the current or 
 magnet, the less becomes the force. A closed circuit in 
 
TRANSFORMATIONS OF ENERGY. 
 
 337 
 
 which no battery is included, placed in such a field will 
 have a current induced in it whenever the force in the 
 field acting on the circuit varies in strength. When- 
 ever, therefore, the inducing current is made or broken, 
 or the inducing magnet made or unmade, or when 
 their strength is simply varied. Relative motion of the 
 inducing current or magnet and the closed circuit will 
 
 PIG. 236. 
 
 Current Induction. 
 
 also vary the force acting on the closed circuit and pro- 
 duce in it an induced current. At every such change a 
 momentary current is induced in the closed circuit. 
 Let the coil P (Fig. 236), which forms a closed circuit 
 with the battery, be thrust into the coil 7, which forms 
 a closed circuit with the galvanometer. Instantly the 
 needle turns, and then comes back to rest. Suddenly 
 withdraw P. The needle turns in the opposite direc- 
 tion, and again comes back to rest. P is called the 
 primary coil ; 7, the secondary or induction coil, because 
 
338 ELECTRICITY. 
 
 currents in it are induced by the motion of P. The 
 current in /is opposite in direction to that of Pwhen 
 this is thrust in, and in the same direction when P is 
 withdrawn. Similar effects are obtained if a magnet 
 be substituted for P, or if in the first experiment the 
 current be alternately made and broken in P. 
 
 8. Ruhmkorff ' s Induction Coil is provided with 
 an automatic circuit-breaker, consisting of an electro- 
 
 FIG. 237. 
 
 Induction Coil. 
 
 magnet whose current passes through a spring to 
 which the armature is attached. When there is no 
 current this spring touches a " contact point " which 
 forms part of a circuit. On dipping the zinc into the 
 acid of the battery, the current excites the electro- 
 magnet. This attracts the armature, and thus removes 
 the spring from the contact point. The circuit is hence 
 broken, and the spring draws back the armature, mak- 
 ing contact again. By this device, the current in the 
 primary is automatically made and broken with great 
 rapidity, and currents alternately in opposite directions 
 are induced in the secondary. The intensity of these 
 currents may be greatly increased by placing a bundle 
 of soft iron wires in the axis of the primary. The in- 
 ductive effect due to the magnetizing and demagnet- 
 
TRANSFORMATION'S OF ENERGY. 339 
 
 izing of the iron wires by the making and breaking of 
 the current in the primary is thus added to the induc- 
 tive effect of the latter. The insulated wire of the 
 secondary coil is long and fine, sometimes a hundred 
 miles or more in length. The electro-motive force of 
 the secondary current is enormously greater than that 
 of the primary.* Connected with the poles of the bat- 
 tery is a condenser, which still further heightens the 
 effect of the coil. 
 
 The induction coil is used for many purposes re- 
 quiring high electro-motive force, and is usually 
 more reliable than any machine generating electri- 
 city by friction. Beautiful effects are obtained by 
 passing sparks from it through Q-eissler tubes. 
 These are made of glass, and contain rarefied gases or 
 vapors. The spark when passing through rarefied hy- 
 drogen assumes a brilliant red tint ; through nitrogen, 
 a gorgeous purple. With the proper degree of rarefac- 
 tion it becomes stratified into bands across the tube. 
 
 9. The Telephonef is an instrument for utiliz- 
 ing magneto-electric currents and reproducing speech 
 by their aid. Within a handle of vulcanite is a per- 
 
 * The largest induction coil ever constructed was made for Mr. Spottis- 
 woode, an English physicist. Its secondary coil contained 280 miles of 
 wire, wound in 340,000 turns, and its resistance exceeded 100,000 ohms. 
 When worked with a Grove battery of 30 cells, it gave a spark 42 inches 
 long, or considerably more than a yard in length. Coils containing 50 miles 
 of wire are not uncommon ; 1;hey yield sparks a foot or more in length. A 
 Leyden jar interposed in the secondary circuit of such a coil is charged 
 and discharged so rapidly as to make almost a continuous sound. 
 
 The action of the condenser is not easily explained in a few words to 
 the elementary student. Consult Thompson's " Lessons in Electricity and 
 Magnetism," pp. 363-365. 
 
 t A telephone in parts, ready to be put together by the experimenter 
 is sold by the apparatus dealers. A simple but effective instrument can be 
 
340 
 
 ELECTRICITY. 
 
 FIG. 238. 
 
 manent magnet (Fig. 238), around one pole, N, of 
 which is an insulated coil, C, connected with the 
 binding posts at the other end. A thin disk of soft 
 iron, BB, is fixed across near the encircled magnet 
 pole, and a mouth-piece, A, serves to direct the sound 
 of the voice against the disk, which is thus made to 
 
 vibrate. Distur- 
 bances are pro- 
 duced in the 
 strength of the 
 magnet, and cor- 
 responding cur- 
 rents traverse 
 the wire. Pass- 
 ing through the 
 coil of the dis- 
 tant telephone 
 they vary the 
 strength of its magnet. Minute clicking sounds are 
 produced as the molecules of the magnet yield to these 
 disturbances. The disk re-enforces these like a sound- 
 ing-board, and gives out vibrations to the air, with 
 such rapidity as to constitute a faithful reproduction 
 of what was talked into the transmitting telephone.* 
 
 made, at a slight expense, by a pupil with ordinary mechanical ability. 
 One process, with illustrative drawings, is given in the " Popular Science 
 Monthly, 1 ' March, 1878, and another in the " Scientific American," Vol. 
 XXXIX., No. 5. In Vol. XXXIX., No. 16, is also described a method of 
 constructing a microphone ; and in the "Scientific American Supplement," 
 No. 133, is an account of a home-made phonograph. These numbers can be 
 procured by any newsdealer. In using the telephone, two instruments 
 exactly alike are employed. One is held to the mouth of the speaker, and 
 the other to the ear of the listener. 
 
 * It should be observed that the presence of a disk is not necessary for 
 the perception of sound from the receiving telephone. The motion is prob- 
 
 The Telephone. 
 
TRANSFORMATIONS OF ENERGY. 341 
 
 1C. The Microphone is a modification of the tele- 
 phone transmitter. It consists of a rod of gas car- 
 bon whose ends rest loosely in cups hollowed out of 
 the same material, and these in turn fixed upon a 
 sounding-board. The current from a battery passes 
 through the microphone carbons and through a re- 
 ceiving telephone. If the sounding-board be made to 
 vibrate in the least, whether by sound-waves or by 
 slight mechanical motion, va- 
 
 7 FIG. 239. 
 
 riations are produced in the 
 pressure of the rod against 
 its cups. Two pieces of carbon 
 firmly in contact conduct elec- 
 tricity moderately well; but 
 if the pressure between them 
 is diminished, the resistance is 
 increased and the current 
 becomes fainter. The micro- 
 phone is thus the last refine- 
 
 Microphone. 
 
 ment of the telegraph com- 
 bined with the telephone receiver. The sound of the 
 voice, the patter of a fly's foot in walking over the 
 sounding-board, or the gentlest ticking of a watch 
 rested upon it, are thus made audible in a telephone 
 many miles away. 
 
 ably among the molecules of the steel magnet, and conducted from them 
 to the disk if this be added. 
 
 The telephone described in the text is the simplest that can be made. 
 Many improvements have been effected in the instrument. The transmit- 
 ting telephone is now generally, made in such manner as to send an in- 
 duced current, like that of the Buhmkorff coil, through the line wire to 
 the receiver. 
 
342 
 
 ELECTRICITY. 
 
 11. The Magneto-electric Machine. The produc- 
 tion of a current in a closed circuit by moving it in 
 the field of a magnet (p. 336, 7) is realized in the 
 construction of the magneto-electric machine. This 
 machine consists of a powerful horseshoe magnet in 
 front of which a pair of coils is made to rotate (Fig. 
 240). This pair is called the armature. Each coil 
 contains a core of soft iron, which acquires and then 
 
 FIG. 240. 
 
 The Magneto-electric Machina 
 
 loses magnetism, as it approaches, passes, and then 
 recedes from a pole of the permanent magnet. In 
 the coil these rapid variations of magnetic strength 
 produce alternating currents whose electro-motive 
 force is determined by the speed of rotation and the 
 strength of the magnetic field. The two ends of the 
 coil are connected with insulated plates of metal on 
 opposite sides of the axle. On each of these a con- 
 ducting spring presses, which carries the currents to 
 the handles, H. This arrangement, called a commuta- 
 tor, is so adapted as to secure but one direction to 
 
TRANSFORMATIONS OF ENERGY. 343 
 
 the currents in the main wires. On taking hold of 
 the handles while the shaft is rotated rapidly, a series 
 of convulsive shocks is experienced.* 
 
 12. The Dynamo-electric Machine. For the gen- 
 eration of currents to be employed in electric light- 
 ing it is necessary that they shall be continuous 
 rather than intermittent. The name dynamo is ap- 
 plied to a development of the magneto-electric ma- 
 chine that accomplishes this -result. The armature 
 coils, in one type of these machines, are wound 
 lengthwise upon a drum or cylinder, Fig. 241, which 
 is revolved between the poles of a powerful electro- 
 magnet called the field-magnet. On this drum a 
 large number of coils may be wound, each with its 
 own pair of commutator plates, these being so close 
 together that the interval between two successive 
 
 * The machine represented in Eig. 240 is known as Clarke's machine, 
 and was one of the first of its kind invented. Many improvements have 
 been subsequently made. In 1866, Mr. Wilde discovered that if the in- 
 duced current be passed through the coil of an electro-magnet, the 
 strength it produces in this is far greater than that qf the permanent 
 magnet employed. An additional and larger armature was maae to rotate 
 in front of this electro-magnet, and the current induced in it was made to 
 excite a second and still larger electro-magnet, whose armature then gen- 
 erated currents greatly stronger than any previously known. Such a ma- 
 chine, driven by a steam-engine of 15-horse power, produces an electric 
 light dazzling as the noonday sun, throwing the flame of the street-lamps 
 into shade at a quarter-mile distance. Its heat is sufficient to fuse a J-inch 
 bar of iron fifteen inches long or 7 feet of No. 6 iron wire." A Yankee 
 once threw the industrial world of Europe into a wonderful excitement by 
 announcing a new theory of perpetual motion based on the magneto-electric 
 machine. He proposed to decompose water by the current of electricity ; 
 then burn the hydrogen and oxygen thus obtained. In this way he would 
 drive a small steam-engine, which, in turn, would keep the magneto-electric 
 machine in motion. This would certainly be a splendid discovery. It 
 would be a steam-engine which would prepare its own fuel, and, in addition, 
 dispense light and heat to all around." HELMHOLTZ. 
 
344 
 
 ELECTRICITY. 
 
 currents is imperceptible. 
 
 FIG. 241. 
 
 Drum Armature and Four-part Commu- 
 tator. 
 
 FIG. 242. 
 
 Diagram of a Series Dynamo. 
 
 In Fig. 242, the end of the 
 cylinder and of the group 
 of commutator plates are 
 seen between the large 
 pole-pieces, JVand S, of the 
 field-magnets. The cur- 
 rent is conducted off by 
 the springs or "brushes," 
 and passes through the 
 coils of the field-magnet 
 before reaching the main- 
 line-wire. The pole-pieces 
 never quite lose their mag- 
 netism, even after the 
 machine is at rest. The 
 energy of the induced 
 current is at first wholly 
 absorbed in exciting the 
 field-magnet. This action, 
 even though almost in- 
 finitely weak at first, in- 
 creases until the magnet 
 is as strong as possible ; 
 after which the energy is 
 expended in doing work 
 on the main circuit.* 
 
 * In the frontispiece is a pic- 
 ture of the Weston Dynamo, such 
 as is used for producing the electric 
 lights on the great bridge between 
 New York and Brooklyn. Each 
 pole-piece is attached to two coils 
 which are so wound that both have 
 the same effect on it. The end of 
 
TRANSFORMATIONS OF ENERGY. 345 
 
 13. Electric Motors. The action of a dynamo 
 is reversible. By revolving its armature we ob- 
 tain a current, and by sending a current through 
 its armature we cause it to revolve. The same ma- 
 chine may, therefore be used either as a dynamo or as 
 a motor. The dynamo converts the mechanical energy 
 of the source of power into the energy of the electric 
 current ; the motor converts the energy of the electric 
 current into mechanical energy. On this principle 
 depends the transmission of power. 
 
 14. The Electric Light. According to the method 
 adopted in winding the armature, a dynamo-machine 
 may give currents of high or of low electro-motive force. 
 Two corresponding kinds of lamp are used. To produce 
 the arc light, a current of high electro- motive force is 
 sent through a pair of carbon rods, which are then drawn 
 slightly apart. Particles of carbon are made white-hot, 
 and even turned into vapor, which is thrown from one 
 rod to the other in the direction of the current. The 
 path of the glowing vapor is curved, and hence this is 
 called the voltaic arc. It is the most brilliant artificial 
 light known, but unsteady because the arc leaps from 
 side to side as the carbons become wasted away. An 
 automatic regulator is employed to keep them at the 
 proper distance apart. The arc light is excellent for 
 lighting streets, halls, and other large public places. 
 For domestic use, the incandescence lamp is better. 
 
 the drum armature is covered with, radiating conductors, which connect 
 the coils with the commutator plates. One of the brushes is seen pressing 
 on these plates, and is connected with the insulated wire that conducts 
 the current away. 
 
346 
 
 ELECTRICITY. 
 
 In this a current of low electro-motive force passes 
 through a filament of carbon inclosed in a globe 
 from which most of the air has been withdrawn. 
 The filament glows with a soft and steady light, 
 which is much inferior in brilliancy to the arc light. 
 Although only J-OTTD-FOT} of the air remains in the 
 globe, the filament is slowly burned away, and has 
 to be replaced with a new one. 
 
 15. Thermo-electricity. In the electric lamp the 
 energy of a current is changed into heat and light. 
 Conversely, heat and light (radiant energy) may be 
 changed into electric energy. If the end of an iron 
 wire be connected with that of a copper or German- 
 silver wire, the other ends being attached to a gal- 
 vanometer, the needle will swing aside when the 
 joined ends are heated. This effect is increased if 
 the junction be made with bismuth and antimony. 
 A current flows at the junction from bismuth to 
 antimony, thence through the galvanometer back to 
 the bismuth. 
 
 A THERMO-ELECTRIC PILE consists of alternate bars 
 
 of antimony and bismuth 
 soldered together, as shown 
 in Fig. 243. When mount- 
 ed for use, the couples are 
 insulated from each other 
 and inclosed in a copper 
 frame, P. If both faces of the pile 
 are equally heated, there is no cur- 
 rent. The least variation of temper- 
 ature, however, between the two is indicated by the 
 
 Fig. 243. 
 
 Fig. 244. 
 
 Thermopile. 
 
TRANSFORMATIONS OF ENERGY. 347 
 
 flow of electricity. Wires from a, the positive pole, 
 and 6, the negative, connect the pile with the gal- 
 vanometer. This furnishes a test of change of tem- 
 perature. A fly walking over the face of the pile by 
 its warmth will move the needle, if the galvanometer 
 be very delicate. When skillfully used, the thermo- 
 pile serves as a very sensitive thermometer. 
 
 THE BOLOMETER is an instrument devised for the 
 detection of very faint variations of temperature. A 
 platinum or iron wire opposes much more resistance 
 to the passage of an electric current when hot than 
 when cold. The current is made to divide between 
 two conductors. These are connected by a cross- 
 wire, or " bridge," with a galvanometer interposed. 
 If the current in the two branches be equal, the gal- 
 vanometer is not affected ; but, if unequal, a cross- 
 current deflects the galvanometer needle. By heat- 
 ing one branch slightly the balance is disturbed, and 
 the difference of temperature is read in the deflection 
 of the needle.* 
 
 16. Animal Electricity. The human body is often 
 electrified. Many animals, especially when angry or 
 otherwise excited, give evidence of being electrified. 
 
 * This instrument was invented by Professor Langley at the Alleghany 
 Observatory near Pittsburg. It was used in examining the invisible parts of 
 the solar spectrum, where lines and bands were discovered whose presence 
 could not be detected with the most delicate thermopile. The invisible 
 part of the spectrum was thus found to be much more extensive than the 
 visible part, while the most intense heat as well as light is found in the 
 region colored greenish-yellow. The bolometer is capable of revealing a 
 change of temperature of .00001 C. Professor Langley has discovered by 
 this means that the highest temperature of the moon scarcely, if at all, 
 exceeds that of the human body, and that the temperature of outer space 
 is nearly as low as the absolute zero of temperature, 273 C. 
 
348 ELECTRICITY. 
 
 Certain fish have the property of giving, when 
 touched, a shock like that from a Leyden jar. The 
 torpedo and the electrical eel are most noted. The 
 former is a native of the Mediterranean, and its 
 shock was anciently prized as a cure for various dis- 
 eases. The latter is abundant in certain South 
 American waters. A specimen of this fish, forty 
 inches in length, was estimated by Faraday to emit 
 a spark equal to the discharge of a battery of fifteen 
 Leyden jars of 3,500 square inches surface. 
 
 SUMMARY. 
 
 ELECTRICITY is a form of energy that may be manifested as 
 an accompaniment of friction, of chemical action, of the motion 
 of magnets, of variations in temperature, or of animal excite- 
 ment. It exhibits a certain kind of duality in its effects, and 
 hence the names positive and negative electricity are used to 
 express the contrast. Many considerations point to the conclu- 
 sion that the molecules of a charged body are in a condition of 
 strain. This condition can be communicated by induction 
 through a "dielectric," which itself becomes strained while thus 
 acting as a medium. By taking advantage of a proper dielec- 
 tric, such as glass, electrical energy may be stored up for subse- 
 quent use, as in the Leyden jar. 
 
 Voltaic electricity has its origin in chemical action, or in 
 contact of different metals, or in both. The essentials of an 
 ordinary battery for its development are two substances, which 
 are unequally affected by a chemical agent. One of these is at 
 higher potential than the other, and neutralization is effected by 
 the passage of a current, continually renewed, from the body 
 at high potential, through the best conductor, to the body at 
 low potential. This difference of potential, however, is very 
 slight in comparison with that developed by friction and induc- 
 tion, as in the Holtz or Voss machine. Voltaic electricity is 
 more manageable, more reliable, more convenient, more gener- 
 
HISTORICAL SKETCH. 349 
 
 ally available than frictional electricity. Electricity may be 
 transformed, under appropriate conditions, into mechanical 
 motion, magnetism, sound, heat, or light. Among its most im- 
 portant applications to the purposes of practical life are the 
 telegraph, the electrotype, the telephone, distribution of power, 
 and the electric light. 
 
 HISTORICAL SKETCH. 
 
 THALES (6th cent. B. c.), one of the seven wise men, knew 
 that when amber is rubbed with silk it will attract light bodies, 
 as straw, leaves, etc. This property was considered so marvel- 
 ous that amber was supposed to possess a soul. From the 
 Greek name of the substance (elektron) our word electricity is 
 derived. This simple phenomenon constituted all that was 
 known until the 16th century, when William Gilbert, physician 
 to Queen Elizabeth, made many valuable experiments. He dis- 
 covered that amber was by no means the only substance which 
 can exhibit electrical manifestations when rubbed, and he exam- 
 ined into the conditions favorable to electrical phenomena. 
 Among the most important of these he found to be the dryness 
 of the atmosphere. Francis Hawksbee called attention to the 
 resemblance between the electric spark and lightning, and in- 
 vented an electric machine, in which the hands were used as 
 rubbers. Stephen Gray, in the 18th century, discovered the dif- 
 ference between conductors and non-conductors, that an electric 
 charge is at the surface, and that the human body can be elec- 
 trified. Dufay discovered that there are two manifestations of 
 electricity, which he called vitreous and resinous, and considered 
 them to be fluids. Kinnersley, the friend and associate of 
 Franklin, recognized that these two electricities were nothing 
 else than what Franklin had already called positive and nega- 
 tive charges. The Leyden jar was invented in 1745, probably 
 by several persons about the same time ; it was first exhibited 
 and used in experiment by Muschenbroeck, at Leyden, in Hol- 
 land. By the use of it students of electricity were able to 
 gather the mysterious "virtue" or "effluvia" in much larger 
 quantities, and to produce effects never imagined before, such 
 as the firing of gunpowder. Experiments were made about this 
 
350 ELECTRICITY. 
 
 time to ascertain the rate of transmission of electricity from a 
 Leyden jar through a metallic conductor. A wire more than 
 two miles long was employed ; through this the discharge ap- 
 peared to be absolutely instantaneous. 
 
 In 1749, Benjamin Franklin wrote from Philadelphia to 
 Peter Collinson at London, as follows : 
 
 "Chagrined a little that we have hitherto been able to 
 produce nothing in this way of use to mankind, and the hot 
 weather coming on, when electrical experiments are .not so 
 agreeable, it is proposed to put an end to them for this season, 
 somewhat humorously, in a party of pleasure on the banks of 
 the SkuylkiL Spirits, at the same time, are to be fired by a 
 spark sent from side to side through the river, without any 
 other conductor than the waber ; an experiment which we some 
 time since performed, to the amazement of many. A turkey is 
 to be killed for our dinner by the electrical shock, and roasted 
 by the electrical jack before a fire kindled by the electrical 
 bottle (Leyden jar); when the healths of all the famous elec- 
 tricians in England, Holland, France, and Germany are to be 
 drank in electrified bumpers, under the discharge of guns from 
 the electrical battery." 
 
 About 1752, Franklin proved the identity of lightning and 
 frictional electricity by means of a kite made of a silk hand- 
 kerchief and with a pointed wire at the top. He elevated this 
 during a thunder-storm, tying at the end of the hemp string a 
 key, and then insulating the whole by fastening it to a post 
 with a long piece of silk lace. On presenting his knuckles to 
 the key, he obtained a spark. He afterward charged a Leyden 
 jar, and performed other electrical experiments in this way. 
 These attempts were attended with very great danger. Prof. 
 Kichman, of St. Petersburg, drew in this manner from the 
 clouds a ball of blue fire as large as a man's fist which struck 
 him lifeless. Shortly after the famous experiments of Franklin, 
 the Frenchman, Coulomb, established the law of electric attrac- 
 tion and repulsion, showing that it was the same as that of 
 gravitation, light, and heat, the law of inverse squares. 
 
 In the year 1790, Galvani was engaged in some experiments 
 on animal electricity. For this purpose he used frogs' legs as 
 electroscopes. He had hung several of these upon copper hooks 
 
HISTORICAL SKETCH. 351 
 
 from the iron railing of the balcony, in order to see what effect 
 the atmospheric electricity might have upon them. He noticed, 
 to his surprise, that when the wind blew them against the iron 
 supports, the legs were convulsed as if in pain. After repeated 
 experiments, Q-alvani concluded that this effect was produced 
 by what he termed animal electricity, that this electricity is 
 different from that caused by friction, and that he had discov- 
 ered the agent by which the will controls the muscles. Volta 
 rejected the idea of animal electricity, and held that the contact 
 of dissimilar metals was the source of the electricity, while the 
 frog was "only a moist conductor, and for that pur- 
 pose was not as good as a wet rag." He applied this 
 view to the construction of "Volta's pile," which is 
 composed of plates of zinc and copper, between which 
 are laid pieces of flannel moistened with an acid or a 
 saline solution (Fig. 245). This theory is substantially 
 the one held at the present time, though we now know 
 that there must be chemical action to continue the 
 supply. 
 
 From the earliest times in which the knowledge of 
 electricity began to be definite, impostors and half- 
 educated people circulated marvelous stories about its value as 
 a panacea for all kinds of disease. Many supposed that deaf- 
 ness and dimness of sight might be cured by the use of the 
 electric spark. Franklin remarked of this, "it will be well if 
 perfect blindness be not the consequence of the experiment." 
 In the hands of experienced physicians electricity has been used 
 with good effect, but to-day, as in Franklin's time, the name 
 often serves as a cloak for ignorance or trickery. 
 
 Electricity and magnetism were studied as distinct branches 
 until 1820, when Oersted of Copenhagen discovered the phe- 
 nomenon shown in Fig. 224. This was published every-where, 
 and excited the deepest interest of scientific men. In the fruit- 
 ful mind of Ampere the experiment bore abundant fruit. He 
 discovered that two parallel wires conveying an electric cur- 
 rent in the same direction attract each other, and when in 
 opposite directions, repel each other. From this he generalized 
 the entire subject. Prof. Henry next exhibited the wonderful 
 power of the electro-magnet, and invented the electro-magnetic 
 
352 ELECTRICITY. 
 
 engine. Scientific men in all parts of the world were now 
 gathering the material necessary for the invention of the elec- 
 tric telegraph. It fell to Samuel F. B. Morse to make this 
 knowledge practical, and in 1837 he exhibited in New York a 
 working instrument. An experimental line between Washing- 
 ton and Baltimore was completed in 1844, and, on May 27th of 
 that year, was sent the first message ever forwarded by a re- 
 cording telegraph. 
 
 Consult Maxwell's "Electricity and Magnetism"; Tyndall's 
 "Lessons in Electricity"; "Faraday's "Lectures on the Phys- 
 ical Forces" and "Researches in Electricity"; Noad's "Manual 
 of Electricity"; Art. on the Microphone, in "Scribner's Monthly," 
 Vol. XVI., p. 600 ; Prescott's " The Speaking Telephone, Talk- 
 ing Phonograph," etc.; Foster's "Electrical Measurements," in 
 "Science Lectures at South Kensington," Vol. I., p. 264 ; Thom- 
 son's "Papers on Electrostatics and Magnetism"; Guillemin's 
 "The Forces of Nature" and "The Applications of Physical 
 Forces"; "American Cyclopedia," Articles on Electricity, Mag- 
 netism, Electro-magnetism, etc.; Smith's "Manual of Teleg- 
 raphy"; Jones' " Historical Sketch of Electric Telegraph"; Watts' 
 "Electro-metallurgy"; "Barnes' Hundred Years of American 
 Independence," Sec. on Morse, p. 442 ; " Fourteen Weeks in 
 Zoology," Sec. on Torpedo, p. 186; Gordon's " Electricity and 
 Magnetism"; Hospitalier's " Modern Applications of Electricity" 
 (specially commended for latest discoveries); Urbanitzki's " Elec- 
 tricity in the Service of Man"; Mendenhall's "A Century of 
 Electricity"; Thompson's "Dynamo-electric Machinery"; Dan- 
 iell's "Principles of Physics," and Anthony and Brackett's 
 "Text-book of Physics." 
 
CONCLUSION. 
 
 "Science is a psalm and a prayer." PARKER, 
 
 NOWHERE in nature do we find chance. Every 
 event is governed by fixed laws. If we would ac- 
 complish any result or perform any experiment, we 
 must come into exact harmony with the universal 
 system. If we deviate from the line of law by a 
 hair's breadth, we fail. These laws have been in 
 operation since the earliest beginnings of the devel- 
 opment of our world, and all the discoveries of sci- 
 ence prove them to extend to the most distant star 
 in space. A child of to-day amuses itself with cast- 
 ing a stone into the brook and watching the widen- 
 ing curves ; little children of ten thousand years ago 
 may have done the same. A law of nature has no 
 force of itself ; it is but the manner in which force 
 acts. 
 
 We can not create force. We find it every-where 
 in Nature ; so that matter is not dumb, but full of 
 inherent energy. A tiny drop of dew sparkling on 
 a spire of grass is instinct with power : Gravity 
 draws it to the earth ; Chemical Affinity binds to- 
 gether the atoms of hydrogen and oxygen ; Cohesion 
 holds the molecules of water, and gathers the drop 
 into a globe ; Heat keeps it in the liquid form ; Ad- 
 hesion causes it to cling to the leaf. If the water 
 
354 CONCLUSION. 
 
 be decomposed, Electricity will be set free ; and from 
 this, Heat, Light, Magnetism, and Motion can be 
 produced. Thus the commonest object becomes full 
 of fascination to the scientific mind, since in it reside 
 the mysterious forces of Nature. 
 
 These various forces can be classified either as 
 attractive or repellent. Under their influence the 
 atoms or molecules resemble little magnets with 
 positive and negative poles. They approach or re- 
 cede from one another, and so tend to arrange 
 themselves according to some definite plan. "The 
 atoms march in time, moving to the music of law." 
 A crystal is but a specimen of "molecular archi- 
 tecture" built up by the forces with which matter 
 is endowed. Forces continually ebb and flow, but 
 the sum of energy through the universe remains 
 the same. In time all the possible changes may be 
 rung, and the various forms of energy subside into 
 one uniformly-diffused heat-quiver, but in that will 
 exist the representation of all the forces which now 
 animate creation. 
 
XI. 
 
 APPENDIX. 
 
APPENDIX. 
 
 QUESTIONS. 
 
 THE following questions are those which the author has 
 used in his classes, both as a daily review and for examina- 
 tion. A standing question, which has followed every other 
 question, has been: "Can you illustrate this?" Without, there- 
 fore, a particular request, the pupil has been accustomed to 
 give as many practical examples as he could, whenever he has 
 made any statement or given any definition. 
 
 I. Introduction. Define matter. A body. A substance. 
 Name and define the two kinds of properties which belong to 
 each substance. State the suppositions of the Atomic Theory. 
 What is a molecule ? An atom ? 
 
 Describe the two kinds of change to which matter may be 
 subjected. What is the principal distinction between Physics 
 and Cheirfistry? Mention some phenomena which belong to 
 each. Why are these branches intimately related? 
 
 Name the general properties of matter. Define magnitude 
 Size. Distinguish between size and mass. Why are feathers 
 light and lead heavy? Why is it necessary to have a standard 
 of measure? What are the French and English standards? 
 Give the history of the English standard. Is the American 
 yard an exact copy of the English? Give an account of the 
 French system. By what name is this system commonly 
 known? Is either of these systems founded on a natural stand- 
 ard? Why is it desirable to have such a standard? 
 
 Define Impenetrability. Give some apparent exceptions, 
 and explain them. Define Divisibility. Is there any limit to 
 the divisibility of matter? Define Porosity. Is the word porous 
 
358 APPENDIX. 
 
 used here in its common acceptation? What practical use is 
 made in the arts of the property of porosity? Describe the 
 experiment of the Florence academicians. 
 
 Define Inertia. Does a ball, when thrown, stop itself? Why 
 is it difficult to start a heavy wagon? Why is it dangerous to 
 jump from the cars when in motion? (Compare First Law of 
 Motion.) Define Indestructibility. Has the earth at all times 
 contained the same quantity of matter that it does now? 
 
 Name the specific properties of matter. Define Ductility. 
 How is iron wire made? Platinum wire? Gilt wire? Define 
 Malleability. 
 
 Describe the manufacture of gold-leaf. Is copper malleable? 
 Define Tenacity. Name and define the three kinds of Elas- 
 ticity. Illustrate the elasticity of compression as seen in solids. 
 In liquids. In gases. What is said about the relative com- 
 pressibility of liquids and gases? 
 
 Illustrate the elasticity of expansion as seen in solids, liquids, 
 and gases. Define Elasticity of Torsion. What is a Torsion 
 balance? Define Hardness. Does this property depend on 
 density? Define Density. Define Brittleness. Is a hard body 
 necessarily brittle? Name a brittle and a hard body. 
 
 II. Motion and Force. Define motion, absolute and relative. 
 Best. Velocity. Force. What are the resistances to motion? 
 Tell what you can about friction. Why does oil diminish fric- 
 tion? What uses has friction? What law governs* the resist- 
 ance of air or water? Define Momentum. 
 
 Show that motion is not imparted instantaneously. State 
 the three laws of motion and the proof of each. If a ball be 
 fired into the air when a horizontal wind is blowing, will it rise 
 as high as if the air were still? Describe the experiments with 
 the collision balls. Give practical illustrations of action and 
 reaction. If a bird could live, could it fly in a vacuum? De- 
 fine compound motion. 
 
 Define the so-called " parallelogram of forces." The result- 
 ant. How can the resultant of two or more forces be found? 
 Name some practical illustrations of compound motion. What 
 is the "resolution of forces"? 
 
 Show how one vessel can sail south and another north, 
 
QUESTIONS. 359 
 
 driven by the same westerly wind. Explain how a kite is 
 raised. Explain the "split-shot" in croquet. 
 
 Explain the towing of a canal-boat. Describe how motion 
 in a curve and circular motion are produced. Explain the cen- 
 tripetal and centrifugal forces. 
 
 Show when the centrifugal force becomes strong enough to 
 overcome the force of Cohesion. Of Adhesion. Of Gravity. 
 Apply the principle of circular motion to the revolution of the 
 earth about the sun. What effect does the revolution of the 
 earth on its axis have upon all bodies on the surface? 
 
 What would be the effect if the rotation were to cease? De- 
 scribe the action of the centrifugal force on a hoop rapidly re- 
 volved on its axis. Define reflected motion. Give its law. 
 
 What is Energy, in the Physical sense of the word? To 
 what ts it proportional? Name and define the two forms of 
 energy. How may one form be changed into the other? 
 
 What is the law of the Conservation of Energy? What did 
 Faraday say with regard to this law? 
 
 III. Attraction. 1. MOLECULAR FORCES. Define a molecular 
 force. What two opposing forces act between the molecules of 
 matter? How is this shown? What is the repellent force? 
 Name the attractive forces. Which of these belong to Physics? 
 
 1. Cohesion. Define. What are the three states of matter? 
 Define. How can a body be changed from one state to an- 
 other? 
 
 Show that cohesion acts only at insensible distances. Ex- 
 plain the process of welding. Why do drops of dew, etc., take 
 a globular form ? Why do not all bodies have this form ? Illus- 
 trate the tendency of matter to a crystalline structure. 
 
 Has each substance its own form? Why is not cast-iron 
 crystalline? Why do cannon become brittle after long use? 
 
 Describe the process of tempering and annealing. Explain 
 the Rupert's Drop. How is glassware annealed? 
 
 2. Adhesion. Define. What is the theory of filtering through 
 charcoal ? Of what use is soap in making bubbles ? Define Cap- 
 illary Attraction. Why will water rise in a glass tube, while 
 "mercury will be depressed? Is a tube necessary to show capil- 
 lary attraction? What is the law of the rise in tubes? 
 
360 APPENDIX. 
 
 Give practical illustrations of capillary action. Why will 
 not old cloth shrink as well as new, when washed ? What is the 
 cause of solution? Why is the process hastened by pulveriz- 
 ing? 
 
 Tell what you can about gases dissolving in water. Why 
 does the gas escape from soda-water as soon as drawn? Why 
 do pressure and cold favor the solution of a gas ? Describe the 
 diffusion of liquids. Of gases. 
 
 Describe the osmose of liquids. Of gases. Why do rose- 
 balloons lose their buoyancy? What is the difference between 
 the osmose and the diffusion of gases? 
 
 2. GRAVITATION. How does Gravitation differ from Cohesion 
 and Adhesion? What is the law of gravitation? Why does a 
 stone fall to the ground? Will a plumb-line near a mountain 
 hang perpendicularly ? Why do the bubbles in a cup of tea 
 gather on the side ? How is the earth kept in its place ? Define 
 Gravitation. Gravity. Weight. 
 
 State the three laws of weight. What is a vertical or plumb- 
 line? 
 
 Describe the " guinea-and-f eather experiment." What does 
 it prove? Describe Atwood's machine. Deduce the formulas 
 for falling bodies. 
 
 How can the time of a falling body be used for determining 
 the depth of a well? How does gravity act upon a body 
 thrown upward? What velocity must be given to a ball to ele- 
 vate it to any point? How high will it rise in a given time? 
 When it falls, with what force will it strike the ground ? Define 
 the Center of Gravity. The line of direction. The three states 
 of equilibrium. 
 
 How may the center of gravity be found ? Give the general 
 principles of the center of gravity. Describe the leaning tower 
 of Pisa. State some physiological applications of the center of 
 gravity. Why do fat people generally walk so erect? 
 
 Define the Pendulum. Arc. Amplitude. What are isochro- 
 nous vibrations? State the three laws of the pendulum. Who 
 discovered the first law? What is the center of oscillation? 
 How is it found? What is the center of percussion? 
 
 Describe the pendulum of a clock. How is a clock regu- 
 lated? Does it gain or lose time in winter? Describe the 
 
QUESTIONS. 361 
 
 gridiron pendulum. The mercurial pendulum. Name the 
 various uses of the pendulum. Describe Foucault's experi- 
 ment. 
 
 IV. The Elements of Machines. Name and define the ele- 
 ments of machinery. Do the " powers," so called, produce en- 
 ergy? What is the law of mechanics? Illustrate the law. 
 What is a lever? Describe the three classes of levers. The 
 law of equilibrium. 
 
 What is the advantage peculiar to each class? Describe the 
 steelyard as a lever. What effect does it have to reverse the 
 steelyard? Describe the arm as a lever. (See "Hygienic 
 Physiology," p. 34.) Would a lever of the first class answer 
 the purpose of the arm? Describe the compound lever. 
 
 Describe the hay scale. The wheel and axle. Its law of 
 equilibrium. Describe a system of wheel-work. At which arm 
 of the lever is the P applied? 
 
 Describe the various uses of the inclined plane. Its law of 
 equilibrium. What velocity does a body acquire in rolling down 
 an inclined plane? Give illustrations. 
 
 Describe the screw. Its uses. Its law of equilibrium. 
 How may its power be increased? What limit is there? 
 Describe the wedge. Its uses. Its law of equilibrium. How 
 does it differ from that of the other powers? Describe the 
 pulley. The use of fixed pulleys. Is there any gain of P in 
 a fixed pulley? 
 
 What is the use of a movable pulley? Describe a movable 
 pulley as a lever. Give the general law of equilibrium in a 
 combination of pulleys. What are cumulative contrivances*/ 
 Is perpetual motion possible? Why? 
 
 V. Pressure of Liquids and Gases. 1. HYDROSTATICS. De- 
 fine. What liquid is taken as the type? What is the first law 
 of liquids? Explain. Illustrate the transmission of pressure 
 by water. Show how water is used as a mechanical power. 
 Describe the hydrostatic press. Give its law of equilibrium. 
 
 What are the uses of this press? What pressure is sus- 
 tained by the lower part of a vessel of water, when acted on 
 by gravity alone? How does this pressure act? State the four 
 laws which depend on this principle, and illustrate them." What 
 
362 APPENDIX. 
 
 is the weight of a cubic foot of sea water? Fresh water? 
 What is the pressure at two feet? Give illustrations of the 
 pressure at great depths. Describe the hydrostatic bellows. Its 
 law of equilibrium. What is the "hydrostatic paradox"? Give 
 illustrations. Give the principle of fountains. How high will 
 the water rise? How do modern engineers carry water across 
 a river? Did the ancients understand this principle? Give 
 the theory of the Artesian well, and of ordinary wells and 
 springs. 
 
 Give the rule for finding the pressure on the bottom of a 
 vessel. On the side. Define the water level. Is the surface 
 of water horizontal? If it were, what part of an approaching 
 ship would we see first ? Describe the spirit-level. Define spe- 
 cific gravity. What is the standard for solids and liquids? For 
 gases? Explain the buoyant force of liquids. 
 
 What is Archimedes' law? Describe the " cylinder - and- 
 bucket experiment." What does it prove? Give the method of 
 finding the specific gravity of a solid. A liquid. 
 
 Is it necessary to use a specific gravity flask holding just 
 1,000 oz., or would any size answer? Suppose the solid is 
 lighter than water and will not sink, what can you do? Ex- 
 plain the hydrometer. How can you find the weight of a given 
 volume of any substance? The volume of any given weight? 
 The exact volume of a body? Illustrate the action of dense 
 liquids on floating bodies. Why will an iron ship float on 
 water? Where is the center of gravity in a floating body? 
 How do fish sink at pleasure? 
 
 2. HYDRODYNAMICS. Define. To what is the velocity of a jet 
 equal? How is the velocity found? Give the rule for finding 
 the quantity of water which can be discharged from a jet in a 
 given time. What is the effect of tubes? Tell something of 
 the flow of water in rivers. 
 
 Name and describe the different kinds of water-wheels. 
 Which is the most valuable form? Describe Barker's Mill. 
 How are waves produced? Explain the real motion of the 
 water. How does the motion of the whole wave differ from 
 that of each particle? How is the character of waves modified 
 near the shore? What is the extreme height of "mountain 
 waves " ? Define like phases. Unlike phases. A wave-length. 
 
QUESTIONS. 363 
 
 What is the effect if two waves with like phases coincide? 
 With unlike phases? What is this termed? 
 
 3. PNEUMATICS. Define. What principles are common to 
 liquids and gases? What gas is taken as the type? Describe 
 the air-pump. Can a perfect vacuum be obtained in this way? 
 What is the condenser? Its use? Prove that the air has weight. 
 
 Show its elasticity and compressibility. Describe the bottle- 
 imps. What principles do they illustrate? Show the expansi- 
 bility of the air. 
 
 Describe the experiments with the hand-glass. The principle 
 of Hiero's fountain. The Magdeburg hemispheres. What^do 
 they prove? Show the upward pressure of the air. 
 
 The buoyant force of the air. Would a pound of feathers 
 and a pound of lead balance, if placed in a vacuum ? On what 
 principle does a balloon rise ? What is the amount of the press- 
 ure of the air? Describe the experiment illustrating this. 
 Where do these figures apply? 
 
 Describe how the pressure of the air continually varies. 
 Explain Mariotte's (called also Boyle's) law. Describe the ba- 
 rometer. Its uses. Are the terms "fair," "foul," etc., often 
 placed on the scale, to be relied upon? Why is mercury used 
 for filling the barometer? Describe Otto Guericke's barometer. 
 
 Describe the action of the lifting-pump. The force-pump. 
 The fire-engine. Compare the action of the lifting-pump with 
 that of the air-pump. What is the siphon? Explain its theory. 
 
 Describe the pneumatic inkstand. The hydraulic ram. The 
 atomizer. Show how a current of air drags with it the still 
 atmosphere. What opposing forces act on the air? How high 
 does the air extend? How does its density vary? 
 
 VI. Acoustics. Define. Name and define the two senses of 
 this word. May not the terms "light," "heat," etc., be used in 
 the same way ? Illustrate the formation of sound by vibrations. 
 
 Show how the sound of a tuning-fork is conveyed through 
 the air. The report of a gun. The sound of a bell. The human 
 voice. Define a sound-wave. In which direction do the mole- 
 cules of air vibrate? In what form do the waves spread? Can 
 a sound be made in a vacuum? Can a sound come to the earth 
 from the stars? 
 
364 APPENDIX. 
 
 How do sounds change as we pass above or below the sea- 
 level? Upon what does the velocity of sound depend? Why is 
 this ? At what rate does sound travel in the air ? In water ? In 
 the metals? In iron? What effect does temperature have on 
 the velocity of sound? 
 
 Do all sounds travel at the same rate? How does the ve- 
 locity of sound enable us to determine distance? Upon what 
 does the intensity of sound depend? At what rate does it 
 diminish ? Why ? 
 
 Explain the speaking-tube. The ear-trumpet. Describe Biot's 
 experiment in the water-pipes of Paris. The speaking-trumpet. 
 What is the refraction of sound? 
 
 Define reflection of sound. What is the law? Give some 
 curious instances of reflection. What is the shape of a whisper- 
 ing-gallery ? Illustrate the decrease of sound by repeated reflec- 
 tion. Why are sounds more distinct at night than by day? Is 
 it desirable to have a aoor or a window behind a speaker? 
 What causes the "ringing" of a sea-shell? How are echoes 
 produced? When is the echo repeated? Illustrate the decrease 
 of sound by reflection. What are acoustic clouds?* 
 
 * "The influence of wind on the intensity of sound seems due to the 
 fact that, owing to obstructions opposed by the ground, there is a consider- 
 able difference between the velocity of the wind close to the ground and 
 the velocity at the height of a few feet above the ground. Thus in a 
 meadow the velocity of the wind at one foot above the surface may be 
 only half what it is at eight feet above the surface. Let us take the 
 velocity of sound at 1,100 feet per second, and suppose that the velocity of 
 a contrary wind is ten feet per second at the surface, and twenty feet per 
 second at the height of eight feet above the surface. Thus, considering 
 this circumstance alone, the wave of sound at the end of a second would 
 be at the surface ten feet in advance of its position at eight feet above the 
 surface ; so that the front of the wave, instead of being a vertical plane, 
 would be inclined to the horizon. Thus the sound, instead of proceeding 
 horizontally, becomes turned upward. It only remains to add that this tilt- 
 ing of the front of the wave is not delayed until the end of a second, but 
 begins at the origin of the sound and increases gradually. Hence a ray of 
 sound, so to speak, instead of traveling horizontally is curved upward, and 
 thus passes over the head of a person stationed at a distance from the 
 origin. A contrary wind then diminishes the intensity of sound by lifting 
 the sound off the ground, and the amount of this lifting increases as the 
 distance from the origin increases. The various consequences which may 
 be deduced from the preceding theory have been verified by experiments. 
 
QUESTIONS. 365 
 
 What is the difference between noise and music? Upon 
 what does pitch depend? Describe the siren. How is it used 
 
 Thus it follows that a listener when the wind is contrary may expect to 
 recover a sound, which he has lost at a certain distance from its origin, by 
 ascending to some height above the surface. Also the influence of a wind 
 will be but small if the surface be very smooth; thus sounds are heard 
 against the wind much farther over calm water than over land. Again, 
 suppose the origin of the sound to be elevated above the surface : then if 
 the listener be also raised above the surface he may hear a very loud sound 
 made up of two parts, namely, that which has traveled horizontally, and 
 that which has been tilted upward from the ground by the action of the 
 contrary wind. Next, suppose the wind to be favorable instead of contrary. 
 In this case the higher part of the wave of sound moves more rapidly than 
 the lower, and so the plane front of the wave is tilted forward, and the 
 rays of sound are bent downward to the advantage of the listener on the 
 ground. Then the influence of the wind on sound has been shown to de- 
 pend on the circumstance that when the wind is blowing, the velocity of 
 sound is different at different heights above the ground ; similar effects 
 will therefore follow if this difference of velocity is produced by any other 
 cause instead of by the wind. Now change of temperature affects the ve- 
 locity of sound : if the temperature rise one degree of Fahrenheit's ther- 
 mometer, the velocity increases by about a foot per second. In general, as 
 we ascend in the air during the day the temperature decreases, and therefore 
 so also does the velocity of sound. Thus the result is the same as in the 
 case of a contrary wind ; the ray of sound is lifted over the head of a per- 
 son on the ground, so that the audibility of the sound is diminished. The 
 presence of vapor in the atmosphere also affects the propagation of sound ; 
 the velocity increases as the quantity of vapor increases. The direct effect, 
 however, is very slight, bub indirectly the vapor is of consequence, for it 
 gives to the air a greater power of radiating and absorbing heat, and so 
 promotes inequality of temperature. The variation of temperature is 
 greatest when the sun is shining, so that it is greater by day than by 
 night, and greater in summer than in winter. Hence, according to the 
 theory now explained, sounds ought to be heard* more plainly by night 
 than by day, and more plainly in winter than in summer. That sounds 
 are heard more plainly by night than by day is a well-known fact. We 
 have supposed that the temperature decreases as we ascend in the atmos- 
 phere ; but it may happen on some occasion that the temperature at the 
 surface is lower than it is a little above the surface. This may be the case, 
 for instance, over the surface of the sea in the day-time, and over the sur- 
 face of the land by night. Thus the effect on sound will be similar to that 
 of a favorable wind. It is obvious that by the combined influence of wind 
 and temperature the results produced may vary much as to degree ; for 
 instance, the operation of a contrary wind may be neutralized by that of 
 the temperature rising as we ascend above the surface." See "Proceedings 
 of the Royal Society of Great Britain," volumes XXTL and XXIV. 
 
366 APPENDIX. 
 
 to determine the number of vibrations in a sound? How is the 
 octave of any note produced ? How can we ascertain the length 
 of the wave in sound? What length of wave produces the low 
 tones in music? The high tones? Give the illustration of the 
 locomotive whistle. When are two tones in unison? How can 
 we find the length 'of the wave in any musical sound ? What is 
 meant by the super-position of sound-waves? 
 
 How can two sounds produce silence? What is this effect 
 termed? Illustrate interference by means of a tuning-fork. 
 What are "beats"? Describe the vibration of a cord. 
 
 Describe the sonometer. What is the object of the wooden 
 box? Give the three laws of the vibration of cords. What is a 
 node? Describe the experiments illustrating the formation of 
 nodes. What are acoustic figures? Nodal lines? 
 
 What is the fundamental tone of a cord? A harmonic? 
 What causes the difference in the sound of various instruments ? 
 Does a bell vibrate in nodes? The violin-case? A piano sound- 
 ing-board ? State the fractions representing the relative rates of 
 vibration of the different notes of the scale. How is the sound 
 produced in wind-instruments? How is the sound-wave started 
 in an organ-pipe ? In a flute ? What determines the pitch ? 
 What are sympathetic vibrations ? Describe the resonance globe. 
 What is a sensitive flame? 
 
 A singing flame? Describe the phonograph. The ear. What 
 is the office of the Eustachian tube? Is there any opening; be- 
 tween the external and internal ear? What effect does it have 
 on the hearing to increase or diminish the pressure of the air? 
 How does a concussion sometimes cause temporary deafness? 
 How can this be remedied? What are the limits of hearing? 
 Does the range vary in different persons? What sounds are 
 generally heard most acutely? Are there probably sounds in 
 Nature we never hear? What causes the "whispering of the 
 pines " ? 
 
 VII. Optics. Define. A luminous body. A non-luminous 
 body. A medium. A transparent body. A translucent body. 
 An opaque body. A ray of light. Show that neither air nor 
 water is perfectly transparent. Why is the sun's light fainter 
 at sunset than at midday? Define the visual angle. Show how 
 distance and size are intimately related. 
 
QUESTIONS. 867 
 
 State the laws of light. Do they resemble those of sound ? 
 What is the velocity of light? How is this proved? Explain 
 the undulatory theory of light. 
 
 How does light-motion differ from sound-motion? What is 
 diffused light? Why are some objects brilliant and others dull? 
 
 Why can we see a rough surface at any angle, and an im- 
 age in the mirror at only a particular one? Would a perfectly 
 smooth mirror be visible? How does reflection vary? Define 
 mirrors. Name and define the three kinds. 
 
 What is the general principle of mirrors ? Why is an image 
 in a plane mirror symmetrical? Why is it reversed right and 
 left? Why is it as far behind the mirror as the object is 
 before it? 
 
 Why can we often see in a mirror several images of an ob- 
 ject? Why can we see these best if we look into the mirror 
 very obliquely? Why is an image seen in water inverted? 
 When the moon is near the meridian, why can we see the im- 
 age in the water at only one spot? When do we see a trem- 
 ulous line of light? What is the action of a concave mirror on 
 rays of light? Define the focus. Center of curvature. Focal 
 distance. Describe the image seen in a concave mirror. What 
 are conjugate foci ? Describe the image seen in a convex mir- 
 ror. Why is it smaller than life? Why can it not be inverted 
 like one seen in a concave mirror? 
 
 Define Refraction. Does the partial reflection of light as it 
 passes from one medium to another of different density have a 
 parallel in sound? Why is powdered ice opaque while a block 
 of ice is transparent? Give illustrations of refraction. 
 
 Why does an object in water appear to be above its true 
 place? What is the general principle of refraction? .State the 
 laws of refraction. Explain total reflection. What is the critical 
 angle? Describe the path of a ray through a window-glass. Is 
 the direction of objects changed? Describe the path through a 
 prism. 
 
 Name and describe the different kinds of lenses. What is 
 the effect of a double-convex lens on rays of light? What is 
 this kind of lens often called? Describe the image. Why is it 
 inverted after we pass the principal focus ? Why is it decreased 
 in size? What is the effect of a double-concave lens on rays of 
 
368 APPENDIX. 
 
 light ? Describe the image. Why can it not be inverted like one 
 through a double-convex lens? Describe the images seen in the 
 large vases in the windows of drug-stores. What is Aberra- 
 tion ? * 
 
 What is mirage? Give its cause. 
 
 How is the solar spectrum formed ? Name the six principal 
 colors. Show that these six will form white light. Why are 
 the rays separated? What is meant by the dispersive power of 
 a prism? What apparatus possesses this property in a high de- 
 gree? Ans. A triangular bottle filled with a liquid called car- 
 bon disulphide ("Popular Chemistry," p. 110). Why does the 
 window of a photographer's dark room sometimes contain yel- 
 low glass? 
 
 Describe the three kinds of spectra. The spectroscope. What 
 are its uses? Describe rainbows primary and secondary. Why 
 is the rainbow circular? How is the rainbow formed? Why 
 must it rain and the sun shine at the same time, to produce the 
 bow ? Why is the bow in the sky opposite the sun ? How many 
 refractions and reflections form the primary bow? The second- 
 ary? How many colors can one receive from a single drop? 
 Define complementary colors. How can they be seen? What 
 
 * To prevent spherical aberration the pupil of the eye can be made very 
 small. The photographer reaches the same result by the use of a diaphragm 
 with a small aperture. " The power of a small orifice to correct the greatest 
 amount of distortion from interfering rays is shown by a simple experi- 
 ment. The normal eye of an adult can not see to read small print nearer 
 than six inches. Within that distance the type becomes more indistinct 
 the closer it approaches the eye. But if we make a pin-hole through a card 
 and place it close to the eye, we can see to read printed matter of any size 
 even as near as half an inch from the eye. At that distance we can see 
 even the texture of fine cambric with microscopic definition. The cause of 
 this is easily explicable. The rays striking the lens perpendicularly on the 
 center suffer no refraction. The effect of the pin-hole is to exclude aU rays 
 but those that impinge perpendicularly on the center of the eye lenses. 
 Hence the image of the object close in front of the eye is pictured on the 
 retina without the interference of the surrounding rays, which would fall 
 obliquely on the lens, and being refracted out of focus would blur the pict- 
 ure. Observation of the effect of a small orifice in correcting aberrant rays, 
 and of the fact that the pupil contracts in near vision, led Haller and some 
 other physiologists to believe that contraction of the pupil was the sole fac- 
 tor in near accommodation. But this view has been sufficiently refuted by 
 other observers." Dr. Dudgeon's "Human Eye" p. 76. 
 
QUESTIONS. 369 
 
 is the effect of complementary colors when brought in contrast ? 
 (In Fig. 163 opposite colors are complementary.) Why do colors 
 seen by artificial light appear differently than by daylight as 
 yellow seems white, blue turns to green, etc. 
 
 Describe Newton's rings. How are these explained according 
 to the wave theory? What causes the play of color in mother- 
 of-pearl ? In soap-bubbles ? In the scum on stagnant water ? In 
 thin layers of mica or quartz ? 
 
 What can you say about the length of the waves ? State the 
 analogy between color and pitch in music. Why is grass green ? 
 When is a body white? Black? What is color-blindness? 
 
 What is double refraction? What are the two rays termed? 
 What is polarized light? How does a dot appear through Ice- 
 land spar? What other methods are there of polarizing light? 
 State some illustrations and practical uses of polarized light. 
 
 What is the meaning of the word microscope? Describe the 
 simple microscope. The compound microscope. How is the 
 power of a microscope indicated ? Do we see the object directly 
 in a microscope? Why is the object-lens made so small and so 
 convex ? 
 
 What is the meaning of the word telescope? Describe the 
 reflecting telescope. The refracting telescope. What is the use 
 of the object-lens ? The eye-piece ? Is the image inverted ? De- 
 scribe the opera-glass. 
 
 The stereoscope. The projecting lantern. How are dissolv- 
 ing views produced? 
 
 Describe the Camera. The structure of the eye.* The for- 
 
 * " In the skate's eye, and generally in the eyes of fishes, the cornea is 
 nearly flat, the aqueous humor is insignificant, and there is virtually no an- 
 terior chamber, for the crystalline lens comes up close to the cornea. A 
 convex cornea filled by an aqueous humor would be of no use in the water, 
 the refractive index of the water being identical with that of the aqueous 
 humor. Accordingly, the refraction of the rays of light has to be effected 
 entirely by the crystalline lens, which is nearly spherical, and of much 
 greater refractive power than the corresponding organ in animals which 
 pass their lives in the air. The crystalline lens being so nearly spherical in 
 shape and of such high refractive power, the axis of the eye is short. The 
 eye of the turtle, which is so much in the water, is very similar to that of 
 the fish. The crystalline lens is very near the cornea. The lens is smaller 
 proportionally than that of the skate, nor is it nearly so spherical ; and its 
 
370 APPENDIX. 
 
 mation of an image on the retina. The adjustment of the eye. 
 The cause of near and over sightedness. The remedy. Why do 
 old people hold a book at arm's length? Illustrate the duration 
 of an impression. What is the range of the eye? 
 
 VIII. Heat. Define solar energy. In what ways may it be- 
 come manifested? What is a diathermanous body? Cold? 
 Gases and vapors? Show the intimate relation between light 
 and heat. What is light? What is the theory of heat? Why 
 can we not see with our fingers or taste with our ears? At 
 what rate does nerve-motion travel? (See "Hygienic Physiol- 
 ogy," p. 177.) How long does it take a man, six feet in height, 
 to find out what is going on in his foot? 
 
 Name the sources of heat. Describe and illustrate each of 
 these. Can force be destroyed? If apparently lost, what be- 
 comes of it? What is Joule's law? Define latent, sensible, and 
 specific heat. Explain the paradox, " that freezing is a warm- 
 ing process and thawing a cooling one." Why does "heat ex- 
 pand and cold contract"? What do you say as to the expan- 
 sion of solids, liquids, and gases? Illustrate the expansion of 
 solids. Is it better to buy alcohol in summer or in winter? 
 What is the thermometer? Describe it. Describe the process 
 of filling and grading. The F., C., and B. scales. Tell what 
 you can about liquefaction. Of a solid. Of a gas. In one 
 case sensible heat becomes latent, in the other latent heat be- 
 comes sensible why is this? 
 
 Explain how a freezing mixture "makes ice-cream." State 
 the theory of vaporization. Of distillation. Since rain comes 
 from the ocean, why is it not salt? Describe the theory of 
 boiling. What is the boiling-point? Do all liquids boil at the 
 same temperature ? What would be the effect, if this were the 
 
 density, and consequently its refractive power, is somewhat less. Hence it 
 has proportionally a longer focus. The cornea is more convex than that of 
 the skate. The fish having no eyelids nor any lachrymal apparatus, its cor- 
 nea will be apt to become dim by exposure to the air, but the turtle is well 
 supplied with the requisite apparatus for maintaining the transparency of 
 the eye in air. Ophidian reptiles have no eyelids or lachrymal apparatus, 
 but they do not require them, as their cornea is transparent though dry." 
 Dr. Dudgeon's ''Human Eye," p. 50. 
 
QUESTIONS. 371 
 
 case? Upon what does the boiling-point depend? Why does 
 pressure raise the melting-point of most substances but lessen 
 that of ice? Why does salt-water boil at a higher temperature 
 than f ruuh-water ? Why will milk boil over so easily? Why 
 will soup keep hot longer than boiling water? Does the air, 
 dissolved in water, have any influence on the boiling-point? 
 Can you measure the height of a mountain by means of a tea- 
 kettle and a thermometer? Show how cold water may be used 
 to make warm water boil. At what temperature will water 
 boil in a vacuum? Why? Can we heat water in the open air 
 above the boiling-point? What becomes of the extra heat? 
 What is the latent heat of water? Upon what principle are 
 buildings heated by steam? Have you ever seen any steam? 
 
 Define evaporation. Does snow evaporate in the winter? 
 What can be done to hasten evaporation? Why is a saucepan 
 made broad ? Why do we cool ourselves by fanning ? Why does 
 an application of spirits to the forehead allay fever ? Why does 
 wind hasten the drying of clothes? Describe a vacuum-pan. 
 Why is evaporation hastened in a vacuum? Why is evapora- 
 tion a cooling process ? How is ice manufactured in the tropics ? 
 What is the spheroidal. state? 
 
 Name and define the three modes of communicating heat. 
 Give illustrations showing the relative conducting power of 
 solids, liquids, and gases. What substances are the best con- 
 ductors? Is water a good conductor? Air? What is the 
 principle of ice-houses? Fire-proof safes? Why do not flannel 
 and marble appear to be of the same temperature? Is ice 
 always of the same temperature? Describe the convective cur- 
 rents in heating water. Where must tfhe heat be applied? 
 Where should ice be applied in order to cool water? Describe 
 the convective currents in heating air. Upon what principle 
 are hot-air furnaces constructed? Ought the ventilator at the 
 top of a room to be opened in winter? At the bottom? Is 
 interplanetary space warmed by the sunbeam? 
 
 Does the heat of the sun come in through our windows? 
 Does the heat of our stoves pass out in the same way? Show 
 how the vapor in the air helps to keep the earth warm. Ex- 
 plain the Radiometer. The relation between absorption and 
 reflection. 
 
372 APPENDIX. 
 
 What is the elastic force of steam at the ordinary press- 
 ure of the air? What is the difference between a high-press- 
 ure and a low-pressure engine? Which is used for a locomo- 
 tive? Why? Describe the governor. What is the object of 
 a fly-wheel? 
 
 How does the capacity of the air for moisture vary? What 
 is the principle on which dew, rain, etc., depend? Show that 
 a change in density produces a change in temperature. What 
 effect does this have on the temperature of elevated regions? 
 Does an ounce of air on a mountain-top contain the same 
 quantity of heat as the same weight at the foot? How is dew 
 formed ? 
 
 Upon what objects will it collect most readily? Why will 
 it not form on windy nights? Why is rice-straw used in Ben- 
 gal in making ice? What is a fog? Why do fogs form over 
 ponds in the early evening? Cause of fogs over the Newfound- 
 land banks? How does a fog differ from a cloud? Why do 
 clouds remain suspended in the air? Describe the different 
 kinds of clouds. Describe the formation of rain. Snow. 
 
 How are winds produced? Land-and-sea breezes? Trade- 
 winds? Oceanic currents? Tell about the Gulf Stream. Ex- 
 plain the influence which water has on climate. Of what prac- 
 tical use is the air in water? Describe the apparent exception 
 which exists in the freezing of water. Describe the two proc- 
 esses by which pure water can be obtained. How is an ex- 
 cessive deposit of dew prevented? 
 
 IX. Magnetism. Define Magnetism. A Magnet. A natural 
 magnet. An artificial one. A bar-magnet. A horseshoe 
 magnet. The poles. The magnetic curves. Describe a mag- 
 netic needle. What is the law of magnetic attraction and re- 
 pulsion? Define magnetic induction. Explain it. 
 
 When is a body polarized? Give some illustrations of in- 
 duced magnetism. Does a magnet lose any force by induction? 
 How do you explain the fact that if you break a magnet each 
 part will have its N. and S. poles? 
 
 Describe the process of making a magnet. On what prin- 
 ciple will you explain this? Describe the compass. Is the 
 needle true to the pole? What causes it to vary? What is the 
 
QUESTIONS. 373 
 
 line of no variation? Declination? Why does the needle point 
 N. and S. ? What is a dipping-needle? Explain. How is a 
 needle balanced? 
 
 Where is the N. magnetic pole? How would one know 
 when he reached it ? Does the earth induce magnetism ? Which 
 end of an upright bar, in the United States, will be the S. 
 pole? 
 
 X. Electricity. Define frictional electricity. The electro- 
 scope. Difference between static and dynamic electricity. Show 
 the existence of two manifestations of electricity. Give the 
 names applied to each. 
 
 State the law. What is the theory of electricity? Define a 
 conductor. An insulator. 
 
 What is the best conductor? Best insulator? Is a poor con- 
 ductor a good insulator? When is a body said to be insulated? 
 Can electricity be collected from an iron rod? Describe a plate- 
 glass electrical machine. What is the use of the chain at the 
 negative pole? Define electrical induction. State Faraday's 
 theory. 
 
 What is the relation between induction and attraction and 
 repulsion? Describe the electric chime. Explain. Describe the 
 dancing images. The Leyden jar. What gives the color to the 
 spark? How is the jar discharged? 
 
 What are the essentials of a Leyden jar? What is the ob- 
 ject of the glass? The tin-foil? State the theory of the charg- 
 ing of the jar. Can an insulated jar be charged? Is the elec- 
 tricity on the surface or in the glass? Can the inner molecules 
 of a solid conductor be charged? Will a rod contain any more 
 electricity than a tube? Why is the prime conductor of an 
 electrical machine hollow? What is the effect of points? De- 
 scribe the electric whirl. Explain the existence of electricity in 
 the atmosphere. What is the cause of lightning? Thunder? Is 
 there any danger after you once hear the report? Describe the 
 different kinds of lightning. Tell how Franklin discovered the 
 identity of lightning and frictional electricity. 
 
 Tell what you can about lightning-rods. In what consists the 
 main value of the rod? Does the lightning ever pass upward 
 from the earth? Ans. It does, both quietly and by sudden 
 
374 APPENDIX. 
 
 discharge. Has nature provided any lightning-rods? What is 
 St. Elmo's fire? What is the velocity of electricity? Illustrate 
 its instantaneousness. Explain the action of the Voss elec- 
 trical machine. 
 
 Name some of the effects of Motional electricity (1) Phys- 
 ical, (2) Chemical, (3) Physiological. How are voltaic electricity 
 and chemistry related? Why is voltaic electricity thus named? 
 Tell the story of Galvani's discovery. What was his theory? 
 Give an account of Volta's discovery. How can we form a 
 simple pile? Describe the simple voltaic circuit. 
 
 Define the poles. Electrodes. Closing and breaking the cir- 
 cuit. What is necessary to form a voltaic pair? Describe the 
 chemical change. Why does the hydrogen come off from the 
 copper? Tell what you can about the current. 
 
 What really passes along the wire ? How is this force trans- 
 mitted ? Will a tube, then, convey as much electricity as a rod ? 
 Explain the term electric potential. 
 
 Describe Smee's battery. Grove's battery. The chemical 
 change in this battery. What are the advantages of Grove's 
 battery? Describe Bunsen's battery. Daniell's battery. The 
 Potassium Bichromate battery. Compare frictional and voltaic 
 electricity. i 
 
 State the effects of voltaic electricity, (1) Physical heat and 
 light ; (2) Chemical decomposition of water, electrolysis, electro- 
 typing, electro-plating, etc.; (3) Physiological. 
 
 What is the effect of a voltaic current on a magnetic needle ? 
 What is a galvanometer? An electro-magnet? Show how a 
 coll can be magnetized. How are bar-magnets made ? Describe 
 the magnetic telegraph. How is a message sent? How is one 
 received? What is a sounder? What is the general principle 
 of the telegraph? Describe the relay. Name the use of each 
 instrument. Describe a magneto-electric machine. Describe 
 Wilde's machine. What are induced currents? Describe the 
 Telephone. The Microphone. What is the difference between 
 the acoustic and the magnetic telephone? Explain Ruhmkorff's 
 coil. Thermal electricity. A thermo-electric pile. Describe the 
 electric fish. 
 
INDEX. 
 
 The index figures denote the page. 
 
 Aberration, 208, 219. 
 Achromatic, 220. 
 Acoustics, 153. 
 Acoustic clouds, 165. 
 figures, 174. 
 
 Action and reaction, 26. 
 Adhesion, 49. 
 Air, 129. 
 Air-pump, 129. 
 Alcoholmeter, 116. 
 Amplitude, 68. 
 Analyzer, 224. 
 Annealing, 47. 
 
 Archimedes, 97, 135, 149, 239. 
 Law of, 115, 149. 
 Aristotle, 15, 98, 277. 
 Artesian wells, 111. 
 Atmosphere, 136, 145. 
 Atomic theory, 3. 
 Atomizer, 143. 
 Attraction, 41. 
 
 " of adhesion, 49. 
 
 *' of cohesion, 43. 
 
 " Capillary, 49. 
 
 Gravitation, 55. 
 Avogadro's law, 16. 
 
 Bacon, 15, 277. 
 Barker's null, 125. 
 Barometer, 138. 
 Battery, Bunsen's, 320. 
 
 " Grove's, 320. 
 
 u Daniell's, 319. 
 
 Battery, Potassium Bichromate, 319. 
 
 Thermo-electric, 346. 
 Beats, 171, 186. 
 Bell, 156, 175. 
 Boiling, 252. 
 Bolometer, 347. 
 Brittleness, 14. 
 Britannia Bridge, 105. 
 
 Caloric, 277. 
 Camera, 230. 
 Capillarity, 49. 
 Capstan, 87. 
 Cartesian diver, 131. 
 Caustic, 199. 
 Center of gravity, 57. 
 
 " " oscillation, 69. 
 
 ** " percussion, 70. 
 Centrifugal force, 31. 
 Chemical affinity, 43. 
 
 " change, 4. 
 Chromatic aberration, 219. 
 Clepsydra, 78. 
 Clock, 71. 
 Clouds, 266. 
 Cohesion, 43. 
 Coils, Induction, 337. 
 Color, 216. 
 
 " blindness, 216. 
 
 " Complementary, 216. 
 Columns of air, 177. 
 Compass, 287. 
 
 Compensation pendulum, 248. 
 Condenser, 130. 
 
376 
 
 INDEX. 
 
 Conductors, 299. 
 Conservation of energy, 35. 
 Cords, Vibration of, 171. 
 Correlation of forces, 278. 
 Co-vibration, 158, 179. 
 Crystals, 46. 
 
 Cumulative contrivances, 93. 
 Current, Electric, 315. 
 of rivers, 123. 
 Curves, Magnetic, 285. 
 
 Declination, 285. 
 Democritus, 16, 277 
 Dew, 265. 
 
 Diathermancy, 275. 
 Dichroic, 217. 
 Diffraction, 221. 
 Diffusion of liquids, 52. 
 
 " gases, 52. 
 Dissolving views, 229. 
 Distillation, 251. 
 Divisibility, 8. 
 Double refraction, 222. 
 Ductility, 10. 
 Dynamo-electric machine, 343. 
 
 E 
 
 Ear, The, 181. 
 Ear of Dionysius, 185. 
 Ear-trumpet, 161. 
 Echoes, 164. 
 Elasticity, 12. 
 Electric battery, 318. 
 chime, 304. 
 light, 345. 
 " potential, 297. 
 " telegraph, 330. 
 
 whirl, 300. 
 
 Electrical machine, Plate, 302. 
 Voss, 308. 
 Electricity, 292. 
 
 Animal, 347. 
 Frictional, 294. 
 Voltaic, 313. 
 Electrophorus, 296. 
 Electro-gilding, 325. 
 
 Electro-magnets, 330. 
 
 " negative and positive sub- 
 stances, 323. 
 
 " plating, 325. 
 Electrolysis, 322. 
 Electromotive force, 316. 
 Electroscope, 294. 
 Energy, 4, 34. 
 
 Kinetic, 35, 65. 
 
 " Potential, 35. 
 Radiant, 243. 
 
 " Solar, 212. 
 Equilibrium, 65. 
 Eustachian tube, 182. 
 Evaporation, 254. 
 Expansion, 247. 
 Extraordinary ray, 223, 
 Extension, 6. 
 
 Eye, The, 230. 
 
 Falling bodies, 58. 
 Faraday, 40, 279, 300. 
 Fire-engine, 140. 
 Fish, 119. 
 Flames, Sensitive, 179. 
 
 Singing, 180. 
 Floating bodies, 118. 
 Fly-wheel, 93. 
 Focus, 198, 205. 
 Fogs, 265. 
 Force, 21. 
 
 " pump, 140. 
 
 " Centrifugal, 31. 
 Centripetal, 31. 
 
 " Molecular, 43. 
 Forces, Parallelogram of, 27. 
 Triangle of, 28. 
 
 " Polygon of, 28. 
 
 " Composition of, 27. 
 
 " Resolution of, 28. 
 Foucault, 72. 
 Fountains, 110. 
 Franklin, 310. 
 Fraunhofer's lines, 214. 
 Freezing mixture, 250. 
 " of water, 271. 
 
INDEX. 
 
 377 
 
 Friction, 22. 
 Frost, 265. 
 Fulcrum, 82. 
 
 Galvanometer, 329. 
 Galileo, 40, 77, 98, 149, 187. 
 Gases, 44. 
 
 Adhesion of, 52, 144. 
 
 " Buoyancy of, 135. 
 
 " Compressibility of, 13, 137. 
 
 " .Diffusion of, 52. 
 
 " Elasticity of, 13, 137. 
 
 " Osmose of, 53. 
 
 " Pressure of, 133. 
 Geissler's tubes, 339. 
 Gold-leaf, Making of, 11. 
 Governor, The, 262. 
 Gravitation, 55. 
 Gravity, 56. 
 
 Center of, 57. 
 Specific, 113. 
 Guericke, 134, 138. 
 Gulf Stream, 270. 
 
 H 
 
 Halos, 219. 
 Hardness, 14. 
 Harmonics, 175. 
 Hay-scales, 86. 
 Heat, 243. 
 
 " affected by rarefaction, 264. 
 
 " Absorption of, 260. 
 
 " Conduction of, 257. 
 
 " Convection of, 258. 
 
 " Expansion by, 247. 
 
 " Latent, 250. 
 
 " Mechanical equivalent of, 246. 
 
 " Physical effects of, 247. 
 
 " Radiation of, 258. 
 
 u Reflection of, 260. 
 
 " Specific, 256. 
 
 " Theory of, 244. 
 
 " unit, 249. 
 Heating by steam, 270. 
 Helix, 328. 
 Helmholtz, 188. 
 
 Hiero's fountain, 133. 
 Holtz's machine, 308. 
 Horse-power, A, 95. 
 Huygens, 40, 70, 240. 
 Hydrodynamics, 121. 
 Hydraulic ram, 142. 
 Hydrometer, 116. 
 Hydrostatics, 101. 
 Hydrostatic bellows, 108. 
 
 " paradox, 109. 
 
 "' press, 104. 
 
 I 
 
 Ice-crystals, 46, 268. 
 Iceland spar, 222. 
 Imbibition, 50. 
 Impenetrability, 7. 
 Inclined plane, 88. 
 Indestructibility, 10. 
 Index of refraction, 204. 
 Induction, 212, 223. 
 Inertia, 23. 
 Insulators, 219. 
 Interference, 128, 169.1 
 Isochronous, 68. 
 
 Joule's law, 246. 
 
 K 
 
 Kaleidoscope, 197. 
 Kite, 30. 
 
 Lantern, 228. 
 Le Conte, 278. 
 Lenses, 205. 
 
 Land-and-sea breeze, 269. 
 Lever, 82. 
 Leyden jar, 225. 
 Light, 191. 
 
 " Composition of, 210. 
 
 " Diffraction of, 221. 
 
 " Interference of, 220. 
 
 " Laws of, 192. 
 
 " Polarization of, 221. 
 
 " Reflection of, 194. 
 
378 
 
 INDEX. 
 
 Light, Refraction of, 202. 
 " Theory of, 193. 
 " Total reflection of, 208. 
 " Velocity of, 192. 
 " Waves of, 193. 
 Lightning, 310. 
 Lines of Force, 285. 
 Liquids, Buoyancy of 118. 
 " Cohesion of, 43, 44. 
 " Compressibility of, 13. 
 " Diffusion of, .52. 
 " Elasticity of, 13. 
 " Osmose of, 53. 
 " Pressure of, 105. 
 " Specific gravity of, 116. 
 " Surface tension of, 45. 
 " tend to spheres, 45. 
 Liquefaction, 250. 
 
 of gases, 276. 
 Lissajous, 188. 
 Locke, 277. 
 
 M 
 
 Machinery, 81. 
 
 Magdeburg hemispheres, 133. 
 
 Magnetic curves, 285. 
 
 Magnetism, 281. 
 
 Magneto-electric machine, 342. 
 
 Magneto-induction, 339. 
 
 Magnets, 281. 
 
 Malleability, 11. 
 
 Mariotte's law, 148. 
 
 Mass, 14, 23. 
 
 Measure, Standards of, 16, 114. 
 
 Mechanical powers, 81. 
 
 Mechanics, Principle of, 81. 
 
 Meteorology, 264. 
 
 Meter, 6. 
 
 Metric system, 17. 
 
 Microphone, 341. 
 
 Microscopes, 225. 
 
 Mirage, 209. 
 
 Mirrors, 194. 
 
 Molecules, 3. 
 
 Molecular forces, 43. 
 
 Moment of a force, 83. 
 
 Momentum, 23. 
 
 Motion, 21. 
 
 " Circular, 30. 
 
 " Communication of, 21. 
 
 " Composition of, 27. 
 
 " Laws of, 22. 
 
 " Perpetual, 94. 
 
 " Reflection of, 33. 
 
 " Resistance to, 22. 
 Multiple images, 196. 
 Music, 165. 
 Musical scale, 176. 
 
 N 
 
 Near-sightedness, 232. 
 Needle, Magnetic, 285. 
 
 Dipping, 288. 
 Newton, 40, 77, 239. 
 Newton's rings, 220. 
 Nicol's prism, 224. 
 Nodal lines, 174. 
 Nodes, 173. 
 Noise, 165. 
 
 Ocean currents, 269. 
 Octave, 168. 
 Opera-glass, 228. 
 Optics, 191. 
 
 Optical instruments, 225. 
 Ordinary ray, 223. 
 Organ pipes, 178. 
 OsciUation, Center of, 69. 
 Osmose of gases, 53. 
 
 " liquids, 53. 
 Oversightedness, 232. 
 Overtones, 175. 
 
 Pascal, 103, 150. 
 Pendulum, 68. 
 Percussion, Center of, 71 
 Perpetual motion, 94. 
 Phonograph, 180, 188. 
 Pinion, 88. 
 Pisa, Tower of, 67. 
 Pitch, 166. 
 Platinum wire, 11. 
 
INDEX. 
 
 379 
 
 Plato, 15. 
 Plumb-line, 56. 
 Pneumatics, 129. 
 Pneumatic inkstand, 142. 
 Polariscope, 223. 
 Polarization of light, 221. 
 
 " Electric, 301, 318. 
 
 Magnetic, 284. 
 Polarizing angle, 223. 
 Porosity, 8. 
 Pressure of air, 133. 
 Prince Rupert's drop, 48. 
 Prisms, 204. 
 Projecting lantern, 228. 
 Pulley, 91. 
 Pumps, 139. 
 
 Air, 129. 
 
 " Force, 140. 
 
 " lifting, 139. 
 
 " Sprengel's air, 148. 
 Pythagoras, 186. 
 
 Radiometer, 258. 
 Rain, 267. 
 Rainbow, 217. 
 Reaction, 25. 
 Reaction-wheel, 125. 
 Reflected motion, 33. 
 Reflection, Total, 208. 
 Refraction, Index of, 204. 
 Relay, 334. 
 Resonance, 179. 
 Rivers, 123. 
 Ruhmkorff's coil, 337. 
 Rumford, Count, 278. 
 Rupert's drop, 48. 
 
 S 
 
 St. Elmo's fire, 311. 
 Screw, 90. 
 
 Sensitive flames, 179. 
 Ship, Sailing of, 29. 
 Singing flames, 179. 
 Siphon, 141. 
 Siren, 166. 
 Size, 14. 
 
 Snow, 267. 
 Solution, 51. 
 Sonometer, 172. 
 Sound, 153. 
 
 " Intensity of, 160. 
 
 " in a vacuum, 156. 
 
 " Interference of, 169. 
 
 " Ixmdness of, 160. 
 
 " Production of, 153. 
 
 " Reflection of, 162. 
 
 " Refraction of, 161. 
 
 " Transmission of, 154. 
 
 " Velocity of, 158. 
 Sounding-boards, 172. 
 Sound-waves, 154. 
 Speaking-tubes, 161. 
 
 " trumpet, 161. 
 Specific gravity, 113. 
 
 flask, 116. 
 Spectroscope, 213. 
 Spectrum, Prismatic, 210. 
 Normal, 212. 
 Interruptions in, 213. 
 Kinds of, 214. 
 Analysis, 214. 
 Spherical aberration, 208. 
 Spheroidal state, 256. 
 Steam, 252. 
 
 " engine, 261. 
 Steelyard, 85. 
 Stereoscope, 234. 
 Stringed instruments, 172. 
 Surface tension, 45. 
 
 Tacking, 30. 
 Tackle-block, 93. 
 Telegraph, 330. 
 Telephone, 340. 
 Telescope, 226. 
 Temperature, 248. 
 Tempering, 47. 
 Tenacity, 12. 
 Thermo-electricity, 346. 
 Thermometers, 248. 
 Thunder, 310 
 Torricelli, 135, 149. 
 
380 
 
 INDEX. 
 
 Torsion pendulum, 13. 
 Total reflection, 208. 
 Tourmaline, 222. 
 Trade-wind, 269. 
 Tubes, 122. 
 Turbine wheel, 124. 
 Tyndall, 188. 
 
 Vaporization, 251. 
 Velocity, 21. 
 Velocity of heat, 243. 
 " light, 192. 
 " sound, 158. 
 Vibration, 68. 
 Vibrations of air, 154. 
 
 " cords, 171. 
 " ether, 193. 
 u " pendulum, 68. 
 
 " Sympathetic, 179. 
 
 Virtual velocity, 98. 
 Vision, 232. 
 
 " Binocular, 233. 
 Visual angle, 191. 
 Vocal Memnon, 185. 
 Voltaic arc, 321. 
 " battery, 318. 
 
 Voltaic electricity, 313. 
 " pair, The, 314. 
 Volume, 6. 
 Voss' machine, 308. 
 
 W 
 
 Watches, 78. 
 Water, 270. 
 
 " barometer, 138. 
 
 level, 112. 
 
 wheels, 123. 
 Waves, 126, 154. 
 Wave motion, 126. 
 Wedge, 91. 
 Weight, 57, 58. 
 Welding, 44. 
 Wells, 111. 
 Wheel and axle, 86. 
 Wheel-work, 88. 
 Whirligig, 125. 
 Wilde's machine, 343. 
 Winds, 268. 
 Wind instruments, 177. 
 
 Young, 240. 
 Youmans, 278. 
 

YB 360C4 
 
 
 M187542 
 
 THE UNIVERSITY OF CALIFORNIA LIBRARY