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Kekl's Comprehensive Grammar. Rece ntly issued : Kerl's Composition and Rhetoric. — A simple, concise, progressive, thor- ougn, and practical worls on a new plan. Kerl's Shorter Course in English Grammari- — Designed for Schools where only one text-book is used. IVe a ho publish : Sill's New Synthesis; or, Elementary Grammar. Sill's Blank; Parsing Book. — To accom- pany above. ■jells' (W. H.) School Grammar. Wells' Elementary Grammar, GRAY'S BOTANICAL TEXT-BOOKS. These standard text-books are recognized throughout this country and Europe as the most complete and accurate of any similar works published. They are more extensively used than all others combined. Grays "How Plants Grow." Gray's Lessons in Botanv. 302 Draw- ings. Gray s School and Fisld Booic of Botany. Gray's M.^nual of Bot,\ny. 20 Plates. Gray's Lessons and Manual. Gray's Manual with Mosses, &c. Illus- trated. Gray's Field, Forest and G.\I\DKN Botany. Gray's STRUCTURAL AND SySTEILATIC Botany. Flor.\ of the Southern States. Cray's Botanist's Microscope. 2 Lenses. 3 " WILLSON'S HISTORIES. Famous as being the most perfectly graded of any before the public. Primary American History. History of the United States. American History. School Edition. Outlines op GiiNERAL History. School Edition, Outlines of General History. Uni- versity Edition. Willson's Ch.vrt of American His- tory. P.arley's Univees,\i. History, WELLS' SCIENTIFIC SERIES. Containing the latest researches in Physical science and their practical appli- cation to every-day lite, and is still the best Science of Common Things. Natural Philosophy. Principles OF Chemistry. First Principles of Geology. Also: HifcJicock's Anatomy and Physiology. Hitchcock's Elementary Geology. Eliot & Siori-r's Chemistry, PASQUELLE'S ERENCH COURSE Has had a success unrivaled in this country, having passed through more than ^yty editions, and is still the best. FasgueUe s Intrnductorv French Course. l'asqtieUe's\.2s^(ix French Course. Re- vised. Fasqitelle's Key to the Above. Fasquelle's Colloquial Frcuch Reader. FasquelW s Telemaque, Fasquelle' s Dumas' Napoleon. Fasqtielle' s Racine. Fasqtie lie's Manual of French Convers- ation. Howard's Aid to French Composition. Talbot's French Pronunciation. c WELLS'S ^^s.--^ NATURAL PHILOSOPHY; USE OF SCHOOLS. ACADEMIES. AND PLIYATE STUDEHS. Dmosucisa THE LATEST RESULTS OP SCIE-VTITIC DBCOVERT AKD RESKAfiCH; JLHRAITGED WITH SPECIAL REFEEEifCE TO THE PRACTICAL AJTUCATIOX OF PHYSICAL SCIEKCE TO THB ABTS ASD TUB EIPERIESCE3 OF ETEBY-DAT LITE. AiTH THREE HUNDRED AND SEVENTY- FIVE ENGRAVINES DAVID A. WELLS, A.M., ATTBOB 07 " Tin: BCTSSCZ OF COStMOJ* THr«fG»," EDITOB OF TffE " AXTTOAL 01 aClESTinO DlflOOVIBT," " K.V0»-LEIXJE 18 P0Wi3," ETC upteexth editioh eetised. XEW TOEK: IVISON, BLAKEMAX, TAYLOR, & COMPACT?, 138 & 140 GRA^-I> Street. CHICAGO: 133 & 135 STATE STREET. 1873. Entered, according to Act of Congress, in the year 185T, by IVISON & PHINNEY, In the Clerk's OiBoe of the District Court for the Southern District of New York. THOMAS B SMITH". 82 & 84 Beekman-street, N. T. PREFACE. The constant progress made in every department of *^^— ^^^ physical science, is a sufficient apology for the prepara- tion and publication of a new elementary text-book on Natural Philosophy. The principles of physical science are so intimately connected with the arts and occupations of every-day life, with our very existence and continuance as sentient beings, that public oj)iuion, at J;he present time, impera- tively demands that the course of instruction on this subject should be as full, thorough, and complete opportunity and time will permit. With this view, the author has endeavored to render the work, in all its arrangements and details, eminently practical, and, at the same time, interesting to the student. The illustra- tions and examples have been multiplied to a greater extent than is usual in works of like character, and have been derived, in most cases, from familiar and common objects. Great care has been also taken to render the work eom-» plete and accurate, and in full accordance with the latest results of scientific discovery and research. In the arrangement of the subjects treated of, and in *^ the incorporation of questions with the text, the most ap- ^ proved methods, it is believed, have been followed. The ^ IV PREFACE. teacLer Xvill also observe that the principles and import- ant propositions are presented in large and prominent type, and the observations and illustrations in smaller letters. The advantage of this to the learner is most evident. Heat, which is often considered as belonging more especially to chemistry, has been discussed at length, and the familiar application of its principles in the industrial arts, in warming and ventilation, in the production of dew, etc., carefully explained. A full and complete outline of the subject of Meteorology has also been given. On the other hand, Astronomy, which is often included in text-books on Natural Philosophy, has been omitted, as rightfully and properly forming the subject of a separate treatise. An elementary work on physical science can have little claim to originality, except in the arrangement and classi- fication of subjects, and the selection of illustrations. In this respect the author makes no pretensions, and ac- knowledges his indebtedness to the very superior French treatises of Ganot, Delaunay, Archambault, and to the writings of Miiller, Arnott, Lardner, Brewster, and others. The engravings in the present volume are of a superior character, and have been prepared, in part, from new and original designs. New Tobk, August, 185Y. CONTENTS. PAOK iKTRODUCnON 9 CHAPTER I. MaTTEB, Am) ITS GrEKERAL FbOPERTIES. 11 CHAPTER II. Force 21 CHAPTER III. Intebkai^ or Molecular Forces 22 CHAPTER IV. Attractiok of Gravttatioit 30 Section I. — ^Weight 32 " II. — Specifio Gravity, or "Weight 37 " m — Center of Gravitt 45 •' rv. — Effects of Gravity as displayed by Fallino Bodies 63 CHAPTER V. Motion 62 Section I. — Action and Reaction 66 " 11. — ^Reflected Motion 71 " IIL — Compound Motion 72 Vi CONTENTS. CHAPTER VI. PAGB Application of Force 87 Section I. — The Elemexts of MAcui>fERY 93 " II.— Friction 112 CHAPTER YII. On the Strength of Materials used in the Arts, and their Appli- cation TO Architectural Purposes 116 Section I. — On the Strength of Materials 115 " IL — ^Application of Materials to Structural Pur- poses .' 119 CHAPTER VIII. Hydrostatics 123 Section I. — Capillart Attraction 142 CHAPTER IX. Htdbauucs 148 CHAPTER X. Pneumatics 163 CHAPTER XI. Acoustics 183 Section I. — Musical Sounds 194 " IL — Reflection of Sound 197 « III. — Organs of Hearing, and the Voice 201 CHAPTER XII. Heat •_ 205 Section I. — Sources of Heat 208 " II. — Communication of Heat 216 •' III. — Effects of Heat 227 «< IV.— The Steam-Engine 251 « Y. — Warming and Ventilation 260 CONTENTS. Vn CHAPTER XIII. FAQK Meteoeologt 266 Section I. — Phenomena axd Production of Dew 270 " II. — Clouds, Rain, Ssow, and Hail. 273 " III. — Winds 281 " IV. — Meteoric Phenomena 288 " Y. — Popular Opinions concerning the Weather 291 CHAPTER XIV. Light 292 Section I. — Reflection of Light 301 " II. — Refraction of Light 312 " IIL — The An.altsis of Light 325 « IV. — The Eye, and the Phenomena of Vision. 347 " V. — Optical Instrujients 360 CHAPTER XV. Electricity 369 Section L — Atmospheric Electricitt 391 CHAPTER XVI. Galvantsm 398 CHAPTER XVII. Theemo-Electricity 416 CHAPTER XVIII. Magnetism 417 CHAPTER XIX. Electbo-Magnetism 429 NATURAL PHILOSOPHY. INTRODUCTION. Ural Philoso phy? What is Nat- ^' Natural PHILOSOPHY, Or Physics, is that department of science which treats of all those phenomena observed in masses of matter, in which there is a sensible change of place. 2. Chemistry, on the contrary, treats of all c^e^st^yT those phenomena observed to take place in minute particles, or portions of matter, in which there is a change in the character and composition of the matter itself, and not merely a change of place. 3. A falling body, the motion of our limbs Miptes^ol-fhe ^^ °^ machinery, the flow of liquids, the occur- Natu^rJ^'m- '■^^ce of sound, the changes occasioned by the losophyf action of heat, light, and electricity, are all ex- amples of phenomena which come under the consideration of Natural Philosophy. Strictly speaking, we have no right, in Natural Philosophy, to conceive or imagine any thing, for the truths of all its laws and principles may be proved by direct observation, — that is, by the use of our senses. When we conceive, reason, or imagine concerning the properties of matter, we have in reality passed beyond the limits of Natural Philosophy, and entered upon the applica- tion of the laws of mind or of mathematics to the principles of Natural Philos- ophy. Practically, however, no such division of the subject is ever made. The truths and operations of Chemistry, in contradistinction to the trutlis and operations of Natural Philosophy, can not all be proved and made evident by direct observation. Thus, when we unite two pieces of machinery, as two wheels, or when we lift a weight with our hands, or move a heavy body by a lever, we are enabled to see exactly how the different substances come in 1* 10 INTEODUCTION. • contact, how they press upon one another, and how the power is transmitted from one point to another : these are experiments in Natm-al Pliilosophy, in which every part of the operation is clear to our senses. But when we mix alcohol and water together, or burn a piece of coal in a fire, we see merely the result of these processes, and our senses give us no dkect mformation of the manner in which one particle of alcohol acts upon another particle of water, or how the oxygen of the air acts upon the coal. These are experi- ments in Chemistry, in which we can not perceive every part of the operation by means of our senses, but only the results. Had there been but one kind of substance or matter in the universe, the laws of Natural Philosophy would have explained all the phenomena or changes which could possibly take place ; and as the character, or composition of this one substance, could not bo changed by the action of any difterent substance upon it, there could be no such department of knowledge as Chemistry. 4. The term Physics is often used instead b^'lheTerm ^f tlic teiiB Ncitural Philosophy, both having Physics? ii^Q gjinie general meaning and signification. It is also customary to speak of " Physical Laws/' " Physical Phenomena," and " Physical Theories," instead of saying the laws, phenomena, and theories of Natural Philosophy. 5. A Physical Law is the constant relation Physical Laws wliich cxlsts bctwecn any phenomenon and its and Theories ? j -r-> m • • i • cause. A Physical Theory is an exposition of all the laws which relate to a particular class of phenomena. Thus, when we speak of the "theory" of heat, ov of electricity, we have reference to a general consideration of the whole subject of heat, or light, or electricity; but when we use the expression a "law"' of heat, of light, or of electricity, we have reference to a particular department of the whole subject. CHAPTER I. ^ i MATTER, AXD ITS GEXERAL PROPERTIES. 1. Matter is the general name which has what^s^Mat- \jqqj^ given to that substance which, under an infinite variety of forms, affects our senses. "We apply the term matter to every thing that occupies space, or that has length, breadth, and thickness. How do we ^- ^^ ^^ ^^^y through the agency of our five know that any scuscs (hearins", seeing, smelliniT, tasting, and thing exists .' ^ <-" •^' ~' ~ feeling), that we are enabled to know that any matter exists. A person deprived of all sensation, could not be conscious that he had any material existence. wTiat is a 3. A BODY is any distinct portion of matter body? existing in space. What are the ^- ^^^^ properties, or the qualities of matter, properties of ^j-q ^]^q powcFS belonging to it, which are capa- ble of exciting in our mind certain sensations. It is only through the different sensations -which different substances ex- cite in our minds, or, in other -words, it is by means of their different properties, that -we are enabled to distinguish one form or variety of matter from another. The forms and combinations of matter seen in the animal, vegetable, and mineral kingdoms of nature, are numberless, yet they are all composed of a very fe-w simple svibstances or elements. _. ^. . 5. By a simple substance we mean one Wnatisasitn- _ •' i pie substance? ivbich has ucvcr been derived from, or sepa- rated into any other kind of matter. Gold, silver, iron, oxygen, and hydrogen, are examples of simple sub. stances or elements, because we arc unable to decompose them, convert them into, or create them from, other bodies. -^ ^ . ,, 6. The number of the elements or simple What 18 the _ ^ number of the gubstauces with which we are at present ac- elements? ... quainted, is sixty-two. '' ^^ 12 WELLS'S NATUKAL PHILOSOPHY. i 7. These substances are not all equally < ^"'ate^df'^^' distributed over the surface of the earth : most of them are exceedingly rare, and only known to chemists. Some ten or twelve qnly make up the great bulk or mass of all the objects we see around us. All the different forms and varieties of matter are in some respects alike — that is, they all possess certain general properties. Some of these prop- erties are essential to the very existence of a body; others are non- essential, or a body may exist -without them. Thus it is essential to the existence of a body that it should occupy a certain amount of space, and that no other body should occupy the same space at the same time ; but* it is not necessary for its existence that it should possess color, hardness, elas- ticity, malleability, and the like non-essential properties. 8. The following are the most important of molt import* tlic gcucral propcrtics of matter — Magnitude ofufauer?''^" or Extension, Impenetrability, Divisibil- ity, Porosity, Inertia, Attraction, and In- destructibility. 9. By Magnitude we mean the property ^^nuudo?''^' of occupying space. We can not conceive that a portion of matter should exist so minute as to have no magnitude, or, in other words, to occupy no space. The SURFACES of a body are the external limits of its magnitude; the SIZE of a bod}"- is the quantity of space it occupies ; the area of a body is its quantity, or extent of surface. The FIGURE of a body is its form or shape, as expressed by its bound- aries or 'terminating extremities. The volume of a body is the quantity of space included within its external surfaces. The figure and volume of a body are entirely independent of each other. Bodies having very different figures may have the same volume, or bodies of the same figure may have very different volumes. Thus a globe may have ten times the volume of another globe and yet have the same figure, or a globe and a cylinder may have the same volume, that is, may contain the same amount of matter within their surfaces, but possess very different figures. 10. By Impenetrability we mean that i^etrJbiuty? property or quality of matter, which renders it impossible for two separate bodies to occupy • the same space at the same time. MATTER, AND ITS GENERAL PROPERTIES. 13 There are many instances of apparent penetration of matter, but in all of ♦them the particles of the body which seem to be penetrated are merely displaced. When a nail is driven into a piece of wood, the particles ol wood are not penetrated, but merely displaced. If a needle be plunged into a vessel of water, all the water which previously fiDed the space into which It entered, will be displaced, and the level of the water in the vessel will rise to the same height as it would have done, had we added a quantity of water equal in volume to the bulk of the needle. "When we walk through the at- mosphere, we do not penetrate into any of the particles of which the air is composed, but we merely push them aside, or displace them. If we plunge an inverted tumbler into a vessel of water, the air contained in it will pre- vent the water from rising in the glass — and notwithstanding the amount of pressure we may exert upon the tumbler, it cannot be fiUed with water until the air is removed from it. 11. By Divisibility we mean that property ^sibiiity?'" "^hich raatter possesses of being divided, or separated into parts. It has until quite recently been taught that matter was infinitely divisible; that is, a body could bo separated into smaller and still smaller particles without Umit. So far as our senses inform us, this is true. So long as we can perceive the existence of a portion of matter by our sense of siglit, of feeling, of taste, or of smell, so long we can continue to divide it. Eeyond this our senses give us no information. But flie recent discoveries and inves- tigations in chemistry, have proved beyond a doubt, that all bodies are ulti- mately composed of exceedingly minute particles, which can not be subdi- vided. 12. To such an nltiraate portion of matter ^^Atom? *^ ^^ ^^ ^^ longer separable into parts, we apply the term Atom. The extent to which matter can be divided and yet perceived Extent to ^j ^j^g senses is most wonderful. which matter ■' can he divid- A grain of musk has been kept freely exposed to the air of ^' a room, of which the door and windows were constantly kept open, for a period of two years, during all which time the air, though constantly changed, was densely impregnated Avith the odor of musk, and yet at the end of that time the particle was found not to have greatly diminished in weight. During all this period, ever}' particle of the atmos- phere which produced the sense of odor must have contained a certain quan- tity of musk. In the manufacture of silver-gilt wire, used for embroidery, the amount of gold employed to cover a foot of wire" does not exceed the 720,000th part of an ounce. The manufacturers know this to be a fact, and regulate the price of their wire accordingly. But if the gold which covers one foot is the '120,000th part of an ounce, the gold on an inch of the same -wire will be only 14 WELLS's NATURAL PHILOSOPHT. the 8,G40,000th part of an ounce. "We may divide this inch into one hundred pieces, and yet see each piece distinctly without the aid of a microscope : iu other words, we see the 864,000,000th part of an ounce. If we now use a microscope, magnifjing five hundred times, we may clearly distinguish the 432,000,000,000th part of an ounce of gold, each of which parts will bo found to have all the characters and qualities which are found in the largest masses of gold. Some years since, a distinguished English chemist made a series of experi- ments to determine how small a quantity of matter could be rendered visible to the eye, and by selecting a peculiar chemical compound, small portions of ■which were easily discernible, he came to the conclusion that he could dis- tinctly see the billionth part of a grain. In order to form some conception of the extent of this subdivision of mat- ter, let us consider what a billion is. We may say a billion is a million of ' miUions, and represent it thus, 1,000,000,000,000; but the mind is incapable of conceiving any such number. If a person were to count at the rate of 200 in a minute, and work without intermission twelve hours in a day, he would take, to count a billion, G,944,944 days, or more than 19,000 years. But this may be nothing to the division of matter. There are living creatures so mi- nute, that a hundred millions of them may be comprehended in the space of a cubic inch. But these creatures, until they are lost to the sense of sight, aided by the most powerful instruments, are seen to possess arrangements fitted for collecting their food, and even capturing their prey. They are there- fore supplied with organs, and these organs must consist of parts correspond- ing to those in larger animals, which in turn must consist of atoms, or little particles, if we please so to term them. In reckoning the size of such atoms, we must not speak of billions, but of billions of billions. Such a number can be represented thus, 1,000,000,000,000,000,000,000,000, but the mind can form no rational conception of it.* 13. We use the term Molecules, or Pak- I^t'o^plrH- TiCLES of matter to designate very small quan- cies of Matter? ^-j^jgg ^f g^ substaiice, Hot meaning, however, the ultimate atoms. A molecule, or particle of matter may- be supposed to be formed of several atoms united to- gether. 14. No two atoms of matter are supposed to touch, or be in actual contact with each other, and the openings or spaces which exist between them are called Pores. This property of bodies, according to which their atoms are thus separated by vacant places^ What U^Poros- ^^ ^^jj PoROSITY. * The billion is here used according to the English notation. — Viae Webstar. MATTER, AND ITS GENERAL PROPERTIES. 15 What Is the evidence of the existence of Pores in all matter ? Jio. 1. If vre suppose the atoms of matter to consist of minute spheres or globes, it is obvious that it will be impossible for them to come into perfect contact at all points : so that there must be small spaces be- tween them, where they do not touch each other. Fig. 1 represents the manner in which we may im- agine a collection of such atoms to be arranged to form a crystal. 15. The reasons for believing that the atoms or particles of matter do not ac- tually touch each other, are, that every form of matter, so far as we are ac- quainted with it, can by pressure be made to occupy a smaller space than it origin- ally filled. Therefore, as no two particles of matter can occupy the same space at the same time, the space, by which the size or volume of a body may be diminished by pressure, must, before sucb diminution took place, have been filled with openings, or pores. Again, all bodies expand or contract under the influence of heat and cold. Now, if the atoms were in ab- solute contact with, each other, no such movements could take place. The porosity of bodies is sometimes illustrated and explained by reference to a sponge, which allows the cavities which per- vade it to be filled with water, or some other fluid. Such an illustration is not strictly correct. The cavities of a sponge are not really its pores, any, more than the cells of a honey-comb are the pores of wax. In common speech, however, the term pore is often used to designate those openmgs which exist naturally in the substance of a body, which are sufficiently large to admit of the passage of fluids like water, and gases like air. Several very important properties of matter are dependent on porosity ; or, In other words, they owe their existence to the fact, that the particles of mat- ter do not actually touch each other. The principal of these are Density, Compressibility, and Expansibility. These properties of matter belong to all bodies, but not to all alike. 16. By Density we mean the proportion which exists between the quantity of matter contained in a body and its magnitude, or size. Thus, if of two substances, one contains twice as much What is gen- erally meant by the term Pores? What is Dens- ity? 16 WELLS'S NATURAL PHILOSOPnT. matter in a given space as the other, it is said to be twice as dense. There is a direct connection between the density of a body and its porosity. A body will be more or less dense, according as its particles are arranged closely together, or are separated from each other ; and hence it is clear, that the greater the density the less the porosity, and the greater the porosity the less the density. 17. If the particles of a body do not touch each other, then, if it is subjected to pressure, they may be forced nearer, and made to occupy less space. This wo find to be the fact. All matter may be compressed. The most solid stone, when loaded with a considerable weight, is found to be com- pressed. The foundations of buildings, and the columns which sustain great weights in architecture, are proofs of this. Metals, by pressure and hammer- ing, are made more compact and dense. Air, and all gases, are susceptible of great compression. "Water, and all liquids, are much less easily compressed than either soUd or gaseous bodies. 18. By Compressibility, therefore, we mean pressibim™? that property of matter in virtue of which a body allows its volume or size to be diminished, without diminishing the number of the atoms or particles of which it is composed. 19. Again, if the particles of matter of which pan^biiityT ^ body is composcd do not touch each other, it is clear that they may be forced further apart. This we find to be the case with all matter. Expansibility is, therefore, that property of matter in virtue of which a body allows its volume or size to be increased, without in- creasing the number of the atoms or particles of which it is composed. All bodies, when submitted to the action of heat, expand, and Illastrations occupy a larger space than before. To this increase in dimen- bility ? sions there is no limit. Water, when sufBcieutly heated, passes into steam, and the hotter the steam the greater the space it will occupy. All bodies, if subjected to a sufficient degree of heat, will pasr from the state of solids or liquids, into the state of vapor, or gases. 20. Inertia signifies the total absence in a ^*rJa? ^"' ^otly of all power to change its state. If a body is at rest, it can not of itself commence moving ; and if a body be in motion, it can not of it- self stop, or come to rest. The motion, or cessation of MATTER, AND ITS GENERAL PROPERTIES. 17 motion in a body, requires a power to exist independent of itself. It is obvious, from the definition given, that when a body is once put in motion, its inertia will cause it to continue to move until its movement is de- Btroyed, or stopped, bj' some other force. A ball fired from a cannon would move on forever, were it not for the re- sistance or friction of the air, and the attraction of the earth. 21. By Friction, we mean the resistance ^"^uonf"*^" which a moving body meets with from the surface on which it moves. A marble rolled upon a carpet will move but a short distance, on account of the roughness and unevenness of the surface. Its motion would bo con- tinued much longer on a flat pavement and longer still on fine, smooth ice. If friction, the attraction of the earth, and the resistance of the air, were en- tirely removed, the marble would move on forever. Owing to the property of inertia, or the indifference of mat- Whatare Ei- ter to change its state, we find it difficult, in running, to stop ertia ? ^^ "^^ once. The body tends to go on, even after we have ex- erted the force of our muscles to stop. We take advantage of this property, by running a short distance when we wish to leap over a ditch or chasm, in order that the tendencj^ to move on, which we acquire by run- ning, may help us in the jump. For the same reason, a running-leap is al- ways longer than a standing one. Many of the most frightful railroad accidents which have happened, are due to the laws of inertia. The locomotive, moving rapidly, is suddenly checked by an obstruction, collision, or breakage of machinery ; but the train of cars, in virtue of the velocity previous^ acquired, continue to move, and in conse- quence are driven into, or piled upon each other. For the same reasons the wheel of an engine continues to pursue its courso for a time after the driving force has stopped. This property is taken advan- tage of to regulate the motions of machinery. A large, heavy wheel is used in connection vnth the machinery, called a fly-wheel. This hea^y wheel, when once set in motion, revolves with great force, and its inertia causefs it to move after the force which has been imparted to it has ceased to act. A water-wheel or a steam-engine rarely moves perfectly uniformly, but as it is not easy, on the instant, either to check or increase the movement of the heavy wheel, its motion is steady, and causes the machinery to which it is attached to work smoothly and without jerking, even if the action of the driv- ing force be less at one moment than at another. 22. Attraction is that tendency which all ^actionT^*" ^^^ particles of matter in the universe have to approach to each other.* • As Attraction, in its various forms and relations to matter, is so comprehensive and Important, it is treated separately in advance. 18 WELLS'S NATURAL PHILOSOPHY. ' The force which holds the particles of a stone, a piece of "What are Ex- •vrood, or metal together, the falling of a body to the earth, the tract^n*? " tendency which a piece of iron or steel has to adhere to a mag- net, are all familiar examples of the dilTerent forms of attraction. 23. All the researches and investigations of dest^uctibiiT modern science teach us, that it is impossi- ble for any finite agent to either create or de- stroy a single particle of matter. The power to create and destroy matter belongs to the Deity alone. The quantity of matter which exists, in and upon the earth has never been diminished by the annihilation of a single atom. When a body is consumed by fire, there is no destruction of matter : it has only changed its form and position. When an animal or vegetable dies and decays, the original form vanishes, but the particles of matter, of which it was once composed, have merely passed off to form new bodies and enter into new combinations. PRACTICAL QUESTIONS ON THE PROPERTIES OF MATTER. 1. Why will water, or any other liquid, when poured into a tunnel closely inserted into the mouth of a bottle, run over the sides of the bottle ? Because the bottle is filled with air, which, having no means of escape, prevents the water from entering, since no two bodies can occupy the same space at the same time. Ifj however, the tunnel be lifted from the bottle a little, so as to aftbrd the air an opportunity to escape, the water will then flow into the bottle in an uninterrupted stream. 2. Are the pores of a body entirely empty, vacant spaces ? The pores of a body are often filled with another substance of a different nature. Thus, if the pores of a bod}^ be greater than the atoms of air, such a body being surrounded by the atmosphere, the air will enter and fill its pores. 3. When a sponge is placed in -water, that liquid appears to penetrate it. Does the water really enter the solid particles of the sponge? It does not ; it only enters the pore-?, or vacant spaces between the par- ticles. 4. When we plunge the hand into a mass of sand, do we penetbate the sand ? We do not ; we only disjylace the particles. 6. Why do bubbles eise to the surface when a piece of sug:ar, wood, or chalk is plunged under water ? Because the air previously existing in the pores becomes displaced by the water, and rises to the surface as bubbles. 6. What occasions the BJTArpiNG of wood or coal when laid upon the fire ? MATTER, AND ITS GENERAL PROPERTIES. 19 Because the air or liquid contained in the pores becomes expanded bj heat, and bursts the covering in which it is confined. 7. "Why does light, poaors wood, like chestnut or pine, make more snapping in burn- ing than any otuzb kind ? Because the porea are very large, and contain more air than wood of a closer grain, like oak, etc. 8. How is water, or any other liquid, made fube by filtering through paper, cloth, a layer of sand, rock, etc. ? The process of filtration depends on the presence of pores in the substance 'used as a filter, of such magnitude as to allow the particles of hquid to pasa freely, but not the particles of the matter contained in it, which we wish to separate. 9. Why is not the substance suitable for the filtration of oxe liquid equaDy adapted for the filtration of all liquids ? Because the magnitude of the pores in different substances and of the im- purities in hquids is diflferent ; and no substance can be separated from a liquid by filtration, except one whose particles are larger than those of the liquid- 10. Gold and lead are metals of great density ; their pores are not visible. Is there any FBOOF of their existence beside the fact that they can be compressed ? 'U'ater can be forced mechanically through a plate of lead or gold without rupturing any portion of the metal. Mercury, or quicksilver, confined in a dish of lead or gold, will soak through the pores, and escape at the bottom. An interesting experiment was tried at Florence, Italy, nearly two centu- ries ago, which furnished a striking illustration of the porosity of so dense a substance as gold. A hoUow ball of this metal was filled with water, and the aperture exactly and firmly closed. The globe was then submitted to a very severe pressure, by which its figure was sUghtly changed. Xovr, it is proved in geometry, that a globe has this peculiar property — that any change what- ever in its figure necessarily diminishes its volume, or capacity. The result was, that the water oozed through the pores, and covered the surface of tlio globe, presenting the appearance of dew, or steam cooled by the metal This experiment also proved that the pores of the gold are larger than the element- ary particles of water, since the latter are capable of passing through them. 11. When a casbiagd is in motion, drawn by nossES, why is the same exertion of power in the horses required to stop it, as would be necessary to back it, if it were at rest ? Because, according to the laws of inertia, the /orce required to destroy mo- tion in one direction is equal to that required to produce as much motion in the opposite direction. 12. If a carriage, railroad-car, or boat, moving with speed, be suddenly stopped or be- tabdei), from any cause, why are the passengers, or the baggage carried, precipitated from their places in the dibectios of the motion ? Because, by reason of their inertia, they persevere in the motion which they shared in common with the body that transported them, and are not deprived of that motion by the same cause. 20 WELLS'S NATUKAL PHILOSOPHY. 13. Why will a peebox, leaping from a carriage in rapid motion, fall in the direction in which the carriage is moving at the moment his feet meet the ground f Because his entire body, on quitting the vehicle and descending to the ground, retains, by its inertia, the progressive motion which it has in common with it. When his feet reach the ground, they, and they alone, will be sud- denly deprived of this progressive motion, by the resistance of the earth, but the remainder of his body will retain it, and he wiU fall as if he were tripped. 14. Why is a man standing carelessly in the stebn of a boat liable to fall Into the wat«r behind, when the boat Degins to move ? Because his feet are pulled forward while the inertia of his lody keeps it a the same position, and, therefore, behind its support. For a similar reason, wheu the boat stops, the man is Uable to fall forward. 15. When the sails of a ship are first spread to receive the fobce or impuibe of the wind, why does not the vessel acquire her full speed at once ? Because it requires a httle time for the impelling force to overcome the tn- trtia of the mass of the ship, or its disposition to remain at rest. 16. Why, when the sails are taken in, does the vessel continue to move for a considerable time? Because the inertia of the mass is opposed to a change of state, and the ves- sel will continue to move until the resistance of the water overcomes the op- position. 17. Why do we kick against the door-post to shake the snow or dust from our shoes ? The forward motion of the foot is arrested by the impact against the poet ; but this is not the case with respect to the particles of dust or snow which are not attached to the foot, and are free to move. According to the laws of inertia, they tend to persevere in the direction of the original motion, and when the foot stops, they move on, or fly off. 18. Why do we beat a coat or carpet to expel the dust? The cause which arrests the motion imparted to the coat or carpet by the blow does not arrest the particles of dust, and their motion being continued, they fly oS. 1 CHAPTER II. FORCE. 23. Matter is constantly changing its form B^ntlych^g- 9'iid place. The most solid substance will in ^^' time wear away. The air about us is never per- fectly still. We see water sometimes as ice, sometimes as a liquid, sometimes as a vapor, in steam or clouds. The earth moves sixty-eight thousand miles every hour. An animal or vegetable dies, decays, and its form vanishes from our sight. „ ^ , 24. As the cause of aU the changes observed To what cansa • ^ , o do we attribute to take placc in the material world, we admit the changes ob- , ^ • n i • i served in mat- the cxistencc of Certain forces, or agents, which govern and control all matter, whatia 25. Force is whatever produces, or opposes '"^'^ motion in matter. What is Mo- 26. Mobility, or the susceptibility of mo- biuty? iJQjj^ is that property whereby a body admits of change of place. What are the 2*^- "^^^ *^^ great forccs, or agents in nature, StMoT*^*"* those which produce, or are the cause of all the changes which take place in matter, may be .enumerated as follows : Internal, or Molecular Forces, the Attraction of Gtravitation, Heat, Light, the At- tractive and Repulsive Forces of Magnetism and Elec- tricity, and, finally, a force or power which only exists in living animals and plants, which is called, Vital Force. Concerning the real nature of these forces, we are entirely What do we ignorant. "We suppose, or say, they exist, because we see know of the ° . „ ^ , ■' ^ . nature of their enects upon matter. In the present state of science, it i3 these forces? impossible to know whether they are merely properties of matter, or whether they are forms of matter itself, existing in an exceedingly minute, subtile condition, without weight, and diflFused through- out the whole universe. The general opinion, however, among scientific men. 22 WELLS'S NATURAL PHILOSOPHY. at the present day, is, that these forces, or agents, are not matter, but prop- erties, or qualities, of matter. We see a stone fall to the ground, and say that the cause of it is the at- traction of gravitation ; — we observe an object at a distance, and say that w© see it through the action of hght on the eye ; — we notice a tree shattered by lightning, and say it is the effect of electricity ; — we observe an animal or plant to grow and flourish, and ascribe this to the action of the vital force. But if it is asked, What is the original cause of gravitation, light, electricity, and vital force? — the wisest man can give no satisfactory answer. If tba Creator governs matter through the agency of instruments, these forces may be called his agents, or tus instruments. ' CHAPTER III. INTERNAL. OR MOLECULAR FORCES. What is an 28. An INTERNAL, Or MOLECULAR FoRCE, 18 MoiJ"uiar°'^ one that acts upon the particles of matter only v^^zoa'i at insensible distances. This variety of force differs from all others in this respect. What is At- 29. The various changes which matter un- traction and dorcToes, rcudcr it certain that the atoms, or Repulsion? . . particles of all bodies are acted upon by two distinct and opposite forces, one of which tends to draw the atoms, or particles, close together, while the other tends to separate them from one another. The first of these forces we call Attraction, the second Repulsion, both acting at insensible distances. A blade of steel, or a thin piece of wood, when bent within Give an ex- i r- -i ^ ample of At- a certain limit, will, when the restraint is removed, restore it- acting °at an ^^^ *° ^'^ original form. This takes place through the agency insensible dia- of an internal force, attracting the particles together, and tend- ing to keep them in their original place. whatisEias- 30. ELASTICITY is that property of matter •^"'y^ which disposes it to resume its original form and shape, after having been bent or compressed by some external force. Elasticity, therefore, is not so much a distinct property of matter, as ia usually stated, as it is a phenomenon of attractive and repulsive forces. INTERNAL, OR MOLECULAR FORCES. 23 Do all bodies "^^ bodies possess the property of elasticity, but in very possess elas- different degrees. Tliere are some in wliicii tlae atoms, after " ^ bending, or displacement, almost perfectly resume their former position. Such bodies are especially termed elastic, as tempered steel, India- rubber, ivory, etc. Other bodies, like iron, lead, etc., are elastic in a limited degree, not being able to bear any great displacement of their atoms without breaking, or permanent disarrangement. Putty, moist clay, and similar bodies, possess a very slight degree of elasticity. 31. If we compress a certain quantity of gas, as common ^n71o *of r^- ^^^' ^^^ i^^QT^ aUow it to dilate, by removing all restraint, it pulsion acting will expand without limit, and fill every really empty space bie^diatance!'" ^bich is open to it. This takes place through the agency of an internal forco which tends to drive the particles from one another. There are many reasons which lead us to suppose that the repuls- ive force which tends to keep the particles of matter asunder is the agent known as heat. Gases may be considered as perfectly elastic. 32. Accordino; as the attractive or repulsive In what three *-' , ,. .,, forms or con- forcBS prevail, all bodies will assume one of ditions does to i ■ • i all matter ei- three lorms or conditions — the solid, the ist? ... ' LIQUID, or the aeriform,-" or gaseous con- dition. What is a 33. A SOLID body is one in which the par- ^"^^^ tides of matter are attracted so strongly to- gether, that the body maintains its form, or figure, under all ordinary circumstances. What is a 34. A LIQUID body is one in which the par- Liquid? tides of matter are so feebly attracted together, that they move upon each another with the greatest facility. Hence a liquid can never be made to assume any particular form, except that of the vessel in wliieh it is inclosed. .^^^j. . ^ 35. An aeriform, or gaseous body is one Gaseous in which the particles of matter are not held together by any force of attraction, but have a tendency to separate and move off from one another. A gaseous body is generally invisible, and, like the air sur- properties of a rounding US, affords to the sense of touch no evidence of its Gaseous existence when in a state of complete repose. Gaseous bodies may be confined in vessels, from whence they exclude liquid.'^ • Aeriform, having the form, or resemblance, of air. 24 WELLS'S NATURAL PHILOSOPHY. or other bodies, thus demonstrating their existence, though invisible, and also their impenetrability. 36. Most substances can be made to assume circumstances succcssivelj the foi'm of a solid, a liquid, or a Bumeth°e^rm gas. lu solids, the attractive force is the Liquid," or' a strongcst ; the particles keep their places, and ^^^ the solid retains its form. But if we heat the golid to a sufficient degree, as, for example, a piece of iron, we gradually destroy the attractive force, and the repul- sive force increases ; the particles become movable, and we say the body melts, or becomes a liquid. In liquids, the attractive and rejDulsive forces are nearly balanced, but if we supply an additional quantity of heat, we destroy the attractive force altogether, and the hquid changes to a gas, in which the repulsive force prevails, and the particles tend to fly off from each other. By the withdrawal of heat {i. e., by the application of cold), we can diminish, or destroy the repulsive force, and allow the attractive force to again predominate. Thus steam, when cooled, becomes a Fig. 2. liquid, water ; and this in turn, by the withdrawal of an additional amount of heat, becomes a solid, ice. The power of the repulsive force is strik- ingly illustrated by the conversion of water into steam. In a cubic inch of water con- verted into steam, the particles will repel each other to such an extent, that the space occupied by the steam wUl be 1700 times greater than that occupied by the water. Fig. 2 illustrates the comparative difference between the bulk of steam and the bulk of water. 37. The term Fluid is applied to those bodies whose particles move easily among themselves. It is used to designate either liquids or gases. . ^ What are the ^8. Wc distiuguish FOUR kiuds of molecular moLcuu/a^^ attraction, or attraction acting upon the par- traction? ticlcs of bodics at insensible distances. These INTERNAL, OR MOLECULAR FORCES. 25 are, Cohesion, Adhesion, Capillary Attraction, and Affinity, „ 39. Cohesion, or Cohesive Attraction, is What is Co- i • i i • i a i hesiveAttrac- that force wliich binds together atoms of the same kind to form one uniform mass. The force which holds together the atoms of a mass of iron, wood, or stone, is cohesion, and the atoms are said to cohere to each other. What is Ad- 40. Adhesion is that form of attraction hesion? -^hich cxists between unlike atoms, or particles of matter, when in contact with each other. Dust floating in the air sticks to the wall or ceiling, through the force of axihesion. When we \vrite on a wall with a piece of chalk, or charcoal, the particles, worn oiF from the material, stick to the wall and leave a mark, tlirough the force of adhesion. Two pieces of wood may be fastened together by means cif glue, in consequence of the adhesive attraction between the par- ticles of th(j wood and the particles of glue. 41. Capillary Attraction is that form of WTiat is Ca- piiiaryAttrac- attraction which, exists between a liquid and the interior of a solid, which is tubular, or porous. When one end of a sponge, or a lump of sugar is brought into contact with water, the li()uid, by capillary attraction, will rise, or soak up above its level, into the interior of the sponge, or sugar, until all its pores are filled.* What is Af- 42. Affinity is that form of attraction which *°*'^' unites atoms of unlike substances into com- pounds possessing new and distinct properties. Oxygen, for example, unites with iron, and forms iron-rust, a substance different from either oxygen oi' iron. The consideration of the attraction of Affinity belongs wholly to Chemistry. How does the 43. Thc forcc, or strength of Cohesive At- Bi'vrAttTac-^' traction varies greatly in different substances, tionvary? accordlng as the nature, form, and arrange- ment of the atoms of which they are composed vary. 44. These modifications of the force of At- What proper- . . . ., , - . - Kes of bodies tractiou, actmg at insensible distances between variation of thc atoms of different substances, give rise to Attraction? . . . . , , . , ■ , certain important properties m bodies, which are designated under the names of Malleability, Duc- • Capillary Attraction is treated of more fully under the department of Hydrostatics and Hydraulics. 2 26 WELLS'S NATURAL PHILOSOPHY. TILITT, PLIABILITY; FLEXIBILITY, TENACITY, HaBDNESS, and Bkittleness. These are not, as is often taught, distinct, independent properties of matter, like magnitude, porosity, inertia, etc., but modifications of the force of attraction. What is Mai- 45 MALLEABILITY is that property in virtue leabiiity? ^£ whicli a suLstance can be reduced to the form of thin leaves, or plates, by hammering, or by means of the intense pressure of rollers. ' In malleable bodies, the atoms seem to cohere equally in whatever relative situations they liapper. to be, and therefore readily yield to force, and change their positions without fracture, almost like the atoms of a fluid. The ^rope^ty of malleability is possessed in the most eminent What are ex- degree by the metals ; gold, silver, iron, and copper being the Malleability? most malleable. Gold may be hammered to such a degree of thinness, as to require 360,000 leaves to equal an inch ia thickness. What is Due- 46, Ductility is that property in virtue tihty? ^£ ^jjjgj^ ^ substance admits of being drawn into wire. "We might suppose that ductility and malleability would belong to the same substances, and to the same degree, but they do not. Tin and lead are highly malleable, and are capable of being reduced to extremely thin leaves, but they are not ductile, since they can not be drawn into fine wire. Some substances are both ductile and malleable in the highest degree. Gold has been drawn into wire so fine, that an ounce of it would extend fifty miles. -^ , 47. Flexibility and Pliability are those What are fnd'^piiabu properties which permit considerable motion "y? of the particles of a body on each other, with- out breaking. What is Te- 48. TENACITY is that property in virtue of nacity? -vvhicli a body resists separation of its parts, by extension in the direction of its length. ■wTiatis 49. Hardness is a property in virtue of Hardness? which the particlcs of a body resist impression, separation, or the action of any force which tends to change their form, or arrangement. When is a 50. A body, whose particles can be removed, bodyboft? ^^^ changed in position, by a slight degree of force, is said to be soft. Softness is, therefore, the oppo- site of hardness. INTERNAL, OR MOLECULAR FORCES. 27 The property of Hardness 13 quite distinct from Density. Gold and lead possess great density, yet tliey are among the softest of metals. What is Brit- 51. Brittleness is a property in virtue of tieness? wliicli Lodics are easily broken into fragments. It is a characteristic of most hard substances. In a brittle body, the attractive force between the atoms exists within such narrow limits, that a very slight change of position, or increase of distance among them, is sufiBcient to overcome it, and the body breaks. ' 52. The modifications of the force of cohesive attraction between, the par- ticles of matter, which give rise to tlie properties of malleability, ductility, flexibility, pliability, hardness, and brittleness, seem to be intimately con- nected with, or depend upon the particular form of the atoms of the sub- stance, and the particular manner in which they are arranged. Every one knows that it is easier to split wood lengthwise than across the fibers ; hence, the force whicli binds tlie particles of the wood together is ex- erted in a less degree in one direction than in the other. Explain how -^^ changing the form or arrangement of the atoms of a the force of substance, we can in many instances apparently renew or de- pends on the stroy the various modifications of the attractive force. The arrangement following is a familiar illustration of this principle : Steel, when heated and suddenly cooled, is rendered not only veiy havd, but very brittle ; but if heated and cooled gradually, it be- comes soft and flexible. We may suppose that when the atoms of steel are expanded — forced apart from each other by the action of heat, and then sud- denly caused to contract — forced in upon each other — by cooling, that no op- portunity is afforded them for airangement in a natural manner. But when the steel is cooled slowly, each atom has an opportunity to take the place best adapted for it, ^Wthout interfering witli its neighbor. According to one ar- rangement of the atoms, the steel is brittle, or the atoms will not admit of any motion among themselves without breaking ; but according to a difiercnt arrangement, the attractive force is modified, and the steel is soft and flexible. In a similar manner, bricks stacked up irregularly, may be made to fall easily, but if piled in a regular manner, the}' retain their stability. It is a very singular circumstance, that the same operation of heating and cool- ing suddenly, whicli hardens steel, should soften copper. A piece of steel which has been hardened in this way is not c ondensed — made smaller — as we might have supposed it would be, but is actually expanded, or made larger. This proves that the arrangement of the atoms, or particles, has been changed. Any ono may satisfy himself of this by taking a piece of steel, fitting it exactly into a guage, or between two fixed points, and then hardening it. It will then bo found that the steel will not go into the guage, or between the fixed points. What is An- 53. The process of rendering metals, glass, neaiing? ^^^^^ ^^^^ ^^^ flcxiblo by heating and gradually cooling, is called Annealing, and is of great importance in the arts. 28 WELLS'S NATURAL PHILOSOrHT. For example, the workman, in fashioning and slaaping a steel instrument, requires it to bo soft and flexible ; but in usino; it after it has been constructed, as for the cutting- of stone, wood, etc., it is necessary that it should be hard. Tills is accomplished by making the steel soft by auneahug, and then render- ing it hard by heating and cooling quickly.* vrhcn will a 54. WliGQ WG beiid or compress a body so compVesled,"'^ tliat its particlcs are separated beyond a certaia break? limited distance, the force of cohesive attrac- ' • • • • ( tion existing between tliem ceases to act, or is destroyed, and the body falls apart, or breaks. 55. When the Attraction of Cohesion between C.in ■we Tc- Etorc the at- the pai'ticles of a substance is once destroyed, traction of co- , , -^ . , . " ho«ion when it IS generally impossible to restore it. Hav- destroycd? . D 7 l mg once reduced a mass of wood or stone to powder, we can not make the minute particles cohere again by pushing them into their former position. In some instances, however, this can be accomplished by resorting to va- rious expedients. The particles of the metals may be made to again cohere by melting. Two pieces of perfectly smooth plate-glass, or marble, laid upon each other, unite together with such force, that it is impossible to separate thera without breakage. In the manufacture of looking-glass plates, this at- traction between two stnooth surfaces is particularly guarded against. * There are many practical illustrations in the arts, of the principle, that the modifica- tions of the attractive force which unites the atoms of solid bodies together, are dependent in a great degree upon the forms, or arrangement of the atoms themselves. If we submit apiece of metal to repeated hammering, or jarring, the atoms, or particles of which it is composed, seem to take on a new arrangement, and the metnl gradually loses all its te- nacity, flcMbility, malleability, and ductility, and becomes brittle. The coppersmith who forms vessels of brass and copper by the hammer alone, can work on them only for a short time before they require annealing ; otherwise they would crack and fly into pieces. For this reason, also, a cannon can only be fired a certain number of times before it will burst, and a cannon which has been long in use, although apparently sound, is always condemned and broken up. A more important Illustration, and one that more closely affects our interests, is the liability of railroad car-axles and wheels to break from the same cause. A car-aile, after a long lapse of time and use, ia almost certain to break. That these phenomena are due to changes l;i the manner of the arrangement and the form of the particles, or atoms, of matter, was conclusively proved by an experiment made a few years since in Franco : — An accident having occurred upon a railroad, by the break- ing of an axle, by which many lives were lost, the attention of scientific men was called to the fact, that the iron composing the axle, when first used, was strong, and capable of standing a test, but after use in locomotion for a certain period, could be broken by a force far inferior to that by which it had formerly been tested. Many suppositions ware made to account for this phenomenon, when finally a person took a series of rods .about the eize of pipe-stems, all strong and tough, and, with great patience, allowed thjm to fall for hours and hours upon an anvil, thus producing rapid strokes and vibrations. After subjecting them for a long time to this treatment, he found tliat the rods could be snap- ped aa L broken iuto fragmcutu almost as easily as rotten wood. INTERNAL, OR MOLECULAR FORCES. 29 wTiat is 56. Iron may be made to cohere to iron by ■Welding? beating the metal to a high degree, and ham- mering the two pieces together. The particles are thus driven into such intimate contact, that they cohere and form one uniform mass. This property is called Weld- ing, and only belongs to two metals, iron and platinum. J-RACTICAL QUESTIONS ON THE INTERNAL, OR MOLECULAR FORCES. 1. In ■what respect does a gas diffeb from a liquid ? A liquid, like ■water, milk, syrup, etc., can be made to flo^w regularly down a slope., or an inclined plane, but a gas can not. 2. Why is a bar of raox stronger than a bar of ivoo© of the same size ? Because the cohesion existing between the particles of iron ia greater than that existing between the particles of A^-ood. 3. Why are the particles of a liquid more easily separated than those of a eoLm? Because the cohesive attraction which binds together the particles of a liquid ia much loss strong than that which binds together the particles of a solid. 4. Why TvlU a small needle, carefully laid upon the surface of water, float ? Because its ■v\^eight is not sufficient to overcome the cohesion of the particles of water constituting the surface ; consequently, it can not pass through them and sink. 5. If Tou drop watrr and laudanum from the same vessel, why will sixtt drops of the waAdT fll the same measure asoxE iiundeed drops of laudanum? The cohesion between the particles of the two liquids is different, being greatest in the water. Consequently, the number of particles which will ad- here together to constitute a drop of water, is greater than in the drop of laudanum. 6. Why is the prescription of medicine by r>K0P8 an unsafe method ? Because, not only do drops of fluid from the same vessel, and often of tho same fluid from different vessels, dilfer in size, but also drops of the same fluid, to the extent of a third, from different parts of the hp of the same vessel I 7. Why arc cements and mortars used to fasten bricks and stone together? . Because the adhesive attraction between the particles of brick and stone and the particles of mortar, is so strong, that they unite to form one soUd mass. 8. How may the efficacy of a locomotive engine be said to depend upon the force of adhesion ? If there were no adhesion, or even insufficient adhesion, between the tire of the driving-wheel of the locomotive, and the rails upon which it presses, the wheel would turn without advancing. This actually happens whea the rails are greasy, or covered with frost and 30 WELLS'S NATURAL PHILOSOPHY. ice. The contact is thus interrupted, and the adhesion between the rail and Tvheel is impaired. 9. When a liquid adheres to a solid, what term do vre apply to designate the act of adhesion 1 "Wetting. It is necessary that a liquid should adhere to the surface of a solid before it can bo wet. Water falling upon an oiled surface does not wet it, because there is no adhesion between the particles of the oil and the partidea of the water. 10. Why are drops of rain, of tears, and of dew upon tho leaves of plants, generally •pherical, or globular ? The force of cohesion always tends to cause the particles of a liquid, when unsupported, or supported on a surface having little attraction for it, to as- sume the form of a sphere — a globe, or sphere, being the figure which will contain the greatest amount of matter within a given surface. This property of fluids is taken advantage of in the arts, in the manufacture of shot. The melted lead is made to fall in a shower, from a great elevation. In its descent the drops become globular, and before they reach the end of their fall become hardened by cooling, and retain their form. CHAPTER IV. ATTRACTION" OF GRAVITATION. 57. The Attraction of Gravitation is traedon' of tliat form of attraction, by which all bodies at Graviution? sensible distances, tend to approach each other. Electricity and Magnetism attract bodies at sensible dis- Gravitation tances also, but their influenca upon difterent classes of bodies differ from varies, and is Mmited by distance. Molecular, or Internal At- other forms . ..,,,. mi . • * of attraction? traction, acts only at insensible distances. The Attraction of Gravitation acts at all distances, and upon all bodies. 58, Every portion of matter in the universe What is the i • • i i> great law of attracts cvcry other portion, with a lorce pro* the attraction • i i- i • • -i of Gravita- portioncd dircctly to its mass, or quantity, and inversely as the square of the distance. Thia is the great general law of the Attraction of Gravitation. By the Attraction of Gravitation being directly proportional to the mass of a body, we mean, that if of two bodies, the mass of one be twice as large as tliat of the other, its force of attraction will be twice as great : if it is only half as large, its attraction will be only half as great. By the Attraction of Gravitation being inversely proportioned to the square ATTRACTION OF GRAVITATION. 81 of the distance, we mean, that if one body, or substance, attracts another body with a certain force at the distance of a mile, it will attract with four times that forc^ at half a mile, nine times the force at one third of a mile, and so on, in like proportion. On the contrary, it will attract with but one fourth of the force at two miles, one ninth of the force at three miles, one sixteenth of ttLQ force at four miles, and so on, as the distance increases. Pj(j 3 This law may be further illustrated by reference to 9 Fig. 3. Let C be the center ^~~*~;^"— / of attraction, and let the four / . -f^-~r:::r.v ~.. dotted lines diverging from 0 /j __ ..n ■::Z'-~^''^'^^'^^*^ represent lines of attraction. — y " „_ — -t^-' ^t a certain distance from C ^-J^-"""" they will comprehend the small square A ; at twice that distance they will include the large square B, four times the size of A ; and since there is only a certain definite amount of attraction included within these lines, it is clear that as B is four times as great as A, the attraction ex- erted upon a portion of B equal to A, will be only one fourth that which it would experience when in the position marked 1, just half as far from C. As gravitative attraction is the common property of all alUwdfes up- bodies, it may be asked, why all bodies not fastened to the on the earth's earth's surface do not come in contact? They would do. so, in contect'?™^ were it not for the overpowering influence of the earth's at- traction, which in a great measure neutralizes, or overcomes, the mutual attraction of smaller bodies on its surface. Does a feather ^® throw up a feather into the air, and it falls through the attract the influence of the earth's attraction ; but as all bodies attract ^^ each other, the feather must also attract, or draw up, the earth, in some degree, toward itself This it really does, with a force pro- portioned to its mass ; but as the mass of the earth is infinitely greater than the mass of the feather, the influence of the feather is infinitely small, and we are unable to perceive it. In some instances, where bodies are free to move, the mu- lustrations of tual attraction of all matter exhibits itself. If we place upon fa^^K'^' ? '^'^ water, in a smooth pond, two floating bodies at certain dis- tances from each other, they will eventually approach, the con- ditions affecting the experiment being alike for each. Two leaden balls sus- pended by a string near each other, are found, by delicate tests, to attract each other, and therefore not to hang quite perpendicular. A leaden weight suspended near the side of a mountain, inclines toward it to an extent pro- portionate to the magnitude of the mountain. What is the '^^^ earth attracts the moon, and this in turn attracts the cause of earth. The solid particles of matter upon the earth's surface, ' ®* ■ not being free to move, do not sensibly show the influe«ce of the moon's attraction ; but the particles of water composing the ocean, being 32 WELLS'S NATURAL PHILOSOPHY. free to move, furnish us evidence of this attraction, in tlie phenomena of the tides. "When, by the revolution of the earth, a certain portion of its surtace is brought witliin the direct influence of the moon's attraction, the siirface of the ocean is attracted, or drawn up, to form a wave. This wave, or elevation of the surface of the water, occurring uniformly, is called a tide ; when tha moon is the nearest to the earth, its attraction is the greatest, and at these periods we have high tides, or " high water." ..^ , . „ 59. All bodies upon the earth are attracted What IS Ter- , ^ , restriai Gray- toward its Center. This we call Terrestrial ' Uravitation. What is the The attraction of the earth is not the same etrth's^attrac- at all distanccs from the center, being greatest **°"^ at the surface, and decreasing upward as the square of the distance from the center increases, and down- ward simply as the distance from the center decreases. SECTION I. "WEIGHT. „ . ^ , 60. When a body falls to the earth, it de- How is a body , . •' ' at rest upon sccuds becausc it IS attracted toward the center tbesurfaeoof . r- /> the earth at- of the earth. When it reaches the suiiace oi tracted? i i i • > i the earth, and rests upon it, its tendency to continue to descend toward the center is not destroyed, and it presses downwards with a force proportioned to the degree by which it is attracted in this direction. Thia pressure we call Weight. What is 61. Weight is, therefore, the measure of Weight? £^^^g ^-^j^ which a body is attracted by the earth. In ordinary language, it is the quantity of matter contained in a body, as ascertained by the balance. "Weight being, then, the measure of the earth's attraction, it How does follows that as the attraction of the earth varies, weight must Weight vary? . , also vary, or a body will not have the same weight at all places. The weight of a body will be greatest at the surface of the Where ■^■'1 a earth, and greatest at those points upon the surface which are the most, and nearest the center. where the ^g ^^^ ^^^-^^ jg ^^^ ^ perfect sphere, but flattened at the poles, the poles are nearer the center than the equator. A WEIGHT. 33 body, therefore, will be attracted most strongly, that is, will weigh the most, at the polea, or at that portion of the earth's surface which is nearest the center, and weigh the least" at the equator, or at that portion of the earth'a surface which is most remote from the center. A ball of iron weighing one tliousand pounds in the latitude of the city of New York, at the level of the sea, will gain three pounds in weight, if re- moved to the north pole, and lose about four pounds if conveyed to the equator. Bw does 62. If a body be lifted above the surface of as we'^aslend the cartb, its weight will decrease in accord- ed th'8''''6ur- ance with the law, that the attraction of face? gravitation decreases upward from the surface, as the square of the distance from the center of the earth increases. The weight of a body, therefore, will bo four times greater at the earth'a surface, than at double the distance of the surface from the center ; or a body weighing one pound at the earth's surface, will have only one fourth of that weight, if removed as far from the surface of the earth, as the surface is from the center. How does ^"^- As the attraction of gravitation decreases weight vary dowuward from the suitace to the center of the as we descend fr^°i thesur- earth, simply as the distance decreases, weight will decrease in like manner. A body weighing a pound at the surface of the earth, will weigh only half a pound at one half tho distance from the surface to the center. Where will ^^' ^^ *^® ceutcr of thc earth a body will body have no neccssarilv lose all wei<2:ht, since, being sur- weight? •' , & 5 5 ?3 rounded on all sides by an equal quantity of matter, it will be attracted equally in all directions, and, therefore, can not exert a pressure greater in one direction than in another. What are -^^ ^^^® attractive force which the earth exerts upon a body heavy and is proportioned to its mass, or to the quantitv of matter con- li'^ht bodies ? ■ . . ^ j tained in it, and as weight is merely the measure of such at- traction, it follows that a body of a large mass will be attracted strongly, and possess great weight, while, on the contrary, a body made up of a small quantity of matter, will be attracted in a less degree, and possess less weight. "We recognize tliis difference of attraction by calling the one body heavy and the other light. If, as is represented in Fig. 4, we place a mass of lead, a, at one extremity of a well-balanced beam, and a feather, 6, at the other, we shall find that the 2* 34 WELLS'S NATURAL PHILOSOPHY. "VNHiat is a Sys- tem ofWeiiihts and Measures? lead is drawn to the earth with a force exactly equal to the superiority of its mass over that of the feather. If, however, we tie on a suflficient number of feathers to make up a quantity of matter equal to that of the lead, the equilibrium is re- stored— the two quantities are attracted with equal force, and the beam is supported in a hori- zontal position. 65. In all the opera- \ tions of trade and com- \ merce, we sell, or ex- Oi change a given quantity \ / of one article or substance "> — '' fQj. a^ certain quantity of Bome other article or substance — so much flour for so much sugar, or so much sugar and flour for so much gold. Hence the necessity, which has existed from the earliest ages, of having some fixed rules or standards, according to which different quanti- ties of difierent substances may be compared. A set, or series, of such rules or standards of comparison, is called a System of Weights and Measures. Various nations adopt different standards, but in the civil- ized and commercial world, but two great Systems of Weights and Measures are generally recognized. These are known as the English, and the French Systems. In the English System, which Ls the one used in the United States, there are two systems of weights — Troy and Avoirdu- pois "Weight Troy Weight is principally used for weighing gold and silver ; Avoirdupois for weighing merchandize, other than the precious metals. It derives its name from the French avoirs (averia\ goods or chattels, and pouls, weight. The smallest weight made use of ia the English Sy.stem is a grain. By a law of England enacted in 1286.lt was ordered that 32 grains of wheat, well dried, should weigh a pennyweight Hence the name grain applied to this measure of weight. It was afterward ordered that a pennyweight should be divided into only 24 grains. Grain weights for practical purposes, are made by weighing a thin plate of metal of uniform thickness, and cutting out, by measurement, such a proportion of the whole as should give one grain. In this way, weights may be obtained for chemical purposes, which weigh only the 1,000th part of a grain. What are the two great SyB- tems of Weights and Measures ? What are the ppculiarities of the English System f WEIGHT. 35 How do we Ob- 66. In constructing a System of Weights i!l-dof weight's ^^^ Measures, it is necessary, in the first place, and Measures? ^q g^ uijou somc dimension which shall forever serve as a standard from which all other weights and measures may be derived, and by which they may be com- pared and verified. If an artificial standard were taken, it is evident that it might be falsified, or even entirely lost or destroyed, thus creating great confusion. It is, there- fore, necessary to fix upon some unchanging and invariable space or size in nature, which will always serve as a stand- ard, and which the art of man can not affect. In th^ English System of Weights and Measures, such an un- varying dimension, or standard, is found in the length of a pendulum. Describe the ^7. A peudulum is a heavy body, suspended Pendulum. from a fixed point by a wire or cord, in such a manner that it may swing freely backward and forward. The alternate movements of a pendulum in opposite di- rections are called its vibrations, or oscillations, and the part of a circle over which it moves is called its akc. In Fig. 5, A B represents a pendulum ; D C, the arc in which it vibrates. Now, it has been found that a pendulum, of any weight, which in the latitude of Lon- don will vibrate, or swing over the same arc, or from the highest point on one side, to the highest point on the other side, in one second of time, will always, under the same circumstances, have the same length. The length of this pendulum (the part A B, Fig. 5) is divided into 391,393 equal parts. Of these parts, 1 0, 000 are called an inch, twelve of which make one foot, thirty-six of them one yard. Thus we ob- tain standards of linear measure. „ . ^ To obtain a Standard of 'Weight, a cubic inch (accuratelii ob- How do wc ob- \ i. 1 J /. tain a Standard tained from the pendulum) of distilled water, of the temperature of Weight? ^f g20 Fahrenheit's thermometer, is taken and weighed. This weight is divided into 252,458 equal parts; and of these, 1,000 will be a grain. The grain multiplied, gives ounces, pounds, etc. How does the Pendulum fur- nish a Stand- ard of Meas- ures of Length ? Fig. 5. A 3S WELLS'S NATUKAL PHILOSOPHY. Explain the construction of the French System of Weights and Measures. „ , , To obtain standards of Liquid Measure, ten pounds, or 7,000 HowaoT7eoD- .„,..„, , •, tain Standards grains of distilled water, at the same temperature, are made of Liquid ^Q constitute a gallon. The gallon, by division, gives quarts, pints, and gills. 68. The French System of Weights and Measures is constructed on a different plan, and originated in the fol- lowing manner : In 1788, the French Government, feeling the necessity of having some standard by which all weights and measures might be compared and made uniform, ordered a scientific in- quiiy to be made ; the result of which was the establishment of the present system of French Weights and Measures, which, from its perfect accuracy and simplicity is superior to all other systems. It is sometimes called the Decimal System, all its divisions bemg made by ten. The French standard is based on an invariable dimension of the globe, viz., a fourth part of the earth's meridian, or the fourth part of the largest circle pass- ing through the poles of the earth. In Fig. 6, the circle N E S "W repre- sents a meridian of the earth ; and a fourth part of this circle, or the distance N E, con- stitutes the dimension on which the French System is founded. This distance, w-hich was accurately measured, is divided into ten milUon equal parts ; and a single ten million til part adopted as a measure of length, and called a metre. The length of the metre is about 39 English inches. By multiplying or dividing this quantity by ten, the other varieties of weights and measures are obtained. 69. In the United States, Standards of Weights and Measures, prepared according to the English System hy order of the Government, are to be found at Washington, and at the capital of every State. Fig. 6. PRACTICAL PROBLEMS ON THE ATTRACTION OF GRAVI- TATION. 1. Suppose two bodies, one weighing 30 and the other 90 pounds, situated ten miles apart, were free to move toward each other, under the influence of mutual attraction: what space would each pass over before they came in contact ? The mutual attraction of any two bodies for each other is proportional to the quantity of matter they contain. 2. A body upon the surface of the earth weighs one pound, or sixteen ouices: if by SPECIFIC GRAVITY, OR WEIGHT. 37 any means we could carry it 4,000 miles abore the earth's surface, irhat would be its weight ? Solution: The force of gravity decreases upward, as the square of the distance from the ceuter increases : weight, therefore, will decrease in like proportion. The ilistancc of the body upon the surface of the earth, from the center, is 4,000 miles. Its distance from the center, at a point 4,000 miles above the surface, is S.vOO. The square of J.iM'i ji 16,000,000 ; the square of 8,000 is 64,iiOi,000. The weight, therefore, will be diminiehed in the proportion that sixty-four bears to sixteen ; that is, it will be diminished 4ths, or weigh ^th of a pound, or 4 ounces. 3. What will be the weight of the same body removed 8,000 miles from the earth's gtirface ? 4. A body on the surface of the earth weighs ten tons : what would he its weight if elevated '2,000 miles above the surface ? 5. How far above the surface of the earth must a pound weight be carried, to make it weigh one ounce avoirdupois ? 6. What would a body weighing 800 pounds upon the earth's surface, weigh 1,000 miles below the surface ? The force of gravity decreases as we descend from the surface into the earth, simply as the distance downward increases. — weight being the measure of gravity, it therefore decreases in the same proportion. The distance from the surface of the earth to the center may be assumed to be 4,000 mUes : 1.000 miles is one fourth of 4,000. The dis- tance being decreased one fourth, the weight is diminished in like proportion, and the body will lose '200 pounds, or its total weight would be 600 pounds. 7. Suppose a body weigViing Sno pounds upon the stirface of the earth were sunk 3,000 miles below the surface : what would be its loss in weight ? 8 If a mass of iron ore weighs ten tons upon the earth's surface, what would it weigh at the bottom of a mine a mile below the surface ? 9. "What wiU be the weight of the same mass at the bottom of a mine one half a mile below the earth's surface T SECTION II. SPECinC GKATITT, OE WEIGHT. 70. A piece of iron sinks in water, and floats upon quick- senses may the silver. In the first instance, we say the iron sinks because it hl^ d^^°^' ^ heavier than water ; and in the second, it floats, because it is hghter than quicksilver. Iron, therefore, is a heavy body compared with water, and a light body compared with mercury. But in or- dinary language, we always consider iron as a heavy body. The term weight may, therefore, be used in two very different senses, and a body may be at once very light or very heavy according to t'iC sense in which the terms are used. A mass of cork which weighs a ton is very heavy, because its ab- solute weight as indicated by the balance, viz.. 2,000 pounds, is considerable. It is, however, in another sense, a light body, because if compared, bulk f jr bulk, with most other solid substances, its weight is very small. Hence we make a distinction between the absolute, or real weight of a body, and its specific, or comparative weight 38 WELLS'S NATURAL PHILOSOPHY. What is Ab- 71. The Absolute Weight of a body is solute Weight f ||^g^^ ^^ j^g entire mass, without any reference to its bulk, or volume. What IB spe- "^2- The Specific Weight, or the Specific cific Weight? Oravity of a body, is the weight of a given bulk, or volume of the substance, compared with the weight of the same bulk, or volume, of some other substance. The term " Specific" Weight, or Gravity, is used, becauaa the^t^rm^^Spey bodies of different species of matter have different weights cific," as ap- under equal bullis, or volumes. Thus, a cubic inch of cork, Vei^htT ^^^ ^ different weight from a cubic inch of oak, or of gold, and a cubic inch of water contains a less weight than a cubic inch of mercury. Hence we say that the specific gravity, or specific weight, of cork is less than that of oak or gold, and the specific gravity of mercury ia greater than that of water. 73. Specific Gravity, or Weight, being merely the compara- What is the ^^^q rrravity, or weight, it is convenient that some standard Standard for , 7, , , ,,.,„, estimating the should be selected, to which all other substances may be re- Specific Grav- ferred for comparison. Distilled water has accordingly been taken, by common consent, as the standard for comparing the weights of all bodies in the soUd, or hquid form. The reason for using dis- tilled water is, that we may be certain of its purity. Water, therefore, being fixed upon as the standard, we determine the spe- cific gravity of a body, or we ascertain how much heavier or ligliter a sub- stance is than water, by the following rule: — How do we 74. Divide the weight of a given bulk of the ci^c '^ra^^ substance, by the weight of an equal bulk of of bodies? water. Explain the Suppose we take five vessels, each of which would contain application of exactly One hundred grains of water, and fill them respectively IS ru e. ^j^^ spirits, ice, water, iron, and quicksilver. The following differences in weight will be found : — The vessel filled with spirits would weigh 80 grains; wiih ice, 90 grains; with water, 100 grains; with iron, 750 grains; with quicksilver, 1,350 grains. Water having been selected as the standard for comparing these different weights, the question to be settled is simply this: How much lighter than w.ater are spirits and ice, and how much heavier than water are iron and quicksilver ; or, in other words, how many times is 1 00 contained in 80, 90, 750, and 1,350? The weights of the different substances fiUing the vessel are, therefore, to be divided by 100, the weight of the water ; and there is found for spirits the weight 0'80. one ifth lighter than water ; for the ice, 0-90, one tenth lighter than water; for the iron, 7'50, or seven and a half times heavier than water; for the quicksilver, 13-50, or thirteen and a half times SPECIFIC GRAYITY, OR WEIGHT. 39 Tlo-w do we ob- tain the Spe- cific Gravity of Liquid bodies 1 heavier than water. These numbers, therefore, are the specific gravities of +he spirits, ice, iron, and quicksilver. For obtaining the specific gravity of Liquids the method above described is substantially the one usually adopted in the arts. A bottle capable of holding exactly 1,000 grains of distilled water, at a temperature of 60° Fahrenheit, is ob- tained, filled with water, and balanced upon the scales. The water is then removed, and its place supphed with the fluid whose specific gravity we wish to determine, and the bottle and contents agaui weighed. The weight of the fluid, divided by the weight of the water, gives the specific gravity required. Thus a bottle holding 1,000 grains of distilled watpr, will hold 1,845 grains of sulphuric acid; 1,845-^1,000 = 1.845, or, the sulphuric acid is 1.845 times heavier than an equal bulk of water. . For obtainino: the specific gravity of sohd bodies, a different When we im- , ^ , ^ ^^, " . u j • * merse a body method 13 adopted. When we unmerse a body m water, in water, what j^ displaces a quantity of water equal to its own bulk. (In occurs? ^ 1 .,, , ,.-r.-i,-l Fig. 7, the space occupied by the cube A B is obviously equal to a cube of water of the same size.) The Fig. T. water that before occupied the space which the body now fills was supported by the pressure of the other particles of water around it. The same pressure is exerted on the substance which we have immersed in the water, and, consequcnth", it will be supported in a like degree. .„ If the body weighs less than an When will a , , „ %. body sink, and equal bulk of water, the pressure when float, in ^f ^]^Q ^.^ter will sustain it entirely, ♦rater ? and the body will float ; if, on the contrary, it is heavier than an equal bulk of water, the pressure of the particles of water will bo un- able wholly to sustain it, and, yielding to the at- traction of gravitation, it descends, or sinks. But to whatever extent a body may be supported in water, to the same extent it will cease to press downward, or its weight will diminish. TTe ac- cordingly find, that a solid body, when immersed in water and weighed, will weigh less than when weighed in air, and the difference between these two Weights wUl be equal to the weight of a quantity of water of the same size or bulk as the solid body ; all bodies of the same size, therefore, lose the sam9 quantity of tlieir weight in water. To find the Specific Gravity of SoUds heavier tlian water, or their weight compared with the weight of an equal bulk of water, we have the following rule : 75. Ascertain the wei^rht of the body in Divide the weight in and the How do we de- terming the Specific Grav- ity of SoUds heavier than WAter? water, and also in air. air by the loss of weio;ht in water, quotient will be the specific gravity required. 40 WELLS'S NATURAL PHILOSOPHY. How do yre find the Spe- cific Gravity of a body lighter than water 7 j-jg g Suppose a piece of gold -n-eighs in the air 19 grains, and in Tvater 18 grains ; the loss of weight in -water will be 1; 19-7-1 = 19, the specific gravity of gold. Fig. 8 represents the arrangement of the balance for taking specific gravities, • and the manner of suspending the body in water from the scale pan, or beam, by means of a fine thread, or hair. 76. To find the specific gravity of a body lighter than water, tie it to some substance sufficiently heavy to sink it, whose weight in air and water is known. Weigh the two together, both in air and. water, and ascertain the loss in weight. This loss will be the weight of as much water as is equal in bulk to the two solids taken together. Subtract the loss of the heavy body weighed by itself in water, previously known, from the loss sus- tained by the combined solids. The remainder will be the weight of as much water as is equal in bulk to tho lighter body, Di\"ide the weight of the lighter body in air by this remainder, and the quotient will be the spe- cific gravity required. Thus, for example, let the weight of the ligliter solid be 3 ounces, and that of the heavier soUd 15 ounces. Let the weight which the two together lose when submerged in water, be 5 ounces, and let the weight which the heavier alone loses when immersed be 1 ounce. Subtracting the loss of weiglit of tho heavier body, in water, 1 ounce, from the combined loss of the two in water, 5 ounces, we have 4 ounces as the weight of a mass of water equal in bulk to the lighter body. But the weight of the hghter body in air is 3 ounces; 3-j.4=0.75=:|-. It will, therefore, weigh three quarters of its own volume of water, or have a specific gravity 0.75. 77. The specific gravity of Liquids may also be found by ths balance in the following manner : Weigh a sohd body 'n water, as well as in the liquid whose specific gravity is tn be de- termined ; then the lo.ss in each case will be the re«;pective weights of equal bulks of water and hquid. We have, there- fore, the following rule : 78. Divide the loss of weight in the liquid by the loss How may we find the Spe- cific Gravity directly by the balance ? SPECIFIC GRAVITY, OR WEIGHT. 41 of "weiglit in water ; the quotient will give the specific gravity of the liquid. ' Thus a solid hodj (a piece of glass is generally used) loses 20 grains when •weighed in water, and 30 grains when weighed in acid; 30-i-20 = 1.5, the spe- cific gravity of the acid. 79. There are various other methods of obtaining the specific gravity of solids and liquids.* Those we have described are the ones most generally adopted. Bpwdoweob- 80. For obtaining the specific gravity of ^ *G^rav?b7" gases, air instead of water is adopted as the •fa Gas? standard of comparison. The weight of a given volurae or measure of a gas is compared with the weight of an equal volume of pure atmospheric air, and the weight of the gas divided by the weight of the air, will express the specific gravity of the gas. 81. The following table exhibits the specific gravity of various solid, liquid, and gaseous bodies ; pure water, having a temperature of 60 degrees Fahren- heit's thermometer, being assumed as the standard of comparison for sohds and hquids, and pure, dry air, having the same temperature, being assumed as the standard of comparison for gases. The metal platinum has the greatest specific gravity of any sohd body, being 21.50 times heavier than an equal bulk of water ; and hydrogen gas the least specific gravity of any of the gases, being 14.4 lighter than an equal bulk of air, and 12.000 lighter thanan equal bulk of water. The.se two substances are respectively the heaviest and light- est forms of matter with which we are acquainted. SOLIDS AKD LIQUIDS. Distilled water 1.000 Platinum • 21.500 Gold 19.360 Mercury 13.600 Lead 11.450 Silver 10.500 Copper 8.870 Iron 7.800 Flint Glass 3.320 Marble 2.830 Anthracite coal 1.800 Box-wood 1.320 Sea-water 1.020 Whale oil 0.920 Pitch-pine wood 0.060 • See Hydrometer. 42 WELLS'S NATURAL rHILOSOPHT. "White pino 0.420 Alcohol 0.800 Ether 0.720 Cork 0.240 GASES. Pure, dry atmospheric air 1.000 Carbonic acid gaa 1.520 Oxygen 1.100 Nitrogen 0.970 Ammoniacal gas 0.580 Hydrogen 0.070 How can we ^^' "^ ^"^^'^ ^°°* °^ 'water weighs almost exactly 1,000 determine the ounces avoirdupois, or 62^ pounds. If^ therefore, the specific of™'bod7fi-om S^^'^'^'^y of water be represented by the number 1,000, the its Specific numbers which express the specific gravity of all other solids ravi y ^^^ liquids, will also express the number of ounces contained in a cubic foot of their dimensions. Thus, the specific gravity of gold being 19.360, it follows that a cubic foot of gold will weigh 19,360 ounces; and the specific gravity of cork being 0.240, the weight of a cubic foot of cork will bo 240 ounces. By means of a table of specific gravities, therefore, the weight of any mass of matter can be ascertained, provided we know its cu- bical contents, by the following rule : 83. Multiply the weight of a cubic foot of water by t^e specific gravity of a substance ; the product will be the weight of a cubic foot of that substance. Thus, anthracite coal has a specific gravity of 1.800. This, multiplied by the weight of a cubic foot of water, 1,000 ounces, gives 1,800 ounces, which is the weight of a cubic foot of coal. How can we 84. Thc volume, or bulk, of any givcH Weight bink o^f "a Bu^b- ^^ ^ substance can also be readily calculated, spTcific'^Grav! ^Y dividing the number expressing the weight "^' in ounces by the number expressing the spe- cific gravity of the substance, omitting the decimal points; the quotient will express the number of cubic feet la the volume, or bulk. Thus, for example, if it be desired to ascertain the bulk of a ton of iron, It is onlj necessary to reduce the ton weight to ounces, and divide the number of ounces by 7.800, the specific gravity of iron ; the quotient will be the .,,.. ,. , number of cubic feet in the ton weicrht. If ths particles t^ i • i n it of matter were 85. If the particles 01 all matter were per- iree io move, ni/> iii» how would lectly tree to move among themselves, their th^-Vver?^^ arrangement in space would always be in ex- SPECIFIC GRAVITY, OR WEIGHT. 43 act accordance with their different specific gravities : in other words, light bodies, or those having a small specific gravity, would rest upon, or rise above all heavier bodies, or those possessing a greater specific gravity. .. In the case of different liciuids, the particles of which are Wliat are illus- , , , , . trations of this free to move among themselves, this arrangement always ex- prmciple? jg^ gQ iQ^g ^g ^Jjq different substances do not combine to- gether, by the force of chemical attraction, to form a compound substance. Thus, water floats upon sulphuric acid, oil upon water, and alcohol upon oil, and by carefully pouring each of these liquids successively upon the surface of the other, they may be arranged in a glass in layers. Carbonic acid gas is heavier than atmospheric air. We accordingly find that it accumulates at the bottom of deep pits, wells, caverns and mines. This principle also explains certain phenomena which at first seem opposed to the law of terrestrial gravity, that all matter is attracted toward the center of the earth. We ob- serve a balloon, a soap-bubble, or a cloud of smoke or steam to ascend'; and a cork, or other light body, placed at the bot- tom of a vessel of water, rises through it, and swims on the surface. These phenomena are a direct consequence of gravitation ; the attraction of which, increasing with the quantity of matter, draws down the denser air and water to occupy the place filled by the lighter bodies, which are thus pushed up, and compelled to ascend. Why does a balloon ascend, or a cork rise to the surface of water ? Fig. 9. Suppose a, Fig. 9, a ball of wood so loaded with lead that it wiU float exactly in the middle of a vessel of water. The weight of the wood and the upward pressure of the water have such a relation to each other, that the ball is balanced in this position. If now we add a few drops of strong salt and water, we shall see, as it sinks and mixes with the water, that the ball, a, is forced to the top of the fluid, because the attraction of gravitation on the denser f uid draws it down, and compels it to occupy the place The principle that the particles of hquids arrange them- selves according to their specific gravitie.", has been taken advantage of in the West Indies by the slaves, in order to enable them to steal rum from casks. The long neck of a bottle filled with water, is inserted through the bung of the cask into the rum. The water falls out of the bottle into the cask, while the lighter rum rises to take its place. The principle of specific gravity admits of many valuable applications in the art.s. It offers a very sure and quick method of determining whether a substance is pure or adul- terated. Thus, silver may be mixed with gold to a consider- able extent, without changing, to any great degree, the ap- Mpntion some of the practical applications of specific gray- ity. 44 WELLS'S NATURAL PHILOSOPHY. pearanco of the gold. The specific gravity of pure gold being 19, and of pure silver 10, it is obvious tliat a mixture of the two -will have a specific gravity less than pure gold, and greater than pure silver, the diflerence being propor- tioned to the amount of adulteration. In the same way we can determine whether cheap oils have been mixed with expensive oils, cheap and poor il- luminating gas, with expensive and brilliant gas. In any case it enables U3 to ascertain the exact size or solid bulk of a mass, however irregular — evcQ of a bundle of twigs.* PRACTICAL PROBLEMS RELATING TO SPECIFIC GRAVITY. 1. The weight of a solid body is 200 grains, but its weight in water is only 150 grakis; -what is the specific gravity of the body ? Solution: 50 grains = loss of weight in water; 200 grains (weight in air)-^50— 4, spe- cific gravity required. 2. A body weighed in the air 28 pounds, and in water 2-1 pounds ; what is its specific gravity ? 3. An irregular fragment of stone weighed in air 73 grains, but lost 30 upon being weighed in water; what was the specific gravity of the stone? 4. A piece of cork weighed in the air 4S grains, and a piece of brass 560 grains ; the brass weighed in water 4S3 grains, and tts brass and cork when tied together weighed in water 336 grains. What was the specific gravity of the cork ? 5. How much more matter is there in a cubic foot of sea-water, than in a cubic foot of fresh water ? 6. Would a piece of steel sink or swim in melted copper? 7. Wlien alcohol and whale-oil are put in the same vessel, which of these two sub- stances will occupy the top, and which the bottom part of the vessel? 8. If a cubic foot of water weigh 1,000 ounces, what will be the weight of a cubic foot of lead ? 9. What will be the weight of a cubic foot of cork, in ounces and in pounds 7 • The attempt to ascertain whether a particular body had been adulterated led Archi- medes, it is said, to the discovery of the principle of specific gravity. Iliero, King of Syracuse, having bought a crown of gold, desired to know if it were formed of pure mctfll ; and as the workmanship was costly, he wished to accomplish this without defacing it. The problem was referred to Archimedes. The philosopher for some time was unable to solve it, but being in the bath one day, he obseived that the water rose in the bath in ex- act proportion to the bulk of his body beneath the surface of the water. He instantly per- ceived that any other substance of equal siz3, would raise the water just as much, thongh one of equal weight and loss size, or bulk, could not produce the same effect. Convinced that he could, by the application of this principle, determine whether Hiero's crown had been adulterated, and moved with admiration and delight, he is said to have leaped from the water and rushed naked into the street, crying " Evpi7ii exerted on a body concen- trated at its Center of Grav- ity? What is the CenterofMag- nituda? Wliere is the Center of Grav- ity of a body ? 46 WELLS'S NATURAL PHILOSOPHY. the center of gravity no longer corresponds with the center of magnitude, or the central point of the bulli of the body. Fig. 11. Thus, in a sphere, a cube, or a cylinder, the center of grav- ity is the same as the center of the body. In a ring of uni- form size and density, the center of gravit}- is the center of the space inclosed in the ruig (see Fig. 11). This example shows that the center of gravity is not necessarily included in that portion of space occupied by the matter of the body. In a w'lieel of wood of unilbrm density and thickness tho •enter of gravity will be the center of the wheel, but if a part of the rim be made of iron, the center of gravity will be removed to some point aside from the center. "WTien two bodies are connected together, they may be regarded as one body, having but one center of gravity. If the two bodies be of equal weight, the center of gi-avity will be in the middle of the line which unites them ; but if one be heavier than the other, the center of gravity will be as much nearer the heavier body, as the heavier exceeds the lighter one in weight Pj(j_ ]^2. Thus, if two balls, each weighing four pounds, be connected together by a bar, the center of gravity will be a point on the bar equally distant from each. But if one of the balls be heavier than the other, then the center of gravity will, in propor- tion, approach the larger ball. This is illustrated by reference to Fig. 12, in which the center of gravity about which the two balls support themselves, is seen to be nearest to the heavier and larger ball. 89. The center of gravity of a body being regarded as the point in which the sum of all tho forces of gravity acting upon the separate particles of the body are concentrated, it be considered as influenced by the attraction of the earth in a greater degree than any other portion of the body. It follows, therefore, that if a hody has freedom of motion, it can not be brought into a position of permanent equilibrium, until its center of gravity occupies the lowest situation which the support of the body will allow; that is, the center of gravity will descend as far toward the center of the earth as possible. _ 90, By Equilibrium we mean a state of rest What do we J ^ nieanbyEquui- produccd by the counterpoise, or balancing, of opposite forces, I Thus when one force tending to produce motion in one direction, is oppa'sed by an equal force tending to produce motion in an exactly opposite direction, the two balance each other, and no motion results. To produce any action, there miat be an inequality in the condition of one of the forces. J, , The truth of this principle may be illustrated by certain ex- periment can perimonts which at first seem to be contradictory to it. Thus This priQci^e ? '^ cylinder may be made to roll up an inclined plane. Fix a piece of lead, I, Fig. 13, on one side of the cylinder a, so that When will the Center of Grav- ity be in perma- nent rest, or equilibriuiu '! CENTER OF GRAVITY. 47 Fig. 13. Illustrate first case. the the center of gravity of the cylinder will be at the point I, wiule its center of magnitude is at c. The cylinder will tlien roll up the mclined plane to the position a I, because the center ic ] of gravity of the mass, Z, \viU endeavor to descend to its lowest point. 91. A prop that supports the center of gravity sup- ports the whole hody. This support may be applied iu T u . .V three different ways : In what three •' center"if Grat ^' "^^^ poiut of support may he applied di- ity be support- rcctly to the center of gravity of the body. 2. The point of support may have the cen- ter of gravity immediately below it. 3. The point of support may have the center of gravity immediately above it. In the first case, where tlie point of support is applied di, rectly to the center of gravitj', the body will remain at rest u\ any position ; this is illustrated in the case of a common wheel, where the center of gravity is also the center of the figure, and this being Pig. 14, supported on the axle, the wheel rests indifferently in any position. In Fig, 14, let a, the center of the wheel, which =■ 0 is also its center of gravity, be supported by an axle; — the wheel rests, no matter -a- to what extent we turn it. In the second case, where the point - C of support is above the center of gravity, the body, if it is allowed freedom of mo- tion, will not rest in perfect equilibrio tmtil its center of gravity has descended to the lowest position, which in all cases ■noU be immediately beneath the point of suspension. •ewmd case. ° '^^^' ^ ^'g- I'*) ^^^ the wheel, the center of gravity of which is at a, be suspended from the pomt b, by a thread, or hung upon an axle, having freedom of motion on that point. However much wa may move it, either right or left, toward m or n, as shown by the dotted lines, am and an, it swings back again, and is only at rest when b and a are in the same perpendicular line. In the third case, where the point of support has the cen- third caTC. * '■°'' °^ gravity above it, a body will remain at rest only so long as the center of gravity is in a vertical line, above the point of stipport. In Fig. 14, suppose the wheel to be supported at the point c, sit- uated iu a vertical line a c, immediately below the center of gravity, a; bo 48 WELLS'S NATURAL PHILOSOPHY. What is Indif- ferent Equili- brium ? "VATiat is Stable Equilibrium ? long as this position is maintained, the wheel will remain at rest, but the mo- meat the center of gravity, a, is moved a little to the right or left, so as to throw it out of the vertical line joining a and c, the wheel will turn over, and assume such a position as to bring the center of gravity immediately beneath the point of support, as in the second case. Upon what 92. The stability of a body, therefore, de- puty oflbod^^ pends upon the manner in which it is sup- djpend? ported, or in other words, upon the positica of its center of gravity. What are the 93, As a body may be supported in three timsVE^quUi- positions, we have, as a consequence, three brium? conditions of equihbrium, viz., Indifferent, Stable, and Unstable Equilibrium. Indifferent Equilibrium occurs when a body is supported upon its center of gravity ; for then it remains at rest indiffer- ently in every position. Stable Equilibrium occurs when the point of support is above the center of gravity. If a bod}' be moved from this position, it swings backward and forward for a time, and finally returns to its original situation. . „ Unstable Equilibrium occurs when the point of support 13 What 13 Un- . ,<>,/. Bt:ible Equili- beneath the center of gravity. The tendency ot the center of bnum ? gravity in such cases is to change, and take the lowest situation the support of the body will allow. 94. The principle that when a body is suspended freely, it will have its center of gravity in a vertical line, immediately below the point of support, has been taken advantage of to determine experimentally the position of the center of gravity, in irregular shaped bodies. Sujipose we suspend, as in Fig. 15, an irregular piece of board by means of cord. A plumb-line let fall from the point of support, or the prolongation of the cord, wUl pass through the center of gravity, G. If we now attach the cord to another point, and suspend the body anew, tha prolongation of the cord in this instance, also, will pas3 through the center of gravity, G. The intersection of these two lines will be the center of gravity, and tha board, if suspended by a cord attached to this point, will hang evenly balanced. 95. A line which connects the center of gravity of a body with the center of the earth, or, in other words, a line drawn from the center of gravity perpendicularly downward, is called the LixE of Direction. It is called the Line of Direction, How may we determine the center of grav- ity ill irrejjular bodies ? Fig. 15. CENTER OF GRAVITY, 49 What is the Line of Direc- tion? because ■svben a solid body falls, its center of gravity moves along tbis line until it reacbes tbe ground. Wben bodies are supported upon a basis, their stability depends on the position of their Line of Direction. 96. If the line of direction falls within the base upon which the body stands, the body remains supported ; but if it falls without the base, the body overturns. When will a body stand, »nd when will It faU ? Fig. 16. Fig. 17. Thus, in Fig. 16, the line directed Terticallv from the center of gravity, G, falls within the base of the body, and it remains standing; but in Fig. 17 a similar line falls Ts-ithout the base, and the body, consequently, can not bo maintained in an upright position, and must faU. A wall, or tower stands securely, so long as the perpendicular line drawn through its center of gravity falls within its base. The celebrated leaning-tower of Pisa, 315 feet high, wliich inclines 12 feet from a per- fectly upright jwsition, is an example of this principle. For instance, the line in Fig. 18, falling from the top of the tower to the ground, and passing through the center of gravity, ' falls within the base, and the tower stands securely. If) however, an attempt had been made to build tho tower a little higher, so that the per- pendicular hne passing through the center of gravity, would have fallen beyond the base, the structure cc -dd no longer have supported itselfl 97. The broader, or larger 60 WELLS'S NATURAL PHILOSOPHY. the "base of a body, and the nearer its principal mass is to the base, or, in other words, the lower its cen- ter of gravity is, the firmer it will stand. A pyramid, for this reason, is the firmest of all structures. The base upon which the human body rests, or is supported, is the two feet and the space included between them. The advantage of turning out the toes when we walk is, that it increases the breadth of the base supportmg the body, and enables us to stand more securely. In every movement of the body, a man adjusts his position unconsciously, in such a way as to support the center of gravity, and cause the line of di- rection to fall within the base. Why does a ^ person carrying a load upon his back, bends forward in fng'a load up- order to bring the center of gravity and his load over hia on his back feet l>end over ? When will body stand most firmly ? What is the advantage of turning out the toes in walk- ing? Fig. 19. Fig. 20. Why does a 'person lean for- ward in ascend- ing a hill, and backward in descending ? Why is a high carriage more liable to over- turn than a low one? Fig. 21. If he carried the load in the position of A, Fig. 19, he would be liable to fall backward, as the direction of the center of gravity would fall beyond his heels ; to bring the center of gravity over his feet, he assumes the position indicated by B, Fig. 20. For the same reason, when a man ascends a hill he leans for- ward, and when he descends he leans backward. See Fig. 21. A high carriage is much more liable to be overset by an irregu- larity in the road than a low one ; because the center of gra\aty being high, the line of direction is easily thrown without the base. This will appear evident from the following illustration, Fig. 22. CENTER OF GRAVITY. 51 Fig. 23. ' Let A represent a coach standing on a level ; B, a cart loaded with stones on a slope ; C, a wagon loaded with hay on a slope ; a a a the centers of gravity ; a b, line of direction ; c d, base. Here it is obvious that the hay-wagon must upset, because the line of di- rection falls without the base ; that the coach is very secure, because the line of direction falls far within the base ; and the stone-cart, though the center of gravity is low down, is not very secure, because the line of direction falls very near the outside of the base. The effect on the stability of a body occa- sioned by placing its center of gravity in a very low position, is shown in an amusing toy for children, represented by Fig. 23. The horse, with his rider, is firmly supported on his hind feet, because, by means of a leaden ball attached to the bent wire, the center of gravity is brought below the point of support. Ty, ... If a body be placed on an in- body slide and clined Surface, it will slide down when its line of direction falls within the base : but it will roll when roll down a slope i Fig. 24. down when it falls with- out the base. Thus the body, e, Fig. 24, having its line of direction e a, with- in the base, will slide down the inclined surface, c d; but the body h a, will roll down, since its line of di- rection, b a, falls without the base. PRACTICAL QUESTIONS ON THE CENTER OF GRAVITY. 1. Why does a person in rising from a chair bend forward ? "When a person is sitting, the center of gravity is supported by the seat; In an erect position, the center of gra\ity is supported by the feet; therefore, before rising it is necessary to change the center of gravity, and, by bending forward, we transfer it from the chair to a point over the feet. 52 WELLS'S NATURAL PHILOSOPHY. 2. Why is a turtle placed on its back unable to move ? Because the center of gravit\- of the turtle is, m this position, at the lowest point, and the animal is unable to change it ; therefore it is obliged to remain at rest. 3. Why do very fat people throw back their head and shoulders when they walk ? In order that they may eflectually keep the center of gravity of the body over the base formed by the soles of the feet. 4 Why can not a man, standing with his heels close to a perpendicular wall, bend over sufficiently to pick up any object that lies before him on the ground, without falling? Because the wall prevents him from throwing part of his body backward, to counterbalance the head and arms that must project forward. 5 What is the reason that persons walking arm-in-arra shake and jostle each other, unless they make the movements of their feet to correspond, as soldiers do in marching? "When we walk at a moderate rate, the center of gravity comes alternately over the right and over the left foot. The bodj' advances, therefore, in a wav- ing line; and unless two persons walking together keep step, the waving mo- tion of the two fails to coincide. 6 In what does the art of balancing or walking upon a rope consist ? In keeping the center of gravity in a' line over the base upon which tho body rests. 7. Why is it a very difficult thing for children to learn to walk ? In consequence of the natural upright position or the human body, it is constantly necessary to employ some exertion to keep our balance, or to pre- vent ourselves from falling, when we place one foot before the other. Chil- dren, after they acquire strength to stand, are obliged to acquire this knowl- edge of preserving the balance by experience. AVhen the art is once ac- quired, the necessary actions are performed involuntarily. 8. Why do young quadrupeds learn to walk much sooner than children ? Because a body is tottering in proportion to its great altitude and narrow "base. A child has a body thus constituted, and learns to walk but slowly be- cause of this difficulty (perhaps in ten or twelve months), while the young of quadrupeds, having a broad supporting base, are able to stand and move about almost immediately 9. Are all the limbs of a tall tree arranged in such a manner, that the line directed from the center of gravity is caused to fall within the base of the tree ? Nature causes the various limbs to shoot out and grow from the sides with as much exactness, in respect of keeping the center of gravity withia the base, as though they had been all arranged artificially. Each limb grows, in respect to all the others, in such a maimer as to preserve a due balance be- tween the whole. LA"WS OF TALLIN; G BODIES. 53 SECTION IV. EFFECTS OF GRAVITY AS DISPLAYED BY FALLIN'O BODIEa What is a Ver- tical Line ? Wliat is a Plumb Line? Fig. 25. 98. Wiien an unsupported body falls, its motion will be in a straight line toward the center of the earth. This line is called a Vertical Line. 99. If a body be suspended by a thread, the thread will always assume a vertical direction, or it will represent that path in which the body would have fallen. A weight thus suspended by a thread, is called a Plumb-Line,* Fig. 25, and is used by carpenters, masons, etc., to ascertain by comparison, whether their work stands in a vertical or perpendicular position. What is a 100. A plumb-line is always Level Surface? perpcudicular to the surface of water at rest. The position of such a sur- face we call Level. No two plumb-lines upon the earth's surface will be parallel, but will incline toward each other, since no two bodies from different points can approach the center of a sphere in a parallel direction. If their distance apart be one mile, this inclination will amount to one minute, and if it be sixty miles, to one degree. In Fig. 26, let E E be a portion of the earth's surface, and D its center; the bodies A, B, and C, when allowed to drop, will fall in the direction A D, B D, and C D. 101. As the attraction of e- the earth acts equally and independently on all the particles composing a body, it is clear that they must all fall with equal velocities. It makes no difference whether the sev- eral particles fall singly, or whether they fall compacted together, in the form of a large or a small body. • Plumb Line, so called from the Latin word plumbum, lead, the weight usually at- tached to the string. aA Will all bodies, under the in- fliienccofgi'av- ity alone, fall with equal ve- locities ? 54 WELLS'S NATURAL PHILOSOPHY. If ten or a hundred leaden balls be disengaged together, they -will fiHl in the same time, and if they be molded into one ball of great magnitude, it will still fall in the same manner. 102. Hence all bodies under the influence of gravity alone, must fall with equal velocities.'"' „ , . There are some familiar facts which seem FiQ. 27. By what ex- , . , periment can to be Opposed to this law. "When we let go ^^ oil prove this law? a feather and a mass of lead, the one floats in the air, and the other falls to the ground very rapidly. But in this case, the operation of gravity is modified by the resistance of the air ; the feather floats because the air opposes its descent, and it can not overcome the resistance offered. But if we place a mass of lead and a feather in a vessel exhausted of air, and Uberate them at the same time, they will fall in equal periods. The experiment is easily shown by taking a glass tube. Fig. 27, closed at one end, and Bupphed with an air-tight cap and screw-cock at the other. A feather and a piece of metal are previously inclosed in the tube. The tube being filled with air, and inverted, the metal will fall with greater speed than the feather, as might be ex- pected. If the tube be now exhausted of air by means of an air-pump and the screw-cock, and in this condition in- verted, the feather and the metal will faU from end to end of the tube with equal velocity. 103. If a man leap from a chair or table, he will strike the ground without injury. If the same man leap from the top of a high house, he will probably be killed. These, and many like instances, prove that the force with which a falling body strikes the ground depends upon the height from which it falls. But the force depends on the velocity of the body the moment it touches the ground ; therefore, the velocity \nth which a body falls depends also upon the height from which it descends. Upon what do the force and velocities of falling bodies depend 1 • Previous to the time of Galileo, the philosophers maintained that the velocity of i falling body was in proportion to its WL'ight, and that if two bodies of unequal weights, were let fall from an elevation, at the same moment, the heavier would reach the ground as much sooner than the lighter, as its weight exceeded it. In other words, a body weigh- ing two pounds would fall in half the time that would be required by a body weighing ono pound. G.ilileo, on tlie contrary, asserted that the velocity of a falling body is iudependaut of its weight, and not affected by it. The dispute running high, and the opinion of the public being generally averse to the views of Galileo, he challenged his opponents to test the matter by a public experiment. The challenge was accepted, and the celebrated leaning- tower of Pisa agreed upon as the placi of trial. In the presence of a large concourse, two balls were selected, one having exactly twice the weight of the other. The two were then dropped from the summit of the tower at the same moment, and, in exact accordance with the asaertions of Galileo, they both struck the ground at the same instant. LAWS OF FALLING BODIES. 55 Aov doea 104 -'WhGn a body falls, it is attracted by gravity during f^ijH^body" ^^^ '^^^^^ *""® °^ i*3 falling. Gravity does not merely set the body in motion and then cease, but it continues to act. During the first second 'of time, the force of gravity will cause the body to descend through a certain space. At the end of this time, the body would continue to move, with the motion it has acquired, without the action of any further force, merely on account of its inertia. But gravity continues to act, and will add as much more motion to the falling body during the second second of time, as it did during the first second, and as much again during the third second, and so on. I _, , . ,^ 105. Fallinoj bodies, therefore, descend to what IS the . . jaw of fauing tliG cartli witli ft uniform accelerated motion. bodies ? • n A body falling from a height will fall 16 feet in the first second of time/^ three times that distance in the second, five times in the third, seven in the fourth, the spaces passed over in each second increasing as the odd numbers 1, 3, 5, 7, 9, 11, etc. How does the 106. The entire space passed over by a body over aifd ^tht i° ftiUing is as the square of the time ; that is, Ji»\^^^^!fi'' in twice the time it w^ill fall through four times ing body com- o p*'^^ the space ; in thrice the time, nine times the space. f The time occupied in falling, therefore, being known, the height from which a body falls may be calculated by the following rule : 107. Multiply the square of the number of Time being / •' •»■ . ^ , . giTun, how- can sccouds of time consumed in falhng, by the tlieheightfrom •n /> ii • i x> which a body distancB wIl zh. a body will fall in one second oi falls be found 1 . •' time. Thus, a stone is five seconds in falling from the tep of a precipice ; the square of five seconds is 25; this multipUed by 16, the number of feet a body will MI in one second, gives 400 — the height of the precipice. How do the 108. As the effect of gravity is to produce a Hm^esoffaiung uuiform accelcratcd motion, the velocity of % iomparer falling body will increase as the time increases. • The spaces described by falling bodies are here given in round numbers, the fractions being omitted. The space described by a falling body during the first second is 16 1-lOth feet. t The resistance of the air essentially modifies the laws of the motions of falling bodies, as here stated, and with a certain velocity, will become equal to the weight of the falling body. After this takes place, the body will descend with a uniform velocity. There Is, therefore, a limit to the velocity which a body can acquire by falling through the «tmosphrve. 56 WELLS'S NATURAL PniLOSOPnT. Thus, at the end of two seconds, the velocity acquired by a falling body will be twice as great as at the end of one second, thrice as great at the end of the third second, and so on. How are bodies ^^^' Bodies projected directly upward, will ^TrSuenTd ^^ influenced by gravitation in their ascent, as by gravity? y^,g|j g^g ^^ their dcscent, but in a reversed order ; producing continually retarded motion while they are rising, and continually increasing motion during their fell. Thus, a body projected up perpendicularly into the air, if not influenced by the resistance of tho air, would rise to a height exactly equal to that from which it must have fallen to acquire a final velocity the same as it had at the first instant of its ascent. 110. To determine the height to which a body projected upward will rise, with a given velocity, ascertain the height from wliich a body would fall to acquire the same velocity. The answer in one case will be the answer in How can we determine the height which a body projected upward with a given velocity will ascead ? the other. How do the times of ascent and descent compare i 111, The time, also, which the ascending body would require to attain its greatest height, would be just equal to the time it would require to fall to the ground from that height. The following table exhibits an analysis of the motions of a falling body; the spaces passed over in each interval of time of falling, increasing as tho odd numbers 1, 3, 5, 7, 9, etc. ; the velocities acquired at the end of each in- terval increasing directly as tho times ; and uhe whole space passed over being as tho squares of tho times. Number of Seconds in the Fntl, counted from B Stftle of Rest. • Spaces fallen throuffh in each Buccessive Second. Veloci:Ies ftciiuired at the End of Number of Seconds expressed in First Column. T..lal Hei-ht fallen through from Rest in the Number of Seconds expressed in First Column. 1 1 2 1 2 3 4 4 3 5 6 9 A 7 8 16 5 9 10 25 6 11 12 36 7 13 U 49 8 15 16 64 9 17 18 81 10 19 20 100 Where extreme accuracy is not required, most of the problems connected with the descent of falling bodies, may be worked with great readiness — 16 LAWS OF FALLING BODIES, 57 feet, the spe • ' ^ . \ the velocity of diculai'lv, be made to roll down an inclined a bodv falling « , p . , down an in- plane, free from friction, the velocity acquired cUaed plane? , . . p- i -Hi i at the termmation oi its descent, ■vsill be equal to that it would acquire in falling through the perpen- dicular height of the inclined plane. Fig. 28. Thus, the velocity acquired in rolling down the whole length of A B, Fig. 28, is equal to that it would acquire by lalling down the perpendicular height A C. 113. The great Itahan philosopher Gahleo, during the .B ^^^^y P^^' of the ITth century, had his attention directed, while in a church at Florence, to the swmging of the chandeliers suspended from the lofty ceihng. He noticed tnat when they How, and by ^^'^^® moved from theu- natural position t»y any disturbing whom was the cause, they swung backward and forward m a curve, for a covered" ^^°S time, and with great uniformity, nsmg and faUing alter- nately in opposite directions. His inquiry mto the cause of thes3 motions led to the invention of the pendulum, the tneory of which may be explamed as follows : Explain the ^^^- ^^^ bodies will have their motion as much accelerated theory of the whilst descending a curve, as retarded whilst ascendme. Let pendulum. C A B be a curve, Fig. 29. If a ball be placed at C, the attraction of gravitation will cause it to descend to A, and in so doing it will acquire velocity sufficient to carry it to B, all opposing obstacles being removed, such as friction and resistance of the air. Gravitation 3* Fig. 29. 58 WELLS'S NATURAL PHILOSOPHY. Fig. 30. A How do the times of the vi- brations of a pcTiduliim com- pare with each other ? Explain the reason of tliis law. will once more bring it down to A ; it will then rise again to C, and so con- tinue to oscillate backward and forward. If we now suspend the ball by a string, or wire, in such a manner that it will swing freely, its motions will be the same as that of the ball rolling upon the curve. A body thus suspended is called a Pendulum. In Fig. 30, D C, the part of the circle through which the pendulum moves, is called its arc, and the whole movement of the ball from D to C is called an oscillation. 115. The times of the vibrations of a pen- dulum, are very nearly equal, whether it moves much or little ; or, in other words, through a greater, or less part of its arc. The reason that a large vibration is performed in the same time as a small one, or, in other words, the reason the pendu- lum alwaj'S moves faster in proportion as its journey is longer, is, that in proportion as the arc described is more extended, the steeper are the declivities through which it falls, and the more its motion is accelerated. Thus, if a pendulum, Fig. 30, begins its motion .at D, the accelerating force is twice as great as when it is set free at b ; and if we take two pendulums of equal lengths, and liberate one at D and another at b at the same time, they will arrive at the same moment at E. 116. This remarkable property of the pendulu^ enables us to employ it as a register, or keeper of time. A pendulum of invariable length, and in the same location, will always make the same number of oscillations in the same time. Thus, if we arrange it so that it will oscillate once in a second, sixty of these oscillations will mark the lapse of a minute, and 3,600 an hour. A common clock is, therefore, merely an arrangement for molfclockT™" registering the number of oscillations which a pendulum makes, and at tlie same time of communicating to the pendu- lum, by means of a weight, an amount of motion sufficient to make up for what it is continually losing by friction on its points of support, and by the resistance of the air. The wheels of the clock turn round by the action of the weight, but they are so connected with the pendulum, that with every double oscillation a tooth of the last wheel is allowed to pass. If, now, tin's wheel has thirty teeth, as is common in clocks, it will turn round once for every sixty vibrations. And, if the axis of this wheel project tlirough the dial-plate or face of a clock, with a hand fastened on it, tliis hand will be the second hand of the clock. The other wheels arc so connected with the lirst, and the number of teeth so pro- HoTT- does this property of the pendulum en- able us to reg- ister time? n n 1 . ^ A \ F=t -B ^ ^ 1. ^ «.i=i_ |-. ^ "^1 LAWS OF FALLING BODIES. 59 portioned, that the second one turns sixty times slower than the first, and this will be the minute hand ; a third wheel moving twelve times slower than the last will constitute the hour hand. „ J A watch ditl'ers from a clock in havina; a vibrating whed in- How does * , ~ o J watch differ stead of a vibrating pendulum. This wheel, called the baHance- fromaclock? wheel, is moved by a .^^rin.^ which is always forcing it to a middle position of rest, but does not fix it there, because the velocity ac- YlQ.^ 31, quired during its approach from either sidt- to the middle position, carries it just as far past on th* other side, and the spring has to begin its work again. The balr ance-ioheel at each vibration allows one tooth of the adjoining wheel to pass, as the pendulum does in a clock, and the record of the beats is pre- served by the wheels wliich follow, as alreadj^ explained for the clock. Fig. 31 represents the arrangement used to keep up the motion in a watch. The barrel, or wheel A, incloses a spring, which, when compressed by wind- ing up, tends to liberate itself^ or unwind, in virtue of its elasticity. This effort to unwind, turns the barrel upon its axis, and thus, by means of a chain coiled round it, motion is communicated to the other wheels of the watch. . „ 117. The leno'th of a pendulum influences •What influ- . „ . ^., . •'^ . - ence has the the time 01 its vibi'ation : the Ioniser the pen- lengthof apen- , .... ■'• duium on its clulum tho slowei' are its vibrations. tiou? The reason why long pendulums vibrate more slowly than short ones is, that in corresponding arcs, or paths, the ball of the long pendulum has a greater journey to perform, without having a steeper Une of descent. ■What is th ^^^' ^^'^^ *^^® ^ pendulum rod, Fig. 32, A D, having balls centpf of oscii- upon it at C and D, and cause it to vibrate, the ball, B, being ktion in a pen- nearer to the point of suspension, will tend to perform its oscillations more quickly than the ball C. In like manner, every other point on the pendulum rod tends to complete its oscillations in a different time ; but as they are all connected together inflexibly, all are compelled to perform their oscillations in the same time. But the action of the portions of the rod near to the ball, B, is to accelerate the motion of the pendulum, and the action of the fi q portions of the rod near to the ball C, is to retard it ; therefore a point mny he found where all these counteractions will balance one an- other, or be neutralized, and this point is termed the Centeu of Os- cillation, and the sum of the momenta of all the portions of the g A rod on each side of this point will balance. The center of oscillation docs not correspond ■vsnth the center of gravitv, but is always a little below it ; the practical method of bringing them near together, is to make the rod hght, and the termination of the pendulum heavy. ^ Aq GO "WELLS'S NATURAL rniLOSOPHT. Why do clocks go faster in winter tiian in Eumnier ? How fire the changes in the length of pen- dulums coun- teracted ? Fig. 33. .\ G 119. As heat expands, and cold contracts all metals, a pendulum rod is longer in -warm than in cold "sveathcr ; hence, clocks gain time in winter, and lose in the summer. As the smallest change in the length of a pendulum alters the rate of a clock, it is highly important, for the maintaining of uniform time, that the expansion and contraction of pendu- lums, caused bj changes in temperature, should bo counteracted. For this purpose various contriv- ances have been employed. The one most commonly em- ployed at the present time is the mercurial pendulum, -which is constructed 03 foUo\^-s : Tlio pendulima rod, A B, Fig. 33, supports a glass jar, G H, containing mercury, inclosed in a eteel frame-work, F C D E. "U'hen the vreather is warm, the Fig 34. steel rod and frame-work expand, and thus in- crease the length of the pendulum, and de- press the center of oscillation. But, at the same time, the mercury contained in the jar also expands, and rises upwaid; and thus, by a proper adjustment, the center of oscillation is carried as far upward in one direction, as do-mi- ward in the opposite direction, or the expansion in both directions is equal, and the vftrations of the pendulum remain uualterLd. Another form of pendu« lum, called the "gridiron pendulum," Fig. 34, is composed of rods of different metals, which expand unequally under the same changes of temperature, and, by counteraction, keep the length of the pendultmi constant. 120. As the force of gravity determines how long the pendulum shall he in falling down its arc, and the time also of its rising in the op- posite direction (since the ball of the pendu- Vim, as already stated, may be considered as a body de- scending by its weight on a slope), it follows, that the time of vibration of a pendulum will var}' as the attraction of gravity varies. The same pendulum will vibrate more slowly at the equa- tor than at the poles, because the attraction of gravitation is less powerful at the equator. Tlierefore a pendulum to vi- brate once in a second, must be shorter at the equator than at the poles. Corresponding results take place wlien a pen- dulum is carried to a moimtain-top, away from the center of the earth, wMcIi How do the variations in the force of gravity aff>ict the vibrntions of a pendulum? Where will a pendulum of a given lencth vibrate slow- est, and where the fastest ? LAWS OF FALLING BODIES. 61 is tho center of attraction, or when carried to the bottom of a mine, -where it is attracted both bj matter above it and below it. •What is the ^-1- The length of a pendulum that wi 1 o'irdf^°pcndu.' describe sixty oscillations in a minute, each '"™' oscillation having the duration of a second, is, in the latitude of Greenwich, England, 39.1393 inches in length ; one to vibrate in half seconds must measure 1.7848, or rather more than 9^ inches. At the pole it would require to be somewhat longer ; at the equator somo- what shorter. A pendulum that ■s^ibratcd seconds at Paris, was found to re- quire lengthening .09 of an inch iu order to perform its vibrations in the same time at Spitzbergen. How may the 122. The Icugth of B pcndulum vibrating In'lfs'^^pe^du- seconds being always invariable at the same L"?ti'ndard^ of P^^ce, siuco thc attraction under the same measure? circumstances is always the same, it may be used as a standard of measure. This application has already been described underthe section "Weight (§ 67). The duration of the oscillation of a pendulum is not aflected by altering tho ■weight of the baU, since all bodies moving over the same space, under tho influence of gravitation, acquire equal velocities. How do the 1-3. The lengths of diiferent pendulums, dThmis vibi^a't- vibrating in unequal times, are to each other tiSiscl^mpar"'? ^^ *^^ squarcs of the times of their vibration. Thus a pendulum, to vibrate once in two seconds, must have four times the length of one that vibrates once in one second ; to vibrato once in three seconds, it must have nine tunes the length, etc. — the duration of the oscillation bemg as the whole numbers, 1, 2, 3, 4, 5, 6, 7, 8, 9. The length of the pendulum will be as their squares. 1, 4, 9, 16, 25, 36, 49. 64, 81. . A pendulum, therefore, that will vibrate once in nine seconds, must have t length of 81 times greater than one vibrating once in one second. •» \ PRACTICAL PROBLEMS ON THE THEORY OF FALLING BODIES. 1. A stone let fall from the top of a tower struck the earth in two seconds ; how high ■was the tower ? 2. How far will a body acted upon by gravity alone, fall in ten seconds? 3. How deep is a well, into which a stone bcin;? dropped, reaches the surface of the water in two seconds, the depth of the water in the well being ten feet ? 62 WELLS'S NATURAL PHILOSOPHY. 4. If a body be projected do-irnward with a velocity of twenty-two feet in the first sec- ond of time, liow far will it fjiU in eight Beconds ? The multiple in this case will be the distance fallen through in the first second. 5. What space will a body pass through in the fourth second of its time of falling T 6. A body falls to the ground in eight seconds ; how largo a space did it pass over dur- ing the last second of its descent? 7. A body falls from a height in eight seconds ; with what velocity did it strike the ground ? 8. A cannon-ball fired upward, continued to rise for nine seconds ; what was its velocity during the first second, or with what force was it projected? 9 Suppose a bullet fired upward from a gun returned to the earth in sixteen seconds; how high did it ascend ? The time occupied in ascending and descending being equal, the body rose to such a heii^ht that it required eight seconds to descend from it. The square of 8=64. This multiplied by the space it would fall in the first second, IC feet = 924 feet. 10. A bird was shot while flying in the air, and fell to the ground in three seconds. How high up was the bird when it was shot ? 11. What must be the length of a pendulum to vibrate once in seven seconds ? 12. If the length of a pendulum to vibrate seconds at Washington is 39.101 inches, hovr long must it be to vibrate half seconds? IIow long to vibrate quarter seconds? CHAPTER V. MOTION. What is Mo- 124. Motion is the act of changing place. t'on ? If no motion existed, the universe would be dead. There would be no alternation of the seasons, and of day and night ; no flow of water, or change of air ; no sound, light, heat, or animal existence. 125. Motion is Absolute or Kelative. solute and Rei"- ABSOLUTE MoTiON is a changc of position in ative Motion ? • i i • 1 1 / /• , space, considered without relerence to any other body. Relative Motion is motion considered in relation to some other body, which is either in motion or at rest. Thus the motions of the planets in space are examples of Absolute Motion, but tlic motion of a man sitting upon the deck of a vessel, while sailing, 13 an example of Eelative Motion, since he is in motion as respects the land, but at rest as regards the parts of the vessel. Rest, which is the opposite cf motion, so far as we know, exists only relatively. We say a body on the surface of the earth is at rest, when it maintains a constant position as re- gards some other body ; but at the same time that it is thus at rest, it partakes MOTION. b' / of the motion of the earth, which is always revolving. "We do not, therefore, really know any body to be in a state of absolute rest. Define uni- 126. A luoving body may have a Uniform rl^ble Mouir ^T a VARIABLE MoTION. UNIFORM MoTION is the motion of a body moving over equal spaces in equal times. Variable Motion is the mo- tion of a body moving over unequal spaces in equal times. What is Ac- 127. When the spaces passed over in equal Regarded Ho- ti^es iucrease, the body is said to possess Ac- *'°°' CELERATED MoTiON ; wheu they diminish, the body is said to possess Retarded Motion. A stone Hilling through the air is an example of Accelerated Motion, since, acted upon by the force of gravity, its rate of motion constantly increases ; while the ascent of a stone projected from the hand, is an example of Re- tarded Motion, its upward motion continually decreasing. What is Power 128. When a body commences to move from MM? ^''"^'' a state of rest, we assign some force as the cause of its motion ; and a force acting in such a manner as to produce motion, is generally termed " Power." On the contrary, a force acting in such a way as to retard a moving body, destroy its motion, or drive it in a contrary direction, is termed Resistance. The chief forces which tend to retard or destroy the motion of a body are Gravitation, Friction, and Resistance of the Air. What is ve- 129- The speed, or rate, at which a body locity? moves, is termed its Velocity. Moving bodies pass over their paths with different degrees of speed ; one may pass through ten feet in a second of time, and another through a hun- dred feet in the same period. We say, therefore, that they have diflerent velocities. The velocity of a moving body is estimated by the time it occupies ia moving over a given space, or by the space passed over in a given time. Tho less the time and the greater the space moved over in that time, the greater the velocity. Hov do we 130. To ascertain the Velocity of a mov- veTocfty of^a ^^S ^ody, dividc the space passed over by the moving body? ^^jj^g cousumed iu moving over it. 64 WELLS'S NATUr.AL PHILOSOPHY. Thus, if a body moves 10 miles in 2 hours, its velocity is found by di- viding the space, 10, by the time, 2 ; the ansvs'er, 5, gives the velocity per hour. How can we l^l. To RscGrtain the Space passed over by space''"paFsed ^ moving bocly, multiply the velocity by the over by a body f i tyi p in motion? time. Thus, if the velocity be 10 miles per hour, and the time 15 hours, the space will be 10 multiplied by 15, or 150 miles. How is the 13^- To ascertain the Time employed by a by'l bmly 'la l>ody in motion, divide the space passed over "ainlT? ""'"' ^y the velocity. Thus, if the space passed over be 150 miles, and the ve- locity 10 miles per hour, the whole time empjloyed will be 150 divided by 10 — 15 hours. ■What is Mo- 133. The Momentum of a body is its quan- mentumf ^j^y ^f motioH. Momcntum expresses the force with, which one body in motion would strike against another. That a mass of matter moving in any manner exerts a cer- IlluBtrations of ^^j^j f^j.^^ against any object with which it may come in eon- tact, is a principle of Natural Philosophy which experience teaches us most frequently and most readily. The child has hardly emerged from the nurse's arms, before it becomes conscious of the force with which it would strike the ground if it fell. We take advantage of momentum, or the force of a moving body, in almost all mechanical operations. The mov- ing mass of a hammer-head drives or forces in the nail, shapes the iron, breaks the stone ; the force of a moving mass of water gives strength to a torrent, and turns the wheel ; the force of a moving mass of air gives strength to the wind, carries the ship over the ocean, forces round the arms of a wind-mill. Is motion im- 134. Whcu a body is caused to move, the t'hr'V'tici''cs motion is not imparted simultaneously to of a body at gyerv particlc of the body, but at first only to the same in- J l -J ' J Btant? the particles which are directly exposed to the influence of the force — for instance, of a blow. From these particles, it spreads to the rest. A slight blow is sufficient to smash a whole pane of glass, How can you x a 7 illustrate this while a bullet from a gun will only make a small round hole ''"^'^ in it, because, in the latter case, the particles of glass that re- ceive the blow are torn away from the remainder with such rapidity, that the motion imparted to them has no time to spread further. A door standing open, which would readily yield on its hinges to a gentle push, is not moved by a caauon-ball passing through it. The ball, in passing through, overcomes the MOTION. 65 whole force of cohesion among the atoms of wood, but its force acts for so short a time, owing to its rapid passage, that it is not sufficient to affect tlio inertia of the door to an extent to produce motion. The cohesion of the part of tlie wood cut out by the ball would have borne a very great weight laid quietly upon it; but supposing the ball to fly at the rate of 1200 feet in a second, and the door to be one inch thick, the cohesion being allowed to act for only the minute fraction of a second, its influence is not perceived. It is an effect of this same principle, that the, iron head of a hammer may bo driven down on its wooden handle, by striking the opposite end of tho handle against any hard substance with force and speed. In this very simple operation, the motion is propagated so suddenly through the wood of the han- dle, that it is over before it can reach the iron head, which therefore, by its own inertia, sinks lower on the handle at every blow, which drives the han- dle up. How 13 the Mo- 135. The Momentum, or force, which a mov- "ody^^c^ca- ing body exerts, is estimated by multiplying lated? j|-g mass or quantity of matter by its velocity. Thus, a body weighing 10 pounds, and moving with a velocity of 500 feet in a second, will have a momentum of (10X500) 5,000. What nn . ^^^' ^^^^ vclocity being the same, the mo- betwein ^''T ^^Qeutum, Or moviug force of a body, will be Momentum of dircctly proportionate to the mass, or weight ; weight and ve- and the mass or weight remainino: the same, locityf Ml 1 T 1 . the momentum will be directly proportionate to the velocity. Thus, if 2 leaden balls, each of 5 pounds' weight, move with a velocity of 6 miles per minute, the momentum, or striking force of each, wiU be 25 ; if now the two balls, molded into one of 10 pounds' weight, move with the same velocity of 5 miles per minute, the momentum, or striking force, will be 50, since with the same velocity the mass, or weight, will be doubled. I^ on the contrary, we double the velocity, allowing the weight to remain the same, f .e same effect will be produced ; a ball of 5 pounds, with a velocity of 5, will have a momentum, or striking force, of 25 ; but a ball of 5, with a velocity of 10, will have a momentum of 50. How can a 1^7. A Small, or hght body, may be made ^tk.n^be'^raide ^^ striko with a greater force than a heavier ^mXrcelsl ^'^dy, by giving to the small body a sufficient large one ? VClocity. Illustrations of these principles are most familiar. Hail-stonea, of small mass and great velocity, strike with sufficient force to break glass, and de- stroy standing grain ; a ship of huge mass, moving with a scarcely percept- ible velocity, crushes in the side of the pier with which it comes in contact. G6 WELLS'S NATURAL PHILOSOPHY. SECTION I. ACTION AND REACTION. ^ . 138. When a body communicates motion Wliat IS meant , •' i /> • by Action and to anothei body, it loses as much of its own Reaction ? •' ' . . momentum, or force, as it gives to the other body. We apply the term Action to designate the power which a body in motion has to impart motion, or force, to another body ; and the term Reaction to express the power which the body acted upon has of depriving the acting body of its force, or motion. What is the 139. There is no motion, or action, in the Ac'tionindRe- univeisc without a corresponding and oppo- action ? gj^g action of equal amount ; or, in other words, Action and Reaction are always equal and opposed to each other. ^y^ . - If a person presses the table with his finger, he feels a re- lustrations of sistance arising from the reaction of the table, and this coun- actio'n?*"'^ ^°" ter-pressure is equal and contrary to the downward pressure. "When a cannon or gun is fired, the explosion of the powder which gives a forward motion to the ball, gives at the same time a backward motion, or "recoil," to the gun. A man in rowing a boat, drives the water astern with the same force that he impels tlie boat forward. Towh.itisthe I'^O- The quantity of motion in a body is Sin ^ in *^ a mcasurcd by the velocity and the quantity of uonatef"^"'' matter it contains. A cannon-ball of a thousand ounces, moving one foot per second, has the same quantity of motion in it as a musket-ball of one ounce, leaving the gun with a velocity of a thousand feet per second. The momen- tum, or quantity of motion, in the musket-ball being, however, concentrated in a very small mass, the effect it wiU produce will be apparently much greater than that of the cannon-ball, whose motion is diffused through a very large mass. This explanation will enable us to understand some phenomena which at first appear to contradict the law, that action and reaction are always equal, and opposed to each other. Thus, when we fire a bullet fi-om a gun, the gun recoils back with as much force as the bullet possesses, proceeding in an opposite direction. The reason the effects of the gun are not equally apparent \\-ith those of the ball, is that the motion of the gun is diffused through a great mass of matter, with a ^maU velocity, and is, therefore, easily checked ; but in the ball the motion ACTION AND REACTION, 67 is concentrated in a very small compass, with a great velocity. A gun recoila more with a charge of fine shot, or sand, than with a bullet. The explanation of this is, that with a ball the velocity is communicated to the whole mass at ance, but with small shot, or sand, the velocity communicated by the ex- plosion to those particles of the substance immediately in contact with the powder, is greater than that received at the same instaiit by the outer particles ; con- sequently, a larger proportion of explosive force acta momentarily in an oppo- site direction. FiQ. 35. We have an illustration of this same principle, when we attempt to drive a nail into a board having no support behind it, or not sufficiently thick to oCFer the necessary resistance to the moving force of the hammer, as is repre- sented in Fig. 35. The blows of the hammer will cause the board to unduly yield, and if strong enough, will break it, but will not drive in the nail. The object i3 attained by applying behind the board, as in Fig. 3G, a block of wood, 68 WELLS'S NATURAL PHILOSOrHY. or metal, against which the blows of the hammer will be directed- liy adopting this plan, however, no increased resistance is opposed to the blows of the hammer, the momentum, or moving force of which is equally imparted iu both cases ; but in the first case, the momentum is received bj^ the board alone, which, having little weight, is driven by it through so great a space as to produce considerable flexure, or even fracture ; but iu the second case, the same momentum being shared between the board and the block behind it, will produce a flexure of the board as much less as the weight of the board and block applied to it together, is greater than the weight of the board alone. The same principle serves to explain a trick sometimes exhibited in feats of strength, where a man in a horizontal position, his legs and shoulders being supported, sustains a heavy anvil upon his chest, which is then struck by sledge-hammers. The reason the exhibitor sustains no injury from the blows, is that the momentum of the sledge is distributed equally through the great mass of the anvil, and gives to the anvil a down- ward motion, just as much less than the motion of the sledge, as the mass of the sledge is less than the mass of the anvil. Thus, if the weight of the an- vil be 100 times greater than the weight of the sledge, its downward motion upon the body of the exhibitor will be 100 times less than the motion with which the sledge strikes it, and the body of the exhibitor easily yielding to BO slight a movement, and also resisting it by means of the elasticity of the body, derived from its peculiar position, escapes without injury. When is the ^^l. When two bodics come in contact, the bodic8°"a[d'to collision is said to be direct, when a right line be direct? passing tlirough their centers of gravity jDasses also through the point of contact. The center of gravity in such cases corresponds with the center of col- lision ; and if such a center come against an obstaclCj the whole momentum of the body acts there, and is destroyed ; but if any other part hit, the body only loses a portion of its momentum, and revolves round the obstacle as a pivot, or center of motion. When two in- 142. When two non-elastic bodics, moving come'into''ciri! in opposltc dircctions, come into direct collision, fcion,whatoc- ^^qj ^.jjj ^q^^]^ jQgg ^n cqual amount of mo- ( mentum. Hence, the momentum of both after contact, will be equal to the difference of the momenta of the two before contact, and the velocity after contact will be equal to the diflerence of the momenta divided by the whole quantity of matter. Let the quantity of matter in A be 2, and its velocity 12 ; its mo- mentum is, therefore, 24. Let the quantity of matter in B be 4, its velocity 3; its momentum will be 12. The momentum of the mass after contact, on the supposition they move in opposite directions and come in direct col- lision, will be the difl"erence of the two momenta, or 12 ; and the velocity of ACTION AND BEACTION, G9 Fig. 37. To what will the shock of collision of two bodies coming in contact be equivaleut ? the mass will be its momentum divided hv the quantity of matter, or 1 2 di- vided bv 6, which is 2.* If two non-elastic bodies, as A and B, Fig 37, be suspended from a fixed point, and the one be raised toward Y, and the other toward X, an equal amount, they will acquire an equal force, or momentum, in falling down the r , . .1, arc, provided their masses are equal ; txplain the re- . ~i > Buitsof thecoi- and will bv contact destroy each lie bodiir^' otl^^'^'s motion, and come to rest. If their momenta are unequal, they wUl, after contact move on together, in the direction of the body having the largest quantity of motion with a momentum equal to the difierence of the momenta of the two before collision. 143. The force of the shock produced by two equal bodies coming in, contact with equal velocity, will be equal to the force which either, being at rest, would sustain, if struck by the other moving with double the velocity ; for reaction and action being equal, each of the two will sustain as much shock from reaction as from ac- tion. If a person running, come in contact with another who is standing, both receive a certain shock. If both be running at the same rate in opposite directions, the shock is doubled! In combats of pugilists, the most severe blows are P oo those struck by fist against fist, for the force sustained by each in such cases, is equal to the sum of the forces exerted by the two arms. If two ships, mov- ing in contrary directions at the rate of 20 miles per hour, come in collision, the shock will be the same as if one of them, being at rest, were struck by the other, moving at 40 miles per hour. 144. If we suspend two balls of some non-elastic substance, as clay or putty, by strings, so that they can move freely, and allow one of the balls to fall upon the other at rest, it wiU communicate to it a part of its motion, and both balls, after collision, will move on together. Illustrate this principle. If one inelastic body comes i:i contact with another at rest, what occurs "? • This whole subject, nsnally considered dry and nninteresting. will be found to possess a new interest, if the student will mike himself a few simple experiments, by suspending leaden balls by the side of a graduated arc. as in Fig. 37, and allow them to fall under different conditions. The length of the arc through which they fall will be fouud to be an exact measare of the furcu with which they will strike. 70 WELLS'S NATURAL THILOSOPHY. The quantity of motion will remain unchanged, the one having gained as much as the other has lost ; so that the two, if equal, will have half the ve- locity after collision that the moving one had when alone. Fig. 38 represents two balls of clay, E and D, non-elastic, of equal-weight, suspended by strings. If the ball D be raised and let fall against the ball E, a part of its motion will be communicated to E, and both together will move on to e d. When two I'lS- If ■^^'e suspend two balls, A and B, Fig. 39, of some eliistic bodies elastic substance, as ivory, and allow them to fall with equal come into col- i ./> -i lision, wliatoc- masses and velocities from the points X and Y on the arc, ""^^^ they will not come to rest after collision, but will recede Fig. 39. What occa- Gions the dif- ference in the results of the collision of eListic and non- elastic bodies? from each other with the same velocity which each had before contact. The reason of this movement in highly elastic bodies, contrary to what takes place in non-elastic bodies, is this: the elastic sub- stances are compressed by the force of the shock, but instantly recover- ing their former shape in virtue of their elasticity, they spring back, as it were, and react, each giving to the other an impulse equal to the force which caused its compression. Suppose the ball A, however, to strike upon the ball B at rest ; then, after impact, A will remain at rest, but B will move on with the same velocity as A had at the moment of contact. In this case the reaction of elasticity causes the ball A to stop, and the ball B to move forward with the motion which A had at the instant of contact. Fig. 40. ~Mr CXXDOOOO ^ J3 C n EF G- The same fact may be illustrated by suspending a number of elastic balls of equal weight, as represented in Fig. 40. If the ball H be drawn out a certain distance, and let fall upon G, the next in order, it vnii communicate its motion to G, and receive a reaction from it, which will destroy its own motion. But the ball B can not move without moving F; it will, therefore, communicate the motion it received from G to F, and receive from F a reaction which will stop its motion. In like manner, the motion and reaction are received by each of the balls E, D, C, B, A, until the last ball, K, is reached ; but there being no ball beyond K to act upon it, K will fly off as far from A^ as H was drawn apart from G. REFLECTED MOTION. 71 SECTION II. In what man- ner way a moving body be reflected 1* What is the Angle of Inci- dence? What is the Ans;le of Re- flection ? REFLECTED MOTIOK. What is Re- 146. When any elastic body, as an ivory fleeted Motion? |^^jj^ jg thrown agaiDst a hard smooth surface, Itie reaction will cause it to rebound from such surface, and the motion it receives is called Reflected Motion. 147. If the ball be projected perpendicu- larly, it will rebound in the same dii'ection ; if it be projected obliquely, it will rebound obliquely in an opposite direction, making the angle of incidence equal to the angle of reflection. 148. The Angle of Incidence is the angle formed by the line of incidence with a perpen- dicular to any given surface. 149. The Angle of Reflection is the angle formed by the line of reflection with a perpendicular to any given surface. FlQ. 41, Thus, in Fig. 41, let B E be a smooth, flat surface. If the ball, A, be projected, or thrown Y upon this surface, in the direction A C, it will rebound, or be reflected, in the direction C F. In this case, the line A C is the line of inci- dence, and the angle A C D, which it makes with a perpendicular D C, is the angle of inci- dence. In like manner the line C F is the lino of reflection, and the angle D C F the angle of reflectioa If the ball be projected against the surface, B C, in the direction D C, perpendicular to the surface, it will be reflected, or rebound back in the same straight hne. 150. The Angles of Incidence and Re- flection are always equal to one another. Thus, in Fig. 41, the angles A C D and F C D are equal. 151. An Angle is simply the inclination of the lines which meet each other in a point. its Bize depend? q^\^Q gjge of the anglc depends upon the open- ing, or inclination, of the lines, and not upon their length. What propor- tion exists be- tween the an- glesofincidence and reflection ? What is an Angle, and up- on what does 72 WELLS'S NATURAL PHILOSOPHY. In TThat cnn- The skill of the plaj'cr of billiards and bagatelle depends of the Gamu of upon his dexterous application of the principles of incident Billiards ? and reflected motion, which he has learned by long-continued experience, viz., that the angle of incidence is always equal to the angle of reflection, and that action and reaction are equal and contrary. An illus- tration of the skillful reflection of billiard balls is given in Fig. 42, which rep- resents the top of a billiard-table. The ball, P, when struck by tho stick, Q, Fig. 42. is first directed in the line P 0, upon the ball P', in such a manner, that being reflected from it, it strikes the four sides of tho table successively, at the points marked 0, and is finally reflected so as to strike the third ball, P". At each of the reflections from the ball P', and the four points on tho side of the table, the angle of incidence is exactly equal to the angle of reflection. 152. Imperfectly elastic bodies oppose the momentum of bodies in motion more perfectly than any others, in consequence of their yield- ing to the force of collision without reacting ; opposing a gradual resistance instead of a sud- den one. Hence a feather-bed, or a sack of wool, will stop a bullet much more ef- fectually than a plate of iron, from its deadening, as it is popularly called, the force of the blow. Why are Im- pprfictly elas- tic bi)(1ies yi.cii- liarly fiUerl to oppose and de- stroy momeu- tuiu? SECTION III. COMPOUND MOTION. What is Sim- ple Motion ? 153. A body acted upon by a single force, moves in a straight line, and in the direction of that force. Such motion is designated as Simple Mo- tion. COMPOUND MOTION. 73 niustra pie M. " Sim- What is the course of a body acted up- on by two forces called t Fig. 43. A body floating upon the water is driven exactly south by a wind blowing south. A ball fired from a cannon takes the exact direction of the bore of the cannon, or of the force which impels it. A\iiat is Com- l^^. When a body is acted upon by two pound Motion f f^^^^^ ^^ ^j^g ^^^^ ^^^^^ ^^^ ^^ different di- rections, as it can not move two ways at once, it takes a middle course between the two. Such motion is termed Compound Motion. 155. The course in which a body, acted upon by two or more forces, acting in different directions, will move, is called the Resultant, or the Resulting Direction. In Fig. 43, if a body, A, be acted upon at the same time by two forces, one of which would cause it to move in the di- rection A y, over the space A B, in one second of tune, and the other cause it to move in the direction A X, over the space A C, in one . second ; then the two forces, acting upon it at the same instant, will cause it to move in a Resultant Direction, A D, in one second. This direction is the diagonal of a parallelogram, which has for its sides the Unes A B, A C, over Vvhich the body would move if acted upon by each of the forces separately. 156. The operations of every-day hfe afford numerous exam- What are fa- , „^ , ,, . -r^ . , ^ miliar Eiam- ples of Resultant Motion. If we attempt to row a boat across pli-s of Kesiilt- ^ rapid river, the boat will be subjected to action of two forces; ant Motion ? ^ ' ..... , viz., the action of the oare, which tend to drive it across the river in a certain time, as ten minutes, in a straight line, as from A to B, Fig. 43, and the action of the current, which tends to carry it down the stream a certain distance in the same time, as from A to C. It will, therefore, under the influence of both these forces, move diagonally across the river, or in the direction A D, and arrive at D at the expiration of the ten minutes. When we throw a body from the deck of a boat in motion, or from a railroad ca:; the body partakes of the motion of the boat or the car, and does not strike at the point intended, but is carried some distance beyond it. For the same rea» son, in firing a rifle from the deck of a vessel moving rapidly, at some object at rest upon the bank, allowance must be made for the motion of the vessel, and aim directed behind the object. 157. The principles of the composition and Science of Pro- rcsolution of different forces acting upon a jec es body to produce motion, constitute the basis 74 WELLS'S NATURAL PHILOSOPHY. Fig. 44. of the Science of Projectiles, or that department of Natural Philosophy which considers the motion of bodies, thrown or driven by an impelling force above the surface of the earth. What iaa Pro- 1^^- ^ PROJECTILE Is a body thrown into jectiie? ^]jQ g^^j. jjj ^j^y direction ; as a stone from the hand, or a ball from a gun, or cannon. Wh f th d" If '"'6 project a body perpendicularly downward, or upward, rection of a it will move in a perpendicular line with a uniform accelerated ^°^y tl'rown Qj. retarded motion, since the force of gravity and that of pro- jection are in the same hne of direction. But if a body is thrown in a direction oblique to the perpendicular, it is acted upon by two forces,* the projectile force which tends to impel it forward in a straight line, and the force of gravity, which tends to bring it 'o the earth. Instead, there- fore, of following the direction of the projectile force, the path of the body will be a curve, the resultant of the two forces. Such a curve ia called a Pakabola. If a cannon-baU is fired Irom A to- ward B, Fig. 44, in an upward direction, instead of moving along the line A B, it is, by the influence of the earth's at- traction, continually drawn downward, and its path is along a line which is in- dicated by the parabolic curve A C ; and although it has been moving on- ward from the impulse it has received from the force of the gunpowder, it oc- cupies exactly the same time in falling to the point C, as if the ball had been allowed to drop from the hand at A, and faU to D. 159. If a ball be projected from the mouth of a cannon in a horizontal direction, it will reach the earth in precisely the same time as a ball dropped from the mouth of the gun. The force of gravity is neither increased or diminished by the force cf projection. T)ie same fact may be strikingly illustrated by placing a number of marbles at unequal distances from the edge of a table and sweeping them off with a ruler, or stick : those which are rolled along the farthest will be projected tbo farthest; yet all wiU strike the floor at the same time. ♦ The theoretical laws governing the motion of projectiles, as herewith given, are In practice esaentially modified by the resistance of the air. What effect has the projectile force on the action of grav- ity? COMPOUND MOTION. 75 Fig. 45. Suppose from the point A, Fig. 45, about 240 feet above the earth, a ball to be projected in a perfectly horizontal line, A B ; instead of traversing this line, it would, at the end of the first second, be found that the ball had fallen 15 feet, at the same time it had moved onward in the direction of B. Its true position would be, therefore, at a ; at the end of the second second, it would have passed onward, but have fallen to b, 60 feet below the horizontal hne; and at the end of the third second, it would hav& fallen 135 feet below the line, and be at c; and thus it would move forward and reach the earth at d 240 feet, in precisely the same time it would have occupied in falling from A to C. An oblique, or horizontal jet of water, is an YlQ. 46. instance of the curve described by a body act- ed upon by gravity and the force of projection. See Fig. 46. 160. The Range of a projectile, is the horizontal distance to which it can be thrown. 161. According to theory, the range is greatest when the angle of elevation is 45^ ; and is the same for elevations equally above and below 45° ; as lor example 70"" and 20°. See Fig. 47. These conclusions are, however, found to be essentially modified in practice by the resistance of the air, which not only changes the path but the velocity of the projectile. "With great velocities, as in the case of a cannon-ball, the greatest range corres- ponds with an elevation of about 30*, but for slow motions it is near 45°. 162. The laws of projectiles are es- pecially regarded in the art of gunnery. By knowing the force of the powder which drives the ball, the engineer is enabled to direct the cannon, or mortar, in such a manner as to cause the ball, or bomb, to fall What is the Kange of a Projectile ? How can the greatest Range be obtained ? Fig. 47. Bow are the Laws of Pro- jectiles practi- ciilly applied in military en- gineering ? 7(5 WELLS'S NATURAL PHILOSOPHY. upon a particular spot in the distance ; thus producing a desired effect without a wasteful expenditure of ammuni- tion. Fig. 48. Fig. 48 represents a bombardment, and the three lines indicate the curves made by the balls. If the bombardment had been conducted from an eleva- tion, instead of the level surface, the balls would have gone beyond the city, ns shown by the familiar fact, that we can throw a heavy body to a greater distance from an elevation, as the steep bank of a river, than on a plain, or level ground. It was on this principle that Napoleon bombarded Cadiz, at the distance of five miles, and from a greater elevation, the balls could have been thrown to a still greater distance.* * The following facts respecting the explosive force of gunpowder, and its application to projectiles, will be found interesting and instructive in this connection. The estimated force of gunpowder when exploded, is at least 14,750 pounds upon every square inch of the surface which confines it. Count Runiford showed, by his experiments made about 60 years ago, that if the powder were placed in a close cavity, and the cayity two thirds filled, its dimensions being at the same time restricted, the force of ezplosioa would ex- ceed 150,000 pounds upon the square inch. The force of gunpowder depends upon the fact, that when brought in contact with any Ignited substance, it explodes with great violence. A vast quantity of r?as, or elastic Jluid, is emitted, the midden production of which, at a high temperature, is the cause of the Tiolent effects which are produced. The reason that gunpowder is manufactured in little grains, is that it may explode moro quickly, by facilitating the passage of the flame among the particles. In the form of dust, the particles would be too compact. The velocity of balls impelled by gunpowder from a musket with a common charge, has been estimated at about 1,650 feet in a second of time, when first discharged. The utmost Velocity that can be given to a cannon-ball is 2,000 feet per second, and this only at th« moment of its leaving the gun. In order to increase the velocity from 1,650 to 2,000 feet, one half more powder is re- quired ; and even then, at a long shot, no advantage is gained, since, at the distance of 500 yards, the grea. 'it velocity that can be obtained is only 1,200 or 1,300 feet per second. Great charges of powde. ■^re, therefore, not only useless, but dangerous ; for, though they give little additional force to 'he ball, they hazard the lives of many by their liability to burst the gun. The velocity is greater with long than with short guns, because the influ- ence of the powder upon the ball is longer continued. The essential properties of a gun are to confine the elastic fluid generated by the explo- sion pf the powder aa completely as possible, and to direct the course of the ball in a COMPOUND MOTION. ■ 77 According to the lawa which govern the motion of project- ^Tbe'^a^med '^^^ ^^ '^ evident that f gun must be aimed, in order to hit to hit an ob- an object, in a direction above that of the object, more or less, distance'r^'^''^ according to the distance of the object and the force of the charge. "With an aim directed, as in Fig. 49, at the object, the ball, moving in a curved path, must necessarily fall below it. straight, or rectilinear path. A rifle sends a ball more accurately than a musket, bccanse the ball is in more accurate contact with the sides of the barrel than in the case of a com- mon musket The space produced by the difference of diameter between the ball and tho bore of the gun, greatly diminishes the effuct of the powder, by allowing a part of tli« elastic fluid to escape before the ball, and also permits the ball to deviate from a straight line. The peculiarity and superiority of the new rifle, called the " Miniu rifle," is to be found in the construction of the ball, which, by the act of firing, is made to fit completely the barrel, or bore, of the gun. This is accomplished by making the ball of an oblong shape and a conical point, with an opening in the base extending up for two thirds the length of the ball. Into the opening of this internal cylinder there is placed a Bm;ill con- cave section of iron, which the powder, at the moment of firing, forces into the leaden ball with great power, spreading it open, and causing it to fit tightly to the cavity of tho barrel in its course out, thus giving it a perfect direction. Cannon of different sizes are named according to the weight of the ball which they are capable of discharging. Thus, we have 6S-pounders, 24-poundcrB, IS-pounders, and tha lighter field-pieces, from 4 to 12-pounders. The quantity of powder generally used for discharging common iron or brass cannon, is one third the weight of the ball. In gen- eral warfare, the effective distance at which artillery can be used is from 500 to 600 yards, or from a quarter to half a mile. At the battle of Waterloo, the brigades of artillery were stationed about half a mile from each other. Cannon-balls and shells can be thrown with effect to the distance of a mile and a half to two miles. The distance to which a ball may be thrown by a 24-pounder, with a quantity of powder equal to two thirds the weight of the ball, is about four miles. Its effective range is, how- ever, much less. Were the resistance of the air entirely removed, the same ball would be thrown to about five times that distance, or twenty miles. It has been found that, by the firing of .in 18-pound shot into a butt, or target, made of beams of oak, when the charges were C pounds of powder, 3 pounds, 2i pounds, aruL 1 ^und, the respective depths of the penetration vere 42 inches, 30 inches, 23 inches, and 15 inches; and the velocities at which the balls flew, were 1,600 feet in a stcond, 1,140 feet, l,02i feet, and 6,56 /ee(. When the cannon is so pointed that the ball goes perfectly straight toward the object aimed at, the direction is said to be point-blank. Ricochet firing Is when the ball is dis- charged in such a manner that it goes bounding aTul shipping along the surface of tho ground. In this way a ball can be thrown moro effectively, and for a greater distance, than in any other way. There are several substances known to chemists which possess a greater exploeivo power than gunpowder. It has not, however, been considered possible to increase the range and effect of a projectile fired from a gun, or cannon, by using any of them. Sup- posing that the guns could be made indefinitely strong, and the gunpowder indefinitely powerful, the point would soon be reached where the resistance which the air opposes to a body moving very rapidly would balance the force derived from the explosive compound, which drives the projectile forward. Beyond this point no increase of impulsive force would urge the projectile farther ; and this limit is considerably within the range of power that can be exercised by common gunpowder. Beside this, the strength of mate- rials of which guns are made is limited. Practical experience has fully demonstrated that the largest piece of ordnance which can be cast perfect, sound, and free from flaws, is a mortar 13 inches diameter; and even this weighs five tons. The French, at the siege of Antwerp, constructed a mortar having a bore of uo less than 20 inches diameter, but ik burst on the uinth time of firing. 78 WELLS'S NATURAL PHILOSOPHY. Fig. 49. Until quite recently, the muskets placed in the hands of soldiers were usu- ally aimed so that the line of sight was parallel to the barrel, and directed to the object, as in Fig 49. So long as the range of the musket was of limited extent, and great precision was not expected, the deviation of the ball from a straight line was not taken into account ; but with the introduction of rifles throwing a ball to a great distance, the drop of the ball occasioned by the curvature of the path of the projectile, was found to deprive the weapon of the necessary precision. On all modem guns, therefore, a double sight is provided, by which the elevation necessary to secure accurate aim can always be given to the barrel This is exhibited in Fig. 50, where one of the sights, B, is fixed, in the usual manner, at one extremity of the barrel, while the other is located nearer the breach. This last sight is often graduated and provided with an adjustment, by which it can be adapted to objects at dif- ferent distances, so as to hit them exactly. Fig. 50. What is Cir- cular Motion? 163. duced central point. 164. Circular Motion is the motion pro- by the revolution of a body about a Hoir if Circn. lar Motion pro- duced} Circular Motion is a species of com- pound motion, and is caused by the continued operation of two forces ; — one the force of projection, which gives the body motion, tends to cause it to move in a straight line ; while the other is continually deflecting it from a straight course toward a fixed point. ninstrate the '^^ ^^^^ ^^ illustrated by the common sling, or by swinging production of a heavy body attached to a string round the head. The body, in this case, moves through the influence of two forces, the force of projection, and the string which confines it to the hand. These two forces act at right angles to one another, and according to Circular tion. COMPOUND MOTION. 79 ihe statements already made (§ 155), the path of the moving body will be a resultant of the two forces, or the diagonal of a parallelogram. How then, it may be asked, does the body attached to the string and whirled round tho head, move in a circle ? This will be clear, if we consider tnat a circle is made of an in- finite number of httle straight lines (diagonals of parallelo- grams) and that the body moving in it, has its motion bent at every step of its progress by the action of the force which confines it to the hand. This force, however, only keeps it within a certain distance, without drawing it nearer to the hand. The two forces exactly balancing each other, the course of the whirling body will be circular. 165. The two forces by which circular mo- tion is produced, are called the Centrifugal'^'* and Centripetal Forces.f 166. The Centrifugal Force is that force which impels a body moving in a curve to move outward, or fly off from a center. 167. The Centripetal Force is that force which draws a body moving in a curve toward the center, and assists it to move in a bent, or curvelinear course. In Circular Motion the Centrifugal and Centri- petal Forces are equal, and constantly balance each other. If the Centrifugal Force of a body revolving in a circular path be destroyed, the body will immediately approach the center ; but if the Centripetal Force be destroyed, the body will fly off in a straight hne, called a tangent. Thus, in whirUng a ball attached by a string to tho fin- ger, the propelling force, or the force of projection, is given by the hand, and the Centripetal Force is exhibited in the stretching, or tension of the string. K the string breaks in whirling, the Centripetal Force no longer acts, and the ball, by the action of the Centrifugal Force, generated by tho whirUng motion, flies off in a tangent, or straight hne, as is represented in Fig. 51. If, on the contrary, tha whirling motion is too slow, the Centripetal Force pre- ponderates, and the ball falls in toward the finger. Familiar examples of the effects of Centrifugal Force are common in the experience of every-day hfe. ,.., , , The motion of mud flving fit)m the rim of a coach-whceL A\'i-t are fa- . r> -i miliar iiiiistra- movmg rapidly, is an illustration of Centrinigal Force. Fig. fu*'!a'ForceV'' ^"^ represents a coach-wheel throwing off mud ; o tho point at which the mud flies off; ab, the straight hne in which it • CentTifnf»al, compounded of center, and "fugio" to fly oft t Centripetal, compounded of center and "peto," to seek. Kow may the curve of a cir- cle be consid- ered as equiva- lent to the diagonal of a parallelogram T What are the two forces which produce Circular Mo- tion called 1 What is Cen- trifugal Force T What is Cen- tripetal Force 7 ■WTiat follows if the Centri- fajfal or Cen- trip'jtal Forces are destroyed f Fig. 51. 80 WELLS'S NATURAL PHILOSOPHY. would move but for the action of the two forces, which compel it to follow the parabolic curve, a c. Fig. 52. h The mud sticks to the wheel, in the first instance, through the force of ad- hesion ; but this force, being very weak, is overcome by the Centrifugal Force, and the particles of mud fly off. The particles which compose the wheel it- self would also fly otf in the same mamncr, were not the force of cohesion which holds them together stronger than the Centrifugal Force. The Centrifugal Force, however, increases with the velocity of revolution, so that if the velocitj' of the wheel were contin- ually increased, a point would at last be reached, when tlie Centrifugal Force would be more powerful than the force of cohesion, and the wheel would then fly in pieces. In this way almost all bodies can be broken by a sufficient rotative velocity. Large wheels and grind- Under TThat circumstances will the (Jen- trifugal Force overcome the Force of Cohe- ■ionf Fig. 53. Btones, revolving rapidly, not infre- quently break from this cause, and the pieces fly off with immense force anci velocity. "When we whirl a mop, the water flies otf from it by the action of the Centrifugal Force. The fibers, or threads, which compose the mop, also tend to fly off", but being confined at one end, they are unable so to do. They, therefore, assume a sphericiil form, or shape. The fact that water can be made to fly off from a mop, by the action of the Centrifugal Force produced by whirling it, has been most ingeniously applied in a machine for drying cloth, called COMPOUND MOTION. 81 Fig. 54, the hydro-extractor (water-extractor), Fig. 53. The machine consists of a large hollow wheel, or cylinder, A A, turning upon an axis, B. The eidca and bottom of the wheel are pierced with holes like a sieve. The wet cloths being in and around the sides, A, the wheel is caused to revolve with great rapidity, and the water contained in the material, by the action of tho Centrifugal Force^ flies out, and escapes through tho apertures left in the sides of the wheel A rotation of 1500 times per minute, is sufficient to almost en- tirely dry the cloth, no matter how wet it may have been originally. When a bucket of water, attached to a string, is whirled rapidly round, the water does not fall out when the mouth is presented downward, since the Centrifugal Force imparted to the water by rotation, tends to causo it to fly oflF from the center, and this overcomes, or bal- ances, the attraction of gravitation, which tends to causo the water to fall out, or toward the center. Thus, in Fig 54, the water contained in the bucket which is up- side down, has no support under it, and if the bucket were kept still in its inverted position for a single mo- ment the water would fall out by its own weight, or, in other words, by the attraction of gravitation, which rep- resents a Centripetal Force ; but tho Centrifugal Force, which is caused by the whirling of the bucket in the di- rection of the arrow, tends to drive the water out through the bottom and side of tho vessel, and as this last forco overcomes, or balances the other, the water retains its place, and not a drop is spilled. When a carriage is moved rapidly round a comer, it is yery liable to be overturned by the Centriftigal Forco called into action. The inertia carries the body of tho vehicle forward in the same line of direction, while tho wheels are suddenly pulled around by the horses into a new one. Thus a loaded stage running south, and sud- denly turned to the east, throws out the luggage and passengers on the south side of the road. When railways form a rapid curve, the outer rail is laid higher than the inner, in order to counteract the Centri- fugal Force. ' An animal, or man, turning a corner rapidly, leans in toward the comer or center of the curve in which he is moving, in order to resist the action of the Centrifugal Force, which tends to throw him away from the center. In all equestrian feats exhibited in the circus, it will be observed that not only the horse, but the rider, inclines liis body toward the center. Fig. 55, and according as the speed of the horse round the ring is increased, this inclina- tion becomes more considerable. When the horse walks slowly round a large 82 WELLS'S NATURAL PHILOSOPHY. ring this inclination of his body is imperceptible ; if he trot, there is a visible inclination inward, and if ho gallop, ho inclines still more, and when urged to full speed he leans very far over on his side, and his feet will be heard to strike against the partition which defines the ring. The explanation of all this is, that the Centrifugal Force caused by the rapid motion around the ring tends to throw the horse out of; and away from, the circular course, and this ho counteracts by leaning inward. Fig. 65. How do tha motions of the Bolar system il- lustrate the ac- tion of Centri- fugal and Cen- tripetal Forces T The most magnificent exhibition of Centrifugal and Centri- petal Forces balancing each other, is to be found in the ar- rangements of the solar system. The earth and other planets are moving around a center — the sun, with immense veloci- ties, and are constantly tending to rush off into space, by the action of the Centrifugal Force. They are, however, restrained within exactly determined limits by the attraction of the sun, which acts as a centripetal power drawing them toward the center. What is the 168. The Axis of a body is the straight line, Axisof a body? j.g^| ^^ imaginary, passing through it, on which it revolves, or may revolve. 169. When a body rotates upon an axis, all its parts revolve in equal times. The velocity of each particle of a revolving body increases with its perpendicular distance from the axis, and as its velocity increases, its Centrifugal Force in- creases. A moment's reflection will show, that a point on the outer part, or rim, of a wheel, moves round the axis in the same time as a point nearer the center, «a upon the hub. But the circle described by the revolution of the outer part Whpn a body ri'.YcAveB round its Axis, what peculiarities do its several parts exhibit t COMPOUND MOTION, 83 What effect does the action of Contrifugal Force have up- on the figure of »body? Fia. 56. of tho -wheel is much larger than that described by the inner part, and as both move round the center in the same time, the outer part must move with a greater velocity. 170. If the particles of a rotating body have freedom of motion among themselves, a change in the figure of the body may be occasioned by the difference of the Centrifugal Force in the different parts. A ball of soft clay, with a wire for an axis, forced through its center, if macia to turn quickly, soon ceases to be a perfect ball It bulges out in the middle, where the Centrifugal Force is, and becomes flattened toward the ends, or whsre the wire issues. This change in the form of revolving bodies may be illus- trated by an apparatus repre- sented in Fig. 56. This con- sists of an elastic circle, or hoop, &stened at the lower side on a vertical shaft, v.hile the upper side is free to move. On turn- ing the wheels, so arranged aa to impart a very rapid motion to the shaft and hoop, the hoop win be observed to bulge out in the middle (owing to the Centrifugal Force acting with greater intensity upon those parts furthest removed from the axis) and to be- come flattened at the ends. .„^ . . 171. The earth itself is an example of the operation of this what 18 the „ ^ ,. , ■ , ^ ^ ^ • -, cause of the force. Its diameter at the equator is about twenty-six miJes of «f "earth''^ greater than its polar diameter. The earth is supposed to have assumed this form at the commencement of its revolu- tion, through the action of the Centrifugal Force, while its particles were in a semi-fluid, or plastic state. In Fig. 57 we have a representation of the general figure of the earth, in which N S is the jxjlar diameter, 6nd also the axis of rotation, and E W the equatorial diameter. 172. At the equator the amount of Ccif- Centrifugal Force of a part icle tripptal Force ^f matter is l-290ths of its At the equator T . ,. . . , gravity. This diminishes as •we approach tl.e poles, where it becomes 6. If the earth revolved 17 times faster than it now docs, or in 8-i minutes instead of 24 Fig. 57. 84 WELLS'S NATURAL PHILOSOPHY. What would be hours, the Centrifugal Force -would be equal to the attraction Telocity of ro- o*" gravitation, which may bo considered as the Centripetal tation of the Force, and aU bodies at the earth's equator would be deprived earth were »n- ^ . , . . ^, ,, , , , creased ? 01 weight, since they would have as great a tendency to leave the surface of the earth as to descend toward its center. If the earth revolved on its axis in less time than 84 minutes, terrestrial gravita- tion would be completely overpowered, and all fluids and loose substances •would fly from its surface. 173. There appears to be a constant tendency to rotary motion in moving bodies free to turn upon their axes. The earth turns upon its axis, as it moves in its orbit ; a ball projected from a cannon, a rounded stone thrown from the hand, all revolve around their axes as they move. TlQ. 58. This phenomenon may be very prettily illustrated by placing a watch-glass upon a smooth plato of glass, Fig. 58, moistened suf- ficiently to insure slight adhesion, and fixed at any angle. As it ^*— begins to move toward the bot- tom of the inclined plane, it will exhibit a revolving motion, which uniformly increases with the acceleration of its downward movement. PRACTICAL QUESTIONS AND PROBLEMS ON THE PRINCIPLES AND COMPOSITION OF MOTION. 1. The STmrACK of the eabth at the equatob mores at the rate of about a TnoTJBAirr MILES in an houk : why are men not sensible of this rapid movement of the earth ? Because all objects about the observer are moving in common with him. It 13 the natural uniformity of the undisturbed motion which causes the earth and all the bodies moving together with it upon its surface to appear at rest. 2. How can you easily see that the eabth is in motion? By looking at some object that is entirely unconnected with it, as the sun or the stars. "We are here, however, liable to the mistake that the sun or atara are in motion, and not we ourselves with the earth. 3. Does the bun really eise and bet each day ? The sun maintains very nearly a constant position ; but the earth revolves, and is constantly changing its position. Really, therefore, the sun neitJier rises nor sets. 4. Why, to a pebson sailing in a boat on a smooth stream, or ooino bwiftlt- in a OABBIAG on a smooth road, do the trees or buildings on the banks or roadside appear to move in an opposite dieection ? The relative sUiiation of the trees and buildings to the person, and to each COMPOUND MOTION. 85 other, ig actually changed by the motion of the observer ; but the mind, in judging of the real change in place by the difference in the position of tho objects observed, unconsciously confounds tho real and apparent motion. 5. Why will a tallow candle fired from a gun pierce a board, or target, in the same manner as a leaden bullet will, under the same circumstances? When a candle starts from the breach of a gun, its motion is gradually in- creased, until it leaves the muzzle at a high velocity ; and when it reaches the board, or target, every particle of matter composing it is in a state of great Telocity. At the moment of contact, the particles of matter composing tho target are at rest ; and as tho density of tho candle, multiplied by the velocity of its motion, is greater than the density of the target at rest, the greater force overcomes the weaker, and the candle breaks through and pierces a hole in the board. 6. Why, with an enormouB pressure and slow motion, can you not force a candle through a board f Because the candle, on account of its slow motion, does not possess suffi- cient momentum to enable the density of its particles to overcome the greater density of the board ; consequently the candle itself is mashed, instead of piercing the board. 7. Why will a large ship, moving toward a wharf with a motion hardly perceptible, crush with great force a boat intervening ? Because the great mass and weight of the vessel compensates for its want of velocity. 8. Why can a person safely skate with great rapidity over ice which would not bear his weight standing quietly 7 Because time is required to produce a fracture of the ice ; as soon as tho weight of the skater begins to act upon any point, the ice, supported by the water, bends slowly under him ; but if the skater's velocity be great, ho passes off from tho spot which was loaded before the bending has reached the point at which the ice would break. 9. A HBAVT COACH and a light tvagok came in collision on the road. A suit for damages was brought by the proprietor of the wagon. How wasit shown that oxBof tho VEHICLES was moving at an unsafe velocitt ? On trial, the persons in the wagon deposed that the shock, occasioned by coming in contact, was so great, that it tfirew them over the head of their horse; and thus lost their case by proving that the faulty velocity was their own. 10. Why did the fact that they were thbown over tho head of the hobbg by coming In contact with the coach, prove that their velocity was obeateb than it ought to hare been f The coach stopped the wagon by contact with it, but the bodies of the per- sons in the wagon, having the same velocity as the wagon, and not fastened to it, contintied to move on. Had the wagon moved slowly, the distance to which they would have been thrown would have been slight To cause them to be thrown a* far as over the head of the horse, would require a great velocity of motion. 86 WELLS'S NATURAL PHILOSOPHY. 11. 'When TWO PEB80N8 BTHiKE their BEADS together, one being In motion and the other «t BEST, why are both equally hurt ? Because, when bodies strike each other, action and reaction are equal ; the head that is at rest returns the blow with equal force to the head that strikes. 12. When an elastic baxi. la thrown against the side of a house with a oebtaim foboi!, why does it rebound t Because the side of the house resists the ball with the same force, and the ball being elastic, rebounds. 13. When the same ball is thrown against a pans of glass with the same force, it goes through, breaking the glass ; why does it not rebound as before ? Because the glass has not sufficient power to resist the full force of the ball: it destroys a part of the force of the ball, but the remainder continuing to act, the ball goes through, shattering the glass. 14. Why did not the man succeed who undertook to make a faib wind for his pleas- CEE-BOAT, by erecting an immense bellows in the stebn, and blowing against the bails f Because the action of the stream of wind and the reaction of the sails were exactly equal, and, consequently, the boat remained at rest. 15. If he had blown in a contsaby dibection from the sails, instead of against them, would the boat have moved ? It would, with the same force that the air issued from the bellows-pipe. 16. Why can not a man raise himself over a fence by pulling upon the stuapb of his BOOTS? Because the action of the force exerted by the muscles of his arms is coun- teracted by the reaction of the force, or, in other words, the resistance of his whole body, which tends to keep him down. 17. Why do watee-dogb give a bemi-eotaby movement to free themselvcB from water f Because in this way a centrifugal force is generated, which causes the drops of water adherent to them to fly off. 18. Why is the coDBSS of rivers rarely BTB.vionT, but bebpenttne and windino? "When, from any obstruction, the river is obliged to bend, the centrifugal force tends to throio aivay the tuater from the center of the curvature, so that when a bend has once commenced, it increases, and is soon followed by others. Thus, for instance, the water being thrown by any cause to the left side, it wears that part into a curve, or elbow, and, by its centrifugal force, acts con- etantly on the outside of the bend, until the rock, or lugher land, resists ita gradual progress ; from this limit, being thrown back again, it wears a simUat bend to the right band, and after that another to the left, and so on. 19. A locomotive passes over a railroad, 200 miles in length, in 5 hours ; what is ita velocity per hour ? 20. If a bird, in flying, passes over a distance of 45 miles in an hour, what is its ve- locity per minute ? 21. The flash of a cannon three railos off w.is seen, and in 14 seconds afterward the sound was heard. How many feet did the sound travel in one second f APPLICATION OF FORCE. 87 22. The sun Is 95 millions of miles from the earth, and ft requiiea 8 J minutes for Its light to reach the earth ; with what velocity per second does light move J 23. If a vessel sail 90 miles a day for 8 days, how far will it sail in that time ? 24 A gentle wind is observed to move 1,250 feet in 16 minutes : how far would it move in 2 hours, allowing 5,000 feet to the mile f 25. What distance would a bird flying uniformly at the velocity of 60 miles per hour, pass over in 12^ hours f 26. Suppose light to move at the rate of 192,000 miles in a second of time, how lonj^a time will elapse in the passage of light from the sun to the earth, the distance being 05 millions of miles 7 27. What is the momentum of a body weighing 25 pounds moving with the velocity •f 30 feet per second f 28. A cannon-ball weighing 520 pounds, struck a wall with a velocity of 45 feet per second : what was its momentum, or with what force did it striltc ? 29. A locomotive and train of cars weighing 180 tons (403,200 pounds), and moving at the rate of 40 miles per hour, came in collision with another train weighing 160 tons, and moving at the rate of 26 miles per hour : what was the momentum, or force of collision ? 30. A stone thrown directly at an object from a locomotive, moving at the rate of 3,520 feet per minute, was 2 seconds in the air ; at what distance beyond tho object did it strike f CHAPTER VI. APPLICATION OF FORCE. What are the 174. The principal agents from whence we Sf powe^'ifThl obtain power for practical purposes, are Men •"■'si' and Animals, Water, Wind, Steam, and Gunpowder. The power of all these may be ultimately resolved into somo ere ™ Natural °'^® ^^ more of the great natural forces, or primary sources of forces are these power, viz., vital force, producing muscular energy, or strength derived? ^° ^^^ ^^^ animals; gravitation, causing the flow of water; heat and molecular forces, the agents producing the power ex- hibited by wind, steam, and gunpowder. Magnetism and electricity when called into action, and Are there any ° •' . ^ r other agents of capillary attraction, are also agents of power; but none oi power? these are capable, as yet, of being used to any great extent. for tho production of motion. „ . 175. Muscular enerojy in men and animals ITowiamuseu- . n ■> • /• i lar energy ex- IS cxcrtcd Dv mcans of the contractiou 01 the crted t fibers which constitute the muscles of the 88 WELLS'S NATURAL PHILOSOPnT. body ; the bones o:N;lie body facilitate and direct the ap^ plication of this force. Beasts of prey possess the greatest amount of muscular power ; but somo very small animals possess muscular power in proportion to their bulk, in- comparably greater than the largest of the brute creation. A flea, considered relatively to its size, is stronger than an elephant, or a lion. A man can exert his greatest active strength in pulling up- exert his great- ward from his feet, because the strong muscles of the back, ebt strgngth f ^^^ those of the upper and lower extremities, are then brought most advantageously into action. The comparative effect produced in the different methods of applying the force of a man, may be indicated as follows : in tho action of turning a crank, or handle, his force maybe represented by the number 17; in working a pump, by 20 ; in pulling downward, as in ringing a bell, by 39 ; and in pull- ing upward from the feet, aa in the action of rowing, by 41. What is the 1'7^6. The estimate of the uniform strength efre"gth*of « ^^ ^^ Ordinary man, for the performance of or- °"'°' dinary daily mechanical labor is, that he can raise a weight of 10 pounds to the height of 10 feet once in a second, and continue to do so for 10 hours in the day. r^ . , .u 177. The estimated strength of a horse is, What i« the , , f, , estimntpd that hc Can raise a weight of 33,000 pounds •trength of a ,•,., , n • • oi horse, or & to the hcicrht ot one foot m a mmute. feucn "horse-power?" ^ n n . -iii ct a measure of force is called a horse- POWEB." The strength of a horso is considered to be equal to that of five men. The average strength which a horse can exert in drawing is about 1600 pounds. What is water- 178. Water-power is the power obtained power T y^^ ^^iQ action of water falling perpendicularly, or running down a slope, by the influence of gravity. What is the 179. When work is performed by any agent, fomjarfng the thcrc Is always a certain weight moved over a perfo"4ed''by'' Certain space, or a resistance overcome ; the different forces? amouut of work performed, therefore, will de- pend on the weight, or resistance that is moved, and the space over which it is moved. For comparing different quantities of work, done by any force, it is necessary to have some standard ; and this standard is the power, or APPLICATION OF FOBCE. 89 labor, expended in raising a pound weight one foot tigh, in opposition to gravity. How is the ef- ^^^' The sffcct produced by a moving power fng powe"es'- ^^ always expressed by a certain weight raised pressed? ^ Certain height. To find, therefore, the effect of a moving power, or to find the power ex- pended in performing a certain work, we have the following rule : — How may the ^^l. Multiply the weight of the body moved Sd'^in'w^rbe' i° pounds by the vertical space through which *~*'^^«'^' it is moved. Thus, for example, if a horse draw a loaded wagon, with a fonce by which the traces are stretched to as great a degree as if 200 pounds v> re suspended vertically fi-om them, and if the horse thus acting draws the wagon over a space of 100 feet, the mechanical effect produced is said to be 200 pounds raised 100 feet; or, what is the same thing, 20,000 pounds raised 1 foot. "When a horse draws a carriage, the work he performs is expended in over- coming the resistance of friction of the road which opposes the motion of tho carriage ; but friction increases and diminishes as the weight of the load in- creases or diminishes. The work performed will, therefore, be estimated by multiplying the total resistance of friction, as expressed in pounds, by the space over which the carriage is moved. J.. , The following examples will illustrate how we are enabled, manner of esti- by the above rules, to calculate the amount of power required mating power? ^ perform a certain amount of work: — Suppose we wish to know the amount of horse-power required to lift 224 pounds of coal from tho bottom of a mine 600 feet deep. The weight, 224, multiplied into space moved over, 600 feet, equals 134,400, the amount of work to be performed each minute; a horse-power equals 33,000 pounds raised 1 foot per minute: therefore, 134,400-^-33,000=4.07, horse-power required. If we wish to per- form the same work by a steam-engine, we would order an engine of 4.07 horse-power, and the engine-builder, knowing the dimensions of the parts of an engine essential to give one horse-power, can build an engine capable of performing the requisite work. Again. Suppose a locomotive to move a train of cars, on a level, at tho rate of 30 miles per hour, the whole weighing 25 tons, with a constant re- sistance from friction of 200 pounds, what is the horse-power of the engine 7 30 miles per hour equals 2,640 feet per minute ; this space multiplied by 200 pounds, the resistance to be overcome, equals 528,000, the work to be done every minute; which, divided by 33,000 (one horse-power), equals 16, tho horse-power of the locomotive. What is a Dy- 182. Au instrument for measuring the rela- namometer? ^j^.g strength of meu and animals, and also of the force exerted by machinery, is called a Dynamometer. 90 WELLS'S NATURAL PHILOSOPHY. p gg Fig. 59 represents one of the most common forms of the dynamometer, consisting of a band of steel, bent in the middle, so as to have a certain de- gree of flexibility. To the expanded extremity of each limb is fixed an arc of iron, which passes freely through an opening in the other limb, and terminates outside in a hook or ring. One of these arcs is graduated, and represents in pounds the force required j-j^ gjj^, to bring the two limbs nearer together. Thus, if a horse were pulling _^ • a rope attached to a body which he had to move, we may imagine the l) rope to be cut at a certain point, and the two ends attached to the j| ends of the c^rcs, as represented in Fig. 59 ; the force of traction ex- erted by the animal would be seen by the greater or less bringing together of the ends of the instrument. In another form of dynamometer, Fig. 60, which is also used as a spring balance in weighing, the force is measured by the collapsing of a steel spring, contained within a cyhndrical case. The construc- tion and operation of this instrument will be easily understood from an examination of the figure. What is a Ma- 183. A Machine is an instrument, or chine T apparatus, adapted to receive, distribute, ^k and apply motion derived from some external force, (9 in such a way as to produce a desired result. A steam-engine and a water-wheel are examples of machines. They re- ceive the power of steam in the one case, and the power of falling water in the other, and apply it for locomotion, sawing, hammering, etc. Do wc produce 184. A MACHINE cau not, under any cir- use"^ of ^ ma! cumstances, create power, or increase the chines? quantity of power, or force, applied to it. A machine will enable us to concentrate, or divide, any quantity of force which we may possess, but they no more increase the quantity of force applied than a mill-pond increases the quantity of water flowing in the stream.* _ Machines, in fact, do not increase an applied force, but they ehines in reality diminish it, or, in Other words, no macliine ever transmits the diminish force? ^±oIq amount of force imparted to it by the moving poweijf since a part of the power is necessarily expended in overcoming the inertia of matter, the friction of the machinery, and the resistance of the atmosphere. • " Power is always a product of nature. God has not vouchsafed to man the means of its primary creation. He finds it in the moving air and the rapid cataract ; in the burn- ing coal and the heaving tide. He transfers it from these to other bodies, and renders it the obedient servant of his will — the patient drudge which, in a thousand ways, adminis- ters to his wants, his convenience, and his luxuries, and enables hira to reserve his own energy for the higber purposes of the development of his mind and the expression of his thoughtB."— Pro/. Henry. APPLICATION OF FORCE. 91 Is Perpctnal 1^5. PERPETUAL MoTION, Or the COnstrUC- cMne^'poTsi' ^^^^ of niachines which shall produce power *''*' sufficient to keep themselves in motion con- tinually, is, therefore, an impossibility, since no combi- nation of machinery can create, or increase, the quantity of power applied, or even preserve it without diminution. What era 1 ^^ nature we have an example of continued and undimin* of continued ished motion in the revolution of the earth upon its axis, and we ia natill-e7 °^ ^^^ planets around the sun. These bodies have been mov- ing with undiminished velocity for ages past, and, unless pre- vented by the agency which created them, will continue so to do for ages to come. „ ^ , 186. We derive advantasres from machines Ilow do ire de- , '-' rive advantages in thrcc different ways; Ist, from the addi- from machines ? i rt i r- tions they make to human power ; 2d, from the economy they produce of human time ; 3d, from the conversion of substances apparently worthless and com- mon into valuable products. How do ma- 18T. Machiucs make additions to human additioni™"^o power, because they enable us to use the human power? power of natural agents, as wind, water, steam. They also enable us to use animal power with greater ef- fect, as when we move an object easily with a lever, which we could not with the unaided hand. How do ma- 1^8. Machiucs produce economy of human economVofiir- tlmc, becausc they accomplish with rapidity man ume t what would rcquire the hand imaided much time to perform. A machine turns a gun-stock in a few minutes ; to shape it by hand would be the work of hours. 189. Machines convert objects apparently chines convert worthless iuto valuablc products, because by worthless ob- - . - • t , '/• jccts into vai- their great power, economy, ana rapidity of epro ucs ^^^Iqj^^ ^\^qj make it profitable to use objects for manufacturing purposes which it would be unprofit- able or impossible to use if they were to be manufactured by hand. Without machines, iron could not be forged into sliafls for gigantic engines; fibers could not be twisted into cables ; granite, in large masses, could not be transported from the quarries. $% WELLS'S NATURAL PHILOSOPHY. Define Power, 19^- ^^ machinery, we designate the mov- wor^ng Point, ^^g foFce as thc Power ; the resistance to be ^cwK *" overcome, whatever may be its nature, as the Weight ; and the part of the machine im- mediately applied to the resistance to be overcome, as the Working Point. What ii the 191. The great general advantage that we S.'ivHntat4"^''of obtain from machinery is, that it enables us machinery r jq exchaugc time and space for power. Thus, if a man could raise to a certain height two hundred pounds in one minute, with the utmost exertion of his strengtli, no arrangement of machinery could enable him unaided to raise 2,000 pounds in the same time. If he de- sired to elevate this weight, he would be obliged to divide it into ten equal parts, and raise each part separately, consuming ten times the time required for lifting 200 pounds. The application of machinery would enable him to raise the whole mass at once, but would not decrease the time occupied in doing it, which would still be ten minutes. Again. A boy who can not exert a force of fifty pounds may, by means of a claw-hammer, draw out a nail which would support the weight of half a ton. It may seem that the use of the hammer in this case creates power, but it does not, since the hand of the boy is required to move through per- haps one foot of space to make the nail rise one quarter of an inch. But it baa been already shown that the force of a small body moving with great velocity may equal the force of a large body with a slight velocity. On the same prin- ciple, the small weight, or power, exerted by the boy on the end of the ham- mer handle, moving through a large space with an increased velocity, ac- quires sufficient momentum to overcome the great resistance of tlie nail. In both of these examples space and time are exchanged for power. „ . .. 192. The mechanical force, or momentum, of a body, is as- How IS the me- ' ' "' chanical effect certained by multiplying its weight by the space through termined" ^''' '"'li'ch it moves in a given time, that is to say, by its velocity. The mechanical force, or momentum, of a power may also be found, by multiplying the power, or its equivalent weight, by its velocity. What is the 193. The power, multiplied by the space {rrium*^ of "all through which it moves in a vertical direction, inaciiinesf jg equal to thc weight multiplied by the space through which it moves in a vertical direction. This is the general law which determines the equilibrium of all machines. „ , 194. The power will overcome the resistance Under what ^ -"^ . -n , i conditions wiu of thc wcijirht, and motion will take place in a motion take ii i • • n place in » ma- machmc, when the product arising from the chine t ^ , ® . power multiplied by the space through which THE ELEMENTS OF MACHINERY. 93 it moves in a vertical direction, is greater than the pro- duct arising from the weight multiplied by the space through which it moves in a vertical direction. «rj) f J g t Practical men express the principle of equilibrium in ma- ty the ex- chinery by saying " that what is gained in power is lost in w * gained*''»t^ time." Thus, if a small power acts against a great resistance, the expense of the motion of the latter will be just as much slower than that '™® of the power, as the resistance, or weight, is greater than tha power ; or if one pound be required to overcome the resistance of two pounds, the one pound must move over two feet in the same time that the resistance,, two pounds, requires to move over one. SECTION I. THE ELEMENTS OP MACHINERY. 195. All machines, no matter how complex How many rim- ... , . . , pie machinei and mtricatc their construction, may be re- duced to one or more of six simple machines, or elements, which we call the " Mechanical Powers." , ^ 196. They are the Lever, the Wheel and Enumerate the '' ' Bii elementary AXLE, the PULLEY, the INCLINED PlANE, the Wedge, and the Screw. These simple Machines may be further reduced to three — the lever, the pulley, and the incUned plane ; since the wheel and axle, the screw and the wedge, may be regarded as modifications of them. The name " mechanical powers ' which has been applied to the six ele- mentary machines, is unfortunate, since it serves to convey an idea that they are really powers, when in fact they possess no power in themselves, and aro only instruments for the application of power. What it a 197. A Lever consists of a solid bar, straight Lever » ^^ bent, tuming upon a pivot, prop, or axis. What are the 1^8. The Arms of the IcvcF are those parts. Anns of a Le- ^f ^j^g ^jg^j. extending on each side of the axis. What Is the 199. The Fulcrum, or prop, is the name Fulcrum applied to the axis, or point of support. „ 200. Levers are divided into three kinds, or How many ... kinds ofieveri classcs, accordin^T to the position which the *re there t n ^ ^ • ^ • i luicrum has in relation to the power and the weight. ^4 WELLS'S NATURAL PHILOSOPHY. What are the relative posi- tions of the power, fulcrum and weight in the three kinds of levers i £" Fig. 61. I What are ex- amples of le- vers of the first class t Wc ^ir 201. In the first class the fulcrum is be- tween the power and the weight ; in the sec- ond class, the fulcrum is at one end of the lever, and the weight is between the fulcrum and the power ; in the third class, the fulcrum is at one end of the lever, and the power is between the fulcrum and the weight. Fig. 61 represents the three classes of ^vers, numbered in their order, 1. 2, 3. P is the power, W the weight, and F tho fulcrum. A crowbar applied to elevate a stone, is an ex- ample of a lever of tho first kind. In Fig. 62, which represents a lever of this class, a indicates the fulcrum which suppports the bar, b the power appUed by the hand at the end of the longest arm, and c tho weight, or stone, raised at the end of the short arm. A poker appUed to stir up the fuel of a grate i3 a lever of th© first class, the fulcrum being th« bars of the grate ; the break, or or handle of a pump, is also a fa- miliar example. Scissors, pin- cerg, etc., are composed of two levers of the first kind, the ful- crum being the joint, or pivot, and tho weight tho resistance of the substance to be cut, or seized. The power of the fingers is appUed at tho other end of the levers. What is the 202, A lever will be in equilibrium, when brhim^ o?"the t^s power and the weight are to each other lever? inversely as their distances from the fulcrum. Thus, if in a lever of tho first class the power and the weight are equal, and are required to exactly balance each other, they must be placed at equal distances from tho fulcrum. If the power is only half the weight, it must be at double the distance from the fulcrum; if one third of tho weight, three times the distance. If we suppose, in Fig. 62, c to represent a weight of 300 pounds, placed two feet from the fulcrum a, and b a power of 100 pounds placed six feet fi-om a, then c and 6 will be in equilibrium, for (300X2) = (100X6). 203. When the weight and lengths of the two arms Fig. 62. THE ELEMENTS OF MACHINERY. 95 "Weigh t, and the length of the arms of a lever being given, how wa find the equiv- alent power ? What are ex- amples of le- VTS of the aecond class ? of a lever are given, the power requisite to balance the weight may be ascertained, by dividing the product of the weight multiplied into its distance from the fulcrum, by the dis- tance of the power from the fulcrum. 204. Cork, or lemon-squeezera, Fig. 63, are examples of the levers of the second class, which have the fulcrum at one end, and the weight, or resistance to be overcome, between the fulcrum and the power. An oar is a lever of the second (daas, in which the reaction of the water against the blade is the fulcrum, the Fig. 63. ^o^t tbe weight, and the hand of the boatman the power. A door moved on its hinges is another example. A wheel-barrow is a lever of the second class, the ful- crum being the point at which the wheel presses upon the ground, the barrow and its load the weight, and the hands the power. Nut- crackers are two levers of the second class, the hinge which unites them being the fulcrum, the resistance of the shell placed between them the weight, and the hand the power. What are ex- ^03. A pair of sugar-tongs rep- p^j, 64^ amples of le- resents a lever of the third class, third cufsa ?^* ^ which the power is appUed be- tween the fulcrum and the resist- ance, or weight. In Fig. 64, the fulcrum is at a, the resistance is the piece of sugar to be lifted at 5, and the power is the fingers applied at c. When a man raises a ladder against a wall, he employs a lever of the third class ; the fulcrum being the foot of the ladder resting upon the ground, the power being the hands applied to raise it, and the resistance being the weight of the ladder. 206. In levers of the third class, the power, being between the fulcrum and the weight, will be at a less distance from the fulcrum than the weight ; and, consequently, in this form of lever the power must be always greater than the weight. What is the re- lation betireen the power and the weight in levers of the third class f Thus (in No. 3, Fig. 01), if the length from the point where the weight, "W, is suspended to F bo three times the length of P F, then a weight of 100 pounds suspended at "W will require a power of 300 applied at P to sustain it. 96 WELLS'S NATURAL PHILOSOPHY. Owing to its mechanical disadvantages, this class of levers circumstancL^*^ ^^ rarely used, except where a quick motion is required, rather do we employ than great force. The most striking examples of levers of the thirrclafs ?"'^ third class are found m the animal kingdom. The Umbs of animals are generally levers of this description. The socket of the bono, a, Fig. G5, is the fulcrum ; a strong muscle attached to the bone near the socket, c, and extend- ing to d, is the power ; and the weight of the limb, together with whatever resistance, w, is opposed to its motion, is the weight. A very slight con- traction of the muscle in this case gives considerable motion to the limb. The leg and claws of a bird, are examples of the third class of levers, the whole arrange- ment being admirably adapted to the wants of the animal. "When a bird rests upon a perch, its body constitutes the weight, the muscles of the leg the power, and the perch the fulcrum. Now, the greater the weight of the body, the more strain it exerts upon the muscles of the claws, which, in turn, grasp the perch more firmly: consequently, a bhd sits upon its perch with the greatest ease, and never falls off in sleeping, since the weight of the body is instrumental in sustaining it. 207. A Compound Lever is a combination Compound Le- of scveral simplc levers, so arranged that the shorter arm of one may act upon the longer arm of another. In this way, the power of a small force in overcoming a large resistance is greatly multiplied. Fig. 66. An arrangement of compound levers is shown in Fig. 66. Here, by means of three simple levers, 1 pound may be made to balance 1000; for if the long arm of each of the levers is ten times tlie length of the short one, 1 pound at the end of the first one will exert a force of 10 pounds upon the end of the second one, which will in turn exert ten times that amount, or TOO pounds, upon the end of the third one, which will balance ten times that amount, or 1000 pounds, at the other extremity. THE ELEMENTS OF MACHINERY. 97 TVTiat are the disadvantages of a compound lever ? Describe the common steel- yard. 208. The disadvantasre of a compound lever is, that its exercise is limited to a very small sj)ace. 209. The different varieties of weighing machines are varie- ties or combinations of levers. The common steel-yard is a lever of unequal arms, belonging to the first class. It consists of a bar (Fig. 67) marked with notches to indicate pounds and ounces, and a weight wliich is movable along the notches. The bar is furnished with threa hooks, or ringsj on the lai'gest of which the article to be weighed is always liung. The other hooks serve to support the instrument when it is in use, and the pivot by which they are attached to the bar serves as the fulcrum. The weight, Q, shding upon tlie bar, balances the article, P, which is to be weighed, it being evident that a pound weight at D wiU balance as many pounds at P as the distance A C is contained in the space D C. Tig. 67, It may happen that when the weight Q is moved to the last notch upon the bar B C, that the article P will still preponderate. In tliis case, the steel-yard is held by the hook or ring nearer to A, which hangs down in the figure, and the steel-yard turned over, it being furnished with two sets of notches on opposite sides of the bar. Ey this means the distance of P, the article weighed, from the fulcrum is diminished, and the weight Q, at the given distance upon the opposite side of the fulcrum, wOl balance a proportionally greater resist- ance, or weight. Describe the ^^^' '^^^ Ordinary balance is a lever of the first class, with ordinary bal- equal arms, in which the power and the weight are neces- *"''*■ sarily equal Fig. G8 shows the common form. The fulcrum or axis, is made wedge -like, with a sharp kuife-like edge, and rests upon a 98 WELLS'S NATUKAL PHILOSOPHY. Fig. G8. surface of hardened steel, oi agate, in order that the beam may turn easily. The scale- pans are suspended by chains from points precisely at equal distances from the fulcrum, and being themselves adjusted so as to have precisely equal ■weights, the two sides will perfectly balance when the pans are empty. 211. If the two arms of a scale-beam be not of perfectly equal length, a smaller weight at the end of the larger arm will balance a greater weight at the end of the shorter. An excess of half an inch in the length of the arm of the beam, to which merchandise is attached, where the arm should be eight inches long, would cheat the buyer exactly one ounce in every pound. This fraud, if suspected, might be detected instantly, by transposing the weight and the article balanced ; the lightest would then be at the end of the short arm, and would appear lighter than it actually is. Fig. 69. Under what circumstances ■will a balance indicate false weights ? What is the 212. Platform scales, and scales intended construcu™^ for wciglilng hay, etc., are usually compound scales? levers, and are constructed in very various forms, but all depend on tlie principles above explained. I'ig. 69 represents one of the varieties, and Fig. 70 a sec- FiG. 70. THE ELEMENTS OF MACHINERY. 99 tion of tlie same, showing the arrangement and combination of the levers. 213. "When a lever is applied to raise a ■weight or overcome What Wheel Axle? stances limit ^ resistance, the space through which it acts at any one time the utility of jg small, and the work must be accomplished by a succession the leyer? of short and intermitting eflforts. These circumstances, there- fore, limit the utility of the common lever, and restrict its use to those cases only in which weights are required to be raised through small spaces. ^ 214. When, however, a continuous motion is required, as in How 13 contin- . . . , ^ . nous motioa raising ore from a mine, or in lilting the anchor ot a ship, obtained? j^ order to remove the intermitting action of the lever, and render it continual, we employ the simple machine known as the wheel and axle, which is only another form of the lever, in which the power is made to act without intermission. 215. The form of the simple machine de- and nominated the Wheel and Axle, consists of a cylinder, termed an axle, revolving on an axis, and having a wheel of larger diameter immovably at- tached to it, so that the two revolve with a common motion. Describe the In Fig. 71, A represents j-j^, -ji_ action of the the axle with a wheel im- wheel and niovably attached to it, and axle. •' the wheel turning on pivots inserted into the ends of the axle. Around this axle is wound a rope, to which is at- tached the weight W, and around the wheel is another rope, to which the power, P, is applied. It is evident that one turn of the wheel will unwind as much more rope from the wheel than it winds on the axle, as its circumference is greater. The power, P, will therefore pass over a much greater space than the weight "W. The weight on the axle, which may be considered as acting on the short arm of a lever which is the radius* of the axle, may be much heavier than the power which acts at the long arm of a lover, which is the radius of the wheel. Hence the advantage gained in the wheel and axle is equal to the numbet of times that the radius of the axle is contained iu the radius of the wheel, and to estimate the mechanical advantage gained by the wheel and axle, wo have the following rule : How do wo 216. The power is to the weight, as the advTntal'e of diameter of the wheel is to the diameter of the whoc'l and .i _i axle? the axle. • The radius of a wheel, or cylinrlcr, is its Sfnii-diainoter, or a line drawn from its cen- ter to its circuml'oreace, Tiie spoke of a can-iaije wheel represents its radius. 100 WELLS'S NATURAL PHILOSOPHY. Fig. 12. Fig. 72 represents a section of the wheel and axle, showing the radius of the axle, b c, and the radius of the Avheel, a c. The two being in a straight line, the weights hanging in opposition are always as if they were connected by a horizontal lever, a c b, turning on a fulcrum at c. If the radius of tho wheel, or tho length of the longer arm of the lever, a c, bo 24 inches, and the radius of the axle, or the length of the shorter arm, c 6, be 3 inches, then the advantage gained would be 24-=-3 = 8, and a power of 100 pounds applied to the wheel would balance a weight of 800 ap- plied to the axle. „ . 217. The methods of applying power How do we ap- ^^ •' " *^ ply power in in the Wheel and axle are very various, axle^f '''^'^' ^^^ ^* ^°^ being essential that the power should be applied by a rope. Tlie axle is sometimes placed in a vertical or upright position, and the power applied by means of levers, or bars, inserted into holes FiCt, 73. in one end of the axle. A capstan of a ship, Fig. 73, is an example of this. ^'*'*%s>^^^^,^*'****^ In the windlass, a handle, or winch, is sub- stituted in the place of a wheel. (See Fig. 74.) In this case, tho advantage gained is equal to tho number of times that the length of handle i3 greater than the radius of the axle. Thus, if the handle is 20 inches and the radius of the axle is 2 inches, then the advantage would be 10, and a power of 50 pounds applied at the handle would just raise a weight of 10 times 50, or 500 pounds. When a weight, or resistance, of comparatively great amount is to be raised by a very small power, by means of the simple wheel and axle, cither of two inconveniences would ensue ; either the diameter of the axle would become too small to support the weight, or the diameter of the wheel would becorao BO great as to be unwieldy. This has been remedied by a very simple ar- FlG. 74, rangement, called the double axle. Fig. 74. The axle of the windlass hero consists of two parts of unequal diameters, and the rope winds around them in different direc- tions ; tlierefore, every turn of the wind- lass, cr handle, winds up a portion equal to the circumference of the one, but un- winds a portion equal to the circumference of tlie other, and if the two be nearly equal, tho weiglit moves very slow. If the weight rise 1 inch while the handle describes 100 inches, 1 pound at the handle will balance 100 attached to the rope. In this arrangement space and time are exchanged for power in a most convenient manner. THE. ELEMENTS OF MACHINERY. 101 What is the most frequent method of transmitting motion through ft conibinatioa o; wheels? When great power is required, wheels and axles may be combined to- gether in a manner similar to that of the compound lever already explained (§ 207). By such a combination we gain the advantage of using a very large wheel with a small axle, without their inconveniences. 218. The most frequent metliod of transmitting motion through a combination of wheels, is by the construction of teeth upon their circumference, so that the teeth of each wheel falling between those of the other, the one necessarily pushes forward the other. When teeth are thus afiBxcd to the circumference of a wheel, they are termed cogs; upon an axle, they are termed leaves, while the axle itself is called a pinion. jj, ^- Fig. 75 represents a combination of wheels and axles for the trans- mission of power. If the teeth on the axle of the wheel c act on six times the number of teeth on the circumference of the second wheel, the second will turn only once for every six turns of the first. In the same manner the second wheel, by turning six times, turns the third wheel once ; consequently, if the proportion between the wheels and their axles be preserved in all three, the third turns once, the second six times and the first thirty-six times. Now, as the wheel and axle act in all respects like a simple lever, and a combination of wheels and axles as a combina- tion of levers, there is no difficulty in understanding how a mechanical ad- vantage is gained by this contrivance. The power is to the weight as the product of the diameter of all the axles is to the product of the diameter of all the wheels. Thus, if the diameter of all the axles be expressed by the numbers 2, 3, and 4, and the diameters of the wheels, c, /, and g, be expressed by the numbers 20, 25, and 30, then power will be to the weiyht as 2X3X 4=24, is to 20X25X30=^15,000 ;— or a power of 24 at the first wheel will balance 15,000 at the axle of the last wheel. 219. One of the most familiar instances of combined wheel- work is exhibited in clocks and watches. One turn of the axle on which the watch-key is fixed, is rendered equivalent, by a train of wheel-work, to about 400 turns, or beats, of the bal ance-whcel ; and thus the exertion, during a few seconds, of the hand which winds up, gives motion for twenty-four, or thirty hours. By What arc fa- miliar illustra- tions of com- pound wheel work? FlQ. 76. Fig. 77. as by teeth, turoing-lathe. increasing the number of wheels, time-pieces are made which go for a year, or a greater length of time. "Wheels may be connected and motion communicated from one to the other, by bands, or belts, as well This principle is seen in the spinning-wheel and common A spiuamg- wheel, as a c, Fig. 70, of thirty inches in circum- 102 WELLS'S NATURAL PHILOSOPHY. Fig. 78. ference, turns by its band a smaller wheel, or spindle, b, of half an inch, sixty times for every revolution of a c. When the wheel is intended to revolve in the same direction -^^ith tlie one from which it receives its motion, the band is attached as in Fig. 76; but •when it is to revolve in a contrary direction, the band is crossed, as hi Fig. 77. In many wheels power is communicated by means of a weight apphed to the ch-cumference. In the tread-mill (Fig. 78) a number of persons stepping upon the circumference of a wheel cause it to revolve. Similar maclunes are often adopted in ferry-boats, moved by horses, and called "horse- boats." In most water-wheels, power is obtained by the action of water apphed to the circumference of the wheel, which is caused to revolve, either through the weight, or pressure of the water, or by both conjointly. Nvhat is a 220. TliG PuLLEY IS a Small wheel fixed in Pulley? Fig. 79. How many kinds of pul- leys ftre there ? What is a fix- ed pulley ? Describe the working and advantage of the fixed pulley. a block, and turning on an axis, by means of a cord, which runs in a groove formed on the edge of the wheel. This simple machine is represented in Fig. 79. 220. Pulleys are of two kinds ; — fixed and movable, 221. By a fixed pulley we mean one that merely revolves on its axis, but does not change its place. Figs. 79 and 80 are illustrations of fixed puUeys. In Fig. 80, C is a small wheel turning upon its axis, around which a cord passes, having at one end the power P, and at the other, the resistance, or weight, W. It Fig. 80. is evident tliat by pulling the cord at P, the weight, W, must ascend as much and as fast as the cord is drawn down As, therefore, the power and the weight move with the same velocity, it is clear that they balance one anothei; and that no mechanical advantage is gained. In all the applications of power there are always soma directions in which it may be exerted to greater advai> tage and convenience than others; and in many cases the power is capable of acting in only one particular di- rection. Any arrangement of machinery, therefore, which ^ill enable us to render power more available, by apply- ing it in the most advantageous direction, is as convenient and valuable as one which enables a small power to balance or overcome a © © THE ELEMENTS OF MACHINERY. 103 great weight. Thus, if we wish to apply the strength of a horse to Uft a heavy weight to the top of a building, we should find it a difficult matter to Fig. 81. accomplish directl}', since the horse exerts his strength mamly, and to the best advantage, in drawmg horizontally ; but by changing the di- rection of the power of the horse, by an ar- rangement of fixed pulleys, as is represented in Fig. 81, the weight is lifted most readily, and the horse exerts his power to the best ad- vantage. 223. A fixed pulley is most ^ifr a"pllS: 'useful for changing the direc- tions of fixed tion of power, and for apply- pulleys ? *^ ' mg power advantageously. By it a man standing on the ground can raise a weight to the top of a builduig. A curtain, a flag, or a sail, can be readily raised to an elevation by a fixed pulley, without ascending with it, by draw- ing down a cord running over the pulley. whatisamov- 224. A MoVABLE PuLLEY differs from a able puuey? g^g^j puUej in being attached to the weight ; it therefore rises and falls with the weight. Fig. 82 represents a movable pulley, B, associated, as it Fig. 82. most commonly is, with a fixed pulley, C. The movable pulley, E, is often called a " Runner." 225. In the fixed pulley, Fig. 80, it wUl be readily seen that to move the weight, "W, at one end of the cord, passing over the pulley, a greater weight must be applied at P, for if P is only equal to W, they will balance one an- other. I^ however, we fasten one end of the cord to a fixed support, as at F, Fig. 82, and pass it under the groove in the movable pulley B, to which the weight, W, we desire to raise is attached, and then carry it over the fixed pulley C, we may lift a force of 100 pounds at W by an application of 50 pounds at P. To understand tliis, we must remember that the weight W is supported by the cords B F and B C on each side of the movable pulley B; and as each are equally stretched, the weight must be equally divided be- tween them ; or, in other word.«;, the point of support, F. sustains half tha weight, and the power, P, the other half A person, therefore, puUing at I^ ' will raise the weight by exerting a force equal to its half But the cord at P must move through two feet to raise the weight W one foot. "When still greater power is required, pulleys are compounded into a system containing two more single pulleys, called Blocks, and these again are com- bined in a compound svi^tem of fixed and movable pulleys. A single movable pifiley may be so arranged that the power will sustain three times its own weight Such an arrangement is represented in Fig. 83. What is the ad- vantage gained by tlie use of a movable pul- ley? 104 WELLS S NATURAL PHILOSOPHY. Bi\v is poirer gjiined at the expense of time in a sys- tem of pul- leys? In this we have four cords, one employed in sustaining the FiG. 83. power, P, and the other sustaining the weight; conse- quently the power will be to the weiglit as 1 to 3. In Fig. 84, we have two blocks, each containing two single pulleys. The rope is thus divided into five portions, each equally stretched; one is employed in supporting the power P, and four sustam the weight. With this system a power of 1 will balance a weight of 4. 226. In all these arrangements of pul- leys, the increase of power has been gained at the expense of time, and the space passed over by the power must be double the space passed over by the weight, mul- tiplied by the number of pulleys. That is, in the case of the single pulley, the power must pass over two feet to raise the weight one foot; and with two movable pulleys, as in Fig. 84, the power must fall four feet to raise the weight one foot. Instead of folding the string on the pulleys entire, it ia sometimes doubled into separate portions, each pulley hanging by a separate cord, one end of which is attached to a fixed support. Here a very great mechanical advan- tage is gained, attended, however, with a corresponding loss of time. In an arrangement of such a character, re- presented in Fig. 85, the weight W, is supported by the two parts of the cord passing round the movable pulley, C ; and as each of these parts is equally stretched, the fixed support will sustain one half the weight, and the next pulley in order above C, namely B, may be consid- ered as sustaining the other half. But the two parts of the string which support the pulley B, again divide the weight, so that th -> pulley A, which is attached to one of them, only sustains one quarter of the first weight, W. The string which passes around A again divides this weight, so that each part of it sustains only one eighth of W. The fixed pulley serves merely to change the direction of the motion. In this system, therefore, a power of 1 will balance a weight of 8. 227. In general, the advan- tage gained by pulleys is found by multiplying the number of movable pulleys by two, or by multiplying the power by the number of folds in the rope which sustains the weight, where one rope runs through the whole. Fig. 85. JTow may the .Tflvantige enined by pul- leys be ascer- tained ? THE j:lements of machinery. 105 Thus a -n-eight of 72 pounds may be balanced by four movable pulleys by a T>'eight or power of 9 pounds; with two pulleys, by a power of 13 pounds, with one movable pulley, by a power of 36 pounds. These rules apply only to movable pulleys in the same block, when the parts of the rope which sustain the weight are parallel to each. The mechan- ical advantage which the pulley appears to possess in theory, is considerably diminished in practice, owing to the stiffness of the ropes, and the friction of the ropes and wheels. From these causes it is estimated that two thirds of the power is lost. When the parts of the cord are not parallel, the strength of the pulley is very greatly diminished. matareCranes J'^' J'""'^ "°^ "^^^i ^IG. 86. and Derricks, able pulleys are arranged Tackle and FaU? j^ ^ g^eat variety of forms, but the principla upon which all are constructed is the same. What'is called a " tackle and fall," or " block and tackle,"' is nothing but a pulley. Cranes and derricks are pieces of mechanism usually consisting of combinations of toothed wheels and pulleys, bj^ means of which materials are lifted to different elevations — as goods from vessels to the wharves, building materials from the ground to the stage where the builders are en- gaged, and for similar purposes. One of the most simple forms of movable cranes is represented in Fig. 86. It consists of a strong triangular ladder, at the top of which is a fixed pulley, C, over which the rope attached to the ob- ject to be elevated passes, and is carried down to the cylindrical axle, T, upon which it is wound by means of bars in- serted in holes, or by a crank. This ladder is inclined more or less from the upright position by means of a rope, C D, which is attached to some fixed point at a distance. 229. The Inclined Plane consists of a hard plane surface, inclined at an angle. What is an In- clined Plane ? Illustrate the use of an In- clined Plane. In Fig. 87, a 6 e repre- sents an inchned plane. 230. If wc attempt, for instance, to raise a cask, or any other heavy body into a wagon, we may find that our strength is unequal to lifting it a* Fig. 87. 106 WELLS'S NATURAL PHILOSOPHY. How do we derive a me- clianical ad- vantage from an Inclined plane ? How can we eBtimate the advantage gain- ed by the use of the inclined plane ? directly, while to haul it up by pulleys would be very inconvenient, if not impossible. We may, however, accomplish our object with comparative ease by rolling the cask up an inclined plank, and exerting our force in a direction parallel to the incUned surface of the plank. The plank, in this instance, forms an inclined plane, and we gain a mechanical advantage, because it supports a part of the weight. If we place a body upon a horizontal plane, or surface, it ia evident that the surface will support its whole weight ; if we incline the surface a little, it will support less of the weight, and as we elcvata it more, it will continue to support less and less, until the surface becomes perpendicular, in which case no support will bo afforded. 2;;i. The advantage gained by the use of the inclined plane may be esti- mated by the following rule : 232. The power is to the weight as the per- pendicular height of the plane is to its length. From this it will appear that the less the height of the in- chned plane, and the greater its length, the greater will be the mechanical advantage. Thus, in Fig. 88, if the plane, c d, bo twice as long as the height, e d, FiG. 88. one pound at p, acting over the pulley, would balance two pounds any where between c and d. If the plane, c d, were three times the length of d e, then one pound at p would balance three pounds any where on the plane, c d, and so of all other quantities and proportions. 233. Roads which are not level may be considered as in- clined planes, and the inclination of a road is estimated by the height which corresponds to some proposed length. Thus, we say a road rises one foot in twenty, or one in fifty, mean- ing that if twenty or fifty feet of the road be taken, as the length of an in- clined plane, the corresponding height of such a plane would be one foot, and the difference of level between the two extremities of such a length of road would be one foot. According to this method of estimating the inclination of roads, the power required to sustain, or draw up a load, fric- tion not considered, is always proportioned to the rate of ele- vation. On a level road, the carriage moves when the horse exerts a strength BufQcicnt to overcome the friction and resistance of the atmosphere ; but in going up a hill, where the road rises one foot in twenty, the horse, beside these impediments, is obliged to exert an extra force in the proportion of one to twenty, or, in other words, he is obliged to lift one twentieth of the load. It is, therefore, bad policy ever to construct a road directly over the summit of a hill, when it can be avoided, because, in addition to the force necessary How do we es- timate the in- clination of roads f IIow ought roads to be constructed ? THE. ELEMENTS Or MACHINERY. 107 to overcome the friction in drawing a heavy load up the steep incline, we must add additional force to overcome the gravity, which acts parallel with the inchned plane of the road, and tends constantly to make the load roll back to the bottom of the slope. This force increases most rapidly with the steepness, and consequently requires an immense expenditure of power. An equal power expended on a road gently winding round the hill, with an increase of speed, would gain the same elevation in much less time. An inteUigent driver, in ascending a steep hill on which there is a broad road, winds from side to side, since by so doing he diminishes the abruptness of the ascent (the plane being made longer in proportion to its height), and thus favors the horses. Our common stairs are inclined planes, the steps being merely for the pur- pose of obtaining a good foot-hold. 234. In the inclined plane, as in all other simple machines, a gain in power is attended \Yith a corresponding loss of time. A body, in ascending an inclined plane, has a greater space to pass over than if it should rise perpendicularly. The time, therefore, of its ascent will be greater, and it will thus oppose less resistance, and consequently require less power. What Is a 235. The Wedge is a movable Wedge? inclined plane. It is also defined to be two inclined planes united at their bases, as A B, Fig. 89. In the inchned plane, the weight moves upon the plane, which remains stationary ; but in the wedge, the plane itself is moved under the weight. 236. The cases in which wedges are most In what cases „ , . , , . , . , are Wedges generally used m the arts, are those m which ns(!d ia the j^^ intense force is required to be exerted through a very small space. It is, therefore, used for splitting masses of wood, or stone, for blocking up buildings, raising vessels in docks, and pressing out the oil from seeds. In this last instance, the seeds are placed in bags, between two surfaces of hard wood, which are pressed together by wedges. How is power gained at the expense oftime in the inclined plane ? 237. The usefnlness of the wedge depends on friction ; for if there were no friction, the wedge would fly back after each stroke of the driving force. 238. The power of the wedge increases as the length of its back, compared with that of its sides, is diminished. Hence, it follows that the power of the wedge is in proportion to its sharpness. The power commonly used in the case of the wedge, is not pressure, but percussion. Its edge being inserted mto a fissure, the wedge is driven in by Upon what does the influ- ence of the Wedge de- pend? How does the power of the Wedge ia. crease? 108 WELLS'S NATURAL PHILOSOPHY. What are fa- miliar exam- ples of tliL' use or applicatioa of the Wedge in the arts ? Pig. 90. What is the Thread of a Screw ? blows upon its back. The tremor produced when the wedge is struck with a violent blow, caus3S it to insinuate itself much more rapidly than it other- wise would. 239. The edges of all cutting and piercing instruments, such as knives, razors, chisels, nails, pins, etc., are wedges. The angle of the wedge in all these cases is more or less acute, according to the purpose to which it is applied. Chisels intended to cut wood have their edges at an angle of about 30° ; for cutting iron from 50° to 60°, and for bra.ss about 80° to 90°. In general, tools which are urged by pressure admit of being sharper than those which are driven by percussion. The softer, or more yielding the sub- stance to be divided is, the more acute the wedge may be constructed. Avhat is the 240. The Screw is an inclined plane wind- screwf ^jjg round a cylinder. This may be illustrated by cutting a strip of paper in such a way as to represent an in- clined plane, and then winding it round a cylinder, or common lead pencil, as is repre- sented in Fig. 90. 241. The edge of the inclined plane winding about the cylinder, or the coil of the spiral line which it describes upon the cylinder, con- stitutes the Thread of the screw, and the distance between the suc- cessive coils is called the distance between THE THREADS. The screw, surrounded by its spiral line is represented in Fig. 91. The screw is not applied directly to the resistance to be FiG. 91. overcome, as in the case of the inclined plane and wedge, but the power is transmitted by means of what is called the NuT. What is the 242. The Nut of a screw is a Nutofascrewf ijjock, with a Cylindrical cavity, having a spiral groove cut round upon the Burface of this cavity corresponding with the thread of the screw. In this groove the thread of the screw will move by causing the screw to rotate. Each turn of the screw in the nut will cause it to advanco or recede a distance just equal to the interval between the threads. Is the Screw, Generally, the nut is stationary and the screw movable, but movaWe?^"'' *^® ^^^ ™^^ ^® movable, and the screw stationary. THE .ELEMENTS OF MACHINERY. 109 How is power ^'*^' ^°'^®'" ^^ commonly applied to the screw by means of appUed to the a lever, either attached to the nut, or to the head of the screw, Screw ? ^ ggpjj Jq Yig. 92. By varying the length of this, the power may be indefinitely increased at the point of resistance. The screw, there- fore, acts with the combined power of the lever and the inclined plane. Thus, in Fig. 92, /d is the lever, c the nut, a d the screw, and e the block upon which the substance to be pressed is placed. As in all the other simple machines, the advantage in this ia estimated by the relative distances passed over by the power and the weight. If the distance of the spiral threads of the screw is 1 inch, and the handle of the screw, that is the lever, is 2 feet in length, then the extremity of the lever will describe a circle of over 12 feet in turning once round, but the screw will only advance 1 inch. The ratio between the power and the weight will be, therefore, as 1 inch to 1 2 feet, or as 1 to 144. Consequently, if a man is capable of exerting a force of 60 pounds at the end of the lever, the screw will ad- vance with a force of 8,640 pounds. If the distance of the threads had been i an inch, the power exerted by the screw would have been doubled. In this illustration friction has not been taken into account ; this will diminish the total effect nearly one fourth. How is the ad- 244. The advantage gained by the screw is by'ttfe^ scl°ew ^^ proportion as the circumference of the circle, estimated? described by the power (that is by the handle of the lever) exceeds the distance between the threads of the screw. Hence the enormous mechanical force exerted by the screw is rendered evident. There is no limit to the smallncss of the distance between the threads except the strength it is necessary to give them ; and there is no limit to the magnitude of the circumference to be described by the power, except the necessary facOity for moving it. FiG. 93. ■nn,»i. „ f 245. The screw is what are fa- miliar applica- generally used where Sct!w?^ ^^^ great pressure is to be exerted through small spaces ; hence its application in presses of ail kinds; for extracting the juices of seeds and fruits, in compressing cot- ton, hay, etc., as also for coining and punching. For the two latter opera- tions it is caused to act with enor- 110 WELLS'S NATURAL PHILOSOPHY. Fig. 94. Describe the construction and advantage of Hunter's Screw. Fig. 95. mous energy by means of tlie momentum of two heavy balls attached to the end of a long lever, or handle, as is represented in Fig. 93. A force of sev- eral tons may thus be applied at one ellbrt. When the thread of a screw ^dlesslcr^w f ^^^ks in the teetii of a wheel, as is shown in Fig. 94, it constitutes what is called an endless screw. Such a con- trivance is oftentimes a very convenient method of applying power. 246. The efficacy of a screw increases with the fineness of the thread ; but a practical limit is soon attained, for if the thread be made too fine, it will become weak, and be liable to be torn off. To obtain an indefinite increase of the strength of the screw ■without diminishing the strength of the thread, we have a contrivance known as " Hunter's screw," rep- resented in Fig. 95. It consists of a screw, A, work- ing in a nut. To a movable bottom-board, D, a sec- ond screw, B, is affixed. This second screw works in the interior of A, which is hollow, and in which fe corresponding thread is cut. When, therefore, A is screwed downward, the threads of B pass upward, and the movable piece, D, urged forward by the screw •^vhich has the greater thread, it is drawn back by that •which has the less ; so that during each revolution the screw instead of being advanced through a space equal to the breadth of either of the threads, moves through a space equal to their difference. Suppose the distance between the threads of A to be l-20th of an inch, and of B 1-2 1st of an inch ; then in turning the screw A once, the board D will descend a distance equal to the difference between l-20th and l-2l3t, or the l-420th of an inch. Hence, if the circle described by the han- dle be 26 inches while the screw advances l-420th of an inch, the power will be to the weight as 1 to 8,400. 247. All macliines, however complicated, are made up of combinations of the six simple machines. If we examine the construction of any complex ma- chine, as a steam-engine, a loom, a spinning machine, or a time-piece, we shall find that they are composed of simple levers, wheels and axle.« screws, etc., connected together in an endless variety of forms, to form a complete whole. In the practical application of machinery, it rarely or never happens that the moving force is capable of producing directly, the particular kind of motion required by the machine to per- form the work to which it is adapted. Expedients must therefore be resorted to, by means of which the motions which the moving Is the moving force in ma- chinery ap- plied directly ? How many- kinds of mo- tion are con THE, ELEMENTS OF MACHINERY. Ill power is capable of exerting directly can be converted into those which aro necessary for the purposes to whicli the machine is applied. 248. The varieties of motion wliich occur in machinery are divided into two classes, viz. : chinef y""""" KoTARY and Kectilinear Motion. wuntisRota. 249. In Rotary Motion, the several parts ry Motion? rgvolve round an axis, each performing a com- plete circle, or similar parts of a circle, in the same time. 250. In Rectilinear Motion, the several parts What is Rec- , . ^^ ^ ,. • \ j. tiiinear Mo- of a mQvmg Dody proceed in parallel straignt lines Avith the same speed. Examples of rotary motion are seen in all kinds of wheel work, and exam- ples of rectihnear motion in the rod of a common pump, the piston of a steam- engine, the motion of a straight saw. „„ .. „ . In rotary and rectilinear motion, if the parts move con- WhatisRecip- •' . • n j • i rocating Mo- staHtly in the same direction, the motion is called contumed *■""*' rotary, or continued rectUinear motion. If the parts move alternately backward and forward in opposite directions, passing over the same spaces from end to end continually, the motion is called reciprocating motion. How are rota- 251. The method by which a power having one of these ry and recipro- motions may be made to communicate the same or a different conv7rted°ia'to kind of motion, involves a lengthy description of a great each other? variety of machinery; butlhe most simple and common plan of converting rotary motion into rectilinear, and rectihnear motion back agaui into rotary, is by means of what is called a Crank. y^^f. is a 252. The Crank is a double winch, or han- crank? ^^e, and is formed by bending an axle so as to form four right angles, facing in opposite directions. It is represented complete in Fig. 96. Attached „ to the middle of C D, by a joint, G, is a rod, H, which is the means of imparting power to the crank. This rod is driven by an alternate motion, like the brake of a pump. The bar C D is turned with a — ^ circular motion round the axle A F.* ~~~ .^^ ^ ,, . The disadvantage attending the What disad- " " vantages at- use of the crank is, that it is incapa- ofthe crlnkT ^^® ^^ transmitting a constant force to the resistance. This is illustrated in Fig. 97. In No. 1, * The terms axis, axle, arbnr, and shaft, in mpohanics, are generally understood to mean the bar, or rod, which passes throu2:h the center of a wheel. A gudgeon is the pin, or support, on which a horizontal shaft turns ; the plus upon which an upright shaft turns are called pivots. 112 WELLS'S NATURAL PHILOSOPHY. Fig. 97. •where the arm of the crank is horizontal, the power from the rod acts with tlie greatest advantage, an at the extremity of a lever. But when the rod which communicates motion stands perpendicular with the arm of the crank, as in No. 2, which is the case twice during every revolution, the power, however great, can exert no effect upon the resist- ance, the whole force being expended in producing pressure upon the axle and pivots of the crank. Such a situation of the rod and the arm of the crank is called the dead point, and when the ma- hinery stops, as is often the case, it is said to bo "set," or "caught on its center." The difficulty is generally overcome by the employment of a fly- wheel (§ 21), which, by ita inertia, keeps up the motion. SECTION II. FRICTION. What propor- 253. TliG most sGrious obstacle to the per- ma°h&" is fection of machinery is Friction ; and it is lost by friction ? ^^gyally consiclcred to destroy one third of the power of a machine. 254. Friction is of two kinds : shding and kinds of"fnv rolling. Sliding friction is produced hy the ticn are there ? , . , . , . ^ /> ^ , sliding, or dragging ot one surlace over another ; rolling friction is caused by the rolling of a circular body upon the surface of another.* Friction increases as the weight, or pressure increases, aa tioTincrease ?' ^^"^ surfoces in contact are more extensive, and as the rough- ness of the surfaces increase. With surfaces of the samo material, friction is nearly proportional to the pressure. Friction diminishes as the weight or pressure is less, as the tioTdimhifsh ? polish or smoothness of the moving surfaces is more perfect, and as the surfaces in contact are smaller. It may also be diminished by applying to the surfaces some unguent, or greasy material: oils, tallow, black-lead, etc., are commonly used for this purpose ; they dimin- ish friction by filling up the minute cavities and smoothing the irregularities that exist upon the surface.* Oils are the best adapted for diminishing the friction of metals, and tallow the friction of wood. • All hollies, however much they may he polished, appear rough and uneren when examined with a microscope. FRICTION. 113 _, ^ .. 255. Friction, altlious^i an obstacle in the workiner of ma- What are the , & • advantages of chinery generally, is not without some advantages. "Without frictionr friction, the Stones and bricks used in building would tend to fall apart from one another. When nails and screws are driven into bodies, ■with a view of holding thera together, it is friction alone that maintains them in their places. The strength of cordage depends on the friction of the short fibers of the cotton, flax, or hemp, of which it is composed, which prevents them from untwisting. In walking, we are dependent on friction for our tiothold upon the ground: the difficulty of walking upon smooth ice illus- trates this most clearly. Without friction we could not hold any body in the hand; the difficulty of holding a lump of ice is an example of this. Without friction, the locomotive could not propel its load ; for if the the of the driving wheel and the rail were both perfectly smooth, one would sHp upon the other without affording the requisite adhesion. ^ , . 256. Experiments seem to show that the friction of two How does fnc- ^ / , tion between surfaces 01 the same substance is generallj- greater than the the same and friction of two unlike substances. The friction of polished different sub- -"^ Btances com- steel against polished steel, is greater than that of polished P^'®' steel upon copper, or on brass. So of wood and various other metals. 257. For transporting very heavy timbers, or large castings, ^eel"^ used 'vvheels of great size are used, as by their use the weight is for transport- moved with greater facility, and the roughness of the road ▼ei<'hts? ^^^ more easily overcome than with small wheels. The reason of this is, that the large wheels bridge over the cavities of the road, instead of sinking into them ; and in surmounting an obstacle, the large circumference of the wheel, causes the load to rise very gradually. The resistance of sliding friction is much greater than that of rolling fric- tion. In the wheel of a carriage there is rolling friction at the circumference of the wheel, but sliding friction at the axles. In a locomotive, the so-called driving wheels are turned by the force of the steam-engine ; the whole car- riage rolls on in consequence of this rotation ; for if the locomotive were to remain at rest, the wheels could not revolve without sliding on the rails, and overcoming a great amount of shding friction ; but by rolling, the wheels have only the much smaller rolling friction to overcome. The machine, therefore, moves onward, this being the direction in which its motion will experience the least resistance. The load which a locomotive is capable of drawing depends, not only upon the force of its steam power, but also upon the weight of the engine, or, in other words, upon the pressure of the driving wheels upon the rails, the fric- tion increasing with the pressure. If we assume that two locomotives have equally strong engines, but that one is heavier than tiio other, a greater •weight will be propelled by the heavier of the two. Friction is generally resorted to as the most convenient method cf retard- ing the motion of bodies, and brinpring them to rest. The different modifica- tions of machinery employed for this purpose are termed Brakes. 114 WELLS'S NATURAL PHILOSOPHY. PRACTICAL PROBLEMS IN MECHANICS. 1. What must be the horse-power of a locomotive engine which moves at the constant Bpced of '25 miles per hour, on a level track, the weight of the train being 60 tons, and the resistance from friction being equal to 430 pounds ? "^ W ^' - 2. If a lever, twelve feet long, have its fulcrum 4 feet from the weight at one end, and this weight be 12 pounds, what power at the other end will balance? 3. In a lever of the first class a power of 20 at one end balances a weight of 100 at the other : what is the comparative length of the two arms ? 4. In a lever of the first class, 6 feet in length, the power is 75, and the weight 150 pounds : where must the fulcrum be placed in order that the two may balance 1 5. Two persons carry a weight of -00 pounds suspended from a pole 10 feet long; one of them being weak can carry only 75 pounds, leaving the rest of the load to be carried by the other : how far from the end of the pole must the weight be suspended ? 6. A lever of the second class is 20 feet long : at what distance from the fulcrum must a weight of SO pounds be placed in order that it may be sustained by a power of 60 pounds ? 7. In a lever of the third class, 8 feet long, what power will be required to balance a, weight of 100 pounds, the power being applied at a distance of 2 feet from the fulcrum ? 8. A power of 5 pounds is required to lift a weight of 20, by means of the wheel and axle: what must be the proportionate diameters of the wheel and axle? 9. A power of 60 acts on a wheel 8 feet in diameter : what weight Buspended from a rope winding round an axle 10 inches in diameter will balance this power? 10 In a set of cog-wheels the diameters of wheel and axle are, first T and 2, Gccond 8 and 1, third 9 and 1 : a power of 25 being applied at the circumference of the first wheel, what weight will be sustained at the axle of the third ? 11. What weight will a power of 3 sustain with a system of 4 movable pulleys, one cord passing round all of them ? 12. Suppose a power of 100 pounds applied to a set of 2 movable pulleys, what weight will it sustain, allowing a deduction of two thirds for friction ? 13. If a man is able to draw a weight of 200 pounds up a perpendicular wall 10 feet high, how much will he be able to draw up a plank 40 feet long, sloping from the top of the wall to the ground, no allowance bei:ig made for friction ? Solution. — In this the height (10) is to the length (40) as the weight (200) is to the re- quired weight. 14. If a man has just strength enough to lift a cask weighing 106 pounds perpendicu- larly into a wagon .3 feet high, what weight could he raise by means of a plank 10 feet long, with one end resting upon the wagon, and the other on the ground? 15. The length of a plane is 12 feet, the height is 4 feet : what is the proportion of tha power to the weight to be raised ? 16. The distance between the threads of a screw beinc; half an inch, and the circumfer- ence described by the power 10 feet, wh»t proportion will exist between the power and the weight ? Sol'Uion. — The power will be to the weight as h;ilf an inch, the distance between the threads, is to 10 feet (240 half inches), the circumference described by the power =1 to 240. 17. A power of 20 pounds acting at the end of a lever attached to a screw describes a circle of 100 inches: whnt resistance will the power overcome, the distance between the threads of the screw being 2 inches ? CHxiPTEE VII. ox THE STRENGTH OP MATERIALS USED IN THE ARTS, AND THEIR APPLICATION TO ARCHITECTURAL PURPOSES. SECTION I. ON THE STRENGTH OF MATERIALS. rponwhatdoes 258. When materials are employed for I'^materi'^'def niechanical purj)oses, their power, or strength, pend? f^^J. resisting external force, apart from the na- ture of the material, depends upon the shape of the material, its bearing, or manner of support, and the nature of the force applied to it. Under what cir- 259. A beam, or bar, will sustain the greatest rbcanTsustTi!! appHcatiou of force, when the strain is in the force f '"'''" direction of its length. 260. The strongest of all metals for resisting tension, or a BtrenRth o^f dif- direct pull, is iron in the condition of tempered steel. The fereiit substan- strength of mctals is affected by their temperature, being * ^ dimiuislied, in general, as their temperature is raised. Wood of the same kind is subjected to very great variations of strength. Trees that grow in mountainous or windy places, have greater strength than those which grow on plains ; and the different parts of a tree, such as the root, trunk, and branches, possess different degrees of strength. Cords of equal thickness are strong in proportion to the fineness of their strands, and also to the fineness of the fibers of these strands. Ropes which are damp, are stronger than those which are dry ; those which are tarred than the un- tarred, the twisted than the spun, the unbleached than the bleached. Other things being equal, a rope of silk is three times stronger than a rope of flax. How does the 261. Of two bodics of similar shape, but of afffct'^ Hs''"'^^ different sizes, the larger is proportionably the Ktr^ngth? weaker.* • A knowledge of the strength of various materials in resisting the action of forces ex- erted in diflferent directions, is of prent importance in the arts. In the following tables are collected the results of the most recent and extensive expcTiraents upon this subjf^ct. The bodies subjected to czperimeat are supposed to be in the form of long rods, the cross- 116 WELLS'S NATURAL PHILOSOPHY. In what posi- tion is a rec- t;ingular beam the strongest ? That a large body may have the proportionate strength of a smaller, it must contain a greater proportionate amount of material ; and beyond a certain limit, no proportions whatever will keep it together, but it will fall to pieces by its own weight. This fact limits the size, and modifies the shape of most productions of nature and art — of trees, of animals, of architectural or mechan- ical structures. 262. The strength of a rectangular beam, or a beam in the form of a parallelogram, when its narrow side is horizontal, is greater than when its broad side is horizontal, in the same proportion that the width of its broad side is greater than the width of its narrow side. Hence, in all parts of structures where beams are subjected to transverse strain, as in the rafters of roofs, floors, etc., they are always placed with their naiTow sides horizontal, and their broad sides vertical. section of which measures a square inch ; in the second column is given the amount of breaking weights, which arc the measure of their strcngtli in resisting a direct pull. Name. lbs. i 1st. Metals ; — Steel, tempered from 114794 to 1.53471. Tin, cast from Iron, bar. — plate, rolled.. — wire — • Swedish mal- leable — English do. . — cast Silver, cast Copper, do — hammered. Brass, cast — wire — plate Gold Tin 531S2 — 84G11 530-:0 5ST3C —112905 72064 55S7-3 IG'243 — 194G4 40997 20.'320 — 373S0 37770 — 899G3 17947 — 1947-2 47114— 5S931 6-2-240 204;)0 — 65237 3228 — 6GGG Metals ; — Tin, cast. . . . Zinc Lead, wire. . 2d. Woods ;— Teak Sycamore. . . Beech Elm Larch Oak Alder Box Ash Pine Fir 4736 '.-820 2&43 to 12915— 1M05 9630 12225 9720 — 15040 10240 103G7 — 25851 114.53 — 21730 14210— 2404.S 13480- 23455 10038 — 149G5 6991 — 12870 The following table shows the average weights sustained by wires of different metals, each having a diameter of about one twelfth of an inch ; Lead 27 pounds. Tin .34 Zinc 109 " Gold 150 " Silver 187 pounds. Platinum 274 " Copper 303 " Iron 549 " Cords of different materials, but of the same diameter, sustained the following weights : Common flax 1175 pounds. I New Zealand flax 2380 pounds. Hemp 1633 " | SUk 3400 The following table shows the weights necessary to crush columns or pillars composed of different metals; the numbers expressed in the second column being the total crush- ing weight in lbs. per square inch : Nnme. lbs. lbs. 2d. Woods:- Oak. from 38G0 to 514T Pine " 1928 Elm " 1284 3d. Stones :— Granite " ^70 Sandstone " 2556 Brick, weU baked " 1092 Nun lbs. lbs. 1st, Metals: Cast iron from 11,5813 to 177776 Bra^s, fine " 1G4SG4 Copper, molten " 11 7088 — hammered. " 10S040 Tin, molten " 1,^456 Lead, molten " 7728 ON THE STRENGTH OF MATERIALS. 117 The strength of a structure depends, in a very great degree, on the manner in which the several parts are joined together, and by a skillful combination, or interlocldng, verj- weak and fragile materials may be made to resist the action of powerful forces. Examples of this occur in the manufacture of ropes, strings, thread, etc. : in the weaving of baskets, and especially in the structure of cloth ; in this last instance, a series of parallel threads called the jijQ gg woof, is made to interlock with another series of threads called — ^D-T n^'T^M ^"^'^^'"T *^® warp, running transversely — '[— nTprTZJ~piI~rJ~rL |_LL||_iL|[-. across, and passing alternately "iNf 1^ ^TT In^ In^ i^:l^iHr ^^^'' ^^'^ under the first series. 7P1 1^. 1^ i^ 1^. I^rponlf Fig. 98 represents the appear- j]^£|~irpr]^J~bt]~^^ ance of a piece of plain cloth itpH^ nprx JTT- Inr In^' 'n~hT^ seen through the microscope; dtjULpOp brgprT. iTpJinl ~ the alternate mtersections of ■ 11 II n II II iml II iririml ii ilii the threads are seen in the •=s%*^Sn«.*:^«^«.«,^=^ «/i=^..-^^-.-« « <^ lower figure, the dots repre- Bcntmg the ends of the warp threads, and the cross line the wooC 263. When a single beam can not be found deep enough to have the strength required in any particular case, several beams may be joined together, in a variety of vrays, so that very great strength is obtained without a very great increase of bulk. Such methods of joining timber are known as scarfing and interlocking, tonguing, dovetailing, mortis- ing, etc. 'OJ Fig. 99. 264. Scarfing and interlocking is the methoi ing and inter- of inscrtlou iu whicli the ends of pieces over- locking lay each other, and are indented together, so as to resist the longitudinal strain by extension, as in tie bearers and the ends of hoops. (See Fig. 99.) 265. Tong-uino; is that method of inbcrtion in which the 118 WELLS'S NATURAL PHILOSOPHY. What Is tonguin" ? ■^^'^lat is dove- tailing? which the edges of boards are wholly, or partially received by channels in each other. 266. Dovetailing is a ^lo- loo. method of insertion in parts are connected hy ^^^g^r ivedge-sliaped indentations which per- mit them to be separated only in one direction. (See Fig. 100.) What is mor- 267. Mortising is a method of insertion in usjng? which the projecting extremity of one timber is received into a perforation in another. (See Fig, 101.) The Fig. 101. opening or hole cut in one piece of wood to re- ceive or admit the pro- jecting extremity of an- other piece, is called a mortise ; and the end of the timber which is re- duced in dimensions so as to be titted into a mor- tise, for fastening two timbers together, is called a tenon. 268. The form in which a given quantity of matter can be arranged in order to oppose the greatest resistance to a bending force, is that of a hollow tube, or cylinder ; and the strength of a tube is always greater than the strength of the same quantity of matter made into a solid rod. The most beautiful and sti'iking illustrations of this princi- ple occur in nature. The bones of men and animals are hol- low, and nearly cylindrical, because they can in this form, with the least weight of material, sustain the greatest force. The stalks of numerous species of vegetables, especially the grain-bearing plants, as wheat, rice, oats, etc., which are required to bear the weight of the ripened ear of grain, or seed, are hollow tubes, and their strength, compared with their lightness, is most remarkable. In this form they not only sustain the crush- ing weight of the ear which they bear at the summit, but also the force of the wind. Iq the construction of columns for architectural purposes, especially those made of metal, this principle is taken advantage of* • In that most gigantic work of modem engineering, the Britannia Tubular Bridge. 'wm In what form can a given quantity of matter be ar- ranged to op- pose the jjreat- est resistance ? What are il- lustrntions of this principle ? MATERIALS FOR ARCHITECTURAL PURPOSES. 119 269. A beam, supported at its two ends, when bent bv its "Why IS a beam . , . , . , ,, , . ,.,.,• i i , ' • beut in the weight m the middle, has its habihtj' to break greatly in- r^'f '^'"k ,^'**''^ creased, because the destroying force acts with the advantage of a long lever, reaching from the end of the beam to the cen- ter ; and the resisting force or strength acts only with the force of a shart lever from the side to the center ; at the same time, a httle only of the beam on the under side is allowed to resist at alL Tliis last circumstance is so remarkable, that the scratch of a pin on the under side of a beam, resting as here supposed, will sometimes suffice to begin the fracture. SECTION II. APPLICATION OF MATERIALS FOR ARCniTECTCRAL OR STRUCTURAL PURPOSES. whatisArchi- 270. Architecture, in its general sense, is the " "^^ art of erecting huilJings. In modern use, the name is often restricted to the external forms, or styles of buildings. ltd '^^^® diflferent varieties of architecture undoubtedly owe theur the different origin to the rude structures which the cUmate or materiiils of architecture "' ^^^ countrj' obliged its early inhabitants to adopt for tempo- ppjbablj' owe rary shelter. These structures, with all their prominent fea- eir origin lures, have been afterward kept up by their refined and opulent posterity. Thus the Egyptian slj'le of architecture had its origin in the cavern, or mound. The Chinese architecture is modeled from a tent ; the Grecian is modeled from the wooden cabin ; and the Gothic, it has been sug- gested, from the bower of trees. On what does 271. Tho Strength of a building will princi- abniidinfprhf- P^^lj dopcud ou the walls being laid on a good cipaiiydepend? j^^j ^^^^ foundation, of Sufficient thickness at the bottom, and standing perfectly perpendicular. Its usefulness will depend upon a proper arrangement of it3 parts. crossing the Menai Straits, which separate the island of Anfrlesea from the mainland of Great Britain, advantage has been taken of the strength of matter arranged in the form of a tube or hollow cylinder. The entire bridge is formed of immense rectangular tube* •f iron, 56 feet hicjh in the center, 14 feet wide, and having an entire length of 1513 feet, with an elevation above the water of more than 100 feet. The sides of the tubes are also composed of smaller tubes, united together in a peculiar maimer, so as to obtain the maximum of strength from the form of structure; and so great is this strength, that a train of loaded cars, weighing 2S0 tons, and impelled with great velocity, deflects the tubes in their centers less th.in three fourths of an inch. The entire weight of the tubes composing this bridge is upward of lO.,50O tons, the length of two of the spans, or dist.ances between the points of support, being -tGO feet each. The same amount of iron in the fbna of a solid rod or beam, would not probably have sustained its own weight. 120 WELLS'S NATURAL PHILOSOPHY. _ 272. A PILE, in architecture and engineei- Whatisapile? . . t i p i ing, IS a cylinder of wood or metal pointed at one extremity, and driven forcibly into the earth, to serve as a support or foundation of some structure. It is gen- erally used in marshy or wet places, where a stable found- ation could not otherwise be obtained. ,„, . In constructing columns for the support of the various parts ii.nns support- of a building, or of great weights, thej are made smaller at 1 'f-^er ^T^t'he *^® *'*^P *'''^° ^* ^^^^ bottom, because the lower part of the b'lttnm than column must sustain not only the weight of the superior part, "^ °^ but also the weight which presses equally on the whole column. Therefore the thickness of the column should gradually decrease from bottom to top. What is an 273. An ARCH is a concave or hollow struct- arch? ^^j.g^ generally of stone or brick, sui^ported by its own curve. The base of an arch is supported by the support upon which it rests, while all the other parts constituting the curve are sustained in their positions by their mutual pressure, and by the adhesion of the cement interposed between their surfaces. A continued arch is termed a vault. .„, . , An arch is capable of resisting a much greater amount of Why is anarch , , . , , stronger than pressure than a horizontal or rectangular structure constructed a horizontal qJ- ^j^g same materials, because the arrangement of the mate- structure ? , rials composmg the arch is such, that the force which would break a horizontal beam or structure is made to compress all the particles of the arch alike, and they are therefore in no danger of being torn or overcome separately. 2H- The vertical wall which sustains the base of an arch abutment? 13 termed an abutment: when there are two contiguous arches, the intermediate supporting wall is called a pier. A beautiful application of the principles of the arch exists lustrations of ^ the human skull, protecting the brain. The materials are the principles jjej-e arranged in such a way as to aftbrd the greatest strength with the least weight. The sliell of an egg is constructed upon the principle of the arch ; and it is almost impossible to break an egg with the hands, by pressing directly upon its ends. A thin watch-glass, lor the same reason, sustains great pressure. A dished or arched wheel of a carriage is many times stronger to resist all kinds of shocks than a perfectly flat wheel. A full cask may fall without damage, when a strong square box would be dashed to pieces. What is an 275. By an order in architecture we under- order in archi- , -t .• ij? • ii lecture? Stand a certain mode oi arranging and decor- MATERIALS FOR ARCHITECTURAL PURPOSES. 121 ating a column, and the adjacent parts of the structure which it supports or adorns. How many or- 276. Fivc orders are recognized in architec- fe^Jturr ""^"are ^^^6 — the Doric, lonic, and Corinthian, de- there? rived from the Greeks ; to these the Komans added two others, known as the Tuscan and Composite. What is a Pi- 277. A Pilastcr is a square column gener- lasterf g^jjy. ^^^ within a wall, and not standing alone. What is a For- 278. A Portico is 'd contiuued range of col- umns, covered at the top to shelter from the weather. What are Bai- 279. Balustcrs are small columns, or pillars ustersf q£ wood, stone, etc., used in terraces or tops of buildings for ornament ; also to support a railing. When continued for some distance, they form a balustrade. Into what two 280. Au ordcr, in architecture, consists of ^d™r''fnVrch^ ^^^ principal members — the column and the tecture divided? eutabluture — each of which is divided into three principal parts. 281. The Entablature is the horizontal con- tinuous portion which rests upon a row of columns. Into how many It is divided into the architrave, which is the lower part of parts is the En- ^j^q Entablature : the frieze, -which is the middle part ; and tablature di- . '^ ' vided ? • the cornice, which is the upper, or projecting part. 282. The column is divided into the base. Into how many i i /. i i • i parts is the the shaft, and the capital. column divided ? ,• n The base is the lower part, oistinct from' the shaft ; the shaft is the middle, or longest part of the column ; the capital is the upper, or ornamental part resting on the shaft. The height of a column is always measured in diameters of the columa Itself; taken at the base of the shaft. Thus we say the height of the Doric column is six times its diameter, and the height of the Corinthian, ten diam- eters. Fig. 102 represents the various parts of an order of architecture. What is the 283. The Fa9ade of a building is its whole Facade of a •Trnnf Building? ironr. Architecture ought to be considered as a useful, and not as a fine art. It is degrading the fine arts to make tliem entirely subservient to utihty. It is out of taste to make a statue of Apollo hold a candle, or a fine 6 What is the Entablature ? 122 WELLS'S NATURAL PHILOSOPHY. painting stand as a fire-board. Our houses are for use, and architecture is, therefore, one of the useful arts. In building, we should plan the inside first, and then the outside to cover it. It is in bad taste to construct a dwelling- house in the form of a Grecian temple, because a Grecian temple was intended for external worship, not for a habitation, or a place of meeting.* Fig. 102. Entablature. . . Cornice. . Frieze. .Architrave. .Capital, .Shaft Stylobate, or Pe- ^ den"quantity the bottoiu aud sldcs of a vessel which con- pL'^wUh^us tains a liquid, is always greater than the weight? weight of the liquid. In a cubical vessel, for example, the pressure upon the bottom will be equal to the weight of the liquid, and the pressure on each of the four sides will be equal to one half the weight ; consequently the whole pressure on the bottom and sides will be equal to three times the weight of the liquid. inwhatcondi- 301. The surfacc of a liquid when at rest is tion is the sur- , _-_ _ face of a liquid alwajS HORIZONTAL, Of LeVEL. The particles of a hquid having perfect freedom of motion wirface'of aiu among themselves, and all being equally attracted by gravita- quid at rest tion, the whole body of liquid will tend to arrange itself in such a manner that all the parts of its surface shall be equally distant from the earth's center, which is the center of attraction. _^ t ■ th "^ perfectly level surface really means one in which every true definition part of the surface is equally near the center of the earth ; it Burface?**'^'*^^' must be, therefore, m fact, a spherical surface. But so largo is the sphere of which such a surface forms a part, that in reservoirs and receptacles of water of limited extent, its sphericity can not be noticed, and it may be considered as a perfect plane and level ; but when the surface of water is of great extent, as in the case of the ocean, it exhibits this rounded form, conforming to the figure of the earth, most perfectly.* This sphericity of the surface of the ocean is illustrated by the fact, that the masts of a sliip appproaching us at sea, are visible long before the hull of the fjQ ii>j vessel can be seen. In Fig. B 'fesei ^^^ ^^^y ^^^^ P^^ °^ *^^ J^%^''^^^J^^^Sf!^^^^^^^^ ship above the line A C can ^^^^f "^~^^^"^^^^^^^^ ^^ ^^^^ hy the spectator at >^^^^^^^^^^^^ '^^^k. -^ because the rest of the ~ ^^^ vessel is hidden by the swell of the curve cf the surface of the ocean, or rather of the earth, D E. In what man. 302. Water, or other liquids will always rise "id rise "in^^a to au cxact Icvel in any series of different scries of tubes , •. • , -i i • , • orvesseiscom- tubps, jjipcs, or othcr vcsscls communicating municatinf; . ,-> i ,i with each other? With eacu othcr. • A hoop Burrounditi!; the earth would bend from a perfectly strnight linceishtinehes In a mile. Consequently, if a segnient of the surface of the earth, a mile long were cut off, and laid on a perfect plane, the center of the segment would he only four Inches hiirher than the edges. A small portion of it, therefore, for all ordii ary purposes, may be considered as a perfect plane. 134 WELLS'S NATURAL PHILOSOPHY. Fig. 118. On what prin- ciple are we enabled to con- voy water in aqueducts over uneven sur- faces ? This fact is sufficiently illustrated by reference to Fig. 118. 303. It is upon the application of the principle that water in pipes will always rise to the height, or level of its source, that all ^ arrangements for conveying water over uneven surfaces in aqueducts, or closed pipes depend. The water brought from any reservoir or source of supply, in or near a town or building, may bo delivered by the efl'ect of gravity alone to every location beneath the level of the reservoir; the result not being affected by the inequalities of the surface over which the water pipes may pass in their connection between the reservoir and the point of delivery. So long as they do not rise above the level of the source of supply, so long will the water continue to flow. Fig. 119 represents the line of a modern aqueduct: — a a a represents the water level of a pond or reservoir upon elevated ground. From this pond a line of pipe is laid, passing over a bridge or viaduct at d, and under a river at c. The fountains at b b, show the stream rising to the level of its source in the pond a, at two points of very different elevation. Fig. 119. The ancients, in constructing aqueducts, do not seem to have ever practi- cally applied this principle, that water in pipes rises to the level of its source. When, in conducting water from a distant source to supply a city, it became necessary to cross a ravine or valley, immense bridges, or arches of masonry were built across it, with great labor and at enormous expense, in order that the water-flow might be continued nearly horizontally. At the present day ' the same object is effected more perfectly by means of a simple iron pipe^ bending in conformity with the inequalities of surface over which it passes. In the construction of pipes for conveying water, it is neces- sary that those parts which are much below the level of the reservoir, should have a great degree of strength, since they sustain the bursting pressure of a column of water whose height is equal to the difference of level. A pipe with o- diameter of 4 inches, 150 feet below the level of a reserv^oir, should have suf- In what map- ner should pipes for the conveyance of water he con- structed ? HYDROSTATICS. 135 Fig. 120. What is an Ar- tesian Well ? 304. ficient strength to bear with security a bursting pressure of nearly 5 tons for each foot of its length. Upon the principle that water tends to rise to the level of its source, orna- Biental fountains may be constructed. Let water spout upward through a pipe communicating with the bottom of a deep vessel, and it will rise nearly to the heiglit of the upper sur- face of the water in the vessel. The resistance of the air, and the falling drops, prevent it from rising to the exact level. Let A, Pig. 120, represent a cistern filled %vith water to a constant height, B. If four bent pipes be inserted in the side of the cistern at different distances below the surface, the water will jet upward from all the orifices to nearly the same level The phenomena of Artesian "^ells, and the plan of boring for water, depend on the same principle. An Artesian Well is a cylindrical excavation formed by boring into the earth with a species of auger, until a sheet or vein of water is found, when the water rises through the excavation. Such excavations are called Artesian, because this method was employed for obtaining water at Artois in France. ,„^ , ,^ The reason that the water rises in Artesian, and sometimes Why does the water rise in m ordinaiy wells, to tne surface, is as follows : The surface w ,j,'^''®^'*'^ of the globe is formed of different layers, or strata, of diflerent materials, such as sand, gravel, clay, stone, etc., placed one upon the other. In particular situations, these strata do not rest horizontally upon one another, but are incUned, the different strata being like cups, or basins placed one within the other, as in Fig. 121. Some of these strata are composed of materials, as sand or gravel, through which water will soak most readily; while other strata, like clay and rock, will not allow the water to pass through them. IfJ now, we suppose a stratum like sand, pervious to water, to be included as at a a, Fig 121, between two other strata of clay or rock, the water falling upon the un- covered margin of the sandy stratum a a, will be absorbed, and penetrate through its whole depth. It will be prevented from rising to the surface by the im- pervious stratum above it, and from sinking lower, by the equally impervious Btratum below it. It will, therefore, accumulate as in a reservoir. If, now, we Fig. 121. Fig. 122. 136 WELLS'S NATURAL PHILOSOPHY. bore do^-n through the upper stratum, as at b, until we reach the stratum containing the water, the water will rise in the excavation to a certain height, proportional to the height or level of the water accumulated in the reser- vok a a from which it tlows.* .„^ . ^ . 305. The rain which falls upon the surface of the earth What IS the on- . , , i ^i , , , gin of springs ? smks Qownward through the sandy and porous soil, un- tQ a bed of clay or rock, through which the water can not penetrate, is reached. Here it accumulates, or running along the surface of tlie impervious stratum, bursts out in some lower situation, or at some point ■where the impervious bed or stratum comes to the suiface in consequence of a valley, or some depression. Such a flow of water consti- tutes a spring. Suppose o, Fig. 122, to be a gravel hill, and b a stratum of clay or rock, impervious to water. The fluid percolating through the gravel would reach the impervious stratum, along which it would nm until it found an outlet at c, at the foot of the hill, where a spring would be formed. „^ , 306. If there are no irregularities in the surface, so situated Aivhv does water " . . coik-ct in an or- as to allow a spring to burst forth, or ii a sprmg issues out dinary well ? ^t some point of the porous earth considerably above the sur- face of the clay, or rock, upon which at some depth all such earth rests, the water soaking downward will not aU be drained off, but will accumulate, and rise among the particles of soil, as it would among shot, or bullets, in a water- tight vessel If a hole, or pit, be dug into such earth, reaching below the level of the water accumulated in it, it will soon be filled up with water to this level, and ^•iU constitute a well The reason why some wells are deeper than others, is, that the distance of the imper^nous stratum of clay below the surface is different in different localities. „ . , 307. All wells and springs, therefore, are merely the rain- Frora what , . , , Bonrce do all water which has sunk into the earth, appearing again, and MTrinsrs^d'^rive gradually accumulating, or escaping at a lower level. their water? 308. The property of liquids to assume a horizontal sur- "WTiat is a ^^<^^ '^^ practically taken advantage of in ascertaining whether Water, or ^ surface is perfectlv horizontal, or level, and is accomplished Spirit Level? , „ . ' , „ -rrr „ by means of an instrument known as the Water or 'Spirit Level." This consists of a small glass tube, b c, Fig. 123, filled with spirit, or water, except a small space occupied with air, and called • In the great Artesian wells of Grenelle, near Paris, and of Kissinpen, in Bavaria, the water risfs from depths of 1,900 .ind 1.900 feet to a considerable height above the surfaca of the earth. The well of P.iris is capable of snpplyinr wnter at the rate of 14 millions of gallons per day. The region of country in which this water fell, from the curvature of the layers, or strata of material through which the excavation was made, must hara been distant two hundred miles or more. HYDROSTATICS. 137 _ - the air-bubble, a. In whaterer position the tube mav be placed, the bubble of air will rest at the high- est point If the two ends of the tube are level, or /' V ^ -V cot UViUU -tA LUC L»V CLU^O \JX lliC lUl-'t' di V A^ * ^-J, yJL \2 t^ perfectly horizontal, the air-bubble will remain in the center of the tube ; but if the tube inclines ever 80 little, the bubble rises to the higher end. For practical use the glass-tube is inclosed in a wood, or brass case, or box. 309. The method of conducting' a canal through a countrv, L pon what pnn- , , - , . , . . , , . , , , ,' ci.jle are canals the surface of which is not pertectlv horizontal, or level, de- et'.istructed and ^Qjxda upon this same property of hquids. In order that boats may saU with ease in both directions of the canal, it is neces- sary that the surface of the water should be level. If one end of a canal were higher than the other, the water would run toward the lower extremity, overflow the banks, and leave the other end dry. But a canal rarely, if ever, passes through a section of country of any great extent, which is not incUned, or irregular in its surface. By means, however, of expedients called Locks, a canal can be conducted along any declivity. In the forma- tion of a canal, its course is divided into a series of levels corresponding with the inequaUties of the surface of the country through which it passes. These levels communicate with each other by locks, by means of which boats passing in any direction can be elevated, or lowered with ease, rapidity, and safety. Fig. 124. Fig. 124 represents a section of a lock, and Fig. 125 the constnic- tion of the Lock Gates, The sec- tion of Fig. 125 represents a place •where there is a sudden fall of the ground, along which the canal has to pass. A B and C D are two gates which completely intercept the course of the water, but at the same time admit of being opened and closed. A H is the level of the water in that part of the canal lying above the gate A B, and E F and F G the levels below the gate A B. The part of the canal included between two gates, as E F, is called a lock, because when a vessel is let into it. it can be shut by closing both pair of gates. If now it is required to let a boat down from the higher level, A H, to the lower level, E G. the gates C D are closed tightly, and an opening made in the gates A B (shown in Fig. 125), which allows the water to flow gradually from A H into the lock A E F C, until it attains a common level, H A C. The gate A B is then opened, and the boat floats into the lock A B C D. The gates A B are then closed, and an opening made in gates C D, which allows the water to flow from the space A E F C, until it comes to the common level, E F G. The gate C D is then opened, and the boat floats out of the locks into the continuation of the canal. To enable a boat to pass from the lower level, E F G, to the superior level, A H, the process here described is reversei •^ H |A C^ 133 WELLS'S NATURAL PHILOSOPHY. Fig. 125. With what force is a iloat- iiigbody iireBs- cd upward ? How mnch water will a Bolid immersed ia it displace ? "What is Buoy, ancy ? 310. When a solid is immersed in a liquid it will be pressed upward with a force equal to the weight of the liquid it displaces. 311. A solid immersed in water will displace as much of the liquid as is equal in volume to the part immersed. 312. Buoyancy is the name applied to the force by which a solid immersed in a liquid is heaved, or pressed upward. The resistance offered when we attempt to sink a body hghter than water in that Hquid, proves that the water presses with a force upward as well aa downward. Upon this fact the laws of floating bodies depend ; and for this reason the bottoms of large ships are constructed with a great degree of Btrength. 313. A body floating upon a liquid is main- tained in EQUiLiBRio by the operation of grav- ity drawing the mass downward, and by the pressure of the particles of the liquid upon which it rests, pressing it upward. whatisessen- ^14. lu ordcr that a body may float with sta- buitj^ofaflolt ^ility, it is necessary that its center of gravity ingbodyf should bc sltuatcd as low as possible. How is a body floating upon a liquid main- tained in equi- librio ? HYDROSTATICS. 139 When ia & floating body in its most sta- ble position ? When -will a Bolid float, and when sink ? Fig. 126. What is the For this reason, all vessels which are light in proportion to i"n veMels?*'*^' *^®''" ^^^^ require to be ballasted by depositing in the lowest portions of the vessel, immediately above the keel, a quantity of heavy matter, usually iron or stone. The center of gravity may thus be brought so low that no force of the wind striking the vessel sideways can capsize it. By raising the center of gravity, as when men in a boat stand upright, the equdibriura is rendered unstable. A body floating is most stable when it floats upon its great- est surface : thus a plank floats with the greatest stability when placed flat upon the water; and its position is unstable when it is made to float edgewise. A soUd can never float that is heavier, bulk for bulk, than the liquid in which it is immersed. If the weight of a solid bo exactly equal to the weight of an equal bulk of liquid, it will sink in it until it is entirely immersed ; but when once it is entirely immersed, then, the upward and downward pressure being equal, the soUd will neither sink or rise, but will remain suspended at any depth at which it may be placed. Let A B, Fig. 126, be a cube of wood floating in water; then the weight of the water displaced, or the weight of a volume of water equal to A B, is equal to the whole weight of the wood ; since the upward pressure on the bottom of A B is the same as that which would support a portion of water equal in bulk to the displaced water, or to the cube A B ; and as the downward pressure of the body is equal to the upward pressure of the liquid, it fol- lows that the weight of the cube is equal to the weight of the water displaced. Hence A B will neither sink or rise. A mass of stone, or any other heavy substance beneath the surface of water is more easily moved -1 than upon the land because, when immersed in the water, it is hghter by the weight of its own bulk of water than it would be on laud. A boy will often wonder why he can lift a stone of a certain weight to the surface of water, but can carry it no farther. The least force will lifl; a bucket immersed in water to the surface ; but if it be lifl;ed farther, its weight is felt just in proportion to the part of it which is above the surface. The weight of the human body does not diflermuch from the weight of its own bulk of water; consequently, ^\'hcn bathers walk in water chin-decji, their feet scarcely press upon the bottom, and they have not sufficient hold upon the ground to give them stability ; a current, therefore, will easily take them off tlieir feet The facility with which different persons are able to float or swim, depends upon the physical constitution of the body. Corpulent people are lighter. 140 WELLS'S NATURAL PHILOSOPHY. bulk for bulk, than those of sparer habits : and as fat possesses a less specific gravity than water, a fat person will swim or float easier than a thin one. 315. It is not, however, necessary, in order that a body should float upon a liquid, that the materials of wliich it is composed should be specifically lighter than the hquid. If the entire mass of a solid is fighter than an equal volume of the liquid, it wiU float. A thick piece of iron, weighing half an ounce, loses in water nearly one eightli of its weight ; but if it is hammered into a plate or vessel, of such a fjrm that it occupies eight times as much space as before, it will then weigh Joss than an equal bulk of water, and will consequently float, sinking just to the brim. If made twice as large, it will displace one ounce of water, conse- quently, twice its own weight; it will then sink to the middle, and can be loaded with half an ounce weight before sinking entirely. How can a 316. A body composed of any material, how- t'han anT^ti ^ver heavy, can be made to float on any liquid, be'^'mldl^'to liowever light, by giving it such a shape as floa" will render its bulk or volume lighter than an equal bulk of water. Iron ships and boats are Ulustrations of this principle. A ship carrying a thousand tons' weight will displace just as much water, or float to the same depth, whether her cargo be feathers, cotton, or iron. A ship made of iron floats just as high out of water as a ship of similar form and size made of wood, provided that the iron be proportionally thinner than the wood, and therefore not heavier on the whole. The buoyancy of hollow solids is frequently used for lifting or supporting heavy weights in water. Life-preservers, which are inflated bags of India- rubber, are an example. Hollow boxes, or tanks, are used for the purpose of raising sunken vessels. These boxes are sunk, fiUed with water, and attached to the side of the vessel to be raised. The water, by a connection of pipes, is then pumped out of them, when the upward pressure of the liquid becoming greater than the gravity or weight of the entire mass, the whole will rise and float. To what is the ^17. The buoyaucy of liquids is in propor- liquIdBpropor- tioii to their density or specific gravity, or, in tionai? other words, a solid is buoyant in a liquid, in proportion as it is light, and the liquid heavy. Thus quicksilver, the heaviest, or most dense fluid known, supports iron upon its surface; and a man might float upon mercury as easily as a cork floats upon water. Many varieties of wood which will sink in oil, float readily upon water. 318. The principle that the buoyancy of liquids varies in proportion as their specific gravity varies, furnishes a very ready method of determining the spe- cific gravity of a liquid. This ia done by means of an instrument called the hydrometer. HYDROSTATICS. 141 What is a Hy- drometer ? How may the specific grav- ity of a liquid be determined by the Hy- drometer 7 319. The Hydrometer con- Fig. 127. sists of a hollow glass tube, oa the lower part of which a spherical bulb is blown, the latter being filled with a suitable quantity of small shot, or quicksilver, in order to cause it to float, in a vertical position. The upper part of the tube contains a scale graduated into suitable divisions. (See Fig. 127.) It 13 obvious that the hydrometer will sink to a greater or less depth in difierent liquids ; deeper in the ligliter ones, or those of small specific gravity, and not so deep in those which are denser, or which have great specific gravity. The specific gravity of a Uquid may, therefore, be estimated by the number of di- visions on the scale which remain above the surface of the hquid. Tables are constructed, so that, by their aid, when the number on the scale at which the hydrometer floats in a given liquid is determined by experiment, the spe- cific gravity ia expressed by figures in a column directly opposite that number in the table. There are various forms of the hydrometer especially adapted for determin- ing the density, or specific gravity, of spirits, oils, syrups, lye, etc. It afibrds a ready method of determining the purity of a liquid, as, for instance, alco- hol. The addition of water to alcohol adds to its density, and therefore in- creases its buoyancy. The addition of water, therefore, will at once be shown by the less depth to which the hydrometer will sink in the liquid. The adulteration of sperm oil with whale, or other cheaper oils, may be shown in the same manner. 320. For the reason that the buoyancy of a liquid is proportioned to its density, a ship will draw less water, or sail lighter by one thirty-fifth in the heavy salt water of the ocean, than in the fi-esh water of a river ; for the same reason it is easier to swim in salt than in fresh water.* • " A floating bodr sirks to the same depth whether the mass of liquid supporting it be great or small, as is seen when an earthen cup is placed first in a pond, and then in a second eup only so much larger than itself, that a very small quantity of water will suffice to fill up the interval between them. An ounce of water in this way may be made to float substances of much greater weight. And if a large ship were received into a dock, or case, so exactly filling it that there were only half an inch of interval between it and the wall, or side of the containing space, it would float as completely when the few hogsheads of water required to fill this little interval up to its usual water-mark were poured in, as if it were on the high seas. In some canal locks, the boats just fit the place in which they have to rise and fall, ahd thus diminish the quantity of water necessary to Eupply th« loci.."— A niott. 142 "WELLS'S NATURAL PHILOSOPHY. SECTION I Explain the phenomena ob- Berved when the hand is l^hmged into different liq- uids. CAPILLARY ATTRACTION. 321. If we plunge the hand into a vessel of water, and withdraw it, it is said to be wet ; that is, it is covered with a thin film, or coating of water, which adheres to it, in opposi- tion to the tendency of the attraction of gravitation to mako it fall off. There is, therefore, an attraction between the par- ticles of the water and the hand, which, to a certain extent, is stronger than the influence of gravitation. If now we plunge the hand into a vessel of quicksilver, no adhesion of the particles of the mercury to the hand will take place, and the hand, when withdrawn, will be perfectly dry. If we plunge a plate of gold, however, into water and quicksilver, it will be wet equally by both, and will come out of the quicksilver covered with a white coating of that liquid. It is, therefore, obvious that a certain molecular attraction exists between certain liquids and certain solids, which does not prevail to the same extent between others. 322. That variety of molecular force which manife.sts itself ] jet ween the surfaces of solids and liquids is called Capillary Attraction, This name oiigiuatcs from the circumstance, that this class of phenomena was first observed in small glass tubes, the bore of which was not thicker than a hair, and which were hence called Capillary Tubes, from the Latin word ca])illus, which signifies a hair. „ „ 323. If we take a series of glass tubes of very fine bore, How may Ca- ° piUary Attrac- but of different diametei-s, and place them m a vessel oi water, tion bo illuB- -wiiich has been colored in order to show the effect more strik- trated ? ingly, we shall see that the water will rise in the tubes to various heights, attaining the greatest degree of elevation in the smallest tube. What 18 Ca- pillary Attrac- tion? What is the origin of the term ? FlO. 128. The height at which the same Uquid will rise in any given tube is always uniform, but it varies for different liquids. Pig. 123 is an enlarged representation of tho manner in wliich water will rise in tubes of diQer- ent diameters. The simplest method of exhibiting capillary at- traction is to immerse the end of a piece of ther. moraeter tube in water (see Fig. 12D) which has been tinted with ink. The liquid will be seen to ascend, and will remain elevated in the tube at a considerable height above the surface of the liquid in the vessel The ordinary definition of capillary attraction ia, that form of attraction which CAPILLARY ATTRACTION. 143 causes liquids to ascend above their level in capillary tubes. Fig. 129. It, however, is not strictly correct, as this force not only acts in elevating but in depressing liquids in tubes, and is at work wherever liquids are in connection with solid bodies. 324. If a Uquid be poured into a vessel, as water in glass, whose sides are of such a naturo as to be wetted by it, the liquid will be elevated above the general level of its sui'face at the points where it touches the sides of the ves- sel This is shown in Fig. 130. If, however, the liquid is poured into a vessel whose sides are of such a nature that they are not wetted by it, as in the case of quicksilver in a glass vessel, then the liquid will be depressed below the general level of its surface at the What will be the condition of the surface of a liquid which wets the sides of the ves- gel containing iJ.' When the liq- uid does not wet the sides of the vessel, what will be the condition of its surface ? Fig. 130. Fig. 131. points where it comes in con- tact with the sides of the ves- sel. This is shown in Fig. 131. 325. If two plates of glass, A and B, Fig. 132, be plunged into water at their lower ex- tremities, with their faces ver- tical and parallel, and at a cer- tain distance asunder, the water will rise at the points m and n, where it is in contact with the glass; but at Fig. 13... all intermediate pomts, beyond a small distance from the plates, the general level of the surfaces E, C, and D, will correspond. If the two plates, A and B, are brought near to each other, as in Fig. 133, the two curves, "~ — . m and n, will unite, so as to form a concave surface, and the water at the same time between them will be raised above the general level, E and D, of the water in the vesseL If the plates be brought still nearer together, as in Fig. 134, the water between them will rise still higher, l.ie force which sustains the column being in- creased as the distance between the plates is diminished 326. The heiglit to which water will rise in — • capillary tubes is ia proportion to the small- ness of their diameters. To what is the elevation of water in capil- lary tubes pro- portioned ? Ill« F IG. 133. ^=^ =E^ —^- 1 - — 141 WELLS'S NATURAL PHILOSOPHY. Fig. 13-L Fig. 135. Thus ia two tubes, one of which is double the diameter of the other, the fluid will rise to twice tiie height in the small tube that it will in the larger. The tmth of this principle can be made evident by the fol- lowing beautiful and simple experiment. Two square pieces of plate-glass, C and B, Fig. 135, are arranged so that their sur- faces form a minute angle at A. This po- sition may be easily given them by fasten- ing with wax or cement. When the ends of t'.ie plates are placed iu the water, as shown in the figure, the water rises in the space between them, forming the curve, which is called an hyperbola. The elevation of the water between the two surfaces will be the greatest at the points where the distance between the plates is tlie least. 327. The figure of the surface which bounds a liquid in a capillary tube wiU depend upon the extent of the attraction which exists between the particles of the hquid and the surface of the tube. Thus, a column of water contained in a glass capillary tube will have a conca\*e fonn of surface, as in Fig. 136, since the attraction of glass for water exceeds the attraction of the particles of water for each other ; a surface of mercury, on the contrary, iu a similar tube, will be convex, see Fig. 137, since the attraction of glass for mercury is less than the mutual attraction of the particles of mercury. 328. In a capillary tube a liquid will ascend above its general level, wben it wets the tube ; and is depressed below its level wheq it does not wet it. 329. If the surface of a body repels a liquid, such a body, though heavier, bulk for bulk, than the hquid, may, under some circumstances, float upon it ; and so present an apparen; exception to the general hydrostatic law by which sohd^ ■which are heavier than liquids, bulk for bulk, will sink in them. An exam- ple of this may be shown by slightly greasing a fine sewing-needle, and then placing it carefully in the du-ection of its length upon the surface of water. The needle, although heavier, bulk for bulk, than water, wiU float The power of certain insects to walk upon the surface of water without sinkine. has been explained upon the same principle. The feet of these in- sects, like the greased needle, have a capillary repulsion for tho water, and Fig. 136. Fig. \Z When will a liquid be ele- vated and when depressed in a capillary tu.be 1 How may a needle be made to float upoQ water ? CAPILLARY ATTRACTION. 145 What is a " Kope Pump ?" Fig. 138. when they apply them to the surface of water, mstead of sinking in it, they produce depressions upon it. For a like reason, water will not flow through a fine sieve, the wires of which have been greased. ,^ .„ 330. A liquid will not wet a solid when the When will a -*• . liquid fail to force of adhesion developed between the par- wet a Golia ' , . 1 „ . . . ticlcs of the liquid and the surface of the solid, is less than half the cohesive furce which exists hetweea the particles of the liquid. 331. The fact of the strong adhesion wliich exists between water and the fibers of a rope, has been taken ad- vantage of in the construction of a kind of pump, called the " Rope," or " Yera's'' Pump, Fig. 138. It con- sists of a cord passing over two wheels, a and b, the lower one of which is immersed in water. A rapid motion is given to the wheels by means of the crank d, and the water, by adhering, follows the rope in its movements, and is discharged into a receptacle above. „^ , , Illustrations of capillary attraction AVhat are fa- ' •' miliar iiiustra- are most famUiar m the experience of ISattracti'o^? everyday hfe. The wick of a lamp, or candle, lifts the oil, or melted grease which supphes the flame, from a surface often two or three inches below the point of combustion. In a cotton-wick, which is the material best adapted for this purpose, the mi- nute, separate fibers of the cotton themselves are capillary tubes, and the in- t'^rstiees between the filaments composing the wick are also capillary tubes ; in these the oil ascends. The oil, however, can not be lifted freely beyord a certain height by capillary attraction : hence, when the suriace of the oil id low in the lamp, the flame becomes feeble, or expires. If the end of a towel, or a mass of cotton thread, be immersed m a basin of water, and the remainder allowed to hang over the edge of the basin, the water wiH rise through the pores and interstices of the cloth, and gradually wpt the whole toweL In this way the basin may be entirelj- emptied. If sand, a lump of sugar, or a sponge, have moisture beneath and slightly in contact with it, it will ascend through the pores by the agency of capillary attraction in opposition 1o gravity, and the entire mass will become wet. The lower story of a hou'^e is sometimes damp, because the moisture of the ground ascends through the pores of the materials constituting the walls of the building. "Wood imbibes moisture by the capillary attraction of its pores, and expands or swells in consequence. This fact has been taken advantage of for splitting stones ; wedges of dry wood are driven into grooves cut in the stone, and on being moistened, swell with such irresistible force fia to split the block in a direction regulated by the groove. 7 146 WELLS'S NATURAL PHILOSOPHY. Pig. 139. An immense weight suspended by a dry rope, may be raised a little way, by merely wetting the rope; the moisture imbibed by capillary attraction mto the substance of the rope causes it to swell laterally and become shorter. Capillary attraction is also iustrumeutal in supplying trees and plants with moisture through the agency of the roots and underground fibers. What are the ^^2. The terms ExosMOSE and Endosmose Exosn^srand ^'^6 applied to tliosG currents in contrary direc- Endosmose? tions wbich arc established between two liquids of a different nature, when they are separated from each other by a partition composed of a membrane, or any porous eubstance. The name Endosmose, derived from a Greek word, signifies going in, and is ap- plied to the stronger current; while the name Exosmose, signifying going out, is applied to the weaker current. The phenomena of Endosmose and Ex- osmose, which are undoubtedly dependent on capillary attraction, may be illustrated by the following simple experiment : — If we take a small bladder, or any other mem- branous substance, and having fastened it on a tube open at both ends, as is repre- sented in Fig. 139, fill the bladder with alcohol, and immerse it, connected with the tube, in a basin of water, to such an extent that the top of the bladder filled with alcohol corresponds with the level of the water in the vessel, in a sliort time it will be observed, that the liquid is rising in the tube connected with the bladder, and will ultimately reach the top and flow over. This' rising of the al- cohol in the tube is evidently due to tho circumstance that the water permeates through the bladder, with a certain de- gree of force, producing the phenomena which we call endosmose, "going in;" the eflect being to elevate the alcohol to a considerable height in the tube. At the same time, a certain quantity of the alcohol has passed out through the pores of the bladder, and mixed with tho water in the external vessel. This outward passage of the alcohol we call exosmose, '' going out." A less quantity of the alcohol will pass out of the bladder in a given time to mingle with the water, than of the water will pasg in, and consequently the bladder containing the alcohol having more liquid in it than at first, becomes strained, and presses the liquid up in the tube. CAPILLARY ATTRACTION'. 14T If we have a box divided bv a partition of porous clay, or any other sub- stance of like nature, and place a quantity of syrup on one side, and water on tlie otlier, or any otlier two liquids of different densities which freely mix with one another, currents will be established between the two in opposite direc- tions through the porous partition, until both are thoroughly mingled with each other. 333. If a liquid is placed in contact with a surface of the body, divested of its epidermis, or outer skin, or in contact with a mucous membrane, the liquid will be ab- sorbed into the vessels of the body through the force of endosmose. PRACTICAL QUESTIONS AND PROBLEMS IN HYDROSTATICS. 1. Why are stones, gravel, and sand so easily moved hy waves and currents ? Because the moving water has only to overcome about half the weight of the stone. 2. Wliy can a stone which, on land, requires the strength of two men to lift it, ha lifted and carried in water by one man ? Because the water holds up the stone with a force equal to the weight of the volume of water it displaces. 3. Why does cream rise upon milk ? Because it is composed of particles of oil}-, or fatty matter, which are Ughtei than the watery particles of the milk. 4. How are fishes able to ascend and descend quickly in water 7 They are capable of changing their bulk by the voluntary distension, or contractioa of a membraneous bag, or air bladder, included in their organiza- tion ; when this bladder is distended, the fish increases in size, and being of less specific gravitj', i.e., lighter, it rises with facilit\'; when the bladder i3 contracted, the size of the fish diminishes, and its tendency to sink is increased. 5. Why does the body of a drowned porsor generally rise and float upon the surfaca several days after death ? Because, from the accumulation of gas within the body (caused by incipient putrefaction), the body becomes specifically lighter than water, and rises and floats upon the surface. 6 How are life-boats prevented from sinking ? They contain in tlicir sides air-tight colls, or boxes, filled with air, which by their buoyancy prevent the boat from sinking, even when it is filled with water. 7. Why does blotting-paper absorb ink ? The ink is drawn up between the minute fibers of the paper by capillary attraction. 8 Why will not writing, or sized paper, absorb ink ? Because the sizing, being a species of glue into which writing papers aro 148 WELLS'S NATURAL PHILOSOPHY. dij^ped, fills np the little interstices, or spaces, betivecn the fibers, and in this way preveuta all capillary attraction. 9. Why is vegetation on the margin of a stream of water more luxuriant than in an open field ? Because the porous earth on the bank draws up water to the roots of tho plants by capillary attraction. 10. Why do persons who water plants in pots frequently pour the water into the sau- cer in which the pot rests, and not over the plants"? Because tho water iu the saucer is drawn up by capillary attraction through the little interstices of the mold with which the pot is filled, and is thus pre- sented to the roots of the plant. IL Why does dry wood, immersed in water, swell ? Because the water enters tlie pores of wood by capillary attraction, and forces the particles further apart from each other. 12. WHiy will water, ink, or oil, coming in contact with the edge of a book, soak fur- ther in than if spilled upon the sides ? Because the space between the leaves acts in the same manner as a small capillary tube would — attracts the fluid, and causes it to penetrate far inward. The fluid penetrates with more difficulty upon the side of the leaf, because the pores in the paper are irregular, and not continuous from leaf to leaf 13. In a hydrostatic press, the area of the base of the piston in the force-pump is one square inch, and the area of the base of the piston in the large cylinder is fourteen square inches ; what will be the force exerted, supposing a power of eight hundred pounds ap- plied to the piston of the force-pump ? 14. A flood-gate is five feet in breadth, and sixteen feet in depth : what will be tho pressure of water upon it in pounds ? 15. What pressure will a vi'sscl, having a superficial area of three feet, sustain when lowered into the sea to the depth of five hundred feet ? 16. What pressure is exerted upon the body of a diver at the depth of sixty feet, sup- posing the superficial area of his body to be two and a half square yards ? 17. "Wliat will be the pressure upon a dam, the area of tho side of wliich is one hun- drcd and fifty superficial feet, and the height of the side fifteen feet, the water rising even with the top ? CHAPTER IX. HYDRAULICS. _ , , 334, Hydr.\ulics is that department of inniat is trie science of Hy- phvsical sciciice which treats of the laws and draulics ? ^ -^ „,..,. phenomena or hquids in motion."-' Hydraulics considers the flow of liquids in pipes, through orifices in the sides of reservoirs, in rivers, canals, etc., and the construction and operation of all machines and engines which are concerned in tho motion of liquids. • From iiiojfi (hudor), water, aad uuXiSf (aulos), a pipe. HYDKAULICS. 149 Upon what docs What is the ve- locity of a liq- uid flowinsfrom a reservoir equal to ? 335. When an opening is made in a rescr- flowTng^ifqum "^^i'' containing a liquid, it will jet out with a depend? velocitv x^r^poi'tiuncd to the depth of thcai^cr- ture below the surface. Fig. 140 Supposing the stirface of water in a vessel, D, Fig. 140, to be kept at a constant height by the water flowing into it, and that the water flows out through oppningg in the side of precisely the same size ; then a quart measure would be filled from the jet issuing from B as soon as a pint measure from the upper opening, A. As the flow of liquids is in consequence of the at- traction of gravity, and as the pressure of a liquid in /*; equal in all directions, we have the following princi- ple estabhshed: — 336. The velocity which the particles of a liquid acquire when issuing from an orifice, whether sideways, upward, or downward, is equal to that which they would have acquired in falling perpendicularly through a space equal to the depth of the aperture below the surface of the liquid. Thus, if an aperture be made in the bottom, or side, of a vessel containing water, 16 feet below the surface, the velocity with which the water will jet out will be 32 feet per second, for this is the velocity which a body acquires in falling through a space of 16 feet. As the velocity acquired by a falling body is as the square root of the space through which it f;ills, the velocitj' with which water will issue from an aper- ture may be calculated by the following rule : — 337. The velocity with which water spouts out from any aperture in a vessel is as the square root of the depth of the aperture below the surface of the water. The water must, therefore, flow with ten times greater velocity from an opening 100 inches below the level of the liquid, than from a dejjth of only one inch below the same level. 338. The theoretical law for determining the quantity of water discharged from an orifice is as follows : — The quantity of water discharged from an ori- fice in each second may be calculated by multi- plying the velocity by the area of the aperture. The above rules for calculating the velocity and quanthy of water flowing from orifices, are not found strictly to hold good in practice. The friction of water against the sides of vessels, pipes, and apertures, and the formation How may the velocity of a liquid flowing fiom a reser- voir be calcu- lated ? What is the f!ieorpticall:iw for determin- ing the qnan- tit of water discharged from an aper- ture ? 150 WELLS'S NATURAL PHILOSOPHY. of what is called the "contracted vein," tend very much to diminish the mo- tion and discharge of water. "When water flows throiicrh a circular aperture What 13 the . , , ■,. ", , . . " contracted m a vessel, the diameter of the issumg stream vain" 11 a (•>"■- j^ contracted, and attains its smallest dimensions rent of water ? ' at a distance from the orifice equal to the diam- eter of the orifice itself The section of the jet at tliis point. Fig. 141, 5 s', will be about two tliirds of tlie magnitude of the onfiue. This point of greatest contraction is called tlie vena contracia, or con'cracted van. ,„, , . „ This phenomenon arises from the circumstance that a liquid What IS the '■ ... cause of this contained in a vessel rushes from aU sides toward an orihce, phenomenon? g^ ^^ ^^ j-^j.^^ ^ sj-stem of converging currents. Tliese issuing out in oblique directions, cause tlie shape of the stream to change from the cjdindrical form, and contract it in the manner described. How may the ^^ *^'^ attaclunent of suitable tubes to the aperture, the effect of the effect of the contracted vein may be avoided, and the quan- vei'n' be' avoid- tity of flowing water be very greatly increased. A short pipe ed? xvill discharge one half more water in tlie same time, than a simple orifice of the same dimensions. The tube, liowever, must be Fig 142 entirely without the vessel, as at B, Fig. 142, for if co' • tinned inside, as at A, the quantity of liquid discharged will be diminished instead of augmented. The rapidity of the discharge of the water will also depend much on the figure of tlie tube, and that of the bottom of the vessel, since more water will flow through a conical, or bell-shaped tube, as at C, Fig. 142, than through a cylindrical tube. A still further advantage may be gained by hav- ing tlie bottom of the vessel rounded, as at D, and the tube bell-shapod. An inch tube of 200 feet in length, placed horizontally, will discharge only one fourth as much water as a tube of the same dimensions an inch in length ; hence, in all cases where it is proposed to convey water to a distance in pipes, there will be a great disappointment in respect to the quantity actu- ally delivered, unless the engineer takes into account the friction, and the turnings of the pipes, and makes large allowances for these circumstances. If the quantity to be actually delivered ought to fill a two inch pipe, one of three inches will not be too great an allowance, if the water is to be conveyed to any considerable distance. In practice, it will be found that a pipe of two inches in diameter, one hun- dred feet long, will discharge about five times as much water as one of one inch in diameter of the same length, and under the same pressure. This dif- ference is accounted for, bj' supposing that both tubes retard the motion of the fluid, by friction, at equal distances from their inner surface.?, and conse- quently, the effect of this cause is much greater in proportion, in the small tube, than in the large one. W i,-J L;J HYDRAULICS. 151 \Vhat wiU be the difference in the flow of B liquid when the vessel is kept full and when it is al- lowed to emp- ty itself? What is the principle and construction of the water- clock? Fig. 143 A As the velocity with which a stream issues depends upon the height of the cohimn of fluid, it follows that when a liquid flows from a reservoir which is not replenished, but the level of wliich constantly descends, its velocity will be uniformly retarded. The following prmciple has been established : — 339. If a vessel be filled with, a liquid and allowed to discharge itself, the quantity issu- ing from an orifice in a given time, will be just one hiilf what would be discharged from the same orifice in the same time, if the vessd was kept constantly full. 340. Before the invention of clocks and watches, the flow of water through small ori- fice's was applied by the ancients for the meas- urement of time, and an arrangement for this purpose was called a Cl'qysydra, or water-clock. One form of this instrument consisted of a cylindrical vessel filled with water, and furnished with an orifice which would discharge the whole in twelve hours. If the whole depth through which the water in the vessel would sink in this time be divided into 144 parts, it will sink through 23 in the first hour, 21 in the second, 19 in the third, and so on, according to a series of odd numbers : this diminishing rate depending on the constantly decreasing height and pressure of the column above the point of discharge. The spaces indicated upon a scale attached to the side of the vessel and compared with the position of the descending column, marks the time. Fig. 143 represents the form of the water clock. 3-il. The force of currents, whe- ther in pipes, canals, or rivers, is more or less resisted, and their velocity re- tarded, by the friction which takes place be- tween those surfaces of the liquid and the solid which are in contact. This explains a fict which may be observed in all rivers: that the velocity of a stream is always greater at the center than near the bank, and the velocity at the surface is greater than the velocity at the bottom. 342. If a given quantity of liquid must pass through pipes or channels of unequal section in the same time, its velocity will increase as the transverse section diminishes, and dimin- ish as the area of the section increases. How is the ve- locity of water in pipes and rivers retard- ed ? At what part of a Btrenni is the velocity greatest? In a channel of iinoqual sec- tion, how will the velocity of a current be affected ? 152 WELLS'S NATURAL PHILOSOPHY. This fact is familiar to every one -who obsen^es tlie course of brooks or rivers: wnerever the bed contracts, tlie current becomes rapid, and on tho contrary if it widens, the stream becomes more sluggish. 343. A very slight declivity is sufficient to give motion to ^^*is'"*siiffi- running water. Three inches to a mile in a smooth, straight ciont to give channel, gives a velocity of about three miles per hour. j!]g'^^tg° "'°" The river Ganges, at a distance of 1,800 miles from it3 mouth, is only 800 feet above the level of the sea. Tho i-vcrage rate of inclination of the surface of the Mississippi is 1.80 for the first 1 indred miles from the Gulf of Mexico, 2 inches for the second hundred, 2.30 lor the third, and only 2.57 for the fourth. ^ . . The velocity of rivers is extremely variable; the slower class jiveraiTR vcloci- moving from two to three miles per hour, or three or four feet ty of rivers ? pgj. ggcond, and the more rapid as much as six feet per second. The mean velocity of the Mississippi, near its mouth, is 2.26 miles per hour, or 2.95 feet per second.* The quantity of water which passes over the beds of rivers in a given time is very various. In the smaller class of streams it amounts to from 300 to 350 cubic feet per second. In tho smaller class of navigable rivers, it amounts to from 1,000 to 1,200 cubic feet; and in the larger class to 14,000 cubic feet and upward. It is estimated that tho Mississippi discharges 12 billions of cubic feet of water per minute.f • In the construction of water-channels for drainage, the regulation of inclination neces- sary to produce free flowage of the water, is a matter of great importance. Thisinclinntion varies greatly with the size of the stream of water to he conducted off. Large and deep rivers run sufficiently swift with a fall of a few inches per mile ; smaller rivers and brooks require a fall of two feet per mile, or 1 foot in 2,.500. Small brooks hardly keep an open course under 4 feet per mile, or 1 in 1,200; while ditches and covered drains require at least S feet per mile, or 1 in GOO. Furrows of ridges, and drains partially filled with loose materials, require a much greater inclination. t A question of some interest relative to the course and flow of rivers, may, perhaps, be appropriately considered in this connection. The question is as follows: Do the Mississippi, .and other rivers whose courses are northerly and southerly, flow up hill or down hill? The Mississippi runs from north to south. If its source were at the pole and its mouth at the equator, the elevation of the mouth would be thirteen miles higher than its source, as this is tlie difference between the equatorial and the polar radii of the earth. On this principle, the mouth of the Mississippi is two and a half miles more ele- vated than its source. Does it run up hill, and if so, how has its course and motion originated? The problem, although apparently one of difficulty, admits of an easy Bolution. The centrifugal force, caused by the rotation of the earth, has changed the form of our planet from that of a perfect sphere to that of an ellipsoid, or a sphere flattened at the poles, in which the length of the largest radius, e.-jceeds the shorter by thirteen miles, the present form being the figure of equilibrium under the present conditions. The cohesion of the solid particles of the earth has resisted, and does resist, to a limited e.xtent, the influence of the centrifugal force which has changed the original figure ; but the par- ticles of liquid on the eartli's surface, being perfectly free to move, yield to the influence, and are at rest only so long as the condition of equilibrium is undisturbed, and always move in such a way as tcf restore it when it is disturbed. Water, consequently, always flows from places which are above the figure of equilibrium, to those which are below it. Kow the mouth of the Mississippi is two and a half miles more distant from the center of HYDRAULICS. 153 How are waves 344. When OHG porlioQ of a liquid is dis- Burfaces form- tui'bed, the dlsturbauce (in consequence of tlie '^^- freedom with which the particles of a liquid move upon each other) is communicated to all the other portions, and a wave is formed. This wave propagates itself into the unmoved spaces adjoining, continually en- larging as it goes, and forming a series of undulations. ^ 345. Ordinary sea waves are caused by^he What is the .J c C ciigin of sea wmd prcssin^; unequally upon the suriace or waves ? ■*■ ,^. the water, depressmg one part more than an- other : every depression causes a corresponding elevation. Where the water is of sufficient depth, waves have only a Does the sub- vertical motion, i. e., up and down. Anv fioatin'^ body, as a stance of the i i r . o ji wave actually buoy, floating on a wave, is merely elevated and depressed if sta"ionary r alternately; it does not otherwise change its place. The apparent advance of waves in deep water is an ocular decep- tion : the same as when a corkscrew is turned round, the thread, or spiral, appears to move forward. 346. A wave is a form, not a thing; the form advances, but Why do waves , , ^ , -rr-. , , ■ always break not the substance of the wave. U hen, however, a rock rises against tha ^q ^\^f^ surface, or the shore bv its shallowness prevents or re- shore ? - ^ tards the oscillations of the water, the waves forming in deep water are not balanced by the shorter undulations in shoal water, and they consequently move forward and form breakers. Thus it is that waves always break against the shore, no matter in what direction the wind blows. When the shore runs out very shallow for a great extent, the breakers are distinguished by tha name of surf On the Atlantic, during a storm, the waves have been observed to rise to a height of about forty-three feet above the hollow occupied by a ship ; tlie total distance between the crests of two large waves being 559 feet, which distance was passed by the wave in about seventeen seconds of time. the earth (i. e., the center of figure) than the source is. But if it had not been for the restraining influence of the cohesive force prevailing among the solid particles, it would have been, through the action of the centrifugal force, three miles higher, instead of two and a half It is therefore below the surface of equilibrium, and the water flows south to fill up the proper level. The question as to whether the river flows up, or down, depends on the meaning we attach to the words used. If by row>" we mean toward the earth's center of figure, or toward that part nf the earth's surface where the attraction of gravity is the greatest, as at the poles, then the Mississippi runs up hill. If, on the contrary, down means below the surface of equilibrium, and w means above the surface of equilibrium, then the Mis- sissipi flows downward. If the earth were a perfect sphere, and without rotlst a liq- ished ; the quantity of liquid displaced will bo less, and the surface, acting as a wedge, or inclined plane, will possess a mechanical advantage, since in displacing the liquid it pushes it aside, instead of driving it forward. The determination of the particular form which should be given to a mass of matter in order that it may move through a hquid with the least resistance, is a problem of great complexity and celebrity in the history of mathematics, inasmuch as it is connected with nearly all improvements in navigation and naval architecture. The principles involved in this problem require that the length of a vessel should coincide with the direction of the motion imparted to it ; and they also determine the shape of the prow and of the surfaces be- neath the water. Boats which navigate still waters, and are not intended to carry a great amount of freight, are so constructed that the part of the bot- tom immersed moves against the liquid at a very obUque angle. Vessels built for speed should have the greatest possible length, with merely the breadth necessary to stow the requisite cargo. The form and structure of the bodies of fishes in general, are such as to en- able them to move through the water with the least resistance. _,_ ., 348. In the paddles of steamboats, that one is only com- Wnen are the ^ "^ paddles of a pletely effectual in propelling the vessel which is vertical in effecTivr?'^™''^* the water, because upon that one alone does the resistance of the water act at right angles, or to the best advantage. In the propulsion of steamboats, it is found that paddle-wheels of a given diameter act with the greatest effect when their immersion does not exceed the width, or depth, of the lowest paddle-board ; their effect also increases with the diameter of the wheel. J. The amount of power lost by the use of the paddle wheel wheel an ad- as a means of propelhng vessels is very great, since, in addi- vantafreous ^j ^^ ^^10 fact that onlv the paddle which is vertical in the motnod of ap- - '^ rlyinj? power water is fully effective, the scries of paddles in descending vessels"?^*'^^ ^^^ *^^ water, are obliged to exert a downward pressure, which is not available for propulsion, and in ascending, to hft a considerable weight of water that opposes the ascent, and adheres to the paddles. The rolling of the vessel, also, renders it impossible to maintain the paddles at the requisite degree of immersion necessary to give them their greatest efficiency ; one wheel on one side being occasionally immersed too HYDRAULICS. 155 deeply, while the other wheel, on the other side may be lifted entirely out of water. 349. To remedy in some degree these causes of inefBcienoy and waste, the submerged propelling-wheel, known as the screw- iwopelltr, has been introduced within the last few years. The screw-propeller consists of a wheel resembUng in its form the threads of a screw, and rotating on an axle. It is placed in the stem of the vessel, below the water-line, immediately in front of the rudder. Fig. 144 represents one form of the screw-propeller, and its locatioa in reference to the other parts of the vesseL Describe the construction and .Tction of the scruw-pro- pcller. Fig. 144. The manner in which the screw-propeller acts in impelling the vessel for- ward, may be understood by supposing the wheel to be an ordinary screw, and the water surrounding it a solid substance. By turning the screw in ono du'ection or the other, it would move through the water, carrying the vessel with it, and the space through which it would move in each revolution would be equal to the distance between two contiguous threads of the screw. In fact, the water would act as a fixed nut, in which the screw would turn. But the water, although not fixed in its position as a solid nut, yet offers a considerable resistance to the motion of the screw-wheel ; and as the wheel turns, driving the water backward, the reaction of the water gives a propul- sion to the vessel in a contrary direction, or forward. The great advantage of the screw-propeller is, that its ac- tion on the water will be the same, no matter to what degree it may be inunersed in it, or how the position of the vessel on the surface of the water may be changed. 3.50. The application of the force of water in motion for im- pelling machiuery, is most extensive and familiar. The sim- plest method of applying this force as a mechanical agent, i3 by means of wheels, which are caused to revolve by tha Wliat Is the preat advan- tatre of thR screw-propelliT over the pad- dle-wheel ? What is the simplest meth- od of using water as a mo- tive power f 156 WELLS'S NATURAL PHILOSOPHY. ■weight, cr pressure, of the water applied to their circumferences. These wheels are mounted upon shafts, or axles, which are in turn connected with the machinery to which motion is to be imparted. intoho^rmany ^^^- Tlie water-wheels at prcsciit most'geii- watpr-wheers eKilly usGcl may be divided into four classes — divided? ^i^Q Undershot, the Overshot, the Breast Wheel, and the Tourbine Wheel. D scribe the C retraction of t 1 Undersliofc Wheel. 352. The Undershot FiG. 145. "Wheel consists of a wheel, on the circumference of which are fixed a number of flat boards called '■'■ flvat-boards" at equal distances from each other. It is placed in Buch a position that its lower floats are im- mersed in a running stream, and is set in motion by the impact of the water on the boards as they successively dip into it. A wheel of this kind will revolve in any stream which fiirnishes a current of suffi- cient power. Fig. 145 represents the construction of the undershot wheel. This form of wheel is usually placed in a "race-way," or narrow passage, in such a manner as to receive the fuU force of a current issuing from the bottom of a dam, and striking against the float-boards. And it is important to re- member, that the moving power is the same, whether water foils downward from the top of a dam to a lower level, or whether it issues from an opening made directly at the lower level. This will be obvious, if it is considered that the force with which water issues fi-om an openinf? made at any point in the dam will be equal to that which it would acquire in falling from the sur- face or level of the water in the dam down to the same point. The undershot wheel is a most disadvantageous method of applying the power of water, not more than 25 per cent, of the moving power of the water being rendered avaUable by it. 353. In the Overshot "Wheel, the water is received into cavities or cells, called "buckets," formed in the circumference of the wheel, and so shaped as to retain as much of the water as possible, until they arrive at the lowest part of the wheel, where they empty themselves. The buckets then ascend empty on the other side of the wheel to be filled as ^^^ before. The wheel is moved by the weight of tb.o water contained in the buckets on the descending side. Fig. 146 repre- sents an overshot wheel What propor- tion of power is lost by the un- dershot wheel ? Describe the construction of the Overshot Wheel. Fig. 146. HYDRAULICS. 157 Tho overshot wheel is one of the most effective varieties of ■water-wheels, and receives its name from the circumstance that the water shoots over it. It requires a fall in the stream, rather higher than its own diameter. "Wheels of this kind, when well constructed, utilize nearly three fourths of the mov- force of the water. 354. The Breast TVheel may be considered as a variety intermediate between the overshot and the undershot wheels. In this, the water, instead of falling on the wheel from above, or passing entirely beneath it, is deUvered just below the level What propor- tion of tho moving power is utilized by tlie overshot irhcel ? ins Describe the construction of the Breast- ■»Tieel. Fig. 147. of the axis. The race-way, or passage for the water to descend upon the side of tho wheel, is built in a circular form, to fit the circumference of the wheel, and the water thus inclosed acts partially by its weig'..t, and partially by its impulse, or momentum. Fig. 147 represents a breast-wheel, with it3 circular race-way. The breast-wheel, when well constructed, will utilize about 65 per cent, of the mov- ing power of the water. It is more efBcient than the undershot wheel, but less than tho overshot. It is therefore only used where the fall happens to be particidarly adapted for it. 355. The fourth class of water-wheels, the " Tour- ^^^- ^'^^• bine," or " Turbine,"' is a wheel of modem invention, and is the most powerful and economical of all water- engines. The principles of the construction and action of the Tourbine wheel may be best understood by a previous examination of the construction of another water- engine known as "Barker's MiU." (See Fig. 148.) „ ., „ This consists of an upridit lube or Descnbe the ,. , ^ . , , • , i, construction of cvhnder, furnished with a smaller Barker's MUl. cross-tube at the bottom, and en- larged into a funnel at the top. The whole cylinder is so supported upon pivots at the top and bottom, that it revolves freely about a vertical axis. It is evident if there are no openings in the ends of tho cross-tubes, and the whole is filled with water, that the entire arrangement will be simply that of a close vessel filled with water, without any tendency to motion. If, however, the ends of the arms, or cross-tube, have openings on the sides, opposite to one another, as is represented in the figure, the sides of the tube on which the openings are, will be relieved from the pressure of the column of water in the Upright tube by tho water flowing out, while the pressure on the sides oppo- 158 WELLS'S NATURAL THILOSOPHT. Fia. 149. Bite to them, which have no openings, will remain the same. The machine, therefore, will revolve in the direction of the greater pressure, that is, in a direction contrary to that of the jets of water. A supply of water poured into the funnei-head, keeps the cylinder full, and the pressure of the column of water constant. - « The action of this machine may also be explained according to another view : the pressure of the column of water in the upright tube, will cause the water to be projected in jets from the openings at the ends of the arms in opposite directions ; when the recoil, or reaction of these jets upon the ex- tremities of the cross-tubes, gives a rotary motion to the whole machine upon its vertical axis. ^ ., , The Tourbine wheel derives its motion, like the Barker's Describe the .„ „ , . „ , ^ i c i constnidtion mill, from the action of the pressure of a column ol water. and action of j^. consists of a flxed, horizontal cyhnder, A B, Fig. 149, in Wheel. the center of which the water enters from an upright tube or cylinder, corresponding in position to the upright cyhnder of a Bark- er's mill. The water descend- ing in the tube diverges from the center in every direction, through curved water-channels, or com- partments, A and B, formed in the horizontal cylinder, and escapes at the circumference. Around the fixed horizontal cylinder, a hori- zontal wheel, D, in the form of a ring or circle, is fitted, with its rim formed into compartments exactly similar to the comijartments of the fixed cylinder, with the exception that their sides curve in an oppo- site direction. The water issuing from the guide-curves A B, strikes against the curved compartments of the wheel C B, and causes it to revolve. The wheel, by attachments beneath tlio fixed cylinder A B, is connected with a shaft, E, which passes up tlirough the fixed and upright cylinder, and by which motion is imparted to machinery. The Tourbine wheel may be used to advantage with a fall Sency "of of Water of any height, and will utUize more of the force of Hie Tourbine the moving power than any other wheel — amounting, in som wheel ? instances, as at the cotton factories at Lowell, Mass., to up- ward of 95 per cent, of the whole force of the water. Is it possible 356. It may appear strange to those unacquainted with the to construct a action of hydraulic engines, that so much of the power exist- which will rcn- ing in the agent we use for producing motion, as running dcT the whole water, should be lost, amounting in the undershot wheel to pojrer ava a- ^^ ^^^ ^^^^^ ^^ ^^^ ^hole power. This is due partiaUy to the HYDRAULICS. 159 friction of the water against the surfiices upon which it flows, and to the fric- tion of the wlieel wliich receives the force of the current i'orce is also lost bj changing the direction of the water in order to convej^ it to the machinery ; iu the sudden change of velocity which the water undergoes when it tirst strikes the wheels ; and more than all, from the feet that a considerable amount of force is left unemployed in the water which escapes with a greater or less velocity from every variety of wheel. It may be considered as practically impossible to construct any form of water-engine which will utihze the whole force of a current of water. 357 Water, although one of the most abimdant substances in nature, and a universal necessity of life, is not always found in the location iu which it is desirable to use it. Mechanical arrangements, therclbre, adapted to raise water from a lower to a higher level, have been among the earliest inventions of every country. What were ^^S. The application of the lever, in the rangeraentsf'; ^rm of the old-fashioned well-sweep (still raising water ? ^^gyj -j^ many pai'ts of this country, and throughout Eastern Asia), of the pulley and rope, and the wheel and axle in the form of the windlass, were un- doubtedly the earliest mechanical contrivances for raising water. The screw of Archimedes, invented by the philosopher whose name it bears, is a contrivance for raising water, of great antiquity. This machine, represented in Fig. 150, consists of a tube wound in a spiral form about a solid cylinder, A B, which is made to revolve by turn- ing tlie handle H. This cylinder is placed at a certain inclination, with its lower extremity resting in the water. As the cylinder is made to ^^\ revolve, the end of the tube dijis into the water, and a certain portion er- ters the orifice a. By continuing the revolution of the cylinder, the water flows down a series of inclined planes, or to the under side of the tube, and if the inclination of the jube be not too great, the water will finally flow out at the upper orifice into A proper receptacle. The foUowing diagram, Fig. 151, representing the curved tube in two opposite positions, will illustrate the action of the Archimedes screw. Suppose a marble dropped into the tube at a, fig. 1, : if it was kept stationary iu the Descnbe the Archimedes screw. Fig. 150. 160 WELLS'S NATURAL PHILOSOPHY. Fig. 151. ^adually roll forward to c. When W.1S the tube until it was turned half round, as in the position, tig. 2, the marble would be at a'; now, if at liberty to move, it would roll down to b'. but this effect, which we have supposed accom plished all at once, is really, gradually performed and a rolls down toward 6' by the gradual turn ing of the tube, and reaches 6' as soon as the screw comes into the position marked in fig. 2 another half turn of the screw would bring it into its first position, and the marble would 359. The common suction-pump is a later discovery than the screw of Archimedes, and is supposed to have been invented by Ctesibius, an Athenian engineer who lived at Alexandria, in Egypt, about the middle of the second century before the Christian era.* common pump invented ? Pescribe the constniotion of the chain-pump Fig. 152. SCO. The chaiu-pump consists of a tube, or cyl- inder, the lower part of which is immersed in a well or reser- voir, and the upper part enters the bot- tom of a cistern into which the water is to be raised. An endless chain is car- ried round a wheel at the top, and is furnished at equal distances with flat discs, or plates, which fit tightly in the tube. As the wheel revolves, they suc- cessively enter the tube, and carry the water up before them, which is dis- charged into the cistern at the top of the tube. The machine may be set in mo- tion by a crank attached to the upper wheel. Fig. 152 represents the construction and arrangement of the chain-pump. , . ^ ., The chain-pump will In wn.at sitna- '■ ^ lions is this act with its greatest ef Chnin-pump f . , ^^ cyWudCT generally used ? ' in which the plates and chain move, can be placed in an inclined position, instead of vertically. It is used generally on board of ships and in sit- uations where the height through which the water is to be elevated is not very great, as iu cases where the founda- tions of docks, etc., are to be drained. * The Buction-pump, and other machines for raising -n-nter which depend upon the pressure of the atmosphere, are described under the head of Pacumatics. \ i>S^~\^ N\^$$J;^S^iilMW HYDRAULICS. 161 For irhat other purposes than raising water is the chain- pump used ? ■^^^latisan Hy draulic Kam? Describe the construction of the Hydraulic Kam. This machine i.s not, however, used exclusively for raising water. Its application, in principle, may be seen in any grist- mill, where it conveys the flour discharged from the stones, to an upper part of the building, where it is bolted. Dredg- ing machines for elevating mud from the bottom of rivers, are also constructed on the same principle. 361. The Hydraulic Ram is a niacliine constructed to raise water by taking advantage of the imijulse, or momentum, of a current of water sud- denly stopped in its course, and made to act in another direction. The simplest construction of the hydraulic ram is repre- sented in Fig. 153, and its operation is aa follows: — At the end of a pipe, B, connected with a spring, or reservoir, A, somewhat elevated, from which a supply of water is derived, is a valve, E, of such weight as just to fall when the water is quiet, or still, Fig. 153. within the pipe; this pipe is con» nected with an air-chamber, D, from which the main pipe, F, leads ; 1^J^ this air-chamber is provided with I ^^^ a valve opening upward, as shown i iflA in the cut. Suppose now, the water being still within the tube the valve E to open by its own weight; immediately the stream begins to run, and the water flow- ing through B soon acquires a momentum, or force, sufBcient to raise the valve E up against its seat. The water, being thus .suddenly ar- rested in its passage, would by its momentum burst the pipe, were it not for the other valve in the air-chamber, D, which is pressed upward, and allows the water to escape into the air-chamber, D. The air contained in the chamber D is condensed by the sudden influx of the water, but immediately reacting by means of its elasticity, forces a portion of the water up into the tube F. As soon as the water in the pipe B is brought to a state of rest, the valve of the air-chamber closes, and the valve E falls down or opens ; again the 6tream commences running, and soon acquires sufRcient force to shut the valve E ; a new portion is then, by the momentum of the stream, urged into the air-chamber and up the pipeF; and by a continuance of this action, water will be continually elevated in the pipe F. Fig. 154 represents a more improved construction of the ram, in which by the use of two air-chambers, C and F, the force of the machine is greatly in« creased. A represents tlie main pipe, B the valvo from whence the water escapes, G the pipe m which it is elevated. 162 WELLS'S NATUKAL PHILOSOPHY. Fig. 154 piHiHi|inmii 1 I i^tiii II nil] ili'll,, li'J|ll|l|l|'||'Hllllll|IIH_ T ! 1 ^te^i^^^^^^^'^^*'^-'^''^^ ^ "^^^ ^ ^ "^^ '^ As this machine produces a kind of intermitting motion from the alternate flux and reflux; of the stream, accompanied with a noLse arisinsr from the shock, its action has been compared to the butting of a ram ; and hence the name of the machine. It will be seen from these details, that a very insignificant pressing column of water, running in the supply pipe, is capable of forcing a stream of water to a very great height, so that a sufficient fall of water may be obtained in any running brook, by damming up its upper end to produce a reservoir, and then carrying the pipe down the channel of the stream until a sufficient fall is obtained. A consideralile length of descending pipe is desirable to insure tlio action of tiie stream, otherwise the water, instead of entering the air-vessel, may be thrown back, when the valve is closed, into the reservoir. science of Pneumatics ? CHAPTETv X. PNEUMATICS. What is the ^^2. Pneumatics is that department of physical science which treats of the motion and pressure of air,* and other aeriform, or gaseous substances. Into what uvo . ^^^^ Acriform, or gaseous bodies, may be classes may au divided iuto Iwo cUisses, viz., the permanent aeriform sub- ' ' ^ stances be di- gases, or ihosc wliicii undcr ail ordinary cir- cumstances of temperature and pressure are always in the gaseous state, as common air ; and the va- pors, wdiich may readily be condensed by pressure, or the diminution of temperature, into liquids, as steam, or the vapor of water. 364. Atmospheric air is taken as the type, or representative, of all perma- nent gases, and steam as the type of all vapors, because these substances pos- sess the general properties of gases and viipors in the utmost perfection. What is the ^^^- The atmosphere is a thiu, transparent fttmospherc? fluid, Or aeriform substance, surrounding the earth to a considerable height above its surface, and which by its peculiar constitution sup^iorts and nourishes all forms of aniaial or vegetable life. • Atmospheric air is conijioseil of oxygen and nitrogen mixed tngether in the proportion of seventy-nine parts of nitrogen and twenty-one of oxygen, or about four-fiftlis nitrogen to one-fifth oxygen. These t\ro gasos existing in the atmosphere are not chemiciilly com- bined with each other, but merely mixed. Beside these two ingredients there is always in the air, at all places, carbonic acid gas »nd watery vapor, in variable proportions, and sometimes also the odoriferous matter of flowers, and other volatile substances. The air in all regions of the earth, and at all elevations, never varies in compositior, so fai as regards the proportions of oxygen and nitrogen which it contains, no matter whether It le collected on the top of hi;^h mountains, over marshes, or over deserts. It is a wonderful principle, or law of nature, that when two gases of different weights, or specific gravities, are mixed together, they can not remain separate, as fluids of differ- ent densities do, but diffuS3 th'-mselvcs uniformly throucrhout the whole sp.ace which both occupy. It is, therefore, by this law that a vapor, arising by it? own elasticity from a volatile substance, is caused to extend its influence and mingle with the surrounding at- mosphere, until its efffcts become so enfeebled by dilution as to be imperceptible to the Bcnsos. Tbus wo are enabled to enjoy and perceive at a di".tance the odor of a flower- Carden, or a perfume which has been exposed in an apartment. 164 WELLS'S NATURAL PHILOSOPHY. The atmosphere is not, as is generally regarded, innsiblo. Is the atmos- "^Jien seen through a great extent, as when we look upward p he re visible ? o o i i- in the sky on a clear day, the vault appears of an azure, or deep blue color. Distant mountains also appear blue. In both these instances the color is due to the great mass of air through which we direct our vision. The reason that we do not observe this color in a small quan- a small quanti- tity of air is, that the portion of colored light reflected to the K) •"/ T"^ ?^^' ^^'° ^^ * limited quantity is insufficient to produce the requis- ite sensation upon the eye, and in this way excite in the mind a perception of the color. Almost aU slightly transparent bodies are exam- ples of this fact. If a glass tube of small bore be filled with sherry wine, or wine of a simi- lar color, and looked at through the tube, it will be found to have all the appearance of water, and be colorless. If viewed from above, downward, in the direction of its length, it will be found to possess its original color. In the first instance, there can be no doubt that the wine has the same color as the liquid of which it originally formed a part ; but in the case of small quantities, the color is transmitted to the eye so faintly, as to be inadequate, to produce perception. For the same reason, the great mass of the ocean appears green, while a small quantity of the same water contained in a glass is perfectly colorless. Poes air pos- ^^^- -^^'''j ^^ commoii witli otliGr material Imtui 'quau- substaiices, possesses all the essential quali- ties of matter? ^jgg of matter, as impenetrability, inertia, and weight. 367. The impenetrability of air may be sIio-rti by taking a ■UTiat are , „ , ^ , / , , , . . / . ^ proofs of the hollow vessel, as a glass tumbler, and immersing it in water impcnetrabiU- ^j^^ jjg niouth downward ; it will be found that the water ty of air / ' will not fill the tumbler. If a cork is placed upon the water under the mouth of the tumbler, it will be seen that as the tumbler is pressed down, the air in it will depress the surface of the water on which the cork floats. The diving-bell is constructed on the same principle. 3G8. The inertia of the air is shown by the resistance which "What are proofs of the it opposes to the motion of a body passing through it. Thus, inertia of air ? jf ^^q open an umbrella, and endeavor to carry it rapidly with the concave side forward, a considerable force wUl be required to overcome the resistance it encounters. A bird could not fly in a space devoid of air, even if it could exist without respiration, since it is the inertia, or resistance of the particles of the atmosphere to the beating of the wings, which enables it to rise. The wings of birds are larger, in proportion to their bodies, than the fins of fishes, because the fluid on which they act is less dense, and has proportionally less inertia, than the water upon which the fins of fishes act. To what ex- ^69. Air is highly compressible and perfectly tent is air plnctiV compressible? '-iclt'LH^. By these two qualities air and aU other gaseous substances PNEUMATICS. 165 are particularly distinguished from liquids, which resist compression, and pos- sess but a small degree of elasticity. Illustrations of the compressibility of air are most familiar. A quantity of air contained in a bladder, or India-rubber bag, may be easily forced by the pressure of the hand, to occupy less space. There is, indeed, no theoretical limit to the compression of air, for with every additional degree of force, an additional degree of compression may be obtained. ^ . The elasticitv, or expansibility of air, also manifests itself Does air pos- . . > r- j \ Bess any con- in an unlimited degree. Air cannot be said to have any volume ?^^ °^ original size or volume, for it always strives to occupy a larger space. What are illus- When a part of the air inclosed in any vessel is withdrawn, trations of the ' expansibility that which remains, expanding by its elastic property, always of air ? gjls the dimensions of the vessel as completely as before. If nine tenths were withdrawn, the remaining one tenth would occupy the same space that the whole did formerly. This tendency of air to occupy a larger space, or in other words, to increase its volume, causes it when confined in a vessel, to continually press against the inner surfltce. If no corresponding pressure acts from the outer surface, the air will burst it, unless the vessel is of considerable strength. This fact may be shown by the experiment of placing a bladder partially filled with air be- neath the receiver of an air-pump, and by exhausting the air in the receiver the pressure of the external air upon the outer surface of the bladder is re- moved. The elasticity of the air contained in the bladder being then unre- sisted by any external pressure, will dilate the bladder to its fiillest extent, and oftentimes burst it. v, . . ^ . 369. Air, as well as all other g-ases and va- .HasaiT weight? ' , ° pors, possesses weight. The weight of air may be shown by first weighing a suitable vessel fiUed with air ; then exhausting the air from it by means of an air-pump, and weigh- ing again. The difference between the two weights will be the weight of the air contained in the vessel. The weight of 100 cubic inches of air is about 31 grains. To what is the ^'^^^ "^^^ elasticity, or expansion of air is due ^rlulT °^ **^ ^^^ peculiar action of the molecular forces among its particles, which manifest themselves in a very different manner from what they do in solid and liquid bodies. In solid bodies, these forces hold the molecules, or particles together so closel3', that they can not change their respective positions; they also hold together the particles of liquid bodies, but to such a limited extent only, as to enable the particles to move upon each other with perfect freedom. But in gases, or aeriform substances, the molecular forces act repulsively, and give to the particles a tendency to move away from each other; and this to so great an ex- tent, that nothing but external impediments can hinder their further expansion. 166 WELLS'S NATURAL PHILOSOPHY. What law reg- ulates till' den- sity of the at- mosphere ? Fig. 155. . . The question, therefore, naturally occurs in this connection, What limits . ^^. ,,..,,, • , , , the atmosphere Tiz. : II air exijands unlimitedly, when unrestricted, wliy does to the earth? j^^j- q^j. atmosphere leave the earth and difluse itself through- out space indefinitely ? Tiiis it would do were it not for the action of gravi- tation. Tiie particles of air, it must be remembered, possess weight, aud by gravity are attracted toward the center of the earth. This tendency of gravity to condense the air upon the earth's surface, is opposed by the mutual repulsion existing between the particles of air. These two forces counterbalance eaoh ether : the atmosphere will therefore expand, that is, its particles will separata from one anotlicr, until the repulsive force is diminished to such an extent as ta render it equal to the weight of the particles, or what is the same thing, tc the force of the attraction of gravitation, when no further expansion can take place. "We may therefore conceive the particles of air at the upper surface of the atmosphere resting in equilibrium, under the influence of two opposite forces, viz., their own weight, tending to carry them downward, and the mutual repulsion of the particles, which constitutes the elasticity of air, tend, ing to drive them upward. 371. The density of the air, or the quantity contained in a given bulk, decreases with the altitude, or height above the surface of the earth. This is owing to the diminished pressure of the air, and the decreasing force of gravity. Those portions directly incumbent upon the earth are most dense, because tliey bear the weight of the superincumbent portions; thus, the hay at the lower part of the stack bears the weight of that above, and is therefore more compact and dense. (See Fig, 155.) This idea may be conveyed by the gradual shading of the figure, which indicates the gradual diminution in the density of the atmosphere in proportion to its altitude. wiien is air 372. Air is Said to be rarefied said to be rare- -, . , . , , , -, ficd? when it IS caused to expand and occupy a greater space. Generally, when we speak of rarefied air, we mean air that is expanded to a greater degree, or is thinner, than the air at the immediate surface of the earth. 373. The great law governing the compressibility of air, which is known irom its discoverer as " Mariotte's Law," may be stated as follows: Tlie vohime of space which air occupies is in- versely as the pressure upon it. If the compressing force be doubled, the air which is compressed ■will occupy one half of the space: if the compressing force be increased in a three- fold proportion, it will occupy one third the space ; if fotiifold, one fourth the space, and so on. ■What is Ma- riotte' s Law T PNEUMATICS. 167 The relation between the compressibility of air, and its elasticity and dens- ity, also obeys a certain law whicli may thus be expressed : — 374. The density and elasticity of air are directly as the force of compression. Tliis relation is clearly exliibited by the following table : — With the same amount of air, occupying the space of What relation exists between the compressi- bility of air and its elastic- ity and density ? 1, 2) 3> 4; 3) t^) I on> the elasticity and density will be 1, 2, 3, 4, 5, 6, 100. Hence by comprcssinsr air into a yery small space, by means What are il- „ •' ^ " •, i 5 r * u lustrations of of a proper apparatus, we can mcrease its elastic lorce to sucn the elastic force ^^ extent as to apply it for the production of very powerful effects. The well-known toy, tlie pop-gun, is an example of Ihe application of this power. The space A of a hollow cylinder, Fig. 156, is inclosed by the stopper, p, at one end, and by the end of the rod, S, at the other end.' This rod being pushed further into the cylinder, the air contained in the space. A, is compressed until its elastic force becomes so great as to drive out the stopper, p, at the other end of tlie cyhnder with great force, Fig. 156. accompanied with a report, BimUar principle. 375. The laws of Mariotte may be lustratc the illustrated and proved by the following laws ofMariotte. experiment : let A B G D be a long, bent glass tube, open at its longer extremity, and fur- nished with a stop-cock at the sliortcr. Tlie stop-cock being open so as to allow free communication with the air, a quantity of mercury is poured into the open end. The surfaces of the mercury will, of course, stand at tlie same level, E F, in both legs of the tube, and will both sustain the weight of a col- umn of air reaching from E and F to the top of the atmosphere. If we now close the stop-cock, D, the effect of the weight of the whole atmosphere above that point is cut off, so tliat the surface, F, can sustain no pressure arising from the weight of the atmosphere. Still, the level of the mercury in both legs of the tube remains the same, because the elas- ticity of the air inclosed in F D is precisely equal, and BuEQcient to balance the weight of the whole column The air-gun is constructed and operated on a Fig. 157. 168 WELLS'S NATURAL PHILOSOPHY. of atmosphere pressing upon the surface, E. If this were not the case, or if there were no air in F D, then the weight of the atmosphere pressing upon the surface E would force the mercury, E B C F, up hito the space, F D. The elasticity of air is, therefure, directly proportionate to the force, or compression, exerted upon it. It is evident that the pressure exerted upon the surface, E, Fig. 157, what- ever may be its amount, is that of a column of air reacliing friSm E to the top of the atmosphere, or, as we express it, the weight of one atmosphere. The amount of this pressure, aceurat.ly detenniued, is equal to the weight, or pressure, which a column of mercury 30 inches high would exert on iho same surface. If then, we pour into the tube, A E, Fig. 157, as much mercury as will raise the surface in the leg A B 30 inches above the surface of the mercury in the leg D C. we shall have a pressure on the surlace of E equal to two atmospheres; and since liquids transmit pressure equally in all directions, the same pressure will be exerted on the air included in the leg D F. This will reduce it in volume one halfj or compress it into half the space, and the mercury will rise in the log D F from F to F'. This weight of two atmospheres reduces a given quantity of air into one half its volume. In the same manner, if mercury be again poured into the tube^ E until the surface of the column in A E is GO inches above the level of the mercury in D F, then the air in D F will be compressed into one third of its original volume. In the same manner it could be shown, by continuing these experiments, that the diminution of the volume of air will always be in the exact proportion of the increase of the compressing force, and its volume can also be increased in exact proportion to the diminution of the compressing force. In fact this law has been verified by actual experknent, until the air has been condensed 27 times and rarefied 112 times. Air has been allowed to expand into more than 2,000 times its bulk, and it would have expanded still more if greater space had been allowed. Air has also been compressed into less than a thousandth of its usual bulk, so as to become denser than water. In this state it still preserved its gaseous forni and condition. 376. The fact that air possesses weight, and consequently Was the weight , ' ., , '. , ■, , of air known exerts pressure, was not known until about two hundred years to the an- g^gQ_ jj^^ ancient philosophers recognized the fact, that air was a substance, or a material thing, and they also noticed that when a solid, or a liquid, was removed, that the air rushed in and filled up the space that had been thus deserted. But when called to give a reason for this phenomenon, they said " that nature abhorred an empty space," or a " vacuum," and therefore filled it up with air, or some liquid, or solid body. What is a 377. A vacuum is a space devoid of matter; vacuum? -^^ general, we mean by a vacuum a space de- void of air. No perfect vacuum can be produced artificially ; but confined spaces can be deprived of air sufficiently lor all experimental and practical purposes. PNEUMATICS. 169 "We do not know, moreover, that any vacuum exists in nature, although there is no conclusive evidence that the spaces between the planets are filled with any material substance. If we dip a pail into a pond, and fill it with water, a hole (or vacuum) is made in the pond aa big as the pail; but the moment the pail is drawn out, the hole is filled up by the water around it. In the same manner air rushes in, or rather is pressed in by its weight, to fill up an empty space. „ , When we place one end of a straw, or tube in the mouth. How does ' ' ' water rise in a and the other end in a liquid, we can cause the liquid to riso tionT ^^ ""''" ^ *^^® straw, or tube by sucking it up, as it is called. We, however, do no such thing ; we merely draw into the mouth the portion of air confined in the tube, and the pressure of the external air which is exerted on the surface of the liquid into which the tube dips, being no longer balanced by the elasticity of the air in the tube, forces the hquid up into the mouth. If, however, the straw were gradually increased in length, we should find that above a certain length we should not be able to raise water into the mouth at all, no matter how small the tube might be in diam- eter ; or, in other words, if we made the tube 34 feet long, we should find that no power of suction, even by the most powerful machinery instead of the mouth, could raise the water to that height. The water rises in the com- mon pump in the same way that it docs in the straw ; but not above a height of 33 or 34: feet above the level of the reservoir. „ ^. 378. The reason why water thus rises in a straw, or pump, How was the ■' > i ri ascent of water remained a mystery until explamed and demonstrated by Tor- Hon'^'firit^Tx" ™^^'' » pupil of Galileo. It is clear that the water is sus- pluined uiid de- taincd in the tube by some force, and Torricelli argued that monstrate whatever it might be, the weight of the column of water sus- tained must be the measure of the power thus manifested ; consequently, if another liquid be used, heavier or lighter, bulk for bulk, than water, then the same force must sustain a lesser or greater column of such liquid. By using a much heavier liquid, the column sustained would necessarily be much shorter, and the experiment in every way more manageable. Torricelli verified his conclusions in the following manner: — He selected for his experiment mercury, the heaviest known UquitL As this is 13^ times heavier than water, bulk for bulk, it followed that if the force imputed to a vacuum could sustain 33 feet of water, it would necessarily sustain 13^ times less, or about 30 inches of mercury. Torricelli therefore made the following experiment, which has since become memorable in the history of ecience : — / He procured a glass tube (Pig. 158) more than 30 inches long, open at ond end, and closed at the other. Filling this tube with mercury, and applying his finger to the open end, so as to prevent its escape, he inverted it, plung- ing the end into mercury contained in a cistern. On removing the finger, he observed that the mercury in tlie tube fell, but did not fall altogether into the cistern ; it only subsided until its surface was at a height of about 30 inches above the surface of the mercury in the cistern. The result was ^vhat Tor- 8 170 WELLS'S NATURAL PHILOSOPHY. How was the conclusion of Torricelli fur- ther verified ? ricelli expected, and he soon FiG, 153, perceived the true cause of the phenomenon. The weight of the atmosphere acting upon the surface of the mercury in the vessel, supports the hquid in the tube, this last being protected from the pressure of the atmosphere by the closed end of the tube. 379. The fact that the col- umn of mer- cury in the tube was sustained by the pressure of the atmosphere, was further verified by an ex- periment made by Pascal in France. He argued, that if the cause which sustained the column in the tube was the weight of the atmosphere act- ing on the external surface of the mercury in the cistern, then, if the tube was trans- ported to the top of a high mountain, where a less quan- tity of atmosphere was above it, the pressure would be less, and the length of the column less. Tlie experiment was tried by carrying the tube to the top of a mountain in the interior of France, and correctly noting the height of the column during the ascent. It was noticed that the height of the column graduaUy diminished as the elevation to which the instrument was carried increased. The most simple way of proving that the column of mercin-y contained in the tube, as in Fig. 158, is only balanced against the equal weight of a column of air, is to take a tube of sufficient length, and having tied over one end a bladder, to fill it up with mercury, and invert it in a cup of the same hquid; the mercury will now stand at the height of about 30 inches ; but if with a needle we make a hole in the bladder closing the top of the tube, the mer- cury in the tube immediately falls to the level of that in the cup. These experiments by Torricelli led to the invention of the Barometer. It was noticed that a column of mercury sus- tained in a tube by the pressure of the atmosphere, the tube being kept in a fixed position, as in Fig. 159, fluctuated from day to day, within certain small hmits. This effect waa IIow did the experiment of Torricelli lead to the inven- tion of the Ba- rometer ? PNEUMATICS. 171 Why should t'le presence of c indensed va- por of water in t!i3 atmos- phere affect its pressure ? naturally attributed to the variation in the weight or pres- ] sure of the incumbent atmosphere, arising from various me- teorological causes. Thus, when the air is moist or filled with vapors, it is lighter than usual, and the column of mercury stands low in the ^ tube ; but when the air is dry and free from vapor, it is heavier, and supports a longer column of mercury. So long as the vapor of water exists in the atmosphere, as a constituent part of it, it con- tributes to the atmospheric pressure, and tlius a portion of the column of mercury in the ba- rometer tube is sustamed by the weight of the vapor ; but when the vapor is condensed, and takes on a visible form, as clouds, etc., then it no longer forms a constituent part of the atmospliere, any more than dust, smoke, or a balloon floating in it does, and the atmospheric pressure being diminished, tlie mercury in the tube falls. In this way the barometer, by showing variations in the weight of the air, indicates also tlie changes in the weather. 380. The space above the mercury in the barometer tube, A D, Fig. 159, is called the Tmricellian vacuum, and is the nearest approach to a perfect vacuum that can be procured by art; for upon pressing the lower end deeper in the mercury, the whole tube becomes completely filled ; the fluid again falling upon elevating the tube, it is therefore a per- fect vacuum, with the exception of a small portion of mercurial vapor. 381. Barometers are constructed in very different forms — the principle remaining the same, of course, in all. The first barometer constructed was simply a tube closed at one end, fiUed with mercur3-, and' inverted in a vessel containing mercury, as in Fig. 159. __ ^ . ,^ A very common form of barometer, What IS the ■' construction of called the "Wheel-Barometer, con- ^^rome'twr'" ^^*^ °^ ^ ^^^^^ ^^^' ^^^^ ^^ ^^® ^°*' torn, and filled with mercury. (See Fig. 160.) The column of mercury in the long arm of the tube is sustained by the pressure of the atmos- phere upon the surface of the mercury in the shorter arm, tlie end of which is open. A small float of iron or glass rests upon the mercury in the shorter arm of the tube, and is suspended by a slender thread, which is passed round a wheel carrying an index, or pointer. As the level of the mercury is altered by a variation of the pressure of the atmosphere, the float resting What is the most perfect v.icuuni with which we are acquainted ? Fig. IGO. 172 WELLS'S NATUKAL PHILOSOPHY. upon the open surflice, is raised or lowered in the tube, moving the index over a dial-plate, upon which the various changes of the weather are lettered. Fig. 160 represents the internal structure of the wheel-barometer, and Fig. 161 its external appear- ance, or casing, with a thermometer attached. _ ., , A very curious barometer, called Aneroid Ba- the "Aneroid Barometer," has been rometer. invented and brought into use withm the last f?w years. Fig. 162 resprcsents its ap- pearance and construction. Its action is dependent en the effect produced by atmospheric pressure on a Fig. 162. metal box, from which the air has been ex- hausted. In the interior of the box is a circu- lar spring of metal, fastened at one extremi- ty to the sides of tlie box, and attached at the otlier extremity by a suitable ar- rangement to a pointer, which moves over a dial-plate, or scale. The in- terior of the box being deprived of air, the atmospheric pressure upon tho external surfaces of the metal sides is very great, and as the pressure varies, these surfaces will be elevated and depressed to a sliglit degree. Tliis motion is communicated to the spring in the interior, and from thence to the pointer, which, moving upon the dial, tlius indicates the changes in the weather, or the variation in the pressure of tho atmosphere. Water, or some other liquid than mercury, may be used for peculiarities of filling the tube of a barometer. But as water is 13-J^ times lighter than mercury, the height of the column in the water- barometer supported by atmospheric pressure, will be 13^- times greater than that of mercury, or about 34 feet high ; and a change wliieh would produce a variation of a tenth of an inch in a column of mercury, would produce a variation of an inch and a third in tlie column of water. Tho Water-barometer is rarely used, for various reasons, one of whioli is, that a barometer 34 feet high ia unwieldy and diJScult to transport. tlie water-ba- rometer ? PNEUMATICS. 173 3S2. The ordinary use of the barometer on land as a weather value of the indicator is extremely limited and uncertain. It has been barometer as a already Stated that the weio-ht of 100 cubic inches of air is weathtr indi- , ^ ../^ m , , • , • , . . cator? about 60 grams, lo obtam this result, it is necessary that tlio experiment should be performed at the level of the sea, and it is also requisite that the temperature of the air should be about 60° Fahren- heit's thermometer, and that the height of the column of mercury in the ba- rometer tube should be 30 inches. As these conditions vary, the weight, or pressure of the atmosphere, and consequently the height of the mercury in the barometer tube must also vary. Especially will the lieight of the mer- curial column vary with every change in the position of the instrument as regards its elevation above the level of the sea. A barometer at the base of a lofty tower will be higher at the same moment than one at the top of the tower, and consequently two such barometers would indicate different com- ing changes in the weather, though absolutely situated in the same place. No correct judgment, thertfore, can be formed relative to the density of the at- mosphere as affecting the state of the weather, without reference to the situ- ation of the instrument at the time of making the obser\'ation. Consequently, no attention ought to be paid to the words "fair, rain, changeable," etc., fre- quently engraved on the plate of a barometer, as they will be found no cer- tain indication of the correspondence between the heights marked, and the state of the weather. _ . . The barometer, however, may be generally relied on for To what extent „ . , . . ,. . -. , /. , , may the ba- fiimishing an indication of the state oi the weather to this ex- roiTieter here- tent: — that a fixll of the mercury in the tube shows the ap- licd on for fore- ' •' ^ tcUinj:; changes proach of foul weather, or a storm; while a rise indicates in the weather ? ^,^^ approach of fair weather. At sea, the indications of the barometer respecting the weather, are gener- ally considered, from various circumstances, more reliable than on land: the great hurricanes which frequent the tropics, are almost always indicated, some time before the storm occurs, by a rapid fall of the mercury. „ , 383. If a barometer be taken to a point elevated above the How mar the . , i -n /• ,i i barometer be surface of the earth, the mercury in the tube will fall ; because mMng'^ ^°\he ^ ^® ascend above the level of the sea, the pressure of the height of atmosphere becomes less and less. In this way the barometer mouutams? maybe used to determine the heights of mountains, and tables have been prepared shov\-ing the degrees of elevation corresponding to the amount of depression in the column of mercury. vTh^t is the 384. The absolute height to which the at- ofThrltmlfs-' ninsphere extends above the surface of the phere? earth is not certainly kno\vn. There are good reasons, however, for believing that its height does not exceed fifty miles. This envelope of air is about as thick, in proportion to the whole prlnhe, as 174 WELLS'S NATURAL PHILOSOPHY. the liquid layer adhering to an orange after it has been dipped in water, ia to the entire mass of the orange. Of the whole bulk of the atmosphere, the zone, or layer which surrounds the earth to the height of nearly 2 3-4 miles from its surface, is supposed to contain one half. The remaining half being relieved of all superincumbent pressure, expands into another zone, or belt, of unknown thickness. Fig. 163 will convey an idea of the proportion which the highest mountains bear to the curvature of the eartli, and the thickness of the atmosphere. The concentric lines divide the atmosphere into six layers, containing equal quantities of air, showing the great compression of the lower layers by the weight of those above them. Fig. 163, ¥hW/jiii//'''M'''i'''/^'0>m'mmM'ii^^^^^^^^^^^ HIMiXATAB. Water is about 840 times the weight of air, taken bulk for bulk, and the weight of the whole atmosphere enveloping our globe has been estimated to be equal to the weight of a globe of lead sixty miles in diameter. If the whole air were condensed, so as to occupy no more space than the same weight of water, it would extend above the earth to an elevation of thirty-four feet. 385. All aeriform, or gaseous substances, like liquids, transmit pressure in every direc- tion equally ; therefore, the atmosphere presses upward, downward, laterally, and obliquely, with the same force. 386. The amount of pressure which the at- mosphere exerts at the level of the ocean is equal to a force of 15 pounds for every square inch of surface. The surface of a human body, of average size, measures about 2,000 square inches. Such a body, therefore, sustains a pressure from the atmosphere amounting to 30,000 pounds, or about 15 tons. The reason we are not crushed beneath so enormous a load, is because the atmosphere presses equally in all directions, and our bodies are filled with liquids capable of sustaining pressure, or with air of the same density as the external air ; What is the comparative weight of the atmosphere ? Hoir is the pressure of aeriform sub- stances exert- ed? What is the amount of pressure ex- erted by the atmosphere ? What pressure is sustained by the human body J Why are we not crushed by the pressure of the atmosphere? PNEUMATICS. 175 so that the external pressure is met and counterbalanced by the internal re- sistance. If a man, or animal were at once relieved of all atmospheric pressure, all the blood and fluids of the body would be forced by expansion to the surface, and the vessels would burst ,„, „ , . Persons who ascend to the summits of very high mountains, What effect is , . ,..,„, • ^ experienced in or who rise to a great elevation m a balloon, have expenenced rising to great ^.j^g most intense sufFeriner from a diminution of the atmos- elevatioas ? ° pheric pressure. The air contained in the vessels of the bodj; being relieved in a degree of the external pressure, expands, causing intense pain in the eyes and ears, and the minute veins of the body to swell and open. Travelers, in ascending the high mountains of South America, have noticed the blood to gush from the pores of the body, and the skin in many places to crack and burst. • We become painfully sensible of the effect of withdrawing principle of the external pressure of the atmospb.ere from a portion of the " c"PPi"S • skin of the body in the operation of cupping. This is efiected in the following manner : a vessel with an open mouth is connected with a pump, or apparatus for exhausting the air. The mouth of the vessel is ap- plied in air-tight contact with the skin ; and by working the pump a part of the air is withdrawn from the vessel, and consequently the skin within the vessel is reUeved from its pressure. All other parts of the body being still subjected to the atmospheric pressure, and the elastic force of the fluids con- tained in the body having an equal degree of tension, that part of the skin which is thus reheved from the pressure swells out, and will have the ap- pearance of being sucked into the cupping-glass. If the lips be applied to the back of the hand, and the breath drawn in so as to produce a partial vacuum in the mouth, the skin will be drawn, or sucked in — not from any force resident in the lips or the mouth drawing the skin in, but from the fact that the usual external pressure of air is removed, and the pressure from within the skin is allowed to prevail The sense of oppression and lassitude experienced in sum- ten feel op- mer previous to a storm, is caused by v -ica S'^storat'^^"" a diminished pressure of the atmosphere. ^^^- ^^^ The external air, in such instances, be- comes greatly rarefied by extreme heat and by the con- densation of vapor, and the air inside us (seeking to become of the same rarity) produces an oppressive and suffocating feeUng. „ . ^ 387. The direct effects of atmospheric Describe tne , .,, , , common suck- pressure may be illustrated by many *'■• practical experiments. If a piece of moist leather, called a sucker, Fig. 1G4, be placed in close contact with any heavy body, such as a stone, or a piece of metal, it will adhere to it, and if a cord be at- tached to the leather, the stone, or metal, may be raised 17( WELLS'S NATURAL PHILOSOPHT. tJpon what p inciple are fl s enabled to w Ik upon the ceiling, etc.? leet. Explain the principle and construction of the exhausting Byrin:^e and air- pump ? Fig. 1G5. by it. The effect of the sucker arises from the exclusion of the air between the leather and the suri'ace of the stone. The weight of the atmosphere presses their surfaces together with a force amounting to 15 pounds on every square inch of the surface of contact. If the sucker could act with full eliect, a disc an inch square would support a weight of 15 pounds; two square inches, 30 pounds, etc. The practical effect, however, of the sucker is much less. 388. The power of flies and other small insects to walk on ceilings, and surfoces presented downward, or upon smooth panes of glass, in opposition to the gravity of their bodies, is gL-ucrally refcred to a sucker-like action of the p;dms of their Kecent investigations have, however, proved, that the effect is rather due to the mechanical action of certain minute hairs growing upon the feet, which are tubular and excrete a sticky liquid. 389. For the purpose of exhibiting the effects produced by the atmosphere in different conditions, and for various practi- cal purposes, instruments have been contrived by which air may be removed from the interior of a vessel, or condensed into a small space to any extent, within certain limits. Tho first of these requirements may be obtained by the use of the instruments known as the exhausting springe and the air-pump. Tlie exhausting syringe consists of a hollow cylinder, generally of metal, B C, Fig. 165, very truly and smoothly bored upon the inside, and having a piston moving in it air-tight. This cylinder communicates by a screw and pipe at the bottom, with any ves- sel, generally called a receiver, from which it is desirable to with- draw the air. The piston has a valve at E, opening upward, and at the bottom of the cylinder another valve precisely similar is placed, which also opens upward, shown at A. Suppose now the piston to be at the bottom of the cylinder and the re- ceiver to be in proper connection — upon raising the piston by the handle, D, a vacuum is made in the cylinder ; immediately the air in the receiver expands, passes through the valve A at the bottom of the cylinder, and fills its interior ; upon depressing the piston, the valve E opening upward permits the air to pass through, and the valve A at the bottom of the cylinder closing, prevents it from passing back into the receiver. Upon again raising the piston, a further portion of air expanding from the receiver, enters the interior of the syringe, and upon depressing the piston, passes out through its valve. It is evident that this operation may be con- tinued as long as the air within the receiver has elasticity sufiBcicnt to force open the valves. The process of removing air from a vessel, or receiver, by means of the ex- hausting syringe is slov.' and tedious, and more powerful instruments, known as air-pumps, are generally employed for this purpose. The modern form of constructing the air-pump is represented by Fig. 166. The principle of ita PNEUMATICS. 17? construction is the same as Fio. 166. that of the exhausting sy- ringe, the piston being work- ed by a lever or handle, as in the common pump, the valves opening and closing with great nicety and perfection. ._,T, , . ,. 381. When VThat 18 the construction of the density the condensing ^f ^^Q ^^j^ jg •ynnge? required to bo increased, the condensing syringe, the converse of the exhausting syringe, is em- ployed. It consists merely of an exhausting syringe, or air-pump, reversed, its valves being so arranged as to force air into a chamber, instead of drawing it out. For this purpose, the valves open inward in respect to the interic of the cylinder, while in the exhausting syringe and air-pump, they open o'ltward. 382. That the air in the inside of vessels is the force which resists and counterbalances the great pressure of the external atmosphere, may be proved by the follo's^'ing experiment : A strong glass vessel. Fig. 167, is provided, open both at top and bottom, and having a diameter of four or five inches. Upon one end is tied a bladder, so as to be completely air-tight, while the other end is placed upon the plate of an air-pump. Upon exhaust- ing the air irom beneath tho bladder, it will be forced inward by the pressure of the air outside, and when the exhaustion has been carried to such an extent that the Btrengtli of the bladder is less than this pressure, it vdM burst with a loud report. What is the ^^^- '^^® air-pump was invented, in experiment of the year 1654, by Otto Guericke, a Ger- ilmisphercsT ™'^°' ''^"^ ^* "^ ^''^''^t P^b"« exhibition of its powers, made in the presence of the emperor of Germany, the celebrated experiment known as the " Magdeburg Hemispheres," was first shown. The Magdeburg Hemispheres, so called from tho city where Guericke resided, consist of two hollow hemispheres of 8* What is an er- perimental proof of the crushing force of the atmos- phere ? Fig. 167. Fig. 168. 178 "WELLS'S NATURAL PHILOSOPHY. Fig. 169. brass, Fig. 168, which fit together air-tight. By exhausting the air in their interior, by means of the air-pump, and a stop-coek arrangement affixed to one of the hemispheres, it will be found that they can not be pulled apart without the exertion of a very great force, since they will be pressed to- gether with a force of 15 pounds for every square inch of their surface. In the exhibition above referred to, given of these hemispheres by Guericke, the surfaces of a pair constructed by him were so large, that thirty horses, fifteen upon a side, were unable to pull them apart. By admitting the air again to their interior, the Magdeburg hemispheres fall apart by their owa weight. Another interesting example of atmospheric pressure is, to fill a wine-glass, or tumbler with water to the brim, and, having placed a card over the mouth, to invert it cautiously. If the card be kept in a horizontal position, the water will be supported in the glass by the pressure of the air agauast the urface of the card. (See Fig. 169.) 384. In a like manner, if we take a principle and jar, and having filled it with water, in- construction of ^^j.^ j^. jjj ^ reservoir or trough, as is rep- resented in Fig. 170, it will continue to bo completely filled with water, the It quid being sustained in it by the pres- the gasometer. Fig. 170. sure of the atmosphere upon the water in the vessel. Such an arrangement enables the chemist to collect and pre- serve the various gases without admix- ture with air; for if a pipe or tube through which a gas is passing be depressed beneath the mouth of the jar, so that the bubbles may rise into it, they will displace the water, and be collected in the upper part of the jar, free of all admixture. The gasometers, or large cylindrical vessels in which gas is collected in gas-works for general distribution, are constructed on this principle. They consist, as is shown in Fig. 171, of a liarge cylindrical reservoir suspended with its mouth downward, and plunged in a cistern of water of somewhat greater diameter. A pipe which leads from the gas-works is carried through the water, and turned upward, so as to enter the mouth of the gasometer. The gas, flowing through the pipe, rises into the gasometer, filling the upper part of it, and pressing down the water. Another pipe, descending from the gasometer through the water, is continued to the service pipes, which supply the gas. The gasometer is balanced by counter weights supported by chains, which pass over pulleys, and just such PNEUMATICS. 179 a preponderance is allowef^. lo it as is sufficient to give the gas contained in it the compression necessary to drive it through the pipes to the remotest part of the district to be illuminated. Fig. 171. Why will not a liquid flow from a tight cask with only one opening? 385. A liquid will not flow continuously from a tight cask after it has been tapped or pierced, unless another opening is made as a vent-hole, in the upper part of the cask. The cask being air-tight, with the exception of a single opening the surface of the liquid in the vessel will be excluded from the atmospheric pressure,- and it can only flow out in virtue of its own weight But if the weight of the liquid be less than the force of the air press- ing upon the mouth of the opening, the liquid can not flow from the cask ; the moment, however, that the air is enabled to act through the vent-hole in the upper part of the cask, the pressure below is counterbalanced, and the hquid descends and runs freely tlirough the opening by its own weight,. If the lid of a tea-pot or kettle be air-tight, the liquid wUl not flow freely from the spout, on account of the atmospheric pressure. This is remedied hj making a small hole in the Ud, which allows the air to enter from without. The Pneumatic Ink-stand, de- signed to prevent tlie ink from thickening, by the exposure of a small surface only to the air, is constructed upon the principles of atmospheric pressure. It consists of a close glass vessel, represented in Fig. 172, from the bottom of wliich a short tube proceeeds, the depth of which is sufficient for the immersion of the pen. By filling the ink-stand in an inclined position, we exclude the What is the principle and construction of the Pneumatic Ink-Etand ? Fig. 172. 180 WELLS'S NATURAL PHILOSOPHY. air in great part from tho interior, and on replacing it in an upriglit position, the ink will be prevented from rising in the small tube and Ho wing over, on account of the atmospheric pressure upon the exposed surface of tlie ink in the small tube, which is much greater than the pressure of the column of liquid in the interior of the vessel. As the ink in the small tube is consumed by use, its surface will gradually fall ; a small bubble of air will enter and rise to the top of the bottle, where it will exert an elastic pressure, which causes the surface of the ink in the short tube to rise a little higher, and this ©fleet will be repeated until all the ink in the bottle has been used. 386k The peculiar gurgUug noise produced when liquid is tittfe ^gurgle freely poured from a bottle, is produced by the pressure of the/ •when a liquid atmosphere forcing air into the interior of the bottle. In the ly out'ofitr^" fii'st instance, the neck of the bottle is filled with liquid, so as to stop the admission of air. When a part has flowed out, and an empty space is formed within the bottle, the atmospheric pressure forces in a bubble of air through the liquid in the neck, which by rushing suddenly into the interior of the bottle, produces the sound. The bottle will continue to gurgle so long as the neck continues to be choked with liquid. But as the contents of the bottle are discharged, the liquid, in flov/ing out, only partially fills the neck ; and, while a stream passes out through the lower half of the neck, a stream of air passes in through tlie upper part. The flow being now continued and uninterrupted, no sound takes place. 387. "Water, and most liquids exposed to the air, absorb a ^iu^water ?^^* greater or less quantity of it, which is maintained in them by the pressure of the atmosphere acting on their surfaces. Boiled water is fiat and insipid, because the agency of heat expels the air which the water previously contained. Fishes and other marine animals ,^ald not live in water deprived of air. The presence of air in water may be shown by placing a presence of air tumbler containing this liquid under the receiver of an air- ihown'^r'^ ^^ pump, and exliausting the air. The pressure of the air being removed from the surface of the water, minute bubbles will ^ake their appearance in the whole mass of the water, and rising to the sur- lace, escape. ,-,_ , The reason that certain bottled liquors froth and sparkle why do some '■ ' tottlcd liquids when uncorked and poured into an open vessel is, that when froth and spar- ^jj^y ^^^ bottled, the air confined under the cork is condensed, and exerts upon the surface a pressure greater than that of the atmosphere. This has the effect of holding, in combination with the Jiquor, air or gas, which, under the atmospheric pressure only, would escape. If any air or gas rise from the liquor after being bottled, it causes a still greater condensation, and an increased pressure above its surface. When the cork is drawn from a bottle containing liquor of this kind, the air fixed in the liquid, being released from the pressure of the air which was condensed under the cork, instantly makes its escape, and rising in bubbles, produces efisrvea- cence and froth. PNEUMATICS. 181 It sometimes liappens that the united force of the air and gases thus con- fined in the bottle, becomes greater than the cohesive strength of the parti- cles of matter composing the bottle ; the sides of the bottle in such cases give way or burst. Those liquors only froth which are viscid, glutinous, or thick, like ale, por- ter, etc., because they retain the little bubbles of air as they rise ; whOe a thin liquor, like champagne, which sufters the bubbles to escape readilj*, sparkles. . 388. The pressure of the atmosphere is connected with tho pressure of the action of breathing. The air enters the lungs, not becauso s mosphere ^j^g^ draw it in, but by the weight of the atmosphere forcing the act of it into the empty spaces formed by the expansion of the air- reat ng . ^^jjg ^j. ^-^^ lungs. The air in turn escapes from the lungs by means of its elasticity ; the lungs, by muscular action, compress the air con- tained in them, and give to it by compression a greater elasticity than the air without. By thia excess of elasticity it is propelled, and escapes by the mouth and nose. 389. It has been proposed to take advantage of the pressure "Wliat is the ^f ^j^g atmosphere for the construction of an atmospheric tele- proposed con- '■ '■ structionofthe graph, or apparatus for conveying tho mails and other matter tele-^aph"*^ °'^^^ great distances with great rapidity. The plan proposed is as follows ; — a long metal tube is laid down, the interior surface of which is perfectly smooth and even. A piston is fitted to the tube in such a manner as to move freely in it and yet be air-tight. To one sido of this piston the matter to be moved, made up in the form of a cyhndrical bundle, is attached. A partial vacuum is then made in the tube before the piston, by means of large air-pumps, worked by steam-power, located at tho further end of the tube, when the pressure of the atmosphere on the other side of the piston impels it forward through the whole length of the exhausted tube. It has been estimated tliat a piston, drawing after it a considerable weight of matter, could in this way be forced tlirough a tube at the rate of 600 miles per hour. 390. The pressure of the atmosphere is taken advantage of in the con- struction of a great variety of machines for raising water ; the most important and familiar of which is the common, or suction pump. Descrihe the The comiiion, or suction pump, consists thr'"common ^f a hollow Cylinder, or barrel, open at both pump. ends, in which is worked a movable piston, which fits the bore of the cylinder exactly, and is air-tight. The pump is further provided with two valves, one of which is placed in the piston, and moves with it, while the other is fixed in the lower part of the pump-barrel. These valves are termed boxes. Fig. 173 represents the construction of tho common pump. The body con- sists of a cyUnder, or barrel, b, the lower part of which, called the suction- 182 WELLSS NATURAL PHILOSOPHY. pipe, descends into the water -vrhich it is designed to FiG. 173. raise. In tlie barrel works a piston containing a valve, p, opening upward. A similar valve, g, is fixed in the bony of the pump, at the top of the suction-pipe. S is a spout from which the water raised by the working of the piston is discharged. The operation of the pump in raising water is as fol- lows ; — when the piston is raised from the bottom of the cjiinder, the air above it is drawn up, leaving a vacuum below the piston ; the water in the well then rushes up through the valve g, and fills the cylinder ; the piston ia then forced down, shutting the valve, g, and causing the water to rise through the piston-valve, p ; the piston is then raised, closing its valve, and raising the water above it, which flows out of the spout, S. ,^ , 391. Water rises in a pump Whv does . . ^ water rise in a siinplv and entirely by the common pump f '■ " n i ' ^ pressure of the atmosphere (15 pounds on every square inch), which pushes it up into the void, or vacuum left by the up- drawn piston. To what height will water rise in the common 392. The common, or suction pump, can will water rise j^^^ j,g^jgQ 'watcr bcvond the point of height at pump? which the column of water in the pump tube is exactly balanced by the weight of the atmosphere. The utmost limit of this does not exceed 34 feet. The height to which water is thus forced up in a pump is simply a question of balance ; 15 pounds' pressure of the atmosphere can support onlj- 15 pounds' weight of water ; and a column of water, one inch square and 34 feet high, will weigh 15 pounds. As the pressure of the atmosphere is subject to va- riations, and as the mechanism of the pump is never absolutely perfect, the length of the pipe through which water is to be elevated ought never to exceed in practice 30 feet above the level of the water in the well, or reser- voir. What is Valve? 393. A valve, in general, is a contrivance by which water or other fluid, flowing through a tube or aperture, is allowed free passage in one direction, but is stopped in the other. Its structure is such, that, while the pressure of fluid on one side has a tendency to close it, the pressure on the other side has a tendency to open it. PNEUMATICS. 183 Figa. 174, 175, and 176, represent the various forms of valves used in pumps, water-engines, etc. Ft^.. IT''.. Fig. 174. Fig. 175. 394. "When it is desired to raise water to a greater height than 34 feet, a modification of the pump, called the forcing-pump, is employed. ■xvhat is a The Forcing-Pump is an apparatus wliich Forcing-Pump? pgjggg vvater from a reservoir, on the principle of the suction-pump, and then, by the pressure of the piston on the water, elevates it to any required height. Fig. 177 represents the principle of the construction of the forcing-pump. There is no valve in the pis- ton c (Fig. 177), but the water raised through the suc- tion-pipe a, and the valve g, by the elevation of the piston, is forced by each depression of the piston up through the pipe e e, which is furnished with a valve to prevent the return of the liquid. The forcing-pump, as constructed in Fig. 177, ejects the water only at each stroke of the piston, in the manner of a syringe. TVhen it is desired to make the flow of the water continuous as in a fire-engine, an air, chamber is added to the force-pump, as is represented at A, Fig. 178. The water then, instead of immediately passing off through the discharging-pipe, partially fills the air vessel, and by the action of the pLston in the pump, compresses the air contained in it. The elasticity of the air, thus compressed, being in- creased, it reacts upon the water, and forces its ascent in the discharge, or force-pipe. When the air in the chamber is condensed into half its original bulk, it wiU act upon the surface of the water with double the atmospheric pressure, while the water in the force-pipe, being subject to only one atmospheric pressure, there will be an unrestricted force, press- FiG. 178. 184 WELLS'S NATURAL PHILOSOPHY. ing the water up, equal to ono atmosphere: consequcntlj^, a column of water will be sustained, or projected to a height of 34 feet. When the air is con- densed into ono third of its bulk, its elastic force will be increased three- fold, and it will then not only counterbalance the ordinary atmospheric pressure, but will force the Avater upward with a pressure equal to two at- mospheres, or 64 feet, and so on. The ordinary fire-engine is simply a conve- nient arrangement of two forcing-pumps, furnished with a strong air-chamber, and which are worked successively by the elevation and depression of two long levers called brakes. What is a 395. The Syphon is an apparatus by which Syphon? g^ liquid can be transferred from one vessel to another without inverting, or otherwise disturbing the l^osition of the vessel from which the liquid is to be re- moved. In its simplest form, the syphon consists of a bent tube, A B C, Fig. 179, having one of its branches longer than the other. If we immerse the short arm in a vessel of water, and by applying the mouth to the long arm, as at C, exhaust the air in the tube, the water will bo pressed over by atmospheric pres- sure, and continue to flow so long as the end of the lower arm is below the level of the water in the vessel „ The action of the syphon is readily principle does explained: the column of liquid in the syphon act? ^j,^ ^^^^^^ ^^^^ ^^^ ^,,3^ reaching in the shorter arm from the top of the curve or bend to the surflice of the liquid in the vessel, have both a tendency to obey the attraction of gravity and fall out of the tube. This tendency is opposed, however, on both sides, by atmospheric pressure, acting on one side at the opening C, and upon tho other upon the surface of the liquid in the vessel, thus preventing, in tlie interior of the tube, tho formation of a vacuum, wliich would take place at tho curve, if the two columns ran down on both sides. But tho column on one side being longer than upon the other, the weight of the long column overbalances the short one, and determines the direction of tho flow; and in proportion as the liquid escapes from the long arm, a fresh portion is forced into the short arm on the other side by the pressure of tho air. The syphon is, therefore, kept full by the pressure of the atmos- phere, and kept running by the irregularity of the lengths of tho columns in its branches. A SMction-tubo is sometimes attached to the syphon to make it more use- ful and efficient, as is represented in Fig. 180. By this means we may fill the whole syphon without the liquid entering the mouth, by sucking at the end of the suction-tube, and temporarily closing the end of the longer arm. In order that tho discharge of a liquid by means of the syphon should bo PNEUMATICS. 185 perfectly constant, it is nec- casary that the difference of lengths of the columns of liquid in both branches should be immovable. Thia may be effected by connecting the sj^Dhon with a float and pulley, aa is ■pepresented in Fig. 181. The curi- Fio. 181. Explain the phenomenon of intermitting springs. ous pheno- menon of intermitting springs may be explained upon the principle of the syphon. These springs run for a time and then stop ^-: altogether, and after a time run again, and then stop. If we suppose a reservoir in the interior of a hiU or mountain, with a syphon- like channel running from it, as in Fig. 182, then as soon as the water collecting in the reservoir rises to the height shown by the dotted line, the stream will be- gin to flow, and continue flow- ing till the reservoir is nearly emptied. Again, after an in- terval long enough to fill the reservoir to the required height, it will again flow, and so on. When wui a ^^^- ^^ ^ solid substancG have the same MsJendeT'^iS density as atmospheric air, it will, when im- the air? merscd in air, lose its entire weight, and will remain suspended in it in any position in which it may be jjlaced. ^ .„ 397. If a solid body, bnlk for bulk, be lighter When mil a .... , it. hody rise in than atmosplieric air, it is pressed upward by the surrounding particles of air, and rises, upon the same principle as a cork rises from the bottom of a vessel of water. (See § 85.) 186 "WELLS'S NATUEAL PHILOSOPHY. As the density of the air continuaUj diminishes as we wiUan^sSnd- ^cend from the surface of the eartli, it is evident that such ing body re- a body, as it goes up. will finaUv attain a height where the air ^jy , °' will have the same density as itsel£ and at such a point the body will remain stationary. Upon this principle clouds, at different times, float at different degrees of elevation. It is also upon these principles that aerostation, or the art of navigating the *ir, depends. What are Bai- 398. BallooHS Ere macliines which ascend loons? through the atmosjjhere, and float at a certaia height, in virtue of being filled with a gas or air lighter than the same bulk of atmospheric air. -^ ^ ^^ Balloons are of two kinds. Montgolfier, What are the ' two varieties or rarefied air balloons, and Hydrogen gas of balloons? in rn balloons. The first are filled with common air rarefied by heat, and thus made lighter than the surrounding atmosphere ; while the second are filled with hydrogen, a gas about fourteen times lighter than air. Describe the '^^ rarefied air-balloon was invented by Montgolfier, a Montgolfier. or French gentleman, in 1782, who first tilled a paper bag with l^ll^^^ "^'^ ^^^' tieated air, and allowed it to pass up a chimney. He after- ward constructed balloons of silk, of a spherical shape, with an aperture formed in the lower surface. Beneath this opening a hght vriie basket was suspended, containing burning material The hot air arising from the burning substances, enters the aperture, and rendering the balloon specific- ally Hghter than the air, causes it to ascend with considerable velocity. Small balloons of a similar character are frequently made at the present day of paper, the air within them being rarefied by means of a sponge soaked in alcohol, suspended by a wire beneath the mouth, and icmited. Describe the "^^^ hydrogen gas balloon consists of a light silken bag, hj-drogen gas filled either with hydrogen, or common illuminating gas. The balloon. difference between the specific weight of either of these gases and common air is so great, that a large balloon filled with them possesses ascensional power sufficient to rise to great heights, carrying with it consid- erable additional weight. The aeronaut can descend by allowing the gas to escape by means of a valve, thereby diminishing the bulk of the balloon. To enable him to rise again, ballast is provided, generally consisting of bags of Band, by thro^ving out which, the balloon is Hghtened, and accordingly rises. By means of one of these machines Gay Lussac, au eminent French chem- ist, ascended in 1804. for the purpose of making meteorological observations, to the great height of 23,000 feet. PNEUMATICS. 187 Do the laws of ^99. Air obeys the laws of motion which t^°ik? "^^'^ ^rs commoa to all other material and ponder- able substances. Howisthemo- 400. The momentura of air, or the amount Si^ijated^? ^"^ of force which it is capable of exerting upon bodies opposed to it, is estimated in the same way as in the case of solids, viz., by multiplying its weight by its velocity. __T, ^ ... The momentum of air is usefully employed as a mechanioai WTiat are illus- -^ ^ •' tratious of the agent in imparting motion to wind-mills and to ships. Its niomentum of ^^^^^ strikmg eflects are seen in the force of wuid, which oc- casionaUy, in hurricanes and tornadoes, acts with fearfiil power, prostrating trees and buildings. Such results are caused by the mo- mentum of the air being greater than the force by which a building, or a tree is fastened to the earth. 401. Any force acting suddenly upon the air from a center, the rings of imparts to it a rotary movement. A very beautiful illustra- snioke observ- ^^^^ ^f- ^j^jg j^ ggg^ ^ ^j^g rinrra of smoke which are produced ed in smoking ° "^ and in the dis- by the mouth of a skilful tobacco-smoker, and frequently also non"?^^ °^ '^°" upon a much larger scale by the discharge of cannon, on a still day. In these cases a portion "Ftp 1 S^ of air acted upon suddenly from a center is caused to rotate, and the particles of smoke render the mo- vTv^T^'^'K^l*^^^ tion visible. The whole circumference of each \f^i^ ^'^-^ circle is in a state of rapid rotation, as is shown by vl./TT'/l' y^f^^h ' the arrows in Fig. 183. The rapid rotation in short, confines the smoke within the narrow limits of a circle, and causes the rings to be well defined. PRACTICAL PROBLEMS IN PNEUMATICS. 1. If 100 cubic Inches of air weigh 31 grains, what will be the weight of one cubic foot T 2. If the pressure of the atmosphere be 15 pounds upon a square inch, what pressure will the body of an animal sustain, whose superficial surfiice is forty square feet ? 3. When the elevation of the mercury in the barometer is 2S inches, what will be tho height of a column of water supported by the pressure of the atmosphere ? Solution: Column of mercury supported by the atmosphere = 29 inches. Mercury being l.lj^ times heavier than water, the column of water supported by the atmosphere = 13ix23=31 feet. 4. When the elevation of the mercury in the barometer is 30 inches, what will be the b«ight of a column of water supported by the atmosphere ? 5. To what height may water be raised by a common pump, at a place where the ba- rometer stands at 24 inches ? 6 If a cubic inch of air weighs .30 of a grain, what weight of air will a vessel whose capacity is (0 cubic inches, contain f CHAPTER XI. ACOUSTICS. 402. Acoustics is that department of ohvs- WTifit is tiiG X ■ %f science of ical scieiice which treats of the nature, phe- nomena, and laws of sound. It also includes the theory of musical concord or harmony. 403. Sound is the sensation produced on the What is Sound? /• i • i organs oi hearmg, when any sudden shock or impulse, causing vibrations, is given to the air, or any other body, which is in contact, directly or indirectly'-, with the ear. underwhatcir- 404. "Whcu au clastic body is disturbed at ^bratorymove- ^^J point, its particles execute a series of vi- mcnts anse f bratory movcments, and gradually return to a position of rest. Thus when a glass tumbler is struck by a hard body, a tremulous agitation is transmitted to its entire mass, whicli movement gradually diminishes in force until it finally ceases. Such movements in matter are termed vibra- tions, and when communicated to the ear produce a sensation of sound. The nature of these vibratory movements may be illustrated by noticing the visible motions whicli occur on striking or twitching a tightly extended Fio. 184. cord, or wire. Suppose such a cord, repre- sented by the central line in Fig. 18-4 to be forcibly drawn out to A, and let go ; it would immediately recover its original posi- tion by virtue of its elasticity ; but when it reached the central point, it would have ac- quired so much momentum as would cause it to pass onward to a ; thence it would vi- brate back in the same manner to B, and back again to b, the extent of its vibration being gradually diminished by the resistance of the air, so that it would at length return to a state of rest. In vibratory movements of this kind all the separate par- ticles come into motion at the same time, simultaneously pasa tion, the point of equilibrium, or rest, simultaneously reach the maximum of their vibration, and simultaneously begin their retrograde mo- tion. Such vibrations are therefore called stationary, or fixed vibrationa Describe the nature of a sta- tionary vibra- ACOUSTICS. 189 Tlow may the Bound-vibra- tions ill solid bodies be ren- dered visible ? ^ ., ,^ If, however, the motions of the vibrating body are of such a Dpscnbe the , ^ • , Hill 11 re of a character that the agitatiou proceeds froin one particle to au- pr.igrcssiTe vi- other, SO that each makes the same vibration, or oscillation, as the preceding one, with tlie sole exception of the motion beginning later, we liave what is called progressive vibrations. Tims if we fasten a cord at one end, and move the other end up and down, a wave, or progressive vibration, is produced. As the clearest conception can be formed of vibrations by comparing them to tho waves prod^iced by throwing a stone into smooth water, the term ul- dulatory, or wave movement, has been adopted in general to express tha phenomena of vibrations. 405. Daily experience teaches us that almost every motion of bodies in our vicinity is accompanied by a noise perceptible to our ears. All such sounds are the result of the vibrations of a portion of matter, and the nature of the tone, or sound, depends only on the manner in which these vibrations originate. 406. Sound-vibrations in solid bodies may be rendered vis- ible by many simple contrivances. If we attach a ball by means of a string to a bell, and strike the bell, the baU will vibrate so long as the bell continues to sound. When a bell is sounding, also, the tremiolous motion of its particles may be perceived by gently touching it with the finger. If the finger is pressed firmly against the bell, the sound is stopped, because the vibrations are in- terrupted. When sounds are produced by drawing the wet finger around the edge of a glass containing water, waves wiU be seen undulating from the sides toward the center of the glass. When a tuning-fork is struck and made to sormd, its vibrations Fig. 1S5. are clearly visible, both branches alternately approacliing and re- receding from each other, as is represented in Fig. 185. If we strike a tuaing-fork, and then touch the surface of mercury with one of its extremities, the surface of the mercury will exhibit little undulations or waves. „ ^. The most interesting method of exhibiting the IIow are the ° ° so-eaiicd acous- character of sound is by means of the so-called duced'?"^^ ^'^°' "acoustic figures," which may be produced in the following manner: — Sprinkle some fine sand over a square or round piece of thin glass or metal, and holding the plate firmly by means of a pair of pincers, draw a vioUn bow down the edge ; the Band is put in motion, and finally arranges itself along those parts of the surface which have the least vi- Fig. 186. assume various interestmg figures, as is represented in Fig. 186. bratory motion. By chang- ing the point by which the plate is held, or by varying the parts to which the viohn bow is applied, the sand may be made to 190 WELLSS NATURAL PHILOSOPHY. ,^ . . .,. 407. Air is the usual medium tliroush ■which What IS the . °. usual medium sound is coDveved to the ear. The vibrating' through which i i . i ... Bound is propa- bodv impErts to the au- m contact with it an gated f >/ ± undulatory, or wave-like movement, which, propagating itself in every direction, reaches the ear, and produces the sensation of sound. What are eon- 408. Vibrating bodies which are capable of •reus bodies? ^^^^ imparting undulations to the air, are termed sounding, or sonorous bodies. The aerial vibrations, or undulations thus caused, propagate themselves from the center of disturbance in concentric circles, in the same way that waves spread out upon the smooth surface of water. If such waves of water, propagated from a center, encounter any obstruction, as a floating body, they will bend their course round the sides of the obstacle, and spread out obUquely beyond it. So the undulations of air, if interrupted in their progress by a high wall or other similar impediment, will be continued over its summit and propagated on the opposite side of it. In a sound-wave or imdulation of the air, as in a wave of water, there is no permanent change of place among the particles, but simply an agitation, or tremor, communicating from one particle to another, so that each particle, like a pendulum which has been made to oscillate, recovers at length its original position. This motion may be best illustrated by comparing it to the motion pro- duced by the wind in a field of grain. The grassy waves travel visibly over the field in the direction in which the wind blows ; but this appearance of an object moving is only delusive. The only real motion is that of the heads of the grain, each of which goes and returns as the stalk stoops or recovers itself This motion affects successively a line of ears in the dfrection of the wind, and affects simultaneously all the ears of which the elevation or de- pression forms one visible wave. The elevations and depressions are propa- gated in a constant direction, while the parts with which the space is filled only vibrate to and fro. Of exactly such a nature is the propagation of sound through air. Under what ^^^- ^^ °^ substaucc intervenes between the sho'^uid^wrbe "^i^i'3'ting body and the organs of hearing, no ssoundV ^^''^ sensation of sound can be produced. This is readily proved by placing a bell, rung by the action of clock-work, beneath the receiver of an air-pump, and exhausting the air. No sound will then be heard, although the striking of the tongue upon the bell, and the vibration of the bell itself, are visible. Now, if a little air be admitted into the receiver, a faint sound will begin to be heard, and this sound will become gradually louder in proportion as the air is gradually read- mitted, imtil the air within the receiver is in the same condition as that without. ACOUSTICS, 191 Sound, therefore, cannot be propagated through a vacuum. "The loudest sound on earth, therefore, cannot penetrate beyond the limits of our atmosphere ; and in the same manner, not the faintest sound can reach our earth from any of the other planets. Thus the most fearful explosions might take place in the moon, without our hearing anything of them." How does the 410. The powei of air to transmit sound ^unT''in°°air^ varies with its uniformity, its density, and its ^y^ humidity. Whatever tends to agitate or disturb the condition of the atmosphere, affecta the transmission of sounds. "When a strong wind blows from the hearer to- ward a sounding body, a sound often ceases to be heard which would be audible in a calm. Falling rain, or snow, interferes with the undulations of sound-waves, and obstructs the transmission of sound. The fact that we hear sounds with greater distinctness by ^I ^Bounds ^o^it than by day, may be, in part, accounted for by the cir- more distinctly cumstance, that the different layers or strata of the atmosphero by day?* '''*° ^^^ 1*^^ ^i^^^® ^^ variations in density and to currents, caused by changes of temperature, at night than by day. The air at night is also more still, from the suspension of business and hum of men. Many sounds become perceptible during the night, which during the day are completely stifled, before they reach the ear, by the din and discordant noises of labor, business, and pleasure. Sound of any kind is transmitted to a greater distance in cold and clear weather than in warm weather, the density of air being increased by cold and diminshed by heat. On the top of high mountains, where the air is greatly rare- mat are a- ggj ^Yie sound of the human voice can be heard for a short lustrations of ' . the variation distance only ; and on the top of Mont Blanc, the explosion of^^sound in ^f ^ pj^^^j appears no louder than that of a small cracker. When persons descend to any considerable depth in a diving- bell, the air around them is compressed by the weight of a considerable column of water above them. In such circumstances, a whisper is almost as loud as a shout in the open air; and when one speaks with ordinary force it produces an effect so loud as to be painfuL Is air neces- 411. Air is Dot Deccssary to the production production '^of ©^ sound, although most sounds are transmitted sound? i^y j^g vibrations. Sound can he produced un- der water, and all bodies are more or less fitted, not only to produce, hut also to transmit soimds. whatsuhstan- 412. Sound is communicated more rapidly ^10*"°°'^^ ^D^ more distinctly through solid bodies than mostreaduy? through either liquids or gases. It is trans- 102 WELLS'S NATURAL PHILOSOPHY. mitted by water near four times more rapidly than by air, and by solids about twice as rapidly as by water. If wo strike two stoucs together under water, the sound will be as loud as if they had been struck in the air. When a stick is lield between the teeth at one extremity, and the other is placed in contact with a table, the scratcli of a pin on the table may be heard ■with great distinctness, though both ears be stopped. The earth often conducts sound, so as to render it sensible to the car, when the air fails to do so. It is well known that the approach of a troop of horse can be heard at a distance by putting tlie ear to the ground, and savages practice this method of ascertaining the approach of persons from a great dis- tance. The principle that solids transmit sounds more perfectly than air, has been applied to the constniction of an instrument called the "stethoscope." ^ . The stethoscope consists of a hollow cylinder of wood, somo- Stetlioscope. what resembling in form a small trumpet. The wide mouth is applied lirmly to the breast, and the other is held to the ear of the medical examiner, wlio is thus enabled to hear distinctly the action of the organs of respiration, and judge whether they are in a healthy condition, or the reverse. How is the in- 413, Souud dccreascs in intensity from the affbcted°b'''di'^- center where it originates, according to the tance? same law by which the attraction of gravita- tion varies, viz., inversely as the square of the distance. That is to say, at double the distance it is only one fourth part as strong ; at three times the distance, one ninth, and so on. This law applies with its full force only when no opposing currents of air, or otlier obstacles, interfere witli the wave movements, or undulations. By confining the sound undulations in tubes, whicli prevent their spreading, tho force of sound diminislies mucli less rapidly. It will, therefore, under such circumstances, extend to much greater distances. This ^jrinciple is taken ad- vantage of in the construction of speaking-trumpets. Sounds can generally be heard, especially on a calm day, at h^^niTnorT.u's- ^ greater distance upon water than upon land. The plane tinotly upon surface of water, as a smooth wall, prevents the lateral spread- Und f '^^ '^ ill? ^^d dispersion of the sound-waves, although on only one side. The air over water, owing to the presence of moist- ure, is also generally more dense, and the density more uniform than over the land. Water, in addition, is a better conductor of sound than the earth. Tho transmission of sound from one apartment to another may be prevented by fining up the spaces between the partition walls with sliavings, or any porous substances. The number of media tlu-ough which the sound must ACOUSTICS. 193 pass is thus greatly increased, and every change of medium diminishes tho strength of sound- waves. 414. The velocity of the sound undulations What law gov- . . ^ . , . , . erns the veioc- IS uniiorm, passinfiT over equal intervals m ily of sound? . ' ^ ° ^ equal times. The softest whisper, therefore, flies as fast as the loudest thunder. With what ye- ^15. Souuds of every kind travel, when tho S travel? tcmperaturo Is at 62° Fahrenheit's thermom- eter, at a rate of 1,120 feet per second, or about 13 miles per minute, or 765 miles per hour. The velocity of sound increases or diminishes at the rate of 13 inches for every variation of a degree in temperature above or below the temperature of 62° Fahrenheit. ^, , When a e;un is fired at some distance, we see the flash a Why ■, „ reflected, or rebound irom that surface, and the angle of reflection is equal to the angle of incidence. This law governing the reflection of sound is the same as that which gov- erns the reflection of all elastic bodies, and also, as will be shown hereafter, the imponderable agents, heat and light. ^ What is an ^29. Au EcHO is a repetition of sound caused Echo ? -jjy. ^Ijq reflection of the sound waves, or undu- lations, from a surface fitted for the purpose, as the side of a house, a wall, hill, etc. ; the sound, after its first pro- duction, returning to the ear at distinct intervals of time. Thus if a body placed at a certain distance from'a hearer produces a sound, this sound wou'd be heard first by means of the sonorous undulations which produced it, proceeding directly and uninterruptedly from the sonorous body to the hearer, and afterward by sonorous undulations which, after striking on reflecting surflices, return to the ear. These last constitute an echo. In order to produce an echo, it is requisite that the reflecting body should be situated at such a distance from the source of sound, that the interval be- 198 WELLS'S NATURAL PHILOSOPHY. tween the perception of the original and reflected sounds may be sufficient to prevent them from being blended together. "When the original and retlected sounds are blended together, the effect produced is caUed a resonance, and not an echo. Thus, the walls of a room of ordinary size do not produce an echo, because the reflecting surface is so near the source of sound that the echo is blended with the original sound ; and the two produce but one impression on the ear. Large halls, spacious churches, etc., on the contrary, often reverberate or re- p3at the voice of a speaker, because the walls are so far otT from the speaker, that the echo does not get back in time to blend with the original sound; and therefore each is heard separately. The shortest interval sufficient to render sounds distinctly appreciable by the ear, is about l-9th of a second ; therefore when sounds follow at shorter intervals, they wiU form a resonance instead of an echo ; so that no reflecting surface wQl produce a distinct echo, unless its distance from the spot where the sound proceeds is at least 62i feet ; as the sound will in its progress in passing to and from the reflecting surface, at the rate of 1,125 feet per sec- ond, occupy l-9th part of a second, passing over 621X2 = 125 feet. •When is (in ^^^' Where separate surfaces are so situ- *"ikid? ™"^"' ^^^^ ^^^^^ ^^^^y receive and reflect the sound from one to the other in succession, multiplied echoes are heard. Fig. 189. An echo in a build- ing near Milan, Italy, repeats a loud sound 30 times audibly. A river bounded by perpen- dicular walls of rock, where the sound is re- flected backward and forward over the sur- face of still water, is a favorable situation for the production of re- peated echoes. Pig. 189 represents the manner in which the sounds rebound, in such situations, as at 1, 2, 3, 4, from side to side. It is not necessary tliat the surfiice producing an echo should be either hard or polished. It is oflen observed at sea that an echo proceeds from the surface of clouds. An echo at sea, however, or on an extensive plane, is heard but rarely, there being no surfaces to reflect sound. To insure a perfect echo, the reflecting surface must be tolerably smooth, and of some What condi- tions of surface are requisite to produce a per- fect echo ? fiEFLECTION OF SOUND. 199 rejfuiar form. An irregular surface must break the echo ; and if the irregu- laricy be very cousiderable, there can be no distinct or audible reflection at all. i or tnia reason an echo is much less perfect from the front of a house wliich has wmdows and aoors, than from the plane end, or any plane wall of the same magnituae. How is sound 431 . If the surface upon wliicli the sound- c^ved^ ^6^ waves strike be concave, it may concentrate ^'^^' sound, and reflect all that falls upon it to A point at some distance from the surface, called the focus. Y^Q IQQ Thus, in Fig. 190, if the sound waves proceeding in right hues from the points d, e, /, g, h, strike upon the concave sur- ^ face, ABC, they will all be reflected to f the focus, F, and there concentrated in jf such a way as to produce a most powerful ll eSect. It is upon this principle that whisper- ing galleries are constructed, and domes and vaulted ceUings often exhibit the same curious phenomena. In these instances a whisper uttered at one point is reflected from the curved surface to a focus at a distant point, at which situation it may be distinctly heard, while in aU other positions it will be in- audibla ._., ^ . All are famihar with the resonance produced by placing a Wtiat occasions ^ j r a the noise heard sea-shell to the ear — an effect which fancy has likened to the in a sea-sheU ? u j-q^j. qJ- ^j,g ^^^y rjj^jg jg caused by the hollow form of the shell and its polished surface enabling it to receive and return the beatings of all sounds that chance to be trembhug in the air around the sheU. 432. Speakmg-tubes and speaking-trumpets depend on the principles of the reflection of sound. Fig. 191. 433. A SpEAKreo-TRUMPET (Fig. 191) is i speaking-Trum. liollow tubc SO coustructed that the rays of ^ sound (proceeding from the mouth when ap- plied to it), instead of diverging, and being scattered through the surrounding atmosphere, are reflected from the sides, and conducted forward in straight lines, thus giving great additional strength to the voice. 200 WELLS'S NATURAL THILOSOPHY. Fig. 192. The course of the rays of souud proceeding from the mouth through this instru- ment, may be shown by Fig. 192. The trumpet be- ing direetfid to any point, a coliection of p.irallel rays of sound moves toward such point, and they rea/^h the ear in much greater number than would the divergmg rays which would pro- ceed from a speaker without such an instrument. \vh.it is an 434. An Ear-Trumpet is, in form and appli- Ear-Trumpet? cution, tliG TeversG of a speaking-trumpet, but in principle the same. The rajs of sound proceeding from a speaker, more or less distant, enter the hearing-trumpet and are reflected in such a manner as to concentrate the sound upon the opening of the ear. Fig. 193 represents the form of the ear-trumpet gen- erally used by deaf persons. The aperture A is placed within the ear, and the sound which enters at B is, by a series of reflections from the interior of the instrument, concentrated at A. In the same manner persons hold the hand concave behind the ear, in order to hear more distinctly. The concave hand acts, in some respects, as an ear-trumpet, and reflects the sound into the ear. Most of the stories in respect to the so-called " haunted houses" can be all satisfactorily explained by reference to the principles which govern the re- flection of sounds. Owing to a peculiar arrangement of reflecting walls and partitions, sounds produced by ordinary causes are ofl:en heard in certain localities at remote distances, in apparently the most unaccountable manner. Ignorant persons become alarmed, and their imagmation connects the phe- nomenon with some supernatural cause. 435. A right understanding of the principles which govern the reflection of sound is often of the utmost importance in the construction of buildings intended for public speaking, as halls, churches, etc. Experience shows that the human voice is capable of fllling a larger -space than was ever probably inclosed within the walls of a single room. The circumstances which seem necessary in order that the human voice should be heard to the greatest possible distance, and with the greatest distinctness, seem to be, a perfectly tranquil and uniformly dense atmosphere, the absence of all extraneous sounds, the absence of echoes and reverberations^ and the proper arrangement of the reflecting surfaces. "What circum- stances are nec- essary to in- sure the utmost distinctness in hearins' ? f - !tANS OF HEARING AND OF THE VOICE. 201 A pure atmosphere in a room for speaking, being favorable pu'rl atmos* to the speaker's health and strength, will give him greater phereinaroom power of voice and more endurance, thus indirectly improv- a o • .^^ ^^^ hearing by strengthening the source of sound, and also by enabhng the hearer to give his attention for a longer period undisturbed. In constructing a room for public speaking, the ceding How should a , . , , -.nn^n-c^.-r.-uj. room for pub- Ought not to exceed 30 to 3o leet m height. lie speaking be ^fj^Q reason of tliis may be explained as follows: — If wo constructed 7 , ,, , , t ■ ^ ^ ^ advance toward a waU on a calm day, producmg at each step "^^"^^ ^LJ^t some sound, we will find a point at which the echo ceases reasoaof taisr ' ' . j mi j- to be distinguishable from the origmal sound. Ihe distance from the wall, or the corresponding interval of time, has been called the limit of perceptibility. This limit is about 30 to 35 feet; and if the ceiling of a building for speaking be arranged at this limit, the sound of the voice and the echo will blend together, and thus strengthen the voice of the speaker. If the ceiling be constructed higlier than this limit of perceptibihty, or higher than 30 or 35 feet, the direct sound and the echo will be heard sepa- rately, and will produce indistinctness. „ Echoes from walls and ceilings may, to a certain extent, How may , ■ , j echoes in a- be avoided by covering their surfaces with thick drapery, partments to ^j^j^j^ absorbs sound, and docs not reflect it. Bonie extent ' be avoided ? If the room is not very large, a curtain behind the speaker impedes rather than assists his voice, by^^the^key- ^^^- ^'^ ^^^^J apartment, owing to the peculiar arrange- note of an ment of the reflecting surfaces, some notes or tones can be heard with greater distinctness than others; or, in other words, every apartment is fitted to reproduce a certain note, called the key- note, better than any other. If a speaker, therefore, wOl adapt the tones of his voice to coincide with this key-note, which may readily be determined by a little practice, he will be enabled to speak -with greater ease and distinct- ness than under any other circumstances. In a large room nearly square, the best place to speak from is near one cor- ner, with the voice directed diagonally to the opposite corner. In most cases, the lowest pitch of voice that will reach across the room wiU be the most audible. In all rooms of ordinary form, it is better to speak along the length of a room than across it. It is better, generally, to speak from pretty near a wall or pillar, than far away from it. SECTION III. ORGANS OF HEARING AND OF THE VOICE. 437. The Ear consists, in the first instance, of a funnel, construction of shaped mouth, placed upon the external surface of the head, the human ear. jj^ many animals this is movable, so that they can direct it to the place from whence the sound comes. It is represented at a, Fig. 194. 9* 202 WELLS'S NATURAL PHILOSOPHY. Fig. 194. Proceeding inward from this external por- tion of the ear, is a tube, something more than an inch long, terminating in an oval-shaped opening, h, across which is stretched an elas- tic membrane, like the parchment on the head of a drum. This oval-shaped opening has re- ceived the name of the tympanum, or drum of the ear, and the membrane stretched across it is called the "membrane of the tympanum, or drum of the ear." The sound concentrated at the bottom of the ear-tube falls upon the mem- brane of the drum, and causes it to vibrate. That its motion may be free, the air contained within and behind the drum has free communication with the external air by an open passage, / called the eustachian tube, leading to the back of the mouth. A degree of deafness ensues when this tube is ob- structed, as in a cold; and a crack, or sudden noise, with immediate return of natural hearing, is generally experienced when, in the eflbrt of sneezing or otherwise, the obstruction is removed. The vibrations of the membrane of the drum are conveyed further inward, through the cavity of the drum, by a chain of four bones (not represented in the figure on account of their minuteness), reaching from the center of tho membrane to the commencement of an inner compartment which contains tho nerves of hearing. This compartment, from its curious and most intricate Btructure is called the Labyrinth. Fig. 194, c e d. The Labyrinth is the true ear, all the other portions being merely accessories by which the sonorous undulations are propa- gated to the nerves of hearing contained in the labyrinth, which is excavated in the hardest mass of bone found in the whole body. Fig. 195 represents the labyrinth on an enlarged scale, and partially open. The labyrinth is filled with a liquid sub- stance, through which the nerves of hearing are distributed. "When the membrane of the drum of the ear is made to vibrate by the undulations of sound striking against it, the vibrations are communicated to the little chain of bones, which, in turn, striking against a membrane which covers the external opening of the labyrinth, compresses tho liquid contained in it. This ac- tion, by the law of fluid pressure, is communicated to the whole interior of the labyrinth, and consequently to all portions of the auditory nerve dis- tributed throughout it: the nerve thus acted upon conveys an impression to the brain. The several parts of the labyrinth consist of what is called the vestibule, e. Fig. 194, three semicircular canals, c, imbedded in the hard bone, and a winding cavity, called the cochlea, d, like that of a snaU-shell, in which fibres, Fig. 195. ORGANS OF HEARING AND OF THE VOICE. 203 stretched across like harp-strings, constitute the lyra. The separate uses of these various parts are not yet fully known. The membrane of the tym- panum may be pierced, and the chain of bones may be broken, -nithout en- tire loss of hearing. 438. In the hearing apparatus of the lower orders of cuUarittes ^of animals, all the parts belonging to the human ear do not the hearing ap- exist. In fishes, the ear consists only of the labyrinth; and LweranimsJ^? "^ lower animals the ear is simply a Uttle membranous cavity fiUed with fluid in which the fibres of the nerves of hearing float. 439. All persons can not hear sounds alike, ^on" "^^ hZr In different individuals the sensibility of the Bound alike? auditorj nerves varies greatly. 440. The whole ranire of human hearing, VThat is the „ , ^ t> . i i • i range of hn- from the lowcst uotc 01 the organ to the high- est known cry of insects, as of the cricket, in- cludes about nine octaves. „ 441. In the human svstem, the parts con- What are the •• • i ^ • " r i i organs of ccmcd m the production ot speech and music, voice? . . are three : the wind-pipe, the larynx, and the glottis. What ia the ^42. The Wind-pipe is a tube extending wiud-pipc? £j.Qj^^ Qjjg extremity of the throat to the other, which terminates in the lungs, through which the air passes to and from these organs of respiration. What is the ^•^- The Larynx, which is essentially the Laryni? Qrgau of spcecli, is an enlargement of the up- per part of the wind-pipe. The Larynx terminates in two lateral membranes which approach near to each other, having a little narrow opening between them called the glottis. The edges of these membranes form what is called the vocal chords. How is voice '^^- III order to produce sound, the air ex- produced? pired from the lungs passes through the wind- pipe and out at the larynx, through the opening between the membranes, the glottis : the vibration of the edges of these membranes, caused by the passage of air, produces sound. 204 -WELLS'S NATURAL PHILOSOPHY. „ „ By the action of muscles vre can vary the tension of these How can the ^ , , , . , , , tones of the membranes, and make the openmg between them large or voice he ren- gjuaH, and thus render the tones of the voice crave or acute.* aerea grave or " »cute? 445. The loudness of the voice depends Upon what mainlv upon the force with which the air is does the loud- •' i- ness of the voice exDclled from the luno;s. depend? J- _ o The force which a healthy chest can exert in blowing ia about one pound per inch of its surface ; that is to say, the chest can con- dense its contained air with that force, and can blow through a tube the mouth of which is ten feet under the surface of water. What is the vo- 446. In coughing, the top of the windpipe, coughing? °^ or ^^^^ glottis, is closed for an instant, during which the chest is compressing and condensing its contained air ; and on the glottis heing opened, a slight explosion, as it w^crc, of the compressed air takes place, and hlows out any irritating matter that may be in the air-passages. 447. Sound, to some ertent, appears to always accompany generally ac- the liberation of compressed air. An example of this is seen lib™^'l"(T '^f ^^^ *^^*^ report which a pop-gun makes when a paper-bullet compressed air ? is discharged from it. The air confined between the paper bullet and the discharging-rod is suddenly liberated, and strikes against the surrounding air, thus causing a report in the same man- ner as when two solids come into collision. In like manner an inflatea blad- der, when burst open with force, produces a sound like the report of a pistol. „ , . ., 448. The sound of falling water appears in a great mcasuro To what is the . , ^ .^ , , . ^ , Bound of falling to be owmg to the lormation and burstmg of bubbles. U hen water due? ^j^g distance which water falls is so Umitcd that the end of • The power which the will possesses of determining with the most perfect precision the exact degree of tension which these membranes of the glottis, or vocal chords shall receive, is extremely remarkable. Their average length in man is estimated at 7o-10Cths of an inch in a state of repose, while in the state of greatest tension it is about 93-lOOtha of an inch. The average length of the membranes in the female is somewhat less. Each interval, or variation of tone which the human voice is capable of producing is occasioned by a different degree of tension of these membranes ; and as the least estimated number of variations belonging to the voice is 240, there must be 240 different states of tension of the vocal chords, or membranes, every one of which can be at once determined by the will. Their whole variation in length in man being not more than one fifth of an inch, the variation required to pass from one interval of tone to another will not be more than l-I '200th of an inch. It is on account of the greater length of the vocal chords, or membranes of the glottis, that the pitch of the voice is much lower in man than in woman : but the difference does not arise until the end of the period of childhood, the size of the larynx in both sexes being about the same up to the age of 14 or 15 years, but then changing rapidly in the malo Bex, and remaining nearly stationary in the female. Hence it is that boys, as well as girls and women, sing treble ; while men sing tenor, which is about an octave lower than treble, or bass which is lower still. — Dr. Carpenter, EEAT. 205 the stream docs not become broken into bubbles and drops, neither sound or air-bubbles will be produced ; but as soon as the distance becomes increased to a sufficient extent to break the end of the column into drops, both air-bub- bles and sounds will be produced. whatissneez- ^^- SneeziDg IS a phenomenon resembling "'°' cougli ; only the chest empties itself at one effort, and chiefly through the nose, instead of through the mouth, as in cougliing. •What is laugh- ^oO. Laughing consists of quickly- repeated ^^^ expulsions of air from the chest, the glottis being at the lime in a condition to produce voice ; hut there is not hetAveen the expirations, as in coughing, a complete closure of the glottis. , •wTiat is crying? 451. Crying differs from laughing almost solely in the circumstance of the intervals be- t'^een the gusts or expirations of air from the lungs being longer. Children laugh and cry in the same breath. Insects generally excite sonorous vibrations by the fluttering of their wings, or other membraneous parts of their structure. PRACTICAL QUESTIONS IX ACOUSTICS. 1. The flash of a cannon was seen, and ten seconds afterward the report was heard: how far oflF was the cannon ? 2. At what distance was a flash of lightning, when the flash was seen seven seconds before the thunder was heard ? 3. How long after a sudden shout will an echo be returned from a high wall 1,120 feet distant? 4. A stone being dropped into the mouth of a mine, was heard to strike the bottom in two seconds ; how deep was the mine ? 5. A certain musical string vibrates 100 times in a second : how many times must it Tibrate in a second to produce the octave ? CHAPTER XII. HEAT. 452. Heat is a physical accent, known onlV What is heat? , . /« t i- ■, by its eiiects upon matter. In ordmary lan- guage we use the term heat to express the sensation of warmth. 206 WELLS'S NATURAL PHILOSOPHY. _ 453. Caloric is the general name given to the physical agent whicli produces the sensa- tion of warmth, and the various effects of heat observed in matter. HoTT- Is heat 454. The quantity of heat observed in dif- measured? fereut substanccs is measured, and its effects on matter estimated, only by the change in bulk, or ap- pearance, which different bodies assume, according as heat is added or subtracted. What Is tem- 455. The degree of heat by which a body perature? jg affcctcd, or the sensible heat a body con- tains, is called its Temperature. 456. Cold is a relative term expressing only %Vhat is cold ? . -^ . ° , the absence of heat m a degree ; not its total absence, for heat exists always in all bodies. What distin- 457. Heat possesses a distinguishing char- ncteristfc'^docs ^ctcristic of passing through and existing in heat possess? ^^^ kiuds of matter at all times. So far as we know, heat is everywhere present, and every body that exists contains it without known limits. Ice contains heat in large quantities. Sir Humphrey Davy, by friction, ex- tracted heat from two pieces of ice, and quickly melted them, in a room cooled below the freezing-point, by rubbing them against each other. In what man- 458. Tlio tcudency of heat is to diffuse, or diffufe,"^ oT' spread itself among all neighboring substances, spread itself? until all havc acquired the same, or a uniform temperature. A piece of iron thrust into burning coals becomes hot among them, because the heat passes from the coals into the iron, until the metal has acquired aa equal temperature. When do we 459. Wlicn thc hand touches a body having caiia body hot r ^ higher temperature than itself, we call ifc hot, because on account of the law that heat diffuses itself among neighboring bodies until all have acquired the same temperature, heat passes from the body of higher temperature to the hand, and causes a peculiar sensation, which we call warmth. 400. When we touch a body having a temperature HEAT. 207 When do we lower than that of the hand, heat, in accord- caiiabodycoid? q^^^q ,^yj|-jj ^|^q saiDC lavv, passcs out from the hand to the body touched, and occasions the sensation which we call cold,* 461. Sensations of heat and cold are, therefore, merely degrees of temperature, contrasted by name in reference to the peculiar temperature of the individual speaking of them. A body may feel liot and cold to the same person at the circumstances Same time, since the sensation of heat is produced by a body may abody feel colder than tlie hand, provided it be less cold than the body the same per- touched immediately before ; and the sensation of cold is tUne?*^ ^^^^ produced under the opposite circumstances, of touching a comparatively warm body, but which is less warm than somo other body touched previously. Thus, if a person transfer one hand to com- mon spring water immediately after touching ice, to that hand the water would feel very warm ; while the other hand transferred from warm water to the spring water, would feel a sensation of cold. Has heat 462. Heat is imponderable, or does not pos- weight? gggg g^j^y. perceptlblc weight. If we balance a quantity of ice in a delicate scale, and then leave it to melt, the equilibrium will not be in the sliglitest degree disturbed. If we substitute for the ice boiling water or red-hot iron, and leave this to cool, there will be no diiferenco in the result. Count Rumford, having suspended a bottle containing water, and another containing alcohol to the arms of a balance and adjusted them so as to be exactly in equilibrium found that the balance remained undisturbed when the water was completely frozen, though the heat the water had lost must have been more than suEBcient to have made an equal weight of gold red hot. What do we 463. The nature, or cause of heat is not nat'uTeofhe*a\^? clcarly Understood. Two explanations, or theories have been proposed to account for the various phenomena of heat, which are known as the me- chanical and vibratory theories. Explain theme- ^64. Tlic mcclianical theory supposes herb chanicai theory. ^^ ^^ ^^^ cxtrcmcly subtilc fluid, or etherial • Thore r.in not be a more fallacious means of estimating heat than by the touch. Thus, in the onii^iary sfate of an apartment, at any season of the year, the objects which are in it have all the samp tpmpcr.iture ; and • et to the touch tliey will fi-el warm and cold in difforent degrees. The metallic objects will be the coldest ; stone and marble h'ss so ; wood still IcKS ; and carpeting and woolen objects will feel warm. Now all these objects are at exactly the same temperature, as ascurtuiucd by the thennumeter. 208 WELLS'S NATURAL PHILOSOPHT. kind of matter pervading all space, and entering into combination in various proportions and quantities, with all bodies, and producing by this combination all the va- rious effects noticed. Explain theri- ^^'^- ^^^^ vibratorj thcorv, on the contrary, bratory theory, gupposcs hcat to be merely the effect of a spe- cies of motion, like a vibration or undulation, produced either in the constituent particles of bodies, or in a subtle, imponderable fluid which pervades them. When one end of a bar of iron is thrust into the fire and heated, the other end soon becomes hot also. According to the mechanical theory, a subtilo fluid coming out of the fire enters into tho iron, and passes from particle to particle until it has spread through the whole. When the hand is applied to the bar it passes into it also, and occasions the sensation of warmth. Ac- cording to the vibratory theory, the heat of the fire communicates to the par- ticles of the iron themselves, or to a subtile fluid pervading them, certain vi- bratory motions, which motions are gradually transmitted in every direction, and produce tho sensation of heat, in the same way that the undulations or vibrations of air, produce the sensation of sound. „ , There seems to bo but little doubt at the present time among Ho-w are these . .r. , , , i • i -i xi -u two theories scientific men, that the theory which ascribes the pneuome- generalijr re- ^^ ^f jjg^^^ ^g 3^ series of vibrations, or undulations, either in garded i ' ' matter, or a fluid pervading it, is substantially correct. At the same time it is not wholly satisfactory, and neither theory will perfectly explain all tho facts in relation to heat with which wo are acquainted. For the purpose of describing and explaining the phenomena and effects of heat, it is convenient, in many cases, to retain tho idea that heat is a substance. The fact that naturo nowhere presents us, neither has art What are pvi- succeeded in showing us, heat alono in a separate state, Gcnccsiu favor o i r i of the respect- is a Strong ground for believing that heat has no separate heat*? '^""'^^ material existence. Heat, moreover, can be produced without limit by friction, and intense heat is also produced by the ex- plosion of gunpowder. On the contrary, as arguments in favor of the material existence of heat, we have the foct, that heat can be communicated very readily through a vacuum ; that it becomes instantly sensible on the condens- ation of any material mass, as if it were squeezed out of it : as when, on re- ducing the bulk of a piece of metal by hammering, we render it very hot (the greatest amount of heat being emitted with the blows that most change its bulk) ; and, finally, that the laws of the spreading of heat do not resemble those of the spreading of sound, or of any other motion known to us. 4G6. The relation between heat and light is a very intimate U^'the'"ri''*be"- one. Heat exists without light, but all the ordinary sources tween heat and of light are also sources of heat; and by whatever artificial means natural light is condensed, so as to increase its splen. SOURCES OF HEAT. 209 dor, Iho heat which it produces is also, at the same time, rendered more intense. 467. When a body, naturally incapable of When is a . . , • i " i ^ . bodyincandes- emittinsr li2:ht, IS heated to a siimcient extent ceat or ignited? , '=' ^ '. . . • i - i • i to become luminous, it is said to bo incandes- cent, or ignited. What is flame 468. Flame is ignited gas issuing from ^ and fire? ^ bumiug bodj. Fire is the appearance of heat and light in conjunction, produced by the combus- tion of inflammable substances. The ancient philosophers used the term fire as a characteristic of the nature of heat, and regarded it as one of the four elements of nature ; air, earth, and water being the other three. Heat and the attraction of cohesion act constantly in opposition to each other ; hence, the more a body is heated, the less will be the attractive force between the particles of which it is composed. SECTIOX I. SOURCES OF KEAT. ^nd ai^ '^° 469. Six great sources of heat are recognized, sourcesofheat? They are — 1. The sun ; 2. The interior of the earth ; 3. Electricity ; 4. Mechanical action ; 5. Chemical action ; 6. Vital action. What is the 470. The greatest natural source of heat is ?Ii^''sourcr*of the sun, as it is also the greatest natural source ^•^'^ of light. Although the quantity of heat sent forth from the sun is immense, its rays, falling naturally, are never hot enough, even in the torrid zone, to kindle -, combustible substances. By means, however, of a burning-glass, the heat of the sun's rays can be con- centrated, or bent toward one point, called a focua, in sufficient quantity to set fire to substances sub- mitted to their action. Fig. 196 represents the manner in which a burning- glass concentrates or bends down the rays of heat until they meet in a focus. Two opinions, or theories, have been entertained in order to account for the production of heat and light by the sun ; one supposes that the sun is an intensely-heated mass, which throws off its light and heat like an intensely- heated mass of iron : the other, based on the ground that heat is occasioned 210 WELLS'S NATURAL PHILOSOPHY. by the vibrations of an ethereal fluid occupying all space, supposes that the sun may produce the phenomena of light and heat without waste of its tem- perature or substance, as a bell may constantly produce the phenomena of sound. Whatever may be the true theory, a series of experiments, made some years since by Arago, the eminent French astronomer, seem to prove that the tem- perature at the surface of the sun is much more elevated than any artificial heat we are able to produce. The experimental reasons which lead to this opinion are as follows : — There are two states in which light is capable of existing — the ordinarf state, and the state of polarization.* It has been proved that all bodies, in i solid or liquid state, which are rendered incandescent by heat, emit a polar- ized light, while bodies that are gaseous, when rendered incandescent, inva- riably emit Hght in its ordinary state. Thus the physical condition of a body may be distinguished when it is incandescent by examining the light which it affords. On applying the test to the direct light of the sun, it was found to be in the unpolarized or ordinary condition of Ught. Hence it has been in- ferred by Arago that the matter from which this light proceeds must be in the gaseous state, or, in other words, in a state of flame. From other experi- ments and observations, Arago was led to the conclusion that the sun was a solid, opaque, non-luminous body, invested with an ocean of flame. _. . ^ , 471. Ovvinoj to the position of the earth's ative heat of axis, the relativ^c amount of heat received from the sun always i • i • • /> greater in some the suii IS alwavs greater in some portions of portions of the , i i i • ^ n t earth than at the oarth than at others, smce the rays oi the others? , />n ti i i sun always lall more directly upon the central portions of the earth than they do at the poles, or extremi- ties ; and the greatest amount of heat is experienced from the rays of the sun when they fall most perpendicularly. Why is the ^'^'^' "^'^^ '^^^^ ^^ ^^^® ^^'^ ^^ greatest at noon, because for heat of the sun the day the sun has reached the highest point in the heavens, Boon^r' *' and its rays fall more perpendicularly than at any other time. What occasions for a Uko reason we experience the extremes of tempera- the diiference . j. ■ . in temperature ture, distinguished as summer and winter. In summer tlie win"erT''^ ^""^ position of the sun in relation to the earth is such, that al- though more remote from the earth than in winter, its rj-fj-a fall more perpendicularly than at any other season, and impart the greatest amount of heat ; while in winter the position of the sun is such that its rays fall more obliquely than at any other time, and impart the smallest amount of heat. The sun, moreover, is longer above the liorizon in summer than in winter, which also produces a corresponding effect. The reason why a difference in the inelination of the sun's rays falling upon • For explanation of the term polarization, see chapter on Light SOURCES OF HEAT. 211 the surface of the earth occasions a difference in their heating effect is, that the more the rays are inclined, the more tlier are diffused, or, in other words, the larger the space they cover. This may be rendered apparent by reference to Fig. 197. pjg jg'j Let us suppose A B C D to represent a portion of the Pun's rays, and C D a ^ portion of the earth's surface upon which the rays fall perpendicularly, and C E portions of the surface upon which they ^ fall obliquely. The same number of ■^ rays will strike upon the surfaces C D and C E, but the surface C E being greater than C D, the rays will necessarily fall more densely upon the latter ; and as the heating power must be in proportion to the density of the rays, it is obvious that C D will be heated more than C E, in just the same propor- tion as the surface C E is more extended. But if we would compare two surfaces upon neither of which the sun's rays fall perpendicularly, k't us take C E and C F. They fall on C E with more obhquity than on C F; but C B is evidently greater than C F, and therefore the rays being diffused over a larger surface are less dense, and therefore less effective in heating. What is the 473, The greatest natural temperature ever ^ueSe?atur'e authentically recorded was at Bagdad, in 1819, ever observed? when thc thermometer (Fahrenheit's) rose to 120° in the shade. On the west coast of Africa the ther- mometer has been observed as high as 108° F. in the shade. Burckhardt in Egypt, and Humboldt in South Americaj observed it at 117° F. in the shade. 474. About 70° below the zero of Fahrenheit's lowest tempe- thermomctcr is the lowest atmospheric temper- rature observed? , . ii.ia,* • ature ever experienced by the Arctic navigators. Towhatextent 475. Tho grcatcst artificial cold ever pro- ^cZ beefprol ^uced was 220° F. below zero. *"c« ii'^ver, however, ex- ceeding the latter distance. 212 WELLS'S NATURAL THILOSOPHT. HoTf do we Independently of the sun, however, the earth is a source of know that the heat. The proof of this is to be found in the fact, that as we of'h^'tr^'""^'^ descend into the earth, and pass beyond the hmitsof the mfiu- ence of the solar heat, the temperature constantly rises. At what rate 477. TliG increasG of temperature observed as peratureoA'he wG (lesceiid iiito the earth, is about one degree earth increase f. ^^ ^j^^ thermometerfor every fifty feet of descent. Supposing the temperature to increase according to this ratio, at the depth of two miles water would bo converted into steam ; at four miles, tin would be melted ; at five miles, lead ; ana at thirty miles, almost every earthy sub- stance would be reduced to a fluid state. The internal heat of the earth does not appear to have any sensible effect upon the temperature at the surface, being estimated at less than 1-3 0th of a degree. The reason why such an amount of heat as is supposed to exist in the interior of the earth does not more sensibly affect the surface is because the materials of which the exterior strata or crust of the earth is composed, do not conduct it to the surface from the interior. Under what ^78. When electricity passes from one sub- ii de^Hcity^a stancG to anothcr, the medium which serves to source of heat? conduct it is vGry frequently heated ; but in what manner the heat is produced we have no positive in- formation. The greatest known heat with which wo are acquainted, is thus produced by the agency of the electric or galvanic current. All kno\vn substances can be melted or volatilized by it. Heat so developed has not been employed for practical or economical pur- poses to any great extent ; but for philosophical experiments and investiga. tions it has been made quite useful. How is chem- 479. Many bodies, when their original con- '^Vceof hLit^? stitution is altered, either by the abstraction of some of their component parts, or by tlie addition of other substances not before in combination with them, evolve heat while the change is taking place. In such cases, the heat is said to be due to chemical ac- tion. What is chem- 480. We apply the term chemical action to icai action? thosc Operations, whatever they may be, by which the form, solidity, color, taste, smell, and action of substances become changed ; so that new bodies, with quite different properties, are formed from the old. A familiar illustration of the maimer in which heat is evolved by chemical SOURCES OF HEAT. 213 action 'm to be found in the experiment of pouring cold water upon quick- lime. The water and tlie lime combme together, and in so doing liberate a great amount of heat, sufficient to set lire to combustible substances. How is heat 481. Hcat is always evolved when a fluid is ciwng'ifof form transformed into a solid, and is always ab- ia matter? sorbed wben a solid is made to assume a fluid condition. As water is changed from its liquid form when it is taken up by quicklime, therefore heat is given ofi". The heat produced by the various forms of combustion, is the result of chem- ical action. , , , 482. Heat exists in two verv difi'ercnt con- In what two conditions dots ditious, as I'ree, or Sensible Heat, and as heat enst ? ' ' Latent Heat.* whatissensi- 483. Whcu thc hcat retained or lost by a bieheat? Lody is attended with a sense of increased or diminished warmth, it is called sensible heat. ■What is latent 484. Wheii the heat retained or lost by a ^^^'^ body is not perceptible to our sense, it is called latent heat „ , Every substance contains more or less of latent heat. Al- How do we ■' know heat to though our senses give us no direct information of its pres- exist in^a body^ ence, we may, by a variety of experiments, prove that it ex- percelye it? ists. Thus, the temperature of ice is 32° by the thermometer, but if ice be melted over a fire and converted into water, the water will be no hotter than the ice was before, although in the operation 140 degrees of heat have been absorbed by the water. "VThen, on the contrary, water passes into ice, a large amount of hcat which was before latent in the water, passes out of it, and becomes sensible.^ 485. Another important source of heat is How is me- i • i • i i • chanir-ii action mechanical action, heat being produced by a source of heat ? p.. I'l ^ . iriction and Dy the condensation, or compres- sion of matter. What are illus- Savage nations kindle a fire by the friction of two pieces trations of the of dry wood ; the axles of wheels revolving rapidly frequent'y hoat"by°"fric- ^e'^O'"© ignited ; and in the boring and turning of metal tha tiou? chisels often become intensely hot. In all these cases the friction of the surfaces of wood or of metal in contact, dis- turbs the latent heat of these substances, and renders it sensible. The following interesting experiment was made by Count Rumford, to il- • Latent, from the Lntin word lltfo, to be hid. t The phenomena of latent heat are further considered under the head of liquefaction. 214 WELLS'S NATURAL PHILOSOPHY. lustrate the effect of friction in producing heat: — A borer was made to re- volve in a cylinder of brass, partially bored, thirty-two times in a minute. The cylinder was inclosed in a box containing 18 pounds of water, the tem- perature of which was at first G0° but rose in an hour to 107°; and in two hours and a half the water boiled. Is air necessary ^^^ ^°®^ "°'' ^^P^^^ ^0 be necessary to the production of for the produc- heat by the friction of solid bodies • suice heat is produced frictiou?''''' ^ ^y ^''''-■ti"" ^vi^l"^ a vacuum. To whatever extent the operation may be carried, a body D3ver ceases to give out heat by friction, and this fact is considered as a strong argument in favor of the theory that heat is not a substance, but merely a property of matter. It was formerly supposed that solids alone could develop heat by friction, but recent experiments have proved, beyond a doubt, that heat is also gener- ated by the friction of fluids. The heat excited by friction is not m proportion to the hardness or elas- ticity of the bodies employed ; on the contrary, a piece of brass rubbed with a piece of cedar- wood produced more heat than when rubbed with another piece of metal ; and the heat was still greater when two pieces of wood were employed. The reduction of matter into a smaller compass by an exter- tratronTof^'the °^^ °^ mechanical force, is generally attended with an evolu- production of tion of heat. To such an act of compression we apply the heat by con- • . i j.- densation ? term condensation. Heat may be evolved from air by condensation. This may be shown by placing a piece of tinder in a tube, and suddenly compressing the air contained in it by means of a piston. The air being thus condensed, parts with its latent heat in sufficient quantity to set fire to the tinder at the bot- tom of the tube. Another famiUar experiment of the evolution of heat by condensation, is the rendering of iron hot by striking it with a hammer. The particles of the iron being compressed by the hammer, can no longer contain so much heat in a latent state as they did before : some of it therefore be- comes sensible, and increases the temperature of the metal, and the striking may be continued to such an extent as to render the iron red-hot. "When a match is drawn over sand-paper or other rough substance, cer- tain phosphoric particles are rubbed off, and being compressed between the match and the paper, their heat is raised sufficiently high to ignite them and fire the match. If the match be drawn over a smooth surface, the compres- sion must be increased, for the temperature of the whole phosphoric mass must be raised in order to cause ignition. The fulminating substance of a percussion-cap explodes when struck by a hammer, because the blow occasions a compression of the particles, by which a sufficient amount of latent heat is liberated to produce ignition. What is meant ^^^- Most livinc^ animals possess the property by vital heat? ^£ maintaining in their system an equable tern- SOURCES OF HEAT. 215 perature, whether surrounded by bodies that are hotter or colder than they are themselves. The cause of this is due to the action of vital heat, or the heat generated or ex- cited by the organs of a living structure. The following facts illustrate this principle : — The explorers of the Arctic regions, during the polar winter, while breathing air that froze mercury, still had in them the natural warmth of 98° Fahrenheit above zero; and the inhabitants of India, where the same thermometer sometimes stands at 3; 5° in the shade, have their blood at no higher temperature. Again, the temperature of birds is not that of the atmosphere, nor of fishes that of the sea. 487. The cause of animal heat is undoubt- cause of vital cdly duc to cliemical action ; — the result of respiration and nervous excitation, jjg . jj. Growing vegetables and plants also possess, in a degree, Bess this prop- the property of maintaining a constant temperature within '■"'^^ their structure. The sap of trees remains unfrozen when the temperature of the surrounding atmosphere is many degrees below the freez- ing point of water. This power of preserving a constant temperature in the animal structure is limited. Intense cold suddenly coming upon a man who has not sufficient protection, first causes a sensation of pain, and then brings on an almost irre- sistible sleepiness, which if indulged in proves fatal. A great excess of heat also can not long be sustained by the human system. Each species of animal and vegetable appears to have a temperatare natural and peculiar to itself, and from this diversity different races are fitted for dif- ferent portions of the earth's surface. Thus, the orange-tree and the bird of Paradise are confined to warm latitudes; the pine-tree and the Arctic bear, to those which are colder When animals and plants are removed from their peculiar and natural dis- tricts to one entirely diSerent, they cease to exist, or change their character in such a way as to adapt themselves to the climate. As illustrations of this, we find that the wool of the northern sheep changes in the tropics to a spe- cies of hair. The dog of the torrid zone is nearly destitute of hair. Bees transported from the north to the region of perpetual summer, cease to lay up stores of honey, and lose in a great measure their habits of industry. Man alone is capable of living in all cUmates, and of migrating fi-eely to all portions of the earth. Of all animals, birds have the highest temperature ; mammalia, or those which suckle their yountr. come next ; then ampliibious animals, fishes, and certain insects. Shell-fish, worms, and tlie like, stand lowest in the scale of temperature. The common mud-wasp, in its chrysalis state, remains unfrozen during the most severe cold of a northern winter ; the fluids of the body in- stantly congeal, however, in a freezing temperature, the moment the case, or shell which incloses it, is crushed. 216 WELLS'S NATURAL PHILOSOPHY. SECTION II. COMMUNICATION OF HEAT. How may heat 433 jjcat mav be commutiicated in three be commum- J cated? ways: by Conduction, by Convection, and by Kadiation. 489. Heat is communicated by conduction How is heat . 1/. • ^ • ^ r> ^ communicated wbcn it travols irom particle to particle of the byconductioQ? ^1 i r- 1 • 1 substance, as Irotn the end ot the iron bar placed in the fire to that part of the bar most remote from the fire. What is con- 490. Whcu hcat is communicated by being vecuoa? carried by the natural motion of a substance containing it to another substance or i)lace, as when hot water resting upon the bottom of a kettle rises and carries heat to a mass of water through which it ascends, the heat is said to be communicated by convection, whatisradia. 491. Hcat is communicatcd by radiation tioaofheat? "vvhen it leaps, as it were, from a hot to a cold body through an appreciable interval of space ; as when a body is warmed by placing it before a fire removed to a little distance from it. How does a 492. A hcatcd body cools itself, first by giv- cooi uscif'?°'^^ ing oif heat from its surface, either by conduc- tion or radiation, or both conjointly ; and sec- ondly, by the heat in its interior passing from particle to particle by conduction, through its substance to the sur- face. A cold body, on the contrary, becomes heated by a process directly the reverse of this. Do all bodies 493. Different bodies exhibit a very great equluy'weuf degree of difference in the facility with whic h they conduct heat : some substances oppose very little impediment to its passage, while through others it is transmitted slowly. What are con- 494, All bodics are divided into two classes non-conduct"or8 ^^ rcspcct to their conduction of heat, viz., of heat? jj^j-Q conductors and non-conductors. The for- COMMUNICATION OF HEAT. 217 Fig. 198. mer are sucli as allow heat to pass freely through them ; the latter comprise those which do not give au easy pass- age to it. Dense solid bodies, like the metals, are the best con- ductors of heat ;* light, porous substances, more esi^ecially those of a fibrous nature, are the worst conductors of heat. The different conducting power of various solid substances may be strik* icgly shown by taking a series of rods of equal length and thickness, coating one of their extremities with wax, and placing the other extremities equally, in a source of heat The wax will be found to entirely melt off from some of the rods before it has hardly softened upon others. What is the 495. Liquids are almost tower"""/ uq- absolutc non-conductois of •""i^- heat. If a small quantity of alcohol be poured on the sur- face of water and inflamed, it will continue to bum for some time. (See Fig. 198.) A thermometer, immersed at a small depth below the common sur- face of the spirit and the water, will fMl to show any increase in temperature. Another and more simple experiment proves the Game fiict ; as when a blacksmith immerses his red- hot iron in a tank of water, the water which sur- rounds the iron is made boding hot, whde the water not immediately in contact with it remains quite cold. Fig. 199. If a tube nearly filled with water is held over a spirit lamp, as in Fig. 199, in such a manner as to direct the flame against the upper layers of the water, the water will be observed to boil at the top, but remain cool below. If quicksilver, on the contrary, be so treated, its lower layers ^ill speedily be- come heated. The particles of mercury will communicate the heat to each Jther, but the particles of water will not do so. A stone, or marble hearth in an apartment, feels colder to btone, or°mar" the feet than a woolen carpet, or hearth-rug, not because tho ble hearth feel Q^e is hotter than the other, for both are really of the sa;; 3 but because the stone and marble are good colder than a carpet? temperature, • The following table exhibits the relative conducting power of different substances, ihe ratio expressing the conducting power of gold being taken at 100 ; Oold . 100-00 Tin . . 30-33 Platinum . 9S-10 Lp.id . . iT-ne Silver . 07 no Marble . 2-.^t Copper . . STS-i Porcelain . . 1-2-2 Iron . 37-41 Brick earth . 113 Zinc . 36-3T 10 218 WELLS'S NATUEAL PHILOSOPHY. conductors, and the -woolen carpet and hoarth-rug very bad conductors. The action of the two substances is as follows: — As soon as the hearth- stone has absorbed a portion of heat from the feet, it instantly disposes of it by conducting it away, and calls for a fresh supply ; and this ac- tion will continue until the stone and the foot have established an equili- brium of temperature between them. The carpet and the rug also absorb heat from the feet in like manner, but they conduct or convey it away so slowly, that its loss is hardly perceptible. Most varieties of wood are bad conductors of heat ; hence, though one end oi a stick is blazing, the other end may be quite cold. Cooking vessels, for this reason, are often fiirnished with wooden handles, which conduct the heat of the vessel too slowly to render its influx into our hands painful. For tho same reason we use paper or woolen kettle- holders. To what extent ^^^- Bodies in the gaseous, or aeriform con- bodieslonXct dition aro more imperfect conductors of heat ^'^*'^ than liquids. Common air, especially, is one of the worst conductors of heat with which we are ac- quainted. How is air ^97. Air is, however, readily heated hy con- heatedf vection. Tlius, when a portion of air by com- ing in contact with a heated body has heat imparted to it, it expands, and becoming relatively lighter than thtr other portions around it, rises upward in a current, carry^ ing the heat with it ; other colder air succeeds, and (being heated in a similar way) ascends also. A series of cur- rents are thus formed, which are called " convective cur- rents." In this way air, which is a bad conductor, rapidly reduces the temperature of a heated substance. If the air which encases the heated substance were to remain perfectly motionless, it would soon become, by contact, of the same temperature as the body itself, and the withdrawal of heat would be checked : but as the external air is never perfectly at rest, fresh and colder portions continually replace and succeed those which have become in any degree heated, and thus the abstraction of heat goes on. For this reason a windy day always feels colder than a calm day of the same temperature, because in the former case the particles of air pass over us more rapidly, and every fresh particle takes some portion of heat. How may the 498. Tho couductiug power of all bodies is Towe'r^dies diminished by pulverizing them, or dividing bedin.inishcd? i[yQ^-^ into fine filaments. Thus saw-dust, when not too much compressed, is one of the most perfect COMMUNICATION OF HEAT. 219 non-conductors of heat. "\iVool, fur, hair and feathers, are also among the worst conductors of heat. "VToolens and furs are used for clothmer in cold weather, not VThy are furs . ,,],,_ i and woolens because they impart any neat to the body, but because they used for cloth- ^^g ygj.™ ^y^^ conductors of heat ; and therefore prevent tho ing? •' •warmth of the body from being di-avm otf by the cold air. The heat generated in the animal system by vitiJ action has constantly a tendency to escape, and be dissipated at the surfoce of the body, and the rat© at which it is dissipated depends on the difference between the temperatiu^ o.' the surface of the body aud the temperature of the surrounding medium. "By interposing, however, a non-conducting substance between the sm-faco of the body and the external atmosphere, we prevent the loss of heat which would otherwise take place to a greater or less degree. The non-conducting properties of fibrous and porous sub- To what are gtances are due almost altogether to the air contained in their the non-con- " j- j ducting prop- interstices, or between their fibers. These are so disposed as subsunce^^due? ^ receive and retain a large quantity of ah: without permittmg it to circulate. The warmest clothing Ls that which fits the body rather loosely, because more hot air -will be confined by a moderately loose garment than by one which fits the body tightly. Blankets and warm woolen goods are always made with a nap or projec- tion of fibers upon the outside, in order to take advantage of this principle. The nap or fibers retain air among them, which, from its non-conducting properties, serves to increase the warmth of the material. ,^ . „ The finer the fibers of hair, or wool, the more closelv they What luflu- . , . , ,.,.., 1 ., • " ence has the retain the air enveloped within them, and the more imperme- fineness of the j^^^jg ^-jjgy ijecome to heat In accordance with this princi- fibers upon the • . warnuli of a pie, the external coverings of animals vary not only with the material ? cUmate which the species inhabit, but also in the same indi- vidual they change with the season. In warm climates the furs are generally coarse and tliin, while in cold countries they are fine, close, hght, aud of uni- form texture, almost perfect non-couductors of heat. We have illustrations of this principle also in the vegetable kingdom. The bark of trees, instead of being compact and hard like the wood it envelops, is porous and formed of fibers, or layers, which, by including more or less of air between their surfaces, are rendered non-conductors, and prevent the escape of heat from the body of the tree. An apartment is rendered much warmer for being furnished with double d lors and windows, beaiuse tlie air confined between the two surfaces op- poses both the escape of heat fi'om within, and the admission of cold from vithout As a non-conducting substance prevents the escape of heat from within a body, so it is equally efficacious in preventing the access of heat from without. In an atmosphere hotter than our bodies, the cSect of clothing would be to keep the body cooL Flannel is one of tho warmest articles of dresa, yet WQ 220 WELLS'S NATUEAL PHILOSOPHY. can not preserve ice more effectually in summer than by enveloping it iu its fjlds. Firemen exposed to the intcuso heat of furnaces and steam-boilers, iu- variablj' protect themselves with flannel garments. Cargoes of ice shipped to the tropics, are generally packed for preservation in sawdust : a casing of sawdust is also one of the most effectual means of preventing the escape of heat from the surfaces of steam-boilers and steam- pipes. Straw, from its fibrous character, is an excellent non-conductor of heat, and is for this reason extensively used by gardeners for incasing plants and trees which are exposed to the extreme cold of winter. Snow protects the soil in winter from the effects of cold in protect the the same way that fur and wool protect animals, and cloth- earth from jjjg, man. Snow is made up of an infinite number of little cold? ° . ..... crj^stals, which retam among their interstices a large amourit of air, and thus contribute to render it a non-conductor of heat. A covering of snow also prevents the earth from throwing off its heat by radiation. The temperature of the earth, therefore, when covered with snow, rarely descends much below the freezing-point, even wlien the air is fifteen or twenty de- grees colder.* Thus roots and fibers of trees and plants, ai'e protected from a destructive cold. 499. Clothing is considered warm or cool ac- circumstances cordlng as it impedcs or facilitates the passage «idfrua"trarm of hcat to or from the surface of our bodies. The finer the cloth, the more slowly it con- ducts heat. Fine cloths, therefore, are warmer than coarse ones. Woolen substances are worse conductors of heat than cotton, cotton than silk, and silk than linen. A flannel shirt more effectually intercepts heat than cotton, and a cotton than a linen one. The sheets of a bed feel colder than the blankets, because they are better conductors of heat, and carry off the heat more rapidly from the body, the actual temperature of both, however, is the same. For the same reason, a linen handkerchief is cooler and more agreeable to the face than a cotton one. Cellars feel cool in summer, and warm in winter, because the external air • " Few can realize the protecting value of the warm coverlet of snow. No eider-down Jn the cradle of an infant is tucked in more kindly than the sleeping-dress of winter about the feeble flower-life of the Arctic regions. The first warm snows of August and Septem- ter, falling on a thickly-blended carpet of grasses, heaths and willows, enshrine the flowery growths which nestle around them in a non-conducting air-chamber; and as each successive snow increases the thickness of the cover, we have, before the intense cild of winter sets in, a light cellular bed, covered by drift, six, eight, or ten feet deep, iu which tlie plant retains its vitality. The frozen sub-soil does not encroach upon this nar- row cover of vegetition. I have found, in mid-winter, in the high latitude of 7S°, the Burface so nearly moist as to be friable to the touch ; and on the ice-floes commencing with a Burface temperature of 30° below zero, I found, at two feet deep, a temperature of 8° below zero, at four feet 2° above zero, and at eight feet 26° above zero. My experi- jnents prove that the conducting power of snow is proportioned to its compression by winds, ruins, drifts, and congelutiou." — De. Kane'b Secund Arctic Espedition. COMMUNICATION OF HEAT. 221 has not free access into them ; in consequence of which they remain almost at an even temperature, wliich in summer is about 10 degrees colder, and in winter about 10 degrees warmer than the external air. „ , 500. Refrigerators, used for the preservation of animal and Upon what , , , . , , , , ,, , principle are vegetable substances in warm weather, are double-walled refrigerators boxes, with the spaces between the sides filled with powdered and fire-proof ^ c i safes con- charcoal, or some other porous, non-conducting substance. Btructed? rpj^^ so-called "fire-proof" safes are also constructed of double or treble walls of iron, with intervening spaces between them filled with gjT^sum, or " Plaster of Paris." This lining, which is a most perfect non-conductor, prevents the heat from passing from the exterior of the safe to the books and papers within. The idea of applying " Plaster of Paris" in this way for the construction of safes, originated, in the first instance, from a workman attempting to heat water in a tin basin, the bottom and sides of which were thinly coated with this substance. The non-conducting proper- ties of the plaster were so great as to almost entirely intercept the passage of the heat, and the man, to his surprise, found that the water, although directly over the fire, did not get hot. 501. It has been already stated that liquids and gases are non-conductors of heat, and can not well be heated, like a mass of metal, or any solid, by the communication of heat from particle to particle. Why can not This pcculiarity is owing to the mobility gHses'^be hMN whicli subslsts among the particles of all fluids, mannerls's™!! ^^^ ^0 ^^^ chaugc iu the slzc of the particles, "*^ which is invariably produced by a change in their temperature. The constituent particles of solid bodies being incapable of changing their relative position and arrangement, the heat can only pass through them, from particle to particle, by a slow process ; but when the particles forming any stratum of liquid are heated, their mass, expanding, becomes lighter, bulk for bulk, than the oolder stratum immediately above it, and ascends, allowing the superior strata to descend. How is water 502. When the heat enters at made hot? ^|^g bottom of a vesscl containing water, a double set of currents is immediately established — one of hot particles rising to- ward the surface, and the other of colder par- ticles descending to the bottom. The por- tion of liquid which receives heat from below is thus continually diffused through the other 222 WELLS'S NATUKAL PniLOSOPHY. parts, and the heat is communicated by the motion of the particles among each other. These currents take place so rapidly, that if a thermometer be placed at the bottom and another at the top of a long jar, the fire being applied below, the upper one will begin to rise almost as soon as the lower one. The movement of the particles of water in boiling will be understood by reference to Fig. 200. They may be rendered visible by adding to a flask of boiling water a few small particles of bituminous coal, or flowers of sulphur. Air and other gases are heated in precisely the same manner aa wateij and this method of communicating heat is termed convection. Heat, however, passes by conduction between the particles of both liquids and gases, but to such a slight extent, that they were for a long time re- garded as entirely incapable of conducting heat. In what man- ^^^- Tlic proccss of cooHng in a h'quid is cooild? "'^"''^ directly -the reverse of that of heating. The particles at the surface, by contact with the air, readily lose their heat, become heavier, and sink, while the warmer particles below in turn rise to the surface. To heat a liquid, therefore, the heat should be applied at the bottom of the mass ; to cool it, the cold should be applied at the top, or surface. The facility with which a liquid may be heated or cooled depends in a great degree on the mobility of its particles. "Water may be made to retain its heat for a long time by adding to it a small quantity of starch, the particles of which, by their viscidity or tenacity, prevent the free circulation of tho heated particles of water. For the same reason soup retains its heat longer than water, and all thick liquids, like oil, molasses, tar, etc., require a consid- erable time for cooling. p 504. "When the hand is placed near a hot body suspended piienomena of in the air, a sensation of warmth is perceived, even for a radiation, considerable distance. If the hand be held beneath the body, the sensation will be as great as upon the sides, although the heat has to shoot down through an opposing current of air approacliing it. This effect does not arise from the heat being conveyed by means of a hot current, since all the heated particles have a uniform tendency to rise ; neither can it de- pend upon the conducting power of the air, because aeriform substances pos- sess that power in a very low degree, while the sensation in the present case is excited almost on the instant. This method of distributing heat, to did" tinguish it from heat passing by conduction, or convection, is called radiation, and heat thus distributed is termed radiant, or radiated heat. T^ .. V :.. 505. All bodies radiate heat in some meas- Do all bodies radiate heat urc, but uot all CQuallv well ; radiation bemar squally well ? ' ii ^i t- m proportion to the roughness of the radiatmg surface. All dull and dark substances are, for the most COMMUNICATION OF HEAT. 223 part, good radiators of heat ; but bright and polished sub- stances are generally bad radiators.* Color, however, alone, has no elFect on the radiation of heat. If a metal surface be scratched, its radiating power is increased. A liquid contained iu a bright, highly-polislied metal pot, will retain its beat much longer than in a dull and blackened one. This is not due to the polish or briglitness of the surface, but to the foct that, by polishing, the surface is ren- dered dense and smooth, and such surfaces do not allow the heat to escape readily. If we cover the polished metal surface with a thin cotton or linen cloth, so as to render the surface less dense, the radiation of heat, and conse- quent cooling, will proceed rapidh'. Black lead is one of the best known radiators of heat, and on this account is generally employed for the blackening of stoves and hot-air flues. As a high polish is unfivorable to radiation, stoves should not be too highly polished with this substance. Heat radiated from the sun is all radiant heat. 506. Heat is propagated throug;!! space by How is heat ... . \ , '^ ^. , ? .' . ■^ propagated by radiatiou in straitrht lines, and its intensity radiation? . T * *1 1 1 ' T vanes according to the same law which governs the- attraction of gravitation, that is, inversely as to the square of the distance.f Thus the heating effects of any hot body is nine times less at three feet than at one ; sixteen times less at four feet ; and twenty-live times less at five. The velocity with which radiant heat moves through space l^Tdty does la- is, in all probabihty, the same as the velocity of light. Somo dirint heat authorities, however, consider it to be only four fifths of that of light, or about 164,000 miles in a second of time. Does radiation 507. Thc radiatiou of heat goes on at all mTiuiyfromrii tiuies, aud from all surfaces, whether their ""^'^^^ temperature be the same or different from that of surrounding objects : therefore the temperature of a body falls when it radiates more heat than it absorbs; its temperature is stationary when the quantities emitted and received are eo^^ual ; and it grows warm when the ab- sorption exceeds the radiation. • The action of a blackened surface of tin being assumed as TOO, it has been found that that of a steel plate was 15 ; of clean tin, 12 ; of tin scraped bright, 16 ; wlien scraped with the edge of a fine file in one direction, -6; wlien scraped again across, al)oiit IH; a sur- face of clean lead, 19 ; covered with a gray crust, 45 ; a tliin crust of isinglass, 80 ; rosin 96 ; wrjting-paper, 9$ ; ice. 85. t It is an exceedingly curious fact that this liw applies to all physical influencos that spread from a center, such as gravitation, heat, light, electrical forces, magnetism and sound ; and to all central forces, when not weakened by any resistance or opposing force. 224 WELLS'S NATUKAL PHILOSOPHY. If a body, at any temperature, be placed among other bodies, it will affect their condition of temperature, or as we express it, thermally ; just as a candle brought into a room illuminates all bodies in its presence ; with this difference, however, that if the candle be extinguished, no more liglit is dift'used by it ; but no body can be thermally extinguished. All bodies, however low be their temperature, contain heat, and therefore radiate it. If a piece of ice be held before a thermometer, it will cause tticrmofneter* *''® mercury in its tube to fall, and hence it has been sup- sink when posed that the ice emitted rays of cold. This supposition is lceT° ^^^ erroneous. The ice and the thermometer both radiate heat, and each absorbs more or less of what the other radiates to- ward it. But the ice, being at a lower temperature than the thermometer, radiates less than the thermometer, and therefore the thermometer absorbs less than the ice, and consequently falls. If the thermometer placed in the presence of the ice had been at a lower temperature than the ice, it would, for like reasons, have risen. The ice in that case would have wanned the thermometer. What do we ^^'^' I^'^^diations, or effects which are propagated in straight mean by rays lines only (such as light and radiant heat), are most conve- Ueht?^* °^ niently considered by dividing them into innumerable straight lines, or rays; not that there are any such divisions in nature, but they enable us more readily to comprehend the nature of the phenomena with which these principles are concerned. When radiant 510. When rajs of heat radiated from one ?hf sm-faTo? ^odj fall upon the surface of another body, Ihey may^it^be di^ maybe disposed of in three ways : 1. They posed of? jjj.-^y rebound from its surface, or be reflected ; 2. They may be received into its surface, or be absorbed ; 3. They may pass directly through the substance of the body, or be transmitted. 511. A ray of beat radiated from the sur- In what man- „ /> i i i • 'it -i ner is heat re- tacc 01 a Dody procccds m a straiiijht line until fleeted' " it meets a reflecting surface, from whicb it rebounds in another straight line, the direction of which is determined by the law that the angle of incidence is equal to the angle of reflection. The manner in which heat is reflected is strikingly shown by taking two concave mirrors, M and N, Fig. 201, of bright metal, about one foot in diameter, and placing them exactly opposite to each other at a distance of about ten feet. In the focus of one mirror, as at ^A , is placed a heated body, as a mass of red hot iron, and in the focus of the other mirror, as at B, a small quantity of gunpowder, or apiece of phosphorus. The rays of heat, radiated in diverg- ing lines from the hot metal, strike upon the surface of the mirror M, and aro COMMUNICATION OF HEAT, 225 reflected by it in parallel lines to the surface of the opposite mirror, N, where they will be caused to converge to its focus, B, and ignite the powder or phosphorus at that point. Fig. 201. 512. Polished metallic surfaces constitute reflectors of the bcst reflcctors of heat ; but all bright and light colored surfaces are adapted for this pur- pose to a greater or less degree.* Water requires a longer time to become hot in a bright tin vessel than in a dark colored one, because the heat is reflected from the bright surface, and does not enter the vessel How does the ^l^- Thc powcr of absorbing heat varies 6orT.ing°*^heat ^^^^ almost cvery form of matter. Surfaces ^^'T? are good absorbers of heat in proportion as they are poor reflectors. The best radiators of heat also are the most powerful absorbers, and the most imperfect reflectors. Dark colors absorb heat from the sun more abundantly than light ones. Tliis may be proved by placing a piece of black and a piece of white cloth upon the snow exposed to the sun ; in a few hotirs the black cloth will havo melted the snow beneath it, while the white cloth wiU have produced little or no effect upon it. The darker any color is, the warmer it is, because it is a better absorbent of heat. The order may be thus arranged : 1 , black (warmest of all) ; 2, violet ; 3, indigo ; 4, blue ; 5, green ; 6, red ; 7, yellow ; and 8, white (coldest of all). • Of 100 rays falling at an angle of G0° from the perpendicular, polished gold will reflect 76 .• silver 62 • brass, 62; brass \rithout polish. 52 ; polished brass varnished. 41 ; looking- glass, 20 ; glass plate blackened on the back, 12 ; and metal plate blackened, 6. 10* 226 "WELLS'S NATURAL PHILOSOPHY. A piece of brown paper submitted to the action of a burning-glass, ignites much more quickly than a piece of white paper. The reason of this is, that the white paper reflects the rays of the sun, and though but slightly heated appears highly lummous ; while the brown paper, which absorbs the rays, readily becomes heated to ignition. For the same reason a kettle whose bot- tom and sides are covered with soot, heats water more readily than a kettle whose sides are bright and clean. Light-colored fabrics are most suitable for dresses in summer, since they reflect the direct heat of the sun, and do not absorb it ; black outside gar- ments, on the contrary, are most suitable for winter, as they absOTb heat readily, but do not reflect it. Hoar-frost, in the spring and autumn, may be observed to remain longer in the presence of the morning sun, on liglit-colored substances than upon the dark-colored soil, etc. ; the former do not absorb the heat, as the dark-colored bodies do, but reflect it, and in consequence of this they remain too cold to thaw the frost deposited upon their surfaces. How is the at- ^^"^ '^^ absorbs heat very slowly, and does not readily xnosphere heat- part with it. Air rarely radiates heat, and is not heated to ^^^ great extent by the direct rays of the sun. The spn, however, heats the surface of the earth, and the air resting upon it is heated by contact with it, and ascends, its place being supplied by colder portions, which in turn are heated also. This reluctance of air to part with its heat occasions some very curious dif- ferences between its burning temperature and that of other bodies. Metals, which are generally the best conductors, and therefore communicate heat most readily, can not be handled with impunity when raised to a temperature of more than 120° F. ; wa,ter becomes scalding hot at 150° F. ; but air ap- phed to the skin occasions no very painful sensation when its heat is far be- yond that of boiling water. Some curious experiments have been made in reference to the power of the human body to withstand the influence of heated air. Sir Joseph Banks en- tered an oven heated 52° above the boiling point, and remained there some time without inconvenience. During the time, eggs, placed on a metal frame, were roasted hard, and a beefsteak was overdone. But though he could thus bear the contact of the heated air, he could not bear to touch any metal- lic substance, as a watch-chain, money, etc. Workmen, also, enter ovens, in the manufacture of molds of plaster of Paris, in which the thermometer stands 100° above the temperature of boiling water, and sustain no mjury. In what man- 515. HGcat, ill passing through most sub- tr'iv'nBmitted^''* stancGs, 01 media, is retained, or intercepted Inrsub^t'ln^es"? ^^ ^^^ passagc in a greater or less degree. The capacity of solids and liquids for transmitting heat is not always in proportion to their transparency, or capacity for transmitting light. THE EFFECTS OF HEAT. 22T 516. The heat of the bud passes through transparent ujdies without loss ; but heat from terrestrial sources is in great part arrested by many substances which allow light to pass freely, — such as water, alum, glass, etc. Thus, a plate of glass held between one's face and the sun mil not protect it, but held between the face and a fire, it will intercept a large proportion of 0 heat 517. Those substances which allow heat to pass freely through them, are called d'latliermanous, and those which retain nearly all the heat they receive, are called ather- manous. Rock-salt allows heat to pass through it more readily than any other known substance ; while a thin plate of alum, which is nearly transparent, almost entirely intercepts terrestrial heat Heat, indeed, will pass more readily through a black glass, so dark that the sun at noon is scarcely discernible through it, than through a thin plate of clear alum. Water is one of the least diathermanous substances, although its transparency is nearly perfect li, therefore, it is desired to transmit light without heat, or with greatly dimin- ished heat, it is only necessary to let the rays pass through water, by which they will be stramed of a great part of their heat. It has been found that the power of heat to penetrate a dense, transparent substance, is increased in proportion as the temperature of the body from which it is radiated is increased. Heat, also, accompanied by light, is transmitted more readily than heat without light 518. Heat and light come to us conjointly from the sun. "When a ray of light is caused to pass through a prism it is analyzed or separated into seven brilliant colors, or element- ary parts. If the heat ray which accompanies the light ia treated in a similar manner, our organs of sight are so constituted that we ao not discover any separation to have taken place in it. It is, however, es- tablished beyond a doubt that in the same manner as a ray of white light can be modified and divided, so a ray of radiant heat can be separated into parts possessing qualities corresponding to the various colors. How flops the tcini)iT;itureof a body raduit- iiv,' hiat aff.ict its truusmis- Bioii ? Is a ray of solar heat simple or compound in its nature ? What effect does heat pro- duce upon all bodies '/ SECTION III. THE EFFECTS OF HEAT. 519. The general and most ob\aous effect (-^ heat upon material substances, is to expand them, or increase their dimensions. 228 WELLS'S NATURAL PHILOSOPHY. Is the form of 520. The form of all bodies appears to be pendcur'upon entirely dependent on heat ; by its increase *'^" solids are converted into liquids, and liquids into vapor ; by its diminution vapors are condensed into liquids, and these in turn become solids. If matter ceased to be influenced by heat, all liquids, vapors, and doubtless even gases, would become permanently solid, and all motion on the surface of the earth would be arrested. What are the ^^1. Tho tln'ce most apparent effects of J)arent°°effects ^^^at, SO far as relate to the form and dimen- ofheat? sions of bodics, are Expansion, Liquefaction, and Vaporization. Heat operates to produce expansion by introducing a repulsive force among the particles of the body it pervades. This repulsive force, in the case of solids, weakens or overcomes the attraction of cohesion, and gives to the par- ticles of all matter a tendency to separate, or increase their distance from one another. Hence the general mass of the body is made to occupy a larger space, or expand. In what bodies ^^2, The cxpausion occasioned by heat is ducethe^'^maN g^eatcst lu thosc bodies which are the least in- est expansion? fl^enced by the attraction of cohesion. Thus the expansion of solids is comparatively triflinj^, that of liquids much greater, and that of gases very consid- erable. Do bodies con- 523. Thc expausion of the same body will "rnd as° lo^^n " coutinue to iucreasc with the quantity of heat as heat enters ^\^^ entcrs it, SO loug as the form and chemi- tuem I . cal constitution of the body is preserved. 524. Among solids the metals expand the most ; but an iron wire increases only 1-282 in bulk when heated from 32° of the thermometer up to 212. Solids appear to expand uniformly from the freezing point of water up to 212°, tlie boiling point of water ; — that is to say, the increase of volume which attends each degree of temperature which the body receives is equal. When solids are elevated, however, to temperatures above 212°, they do not dilate uniformly, but expand in an increasing ratio. The expansion of solids by heat is clearly shown by the following experi- ment, Fig. 202 : m represents a ring of metal, through which, at the ordinary temperature, a small iron or copper ball, a, will pass freely, this ball being a little less than the diameter of the ring. If this ball be now heated by the THE EFFECTS OF HEAT. 229 "What applica- tions of the expansion of Bolids by heat arc made in the arts? ._-J^i j^T-^qiggcgwcra With what de- gree of force do bodies ex- pand and con- tract ? flame of an alcohol lamp, it will become so FiG. 202. for expanded by heat as no longer to pass through the ring. The expansion of solids by heat is made applicable for many useful purposes in the arts. The tires of wheels, and hoops surrounding ■water- vats, barrels, etc., are made in the f rst instance somewhat smaller than the frame- work they are intended to surround. Tliey are then heated red hot and put on in an expanded condition ; on cooling, they contract and bind together the several parts with a greater force than could be conveniently applied by any mechanical means. In hke manner, in con- structing steam-boilers, the rivets are fastened while hot, in order that they may, by subsequent contraction, fasten the plates together more firmly. 525. The force with which bodies expand and contract under the influence of the in- crease or diminution of heat, is apparently- irresistible, and is recognized as one of the greatest forces in nature. The amount of force with which a solid body wUl expand or contract is efiual to that which would be required to compress it througli a space equal to its expansion, and to that which would be required to stretch it through a space equal to its contraction. Thus, if a pillar of metal one hundred inches in height, being raised in temperature, is augmented in height by a quarter of an inch, the force with which such increase of height is produced is equal to a weight which being placed upon the top of the pillar would compress it so as to diminish its height by a quarter of an inch. In the same manner, if a rod of metal, one hundred inches in length, be cofttracted by diminished temperature, so as to render its length a quarter of an inch less, the force with which this contraction takes place is equal to that which being applied to stretch it would cause its length to be increased by a quarter of an inch. This principle is sometimes practically applied when great mechanical forc« is required to be exerted through small spaces. Thus walls of buildings •which, from a subsidence of the foundation, or an unequal press\u"e, have been thrown out of their perpendicular position, and are in danger of falling, may be restored in the following manner : A series of iron rods are carried across the budding, passing through holes in the walls, and secured by nuts on the outside. The rods are then heated by lamps until they expand, thereby causing their ends to project beyond the building. The nuts with which these extremities are provided are then screwed up until they are in close contact with the outside wall, the lamps arc then withdrawn and the roda 230 WELLS'S NATURAL PHILOSOPHY. allowed to cool. lu cooling thej gradually contract, and by their contrac- tion draw up the waUs. On account of the expansion of metal by heat, the successive rails which compose a hne of railway can not be placed end to end, but a small space is left between their extremities for expansion. A stove snaps and crackles wlien a fire is first kindled in it, and also when the fire in it is extinguished. This noise is occasioned by the expansion and contraction of the several parts consequent on the increase and diminution of heat. A glass or earthen vessel is Uable to break when hot water is poured into it, on account of the unequal expansion of the inner and outer surfaces. Glass and earthen ware being poor conductors of heat, the inner surfaces m contact with the hot water become heated and expand before the outer aro afiectod ; the tendency of this is to warp or bend the sides unequally, and as the brittle material can not bend, it breaks. Nails in old houses are often loose and easily drawn out ; the iron expands in summer and contracts in winter more than the wood into which it has been driven, and thus in time the opening is enlarged. When the stopper of a decanter or smelling-bottle sticks, a cloth dipped in hot water, and applied to the neck of the bottle will frequently loosen it, since by the heat of the cloth its dimensions aro expanded and enlarged. The tone of a piano is higher in a cold than in a warm room, for the reason that the strings, being contracted by cold, are drawn tighter. 526. Liquids expand through the ao;ency of To what extent -■■ n i i do liquias ox- heat more unequally, and to a mucli cjreater pandbyheatr , i i-i degree than solids. A column of water contained in a cylindrical glass vessel will expand aV in length on being heated from the freezing to the boiling point, while a column of iron, with t£e same increase of temperature, will expand only -,h. A familiar illustration of the expansion of water by heat is seen in the over- flow of full vessels before boiling connnences. Diflerent liquids expand very unequally with an equal increase in temperature. Spirits of wine, on being heated from 32° to 212°, increase one ninth in bulk; oil expands about one twelfth ; water, as before stated, about one twenty-third. A person buying oil, molasses and spirits in winter, will obtain a greater weight of the same material in the same measure than in summer. Twenty gallons of alcohol bought in January, will, with the ordinary increase of temperature, become, by expansion, twenty-one gallons in July. What pccuii- 527. Water, as it decreases in temperature paasion°^doe8 toward the freezing point, exhibits phenomena water exhibit? yf\^[Q\^ q^j-q wholly at variauce with the general THE EFFECTS OF HEAT. 231 law that bodies expand by heat and contract by cold, or by a withdrawal of heat.* As the temperature of water is lowered, it continues to contract until it arrives at a temperature of 39° F., when all further contraction ceases. The volume or bulk is observed to remain stationary for a time, but on lowering the temperature still more, instead of contraction, expansion is produced, and this expansion continues at an increasing rate until the water is congealed. At the moment also of its conversiou into ice, it undergoes a still further expansion. 528. Water attains its greatest density, or of the greatest the greatest quantity IS contained in a given bulk, at a temperature of 39' F. As the temperature of water continues to decrease below 39°, the point of its greatest density, its particles, from their expansion, necessarily occupy a larger space than those which possess a temperature somewhat more elevated. The coldest water, therefore, being lighter, rises and floats upon the surface of the warmer water. On the approach of winter this phenomenon actually takes place in our lakes, ponds and rivers. "^Vhen the surface-water becomes sufQcientl}'- chilled to assume the form of ice, it becomes still lighter, and con- tinues to float. By this arrangement, water and ice being almost perfect non-conductors of heat, the great mass of the water is protected from the influence of cold, and prevented from becoming chillod throughout. If water constantly grew heavier as its temperature diminished (as is the case with most liquids), the colder particles at the surface would constantly sink, until the whole body of water was reduced to the freezing point. Again, if ice was not lighter than water, it would sink to the bottom, and by the continuance of this operation, a river or lake would soon become an immense solid mass of ice, which the heat ot' summer would be insufficient to dissolve. The temperate regions of the earth would thus be rendered uninhabitable. Among all the phenomena of the natural world, there is no more striking illustration of the wisdom of the Creator, and of the evidences of design, than in this wonderful exception to a great general law. ,^ , The expansion of water at the moment of freezing is attrib- Why does wa- ' , . . ? , _ ter expand in uted to a new and peculiar arrangement of its particles. Ice freezing ? jg^ jj^ reaUty, crystallized water, and during its formation the particles arrange themselves in ranks and lines which cross each other at angles of 60° and 120°, and consequently occupy more space than whea liquid. This may be seen by examining the surface of water in a saucer whOe freezing. A beautiful illustration of this crystallization of water in freezing is seen in the frost-work upon windows in winter, caused by the congelation of the vapor of the room when it comes in contact with the cold surface of the glass. • A few other liquids besides watgr expand with a reduction of temperature. Fused Iron, antimony, zinc, and bismnth, are examples of such expansion. Mercury is a re- markable instance of the reverse, for when it freezes, it suffers a very great contraction. 232 WELLS'S NATURAL rHILQSOPHY. All these frost-work figures are limited by the laws of crystallization, and the lines which bound them, form among themselves no angles but those of 30°, 60° and 120°.. If we fracture thin ice, by allowing a pole or weight to fall upon it, the fracture will have more or less of regularity, being generally in the form of a star, with six equi-distant radii, or angles of 60° 529. The force exerted by the expansion of water in the force does wa- ^'^^ ^^ freezing is very great. As an illustration, the following ter expand in experiment may be quoted : — Cast-iron bomb-shells, thirteen inches in diameter and two inches thick, were filled with wa- ter, and their apertures or fuse-holes firmly plugged with iron bolts. Thus prepared, they were exposed to the severe cold of a Canadian winter, at a temperature of about 19° below zero. At the moment the water froze, the iron plugs were violently thrust out, and the ice protruded, and in some instances the shells burst asunder, thus demonstrating the enor- mous interior pressure to which they were subjected by water assuming a solid state. The rounded and weather-worn appearance of rocks is mainly due to tho expansion of freezing water, which penetrates into their fissures, and is ab- sorbed into their pores by capillary attraction. In freezing, it expands and detaches successive fragments, so that the original sharp and abrupt outline ia gradually rounded and softened down. The bursting of eartlien water vessels, and of water pipes, by the freezing of water contained in them, are familiar illustrations of the same principle. By allowing the water to run in a service-pipe, we prevent its freezing, be- cause the motion of the current prevents the crystals from forming and attaching themselves to the sides of the pipe. 530. The ordinary temperature at which water freezes is perature does 32°, Fahrenheit's thermometer. This rule applies only to water freeze? fresh water; salt water never freezes until the surface is cooled dowTito 27°, or five degrees lower than the freezing point of winter. Under some circumstances pure water may be cooled down to a tempera- ture much below 32° without freezing. Thus, if pure, reccntly-boUed water, be cooled very slowly and kept very tranquil, its temperature may be low- ered to 21° without the formation of ice; but the least motion causes it to congeal suddenly, and its temperature rises to 32°. . 531. The ice produced by the freezing of sea or salt water ice produced is generally ft-esh and free from salt, since water in freezing, by the freez- y sufficient freedom of motion be allowed to its particles, ex- ing of salt wa- ^ ter free from pels all impurities and coloring matters. The ice formed m ■^'' ^ the congelation of a solution of indigo is colorless, since tho water in which the indigo was dissolved expels the blue coloring matter in freezing. .„^ , . , Blocks of ice are generally filled with minute air-bubbles ; What IS the „ , , . ,. 1 XL origin of the this is owing to the fact that the water in freezing expels the minute bubbles j^j^ contained in it, and many of the liberated bubbles become Been in ice r ' "^ „ . , lodged and imbedded in the thickening fluid. THE EFFECTS OF HEAT. 233 In what man- ^^2. GasGs and aeriform substances expand e^an^dby^hSTtr l-490th of the bulk which they possess at 32° lor every degree of heat which they receive above that point, and contract in the same proportion for every degree of heat withdrawn from them. Thus, 490 cubic inches of air at 32° would so expand as to occupy an inch more space at 33°, and by the addition of another degree of heat, raising its temperature to 34°, it would occupy an additional inch, and so on. In a like manner, by the ■wnthdrawal of heat, 490 cubic inches of air would occupy an inch less space at 31° than at 32° ; two inches less at 30° and so on. The same law holds good for all other gases, and for vapors and steam. Illustrations of the expansion of air by heat are most familiar. If a bladder partially fiUed with confined air be laid before the fire, the air contained in it may be expanded to a degree sufficient to burst the bladder. Chestnuts laid upon a heated surface, burst with a loud report on account of the expansion of the air within their shells. The process of warming and ventilating build- ings depends entirely upon the application of this principle of the expansion and contraction of air by the increase and diminution of heat. How may the 533. As the magnitude of every body changes c^tSn^Jff with the heat to which it is exposed, and as l\^T to^ fhe ^^^ same body, when subjected to calorific in- Shl^t?"^"* fluences under the same circumstances has al- ways the same magnitude, the expansions and contractions which are the constant effects of heat, may be taken as the measure of the cause which produced them. -What are the ^34. Thc instrumcuts for measuring heat meLTuring'h^t ^^c Thermomcters and PjTometers. The for- •^^•^^ mer are used for measuring moderate tempera- tures ; the latter for determining the more elevated de- grees of heat. Liquids are better adapted than either solids or gases for measuring the effects of heat by expansion and contraction ; since in solids the direct ex- pansion by heat is so small as to be seen and recognized with difficulty, and in air or gases it is too extensive, and too liable to be affected by variations in the atmospheric pressure. From both of these disadvantages liquids are free. The liquid generally used in the construction of thermometers is mercury, or quicksilver. Mercury possesses greater advantages for this purpose than l^e^e^lW any other liquid. It is, in the first place, eminently dis- ada'pted for the tinguished for its fluidity at all ordinary temperatures ; it thenncmeters r is, in addition, the only body in a liquid state whose va- 234 WELLS'S NATURAL PHILOSOPHY. nations in volume, or magnitude, through a considerable range of tempe- rature are exactly uniform and proportional with every increase and dim- inution of heat. Mercury, moreover, boils at a higher temperature than any other liquid, except certain oils ; and, on the other hand, it freezes at a lower temperature than all other liquids, except some of the most vola- tile, such as ether and alcohol. Thus a mercurial thermometer will have a wider range than any other liquid thermometer. It is also attended with this convenience, that the extent of temperature included between melting ice and boiling water stands at a considerable distance from the limits of ita range, or its freezing and boiling points. Describe the ^35. TliG mercurial thermometer consists es- Someter *'""" sentially of a glass tube with a bulb at one end, partially filled with mercury. The mer- cury introduced through an opening in the end of the tube is afterward boiled, so as to expel all air and moist- ure, and fill the tube with its own vapor. The open end of the tube is then closed, by fusing the glass, and as the mercury cools it contracts, and collects in the bulb and lower part of the tube, leaving a vacuum above, through which it may again expand and rise on the application of heat. In this condition the thermometer is complete, with the exception of graduation. 536. As thermometers are constructed of different dimen- IToT7 are tner- . .... ^ j i momcters gra- sions and capacities, it IS necessary to nave some nxed rules duated ? f-Qj. graduating them, in order that they may always indicate the same temperature under the same circumstances, as the freezing-point, for example. To accomplish this end the following plan has been adopted : — The thermometers are first immersed in melting snow or ice. The mercury will be observed to stop in each thermometer-tube at a certain height ; these heights are then marked upon the tubes. Now it has been ascertained that at whatever time and place the instruments may be afterward immersed in melting snow or ice, the mercury contained in them will always fix itself at the point thus marked. This point is called the freezing point of water. Another fixed point is determined by immersing the instruments in boiling •water. It has been found that at whatever time or place the instrumenta are immersed in pure water, when boiling, provided the barometer stands it the height of thirty inches, the mercury will always rise in each to a certain height. This, therefore, forms another fixed point on the scale, and is called the boiling point. Thus far all thermometers are constructed alike. In the th"7raometM° thermometer most generally used, and which is known as of Fahrenheit Fahrenheit's, the intervals on the scale, between the freezing graduated? ^^^ boiling points, are divided into 180 equal parts. This THE EFFECTS OF HEAT. 235 division is similarly continued below the freezing point to tlio place 0, called zero, and each division upward from that is marked with the successive numbers 1, 2, 3, etc. The freezing point will now be the 32d division, and the boiling point will be the 212th division. These divisions are called degrees, and the boiling point will therefore be 212°, and the freezing temperature, 32°. Fig. 203 represents the usual form of thermometer, with its graduated scale. Thermometers of this character are called Fahrenheit's, from a Dutch philosophical instrument-maker who first intro- duced this method of graduation in the year 1724. "WTiat other 537. In addition to Fahrenheit's besi™B\lhrJn- theimometer, two others are ex- heifsareused? tensivclj used, which are known as Reaumur's, and the Centigrade thermom- eter, or thermometer of Celsius. The only difference between these three kinds of- thermometers is the difference in graduating the interval between the freezing and boiling points of water. Reaumur's is di- vided into eighty degrees, the Centigrade into one hundred, and Fahrenheit's into one hundred and eighty. According to Reaumur, water freezes at 0°, and boils at 80°; oceording to Centigrade, it freezes at 0°, and boils at 100° ; and according to Fahrenheit, it freezes at 32°, and boils at 212°; the last, very singularly, commences counting, not at the freezing pouit, but 32° below it. The difference between these 203. What consti- tutes the dif- ference be- tween the dif- ferent varieties of the ther- mometer ? instruments can be easily seen by reference to Fig. 204. In England, Holland, and the United States, the thermometer ^^-a^,^^^ most generally used is Fah- renheit's. Reaumur's scale is used in Ger- many, and the Centigrade in France, Sweden, and some other parts of Europe. The scale of the Centigrade is by far the simplest and most rational method of graduation, and at the present it is almost universally adopted for scientific purposes. 538. The thermometer was invented about the year 1600; but, like many other inven- tions, the merit of its discovery is not to bo ascribed to one person, but to be distributed among many. 236 "WELLS'S NATURAL PHILOSOPHY. „ . J . 539. As the temperature is lowered, the mercury in Fah- great Intensity renheit's thermometer gradually sinks, until it reaches a point indicated? ggo below zero, where it freezes. Mercury, therefore, can not be made available for measuring cold of a greater intensity. This difficulty is, however, obviated by using a thermometer filled with alcohol colored red, as this fluid, when pure, never fi-eezes, but will continue to sink lower and lower in the tube as the cold increases. Such a thermometer is called a spirit thermometer. _ . ^ , , 540. If a Fahrenheit's thermometer be heated, the mercury How IS neat of "^ great intensity contained in it will rise in the tube until it reaches 660°, at nieasured ? ■v\-liieh temperature it begins to boil. A slight additional heat forms vapor sufficient to burst the tube. Mercury, therefore, can not be used to measure degrees of heat of greater intensity than 660° F. Temperatures greater than this are determined by means of the expansion of solids ; and instruments founded upon this principle are commonly called pyrometers. Fig. 205. Fig. 206. The construction of the pyrometer is represented in Fig. construction of 205 A represents a metallic bar, fixed at one end, B, but the pyrometer. |g{^ f^ee at the other, and in contact with the end of a pointer K, moving freely over a graduated scale. If the bar be heated by the flame ot alcohol, the metal expands, and pressing upon the end of the pointer, moves it, in a greater or less degree. In this manner, the effect of heat, applied for a given length of time, to bars of different metals, having the same length and diameter, may be determined. 541. The first thermometer air-thermome- used consisted of a column of **'' * air confined in a glass tube over colored water. Heat expands the air and in- creases the length of the column downward, pushing the water before it : cold produces a contrary effect. The temperature is thus indi- cated by the height at which the water is ele- vated in the tube. Fig. 206 represents the prin- ciple of the construction of the air- thermometer. THE EFFECTS OF HEAT. 237 ^ A thermometer does not inform us how much heat any sub- Does a ther- . , . , . . ^ ^, ,.„• ■ ^i moineter in- stance contams, but it merely pouits out the diUerence m tuo form us bow temperature of two or more substances. All we learn by the iniich heat a ^ , ,. , j • i. substance con- thermometer 13 whether the temperature of one body is greater '^'"^ ^ or less than that of another ; and if there is a difference, it is expressed numerically — namely, by the degrees of the thermometer. It must be remembered that these degrees are part of an arbitrary scale, selected for convenience, without any reference whatever to the actual quantity of heat present in bodies. After the ex- ^42. The first cfifect produced by heat upon soudl^b heat solids is expansion. If the heat be augmented, f[>ct''is°nextolf" ^^^^J changc their aggregate state and melt, Kcrved? Qj. Ibecome liquid. Many solids become soft before melting, so that they may be kneaded ; for instance, wax, glass, and iron. In this position, glass can be bent and molded with facility, and iron can be forged or welded. whatisLique- ^43. By Liqucfactiou we understand the faction? conversion of a solid into a liquid by the agency of heat, as solid ice is converted into water by the heat of the sun. Heat is supposed to convert a soHd into a liquid, by forcing its constituent particles asunder to such an extent that the force of cohesion is overcome or destroyed. What is soiu- 544. When a solid is immersed in a liquid, ''""^ and gradually disappears in it, the process is termed solution, and not liquefaction. A solution is the result of an attraction or affinity between a solid and a fluid ; and when a solid disappears in a liquid, if the compound exhibits perfect transparency, we have an ex- ample of a perfect solution. ^„, . , When a fluid has dissolved as much of a solid as it is When IS a solu- tion said to be Capable of doing, it is said to be saturated ; or, m other words, caturated ? ^^iQ affinity or attraction of the flui 1 for the solid continues to operate to a certain point, where it is overbalanced by the cohesion of tl.« solid; it then ceases, and the fluid is said to be saturated. A solution is a complete union : a mixture is a mere me- llow does a , . , . ^ ,. solution differ chanical union of bodies. from a mix- j^ most cases, the addition of heat to a liquid greatly in- creases its solvent properties. Hot water will dissolve much more sugar than cold water ; and hot water will also dissolve many things which cold water is unable to affect 238 WELLS'S NATURAL PHILOSOPHY. What is va- ^45. If heat be imparted in sufficient qiian- porization? ^-^^^ ^^ ^ ]-,qJ^. -j^ .^ ^-^^^^-^j ^^.^^^^ -^ ^^.-^ ^^^^^g -^^^ a state of vapor. Thus, water being heated sufficiently will pass into the form of steam. This change is called Vaporization. What is Con- ^46. If a bodj in a state of vapor lose heat densatiou? jj^ siifficicnt quautitv, it will pass into a liquid Etate. Thus, if a certain quantity of heat be abstracted from steam, it will become water. This change is called Condensation. The change from a state of vapor to a liquid is termed condensation, be- cause, in so doing, the body always undergoes a very considerable diminution of volume, and therefore becomes condensed. Most solids become liquefied before they vaporize ; but some pass at once, on the application of heat, from the state of a solid to that of a vapor, without assuming the liquid condition. 547. The meltinc^ of a solid, or its conver- Isanyparticu- . . ,..,, , , -., lar tempera- siou luto a liquid, ouly occurs when the solid ture requisite . , - • r« i • i i for the forma- IS hcatcd Up to a Certain fixed point ; but tiie tion of vapors? . /• t • i • , , ^ i conversion ot a liquid into a vapor takes place at all temperatures. If in a hot day we expose a vessel filled with cold water to the open air» we find that the quantity of water rapidly diminishes, that is, it evaporates, which means that it is converted into vapor and ditiused thi-ough the air. What ia the ^48. Thc vapor of water, and all other va- vapor^'™'"^ °^ pors, are invisible and transparent. The water which has become diffused through the air by evaporation only becomes visible when, on returning to its fluid condition, it forms mist, cloud, dew, or frost. Steam, which is the vapor of boiling water, is invisible, but when it comes in contact with air. which is cooler, it becomes condensed into small drops, and is thus rendered visible. The proof of this may be found in examining the steam as it issues from an orifice, or the spout of a boiling kettle : for a short space next to the open- ing no steam can be seen, since the air is not able to condense it ; but as it spreads- and comes in contact with a larger volume of air, the invisible vapor becomes condensed into drops, and is thus rendered visible. The visible matter popularly called steam, should be, therefore, distin- guished from steam proper, or the aeriform state of water. The cloud, or smoke-like matter observed, is really not an air or vapor at all but a collec- tion of minute bubbles of water, wafted by a current either of true steam, or, more frequently, of mere moist air. THE EFFECTS OF HEAT. 239 Is a boiling temperature reiMisite for tile production 9\ iieam ? Is vapor al- ways present Fig. 207, The myriads of minute globules of water into which the steam is condensed are separately invisible to the naked eye, but each, nevertheless, reflects a minute ray of white light. The multitude of these reflecting points, there- fore, make the space through which ihey are difl'used appear like a cloudy body, more or less white, accordmg to their abundance. The surface of any watery liquid, whose temperature is about 20° warmer than any superincumbent au", rapidly gives off true steam. It is not necessary, therefore, for the produc- tion of steam that water should be raised to the boihng tem- perature. 549. Air without vapor (tlieoretically called dry air) is not known to exist in nature, and is probably not producible by art. 550. Liquids in passing into vapors occupy a much greater space than the substances from which they are produced. Water, in pass- ing from its point of greatest density into steam, expands to nearly 1700 times its volume. Fig. 207 represents the comparative volume of water and steam. 551. Vapors are of all degrees of density. The va- por of water may be as thin as air, or almost as dense as water. The opinion formerly prevailed that va- pors could not exist by themselves as Bucli, but that they were dissolved in the air in the same way as salt is dissolved in water. The fallacy of this idea is proved by the fact that evaporation goes on more rapidly in a vacuum, where no substance whatever is present, than in the air. 552. Evaporation takes place from the sur- faces of bodies only, and is influenced in a great degree by the temperature, dryness, still- ness, and density of the atmosphere. The effect of temperature in promoting evaporation may bo perature influ- illustrated by placing an equal quantity of water in two sau- cers, one of which is placed in a warm and dry, and the other in a cold and damp, situation. The former will be quite dry before the latter has suffered an appreciable diminution. in air f What is the relative space occupied by liquids and va- pors? Is the density of vapors uni- form? What circum- St;inces iiirtu- ence evapora- tion ? ence evapora- tion? 24.0 WELLS'S NATURAL PHILOSOPHY. „ , ., "When water is covered by a stratum of dry air, the evapo- Tlow does the •' j i i Btate of the air ration is rapid, even when its temperature is low ; whereas it oratr'nT '^^'^^' goes on very slowly if the atmosphere contains much vapor, even though the air be very warm. Evaporation is far slower in still air than in a current. The air imme- diately in contact witli the water soon becomes moist, and thus a check is put to evaporation. But if the air be removed by wind from the surface of the water as soon as it has become charged with vapo*-, and its place supplied with fresh air, then the evaporation continues on without inter- ruption. Evaporation is by no means confined to the surface of liquids ; but takes place from the surface of the soil, and from all animal and vegetable produc- tions. Evaporation takes place to a very considerable extent from the sur- face of snow and ice, even when the temperature of the air is far below the freezing point. „„ . , . 553. A very singular circumstance is connected with the What singular ,.„, , , , •,, circumstance diffusion of vapors throughout the atmosphere, viz. : that the '*in ti'""rt''ff ** vapors of aii ».'odic? arise into any space filled with air, in Bioa 01 vapors? the same manner as if air were not present, the two fluids seeming to be independent of each other. Thus as much vapor of water can be forced into a vessel filled with air as into one from which the air has been exhausted. 554. When a drop of water falls upon a surface highly phenomena of heated, as of metal, it will be observed to roll along the sur- *° " p^J^f"'"^" face without adhering, or immediately passing into vapor, liquids. The explanation of this is, that the drop of water does not in reality touch the heated surface, but is buoyed up and sup- ported on a layer of vapor which intervenes between the bottom of the drop and the hot surface. This vapor is produced by the heat which is radiated from the hot substance, before the liquid can come in contact with it, and being constantly renewed, continues to support the drop. The drop generally rolls because the current of air which is always passing over a heated sur- face drives it forward. The drop evaporates slowly, because the layer of vapor between the hot surface and the liquid prevents the rapid transmis- sion of heat. The liquid resting upon a cushion of steam continually evolved from its lower surface by heat, assumes a rounded, or globular shape, as tha result of the gravity of its particles toward its own center. The designation which has been given to the condition which water and other liquids assume when dropped upon very hot surfaces, is that of the " spheroidal state." If the surface upon which the liquid rests is cooled down to such an ex- tent that vapor ie not generated rapidly, and in sufficient quantity to sup- port the drop, it will come in contact with the surface, and heat being com- municated by conduction, will transform it instantly into steam. This is the explanation of the practice adopted by laundresses of touching a flat-iron with moisture to ascertain whether the surface is sufficiently hot. THE EFFECTS OF HEAT. 241 If the temperature of the iron is not elevated sufEciently, the moisture wets the surface, and is evaporated ; but at a higher degree of temperature, the moisture is repelled. The phenomenon of the spheroidal condition of water furnishes an explana- tion of the feats often pertbrnied by jugglers, of plunging the hands with im- punity into molten lead, or iron. The baud is moistened, and when passed into the liquid metal the moisture is vaporized, and interposes between the metal and the skin a sheath of vapor. In its con\ersion into vapor, the moisture absorbs heat, and thus still further protects the skin. What is ebui- 5^^- When a liquid is heated sufficiently to htion? ^Qj.^^ steam, the production of vapor takes place principally at that part where the heat enters ; and when the heating takes place not from above, but from the bottom and sides, the steam as it is produced rises in bubbles through the liquid, and produces the phenomenon of boiling, or ebullition. wTiat is the ^56. The temperature at which vapor rises bouiug point? -^{lYi sufficient freedom to cause the phenome- non of ebullition, is called the boiling point, la the boiling •'^57. Different liquids boil at different tem- ent'liquiS; pGraturcs. The boiling^ point of a liquid is, **™^''' therefore, one of its distinctive characters. Thus water, under ordinary circumstances, begins to boil when it is heated up to 212° F. ; alcohol at 173°; ether at 96°; syrup at 221°; linseed oil at 0-40°. „ ^ . . The gentle tremor, or undulation, on the surface of water WTiat IS Sim- . mering? which precedes boihng, and which is termed " simmering," is owing to the collapse of the bubbles of steam as they shoot upward and are condensed by the colder water. The first bubbles which form are not steam, but air which the heat expels from the water. As the temperature of the whole mass of the water increases, the bubbles are no longer condensed and collapsed, but rise through to the surface ; and the moment that this takes place boiling commences. The singing of a tea-kettle before boiling is occasioned by the irregular escape of the air and steam ex- pelled from the water through the spout of the tea-kettle, which acts in tho manner of a wind-instrument in producing a sound. ^ , ,^. 558. Liquids, in general, being boiled in open vessels, are How does the i < o i o r pressure of the sub]ected to the pressure of the atmosphere. The tendency f'"tt!i'^^b'^T'^*^' °^ ^^^^ pressure is to prevent and retard the particles of of liquids? water from expanding to a sufficient extent to form steam. Hence if the pressure of the atmosphere varies, as it does at different times and places, or if it bo increased or diminished by artificial means, the boiling point of a Uquid will undergo a corresponding change. 11 242 WELLS'S NATURAL PHILOSOPHY. „ x|^ 559. As we ascend into the atmosphere the pressure is di- temperature at minished, because there is less of it above us ; it thereibro boils^'be "iised Allows, that water at different heights in the atmosphere will for determin- boil at different temperatures, and it has been found by ob- ng e evationg servation, that an elevation of 550 feet above the level of tha sea causes a difference of one degree in its boiliBg point. Hence the boiling point of water becomes an indication of the height of any station above the sea-level, or in other words, an indication of the atmospheric pressure ; and thus by means of a kettle of boiling water and a thermometer, the height of the summit of any mountain may be ascertained with a great degree of ac- curacy. If the water boils at 211° by the thermometer, the height of the place is 550 feet ; if at 210°, the height is 1100 feet, and so on, it being only necessary to multiply 550 by the number of degrees on the thermometer between the actual boiling" point and 212°, to ascertain the elevation. In the city of Quito, in South America, water boils at 194° 2" F. ; its height above the sea-level is, therefore, 9,541 feet. As we descend into mines, the pressure of the atmosphere is increased, there being more of it above us than at the surface of tlie earth. "Water, therefore, must be heated to a higher temperature before it will boU, and it has been found that a descent of 550 feet, as before, makes a difference of one degree. 560. In a hke manner, if by artificial means we increase or toiling point diminish the pressure of the atmosphere on the surface of a chan^e'd'^^rti- ^^1^^'^' ^'^ change its boihng point. If water be heated in a ficiaUy? vacuum, ebullition will commence at a point 140° lower than in the open air. If a vessel of ether be placed under the re- ceiver of an air-pump, and the atmospheric pressure removed from its surface, the vapor rises so abundantly that ebulhtion is produced without any in- crease of temperature. How is su-'ar Several beautiful applications in the arts have been made boiled in the of the principle that liquids boil at a lower temperature when fining^ °^ ^^' ^^^®^ ^^""^ *^^° pressure of the atmosphere than m the open air. In the refining of sugar, if the syrup is boiled in the open air, the tempera- ture of the boiling point is so high that portions of the sugar become decom- posed by the excess of heat, and lost or injured ; the syrup is therefore boiled in close vessels from which the air has been previously exhausted, and in this way the water of the syrup may be evaporated at a temperature so low as to prevent all injury from heat. For cooking, this application could not be carried out. The water might, Indeed, be made to boil at a temperature much less than 212°, but owing to its diminished heat would not produce the desired effect. whatisdista- 561. Distillation is a process by which one ation? hody is separated from another by means of heat, in cases where one of the bodies assumes the form of vapor at a lower temperature than the other ; this first THE EFFECTS OF HEAT. 2-i3 Fig. 203. rises in the form of vapor^ and is received and condensed in a separate vessel. By this means very volatile bodies can be easily separated from less vola- tile ones; as brandy and alcohol from the less volatile water which may be mixed with them. Water of extreme purity can also be obtained by distil- lation, because the non-volatile and earthy substances contained in all spring •waters do not ascend with the vapor, but remain behind in the vessel Distillation upon a small scale is effected by means ol a peculiar-shaped vessel, called a retort. Fig. 208, which is half filled with a volatile liquid and heated ; the steam, as it forms, passes through the neck of the retort into a glass re- ceiver set into a vessel filled with cold water, and is then condensed. When the operation of distillation is conducted on an extensive scale, a large vessel called a " stiW^ is used, and, for con- densing the vapor, vats are constructed, holding serpentine pipes, or "worms," which present a greater condensing sur- face than if the pipe had passed directly through the vat. To keep the coil of pipe cool, the vats are kept filled with cold water. In Fig. 209, a is a furnace, in which is fixed a copper vessel, or still, to contain the liquid. Heat being applied, the steam rises in the head, b, and passes through the worm, d, which is placed in a vessel of water, the refrigerator. The vapor thus generated is condensed in its passage, and passes out as a liquid by the external pipe into a receiver. What is th ^^® difference between drying by heat and distillation is, difference be- that in one case, the substance vaporized, being of no use, is bT'^heat "^^and allowed to escape or become dissipated in the atmosphere ; distillation ? while in the other, being the valuable part, it is caught and condensed into the liquid form. The vapor arising from damp linen, if caught and condensed would be distilled water; the vapor given out by bread while baking, would, if collected, be a spirit like that obtained in the distillation of grain. .^~ j^j . ^ . J. 5G2. As some substances, by the application of heat, pass matiwi ? directly from the solid condition to the state of vapor, so some substances, as camphor, sulphur, arsenic, etc, when vaporized 244 WELLS'S NATURAL PHILOSOPHY. by heat, deposit their condensed vapors in a solid form. This process is termed sublimation. What remark- 563. One of the most remarkable circum- stance "nuend's stances which accompany the phenomena, vaporSonT'^ ^oth of liqucfactioH and vaporization, is the disappearance of the heat which has effected the change. IIow may this Thus, ii' a thermometer be applied to a mass of snow, or ica principle be U- just upon tliG point of meltiug, it will be found to stand at "'' '^''"^ 32° F. If the ice be placed in a vessel over a fire, and tho temperature tested at the moment it has entirely melted, the water produced will have only the temperature of 32°, the same as that of the original ice. Heat, heweper, during the whole process of melting, has been passing rajjidly into the vessel from the fire, and if a quantity of mercury, or a solid of the same size, had been exposed to the same amount of heat, it would have con- stantly increased in temperature. It is clear, therefore, that the conversion of ice, a solid, into water, a licjuid, has been attended with a disappearance of heat. Again : if one pound of water, having a temperature of 174°, be mixed with one pound of snow at 32°, we shall obtain two pounds of water, having a temperature of 32°. All the heat, therefore, which was contained in tho hot water is no longer to be detected by the thermometer, it having been en- tirely used up, or disposed of in converting snow at 32° into water at 32°. Such disappearances always occur whenever a solid is converted into a liquid. If, however, a pound of water at 32°, instead of ice at the same tempera- ture, had been mixed with a pound of water at 174°, we ehall obtain two pounds at 103° a temperature exactly intermediate between the temperatures of the components But if the pound at 32° had been soUd instead of liquid, then the mixture, as before explained, would have had a temperature of 32°. It is evident, therefore, that it is the process of liquefaction, and it alone, which renders latent or insensible all that heat which is sensible when the pound of water at 32° is liquid. „ „ In the same manner heat disappears when a liquid is eon- How may the , . m, , • /-, • 1 • • X absorption of verted mto a vapor. The absorption or heat, in this instance, heat la evapo- ^^^y j^^ easily rendered perceptible to the feelings by pouring dcrud eviUent ? a few drops of some liquid which readily evaporates, such as ether, alcohol, etc., upon the hand. A sensation of cold is immediately ex- perienced, because the hand is deprived of heat, which is drawn away to effect the evaporation of the liquid. On tho same principle, inflammation and fever- ish heat in the head may be allayed by bathing the temples with Cologno water, alcohol, vinegar, etc. If we surround the bulb of a thermometer loosely with cotton, and then moisten the latter with ether, the thei mometer will speedily fall several degrees. Why can not Water when placed in a vessel over a fire, gradually at- Vater impart tains the boiling temperature, or 212°; but afterward, how- oftcr boUing f ever much we may increase the fire, it becomes no hotter, all THE EFFECTS OF HEAT. 245 the heat which is added serving only to convert the water at 212° from a liquid condition into steam, or vapor, at 212°. 564r. If we immerse a thermometer in boiling water, it know that Stands at 212° ; if we place it in steam iramediately above it, steam at 2;-2° jj incjicates the same temperature. "We know, however, that IS hotter than ,,,.,. , .j. water at the Steam contains more heat than bouing water, because u we same tempera- ^^ ^^ ^^^^3 q|- ^^^^^j. at 212° with five and a half ounces of ture .' water at 32°, we obtain six and a half ounces of water at a temperature of about 60° ; but if we mix an ounce of steam at 212° with five and a half ounces of water at 32°, we obtain six and a half ounces of water at 212°. The steam, from which the increased heat is all derived, contains as much more heat than the ounce of water at the same temperature, as would be necessary to raise six and a half ounces of water from the temperature of 60° to 212°, or six and a half times as much heat as would be requisite to raise one ounce of water through about 152° of temperature. This quantity of heat will, therefore, be found by multiplying 152° by six and a half, which will give a product of 983° — the excess of heat contained in an ounce of steam at 212° over that contained in an ounce of boUing water at the same temperature. y^ . 565. In the conversion of solids into hquids, and liquids into of the heat vapors by heat, we may suppose the heat, the sohd, aud the pea'rs in li'nue- ^^^d to have respectiveh" combined together ; — forming a faction and va- liquid in the one case, and a vapor in the other. A liquid, poriza loa . therefore, may be regarded as a compound of a solid and heat, and a vapor as a compound of heat and the liquid from wliich it was formed. The heat wliich disappears in these combinations is called Latent, or CoiiPoujTD Heat. What are '^'^^ absorption of heat consequent on the conversion of freezing mix- solids into liquids, has been taken advantage of in the arts for ^'^^' the production of artificial cold; and the compounds of dif- ferent substances which are made for this purpose, are called f'-eezing mix- tures. Wh does th '^^ most simple freezing mixture is snow and salt. Salt mixture of dissolved in water would occasion a reduction of temperature, producTint^M ^^^ when the chemical relations of two solids are such, that cold? on mixing, both are rendered liquid, a still greater degree of cold is produced. Such a relation exists between salt and snow, or ice, and therefore the latter substances are used in preference to water. "When the two are mixed, the salt causes the snow to melt by reason of its attraction for water, and the water formed dissolves the salt : so that both pass from the solid to the liquid condition. If the operation is so conducted that no heat is supplied from any external source, it follows that the heat absorbed in liquefaction must be obtained from the salt and snow which comprise the mixture, and they must therefore suffer a depression of temperature propor- tional to the heat which is rendered latent. In this way a degree of cold equal to 40° below the freezing point of 246 WELLS'S NATURAL PHILOSOPHY. How great a crater may be obtained. The application of this experiment degree of cold to tlie freezing of ice-creams is familiar to all. can be obtain- _ . . _,,.., , . ed by freezing By mix'ng snow and sulphuric acid together m proper pro- mixtures ? portions, iii temperature of 90° below zero can be obtained without difficulty. y^ . , . The air in the spring of the year, when the ice and snow In spring cold are thawing, is always peculiarly cold and chilly. This is due and chilly ? ^ ^j^g constant absorption of heat from the air by the ice and snow in their transition from a soUd to a liquid state. „^ , A shower of rain cools the air in summer, because the earth, why does a , , . , , . , , . , Bhowcrinsum- and the air both part with their heat to promote evaporation. mer cool the jy g, like manner, the spriukUng of a hot room with water cools it. Why is the The draining of a country increases its warmth, since by warmth of a withdrawing the water, evaporation is diminished, and less country pro- •,,.,, moted by heat IS subtracted from the earth. draining ? rjj-^g danger arising from wet feet and clothes is owing to Why do wet the absorption of heat from the body by the evaporation from feet or clothes the surfaces of the wet materials ; the temperature of the body tend to impair .... ■,,,,. , , -i i , the health of 13 ID this way reduced below its natural standard, and the the body ? proper circulation of the blood interrupted. 566. The absorption of heat in the process by which liquids tin vessel°con- ^^^ converted into vapor, will explain why a vessel containing tainlng water a liquid that is constantly exposed to the action of fire, can fire dMtroyed? never receive such a degree of heat as would destroy it. A tin kettle containing water may be expo.sed to the action of the most fierce furnace, and remain uninjured ; but if it bo exposed, without containing water, to the most moderate fire, it will soon be destroyed. The heat which the fire imparts to the kettle containing water is immediately ab- sorbed by the steam into which the water is converted. So long as water is contained in the vessel, this absorption of heat will continue ; but if any part of the vessel not containing water be exposed to the fire, the metal will be fused, and the vessel destroyed. 567. When vapors are condensed into liq- TJnder what ., ^ ^• • ^ i i* ti i circumstances uids, and liquids are changed into solids, the become sensi- latent hcat containcd in them is set free, or made sensible. If water be taken into an apartment whose temperature is several degrees below the freezing point, and allowed to congeal, it will render the room sen- sibly warmer. It is, therefore, in accordance with this principle that tubs of water are allowed to freeze in cellars in order to prevent excessive cold. It is from this cause that oceans, seas, and other large collections of water ere most powerful agents in equahzing the temperature of the inhabited parts of the globe. In the colder regions, every ton of water converted into ice «»ve3 out and diffuses in the surrounding region as much heat as would THE EFFECTS OF HEAT. 247 raise a ton of water from 32° to 174° ; and, on the other hand, when a riso of temperature takes place, the thawing of the ice absorbs a like quantity of heat: thus, in the one case, supplying heat to the atmosphere when the tem- perature falls ; and, in the other, absorbing heat from it when the temperattiro rises. In the winter, the weather generally moderates on the fall of snow ; snow is frozen water, and in its formation heat is imparted to the atmosphere, and its temperature increased. Steam, on account of the latent heat it contains, is well Why is steam adapted for the warming of buildings, or for cooking. la adapted for passing through a line of pipes, or through meat and vegeta- cookiDgf '""^ ^^^^^ ^* ^ condensed, and imparts to the adjoining surfaces nearly 1000° of the latent heat which it contained before condensation. Steam bums much more severely than boiling water, for the reason that the heat it imparts to any surface upon which it is condensed is much greater than that of boiling water. Is the quantity ^68. All bodles coiitam incorporated with bod^ief the^'^ them more or less of heat ; but equal weights Bamo? of dissimilar substances, having the same sen- sible temperature, contain unequal quantities of heat. Thus if we place a pound of water and a pound of mercury be demon- over a fire, it will be found that the mercury will attain to any Btrated? given temperature much quicker than the water. Or if we perform the converse of this experiment, and take two equal quantities of mercury and water, and having heated them to the same degree of tempera- ture, allow them to cool freely in the air, it will be found that the water will require much more time to cool down to a common temperature than the mercury. The water obviously contains more heat at the elevated tempera- ture than the mercury, and therefore requires a longer time to cooL . . 569. Dissimilar substances require, respectively, different meaning of the quantities of heat to raise their temperatures one degree ; and term specific ^j^g quantity of heat necessary to produce this effect upon a body is termed its specific heat. In like manner, the weight which a body includes under a given volume, is termed its specific gravity. _ , 570. A substance is said to have a greater What IS under- • n ^ t stood bycapac- Or Icss capacitv for heat, according as a greater ityforheatr .-f r X, J - • A 4- A or less quantity of heat is required to produce a definite change of temperature, or an elevation of tem- perature of one degree. n d the ^^ general, the capacity of bodies for heat decreases with capacity for heat their density. Thus mercury has a less tapacity for heat than itanfte vary"?''" '^^tsr, because its density is greater. Air that is rarefied, or thin, has a greater capacity for heat than dense air. This 243 WELLS'S NATURAL rHILOSOPHT. circumstance will explain, in part, the reason of the very low temperatures which exist at great elevations in the atmosphere. Persons ascending high mountains, or in balloons, lind that the cold increases with the elevation. The reason of this is, that as the air expands and becomes rarefied, its capac- ity for heat is greatly increased, and it therefore absorbs its own sensibl© heat. . In all quarters of the globe, the temperature of the air at a limit of per- certain height is reduced so low by its rarefaction, that water peiualsnow? ^^^ ^^^ ^^^^ j^ ^ jj^^jj^j ^^^^^^ rj^j^jg jj^^^jf^ ^^^ height of ■which varies, being the most elevated at the equator, and the most depressed at the poles, is called the line of Perpetual Sxow.* Air forcibly expelled from the mouth feels cool ; in this instance the cold ia due to a sudden expansion of the air, by which its capacity for heat is in- creased. The capacity for heat also increases with the temperature. Thus it requires a greater amount of heat to elevate the temperature of platinum from 212° to 213°, than from 32° to 33°. Of aU known bodies, water has the greatest capacity for heat. There are several diflerent ways by means of which the ca- HoTT may the pacitv of bodies for heat mav be determined. One method capacity for f J heat in differ- consists in inclosing equal weights of different bodies heated be ascertained? *° ^^^® ^^^^ temperature, in closed cavities in a block of ice, and measuring the respective quantities of water which they produce by melting the ice. The same result may also be obtained by what is called the method of mix- tures. Thus, if we mix 1 pound of mercury at 66° with 1 pound of water at 32°, the common temperature will be 33°. Here the mercury loses 33° and the water gains 1°; that is to say, the 33° of the mercury only elevates the water 1°, therefore the capacity of water for heat is 33 times that of mercury ; or, if we call the capacity or specific heat of water 1, then the capacity or specific heat of mercury will be l-33d or .0303. In this way the capacities for heat of a great number of bodies has been determined, and tables constructed in which they are recorded. In these tables water is taken as the unit of comparison. All vapors are elastic, like air. eiisticityofra- The tendency of vapors to expand is unlim- *'°" ited ; that is to say, the smallest quantity of vapor will diffuse itself through every part of a vacant space, be its size what it may, exercising a greater or less degree of force against any obstacle which may have a tendency to restrain it. • The line of perpetual snow at the equator occurs at a height of about 15,000 feet ; at the Straits of Magellan, It occurs at an elevation of only 4,000 feet. THE EFFECTS OF HEAT. 249 The force with which a vapor expands is called its elastic force, or tension. The elasticity or pressure of vapors is best illustrated in the case of steam, which may be considered as the type of all vapors. "When a quantity of pure steam is confined in a close vessel, nerTs rheTla^ i^s elastic force will exert on every part of the interior of the tic force of vessel a certain pressure directed outward, having a tendency steam exerted? . , , , to burst the vessel What is the When steam is generated in an open vessel its elastic forca Bteam fonned ™"^ ^® equal to the elastic force or pressure of the atmos- iu aa open reb- phere ; otherwise the pressure of the air would prevent it from forming and rising. Steam, therefore, produced from boiling wa- ter at 212° F., is capable of exerting a pressure of 15 pounds upon every square inch of surface, or one ton on every square foot, a force equivalent to the pressure of the atmosphere. „ , If water be boiled under a diminished pressure, and there- How may the , , , , • , • , j r- clastic force of fore at a lower temperature, the steam which is produced irom Bteara be in- jj. ^jjj have a pressure which is diminished in an equal de- crensed or di- '^ '■ minished? gree. If, on the contrary, the pressure under which water boUs be increased, the boiling temperature of the water and the pressure of the steam formed will be increased in a like proportion. TTe have, therefore, the following rule : — To what is the 571. Stcam raiscd from Water, boilitig Under stwrn *^^iwayl ^^J givcn prcssure, has an elasticity always equal? cqual to the pressure under which the water boils. _ . , Steam of a high elastic forc3 can onlv be made in close ves- IIow IS steam ° of hi-h elastic sels, or boilers. The water in a steam-boiler, m the first in- force generated? stance, boils at 212°, but the steam thus generated being prevented from escaping, presses on the surface of the water equally as on the surface of the boiler, and therefore the boiling point of the water becomes higher and higher ; or in other words, the water has to grow constantly hot- ter, in order that the steam may form. The steam thus formed has the same temperature as the water which produces it. The temperature of the water in working steam-boilers ia tent can water always much greater than 212°. It should also be borne in be heated un- mind that water, if subjected to sufiBcient pressure, can bo oer pressure ? ■' . heated to any extent without boiling. There is no limit to the degree to which water may be heated, provided the vessel is strong enough to confine the vapor ; but the expansive force of steam is so enormous under these circumstances, as to overcome the greatest resistance which has ever been exerted upon it. If a boiler, containing water thus overheated many degrees beyond the boiling point, be suddenly opened, and the steam allowed to expand, th« 11 250 WELLS'S NATURAL PHILOSOPHY. whole water is immediately blown out of the vessel as a mist by the steam formed at the same instant throughout every part of the mass. To use a common expression, " the water Hashes into steam." Steam, like water, may be heated to any extent when con- tent can Bteam fined and prevented from expanding with the increase of ^e heated un- temperature ; in some of the methods lately introduced for purifying oils, etc., the temperature of the steam, before its application, is required to be sufficiently elevated to enable it to melt lead. Whatissnper- ^72. Steam which has been heated in a heated Steam? geparate statc to a high degree of temperature Tinder pressure, is known as " Superheated Steam." In this condition its mechanical and chemical powers are wonderfully increased. In the manufacture of lard on an extensive scale the carcass of the whole hog is exposed to the action of steam at very high pressure, this acting upon the mass of flesh and bones, breaks up and reduces the whole to a fat fluid mass. Ordinary steam, under the same circumstances, would dissolve nothing. Steam has also been recently applied to the carbonization of wood. For this purpose ordinary steam is conducted through red hot pipes, whereby it attains a very high degree of temperature. It is tlien allowed to pass into a vessel containing wood intended to be converted into charcoal. The heated steam, penetrating into the pores of the wood, drives off the volatile portions, the water, the tar, etc., and leaves the pure carbon alone behind. What is High- 573. Steam generated by water boiling at a pressure steam? ^q^j \^yy[^ tcmperatuie, is known as High- pressure Steam. By this term we mean steam condensed not by withdrawal of heat, but by pressure, just as high- pressure air is merely condensed air. To obtain a double, triple, or greater pressure of steam, we must have twice, thrice, or more steam under the same volume. What relation ^74. Thc sum of thc scusiblc heat of any Mnsibie'""^nd vapor, aud the latent heat contained in it, is latent heat? always the same. It is an established fact that the heat absorbed by vaporization is always less tlie higher the temperature at wliich this vaporization takes place, and just in proportion also as vapor or steam indicates a lower temperature by the thermometer, it contains more latent heat. Thus, if water boils at 312°, the heat absorbed in vaporization will be less by 100° than if it boiled at 212°. And again, if water be boiled under a diminished pressure at 112°, the heat absorbed in vaporization will be 100° more than the heat absorbed by watef boiled at 212°. THE STEAM-ENGINE. 251 SECTION IV. What Is a ^^^- The Steam-EnginG is a mechanical steara-Engine? coDtrivaiice by which coal, wood, or other fuel, is rendered capable of executing any kind of labor.* „ , The substance which furnishes the means of callino; the How much me. =" chanical force powers of coal into activity is water; two ounces of coal, with can be excited a proper arrange'ment will evaporate about one pint of water; tioa of two this will produce 216 gallons of steam, which can exert a ounces of coal? mgdianical force equivalent to raising a weight of 37 tons to the height of one foot. It has been found bv experiment that the greatest amount IIow does the „. ,., ' ^ , .... force of a man of force which a man can exert when applying his strength to compare with ^-^e bc-st advantage throuarh the help of machinery, is equal to erated by the elevating one and a half millions of pounds to the height of combustion of ^^g ^^^^ ^y ^Qj-king on a treadmill continuously for eight hours. A well-constructed steam-engine will perform the same labor with an expenditure of a pound and a half of coaL The average power of an able-bodied man during his active How much coal ,.„ . , . x i ^ ^^ .^ ^ ^i . /. is equivalent to life, Supposing him to work tor twenty years at the rate of the whole ac- eight hours per dav, is represented bv an equivalent of about tive power of ° r . i r . -l a man ? four tons of coal, since the consumption of that amount will evolve in a steam-engine, fully as much mechanical force. The great pyramid of Egypt is five hundred feet high, and weiglis twelve thousand seven hundred and sixty millions of pounds. Herodotus states that in constructing it one hundred thoiteand men were constantly employed for twenty years. At the present time, with the consumption of 480 tons of coal, all the materials could be raised to their present position from the ground in comparatively little time. , . , The greatest work ever known to have been performed by What IS the ° . . . , /. greatest amount a steam-engine, was to raise sixty thousand tons of water a of work evi;r fg^j. jjj„jj ^s^th the expenditure of one bushel of coal. This acconiphshed o i by a steam- work was accomplished bj' one of the engines employed m engine ? ^-^^ ^^^^ ^f Cornwall, England. • "Coals are by It made to spin, weave, dye, print, and dress silks, cottons, woolens, and other cloths; to make paper, and print books upon it when made; to convert com Into flour ; to express oil from the olive, and wine from the grape ; to draw up metals from the bowels of the earth ; to pound and smelt it ; to melt and mold it ; to roll it and fashion it into every desirable form ; to transport these manifold products of its own labor to the doors of those for whose convenience they are produced ; to carry persons and goods over the waters of rivers, lakes, seas, and oceans, in opposition alike to the natural difficulties of wind and water ; to carry the wind-bound ship out of port, to place her on the open deep, ready to commence her voyage ; to transport over the surface of the sea and the land, persons and information from town to town, and from country to country, with a spped as much exceeding the ordinary wind, as the ordinary wind exceeds that of a pedestrian." — Lardner, 252 WELLS'S NATURAL PHILOSOPHY. How is steam 576. Steam is rendered useful for mechan- madc available • ^ • i ^ • , i , • for mechanical ical purposes sunplj Dj its prcssure, or elastic purposes? ^^^^^_ Steam can not, like wind and -vrater, be made to act advantageously by ita Fig. SIO. impulse in the open air, because the momentum of so light a fluid, milcss generated in vast quantities, would be inconsidi,Table. The first attempts, however, to employ steam as a moving power, consisted in direct- ing a current of steam from the mouth of a tube against the floats or vanes of a revolving wheel. A machine of this kind, invented more than 2,000 years ago by Hero of Alexandria, is represented in Fig. 210. It consists of a small hollow sphere, fur- nished with arms at right angles to its axis, and v.iioso ends are bent iu opposite directions. The sphere is suspended between two columns, bent and pointed at their extremities, as represented in the figure : .one of these is hollow, and conveys steam from the boiler below, into the sphere; and the escape of the vapor from the small tubes, by the reaction, produces a rotary motion. In ordpr to render the presstire of steam practically availa- ble in machinery, it is necessary that it should be confined within a cavity which is air-tight, and so constructed that its dimensions or capacity can be enlarged or diminished without impairing its tightness. When the steam enters such a ves- sel, its elastic force pressing agrjnst some movable part, causes it to recede before it, and from this movable part motion is communicated to machinery. „ , The practical arrangement bv which such a now are these , . ,.,,.,". , ,, conditions at- result IS accomplished is by having a hollow tained ? cylinder, A B, Fig. 211, with a movable piston, D, accurately fitted to its cavity. When steam under pressure in a boiler is admitted into the cylinder below the piston, it ■ expands, and acting upon the under surface of the piston, causes it to rise, lifting the piston-rod along with it. Suppose, as in Fig. 212, the cylinder to be connented at the bottom or side with a pipe, E, opening into a steam boiler, and on the other side with a pipe, B, terminating in a vessel of cold water. Suppose the valve in R to bo open, and that in B to be shut; steam then passing into the cylinder from the boiler will force the piston up to the top of the cylinder. Let the valve in E, then be shut, and the valve in B be opened; the steam contained in the cylinder will pass out ' of the pipe B, and coming in contact with cold water, in the vessel connected with it, will bo condensed, and a vacuum fona* To render the pressure of stearu availa- ble in machin- ery, what con- ditions are necessary ? Fia 211. THE STEAM-ENGINE. 253 Pig. 212. beneath the piston. The pressure of the atmosphere then acting upon the other side of the piston, ■will drive it do^-n. The position of the valves in R and B being reversed, the piston may be raised ane'w- by the admis- sion of more steam, to be condensed in its turn, and in this manner the alternate motion may be continued indefinitely. The alternating, or re- ciprocating motion of the piston, is converted, by means of a lever and crank attached to the top of the pis- ton-rod, into a rotary motion, suitable for driving-wheels, shafts, and other machinery. Such an arrangement as described constituted the first practical steam- engine. It received the name of the atmospheric engine, from the fact that the pressure of the atmosphere was employed to press down the piston after it had been elevated by the steam. 577. In modern engines, the pressure of the atmosphere is not employed to drive the piston down. The steam is ad- mitted into the cylinder above the piston, at the same time that it is condensed or withdrawn from below, and thus exerts its expansive force in the returning as well as in the ascending stroke. This results in a gr^cat increase of power. By the condensation or with- draw.al of the steam, a vacuum is created below the piston, and the steam admitted into the cylinder above the piston, forces it through the vacuum with an ease and rapidity far greater than would be possible if atmospheric or other resistance were to be overcome.* The withdrawal or condensation of the steam, in order to produce a vacuum either above or below the piston, is accomplished by opening at the proper time a communication between the cylinder and a strong vessel situated at a distance from it, called the condenser. Into this vessel a jet of cold water is thro■^^^l, which instantly condenses the steam, escaping from the bottom of the cylinder, into water. Wliat is the construction and operation of a condens- ing steam-en- gine ? • "A proof of the extraordinary power obtained in this uray, through the combnstion of fuel, is presented in the following cnlculations : — One cubic inch of water is converti- ble into steam, of one atmospheric pressure, by 15} grains of coal, and this expansion of the water into steam is capable of raising a weight of one ton the height of a foot. Tha one c\ibic inch of water becomes very nearly one cubic foot of steam, or 1,7'2S cubic inches. When a vacuum is produced by the condensation of this ste;im, a piston of one squaro inch surface, that may have been lifted 1,7-8 inches, or 144 feet, will fall with a velocity of a heavy body rushing by gravity down a perpendicular height of ir",5i10 feet. This would give the falling body a velocity, at the termination of its descent, equal to 1,300 feet per gecond, great'^r than that of the transmission of sound. From this we can form some estimate of the strength of the terhpest which alternately blows the piston in its cylinder, when elastic steam of high-pressure is employed." — Prof. U. D, Rogers, 254 WELLS'S NATURAL PHILOSOPHY. A steam-engine of this character is called a condensing steam-engino, bo- cause the steam whicli has been employed in raising or depressing the piston ia condensed, after it has accomplished its object, leaving a vacuum above or below the piston. It is also called a low-pressure engine, because, on ac- count of the vacuum which is produced alternately above and below the piston, the steam, ia acting, does not expend any force in overcoming the pressure of the atmosphere. Steam, therefore, may be used under such condi- tions of low expansive force, or, as it is technically called, of " low-pressure." The practical construction of the piston and cylinder, and the ar- ^IG- 213. rangement of connecting pipes by (^ which the steam is admitted alter- nately above and below the piston, is fully shown in Fig. 213. The valves, which are of various forms, are connected bj^ lovers with tho machinery, in such a way as to open and close with great ac- curacy at exactly the proper mo- ment. ■iiTT, .• -u- i, 5'^8. In some vVnat IS a nign- pressure eu- engines, the appa- ^°® ' ratus for condens- ing the steam alternately above or below the piston, is dispensed with, and the steam, after it has moved the piston from one end of the cylinder to the other, is al- lowed to escape, by the opening of a valve, directly into the air. To accomplish this, it is evident that the steam must have an elastic force greater than the pressure of the atmosphere, or it could not expand and drive out the waste eteam on the otlier side of the piston, in opposition to the pressure of the air. An engine of this character is accordingly termed a "high-pressure" engine. High-pressure engines are generally worked with a pressure of from fifty to sixty pounds per square inch of the piston ; of this pressure, at least fifteea pounds must be expended in overcoming the pressure of the atmosphere, and the surplus only can be applied to drive machinery. One of the most familiar examples of a high pressure engine is the loco- motive used on railroads. The steam which has been employed in forcing the piston in one direction is, by the return movement of the piston, forced out of the cylinder into the smoke-pipe, and escapes into the open au- with irregular puffs. THE STEAM-ENGINE. 255 ^ What are the advantages and disadvantages of high-press- ure engines f High-pressure engines are generally used in all situations where simplicity and lightness are required, as in the case of the locomotive ; also in situations where a free supply of water for condensation can not be readily obtained. As they use steam at a much higher pressure than the condensing en- gines, they are more liable to accidents arising from explosions. High-press- ure engines are less expensive than low-pressure, since all the apparatus for condensing the steam is dispensed with, the only parts necessary being the boiler, cylinder, piston, and valves. _,. . . 579. It is not necessary in the steam-enrino that the steam when IS steam •' ° said to be used should flow continuously from the boiler into the cylinder expansively? during the whole movement of the piston, but it may be cut off before it has fully completed its ascent or descent in the cylinder. The eteam already in the cylinder immediately expands, and completes the move- ment already begun, thus saving a considerable quantity of steam at each movement. Steam employed in this way is said to be used expansively. To carry out this plan to the best advantage, the expansive force of the steam must be greatly in- creased by working it under a high pressure. 580. In many engines the supply of steam to the cylinder is regu- lated by an apparatus called the Governor. This consists, as is rep- 214, of two heavy balls, C and C, connected by jointed rods, D D', with a revolving axis. A. When the axis is made to revolve rap- idly, the centrifugal force tends to make the balls diverge, or separate from one another in the same manner as the two legs of a tongs will fly apart when whirled round by the top. This divergence draws down the jointed rods, but a slower motion of the axis causes the balls, on the contrary, to approach each other, and thus push them up. These movements of the jointed rods in turn raise or lower the end of a bar, E, which acts as a lever, and moves a valve which increases or diminishes the quantity of steam admitted from the boilers into the cyhnder — thus preserv- ing the motion of the engine uniform. In stationary engines, also, a large and heavy fly-wheel is often used, which by its momentum causes the machinery to move uninterruptedly, even if tha pressure of steam be less at one point than at another * • Fig. 215 illnstrates the principal parts of a condensing steam-engine and its mode of action. Upon the left of the fijjure is the cylinder, which receives the steam from the boiler. A part of the side of the cylinder is cut away in order to show the piston, which moves alternately up and down accordinij as the steam is admitted above or below it. By the rod A the piston triin-mits its alternating movements to the walking-beam, L, which is an enormous lever accurately balanced on its center, and supported by four columns. The walking-beam, L, communicates its motion by means of a connecting-rod, I, to the crank. How isthe mo- tion of steam- engines regu- lated ? resented in Fie 256 "WELLS S NATURAL PniLOSOPHT. Fig. 215. 581. Steam-boilers, which, although necessary to the generation of tho power, are quite independent of the engine, are constructed of thick sheets of iron or copper, strongly riveted together. K, by -which a rotary movement is communicated to the »rheel, Y ; from this the poorer iuay be applied by other wheels, or by bands and pulleys, to effect different operations. At the left of the cylinder is an arrancrcment of valves a^d pipes, by which the steam it allowed to act alternately above and below the piston. Aftpr the steam has completed its action by forcing the piston to the extremity of the cylinder, it is necessary that it should be withdrawn, and a vacuum formed in its place. In order tf^ accomplish this, the steam, after having acted, is caused to pass into the cylinder, O, whi?h contains cold water, and is termed the condenser. Here it is condensed, and a vacuum formed in the cylinder above or below the piston, as the case may be. As the cold water of the condenser becomes quickly heated bv the condensed steam withdrawn from the cylinder, it becomes necessary to constantly withdraw the hot water and replace it by cold water, in order that the condensation of the steam nay take placa as rapidly as possible. This is effected by means of two pumps ; the one, F M. which is called the " air-pump," which withdraws the hot water from the condenser, and with it aay air that may be present either in the cylinder or the condenser^ the other, H R. THE STEAM-ENGINE. 257 „^ ^ ^^ The essential requisites of a steam-boiler are, that it should What are the ^ ■ , ■ , , . , essential req- possess sufficient Strength to resist the greatest pressure which nisites of a jg g^gj. ij^ble to occuT from the expansion of the steam, and Eteam-boUer ? ^ ' that it should offer a sufficient extent of surface to the fire to insure tlie requisite amount of vaporization. In common low-pressure boilers, it requires about eight square feet of surface of the boiler to be ex« posed to the action of the fire and flame to boil off a cubic foot of water in an hour; and a cubic foot of water in its convertion into steam equals one- horse power. The strongest form for a boiler, and one of the earliest which was used, is that of a sphere; but this form is the one which offers least surface to the fire. The figure of a cylinder is on many accounts the best, and is now ex- tensively used, especially for engines of higli-pressure. It has the advantage of being easily constructed from sheets of metal, and the form is of equal strength except at the ends. In such a boiler the ends should be made thicker than the other parts. called the "cold-water pnmp," draws from a well or river the cold water to supply the plice of the heated water withdrawn from the condenser by the air-pump. There is also a tliird pump, G Q, which is called the " supply" or " feed-pump," because it pumps into the boiler the hot water which the air-pump withdraws from the condenser, thus econ- omizing the consumption of fuel. The various parts of the engine (as shown in Fig. 215) are illustrated in detail by tha following descriptive explanation : — A — Piston-rod connected with the walking-beam, and transmitting to it the alternating movement of the piston. B, C, D, E — Arrangements of levers and joirits, intended to guide and preserve the pis- ton-rod A in a perfectly rectilinear track during its up-and-down movements. F — Arm or rod of the air-pump, which removes the hot water and air from the con- denser. G — Rod of the " supply" or " feed-pump," which supplies to the boiler the hot water withdrawn from the condenser. H — Rod of the cold-water pump, which supplies the cold water necessary for con- densation. I — Connecting-rod, which transmits the motion of the walking-beam, L, to the crank, K. M — Cylinder of the air-pump in communication with the condenser, O. O — Condenser filled with cold water, in which the steam after acting upon the piston is condensed. P — Piston, movable in the cylinder ; it receives directly the pressure of the steam upon the upper and lower surface alternately, and transmits its movements by means of the rod A to the rest of the machinery. S — Pipe conducting the hot water withdrawn from the condenser to the boiler. T — Pipe discharging the cold water from the cold-water pump into the condenser, O. U — Pipe conducting the steam from the cylinder, after it has acted upon the piston, into the condenser. V— Fly-wheel. Z — Cornecting-rod, which transmits the movements of the eccentric, e, through the lever, Y, to the valves, b. The eccentric is a wheel fixed upon the crank-shaft, as seen at e. It ii called an eccentric from the circumstance of the wheel not being concentric, or having a common center with the crank-shaft upon which it is fixed. It becomes, there- fore, a substitute for a short crank, and transmits a reciprocating movement to the rod Z, which is connected with the valves at 6 by the lever Y. These valves being alternately opened and closed by the movemeat of the rod Z, admit the steam alternately above or below the pibton. 258 WELLS'S NATURAL PHILOSOPHY. What is the construction of a flue-boiler? A very great improvement was FiG. 216. effected in the construction of steam- boilers by placing a cylindrical fur- nace within a cylindrical boiler, thus surrounding the heated surfaces with water upon all sides. By this method, all the heat, except what escapes up the chimney, is communicated to the water. Such boilers are known as " flue-boilers." Their general form and plan of construction are represented in Fig. 216. ___ , The requirements of a boiler suit- peculiarities of able for a locomotive are, that boiier'?™°"^^' *^® greatest possible quantity of water should be evapor- ated with the greatest rapidity in the least possible space. The quantity of fuel consumed is a secondary consideration, as this can be carried in a separate vehicle. The principle by which this has been accom- plished, and the invention of which may be said to have made the present railway system, consists in carrjdng the hot product of the fire through the water in numerous small parallel flues or tubes, thus dividing the heated matter, and as it were filtering it through the water to be heated. In this manner the surfaces, by which the water and the heating gases communicate, axe immensely increased, the whole having a resemblance to the mechan- FiG. 217. ism of the lungs of animals, in which the air and the blood are divided and presented to each other at as many points, and with as little intervening matter between them, as is consistent with their separation. Fig. 217 represents the interior of the fire- box of a locomotive, showing the opening of the tubes, which extend through the whole length of the boiler, and are surrounded with water. The smoke and other products of combustion pass through these tubes, and finally escape up the smoke-pipe. It will be furfher observed by the examination of the figure that the fire-box is double-walled, or rather walled and roofed with a layer of water, leaving onty the bottom vacant, which receives the grate-bars. . 582. The safety-valve is generally a conical lid fitted safety-valve. i^^o the boiler, and opening outward ; it is kept down by a weight, acting on the end of a lever, equal to the pressure which the boiler is capable of sustaining without danger from the steam generated within. If the amount of steam at any time exceeds the pressure, THE STEAM-ENGINE. 259 Fig. 218. How does a diminution of water in boil- ers often oc- casion eiplo- Bions ? it overcomes the resistance of the weight, lifts the valve, and allows the steam to escape. "When sufficient steam has escaped to diminish the pres- sure, the valve falls back into its place, and the boiler is as tight as if it had no such opening. Fig. 218 represents the ordinary construction of the safety-valve. 583. The explosion of steam-boilers, when the safety-val\-9 is in good condition and working order, is sometimes inex- plicable ; but explosions often result from the engineer allow- ing the water to become too low in the boilers. When this occurs, the parts of the boiler which are not covered with water, and are exposed to the fire, become highly overheated. If, in this condition, a fresh supply of water is thrown into the boiler, it comes suddenly into contact with an intensely -heated metal surface, and an immense amount of steam, having great elastic force, is at once generated. In this case the boiler may burst before the inertia of the safety-valve is overcome, and the stronger the boiler the greater the explosion. What is a ^S"^- '^^^ degree of pressure which the steam exerts upon Bteam-guage ? the interior of the boiler, and which is consequently avaQ- able for working the engine, is indicated by means of an instrument called the "steam" or "barometer-guage." It consists simply of a bent tube, A, C, D, E, Fig. 219, fitted into the boiler at one end, and open to the air at the other. The lower part of the bend of the tube contains mercury, which, when the pres- sure of steam in the boiler is equal to that of the external atmosphere, will stand at the same level, H R, in both legs of the tube. "When the pressure of the steam is greater than that of the atmos- phere, the mercury is depressed in the leg C D, and elevated in the leg D E. A scale, G, is attached to the long arm of the tube, and by observing the difference of the levels of the mercury in the two tubes, the pressure of the steam may be calculated. Thus, when the mercury is at the same level in both legs, the pressure of the steam balances the pressure of the atmosphere, and is therefore 15 pounds per square inch. If the mercury stands 30 inches higher in the long arm of the tube, then the pressure of the steam is equal to that of two atmospheres, or is 30 pounds to the square inch, and Fig, 219. 2G0 WELLS'S NATUKAL PHILOSOPHY. _ . As tho pressure of steam increases with its temperature, the pressure of pressure upon the interior of the boiler may also be known by steam be m- means of a thermometer inserted into tlie boiler. Thus it has dicatea by a thermometer? been ascertained that steam at 212° balances the atmosphere, or exerts a pressure of 15 pounds per square inch ; at 250°, 30 pounds; at 275°, 45 pounds; at 294°, 60 pounds, and so on. „ ., ,, 585. The steam-whistle attached to locomotive and other Describe the . . , , , . , . » steam-whistle. engmes is produced by causmg the steam to issue from a . narrow circular slit, or aperture, cut in the rim of a metal cup; directly over this is suspended a bell, formed like the bell of a clock. The Bteam escaping from the narrow aperture, strikes upon the edge or rim of the bell, and thus produces an exceedingly sharp and piercing sound. The size of the concentric part whence the steam escapes, and the depth of the bell part, and their distance asunder, regulate tho tones of the whistle from a shrill treble to a deep bass. SECTION V. WARMING AND VENTILATION. Upon what ^86. lu tliG Warming and ventilation of thl°"warraing Ijuilcllngs, tttG cntiro process, whatever expe- of'^'^buumngs tlients may be adopted, is dependent upon the depend? cxpansion and contraction of air ; or in other words, upon the fact that air which has been heated and expanded ascends, and air which has been deprived of heat, or contracted, descends. What is ven- ^^'^ • Ventilation is the act or operation of tiiation? causing air to j)ass through any place, for the purpose of expelling impure air and dissipating noxious vapors. The theoretical perfection of ventilation is to render it impossible for any portion of air to be breathed twice in the same place. „,. , In the open air, ventilation is perfect, because the breath, as Wnere is ven- , , ■, . \ ,- , , tiiation perfect ? it leaves the body, IS warmer and lighter than the surround- ing fresh air, and ascending, is immediately replaced by an ingress of fresh air ready to be received by the next respiration. .^ . . Common air consists of a mixture of two gases, oxygen an(J once respired nitrogen, in the proportion of one fifth oxygen to four fifths unwholesome ? nitrogen. By all the forms of respiration or breathing, and of combustion, the quantity of oxygen in atmospheric air is diminished and impaired, and to exactly the same extent is air rendered unwholesome and unsuitable to supply the wants of the animal system. WARMING AND VENTILATION. 261 R, ^ mach f^fw^ jir is re- quired ^r hour by a Lcalthy man'? In what man- ner does a heut- «d substance generate a cur. rent of air i Fig. 220. It is calculated that a fuU-gi'own person of average size ab- sorbs about a cubic foot of oxygeu per liour by respiration, and consequently renders Jive cubic feet of air unlit for breath- ing, since every five cubic feet of air contain one cubic foot of oxygen. It is also calculated that two wax or speroi candles absorb a^ much oxygen as an adult. To renaer the air of a room perfectly pure, five cubic feet of fresh air per hour, for each person, and two and a half cubic feet for each candle, should be allowed to pass in, and an equal quantity to pass out. 588. From every heated substance, an up- ward current of air is continually rising. The existence and force of this upward current may bo shown in the case of an ordinar3' stove, by attaching to the Bide of the pipe a wire on which a piece of thicic paper cut in the form of a spiral is suspended, as is represented in Fig. 220. The upward current of liot air striking against the surfaces of the coil causes it to revolve rapidly around the wire. _„ ^ Apart from the consideration of con- Why are stoves ^ and grates venience, it is necessary that stoves and flowf '^^^'^ ^^^ g'"''''^^' intended for warming, should be located as near to the floor of the room as possible ; since the heat of a fire has very little ef- fect upon the air oi" an apartment below the level of the surface upon which it is placed. .,-,, , 589. When a fire is lighted in a stove Why does ° •moke ascend or grate to warm a room, the smoke * ciuniney ? ^j^^^ other gaseous products of combus- FiG. 221. tion, being lighter than the air of the room, ascend, and soon fill the chimney witii a column of air lighter, bulk for bulk, than a column of atmospheric air. Such a col- umn, therefore, will have a buoyancy proportional to its relative lightness, as compared with the external air and the air of the apartment. ' The upward tendency of a column of heated air constitutes the draft of a chim- ney, and tliis draft will be strong and cP fectivc just in the same proportion as th3 column of air in the chimney is kept warm. Fig. 221 represents a section of a grate and chimney. C D represents tlie light __ and warm column of air within the chim- ney, and A B the cold and heavy column 262 WELLS'S NATURAL PHILOSOPHY. of air outside the chimney. The column A B being cold and heavy presses down, the column C D being light and warm rushes up, and the greater the difference between the weight of these two columns, the greater will be tho draft. A chimney quickens the ascent of hot au- by keeping a long ^°mney°quick- Column of it together. A column of two feet high rises, or is en tlie ascent pressed up, with twice as much force as a column of ono bot^a^r ?'™° " ^oot, and so in proportion for all other lengths — just as two or more corks, strung together and immersed in water, tend ■upward wuth proportionably more force than a single cork. In a chimney where a column of hot air one foot in height is one ounce lighter than the same bulk of external cold air, if the chimney be one hun- dred feet high, the air or smoke in it is propelled upward with a force of one hundred ounces. To what is the ^^ ^lie fire be sufficiently hot, the draft of cWmney°Vo* ^^^ chimnej will be proportional to its length. portional ? -poT this reason, the chimneys of large manufacturing estab- lishments are generally very high. How should a A chimney should be constructed in such constructed? ^ a Way that the flue or passage will gradually contract from the bottom to the top, being widest at the bottom, and the smallest at the top. The reason of this will bo evident from the following con- chimney be siderations: — At the base of the chimney, the hot column of constructed in expanded air fills the entire passage ; but as the hot air this manner f ^ . , ascends it gradually cools and contracts, occupymg less space. If, therefore, the chimney were of the same size all the way up, the tendency would be, that the cold external air would rush down to fill up the space left by the contraction of the hot column of air. This action would still further cool the hot air of the chimney and diminish the draft. Some persons suppose that a chimney should be made larger at the top than at the bottom, because a column of smoke ascending in the open air, ex- pands or increases in bulk as it goes up. This, however, is owing, in great part, to the action of currents in the air, and to the fact, that a column of smoke freely exposed to the air, is more rapidly cooled than in a chunney, and losing its ascensional power, tends to float out laterally, rather than ascend perpendicularly. The causes of " smoky chimneys" are various, rirc^umstal^et* A chimney may smoke for want of a sufficient supply of •will a chimney air. If the apartment is very tight, fresh air from without smoke? ^.j^ ^^^ ^^ admitted as fast as it is consumed by the fire, and in consequence a current of air rushes down the chimney to supply the defi- ciency, driving the smoke along with it. A chimney will often smoke when the heat of the fire is not sufQcient to -WARMING AND VENTILATION. 2G3 rarefy all the air in the chimney ; in such cases the cold air (condensed in tho upper part of the flue) will sink from its own weight, and sweep the ascend- ing smoke back into the room. "When the fire is first hghted, and the chimney is filled with cold air, there is often no draft, and consequently the flame and smoke issue into tho room. This, in most cases, is remedied by the action of a " blower." A blower is a sheet of iron that stops up the space above ^of a blower? *^® grate bars, and prevents any air from entering the chim- ney except that which passes through the fuel and produces «ombustion. This soon causes the column of air in the chimney to become heated, and a draft of considerable force is speedily produced through the fire. The increase of draft increases the intensity of the fire. Another frequent cause of smoky chimneys is, that when the tops are commanded by higher buildings, or by a hill, the wind m blowing over them, falls hke water over a dam, and beats down the smoke. The remedy in such cases is, either to increase the height of the chimney, or to fix a bonnet or cowl upon the top. The pMlosophy of this last contrivance consists in the fact that in whatever direction the wind blows, tho mouth of the chimney is averted from it. In a room artificially heated, there are al- What is the , , /• • j? i . • n motion of the wajs two curreuts 01 air ; one oi not air now- artificiaiiyheat- insc out of the room, and another of cold air ed* flowing into the room. If a candle be held in the doorway of such an apartment, near the floor, it will be found that the flame will be blown inward ; but il' it be raised nearly to the top of the doorway, the flame will be blowm outward. The warm air, in this case, flows out at the top, while the cold air flows in at the bottom. 590. An open fire-place diflers greatly from a close stove How does a ., . . , ,, /■ j stove diflfer m respect to ventflation, uiasmuch as the lormer warms ana from an open ventilates an apartment, whUe the latter only warms, and can fire-place in ' ,, i -i • t respect to ven- hardly be said to contribute at all to the ventilation, in a tUation? gjjjgg gtQvg^ no air passes through the room to the flue of the chimney, except that which passes through the fuel, and the quantity of this is necessarily limited by the rate of combustion maintained in the stove. In an open fire-place, a large amount of air is continually rushing up the chimney through the opening over the grate, irrespective of what passes through the fire and maintains combustion. In summer time, when no fire is made in the chimney, the column of air in it is generally at a higher temperature than the external air, and a current will therefore ip such case be established up the chimney, so that the fire- place will still serve, even in the absence of fire, the purposes of ventilation. In very warm weather, however, when the external air is at a higher tem- perature than the air within the building, the eff^jcts are reversed ; and the air in the chimney being cooled, and therefore heavier than the external air, a downward current is established, which produces in the room the odor of soot 264 WELLS'S NATURAL PHILOSOPHY. Fig. 222 represents the lines of the currents descend- FiG. 222 ing the chimney and circulating round an apartment. Tj . A room Ls well ventilated by openinp: IIow IS a room j s. r> best ventUat- the upper sash of a window ; because * tlie hot vitiated air (which always as- cends toward the ceiling) can thus escape more easily. If the lower sash of the window be also partially opened, a corresponding current of cold air, flowing into tho room, is created, and ventilation ■will be so effected more perfectly. ,,^ Open fire-places are ill adapted for tho why are open i i . i fire-places ill economical heating of apartments, be- arlapted for cause the air which flows from the room heating ? to the fire becomes heated, and passes off directly into the chimney, without having an oppor- tunity of parting with its heat for any useful purpose. lu addition to this, a quantity of the air of the room, j^w^w^'w^^r^ ■which has been ■warmed by radiation, is uselessly carried | t away by the draft. j ' I The advantages of a stove over an [* ' ■Wtat are the ^ i /• n i' i advantages and open fire-place are as follows: ^ f disadvantages 1. Being detached from the walls of of StOV6S ? the room, the greater part of the heat "»r-«ac ■>— gg f-T^st ■^' produced by combustion is saved. The radiated heat being thrown into the walls of the stove, they become hot, and in turn radi- ate heat on all sides of the room. The conducted heat is also received by successive portions of the air of the room, ■which pass in contact with the Btove. 2. The air being made to pass through the fuel, a small supply is suffi- cient to keep up the combustion, so that little need be taken out of the room; and 3. The smoke, in passing off by a pipe, parts -with the greater part of its heat before it leaves the room. Houses warmed by stoves, as a general rule, are ill-ventilated. The air coming in contact with the hot metal surfaces is rendered impure, which in> purity is increased by the burning of the dust and other substances which settle upon the stove. The air is, in most cases also, kept so dry as to ren- der it oppressive. 591. The method of warming houses by the common hot- What is the . . . . ,, a ^ i. • i i- ,- method of air furnace is as follows : — A stove, havmg large radiating sur- jranning by faces, is inclosed in a chamber (general) v of masonry). This hot-air fur- ' ^:^ . J / naees? chamber is frequently built with double walls, tliat it may be a better non-conductor of heat. A current of air from ■with- out is brought by a pipe or box, and delivered under the stove. A part of this air is admitted to supply the combustion ; the rest passes upward in the cavity between the hot stove and the walls of the brick chamber, and, after -WARMING AND VENTILATION. 265 becoming thoroughly heated, is conducted tlirough passages in which its hght- ntss causes it to ascend, and be delivered in any apartment of the house. „^ , , In the construction and arrangement of a furnace for heat- What two . ° points are of ing, the two ponits of special importance are, to secure a pcr- spccial import- ^^^^ combustion of the fuel, and tho best possible transmission ance in the con- ' '■ Btructioaoffur- of all the heat formed, into tho air that is to pass into the **'^^^''' rooms of the house. The first of these requisites is obtained by having a good draft and a fire- ■box which is broad and shallow, so that tho coal shall form a thin stratum »nd burn most perfectly. The second requisite is obtained by providing a great quantity of surface "u the form of pipes, drums, or cylinders, through which the smoke and hot gases must pass on tlieir way to the chimney, and to which their heat will be imparted, to be in turn deUvered to the cold and pm-e air of the rooms of tho house. 592. The grrcat advantages of heating by steam are, that What is the , , ° . , „ ,. ... advantage of the heat can be communicated lor a great distance m anj"^ ai- heating by rection — upward, downward, or horizontally. As the tem- steam? i i ' j perature of the heating surfaces, when low-pressure steam m used, never exceeds 212° F-, the air in contact with tliem is never contami- nated by the burning of dust, or the abstraction of oxygen. Under favorable circumstances, one cubic foot of boiler will heat about two thousand cubic feet of suitably inclosed space to a temperature of 70° to 80° F. 593. We. apply the term fuel to any suh- Whatisfucl? tri J •' stance which serves as aliment or food for fire. In ordinary language we mean by fuel the peculiar suh- fitance of plants, or the products resulting from their de- composition, designated under the various names of wood, coal, &c. In recently cut wood, from ono fifth to one half of its weight tion'' of'^^t'he is water ; after wood has been dried in the air for ten or weight of wood twelvemonths, it will even then contain fi-om 15 to 25 per IB water y ' cent, of water. The amount of moisture in wood is greatest in the spring and summer, when the sap flows freely and the influence of vegetation is the greatest. Wood, therefore, is generally cut in the winter, because at that season there is but little sap in the tissues, and the wood is drier than at any other period. "Woods are designated as hard and soft. This distinction is Snated°°as^ grounded upon the facility with which they are worked, and hard and soft ? upon their power of producing heat. Hard woods, as tho oak, beech, walnut, elm, and alder, contain in the same bulk more .solid fiber, and their vessels are narrower and more closely packed than those of the softer kinds, such as pine, larch, chestnut, etc. 12 266 WELLS'S NATURAL PHILOSOPHY. What Is the 594. The weight of wood varies greatly ; weTJiuofwood? from forty-four hundred pounds in a cord of dry hickory, to twenty-six hundred in a cord of dry, soft maple. What is the ^^5. For fuel, the most valuable of the com- vaiurof"wood ^^^ kinds of wood are the varieties of hickory; for fuel? after that, in order, the oak, the apple-tree, the white-ash, the dog-wood, and the heech. The wooda that give out the least heat in burning are the white-pine, the white-birch, and the poplar. , ., -^ , , 596. The remark is sometimes made that " it is economy to Is it profitable •' to bum green bum green wood, because it is more durable, and therefore ■wood? jjj ^y^Q gQ(j more cheap." This idea is erroneous. The con- sumption of green wood is less rapid than dry, but to produce a given amount of heat, a far greater amount of fuel must be consumed. The evaporation of liquids, or their conversion into steam, consumes or ren- ders latent a great amount of caloric. When green wood or wet coal is added to the fire, it abstracts from it by degrees a sufficient amount of heat to con- vert its own sap or moisture into steam before it is capable of being burned. As long as any considerable part of this fluid remains unevaporated, the combustion goes on slowly, the fire is dull, and the heat feeble. ,^^ , 597. Coal and hard wood are not readily ignited by th« Why are coal •' ° •' and hard woods blaze of a match, because on account of their density they are difficult to ig- rendered comparatively good conductors, and thus carry off match? the heat of the kindling substance, so as to extinguish it, before they themselves become raised to the temperature necessary for combustion. Light fuel, on the contrary, being a slow conductor of heat, kindles easily, and, from the admixture of atmospheric air in its pores and crevices, burns out rapidly, producing a comparatively temporary, though often strong heat. CHAPTER XIII. METEOROLOGY. What is Me- ^9^- Meteorology is that department of teoroiogy? physical science which treats of the atmos- phere and its phenomena, particularly in its relation to heat and moisture. 699. By climate, we mean the condition of a place in METEOROLOGY. 267 .v.Tiat do we relation to the various phenomena of the at- tMm"ciimate?* luosphere, as temperature, moisture, etc. Thus, we speak of a warm or cold climate, a moist or dry climate, etc. How is the 60^- The mean or average temperature of rXrf of a'day' ^^^ ^^J ^^ fouod by obscrving the thermometer found f g^^ fixed intervals of time during the twenty- fjur hours, and then dividing the sum of the tempera- tures by the number of observations. At what tim From such a seiiea of observations it has been found that is the tempe- the lowest temperature of the day occurs shortly before sun- da/t'iieWghl^st '"'^^' ^^^ *^® highest a few hours after 12 at noon, somewhat and lowest ? later in summer and somewhat earlier in winter. The mean annual temperature of any par- ticular location is found by taking the average of all the mean daily temperatures throughout the year. The mean daily temperature of any place seems to vary in a regular and constant manner, while the mean annual temperature of the same location i3 very nearly a constant quantity. Thus, by long observations made in Phil- adelphia, it has been found that the mean daily temperature of that locality is cue degree less than the temperature at 9 o'clock, a. m., at the same place ; while the mean annual temperature of Paris varied only 4° in thirteen years. All the result? of observation seem to show that the same quantity of heat is always annually distributed over the earth's surface, although unequally — that is to say, the average annual temperature of each place upon tlie earth's surface is very nearly the same. In our latitude, July is on the average tho hottest month, and January the coldest ; and in reference to particular days, we may on an average consider the 26th of July as the hottest, and the 14th of Januarj' as the coldest day of the year for the temperate zone of the north- em hemisphere. Howdoestem- The avcragc annual temperature of the at- with'The utu mosphere diminishes from the equator toward ^""^"^ either pole. At the equator, in Brazil, the average annual temperature is 84° Fahren- heit's thermometer ; at Calcutta, lat. 22° 35' N., the annual temperature la •iSo F. ; at Savannah, lat. 32° 5' N. the annual temperature is 05° F. ; at London, lat. 51° 31' N., the annual temperature is 50° F. ; at Mclvillo Island, lat. 74° 47' N., the average annual temperature is 1° below zero. .^^ .g |., 601. If the whole surface of the earth were covered by temperature of Water, or if it were all formed of solid plane land, possessing f ""^u^e^ sarao everywhere tlio same character, and having an equal ca- latitude alike ? pacity at all places for absorbing and again radiating heat, tha 2G8 WELLS's NATURAL PHILOSOPHY. temperature of a place would depend only on its geographical latitude, and cousequently all places having the same latitude would have a hke climate. Owing, however, to various disturbing causes, such as the elevation and form of the land, the proximity of the sea, the direction of the winds, etc., places of the same latitude, aud comparatively near each other, have very diflerent temperatures. In warm climates the proximity of the sea tends to diminish the heat ; in cold climates, to mitigate the cold. Islands and peninsulas are warmer than continents ; bays and inland seas also tend to raise the mean temperature. Chains of mountains which ward off cold winds, augment the temperature; but mountains which ward off south and west winds, lower it. A sandy soil, which is dry, is wanner tlian a marshy soil, which is wet and subject to great evaporation. 602. Air absorbs moisture at all tempera- capacity of air tures, and retains it in an invisible state, for moisture? i-m • /• i i • • ± i 'i v This power oi the air is termed its capacity for absorption. The capacity of air for moisture increases with the tem- perature. A volume of air at 32° can absorb an amount of moisture equal to the hun- dred and sixtieth part of its own weight, and for every 27 additional degrees of heat, the quantity of moisture it can absorb at 32° is doubled. Thus a body of air at 32° P. absorbs the 160th part of its own weight ; at 59" R, the 80th; at 8G° F., the 40th ; at 113° F., the 20th part of its own weight in moisture. It follows from this that while the temperature of the air advances in an arith- metical series, its capacity for moisture is accelerated in a geometrical series. When is ai.- -^^^ ^^ ^^i*^^ ^^ ^^ Saturated with moisture Mtedf^^ *'''''' when it contains as much of the vapor of water as it is capable of holding with a given tem- perature. "We say that air is dry when water evaporates quickly, or any wetted sur- face dries rapidly ; and that it is damp when moistened surfaces dry slowly, or not at all, and the sliglitest diminution of temperature occasions a deposit of moisture in the form of mist and rain. These expressions do not, however, convey altogether a correct idea of the condition of the atmosphere, since air which we term " dry," may contain much more moisture than that which we distinguish as " damp." For indicating the true condition of the atmosphere in reference to moisture, we therefore use the terms " absolute" and " relative" humidity. "When we speak of the absolute humidity of the air, we br^blolii'tlfand ^^^^ reference to the quantity of moisture contained in a given relative humid- volume. By relative humidity, we refer to its proximity to * ^ saturation. Relative humidity is a state dependent upon the mutual influeuce of absolute humidity and temperature ; for a given volume METEOROLOGY. 269 priuc nyi^rometers constructed ? of air may be made to pass from a state of dampness to one of extreme dry- ness, by merely elevating its temperature, and tills, too, without altering tlio amount of moisture it contains in the least degree. AVhatareHy- Instruments designed for measuring the groniet«rs? quantity of moisture contained in the atmos- phere, are called Hygrometers."* _ Many organic bodies have the property of absorbing vapor, principle are and thus increasing their dimensions. Among such may bo mentioned hair, wood, whalebone, ivory, etc. Any of these connected with a mechanical arrangement by which tho change in volume might be registered, would furnish a hygrometer. A large sponge, if dipped in a solution of salt, potash, soda, or any other substance which has a strong attraction for water, and then squeezed almost dry, will, upon being balanced in a pair of scales suspended from a steady support, be found to preponderate or ascend according to the relative damp- ness or dryness of the weather. The beard of the wild oat may also serve as a hygrometer, as it twists around, during atmospheric changes from dampness to dryness. If we fix against a wall a long piece of catgut, and hang a weight to the end of it, it will be observed, as the air becomes moist or dry, to alter in length ; and by marking a scale, the two extremities of which are determined by observation when the air is very dry, and when it is saturated with moist- ure, it will be found easy to measure the variations. . An instrument called the " Hair Hygrom- "Hair Hy- eter," is constructed upon this principle. It grometer. consists of a human hair, fastened at one extremity to a screw (see Fig. 223), and at the other pass- ing over a pulley, being strained tight by a silk thread and weight, also attached to the pulley. To the axis of the pulley an index is attached, which passes over a graduated scale, so that as the pullej^ turns, through the shortening or lengthening of the hair, the index movea When the in- strument is in a damp atmosphere, the hair absorbs a con- siderable amount of vapor, and is thus made longer, while in dry air it becomes shorter ; so that the index is of course turned alternately from one side to the other. The instrument is graduated by first placmg it in air ar- tificially made as dry as possible, and the pomt on the scale at which the index stops under these circumstances, is the point of greatest dryness, and is marked 0. The hygrometer is then placed in a confined space of air, which is completely saturated with vapor, and under these cir- cumstances the index moves to the other end of the scale : this point, which is that of greatest moisture, is marked • Hydrometer, from the Greek words v>/)oj (moist) and nirpiv (measure). Fig. 223. 270 WELLS'S NATURAL PHILOSOPHY. 100. The intervening space is then divided into 100 equal parts, -which indicate diflerent degrees of moisture. Sach hygrometers are not, however, considered as altogether reliable. SECTION I. PHENOMENA AND PRODUCTION OF DEW. ^,^ , . „ . 603. Dew is the moisture of the air con- What IS Dew ? densed by coming in contact with bodies colder than itself. What is the 6*^4. The temperature at which the conden- Dew-pomt? gatiou of moisturc in the atmosphere com- mences, or the degree indicated by the thermometer at which dew begins to be deposited, is called the " Dew- Point." , ,, ^ This point is by no means constant or invariable, since dew Is the dew- . ^ -^ , • , point a con- IS only deposited when the air is saturated with vapor, and etant one ? j-|jg amount of moisture required to saturate air of high tem- perature is much greater than air of low temperature. If the saturation be complete, the least diminution of temperature is at- tended with the formation of dew ; but if the air is dry, a body must be several degrees colder before moisture is deposited on its surface ; and indeed the drier the atmosphere, the greater will be the difference between the tem- perature and its dew-point. Dew may be produced at any time by bringing a vessel of production * of ^°'^^ Water into a warm room. The sides of the vessel cool dew be occa- the surrounding air to such an extent that it can no longer time^? * ^"^ retain all its vapor, or, in other words, the temperature of tlio air is reduced below the dew-point ; dew therefore forms upon the vessel. A pitcher of water tmder such circumstances is vulgarly said to " sweat." In the same manner, moisture is deposited upon the windows of a heated apartment when the temperature of the external air is low enough to suffi- ciently cool the glass. .^ As soon as the sun has set in summer, and the earth is no formed in sum- longer receiving new suppUes of heat, its surface begins to ™t ? "^'^"^ ^"°' throw off the heat which it has accumulated during the day by radiation ; the air, however, does not radiate its heat, and, in consequence, the different objects upon the earth's surface are soon cooled down from 7 to 25 degrees below the temperature of the air. The warm vapor of the air, coming in contact with these cool bodies, is condensed and P'fX'ipitated as dew. In a clear summer's night, when dew is depositing, a thermometer laid PHENOMENA AND PKODUCTION OF DEW. 271 upon the grass, vriH sint nearly 20 degrees below one suspended in the air at a little distance above. All bodies hare not an equal capacity for radiating heat, stances is dew but some cool much more rapidly and perfectly than others. deposited most Hence it follows, that with the same exposure, some bodies will be densely covered with dew, while others will remain perfectly dry. Grass, the leaves of trees, wood, eta, radiate heat very freely : but polished metals, smooth stones, and woolen cloth, part with their heat slowly: tha former of these substances will therefore be completely drenched with dew, while the latter, in the same situations, will be almost dry. The surfaces of rocks and barren lands are so compact and hard, that they can neither absorb nor radiate much heat ; and (as their temperature varies but slightly) very little dew deposits upon them. Cultivated soils, on the contrary (being loose and porous) very freely radiate by night the heat which they absorb by day; in consequence of which they are mucii cooled down, and plentifiilly condense the vapor of the air into dew. Such a condition of things is a remarkable evidence of design on the part of the Creator, since every plant and inch of land which needs the moisture of dew is adapted to collect it; but not a single drop is wasted where its refreshing moisture is not required. .... . 605. Dew is deposited most freelv upon a calm, clear night, What circnm- . , '. , • ,. - .7 stances influ- smce under such cu-cumsiances heat radiates irora the earth ence the pro- j^^g^ freelv, and is lost in space. On a cloudv niffht, on the ductiou of dew 7 - ' *^ " , • j contrary, the deposition of dew is almost entirely interrupted, since the lower surfaces of the clouds turn back the rays of heat as they radiate, or pass oflf from the earth, and prevent their dispersion into space ; the surface of the earth is not, therefore, cooled down sufficiently to chill the vapor of the air into dew. When the wmd blows briskly, also, little or no dew is formed, since warm air is constantly brought mto contact with soUd bodies, and prevents their re- duction in temperature. Can dew be ^^^w is alwajs fomied upon the surface of properly said to ^^le material upon which it is found, and does not fall from the atmosphere. Other things being equal, dew is most abundant in situations most e?cposed, because the radiation of heat is not arrested by houses, trees, etc. Little dew is ever observed in the streets of cities, because the objects are necessarily exposed to each other's radiation, and an interchange of heat takes place, which maintains them at a temperature uniform with the air. * , , Dew rarelv falls upon the surface of water, or upon ships Does dew form . " „,, „,..,. , upon the sur- m mid-ocean. The reason of this is, that whenever the face of wate r ? aqueous particles at the surface are cooled, they become heaner than those below them, and sink, while warmer and lighter particles rise to the top. These, in their turn, become heavier, and descend ; and this pro- 272 WELLS'S NATURAL PHILOSOPHY. cess, continuing throughout the night, maintaLos the surface of the water and the air at nearly the same temperature. Although dew does not appear upon ships in mid-ocean, it is freelr depos- ited on the same vessels arriving in the vicinity of land. Thus, navigatora who proceed from the Straits of Sunda to the Coromandel coast, know tliat they are near the end of the voyage when they perceive the ropes, sails, and other objects placed on the deck become moistened with dew during the night The exposed parts of the human body are never covered with dew, because fii3 vital temperature, varying from 9G° to 9S° F., effectually prevents a loss of heat sufficient for its deposition. Dew is produced most copiously in tropical countries, because there is in such latitudes the greatest difference between the temperature of the day and that of the night. The development of vegetation is also greatest in tropica) countries, and a great part of the nocturnal cooUng is due to the leaves which present to the sky an immense number of thin bodies, having large surface, well adapted to radiate heat. Dew rarely falls upon the small islands of the Pacific; the reason is, that the air over the vast ocean in which these islands are situated, preserves a nearly unrform temperature day and night. The islands are comparatively of small extent, and the stratum of air cooled by the contact of the sod is warmed by mixing with the air that is constantly reaching it from the sea. This prevents a depression of temperature in the air sufficient to cause a depo- eition of dew. What is frost? 606. Frost is frozGn dew. "VVhen the temperature of the body upon which the dew is deposited sinks below 32° F., the moisture freezes and assumes a sohd form, constituting what is called "frost." Shrubs and low plants are more liable to be injured by frost than trees of a greater elevation, since the air contiguous to the surface of the ground is the most reduced in temperature. Why does a ^^ exceedingly thin covering of muslin, pro'tect'^oTSs matting, etc., will prevent the deposition of froS?^*^"^ "'■ dew or frost upon an object, since it prevents the radiation of heat, and a consequent cool- ing suflScient to occasion the production of either dew or frost. Fig. 224, in which the arrows indicate the movements of heat, and the numerals the temperatures of the earth and air under diflerent circumstances, will render the explanations of the phenomena of dew and frost more in- telligible. The figures in the middle/jf the diagram represent the temperature of the air at a distance from the surface of the earth ; the figures in the margin, the temperature of the air adjoining the surface of the earth ; the figures below CLOUDS, RAIN, SNOW, AND HAIL. 273 the margin, the temperature of the earth itself. The directions of the arrows represent the radiation and reflection of the heat Fig. 224. ■50° . Hj W I .3-IFrn.ftorjDek-- ■■■^^^■'-^ -,..,>.„-. -—,... -J 1 Hi 63° ' £ill_ .'/;::.*.IAA:^AMI Surface of the earth, 59°. 41°. 1 32°. Dew. 1 Frost. 53°. No dew or frost. 41°. \o dew or frost In the day- time. In clear and serene nights. Cloudy or windy nights. Clear night ; soil protected. What are clouds ? SECTION II. CLOUDS, RAiy, SXOW, AND HAIL. 607. Clouds consist of vapor evaporated from the earth, and partially condensed in the higher regions of the atmosphere. How is mist or When air, saturated with vapor, in imme- fogoccasioued? ^jjg^^g coutact with the surface of the earth is cooled down rapidly, its vapor is condensed ; if the con- densation, however, is not sufficient to allow of its precipi- tation in drops, it floats ahove the surface of the earth as mist or fog. Clouds, fog, and mist diflFer only in one re- spect. Clouds float at an elevation in the air, while fogs and mists come in contact with the surface of the earth. Mist and fog are also formed when the water of lakes and rivers, or tha damp ground, is warmer than the surrounding air wliich is saturated with moisture. The vapors which rise in consequence of the higher temperature of the water, are immediately recondensed, as soon as they diffuse themselves through the colder air. ilist and fog are observed most frequently over rivers and marshes, be- cause in such situations the air is nearly saturated with vapor, and therefor* 12* How do clouds, tf' _^ there is reason to suppose that water rather ^^j^'- ' :^^^^^^^'" ^"i'J- descends from the clouds, as water which ^^^T^^i^^^^^J?"'^^ has fallen from a spout upon the deck of a ■s^^^g^-^"^'^^,^^^;^^' vessel has been found to be fresh. There is ^^^^^ no evidence, furthermore, that a continuous column of water exists within the whirling pillar, SECTION IV. METEORIC PHENOMENA. What are ^^^- Mctcorites arc luminous bodies, which Meteoiites? from time to time appear in the atmosphere, moving with immense velocity, and remaining visible but for a few moments. They are generally accompanied by a luminous train, and during their progress explosions are often heard. What are ^^5. Tlic tcrm aerolltc is given to those Aerolites? stouy masscs of matter which are sometimes Been to fall from the atmosphere.* What is known The weiglit of those aerolites which have been known to respecting the fall from the atmosphere varie? from a few ounces to several weight and ve- , , , , locity of aero- hundred pounds, or even tons. ^'■^' The height above the earth's surface at which they are sup- posed to make their appearance has been estimated to vary from 18 to 80 miles. • Aerolite is derived from the Greek words acp (atmosphere) and AiOoj (a stone). A meteor is distinguished from an aerolite by the fact that it bursts in the atmosphere, but leaves no residuum, -while the aerolite, which is supposed to be a fragment of a meteor, eomes to the ground. METEORIC PHENOMENA. 289 The estimated velocity of tbeso bodies is somewhat more than three hun- dred miles per minute, though one meteor of immense size, which is supposed to have passed within twenty-five miles of the earth, moved at the rate of twelve hundred miles per minute. Owing, however, to the short time the meteor is visible, and its great velocity, accurate observations can not bo made upon it ; and all estimates respecting their distance, size, etc., must be considered as only approximations to the truth. --„.., Very many of the meteorites which have fallen at different Wnat IS known respecting the tunes and m different parts of the globe, resemble each other ^roUtes?°° °^ ^° closely, that they would seem to have been broken from the same piece or mass of matter. Most of them arc covered with a black shining crust, as if the body had been coated with pitch. "When broken, their color is ash-gray, inclining to black. They consist for the most part of malleable iron and nickel but they often contain small quantities of other substances. They do not resemble in composition any other bodies found upon the surface of the earth, but have a character of their own so peculiar that it enables us to decide upon the me- teoric origin of masses of iron which are occasionally fotand scattered up and down the surface of the earth, as in the south of Africa, in Mexico, Siberia, and on the route overland to California. Some of these masses are of immense weight, and ixndoubtedly fell from the atmosphere. ■What is the ^^^- Four hvpotheses have been advanced Bupposedon^n j.^ account foF the origin of these extraordinary- bodies ? bodies : 1, That they are thrown up from ter- restrial volcanoes ; 2. That they are produced in the at- mosphere from vapors and gases exhaled from the earth; 3. That they are thrown from lunar volcanoes ; 4. That they are of the same nature as the planets, either derived from them, or existing independently. The fourth of these suppositions most fully explains the facts connected with the appearance of meteorites, and the third Ukewise has some strong evidence in its favor. Hovr do shoot- ^^"^^ Shooting-stars differ in many respects ftom m"t4'r^'? from mctcors. Their altitude and velocity are greater ; they are far more numerous and fre- quent, and are unaccompanied by any sound or explosion. Their brilliancy is also much inferior to that of the me- teor, and no portion of their substance is ever known to have reached the earth. At what height The altitude of shooting-stars is supposed to vary from sfx do shooting- to four hundred and sixty miles, the greatest number appear- •turs appear . ing at a height of about seventy miles. Owing to their num- 13 290 WELLS'S NATURAL PHILOSOPHY. ber and frequency of occurrence, many careful observations have been made upon them, with a view of determining these facts. Their velocity is supposed to range from sixty to fifteen hundred miles per minute. Some of these meteoric appearances may bo seen every clear night, but they appear to fall in great numbers at certain periodical epochs. The pe- riods when they may be noticed most abundantly are on the 9th and 10th of August, and the 12th and 13th of November.* The majority of shooting stars appear to radiate from a particular part of the heavens, viz., a point in the con- stellation Perseus, undoubtedly far beyond the limits of our atmosphere. __.. .... In order to account for the origin of shooting stars, it has What theories ° ° ' have been pro- been supposed by Prof. Olmstead, that they are derived from posed to ac- ^ body composed of matter exceedingly rare, like the tail of a count for the •' ' a j > origin of shoot- comet, revolving around the sun •svithin the orbit of the earth, ing stars ? ^^ ^ spaco little less than a year ; and that at times the body approaches so near the earth that the extreme portions become detached and drawn to the earth by virtue of its great attraction. It has been further sup- posed that the matter of which these bodies is composed is combustible, and becomes ignited on entering the earth's atmosphere. The nearest approach of the central body to the earth is supposed to be about 2,000 miles. Bodies falling from this distance would enter the earth's atmosphere at a height of at least 50 mUes above the surface, with a velocity generated by the force of gravity above 4 miles per second — a velocity tea times greater than the utmost speed of a cannon-balL "When common air is compressed in a tight cylinder to the extent of ono fifth of its volume, sufficient heat is generated to ignite tinder. If we suppose that the fragments descend with such velocity as to compress the rarefied atmosphere at the height of 30 miles to such an extent only as to make it as dense as ordinary air, the temperature would be raised as high as 46,000° F. — a heat far more intense than can be generated in any furnace. Unless, therefore, the mass of matter comprising the body was very large, it must bo dissipated by heat long before it reaehes the surface of the earth. • Another theory has been proposed by the eminent astronomer Chaldini, who supposes that, in addition to the planets and their satellites which revolve about the sun, there are innumerable smaller bodies ; and that these occa- sionally enter within the atmosphere of the earth, take fire, or descend to ita surface. * They hare also been noticed in unusual abundance on the 18th of October, the 6th and Tth of December, the 2d of January, the 23d and 24th of April, and from the 18th to the 20th of June. Four most remarkable meteoric showers hare been noticed, viz., in 1707, 18.^1, 1832, and 1833, all in the month of November. In the shower of 1833, the meteors, in many parts of the United States, appeared to fall as thick as snow-flakes. POPULAR 0PI2II0NS CONCEENING THE WEATHEE. 291 SECTION T. POPULAB OPIKIONS CONCEENING TKE "WEATHEE. 638. There is no reason to doubt that every Do changes in . .... , the weather chansTC 111 the Weather is in strict accordance cccur in ac- '=' -, r- • ^ • ^ • i • i c.rdance with With soHie dciinite physical ajrencies, which are fixed laws ? X .. o 7 fixed and certain in their operations. We can not, however, foretell with any degree of certainty the character of the weather for any particular time, be- cause the laws which govern meteorological changes are as yet imperfectly understood. There are, however, in all countries, certain ideas and pop- Are the popn- ' . . \, , ■, • n lar ideas re- ular proverbs respecting changes in the weather, the inuu- specting g^^^^ ^f ^j^g moon, the aurora borealis, etc., which are wholly changes in the ' . weather found- erroneous and unworthy of belief; since, when tested by ed on fact? long-continued observations, they are invariably found to be unsupported by evidence. Thus an examination of meteorological records, kept in different countries, through many years, proves conclusively that the popular notions concerning the influence of the moon on the weather has no foundation in any well- established theory, and no correspondence with observed facts. There is, however, some reason for supposing that rain falls more frequently about four days before full moon, and less frequently about four or five days before new moon, than at other parts of the month; but this can not be con- sidered as an established fact. In other respects, the changes of the moon can not be shown to have influenced in any way the production of rain. There is also a current belief among many persons that timber should be cut during the decline of the moon. To test the matter, an experiment, on an extensive scale, was made some years since in France, when it was found that there was no diflerence in the quality of any timber felled in diflerent parts of the lunar month. It is also supposed that bright moonlight hastens, in some way, the putre- lactiou of animal and vegetable substances. The facts in respect to this sup- position are, that on bright, clear nights, when the moon shines brilliantly, dew is more freely deposited on these substances than at other times, and in this way putrefaction may be accelerated. With this result tho moon has no connection. It is a traditional idea with many that a long and violent storm usually accompanies the period of the equinoxes, espociallj' the autumnal ; but tho examination of weather records for sixtj'-four years has shown that no particular day can be pointed out in the month of September (when the " equinoctial storm" is said to occur) upon which there eyer was, or ever will 292 WELLS'S NATURAL PHILOSOPHY. be, a so-called equinoctial storm. The fact, however, should not be concealed, that, taking the average for the five days embracing the equinox for the period above stated, the amount of rain ia greater than for any other five days, by three per cent., throughout the month. Observations recorded for a long period have proved that the phenomenon of the aurora borealis, which is said to precede a storm, is as often followed by fair, as by foul weather. Meteorologioal records, kept for eighty years at the observatory of Green- wich, England, seem to show that groups of warm years alternate with coid ones in such a way as to render it probable that the mean annual tempera- tures rise and fall in a series of curves, corresponding to periods of about four- teen years. There is little doubt that some animals and insects are able to foretell changes in the weather, when man fails to perceive any indications of the same. Thus some varieties of the land-snail only make their appearance be- fore a rain. Some other varieties of land crustaceous animals change their color and appearance twenty-four hours before a rain. For a light, short rain, some trees have been observed to incline their leaves, BO as to retain water ; but for a long rain, they are so arranged as to conduct the water away. The admonition given several thousand years ago, is equally sound in its philosophy at the present day : " He that observeth the winds shall not sow ; and he that regardeth the clouds shall not reap." — Eccles., xi. 4. CHAPTER XIV. LIGHT. whatisLightf ^^^- LiGHT is the physical agent which oc- casions, by its action upon the eye, the sensa- tion of vision. What is the 6'^^- Optics is the name given to that de- sciencd of Op. pgrtment of physical science which treats of vision, and of the laws and properties of light."* Between the eye and any visible object a space of greater or less extent intervenes. In some instances, as when we look at a star, the extent of the space existing between the eye and the object seen is so great, that the mind is unable to form any adequate conception of it. Yet we recognize the ex- istence of objects at such distances, by the physical effect which they produce on our organs of vision. • From the Greek word " Onrofiai" to see. LIGHT. 293 yn. jv • 641. In order to explain how such a result is possible, or of light have in Other words, to account for the origin of light, two theories been proposed? Yiare been proposed, which are called the Corpusculak and the Undulatobt Theories, What is the The Corpuscular Theory supposes that a Theor^"^of distant object becomes visible to us by emit- ^'^'*'' ting particles of matter from its surface, which particles of matter, passing through the intervening space between the visible object and the eye, enter the eye, and striking upon the nervous membrane, so affect it as to produce the sensation of light, or vision. According to this theory, there is a striking analogy or resemblance be- tween the eye and the organs of smelling. Thus, we recognize the odor of an object in consequence of the material particles which pass from the object to the organs of smelling, and there produce a sensation. In the same manner, a visible object at any distance may be supposed to send forth parti- cles of light, which move to the eye and produce vision, by acting mechan- ically on its nervous structure, as the odoriferous particles of a rose produce a sensible effect upon the organs of smelling. What is the I'li^ Undulatory Theory supposcs that Theo"^*?"^^ there exists throughout all space an ethereal, elastic fluid, which, like the air, is capable of receiving and transmitting undulations, or vibrations. These, reaching the eye, affect the optic nerve, and pro- duce the sensation which we call li2:ht. According to this theory, there is a striking analogy between the eye and the ear ; the vibrations, or undulations of the ethereal medium being supposed to pass along the space intervening between the visible object and the eye in the same manner that the undulations of the air, produced by a sounding body, pass through the air between it and the ear. Whi h f th '^^° Corpuscular Theory was sustained by Newton, and waa two theories of for a long time generally believed. At the present day it is ^y'recefved ?' almost entirely discarded, and the Undulatory Theory is now received by scientific men as substantially correct ; since it explains in a satisfactory manner nearly all the phenomena of light, which the Corpuscular Theory does not. If the Corpuscular Theory be correct, a common candle is able to fill for - hours, with particles of luminous matter, a circle four miles in diameter, since it would be visible, under favorable circumstances, in every portion of this space. Light, moreover, has no weight ; the largest possible quantity col- lected in one point and thrown upon the most sensitive balance, does not affect it in the sUghtest degree. 294 WELLS'S NATURAL PHILOSOPHY. What are the ^he cLief souices of light are the sun, the of iightr"'^''^* stars, fire or chemical action, electricity, and phosphorescence. Under the head of chemical action are included all the forms of artificial light which are obtained by the burning of bodies. Examples of light pro- duced by pliosphorescence, as it is called, are seen in the glow of old and d> cayed wood, and in the light emitted by fire-flies and some marine animals. G42. All bodies are either luminous or non-luminous. What is a lu- Luminous bodies are those which shine by mmousbody? ^jjgjj, ^^^.^ light; such, for example, as the BUn, the flame of a candle, metal rendered red hot, etc. All sohd bodies, when exposed to a sufficient degree of heat, become lu- minous. It has been recently proved* that all solids begin to emit light at the same degree of heat, viz., 977° of Fahrenheit's thermometer. As the temperature rises, the brilliancy of the light rapidly increases, so that at a temperature of 2000° it is almost forty times as intense as at 1900°. Gases must be heated to a much greater extent before they begin to emit light. What is a. non- Nou-lumlnous bodlcs are those which pro- luminousbody? ^^^^^ ^^ jjg|j^ thcmselves, but which may be rendered temporarily luminous by being placed in the presence of luminous bodies. Thus, the sun, or a candle, renders objects in an apartment luminous, and therefore visible ; but the moment the sun or candle is withdrawn, they be- come invisible. What are trans- Transparent bodies are those which do not parent bodies? interrupt the passage of light, or which allow other bodies to be seen through them. Glass, air, and water are examples of very transparent bodies. What are Opaquc bodics are those which do not permit opaque bodies? ^ight to pass through them. The metals, stone, earth, wood, etc., are examples of opaque bodies. Transparency and opacity exist in different bodies in very different degrees. We can not clearly explain what there is in the constitution of one mass of matter, as compared with another, which fits the one to transmit light, and the other to obstruct it ; but the arrangement of the particles has undoubt- edly much influence. Strictly speaking, there is no body which is perfectly transparent, or per- fectly opaque. Some light is evidently lost in passing even through space, and still more in tr&versing our atmosphere. It has been calculated that the atmosphere, when the rays of the sun pass perpendicularly through it, inter- • By Prof. J. W. Draper. LIGHT. 295 cept from one fifth to one fourth of their light : but when the sun is near the horizon, and the mass of air through wliicla the solar raya pass is consequently vastly increased in thickness, only l-212th part of their light can reach the surface of the earth. If our atmosphere, in its state of greatest density, could be extended rather more than 700 miles from the earth's surface, instead of 40 or 50, as it is at present, the sun's rays could not penetrate through it, and our globe would roll on in darkness. Bodies, on the contrary, which are considered as perfectly opaque, will, if made sufficiently thin, allow light to pass through them. Thus, gold-leaf transmits a soft, green light. .i 643. Liffht, from whatever source it may be In what man- t • t • i-j.'l_a. ner is light derived, moves, or is propagated in straight propagated ? , . , i i • ' • , i lines, so long as the medium it traverses is uniform in density. If we admit a sunbeam through a small opening into a darkened chamber, the path which the light takes, as defined by means of the dust floating la the air, is a straight line. ,„^ , ^. , It is for this reason that we are unable to see through a What practical applications are bent tube, as we can through a straight one. made of the jjj taking aim, also, with a gun or arrow, we proceed upon movement of o i i o i *- * li'^iuin straight the supposition that light moves in straight lines, and try to ^""^^ ^ make the projectile go to the desired object as nearly as pos- able by the path along which the hght comes from the object to the eye. FiO, 229. Thus, in Fig. 229, the hne A B, which represents the line of sight, is also the direction of a line of light passing in a perfectly straight direction from the object aimed at to the eye of the marksman. A carpenter depends upon this same principle for the purpose of determin- ing the accuracy of his work. If the edge of the plank be straight and uni- form, the light from all points of its surface will come to the eye regularly and uniformly ; if irregularities, however, exist, they will cause the light to ba irregular, and the eye at once notices the confusion and the point which oc- casions it. What is a ray ^44. A ray of light is a line of particles of ofught? light, or the straight line along which light passes from any luminous body. A luminous body is said to radiate its hght, because the light issues from it in every direction in straight linoe. 296 WELLS'S NATURAL PHILOSOPHY. Whea rays of licrht radiate from any lumin- Explain the i i .1 t r \-i divergence of ous Docly, tliGy aiver2;e irom one another, or rays of light. , J5 J ^ J they spread over more siDace as they recede from their source. Fig. 230 represents the manner of the diverg- Fig. 230. ence. What is the law Thc siirfaces covered, or «f divergence? iHuminatcd by rays of light diverging from a luminous cen- ter, increase as the squares of the distances. Thus, a candle placed behind a window will illuminate a certain space on the wall of a house opposite. If the wall is twice as for from the candle as from the window, the space illuminated by it will be four times as large as the window. If the wall be removed to three times the distance, the surface covered by the rays of light will be nine times as large, and so on. A collection of radiating rays of light, as shown in Fig. 230, constitutes what is called a "pencil of light." A thousand, or any number of persons, are able to see the great numher same object at tho same time, because it throws off from its of persons able surface an infinite number of rays in all directions ; and ono to see the same -^ ' object at the person sees one portion of these rays, and another person same time? another. Any number of rays of light are able to cross each other, in the same space, without jostling or interfering. If a small hole be made from one room to another through a thin screen, any number of candles in one room will shino through this opening, and illuminate as many spots in the other room as there are candles in this, all tlieir rays crossing in tho same opening, without hinder- ance or diminution of intensity ; just as sounds of different character proceed through the air and communicate to the ear, each its own particular tone, without materially interfering with each other. Eays of light which continually separate as When are rays *' "^ . ^ x Baid to be di- they procccd irom a luminous source, are called verfing; and Diverging Rays. Rays wliich. continually ap- *"*" ^ proach each other and tend to unite at a com- mon point, are called Converging Rays. Rays which move in parallel lines, are called Parallel Rays. What is a 645. When rays of light, radiated from a shadow? luminous point, through the surrounding space, encounter an opaque body, they will (on account of their transmission in straight lines) be excluded, from LIGHT. 297 the space behind sucli a body. The comparative dark- ness thus produced is called a shadow. ■When tlie light-giving surface is greater than the body casting the shadow. -i^cross section of the shadow thrown upon a plane surface will be less than the body ; and less, moreover, the further this surface is from the body, for the shadowed space terminates in a point. TrVTien the luminous center is smaller than the opaque body casting the shadow, the shadow will gradually increase in size \A'ith the distance, without limit ; thus the shadow of a hand held near a candle, and between a candla and the wall, is gigantic. If the shadow of any object bo thrown on a wall, the closer circumstances the opaque bod}' is held to the light-producing center, as a will thetize of candle, for example, the larger will be its shadow. The rea- a shadow be ' ^ ' ° incrpafted or son of this is, that the rays of light diverge from the center dimims ed ^ straight lines, Uke lines drawn from the center of a circle ; and therefore the nearer the object ■ * ■ is held to the center, the greater the number of rays it intercepts. Thus, in Fig. 231, the arrow A, held close to the candle, intercepts a large number of rays, and produces the shadow B F; while the same ar- row held at C, intercepts a smaller number of rays, and produces only the little shadow D E. "W'heu two or more luminous ob- — ^ ' ■~~-.^ i jccts, not in the same straight line, '~~-J shine upon the same object, each one ^ will produce a shadow. 646. The intensity of li^ht which issues How does the .,...,.■■ intensity of from a luminous point diminishes in the same light vary? . •■■ pit /• proportion as the square oi the distance irom the luminary increases. Thus, at a distance of two feet, the intensity of light will be one fourth of "what it is at one foot ; at three feet the intensity will be one ninth of what it is at one foot. In other words, the amount of illumination at the distance of one foot from a single candle would be the same as that from four, or nina candles at a distance of two or three feet, the numbers four and nine being the squares of the distances two, and three, from the center of illumination. Upon what ^^^- ^his law, therefore, may be made available for meas- principle may urmg the relative intensities of light proceeding from different tensities of sourccs. Thus, in order to ascertain the relative quantities of different lu- jigij^ furnished by two different candles, as, for example, a be ascertained? wax and a tallow caudle, placa two discs or sheets of whit« 13* 298 WELLS'S NATUEAL PHILOSOPHY. paper, a few feet apart on a wall, and throw the light of one candle on one disc, and the light of the other candle upon the other disc. If they are of unequal illuminating power, the candle which affords the most hght must be moved back until the two discs are equally iOuminated. Then, by meas- uring the distance between each candle and the disc it illuminates, the lum- inous intensities of the two candles may be calculated, their relative intensi- ties being as the squares of their distances from the illuminated discs. If, when the discs are equally illuminated, the distance from one candle to its disc is double the distance of the other candle from its disc, then the first candle is four times more luminous than the second ; if the distance be triple, it is nine times more luminous, and so on. Instniments called "Photometers," operating in a similar manner, have also been constructed for measuring the relative intensity of two luminous bodies. Their arrangement and plan of operation is substantially the same as in tho method described. 648. The light of the sun greatly exceeds iu most intense iiitensitv that deiived from any other lumin- Ught known? , ; *' ous body. The most brilliant artificial lights yet produced, are very far inferior to tho Bplendor of the solar liglit, and when placed between the disc of the sun and the eye of the observer, appear as black spots. Dr. Wollaston has calculated that it would require twenty thousand mil- lions of the brightest stars like Sirius to equal the light of the sun, or that that orb must be one hundred and forty thousand times further from us than he is at present, to bo reduced to the illuminating power of Sirius. The light of the full moon has also been estimated as three hundred thou- sand times less intense than that of the sun. During the day the intensity of the sun's light is so great as to entirely eclipse that of the stars, and render them invisible ; and for the same reason, we only notice the light emitted by fire-flies and phosphorescent bodies in the dark. Are the more- ^^9. Light docs Hot pass instantancously Sst"antancofs? through space, but requires for its passage from one point to another a certain interval of time. With what re- The velocity of light is at the rate of about trarrir"^^ ^ One hundred and ninety-two thousand miles in a second of time. .„^ , ., Light occupies about eight minutes in traveling from the What are il- ° ^ ° ^ lustrations of sun to tlie earth. To pass, however, from the planet r'-'ht^?'^"''''^ "^ Uranus to tho earth, it would require an interval of three hours. Tlie time required for light to traverse the space intervening between the nearest fixed star and the earth, has been estimated at SJyears ; and from the farthest nebulae, a period of ssveral hundred years would be requisite, so LIGHT. 290 immense is their distance from our earth. If, therefore, one of the remote fixed Btars were to-dav blotted from the heavens, several generations on the earth •would have passed away before the obliteration could be known to man. The following comparison between the velocity of light and the speed of a locomotive engine has been instituted : — Light passes from the sun to the eartli in about eight minutes ; a locomotive engine, traveling at the rate of a mile in a minute, ■would require upward of one hundred and eighty years to accomplish the same journey. Who first as- 650. The velocity of light was first deter- rei^ityofu<^ht? mined by Yon Roemer, an eminent Danish astronomeij from observations on the satellites of Jupiter. Ex lain th ^^® method by •which Ton Roemer arrived at this result method by may be explained as follows : — The planet Jupiter is sur- l^citT of li^ht rounded by several satellites, or moons, which revolve about was deterniined it in certain definite times. As they pass behind the planet, of'°Jupiter''sKiU ^^^Y disappear from the sight of an observer on the earth, or ellites. in other words, they undergo an eclipse. The earth also revolves in an orbit about the sun, and in the course of its revolution is brought at one time 192 millions of miles nearer to Jupiter than it is at another time, when it is in the most remote part of its orbit. Suppose, now, a table to be calculated by an astronomer, at the time of year when the earth is nearest to Jupiter, showing, for twelve successive months, the exact moment when a particular satellite would be observed to be eclipsed at that point. Six months afterward, when the earth, in the course of its revolution, has attained a point 192 millions of miles more remote from Jupiter than it formerly occupied, it would be found that the echpse of the satellite would occur sixteen minutes, or 960 second?, later than the calculated time. This delay is occasioned by the fact that the light has had to pass over a greater distance before reaching the earth than it did when the earth was in the op- posite part of its orbit, and if it requires sixteen minutes to pass over 192 mil- lions of miles, it will require one second to move over 200,000 miles. "When, on the contrary, the earth at the end of the succeeding six months has as- sumed its former position, and is 192 millions of miles nearer Jupiter, the eclipse will occur sixteen minutes earlier, or at the exact calculated time given in the tables. The velocity of light, therefore, in round nimibers, maybe con- sidered as 200,000 miles per second.* A more exact calculation, founded on perfectly accurate data, gives as the true velocity of hght 192,500 miles per Becond. • The explanation ahove given ■will be made clear by reference to the following dia- gram. Fig. 23'2. S represents the sun, a b the orbit of the earth, and T T' the position of the earth at different and opposite points of its orbit. J represents Jupiter, and E its satellite, about to be eclipsed bv passing within the shadow of the planet. Now the lima of the commencement or termination of an eclipse of the satellite, is the instant at which the satellite would appear, to an observer on the earth, to enter, or emerge from tho 300 WELLS'S NATURAL PHILOSOPHT. Several other plans have been devised for determining the velocity of lights the results of whicli agree very nearly with those obtained by the observationt, on the sateUites of Jupiter.* ■vThen is light 651. When a ray of light strikes against a reflected? surfacc, and is caused to turn back or rebound in a direction different from whence it proceeded, it is said to be reflected. ■What is ab- ^52. "When rays of light are retained upon fr-ht'?'"^ °^ "^^^ surface upon which they fall, they are said to be absorbed ; in consequence of which their presence is not made sensible by reflection. The question as to what becomes of the light which is absorbed by a body, can not be satisfactorily answered. In all probability it is permanently re- tained within the substance of the absorbing body, since a body which absorbs light by continued exposure, does not radiate or distribute it again in any way, as it might do if it had absorbed heat. shadow of the planet. If the transmission of light were instantaneous, it is obvious that an observer at T', the most remote part of the eartli's orbit, would see the eclipse begia and end at the same moment as an observer at T, the part of the e.arth's orbit nearest to Jupiter. This, however, is not the case, but the observer at T' sees the eclipse 960 sec- onds later than the observer at T ; and as the distance between these two stations is 198 millions of miles, we have, as the velocity of light in one second, 192,000,O00-r-96O = 200,000. Fig. 232. ♦ A very ingenious plnn was devised a few years since by M. Fizeau of Paris, by which the velocity of artificial light was determined and found to agree with that of solar light. A disc, or wheel, carrying a certain number of teeth upon its circumference, was made to revolve at a known rate : placing a tube behind these, and looking at the open spaces be- tween the teeth, they become less evident to sight, the greater the velocity of the moving wheel, until, at a certain speed, the whole edge appears transparent. The rate at which the wheel moves being known, it is easy to determine the time occupied while one tooth passes to take the place of the one next to it. A ray of light is made to traverse many miles throngh space, and then passes through the teeth of the revolving disc. It moves the whole distance in just the time occupied iu the movement of a single tooth to theplao» of another at a certain speed. BEFLECTION OF LIGHT. 301 SECTION I. REFLECTIOX OF LIGHT. Tnjat ocmrs ^^3. When rays of li^ix^t fall upon any sur- nponauy'^M! ^^^e, tliGy mav be reflected, absorbed, or '^***' transmitted. Only a portion of the light, however, "which meets any surface is reflected, the remain- der being absorbed, or transmitted. When does a 654. When the portion of light reflected vwteand^en ^^om anv surface, or point of a surface, to the ^'^^ eye is considerable, such surface, or point, ap- pears -white ; when very little is reflected, it appears dark- colored; but when all, or nearly all the rays are absorbed, and none are reflected back to the eye, the surface appears black. Thus, chjwcoal 13 black, because it absorbs all the light which falls upon it, and reflects none. Such a bodj can not be seen unless it is situated near other bodies which reflect light to it. According to a variation in the manner of reflecting light, the same surface •which appears whits to an eye in one position, may appear to be black from another point of view, as frequently happens in the case of a mirror, or of any other bright, or reflecting surface. ■What are good Dcusc bodics, particularly smooth metals, re^flectors of reflect light most perfectly. The reflecting power of other bodies decreases in proportion to their porosity. How are non- ^^5. All bodics uot in thcmselves luminous, J^XTd ^^^' become visible by reflecting the rays of light. It is by the irregular reflection of light that most objects in nature are rendered visible ; since it is by rays which are dispersed from re- flecting surfaces, irregularly and in every direction, that bodies not exposed to direct light are illuminated. If light were only reflected regularly from the surface of non-luminous bodies, we should see merely the image of the lumin- ous object, and not the reflecting surface.* In the day-time, the image of the Bun would be reflected from the surface of all objects around us, as if they ■were composed of looking-glass, but the objects themselves would be invisi- ble. A room in which artificial li:rhts were placed would reflect these lights from the walls and other objects as if they were mirrors, and all that would, be visible would be the multiplied reflection of the artificial lights. • In a very good mirror we scarcely perceive tlie reflecting surface interyening betireen ns and the images it shows us. 802 WELLS'S NATURAL PHILOSOPHY. t ff 1 1 "^^^ atmosphere reflects light irregularly, and every particle the atmosphere of air is a luminous center, which radiates light in every direc- upon the diffu- tj^Q^ -^g^g j^ not for this, the sun's light would only illumi- Bion of light? ' ° •' nate those spaces which are directly accessible to its rays, and darkness would instantly succeed the disappearance of the sun below the horizon, 05Q. Any surface which possesses the power of reflecting light in the highest degree is called "What is Mirror ? a Mirror. Into how many ' classes are mir- rors divided ? Mirrors are divided into three general classes, without regard to the material of which they con- sist, viz.. Plane, Concave, and Convex Mirrors. These three varieties of mirrors are represented in Fig. 233; A, being plane, like an ordinary' looking-glass; B, concave, like the inside of a watch-glass ; and C, convex, like the outside of a watch-glass. AVhat is the 657. Whcu light falls upon fi^r'rcfle'ction » plane and polished surface, ©flight? ^]jQ angle of reflection is equal to the angle of incidence. This is the great general law which governs the reflec- tion of light, and is the same as that which governs the motion of elastic bodies. " Thus, in Fig. 234, let A B be the direction of an inci- dent ray of light, falling on a mirror, F C. It will be reflected in the direction B E. If we draw a line, D B, perpendicular to the surface of the mirror, at the point of reflection, B, it will be found that the angle of incidence, A B D, is precisely equal to the angle of reflection, E B D. The same law holds good in regard to every form of surface, curved as well as plane, since a curve may be supposed to be formed of an infinite num- ber of little planes. Thus, in Fig. 235, the incident ray, E 0, falling upon the concave surface, a C b, will still be reflected, in obedience to the same law, in the direction C D, the angle being reckoned from the perpendicular to that point of the curve where the incident ray falls. The Bame will also be true of the convex surface, A G B. REFLECTION OF LIGHT. 303 whatismeant 658. An image, in optics, is the figure of by an image ? ^jjy object made by rays proceeding from the several points of it. Tvhatisacom 659. A common looking-glass consists of a ^assT *"" '°°' glass plate, having smooth and parallel sur- faces, and coated on the back with an amalgam* of tin and quicksilver. „ ,^ . The images formed in a common looking-jrlass How are the im- ^ " . ages formed in arc mainlv Droduccd by the reflection of the * looking-glass 7 n \ r rays of light irom the metallic surface attached to the back of the glass, and not from the glass itself. The effect may be explained as follows: — A portion of the light incident upon the anterior surface is regularly reflected, and another portion irregu- larly. The first produces a very faint image of an object placed before the glass, whUe the other renders the surface of the glass itself visible. Another, and much greater portion, however, of the liglit falling upon the anterior sur- face passes into the glass and strikes upon the brilliant metallic coating upon the back, from which it is regularly reflected, and returning to the eye, pro- duces a strong image of the object. There are, therefore, strictly speaking, two images formed in every looking-glass — the first a faint one by the light reflected regularly from the anterior surface, and the second a strong one by the light reflected from the metaUic surface ; and one of these images will be before the other at a distance equal to the thickness of the glass. In good mirrors, the superior brilliancy of the image produced by the metallic surfaca will render the faint image produced by the anterior surface invisible, but in glasses badly sQvered, the two images may be easily seen. If the surfaces of the mirror could be so highly polished as to reflect regu- larly all the light incident upon it, the mirror itself would be invisible, and the observer, receiving the reflected light, would perceive nothing but the images of the objects before it. This amount of polish it is impossible to effect arti- ficially, but in many of the large plate-glass mirrors manufactured at the pres- ent time, a high degree of perfection is attained. Such a mirror placed ver- tically against the wall of a room, appears to the eye merely as an opening leading into another room, precisely similar and similarly furnished and illum- inated ; and an inattentive observer is only prevented from attempting to w^alk through such an apparent opening by encoiintering his own image iS he approaches it. 660. A T)lane mirror only chanjojes the direc- In what man- /. , /• i • i i • i . n ner does a tiou 01 tlic rays 01 liglit wliich tail upon it, plane minor .*^ ,.,. . . ' reflect rays of without altering their relative position. If '' they fall upon it perpendicularly, they will be * An amalgam is a mixture or compound of quicksilver and some ether metaL 304 WELLS'S NATURAL PHILOSOPHY. When will the image in a looking-glaRS appear distort- ed? Bow is an ap- parent change of place caused bj reflection f reflected perpendicularly ; if they fall upon it obliquely, they will be reflected obliquely ; the angle of reflection being always equal to the angle of incidence. If the two surfaces of mirrors are not parallel, or uneven, then the rays of light falling upon it will not be reflected regu- larly, and the image will appear distorted. 661. We always seem to see an object in the direction from which its rays enter the eye. A mirror, therefore, which, by reflection; changes the direction of the rays proceeding from an object, will change the apparent place of the object. Thus, if the rays of a candle fall obliquely upon a, mirror, and are reflected to the eye, we shall seem to see the candle in the mirror in the direction in which they proceed after reflection. If we lay a looking-glass upon the floor, with its face uppermost, and place a candle beside it, the image of the candle will be seen in the mirror, bj' a person standing opposite, as inverted, and as much below the surface of the glass as the candle itself stands above the glass. The reason of this is, that the incident rays from the candle which fall upon the mirror are reflected to the eye in the same ^^^- 236. direction that they would have taken, had they really come from a candle situated as much below the sur- face of the glass, as the first candle was above the surface. This fact will be clearly shown by re- ferring to Fig. 236. "When we look into a plane mirror (the common looking-glass) the rays of light whicli proceed from each point of our body before the mirror will, after reflection, proceed as if they came from a point holding a corresponding posi- tion behind the mirror ; and therefore produce the same eflect upon the eye of the observer as if they had actually come from that point. The imago in the glass, consequently, appears to be at the same distance behind the surface of the glass, as the object is before it. Let A, Fig. 237, be anv point of a visible object placed before a looking- glass, M N. Let A B and A 0 be two rays diverging from it, and reflected from B and C to an eye at 0. After reflection they will proceed as if they had issued from a point, a, as far beliind the surflice of the looking-glass as A is before it — that is to say, the distance A N will be equal to the distance N a. REFLECTION OF LIGHT. 305 For this reason our reflection in FiG. 2 ST. a mirror seems to approach us when ■we walk toward it, and to retire from us as we retire. Upon the same principle, when trees, buildings, or other objects are reflected from the horizontal surface of a pond, or other smooth sheet of water, they appear in- verted, since the light of the object, reflected to our eyes from the surface of the water, comes to us with the same direction as it would have done, had it proceeded directly from an inverted object in the water. In Fig. 238, the light proceed- YiQ 238 ^^o ^'"0^ the arrow-head. A, strikes the water Q at F, and is reflected to D, and that from A ^Z the barb, B, strikes the water at E, and is ..■■'X reflected to C. A spectator standing at G will see the reflected rays, E G and F G, as if they proceeded directly from C and D, and the image of the arrow will appear to be lo- cated at C D. It is in accordance with the law that tho angles of incidence are equal to the angles of reflection, that a person is enabled to see his whole figure reflected from the surface of a comparatively small mirror. Thus, in Fig. 239, let a person, C D, Fig. 239. be placed at a suitable distance from a mir- ji J. ror, A B. The rays of hgbt, C A. proceed- ing from the head of the person, fall perpen- dicularly upon the mirror, and are therefore reflected back perpendicularly, or in tho same line; the rays B D proceedmg from the feet, however, fall obliquely upon the mirror, and are therefore reflected obliquely, and reach the eye in the same direction they would have taken had they proceeded from the point F behimd the mirror. 662, The quantity of light reflected from a given surface, is not the same at all angles, or inclinations. When the angle or inclination with which a ray of light strikes upon a reflecting surface is great, the amount of light reflected to the eye will be Is tho Baine quantity of light reflected at all angles? 306 WELLS'S NATURAL PHILOSOPHY. considerable ; when the angle, or inclination is small, the amount of light reflected will be diminished. Thus, for example, when light falls perpendicularly upon the surface of glass, 25 rays out of 1,000 are returned; but when it falls at an angle of 85°, 550 rays out of 1,000 are returned. Thus, a surface of unpohshed glass produces no image of an object by re- flection when the rays fall on it nearly perpendicularly ; but if the flame of a candle be held in such a position that the rays fall upon the surface at a veiy small angle, a distinct image of it will be seen. We have in this an explanation of the fact, that a spectator standing upon the bank of a river sees the images of the opposite bank and the objects upon \t reflected in the water most distinctly, while the images of nearer objects are seen imperfectly, or not at all. Here the rays coming from the distant objects strike the surface of the water very obliquely, and a sufBcient number are reflected to make a sensible impression upon the eye ; while the rays pro- ceeding from near objects strike the water with little obliquity, and the hght reflected is not sufBcient to make a sensible impression upon the eye. This fact may be clearly seen by reference to Fig. 240. Fig. 240. Let S be the position of the spectator ; 0 and B the position of distant objects. The rays 0 R and B R which proceed from them, strike the surface of the water very obliquely, and the hght which is reflected in the direction R S is sufficient to make a sensible impression upon the eye. But in regard to objects, such as A, placed near the spectator, they are not seen reflected, Jjecause the rays A R' which proceed from them strike the water with but little obliquity ; and consequently, the part of their light which is reflected in the direction R' S, toward the spectator, is not sufficient to produce a seo* sible impression upon the eye. What is the 663. If an object be placed between two pa?aiiei°*^pune V^^^^' mirrors, each will produce a reflected mirrors? image, and will also repeat the one reflected by the other — the image of the one becoming the object for the other. A great number of images are thus pro- =^7/ Iff- REFLECTION OF LIGHT. 307 duced, and if the light were not gradually weakened hy loss at each successive reflection, the number would be in- finite. If the mirrors are placed so as to form an angle -with each other, the num- ber of mutual reflections will be diminished, proportionably to the extent of the angle formed by the mirrors. _, .^ ,, The construction of the optical instrument called the Kalei- Descnbe the , . _ Kaleidoscope. doscope IS based simply upon the multiplication of an image by two or more mirrors incUned toward each other. It con- sists of a tube containing two or more narrow strips of looking-glass, which run through it lengthwise, and are generally inclined at an angle of about 60°. If at one end of the tube a number of small pieces of colored glass and other similar objects are placed, they will be reflected from the mirrors in such a way as to form regular and most elegant combinations of figures. An endless variety of symmetrical combinations may be thus formed, sinco every time the instrument is moved or shaken the objects arrange themselves differently, and ^ new figure is produced. _, , ^, Upon the surface of smooth water the sun, when it is nearly Why does the ^ ' ^ Bun appear at vertical, 33 at noon, appears to shine upon only one spot, noon to shine ^U the rest of the water appearing dark. The reason of this point upon the is, that the rays fall at various degrees of obliquity on the water ? Water, and are reflected at similar angles ; but as only those which meet the eye of the spectator are visible, the whole sur- face will appear dark, except at the point where the reflection occurs. jijQ 241 Thus, in Fig. 241, of the rays S A, S B, and S C, only the ray S C meets the eye of the specta- tor, D. The point C, therefore, will appear luminous to the spec- tator D, but no other part of tho surface. Another curious optical pheno- D menon is seen when the rays of the sun, or moon fall at an angle upon the surface of water gently agitated by the wind. A long, tremulous path of light seems to be formed toward the eye of the spectator, while all the rest of the surflice appears dark. The reason of this appearance is, that every little wave, in an extent perhaps of miles, has Bome part of its rounded surface with the direction or obliquity which, accord- ing to the required relation of the angles of incidence and reflection, fits it to reflect the light to the eye, and hence every wave in that extent sends its mo- mentary gleam, which is succeeded by others. 308 WELLS'S NATURAL PHILOSOPHY. Row are paral lei rays reflect ed from a con- cave mirror? Fig. 242. What Is a Con- 664. A concave mirror may be considered as the interior surface of a portion, or segment of a liollow sphere. This is clearly shovra in Fig. 242. A concave mirror may be represented by a bright spoon, or the reflector of a lantern. When parallel rays of light fall upon the surface of a concave mirror, they are reflected and caused to converge to a point half way between the center of the surface and the center of the curve of the mirror. This point in front of the mirror is called the principal focus of the mirror. Thus, in Fig. 242, let 1, 2, 3, 4, etc., be parallel rays falling upon a concave mir- ror; they will, after reflection, be found con verging to the point o, the principal focus, which is situated half way between the center of the surface of the mirror and the geometrical center of the curve of the mirror, a. GQ5. Concave mirrors are some- times designated as "Burning Mirrors," since ' "' the rays of the sun which fall upon them parallel, are re- flected and converged to a focus (fire-place), where their light and heat are increased in as great a degree as the area of the mirror exceeds the area of the focus.'-' 6QQ. Diverging rays of light issuing from a luminous body placed at the center of the curve of a concave spherical mirror, will be reflected back to the same point from which they diverged. • A burning mirror, 2n inches in diameter, constructed of plaster of Paris, gilt and bur- nished, has been found capable of igniting tinder at a distance of 50 f(>et. It is related that Archimedes, the philosopher of Syracuse, employed burning mirrors 200 years before the Christian era, to destroy the besieging nary of Marcellus, the Roman consul ; hi« mirror was probably constructed of a great number of flat pieces. The most remarkable experiments, however, of this nature, were made by Buffon, the eminent French natural- ist, who had a machine composed of 168 small plane mirrors, so arranged that they all reflected radiant heat to the same focus. By means of this combination of reflecting sur- faces he was able to set wood on fire at the distance of 209 feet, to melt lead at 100 feet, and silver at 50 feet. Why are con- cave mirrors called burning mirrors ? ^ /■ A , / /A ' c^ \ ■ ' 'W «-' « \\^ : -^^ 4/ \ In what man- ner are diverg- ing rays re- flected from a concave mir- ror? BEFLECTION OF LIGHT. 309 Fig. 243. Tbus, if A B, Fig. 243, were a concave spheri- cal mirror, of whicli C were the center, rars issu- ing from C would, in obedience to the law that the angle of incidence and reflection are equal, meet again at C. Diverging rays falling on a spheri- cal concave mirror, if they issue from the principal focus, half way between the center of the sur- face and the center of the curve of the mirror, will be re- flected in parallel lines. Thus, in Fig. 244, if F represent a can- ' pj^, 244 die placed before a concave mirror, ABC, half way between the center of its surface, B, and the center of its curve, C, its rajs, falling upon the mirror, will be reflected ^" in the paraUel lines d ef g h. This principle is taken advantage of in the arrangement of the illuminating and reflecting apparatus of light-houses. The lamps are placed before a concave mirror, in its principal focus, and the rays of light proceeding from them are reflected parallel from the surface of the mirror. ■^hen the rays issue from a point, P, Fig. 245, beyond the center, C, of the curve of the mirror, they will, after reflection, converge to a focus, / between the principal focus, F, and the center of the curve, C. On the contrary, if the rays issue from a point between the principal focus, F, and the surface of the mirror, they will diverge afl:er reflection. 667. Images are formed by concave mirrors in the same manner as by plane ones, but they are of difterent size from the object, their gen- being to produce an image larger than the Fig. 245. How are images formed by con- cave mirrors? eral effect object. "When an object is placed between a concave image formed mirror and its principal focus, the image will mirror b^"iSlg! appear larger than the object, in an erect posi- tion and behind the mirror. This will be apparent from Fig. 246. Let a be an object situated within the focus of tiie mirror. The rays from its extremities will fall divergent on the mirror, and be reflected less divergent to the eye at b, 310 •WELLS'S NATURAL PHILOSOPHY. Fig. 2-4G. as though they proceeded from an ob- ject behind the mirror, as at h. To an eye at b also, the image will appear larger than the object a, since the anglo of vision is larger. If the rays proceed from a distant body, as at E D., Fig. 247, beyond the cen- ter, C, of a spherical concave mirror, A B, they will, after reflection, be converged to a focus in front of the mirror, and some- what nearer to the center, C, than the prin- upon any Fig. 24T. cipal focus, and there paint substance placed to receive it, an im- age inverted, and smaller than the object ; this image will be very bright, as all the light incident upon the mirror will be gath- ered into a small space. As the object approaches the mirror, the imago recedes from it and approaches C ; and when situ- ated at C, the center of the curve of the mirror, the image will be reflected as large as the object : when it is at any point between C and / supposing / to be the focus for parallel rays, it will be reflected, enlarged, and more dis- tant from the mirror than the object, this distance increasing, until the ob- ject arrives at/, and then the image becomes infinite, the rays being reflected parallel* 668. When an object is further from the surface of a concave mirror than its principal focus, the image will appear inverted ; but when the object is between the mirror and its principal focus, the image will be upright, and increase in size in proportion as the object is placed nearer to the focus. The fact that images are formed at the foci of a concave mirror, and that by Tarying the distance of objects before the surface of the mirror, we may vary the position and size of the Jmages formed at such foci, was often taken ad- vantage of in the middle ages to astonish and delude the ignorant. Thua, the mirror and the object being concealed behind a curtain, or a partition, and the object strongly illuminated, the ra^s from the object might be reflected from the mirror in such a manner as tc ^>ass through an opening in the screen, and come to a focus at some distance beyond, in the air. If a cloud of smoka * In all the cases referred to, of the reflection of light from concave mirrors, the aper- ture or curvature of the mirror is presumed to be inconsiderable. If it be increased be- yond a certain limit, the rays of light iacident upon it are modified la their reflectioa from its BurfacD. When will the images reflect- ed from a con- cave mirror appear invert- ed, and when erect f REFLECTION OF LIGHT. 311 from burning incense were caused to ascend at this point, an image would be formed upon it, and appear suspended in the air in an apparently supernatural manner. In this way, terrifying apparitions of skulls, daggers, etc., were produced. 669. A Convex Mirror may be considered as any given portion of the exterior surface of Whit is a Con vex JUrror ? a sphere. Where is the principal focus of a convex mirror? The principal focus of a convex mirror lies as far behind the reflecting surface as in con- cave mirrors it lies before it. (See § 664.) The focus in this case is called the virtual focus, because it is only an imaginary point, toward which the rays of reflection appear to be directed. Thus, let a b c d e, Fig. 2-48, be parallel rays incident upon a convex mirror, A B, whose center of curvature is C. These rays are reflected diverg- ent, in the directions a b' c d' e, as though they proceeded from a point, F, behind the mirror, corresponding to the focus of a concave mirror. If the point C be the geometrical center of the curve of the mirror, the point F will be half way between C and the surface of the mirror ; as this focus is only apparent, it is called the virtual focus. Rays of light falling upon a convex mirror, diverging, are rendered still more divergent by reflection from its surface ; and convergent rays are reflected, either parallel or less con- FiG. 249. 670. The general efi'ect of convex mirrors is to produce an image smaller than the object itself. 249, let D E be an object placed before a convex mirror, A B ; the rays proceed- ing from it will be reflected from the convex sur- face to the eye at H K, as though they proceeded from an object, d e, behind the mirror, thus pre- senting an image smaller, erect, and much nearer the mirror than the object. How are di- Terging and conver^ng rays reflected from a convex mirror i vergent. What is the n.iture of the Images formed by convex mir- rors? Thus, in Fij 312 WELLS'S NATURAL PHILOSOPHY. Thus the globular bottles filled with colored liquid, in the window of a drug-store, exhibit all the variety of moving scenery without, sach as car- riages, carts, and people moving in diflerent directions: the upper half of each bottle exhibiting all the images inverted, while the lower half exhibits another set of them in the erect position. Convex mirrors are sometimes called dispersing mirrors, as all the rays of light which fall upon them are reflected in a diverging direction. What is ca- 671. That department of the science of toptrics? optics which treats of reflected light, is often designated as Catoptpjcs. SECTIONII. REFRACTION OP LIGHT. What is meant Light traverscs a given transparent sub- by the retrac- stancc, such as air, water, or o^lass, in a straight tion of light? . ' . ' , line, provided no reflection occurs and there is no change of density in the composition of the medium ; but when light passes obliquely from one medium to an- other, or from one part of the same medium into another part of a difterent density, it is bent from a straight line, or refracted. What is a me- 672. A mcdium, in optics, is any substance, dium in optics ? gQ^jj^ Hquid, or gaseous, through which light can pass. A medium, in optics, is said to be dense or rare, according to its power of refracting light, and not according to its specific gravity. Thus alcohol, olive oil, oil of turpentine, and the like substances, although of less specific gravity than water, have a greater refractive power ; they are, therefore, called denser media than water. 673. The fundamental laws which govern the refraction of light may be stated as follows ; What laws srov- Whcu light passcs from one medium into tion*/u-Mr' another, in a direction perpendicular to the surface, it continues on in a straight line, with- out altering its course. When light passes obliquely from a rarer into a denser medium, it is refracted toward a perpendicular to the surface, and this refraction is in- creased or diminished in proportion as the rays fall more or less obliquely upon the refracting surface. REFRACTION OF LIGHT. 313 Fig. 250. Fig. 251. Wliea liglit passes obliquely out of a denser into a rarer medium, it passes through the rarer medium in a more oblique direction, and further from a perpendicular to the surface of the denser medium. Thus, in Fig. 250, suppose n m to represent the surface of water, and S 0 a ray of hght striking upon its surface. When the ray S 0 enters tho water, it will no longer pursue a straight course, but will be refracted, or bent toward the perpen- dicular line, A B, in the direction S 0. The denser the water or other fluid may be, the more the ray S 0 H will be refracted, or turned toward A B. If, on the contrary, a ray of light, H 0, passes from the water into the air, its direction after leaving the water will be further from the perpendicular A 0, in the direction 0 S. The effects of the refraction of light may be illustrated by the fol- lowing simple experimenc : — Let a coin or any other object be placed at the bottom of a bowl, as at m, Fig. 251, in such a manner that the eye at a can not perceive it, on ac- count of the edge of the bowl which intervenes and obstructs tlie rays of light. If now an attendant care- fully pours water into the vessel, the coin rises into view, just as if the bottom of the basin had been elevated above its real level. This is owing to a refraction by the water of the rays of light proceeding from the coin, whicli are tliereby caused to pass to tho eye in the direction i i. The image of the coin, therefore, appears at n, in tho direction of these rays, instead of at m, its true position. A straight stick, partly immersed in water, appears to be broken or bent at the point of immereion. This is owing to the fact that the rays of light proceeding from the part of the stick contained in the water are refracted, or Fig. 252. caused to deviate from a straight line as they pass from the ■n-ater into the air ; consequently that portion of the stick immersed in the water will appear to be lifted up, or to be bent in such a manner aa to form an angle -with the part out of the water. The bent appearance of the stick in water is represented in Fig. 252. For the same reason, a spoon in a glass of water, or an oar partially immersed in water, always ap- pears bent. On account of this bending of light from objects under water, a person who endeavors to strike a fish with a spear, must, unless directly above the fish, 14 '314 WELLS'S NATURAL PHILOSOPHY. aim at a point apparently below it, otherwise the weapon will miss, by pass- ing too high. A river, or any clear water viewed obUquely from the bank, appears more shallow than it really is, since the hgbt proceeding from the objects at tho bottom, is refracted as it emerges from the surface of the water. Tho depth of water, under such circumstances, is about one third more than it appears, and owing to this optical deception, persons in bathing are liable to get be- yond their depth. Lio;ht, on enterinor tbe atmosphere, is re- Whatisatmos- /. , . -, , . pheric refrac- iractecl Id a greater or less degree, m propor- tion to the density of the air ; consequently, as that portion of the atmosphere nearest the surface of the earth possesses the greatest density, it must also possess the greatest refractive power. _,, ^ _ , , From this cause the sun and other celestial bodies are never Wliat effect has refraction upon seen in their true situations, unless they happen to be verti- tii" J"u v'°" °^ ^^^ ' ^^^ ^^^^ nearer they are to the horizon, the greater will be the influence of refraction in altering the apparent place of any of these luminaries. This forms one of the sources of error to be allowed for in all astronomical observations, and tables are calculated for finding the amount of refraction, depending on the apparent altitude of the object, and the state of the barom- eter and thermometer. When the object is v6rtical, or nearly so, this error is hardly sensible, but increases rapidly as it approaches the horizon ; so that, in the morning, the sun is rendered visible before he has actually risen, and in the evening, after ho has set. For the same reason, morning does not occur at the in- what 13 the gtant of the sun's appearance above the horizon, or night light? set in as soon as he has disappeared below it. But both at morning and evening, the rays proceeding from the sun below the horizon are, in consequence of atmospheric refraction, bent down to the surface of the earth, and thus, in connection with a reflect- ing action of the particles of the air, produce a lengthening of the day, termed twilight. 1 hat man- ^^ ^'''^ density of the air diminishes gradually upward from ner is lii^ht re- the earth, atmospheric refraction is not a sudden change of ataw^phere?*'^ direction, as in the case of the passage of light from air into water, but the ray of light actually describes a curve, being refracted more and more at each step of its progresa This applies to the light received from a distant object on the surface of the earth, which is lower or higher than the eye, as well as to that received from a celestial ob- ject, since it must pass through air constantly increasing or diminishing in density. Hence, in the engineermg operation of leveling, this refraction must bo taken into consideration. REFRACTION OF LIGHT. 315 ^ , . ,^ 674. The application of the laws of refraction of light ac- Explain the .... . , phenomena of count for many curious deceptive appearances in the at- Mir;ige. mosphere, which are included under the general name of Mirage. In these phenomena, the images of objects far remote are seen at an elevation in the atmosphere, either erect or inverted. Thus travelers upon a desert, where the surface of the earth is highly heated by the sun, are often deceived by the appearance of water in the distance, surrounded by trees and. villages. In the same manner at sea, the images of vessels at a great distance and below the horizon, will at times appear floating in the atmosphere. Such appearances are frequently seen with great distinctness upon the great Amer- ican lakes. These phenomena appear to be due to a change in the density of the strata of air which are immediately in contact with the surface of the earth. Thus it often happens that strata resting upon the land may be rendered much hotter, and those resting upon the water much cooler, by contact with the surface, than other strata occupying more elevated positions. Rays, there- fore, on proceeding from a distant object and traversing these strata, will be unequally reflected, and caused to proceed in a cun-ilinear direction ; and in tiiis way an object situated behind a hill, or below the horizon, may be brought into view and appear suspend- ed in the air. This may be readily understood by reference to Fig. 253. Suppose the rays of light from tlis ship, S, below the horizon to reach the eye, after assuming a curvilinear direction by passing through strata of air of varj-ing density; then, as an .V1iat is the course of rays falling upon a double con- cave lens ? * A lens of this character was constnicted many years since in England, three feet in diameter, with a focal distance of si.'c feet eight inches. Exposed to the heat concentrated in the focus of this powerful instrument, the metals were instantly melted, and even vol- atilized, while quartz, flint, and the most refractory earthy substaDCos, were readily liqueiled and caused to boil. REFRACTION OF LIGHT. 323 Fig. 263. therefore, of a concave lens is not real, but virtual, as is the case with a convex mirror. Thus, in Fig. 263, the parallel raj-s, a b cde, etc., falling upoa the double concave lens, L L', are so re- fracted in passing through it, that they are made to diverge, as though proceeding from the point F, be- hind the lens. In a similar man- ner convergent ray nre rendered less convergent, or even parallel. Do convex ^82. Images are formed in the foci of con- vex lenses in the same way as in the foci of concave mirrors. Thus, if we take a convex lens and place behind it, at a proper distance, a s'lcet of paper, there will be depicted upon the pap r beautifully clear and distinct images of all the objects in front of the lens, in an inverted position. The manner in which they are formed is illustrated in Fig. 264:. Thus, let A B convex lenses jrive rise to thfi forma- tion f images? Dcscribfi the formation of ima'^es by the convex lens. Fig. 2G4. represent an ob- ject placed be- fore a double convex len.g, E F. The rays proceeding from A, the top of the object, will be converged by the lens and brought to a focus at D, where they will form an image; the rays pro- ceeding from B, the base of the object, will also be converged and brought to a focus at G ; and so. each point of the object, A B, will have its corre- sponding image between C D. In this way a complete image will be formed.. The image formed by a convex lens will ap- pear inverted, because the rays of light from the several points of the oliject cross each other in proceeding to the corresponding points of the image. Thus, in Fig. 264, the ray, A E, proceeding from the top of the object and filling obliquely upon tlio lens, is refracted into tlie course E D, and in lika manner the ray B F is refracted in the direction F C ; and as these rays cro33 Why are the Images formed hy convex lenses invert- ed? 324 WELLS'S NATURAL PHILOSOPHY. each other, the image of the arrow appears inverted. The central ray of light proceeding from the object in tlie direction of tlie axis G, and falling perpen- dicularly upon the surface of tlie lens, undergoes no refraction, but continues on in a direct course. The images thus formed hy convex lenses maybe rendered ageTforn^dby ''^'sible by being received upon white screens, or any suitable convex lenses objects, or directly by the eye, when placed in a proper posi- be made visi- ^. , • ii jjjgj tion to receive the rays. When, by the employment of the convex lens as a burning- glass, we concentrate on any suitable surface, the sun's rays to a focus, the little luminoiis spot, or circle formed, is really an image, or picture of the sun itself. Why are con- 683. Convex leiises, as ordinarily used, are irMa^Tfy^ng Called Hiagnifying-glasses, because tbey in- Giasses? creasG the apparent size of tbe objects seen througli them. The reason of this is, that the lens so alters, convex lens by rcfraction, the direction of tbe rays of light magni y procccding froiu an object, that tliey enter tbe eye as if they came from points more distant from each other than is actually the case, and hence the object ap- pears larger, or magnified. On the contrary, the concave lens, which Why does a y ■ nn t corcave lens produces an exactly opposite eiiect upon the diminish the •' n i- i i • n j ' . apparent size lavs 01 lifflit, causcs the Huaire ot an obiect of an object? ,i i •- x fl seen through it to appear smaller. On the same principles also, concave mirrors magnify, and convex mirrors diminish the images of objects reflected from their surfaces. Hence the magnifying or diminishing power ■What is said of .- , , ■ n i , i the magnifymg 01 Icuscs IS uot, as IS oitcu popularly supposed, or diminishing , , ' ,. p -l t r power of lenses? ctue merely to the pecuhar nature oi the glass ot which they are made, but to the figure of their surfaces. The double convex lens, inclosed in a convenient setting of metal or horn, if extensively employed by watch-makers, engravers, etc., with whom it passes under the general name of lens. How may con. 684. In additiou to the efiect which convex derdishT^ob- lenses produce by magnifying the images of jects visible? objccts, they are also capable of rendering distant objects visible which would be invisible to the THE ANALYSIS OF LIGHT. 325 Explain more fully tho action of the comtlx leiis in tliis re- spect? naked eye, by causing a greater number of rays of light proceeding from them to enter the eye. The lighb which produces vision, as ■will bo more fully ex- plained herealler, enters the eye through a circular opening called the pupil, which is tho black circular spot surrounded by a colored ring, appearing in the center of tho front of tho eye. Now, as the ra^^s of light proceeding from an object diverge or spread out in every direction, the number which will enter tho cyo frill oe hmited by the size of the pupil. At a great distance from an object, at will be seen in Fig. 2G5, few rays will enter the eye; but if, as in Fig. 2G6, we place before the eye a convex lens of moderate size, a large number of the diverging rays will be coUected and concentrated into a single point or focus behmd it, and thus aBbrd to the eye occupying a proper position suffi- cient light to enable it to see the distant object distinctly. FiCt. 265. Fig. 266. In like manner a concave mirror, by causing divergent rays which fal) upon the surface to become convergent, may be used to produce the same ef- fect, as is shown in Fig. 267. Ftc. 2G7. SECTION III. THE ANALYSIS OF LIGHT. 685. It has, up to this point, been assumed, that light is a simple substance, and tliat all its rays, or parts, are refracted in precisely the same manner, and therefore suffer the same changes when acted upon by transparent media. This, however, is not its constitution. 326 WELLS'S NATURAL PHILOSOPHY. What is the White light, as emitted from the sun, or wMte''iight'? °^ fj'oni any luminous body, is composed of seven different kinds of light, viz., red, orange, yel- low, green, bhie, indigo, and violet. What is the Thc scven different kinds of light produce originofcoior? ^^^^^^ different colors, viz., red, orange, yellow, green, blue, indigo, and violet. These seven colors are called primary colors, since by the union or mixture of some two or more of them, all other colors, or varieties of color are j^roduced. How is light The separation of white light into its sev- auaiyzed? ^^^^ parts is cffccted by means of a prism. When a ray of white light is made to pass through a prism, each of the seven rays of which it is composed are refracted, or bent out of their course differently, and form on an opposite screen or wall an image composed of bands of the seven different coloi's. What is the 686. The image formed by a ray of white Spectrum? ijght passiug througli a prism, is called the Solar, or Prismatic Spectrum. Fig. 268. The separation of a ray of solar light into different colored rays, by refrac- tion, is represented in Fig. 268. A ray of light, S A, is admitted through an aperture in a shutter into a darl^ened chamber, and caused to fall on a prism, P. The ray thus entering would, if allowed to pass unobstructedly, havo moved in a straight line to the point K, on the floor of the room, and thero THE ANALYSIS OF LIGHT. 327 formed a circular disc of white light ; but by the interposition of the prism the ray spreads out in a fan-shape, and forms an obloug colored image on the opposite waiL This image, called the solar spectrum, is divided horizontally into seven colored spaces, or bands, of unequal extent, which succeed each other in an invariable order, viz., red, orange, yellow, green, blue, indigo, violet. uponwhatdoes The Separation of the seven different rays of''wMfe"iigh't composing white light from one another, de- depend? pends entirely upon a difference in their re- frangibility in passing through the prism ; those which are refracted the least falling upon the lowest part of the screen, and those which are refracted the most upon the upper part. Thus the red rays, which are the least refracted, or the least turned from their course by the prism, always occur at the bottom of the spectrum, while the violet, which is the most refracted, occurs at the top ; the remaining colors being arranged in the intermediate space in the order of their refrangibility. What additional Thc sevcu diifcrcnt rays of light, when once ofThe compoli! Separated and refracted by a prism, are not ii-ht?°^ ^^^^° capableof being further analyzed by refraction ; . but if by means of a convex lens they are col- lected together and converged to a focus, they will form ■white light. If the spectrum formed by a prism of glass bo divided into three hundred and sixty parts, it is found that the red ray, or color, occupies forty-five of those par' , the orange twenty-seven, the yellow forty-eight, the green sixty, the blu sixty, the indigo forty, and the violet eighty. If we take a circle of paper and paint upon it in divisions of proportionate size the seven colors of the spectrum, and then cause it to rotate rapidly about a center, the colors by combination wiU impart to it a white appearance.* From this and other experiments, therefore, it is inferred that light which wo call colorless, or white (as that coming immediately from the sun), really con- tains light of all possible colors so mixed as to neutrahzo each other. 687. The separation of the difterent rays of light which lakes place in their passage through a prism, is designated by the term Dispersion. i E lain what '^^^° order of refrangibility of the seven different rays of is meant by the light, or the arrangement of the seven colors in tho spcc- ero^dSi'r'ent t^um, is always the same and invariable, whatever way the Bubstanc«s. prism may be turned ; the lower end of the spectrum being • It is very common to find it stated in hooks of science that by niixinjr powders of tho seven different colors together a white, or grayish-whito compound may be produced. The exporimont, is not, howcTcr, satisfactory 328 WELLS'S NATURAL PHILOSOPHY. red, which passes upward into orange, then into yellow, then green, blue, indigo, and violet, which is at the upper end. Dissimilar substances, however, produce spectra of dilTerent lengths, on ac- count of a diflFerence in their refractive properties. Thus a ray of light tra- versing a prism of llint-glass, will have its red aud violet colors separated on a screen twice as widely as those of a ray passing through a similar prism of crown-glass. This diflcrence is expressed by saying that the dispersive power of the two substances is different, or that flint-glass has twice the dis- persive power of crown-glass. -„ .„ , As a lens may be considered as a modification of the WTiy -will not •' »n ordinary prism, it follows that when light is refracted through a lens, lens produce a jj jg separated into the diQ'erent colors, precisely as by a perfect image? '■ ^ t- j j prism ; and as every ray contained in white light is refracted differently, every lens, of whatever substance made, will have a different focus for every different color. The images, therefore, of such lenses will be more or less indistinct, and bordered with colored edges. This imperfection is termed chromatic aberration. For this reason the focus of a burning-glass, which is an optical image of the sun, is never perfectly distinct, but always confused by a red, or blue bor- der, since the various-colored rays of which sunlight is composed, can not all be brought to the same focus at once. In a like manner, if we point a common telescope at a blue and red hand-bill at a short distance, we shall have to draw out the tube of the instrument to a greater length in order to read the red than the blue letters. These fringes of color are a most serious obstacle to the Kxplam the . . X^ . , . .... . , construction of perfection of optical instruments, especially in astronomical an achromatic telescopes, where great nicety of observation is required ; and to prepare a lens in such a way that it would . fract light ■nnthout at the same time dispersing it into colors, was long consider ;1 an im- possibility. The discovery was, however, made by Mr. Dollond, an Englishman, that by combining two lenses, formed of materials which refract light differently, the one might be made to counteract the effects of the other; on the same principle as by combining two metals together which expand unequally, we may construct a pendu- lum whose length never varies. Such a combination is represented in Fig. 268, whero a convex lens of crown glass is united with a concave lens of flint glass, so as to destroy each the dispersivo power of the other, while at the same time the refract- ing, or converging power of the convex lens is pre- served. A lens of this character is called Achro- matic,* since it produces images in their natural colors. * Achromatic, from a, not, and xp<^f^<^i color. Fig. 268. THE ANALYSIS OF LIGHT. 329 whati3spheri. Lenses are also subject to another imperfec- caiaberraHonr tioD, which is Called splierical abeiratioD. This arises from the fact that the curved surface of a lens is at unequal distances from the object and from the screen which receives the image formed at its focus ; and hence, if one point of the image is perfect, another point is less BO, owing to a difference in the convergence of the rays coming from the center and the edges of the lens. Thus, if tlie image is received on a screen of ground glass, it will be found that when the picture is well defined at the center, it will be indistinct at tho edges ; but by bringing the lens nearer the screen, the edges of the imago will be more sharply defined, but the middle is indistinct. To make the im- age perfect, therefore, the marginal portions of the lens should be covered with a circlet of paper, so as to permit those rays only to pass which lie near the axis of the lens. This plan, however, impairs the brightness of the image. When tho image formed by the lens is small, the efiect of spherical aberration is scarcely noticed, and by combination of lenses of different refractive powers, it may be ahnost entirely overcome. 688. The various ravs composino; solar lijrht AreaUtherays n i " • , . ©flight eqnauy are uot ail equally lummous, that is to say, brilliant? . , i ■,■,-, .,t they do not appear to the eye equally brilliant. The color most visible to the human eye is yellow. The luminous intensity of the different colored rays of light may be ex- pressed numerically as follows: — Red, 94; orange, 640; yellow, 1,000; green, 480; blue, 170; indigo, 31; violet, 6.* 689. According to some authorities, white solar light consists of only three colors — red, yellow and blue, which, by combining, produce the other four colors, orange, green, indigo and violet. w>,.».,»=-,,^o I^ed, vellow, and blue, are, therefore, some- >V n at are some- 7*7 7 / y times called the times Called the simple colors. nmple colors ? i- Thus, by the union of red and yellow, we may produce orange ; by yellow and blue, green ; by blue and red, violet ; indigo being considered as merely a shade of blue. Red, yellow, and blue, on the contrary, can not be produced by the mlngUcg of any two other colors. When blue and yellow powders are mixed together, blue and yellow rays are reflected to the eye from the minute particles, but the two colors are so • It -would appear, from numerous observations, that soldiers are shot during battle according to the color of their dress in the following proportion : — red, VI ; dark green, 7; browTi, 6 ; bluish gray, 5. Eed is therefore the most fatal color, and a light gray tha least so. o^\) WELLS'S NATURAL PHILOSOPHY. mingled that the eye only notices the combined effect, which is green. If we now examine the same mixture with a microscope, the blue and yellow par- ticles will be seen separately, and the green color will disappear. „^ ^ , 690. The natural color which an obiect Why do nat- i -i • i ""■J-u. """f "'^o exhibits when exposed to the light, depends exhibit colors ? ■■■ o 7 r upon the nature and arrangement of the par- ticles of matter of which it is composed, and is not the re- sult of any quality inherent in the object itself Bodies which naturally exhibit color have, by reason of a certain peculiar arrangement of their surfaces, or mole- cular structure, a greater preference for some qualities of light than for others. If the body is not transparent, it will reflect certain rays of light from its surface, and ap- pear of the color of the light it reflects *, if the body is transparent, it will allow only certain rays to pass through its structure, and will consequently appear of the color of the light it transmits. Thus a red body appears red because it reflects or transmits the red ray of solar light to the eye ; and a yellow bod}' appears yellow because yellow light is reflected or transmitted by its surface or structure more powerfully than light of any other color; and so on through all the colors. It is not, however, to be understood that colored bodies reflect or transmit only pure rays of one color, and perfectly absorb all others ; on the contrary, it has been found that a colored body reflects, in great abundance, those rays of liglit which determine its particular color, and also the other rays which make up white light in a greater or less degree, in proportion as they more or less resemble its color in the order of their refrangibility. _,„... Some substances have no preference for any one quality of When IS a body '■ \ colorless, when light more than another, but reflect or absorb them all whin' black?'^ equally ; such are called neutral, or colorless bodies. Those substances wliich reflect all the rays of liglit which fall upon them appear white ; those which absorb all the rays appear black. In the dark there is no color, because there is no hght to be absorbed or reflected, and therefore none to be decomposed. A glass is called red because it allows the red rays of light to penetrc':o through a greater thickness of its substance than tlie other rays ; but at a cer- tain thickness, even the red rays would be absorbed like the rest, and wo Bhould call the glass black. No body, unless self-luminous, can appecr of a color not existing in the light which it receives. This may be proved by holding a colored body in a ray of light which has been refracted by a prism, when the body will appear of the color of the ray in which it is placed ; for since it receives but one col- ored ray, it can reflect no other. ■ THE ANALYSIS OF LIGHT. 331 May the color 691. Bj changing the structure or molecu- cLil'ged^^ by lar arrangement of a body, the color which it moiecuiar'^^*' cxhibits may be often changed also. ® Illustrations of this principle are frequently Been in chem- ical compounda The iodide of mercury is a beautiful scarlet compound, which, when gently heated, becomes a bright yellow, and so remains when undis- turbed. If| however, it is touched, or scratched with a hard substance, ag with the point of a pin, its particles turn over, or readjust themselves, and resume their original red color. Chameleon mineral is a solid substance pro- duced by fusing manganese with potash ; when dissolved in water, it changes, jccording to the amount of dilution, from green to blue and purple. Indigo also, spread on paper and exposed to heat, becomes red. 692. Some bodies have the power of reflecting from their surfaces one color while they transmit another. This is the case with the precious opal. A solution of quinine in water containing a little sulphuric acid, is colorless and transparent to the eye look- ing through it, but by looking at it, it appears intensely blue. An oil ob- tained in the distillation of resin transmits yellow light, but reflects violet light. Smoke reflects blue hght, but transmits red light. These phenomena result from a peculiar action of the surface or outer layer of the substance of the body on some of the rays of light entering it, and have received the name of epipolic, or surface dispersion. Deepness of color proceeds from a deficiency, rather than from an abund- ance of reflected rays : thus, if a body reflects only ft few of the red rays, it will appear of a dark red color. When a great number of rays are reflected, the color will appear bright and intense. If the objects of the material world had been illuminated only with whito light, all the particles of which possessed the same degree of refrangibility, and were equally acted upon by all substances, the general appearance of nature would have been dull, and all the combinations of external objects, and all the features of the human countenance would have exhibited no other variety than that which they posses in a pencil sketch or India-ink drawing. What are com- 693. Any two colors which are able, by com- piementary biuiiig, to produco whitc light, arc termed complementary colors. Each color of the solar ray has its complementary color, for if it be not white, it is deficient in certain rays that would aid in producing white. And these absent rays compose its complementary color. The relative position of complementary colors in the prismatic spectrum may be determined as follows* Thus, if we take half the length of a spectrum by fr pair of compasses, and fix one leg on any color, the other leg will fall upoa 332 WELLS'S NATURAL PHILOSOPHY. its complementary color, or upon the ono which added to the first will pro- duce white light. The complementary color of red is bluish green ; of orange is blue ; of yellow is indigo ; of green is reddish viclet ; of blue is orange red ; of indigo is orange yellow ; of violet is yeUow green ; of black ia white ; of white is black. Complementary colors may be seen by fixing the eye steadily upon any colored object, such as a wafer upon a sheet of white paper. A ring of col- ored light will play round the wafer, and this ring will be complementary to the color of the wafer. A red wafer will give a green ring, a blue wafer an orange-colored ring, and so on. Or if, after having regarded the colored wafer steadily for a few moments, the eye be closed, or turned away, it will retain the impression of the wafer, not in its own, but in its complementary color ; thus a red wafer will give a green ray, and so on. In Uke manner, if we look at a red hot fire for a few minutes, every object as we turn away appears tinged with bluish green. The art of harmonizing and contrasting colors is intimately connected with the principles of complementary colors. How do colors Every color placed beside another color is ifapp^rani^r cbangcd, and appears differently from wliat it does when seen alone ; it equally modifies, moreover, the color with which it is in proximity. As a general rule, two colors will appear to the best advantage when one is complementary to the other. Thus, if a dresa is composed of cloths of two colors, the one complementary to the other, as red and green, orange and blue, yellow and violet, they will mutually heighten the effect of each, and make each portion appear to the best advantage. For this reason, a dress composed of cloths of different colors, looks well for a much longer time, although worn, than one of a single color, the character of the fabric being the same in both instances. A suit of clothes of one color can be worn to advantage only when it is new, because as soon as one portion of the suit loses its freshness from hav- ing been worn longer than another, the difference will increase by contrast. Thus a pair of new black pantaloons worn with a vest of the same color, which is old and rusty, will make the tinge of the latter appear more con- Bpicuous, and at the same time the black of the pants will appear more brilliant. White and other light-colored pantaloons would produce a contrary effect. In printing letters on colored paper, the best effect will be produced when the color of the paper is complementary to the ink ; blue should be put upon orange, and red upon green. Stains will be less visible on a dress of different colors than on one com- posed of only a single color, since there exists in general a greater contrast among the various parts of the first-named dress, than between the stain and the adjacent part, and this difference renders the stain less apparent to the eye. THE ANALYSIS OF LIGHT. 333 In the grouping of flowers in gardens, and in the preparation of bouquets, the most pleasing effects will be produced by placing the blue flowers next to the orange, and the violet next to the yellow. White, red, and pink flowers are never seen to greater advantage than when surrounded with green leaves, or white flowers ; on the otlier hand, we should always separate pink flowers from those that are either scarlet or crimson ; orange, from orange- yellow flowers ; yellow flowers from greenish -yellow flowers ; blue from violet- blue, red from orange, pink from violet. By grouping colors together which are not complementary, or which do not riglitly contrast with each other, we produce a discordant effect upon the eye, analogous to the discord which is produced upon the ear by instruments out of tune. It is always necessary that, if one part of the dress be highly ornamented, or consists of various colors, a portion should be plain, to give repose to the eye. Black being the complementary color of white, the effect of black drapery upon the color of the skin or face is to make it appear pale, or whiter than it usually is. The optical effect of dark and black dresses is to make the figure appear smaller ; hence it is a suitable color for stout persons. On the contrary, whito and light-colored dresses make persons appear larger. Large patterns or de- signs upon dress, make the figure appear shorter : longitudinal stripes, if not too wide, add to the height of the figure ; horizontal stripes have a contrary tendency, and are very ungraceful.* whatisaRain- ^94. The Eainbow is a semicircular band ^"""^ or arch, composed of the seven different colors, generally exhibited upon the clouds during the occurrence of rain in sunshine. How is a rain- Thc raiubow is produced by the refraction bow produced? ^^^j^ reflcctiou of the solar rays in the drops of falling rain. • The following curious facts are known to persons employed in trade: — "AVhen a pur- chaser has for a considerable time looked at a yellow fabric, and is then shown orange or scarlet stufTs, he considers them to be amaranth-red, or crimson, for there is a tendency In the eye, excited by yellow, to see violet, whence all the yellow of the scarlet or orang* cloth disappears, and the eye sees red, or red tinged with scarlet. Again, if there are presented to a buyer, one after another, fourteen pieces of red cloth, he will consider tha last six or seven less beautiful than those first seen, although the pieces be identically the same. Now what is the cause of this error in judgment? It is that the eyes having seen seven or eight red pieces in succession, are in the same condition as if they luid regarded fixedly during the same period of time a single piece of red cloth ; they hava then a tendency to see the complementary color of red, that is to say, green. This tend- ency goes, of necessity, to enfeeble the brilliancy of the red of the pieces seen later. In order that the merchant may not be the sufferer by this failing of the eyes of his cus- tomer, he must take care after h.aving shown the latter seven piecos of red, to present to him some pieces of green cloth, to restore the eyes to their natural state. If the sight of the green be sufficiently prolonged to exceed the normal state, the eyes will acquire a tendency to see red; then the last seven pieces irill appear more beautiful than tb« otiten." — Chevreul on Celor. 334 WELLS'S NATURAL PHILOSOPHY. What experi- ments prove the decomposi- tion of light by drops of wa- ter? C95. Rainbows are also formed when the sun shines upon drops of watei falling in quantity from fountains, waterfalls, paddle-wheels, etc. That the rainbow results from the decomposition of the solai rays by drops of water, may be proved by the following sim- ple experiment: — If we take a glass globe filled with water, and suspend it at a certain heiglit in the solar rays above the eye, a spectator standing with his back to the sun will seo the refraction and reflection of red light; if, then, the globe be lowered slowly, the observer retaining his position, the red light will be replaced by orange, and this in its turn by yellow, and so on, the globe at dif- ferent heights presenting to the eye the seven primitive colors in succession- If now, in the place of the globe occupying different positions, we sub- stitute drops of water, we have a ready explanation of the phenomena of the rainbow. Drops of rain, suspended to grass or bushes, may be frequently found to appear to the eye of a bright red; and by slightly changing the position of tho eye, the colors of the drop may be made to appear successively yellow, green, blue, violet, and also colorless. This also proves that rays of light, faUing in certain directions upon drops of water, are refracted thereby and decomposed into colored rays that become visiblo to the eye when it is situated in the proper direction. Fig. 2G9, The principles of the formation of the rain- bow may be further illustrated by Fig. 2G9. Let A B and C be three drops of rain ; S A, S B, and S C, three rays of the sun. Tho ray S A, by refraction, is divided into three colors ; the blue and yellow are bent above the eye, D, and tha red enters it. The ray, S B, is di- vided into three col- ors ; the blue is bent above the eye, and tho red falls below the eye D, but tho yellow enters it. The ray, S C, is also divided into three colors. The blue (which i3 bent most) enters the eye, and the other two fall below it Thus tho eye sees the blue of C, and of all drops in the position of C ; tha yellow of B, and of all drops in the position of B; and the red of A, and of all drops in the position of A. The same may be also inferred respecting the other four colors of tho spectrum; and thus tha eye 8ee» a rainbow. THE ANALYSIS OF LIGHT. Zoo What are the conditions nec- essary in order to see a rain- bow? Bame seen alike byallper- EOUS? The rainbow can be seen only when it ruins, and in that point of the heavens which is op- posite to the sun. Hence a rainbow is always observed to be situated in the west in the morning, and in the east in the alternoon. It is also necessary for the production of a rainbow that the height of the sun above the horizon should not exceed forty-two degrees. Hence we generally observe this phenomenon in the morning, or toward evening ; and it is only in the winter, when the sun stands very low, that tho rainbow is sometimes seen at hours approaching noon. Is the Bame '^^ ^^® ^'^-^^ °^ ^^^'^* differ greatly in refrangibility, only a rainbow seen single and different-colored ray from each drop will reach tho eye of a spectator ; but as in a shower there is a succession of drops in all positions relative to the eye. the eye is en- abled to receive the different-colored rays refracted at different inclina- tions. This is clearly illustrated in Fig. 270, in which S represents rays of the sun falUng upon suc- cessive drops, E, 0, Y, G, B, I, Y ; but a single colored ray, and a different one for each drop, will reach the eye. As no two spec- tators can occupy exactly the same position, no two can see the same color reflected from the same drop ; and consequently no two persons see the same rainbow. In the formation of a rainbow each colored ray reflcctea bowTcircuUrT' '^°™ ^^ falling drops of rain, enters the eye at a different inclin- ation or angle. But the several positions of those drops, which alone are capable of reflecting the same color at the same angle, to the eye constitute a circle, — and hence the bands of color which make up a rainbow, appear circular. What are pri- Two Tainbows are not unfrequently observed ^'dTr/"^!^- ^t t^^6 same time, the one being exterior to, ^"^^^ and less strongly developed than the other. The inner arch, which is the brightest, is called the pri- mary bow, and the outer, or fainter arch, the secondary bow. The order of colors in the inner bow is also the* re- verse of that in the outer bow. 336 WELLS'S NATURAL PHILOSOPHY. How is the ])rimary rain- bow formed '! Fig. 271. Fig. 272. The inner, or primary rainbow, wliicli is the one ordinarily seen, is formed by two refrac- tions of the solar ray, and one reflection, the ray of light entering the drops at the top, and being reflected to the eye from the bottom. Thus, in Fig. 271, the ray S A of the pri- mary rainbow strikes the drop at A, is re- fracted, or bent to B, the back part of tho inner surface of the drop ; it is then reflected to C, the lower part of the drop, when it is refracted again, and so bent as to come di- rectly to the eye of the spectator. Howisthc Bee- The secondary, or outer rainbow, is produced bow^fJrm/d?"" ^y t'^^'*^ refractions of the solar ray, and two reflections, the ray of light entering the drops at the bottom, and being reflected to the eye from the top. Thus, in Fig. 272, the ray S B of the sec- ondary bow strikes the bottom of the drop at B, is refracted to A, is then reflected to C, is again reflected to D, when it is again refracted or bent, till it reaches the eye of the spectator. The position and formation of the primary and secondary rainbows are represented in Fig. 273. Thus, in the formation of the pri- ^\ mary bow, the ray of light S strikes the drop n at a, is refracted to b, reflected to g, and leaving the drop at this point, is refracted to the eye of tho spectator at 0. In the formation of the secondary bow, the ray S' strikes the drop p at the bottom at the point t, is refracted to d, reflected to /, and thence to e, and refracted from the top of the drop, pro- ceeds to the eye of the spectator at 0. Tho reason the outer bow is paler than the inner is because it is formed by rays which have undergone a second internal reflection, and after every re- flection light becomes weaker. wiiat are Halos are colored rays which are sometimes iiaios? gggjj surrounding luminous bodies, especially the sun and moon. They are occasioned by the refraction and decomposition of light by particles of moisture, or crystals of ice floating in the higher regions of the atmos- phere, and are neyer seen when the sky is perfectly clear. THE ANALYSIS OF LIGHT. Fig. 273. 337 The production of halos may be illustrated experimentally, by crystallizing various salts upon plates of glas.s, and looking through the plates at the sun, or a candle. A few drops of a saturated solution of alum, spread over a glass so as to crystallize quickly, will cover it with an imperfect crust of crys- tals, scarcely visible to the eye. Upon looking at a luminous body through the glass plate, with the smooth side next the eye, three fine halos will be perceived encircling the source of light. The flict that halos, or rings round the moon, are more frequently observed than solar halos, is dependent upon the circumstance th.at the sun's light ia too intense and dazzling to allow the halo to be recognized. Halos may be observed most frequently in the winter S3ason, and in high northern latitudes. 696. The beautiful crimson appearance of the clouds after sunset in the western horizon, is due in a great measure to the fact that the red rays of the solar light are less refrangible than any of the other colored rays, and in conse- quence of this, they are not bent out of their course so much as the blue and yellow s rays, and are the ^^^ last to disappear. '"^^0^ For the same rea- ls What is tha occasion of the red appearance of tho clouds at Eunrise and fiUQset 1 Fig. 333 WELLS'S NATURAL PHILOSOPHY, son they are the first to appear in the morning when tlie sun rises, and impart to the morning clouds red or crim- son colors. Let us suppose, as in Fig. 274, a ray of light proceeding from the sun, S, to enter the earth's atmosphere at the point P. The red rays, which com- pose in part the solar beam, being the least refrangible, or the least deviated from their course, will rea'/h the eye of a spectator at the point A ; while the yellow and blue rays, being refracted to a great^ir degree, will reach tho surface of the earth at the intermediate points B and C. They will, conse- quently, be quite invisible from tlie point A. Tlie red and golden appearance of the clouds at morning and evening is aL?o due in part to the fact, that aqueous vapor on the point of being con- densed, only allows the red and yellow rays of light to pass through it. For this reason, if the sun be viewed through a column of steam escaping from a boiler, it appears of a deep red, or crimson color. The same thing may be noticed during a drought in summer, when the air is filled with dry exhala- tions. ■What is 697. The irregular brilliancy of the stars, Twiakimgr known as twinkling, is supposed to be due to unequal reflections vf light occasioned by inequalities and undulations in the atmosphere. Tio^ i3 color 698. Light, according to the undulatory the unduhitory thcorv, is occasioucd by the vibrations or un- theoryofiisht? Julatious of a certain clastic medium diffused throughout all space, called Ether. Color, according to this theory, depends on the number of vibrations which are made in a certain time ; those vibrations which are the most rapid, producing upon the eye the sensation of violet, and those which are the slowest, the sensation of red. The analogy between sound and light, according to the ^Ihere Ikn ^mdulatory theory, is perfect, even in its minutest circum- tween color and stances. When a certain number of vibrations of a musical music? chord are caused in a given time, we produce a required sound ; as the vibrations of the chord vary from a quick to a slow rate, we produce sounds sharp or grave. So with light ; if the rate at which the ray undulates is altered, a difierent sensation is made upon the organs of vision. The number of aerial vibrations per second required to produce any particu- lar note in music has been accurately calculated, and it is also known that the ear is able to detect vibrations producing sound, through a range com- mencing with 15, and reaching as far as 48,000 in a second. So also in tho case of light, tho frequency of vibrations of the other required for the produc- THE ANALYSIS OF LIGHT. 339 tion of any particular color has been determined, and the length of the waves corresponding to these vibrations. What relation The waves Tequisitc to produce red are the the'^'^ wavet^" largest ; orange comes next ; then yellow, br"aon?o°fM,!e S^'^^^, ^1^^, Indlgo, and violet, succeed each different colors? other, thc wavcs of each being less than the preceding. The rapidity of vibration is in the same order, the waves producing red light vibrating with the least rapidity, and the waves producing violet with the greatest rapidity. To produce red light it is necessary that 40,000 waves or undulations should be comprised within the space of a single inch, and that 480 billions of vibra- tions should be executed in one second of time ; while, for the production of violet, 60,000 waves within an inch, and 720 bilhons of vibrations per second are required.* 699. As two sets of sound-waves or vibra- Can waves of . t r- i light be made tious may SO combmo as to modiiy or destroy each other, and thus produce partial or total silence, so two waves or vibrations of light may be made to interfere and jDroduce various colors, or entire darkness. • It may perhaps be asked, with something of incredulity, how such a resalt could pos- sibly have been arrived at, with any degree of scientific accuracy. The problem, how- ever, is not a difficult one. In the first place, Newton, by a series of perfectly satisfactory and beautiful experi- ments, ascertained the number of waves or andulations of the different colored raya comprised within the space of an inch. Let us now suppose an object of any particular color, a red star, for example, to be viewed from a distance. From the star to the eye there proceeds a continuous line of waves ; these waves enter the pupil, and impinge upon the retina ; for each wave which thus strikes the retina, there will be a separate pulsation of that membrane. Its rate of pulsation, or the number of pulsations which it makes per second, will therefore be known, if we can ascertain how many luminous waves enter the eye per second. It has been already shown that light moves at the rate of about 200,000 miles per second ; it follows, that a length of ray amounting to 200,000 miles must enter the pupil each second ; the number of times, therefore, per second, which the retina will vibrate, will be the same as the number of the luminous waves contained in a ray 200,000 mile* long. Let us take the case of red light In 200,000 miles there are, in round numbers, 3,000,000,000 feet, and therefore 12,000,000,000 inches. In each of these 12,000,000,000 of inches there are 40,000 waves of red light. In the whole length of the ray, therefore, there ore 4S0,u00,000,000,0O0 waves. Since this ray, however, enters the eye in one second, and the retina must pulsate once for each of these wavts, we arrive at the astounding conclusion, that when we behold a red object, the membrane of the oye trembles at the rate of 450,000,000,000,000 of times between every two ticks of a common clock ! In the same manner, the rate of pulsation of the retina corresponding to other tints of colors is determined ; and it is found that when violet is perceive^i it trembles at the rate of 720,000,000,000,000 of times per second.— iardzwr. 340 WELLS'S NATURAL PHILOSOPHY. • ^^ If we stand at the junction of two stream3 of water, it will iiiterferenooof be noticed tliat when the waves from each meet in the samo darkne^s*?*^"'^^ ^^^^^ °^ vibration, the resulting wave will bo equal to the two combined ; if, however, one wave is half an undulation behind the other, the crest of one will meet the hollow of the other, and compara- tively smooth water will be the result. So if two pencil rays of light, radiat- ing from iwo points, reach a point of interference at the same degree of ele- vation, a spot of double the luminous intensity of either will be produced; but if one is half a vibration behind the other, the result will be, that a dart instead oi a light spot will be apparent. How is color The brilliant tints of soap bubbles, and thin tiie'*"inferfe^r- platcs of different transparent bodies, are ex- •nce of light? amples of the interference of light; for the undulations reflected from the first surface interfere with those reflected from the second, and thus produce the various colors. The varying play of colors exhibited by films of oil on the surface of water, and the iridescent appearance of mother-of-pearl, the scales of fishes, and the wings of some insects, are all phenomena resulting from the interference of light. whatis double 700. Doublc rcfractiou is a property which refraction? certain transparent substances possess, of causing a ray of light in passing through them to undergo two refractions ; that is, the single ray of light is divided into two separate rays. Fig. 275. Fig. 276. A very common mineral called " Iceland spar," which is a crystallized form of carbonate of lime, is a remarkable example of a body possessing double refracting properties. It is usually transparent and colorless, and its crystals, as shown in Fig. 275, havo the geometrical form of a rhomb, or rhomboid; — this term being applied to a solid bounded by parallel faces, inclined to each other at an angle of 105°. The manner in which a crystal of phtnomunon of Iceland spar divides a ray of light in- double refrac- ^q ^^q separate portions is clearly Bhown in Fig. 276 ; in which S T represents a ray of light, falling upon a surface of a crystal of Iceland spar, A D E C, in a perpendicular di- rection. Instead of passing througli without any refrac- tion, as it would in case it had fallen perpendicularly upon the surface of glass, the ray is divided iuto two separate rays, the one, T 0, being in the direction of the original ray, and the other, T E, being bent or refracted. The first of these rays, or the one which follows the ordinary THE ANALYSIS OF LIGHT. 341 law of refraction, is called the " ordinary"' ray ; the second, which follows a different law, ij called the "extraordinary" ray. ^ If we look at a small object, a3 a dot, a letter, or a line, through a plate of glass, it appears single ; but if a plate of Iceland spar be sub- stituted, a double imago will be per- ceived, as two dots, two letters, two lines, etc. This result of double re- fraction is represented in Fig. 27 T. Crystals of many other substances, such as mica, the topaz, gypsum, etc., possess the property of double refraction, but not in so remarkable a degree as Iceland spar. __ , , In all these crystals, there are one or more directions along what are the ' . , , axes of double which objects when viewed through them appear single; re.raction? these directions are termed the Imes, or axes of double re- fraction. In the case of Iceland spar, there is one axis of double refraction, i. c, one direction along which objects when viewed appear single ; this is in the direction of the line A B, Fig. 275, which joins the two obtuse three- sided angles. If the summits A and B be ground down and polished, no double refraction will occur in looking through the crystal in this direction. To what is the '^'^^^ ^^^ phenomenon of double refraction is due entirely to phenomenon of the molecular structure of the medium through which light tion due? passes, is proved by taking a cube of regularly annealed glass, which produces but one refracted ray, and heating it unequally, by subjecting it to pressure : a change is thereby affected ui the arrangement of its parts, and double refraction takes place. What is polar- 701. When a ray of light has been reflected ized light? £j.Qj^ ll^g surface of a body under certain special conditions, or transmitted through certain trans- parent crystals, it undergoes a remarkable change in its properties, so that it is no longer reflected and refracted as before. The effect thus produced upon it has been called polarization, and the ray or rays of light thus af- fected are said to be polarized. •What are the The uame poles is given in physics in gen- poiesofabody? ^^^^ ^^ ^l^g gjj^g ^j, ^^j^ of any body which enjoy, or have acquired any contrary properties. Thus, the opposite ends or sides of a magnet have contrary properties, in- asmuch as each attracts what the other repels. The opposite ends of an elec- tric or galvanic arrangement are, for like reasons, denominated poles. So also iu the case of light, the raya which have been reflected or transmitted under 342 WELLS'S NATURAL PHILOSOPHY. peculiar conditions are said to possess poles, because in some positions they can be reflected and in others thej can not, and these positions are at right angles to one another. ^ . , ^ ,. 702, The phenomenon of polarized light was discovered in Explain the dis- ,,,7 . ^ ,. r^ ■ ^ coveryandphe- 1808, by Malus, a young engmeer oiScer oi Fans. On ono iioniena of po- occasion, as he was viewing through a double refracting prism of Iceland spar the light of the sun reflected from a glass window in one of the French palaces, ho observed some very peculiar effects. The window accidentally stood open like a doc* on its hinges at an angle of 54°, and Malus noticed that the light reflected from this angle was entirely altered in its character. Tliis alteration in the character of the light reflected from the glass window, which was thus first observed by Malus, may be made clear by the following experiment : — Suppose we have a cylinder with a mirror at one end of it. If we point this to the sun, and receive the image on a distant screen, we may turn the cyUnder round on its axis, and the reflected ray will be found to revolve constantly with it. But if now, instead of receiving the ray direct from the sun, we allow a beam reflected from a glass plate, at an angle of about 54°, to fall upon the mirror, and then be reflected on the screen, it will be found that the point of light will not have the same properties as that previously examined ; it will bo altered in its degree of intensity as the cylinder turns round ; will have points where it is very bright, and others where it '^^^ ^^ accomplished in the following manner: — A large pris- tion of a pho- matic Spectrum is thrown upon a lens fitted into one side of a Bult'?^"° "^' dark chamber; and as the actinic power resides in great ac- tivity at a point beyond the violet ray, where there is no light, the only rays allowed to pass the lens into the chamber are those beyond the limit of coloration, and non-luminous; these are directed upon any object, and from that object radiated upon a highly sensitive photographic surface. In this way a picture may be formed by radiations which produce no eflect upon the eye. - 706. There are many reasons for supposing that each of the do the three three principles, light, heat, and actinism, included in the solar {""""olar rav ^^^' ^^^rcise a distinct and peculiar influence upon vegeta- fxert on rege- tion. Thus the luminous principle controls the growth and tation coloration of plants, the calorific principle their ripening and fructification, and the chemical principle the germination of seeds. Seedt which ordinarily require ten or twelve days for germination, will germinato under a blue glass in two or three. The reason of this is, that the blue glass permits the chemical principle of liglit to pass freely, but excludes, in a great measure, the heat and the light. On the contrary, it is nearly impossible to make seeds germinate under a yellow glass, because it excludes nearly all the chemical influence of the solar ray. THE EYE, AND THE PHENOMENA OF VISION. 347 SECTION lY, THE EYE, AND THE PHENOMENA OF VISION. If an opening be made in the Bide of a dark chamber how will images of external ob- jects be repre- aeated? 707. If we make a small aperture through the shutter of a darkened room, the images of external objects will be pic- tured indistinctly, and in an inverted position, upon the op- posite walL The reason of this will appear evident from aa inspection of I'ig. 280. It will be seen that the rays of light diverging from the top and bottom of the object cross each other in passing through the aperture, and consequently form an inverted image. This image is rendered more distinct with a small aperture than with a large one, since, in the first case, the rays which proceed from any particu- lar part of the object fall only upon the corresponding part of the image, and are not scattered indiscriminately over tlie whole picture, as they would be if the aperture was larger. Fig. 280. Describe the ^^' '^ ^'^^ place of the room ■\\-ith an aperture in the shutter, construction of We substitute a dark box, with a double-convex lens fitted Obscura.''™^'^* into one side, a picture will be formed on the opposite side of the box, or upon a screen placed at the focal distance of the lens. This picture will represent, with great beauty and distinctness, whatever is in front of the lens, all the objects having their proper relations of light and shadow, and their proper colors. Such an apparatus is called a Camera. Obsccr.\. i' Fig. 281 represents the ordinary construction of the camera obscur.o. It consists of a wooden rectangular box, into wliich the rays of the light penetrato through a convex lens placed at the termination of the tube B. These rays, if unobstructed, will form an image upon the opposite side of the box 0, but if they are received upon a mirror, M, inclined at an angle of 45°, their direc- tion is changed, and the image will be formed upon a screen, or plate of ground glass, N, placed at the top of the box. By placing upon this screen a sheet of tracing paper, the outlines of the image may be readily copied. 348 "WELLS'S NATURAL PHILOSOPHY. Sucli a modiflcatiou of the camera is very convenient for artists and travelers ia sk etching landscapes, etc. Fig. 281, now does the '''08. The mechanical arrangement of the thi "camera ^7® ^^ viau. and thc higher animals is the same obscura? j^g ^j^g^^ ^f j-\^q camera obscura, being simply a double-convex lens, fitted into one side of a spherical chamber, through which the rays of light pass to form an inverted picture upon the back of the chamber.* What is the ^^ man, the organs of vision consist of two urro7th'e"eye liollow sphcrcs, cach about an inch in diam- in man? eter, filled with certain transparent liquids, and deposited in cavities of suitable magnitude and form, in the upper part of the front of the head on cach side of the nose. The sphere of the eye, or the eye-ball, is moved in its socket by muscles attached to different points of its surface, so that it is capable of being moved within certain limits in every direction. • This may be proved by taking the eye of a recently-killed buUock and cutting* Email iole in the upper part of the ball, looking into the interior. How are we enabled to move the eye in different di- rections ? THE EYE, AND THE PHENOMENA OF VISION. 349 Fig. 282. The arrangement of tlieso muscles is shown in Fig. 282, where the external bones of the temple are supposed to bo removed in order to render them visible. The muscle, 1, raises the ej^ehd, and is con- stantly in action while we are awake. During sleep, tho muscle being in repose and relaxed, the eye-lid falls and protects the eye from the ac- tion of light. The muscle, 4, turns the eye upward; 5, downward; 6, outward; and a corresponding one on the in- side, not seen in the figure, turns it inward. Kos. 2 and 10 turn the eye round its axis. The eye consists essentially of four coats, or membranes, called the Sclerotic coat, the Choroid coat, the Cornea, and the Ketina ; and these coats inclose three transparent liquids, called hu- mors— the Aqueous humor, tlie Vitreous humor, and the Crystalline humor, the last of which has the form of a lens. Describe the The Sclcrotic coat is the external coat of the Sclerotic coat, g^g^ ^^^ ^'^^ ^^^ upon ^yhich the maintenance of the form of the eye chiefly depends. It is a stronjr, toush Of what parts does the eye consist ? membrane, and to it the muscles which move the eye are attached. It cov- ers about four fiftlis of tho external surface of the eye-ball, leaving, however, two circular openings, one before and the other be- ^. hind the eye. Its position is shown at i, Fig. 283. The Cornea the clear, trans- FiG. 283. "What is the Conieii ? IS parent coat which 350 WELLS'S NATURAL PHILOSOPHY. forms the front of the eye-ball. It is firmly united to, or fixed in the sclerotic coat, like the glass in the case of a watch. The Cornea is represented at a, Fig. 283, •What is the The Choroid coat is a delicate membrane, Choroid Coat? lining the inner surface of the sclerotic cOat, and covered on the interior with a black pigment. It is represented at k, Fig. 283. What is the Thc Rctiua is a delicate, transparent mem- Retina? brauc wliich spreads over the chief part of the internal surface of the eye-ball, and is situated imme- diately within and close to the choroid coat. The position of the Retina is shown at to, Fig. 283. How is the re- Thc Tctina is formcd by the expansion of a tina formed? jjervc Called the optic nerve, which proceeds from the back of the eye through the bones of the skull into the brain, and conveys to the brain the impressions made by external objects on the organs of vision. If this nerve were divided, notwithstanding the eye might be in other respects perfect, the sense of sight would be de- stroyed. No- 11, Fig. 282, and n, Fig. 283, exhibit the relative position of tho optic nerve. What is the ^^ looking into the eye from without, we irisT perceive a flat, circular membrane, which, in different eyes, is of a black, blue, or gray color. This membrane is called the Iris, and divides the eye into two very unequal portions. The Iria is represented at c d, Fig. 283. The Pupil of the eye is the circular black Pupil of the opening in the center of the iris, and is th» '^"^ space through which light is admitted intd the interior of the eye. The open space between c and d, Fig. 283, represents the pupil. It is, properly speaking, the window of the eye, and appears black, only because the chamber within and behind it is dark. When a small quantity of light enters the eye the pupil widens or expands ; but when a largo quantity enters, it closes or contracts. THE EYE, AND THE PHENOMENA OF VISION. 351 The two parts into which the iris divides the eye are called the anterior and posterior chambers. What are the ^hc anterior chamber, or the space before TitieoSa hu"! ^^^ ^"^j ^^ filled with a fluid resembling pure mora? water, and therefore called the aqueous hu- tnor ; and the posterior chamber, or the space behind the iris, is filled with a thick hquid, somewhat resembling the "white of an egg, called the vitreous humor. In Fig. 283, he represents the aqueous humor, and h the vitreous humor, thie last occupying all the interior of the chamber of the eye. The crystalline lens is composed of a more solid sub- stance than either the aqueous or vitreous humor. It is inclosed within a transparent bag, or capsule, having the form of a double-convex lens, and is suspended imme- 'iiately behind the iris, and between the aqueous and vitreous humors. Its form and position are represented at/ Fig. 283. low do we by ^^'^^ I>'ays of light proceeding from an ob- Ae e?r"pe?- j^^* ^^^ entering the eye, are refracted by the *eive objects? comca and crystalline lens, and made to con- Verge to a focus at the back of the eye, and form an image upon the retina. This image, by producing a sen- sation upon the optic nerve, conveys in some unknown way to the mind a perception and knowledge of the ex- ternal object. Fig. 284 represents the manner in FiG. 28-i. which the image is formed upon the retina in the perfect eye. The curva- ture of the cornea, s s, and of the erystalline lens, c c, is just sufficient to cause the rays of light proceeding from the image, 1 1', to converge to the right focus, m m, upon the retina. When does dig- Distinct vision can only take place in the puc«V"°°'^''* ^ys when the cornea and crystalline lens have such convexities as to bring the rays of light proceeding from an object to an exact focus upon the retina. 352 WELLS'S NATURAL PHILOSOPHY. How is the eye enabled to see objects dis- tinctly at differ- ent distances ? Pig. 285. As the rays of light proceeding from distant objects enter the eye at diflerent angles, they will naturally tend to meet at diflerent foci after refraction by the crystalline lens, and thus form indistinct images. This is remedied by a power which the eye possesses of adapting itself to the direction of the light proceeding from various distances, so that in the healthy eye, rays coming from near and distant objects are all equally converged to a focus on the same point of the retina. How the eye effects this is not certainly known, but it is supposed to be by increasing or diminishing the sphericity of tho ca'ystalline lens and cornea. What is tho ^ person is said to be near-sighted when efghTednersT" ^^^^ curvuture of the cornea and crystallino lens is so great, that the rays of light which form the image are brought to a focus before they reach the retina, or the back part of the eye. The object, there- fore, is not distinctly seen. Fig. 285 represents the manner in which the image is formed in the eye of a near-sighted person. Tho curvature of the cornea, s s, and of tho crystalline lens, c c, is so great that the image is formed at m m, in advance of the re- tina. Short-sightedness is remedied cither by holding the objedt nearer to the eye, or by the employment of spectacles tho glasses of which are concave lenses. In both cases the rays proceediag from the object enter the eye with a greater degree of divergence, and therefore do not converge so soon to a focus. A person is said to be far-sighted when, on account of a flattening of the cornea and the crystalline lens, the rays of light do not con- verge sufficiently to form a distinct image upon the retina. Fig. 286, represents the manner in which the imago is formed in the eye, when the cornea or crys- talline lens is flattened. The per- fect image would be produced at m m, behind the retina, and, of course, beyond the point necessary to secure distinct vision. Long-sightedness may be remedied by the employment of spectacles, tho glasses of which are convex lenses. These, by Xlow is short Eiglitedness remedied J" What is the cause of far- Bightedness ? Fig. 286. How may long, sightedness be remedied ? THE EYE, AND THE PHENOMENA OF VISION. 353 increasing the convergence of rays of light passing through them, bring them sooner to a focus in tlie eye, and thus produce the image upon the right point of the retina.* Most persons of advanced age are troubled with long-sightedness, and are obliged to use spectacles. The reason of tliis is, that as the physical organi- zation of the body becomes enfeebled, the humors of the eye dry up, or are absorbed, and in consequence of this, the cornea and crystalline lens shrink and become flattened. Beside these defects of the eye, a person may have the sense of vision impaired or destroyed by an injury or disease of the optic nerve, or by a dimi- nution of the transparency of the crystalline lens ; the first of these cases is called amaurosis, and is incurable — the second, which is called cataract, may be cured. . ., . The images formed by the rays of light upon the retina are on the retina inverted. It may, therefore, be asked why all visible objects whr do^we'not *^° °°* appear upside down? The explanation of this curious see' them up- point, which has formed the subject of much dispute, appears °^^' to be this: an object appears to be inverted only as it is com- pared with some other objects which are erect If all objects hold the same relative position, none can be properly said to be inverted. Now, since all the images produced upon the retina liold, with relation to each other, the same position, none are inverted ^-ith respect to others ; and as such images alone can be the object of vision, no one object of vision can be inverted with respect to any other object of vision ; and, consequently, all being seen in the same position, that position is called the erect position. . , 710. The optic axis of the eye is a line What is the t i i i 11 n optic axis of drawn perpendicularly through the center of the cornea, and center of the eye-ball. ^s^dTwe^iirt "^^^^ reason why with two eyes we do not see potnt'ofar^ double is, because the axis of both eyes is ject double? turned to one point, and therefore the same impression is made on the retina of each eye. The law of vision for visible objects is entirely different from that for points. A visible object can not, in all its parts, be seen single at the same instant of _ time, but the two eyes converge their axes to the near and the remote parts of it in succession, and thus give an idea of the different distances of its parts. Aaj defect which will prevent the two eyes from moving together conjointly, and from converging their optic axes upon every point of an object in succes- sion, will be fatal to distinct vision. • Birds of prey are enabled to adjus', their eyes bo as to see objects at a great distance, and again those which are very near. The first is accomplished by means of a muscle in the eye, which permits them to flatten the cornea by drawin;^ back the crystalline lens; and to enable them to perceive distinctly very near objects, their ryes are furnished with a tie.xible bony rim, by which the cornea is thrown forward at will, and the eye thus ren- dered near-sighted. 354 WELLS'S NATURAL PHILOSOPHY. „ , Double vision may be produced by pressincr How may don- . . .' x^ o bie vision be slightly from the side upon the ball of either produced? "-" , . . i • i eye while viewing an object ; the pressure of the finger prevents the ball of one eye from following the motion of the other, and the axis of vision in each eye being rendered different, we see two images. Strabismus, or squinting, is caused by the inability of one eye to follow th« motioiia of the other, and persons so affected always see double ; practice^ however, gives them power of attending to the sensation of only one eye at a time. It is from this inability of the eye to fix its optical axis that drunkards see double. „ . 711. We judge of the distance and size of judge of the an object by the relative direction of lines distance and •' •' Bize of an ob- drawn from the object to the eye, and by the angle which the intersection of these lines makes with the eye. This angle is called the angle of vision. Fig. 287. The student will bear in mind that an angle is simply the angie^of vision? inclination of two lines without any regard to their length. Thus, in Fig. 287, the Unes drawn from A and B, C and D, which may be supposed to represent rays of light, meet at the eye, and form an angle at the point of intersection. This angle is the angle of vision. If A B, Fig. 287, represent a man on a distant mountain, or on a church steeple, and C D a crow close by, the angle formed by the inclination of tha lines proceeding from the two objects will be equal, or the line A B, whicli a the height of the man, will subtend the same angle as the line C D, which is the height of the crow ; and therefore the man appears at such a distance no larger than a crow. jT . , The nearer an object is to the eye, the greater must be the an^le of vision inclination of the lines drawn from its extremities to intersect. distance? ^^ ^°*^ ^^^''^ ^^ angle at the eye, and consequently the greater will be its angle of vision. On the contrary, the more remote THE EYE, AND THE PHENOMENA OF VISION. 355 an olject is from the eye, the less will be the inclination of the lines, and tho less the angle of vision. The nearer an object is to the eye, therefore, the larger it will appear. Fig. 288. Thus the trees and houses far down a street or avenue appear smaller than those near by, and tho size of a vessel seen at sea diminishes with the increase of distance, as is shown in Fig. 288. Tho moon, on account of its proximity, appears much larger than any of the stars or planets, although '\t is, in fact, very much smaller. Fia. 289. - Q Let A B, Fig. 289, represent a planet, and C D the moon. The angle of Tision which the planet A B makes with the eye at G, is evidently less than the angle which the moon subtends at the same point. To a spectator at Gr, therefore, A B, though much the larger body, will appear no larger than E F; whereas the moon, C D, will appear as large as the hne C D. When will an "^l^. When ail object is so remote, or so object appear gmaU that llncs drawn from its extremities as a mere ' point r form no appreciable angle at the eye, the ob- ject appears as a mere speck or point. How »mnii an ^^^ ^^^j with an Ordinary amount of light, •bject is visible can 866 an object which occupies in the field to the eye ? '' J- ^ of view a space of only the sixtieth of a de- gree (or one minute). This space is about the 100th of an inch in a circle of twelve inches diameter, the eye being supposed to be in the center of the circle. Now a body smaller than this at six inches from the eye, or any thing, however large, placed so iar from the eye as to occupy in the field of view less space than this, is invis- 356 WELLS'S NATURAL PHILOSOPHY. ible to ordinary sight. At four miles off, a man becomes thus invisible, and a pin-head near by will hide a house on a distant hill.* What do ^e "^l^- When we say we see an object, we ?iy* wr^'see In dean that the mind is taking cognizance of a object? picture or image of the object formed on the retina. The manner in which the sensation is conveyed by the optic nerve to the brain, and a knowledge of tho external object imparted to the mind, is entirely un- known. ^ ^. As the picture, or imaoro on the retina, is formed on a corn- Does the sense ■ ■, n r o of sight give paratively flat surface, the sense of sight can not of itself af- c^'uon'^l^^form ^^^^ ^°^ immediate perception of the distance, size, or position size, position, of external objects. This knowledge we gain by experience * '^' derived from continued observation, and from tlie other senses. A young child has no conception of distance, and grasps at the moon as if it were an object immediately within its reach. Persons born blind and re- stored to sight by surgical operations, although able to see distinctly, can not properly comprehend any object or prospect before them. " I see men as trees walking," said the man bom blind when restored to sight. Individuals thus situated acquire the correct sense of vision only by degrees, like infants . and it is by experience tliat they learn to walk about among the objects around them, without the continual apprehension of striking themselves against every thing they behold. What is Per- Perspective is the name given to that science spective? which tcachcs how to draw on a jilane surface true pictures of objects as they appear to the eye from any distance and in any position. The skill of the artist consists in rightly applying tlie laws and principles of perspective ; and a picture is perfect to the extent in which it agrees with our experience of the objects it represents. 714. Many optical and mental delusions are occasioned in estimating the size, figure, and position of objects, by • " The smallest particle of a white substance distinguishable by the naked ere upon a black ground, or of a black substance upon a white ground, is about the l-400th of an Inch square. It is possible, by the closest attention, and by the most f worable directioa of light, to recognize particles that are only l-540th of an inch square, but without any sharpness or certainty. But particles which strongly reflect light may be seen when not half the size of the least of the foregoing: thus, gold dust of the fineness of 1-1 125th of an inch may be discerned by the naked eye in common daylight. When particles that can not be distinguished by themselves with the naked eye are placed in a row, they become visible ; and hence the delicacy of vision is greater for lines than for single particles. Thus, opaque threads of no more than l-490(1th of an inch across, or about half the diam- eter of the silkworm's fiber, may be discerned wiLh the naked eye when they are held toward the light." — Dr. Cai-pentcr. THE EYE, AND THE PHENOMENA OF VISION. 357 an erroneous application of the experience which in ordi- nary cases supphcs true and accurate conclusions. Thus, to most persons a conflagration at night, however misjudge' the distant, appears as if very near. The explanation of this mis- distance of a ^ j.g ig ag follows: — LUAit radiating from a center rapidly fire in the ° ° . , • /. night f weakens as the distance from the center increases, being, lor instance, only one fourth part as intense at double the dis- tance. The eye learns to make these allowances, and by the clearness and intensity of the light proceeding from the object, judges with considerable ac- curacy of the comparative distance. But a lire at night appears uncommonly brilliant, and therefore seems near. The evening-star rising over a hill-top, appears as if situated directly over the top of the eminence. The reason of this also is, that in judging we make brightness and clearness to depend on contiguity, as it ordinarily does; and as the star is bright, we unconsciously think it near us. In consequence of terrestrial objects being placed in close and moon ap- comparison, the sun and moon appear larger at their rising pear larger ^^^^ setting than at any other time. This illusion is wholly a when nsing and o .' .' setting than at mental one, since the organs of vision do not present to us a other umeb? larger image of the sun or moon in the horizon than when in the zenith, or overhead. ,^^ ^ ,, The moon, although a sphere, appears to be a flat surface, Why does the i ^ f t if > moon, a sphere, since it is SO remote that we are unable to distinguish any appear hke a difierence between the lonGrth of the ravs reflected from the flat surface i ^ - circumference, and those reflected from the center. Thus the says A D and C D, Fig. 290, appear to be no longer than the ray ^ B D ; but if all the rays seem of the same length, the part B I ■u'ill not seem to be nearer to us than A and C ; and there- fore the curve ABC will look like a flat, or horizontal surface. The rays A D and C D are 210,000 miles long. The ray B D is 238,910 miles long. What two '^15- I^ order that the eye may see distinctly, ■emfai foT d1^ the picture formed upon the retina must he tiact vision? illuminated to the right degree, and it must also remain sufficiently long upon the retina to produce a Bcnsation upon the optic nerve. The image of an object on the retina may bo illuminated too much or too little to produce a sensible perception of its form. Thus, wo can gain no idea of the form of the sun by viewing it in the clear skj', because the degree of illumination is so great, that the sense of vision is overpowered, just as sounds arc sometimes so intense as to bo deafening. That it is the intense splendor alono which prevents a distinct perception of the sun's figure, is rendered 358 WELLS'S NATURAL PHILOSOPHY. evident by the fact that when a portion of the liglit is cut off hj a colore(3 glass, or a thin cloud, the image of tlie sua is seen distinctly. On the con- trary, we fail to perceive many stars at night, because the images they pro- duce on the retina are too faintly illuminated to produce sensation. That some light from such stars actually enters the eye, is proved by the fact that if we place a lens before the eye, and collect a greater quantity of their light upon the retina, they at once become visible. Can the eye The Gje posscsscs a limited power of accom- Ses'onuu- modating itself to various degrees of illumi- ttiaation? natioD. Ill tliG dark, the pupil of the eye enlarges its opening, and allows a greater number of rays to fall upon the retina ; in the light, the pupil contracts in proportion to the intensity of the illumination, and diminishes the number of rays falling upon the retina. .yy^ . . This change does not take place instantaneously. "When from the light We leave a brilliantly illuminated apartment at night and go do°we ^find'^u '^^^'^ *^^® ^^^^ Street, wo are unable for a few moments to seo difficult at first any thing distinctly. The reason of this is, that the pupil of thing? ^°^ ^^° "^y^i which has become contracted in the light, is unablo to collect sufficient raj-s from the objects in the dark to see them distinctly. In a few moments, however, the pupil dilates, allows moro rays to pass througli its aperture, and we seo more distinctly. The reverse of this takes place when wo go from the dark into the light. Cats, owls, and some other animals are able to see distinctly in the dark, because they have the power of enlarging the pupils of theur eyes so aa to collect the scattered rays of light. Every impression made by light remains for a certain length of time on the retina of the eye, according to the intensity of its efl'ects, and a measur- able period is necessary to produce a sensation. .^j^^j. . "Wo are unablo, when riding rapidly on a railroad, to count prove the con- the posts of an adjoining fence, because the light from each imager on the ^°^^ ^^^^^ "P^"^ *'^° ^y^ ^^ ^^^^^ ^^P^'^ succession, that the dif- retina after the ferent images become confused and blended, and wo do not •ppeared ? '*'' obtain a distinct vision of the particular parts. If wo rotate a stick, lighted at ono end, somewhat rapidly, it seems to produce a complete circle of fire ; the reason of this is, that tho eye retains the image of any bright object for some little time after the object is withdrawn ; and as the light of the stick returns to each particular point of its path before the image previously formed has faded from the retina, it seems to form a complete circle of fire. w^ . . This continuance of the impression of external objects on dark when we the retina after the light proceeding from them has ceased to ^'" act, is tho reason also why w© are not sensiblo of darknesa when wo wink. THE EYE, AND THE PHENOMENA OF VISION. 359 The apparent motion of certain colored figures in -worsted work, known by tlie name of the "dancing mice," is due to the fact that when the surface is moved in a particular direction, as from side to side, the impression of the color on the retina remains for an appreciable interval after the figures have moved, and this gives to them an apparent motion. This effect will not, however, take place unless the colors of the figures and the ground-work are very brilliant and complementary of each other, as red upon a green ground. When is motion 716. No motion is perceptible to the eye tofureye'?"^ which has a less apparent velocity than one degree per minute. It is for this reason that the motions of the heavenly bodies are invisible, not- withstanding their immense velocity. The apparent motion of the sun, moon, and stars, owing to the revolution of the earth, is one quarter of a degree a minute ; but if the earth revolved on its axis in six hours instead of twcnty- foar, then the celestial bodies would have a motion of one degree per minute, and their movements would be distinctly perceptible. For the same reason, the motions of the hands of a clock are not per- ceptible to the eye. On the contrary, when a body moves with such rapidity from one position to another, that its image does not remain long enough upon one point of the retina to sufficiently impress it, it becomes invisible. Kence it is that a ball discharged from a cannon, and passing transversely across the eye, is not seen. How is appa- Apparent motion is affected by distance, and f^LTbT^t t^e motion of a body which is visible at one taace? distancc may be invisible at another, inasmuch as the angular velocity will be increased as the distance is diminished. Thus, if an object at a distance of 57^ feet from the eye move at the rate of a foot per second, it will appear to move at the rate of one degree per second, inasmuch as a line one foot long at 57^ feet distance subtends an angle of one degree. Now if tlie eye be removed from such an object to a distance of 115 feet, the apparent motion will be half a degree, or thirty min- utes per second ; and if it be removed to thirty times that distance, the ap- parent motion will be thirty times slower. Or if, on the other hand, the eye be brought nearer to the object, the apparent motion will be accelerated ia exactly tlie same proportion as the distance of the eye is diminished. A cannon-ball moving at 1,000 miles an hour transversely to ths lino of vision, and at a distance of fiftv yards from the eye, will be invisible, since it will not remain a sufficient time in any one positi?/a to produce perception. The moon, however, moving with more than double the velocity of the can- non-ball, being at a distance of 240.000 miles, has an apparent motion soalow aa to bo imperceptible to the unassisted eye. 360 WELLS'S NATURAL PHILOSOPHY. SECTION V. Describe the portable cam- era obscura. Fig. 291, OPTICAL INSTRUMENTS, 717. The portable camera obscura, such as is ordinarily used for photographic purposes, consists of a pair of achro- matic double convex lenses, set in a brass mounting (see Fig, 291), into a box consisting of two parts, one of which ,^i| slides within the otlier. The total length of the box la wTt adjusted to suit the focal distance of the lens. In the |i-l back of the box, which can be opened, there is a square ^^ piece of ground glass which receives the images of the -^ objects to which the lens is directed, and hy slidixig the movable part of the box in or out, the ground glass can be brought to the ^IG- 292. precise focus. The interior of the box is blackened all over to extinguish any stray light. The appearance of the camera as described is represented by Fig. 292. What are Spec- 718. SpGCta- '^''^^'- cles consist of two glass or crystal lenses, of such a character as to remedy the defects of vision in imperfect eyes, — mounted in a frame so as to be conveniently supported before the eyes. Spectacles are of two kinds, namely those with convex glasses, which magnify objects, or bring their images nearer to the eyes ; and those with concave glasses, which diminish the ap- parent size of objects, or extend the Hmits of distinct vision. Some persons, in order to protect the eye from excessive light, use blue glasses as spectacles ; they are, however, more mischievous than useful, since they absorb different parts of the spectrum unequally, and transmit the violet and blue rays. What is a Mi- ^^^- ^ Microscopc is any instrument which croscope? magnifies the images of minute objects, and enables us to see them with greater distinctness. This result is produced by enlarging the angle of vision under What are the two varieties of spectacles ? OPTICAL INSTRUMENTS. 361 which the object 13 seen — since the apparent magnitude of every body increases or diminishes with the size of this angle. Microscopes are of two kinds — simple and compound. What are the ^^ *^^ simplc microscopc, the object under two ▼arieties examiuatiou is viewed directly, either by a •f microscopes? . •' - •' Simple or compound convergmg lens. In the compound microscope, an optical image of tha object, produced upon an enlarged scale, is thus viewed. The simple microscope is generally a simple convex lens, in the locus of which the object to be examined is placed. Little spheres of glass, formed by melting glass threads in the flame of a candle, form very powerful microscopes. Fig. 293 represents the mag- nifying principle of the micro- scope. An eye at E would seo the arrow A B, under the visual angle A E B ; but when tho lens, F F' is interposed, it is seen under the visual angle at A' E B', and hence it appears much enlarged, as ahown in the image A' B'. FlQ. 293. Fig. 294 represents the most im- proved form of mounting a simple microscope. A horizontal support, capable of being elevated or depressed by means of a screw and ratch-woik, D, sustains a double-convex lens, A. The object to be viewed is placed upon a piece of glass, C, upon a stand- ard, B, immediately below the lens. As it is desirable that the object to be magnified should be strongly illuminated, a concave mirror of glass, M, is placed at the base of the instru- ment, inclined at such an angle as to reflect the rays of light which fall upon it directly upon the object. What is the 720. TheCom- Zt'ZTouM pound Micro- Microscope ? g^jQpg^ i^ jj.g Jj^^st FiS. 294. 362 WELLS'S NATURAL PHILOSOPHY. simple form, consists of two lenses, so arranged that the second lens magnifies the image formed by the first lens, or simple microscope. In this way the image of the object is examined by the eye, and not the object itself. The first of these lenses is called the object- glass, or objective, since it is always directed immediately to the object, which is placed very near it ; and the latter the eye-glass, or eye-piece, inasmuch as the eye of the observer is applied to it to view the magnified image of the object. Fig. 295. How are the lenses of a compound mi- cr iscope desig- nated ? Fig. 295 illustrates the magnifying principle of the compound microscope. O represents the object-glass placed near the object to be viewed, A B, and G, the eye-glass placed near the eye of the observer, E. The object-glass, 0, presents a magnified and inverted image, a b, of the object at the focus of tho eye-glass, G. The image thus formed, by means of the second lens or eye- glass, G, is magnified and brought to the eye at E, so as to appear under the enlarged visual angle, A' E B'. If we suppose the object-glass, 0, to have a magnifying power of 25 — that is, if the image a h equals 25 A B, and the eye-glass, G, to have a magnifying power of 4 — then the total magnifying power of the microscope will be 4 times 25, or 100; that is to say, tho image will appear 100 times the size of the object. Fig. 296 represents the most approved form of mounting the lenses which compose a compound microscope. The tube, A, which contains in its upper part the eye-glass, slides into another tube, B, in the bottom of ■which the object-glass is fixed ; this last tube also moves up and down in the stand, C, and in this way the lenses in the tubes may be adjusted to tho proper distance from each other and the object. M is a mirror for reflecting light upon tho object, and S a support on which the object to be examined IS placed. OPTICAL INSTRUMENTS. 363 WTiat is I Ttlescope ? How many kinds of tele- scopes are there? T21. A Telescope is any I"ig- 296. instrument which magni- fies and renders visible to the eye the images of distant objects. This result is efiected in the same manner as in the microscope, viz., by enlarging the \isual angle under which the objects are seen. Telescopes are of two kinds, refracting telescopes and reflecting telescopes; the principle of construction in both beins: the same as that of the com- pound microscope. What is a Re- '^^- The Ecfractiug Icope?" ^^^^' Telescope consists essen- tially of two convex lenses, the object-glass and the eye-glass. An inverted image of an object, as a star, is produced by the object-glass, and magnified by the eye-glass. Fig. 297 represents the principle of construction of the astronomical refracting telescope. 0 is an object-glass placed at the end of a tube, which collects the rays proceedinfr from a distant object and forms an inverted imago of tlic s:ime at o o', in tho focus of the eye-glass, G. By this the image is magnified and viewed by tb« eye at K Fig. 297. "Wliat is Equatorial Telescope ? 723. When a telescope is mounted on an axis inclined to the latitude of a place, so that it can follow a star, or planet, in its diurnal revolution, by a single motion, it is called an Equato^ RIAL Telescope. Such an instrument is generally moved by clock-work, an4 i^ accurately 364 WELLS S NATURAL PHILOSOPHY. counterbalanced by an arrangement of weights. A small telescope called tho finder, is attached near the eye end of tho large one ; this is so adjusted that when the object is seen through it, it appears in the field of the large tele- scope, thus saving much trouble in directing the instrument toward any par- ticular object. The mounting and attachments of an equatorial telescope are represented in Fig. 298. Fic. 2D8. What is a Spy. glass ? 724, A spy-i^lass, or terrestrial telescope, differs from an astronomical telescope only in an adjustment of lenses, which enables the observer to see the images of objects erect instead of inverted. This is efiected by the addition of two lenses placed between tho eye and the image. The arrangement of the lenses, and the course of the rays of light, in a common spy-glass, are represented in Fig. 299. 0 is the object-glass, and C L M the eye-glasses, placed at distances from each other equal to double their focal length. The progress of the rays through the object-glass, 0, and tho first eye-glass, C, is tho same as in tho astronomical telescope, and an inverted OPTICAL INSTRUMENTS. 365 image is formed ; but the second lens, L, reverses the image, which is viewed therefore, in an erect position by the last eye-glass, M. Fig. 299. Fig. 300, What is the 725. The common opera-glass, also called tC'^'opera- the Galilean telescope from Galileo, its in- giassr ventor, consists of a single convex object-glass and a concave eye-glass. Fig. 30O represents the construction of this form of telescope. 0 is a single convex object- glass, in the focus of which an inverted image of t-he object would be naturally formed, were it not for the interposition of the double-concave lens, E. Tliis receiving the converging rays of light, causes them to diverge and enter the eye parallel, and form an erect image. wiiatis a Re- "^^G. A Reflecting Telescope consists essen- BcopeT ^^^' tially of a concave mirror, the image in which is magnified by means of a lens. The mirror employed in reflecting telescopes is made of polished metal, and is termed a speculum. The manner in which the rays of light falling upon the concave speculum of a reflecting telescope are caused to converge to a focus is clearly shown in Fig. 301. The image formed at this focus is viewed through a double- convex lens. Fig, 301. Fig, 302 represents one of the earliest forms of the reflecting telescope, called from its inventor, Mr. Gregory, the " Gregorian Telescope." It consists of a concave metallic speculum, A B, with a hole in its center, and a convex eye-glass, E, the whole being fitted into a tube. An inverted image, n' m\ of a distant object is formed by the speculum, A B ; this image ia agaia 366 WELLS'S NATURAL PHILOSOPHY. Fig. 302. reflected by a small mirror, C D, and forma an erect image at n m, wiiich is magnified hy the lens, E, when ob- served hy the eye. Fig. 303. Another form of the reflecting tele- scope, called the Newtonian, is rep- resented in Fig. 303. It consists of a largo concave speculum, A B, set in one end of a tube, and a small plane mirror, C D, placed obliquely to the axis of the tube. The image of a distant object formed by tlie spoculum, A B is reflect- ed by the mirror, C D, to a point, m' n', on the side of the tube, and is there viewed through an eye-glass, E. Fig. 304. Large reflecting telescopes, at the present da\', are so con- structed as to dispense with the small mirror. This is ac- complished by slightly inclin- ing the large speculum, so as to throw the image on on» side where it is viewed by an eye-glass, as is represented in Fig. 304 Fig. 305. OPTICAL INSTRUMENTS. 367 The largest telescope ever constructed is that made by Lord Rosso. This instrumeiit, which is a rellecting telescope, is located at Parsonstown, in Ireland. Its external appearance and method of mounting is represented in Fig. 305. The diameter of tlie speculum is 6 feet, and its weight about 4 tons. The tube in which it is placed is of wood hooped with iron, 52 feet in length, and 7 feet in diameter. It is counterpoised in eveiy direction, and moves between two walls, 2-1: feet distant, 72 feet long, and 48 feet high. The ob- server stands on a platform which rises or fails, or at great elevation upoa Bhding galleries which draw out from the walL This telescope commands an immense field of vision, and it is said that ob- jects as small as 100 yards' cube, can be distinctly observed by it in the mooa at a distance of 240,000 miles.* What is a Magic 727. The Magic Lantern is an optical in- Lantem? strumcnt ada])ted for exhibiting pictures paint- ed on glass in transparent colors, on a large scale, by means of magnifying lenses. Fig. 306. It consists of a metallic box, or lantern, A A', Fig. 306, containing a lamp, L, behind which is placed a metalhc concave mirror, p q. In front of the lamp are two lenses, fixed in a tube projecting from the side of the lantern, one of which, m, is called the illuminator, and the other the magnifier. The objects to be exhibited are painted on thin plates of glass, which are intro- duced by a narrow opening in the tube, c d, between the two lensea The mirror and the first lens, m, serve to illuminate the painting in a high degree^ for the lamp being placed in their foci, they throw a briUiant light upon i^ and the magnifying lens, n, which can slide in its tube a little backward and forward, is placed in such a position as to throw a highly magnified image if the drawing upon a screen, several feet off, the precise focal distance being adjusted by sliding the lens. The further the lantern is withdrawn from the ♦ By the aid of this mighty instrument, "one of the most wonderful contributions of art and science the world has yet seen," what astronomers have before called nebula, on account of their cloud-like appearance, have been discovered to be stars, or suns, analo- gous, in all probability, in constitution, to our own sun. In the constellations Andro- meda and the sword-hilt of Orion, both of which are visible to the naked eye, theso cl<>ud-like p&tches have been seen as clusters of stars. SGS WELLS'S NATURAL PHILOSOPHY. screen, the larger the image will appear ; but when the distance is considera- ble the image becomes indistinct. What are Dis- '^^S. The beautiful optical combinations BoivingViews? jjQown as Dlssolving Views are produced by means of two magic lanterns of equal power, so placed as to throw pictures of precisely equal magnitude on the same part of the same screen. By gradually closing the aperture of one lantern and opening that of the other, a picture formed by the first may seem to be dissolved away and changed into another. Thus, if the picture produced by one lantern represents a day landscape, and the picture produced by the other the same landscape by night, the one may be changed into the other so gradually as to imitate with great exactness the appearance of approaching night. What is a Solar 'i^29. The Solar Microscope is an optical in- Microscope? gtrument constructed on the principle of the magic lantern, but the light which illuminates the object is supplied by the sun instead of a lamp. This result is effected by admitting the rays of the sun into a darkened room, through a lens placed in an aperture in a window shutter, the rays being received by a plane mirror fixed ob- .liquely, outside the shutter, and thrown horizontally on the lens. The object is placed between this lens and another smaller lens, as in the magic lantern ; and the magnified imago formed is received upon a screen. In Fig. 307, which represents ihe construction of the solar microscope, 0 is a plane mirror, A the illuminating lens, and B the magnifying lens. The objects to be magnified are placed between the lenses A and B. In consequence of the superior illumination of the object by the rays of the sun, it will bear to be magnified much more highly than with the lantern. Hence this form of microscope is often employed to represent, on a very enlarged scale, various minute natural objects, such as animalculfc existing in various liquids, crys- tallization of various salts, and the structure of vegetable substances. Pia. 307, CHAPTER XV. ELECTRICITY. ■What is Eiec 730. Electricity is one of those subtle tricity? agents without weight, or form, that appear to be difFused through all nature, existing in all substances without aflecting their volume or their temperature, or giving any indication of its presence when in a latent, or ordinary state. When, however, it is liberated from this repose, it is capable of producing the most sudden and destructive effects, or of exerting powerful influences by a quiet and long-continued action. HoTrmayeiec- ^31. Elcctricity may be excited, or called cUed? ^^ ^^' i*^^^ activity by mechanical action, by chemical action, by heat, and by magnetic influence. "We do not know any reason why the means above enumerated should de- velop electricity from its latent condition, neither do we know whether elec- tricity is a material substance, a property of matter, or the vibration of an ether. The general opinion at the present day is that electricity, Uke light and heat, is the result of vibrations of an ether pervading all space. How is dec- 732. The most ordinary and the easiest way easuy ciciTedl of cxcitiug clcctricity is by mechanical action — by friction. How does eiec- If WO rub a glass rod, or a piece of sealing- by"*^ friction wax, or resin, or amber, with a dry woolen, or manifest Itself? g||^ substancc, thcsc substanccs will imme- diately acquire the property of attracting light bodies, such as bits of paper, silk, gold-leaf, balls of pith, etc. This attractive force is so great, that even at the dis- tance of more than a foot, light substances are drawn to- ward the attracting body. The cause of this attraction is called electricity. Thales, one of the seven wise men of Greece, noticed and recorded tho fact more than two thousand years ago, that amber when rubbed would at- 16* 370 WELLS'S NATURAL PHILOSOPHY. tract light bodies ; and the name electricity, used to designate such pheno- mena has been derived from the Greek word tjXektpov, electron, signiiying amber. What other ef- ^^ *^^ frictioii of the glass, wax, amber, etc., tractiontreno- ^^ vigorous, Small strcams of light will be seen, eiectricitr''b"/ ^ crackling noise heard, and sometimes a re- friction? markable odor will be perceived. whenisabody 733. When, by friction or other means, elec- trified?''^ ^''"'' tricity is developed in a body, it is said to be electrified, or electrically excited. What is electric The tendency which an electrified body has to move toward other bodies, or of other bodies toward it, is ascribed to a force called electric attraction. whatis electric Evcry electrified body, in addition to its at- repuision? tractivc'forcc, manifests also a repulsive force. This is proved by the fact that light substances, after touching an electrified body, recede from it just as actively as they approached it before contact. Such action is as- cribed to a force called electric repulsion. * Thus, if we take a dry glass rod, rub it well Fig 308 j n ^ ' with silk, and present it to a light pith ball, or feather, P, suspended from a support by a silk thread, the ball or feather will be attracted to- ward the glass, as seen at G, Fig. 308. After it has adhered to it a moment, it will fly off, or be repelled, as P' from G'. The same thing will happen if sealing-wax bo rubbed with dry flannel, and a like experiment made ; but with this remarkable difference, that when the glass repels the ball, the sealing-wax attracts it, j-j^ ^q^ and when the wax repels, the glass will attract. Thus if we suspend a light pith baU, or feather, by a silk thread, as in Fig. 309, and present a stick of excited sealing-wax, S, on one side, and a tube of excited glass, G, on the other, the ball will commence vibrating like a pendulum from one to the other, being alternately attracted and repelled by each, the one attracting when the other repels; hence we conclude that the electricities excited in the glass and wax are different. 784. As the electricity developed by the Is there more „ . . -. , , i -i-i i than one kind frictiou 01 glass and other like substances is essentially difierent from that developed by 'electricity. 371 the friction of resla, was, etc., it has been inferred that there are two kinds or states of electricity — the one called vitreous, because especially developed on glass, and the other resinous, because first noticed on resinous sub- stances. What is the The fundamental law which governs the re- &Tc^trfJai*\^ lation of these two electricities to each other, rep^iasToar*"'* and which constitutes the basis of this depart- ment of physical science, may be expressed as follows : — Like electricities repel each other, unlike electricities attract each other. Thus, if two substances are charged with vitreous electricity, they repel each otherj two substances charged with resinous electricity also repel each other ; but if one is charged with vitreous, and the other with resinous elec- tricity, they attract each other. whenisabody '^^^- Wlicu a body holcls its own natural non-electrified? quantity of elcctrlcity undisturbed, it is said to be non-electrified. "When an electrified body touches one that trifled body is non-electrlficd, the electricity contained in non-electrified, the formcr is transferred in part to the latter. what occurs ? Thus, on touching the end of a suspended silk thread with a piece of excited wax or glass, electricity will pass from the wax or glass into the silk, and render it electrified; and the silk will exhibit the effects of the electricity imparted to it, by moving toward any object that may be placed near it. 736. Two theories, based upon the phenom- ories have been eua of attraction and repulsion, have been count for eiec- formcd to account for the nature and origin of electricity. These two theories are known as the theory of two fluids, and the theory of the single fluid ; or the theory of Da Fay, an eminent French electrician, and the theory of Dr. Franklin. 737. The theory of two fluids, or the theory theory of two of Du Fay, supposcs that all bodies, in their natural state, are pervaded by an exceedingly thin subtle fluid, which is composed of two constituents, 372 WELLS'S NATURAL PHfLOSOPHT. or elements, viz., the vitreous and the resinous electrici- ties. Each kind is supposed to repel its own particles, but attract the particles of the other kind. When these two fluids pervade a body in equal quantities, they neutralize each other in virtue of their mutual attraction, and remain in repose ; but •when a body contains more of one than of the other, it exhibits vitreous or resinous electricity, as the case may be. 738. The theory of a single fluid, or the theory of a thcory propoundcd hy Dr. Franklin, supposes Bing e ui ^^^^ existence of a single subtile fluid, without weight, equally distributed throughout nature ; every sub- stance being so constituted as to retain a certain quantity, which is necessary to its physical condition. When a substance pervaded by this single fluid is in its natural state or condition, it offers no evidence of the presence of electricity; but when its natural condition is disturbed it appears electrified. The difference between the electricity developed by glass and that by resin is explained by this theory, by supposing electrical excitation to arise from the difference in tho relative quantities of this principle existing in the body rubbed and the rub- ber, or in their powers of receiving and retaining electricity. Thus one body becomes overcharged by having abstracted this principle from the other. whatareposi- '^^^' Tlic t WO different conditious of elcctric- tive "efeS- i^Jj which wcro called by Du Fay vitreous and *'^^^ resinous electricities, were designated by Dr. Franklin as positive and negative, or plus and minus. Thus a body v/hich has an overplus of electricity is called positive, and one that has less than its natural quantity is called negative. The theory of a single fluid has, until quite recently, been generally adopted by scientific men, and tho terms positive and negative electricities are uni- versally used in the place of vitreous and resinous. Within the last few yars, however, some discoveries have been made which seem to indicate that the theory of two fluids is the one which approaches nearest to the truth. .„^ ^ . x. In addition to these two theories respecting the nature of What 18 Pro- ^ '^ fessor Fara- electricity, another has been proposed by Professor Faraday, eiec'tridty7 °^ °^ England. He considers electricity to be an attribute, or quality of matter, like what we conceive of the attractioc of gravitation.* * It is not easy to perfectly explain to a befrfnner the view which has been talcen by Professor Faraday (who is at present the highest recognized authority on this subject) re- fpecting the nature of electricity. The following statement, as given by a late writer (Robert Hunt), may be sufficiently comprehensive and clear : " Every atom of matter is ELECTRTCITY. 373 740. Light, heat, and electricity appear to have some prop- connection be- erties in common, and each may be made, under certain cir- tween light, cumstances, to produce or excite the other. All are so light, heat, and elec- i i- <.,••., tricity? Subtle, andditfusive, that it has been found impossible to recog- nize in them the ordinary characteristics of matter. Some sup- pose that light, heat, and electricity are all modifications of a common principle. 741. Electricity exists in, or may be excited in all bodies, electrical di- There are no exceptions to this rule, but electricity is de- ^b°ta* "% ^^ veloped in some bodies with great ease, and in others with great difficulty. All substances, therefore, have been divided into two classes, viz.. Electrics, or those which can be easily excited, and Non-electrica, or those which are excited wth difficulty. Such a division is, however, of Uttle practical value in science, and at present is not generally recognized. There is no certain test which ^-ill enable us to determine, previous to ex- periment, which of two bodies submitted to friction will produce positive, and which negative electricity. Of all known substances, a cat's fur is the most susceptible of positive, and sulphur of negative electricity. Between these extreme substances others might be so arranged, that any substance in the list being rubbed upon any other, that which holds the highest place will be positively electrified, and that which holds the lower place negatively elec- trified. For instance, smooth glass becomes positively electrified when rub- bed with silk or flannel, but negatively electrified when excited by the back of a living cat. Sealing-wax becomes positive when rubbed with the metals, but negative by any thing else. Can one elec- I^ HO CRSG Can electricity of one kind be citeii^wfthout excited without setting free a corresponding othwf ''^^^"'^ amount of electricity of the other kind ; hence, when electricity is excited by friction, the rub- ber always exhibits the one, and the electric, or body rubbed, the other. What ara con- 742. Bodics differ greatly in the freedom non!^onductor* ^'^^^ which they allow electricity to pass over of electricity f qj. through them. Those substances which regarded as existing by virtue of certain properties or powers, these being merely pecu- liar affections, which may be regarded as being of a similar nature to vibrations. It it assumed that the electric state is but a mode or form of one of these affections. One par- ticle of matter, having received this form of disturbanoe, communicites it to all contigu- ous particles — that is, those which are next to it, although not in contact— and this com- munication of force takes place more or less readily, the communicating particles assuming a polarized state — which may be explained as a stitc presenting two dissimilar extremities. When the communication is slow, the polarized state is bighpst, and the body is said to be an insulator: insulation being the result. If the particles communicate their condition readily, they are termed conductors : conduction is the result. The phenomena of in. duction, or the production of like effects in contiguous bodies, is, therefore, according t* this view, but something analogous to the communication of tremors, or vibratioos." 374 WELLS'S NATURAL PHILOSOPHY. facilitate its passage arc called conductors ; those tliat re- tard, or almost prevent it, are called non-conductors. No substance can entirely prevent the passage of electricity, nor is there any which does not oppose some resistance to its passage. "What Eiib- 0^' ^^^ bodies, the metals are the most per- comhictort^^'of ^^ct couductors of clcctricity ; charcoal, the electricity? earth. Water, moist air, most liquids, except oils, and the human body, are also good conductors of electricity. What IB the *^^^- The velocity with which electricity tricuyr"^ *''*"'' passes through good conductors is so great, that the most rapid motion produced by art appears to be actual rest when compared to it. Some authorities have estimated that electricity will pass through copper wire at the rate of two hundred and eighty-eight thousand miles in a second of time — a ve- locity greater than that of light. The results obtained, however, by the United States Coast Survey, with iron wire, show a velocity of from 15,000 to 20,000 miles per second. What snh- Gum sliellac and gutta percha are the most conTuct^rsTf' P^rfcct nou-conductors of electricity ; sulphur, electricity? sealing- wax, resin, and all resinous bodies, glass, silk, feathers, hair, dry wool, dry air, and baked wood are also non-conductors. Electricity always passes by preference over the best conductors. Thus, if a metallic chain or -wire is held in the hand, one end touching the ground and the other brought into contact with an electrLfied body, no part of the electricity will pass into the hand, the chain being a better conductor than the flesh of the hand. But if; while one end of the chain is in contact with the conductor, the other be separated from the ground, then the electricity will pass into the hand, and vnll be rendered sensible by a convulsive shock. WTienisabody 7^44. Whcu 0. couductor of electricity is sur- rounded on all sides by non-conducting sub- stances, it is said to be insulated; and the non-conducting substances which surround it are called insulators. ELECTRICITY. 375 whenisabody When a conducting body is insulated, it cwged" with retains upon its surface the electricity com- eiectricity? muuicated to it, and in this condition it is said to be charged with electricity. A .conductor of electricity can only remain electric as long as it is insulated, that is, surrounded by perfect non-conductors. The air is an insulator, since, if it were not so, electricity would bo instantly withdrawn by the atmosphero from electrified substances. "Water and steam are good conductors, conse- quently, when the atmosphere is damp, the electricity will soon be lost^ which, in a dry condition of the air, would have adhered to an insulated con- ductor for a long period of time. Thus a globe of metal supported on a glass pillar, or suspended by a silken cord, and charged with electricity, will retain the charge. If, on the oon- traiy, it were supported on a metallic pOlar, or suspended by a metallic wire, the electricity would immediately pass away over the metaUic surface and escape. In the experiments made with the pith balls (§ 733, Fig. 308), the silk thread by which they were suspended acts as an insulator, and the electricity with which they become charged is not able to escape. 745. When electricity is communicated to Does clectnci- , , , ^ . . , ty accumulate a couductino^ bodv it Fcsidcs merely upon the upon the sur- ii ii face or the in- surfacc, and does not penetrate to any depth terior of bodies? ... wo. withm. _ Thus, if a solid globe of metal suspended by a silken thread, or supported upon an insulated glass pillar, be highly electrified, and two thin hollow caps of tin-foil or gilt paper, furnished with insulating handles, as is represented in Fig. 310, be applied to it, and then withdrawn, it will be found that the electricity has been completely taken off the sphere by means of the caps. An insulated hollow ball, however thin its substance, will contain a charge of electricity equal to that of a solid ball of the same size, all the electricity ia both cases being distributed upon the surface alone. In the case of a spherical body charged with electricity; form of a body *^^ distribution is equal all over the surface ; but when th© Influence iu body to which the electricity is communicated is larger in one dition? direction than the other, the electricity is chiefly found at it3 longer extremities, and the quantity at any point of its sur- fece is proportional to its distance from the center. The shape of a body also exercises great influence in retaining electricity : it is more easily retained l^ a sphere than by a spheroid or cylinder; but it readily escapes from a point, and a pointed object also receives it with tha greatest facility. 376 WELLS'S NATURAL PHILOSOPHY. What is the ''^^^- ^^^ earth is considered as the great of eieJtriciTl' general reservoir of electricity. When by means of a conducting substance a communi- cation is established between a body containing an excess of electricity and the earth, the body will immediately lose its surplus quantity, which passes into the earth and is lost by diffusion. What is dec 747. When a body charged with electricity tricai induction • ^f q^q j^jn^j jg brought into proximity with other bodies, it is able to induce or excite in them, with- out coming in contact, an opposite electrical condition. This phenomenon is called Electrical Induction. _ , . .. This effect arises from the peneral law of electrical attrac- Explain the . , , . . , , . . phenomena of tion and repulsion. A body m its natural condition contains induction. equal quantities of positive and negative electricities, and when this is the case, the two neutralize each other, and remain in a state of equili- brium. But when a body charged with electricity is brought into proximity with a neutral body, disturbance immediately ensues. The electrified body, by its attractive and repulsive influence, separates the two electricities of the neutral body, repelling the one of the same kind as itself, and attracting the other, which is unlike, or opposite. Thus, if a body electrified positively be brought near a neutral body, the positive electricity of the neutral body will be repelled to the most remote part of its surface, but the negative electricity will be attracted to the side which is nearest the disturbing body. Between these two regions a neutral line will separate those points of the body over which the two opposite fluids are respectively distributed. ■p,- „, , Let CAD, Fig. 311, be a metallic X l\i, olX* T -I , 1 ... cylinder placed upon an insulating support, with two pith balls sus- pended at one end, as at D. If now an electrified body, E, be brought near to one end of the cyl- inder, the balls at the other ex- tremity will immediately diverge from one another, showing the pres- ence of free electricity. This does not arise from a transfer of any of llie electric fluid from E to C, for upon withdrawing the electrified body, E, the balls will fall together, and appear unelectrified as before ; but the electricity in E decomposes by its proximity the combination of the two electricities in the cylinder, CAD, attracting the kind opposite to itself toward the end nearest to it, and repelling the same kind to the further end. The middle part of the cylinder. A, whidi intervenes between the two extremities, will remain neutral, and exhibit .either positive nor negative electricity. CAD U_ 1 . r^ Edl^ A 1 0 ELECTRICITY. 377 Fig. 312. If three cylinders aro placed in a row, touching Y ^ ■ . one another, as in Fig. 312, V ^ k X ^ ^ and a positively electrified body, E, be brought in proximity to one extremity, E^i the electricities of the cyl- inders will be decomposed, the negative being accumulated in N, and the positive repelled to P. If in this condition the cyhnder P be first removed, and then the electrified body, the separate electricities -will not be able to unite, as in the former experi- ment, but N will remain negatively, and P positively electrified. Explain the ThesG experiments explain why an electrified lie^nZd^Bu^- surface attracts a neutral, or unelectrified body, face attracts a g^^]^ ^g g^ pj<-]j jjall. It is not that elcctricitv neutral, or un- I •' electrified body, causcs attractious between excited and unex- cited bodies, the same as between bodies oppositely, ex- cited ; but that the pith ball is first rendered opposite by induction, and attracted in consequence of this opposition. A pith ball at a few inches distance from an electrified surface, is charged with electricity by induction ; and the kind being contrary to that of the surface, attraction en- sues ; when the two touch, they become of the same kind by conduction. A person may also receive an electric shock by induction. Thus, if a per- son stand close to a large conductor strongly charged with electricity, ho will be sensible of a shock when this conductor is suddenly discharged. Tliis shock is produced by the sudden recomposition of the fluids in the body of the person, decomposed by the previous inductive action of the conductor. What is an "^^8. Au elcctrical machine is an apparatus, cMnl?^ "*' ^y iiieans of which electricity is developed and accumulated, in a convenient manner for tho purposes of experiment. ofwhatessen- -^.11 clcctrical machincs consist of three an^ ''dectricai priuclpal parts, the rubber, the body on machiM con- "whose surfacc the electric fluid is evolved, and one or more insulated conductors, to which this electricity is transferred, and on which it is accumulated. 378 WSLLS'S NATURAL PHILOSOPHY. Describe the two varieties of electrical machines in commoa use. Electrical machines are of two kinds, the plate and cylinder machines. They derive their names from the shape of the glass em- ployed to yield the electricity. Fig. 313. The plate electrical machine, which is represented in Fig. 313, consists of a large circular plato of glass mounted upon a metallic axis, and supported up- on pillars fixed to a secure base, so that the plate can, by means of a handle, w, be turned with ease. Upon the sup- ports of the glass, and fixed so as to press easily but uniformly on the plate, are four rubbers, marked r r r r in the figure ; and flaps of silk, s s, oiled on ono side, are attached to these, and secured to fixed supports by several silk cords. When the machine is put in motion, these flaps of silk are drawn tightly against the glass, and thus the friction is increased, and electricity excited. The points p p collect the electricity from the glass as it revolves, and convey it to the prime conductor, c, which is insulated and supported by the glass rod, g. The cylinder electrical machine represented by Fig. 314, consists of a glass cylinder, so arranged that it can be turned on its axis by a crank, and supported by two uprights of wood, dried and varnished. F S indicates the position and ar- rangement of the rubber and silk, and Y that of the prime conductor. The principle of the con- struction of the cylinder machine is, in every respect, the same as that of the plate machine. What is the '^^® rubber of an electrical ma- chine consists of a cushion stuffed with hair, and covered with leather, or some substance which readily generates electricity by friction. The efiBciency of the machine is greatly increased by covering the cushioa with an amalgam, or mixture of mercury, tin, and zinc* In the ordinary working of the machine, the rubber is connected by a chain with the ground, from whence the supply of electricity is derived. • The best composUion of the amalgam is two parts, by weight, of zinc, one of tin, and Bis of mercury. The mercury is added to the mixture of the zinc and tin when in a fluid state, and the whole is then shaken in a wooden box until it is cold ; it is then reduced to a powder, and mixed with a sufficient quantity of lard to reduce it to the consistency of paste. A thin coating of this paste is spread over the cushion ; but before this is done, all darts of the machine should be carefully cleaned and warmed. construction of the rubber ? ELECTRICITY. 379 _, ^ , . The receiver of electricitv from an electrical machine is What Is the conductor i.f Called the prime conductor. It usually consists of a thin brass machin^*'"'^ cylinder, or a brass rod, mounted on a glass pillar, or some other insulating material. To put the electrical machine in good order, every part must be dry and clean, because dust or moisture would, by their conducting power, diSiise the electric fluid as fast as accumulated. As a general rule, it is highly essential that the atmosphere should be in a dry state when electrical experiments are made, as the conducting property of moist air prevents the collection of a sui^ ficient amount of electricity for the production of striking effects. In tliO winter, the experiments succeed best when performed in the vicinity of a fire ; and it is advisable to place the apparatus in front of the fire for some lime before it is employed. ^ . . . Electricitv is developed bv the action of an electrical ma- Explam the , . . " . „ , ' ... • , , method in chine m essentially tue same manner as it is m a simple glass which an elec- j^j^g jjy fi-iction. When the glass cvlinder or plate is turned tncil machine ■' c . r develops elec- round by the handle, the friction between the glass and- the tricityf rubber excites electricity; positive electricity being developed upon the glass, and negative upon the rubber. When the points of the prime conductor are presented to the revolving glass plafe or cylinder, the positive electricity is immediately transferred to it, and it emits sparks to any conduct- ing substance brought near. The electricity thus abundantly excited is sup- plied from the earth to the rubber (by means of a chain extending to the ground), and the rubber is continually having its supply drawn from it by the force called into action by friction with the glass. That the electricity is de- rived from this source is evident from tlie fact that but a small quantity of electricity can be excited when the metallic connection between the rubber and the ground is removed. For this reason the chain must always bo attached to the rubber when it is desired to develop positive electricity, and to the prime conductor when negative electricity is required. According to the theory of a single fluid, the excitement of electricity is as follows : — the friction of the glass and silk, by disturbing the electrical equi- librium deprives the rubber of its natural quantity of electricity, and it is therefore left in a negative state, unless a fresh quantity be continually drawn from the earth to supply its place. The surplus quantity is collected on the prime conductor, which thereby becomes charged with positive electricity. On the hypothesis of two electric fluids, the same frictional action causes the separation of the vitreous from the resinous electricity in the nibber, which therefore remains rosinously charged, unless there be a connection with the earth to restore the proportion of vitreous electricity of which the rubber has been deprived. Various other arrangements have been devised for the pro- boUer'be'uspd duction and accumulation of electricity. High-pressure steam as an electrical escaping from a steam-boiler carries with it minute particles of water, and the friction of these against the surface of the jet from which the steam issues produces electricity in great abundance. A 380 WELLS'S NATURAL PHILOSOPHY. Fig. 315. eteam-boiler, properly arranged and insulated, therefore constitutes a most powerful electrical machine ; and by mean3 of an apparatus of this character, constructed some time since in London, flashes of electricity were caused to emanate from the prime conductor more than 22 inches in length. 749. The Insulating Stool, which is a usual Bulating Stool ? appendage to an electrical machine, consists of a board of hard-baked wood, supported on glass legs covered with varnish. (See Fig. 315.) It is useful for insulating any body charged with electricity; and a person standing upon such a stool, and in communication with a prime conductor, will become charged with electricity. n Fig. 316. -er battery? ^j- guijijjate of copper (blue vitriol) is used a3 ..he exciting liquid, is represented by Fig. 339. It consists of two conceuirio jylinders of copper tightly soldered to a copper bottom, ind a zinc cylinder, Z, fitting in between them. The 'jnc cylinder, when let down into the solution, is pre- /ented from touching the copper by means of tlu'oo jieces of wood or ivory, shown in the figure. T>'ro tcrew-cups for holding the connecting vrires are at- ached, one to the outer copper cylinder, and the oiher o the zinc. The principal imperfection of the gal- vanic battery is the want of uniformity in its action. In all the variois forma the strength of the electric current ex- cited continually diminishes from the moment the battery a-'tion commences. In the sulphate of copper battery, especially, tne power if reduced to almost notLirg in a comparatively brief space of time. This i» '.- chiefly owing to the circumstance that the metallic plates soon become ii^ated with the products of the chemical decomposition, the result af the itiamical action, whereby the electricity is developed. This dilEculty is obviated, in a great degree, by the use of a diaphragm, ot f orous pa-'tition, between the two metallic plates, which allows a free contact • It is found that by coating the zhic with mercury, the waste of the zinc is greatly diminished. It is not well understood in what way the raercury contributes to this effect. We have a parallel to it in the rubber of the electrical machine, which, when coated with an amalgam of zinc and tin, acts with greater e£Sciency than under any other ciroum •tauoea. i^at is the jnncipiil im- perfection of :he galvanic uattery ? GALVANISM. 407 Fig. What is the construction of Grove's bat- tery? 3( the liquid on each side, within its pores, but prevents the solid proaucta of decomposiDDu from passing from one plate to the other. Describe Dan- Daniel's constant battery, constructed according to this iel's constant principle, and represented in Pig. 340, maintams an efiective ttery. galvanic action longer than any other ; a is a hollow cylinder of copper; z, a solid rod of amalgam- ated zinc; and e, a porous tube of earthenware separating the two. Diluted sulp'auric is placed in the porous tube, and a saturated solution of sulphate of copper in the copper cylinder. One of the most eflQ- cient batteries is that kno\vn as Grove's bat- tery, from its mventor, and 13 the form generally used for telegraphing and for otlier purposes in which powerful galvanic action is re- quired. It consists of a plain glass tumbkr, in which is placed a cyhnder of amalgamated zinc, with an opening on one side to allow a free circulation of the liquid Within this cyluider is placed a porous cup, or cell, of earthen- ware, in which is suspended a strip of platinimi fetened to the end of a ztac arm projecting from tlie adjoining zinc cylinder. The porous cup containing the platinum is filled with strong nitric acid, and the outer vessel containing the zinc with weak sulphuric Fig. 341. aci(j_ Pig ^^i represents a series of these cups, arranged to form & compound circuit, with their terminal poles, P and Z. This form of battery- is objectionable on account of the corrosive character of the acids employed, and the deleterious vapors tliat arise from it when in ac- tion. What is the dis- 777. The electricity evolved by a single gal- ter"if*^^T^ic vanic circle is great in quaatity, but weak in electricity? intensity. These two qualities may be compared to heat of different temperatures. A gallon of water at a temperature of 100° has a greater quantity of heat than a pmt at 200° ; but the heat of the latter is more intense than that of the former. What is the dis- The electricity, on the contrary, produced ter"of frSc^'ti'o"^ ^y friction, or that of the electrical machine, electricity? jg small iu quautity, but of high tension, o: intensity. 408 WELLS'S NATURAL PHILOSOPHY. . Frictional electricity is capable of passing for a considerable diffureiicc-s In;- distance through or over a non-conducting or insulating sub- tweeii the two gtance, whicli galvanic electricity can not do. Thus, the spark from a prime conductor will leap toward a conducting sub- Btance for some distance through the air, which is a non-conductor; but if '* current of galvanic electricity is resisted by the slightest insulation, or the in- terposition of some non-conducting substance, the action at once stops. Gal- vanic electricity will traverse a circuit of 2,000 miles of wire, rather than makj a short circuit by overleaping a space of resisting air not exceeding one hun- dredth part of an inch. Frictional electricity, on the other hand, v.'ill force ( passage across a considerable interval, in preference to taking a long circuit through a conducting wire, or at least the greater portion of it will pass through the air, though some part of the charge will always traverse the wire. Frictional electricity produces very slight chemical or heating effects ; gal- vanic electricity produces very powerful efliects. A proper and simple arrangement of a zinc plate and a little acidulated water, will produce as much electricity in three seconds of time as a Leyden jar battery charged with thirty turns of a largo and powerful plate electrical machine in perfect action. The shock received by transmitting this quantity of galvanic electricity through the animal system would be hardly perceptible, but received from a Leyden jar, would be highly dangerous, and perhaps fatal. A grain of water may be decomposed and separated into its two ele- ments, oxygen and hydrogen, by a very simple galvanic battery, in a very short time; but 800,000 such charges of a Leyden jar battery, as above re- ferred to, would be required to supply electricity sufficient to accomplish the same result. Such a quantity of electricity sent forth from a Leyden jar would be equal to a very powerful flash of lightning. rpon what does T^iG quantltj of electricity excited in a gal- vanic^'dectrici- vaiiic circuit is directly proportional to the ty depend? amount of cliemical action that takes place — as between the zinc and the acid. By increasing the amount of surface exposed to chemical action, we there- fore increase the quantity of electricity evolved. Hence, gigantic plates have been constructed for the purpose of obtaining^ an immense quantity. The intensity of the electricity evolved de- Upon what does i /> i i i • intensity de- pcnus upon thc niimocrot plates, and isgreat- ^™ est when thc voltaic pile is made up of a great number of small plates. Supposing an equal amount of surflxce of copper and zinc employed, the shock, and other indications of a strong charge, would be greater if it were cut up into many small circles, than if it formed a few large ones. But the actual quantity of excitement would bo greatest with the large plates. GALVANISM. 409 tiow may vol- '^'^^- When the wire from one end of a vol- interr^uptedand ^^^^ battery is conuected with the wire from renewed? ^^q opposite end, voltaic action instantly com- mences ; and it as instantaneously ceases when the con- nection is interrupted. The rapidity with which the elec- tric circuit may be completed and broken has no ascertained limit ; nor does it appear to be controlled by resistance caused by traversing miles of wire. 779. The most ordinary effects produced by most ordinary thc dcvelopcd clectricitY of a larsre g:alvanic effect* of gal- 1 ^ , , ■/ „ r 1 1 -1 vanic electric!- battcry, are the production of sparks and bril- liant flashes of light, the heating and fusing of metals, the ignition of gunpowder and other inflam- mable substances, and the decomposition of water, saline compounds, and metallic oxyds. _ Heat is evolved whenever a galvanic cur- Whendoesfral- . ^ -i ^ vanic eiectrici- rcut passcs ovcr a conductmfi^ body, the amount tyevolTeheat? ., a j ? ^ of which will depend on the quantity and in- tensity of the electricity transmitted, and upon the re- sistance which the body offers to the passage of the cur- rent. The metals differ greatly in their conducting power. Thus, if vre link together pieces of copper, iron, silver, and platinum wire, and pass a galvanic current along them, they will be found to be unequally heated, the platinum being the most, and the copper the least. The easiest method of showing by experiment the heating liwUn™^^ect» power of the galvanic current is to connnect the poles of a of galvanic battery by means of a fine platinum wire. If the wire is very Ulu^stratedf * long it may become hot; shorten it to a certain extent, and it will become red-hot ; shorten it still more, and it will bo- come white-hot, and finally melt. If such a wire is carried through a sma'.l quantity of salt water on a watch-glass, the liquid will boil ; if through alco- hol, ether, or phosphorus, they will bo inflamed ; if through gunpowder, it wiU be exploded. This power has been applied to tho purpose of firing blasts. What practical *^ „ , " . , . , , ^ , application hag or mmcs of gunpowder, an operation which may bo effected ^eii made of ^j^j^ equal facility under water. The process is as follows: — The wires from a sufficiently powerful battery are connected by a piece of fine platinum wire, which is placed in a mass of gunpowder con- tained in a cavity of a rock, or inclosed in a vessel beneath tho surface of water. The wire may bo of any length, but tho moment conr^ectioq ia made 18 410 WELLS'3 NATURAL PHILOSOPHY. with the battery tho current passes, renders the platinum red-hot, and eXs plodea the the powder.* „ , The (greatest artificial heat man has yet succeeded in pro- IIow may the , . , , , , , „ "^ , , . , -^ greatest artifi- ducmg lias been through the agency oi the gaivamc battery. cial heat be ^jj ^)^q metals, including platinum, which can not be fused produced? ' ° ^ ' by any furnace heat, are readily melted. Gold bums with a blueish light, silver with a bright green flame, and the combustion of the other metals is always accompanied with brilliant results. All the earthy minerals may be liquefied by being placed between the poles of a sufficiently large battery. Sapphire, quartz, slate, and lime, are readily melted ; and the diamond itself fuses, boils, and becomes converted into coal. How are the ^^^' The luminous eflects of the galvanic focts'^^of the hattery are no less remarkable than its heating fe^^^manifest- sfFects. A vcrj Small voltaic arrangement is "^^ sufficient to produce a spark of light every time the circuit is closed or opened. If the two ends of wires proceeding from the opposite poles of a battery are brought nearly together, a bright spark will pass from one to the other, and this takes place even under water, or in a vacuum. How may the Tlic most splcudid artificial light known ia raificiari'i^ht produced by fixing pieces of pointed charcoal be produced? ^q ^^q wircs counectcd with opposite poles of a powerful galvanic battery, and bringing them within a short distance of each other. The space between the points is occupied by an arch of flame that nearly equals in dazzling brightness the rays of the sun. This light, which is termed the electric light, differs from electric li"iit ^11 other forms of artificial light, inasmuch as it is independent differ from nil of ordinary combustion. The charcoal points appear to suffer lights? no cliange, and the light is equally strong and brilliant in a vacuum, and in such gases as do not contain oxygen, whera • In the course of the construction of a railway recently in England, it bocame neces- cary to detach a large mass of rock from a cliff on the sea-cuast in order to avoid the ex- pense of a long tunnel. To have done this by the direct application of human labor and the ordinary operations of blasting, would have been attended with an immense expendi- ture of time and money. It was accordingly resolved to blow it up with gunpowder, ignited by the galvanic battery. Nine tons of powder were accordingly deposited in cham- bers at from 50 to 70 feet from tho face of the cliflT, and fired by a conducting wire connected ■with a powerful battery, placed at 1,000 feet from the mine. The explosion detached 600,000 tons' weight of chalk from the cliff. It was proved that this might h.ave been equally effected at the distance of 3,000 feet. This bold experiment saved eight moDtka' labor and $50,000 expense. GALVANISM. 411 all other artificial lights -^vould bo extinguished. It may even be produced under "water. To excite the electricity, however, which occasions this light, zinc or some other mital must be oxydized, or what is the same thing burnt, the same as oil ia our lamps, or coal in the gas retorta for the production of other species of artificial light. The effects of the galvanic battery upon the "VVTiat are the j •■ /^ ,^ • i , physiological ncFves and muscles oi the animal system are .v.uic*eLctricI of the same character as those produced by ^' ordinary electricity. On grasping the two ends of the connecting wires of a battery of some force with wet hands, a peculiar tremor will be felt in the joints of the arm and hand, accompanied by a slight contortion of the muscles, and increasing to a violent shock. This shock is repeated every time a contact between the hand and the wire is broken and renewed. The concussion of the nerves of the body is, therefore, produced by the entrance and exit of the currents of electricity ; for they evidently must pass through the body the moment it forma the connecting link between the two poles. By a particular arrangement, the circuit may be closed or interrupted at pleasure, and in such a manner that the current may be made to pass alter- nately through the wires and the body ; the latter being thus exposed to a series of shocks which are considered particularly adapted for the cure of diseases arising from the injury or derangement of the nervous system. It is, moreover, a highly valuable remedy in cases of suffocation, drowning, paraly- sis, etc. , and numerous arrangements have been at various times proposed for the construction of medico-galvanic machines. The effects of galvanic electricity on bodies recently deprived of life is very remarkable, and it was through an accidental observance of its action upon a dead frog that galvanism was discovered. By connecting the muscles and nerves of recently-killed animals with the poles of a battery, many of the movements of life may be produced. Some remarkable experiments of this character were made some years since upon the body of a man recently executed for murder at Glasgow, in Scotland. The voltaic battery em- ployed consisted of 270 pairs of plates, four inches square. On applying one pole of the battery to the forehead and the other to the heel, tho muscles are d.?scribed to have moved with fearful activity, so that rage, anguish, and despair, with horrid smOes, were exhi'jited upon the counten- ance. 781. Galvanic electricity is a powerful agent in effecting chemical decom-. positions, and in its application to such purposes, it is most practically usefuL Can galvanic Wlieu a currcnt of galvanic electricity ia fecfchemirai "oi^^^ to pass through a compound conducting dLcompositiou? substance, its tendency is to decompose and separate it into its constituent parts. 412 WELLS'S NATURAL PHILOSOPHY. How may wa ter be decom' posed ? Fig. 342. Thus, water is composed of two gases, oxygen and hydro- gen united together. When the wires connecting the polea of a galvanic battery are placed in water, and a sufEciently strong current made to pass tlirough them, the water is decomposed, tho hydrogen being given out at the negative pole of the battery, and the oxygen at the positive pole. Fig. 342 represents a form of apparatus by which tliia experiment can be performed in a very satisfactory manner. It consists of two tubes, 0 and H, sup- ported vertically in a small reservoir of water, and two slips of platinum, p p, which can be con- nected with the poles of a voltaic battery, passing in at the open end of tho tubes. When communi- cation ia effected between the platinum slips and a battery in action, gas rapidly rises in each tube and collects in the upper part. In that tube which is in connection with the positive polo of the battery oxygen accumulates, and in tho other hydrogen. And it will bo noticed that the quantity of the latter ia equal to twice the quantity of tho former gas, since water contains by volume twice as much hydrogen as it does ox3'gen. The explanation of this phenomenon may be briefly given as follows : — All atoms of matter are regarded as originally charged with either positive or negative electricity. In tho case of water, hydrogen is the electro-positive element and oxygen the electro-negative clement. It has been already shown that bodies in opposite electrical states are attracted by each other. Hence, when the poles of a galvanic battery are immersed in water, the nega- tive pole will attract the positive hydrogen, and the positive polo the negative oxygen. If tho attractive force of the two electricities generated by tho bat- tery is greater than the attractive force which unites the two elements, oxygen and hydrogen, together in the water, the compound will be decomposed. Upon tlio same principle other compound substances may be decomposed, by em- ploying a greater or less amount of electricity. In this way Sir Humphrey Davy made tho discovery that potash, soda, lime, and other bodies, were not simple in their nature, as had previously been supposed, but compounds of a metal with oxygen. ; 782. Recent experiments have shown that the electricity which dccomposL'S, and that which is evolved by tho decom* position of a certain quan,tity of matter, are alike. Thus, wat^r i is composed of oxygen and hydrogen ; now, if the electrical power which holds a grain of water in combination, or which causes a grain of oxygen and hydrogen to unite in tho right proportions to form water, could be collected and thrown into a voltaic current, it would be exactly the quantity required to produce tho decomposition of a grain of water or the liberation of its elements, oxygen and hy- drogen. What is tha theory of the decomposing nction of gal- vanic elec- tricity ? Wliat quantity of electricity is necessary to decompose a •ubstance ? GALVANISM. 413 •wTiat is an "^83. For conveniencG in certain experi- Eiectrode? ments, the ends of the copper wires connect- ing the poles of the galvanic battery are frequently terminated with thin strips of platinum, which are called Electrodes. The platinum slip connected with the posi- tive pole forms the positive electrode, and that with the negative pole, the negative electrode. Platinum is used for the reason, that in employing the battery for effecting decompositions, it is frequently necessary to immerse the ends of the con- ducting wires in corrosive liquids, and this metal generally is not affected by them. What is Eiec- '<^84. Electto-metallurgy, or electrotyping, is tro-metauurgf f ^q ^j.|. qj. procGss of depositing, from a metal- lic solution, through the agency of galvanic electricity, a coating or film of metal upon some other substance.* Upon what is ^he process is based on the fact, that when bated ? ^"^""^^^ a galvanic current is passed through a solu- tion of some metal, as of sulphate of copper (sulphuric acid and oxyd of copper), decomposition takes place ; the metal is separated in a metallic state, and attaches itself to the negative pole, or to any substance that may be attached to the negative pole ; while the oxygen or other substance before in combination with the metal, goes to, and is deposited on the positive pole. In this way a medal, a wood-engraving, or a plaster cast, if attached to the negative pole of a battery, and placed in a solution of copper opposite to the positive pole, will be covered with a coating of copper ; if the solution con- tains gold or silver instead of copper, the substance will be covered with a coating of gold or silver in the place of copper. The thickness of the deposit, providing the supply of the metallic solution be kept constant, will depend on the length of time the object is exposed to the influence of the battery. In this way, a coating of gold thinner than the thinnest gold-leaf can bo laid on, or it may be made several inches or feet in thickness, if desired. The usuiU arrangement for conducting the electrotype process is represented • The preneral name of electro-metalhirpy includes aU the various processes and results which different inventors and manufacturers have dcsi what are, 283 Affinity definel, 25 Aim, philosophy of taking, 295 Air, compressibility of, 164 capacity of, for moisture, 268 constitnents of, 163 density of, 165 elasticity of, 165 fresh, how much required for a healthy man, '^Ol heated, why rises, 261 how heated, 213 in spring, why chilly, 246 in water, ISO inertia of, 104 momentum of, 1S7 illustrations of, 187 not necessary for the production of sound, 101 •weight of, 163 when rarefied, 166 when said to be saturated, 263 ■why unwholesome^ after having been respired, 260 pump, construction of, 176, 177 Alphabet, telegraphic, 435 Anemometer, 2Si Angle, defined, 71 of incidence and reflection, 71 Animals foretell changes in weather, 202 Annealing described, "^7 Aqueducts, construction of, 134 Arch, base of, 120 springing of, 120 strength of, 120 what is an, 120 why stronger than a horizontal struct- ure, 120 Archimedes, experiment with the crown, 44 screw of, 159 Architrave, 121 Architecture, 119 orders in, 120 origin of different styles of, 119 Armature of a magnet, 423 Artillery, effective distance of, 7T Artesian wells, 135 Astatic needle, what is an, 430 Atmosphere, composition of, 163 effect of, on diffusion of light, 302 how heated, 226 pressure of, 163 supposed height of, 173 what is, 163 Atmospheric electricity, 391 pressure, effects of, 174, 175 how sustained, 179 refraction, 314 Atom, what is an, 13 Attraction at insensible distances illustrat- ed, 22 cohesive, 25 liow varies, 25 ~ illustration of simple, 13 molecular, four kinds of, 24 mutual, illustrations of, 30 wliat is, 17 Aurora borcalis, cause of, 396 no influence on the weather, 291 Auroras, not local, 397 peculLarities vf, 397 Avoirdupois weight, 34 Axis of a body, what is an, 83 Balusters, 121 Balance, ordinary, described, 97 torsion, 332 when indicates false weights, Ballast, use of, in vessels, 139 Balls, cannon, velocity of, 76 Balloons, varieties of. ISO what are, 136 Balloon, why arises, 43 Barker's mill. 157 Barometer, how invented, 169-171 bow constructed, 171 aneroid, 172 444 INDEX, Barometer, water, 172 wheA-l, ITl how indicates weather changes, 1T3 how used for measuring heights, 1T3 Batteries, thermo-electric, 417 Battery, Daniull's, 4UT galvanic, 401 trrove's galvanic, construction of, 4U(> imperfections of, 407 luminous effects of, 410 Smee's galvanic, 401) sulphate of copper, 406 trough, described, 4o5 Beam, rectangular, strength of, 116 bent in the middle, why liable to break, 119 or bar, when the strongest, 115 Bellows, hydrostatic, 1J9 Bells, electrical, 3S5 Belts, motion communicated by, 101 Billiards, principles of the game of, 72 Blanket, utility of the nap of, 219 Blower, use of, '262 Boats, life, how preventeil from sinking, 147 Bodies, form of, how dependent on heat, 228 form of, how changed by centrifu- gal force, 83 falling, laws of, 5.5 force and velocity depend on what, 54 lighter than water, specific gravity, how determined, 39 non-lumiuous, when rendered viii- ble, 301 when heavy and light, 33 when transparent, 294 when luminous, 294 when appear white, 301 when solid, liquid, or gaseous, 24 when float in air, 185 Body, what is a, 11 when called hot, 206 size of, how affects its strength, 115 when stands most firmly, 50 when rolls, and when slides down a slope, 51 where will have no weight, 33 Boiling-point, depends on what, 241 infl uence of atmospheric pres- sure on, 342 Boiler-flue, 253 Boilers, steam, how constructed, 256 essentials of, 257 locomotive, how constructed, 259 Bones of men and animals, why cylindri- cal, lis Boxes of a pump, 181 Breath, why visible in winter, 274 Breathing, mechanical operation of, 181 Breezes, land and sea, 284 Bridge, Britannia tubular, 118, 119 Brittleness, what is, 27 Bubble, soap, why rises in the air, 43 Buckets of wheels, 156 Building, strengthof a, on what depends, 119 Buildings, how warmed and ventilated, 2U0 Buoyancy, what is, 138 Buruiug-glassee, 209 Caloric, what is, 206 Camera obscura, o47 portable, 360 Canals, how constructed, 137 locks in, lo7 Cannon bursting by firing, 28 varieties of, 77 Capillary Attraction, 25, 142 illustrated, 143 Capstan, construction of, 100 Car axles, why liable to break, 28 Carriage, high, liable to be overturned, 50 Cask, tight, liquids will not flow from, 179 Catoptrics, 312 Cellars, cool in summer, warm in winter, why, 2'JO Center of gravity in irregular bodies how found, 4S when at rest, 46 in what three ways sup- ported, 47 Centripetal Farce, 79 Champagne, why sparkles, 181 Charcoal marks, why stick to a wall, 25 why black, 301 Chemistry, definition of, 9 Children, why difficult to learn to walk, 52 Chimney, draught of, 202 how constructed, 262 how quickens ascent of hot air, 202 Chimneys, when smoke, 262 Chord in music, 196 Chain-pump, construction of, 160 Climate, what is, 207 Circuit, galvanic, 401 Clock, common, described, 58 water, principle of, 151 Clocks, why go faster iu winter than in sum, m.T, 60 Clothing, when warm and when cool, 220 Clouds, average height of, 274 cirrus, 275 cumulus, 275 how differ from fog, 273 how formed, 274 nimbus, 277 stratus, 277 variety of, 275 what are, 273 why appear red at sunset, 337 why float in the atmosphere, 274 Coals, meclianical force of, 251 Coal, equivalent to active power of man, 251 Co.^s on wheels, 101 Cohesion defined, 25 Cold, greatest artificial, 211 natural, 211 what is, 206 Color and music, analogy between, 338 Color, no effect on radiation of heat, 223 origin of, 320 Colors, complementary, 331 dark, absorb any heat, 225 how affect their relative appearance, 332 of natural objects on what depend, 3:;o Bimple, what are, 323 INDEX, 445 Column, height of, kow measured, 121 what is a, 121 Compass, mariner's, 4-4 ordinary, 4-'4 when discovered, 426 Compressibility, what is, 16 Concord in music, 196 Condensation, what is, 238 Conduction of heat, 216 Convection of lieat, "16 Cordage, streTigth of, on what depends, 113 Corit, why floats upon water, 43 Cornea, what is the, ^49 Coulomb's torsion balance, 382 Countries destitute of rain, 279 Coughing, sound of, how produced, 204 Cranes, what are, 105 Cranio defined, 110 Cream, why rises upon milk, 147 Crying, what is, '205 Cuppinji, operation and principle of, 175 Currents, electric, how exert their influence, 4-23 Cylinders, strength of, 118 Dagnerreotjrpes, how formed, 345 Dead point explained, 112 Declination of needle, 426 Density, what is, 15 Derrick, what is a, 105 Dew, circumstances that influence the pro- duction of, '271 does not fall, 271 phenomena and production of, 270 when deposited most freely, 271 Dew-drop, why globular, 30 Dew-point, 270 not constant, 270 Diamagnetic phenomena, 441 Diamagnetism, 441 Dioptics, 313 Directinn, line of, 40 Discord in music, 196 Distillation, '242 Divibibility, 13 Dovetailing, what is, 117, 118 Drainage, principles of, 152 Draught of chimney, 262 Dresses, black, optical effect of, 333 Drops, prescription of medicine by, unsafe, 29 Ductility, what is, 26 Dust, how we free our clothes of by agita- tion, 20 Dynamometer described, 89 E Ear, construction of, 201 , 202 Earth, bodies upon, why not rush together, 30 cause of present form of, 83 centripetal force at equator of, 83 how proved to be in motion, 84 the physical features of, how affect winds, 282 the reservoir of electricity, 376 Earth, telegraphic communication through, 4o7 Earth's attraction, law of, 32 Ebullition, what is, 241 Echo, conditions for the production of, 198 what is, 197 Echoes, when multiplied, 198 where most frequent, 198 Egg-shell, application of the principle of the arch in, 1'20 Elastic bodies, results of collision of, 68 Elasticity defined, 22 Eel, electrical, 391 Electric attraction, 370 currents, how eiert their influence, 429 fluid non-luminous, 88T light, 410 repulsion, 370 shock, 383 spark, duration of, 888 Electrical battery, 3S4_ induction, 377 machines, 378 Electricity a source of heat, 212 atmospheric, S91 conductors and non-condnctort of, 273 Du Fay's theory of, 271 Franklin's theory of, 271 effect of on a conductor, 386 experiments of Franklin with, 392 frictional, distinctive character of, 407 galvanic, how excited, 401 how differs from ordi- nary, 300 how discovered, 388 quantity of, what is, 408 theory of, 402 intensity of, what is, 408 what is, 398 how evolves heat, 409 how excited, 369 how exerts a magnetic force, 431 influence on the form of bodies 376 kinds of, 370 magneto, 437, 438 cf vital action, 391 positive and negative, 272 quantity necessary for decompo- sition, 412 real character of unknown, 403 secondary currents, 437 thermo, what is, 416 velocitv of, how determined, 389 what is, 369 where resides in bodies, 876 Electro-magnetism, 429 magnets, how formed, 432 what are, 432 Electrometer, 381 Electro-mctallur^y, 413 Electrophorns, 380 Electroscope, 381 Electrotyping, 413 Electrodes, what are, 413 446 INDEX. Elements, simple, 11 number of, 11 Elevations, how determined by the boiling point of water, 242 Embankments, why made stronger at the bottom than at the top, 132 Endosmose, what is, 146 Engine, fire, construction of, 183 steam, -51--54 Engraving, how affected by electro-metal- lurgy, 415 Entablature, divisions of, 121 what is, 121 X^uilibrium indifferent, 43 law of, in all machines, 92 stable, 43 unstable, 4S what is, 46 Equinoctial storm, 291 Evaporation, 23.S circumstances influencing, 239 influence of temperature on, 240 Exosmose, what is, 146 Expansibility, what is, 16 illustrations of, 16 Expansion by heat, 228 how measured, 233 Eye, S4T how judges of size and distance, 354 how moved, 34S ' optic axis of, 353 Structure of in man, 343 Facade of a building, 121 Far-sightedness, cause of, 353 Feather attracts the earth, 32 Fibrous substances non-conductors of heat, 219 Filtration defined, 19 Fire, what is, 209 Fishes, structure of the body of, 154 Flame, what is, 209 Flexibility, what is, 26 Flics, how walk upon ceilings, 176 Floating bodies, laws of, 138 Fluid, electric, 403 Fluids, what are, 24 Fly-wheel, use of, 17 Focus, what is a, 322 Form of bodies dependent on heat, 228 Forcing-pump, construction of, 183 Force defined, 21 accumulation of, 87 internal, 22 magnetic, 418 molecular, 22 real nature of, 21 Forces, great, of nature, 21 electro-motive, 401 Fountains, ornamental, principle of con- struction of, 135 Friction, 112 advantages of, 113 how diminished, 112 kinds of. 112 rolling, 112 sliding, 112 Friction, heat produced by, 214 Freezing mixtures, composition of, 245 Frieze in architecture, 121 Frost, origin of, 272 Fuel, what is, 265 Fulcrum defined, 93 Furs, why used for clothing, 219 Furnaces, hot-air, 264 how constructed, 265 G Galvanism, 398 Galvanic action, how increased, 403 battery, 401 heating effects of, 403 phvsiological effects o^ 411 Galvanometer, 430 Gamut, the, 196 Gas, how differs from a liquid, 29 what is, 23 Gases, how expand by heat, 232 specific gravity, how determined, 41 Gaseous bodies, properties of, 23 Gasometcjs, construction of, 179 Gears, in wheel work, 101 Glass, opera, 365 Glasses, sun, 209 Glottis, what is the, 203 Glue, why adhesive, 25 Grain weight, origin of, 34 bearing plants, construction of the stems of, lis Gravitation, attraction of, how varies, 30 defined, 30 terrestrial, 32 Gravity, action of, on a falling body, 55 center of, 45 specific, 37 Green wood, unprofitable to bum, 206 Grindstones, how broken by centrifugal force, SO Guage, barometer, 259 steam, 259 rain, 277 Gun, essential properties of, 76 Gunpowder, effective limit of the force o^ 77 force of 76 Gurgle of a bottle explained, ISO Hail, what is, 280 storms, where most frequent, 231 stones, formation of, 281 Halos, what are, 336 Hardness, what is, 26 Hearing, conditions for distinctness In, 200 range of human, 203 Heat, 205 how diffuses itself, 206 how measured, 206 distinguishing characteristic of, 206 nature of, 207 theory of, 207. 203 and light, relations between, 208 devoid of weight, 209 INDEX. 447 Heat', sources of, 209 influence extends how far into the earth, 211 of cliemical action, 212 greatest artiticial, 212 derived from mechanical action, 213 latent, 213 sensible, 213 of vital action, 214 of friction, 214 conductors and non-conductors of, 216 radiation of, 216 communication of, 216 conducting power of bodies, how di- minished, 218 good radiators of, 222 how propagated, 223 velocity of, 223 how reflected, 224 rays of, what is meant by, 234 absorption of,. 225 expansion by, 223 how transmitted through different substances, 226 effects of, 227 Bolar, compound nature of, 227 force of expansion of, 229 expansion of, practical illustrations of, 229 latent, when rendered sensible, 246 capacity for, 247 quantity of, different in all bodies, 247 specific, 247 Helix, construction of, 432 Horse power defined, 83 Houses, haunted, explanation of, 200 Humidity, absolute and relative, 263 Hurricane, what is a, 285 Hurricanes, where most frequent, 2S5 space traversed by, 286 velocity of, 286 Hydraulics, 148 Hydraulic engines, cause of theloss of power in, 15S, 159 ram, construction of, 161, 163 Hydrometer, what is a, 141 uses of, 141 Hydrostatics, 123 Hydrostatic press, construction of, 126, 127 Hydro-extractor, 80 Hygrometer, how constructed, 269 Ice, origin of bubbles in, 233 heat in, 206 Images, when distorted in mirrors, 303 Impenetrability, 12 illustrations of, 13 Incidence, angle of, 71 Inclined plane described, 105 advantage gained by, 106 Induction, magnetic, 421 Inelastic bodies, results of collision of, 69 Inertia, what is, 10 examples of, 17 Inkstand, pneumatic, 179 Insects, how produce sound, 205 Insulation, 374 Intensity in electricity, what is, 408 Iron, galvanized, what is, 415 how made hot, 206 how rendered magnetic by induction, 421 ships, principle of flotation o^ 140 soft, how magnetized, 421 why stronger than wood, 29 Kaleidoscope, construction of, 307 Key-note, what is, 201 Lakes, salt, origin of, 124 Lamp-wick, how raises oil, 145 Lantern, magic, what is, 361 Larynx, description of, 203 Laughing, what is, 205 Law, physical, definition of a, 10 Lens, achromatic, 3-8 axis of, 321 defined, 319 focal distance of, 321 Lenses, varieties of, 319 Level, spirit, construction of, IBT what is a, 53 Lever, law of equilibrium of the, 94 Levers, arms of, 93 compound, 96 disadvantages of, 9T kinds of, 93 what are, 93 Leydenjar, 382 Light, absorption of, 300 analysis of, 325 chief sources of, 294 corpuscular theory of, 293 divergence of rays of, 296 electric, 410 good reflectors of, 301 how analyzed, 326 how propagated, 295 how refracted by the atmosphere, 314 intensity of, how varies, 297 interference of, 339 moves in straight lines, 295 polarized, 341 polarization of, 343 ray of, what is, 295 refraction of, 312 same quantity not reflected at all angles, 305 three principles contained in, 344 undulatory theory of, 293 velocity of, 298 how calculated, 299 vibrations of, 339 waves of, 339 what is, 292 when totally reflected, 316 white, coinpositi'in of, 326 Lightning, identity of with electricity, 393 mechauical effects of, 306 rods, how constructed, 394 space protected by, 395 when dangerous, 395 448 INDEX. LigUtning, rarieties of, 393 what is, 392 Line, vertical, 53 Liquefaction, what is, 237 Liquid at rest, condition of the surface of a, 133 pressure of a column of, 123 what is a, '23 Liquids, boiling point how changed, 242 flowing from a reservoir, 149 have no particular form, "23 heat conducting power of, 217 how cooled, 2-t move upon each other without fric- tion, 1-4 pressure of, 125 illustrated, 125 ■why some froth, 180 Bpecific gravity how found, 39 spheroidal state of, 240 to what extent expanded by heat, 229 transmit pressure in all directions, 125 when do they wet a surface, 30 Loadstone, what is a, 416, 417 Locks, canal, how operated, 137 Locomotive, efficacy depends on what, 29 Looking-glasses, how formed, 303 M Machine, what is a, 90 Machines diminish force, 90 do not produce power, 90 how make additions to human power, 91 how produce economy of time, 94 motion in, takes place when, 92 simple, 93 Machinery, elements of, 93 general advantage of, 92 magnetic, 433 when caught on a center, 112 Magdeburg hemispheres, 177 Magnet, rotation of a, 431 when traverses, 419 Magnets, artificial, 418 horse-shoe, 420 native, 417 power of artificial, 423 what are poles of, 418 Magnetic induction, 421 meridian, 425 phenomena, how accounted for, 423 polarity, 419 power of a body, where resides, ,' 421 'Magnetism, 417 animal, what is, 413 electro, 429 how excited by electricity, 432 how induced by the earth, 422 why not available for propel- ling force, 433 Magneto-electric machines, 438 Magnifying glasses, 324 Mag^nitude, 12 center of, 45 Malleability, what is, 26 examples of, 26 Man, how exerts his greatest strength, 88 estimated strength of, 88 Mariotte's laws, what are, 166, 167 Matter, cause of changes in, 21 definition of, 11 essential properties of, 12 indestructible, 18 not infinitely divisible, 13 smallest quantity visible to the eye, 14 Materials, strength of, 115 upon what depend, 115 Matting, how protects objects from frost, 272 Mechanical powers, 93 Meniscus, 320 Meridian, magnetic, 425 of the earth defined, 36 Metals, union of, how affects durability, 415 Meteors, how differ from shooting stars, 289 Meteorites, what are, 288 Meteoric bodies, supposed origin of, 289 phenomena, 288 Meteorology, 266 Microscope, compound, 361 solar, 308 what is a, 360 Microscopes, varieties of, 361 Milk, why cream rises upon, 147 Mirage, 315 Mirror, plane, how reflects light, 303 what is a, 302 Mirrors, burning, 303 concave, 308 convex, 311 parallel, effect of, 306 varieties of, 302 Mississippi, does it flow up hill, 152 quantity of water in, 152 Mists and fogs, how occasioned, 273 Moisture in air, how determined, 269 Molecule defined, 14 Momentum, how calculated, 65 what is, 64 illustrations of, 64 Monsoons, theory of, 283 what are, 283 Moon, influence of on weather, 291 Motion, absolute and relative, 62 accelerated and retarded, 03 apparent, affected by dietance, 359 circular, illustrations of, 78 compound, 72 illustrations, 72 imparted to a body not instanlana* ously, 65 perpetual, in machinery, not possi> ble, 91 perpetual, in nature, 91 example of, 91 reflected, what is, 71 * reversion of by belting, 101 rotary. 111 rectilinear. 111 simple, illustrations of, 73 uniform and variable, 63 what IS, 62 whcu imperceptible to the eye, 3S0 INDEX. 449 Mortise, what is a, US Mountains, height of, how determined by tlie barometer, ITS Movements, vibratory, nature of, 1S3 Mud, why flies from wheel of carriage, 79 Muscular energy, how excited, SI Music, scale in, 195 notes in, how indicated, 196 Musical sounds, 194 N Natural Philosophy, definition of, 9 Near-sightedness, cause of, 352 Keedle, magnetic, 423 dipping, 425 diurnal variation of, 428 magnetic, directive power of, how explained, 426 rariations, cause of, 423 Notes, musical, when in unison, 195 in music, how indicated, 196 Ocean, depth of, 123 extent of, 123 Octave in music, 195 Oersted's discovery, 429 Oils, how diminish friction, 113 Opaque bodies, 234 Optical instruments, 360 Optics, medium in, 312 science of, 292 Paddles of a Eteamboat, when most effect- ive, 154 Paper, blotting, why absorbs ink, 147 Parabola defined, 74 Paradox, hydrostatic, 126 Pendulum, center of oscillation of a, 59 compensating, 60 described, 35 influence of length on yibration of, 59 length of a, seconds, Gl times of vibration of, 53 compared, 53 nsed as a standard for measure, 61 Perspective, what is, 356 Photometers, construction of, 293 Physics, definition of, 10 Pilaster, what is a, 121 Pile, in architecture, 120 Zamboni's, explained, 404 Piles, voltaic, 4iU Pipes, rapidity of water discharged from, 150 water, requisite strength of, 134 Pisa, leaning tower of, 49 Pitch, or tone, 195 Pkints, viUl action of, 215 Platform scales, 93 Pliability, what is, 26 Plumb-line, 53 Pneumatics, 1G3 Polarity, magnetic, 419 I Poles, magnetic, where situated, 426 of galvanic battery, what are, 402 Pop-gun, operation of, 167 Pores, defined, 14 evidence of the existence of, 15 Porosity defined, 14 Porter, why froths, ISO Portico, what is a, 121 Power, agents of in nature, S" and resistance defined, C3 and weight in machinery defined, 93 expended in work, how ascertained, 89 mechanical effect of, how estimated, 92 moving, effect ot how expressed, 89 space and time, how exclianged for, 92 P^ess, hydrostatic, 127 Prism defined, 313 Projectile, what is, 74 Projectiles, laws of, 74 range of, 75 Propellers, advantage over paddle-wheels, 155 construction of, 155 Pugilists, blows of, when most severe, 69 Pulley defined, l'>2 kinds of, 102 fixed, described, 102 movable, 103 Pulleys, advantage of, 104 Pump, air, 177 chain, IGO common suction.lSl when invented, 160 forcing, construction of, 183 Vera's, 145 Pyrometers, 233 Quantity in electricity, what is, 403 R Badiation of heat proceeds from all bodies, 223 ruin, what is, 277 why falls in drops, 277 formation of, on what depends, 277 guage, 277 yearly estimated quantity of, 279 where most abundant, 273 Rain-bow, what is a, 333 when seen, 335 why semicircular, 335 Ram, hydraulic, construction and opention of, 161, 162 Range in gunnery, 75 greatest, when attained, 75 Rays of heat, what meant by, 224 Reflection, angle of, 71 Reflectors of heat, best, 225 Refraction, index of, 316 double, 340 how accounted for, 317 Refrigerators, construction of, 221 Regulators of steani-engines, 256 Repulsion, what is, 22 450 INDEX. Repulsion, and attraetion, magnetic, 419 Retina of the eye, S4S Ricocliet firing-, TT Rifle, ilini^, construction of, 77 how sighted, 7S Elvers, why rarely straight, 86 velocity of, 15'2 water discharge of, 153 Roads, inclination of, how estimated, 106 how should he made, 106 Rods, discharging, 3S9 Room, how best ventilated, 264 Rooms for speaking, how constructed, 201 £ope-dancing, art of, 52 8 Safes, fire-proof, how constructed, 221 Sandstones, why ill adapted for architectural purposes, l'2ii Saw-dust, utility of in preserving ice, 220 Scales, hay and platform described, 98 Scarfing and interlocking 117 Scissors, a variety of lever, 94 Screw, advantage gained by, 109 applications of, 109 defined, 108 endless, 110 Hunter's, 110 nut of, 108 of Archimedes, 159 thread of, 109 Screw-Propeller, what is a, 155 Sea, proximity to, mitigates cold, 263 Shadow, what is a, 296 Shadows, how increase and diminish, 297 Shell, sea, cause of the Bound heard in, 109 Ships, copper sheathing of, how protected, 416 iron, why float, 140 Shooting-stars, how accounted for, 290 Sliort-sightedness, cause of, 35'2 Shot, how manufactured, 30 Silver, adulteration of, how detected, 43 Simmering, what is, 241 Skull, human, combines the principle of the arch, 120 Smoke, why rises in the air, 43 why ascends in chimney, 261 rings, origin of, 1S7 Sneezing, what is, 205 Snow crystals, 2S0 flake, composition of, 2S0 how formed, 280 line of perpetual, 243 protective influence of against cold,220 what is, 280 Soft, when is a body, 26 Solar microscope, 3(58 Solid, what is a, 23 Solids, why easily lifted in water, 139 specific gravity, how determined, 39 Solution, what is a. 287 how diflfers from a mixture, 237 when saturated, 237 Sound, conductors of, 192 how decreases in intensity, 192 how propagated, 190 interference of waves of, 194 loudness of, on what depends, 194 Sound, reflection of, 19T velocity of, 193 what is, 183 when communicated most readily, 191 when inaudible, 190 Sounds, musical, 194 not heard alike by all, 203 seem louder by night than by day, 191 Spark, electric, 388 Speaking, rooms suitable for, 201 Specific gravity, 37 how discovered, 44 how found, 33 standard for estimating, 38 practical applications of, 44 Spectacles, what are, 360 Spectrum, solar, 326 Springs, intermitting, 185 origin of, 136 Spy-glass, what is, 364 Stability of bodies, depends on what, 48 Stairs are inclined planes, 107 Stars, shooting, 289 height of, 289 Steel, how tempered, 27 how magnetized, 421 Steel-yard described, 97 Steam, advantages of heating by, 265 elastic force of, 249 superheated, 250 high pressure, 250 formed at all temperatures, 239 guage, 259 how rendered useful, 252 pressure of. how indicated, 260 true, invisible. 238 when used expansively, 255 Steam-boilers, cause of explosion of, 258 whistle, 260 engine, what is, 251 condensing. 253 construction of, 253 high pressure, 2.54 regulators of, 255 greatest amount of work per- formed by, 251 Stethoscope, construction of, 193 Still, construction of, 243 Structure, influence of the parts on the strength of a, 117 Stone for architectural purposes, how se- lected, 122 Stool, insulating. 380 Stove, why snaps when heated, 230 Stoves, how differ from open fire-place, 263 disadvantages of, 264 why placed near the floor, 261 Sublimation, what is, 243 Sucker, the common, 175 Suction, what is, 109 Sugar, how refined, 243 how absorbs water. 145 Sun does not really rise and set, 84 heat of, why greatest at noon, 210 the greatest natural source of heat, 209 nature of the surface of, 210 Surface defined, 12 spherical, definition of a, 133 Syphon, what is a, 184 INDEX. 451 Syphon, action of, 1^ Syringe, principle of, 1T6 Tackle and fall, what is a, 105 Telegraph, atmospheric, ISl Bain's, 4oO chemical, 436 House's, 4:j6 Worse's magnetic^ 434 printing, 43G Telegraphic method, the first proposed, 436 wires, insulation of, 436 Telescope, equatorial, 303 reflecting, 365 refracting, 363 what is a, 363 Temperature, average, how found, 267 greatest natural ever observ- ed, -210 in winter and summer, differ- ence between, 210 meaning of, SiW varies with latitude, 367 Tenacity, what is, 26 Theory, physical, definition of a, 10 Thermometer, 233 how graduated, 234 centigrade, 235 mercurial described, 234 Reaumur, described, 235 Thermometer-air, described, 236 Thermo-electricity, what is, 416 Thunder, cause of, 393 storms, where most prevail, 394 Tides, origin of, 32 Toes, advantage of tnming out in walk- ing, 50 Tone in sound, 195 Tongueing, what is, 117 Tdrnadoes, how produced, 2S7 what are. 2S7 Torpedo, electrical effects of, 391 Torricelli's invention, 109, 170 Trade winds, cause of. 2S3 direction of, 2S3 Transparent bodies, 294 Troy weight, 34 Trumpet, ear, what is an, 200 speaking, construction of, 199 Tubes, capillary, height to which water rises in, 143 Twilisht, 314 Twinkling, what is, 333 Tympanum of the ear, 203 "Vacnnm, what is a, 168 Valve, definition of, 1S3 safety, "258 Variation, lines of, 406 Vapor, always present in air, 239 appearance of, 233 Vapors, elasticity of, 243 formed at all temperatures, 233 Vault, what is a, 120 Velocity defined, 63 Velocitv of moving bodv, how determined, 63 Vena contracta, what is the, 150 Ventilation, what is, 200 when perfect, 260 Vessels of liquid, pressure upon the bot- tom of, 131 Vibrations of sound, nature of, ISS, 1S9 Views, dissolving, 363 Vision, angle of, 354 deceptions of, 357 double, how produced, 354 phenomena of, 347 Vital action, 214 Voice, compass of, 200 how produced, 203 organs of, 201 ^ Voltaic piles, 404 Volume defined, 12 "Walls, how deafened, 192 Warming and ventilation, 260 Warp and woof. 117 Watch, how diff>'rs from a clock, 50 Water as a motive power, 155 boiled, why flat, ISO boiling, temperature of, 241 composition of, 123 compressibility of, 124 decomposition of, 413 elasticity of, 124 expands in freezing, 231 force of expansion of in freezing, 239 freezing temperature of, 232 greatest capacity of all bodies for heat, 248 how high rises in a pump, 1S3 how made hot, 221 illustrations of the pressure of, 130 imparts no additional heat after boil- ing, 244 inclination suflScient to give motion to, 152 level. 1.36 power defined. 83 pressure at different depths, 131 how calculated, 133 sound of falling, how produced, 2(K spouts, what are, 287 supply of towns, 134 to what degree can be heated, 249 velocity of in pipes, how retarded, 151 when has its greatest density, 231 why rises by suction, 109 why rises in a pump, 183 Waters, comparative purity of, 123 Wave, a form, not a thing, 153 Waves, height of, 153 optical dclvisions of, 153 origin of, 153 of sound, 190 Weather, popular opinions concerning, 291 Wedge, what is a, lti6 when used in the arts, 107 how tlie power of increases, 107 Weight, absolute, what is, 3S how determined by spe- cific gravity, 43 452 INDEX, Weight, how varies, 32 of a body, when greatest, 32 "Weights and measures, standards of, 34 Fren "h system of, described, 36 United States standard of, 36 ■Welding described. 29 Well-sweep, old fashioned. 159 Wells, Artesian, construction of, 135 origin of water in, 186 Wetclothjs, why injurious, 246 Wheel and a.xle, action, of 99 spinning, 101 tourbine, advantages of, 153 work, compound, familiar illustra- tions of, 101 Wheels, breast, construction of, 157 cog, 101 overshot, 15T tourbine, 153 undershot, 156 paddle, power lost by, 154 Whirlwinds, how produced, 261 WhisUe, steam, 269 ■\Vlnch, what is a, 99 "Wind, principal cause of, 281 what is, 2S1 Wind-pipe, what is the, 203 Windhiss described, 100 Winds, force of, how calculat€d, 2S3 of United States, 2S5 trade, 238 variable, where prevail, 284 velocity of, 281 Wood, a bad conductor of heat, 218 comparative value of for fuel, 266 hard, why difficult to ignite, 266 made wet, why swells, 143 snapping of, 19 water in, 205 weight of, 266 Woods, when hard and when soft, 265 Wo liens, why used for clothing, 219 Woof of cloth, 117 Work of different forces, standard of m> paring, S3 Working-point in machinery, 92 Jfison, Slakenian, Taylor d' Co. 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