Digitized by the Internet Archive in 2015 https://archive.org/details/schoolcompendiumOOpark_0 OF THE. , SCHOOL COMPENDIUM OF ; NATURAL AND EXPERIMEiNTAL PHILOSOPHI, EMBRACING THE ELEMENTARY PRINCIPLES OF MECHANICS, HYDROSTATICS, HYDRAULICS, PNEUMATICS, ACOUSTICS, PYRONO- MICS, OPTICS, ELECTRICITY, GALVANISM, MAGNETISM, ELECTRO- MAGNETISM, MAGNETO-ELECTRICITY, AND ASTRONOMY. WITH A DESCRIPTION OF THE STEAM AND LOCOMOTIVE ENGINES. BY RICHARD GREEN PARKER, A.M. FftlNCIPAL OF THE JOHNSON GRAMMAR SCHOOL, BOSTON *, AUTHOR OF AIDS TO ENGLISH COMPOSITION, OUTLINES OF GENERAL HISTORY, ETC., ETC., ETC. " Delectniido pariterque monendo, Prodesse qiiam coiispici." TWENTY-FTRST EDITION, WITH ADDITIONS AND IMPROVEMENTS. » NEW YORK: PUBLISHED BY A. S. BARNES & CO., NO. 51 .JOHN-STREET. CINCINNATI: — II. W. DERBY & CO. 1848. At a meeting of the School Committee of the City of Boston, May 8, 1838, Voted, That Parker's Compendium of Natural and Experimental Phi- losophy be adopted and used as a text-book in our public schools. Attestf S. F. McCleary, Secretary. TO THE Hon. SAMUEL ATKHSTS ELIOT, MAYOR OF THE CITY OF BOSTON, AND CHAIRMAN OF THE SCHOOL COMMrtTEE. Sir, The Public Schools of this city are under many obligations to you, for the interest you have taken in them, and for your disinterested exertions for their improvement. This volume, designed to supply a want which they have long felt, affords an opportunity of acknowledging the obliga- tion, which I gladly embrace. The gratification which I feel in seeing you at the head of our municipal institutions, I beg leave to express in borrowed language : — Tibi ut gratuler non est in animo ; sed contra, banc occasionem, mihi sic oblatam, nostram civitatem gratulandi, reniti non possum. Quae omnia solita tua benevolentia ut accipias quaeso. I am, Sir, very respectfully. Your obedient Servant, RICHARD GREEN PARKER Entered, according to Act of Congress, in the year 1848, By a. S. BARNES & Co., In the Clerk's Office of the District Court for the Southern District ol New York. > 5 ^c.^ REMOTE PREFACE. The School Committee of the city of Boston having recently furnished the Grammar Schools with apparatus for exemplifying the principles of Natural Philosophy, the author of this work, who, for twenty years, has been at the head of one of these large establishments, and has felt the want of an elementary treatise unencumbered with extraneous matter^ has been induced to attempt to supply the deficiency. If he is not deceived in the result of his labors, the work will commend itself to notice by the follow- ing features : 1. It is adapted to the present state of natural science ; embraces a wider field, and contains a greater amount of information on the respective subjects of which it treats, than any other elementary treatise of its size. ^ 2. It contains engravings of the Boston School set of philosophical apparatus; a description of the instruments, and an account of many ex- periments which can be performed by means of the apparatus. 3. It is enriched by a representation and a description of the Locomo- C five and the Stationary Steam Engines, in their latest and most approved ^ forms. ^ 4. Besides embracing a copious account of the principles of Electricity ^ and Magnetism, its value is enhanced by the introduction of the science y of Pyronomics, together with the new sciences Electro-Magnetism and ^ Magneto-Electricity. ^ 5. It is peculiarly adapted to the convenience of study and of recitation, by the figures and diagrams being first placed side by side with the lUus- lli trations, and then repeated on separate leaves at the end of the volume. The number is also given, where each principle may be found, to which allusion is made throughout the volume. 6. It presents the most important principles of science in a larger type ; while the deductions from these principles, and the illustrations, are con- tained in a smaller letter Much useful and interesting matter is also 6 PREFACE. crowded into notes at the bottom of the page. By this arrangement, the pupil can never be at a loss to distinguish the parts of a lesson which are of primary importance ; nor will he be in danger of mistaking theory and conjecture for fact. 7. It contains a number of original illustrations, which the author has found more intelligible to young students, than those with which he has met elsewhere. 8. Nothing has been omitted which is usually contained in an elemen- tary treatise. A work of this kind, from its very nature, admits but little originality. The whole circle of the sciences consists of principles deduced from the discoveries of different individuals, in different ages, thrown into common stock. The whole, then, is common property, and belongs exclusively to no one. The merit, therefore, of an elementary treatise on natural science must rest solely on the judiciousness of its selections. In many of the works from which extracts have been taken for this volume, the author has found the same language and expressions without the usual marks of quo- tation. Being at a loss, therefore, whom to credit for some of the expres- sions which he has borrowed, he subjoins a list of the works to which he is indebted, with this general acknowledgment ; in the hope that it may be said of him £is it was once said of the Mantuan Bard, that " he has adorned his thefts, and polished the diamonds which he has stolen." It remains to be stated, that the Questions at the bottom of the page, throughout the volume, were not written by the author, but were prepared by another hand. R. G P Boston, March, 1848. ADVERTISEMENT TO THE SEVENTEENTH EDITION. Ten years have elapsed since this work first appeared in permanent form from the hands of the stereotyper. During this time, the author has been gratified to learn that sixteen editions have been called for by the pub- lic ; but this gratification has been mingled with re- gret that he has been unable from time to time to make such improvements as he knew were needed, and which the progress of science, as well as a more extended ex- perience, seemed imperiously to demand. He gladly avails himself of the present opportunity, afforded by the new publishers into whose liands it has fallen, to make such improvements as in his opinion will render it more worthy of the liberal patronage it has received ; for although it is a long time since the author has had any pecuniary interest in the work, he hopes that it is not true that he has had " no further solicitude." The necessity of a revision of the work at this time will appear from the following statement. By a vote of the School Committee of Boston in 1836, a certain portion of philosophical apparatus was introduced into the Grammar Schools, as an experiment. The appara- tus was designed to unite economy with simplicity, and was confined to the departments of Pneumatics and Electricity. To this apparatus the book was specially adapted, though not wholly confined. By a recent vote of the Committee, (August, 1847,) apparatus of superior 8 ADVERTISEMENT construction, and embracing a much wider field, was substituted for the cheap and defective sets that were at first mtroduced. It becomes necessary, therefore, in a volume prepared with special reference to the wants of the Boston schools, to have regard to the construction and the character of the instruments by which the prin- ciples of physical science are illustrated in these large establishments ; the author has therefore deemed it ex- pedient to make such a revision of the whole work, as will not only render it a convenient manual to accom- pany the new apparatus, but also embrace the recent improvements and discoveries by which the branches of science of which it treats have been enriched. A sched- ule of the new apparatus is subjoined ; and the author mdulges the expectation that the present edition of this work will be found more worthy than its predecessors of the favor which the public have bestowed upon it. Lilac Lodge, Dedham, October, 1847. • LIST OF WORKS WHICH HAVE BEEN CONSULTED, OR FROM WHICH EXTRACTS HAVE BEEN TAKEN, IN THE PREPARATION Of THIS VOLUME. Annals Of Philosophy; Aniott's Elements of Physics ; Bigelow's Tech- BI 'r r !f.^'n''^'''f ' ^h'^n^bers' Dictionary; Enfield's, Olmsted's, Blair s, Bakewell's, Draper's, Grund's, Jones', Comstock's, and Conver- sations on Natural Philosophy; Davis' Manual of Magnetism; Encyclo- piBdia Americana ; Franklin's Philosophical Papers ; Henry's Chemistry • Kings Manual of Electricity ; Lardner on the Steam Engine ; Library o^ Useful Knowledge ; Paxton's Introduction to the Study of Inatly ; pL. bour on Locomotive Engines on Railways; Peschel's Elements of Physics • Philips' Astronomy; Sir John Herschel's Astronomy; SilUman's Jo' na of Science; Singer's Electricity; Scientific Class Book; Scientific Dia- wZi'A^ '''"''T'^'^'^' Year Book; Turner's Chemistry ; r h t Worcester's and the American School Geo^aphy- Lathrop, Mclntire, and Keith on the Globes. ° ' SCHEDULE OF PniLOSOPHICAL APPARATUS FOR THE BOSTON GRAMMAR SCHOOLS.* Adopted by the School Committee, August, 1847. LAWS OF MATTER. Apparatus for illustrating Inertia. Pair of Lead Hemispheres, for Cohesion. Pair of Glass Plates, for Capillary Attraction. LAWS OF MOTION. Ivory Balls on Stand for Collision. Set of eight Illustrations for Centre of Gravity Sliding Frame for Composition of Forces. Apparatus for illustrating Central Forces. MECHANICS. Complete set of Mechanicals, consisting of Pulleys ; Wheel and Axle ; Capstan ; Screw ; Inclined Plane ; Wedge. HYDROSTATICS. Bent Glass Tube for Fluid Level. Mounted Spirit Level. Hydrometer and Jar, for Specific Gravity. Scales and Weights, for Specific Gravity. Hydrostatic Bellows, and Paradox. HYDRAULICS. Lifting, or Common Water-pump. Forcing Pump ; illustrating the Fire-engine. Glass Syphon-cup ; for illustrating intermitting Springs. Glass and Metal Syphons. PNEUMATICS. Patent Lever Air-pump and Clamp. Three Glass Bell Receivers, adapted to the Apparatus. Condensing and Exhausting Syringe. Copper Chamber for Condensed Air Fountain. Revolving Jet and Glass Barrel. Fountain Glass, Cock, and Jet for Vacuum. Brass Magdeburg Hemispheres. * The cost of this apparatus is about two hundred and sixty dollars. It was made by Mr. Joseph M. Wightman, No. 33 Cornhill, Boston, and in an eminent degree unites beauty with durabiiity. 10 SCHEDULE OF PHILOSOPHICAL APPARATUS. Improved Weight-lifter, for upward pressure. Iron Weight of fifty-six pounds and strap, ) Weight-lifter. Flexible Tube and Connectors, J Brass Plate and Sliding Rod. Bolt Head and Jar. Tall Jar and Balloon. Hand and Bladder Glasses. Wood Cylinder and Plate. India-rubber Bag, for expansion of ail. Guinea and Feather Apparatus. Glass Flask and Stop-cock, for weighing aif. ELECTRICITY, Plate Electrical Machine. Pith-ball Electrometer. Electrical Battery of four Jars. Electrical Discharger. Image Plates and Figure. Insulated Stool. Chime of Bells. Miser's Plate, for shocks. Tissue Figure, Ball and Point. Electrical Flyer and Tellurian. • Electrical Sportsman, Jar and Birds. Mahogany Thunder-house and Pistol. Hydrogen Gas Generator. Chains, Balls of Pith, and Amalgam. OPTICS. Glass Prism, and pair of Lenses. Dissected Eyeball, showing its arrangement. MAGNETISM. Magnetic Needle on Stand. Pair of Magnetic Swans. Glass Vase for Magnetic Swans. Horseshoe Magnet. ASTRONOMY. Improved School Orrery. Tellurian, or Season Machine. ARITHMETIC AND GEOMETRY. Set of thirteen Geometrical Figures of Solids. Box of sixty-four one-inch Cubes, for Cube Root, &c. AUXILIARIES. Tin Oiler; Glass Funnel; Sulphuric Acid. Set of Iron Weights for Hydrostatic Paradox. CONTENTS. CHAPTER L Page Divisions of the subject 17 CHAPTER II. Of Matter... 18 CHAPTER III. Of Gravity 25 CHAPTER IV. Mechanics, or the Laws of Motion 30 CHAPTER V. Hydrostatics * 73 CHAPTER VI. Hydraulics 90 CHAPTER VII. Pneumatics • 97 CHAPTER VIII Acoustics CHAPTER IX. Pyronomics ^32 12 CONTENTS. CHAPTER X. p^g^ Optics Electricity , CHAPTER XI. 192 CHAPTER XII. Galvanism, or Voltaic Electricity 2X6 CHAPTER XIII. Magnetism and Electro-Magnetism 230 CHAPTER XIV. Astronomy INTRODUCTION. The term Philosopliy literally signifies, the love of wisdom ; but, as a general term it is used to denote an explanation of the reason of things, or an investigation of the causes of all phe- nomena, both of mind and of matter. When apphed to any particular department of knowledge, the word Philosophy implies the collection of general laws or principles, under which the subordinate facts or phenomena relating to that subject are comprehended. Thus, that branch of Philosophy which treats of God, his attributes and perfec- tions, is called Theology ;^ that which treats of the material world is called Physics, or Natural Philosophy ; that which treats of man as a rational being, is called Ethics, or Moral Philosophy ; and that which treats of the mind is called In- tellectual Philosophy, or Metaphysics.f * The word Theology is derived from two Greek words, the former of which {Qeos) signifies God, and the latter {\oyos) means a discourse ; and these two words combined in the term Theology, Hterally imply a discourse about God. The latter of these two Greek words (\oyos or logos) is changed into logy to form English compounds, and it enters into the composition of many scientific terms. Thus, we have the words mineraZog-y, the science of minerals; meteorology, the science which treats of meteors ; ichthyoZo^y, the science of fishes ; entomoZo^y, the science of insects ; lithoZo^y, of stones ; conchology, of shells, &c. t The word Metaphysics is composed of two Greek words, Meta, (or licra,) which signifies beyond, and phusis, (or (pvais,) which signifies nature, and in composition these words imply something beyond nature. From 14 INTRODUCTION. All material things are divided into two great classes, called organized and unorganized matter. Organized matter is that which is endowed with organs adapted to the discharge of ap- propriate functions, such as the mouth and stomach of animals, or the leaves of vegetables. By means of such organs they enjoy life. Unorganized matter, on the contrary, possesses no such organs, and is consequently incapable of life and volun- tary action. Physical Science, or Physics, with its subdivisions of Natural History, (including Zoology, Botany, Mineralogy, Conchology, Entomology, Ichthyology, &c.,) and Natural Philosophy, inclu- ding its own appropriate subdivisions, embraces the whole field of organized and unorganized matter. The term Natural Philosophy is considered by some authoi s as embracing the whole extent of physical science, while others use it in a more restricted sense, including only the general properties of unorganized matter, the forces which act upon it, the laws which it obeys, the results of those laws, and all those external changes which leave the substance unaffected. It is in this sense that the term is employed in this work. Chemistry, on the contrary, is the science which investigates the composition of material substances, the internal changes which they undergo, and the new properties which they ac- the latter of these words, phusis, ((pvcis,) we obtain the term physics, which in its most extended sense implies the science of natui e nud natural objects, comprehending the study or knowledge of whatever exists. The natural division of all things that exist is into body and mind — things material and immaterial, spiritual and corporeal. Physics relates to material things — Metaphysics to immaterial. Man, as a mere animal, is included in the science of Physics ; but, as a being possessed of a soul, of intellect, of powers of perception, consciousness, volition, reason, and judgment, he be- comes a subject of consideration in the science of Metaphysics, INTRODUCTION. 15 quire by such changes. The operations of Chemistry may be described under the heads of, Analysis or decomposition, and Synthesis or combination. Natural Philosophy may be said, pre-eminently, to treat of motion, while Chemistry particularly relates to change or al- teration. By the former we become acquainted with the con- dition and relations of bodies as they spontaneously arise without any agency of our own. The latter teaches us how to alter the natural arrangement of elements to bring about some particular condition that we desire. To accomphsh these ob- jects m both of the departments of science to which we refer, we make use of appliances called philosophical and chemical apparatus, the proper use of which it is the office of Natural Philosophy and Chemistry respectively to explain. All philo- sophical knowledge proceeds either from observation or experi- ment, or from both. It is a matter of observation that water, by cold, is converted into ice ; but if, by means of freezing mixtures, or evaporation, we actually cause water to freeze, we arrive at the same knowledge by experiment. By repeated observations, and by calculations based on such observations, we discover certain uniform modes in which the powers of nature act. These uniform modes of operation are called laws ;— and these laws are general or particular accord- ing to the extent of the subjects which they respectively em- brace. Thus, it is a general law that all bodies attract each other in proportion to the quantity of matter which they con- tain. It is a particular law of electricity that similar kinds repel, and dissimilar kinds attract each other. The collection, combination, and proper arrangement of such general and particular laws, constitute what is called Science. Thus, we have the science of Chemistry, the science of Geome- try, the science of Natural Philosophy, &c. 16 INTRODUCTION. The terms, art and science, have not always been employed with proper discrimination. In general, an art is that which depends on practice or performance, while science is the ex- amination of general laws, or of abstract and speculative prin- ciples. The theory of music is a science ; the practice of it is an art* Science diflfers from art in the same manner that knowl- edge differs from skill. An artist may enchant us with his skill, although he is ignorant of all scientific principles. A man of science may excite our admiration by the extent of his knowl- edge, though he have not the least skill to perform any opera- tion of art. When we speak of the mechanic arts, we mean the practice of those vocations in which tools, instruments, and machinery are employed. But the science of mechanics ex- plains the principles on which tools and machines are construct- ed, and the effects which they produce. Science, therefore, may be defined, a collection and proper arrangement of the general principles or leading truths relating to any subject; and there is this connection between art and science, namely— ''A principle in science is a rule of art." NATURAL PHILOSOPHY. CHAPTER I. DIVISIONS OF THE SUBJECT. 1. Natural Philosophy is the science which treats of the powers and properties of natural bodies, their mutual action on one another, and the laws and opera- tions of the material world. 2. Some of the principal branches of Natural Philos- ophy are, Mechanics, Pneumatics, Hydrostatics, Hy- draulics, Acoustics, Pyronomics, Optics, Astronomy, Electricity, Galvanism,^ Magnetism, Electro-Magnetism, and Magneto-Electricity. 1. Mechanics is that branch of Natural Philosophy which relates to motion and the moving powers, their nature and laws, with their effects in machines, &c. 2. Pneumatics treats of the nature, properties, and effects of air. * It may perhaps bo questioned whether the subjects of Galvanism, Electro-Magnetism, and Magneto-Electricity, do not more properly fall within the province of Chemistry, as they describe effects dependent on chemical action. As this volume is designed for the diffusion of useful information, a strict adherence to rigid classification has not been deemed so important, as to exclude the notice of subjects so intimately connected with one of the most hiteresting branches of Natural Philosophy. 1. What is Natural Philosophy? 2. What are the principal branches of Natural Philosophy ? What is Mechanics? Of what does Pneumatics treat? NATURAL PHILOSOPHY. 3. Hydrostatics treats of the nature, gravity, and pressure of fluids. 4. Hydraulics treats of the motion of fluids, particularly of water ; and the construction of all kinds of instruments and machines for moving them. 5. Acoustics treats of the nature and laws of sound. 6. Pyronomics treats of heat, the laws by which it is gov- erned, and the effects which it produces. V. Optics treats of light, of color, and of vision, or sight. 8. Astronomy treats of the heavenly bodies, such as the sun, moon, stars, comets, planets, &c. 9. Electricity treats of thunder and lightning, and the causes by which they are produced, both naturally and artificially. 10. Galvanism is a branch of Electricity. 11. Magnetism treats of the properties and eff'ects of the magnet, or loadstone. 12. Electro-Magnetism treats of Magnetism induced by Electricity. 13. Magneto-Electricity treats of Electricity induced by Magnetism. CHAPTER II. OF MATTER* AND ITS PROPERTIES. 3. Matter is the general name of every thing that occupies space, or has figure, form, or extension. * The ancient philosophers supposed that all material substances were composed of Fire, Air, Earth, and Water, and these four substances were called the four elements, because they were supposed to be the simple substances of which all things were composed. Modern science has proved that not one of these is a simple substance, but that there are at least fifty -five simple substances, thirty-two of which are metallic and twenty-four non-metallic. The consideration of those substances which enter into the composition of all matter, in whatever form, belongs to Of what does Hydrostatics treat ? Hydraulics ? Acoustics ? Pyronomics ? Optics? Astronomy? Electricity? Of what is Galvanism a branch? Of what does Magnetism treat? Electro-Magnetism? Magneto-Elec- tricity ? 3. What is Matter? MATTER AND ITS PROPERTIES. 19 4. There are seven essential properties belonging to all matter, namely: 1. Impenetrability, 2. Extension, 3. Figure, 4. Divisibility, 5. Indestructibility, 6. Inertia, and 7. Attraction. 1. These are called essential properties, because no particle of matter can be deprived of them, or exist without them. 2. There are certain other properties existing in different bodies, called accidental properties, because they do not ne- cessarily exist in the bodies themselves, but depend upon their connection with other bodies. Thus, color and weight are ac- cidental properties, because they do not necessarily exist in the bodies that possess them, but depend upon their connection with other things. 3. There are also certain terms used to express the state in which matter exists, such as Porosity, Density, Rarity, Com- pressibihty. Expansibility, Mobihty, Elasticity, Brittleness, Mal- leability, Ductility, and Tenacity. 5. Impenetrability is the power of occupying a cer- tain space, so that where one body is, another cannot be, without displacing it. 1. Impenetrability belongs to fluids as well as sohd bodies. The reason why fluids appear less impenetrable than solid bodies, is, that the particles of which they are composed move the science of Chemistry. Bodies which consist of one simple substance are called homogeneous, while those which consist of two or more sim- ple substances are called heterogeneous. Thus, water is a heterogeneous substance, being composed of two simple or homogeneous aeriform fluids, called Hydrogen and Oxygen. An aeriform fluid is a fluid in the form of air. When the particles of which matter is composed is mentioned, it is to be understood that the smallest imaginable portion is meant, not of the homogeneous substances of which it may be composed, but of the matter itf elf, whether homogeneous or heterogeneous. 4. How many essential properties of matter are there ? What are they ? Why are they called essential properties? What other properties exist in different bodies ? Why are they called accidental properties ? Are color and weight essential or accidental properties? Why? What terms are used in Philosophy to express the state in which matter exists ? 5. What is meant by Impenetrability ? Does impenetrability belong to fluids ? Why do fluids appear less impenetrable than solid bodies ? What is supposed to be the form of the particles of fluids ? 20 XATL'EAL PHILOSOPHY. easily among themselves, on account of their shght degree of cohesion.* 2. Put some water into a tube closed at one end ; and then insert a piece of wood that fits closely the inside of the tube. It will be impossible to force the wood to the bottom of the tube, unless the water be first removed. The same ex- periment may be made with air instead of water ; and proves that water, air, and all other fluids aie equally sohd, or im- penetrable, with the hardest bodies. 3. The impenetrabihty of water was shown by an experi- ment made at Florence many years ago. A hollow globe of gold was filled with water, and submitted to great pressure. The water was seen to exude through the pores of the gold, -and covered it with a fine dew. 4. When an open vial, not inverted, is plunged into a basin of water, the air will iush out in bubbles, to make room for the water ; and if an inveited tumbler or goblet be immersed in water, the water will not lise far in the tumbler unless it be inchned so that the aii- can escape. These are fmther proofs of the impenetrabihty of air. 5. When a nail is driven into wood, or any other substance, it forces the particles asunder, and makes its way between them : and the wood is not increased in size by the addition of the nail, because wood is a porous substance, the particles of which may be compressed, and thus make way for the nail. * It is a well-known fact, that a certain quantity of salt, the particles of which are supposed to be smaller than those of water, can be put into a vessel full of water, without causinof it to overflow ; and as the particles of which su^r is composed are smaller than those of salt, a portion of sugar may be added after the fluid is saturated with salt. This mav be accounted for by supposing that the particles of fluids are round, and therefore touch one another only in a ^■ few points. There will be spaces between the parti- cles in the same manner that there are between larg-e baUs which are piled on one another. Between these spaces other smaller balls may be placed : and these smaller balls, having spaces between them, will ad- mit others still smaller, as may be seen in Figf. 1. "VMiat follows from this ? What figure illustrates this ? ^\liat example can you give to prove the impenetrability of water? What of the air? What of soLds ? MATTER AND ITS PROPERTIES. 21 0. Extension is but another name for bulk, or size ; and it is expressed by the terms length, breadth, width, height, depth, and thickness.* 7. Figure is the form or shape of a body. Two cir- cles or two balls may be of the same shape or figure, while they differ in extension. The limits of extension constitute figure. 8. Divisibility is susceptibility of being divided. To the divisibility of matter there is no known limit. 1. A single grain of gold may be hammered by a gold- beater until it will cover fifty square inches ; each square inch may be divided into two hundred strips ; and each strip into two hundred parts. One of these parts is only one two-mil- lionth part of a grain of gold, and yet it may be seen with the naked eye. 2. The particles which escape from odoriferous objects also afi'ord instances of extreme divisibility. 9. By the Indestructibility of matter is meant that it cannot be destroyed. It may be indefinitely divided, or altered in its form, color, and accidental properties, but it must still continue to exist in some form through all its changes of external appearance. 1. When water disappears, either by boiling over a fire, or * Length is the extent from end to end. Breadth or width is the ex- tent from side to side. Heiglit, depth, or thickness, is the extent from the top to the bottom. The measure of a body from the bottom to the top is called height ; from the top to the bottom is called depth. Thus we speak of the depth of a well, the height of a house, &c. 6. What is meant by Extension? What terms are used to express the size of a body? What is length? Breadth? Height, depth, or thick- ness? What is the difference between height and depth? 7. What is meant by Figure ? May bodies be of the same shape or figure and of different dimensions ? Give an example. What constitutes figure ? 8. W^hat is meant by Divisibility ? Is there any known limit to the divisibility of matter ? Mention some examples of the extreme divisibility of matter. 9. What is meant by the Indestmctibility of matter? May it be changed in form and in external appearance? Give examples of such changes. 22 NATURAL PHILOSOPHY. evaporating by the heat of the sun, or, in other words, when " it dries up/' it rises slowly in the fonii of steam or vapor. This vapor ascends in the air and constitutes clouds ; these clouds again fall to the earth in the shape of rain, snow, or hail, and form spiings, fountains, rivers, ckc. The water on or in the earth, therefore, is constantly changing its shape or situation, but no particle of it is ever actually destroyed. 2. The simple substances of which fuel is composed are not destroyed when the fuel is burnt. Parts of them arise in smoke or vapor, and the remainder is reduced to ashes. A body in burning undergoes remarkable changes ; but the various parts into which it has been separated by combustion, continue in existence, and retain all the essential properties of bodies. 10. Inertia is the resistance which inactive matter makes to a change of state, whether of motion or rest. A body at rest cannot put itself in motion, nor can a body in motion stop itself. A body, when put in motion, will continue to move for- ever, unless it be stopped. When a stone or ball is thrown from the hand, there are two forces which continually operate to stop it, namely, the resistance of the air, and gravitation: all motion which is caused by animal or mechanical power, will be destroyed by Fig. 2. the combined action of these forces. But could these forces be suspended, the body in motion would continue to move forever. Fig. 2 represents the simple apparatus, of Mr. Wightman, for illustrating iner- tia. A ball and a card being placed upon the pillar, motion is given to the card by means of a spring, but the ball remains on the pillar. 11. Attraction is the tendency which different bodies or portions of matter have, to approach or to adhere to each other. 10. What is meaut by Inertia? Can a body at rest pat itself in mo- tion? Can a body in motion stop itself? When a stone or ball is thrown from the hand, how many forces continually operate to stop it? What are they ? How could a body in motion be made to move forever ? Ex plain the apparatus for illustrating inertia. MATTEU AND ITd I'ilOl'ERTIES. Every portion of matter is attracted by every other portion of matter, and tliis attraction mcreases as he quaVitity of matte,' is increased, and d.mmishes as the quantity of matter is dimmished. 12 There are two kinds of attraction belonging to all matter, namely, the attraction of gravitation and the attraction of cohesion. • . j r>f Hif The attraction of gravitation is the tendency ot dit- ferent bodies to approach each other. The attraction of cohesion is that which causes the particles of a body to cohere to each other. By the attraction of gravity, a stone falls to the ground. By the attraction of cohesion, the particles which compose the stone are held together. 1 All matter is composed of very minute particles which are connected together in different bodies by different degrees The mrticles of which bodies are composed absolutely touch one another in few points only. There are small spaces caned pores\ between the particles; and the Foportion of these poves gives rise to the terms density and ranty. A body n which the pores are small and few m number.as called a dense body. When the pores are large and numerous, the body is said to be rare. . , Lemity, therefore, iraplies the closeness and compactness of the * Cohesive attraction is illustrated by nieans of a pair of ispheres, which being pressed together, will be found to cohere. The ex- pTrimen; may be made with equal success with two bullets scraped smooth at the points of contact. t The pores of bodies are generally filled with air. 11. What is attraction ? Is every portion of matter attracted by every other portion of matter 7 How does this attraction increase and dimmish . 12 How many kinds of attraction are there belonging to all matter? Wh!; S the attrlction of gravitation, or gravity 7 What is ^^^^^^^^^^ of cohesion, or cohesive attraction ? What causes a « « ^^'^ ^ ground? By what are the particles which compose the stone held to feZtl Of what is matter composed? Is the cohesive power which mi tes them the same in all bodies? How may cohesive at raction be il- r^ated? Do the particles of matter in bodies absolutely touch each Other? What are the spaces between them called / 24 ^ ^ NATURAL PHILOSOPHY. particles of a hody^ and indicates the quantity of matter contained in it under a given hulk. Rarity is the reverse of density ^ and implies extension of hulk, without increase of quantity of matter. 13. Compressibility implies the reduction of the limits of extension. Of this all substances are susceptible if a sufficient force be applied.* 14. Expansibility is the reverse of compressibility, and implies the increase of the limits of extension. 15. Mobility implies susceptibility of motion. 16. Elasticity is the property which causes a body to resume its shape after being compressed or expanded. Thus, when a bow is bent, its elasticity causes it to resume its shape. India-rubber possesses this property in a remark- able degree, but the gases in a still greater. The elasticity of ivory is very perfect, that is to say, it restores itself after com- pression with a force very nearly equal to that exerted in compressing it. Liquids, on the contrary, have scarcely any elasticity. 17. Malleability implies susceptibility of extension un- der the hammer or the rolling-press. This property be- longs to some of the metals, as gold, silver, iron, copper, &c., but not to all ; and it is of vast importance to the arts and conveniences of life. Gold is the most malle- able of all metals. 18. Brittleness is the reverse of malleability, and im- plies aptness to break into irregular fragments. This property belongs chiefly to hard bodies. * Sir Isaac Newton conjectured, that if the earth were so compressed as to be absolutely without pores, its dimensions might not be more than a cubic inch. What do you understand by density? What by rarity? 13. WTiat is compressibility? Are all substances susceptible of it? 14. What is expansibility ? 15. What is mobility ? 16. What is elasticity ? Wliat substance possesses this property in a remarkable degree ? 17. What is malleability? Does this property belong to all the metals? What metal possesses it in the highest degree ? 18. What is brittleness ? What bodies are most brittle? OF GRAVITY. 25 19. Ductility is thot property which renders a sub- stance susceptible of being drawn into wire. Platina is the most ductile of all metals. It can be drawn into wire scarcely larger than a spider's web. 20 Tenacity impUes a great degree of adhesion among the particles of bodies. The tenacity of bodies consti- tutes their strength, or their capability of sustaining weight. Iron, on account of its fibrous structure,is very tenacious. CHAPTER III. OF GRAVITY. 21. The term Gravity, in Philosophy, expresses the reciprocal attraction of separate portions of matter. All matter possesses this attraction, and all bodies attract each other with a force proportionate to their size and density, when at a given distance from each other. A body unsupported falls to the earth. This is caused by the superior attraction of the earth, arising from its density and size. 22. The attraction of gravitation causes weight. 1. When we say that a body weighs an ounce, a pound, or a hundred pounds, we express, by these terms, the degree of attraction by which it is drawn towards the earth. 2. Weight, therefore, is the measure of the eartKs attraction. As this attraction depends upon the quantity of matter there is in a body, it follows that those bodies which contain the most matter will be most strongly attracted, and will consequently be the heaviest. 19. What is ductility? Which is the most ductile of the metals? 20. What is tenacity? Which is the most tenacious of the metals? 21. What do you understand by the term gravity ? Do all bodies pos- sess this attraction? To what is its force proportional ? If a body be un- supported, will it remain stationary ? Why will it fall ? 22. What causes weight? When you say that a body weighs an ounpe what do you mean by it ? What, therefore, is weight? 2 26 NATURAL PHILOSOPHY. 23 The force of gravity is greatest at the surface of the e'arth, and decreases both upwards and downwards, but in different degrees.* ^ tLp It decreases above the surface as the square of t.e distance from the centre increases, r rom tne surface to the centre it decreases, simply as the aistance in- creases. That is. gravity at the surface of the earth (which is about 4000 miles from che centre) is four times more powerful than it would be at double taat distance, or 8000 miles from the centre. According to the principles just stated, a body which at the surface of the earth weighs a pound, at the centre of the earth will weigh nothing. ... • ^ . r a 1000 miles from the centre it will weigh ^ of a pound, 2000 " 3000 4000 8000 12000 ^- of a pound, I of a pormd, 1 pound. , d:c.t * The force of crravity is absolutely greatest at the centre of the earth ; but at that point it^is exerted in all directions, and consequently a body at that pomt would remain stationary, because there is no superior attraction for it to obey. , . , ^ „„„ + It follows from what has been stated, with regard to weight as a con- sequence of attraction, that if there were but one body in the universe, i would have no weight, because there would be nothing to attract it. But cohesive attraction would still exist, and keep the particles which compose the body united. As the attraction between all bodies is mutual, it loilows that when a stone or any heavy body falls to the earth, the earth will rise to meet it. But as the attraction is in proportion to the quantity of matter each contains, the stone will fall as much farther than the earth rises, as the earth exceeds the stone in mass. Now the earth is one quatnlliou, that is, one thousand miUion millions times larger than the largest bod) which has ever been known to fall through our ^to°^P*>«';^^/"PP^'"f' then, that such a body should fall through a distance of 1000 feet-the earth would rise no more than the hundred billionth part of an inch, a dis- tance altogether imperceptible to our senses. The principle of mutual at- traction is not confined to the earth. It extends to the sun, the planets, comets, and stars. The earth attracts each of them, and each of them 23. Where is the force of gravity greatest ? How does it change' How does it decrease above the surface of the earth 1 How below ? OF GRAVITY. 27 24. The direction in which falhng bodies approach the surface of the eartli, is called a vertical line.* Such lines are everywhere perpendicular to the surface, and when prolonged will meet nearly at the centre of the earth. For this reason no two lines suspended by weights, will be parallel to each other. ^' Even a pair of scales, hanging perpendicular to the earth, are not exactly parallel, because they point to the same spot, namely, the centre of the earth ; but the convergency is so small, that their inclination is not percepti- ble to our senses. [See Fig. 3.] For the same reason no two bodies can fall to the earth in parallel lines. 25. According to the laws of attraction, all bodies at an equal distance from the earth will fall to it in the same space of time, if nothing impede them. But bodies of different density fall with different degrees of velocity, on account of the resistance of the air ; and as heavy bodies overcome this resistance more easily than light ones, the former will fall with the greater velocity. The resistance which the air opposes to the fall of bodies, is proportioned to their surface, not to their weight. attracts the earth, and these mutual attractions are so nicely balanced by the power of God, as to cause the regular motions of all the heavenly bodies, the diversity of the seasons, the succession of day and night, sum- mer and winter, and all the grand operations which are described in as- tronomy. * A vertical line is sometimes called a plumh-line, because it is formed by a weight suspended at rest from a string. As the weight thus employ- ed is usually of lead, the term plumb, from the Latin plumbum, lead, is applied to the line. 24. In what direction will a fairing body approach the surface of the earth? Will the lines of suspension of different bodies ever be parallel? Where will they meet, if sufficiently produced ? 25. Will all bodies at equal distances from the earth, fall to it in the same time? Why not? What bodies falls fastest ? To what is the re- sistance of the air proportional ? 28 N\TURAL PHILOSOPHY. . Heavy bodies caa be made to float in the air, instead of falling immediately to the ground, by making the extent ot h^r^surface counterbalance their weight Thus gold, which is one of the heaviest of all substances, when spread out mto thin leaf, is not attracted by gravity with su&cient force to overcome the resistance of the air; it therefore floats in the air, or falls slowly. 26 All substances are influenced by gravity, in exact proportion to their quantity of matter, and their distance from the central point of attraction. 1 Even air itself, light as it seems, is subject to this attrac- tion' The air* probably extends to a height of more than forty-five miles above the surface of the earth. The pressure of the upper parts of the atmosphere on those beneath ren- ders the air near the surface of the earth much more dense fhan that in the upper regions. This pressure is caused by the attraction of the earth, or, what is the same fj.-r J^jf^* 01 tne air above; and it would cause the air to fall like other bodies completely to the earth, were it not for the elasticity of that portion which is near the surface. _ 2 The air therefore, of which the atmosphere is composed exists in a state of compression, which causes it to be heaviest near the surface of the earth. * We have no means of ascertaining the exact height to which the air extends. Sir John Herschel says, " Laying out of consideration all n.ce ■ questions as to the probable existence of a definite limit to the atmosphere,, bevond which there is absolutely and rigorously speaking no air, it is clear, th^t for all practical purposes we may speak of those regions which are more distant above the earth's surface than the hundredth part of its diameter as void of air, and of course, of clouds, (which are nothing but visible vapors, diffused and floating in the air, sustained by it, and rendering 't t-b.d as mud does water.) It seems probable, from many indications, that the great- est height at which visible clouds over exist does not exceed ten miles ; at which height the density of the air is about an eighth part of what it is at the level of the sea." Although the exact height to which the atmosphere extends has never been ascertained, it ceases to reflect the sun s rays at a greater height than forty-five miles^ 26 In what proportion are all Substances influenced by gravity ? Is air affected by it? How far does the air extend above the surface of the earth ' What causes the air to be more dense at the surlace of the earth ? What causes this pressure? Why does not the air fall to the earth like other bodies ? Where is the air heaviest ? What effect have gravity and elasticitv upon the air? OF GRAVITY. 29 27. The specific gravity of bodies is a term used to xpress the relative weight of equal bulks of different Dodies.* 1. If we take equal bulks of lead, wood, cork, and air, we find the lead to be the heaviest, then the wood, then the cork, and lastly the air. Hence we say that the specific gravity of cork is greater than that of air, the specific gravity of wood is greater than that of cork, and the specific gravity of lead greater than that of wood, &c. 2. From what has now been said with respect to the attrac- tion of gravitation and the specific gravity of bodies, it appears that although the earth attracts all substances, yet this very attraction causes some bodies to rise and others to fall. 3. Tliose bodies or substances, the specific gravity of which is greater than that of air, will fall, and those whose specific gravity is less than that of air, will rise ; or rather, the air being more strongly attracted will get beneath them, and, thus displacing them, will cause them to rise. For the same reason, cork and other light substances will not sink in water, because the specific gravity of water being greater, the water is more strongly attracted, and will be drawn down beneath them. [For a table of the specific gravity of bodies, see Hydrostatics.] 4. The principle which causes balloons to rise, is the same which occasions the ascent of smoke, steam, &c. The mate- l ials of which a balloon is made, are heavier than air, but their extension is greatly increased, and they are filled v/ith an elas- tic liuid of a different nature, specifically lighter than air, so that on the whole, the balloon when thus filled is much lighter than * The quantity of matter in a body is estimated, not by its apparent size, but by its weight. Some bodies, as cork, feathers, &c., are termed hght ; others, as lead, gold, mercury, &c., are called heavy. The reason of this is, that the particles which compose the former are not closely packed together, and therefore they occupy considerable space ; while in the latter they are joined more closely together, and occupy but little room. A pound of cork and a pound of lead, therefore, will differ very much in apparent size, while they are both equally attracted by gravity, that is, they weigh the same. 27. What is specific gravity ? Illustrate this. Does the attraction of the earth cause all bodies to fall? What bodies will fall? What rise? How does the air cause them to rise ? Why do not cork and other light bodiep sink in wate^? Explain the principle upon which balloons rise. 30 NATURAL PHILOSOPHY. a portion of air of the same size or dimensions, and it will con- sequently rise. 5. Gravity, therefore, causes bodies which are lighter than air to ascend, those which are of equal weight with air to re- main stationary, and those which are heavier than air to de- scend; but the rapidity of their descent is affected by the resistance of the air which resistance is proportioned to the extent of the surface of the falling body. CHAPTER IV. MECHANICS, OR THE LAVV^S OF MOTION. 28. Mechanics is that branch of Natural Philosophy which relates to motion and the moving powers, their nature and laws, with their effects in machines, &c. 29. Motion is a continued change of place. On account of the inertia of matter, a body cannot put it- self in motion, nor when it is in motion can it stop itself. 30. The power which puts a body into motion is called ^ force ; and the power which has a tendency to stop or impede motion is called resistance, 31. The motion of a body impelled by a single force is always in a straight line, and in the same direction in which the force acts. 32. The rapidity with which a body moves is called its velocity. What effect has gravity on -bodies Ughter than the air ? What effect on bodies of equal weight ? What effect on those that are heavier ? What affects the rapidity of their descent ? To what is the resistance of the air proportioned ? 28. What is Mechanics? 29. What is motion? Why cannot a body put itself in motion? Why cannot a body stop itself when in motion ? 30. What is force ? What is resistance ? 31. When is the motion of a body in a straight hue ? In what direction will it move ? 32. What is meart by velocity? MECIIANICS. 31 33. The velocity of a given body is proportional to the force by which it is put in motion.* 34. The velocity of a moving body is determined by the time that it occupies in passing through a given space. The greater the space, and the shorter the time, the greater is the velocity. Thus, if one body move at the rate of six miles, and another twelve miles in the same time, the velocity of the latter is double that of the former.f 35. The velocity of a body is measured by the space over which it moves, divided by the time which it em- ploys in the motion. Thus, if a body move one hundred miles in twenty hours, the velocity is one hundred divided by twenty, that is, five miles an hour. * The mean velocity of rivers is about four feet in a second ; of a very rapid stream, about 13 feet ; of a moderate wind, about 10 feet ; of a storm, 54 feet ; of a violent hurricane, 125 feet ; of sound, 1142 feet ; of atmospheric air rushing into a vacuum, 1280 feet ; of a musket-bal!, 1280 feet ; of a rifle-ball, 1600 ; of a cannon-ball of 24 pounds, 2400 feet ; of a point at the surface of the earth, under the equator, 1500 feet ; of the earth's centre in its orbit round the sun, 101,061 feet. The average rate of steamers between New York and Albany, exclu- sive of stoppages, is ten and a half miles per hour ; of the mail trains on the great railroads, about 25 miles an hour ; of the fastest sailing vessel, 15 feet in a second ; of the swiftest race-horse, 42 feet in a second. t Velocity is sometimes called absolute, and sometimes relative. Ve- locity is called absolute when the motion of a body in space is considered without reference to that of other bodies. When, for instance, a horse goes a hundred miles in ten hours, his absolute velocity is ten miles an hour. Velocity is called relative when it is compared with that of another body. Thus, if one horse travel only fifty miles in ten hours, and another one hundred in the same time, the absolute velocity of the first horse is five miles an hour, and that of the latter is ten miles ; but their relative velocity is five miles. 33. To what is the velocity of a moving body proportional? 34. How is the velocity of a moving body determined ? If one body go through six miles in an hour, and another twelve, how does the velocity of the latter compare with that of the former? What is meant by abso- lute velocity ? Give an example. Wheu is the velocity of a body termed relative? Give an example. 35. How is the velocity of a body measured? Illustrate this. 32 NATURAL PHILOSOPHY. 36. The time employed by a body in motion may be ascertained by dividing the space by the velocity. Thus, if the space be one hundred miles, and the velocity five miles in an hour, the time will be one hundred divided by five, which is twenty hours. 37. The space also may be ascertained by multiply- ing the velocity by the time. Thus, if the velocity be five miles an hour, and the time twenty hours, the space will be twenty multiplied by five, which is one hundred miles. 38. There are three terms applied to motion to ex- press its kind ; namely, uniform, accelerated, and re- tarded motion. Uniform motion is that of a body passing over equa, spaces in equal times. Accelerated motion is that in which the velocity con- tinually increases as the body moves. Retarded motion is that in which the velocity de- creases as the body moves. 39. Uniform motion is produced by a force having acted on a body, and then ceasing to act A ball struck by a bat, or a stone thrown from the hand, is in theory an instance of uniform ' motion ; and if both the at- traction of gravity and the resistance of the air could be en- tirely removed, it would proceed onwards in a straight fine, and with a uniform motion forever. But as the resistance of the air and gravity tend to deflect it, it in fact becomes an in- stance first of retarded and then of accelerated motion. 40. Accelerated motion is produced by the continued action of one or more forces. 36. How do you ascertain the time employed by a body in motion? Il- lustrate this. 37. How can you ascertain the space ? IHustrate this. 38. How many terms are applied to motion to express its kind ? What are they? What is uniform motion ? Accelerated? Retarded? 39. How is uniform motion produced? Why is not a ball struck by a bafc, or a stone thrown from the hand, an instance of unifoiTn motion? How can it be made an instance ? 40. How is accelerated motion produced ? MF.CIIANICS. ^ 33 1. Thus, wlien a stone falls from a height, the impulse which it receives from gravity would be suflicient to bring it to tlie ground with a uniform veh)city. But the stone while falhng ac this rate is still acted upon by gravity with an additional force, which contmues to impel it during the whole time of its descent. 2. In the first second it falls sixteen feet, three times that distance in the next, five times in the third, seven times in the fourth, and so Oil, regularly increasing its velocity according to the numher of seconds consumed in falling. 3. The height of a building, or the depth of a well, may thus be measured by observing the length of time which a stone takes in falling from the top to the bottom.^ 41. Retarded motion is produced when a body in motion encounters a force operating in an opposite di- rection. 1. Thus, when a stone is thrown perpendicularly upwards, the force of gravity is continually operating in the opposite direction, and attracting it downwards to the earth. The stone moves upwards slower and slower, until the upward motion ceases, and the body returns with accelerated motion to the earth. It is found that a body thrown perpendicularly up- wards, takes the same length of time in ascending that it takes in descending. 2. Perpetual motion has never yet been produced by art; and the principles of mechanics seem to prove that such a motion is impossible ; for although in many cases of bodies acting upon one another, there is a gain of absolute motion, yet the gain is always equal in opposite directions, so that the * The spaces through which a body falls hi equal successive portions of time, increase as the odd numbers 1, .3, 5, 7, &c. ; that is, a falhng body descends in the 2d second of its fall through 3 times, and in the 3d second through 5 times the space passed over in the first second. But the entire spaces through which a body will have fallen in any given number of sec- onds, increase as the squares of the times. Give an instance of accelerated motion. How far does a stone fall the first second of time? The second? Third? Fourth? How can you measure the height of a building, or the depth of a well ? 41. How is retarded motion produced? Give an example. How does the time of the ascent of a body thrown perpendicularly upwards, compare with that of its descent ? Why cannot perpetual motion b® produced ? 2* 34 NATURAL PHILOSOPHY. quantity of direct motion is never increased. But nature abounds with examples of perpetual motion, as for instance, the motion of the heavenly bodies, described in the science of astronomy. 42. The momentum of a body is its quantity of mo- tion, or the force with which it would strike agamst another body. It, is measured by multiplying its weight by its velocity.* Thus, if a body weighing six pounds move at the rate of two miles in a second of time, its momentum may be repre- sented by six multiplied by tAvo, which is twelve. Hence a small or a hght body may be made to strike against another body with a greater force than a heavy one, simply by giving it sufficient velocity. 43. The action of a body is the effect which it pro- duces upon other bodies. Reaction is the effect which it receives from the body on which it acts. Thus, when a body in motion strikes against another body, it acts upon it, or produces action ; but it also meets with re- sistance from the body which is struck, and this resistance is the reaction of the body. 44. Action and reaction are always equal, but in op- posite directions. 1. Experiments to show the mutual action^ and reaction of bodies, are made with both elastic and non-elastic bodies. Fig. 4 * The quantity of motion communicated to a body does not affect the duration of the motion. If but httle motion be communicated, the body will move slowly. If a great degree be imparted, it will move rapidly. But in both cases the motion will continue until it is destroyed by some external force. 42. What is the momentum of a body? How can the momentum of a body be ascertained? Note. Does the quantity of motion communicated to a body affect the duration of the motion? If but Uttle motion is com- municated, how will the body move ? If a great degree ? How long will the motion continue ? How can a light body be made to have a greater momentum than a heavy one ? Give an instance of this. 43. What is meant by action? Reaction? Illustrate this. 44 How do action and reaction compare ? MECHANICS. 35 represents two ivoiy balls, A and B, of equal ^.^ ^ weio-ht, ike, suspended by tlircads. If the ball A be drawn a little on one side and then let i>-o, it will strike against the other ball B, and drive it off to a distance equal to that through which the first ball fell ; but the motion of A will be stopped, because when it strikes B it receives in return a blow equal to that which it gave, but in a contrary direction, and its motion is thereby stopped, or rather, given to B. Therefore, when a body strikes against another, the quantity of motion communicated to the second body is lost hy the first ; but this loss proceeds, not from the blow given by the striking body, but from the reaction of the body which it struck. 2. Fio-. 5 represents six ivory balls, of equal weight, sus- pended V threads. If the ball A be drawn out of the per- pendicular, and let fall against B, it will communicate its motion to B, and receive a reaction from it which will stop its own motion. But the ball B cannot move without moving C ; it will there- fore communicate the motion which it received from A to C, and receive from C a reaction which will stop its motion. In like manner the motion and reaction are received by each of the balls, D, E, F ; but as there is no ball beyond F to act upon it, F will fly off. ]Sr. B. This experiment can be accurately performed by those bodies only which are perfectly elastic. 3. Fig. 6 represents two balls of clay, (which are not elastic,) of equal weight, suspended by strings. If the ball D be raised and let fall against E, only part of the motion of D will be destroyed by it, (because the bodies are non-elastic,) and the two balls will move on together to d and e, which are less distant from the vertical line than the ball D was before it fell. Still, however, action and reac- tion are equal, for the action on E is only enough to make ii move through a smaller space, but so much of D's motion is now also destroyed.* * Figs. 4 and 5, as has been explained, show the effect of action and reaction in elastic bodies, and Fig. 6 shows the same effect in non-elastic Explain Fig. 4. Fig. 5 Fig. 6. 36 NATURAL PHILOSOPHY. 4. It is -upon the principle of action and reaction, that birds are enabled to fly. They strike the air with their wings, and the reaction of the air enables them to rise, fall, or remain stationary at will, by increasing or diminishing the force of the stroke of their wings. 5. It is likewise upon the same principle of action and re- action, that fishes swim, or, rather, make their way through the water ; namely, by striking the water with their fins.f 6. Boats are also propelled by oars on the same principle, and the oars are lifted out of the water, after every stroke, so as completely to prevent any reaction in a backward direction. 45. Motion may be caused either by action or reac- tion. When caused by action it is called incident, and when caused by reaction it is called reflected motion. J bodies. When the elasticity of a body is imperfect, an intemnediate effect will be produced ; that is, the ball which is struck will rise higher than in case of non-elastic bodies, and less so than in that of perfectly elastic bodies ; and the striking ball will be retarded more than in the former case, but not stopped completely, as in the latter. They will, therefore, both move onwards after the blow, but not together, or to the same distance ; but in this, as in the preceding cases, the whole quantity of motion de- stroyed in the striking ball, will be equal to that produced in the ball struck. Connected with '* the Boston school apparatus" is a stand with ivory balls, to give a visible illustration of the effects of collision. * The muscular power of birds is much greater in proportion to their weight than that of man. If a man were furnished with wings sufficiently large to enable him to fly, he would not have sufficient strength, or mus- • cular power, to put them in motion. t The power possessed by fishes, of sinking or rising in the water, is greatly assisted "by a pecuHar apparatus furnished them by nature, called an air-bladder, by the expansion or contraction of which they rise or fall, on the principle of specific gravity. t The word incident implies falling upon, or directed towards. The word reflected implies turned back. Incident motion is motion directed towards any particular object, against which a moving body strikes. Re- flected motion is that which is caused by the reaction of the body which is struck. Thus, when a ball is thrown against a surface, it rebounds or Upon what principle do birds fly? Explain hov/. Upon what principle do fishes swim? Upon what principle do boats move upon the water? Explain how. 45. How may motion be caused? When caused by action what is it called ? When caused by reaction what is it termed ? MECHANICS. 37 40. The angle* of incidence is the angle fornried by the line which the incident body makes in its passage towards any object, with a line per- ^ pendicular to the surface of the ob- ^ ject. '^'--..^^^ Thus, in Fig. 1, the line ABC repre- p sents a wall, and P B a line perpendicular ^^.--^ to its surface. 0 is a ball moving in the ^ direction of the dotted line, 0 B. The an- o-le 0 B P is the ano^le of incidence. is turned back. This return of the ball is called reflected motion. As re- flected motion is caused by reaction, and reaction is caused by elasticity, it follows, that reflected motion is always greatest in those bodies which are most elastic. For this reason, a ball filled with air rebounds better than one stuffed with bran or wool, because its elasticity is greater. For the same reason, balls made of caoutchouc, or India-rubber, will rebound more than those which are made of most other substances. * As this book may fall into the hands of some who are unacquainted with geometrical figures, a few explanations are here subjoined. 1. An angle is the opening made by two lines which meet each other in a point. The size of the angle depends upon the opening, and not upon the length of the lines. 2. A circle is a perfectly round figure, ev- Fig. 8. ery part of the outer edge of which, called the ^ circumference, is equally distant from a point within, called the centre. (See Fig. 8.) 3. The straight lines drawn from the cen- tre to the circumference are called radii. [The singular number of this word, is radius.] Thus, in Fig. 8, the lines CD, CO, CR, and CA, are radii. 4. The lines drawn through the centre, and terminating in both ends at the circumference, are called diameters. Thus, in the same figure, D A is a diameter of the circle. 5. The circumference of all circles is divided into 360 equal parts, called degrees. The diameter of a circle divides the circumference into two equal parts of 180 degrees each. 46. What is the angle of incidence? (Note.—l. What is an angle? Upon what does the size of an angle depend? 2. W^hat is a circle? 3. What are radii? What lines in Fig. 8 are radii? 4. What are diam- eters? In Fig. 8, what line is the diameter? 5. How is the circumfer- ence of all circles divided ? Into how many parts does the diameter of a circle divide it ? c 38 NATURAL PHILOSOPHY. 47. The angle of reflection is the angle formed by the perpendicular with the Une made by the reflected body as it leaves the surface against which it struck. Thus, in Fig. 7, the angle P B K is the angle of reflection. 48. The angles of incidence and reflection are always equal to one another. I. Thus, in Fig. Y, the angle of incidence, 0 B P, and the angle of reflection P B R, are equal to one another ; that is, they contain an equal number of degrees. 6. All angles are measured by the number of degrees which they con- tarn. Thus, in Fig. 8, the angle R C A, as it includes one quarter of the circle, is an angle of 90 degrees, which is a quarter of 360. And the an- gles R C O and O C D are angles of 45 degrees. 7. Angles of 90 degrees are right angles ; angles of less than 90 degrees, acute angles, and angles of more than 90 degrees are called obtuse angles. Thus, in Fig. 8, RC A is a right angle, OCR acute, and O C A an ob- tuse angle. 8. A perpendicular line is a line which makes an angle of 90 degrees on each side of any other line or surface ; therefore, it will incline neither to the one side nor to the other. Thus, in Fig. 8, R C is perpendicular to D A. 9. The tangent of a circle is a line which touches the circumference, without cutting it when lengthened at either end. Thus, in Fig. 8, the line R T is a tangent. 10. A square is a figure having four equal sides, and four equal angles. These will always be right angles. (See Fig. 9.) II. A parallelogram is a figure whose opposite sides are equal and par- allel. (See Figs. 10 and 11.) A square is also a parallelogram. 12. A rectangle is a parallelogram whose angles are right angles. 13. The diagonal of a square, of a parallelogram, or a rectangle, is a line drawn through either of them, and terminating at the opposite angles. Thus, in Figs. 9, 10, and 11, the line AC is the diagonal of the square, parallelogram, or rectangle. 6. How are all angles measured ? Illustrate this by Fig. 8. 7. How many degrees do right angles contam ? Acute? Obtuse? Illustrate these angles by Fig. 8. 8. What is a perpendicular line ? What line is per- pendicular in Fig. 8 ? 9. What is a tangent? What line is a tangent in Fig. 8? 10. What is a square ? 11. What is a parallelogram? 12. A rectangle? 13. What is a diagonal? What hues are diagonals in Figs- 9, 10, and 11 ?) Explain the angle of incidence by Fig. 7. 47. What is the angle of reflection? Illustrate this by Fig. 7. 48. How do the angles of incidence and reflection compare with each other ? Illustrate this by Fig. 7. MECHANICS. 39 2. From what has now been stated with regard to the angles of incidence and reflection, it follows, that when a ball is thrown perpendicularly against an object which it c^mnot penetrate, it will return in the same direction ; but if it be thrown obliquely, it will return oblicjuely on the opposite side of the perpendicular. The more obhquely the ball is thrown, the more obliquely it will rebound.* COMPOUND MOTION. 49. Compound motion is caused by the operation of two or more forces at the same time. 50. When a body is struck by two equal forces in opposite directions, it will remain at rest. ,51. A body struck by two forces in different direc- tions, will move in a line between them. This line will be the diagonal of a parallelogram, having for its sides the lines through which the body would pass, if urged by each of the forces separately. 1. Let Fig. 9 represent a ball struck by the two equal forces, X and Y. In this figure, the forces are inclined to each other at an angle of 90 degrees, or j,.^ ^ a right angle. Suppose that the force X would send it from C to B, and the force Y, from C to D. As it cannot obey both, it will go between them to A, and the line C A, through which it passes, represents the diagonal of the square, A B C D. The time occupied in its passage from C * It is from a knowledge of these facts that skill is acquired in many different sorts of games, as Billiards, Bagatelle, &c. A ball may also, on the same principle, be thrown from a gun against a fortification, so as to reach an object out of the range of a direct shot. What follows from what has been stated with regard to the angles of incidence and reflection? 49. What is compound motion ? 50. In what direction will a body, struck by two equal forces in oppo- site directions, move ? 51. When struck by two forces inclined to each other, how will it move? What is this fine called ? Illustrate these, first, by Fig. 9, which represents a ball struck by two equal forces in different directions. 40 NATURAL PHILOSOPHY. to A will be the same as the force X would require to send it to B, or the force Y to send it to D. 2. If the two forces acting on a body are unequal, but still operate at right angles to each other, the body will move from C to A as represented in Fig. 10 ; in which it is to be observed that the force Y is as much greater than the force X, as the length of the side C D of the rectangle A B C D, ex- ceeds the length of the side C B. 3. When two forces operate in the direc- tion of an acute angle, (see Fig. 11,) the body will move as represented by C A, in the parallelogram A B C D. 4. If the forces operate in the direction of an obtuse angle, the body will move as represented by D B in the same figure. 52. Circular motion is motion around a central point, and is caused by two forces operating at the same time, by one of which it is projected forward in a straight line, while by the other it is deflected towards a fixed point. The whirling of a ball, fastened to a string held by the hand, is an instance of circular motion. The ball is urged by two forces, of which one is the force of projection, and the other the string which confines it to the hand. The two forces act at right angles to each other, and (according to No. 51) the ball will move in the diagonal of a parallelogram. But, as the force which confines it to the hand only keeps it within a certain distance, without drawing it nearer to the hand, the motion of the ball will be through the diagonals of an indefinite number of minute parallelograms, formed by every part of the circum- ference of the circle. 53. There are three different centres which require to be distinctly noticed; namely, -the centre of magni- tude, the centre of gravity, and the centre of motion. Second, by Fig. 10, which represents a ball struck by two unequal forces, acting at right angles. Third, by Fig. 11, where the forces operate in the direction of an acute angle. Fourth, by Fig. 11, where the forces oi>erate in the direction of an obtuse angle. 52 What is circular motion? How is it caused? Illustrate th s. 53. How many different centres are there which require to be noticed? Define each of them. MECHANICS. 41 The centre of magnitude is the central point of the bulk of a body. The centre of gravity is the point about which all the parts balance each other. The centre of motion is the point around which all the parts of a body move. When the body is not of a size nor shape to allow every point to revolve in the same plane, the line around which it revolves is called the axis of motion.* 54. The centre or the axis of motion is generally sup- posed to be at rest. Thus the axis of a spinning top is stationary, while every other part is in motion around it. The axis of motion and the centre of motion are terms which relate only to circular motion. 55. The two forces by which circular motion is pro- duced, are called central forces. Their names are the centripetal force and the centrifugal force. f 56. The centripetal force is that which confines a body to the centre around which it revolves. The centrifugal force is that which impels the body to fly off from the centre. 57. If the centrifugal force of a revolving body be destroyed, the body will immediately approach the cen- tre which attracts it ; but if the centripetal force be * Circles may have a centre of motion ; spheres or globes have an axis of motion. Bodies that have only length and breadth may revolve around their own centre, or around axes ; those that have the three dimensions of length, breadth, and thickness, must revolve around axes. t The word centripetal means seeking the centre, and centrifugal means flying from the centre. In circular motion, these two forces con- stantly balance each other; otherwise the revolving body will'either ap- proach the centre or recede from it, according as the centripetal or centrif- ugal force is the stronger. 54. Is the centre or axis of motion supposed to be at rest, or does it move? To what do the terms centre of motion and axis of motion relate ? 55. What are the two forces called which produce circular motion? What is the name of each? What do the words centripetal and centrifu- gal mean? 56. Define a centripetal force. Also a centrifugal force. 57. If the centrifugal force be destroyed, to what point will the body tend? 42 NATURAL PHILOSOPHY. destroyed, the body will fly off' in the direction of a tan- gent to the curve which it describes in its motion. Thus, when a mop filled with water is tui-ned swiftly round bv the handle, the threads which compose the head will fiy off from the centre ; but bemg confined to it at one end, they cannot part from it ; while the water they contain, being un- confined, is thrown off in straight lines. 5S. The parts of a body which are farthest from the centre of motion, move with the greatest velocity ; and the velocity of all the parts diminishes, as their distance from the axis of motion diminishes. Fig. 12 represents the vanes of a windmill. The circles denote the paths in which the different parts of the vanes move. M is the centre or axis of mo- tion around which all the parts revolve. The outer part revolves in the circle D E F G, another part revolves in the circle H I J K, and the iimer part in the circle L X O P. Consequently, as they all revolve around M in the same time, the velocity of the parts which re- volve in the outer cuxle is as much great- er than the velocity of the parts which revolve in the mner circle, L X 0 P, as the diameter of the outer circle is greater than the diameter of the inner. 59. As the earth revolves round its axis, it follows, from the preceding illustration, that the portions of the earth which move most rapidly are nearest to the equa- tor, and that the nearer any portion of the earth is to the poles, the slower will be its motion. 60. Curvilinear motion requires the action of two forces ; for, the impulse of one single force always pro- duces motion in a straight line. What would be its direction if the centripetal force were destroyed? Give an example. 58. What parts of a body move with the 2n"eatest velocity ? In what proportion does the velocity of all the parts diminish? What does Fig. 1'2 represent ? 59. What follows, with regard to the motion of the earth, from the il- lustration of Fig. 12 ? 60. Of what is curvilinear motion always the result ? Why? I MECHANICS. 43 Gl. A ball thrown in a horizontal direction is in- fluenced by three forces; namely, first, the force ol projection, (which ,G:ives it a horizontal direct, n ;) second, the resistance of the air through which it passe.-^, which diminishes its velocity, without changing its di- rection ; and third, the force of gravity, which finally brings it to the ground. 62. The force of gravity is neither increased nor di- minished by the force of projection.* Fio\ 13 represents a cannon, loaded with a ball, and placed on the top of a tower, at such a height as to require just three seconds for another ball to descend ^. perpendicularly. Now suppose the ^ tower at the same instant. In this « figure C a represents the perpendicular line of the falling ball. Qb is the curvilinear path of the projected ball, 3 the ho.izontal hne at the base of the tower. During the first second of time, the falling ball reaches 1, the next second 2, and at the end * The action of gravity being always the same, the shape of the curve of every projectile (see No. 63) depends on the velocity of its motion ; but, whatever this velocity be, the moving body, if thrown horizontally from the same elevation, will reach the ground at the same instant. Thus, a ball from a cannon, with a charge sufficient to throw it half a mile, will reach the ground at the same instant of time that it would had the charge been sufficient to throw it one, two, or six miles, from the same elevation. The distance to which a ball will be projected, will depend entirely on the force with which it is thrown, or on the velocity of its motion. If it moves slowly, the distance will be short— if more rapidly, the space pa^ed over in the same time will be greater ; but in both cases the descent of the ball towards the earth, in the same time, will be the same number of feet, whether it moves fast or slow, or even whether it move forward at all, or not. 61. How many forces act upon a ball thrown in a horizontal direction? What are they? Why do bodies fall to the ground? 62. Does the force of gravity either increase or decrease the fore© of projection ? Give an illustration. cannon to be fired in a horizontal direction, and at the same instant the other ball to be dropped towards the ground. They will both reach the horizontal line at thse base of the 44 NATURAL PHILOSOPHY. of the third second it strikes the ground. Meantime, that pro- jected from the cannon, moves forward with such velocity, as to reach 4 at the same time that the falling ball reaches 1. But the projected ball falls downward exactly as fast as the other, since it meets the line 1 4, which is parallel to the hori- zon, at the same instant. During the next second the ball from the cannon reaches 5, while the other falls to 2, both having an equal descent. During the third second the project- ed ball will have spent nearly its whole force, and therefore its downward motion will be greater while the motion forward will be less than before. Hence it appears thai the horizontal motion does not interfere with the action of gravity, hut that a projectile descends with the same rapidity while moving forward that it would if it were acted on by gravity alone. This is the neces- sary result of the action of two forces. 63. A projectile is a body thrown into the air, as a rocket, a ball from a gun, or a stone from the hand. The force of gravity and the liesistance of the air cause projectiles to form a curve both in their ascent and descent ; and in descending, their motion is grad- ually changed from an oblique towards a perpendicular direction. In Fig. 14 the force of projection would carry a ball from A to D, while gravity would bring it to C. If these two forces alone prevailed, the ball d would proceed in the dotted line to B. But as the resistance of the air operates in direct opposition to the force of projection, instead of reaching the ground at B, the ^ ball will fall somewhere about E.^ 64. When a body is thrown in a horizontal direc- tion, or upwards or downwards obliquely, its course will * It is calculated that the resistance of the air to a cannon-ball of two pounds weight, with the velocity of two thousand feet in a second, is more than equivalent to sixty times the weight of the ball. 63. What is a projectile? What lines do projectiles describe? From what cause? Give the illustration. How great is the resistance of the air calculated to be to a cannon-ball of two pounds weight, with the velocity of 2000 feet in a second ? MECHANICS. 45 be in the direction of a curve-line, called Si parabola f {see Fig. 15,) but when it is thvo\vnpe7ye7idicularlyupwa.Ydsordown' wards, it will move perpendicularly, be- cause the force of projection and that of gravity are in the same line of direction. ^ * The science of gunnery is founded upon the laws relating to projec- tiles. The force of gunpowder is accurately ascertained, and calculations are predicated upon these principles, which enable the engineer to direct his guns in such a manner as to cause the fall of the shot or shells in the very spot where he intends. The knowledge of this science saves an immense expenditure of ammunition, which would otherwise be idly wasted without producing any effect. In attacks upon towns and fortifications, the skilful engineer knows the means he has in his power, and can calculate, with great precision, their effects. It is in this way that the art of war has been elevated into a science, and much is made to depend upon skill, which, previous to the knowledge of these principles, depended entirely upon physical power. The force with which balls are thrown by gunpowder is measured by an mstrument called the Ballistic pendulum. It consists of a large block of wood suspended by a rod in the manner of a pendulum. Into this block the balls are fired, and to it they commuiiicate their own motion. Now the weight of the block and that of the ball being known, and the motion or velocity of the block being determined by machinery, or by observation, the elements are obtained by which the velocity of the ball may be found ; for, the weight of the ball is to the weight of the block as the velocity of the block is to the velocity of the ball. By this simple apparatus, many facts relative to the art of gunnery may be ascertained. If the ball be fired from the same gun, at different distances, it will be seen how much resistance the atmosphere opposes to its force at such distances. Rifles and guns of smooth bores may be tested, as well as the various charges of powder best adapted to different distances and different guns. These, and a great variety of other experiments, useful to the practical gunner or sportsman, may be made by this simple means. The velocity of balls impelled by gunpowder from a musket with a common charge, has been estimated at about 1650 feet in a second of time, when first discharged. The utmost velocity that can be given to a can- non-ball, is 2000 feet per second ; and this only at the moment of its leav- ing the gun. In order to increase the velocity from 1650 to 2000 feet, one half more 64. When a body is thrown horizontally, or upwards or downwards obliquely, in what curve will it move ? In what hne will it move when thrown upwards or downwards obliquely ? 46 NATURAL PHILOSOPHY. 65. The random of a projectile is the horizontal dis- tance from the place whence it is thrown, to the place where it strikes. The greatest random takes place at an angle of 45 degrees— that is, w^hen a gun is pointed at this angle with the horizon, the ball is thrown to the greatest distance. Let Fig. 16 represent a gun or a carron- ade, fron? which a ball is thrown at an an- gle of 45 degrees with the horizon. If the ball be thrown at any angle above 45 de- grees, the random will be the same as it would be at the same number of degrees below 45 degrees.* 66. When the centre of gravity of a body is support- ed, the body itself will be supported ; but^ when the centre of gravity is unsupported, the body wiU falLf powder is required ; and even then, at a long shot, no advantage is gained ; since, at the distance of 500 yards, the greatest velocity that can be ob- tained is only 1200 or 1300 feet per second. Great charges of powder are therefore not only useless, but dangerous; for, though they give little ad- ditional force to the ball, they hazard the lives of many by their liability to burst. Experiment has also shown, that, although long guns give a greater ve- locity to the shot than short ones, still, that on the whole, short ones are preferable ; and, accordingly, armed ships are now almost invariably fur- nished with short guns, called carronades. The length of sporting guns has also been gi-eatly reduced, of late years. Formerly, the barrels were from four to six feet in length ; but the best fowUng-'pieces of the present day have barrels of two feet, or two and a half, only, in length. Guns of about this length are now universally em- ployed for such game as woodcocks, partridges, grouse, and such birds as are taken on the wing, with the exceptions of ducks and wild geese, which require longer and heavier guns. * A knowledge of this fact, and calculations predicated on it, enables the engineer so to direct his guns, as to reach the object of attack when within the range of shot. t The Boston School Apparatus contains a set of eight illustrations for 65. What is the random of a projectile ? At what angle does the great- est random take place? 66. When the centre of gravity of a body is supported, will the body stand or fall ? What if the centre be unsupported ? MECHANICS. 47 A line drawn from the centre of gravity, perpendicu- lar to the horizon, is called the line of direction.'* 67. When the line of direction falls within the basef of any body, the body will stand ; but when that line falls outside of the base, the body will fall or be overset. 1. Fig. 18 represents a loaded wagon on the dedivity of a hill. The hne C F represents a horizontal line, D E the base of the wagon. If the w^agon be ^^s- is. loaded in such a manner that tlie centre of gravity be at B, the perpendicular B D will fall within the base, and the wagon will stand. But if the load be altered so that the centre of gravity be raised to A, the perpendicular A C wall fall outside of the base, and the wagon will be overset. From this it follow^s that a wagon, or any carriage, will be most firmly supported when the line of direction of the centre of gravity falls exactly between the w^heeis ; and that is the case on a level road. The centre of gravity in the human body, is between the hips, and the base is the feet. So long as we stand uprightly, the line of direction falls w^ithin this base. When we lean on one side, the centre of gravity not being supported, we no longer stand firmly. 2. A rope-dancer performs all his feats of agility, by dex- terously supporting the centre of gravity. For this purpose he carries a heavy pole in his hands, which he shifts from side to side as he alters his position, in order to throw the weight to the side which is deficient; and thus, by changing the the purpose of giving a clear idea of the centre of gravity, and showing the difference between the centre of gravity and the centre of magnitude. * The line of direction is the line which the centre of gravity would describe if the body were permitted to fall. t The base of a body is its lowest side. The base Fig. 17. of a body standing on wheels or legs, is represent- ed by lines drawn from the lowest part of one wheel or leg, to the lowest part of the other wheel or leg. Thus, m Figs. 17 and 18, D E represents the base of the wagon and of the table. What is the line of direction ? 67. If the line of direction falls within the base, will the body stand or fall? Give an illustration. 48 NATURAL PHILOSOPHY. situation of the centre of grai-ity, lie keeps the hne of dn-ec- tion ^-iihin the base, and he will not fah.^ A spherical body will roll down a slope, because the centre of gravity is not supported. f 68. When a bodv is of uniform density, the centre of gravity is in the same point ^vith the centre of mag- nitude. . 1 r 1_ • When one part of the body is composed ot heavier materials than another part, the centre of gravity (being the centre of the weight of the body) no longer corre- sponds with the centre of magnitude. Thus, the centre of gravity of a cylinder plugged with lead, is not in the same point as the centre of magnitude. Bodies, therefore, consisting of but one kind of substance, as wood, stone, or lead, and whose densities are consequently uniform, will stand more tirmly than bodies composed of a varietv of substances, of different densities. * The shepherds in the south of France alFord an interesting instance of the appUcation of the art of balancing to the common business of hfe. These men walk on stilts from three to four feet high, and their children, when quite voung, are taught to practise the same art. By means of these odd additioi to the length of the leg, their feet are kept out of the water, or the heated sand, and they are also enabled to see their sheep at a greater distance. They use these stilts with great skill and care, and run, jump, and even dance on them with great ease. t A cyhnder can be made to roll up a slope, by plugging one side of it with lead : the body being no longer of a uniform density, the centre of gravity is removed from the middle of the body to some point in the lead, as that substance is much heavier than wood. Xow, in order that the cyhnder mav roll down the plane, as it is here situated, the centre of grav- ity must rise, which is impossible ; the centre of gravity must always de- scend in moving, and ^vill descend by the nearest and readiest means, which wiU be by forcing the cylinder up the slope, until the centre of OTavitv is supported, and then it stops. 4 body also in the shape of two cones united at their bases, can be made to roll up an incUned plane formed by two bars with their lower ends inclined towards each other. This is iUustrated by a simple contrivance m the " Boston School Set," and the fact illustrated is called " the mechani- cal paradox." 68 If a body is of uniform density, what centres will coincide? Give an example in which the centres of magnitude and gravity will not coin- cide MECHANICS. 49 69. Bodies that have a narrow base are easily over- set ; for if they are in the least degree incHned, the hne of direction will fall outside of the base, and their cen- tre of gravity will not be supported.* The broader the base, and the nearer the centre of gravity to the ground, the stronger will be the edifice. For this reason a pyramid,f having a broad base and but little elevation, is the firmest of all structures. 70. When two bodies are fastened together, they are to be considered as forniing but one body, and have but one centre of gravity. If the two bodies be of equal weight, the centre of gravity will be in the middle of the line which unites them. But if one be heavier than the other, the centre of gravity will be as much nearer to the heavier one as the heavier exceeds the light one in weight. 1. Fig. 19 represents a bar with an equal weight fastened at each end: the ^ig- ^9. centre of gravity is at A, the middle of g ^ ^ the bar, and whatever supports this centre will support both the bodies and the pole. 2. Fig. 20 represents a bar with an unequal weight at each end. The cen- tre of gravity is at C nearer to the larger body. * A person can carry two pails of water more easily than one, because the pails balance each other, and the centre of gravity remains supported by the feet. But a s'mc h'rer, the greater is the advantage gained. Thus, a greater w<^ight can be moved by the same power, when applied at IJ, ii\:in when it is exerted at P.'^ 2. Tiie common steelyard, an instrument for weighing av ti- des, is constructed on the principle of the lever of the ii.bt kind. It consists of a rod or bar, marked with notches to designate the pounds and ounces, and a weight which is move- able\long the notches. The bar is furnished with three hook s materials ; and is much used in almost every kind of mechanical operation. Sometimes it is detached from the fulcrum, but most generally the fulcrum is a pin or rivet by which the lever is permanently connected with the iramework of other parts of the machinery. * It is a fundamental principle in mechanics, that what is gained in power is lost in time. To illustrate this principle, (Fig. 25,) W represents the weight, F the fulcrum, P the power, and the bar W F P the lever. To raise the weight W to Fig. 25. ID, the power P must descend to p. But as the radius of the circle in which the power P moves is double that of the radius of the circle in which the weight W moves, the arc P p is double the arc w ; or, in other words, the distance P ^ is double the distance of W to. Now, as these dis- tances are traversed in the same time by the power and the weight respectively, it follows, that the velocity of the power must be double the velocity of the weight ; that is, the power must move at the rate of two feet in a second, in order to move the weight one foot in the same time. This principle applies not only to the lever, but to all the mechanical powers, and to all machines constructed on mechanical principles. When two weights are equal, and the fulcrum is placed exactly in the centre of the lever between them, they will mutually balance each other; or, in other words, the centre of gravity being supported, neither of the weights will sink. This is the principle of the common scale for weighing. To gain power by the use of the lever, the fulcrum must be placed near the weight to be moved, and the power at the greater distance from it. The force of the lever, therefore, depends on its length, together with the power applied, and the distance of the weight from the f ulcrum. What follows from this? What is meant by an indexible bar? Note What is a fundamental principle in Mechanics? Illustrate this by the figure. Does this principle ^ppsy to all the mechanical powers ? NATURAL PHILOSOPHY. on the longest of which, the article to he v:eighed is always to he hung. The other two hooks serve for the handle of the in- stiTiment when in use. The pivot of each of these two hooks serves for the ftilcrtini. When suspended by the hook C, as in Fi^. 26, it is manifest that a pound weight at E will balance as manv pounds at W, as the distance between the pivot of D, and the pivot of C, is contained in the space between the pivot of C and the ring from which E is suspended. MECHANICS. 57 3. The same instrument may be used to weigh heavy arti- cles, by using the middle hook for a luindle, where, as will be seen in the figure, the space between the pivot of F (which in this case is the fulcrum) and tlie pivot of D (from which the w light is suspended) being lessened, is contained a greater lumber of times in the distance between the fulcrum and the notches on the bar. The steelyard is furnished with two sets of notches on opposite sides of the bar. An equilibrium will always be produced when the product of the weights on the opposite sides of the fulcrum into their respective distances from it, are equal to one another. 4. A balance, or pair of scales, is a lever of the first kind, vi h equal arms. Steelyards, scissors, pincers, snuffers, arid a poker used for stirring the r 11 1 ^ 4-U Fig. 28. fire, are all levers oi the ^ ^ c first kind. The longer the handles of scissors, pincers, &c., and the shorter the points, the more easily are they used. A compound lever, represented in Fig. 28, consists of several levers, so arranged that the shorter arm of one may act on the longer arm of the other. Great power is obtained in this way ; but its exercise is limited to a very small space. 82. In a lever of the second kind, the fulcrum is at one end, the power at the other, and the weight between them. 1. Let Fig. 29 represent a lever of the second kind. F is the fulcrum, P the power, and W the weight. j,.^ 29. The advantage gained by a lever of this , kind is in proportion as the distance of the P^/ p power from the fulcrum exceeds that of the . , , weight from the fulcrum. Thus in this fig- I ure, if the distance from P to F, is four times ^ the distance from W to F, then a power of one pound at P will balance a weight of four pounds at W. 2. On the principle of this kind of lever, two persons carry- Explain the common steelyard in Fig. 26. Also in Fig. 27. What is a balance, or pair of scales? Give some examples of levers of the first kind. 82. What is a lever of the second kind? What figure illustrates this? To what is the advantage gained by this lever proportional ? Give some examples of levers of the second kind. 59 NATURAL PHILOSOPHY. iog a beaTV bnrden, suspended on a bar, may be made to bear unequal po'rtions of it, by placing it nearer to the one than the other. 3. Two horses, also, mar be made to draw unequal portions Drs turning on hmges, and cuttincr-kniTes, which are hxed at one end, are constructed up- on the" principle of levers of the second kind.* 83. In a lever of the third kind, the fulcrum is at one end, the weight at the other, and the power is applied between them. 1. In levers of this kind the powar must always axeed the weiqfit, in the .^ime proportion as the distance of the weight from the fulcrum exceeds that of the power from the fulcrum. 2. In Fig. 30, F is the fulcrum, W the weight, and P the power between the fulcrum and the weight ; ^ and the piwer must exceed the weight in the " same proportion that the distance between W p ^ p and F exceeds the distance between P and F. JL . 3. A laddt-r which is to be raised by the stren^^th of a man's arms, represents a lever ^ of this kind, where the fulcrum is that end w which is fixed against the wall : the weight may be considered as at the top part of the ladder, and tb- power is the strength apphed in raismg it. 4. The bDnes of a man's arm, and most of the moveable bones of animals, are levers of the tlurd kind. But the loss of power m hmbs of anim-Js is compensated by the beautv and compactness :: : V e liii: ?. as weU as the increased velocity of their mo:!:::. Tz- -z-^-s m clock and watch work, and in vii-i kin is : : mi ; linerv. mav be considered as levers of * I- 5 - = ~e principle that, in ra^ng a window, the hand ^(mlq v.. ^ 11 idV of the sa^, as it will then be esskj raised ; where - , , ^ iiearer to one ade than the other, the centre cf . will cause the farther side to bear against the XTa-tQc. ana ocis::-i:: ir^f irLrtZ'jii. 83. What is a lever o: ' power exceed the weight mnipies of IcTeis of the thiic nd ? In what propordon mus; the £x{4ain Fig. 30. Give soom ex- ME(;ilAMCS. 59 this kind, when the power that moves tliem ac^s on tlie pinion, near tlie centre of motion, and the resistance to be overcome acts on tlie teeth at the circumference. But liere the advan- tage gained is the change of slow into rapid motion. The sails of vessels are constructed on the principle of the lever/-^ 84. The Pulley is a small wheel turning on an axis, with a string or rope in a groove running around it. There are two kinds of pulleys — the fixed and the moveable. The fixed pulley is a pulley that has no other motion than a revolution on its axis, and it is used only for changing the direction of motion. Fig. 31 represents a fixed pulley. P is a small wheel turn- ing on its axis, with a string running round it in a groove. W is a weight to be raised, F is the force or Fig. 3i. power applied. It is evident that, by pulling the string at F, the weight must rise just as much as the string is drawn down. As, therefore, the velocity of the weight and the power is precisely the same, it is manifest that they balance each other, and that no mechanical advantage is gain-ed. But this pulley is very useful for changing the direction of motion. If, for instance, we wish to raise a weight to the top of a high building, it can be done with the assistance of a fixed pulley, by a man standing below.f ^ A curtain, or a sail, also, can be raised by means of a fixed pulley, without ascending with it, by drawing down a string running over the pulley. 85. The moveable pulley differs from the fixed pulley * It may perhaps assist the memory to retain the relative positions of the weight the power and the fulcrnm in the three kinds of levers, if the initials be presented to the eye as follows: First kind, W. F. P. Second " F. W. P. Third " F. P. W. t The fixed pulley operates on the same principle as a lever of the first kind with equal arms, where the fulcrum being in the centre of gravity, the power and the weight are equally distant from it, and no mechanical advantage is gained. 84. What is a pulley? How many kinds of pulleys are there? What are they? What is a fixed pulley? Explain Fig. 31. What advantage is gained by this pulley ? What is the use of this pulley ? Upon what principle does the fixed pulley operate ? 60 NATURAL PHILOSOPHY. by being attached to the weight ; it therefore rises and falls with the weight. Fig. 32 represents a moveable pulley, Tvitli the weight W attached to it by a hook below. One end^of the rope is fastened at F ; and as the power P draws the weight upwards, the pulley rises with the weight. Now, in order to raise the weight one inch, it Ts evident that both sides of the string must be shortened ; in order to do which, the power P must pass over two inches. As the velocity of the power is double that of the weight, it follows that a power of one pound will balance a weight on the moveable pulley of two pounds. 86. The power gained by the use of pulleys is ascer- tained by multiplying the number of moveable pulleys by 2.* 1. A weight of 12 pounds may be balanced by a power of 9 pounds with four pulleys ; by a power of 18 pounds with two pulleys ; or by a power of 36 pounds with one pulley. But in each case the space passed over by the power must be double the space passed over by the weight, multiplied by the number of moveable pulleys. That is, to raise the weight one foot, with one pulley, the power must pass over two feet, with two pulleys four feet, with four pulleys eight feet. 2. Fig. 33 represents a system of fixed and moveable pulleys In the block F, there are four fixed pulleys, and in the block M there are four moveable pulleys, all turning on their common axis, and rising and falling with the weight W. The moveable pulleys are connected with the fixed ones by a string at- tached to the hook H, passing over the ahernate grooves of the pulleys in each block, forming eight cords, and terminating at the power P. Now to raise the weight one foot, it is evident that each of the eight cords must be shortened one foot, and, consequently, that the power P must descend eight times that distance. The * This rule applies only to the moveable pulleys in the same block. Fig. 33. 85. How does the moveable pulley differ from the fixed pulley ? Ex- plain Fig. 32. 86. How can the power gn.ined by the use of the moveable pulley be as- certained ? What illustration of this is given ? What does Fig. 33 represent 7 MECHANICS. 61 po^er, thei-efore, must pass over eight times the distance that the weiglit moves. 87. Pulleys act on the same principle with the lever, the deficiency of the strength of the power being com- pensated by its superior velocity. Now, as we cannot increase om- natural strength, but can increase the velocity of motion, it is evident that we are enabled, by .pulleys and other mechanical powers, to reduce the resistance -or weight of any body to the level of our strength. 1. Practical use of Pulleys, Pulleys are used to raise goods into warehouses, and in ships, (fee. to draw up the sails. Both kinds of pulleys are in these cases advantageously applied ; for the sails are raised up to the masts by the sailors on dock, by means of the fixed pulleys, while the labor is facilitated by the mechanical power of the moveable ones. 2. Bo' h fixed and moveable pulleys are constructed in a great variety of forms, but the principle on which all kinds are coii- sti-uc.cd, is the same. What is generally called a tackle and fall, or a block and tackle, is nothing more than a pulley. jPulleys have likewise lately been attached to the harness of a horse, to enable the driver to govern the animal with less exertion of sti ength. 3. It may be observed, in relation to the mechanical powers in general, that power is always gained at the expense of time and velocity ; that is, the same power which will raise one pound in one minute, will raise two pounds in two minutes, six pounds in six minutes, sixty pounds in sixty minutes, &c. ; and that the same quantity of force used to raise two pounds one foot, will raise one pound two feet, (fee. And, further, it may be stated that the product of the weight, multiplied by the velocity of the weight, will always be equal to the product of the power muhiplied by the velocity of the power. Hence w^e have the following rule. The power is in the same j^ropor- tion to the lueight as the velocity of the loeight is to the velocity of the povjer. 87. Upon what principle do pulleys act? What advantage is gained by the use of pulleys and other mechanical powers ? What are some of the practical uses of the pulley? What is a tackle and fall? Is there any time or velocity gained hy the power in the mechanical powers? To what is the product of the weight, nialtiplied by its vebclty, always equal ? What rule is given ? 62 NATURAL PHILOSOPHY. 88. The wheel and axle consists of two wheels of different sizes, revolving together around the same cen- tre of motion. The place of the smaller wheel is generally supplied by a cylinder,* which forms the axle. 1 . The wheel and axle, though made in many forms, will easily be understood by inspecting Figs. 34 and 35. In Fig. 34, P represents the larger wheel, where the power is applied ; C the smaller wheel or cylinder, which is the axle, and W the weight to be raised. The advantage gain- ed is- in proportion as the circumference of the wheel is greater than that of the axle. That is, if the circumference of the wheel be six times the circumference of the axle, then a power of one pound applied at the wheel will balance a power of six pounds on the axle. 2. Sometimes the axle is constructed with a winch or handle, as in Fig. 35, and sometimes the wheel has project-, ing spokes, as in Fig. 34. 3. The principle upon which the wheel and axle is con- structed is the same with that of the other mechanical powers, the. want of power being compensated by velocity. It is evident (from the Figs. 34 and 35) that the velocity of the circumference of the wheel is as much greater than that of the axle as it is further from the centre of motion ; for the wheel describes a great circle in the same time that the * A cylinder is a long circular body of uniform diameter, with ex- tremities forming ^qual and parallel circles. 88 Of what does the wheel and axle consist? What is a cylinder? What figures illustrate the wheel and axle? Explain. To what is the advantage gained in proportion ? What does Fig. 34 represent ? Fig. 35 ? Upon what principle is the wheel and axle constructed ? Explain by Figs. 34 and 35. MF.CIIANICS. 63 axle desci ibes a small one ; therefore the power is in- creased in the same pro- portion as the circumfer- ence of the wheel is greater than that of the axle. If the velocity of the wheel be twelve times greater than that of the axle, a power of one pound on the wheel will support a weight of twelve pounds on the axle. 89. The wheel and axle are constructed on the same principle with the lever ; the axle acting the part of the shorter arm of the lever, the wheel that of the longer arm. 1. The capstan,'^ on board of ships and other vessels, is constructed on the principle of the wheel and axle. It consists of an axle placed uprightly, with a head or drum, pierced with holes for the lever, or levers, which supply the place of the wheel. 2. Windmills, lathes, the common windlass,"^ used for draw- ing water from w^ells, cmd the large wheels in mills, are all con- structed on the principle of the wheel and axle. 3. Wheels are a very essential part to most machines ; they are applied in different ways, but when affixed to the axle their mechanical power is always in the same proportion ; that is, as the circumference of the wheel exceeds that of the axle, so much will the power be increased. Therefore the larger the wheel and the smaller the axle, the greater will be the power obtained. 4. Fly-wheels are heavy wheels used to accumulate power * The difference between a capstan and a windlass lies only in the position of the wheel. If the wheel turn horizontally it is called a cap- stan ; if vertically, a windlass. 89. Upon what principle are the wheel and axle constructed? Explain how. Upon what principle is the capstan on board of vessels constructed ? Of what does it consist? What other things are mentioned as constructed upon this principle? Are wheels an essential part to most machines? Aro they applied in more than one way? When they are affixed to the axle, in what proportion is the power increased? 61 NATURAL PHILOSOPHY. and distribute it equally among all the parts of a machine. They are caused to revolve by a force applied to the axle ; and when once set in motion continue by their inertia to move for a long time. As their motion is steady and without sudden jerks, they serve to steady the power, and cause a machine to work vnth regularity. 5. Cranks are sometimes connected with the axle of a wheel, either to give or to receive its motion. They are made by bending the axle in such a manner as to form j,.^ four right angles facing in different directions, as is represented in Fig. 36. This is seen in lathes and many other kinds of machinery. Cranks are often used to change the motion from rectilinear to circular, or from circular to rectilinear. 6. In connexion with the wheel and axle, it is proper to mention the subject of complex wheel- work. It has already been stated that the velocity of the wheel is greater than that of the axle ; and that this velocity is in proportion to the relative size of the wheel compared with that of the axle. Advantage is taken of this circumstance in the construction of machinery, by such an arrangement of the parts as will enable us to increase or lessen the speed at pleasure. For it is evident that if the power be applied to the axle, and machinery attached to the wheel, rapid motion Avill be produced ; and on the contrary, if the power be apphed to the wheel and the machinery to the axle, slow motion will be produced. v. Fig. 37 represents four wheels with their axles, each wheel acting on the axle of the adjoining wheel. F is the power applied to the axle of the wheel d. Now, supposing the circumference of each wheel to be six times the circumference of each axle, it is evident that each time the wheel d revolves it must cause the wheel c to make six revolutions, because the circumference of the wheel d is What are fly-wheels, and for what are they used? How are they made to revolve ? When once set in motion, what causes them to move on for some time? Of what service are they in a machine? For what are cranks sometimes connected with the axle of a wheel ? Hov/ are they made? What does Fig. 36 represent? For what are cranks often used? How does the velocity of the wheel compare with that of the axie? To what is this velocity :n proportion.? Is any advantage taken of this in dri'^^ing machin^-ry where the speed is to be increased or diminished ? M KCII A M K 'r?, 05 A six (inu\s the circuinfcMciU'c of llic nxK^ of c. \\\ like mMrifier the circumferences of llie wlieels c and />, cictin<^ respect iv(;ly on the circumferences of tlie axles of the adjoining wiieel, will communicate a velocity six times tj^realer than their own, and Avhile the Avheel d makes one revolution the wheel c will make six, h thirty-six, and a tw^o hundred and sixteen revolu- tions. 8. Reversing the figure, and applying the power at S which communicates with the circumference of the wheel a, it follows that a must perform six revolutions while /; is performing one, thirty-six while c, and two hundred and sixteen, while d performs one revolution. It will thus be perceived that a rapid or a slow motion may be communicated by various combinations of the wdieel and axle. 9. The usual way of transmitting the action of the axles to the adjoining wheels is by means of teeth or cogs, raised on their surfaces. The cogs on the surface of the wheels are generally called teeth, and those on the surface of the axle are called leaves. The axle itself, when furnished with leaves, is called a pinion. 10. Fig. 38 represents a connexion of coofo^ed wheels. The wheel B being moved by a string around its circumference is a simple wheel with- out teeth. Its axle being furnished with coo^s or o leaves, to which the teeth of the wheel D are fitted, communicates its motion to D, which, in like man- ner, moves the wheel G. The power P and the weight W must be attached to the cir- cumference of the v/heel or of the axle according as a slow or a rapid motion is desired. 11. V/heels are sometimes turned by bands, as in Fig. 39 ; How would rapid motion be produced? Slow motion? Explain Fig. 37. What is the usual way of transmitting the action of the axles to the adjoining wheels ? What are the cogs on the surface of the wheel called ? Those on the axle? What is a pinion? Explain Fig. 38. By what are wheels sometimes turned ? Fig. 38. D C 66 NATURAL PHILOSOPHY. and the motion communiccited may be direct or reversed by attaching the band as repre- sented in Figs. 39 and 40. When the wheel and the axle from which it receives motion are intended to revolve in the same direction, the strap is not crossed, but is applied as in Fig. 39. But when the wheel is to revolve in a direction contrary to the revoltition of the axle, the strap is crossed, as in Fig. 40. Different directions may be given to the motion pro- Fig. 42. Fig. 40. duced by wheels, by varying the position of their axles, and causing them to revolve in different planes, as in Fig. 41 ; or by alteiing the shape and position of the cogs, as in Fig 42. 90. The inclined plane consists of a plain surface in- clined to the horizon, and the advantage gained by the inclined plane is in proportion as the length of the plane exceeds its perpendicular height. 1. Fig. 43 represents an inchned plane, its length, and W a.. weight which is to be moved on it. If the length C B be four times the height C A, then a power of one pound at C will balance a weio'ht of four pounds on the inchned plane C B. C A its heiirht, C B What figure represents one ? In what way can the motion be made direct or reversed? What does Fig. 39 represent ? Fig. 40 ? In what way can different directions be given to the motion produced by wheels ? What dots Fig. 41 represent ? Fig. 42 ? 90. What is an inclined plane ? AVhat figure represents an inclined plane ? Explain the figure. To what is the advantage gained by the use of the inclined plane in proportion? MECHANICS. 67 2. The greater the indinatioii of tlie plane, the greater must be its po'pendicular height, compared with its length ; and, of course, the greater must be the power to elevate a weight ah.)ng its surface. 3. Instances of the appUcation of the inclined plane are vei y common. Sloping planks or pieces of timber leading into a cellar, and on which casks are rolled up and down ; a plank or bo.trd with one end elevated on a step, for the convenience of trundling wheelbarrows, or rolling barrels mto a store, &c., are inclined planes. 4. The advantage gained by the use of the inclined plane, like that of the other mechanical powers, is attended by a loss of time ; for the weight, instead of moving directly up the ascent, must move the whole length of the plane. 5. Chisels and other cuttinof instruments, which are ckam- ferecl or sloped only on one side, are constructed on the prm- ciple of the inclined plane. 91. The wedge consists of two inclined planes united at their bases ; and the advantage gained by the use of the wedge is in proportion as the length exceeds one half the thickness of its converging sides. 1. Fig. 44 represents a wedge. The line a h repre- sents the base of each of the inclined planes of which it is composed, and at which they are united. 2. The wedge is a very important mechanical power, used to split rocks, timber, &c., which could not be effected by any other power. 3. Axes, hatchets, knives, and all other cutting in- struments chamfered, or sloped on both sides, are constructed on the principle of the wedge. 92. The screw consists of an inclined plane, wound round a cylinder, thus producing a circular inclined What follows from the greater or less inclmation of the plane? Give some instances of the application of the inclined plane. Is any time gained by the use of the inclined plane? Upon what principle are chisels and other cutting instruments, which are sloped only on one side, constructed ? 91. Of what does the wedge consist? What does Fig. 44 represent? To what is the advantage gained by the wedge in proportion ? Of what use is the wedge ? Give some examples of the wedge. 92. Of what does the screw consist ? 68 NATURAL PHILOSOPHY. plane, and forming what is called the threads of the screw. . . The advantage gained in the use of the screw is m proportion as th'e circumference described by the handle exceeds the distance between the threads of the screw. The screw is generally composed of two parts, the screw and the nut ; or, as they are generally called, the convex and concave screw\ 1. Fio-. 45 represents the screw and the nut. S is the con- vex screw, (which is an inclined plane wound round a cvlinder,) jS" is the nut, or concave screw, which has a spiral groove, to which the thread of the convex screw is accurately fitted. L is a lever attached to the nut, to which the power is applied. By turning the lever in one direction the nut ascends, and by turning it in the opposite direction, the nut descends on the screw.^ In this fio'ure the screw is fixed, and the nut is moveable. 2. Fig. 46 represents another screw, which is moveable. The nut is fixed to the frame, and ' ' the screw ascends or descends as the lever L is turned. * Although the screw is mentioned as one of the six mechanical powers, it is, in reality a compound power, consisting of a lever and an inclined plane. The power of the screw being estimated by the distance of the threads, the closer the threads the greater is the power ; but here, again, the increase of power is procured by an increase of velocity, for a loss of time. For if the threads be a quarter of an inch apart, the power must move through the wlioie circumference of the circle described by the lever, in order to move the resistance a quarter of an inch. The screw, with its appendage the lever, is therefore used for the purpose of moving large or heavy bodies through small distances. Its power may be increased by lengthening the lever. The screw is applied to presses of all kinds where great power is required, such as bookbinders' presses, cider and wine presses, &c. Of how many parts is it generally composed ? What are they ? What figure represents the screw and the nut ? Explain the figure. How does Fig. 45 difFsr from Fig. 46 ? Note. Is the screw a simple or compound power? How is the power of the screw estimated ? How does the close, ness of the thread afFect the power? What is the use of the screw ? How can its power be increased ? To what is the screw appUed 1 A\ — ^ A. MECHANICS. 69 3. By friction in macliinery is meant the resistance which bodies meet with in rubbing against each other. 4. There are two kinds of friction, the rolhng and the sliding. The rolHng' fiiction is caused by the rolhng of a circular body. The sliding friction is produced by the shding or dragging of a body over a flat surface. The sliding friction is overcome with more difficulty than the rolling. In calcula- ting the power of a machine, an allowance must always be made for friction. It is usually computed that friction de- stroys one-third of the power of a machine.^ 5. Friction is caused by the unevenness of the surfaces which come into contact ;f and it is diminished in proportion as the surfaces are smooth and well pohshed. Oil, grease, black-lead, or powdered soap-stone, is used to lessen faction, * When finely polished iron is made to rub on bell metal, the friction is said to be reduced to about one-eighth. Mr. Babbit of Boston has pre- pared a composition for the wheel-boxes of locomotive engines and other machinery, which it is said has still further reduced the amount of friction. This composition is now much in use. As the friction between rolling bodies is much less than in those that drag, the axle of large wheels is sometimes made to move on small wheels or rollers. These are called friction wheels, or friction rollers. They turn round their own centre as the wheel con- tinues its motion. t All bodies, how well soever they may be polished, have inequalities in their surfaces, which may be perceived by a microscope. When, there- fore, the surfaces of two bodies come into contact, the prominent parts of the one will often fall into the hollow parts of the other, and cause more or less resistance to motion. Friction increases, 1st, as the weight or pres- sure is increased ; 2d, as the areas of the surfaces in contact are increased ; 3d, as the roughness of the surface is increased. Friction may be diminished, 1st, by lessening the w^eight of the body in motion ; 2d, by mechanically reducing the asperities' of the sliding surfaces ; 3d, by lessening the amount of surface of homogeneous bodies in contact with each other ; 4th, by con- verting a sliding mto a rolling motion ; 5th, by applying some suitable unguent. What is meant by friction in machinery? How many kinds of friction are there ? What are they ? How is the rolling friction produced ? The sliding? Which is overcome with the less difficulty, the rolling or sliding? What allowance must always be made, in calculating the power of a ma- chine? What proportion of the power is usually computed to be destroyed by friction? Between which is friction the less, rolling bodies or those that slide? What causes friction ? In what proportion is it diminished? In what manner can it be lessened ? 70 NATURAL THILOSOPHY. because they act as a polish by fiUing up the cavities of the rubbing surfaces, and thus making them shde more easily over each other. 6. From what has been stated with regard to the mechanical powers, it appears that by their aid a man is enabled to per- form works to which his unassisted natural strength is wholly inadequate. But the power of all machines is hmited by the strength of the materials of which they are composed. Iron, which is the strongest of all substances, will not resist a strain beyond a certain limit. Its cohesive attraction may be de- stroyed, and it can Avithstand no resistance which is stronger than its cohesive attraction. Besides the strength of the ma- terials, it is necessary, also, to consider the time which is ex- pended in the apphcation of mechanical assistance. Archimedes is said to have boasted to Hiero, king of Syracuse, that, if he would give him a place to stand upon, he would move the whole world. In order to do this, Archimedes must himself have moved over as much more space than he moved the world, as the weight of the world exceeded his own weight ; and it has been computed that he must have moved with the velocity of a cannon ball for a milhon of years, in order to move the earth the twenty-seven milhonth part of an inch. 7. Wheels are used on vehicles to diminish the friction of the road. The larger the cu*cumference of the wheel, the more readily it will overcome any obstacles, such as stones, or inequahties in the road.'^ 93. A mediumf is the substance, solid or fluid, which surrounds a body. * In descending a steep hill, the wheels of a carriage are often locked, (as it is called.) that is, fastened in such a manner as to prevent their turning ; and thus the rolling is converted into the sUding friction, and the vehicle descends more safely. Castors are put on the legs of tables and other articles of furniture, to facilitate the moving of them ; and thus the sliding is converted into the rolling friction. t The plural of this word is media. What is the use of wheels ? In what proportion do they overcome the obstacles, such as stones, &c., in the road? Why, in descending a steep hill, are the wheels of a carriage often locked ? How do castors, which are put upon furniture, facilitate the moving of it ? How is the motion of all bodies influenced? 93. What is meant by a medium 1 Give examples. MECHANICS. 71 Thus, air is the medium which surrounds a bird when flying ; water is the medium which surrounds the lish when swim- ming, &c. 94. The motion of all bodies is influenced by the medium in which they move ; and the resistance of a medium is in exact proportion to its density. A body falhng through water meets with more resistance than when falhng through air, because water is a denser medium than air. If a machine could be worked in vacuo, (that is, in a vacuum, or a space where there is neither air nor any thing else to impede it,) and without friction, it would be perfect. 95. The main-spring of a watch consists of a long ribbon of steel, closely coiled, and contained in a round box. It is employed instead of a weight, to keep up the motion. 1. As the spring, when closely coiled, exerts a stronger force than when it is partly loosened, in order to correct this in- equality, the chain through which it acts is wound upon an axis surrounded by a spiral groove, (called fusee?) gradually increasing- in diameter from the top to the bottom; so that, in proportion as the strength of the spring is diminished, it may act on a larger lever, or a larger wheel and axis. 2. Fig. 47 represents a spring coiled in a round Fig. 47. A box. A B is the fusee, sur- i .1 rounded by a spiral groove, on which the chain C is wound. When the watch is recently wound, the spring is in the greatest state of tension, and will, therefore, turn the fusee by the smallest groove, on the prmciple of the wheel and axle. As the spring loses its force by being partly unwound, it acts upon the larger circles of the fusee ; and the 94. To what is the resistance of a medium in proportion ? What ilhis- tration is given ? When would a machine be perfect ? 95. Of what does the main-spring of a watch consist ? What is its use ? Does the spring exert a stronger force when closely coiled, or when partly loosened ? What is done in order to correct this inequality ? What does Fig. 47 represent ? Explain. 72 NATURAL PHILOSOPHY. want of strength in the spring is compensated by the me- clianical aid of a larger wheel and axle in the larger grooves. By this means the spring is made at all times to exert an equal power upon the fusee. The motion is. communicated from the fusee by a cogged wheel which turns with the fusee. 96. The name of governor has been given to an in- genious piece of mechanism, invented by Mr. James Watt, which is used to regulate the supply of power in machinery ; as that of steam in steam-engines, and of water in water-mills. Fig. 48 represents a governor. A B and A C are two levers or arms, loaded with heavy balls at their extremities B and C, and suspended by a joint at A upon the extremity of a revolving shaft, AD. At a is a collar, or sliding box, connected with the levers by the rods h a and c a, with joints at their extremities. When the shaft A D re- volves rapidly, the centrifugal force of the balls B and C will cause them to diverge in their attempt to fly off", and thus raise the collar a, by means of the rods h a and c a. On the contrary, when the shaft A D revolves slowly, the weights B and C will fall by their own weight, and the rods h a and c a wih cause the collar a to descend. The steam- valve in a steam-engine, or the sluice-gate of a water-wheel, being connected with the collar a, the supply of steam or water, which puts the works in motion, is thus regulated.'-^ * In manufactures, there is one certain and determinate velocity with which the machinery should be moved, and wliich, if increased or diminish- ed, would render the machine unfit to perform the work it is designed to execute. Now, it frequently happens that the resistance is increased or diminished by some of the machines which are worked, being stopped, or others put on. The moving power, having this alteration in the resistance, would impart a greater or less velocity to the machinery, were it not for the regulating power of the governor, which increases or diminishes the supply of water or of steam, which is the moving power. 96. What is a governor? Explain Fig. 48. What is said in the note of the use of the governor 1 HYDROSTATICS. 73 97. The knee-jomt, or, as it is sometimes called, the toggle-pmt, consists of two rods or bars connected by a joint, and increasing rapidly in power as the two rods approach to the direction of a straight line. 1. Fig. 49 represents a toggle-joint. A C and B C are the two rods connected by a joint C. A moving force applied in the direction C D acts with great and constantly increasing D - power to separate the parts A and B. 2. The operation of the toggle-joint is seen in the iron joints which are used to uphold the tops of chaises. It is also used in various kinds of printing- presses, to obtain the greatest power at the moment of im- pression. CHAPTER Y. HYDROSTATICS.* 98. Hydrostatics treats of the nature, gravity, and pressure of fluids. 99. A fluid is a substance which yields to the slightest pressure, and the particles of which, having but a slight degree of cohesion, move easily among themselves. 100. A liquid differs from a fluid in its degree of com- pressibilityf and elasticity. * Hydrostatics treats of the properties of fluids at rest; Hydraulics treats of fluids in motion. t The experiments made at Florence many years ago seem to prove that some kinds of hquids — water, for instance — are wholly incompressible Later experiments, particularly those of Mr. Jacob Perkins, of Newbury- 97. Of what does the knee-joint, or toggle-joint, consist? In what pro- portion does it increase in power ? What does Fig. 49 represent ? Ex- plain the figure. Give an instance of the operation of the toggle-joint. What is its use in printing-presses? 98. Of what does Hydrostatics treat ? 99. What is a fluid ? Does the attraction of cohesion have much in- fluence on the particles of fluids ? What follows from this ? 100. How do fluids differ from hquids? Can water be compressed; 4 74 NATURAL PHILOSOPHY. 1. Tne panicles of fluids gra^-iiate among themselves in a more perfect manner than solids ; because the strong cohesion of the particles of sohd bodies in some measm-e coimteracts the effecis of gravity. 2. From the shght degree of cohesion in the particles of fluids, it is inferred that they must be small, smooth, and o'lobular ; smooth, because there appears to be no fiiction among them ; and globular, because their touching each other but bv a point will account for the shghtness of their cohesion. 3. Fluids cannot be fonned into ^figures, or preserved in heaps, on account of their want of cohesion. 4. Fluids are subjected to a kind of attraction called capil- lary* attraction, bv which they are raised above their levels in capillary tubes, or tubes the 'bores of which are exceedingly small. ' Thus, if a small glass tube be placed in water, the water on the inside will lise above the level of that on the outside of the tube. 5. The cause of this seems to be nothing more than the or- dinary attraction of the particles of matter for each other. The sides 'of a small oiifice are so near to each other as to attract the panicles of the fluid on then- opposite sides ; and as all attraction is strongest in the direction of the greatest quantity of matter, the water is raised upwards, or in the direction of port, (now in London,) have proved that water is capable of a considerable degree of compression. Fluids, in general, have a voluntary- tendency to esjand when at liberty: but hquids will not expand without a change of temperature. Heat is supposed to be the primary cause of the fluid form of bodies. It insinuates itself between the particles of bodies, and forces theni asunder. Thus, for instance, ice, without heat, is a solid : with heat it becomes water, and, with a greater degree of heat, it expands mto an elastic fluid, called steam. * The word capillary is derived from the Latin word capiUa, (hair,) and it is applied to this kind of attraction, because it is exhibited most prominently m tubes, the bores of which are as fine as a hair, and hence called capillar)' tubes. VThdit is supposed to be the primary cause of the fluid form of bodies ? ^^llat eflect has heat upon bodies ? What illustration is given ? ^^'hy do fluids oravitate in a more perfect manner than sohds ? What is inferred from the slight decree of cohesion in the particles of fluids ? Why smooth ? ^Mly globular ? Why cannot fluids be formed mto flgures, or preserved m heaps ? ^^'hat do' you understand by.capillary attraction ? Explain \he reason of it. HYDROSTATICS. 75 the length of tlie tube. On the outside of tlie tube, the opposite surfaces cannot act on the same cokimn of water and tliei-elore the influence of attraction is here imperceptible m raising the fluid. 6. All porous substances, such as sponge, bread, hnen sugar, &c may be considered as collections of capillary tubes ; and, for this reason, water and other liquids will rise in them, when they are partly immersed. 1. It is on the same principle that the wick of a lamp will carry up the oil to supply the flame, although the flame is several inches above the level of the oil.* If the end of a towel happen to be left in a basin of water, it will empty the basin of its contents. On the same principle, when a dry wedge of wood is driven into the crevice of a rock, as the ram falls upon it, it will absorb the water, swell, and some- times split the rock. In this manner, mill-stone quarries are worked m Germany. 8. A beautiful experiment, dependent on the same principle ot capillary attraction, may be thus performed. Take two * The reason why well-filled lamps will sometimes fail to give light, is. that the wick is too large for its tube, and being thus compressed, the cap- illary attraction is impeded by the compression. The remedy is to reduce the size of the wick. Another cause, also, that prevents a clear light, is, that the flame is too far from the surface of the oil. As capillary attraction acts only at short distances, the surface of the oil should always be within a short distance of the flame. But another reason which requires particular atten- tion, IS, that all kinds of oil usually employed for lamps contain a glutinous matter, of which no treatment can wholly divest them. This matter fills the pores or capillary tubes of the wick, and prevents the ascent of the oil to feed the flame. For this reason the wicks of lamps should be often renewed. A wick that has been long standing in a lamp, will rarely afford a clear and bright light. Another thing to be noticed by those who wish the lamp to perform its duty in the best possible manner, is, that the wick be not of such size as, by its length as well as its thickness, to fill the cup, and thereby leave no room for the oil. It must also be remembered, that al- though the wick when first adjusted may be of the proper, size, the glu- tinous matter of the oil, filling its capillary tubes, causes the wick to swell, and thereby become too large for the tube, producing the same difficulty as has already ^Dcen noticed in cases where the wick is too large to allow th(i free operation of capillary attraction Explain why the same takes place in all porous substances. Explain ah the circumstances attending the burning of a lamp. Explain the ex. peiiment with the glass plates. ■yg NATURAL PHILOSOPHY. nieces of flat glass, ioined together at one side, and separated Ke other by a th>n strip of wood, card, or other substance. When thus prepared, immerse the glass m colored water, having previously wet the inner surface. The water wdl then rtrbetween the pieces of glass, forming a beau iful curve the higher part appearing where the pieces of glass are m contact. This is exemphfied by the glass plates m the Bos- ton School Set." 101 The level, or equilibrium of fluids, is the tendency of the particles so to arrange themselves that every part of the surface shall be equally distant from the Sntre of the earth; that is, from the point to which gravity tends. . . . All fluids have a tendency to preserve this equilibrium Hetce the surface of all fluids, when in a state of res mus Dartake of the spherical form of the earth. This level or Shbrium of flu ds is the natural result of the mdependent ;Satlon of each particle. The particles of a jhd body heina- united by cohesive attraction, if any one ot them be XmS itw ll uphold those also with which it is united^ But when any particle of a fluid is unsupported it is attracted Sown o Uie ll^el of the surface of the fluid ; and the readiness with which fluids yield to the slightest ^^£^.^^5 particle by its own weight, to penetrate the surface of the flmd and mix with it. . 102. Fluids of different densities all preserve their own equiUbrium. , . ^ * • + fi,^ If a quantity of mercury, water, oil and air be put into the same vessel, they will arrange themselves m the orde^^^^^^^^ snecific gravities. The mercury will sink to the bottom, tue Sr win stand above the Kiercury, the od f tl- * and the air above the oil; and the surface of each flmd wi partake of the spherical form of the earth, to which they all respectively gravitate. 103. A vfater-level is an instrument constructed on the ' 101. What is meant by the level or equilibrium of fluids? Have all flu ds a tendency to preserve this equilibrium ? What follows from t^s Of what is this level or equilibrium of fluids the natural result? How does the cavitation of solid bodies difier from that of fluids ? . , 102 Do lids of different densities all preserve their own eqmhbnuml What illustration is given to prove this? HYDROSTATICS. 77 principle of the equilibrium of fluids. It consists of a glass tube, partly tilled with water, and closed at both ends. When the tube is not perfectly horizontal, — that is, if one end of the tube be lower than the other, — the water will run to the lower end. By this means the level of any line to which the instrument is applied may be ascertained. Fig. 50 represents a water-level. A B i^ig- ^o. is a glass tube partly filled with water. C a c b is a bubble of air occupying the space not d ^ ^ ^ b filled by the water. When both ends of the tube are on a level, the air-bubble will remain in the cen- tre of the tube ; but if either end of the tube be depressed, the water will descend and the air-bubble will rise. The glass tube when used is generally set in a wooden or a brass box. It is an instrument much used by carpenters, masons, sur- veyors, &c. 104. Solid bodies gravitate in masses, their parts being so connected as to form a whole, and their weight may be regarded as concentrated in a point called the centre of gravity ; while each particle of a fluid may be considered as a separate mass, gravitating independently. It is for this reason that a body of water, in falling, does less injury than a solid body of the same weight. But if the v/ater be converted into ice, the particles losing their fluid form, and being united by cohesive attraction, gravitate united- ly in one mass. 105. Fluids not only press downwards like sohds, but also upwards, sidewise,* and in every direction. * If the particles of fluids were arranged in regular columns as in Fig. 51, there would be no lateral pressure ; for when one particle is oerpendicularly above the other, it can press only downwards. But if the particles be ar- 103. Upon what principle is a water-level constructed? Of what does it consist? For what is it used? What figure represents a water-level? Explain the figure. 104. In what manner do solid bodies gravitate? What is the centre of gravity? What effect has gravity on the particles of fluids? 105. How is the pressure of fluids exerted ? Fig. 51. 78 NATURAL PHILOSOPHY. So long as the equality of pressure is undisturbed, every particle will remain at rest. If the fluid be dis- turbed by agitating it, the equality of pressure will be disturbed, and the fluid will not rest until the equilibrium is restored. 1 The downward pressure of fluids is shown by making an aperture in the bottom of a vessel of water. Every particle of the fluid above the aperture will run downwards through the opening. , . ^ „, 2 The lateral pressure is shown by making the aperture at the side of the vessel. The fluid will then escape through the aperture at the side. , 3 The upward pressure is shown by takmg a giass tube, open at both ends, inserting a cork in one end, (or stopping it with the finger,) and immersing the other in the water, ihe water will not rise in the tube. But the moment the cork is taken out, (or the finger removed,) the fluid will rise m the tube to a level with the surrounding water. 106. The pressure of a fluid is in proportion to the perpendicular distance from the surface; that is, the deeper the fluid the greater will be the pressure. 1 his pressure is exerted in every direction, so that all the parts at the same depth press each other with equal force. 1 A bladder, filled with air, being immersed in water, will be contracted in size, on account of the pressure of the water ranged as in Fig. 52, where a particle presses between two particles be- neath, these last must suffer a lateral pressure. In whatever manner the particles are arranged, if they be globular, as is suppoed, there must be spaces between them. See Fig. 1, page 20. How long will the particles of fluids remain at rest ? Explani Fig. 51. What does Fig. 52 represent? If the equality of the pressure be undis- turbed, what will follow? If the fluid be agitated, when will it agam come to a state of rest 1 How is the downward pressure of fluids shown .' I he lateral pressure? The upward pressure ? 106 To what is the pressure of a fluid In proportion ? In what direc- tion is this pressure exerted? What illustrations are given to prove this? Why can a bottle, filled with water, or any other liquid, be let down to any depth without injury ? HYDROSTATIC.-^. 79 in all directions; and tlui dcej)cr it is immers(;d, Uio more will it be contract(Hl. 2. An empty bottle, bein;^ corked, and by means of a w(;i;^^lit let down to a certfiin deptli in the sea, will either be broken by tlie pi-essure, or the cork will be driven into it, and the bottJe be lilled with water. This will take place even if the cork be secured with wire and sealed. But a bottle filled with water, or any other liquid, may be let down to any depth without damage, because, in this case, the internal pressure is equal to the external.* * Experiments at sea. — We are indebted to a friend, who has just ar- rived from Europe, says the Baltimore Gazette, for the following experi- ments made on board the Charlemagne : " 26th of September, 183G, the weather being calm, I corked an empty wine-bottle, and tied a piece of linen over the cork ; I then sank it into the sea six hundred feet ; when drawn immediately up again, the cork v/as inside, the linen remained as it was placed, and the bottle was filled with water. " I next made a noose of strong twine around the bottom of the cork, which I forced into the empty bottle, lashed the twine securely to the neck of the bottle, and sank the Vjottle six hundred feet. Upon drawing it up immediately, the cork was found inside, having forced its way by the twine, and in so doing had broken itself in two pieces ; the bottle was filled with water. I then made a stopper of white pine, long enough to reach to the bot- tom of the bottle ; after forcing this stopper into the bottle, I cut it off about half an inch above the top of the bottle and drove two wedges, of the same wood, into the stopper. I sank it six hundred feet, and upon drawing it up immediately the stopper remained as I placed it, and there Vv^as about a gill of water in the bottle, which remained unbroken. The water must have forced its way through the pores of the wooden stopper, although wedged as aforesaid ; and had the bottle remained sunk long enough, there is. no doubi that it would have been filled with water." Similar experiments were made by the author of this work, in a voyage to the West Indies in the year 1839, first with an empty bottle and then with a bottle filled with water from the tanks on the deck ; in both cases the bottle being closely stopped and the cork covered with canvass. The empty bottle was drawn from a depth of six hundred feet filled with water, and the full bottle with brackish water, the water from the tank having been compressed, and water from the depths of the ocean mixing with it. It is the opinion of some philosophers, that the pressure at very great What experiment is mentioned in the note ? What opinion have some philosophers expressed? 80 NATURAL PHILOSOPHY. 107. From what has now been stated, it appears that the lateral pressure proceeds entirely from the pressure downwards, or, in other words, from the weight ot the liquid above ; and that consequently the lower an orihce is made in a vessel containing water or any other liquid, the greater will be the force and velocity with which the liquid will rush out. Fig. 53 represents a vessel of watei', ^itli^orilices at the side at different dis- tances from the surface. The different curves in the figure, described by the liquid in running out of the vessel, show the ef- fects proved bv the force of the pres- sui-e on the liquid at different depths, and the action of gravity. At A the pressure is the least, because there is less weight of fluid above. At B and C the fluid is driven outwards by the weight of that portion above, and «e force will be strongest at C. 108. As the lateral pressure arises solely from the downward pressure, it is not aflfected by the \yidth nor the length of the vessel in which it is contained, but merely bv its depth ; for as every particle acts indepen- dentlv of'the rest, it is only the column of particles above the orifice that can weigh upon and press out the water. 109. The lateral pressure on one side of a cubical ves-. sel will be equal only to half of the pressure downwards ; for every particle at the bottom of the vessel is pressed upon bv a column of the whole depth of the fluid, while the lateral pressure diminishes from the bottom upwards to the surface, where the particles have no pressure. depths of the sea is so great, that the water is condensed into a solid state ; and that at or near the centre of the earth, if the fluid could extend so deeply, this pressure would convert the whole into a sol id mass of fire. 107. What causes the lateral pressure ? What follows from this? Ex- plain Fig. 53. r, J • 108. Does the length or the width of the vessel in which a fluid is con- tained have any effect upon the lateral pressure? By what is it affected? 109. How does the lateral pressure on one side of a cubical vessel com- pare with the pressure downwards ? How would you explain this ? IIYDROSTATJCS. 81 110. The upward pressure of fluids, although ap- parently ill opposition to the principles of gravity, is but a necessary consequence of the operation of that princi- ple ; or, in other words, the pressure upwards as well as the pressure downwards is caused by gravity. When water is poured into a vessel with a spout, (like a tea-pot, for instance,) the water rises in the spout to a level with that in the body of the vessel. The particles of water at the bottom of the vessel are pressed upon by the particles above them, and to this pressure they will yield, if there is any mode of making way for the particles above them. As tliey cannot descend' through the bottom of the vessel they will change their direction '^^s- 5i. and rise in the spout. Fig. 54 represents a tea-pot, and the columns of balls represent the particles of water magnified. From an inspection of the figure it appears that the particle numbered 1, at the bottom, will be pressed laterally, by the particle numbered 2, and by this pressure forced into the spout, where meeting with the particle 3 it presses it upwards, and this pressure will be continued from 3 to 4, from 4 to 5, and so on till the water in the spout has risen to a level with that in the body of the vessel. If water be poured into the spout the water will rise in the same manner in the body of the vessel ; from which it appears that the force of pressure depends entirely on the height, and not on the length or breadth of tlve column of fluid, 111. One principle in hydrostatics, is so remarkable, that it is named the hydrostatic paradox. It is this. That any quantity of fluid ^ however small, may he made to balance and support any other quantity, however large. 110. What causes the upward and downward pressure? Illustrate this by Fig. 54. 111. Upon what does the force of pressure depend ? What is meant by the hydrostatic paradox? What is the use of the hydrostatic bellows? What figure represents the hydrostatic bellows? Explain the figure. What is the fundamental principle of Mechanics ? Is this the principle of the hydrostatic b^'llows ? 4* 82 NATURAL PHILOSOPHY. Fig. 55 Fig. 55 represents the hydrostatic bellows.* A B is a long tube, one inch square. C D E F are the bellows, consisting of two boards, ei(/ht inches square, connected by broad pieces of leather, or india-nibber cloth, in the man- ner of a pair of common bellows. One pound of water poured into the tube will raise 64 pounds on the bellows. If a smaller tube be used the same quantity of water will fill it higher, and consequently will raise a greater weight ; but if a larger tube be used it will of course not fill it so high, and consequently will not raise so great a weight ; because it is the height not the quantity lohich causes the pressure.^ * This is the form of the Hydrostatic bellows in the original " Boston School Set." By means of a straight jet substituted for the tube, it was designed to illustrate a principle in Hydraulics also. t The Hydrostatic bellows may be constructed in a Tariety of forms, the simplest of which consists, as in the figure, of two boards connected together by broad pieces of leather, or india-rubber cloth, in such a man- ner as to allow the upper board to rise and fall like the common bellows. A perpendicular tube is so adjusted to this apparatus, that water poured into the tube, passing between the boards, will separate them by its up- ward pressure, even although the upper board is loaded with a considerable weight. [N. B. A small quantity of water must be poured into the bellows to sep- arate the surfaces before they are loaded with the weight.] The force of pressure exerted on the bellows by the water poured into the tube, is estimated by the comparative size of the tube and the bel- iov>^s. Thus, if the tube be one inch square, and the top of the bellows twelve inches, thus containing 144 square inches, a pound of water poured into the tube will exert a pressure of 144 pounds on the bellows. Now it will be clearly perceived that this pressure is caused by the height of the column of water in the tube. A pound, or a pint of water will fill the tube 144 times as high as the same quantity would fill the bellows. To raise a weight of 144 pounds on the bellows to the height of one inch, it will be necessary to pour into the tube as much water as would fill the tube were it 144 Inches long. It will thus be perceived that the funda- mental principle of the laws of motion is here also in full force ; namely, that what is gained in power is lost either in time or in space; for, while the water in the bellows is rising to the height of one inch, that in the tube passes over 144 inches. HYDROSTATICS. 83 ^ig. 5G is an apparatus* to show that liquids press according ' to the height and not the qiiantltij. A and Fig. 56 V) are two vessels of unequal size but of tli() same length. Tliese may succes- sively be screwed to the apparatus and filled with water. Weights may then be added to the sus- pended scale un- til the pressure is counterbalanced. It will then be per- ceived that al- though A is ten times larger than B, the water Avill stand at the, same height 41 both, because they o.re of the same length. If C be used in- stead of A or B, the apparatus may be used as the hydrostatic oellows. 112. If water be confined in any vessel, and a pressure to any amount be exerted on a square inch of that water, a pressure to an equal amount will be transmit- ted to every square inch of the surface of the vessel in which the water is confined. f * This apparatus, belonging to the " Boston School Set," unites sim- plicity with convenience. Instead of two boards, connected with leather, an india-rubber bag is placed between two boards, and the boards are made to rise or fall as the water runs into or out of the bag. It is an ap- paratus easily repaired, and the bag may also be used for gas, or for ex- periments in Pneumatics. t This property of fluids seems to invest us with a power of increasing the intensity of a pressure exerted by a comparatively small force, without any other limit than that of the strength of the materials of which the en- gine itself is constructed. It also enables us with great facility to transmit ' 112. What fact is mentioned in this number with regard to the pres- sure on water? S4 NATURAL PHILOSOPHY. 1. It is upon this principle that Bramah's hydrostatic press, represented in Fig. 57, is constructed. The main features of this apparatus are as follows : a is a narrow and A a large metallic cyl- inder having com- munication one with the other. Water stands in both the cylin- ders. The piston S carries a strong head P, which works in a frame opposite to a simi- lar plate, R. Be- tween the two plates the sub- stance W to be compressed is placed. In the narrow tube a is a piston p, worked by a lever chd, its short arm c h driving the piston, while the power is applied at d. The pressure exerted by the small piston p on the water at a is transmitted with equal force throughout the entire mass of the fluid, w^hiie the surface at A presses up the piston S with a force proportioned to its area. For instance, if the cylinder a of the force-pump has an area of half an inch while the great cylinder has an area of 200 inches, then the pressure of the water in the lat- ter on the piston S, will be (1 half inch multiplied by 400 half inches) equal to 400 times that on^. 2. Next, suppose the arms of the lever to be to each other as 1 to 50, and that at d, the extremity of the longer arm, a the motion and force of one machine to that of another, in cases where local circumstances preclude the possibility of instituting any ordinary mechan- ical connection between the two machines. Thus, merely by means of water-pipes, the force of a machine may be transmitted to any distance, and over inequalities of ground, or through any other obstructions. Upon what principle is Bramah's hydrostatic press constructed? What figure represents this? Explain the figure. To what uses is this press applied ? HYDROSTATICS. 85 man works with a force of 50 pounds, the- piston p will con- scMiiKMilly descend on the water with a force of 2000 pounds. ])(;(lucliii<^r .1 for the loss of power caused by the different im- jxHliments to motion, and one man would still be able to exeit a force of three (quarters of a million of pounds by means of this machine. This press is used in pressing paper, cloth, hay, gunpowder, K()S'l\\ ricB. 89 If Ji body lii^lilcr (lian w.-ilcr weighs six ounces, and on ho- lug attaclK.'d (o a heavy body, balanced in water, is found to occasion it to lose twelve ounces of its weight, its sj)(H^ili(' o'lavity is determined by dividing its weight (six ounces) by the sum of its Aveight, added to the loss of weight it occasions in the heavy body, namely, six added to twelve, which, in other words, is 6 divided by 18, or whicli is 1. 118. An hydrometer is an instrument to ascertain tlie specific gravity of liquids. 1. The hydrometer is constructed on the principle, that the greater the wx^ight of a liquid, the greater will be its buoyancy. 2. The hydrometer is made in a variety of forms, but it generally consists of a hollow ball of silver, glass, or other material, with a graduated scale rising from the upper part. A weight is attached below^ the ball. When the instrument, thus constructed, is immersed in a fluid, the specific gravity of the fluid is estimated by the portion of the scale that re- mains above the surface of the fluid. The greater the specific gravity of the fluid, the less will the scale sink. ed had been adulterated by the w-orkmen. The philosophei labored at the problem in vain, till going one day into the bath, he perceived that the water rose in the bath in proportion to the bulk of his body. He instantly perceived that any other substance of equal size would raise the water just as much, though one of equal weight and less bulk could not produce the same effect. He then obtained two masses, one of gold and one of silver, each equal in weight to the crown, and having filled a vessel very accu- rately with water, he first plunged the silver mass into it, and observed the quantity of water that flowed over ; he then did the same with the gold, and found that a less quantity had passed over than before. Hence he inferred that, though of equal weight, the bulk of the silver was greater than that of the gold, and that the quantity of water displaced was, in each experiment, equal to the bulk of the metal. He next made trial with the crown, and found that it displaced more water than the gold, and less than the silver, which led him to conclude, that it was neither pure gold nor pure silver. 118. What is an hydrometer? Upon what principle is it constructed? Explain its construction. In what proportion does the scale sink ? 90 NATURAL PHILOSOPHY CHAPTER YI. HYDRAULICS. 119. Hydraulics treats of the motion of fluids, particu- larly of water ; and the construction of all kinds of in- struments and machines for moving them. Water, in its motion, is retarded by the friction of the bottom and sides of the channel through which it passes. For this reason the velocity of the surface of a running stream is always greater than that of any other part.^ 120. A fluid running from an orifice in a vessel is dis- charged wdth the greater rapidity when the vessel from which it flow^s is kept constantly full. This is a necessary consequence of the law, that pressure is proportioned to the height of the column above. 121. When a fluid spouts from several orifices in the side of a vessel, it is throw^n to the greatest distance from the orifice nearest to the centre. t 122. A vessel filled with any liquid will discharge a greater quantity of the liquid through an orifice to which * In consequence of the friction of the banks and beds of rivers, and the numerous obstacles they meet in their circuitous course, their progress is slow. If it were not for these impediments, the velocity which the waters would acquire would produce very disastrous consequences. An inclina- tion of three inches in a mile, in the bed of a river, will give the current a velocity of about three miles an hour. t This is true only on the condition that the vessel be not elevated. If the vessel be elevated, the lowest orifice will discharge the fluid to the greatest distance, but when the vessel is placed low, the fluid will reach the plane before its projectile force is expended. 119. Of what does Hydrauhcs treat ? What retards the motion of water ? Why does the surface of a canal or river have a greater velocity than any other part ? 120. Does the fulness of a vessel from the orifice of which a fluid is running, have any effect upon its velocity ? 121. When a fluid spouts from several orifices in the side of a vessel, from 'which is it thrown to the greatest distance ? HYDRAULICS. 91 a short pipe of peculiar shape is fitted, than through an orifice of the same size without a pipe.* But if the pipe project into the vessel, the quantity discharged will be diminished instead of increased by the pipe. The quantity of a fluid discharged through a pipe or an orifice is increased by heating the liquid ; because heat di- minishes the cohesion of the particles, which exists, to a certain degree, in all liquids. 123. The velocity of a current of v^ater may be as- certained by immersing in it a bent tube, shaped like a tunnel at the end which is immersed. Fig. 59 is a tube' shaped like a tun- nel, with the larger end immersed in an opposite direction to the current. The rapidity of the current is estimated by the height to which the water is forced into the tube, above the surface of the current. By such an instrument the comparative velocity of different streams, or the same stream at different times, may be estimated. f Fig. 59 * This is caused by the cross-currents made by the rushing of the water from different directions towards the sharp-edged orifice. The pipe smooths the passage of the Hquid. t To measure the velocity of a stream at its surface, hollow floating bodif^s are used ; as, for example, a glass bottle filled with a sufficient quaptity of water to make it sink just below the level of the current, and having a small flag projecting from the cork. A wheel may also be cau?ed to revolve by the current striking against boards projecting from the circumference of the wheel, and the rapidity of the current may be estimated by the number of the revolutions in a given time. 122. What effect will a pipe, fitted to an orifice, have with regard to the quantity discharged? What will be the effect if the pipe project into the vessel ? How can the quantity discharged through a pipe or orifice, be increased? Why will heat increase it? 123. How can the velocity of a current of water be ascertained? Wh^. does Fig. 59 represent ? How is the rapidity of the current estimated What is the use of the instrument ? 92 NATURAL PHILOSOPHY. 124. Waves are caused by the friction between air and water.* 125. The instruments used for raising or drawing water or other Hquids, are the common pump,t the chain pump, the forcing pump, the siphon, and the screw of Archimedes. 126. The chain-pump is a ma- chine by which the water is hfted through a box or channel, by boards fitted to the channel and attached to a chain. It has been used prin- cipally on board of ships. Fig. 60 represents a chain-pump. It consists of a square box through which a number of square boards or buckets, connected by a chain, is made to pass. The chain passes over the wheel C and under the wheel D, which is under water. The buckets are made to fit the box, so as to move with little fric- tion. The upper wheel, C, is turned by a crank, (not represented in the Fig.) which causes the chain with the buckets attached to pass through the box. Each bucket, as it en:ers the box, lifts up the water above it, and discharges it at the top. 127. The screw of Archimedes is a machine said to have been invented by the philosopher Archimedes, for * When oil is poured on the windward side of a pond, the whole surface will become smooth. The oil protects the water from the friction of the wind or air. It is said that boats have been preserved in a raging surf, in consequence of the sailors having emptied a barrel of oil on the water. t The common pump, and the forcing pump, will be explained in con- nection with Pneumatics. 124. What causes waves? What is sometimes done to remove this friction ? 125. What instruments are used for ra:sing liquids ? 126. W^here is the chain-pump used ? What figure represents it ? Ex- plain the figure. HYDRAULICS. 93 raising water and draining the lands of Egypt, about 200 years before the Christian era. Fisr. 6 1 represents the „. r A 1 • -J ^^S' 61. screw 01 Archimedes. A single tube, or two tubes, are wound in the form of a screw around a shaft or cylinder, sup- ported by the prop and the pivot A, and turned by the handle n. As the end of the tube dips into the water, it is filled with the fluid, which is forced up the tube by every successive revolution, until it is discharged at the upper end. 128. Springs and rivulets are formed by the water, from rain, snow, &c., which penetrates the earth, and descends until it meets a substance which it cannot pen- etrate. A reservoir is then formed by the union of small streams under ground, and the water continues to accu- mulate until it finds an outlet. Fig. 62. Fig. 62 represents a vertical section of the crust of the earth, a, c, and e are strata, either porous, or full of cracks, 127. What is said of the screw of Archimedes? Explain the use of the screw by Fig. 61. 128. How are springs and rivulets formed ? Explain Fi^. 62. 94 NATURAL PHILOSOPHY. vTaich permit the water to flow through, while h, d, and/, are impervious to the water. Xow according to the laws of hy- drostatics, the water at h will descend and form a natural sprino- at g ; at i it will run with considerable force, formmg a natuml jet ; and at p, and g, artesian wells may be dug, in which the water will rise to the respective heights g h, pK and I m, the water not being allowed to come in contact with the porous soil through which the bore is made, but being brought in pipes to the surface ; at n the water will ascend to about o, and there will be no fountain. This explains also the manner in which water is obtained by digging wells. 129. A spring will rise nearly as high, but cannot rise hio-her than the reservoir from whence it issues. Water may be conveved over hills and valleys in bent pipes and tubes, or "through natural passages, to any height which is not greater than the level of the reservoir Irom whence it flows.* 130. Fountains are formed by water carried through natural or artificial ducts from a reservoir. The water will spout through the ducts to nearlyt the height of the surface of the reservoir. A simple method of making an artificial fountain may be understood by Fig. 63. A glass siphon a 5 c is . immersed in a vessel of water, and the air being exhausted from the siphon, a jet will be produced at a, pro- portioned to the fineness of the bore and the length of the tube. Fig. 63. * The ancient Romans, ignorant of this property of fluids, constructed vast aqueducts across valleys, at great expense, to convey water over them. The moderns effect the same object by means of wooden, metaUic, or stone pipes. t The resistance of the air prevents the fluids from rising to quite the same height with the reservoir. 129. How high will a spring rise ? 1,30. How are fountains formed? How high will the water spout through the ducts? What prevents the fluid from rismg to the same height with the reservoir ? HYDRAULICS. 95 131. The siphon is a tube bent in the form of the letter U, one side being a Httle longer than the other. 1. Fig. 64 represents a siphon. A siphon is used by filhng it witli water or some other fluid, ^^s- 64. then stopping both ends, and in this state im- mersing the shorter leg or side into a vessel con- taining a liquid. The ends being then unstopped, the licpiid will run through the siphon until the vessel is emptied. In performing this experiment, the end of the siphon which is out of the water must always he below the surface of the water. 2. This instrument may also be used to show the equilibrium of fluids. For, if the tube be in- verted and two liquids poured into it, they will rise in each side or leg of the siphon to dif- ferent heights — the higher fluid standing at the highest level. The specific gravity of mercury being thirteen times greater than that of Avater, will balance thirteen times its bulk of water. Consequently water will rise thirteen times as high on one side of the siphon as the mercury on the other. But if one liquid only is poured into the siphon it will rise to the same height in both sides or legs of the siphon. 3. Tantalus' cup consists of a goblet containing a small figure of a man. A siphon is concealed within the figure, which empties the water from the ^5. goblet as fast as it is poured in, so that the glass can never be filled. 4 Fig. 65 represents the cup with the siphon. The figure of the man is omitted, in order that the position of the siphon may be seen. 132. Water, by means of its weight or its force when in motion, becomes a mechanical agent of great power. It is used to propel or turn wheels of different construc- tion, which, being connected with machinery of various kinds, form mills, &c. 131. What is the siphon ? In what manner is the siphon used? How can the siphon be used to show the equilibrium of fluids ? How high will the liquid rise in each side of the siphon ? What is Tantalus' cup ? What does Fig. 65 represent ? 132. How, and for what purposes is water used as a mechanical agent ? 96 NATURAL PHILOSOPHY. There are three kinds of water-wheels, called un- dershot, overshot, and breast wheels. 133. The Overshot wheel is a wheel set in motion by the weight of water flowing upon it. It receives its motion at the top. Fig. 66 represents the overshot wheel. It consists of a wheel turn- ing on an axis, (not represented in the Fig.,) with compartments called buckets, ahcd, &c., at the circum- ference, which are successively filled with water from the stream S. The weight of the water in the buckets causes the wheel to turn, and the buckets being gradually inverted are emptied as they descend. It will be seen, from an inspection of the figure, that the buckets in the descending side of the wheel are always filled, or partly filled, while those in the op- posite or ascending part are always empty until they are agam presented to the stream. This kind of wheel is the most powerful of all the water-wheels. 134. The Undershot wheel is a wheel which is moved by the motion of the water. It receives its impulse at the bottom. Fig. 67 represents the undershot wheel. Instead of buckets at the circum- ference, it is furnished with plane surfaces, called float- boards, ahcd, &c., which receive the impulse of the water, and cause the wheel to revolve. How many kinds of water-wheels are there? What are they? 133. What is the overshot wheel? Where does it receive its motion? Explain Fig. 66. What causes the wheel to turn? How does this wheel compare in power with the other water-wheels? 134. What is the undershot wheel? Where does it receive its motion? What does Fig. 67 represent? How does this wheel differ from the overshot ? rNEi;MATICS. 97 135. The Breast-wheel is a wheel which receives the water at about half its own height, or ^'s- at the level of its axis. It is moved both by the weight and the motion of the w^ater.* Fig. 68 represents a breast-wheel. It is furnished either with buckets, or with float- boards, fitting the water-course. CHAPTER YII. PNEUMATICS. 136. Pneumatics treats of the nature, mechanical properties, and effects of air and similar fluids, distin- guished by the name of aeriform fluids, f 137. The air we breathe is an elastic fluid which sur- rounds the earth, extending to an indefinite distance above its surface, and constantly decreasing upwards in density. J * In all the wheels which have been described, the motion given to the wheel is communicated to other machinery or gearing, as it is called, by )ther wheels or pinions attached to the axis, such as have been described under the head of Mechanics. t An aeriform fluid is a fluid in the form of air, and, like air, generally nvisible. t The terms " rarefaction" and " rarefied" are applied to air when it is axpanded ; and " condensation" or " condensed " when it is compressed. 135. What is the breast-wheel ? How is it set in motion? What figure represents the breast-wheel ? To what is the motion given to the wheels which have been described, communicated? 136. Of what does Pneumatics treat? 137. What is the air which we breathe / How far does it extend above the surface of the earth ? 5 gg NATURAL PHILOSOPHY. It possesses many of tlie properties belonging to liquids in general, besides several others, the result, or, perhaps, the cause of its elasticity. Its specific gravity is eight hundred times less than that of water. 138. Air, steam, vapor, gas, are all elastic fluids pos- sessing the same mechanical properties. f Whatever, therefore, is stated in relation to air, belongs m common to all of these fluids. 139. Aeriform fluids have weight, but no cohesive attraction. 140. Air has two principal properties, namely. Grav- ity and Elasticity-J It has already been stated, that the air near the surface of the earth bears the weight of that which is above it. Being compressed, therefore, by the weight of that above it, it must exist in a condensed form near the surface of the earth, while in the upper regions of the atmosphere, where there is no pressure, it is highly rarefied. This condensation, or pressure, is very similar to that of water at great depths in the sea. * The air is necessary to animal and vegetable life, and to combustion. It is a very heterogeneous mixture, being filled with vapors of all kinds It consists, however, of two principal ingredients, called oxygen and nitro- gen, or azote ; of the former of which there are 28 parts, and of the latter, 72 in a hundred. The air is not visible, because it is perfectly transparent. It may be felt when it moves in the form of wind, or by swinging the hand rapidly backwards and forwards. . . t The chemical properties of liquids, fluids, &c., are not treated m the sciences of Pneumatics, Hydraulics, or Hydrostatics, but belong peculiarly to the science of Chemistry. They are not, therefore, described m this work. But fluids possess all the properties of liquids, and the laws of Hydrostatics and Hydraulics apply to them as well as to liquids. X Besides these two j>rincipal properties, the operations of which pro- duce most of the phenomena of Pneumatics, it will be recollected that as air, although an invisible is yet a material substance, possessing all the Does it possess properties common to liquids in general i How does its specific gravity compare with that of water ? Of what two principal ingre- dients does the air consist ? What is the proportion of these parts to each ""^^38. What other fluids are named belonging to the class of elastic fluids? 139. Have the air and other similar fluids weight? With what power alone has heat to contend in aeriform fluids? 140 What two principal properties has the air ? PNEUMATICS. 99 141. A column of air, having a base an inch square, and reaching to the top of the atmosphere, weighs about fifteen pounds. This pressure, hke the pressure of Hquids, is exerted equally in all directions.* 142. The elasticity of air and other aeriform fluids is that property by which they are increased or diminish- ed in extension, according as they are compressed. f This property exists in a much greater degree in air and other similar fluids than in any other substance. In fact it has no known limit ; for when the pressure is removed from any portion of air, it immediately expands to such a degree that the smallest quantity will diffuse itself over an indefinitely lai-ge space. ^ And, on the contrary, when the pressure is in- creased, it will be compressed into indefinitely small dimen- sions.J common properties of matter, it possesses also the common property of imp enetr ability. This will be illustrated by experiments. * It has been computed that the weight of the whole atmosphere is equal to tnat of a globe of lead sixty miles in diameter, or to five thousand billions of tons. t The pressure of the atmosphere caused by its weight is exerted on all substances, internally and externally, and it is a necessary consequence of its fluidity. The body of a man of common stature has a surface of about 2000 square inches ; whence the pressure at 15 pounds per square inch will be 30,000 pounds. The reason why this immense weight is not felt, is, that the air within the body and its pores counterbalances the weight of the external air. When the external pressure is artificially removed from any part, it is immediately felt by the reaction of the internal air. X Heat insinuates itself between the particles of bodies, and forces them asunder, in opposition to the attraction of cohesion and of gravity ; it there- fore exerts its power against both the attraction of gravitation and the at- traction of cohesion. But as the attraction of cohesion does not exist in aeriform fluids, the expansive power of heat upon them has nothing to contend with but gravity. Any increase of temperature, therefore, ex- pands an elastic fluid prodigiously, and a diminution of heat condenses it. 141. What is the weight of a column of air one inch square at the base, and reaching to the top of the atmosphere ? Is the pressure exerted equally in all directions ? 142. What is meant by the elasticity of the air? How do the aeriform fluids differ from liquids ? When is the air said to be rarefied? When condensed ? Is the air, near the surface of the earth, rare or dense ? 1^00 NATURAL PHILOSOPHY. 143. Air becomes a mechanical agent by nneans of its weight, its elasticity, its inertia, and its fluidity.* 144. A vacuum is a space from which air and every other substance has been removed. The Torricellian vacuum was discovered by Torricelli, and was obtained in the following manner. A tube closed at one end, and about 32 inches long, was filled with mercury ; the open end was then covered with the finger so as to pre- vent the escape of the mercury, and the tube inverted and plunged into a vessel of mercury ; the finger was then removed and the mercury permitted to run out of the tube. It was found, however, that the mercury still remained in the tube to the height of about thirty inches, leavmg a vacuum at the top of aboul two inches. This vacuum, called from the discoverer the Torricelhan vacuum, is the most perfect that has been dis- covered.f » The fiuidity of air invests it, as it invests all other liquids, with the power of transmitting pressure. But it has already been shown, under the head of Hydrostatics, that fluidity is a necessary consequence of the in- dependent gravitation of the particles of a fluid. It may, therefore, be included among the efifects of weight. The inertia of air is exhibited in the resistance which it opposes to mo- tion, which has already been noticed under the head of Mechanics This is clearly seen in its eflect upon falling bodies, as will be exemplified m the experiments with the air-pump. t As this is one of the most important discoveries of the science ot Pneumatics, it is thought to be deserving of a labored explanation. The whole phenomenon is the result of the equilibrium of fluids. The atmo- sphere pressing by its weight (15 pounds ou every square mch) on the sur- face of the mercury in the vessel, counterpoised the column of mercury m the tube when it was about 30 inches high, showing thereby that a column of the atmosphere is equal in weight to a column of mercury of the sarne base, having a height of 30 inches. Any increase or dimmution in the density of the air produces a corresponding alteration in its weight, and con- sequently, in its ability to sustain a longer or a shorter column of mercury. Had water been used instead of mercury, it would have required a height of about 33 feet to counterpoise the weight of the atmospheric column. Other fluids may be used, but the perpendicular height of the column of any fluid, to counterpoise the weight of the atmosphere, must be as much 143. How does the air become a mechanical agent 1 144. What is a vacuum ? PNEUMATICS. 101 145. The barometer is an instrument to measm'e the weight of the atmosj)here, and thereby to indicate the variations of the weather.* 1. Fig. 69 represents a barometer. It consists of a long glass tube, about thirty-three inclies in length, closed at the upper end and filled with mercury. The tube is then inverted in a cup, or leather bag, of mercury, on which the pressure of the atmo- sphere is exerted. As the tube is closed at the top, it is evident that the mercury cannot de- scend in the tube without producing a vacuum. The pressure of the atmosphere (which is capable of supporting a column of mercury of about 30 inches in height) prevents the descent of the mercury ; and the instrument, thus constructed, becomes an implement for ascertaining the weight of the atmosphere. As the air varies in weight or pressure, it must, of course, influence the mer- cury in the tube, which will rise or fall in exact proportion with the pressures When the air is thin and light, the pressure is less, and the mercury will descend ; and when the air is dense and heavy, the mercury will rise. At the side of the tube there is a scale, marked inches and tenths of an inch, to note the rise and fall of the mercury.f g^reater than that of mercury as the specific gravity of mercury exceeds that of the fluid employed. * The elasticity of the air causes an increase or diminution of its bulk, according as it is afFected by heat and cold ; and this increase and diminu- tion of bulk materially affect its specific gravity. The height of a column of mercury that can be sustained by a column of the atmosphere must, therefore, be affected by the state of the atmosphere. The instrument used to indicate these changes is called a barometer, from two Greek words signifying a measure of the weight, that is, of the atmosphere. A Thermometer is a measure of the heat, and a Hygrometer a measure of the moisture of the atmosphere. t Any other fluid may be used as well as mercury, provided the length 145. What is a barometer? What does the word barometer mean? What is a thermometer ? What does the word thermometer mean ? What is a hygrometer? What does the word hygrometer mean? What figure represents a barometer ? Explain its construction. What height of mer- cury is the pressure of the atmosphere capable of sustaining ? What effect has the pressure of the atmosphere on the mercury in the tube ? 102 NATURAL PHILOSOPHY. 2. The pressure of the atmosphere on the mercury, in the bag or cup of a barometer, bemg exerted on the principle of the eqiiihbrium of fluids, must vary according to the situation in which the barometer is placed. For this reason it will be the greatest in valleys and low situations, and least on the top of high mountains. Hence the barometer is often used to ascer- tain the height of mountains and other places above the level of the sea.* of the tube be extended in proportion to the specific gravity of the fluid Thus, a tube filled with water must bo 33 feet long, because the atmo- sphere will support a column of water of that height. Mercury is used, therefore, in the construction of the barometer, because it does not require so long a tube as any other fluid. It may here bo remarked, that the ail is the heaviest in dry weather, and that, consequently, the mercury will then rise highest. In wet weather the dampness renders the air less salubrious, and it appears, therefore, more heavy then, although it is, in fact, much lighter. The greatest depression of the barometer occurs daily at about 4 o'clock, both in the morning and in the afternoon, and its highest eleva- tion at about 10 o'clock morning and night. In summer, these extreme points are reached an hour or two earlier in the morning, and as much later in the afternoon. * As the air diminishes in density, upwards, it follows, that it must be more rare upon a hill than on a plain. In very elevated situations it is so rare that it is scarcely fit for respiration, or breathing ; and the expan- sion which takes place in the more dense air contained within the body is often painful : it occasions distention, and sometimes causes the bursting of the smaller blood-vessels, in the nose and ears. Besides, in such situa- tions, we are more exposed botli to heat and cold ; for, though the at- mosphere is itself transparent, its lower regions abound with vapors and exhalations from the earth, which float in it, and act in some degree as a covering, which preserves us equally from the intensity of the sun's rays, and from the severity of the cold. In what proportion does the mercury rise and fall ? In what way can barometers be made of other fluids? Why is mercury used in preference to any other fluid? Is the air heaviest in wet or dry weather? On what principle is the pressure of the atmosphere on the mercury, in the cup of a barometer, exerted? What follows from this? For what other purpose, besides measuring the pressure of the atmosphere, and foretelling the varia- tions of the weather, is the barometer used ? Is the air the more dense at the surface of the earth or upon a hill? What is a thermometer? What figure represents a thermometer? Explain its construction. I'NEUMATICS. 103 3. The thermometer is an inst*-ument used to 's- indicate the temperature of the atmosphere. In appearance it resembles a barometer, but it is constructed on a difibrent principle, and for a different purpose. It consists of a capillary tube, closed at the top, and terminated with a bulb, which is filled with mercury.* As heat expands and cold contracts most substances, it follows, that in warm weather the mercury must be expanded and will rise in the tube, and that in cold weather it will contract and sink. Hence the instrument becomes a correct measure for the heat and cold of the air. A scalef is placed at the side of the tube, to mark the degree of heat or cold, as it is indicated by the rise and fall of the mercury in the capillary tube. 4 The hygrometer, for measuring the degree of moisture in the air,+ may be constructed ot any thing which contracts and expands ^7 the moisture or dryness of the atmosphere, such as most kinds of wood , cat- gut, twisted cord the beard of wild oats, &c. * Any other liquid which is expanded by heat and contracted by cold, such as spirits of wine, &c., will answer instead of mercury. t There are several different scales applied to the thermometer, of which those of Fahrenheit, Reaumur, Delisle, and Celsius are the princi- pal The tliermometer in common use in this country, is graduated by Fahrenheit's scale, which, commencing with 0, or zero, extends upwards to 212 degrees, the boiling point of water, and downwards to 20 or M de- grees. The scales of Reaumur and Celsius fix zero at the freezing point of water ; and that of Delisle at the boiling point. t Bv the action of the sun's heat upon the surface of the earth, whether land or water, immense quantities of vapor are raised into the atmosphere, supplying materials for all the water which is deposited again m the va- rious 'forms of dew, fog, rain, snow, and hail. Experiments have been made to show the quantity of moisture thus raised from the ground by the heat of the sun. Dr. Watson found that an acre of ground, apparently dry, and burnt up by the sun, dispersed into the air sixteen hundred gal- What effect have heat and cold on most substances? What follows from this 1 Whose scale is generally used in this country ? For what is the hyo-rometer used ? Of what kind of substances may it be constructed What experiment is given in the note to show the quantity of moisture raised from the ground by the heat of the sun? 4 104 NATURAL PHILOSOPHY. 146. The impenetrability of air prevents the ascent of water into any inverted vessel, unless the air is first permitted to escape. 1. If a tube, closed at one end, or an inverted tumbler, be inserted at its open end, in a vessel of water, the water will not rise in the tube or tumbler, to a level with the water in the vessel, on account of the impenetrability of the air within the tube. But if the tube be open at both ends, the water will rise, because the air can escape through the upper end. It is on this principle that the diving-bell (or the diver's bell, as it is sometimes called) is constructed. 2. Fig. 71 represents a diving-bell. It consists of a large heavy vessel, formed hke a bell, (but may be made of any other shape,) with the mouth open. It descends into the water with Ions of water in the space of twelve hours. His experiment was thus made : he put a glass, mouth downwards, on a grass-plot, on which it had not rained for above a month. In less than two minutes the inside was covered with vapor; and in half an hour drops began to trickle down its inside. The mouth of the glass was 20 square inches. There are 1296 square inches in a square yard, and 4840 square yards in an acre. When the glass had stood a quarter of an hour, he wiped it with a piece oi muslin, the weight of which had been previously ascertained. When the glass had been wiped dry, he again weighed the muslin, and found that its weight had been increased six grains by the water collected from 20 square inches of earth; a quantity equal to 1600 gallons, from an acre, in 12 hours. Another experiment, after rain had fallen, gave a much larger quantity. (See No. 9.) When the atmosphere is colder than the earth, the vapor, which arises from the ground, or a body of water, is condensed and becomes visible This is the way that fog is produced. When the earth is colder than the atmosphere, the moisture in the atmosphere condenses in the form of dew, on the ground, or other surfaces. Clouds are nothing more than vapor, condensed by the cold of the upper regions of the atmosphere. Rain is produced by the sudden cooling of large quantities of watery vapor. Snow and hail are produced in a similar manner, and differ from rain only in the degree of cold which produces them. 146. Is air impenetrable, like other substances ? - How is this shown ? Upon what principle is the diving-bell constructed ? What figure repre- sents the diving-bell? Why does not the water rise in the bell? Explain the figure. PNEUMATICS. 105 its mouth downwards. The air within Fig. 71. it having no outlet is compelled, by the order of specific gravities, to ascend in the bell, and thus (as water and air cannot occupy the same space at the same time) prevents the water from rising in the bell. A person, therefore, may descend with safety in the bell to a great depth in the sea, and thus re- cover valuable articles that have been lost. A constant supply of fresh air is sent down, either by means of barrels, or by a forcing-pump. In the Fig., _B represents the bell with the diver in it. C is a bent metallic tube attached to one side and reaching the air within ; and P is the forcing-pump through which air is forced into the bell. The forcing-pump is attached to the tube by a joint at D. When the bell de- scends to a great depth, the pressure of the water condenses the air within the bell, and causes the water to ascend in the bell. This is forced out by constant accessions of fresh air, supphed as above mentioned. Great care must be taken that a constant supply of fresh air is sent down, otherwise the lives of those within the bell will be en- dangered. The heated and impure air is allowed to escape through a stop-cock in the upper part of the bell. 147. Water is raised in the common pump by means of the pressure of the atmosphere on the surface of the water. A vacuum being produced by raising the piston or pump-box * the water below is forced up by the at- mospheric pressure, on the principle of the equilibrium 1^ In order to produce such a vacuum, it is necessary that the piston or box should be accuratelv fitted to the bore of the pump; for if the air above the piston has any means of rushing in to fill the vacuum, as it is produced by the raising of the piston, the water will not ascend. The pis- ton is generally worked by a lever, which is the handle of the pump, not represented in the figure. 147. By what means is water raised in the common pump? How is the pressure removed? i06 NATURAL PHILOSOPHY. of fluids. On this principle the water can be raised only to the height of about thirty-three feet, because the pressure of the atmosphere will sustain a column of water of that height only. Fig. 'Z 2 represents the common pump, called ^^s- 72 the sucking-pump. The body consists of a large r-Q^ tube, or pipe, the lower end of which is to be 1 1 immersed in the water which it is designed to raise. P is the piston, Y a valve* in the piston, which, opening upv/ards, admits the water to rise through it, but prevents its return. Y is a similar valve in the body of the pump, below the piston. When the pump is not in action the valves are closed by their own weight ; but when the piston is raised it draws up the column of water which rested upon it, producing a vacuum between the piston and the lower valve Y. The water below, immediately rushes through the lower valve, and fills the vacuum. When the piston descends a second time, the water in the body of the pump passes through the valve Y, and on the ascent of the piston is lifted up by the piston, and a vacuum is again formed below, f which is immediately filled by the water rushing through the lower valve Y. In this manner the body of the pump is filled with water, until it reaches the spout S, where it runs out in an interrupted stream. f * A valve is a lid or cover, so contrived as to open a communication in one way and close it in the other. Valves are made in different ways, ac- cording to the use for which they are intended. In the common pump, they are generally made of thick leather partly covered with wood. In the air-pump they are made of oiled silk, or thin leather softened with oil. The clapper of a pair of bellows is a familiar specimen of a valve. The valves of a pump are commonly called boxes. t Although water can be raised by tlie atmospheric pressure only to the height of 33 feet above the surface, the common pump is so constructed, that after the pressure of the atmosphere has forced the water through the What figure represents the common pump ? Explain it. Which of the mechanical powers is the handle of the pump? How high can water be raised by the common pump? Why? Why is the common pump some- times called the lifting-pump? FNEUMATICS. 107 148. The forcing-pump differs from the common pump in having a "forcing power added to raise the water to any desired height. 1. Fig. Y3 represents the forcing- pump. The body and lower valve y are similar to those in the com- ^ mon pump. The piston P has no valve, but is sohd ; when, therefore, the vacuum is produced above the lower valve, the water on the de- scent of the piston is forced through the tube into the reservoir or air- vessel R, where it compresses the air above it. The air, by its elas- ticity, forces the water out through the jet J in a continued stream and with great force. It is on this prin- ciple that fire-engines are constructed. 2. Sometimes a pipe with a valve in it is substituted for the air-vessel ; the water is then thrown out in a continued stream, but not with so much force. 149. Wind is air put in motion.* When any portion of the atmosphere is heated, it becomes rarefied, its specific gravity is diminished, and it consequently rises. The adjacent portions immediately rush into its place to restore the equilibrium. This motion produces a current which rushes into the rarefied spot from all directions. This is what we call wind. The portions north of the rarefied spot, valve in the body of the pump, and the descent of the piston has forced it through the valve in the piston, it is lifted up, when the piston is raised. For this reason, this pump is sometimes called the lifting pump. The distance of the lower valve from the surface of the water must never ex- ceed 32 feet ; and in practice it must be much less. * There are two ways in which the motion of the air may arise. It may be considered as an absolute motion of the air, rarefied by heat and condensed by cold ; or it may be only an apparent motion, caused by the superior velocity of the earth in its daily revolution. 148. How does the forcing-pump differ from the common pump? What figure represents the forcing-pump? Explain it. 149. What is wind? In what two ways may the motion of the air be explained? Explain the manner in which the air is put in motion. 108 NATURAL PHILOSOPHY. produce a north wind ; those to the south produce a south wind ; while those to the east and west, in hke manner, form currents moving in opposite directions. At the rarefied spot, agitated as it is by winds from all directions, turbulent and boisterous weather, whirlwinds, hurricanes, rain, thunder and lightning prevail. This kind of weather occurs most frequent- ly in the torrid zone, where the heat is greatest. The air be- ing more rarefied there than in any other part of the globe, is lighter, and, consequently, ascends : that about the polai regions is continually flowing from the poles to the equator, to restore the equilibrium ; while the air rising from the equatoi flows in an upper current towards the poles, so that the polar regions may not be exhausted.^ A regular east wind prevail? * From what has now been said, it appears that there is a circulation of air in the atmosphere ; the air in the lower strata flowing from tht3 poles to the equator, and in the upper strata flowing back from the equator to the poles. It may here be remarked, that the periodical winds are more regular at sea than on the land ; and the reason of this is, that the land reflects into the atmosphere a much greater quantity of the sun's rays than the water ; therefore, that part of the atmosphere which is over the land is more heated and rarefied than that which is over the sea. This occasions the wind to set in upon the land, as we find it regularly does on the coast of Guinea and other countries in the torrid zone. There are cer- tain winds called trade-winds, the theory of which may be easily explained on the principle of rarefaction, affected as it is by the relative position of the different parts of the earth with the sun, at ditTerent seasons of tlie year and at various parts of the day. A knowledge of the laws by which these winds are controlled, is of importance to the mariner. When the place of the sun, with respect to the different positions of the earth at the diff*erent seasons of the year is understood, it will be seen that they all depend upon the same principle. The reason that the wind generally sub- sides at the going down of the sun, is, that the rarefaction of the air, in the particular spot which produces the wind, diminishes as the sun de- clines, and consequently, the force of the wind abates. The great variety of winds in the temperate zone is thus explained. The air is an exceed- ingly elastic fluid, yielding to the slightest pressure ; the agitations in it, therefore, caused by the regular winds, whose causes have been explained, must extend every way to a great distance ; and the air, therefore, in all climates will suflfer more or less perturbation, according to the situation of the country, the position of mountains, valleys, and a variety of other causes. Hence every climate must be liable to variable winds. The How are the north, south, east, and west winds produced ? about the (Mjualoi", caused 1)\' (lie rai'c fact ion of tlu; aii' [)n)- (luct'il l>y (lie sun in his daiK' coui'sc from cast (o wcsl. 'J'his wiiuL I'oinhiniiiL;" \\ilh llial, iVoni (he poh's, caus(!s ji conslanl noi lluast wind, for about tliirty dogrees north of the etjuator, and ;i soutlioast wind at the same distance south of tlie equator. OF THE AIR-PUMP. 150. The air-pump is an instrument for exhaustinf^ the air from a vessel prepared for the purpose. This vessel is called a receiver, and is made of glass in order that the etiects of the removal of the air may be seen. x\ir-pumps are made in a great variety of forms ; but all are constructed on the principle, that when any portion of confined air is removed, the residue immedi- ately expanding, by its elasticity fills the space occupied by the portion that has been withdrav^n. 1. From this statement it will appear that a perfect vacuum can never be obtained by the air-pump as at present construct- ed. But so much of the air within a receiver may be exhausted that the residue will be reduced to such a degree of rarity as to subserve most of the practical purposes of a vacuum. Fig. 74 represents a single barrel air-pump, used both for condensing and exhausting. A D is the stand or platform of the instrument, which is screwed down to the table by quality of winds is affected by the countries over which they pass; and they are sometimes rendered pestilential by the heat of deserts, or the pu- trid exhalations of marshes and lakes. Thus, from the deserts of Africa, Arabia, and the neighboring countries, a hot wind blows, called Samiel, 01 Simoom, which sometimes produces instant death. A similar wind blows from the desert of Sahara, upon the western coast of Africa, called the Harmattan, producing a dryness and heat which is almost insupport- able, scorching like the blasts of a furnace 160. What is an air-pump? What is *,he vessel called from which the air is exhausted ? On what principle are all air-pumps constructed ? Can a perfect vacuum ever be obtained? Describe the air-pump represented bv Fig. 74 110 NATURAL PHILOSOPHY. IICCD Fig 74 means of a clamp, under- neath, which is not repre- sented in the figure. K is the glass vessel or bulbed receiver from which the air is to be exhausted. P is a solid piston, accurate- ly fitted to the bore of the cylinder, and H the handle by which it is moved. The dotted line T, represents the commu- nication between the re- ceiver R and the barrel B ; it is a tube through which the air, entering at the opening I, on the plate of the pump, passes into the barrel, through the exhausting valve e v. c v is the conden- sing valve, communicating with the barrel B by means of an aperture near e, and opening outwards through the condensing pipe 'p. 2. The operation of the pump is as follows : The piston P being drawn upwards by the handle H, the air in the receiver R, ex- panding by its elasticity, passes by the aperture I through the tube T, and through the exhausting valve e v into the barrel. On the descent of the piston, the air cannot return through that valve, because the valve opens upwards only : it must, therefore, pass through the aperture, by the side of the valve, and through the condensing valve c v into the pipe p, where it passes out into the open air. It cannot return through the condensing valve c v, because that valve opens outwards only. By continuing this operation, every ascent and descent of the piston P must render the air within the receiver R more and more rare, until its elastic power is exhausted. The receiver is then said to be exhausted ; and although it still contains a small quantity of air, yet it is in so rare a state that the space within the receiver is considered a vacuum.'^ 3. From the explanation which has been given of the opera- tion *of this air-pump, it will readily be seen that, by removing the receiver R, and screwing any vessel to the pipe p, the air * The only known method of procuring a perfect vacuum, is that pur- sued by Torricelli, which has already been explained. Ill m:\y ])(' condcnscil in (he vessel. Thus (he j)Ui)ij) is in;i(l(i to i'\h;iiisl or to (•( )iHhMise. wiiIkmiI aher.'iliou.'^' t. 'riie (h)iible ait-piiin j) (iilleis tVoin (he sini;le :iii-])um p, in havini;- two barrels aiul two })ist()iis ; which, iiistciJid of beiiii;- moved by the hand, aie worked by iiieiins of a toothed wiui(;l, })lavini;- in notches of the jMston rods. 5. Fio-. 75 repn^sents Wio-htinan's patent levor air-})umj), belonn-ino- lo " ///c Jiosto)) School Set.'" This instrument is of Fig. 75. an improved construction, and differs from others in the facih- ty with which it is worked. In this pump the piston is sta- * Air-pumps in general are not adapted for condensation ; that office being performed by an instrument called " a condensing syringe,^^ which is an air-pump recersed, its valves being so arranged as to force air into a chamber, instead of drawing it out. For this purpose, the valves open inwards in respect to the chamber, while in air-pumps they open outwards. A gauge, constructed on the principle of the barometer, is sometimes adjusted to the air-pump for the purpose of exhibiting the degree of ex- haustion 112 NATURAL PHILOSOPHY. tionaiy, while motion is given to the barrel by means of the lever H. The barrel is kept in a proper position by means of polished steel guides.^ 151. By means of the air-pump, many interesting experiments may be performed, illustrating the gravity, elasticity, fluidity, and inertia of air. Fig. 76. EXPERIMENTS ILLUSTRATING THE GRAVITY OF AIR. 1. Having adjusted the receiver to the plate of the air- pump, exhaust the air and the receiver will be held firmly on the plate. The force which confines it, is nothing more than the weight of the external air, which, having no internal pres- sure to contend with, presses Avith a force of nearly 1 5 pounds on every square inch of the external surface of the receiver. N. B. The exact amount of pressure depends on the degree of exhaustion, being at its maximum of 15 pounds when there is a peifect vacuum. On readmitting the air the receiver may be readily removed. 2. The Magdeburgh Cups or Hemispheres. Fig. 76 repre- sents the Magdeburgh cups or hemispheres. They consist of two hollow brass cups, the edges of which are accurately fitted together. They each have a handle, to one of which a stop- cock is fitted. The stop-cock, being attached to one of the cups, is to be screwed to the plate of the air-pump, and left open. Having joined the other cup to that on the pump, exhaust the air from Avithin them, turn the stop-cock to prevent its re- admission, and screw the handle that had been removed to the stop- cock. Two per- sons may then attempt to draw the cups asunder. It will be found that great power is required to separate them ; but, on readmitting the air betAveen them, by turning the cock, they will fall asunder by * Mr. Wighlman has published a small volume, entitled, A Con panioii to the Air-pump," which will be found a very convenient guide fo the management of the pump, and the skilful performance of experiments 151. Give, in succession, the experiments made by means of the air pump. Of the receiver Of the Magdeburgh cups. PNEUMATICS. 113 Fig. 77. Fig. 78. their own weight. When the air is exhausted from withm them, the pressure of the surrounding air upon the outside keeps them united. This pressure being equal to a pressure of lifteen pounds on every square inch of the surface, it toilows that the larger the cups or hemispheres the more difhcult it will be to separate them.^ 3. The Hand-glass. Fig. 77 is nothing more than a tumbler, open at both ends, with the top and bottom ground smooth, so as to fit the brass plate of the air-pump. Placing it upon the plate, cov.er it closely with the palm of the hand, and work the pump. The air within the glass being thus exhausted, the hand will be pressed down by the weight of the air above it : on read- mitting the air, the .hand may be easily re- moved. 4. The Bladder-glass. Fig. 78 is a bell- shaped glass, covered with a piece of bladder, which is tied tightly around its neck. Thus prepared, it may be screwed to the plate of the air-pump, or connected with it by means of an elastic tube. On exhausting the air from the glass, the external pressure of the air on the bladder will burst it inwards with a loud explosion. 5. The India-rubber Glass. Fig. 79 is a glass similar to the one represented in the last figure, covered with india-rubber. The same experiments may be made with this as were mentioned in the last article, but with difterent results. Instead of bursting, the india-rubber will be pressed inwards the whole depth of the glass. * Otto Giiericke, the inventor of the air-pump, prepared two hem- ispheres, two feet in diameter, and having accurately fitted them together, and exhausted the air, 30 horses harnessed to them were unable to sep- arate them. When more" horses were added, the hemispheres parted with a loud report. Fig. 79. Explain the experiment with the hand-glass. Of the bladder-glass. Of the india-rubber glass. 114 NATURAL PHILOSOPHY. Fig. 81. 6. The Fountain-Glass and Jet. Fig. 80 represents the jet, which is a small brass tube. Fig. 81 is the fountain- glass. The expeiiment with these in- struments is designed to show the pressure of the atmosphere on the surface of liquids. Screw the straight jet to the stop-cock, the stop -cock to the fountain-glass, with the straight jet inside of the fountain-glass, and the lower end of the stop-cock to the plate of the air-pump, and then open the stop-cock. Having^ exhausted the air from the fountain-glass, close the stop- cock, remove the glass from the pump, and, im- mersing it in a vessel of water, open the stop- cock. ^ The pressure of the air on the surface of the water will cause it to rush up into the glass hke a fountain. 7. Pneumatic Scales for weighing Air. Fig. 82 repre- sents the flask or glass vessel and scales for A^^io-hino- air Weigh the flask when full of air : then exhaust the air and weigh the flask again. The difference between its pres- ent and former weight is the weight of the air that was contained in the flask. 8. The Sucker. A circular piece of wet leather, with a string attached to the centre, being pressed upon a smooth surface, will adhere with con- siderable tenacity, when drawn upwards by the string. The string in this case must be attached to the leather so^hat no air can pass under the leather. 9. The Mercurial or Water Tube. Exhaust the air from a glass tube three feet long, fitted mth a stop-cock at one end, and then immerse it in a vessel containing mercury or water. On turning the stop-cock, the mercury will rise to the height of nearly 30 inches; or, if immersed in water, the water will rise and fill the tube, and would fill it were it 30 feet long. This experiment shows the manner in which water is raised to the boxes or valves in common water-pumps. Explain the experiment of the fountain-glass and jet. Explain the Dneumatic scales for weighing air. Explain the sucker. Mercurial tube. PNEUMATICS. 116 EXPERIMENTS SHOWING THE ELASTICITY OF AIR. 1 Place an iiidia-mbber bag, or a bladder, partly inflated, and tio-litly closed, under the receiver, and on exhausting the an% the air within the bag or bladder expanding, fill the bag On readmitting the air, the bag will collapse. The experimeirt may also be made wath some kinds of shrivelled fruit, it tne skin be sound. The internal air expanding v/ill give the truit a fresh and plump appearance, which will disappear on the re- admission of the air. 2 The same principle may be illustrated by the mdia- rubber and bladder glasses, if they have stop-cocks to conline the air. .... i • o 3 A small bladder partly filled wath air may be sunk m a vessel of water by means of a weight, and placed under the receiver. On exhausting the air from the receiver, the air m the bladder will expand, and its specific gravity bemg thus diminished, the bladder with the weight^ will rise. On re- admittino' the air the bladder will sink again. 4 Air contained in Water and in Wood. Place a vessel of water under the receiver, and on exhaustmg the air trom the receiver, the air in the water previously mvisible will make its appearance in the form of bubbles, presenting the semblance of ebullition. . . j • xi 5 A piece of light porous wood being immersed m the water below the surface, the air will be seen issumg m bub- bles from the pores of the wood. 6. The Pneumatic Balloon. Fig. 83 repre- sents a small glass balloon with its car im- mersed in a jar of water, and placed under a receiver. On exhausting the air, the air withm , the balloon expanding, gives it buoyancy, and it will rise in the jar. On readmitting the air the balloon will sink. 7. The experiment may be performed with- out the air-pump by covering the jar with some elastic substance, as india-rubber. By pres- sing on the elastic covering with the finger the^ air will be condensed, the water will rise in the balloon, and it will sink. On removing the pressure, the air in the balloon expand- Fig. 83 Explain the first experiment.— 2d. 3d. 4th. 5th. 6th. 7th. 116 NATURAL PHILOSOPHY. ing, will expel part of the water and the balloon will rise. This is the more convenient mode of performing the experiment, as it can be repeated at pleasm e without resort to the pump."^ 8. The following is a full explanation : — The pressure on the top of the vessel first condenses the air between the cover and the surface of the water; — this condensation presses upon the water below, and as this pressure affects every portion of the water throughout its whole extent, the water, by its upward pressure, compresses the air within the balloon, and makes room for the ascent of more water into the balloon so as to alter the specific gravity of the balloon, and cause it to sink. As soon as the pressure ceases, the elasticity of the air in the balloon drives out the lately entered water, and restoring the former lightness to the balloon causes it to rise. If, in the commencement of this experiment, the balloon be made to have a specific gravity too near that of water, it will not rise of itself, after once reaching the bottom, because the pressure of the water then above it will perpetuate the condensation of the air which caused it to descend. It may even then, how- ever, be made to rise, if the perpendicular height of the water above it be diminished by inclining the vessel to one side. 9. This experiment proves many things ; namely : First. The materiality of air, hj X\lq pressure of the hand * This experiment exhibits the principle on which the well-known glass figure, called the Cartesian Devil, is constructed ; and it may be thus ex- plained : several images of glass, hollow within, and each having a small opening at the heel by which water may pass in and out, may be made to manoeuvre in a vessel of water. Place them in a vessel in the same manner with the balloon, but by allowing different quantities of water to enter the apertures in the images, cause tbem to differ a little from one another in specific gravity. Then, when a pressure is exerted on the cover, the heaviest will descend first, and the others follow in the order of their spe- cific gravity ; and they will stop or return to the surface in reverse order, when the pressure ceases. A person exhibiting these figures to spectators who do not understand them, while appearing carelessly to rest his hand on the cover of the vessel, seems to have the power of ordering their move- ments by his will. If the vessel containing the figures be inverted, and the cover be placed over a hole in the table, through which, unobserved, pressure can be made by a rod rising through the hole, and obeying the foot of the exhibiter, the most surprising evolutions may be produced among the figures, in perfect obedience to the word of command. 8. Explain, in full, the experiment of the glass figures. 9. Explain all that this experiment proves. Explain the condensing jar. PNEUMATICS. 117 on the top being communicated to the water below through the ah' in the upper part of the vessel. Secondly. The compressibility of air, by what happens m the globe before it descends. Thirdly. The elasticity, or elastic force of air, when the water is expelled from the globe, on removing the pressure. Fourthly. The lightness of air, in the buoyancy of the globe. Fifthly. It shows that the pressure of a liquid is exerted m all directions, because the effects happen in whatever position the jar be held. Sixthly. It shows that pressure is as the depth, because less pressure of the hand is required, the farther the globe has de- scended in the water. n -i Seventhly. It exemplifies many circumstances of fluid sup- port. A person, therefore, who is familiar with this experi- ment, and can explain it, has learned the principal truths ot Hydrostatics and Pneumatics. EXPERIMENTS W^ITH CONDENSED AIR. 1. The Condensing and ExHAusTiNa SvEiNaE. The con- densing syringe is the air-pump reversed.^ The exhausting syringe is the simple air-pump without its plate or stand. These implements are used respectively with such parts ol the apparatus as cannot conveniently be attached to the air- pump ; and as an addition to such pumps as do not perform the double office of exhaustion and condensation. In some sets of apparatus the condensing and exhausting syringes are united, and are made to perform each office respectively, by merely reversing the part which contains the valve. 2. The Air-Chamber. The air-chamber, Fig. 83, is a hollow brass globe prepared for the re- ::,eption of a stop-cock, and is de- signed for the reception of con- densed air. It is made in different forms in different sets, and is used by screwing it to a condensing pump or a condensing syringe. 118 NATURAL PHILOSOPHY. 3. Straight and Revolving Jets from Condensed Air. Fill the air-chamber (Fig. 84) partly with water and then con- dense the air. Then confine the air by turning the cock; after which unscrew it from the air-pump, and screw on the straight or the revolving jet. Then open the stop-cock, and the water will be thrown from the chamber in the one case, in a straight continued stream, in the other in the form of a wheel. Figs. 85 and 86 represent a view of the straight and the revolving jets. In the revolving jet the water is thrown from two small apertures made at each end on opposite sides, to assist the revolution. The circular motion is caused by the reaction of the water on the sides of the arms opposite the jets ; for as the water is forced into the tubes, it exerts an equal pressure on all sides of the tubes, and as the pressure is re- lieved on one side by the jet-hole, the arm is caused to revolve in a contrary direction. This experiment performed with the straight jet, illustrates the principle on which " Hero's ball" and Hero's fountain are constructed. 4. The Principle of the Air-gun. With the air-chamber as in the last experiments, a small brass cyHnder or gun-barrel, Fig. 87, may be substituted for the jets," and loaded with a small shot or paper ball. On turning the rig. 87. cock quickly, -the condensed air rushing out will throw _ the shot to a considerable distance. In this way the air-gun operates, an apparatus resembling the lock of a gun being substituted for the stop-cock, by which a 1 1 small portion only of the condensed air is admitted to | j l escape at a time, so that the chamber being once filled • will afford two or three dozen discharges. The force W of the air-gun has never been equal to more than a fifteenth of the force of a common charge of powder, and the loudness of the report made in its discharge is always as great in proportion to its force as that of the common gun. 5. Condensed air may be weighed in the air-chamber; but m estimating its weight, the temperature of the room must al- ways be taken into consideration, as the density of air is materially affected by heat and cold. Explain the jets. The air-gun PNEUMATICS. 119 EXPERIMENTS SHOWING THE INERTIA OF AIR. The Guinea and Feather Drop. The inertia of air is shown by the guinea and feather drop, exhibiting the resistance which the air opposes to falhng bodies. This apparatus is made in different forms, some having shelves j,.^ gg Fig. 88. on w^hich the guinea and feather rest ; ° and when the air is exhausted they are made to fall by the turning of a handle. A better form is that rep- resented in Fig. 88, in which the guinea and feather (or a piece of brass substituted for the guinea) are enclosed, and the apparatus being \ \ screwed to the plate of the pump, the air is exhausted ; a stop-cock ^ turned to prevent the readmission of the air, and the apparatus being then unscrewed, the experiment may be repeatedly showed by one exhaustion of the air. It will then appear that every time the apparatus is inverted, the guinea and the feather will fall simultaneously. The two forms of the guinea and feather drop are exhibited in Figs. 88 and 89, one of which. Fig. 88, is furnished with a stop-cock,^ the other, Fig. 89, with shelves. EXPERIMENTS SHOWING THE FLUIDITY OF AIR. 1. The Weight-lifter. The upward pressure of the air, one of the properties of its fluidity, may be exhibited by an apparatus called the weight-lifter, made in different forms, but all on the same principle. The one represented in Fig. 90 * Most sets of philosophical apparatus are furnished with stop-cocks and elastic tubes, for the purpose- of connecting the several parts with the pump or with one another. In selecting the apparatus, it is important to have the screws of the stop-cocks, and of all the apparatus of similar thread, in order that every article may subserve as many purposes as possible. This precaution is suggested by economy as well as by conve- nience. Explain the experiment with the guinea and feather. Also the weight- lifter. . 120 NATURAL PHILOSOPHY. consists of a glass tube, of large bore, set in a strong case or stand. Fig. 90. supported by three legs. A piston is accurately fitted to the bore of the tube, and a hook is attached to the bot- tom of the piston, from which weights are to be suspended. One end of the elastic tube is to be screwed to the plate of the pump, and the other end attached to the top of this instrument. The air being then exhausted from the tube, the weights will be raised the whole length of the glass. The num- ber of pounds weight that can be raised by this instrument may be estimated by multiplying the num-- ber of square inches in the bottom of the piston by fifteen. 2. The Pneumatic Shower-bath. On the principle of the upward pressure of the air, the pneumatic shower-bath is constructed. It consists of a tin vessel perforated with holes in the bottom for the shower, and having an aperture at the top, which is opened or closed at pleasure by means of a spring valve. [Instead of the spring valve, a bent tube may be brought round from the top down the side of the vessel, with an aperture in the tube below the bottom of the vessel which may be covered with the thumb.] On immersing the vessel thus constructed in a pail of water, with the valve open, and the tube (if it have one) on the outside of the pail, the water will fill the vessel. The aperture then being closed with the spring or with the thumb, and the vessel being hfted out of the water, the upward pressure of the air will confine the water in the vessel. On removing the thumb, or opening the valve, the water will descend in a shower until the vessel is emptied. MISCELLANEOUS EXPERIMENTS DEPENDING ON TWO OR MORE OF THE PROPERTIES OF AIR. 1. The Bolt-head and Jar. Fig. 91, a glass globe with a long neck, called a bolt-head, (or any long-necked bottle,) partly filled with water, is inverted in a jar of water, (colored with a few drops of red ink or any coloring matter, in order that Explain the pneumatic shower-bath. PNEUMATICS. the effects may be more distinctly visible,) and placed under the receiver. On exhausting the air in the receiver, the air in the upper part of the bolt- head expanding, expels the water, showing the elasticity of the air. On readmitting the air to the receiver, as it cannot return into the bolt- head, the pressure on the surface of the water in the jar, forces the water into the bolt-head, showing the pressure of the air caused by its weight. The experiment may be repeated with the bolt-head without any water, and on the re- admission of the air, the water will nearly fill the bolt-head, affording an accurate test of the degree of exhaustion. 2. The Transfer of Fluids from one Vessel to another. The experiment may be made with two bottles tightly closed. Let one be partly filled with water, and the two connected by a bent tube, connecting the interior of the empty bottle with the water of the other, and extending nearly to the bottom of the water. On,, exhausting the air from the empty bottle, the water will pass to the other, and on readmitting the air the water will return to its original position, so long as the lov/er end of the bent tube is above the surface. EXPERIMENTS WITH THE SIPHON. 1. Close the shorter end of the siphon with the finger or with a stop-cock, and pour mercury or water into the longer side. The air contained in the shorter side will prevent the liquid from rising in the shorter side." But if the shorter end be opened, so as to afford free passage outwards for the air, the fluid will rise to an equilibrium in both arms of the si- phon. 2. Pour any liquid into the longer arm of the siphon until the shorter arm is filled. Then close the shorter end, to pre- vent the admission of the air ; the siphon may then be turned in any direction and the fluid will not run out, on account of the pressure of the atmosphere against it. But if the shorter end be unstopped, the fluid will run out freely. Explain" the transfer of fluids. Explain the first experiment with the siphon. Also the second. 6 122 NATURAL PHILOSOPHY. AIR ESSENTIAL TO ANIMAL LIFE. If an animal be placed under the I'eceiver, and the air ex- hausted, it will immediately droop, and if the air be not speedily readmitted it will die. AIR ESSENTIAL TO COMBUSTION. Place a lighted taper, cigar, or any other substance that will produce smoke, under the receiver, and exhaust the air ; the light will be extinguished, and the smoke will fall, instead of rising. If the air be readmitted, the smoke will ascend. THE PRESSURE OF THE AIR RETARDS EBULLITION.* 1. Ether, alcohol, and other distilled liquors, or boiling water, placed under the receiver, will appear to boil when the air is exhausted. 2. The existence of many bodies in a liquid form depends on the weight or pressure of the atmosphere upon them. The same force, likewise, prevents the gases which exist in fluid and solid bodies from disengaging themselves. If, by i-ai efy- ing the air, the pressure on these bodies be diminished, they either assume the form of vapors, or else the gas detaches itself altogether from the other body. The following experi- ment proves this : place a quantity of lukewarm water, milk, or^alcohol, under a receiver, and exhaust the air, and the liquid will either pass off in vapor, or will have the appearance of boilmg. 3. An experiment to prove that the pressure of the atmo- sphere preserves some bodies in the liquid form, may thus be performed : fill a long ^^al, or a tube closed at one end, with water, and invert it in a vessel of water. The atmospheric pressure will retain the water in the vial. Then by means of a bent tube introduce a few drops of sulphuric ether, which, by reason of their small specific gravity, will ascend to the top * Ebullition. The operation of boiling. The agitation of liquor by heat, which throws it up into bubbles. What takes place when an animal is placed under an exhausted re- ceiver? Is air essential to combustion ? How is this proved? How is it shown that air prevents ebullition ? Give the 1st, 2d, and 3d experiments. TNEUMATICS. 123 of the vial, expelling an equal bulk of water. Place the whole under the receiver, and exhaust the air, and the ether will be seen to assume the gaseous form, expanding in proportion to the rarefaction of the air under the receiver, so that it gradually expels the water from the vial, and fills up the entire space itself. On readmitting the air, the ether becomes condensed, and the water will reascend into the vial. 4. A simple and interesting experiment connected with the science of chemistry, may thus be performed by means of the air-pump. A watch-glass, containing water, is placed over a small vessel containing sulphuric acid, and put under the bulbed receiver. When the air is exhausted, vapor will freely rise from the water, and be quickly absorbed by the acid. An intense degree of cold is thus produced, and the water will freeze. 5. In the above experiment, if ether be used instead of the acid, the ether will evaporate instead of the water, and in the process of evaporation, depriving the water of its heat, the water will freeze. These two experiments, apparently similar in effects, namely, the freezing of the water, depend upon two different principles which pertain to the science of chemistry. THE PNEUMATIC PARADOX. An interesting experiment, illustrative of the pneumatic paradox, may be thus performed : — Pass a small open tube, (as a piece of quill,) through the centre of a circular card two or three inches in diameter, and cement it, the lower end pass- ing down, and the upper just even with the card. Then pass a pin through the centre of another similar card, and place it on the former with the pin projecting into the tube to prevent the upper card from shding off. It will then be impossible to displace the upper card by blowing through the quill, on ac- count of the adhesion produced by the current passing between the discs. On this principle, smoky chimneys have been rem- edied, and the office of ventilation more effectually performed. Give the 4th experiment to show that air prevents ebulhtion. Also the fifth. Explain the pneumatic paradox. 121 NATURAL PHILOSOPHY. CHAPTER VIII. ACOUSTICS, 152. Acoustics is the science which treats of the nature and laws of sound. It includes the theory of musical concord or harmony. 153. Sound is caused by a tremulous or vibratory motion of the air. 1. If a bell be rung under an exhausted receiver, no sound can be heard from it ; but when the air is admitted to surround the bell, the vibrations immediately produce sound. 2. Again, if the experiment be made by enclosing the bell in a small receiver, full of air, and placing that under another receiver, from which the air can be withdrawn ; though the bell, when struck, must then produce sound, as usual, yet it will not be heard if the outer receiver be well exhausted, and care be taken to prevent the vibrations from being communicated through any solid part of the apparatus ; because there is no medium through which the vibrations of the bell, in the smaller receiver, can be communicated to the ear. 154. Sounds are louder when the air surrounding the sonorous body is dense, than when it is in a rarefied state. For this reason the sound of a bell is louder in cold than in warm weather; and sound of any kind is transmitted to a greater distance in cold, clear weather, than in a warm sultry day. On the tops of mountains, where the air is rare, the hu- man voice can be heard only at the distance of a few rods ; and the firing of a gun produces a sound scarcely louder than the cracking of a whip. 152. What is that science called which treats of the nature and laws of sound? What does it include? 153. What causes sound? What illustrations are given to prove this? 154. In what proportion are sounds loud or faint ? Why does a bell sound louder in cold than in warm weather? Why is sound fainter on the top of a mountain than near the surface of the earth ? ACOUSTICS. 125 15r>. Sonorous bodies are those which produce clear, distinct, regular, and durable sounds, ^uch as a bell, a drum, wind instruments, musical strings and glasses. These vibrations can be communicated to a distance not only through the air, but also through liquids and solid bodies. 156. Bodies owe their sonorous property to their elasticity.* 157. The sound produced by a musical string is caused by its vibrations ; and the height or depth of the tone depends upon the rapidity of these vibrations. Long strings vibrate with less rapidity than short ones, and for this reason the low tones in a musical instru- ment proceed from the long strings, and the high tones from the short ones. 1. Fig. 92, AB represents a musical string. If it be drawn up to G, its eks- ^. , Fig. 9a ticity will not on- ^ ly carry it back again, but will "''.V'-'^-C iTr'^'^'^^'V*^^., give it a momen- ^ turn which will -d Z^-'^^^^^^ carry it to H, """^--I'^^^ F "^l"""'^' from whence it -^j ""' will successively return to T, F, C, D, &c., until the resistance of the air entirely destroys its motion. 2. the vibrations of a sonorous body give a tremulous mo- tion to the air around it, similar to the motion communicated tC' smooth water when a stone is thrown into it. * Although it is undoubtedly the case that all sonorous bodies are elas- tic, it is not to be inferred that all elastic bodies are sonorous. 155. V^hat are sonorous bodies? 156. To what do sonorous bodies owe their sonorous property? Are all elastic bodies sonorous ? 157. What causes the sound produced by a musical string? Upon what does the height and depth of the tone depend ? Which strings, in a mu^ Bical instrument, produce the low tones? Why? Explain Fig. 92. 126 NATURAL PHILOSOPHY. 158. The science of harmony is founded on the rela- tion which the vibrations of sonorous bodies have to each other. Thus, when the vibrations of one string are double those of another, the chord of an octave is produced. If the vibrations of two strings be as two to three, the chord of a fifth is pro- duced.^ When the vibrations of two strings frequently coin- cide, they produce a musical chord ; and when the coincidence of the vibrations is unfrequent, discord is produced. 159. The quality of the sound produced by strings depends upon their length, thickness, weight, and degree of tension. The quality of the sound produced by wind instruments depends upon their size, their length, and their internal diameter. Long and large strings, when loose, produce the lowest tones ;^but different tones maybe produced from the same string, according to the degree of tension. Large wind in- struments, also, produce the lowest tones ; but different tones may be produced from the same instrument, according to the distance of the aperture for the escape of the wind, from the aperture where it enters. 160. The quality of the sound of all musical instru- ments is affected by the changes in the temperature and specific gravity of the atmosphere. As heat expands and cold contracts the materials of which * When music is made by the use of strings, the air is struck by the body, and the sound is caused by the vibrations ; when it is made by pipes, the body is struck by the air ; but as action and reaction are equal, the effect is the same in both cases. 158. Upon what is the science of harmony founded ? How is the chord of an octave produced? How is the chord of a fifth produced? How is a musical chord produced ? A discord ? 159. Upon what does the quality of the sound produced by strings de- pend? Upon what does that produced by wind instruments depend? What strings produce the lowest tones I How may different tones be pro- duced from the same string ? How may different tones be produced from the same wind instrument ? 160. What, in som.e degree, affects the quality of the sound of all musi- cal instruments? What effect have heat and cold on the materials of which the instrument Is made ? What follows from this ? ACOUSTICS. 127 the instrument is made, it follows, that the strings will have a greater degree of tension, and that pipes and other wind in- struments Avill be contracted, or shortened, in cold weather. For this reason, most musical instruments are higher m tone (or sharper) in cold weather, and lower in tone (or more fiat) ill warm weather. IGl. Sound is communicated more rapidly and with greater power through solid bodies, than through the air, or fluids. It is conducted by water about four times quicker than by air, and by solids about twice as rapidly as by water. 1. If a person lay his head on a long piece of tinaber, he can hear the scratch of a pin at the other end, while it could not be heard through the air. 2. If the ear be placed against a long, dry, brick wall, and a person strike it once with a hammer, the sound will be heard twice, because the wall will convey it with greater rapidity than the air, though each will bring it to the ear. 162. The Stethescope is an instrument depending on the power of solid bodies to convey sound. It consists of a wooden cylinder, one end of which is applied firmly to the breast, while the other end is brought to the ear. By this means the action of the lungs may be distinctly heard. The instrument, therefore, becomes useful in the hands of a skilful physician, to ascertain the state of those organs. 163. Sound, passing through the air, moves at the rate of 1142 feet in a second of time. This is the case with all kinds of sound. 1. The softest whisper flies as fast as the loudest thunder, Why are most musical instruments higher in tone, or sharper, in cold weather ? 161. Through which is sound communicated more rapidly, and with irreater power, through solid bodies, or the air? How fast is it conducted by water? How fast by solids? What examples are given to show that sound is communicated more rapidly through solid bodies than the air or fluids? 162. What is a stethescope? Of what does it consist? For what is it used ? 163. How fast does sound move? Does the force or direction of the v[ nature.* 3. Yentriloquismf is the art of speaking in such a manner as to cause the voice to appear to proceed from a distance. * The reader is referred to Dr. Rush's very valuable work oii the Philosophy of the Huruaii Voice," for plain and jjraetieiii instructions on this Buhject. Dr. Barber's " Grammar of Eloculioji, ' and Purkor's " Pro- gressive Exercises iji Rlielorical j{<'-adinrf," likewise contain the same in- structions in a practical form. To thfj work of Dr. Rush, both of the latter-mentioned works arc jjjrcr. iy ind-'-bl/ d. t The word ventj-iloqiiism lit-'-r-; I i; ijir ^ms, speakinrr from the helhj,'' and it is so defined in Chamljt-rs' Dictionary of Arts and Sciences. The ventriiofjuist, by a sinj^uiar Kvnio/nuuf'nt of tfie voice, seems to have it ia his power " to throuj his voice'' in any direction, so that the sound shall ap- pear to proceed from that spot. The words are pronounced by the or^rans usually employed for that purpose, but in such a manner as to f^ive httle or no motion to the lips, the organs chiefly concerned bein^ tho.:e of the throat and tongue. The variety of sounds which the human voice is ca- pable of thus producing is altogether beyond common belief, and, indeed, is truly surprising. Adepts in this art will mimic the voices of all ages and conditions of human life, from the smallest infant to the tremulous voice of tottering age, and from the intoxicated foreign beggar to the high-bred, artificial tones of the fashionable lady. Some will also imitate the warbling of the nightingale, the loud tones of the whip-poor-will, and the scream of the peacock, with equal truth and facility. Nor are these arts confined to professed imitators ; for in many villages boys may be found, who are in the habit of imitating the brawling and spitting of cats, in such a manner as to deceive almost every hearer. The human voice is also capable of imitating almost every inanimate sound. Thus, the turning and occasional creaking of a grindstone, with the rush of the water, the sawing of wood, the trundling and creaking of a wheelbarrow, the drawing out of bottle-corks, and the gurgling of the flowing liquor, the sound of air rushing through a crevice on a wintry How are the tones varied and regulated? Upon what does tlie manage- ment of the voice depend ? What is ventriloquism ? 132 NATURAL PHILOSOPHY. 4. The art of ventriloquism was not unknown to the an- cients ; and it is supposed by some authors that the famous responses of the oracles at Delphi, at Ephesus, &c., were de- livered by persons who possessed this faculty. There is no doubt that many apparently wonderful pieces of deception, which, in the days of superstition and ignorance, were con- sidered as little short of miracles, were performed by means of ventriloquism. Thus houses have been made to appear haunted, voices have been heard from tombs, and the dead have been made to appear to speak, to the great dismay of the neighborhood, by means of this wonderful art. 5. Ventriloquism is, without doubt, in great measure the gift of nature ; but many persons can, with a httle practice, utter sounds and pronounce words without opening the lips or moving the muscles of the face ; and this appears to be the great secret of the art. CHAPTER IX. PYRONOMICS, OR THE LAV^S OF HEAT. 168. Pyronomics is the science which treats of the laws, the properties, and operations of heat.^ 1. The nature of heat is unknown; but it has been proved that the addition of heat to any substance produces no per- ceptible alteration in the weight of that substance. Hence it is inferred that heat is imponderable. 2. Heat pervades all bodies, insinuating itself, more or less, night, and a great variety of other noises of the same kind, are imitated by the voice so exactly, as to deceive any hearer vvho does not know whence they proceed. * Heat is undoubtedly a positive substance or quality. Cold is merely negative, being only the absence of heat Was this art knov^^n to the ancients ? What is supposed, by some au- thors, concerning the responses at Delphi, Ephesus, &c. ? Is ventriloquism a natural gift, or an acquired one ? 168. What is Pyronomics? What is said in regard to the nature of heat? Is it ponderable or imponderable ? PYRONOMICS. bet^^-een their particles, and forcing them asunder. Heat, and the attraction of cohesion, constantly act in opposition to each otht.r ; hence the more a body is heated, the more its particles will be separated. 3, The effect of heat in separating the particles of different kinds of substances is seen in the melting of solids, such as metals, wax, butter, &c. The heat insinuates itself between the particles, and forces them asunder. These particles then are removed from that degree of proximity to each other with- in which cohesive attraction exists, and the body is reduced to a fluid form. When the heat is removed the bodies return to their former solid state.* 4 . Heat passes through some bodies with more difficulty * Of all the effects of heat, that produced upon water is, perhaps, the mosl remarkable. The particles are totally separated and converted into steam or vapor, and their extension is wonderfully increased. The steam which arises from boiling water is nothing more than portions of the water heated. The heat insinuates itself between the particles of the water, and forces them asunder. When deprived of the heat, the particles will unite in th.e form of drops of water. This fact can be seen by holding a cold plate over boiling water. The steam rising from the water will be con- densed into drops on the bottom of the plate. The air which we breathe generally contains a considerable portion of moisture. On a cold day, this moisture condenses on the glass in the windows, and becomes visible. We see it also collected into drops on the outside of a tumbler or other vessel containing cold water in warm weather. Heat also produces most re- markable effects upon air, causing it to expand to a wonderful extent, while the absence of heat causes it to shrink or contract into very small dimensions. The attraction of cohesion causes the small watery particles which compose mist or vapor to unite together in the form of drops of water. It is thus that rain is produced. The clouds consist of mist or vapor expanded by heat. They rise to the cold regions of the skies, where the particles of vapor lose their heat, and then, uniting in drops, fall to the earth. But so long as they retain their heat, the attraction of cohesion can have no influence upon them, and they will continue to exist in the form of steam, vapor, or mist. W^hat effect has heat upon bodies? What two forces continually act in opposition to each other? In what can the effect of heat be seen? How does it separate the particles? What would be the effect were the heat rem(ved ? Upon what has heat the most remarkable effect ? How does it affect it ? What effect has heat upon air ? How is rain produced ? What is stated with regard to heat ? 134 NATURAL PHILOSOPHY. than tlirougli others ; but there is no kmd of matter which can compietely arrest its progress.^ 169. The principal efFects of heat are three, namely : ist. Heat expands most substances. •2d. It converts them from a sohd to a fluid state. 3d It destroys their texture by combustion.f * The thermometer, an iiistrmiient designed to measure degrees of heat, has already been d.escribed, in connexion with the barometer, under the head of Pneumatics. Heat, under the name of caloric, is properly a sub- ject of consideration in the science of Chemistr}^ It exists in two states, called, respectively, free heat and latent heat. Free heat, or free caloric, is that which is perceptible to the senses, as the heat of a fire, the heat of the sun, &c. Latent heat is that which exists in most kinds of substances, but is not perceptible to the senses, until it is brought out by mechanical or chemical action. Thus, when a piece of cold iron is hammered upon an anvil, it becomes intensely heated ; and when a small portion of sul- phuric acid, or vitriol, is poured into a vial of cold water, the vial and the hquid immediately become hot. A further illustration of the existence of latent or concealed heat is given at the fireside every day. A portion of cold fuel is placed upon the grate or hearth, and a spark is appUed to kindle the fire which warms us. It is evident that the heat given out by the fuel, when ignited, does not all proceed from the spark, nor can we perceive it in the fuel : it must, therefore, have existed somewhere in a latent state. It is, however, the effects of free heat, or free caloric, which are embraced in the science of Pyronomics. The subject of latent heat belongs more properly to the science of Chemistry-. The terms heat and cold, as they are generally used, are merely rela- tive terms : for a substance which in one person would excite the sensation of heat, might, at the same time, seem cold to another. Thus, also, to the same individual, the same thing may be made to appear, relatively, both warm and cold. If, for instance, a person were to hold one hand near to a warm fire, and the other on a cold stone, or marble slab, and then plunge both into a basin of lukewarm water, the liquid would appear cold to the vrarm hand and warm to the cold one. t These eiiects do not take place ia all substances. Some substances Can the progress of heat be arrested ? What is caloric ? In what two states does heat exist ? What is free heat ? Give some examples of free heat. What is latent heat ? Give some examples of latent heat. How are the terms heat and cold generally used ■ What illustration of this is given ? 1G9. What are the three principal effects of heat on bodies to which it is applied? Give an example of each effect. PYRONOMICS. 135 170. Heat tends to diffuse itself equally through all substances. 1. If a heated body be placed near a cold one, the tempera- ture of the former will be lowered, while that of the latter will be raised. 2. All substances contain a certain quantity of heat ; but, on account of its tendency to diffuse itself equally, and the difference in the power of different substances, to conduct it, bodies of the same absolute temperature appear to possess different degrees of heat. 3. Thus, if the hand be successively applied to a woollen garment, a mahogany table, and a marble slab, all of which have stood for some time in the same l Oom, the woollen gar- ment will appear the warmest, and the marble slab the coldest of the three articles ; but if a thermometer be applied to each, ao difference in the temperature will be observed. 4. From this it appears, that some substances conduct heat readily, and others with great difficulty. The reason that the marble slab seems the coldest, is, that marble, being a good conductor of heat, receives the hccat from the hand so readily that the loss is instantly felt by the hand ; w^hile the woollen garment, being a bad conductor of heat, receives the heat from the hand so slowly that the loss is imperceptible. 171. The different power of receiving and conducting heat, possessed by different substances, is the cause of the difference in the warmth of various substances used for clothing. y,re incombustible ; others cannot be transformed to a fluid state by any degree of heat yet produced artificially. The expansive effect of heat has but one known exception. The sources from which heat is derived are — 1st. From the sun in connexion with light ; 2dly. From mechanical operations, such as friction, percussion, and compression ; 3dly. From chemical operations, especially combustion ; 4thly. From hving animals and vegetables. What are the sources of heat ? 170. In what way does heat tend to diffuse itself? Why do bodies of the same absolute temperature appear to possess different degrees of heat ? Wliat illustration of this is given ? What appears from this? 171. What causes the difference in the warmth of substances used for clothing? NATURAL PHILOSOPHY. 1. Thus, woollen garments are warm garments, because they part slowlv with the heat which they acquire from the body, and, consequently, they do not readily convey the warmth of the body to the air ; while, on the contrary, a linen garment is a cool one, because it parts with its heat readily, and as readi- ly receives fresh heat from the body. It is, therefore, con- stantly receiving heat from the body and throwing it out into the air, while the woollen garment retains the heat which it receives, and thus encases the body with a warm covering. 2. For a similar reason ice, in summer, is wrapped in woollen cloths. It is then protected from the heat of the air, and will not melt. 172. Heat is propagated in two ways, namely, by conduction and by radiation. Heat is propagated by conduction when it passes from one substance to another in contact with it. Heat is propagated by radiation when it passes through the air or any other elastic fluid. 173. Different bodies conduct heat with different de- grees of facility. The metals are the best conductors, and among metals silver is the best conductor. 1. For this reason any hquid may be heated in a silver vessel more readily than in any other of the same thickness. The metals stand in the following order, with respect to their con- ducting power ; namely, silver, gold, tin, copper, platina, steel, iron, and lead. 2. It is on account of the conducting power of metals^ that * Metals, on account of their conducting power, cannot be handled when raised to a temperature above 120 degrees of Fahrenheit. Water becomes scalding hot at 150 degrees, but air, heated far beyond the tem- perature of boiling water, may be applied to the skin without much pain. Sir Joseph Banks, with several other gentlemen, remained some time in a room when the heat was 52 degrees above the boiling point ; but, though they could bear the contact of the heated air, they could not touch any metallic substance, as their watch-chains, money, &c. Eggs, placed on 172. In what two ways is heat propagated? When is it propagated by conduction? When is it propagated by radiation? 173. Do all bodies conduct heat with the same degree of facility ? What bodies are the best conductors? In what order do the metals stand with respect to their conducting power'? I'YRONOMICS. 137 the handles of metal tea-pots and coffee-pots are commonly made of wood ; since, if they were made of metal, they would become too hot to be grasped by the hand, soon after the vessel is filled with heated fluid. Wood conducts heat very imper- fectly. For this reason wooden spoons and forks are preferred for ice. Indeed, so imperfect a conductor of heat is wood, that a stick of wood may be grasped by the hand while one end of the stick is a burning coal. Animal and vegetable substances, of a loose texture, such as fur, wool, cotton, &c., conduct heat very imperfectly ; hence their efficacy in preserving the warmth of the body. 174. Heat is reflected from bright surfaces; while black or dark colored bodies absorb the heat that falls on them. 1. This is the reason why the bright brass andirons, or any other bright substances, placed near a hot fire, seldom become heated ; while other dark substances, further removed from the fire, become too hot for the hand. 2. Snow or ice will melt under a piece of black cloth, when it will remain perfectly sohd under a white one. The farmers in some of the mountainous parts of Europe, are accustomed to spread black earth, or soot, over the snow, in the spring, to hasten its melting, and enable them to commence ploughing early. 175. All bodies, when violently compressed or ex- tended, become warm. a till frame, were roasted hard in twenty minutes ; and a beef-steak was overdone in thirty-three minutes. Chantrey, the celebrated sculptor, had an oven which he used for drying his piaster cuts and moulds. The thermometer generally stood at 300 de- grees in it, yet the workmen entered, and remained in it some minutes without difficulty ; but a gentleman once entering it with a pair of silver- mounted spectacles on, had his face burnt where the metal came in con- tact with the skin. Is wood a good conductor of heat ? Why are wool, fur, &c., so effi- cacious in preserving the warmth of the body? What is related in the note with regard to the conducting power of heat? 174. What bod.es reflect the heat? What bodies absorb the heat"? Why do bright bodies, when placed near the fire, seldom become heated? Will snow melt most readily under white or black cloth ? 175. What effect is produced on all bodies when violently compressed or extended ? 138 NATURAL PHILOSOPHY. 1. If a piece of india-rubber be quickly stretched and ap- plied to the lip, a sensible degree of heat will be felt. An iron bar, on being hammered, becomes red-hot ; and even water, when strongly compressed, gives out heat. 2. When air is forcibly compressed by driving down the piston of a syringe, nearly closed at the end, great heat is pro- duced. Syringes have been constructed on this principle for procuring fire, the heat, thus produced, being sufficient to kindle dry tinder. 176. All substances, as they are affected by heat, may be divided into combustible and incombustible bodies.* 177. The pyrometerf is an instrument to show the expansion of bodies by the application of heat. It consists of a metallic bar or wire, with an index connected with one extremity. On the application of heat the bar ex- pands and tui-ns the index to show the degree of expansion. 178. The most obvious and direct effect of heat on a body, is to increase its extension in all directions. 1. Coopers, wheelwrights, and other artificers, avail them- selves of this property in fixing iron hoops on casks, and the tires or irons on wheels. The hoop or tire having been heated, expands, and being adapted in that state to the cask or the wheel, as the metal contracts in cooling, it clasps the parts, very firmly together.]; * Vegetable substances, charcoal, oils, most animal substances, as hair, wool, horn, fat, and all metallic bodies, are combustible. Stones, glass, salts, &LC., are incombustible. t Wedge wood's pyrometer, the instrument commonly used for high temperatures, measures heat by the contraction of clay. t From what has been stated above, it will be seen, that an allowance should be made for the alteration of the dimensions in metallic beams or "What experiments are here related to iliustrdte this ? What is said oi the air when strongly compressed 1 176. Into what classes are all substances, as affected by heat, divided ? What substances are combustible ? What substances are incombustible ? 177. What is a pyrometer? Of what does it consist ? How does Wedge- wood's pyrometer measure high temperatures? 178. What is the most obvious and direct effect of heat on a body What application of this principle is related in the note ? PYRONOMICS. 130 2. The effect of heat and cold,^ in the expansion and con- traction of glass, is an object of common observation; for it is tins expansion and contraction which cause so many accideats wilh glass articles. Thus, when hot water is suddenly poured inlo a cold glass, of any form, the glass, if it have any thick- ness, will crack ; and, on the contrary, if cold water be pou: td into a heated glass vessel, the same effect will be produced. The reason of \vhich is this : heat makes its way but slowly through glass ; the inner surface, therefore, when the hot water"^ is poured into it, becomes heated, and, of course, dis- tended before the outer surface, and the irregular expansion causes the vessel to break. There is less danger of fracture, therefore, when the glass is thin, because the heat readily penetrates it, and there is no irregular expansion.j supporters, caused by the dilatation and contraction effected by the weather. In the iron arches of Southwark bridge, over the Thames, the variation of the temperature of the air causes a difference of height, at different times, amounting to nearly an inch. A happy apphcation of this principle to the mechanic arts was made, some years ago, at Paris. The weight of the roof of a building, in the Conservatory of Arts and Trades, had pressed outwards the side walls of the structure, and endangered its security. The following method was adopted to restore the perpendicular direction of the structure. Several apertures were made in the walls, opposite to each other, through which iron bars were introduced, which, stretching across the building, extended beyond the outside of the walls. These bars ter- minated in screws, at each end, to which large broad nuts were attached. Each alternate bar was then heated by means of powerful lamps, and their lengths being thus increased, the nuts on the outside of the building were screwed up close to it, and the bars were suffered to cool. The pow- erful contraction of the bars drew the walls of the building closer together, and the same process being repeated on all the bars, the walls were grad- ually and steadily restored to their upright position. * Cold is merely the absence of heat ; or rather, more properly speak- ing, inferior degrees of heat are termed cold. t The glass chimneys, used for oil and gas burners, are often broken by being suddenly placed, when cold, over a hot flame. The danger of frac- ture may be prevented (it is said) by making a minute notch on the bot- tom of the tube with a diamond. This precaution has been used in an What is said of the effect of heat and cold on glass? When hot water is suddenly poured into a cold glass, why will the glass crack? When cold water is applied to a heated glass, why will the glass crack? 140 NATURAL PHILOSOPHY. 179. The expansion caused by heat in soHd and liquid bodies differs in different substances ; but aeri- form fluids all expand alike, and undergo uniform de- grees of expansion at various temperatures. The expansion of solid bodies depends, in some degree, on the cohesion of their particles ; but as gases and vapors are destitute of cohesion, heat operates on them without any op- posing power. 180. The density of all substances is augmented by cold, and diminished by heat. There is a remarkable exception to this remark, and that is in the case of water ; which, instead of contracting, expands at the freezing point, or when it is frozen. This is the reason why pitchers, and other vessels, containing water and other similar fluids, are so often broken when the hquid freezes in them. For the same reason, ice floats^ instead of sinking in water ; for as its density is diminished, its specific gravity is conse- quently diminished. 181. Different bodies require different quantities of heat to raise them to the same temperature ; and those establishment where six lamps were lighted every day, and not a single glass has been broken in nine years. * Were it not for this remarkable property of water, large ponds and lakes, exposed to intense cold, would become solid masses of ice ; for if the ice, when formed on the surface, were more dense (that is, more heavy) than the water below, it would sink to the bottom, and the water above, freezing in its turn, would also sink, until the whole body of the water would be frozen. The consequence would be the total destruction of all crea- tures in the water. But the specific gravity of ice causes it to continue on the surface, protecting the water below from congelation. 179. Is the expansion caused by heat in solid and liquid bodies the same in all substances ? How do aeriform fluids differ, in this respect, from solid and liquid bodies ? Upon what does the expansion of solid bodies in some degree depend? Why has heat more power over gases and vapors? 180. What effect has heat and cold upon the density of all substances? What exception is there to this remark ? Why are the vessels, contain- ing water and other similar fluids, so often broken when the liquid freezes in them? Why does ice float upon the water, instead of sinking in it? What is stated in the note with regard to this property of v/ater ? I'VKOXOMICS which iwc hiMl'Ml with mosl (hllicuhy rcUuii their heat the loiii^est. Thus oil hei'onios heated more s])ee(lily tlian water, and it hkewise cools more (juickly. 18\2. When heat is thrown upon a bright or polished surface it is reflected,* and the angle of reflection will he eipial to the angle of incidence. 18:^. When a certain degree of heat is applied to water it converts it into steam or vapor. 184. The temperature of" steam is always the same with that of the liquid from which it is formed, while it remains in contact with that liquid. When closely con- fined, its elastic power is often sufficient to burst the vessel in which it is confined. 185. The elastic force of steam is increased by heat, and diminished by cold. The amount of pressure, therefore, which it will exert depends on the tempera- ture at which it is formed. 186. The great and peculiar property of steam, on which its mechanical agencies d^epend, is its power of * Advantage has been taken of this property of heat in the construction of a simple apparatus for baking. It is a bright tin case, having a cover incHned towards the fire in such a manner as to reflect the heat down- wards. In this manner use is made both of the direct heat of the fire, and the reflected heat, which would otherwise pass into the room. The whole apparatus, thus connected with the culinary department, is called, in New England, " The Connecticut baker" 181. Can all bodies be raised to the same temperature by the same quantities of heat? What bodies retain their heat the longest? 182. What becomes of the heat which is thrown upon a bright or pol- ished surface ? How do the angles of incidence and reflection compare with each other? 183. When is water converted into steam or vapor ? 184. How does the temperature of the stearn compare with that of the liquid from which it is formed while it remains in contact with that liquid? 185. By what is the elasticity of steam increased and diminished ? Upon what does the amount of pressure, which steam exerts, depend ? 186. What is the great and peculiar property of steam, on which its mechanical agencies depend? 142 NATURAL PHILOSOPHY. exerting a high degree of elastic force, and losing it in- stantaneously. 187. The steam-engine is a machine moved by the expansive force of steam.* 188. Steam occupies a space about 1700 times larger than it will when converted into water. If, therefore, the steam in a cylinder be suddenly converted into water, it will occupy a much smaller space, and pro- duce a vacuum in the cylinder. 1. The mode in which steam is made to act, is by causing its expansive force to raise a solid piston accurately fitted to the bore of a cylinder, like that in the forcing-pump. 2. The piston-rod rises by the impulse of expanding steam, admitted into the cylinder below. When the piston is thus raised, if the steam below it be suddenly condensed, or with- drawn from under it, a vacuum will be formed, and the pres- sure of the atmosphere on the piston above will drive it down. The admission of more steam below it Avill raise it again, and thus a continued motion of the piston, up and down, will be produced. This motion of the piston is communicated to wheels, levers, and other machinery, in such a manner as to produce the effect intended.f 3. The celebrated Mr. James Watt introduced two important improvements into the steam-engine. Obser\ang that the cooling of the cylinder by the water thrown into it to condense, the st'eam, lessened the expansibility of the steani ; he con- trived a method to withdraw the steam from the principal cyl- inder, after it had performed its office, into a condensing- * Steam, as it issues into the air, is visible, and resembles smoke in itt appearance, because the coldness of the air instantly condenses it intc minute watery globules ; but while performing its office, it is perfectly dry, that is, it contains no watery particles, but is expanded into so rare a state as to be absolutely invisible. t This is the mode in which the engine of Newcomen and Savery, commonly called the atmospheric engine, was constructed. 187. What is the steam-engine? 188. How much larger space does steam occupy than water? By what mode is steam made to act ? By what impulse does the piston rise? What causes the piston to descend? AVhat improvement did Mr Watt introduce into the steam-engine ? rVRONOMICS. 143 cliamber, where it is reconverted into water, and conveyed back to the boiler. 4. The otlicr improvement consists in substituting the ex- pansive power of steam for the atmospheric pressure. This was performed by admitting the steam into the cyhnder ahove the raised piston, at the same moment that it is removed from heloio it ; and thus the power of steam is exerted in the descend- ing as well as in the ascending stroke of the piston ; and a much greater impetus is given to the machinery than by the former method. From the double action of the steam ahove, as well as below the piston, and from the condensation of the steam, after it has performed its office, this engine is called Watt's double-acting condensing steam-engine. 5. Fig. 93 represents that portion of the steam-engine in which steam is made to act, and propel such machinery as may be connected with it. The principal parts are the boiler, the cylinder and its piston, the condenser, the air- pump, the steam-pipe, the eduction-pipe, and the cistern. In this figure, A represents the boiler, C tht cylinder, with H the piston, B the steam- pipe, with two branches^ communicating with the cylinder, the one above and the other below the piston. This pipe has two valves, F and G, which are opened and closed alternately by machinery connected with the piston. The steam is carried through this pipe by the valves, w^hen open, to the cylinder both above and below the piston. K is the eduction-pipe, having two branches, like the steam-pipe, furnished with valves, &c., which are opened and shut by the same machinery. By the eduction-pipe the steam is led off from the cylinder as the piston ascends and descends. * The steam and the eduction pipes are sometimes made in forms dif- fering from those in the figure, and they differ much in different engines. What does Fig. 93 represent? What are the principal parts? What does A represent? What does C represent? What does B represent? What does K represent? By what is the steam led ofTfrom the cylinder? Fig. 93. X J 144 NATURAL PHILOSOPHY. L is the condenser, and O a stop-cock for the admission of cold water. M is the air-pump. N is the cistern of cold water in which the condenser is immersed. R is the safety-valve. When the valves are all open, the steam issues freely from the boiler, and circulates through all the parts of the machine, ex- pelling the air.* Now, the valves F and Q, being closed,^ and G and P remaining open, the steam presses upon the piston - and forces it down. As it descends, it draws with it the end of the working-beam, which is attached to the piston-rod J, (but which is not represented in the figure.) To this working- beam, (which is a lever of the first kind,) bars or rods are at- tached, which, rising and falling with the beam and the piston, open the stop-cock 0, admitting a stream of cold water, which meets the steam from the cylinder and condenses it, leaving no force below the piston to oppose its descent. At this moment the rods attached to the working-beam close the stop-cocks G and P, and open F and Q. The steam then flows in below the piston, and rushes from above it into the condenser, by which means the piston is forced up again with the same power as that with which it descended. Thus the steam-cocks G and P and F and Q are ahernately opened and closed ; the steam passing from the boiler drives the piston ahernately upwards and downwards, and thus produces a regular and continued motion. This motion of the piston, being communicated to the working-beam, is extended to other machinery, and thus an engine of great power is obtained. The air-pump M, the rod of which is connected with the working-beam, carries the water from the condenser back into the boiler, by a communication represented in Fig. 94. * This process is called blowing out, and is heard when a steamboat is about starting. What does L represent ? What does O represent ? What does M rep resent? What does N represent? What does R represent? When the valves are all open, what becomes of the steam ? When the valves F and Q are closed, and G and P open, upon what does the steam press? What does the cylinder draw with it in its descent? Which of the mechanical powers is this working-beam ? What are attached to this working-beam ? What is their use? What becomes of the steam when the stop-cocks G and P are closed, and F and Q are open? How is the regular and con- tinued motion produced? To what is this motion of the piston communi- cated ] What IS the use of the air-pump M ? For what is the safety- valve R used ? PYRONOMICS. The safety-valve R, connected with a lever of the second ^ind, is made to open v^hen the pressuie of the steam within the boiler is too great. The steam then rushing through the aperture under the valve, removes the danger of the bursting of the boiler. 189. The power of a steam-engine is generally ex- pressed by the power of a horse, which can raise 33,000 lbs. to the height of .one foot in a minute. An engine of 100 horse power, is one that will raise 3,300,000 lbs. to the height of one foot in one minute. 190. The steam-engine* is constructed in various forms ; the principal of which are the high and the low pressure engines ; or, as they are sometimes called, the non-condensing and the condensing engines. 1. The non-condensing or high-pressure engines differ from the low pressure or condensing engines in having no condenser. The steam, after having moved the piston, is let off into the open air. As this kind of engine occupies less space, and is much less compUcated, it is generally used on railroads. 2. In the low pressure or condensing engines, the steam, after having moved the piston, is condensed, or converted into water, and then conducted back into the boiler. * The steam-engine, as it is constructed at the present day, is the re- sult of the inventions and discoveries of a number of distinofuished indi- viduals, at different periods. Among those who have contributed to its present state of perfection, and its application to practical purposes, may be mentioned the names of Somerset, the Marquis of Worcester, Savery, Newcomen, Fulton, and especially Mr. James Watt. To the inventive genius of Watt, tiie engine is indebted for the con- denser ^ the appendages for parallel motion, tiie application of the governor, and for the double action. In the words of Mr. Jeffrey it may be added, that, " by his admirable contrivances, and those of Mr. Fulton, it has be- come a thing alike stupendous for its force and its flexibility ; for the pro- 189. How is the power of a steam-engine expressed? What is an en- gine of 100 horse power I 190. What are the principal forms in which the steam-engine is con- structed? How do they diffei from each other? What becomes of the steam after having moved the piston in the non-condensing engines ? What kind of engines is generally used on railroads? What becomes of the steam after having moved the piston in the condensing engines? 7 146 NATURAL PHILOSOPHY. watt's DOUBLE' acting CONDENSING STEAM-ENGINE. Fig. 94. 3. Fig. 94 represents Watt's double-acting condensing steam- engine, in which A represents the boiler, containing a large quantity of water, which is constantly replaced as fast as por- tions are converted into steam. B is the steam-pipe, convey- ing the steam to the cylinder, having a steam-cock h to admit or exclude the steam at pleasure. C is the cyhnder, surrounded by the jacket cc, a space kept constantly supplied with hot steam, in order to keep the cylinder from being cooled by the external air. D is the digious power it can exert, and the ease and precision and ductility with which it can be varied, distributed, and applied. The trunk of an ele- phant, that can pick up a pin, or rend an oak, is as nothing to it. It can engrave a seal, and crush masses of obdurate metal before it : draw out, without breaking, a thread as fine as gossamer, and lift up a ship of war like a bauble in the air. It can embroider muslin, and forge anchors ; cut steel into ribands, and impel loaded vessels against the fury of the winds and waves," rV llONOiMlCS. 117 0(lucti()n-pij)(\ coniiniinicatiiiL;- belwccii tlic. cylinder nrul tlio coiuloiiscr. K is (lie condenser, with a valve c, calked tlie injection-cock, admitting ii jet of cold water, which meets the si earn the instant that the steam enters the condenser. F is tl\e air-pump, which is a common suction-pump, but is heie called tlie air-pump because it removes from the condenser not only the water, but also the air, and the steam that es(^apcs condensation. G G is a cold-water cistern, which surrounds the condenser, and supplies it with cold water, being filled, by the cold- water pump, which is represented by H. I is the hot well, containing water from the condenser. K is the hot- water pump, which conveys back the water of condensation from the hot well to the boiler. L L are levers, which open and shut the valves in the chan- nel between the steam-pipe, cylinder, eduction-pipe, and con- denser ; which levers ai-e raised or depressed by projections attached to the piston-rod of the pump. M M is an apparatus for changing the circular motion of the working-beam into parallel motion, so that the piston-rods are made to move in a straight line. N N is the working-beam, which being moved by the rising and falhng of the piston, attached to one end, communicates motion to the fly-wheel by means of the crank P, and from the fly-wheel the motion is communicated by bands, wheels, or levers, to the other parts of the machinery. 0 O is the governor. The governor being connected with the fly-wheel, is made to participate the common motion of the engine, and the balls will remain at a constant distance from the perpendicular shaft, so long as the motion of the engine is uniform ; but whenever the engine moves faster than usual, the balls will re- cede farther from the shaft, and, by raising a valve connected with the boiler, will let ofl^ such a portion of the force as to reduce the speed to the rate required. The steam-engine, thus constructed, is apphed to boats to turn wheels having paddles attached to their circumference, which answer the purpose of oars. It is used also in work- What does Fig. 94 represent ? What does A represent ? What does B represent? What does G represent? What does D represent? What does E represent? What does F represent? What does G G represent? What does I represent? What does K represent? What does LL rep- resent? What does M M represent? What does N N represent? What does O O represent ? What is said of the governor ? 148 N A T I ' K A L r I i i L O .< O P 11 Y . sliops, factories, s(Mits the lavs ftoin an ()])j('C'l, ^/ r, ctilci-iiifr an aj)iMtui-(\ 'Vhc v:i\ iVoin (f passes down throuj^"h the ajx'ilure to (/, and the ra\' iVoin j.,^,, k,;^. (' passos up to i), aiul thus these rays, cioss- ing- at tlio apertuiv, I'oiin an inverted iinai^(; on the wall. The room in which this vx- j)oriniont is made should be darkened, and no lio-ht permitted to enter, excepting through the aperture. It then becomes a camera obscura."^ 213. The angle of vision is the angle formed at the eye by two lines drawn from opposite parts of an ob- ject. 1. The angle C, in ^^s- 104. a Fig. 104, represents the angle of vision. The line A C proceed- ing from one extremi- ty of the object meets the line B C proceeding b from the opposite ex- tremity, and forms an angle C at the eye; — this is the angle of vision. 2. Fig. 105 represents the different angles, made by the same object, at different distances. From an inspection of the * These words signify a darkened chamber. In the future description which will be given of the eye, it will be seen that the camera obscura is constructed on the same principle as the eye. If a convex lens be placed in the aperture, an inverted picture, not only of a single object, but of the entire landscape, will be found on the wall. A portable camera obscura is made by admitting the light, into a box of any size, through a convex lens, which throws the image upon an inclined mirror, from whence it is reflected upwards to a plate of ground glass. In this manner a beautiful but diminished image of the landscape, or of any group of objects, is pre- sented on the plate in an erect position. 212. Wbat kind of an image is formed when rays of light, proceeding from an obiect, enter a small aperture'? Illustrate this by Fig. 103. Vv^hat is a camera obscura? How can a portable camera obscura be made? 213. How is the angle of vision formed? Explain Fig. 104. What does Fig. 105 represent? lf>0 NATURAL PHILOSOPHY. figure it is e\i(ient, that the nearer an object is to the eye, the wider must be the opening of the hues to admit the extremities of the object; and, consequently, the larger the angle under which it is seen ; and, on the contrary, that objects at a distance will form small angles of vision. Thus, in this figui-e, the three crosses, F G, D E, and A B, are all of the same size; but A B, being the most distant, subtends the smallest angle'^ A C B, while D E and F G, being nearer to the eye, situated at C, form respectively the larger angles, D C E and F C G. 214. When an object, at any distance, does not sub- tend an angle of more than two seconds of a degree, it is invisible. At the distance of four miles a man of common stature will thus become imisible, because his height at that distance will not subtend an angle of two seconds of a degree. The size of * The apparent size of an object depends upon the size of the angle of vision. But we are accustomed to correct, by experience, the fallacy of appearances ; and, therefore, since we know that real objects do not vary in size, but that the angles under which we see thera do vary with the distance, we are not deceived by the variations in the appearance of ob- . jects. Thus, a house at a distance appears absolutely snaaller than the window through which we look at it ; otherwise w^e could not see it through the window ; but our knowledge of the real size of the house pre- vents our alluding to its apparent magnitude. In Fig. 104 it will be seen that the several crosses, AB, DE, FG, and HI, although very different in size, on account of their different distances, subtend the same angle AC B ; they, therefore, all appear to the eye to be of the same size, while, in Fig. 105, the three objects AB, DE, and FG, although of the same absolute size, are seen at a different angle of vision, and they, therefore, will seem of different sizes, appearing larger, as they approach the eye. It is upon a correct observance of the angle of vision that the art of per- spective drawing is indebted for its accuracy. What effect has the nearness of the object to the eye, on the angle? Illustrate this by the figure. Upon what does the apparent size of an ob- ject depend ? Why do objects appear so large? To what is the ari: Oi perspective drawing indebted for its accuracy ? 214. How large an angle must a body subtend to be visible? OPTICS. 1(31 tlie apparent diameter of the heavenly bodies is generally stated by the angle which they subtend. 215. When the velocity of a moving body does not exceed tw^enty degrees in an hour, its motion is imper- ceptible to the eye. 1. It is for this reason that the motion of the heavenly bodies is invisible, notwithstanding their immense velocity. 2. The real velocity of a body in motion ro nd a point, de- pends on the space comprehended in a degree. The more distant the moving body from the centre, or, in other words, the larger the circle ^^^^ which it has to describe, the larger will be $ the deoTce. -^.A 3. In Fig. 106, if the man at A, and the man at B, both start together, it is manifest that A must move more rapidly than B, to / arrive at C at the same time that B reaches D ; because the arc AC is the arc of a o » |^ e larger circle than the arc B D. But to the eye at E, the velocity of both appears to be the same, be- cause both are seen under the same angle of vision. 216. There are three kinds of mirrors,^ namel}% the plain, the concave, and the convex mirror. Plain mirrors are those which have a flat surface, such as a common looking-glass ; and they neither * A mirror is a smooth and polished surface, that forms images by tlio reflection of the rays of hght. Mirrors (or looking-glasses) are made of glass, with the back covered with an amalgam, or mixtm'e of mercury and tinfoil. It is the smooth and bright surface of the mercury that reflects the rays, the glass acting only as a transparent case, or covering, through which the rays find an easy passage. Some of the rays are absorbed in their passage through the glass, because the purest glass is not free from imperfections. For this reason, the best mirrors are made of fine and highly-polished steel. 215. When is the motion of a body invisible ? Why is the motion of the heavenly bodies invisible ? Upon what does the real velocity of a body, in motion round a point, depend? Explain Fig. 106. Why does the velocity of both, to an eye at E, appear to be the same ? 1:216. How many kinds of mirrors are there? What are plain mirrors? Do tiiey magnify or diminish the object? 162 NATUR ATi PIIILOSDriiY. magnify nor diminish the image of objects reflected from them. A convex mirror is a portion of the external surface of a sphere. Convex mirrors have therefore a convex surface. A concave mirror is a portion of the inner surface oi a hollow sphere. Concave mirrors have therefore a concave surface. In Fig. 107, MIS' lepresents both a convex and a concave mirror. They are both a portion of a sphere of which 0 is the centre. The outer part of M N is a convex, and g the inner part is a concave mirror. Let AB, CD, EF, represent rays falling on / the convex mirror Si N. / As the three rays are ( parallel, they would all be \ perpendicular to a plane or llat mirror ; but no ray cam, fall peiyendicularJy H on a concave or convex mir- ror, ichich is not directed towards the centre of the sphere of ivhich the mirror is a portion. For this reason the ray C D is perpen- dicular to the mirror ; while the other rays A B and E F fall, obliquely upon it. The middle ray therefore falling perpendicu- larly on the mirror, will be reflected back in the same line, while the two other rays falling obliquely will be reflected obliquely ; namely, the ray A B will be reflected to G, and the ray E F to H, and the angles of incidence A B P and EFT will be equal to the angles of reflection P B G and T F H, and since we see objects in the direction of the reflected rays, we shall see the image at L, which is the point at which the reflected rays if continued through the mirror would unite and form the image. This point is equally distant from the surface, and the centre of the sphere, and is called the imaginary focus of the mirror. It is called the imaginary focus, because the rays do . A j • S— E What are convex mirrors? What part of a sphere is a convex mir- ror? What are concave mirrors? What part of a sphere is a concave mirror? In Fig. 107, which part of the sphere represents a convex mir- iror? Which part a concave mirror? Explain the %nre. OPTICS. 163 not really unite at tliat point, but only appear to do so ; for the rays do not pass tlirougli the mirroi-, since they are reflect- ed by it. 217. The image of an object reflected from a convex mirror is smaller than the object. This is owing to the divergence of the reflected rays. A convex mirror converts, hj reflection, parallel rays into divergent rays ; rays that fall upon the mir- Fig. 108, ror divergent, are ^ rendered still more divergent by re- flection, and con- vergent rays are reflected either parallel, or less convergent. If, then, an object, A B, be placed before any part of a convex mir- ror, the two rays A and B proceeding from the extremities, falling convergent on the mirror, will be reflected less con- vergent, and will not come to a focus until they arrive at C ; then an eye placed in the direction of the reflected rays will see the image formed in (or rather behind) the mirror at a h ; and as the image is seen under a smaller angle than the object, it will appear smaller than the object. 218. The true focus of a concave mirror is a point equally distant from the centre and the surface of the sphere, of which the mirror is a portion. 219. When an object is further from the concave mirror than its focus, the image will be inverted ; but when the object is between the mirror and its focus, 217. How does the image of an object reflected from a convex mirror compare with the object? Give the illustration. 218. What is the focus of a concave mirror? 219. When an object is further from the concave mirror than the focus, how will the image appear? When the object is between the mirror and the focus ? 164 NATURAL PHILOSOPHY. the image will be upright, and grow larger in proportion as the object is placed nearer to the mirror.* 220. The image reflected by a concave mirror is lar- ger than the object, when the object is placed between the mirror and its focus. f 1. This is owing to the convergent property of the concave mirror. If the object A B be placed between the concave mirror and its fo- cus /, the rays A Fig- 109. and B from its ex- tremities will fall divergent on the mirror, and, on being reflected, become less di- vergent, as if they proceeded from C. To an eye placed in that sit- uation, namely, at * Concave mirrors have the peculiar property of forming images in the air. The mirror and the object being concealed behind a screen, or a wall, and the object being strongly illuminated, the rays from the object fail upon the mirror, and are reflected by it through an opening in the screen or wall, forming an image in the air. Showmen have availed themselves of this property of concave mirrors, in producing the appearance of appa- ritions, which have terrified the young and the ignorant. These images have been presented with great distinctness and beauty, by raising a fine transparent cloud of blue smoke, by means of a chafing-dish, around the focus of a large concave mirror. t There are three cases to be considered with regard to the efl^ects of concave mirrors : 1. When the object is placed between the mirror and the principal focus. 2. When it is situated between its centre of concavity and that focus. 3. When it is more remote than the centre of concavity. 1st. In the first case, the rays of light div^f^iiig after reflection, but in a How nmst the object be placed that the image may appear upright ? And as the object is removed towards the mirror? 220. If the object be placed between the mirror and the focus, how does the image compare with the object? OPTICS. 1G5 C, the image will appear magnified beliiad the min or, at a h, since it is seen under a larger angle than the object. 2. Tlie following facts result from the operation of the law already stated as the fundamental law of Catoptrics, namely, that the angles of incidence and reflection are always equal. 3. In estimating these angles, it must be recollected, that no hne is perpendicular to a convex or concave mirror, which will not, when sufficiently prolonged, pass through the centre of the sphere of which the mirror is a portion. 4. The truth of these statements may be illustrated by simple drawings ; always recollecting, in drawing the figures, to make the angles of incidence and reflection equal. The whole may also be shown by the simple experiment of placing the flame of a candle in various positions, before both convex and concave mirrors. FACTS WITH REGARD TO CONVEX MIRRORS. 1. Parallel rays reflected from a convex surface, are made to diverge. less degree than before such reflection took place, the image will be larger than the object, and appear at a greater or smaller distance from the sur- face of the mirror, and behind it. The image in this case will be erect. 2d. When the object is between the principal focus and the centre of the mirror, the apparent image will be in front of the mirror, and beyond the centre, appearing very distant when the object is at or just beyond the focus, and advancing towards it as it recedes towards the centre of con- cavity, where, as already stated, the image and the object will coincide. During the retreat of the object, the image will still be inverted, because the rays belonging to each visible point will not intersect before they reach the eye. But in this case, the image becomes less and less distinct, at the same time that the visual angle is increasing ; so that at the centre, or rather a little before, the image becomes confused and imperfect, owing to the small parts of the object subtending angles too large for distinct vision, just as happens when objects are viewed too near with the naked eye. 3d. In the cases just considered, the images will appear erect ; but in the case where the object is further from the mirror than its centre of con- cavity, the image will be inverted. The more distant the object is from the centre, the less will be its image ; but the image and object will coin- cide when the latter is stationed exactly at the centre. What peculiar properties have concave mirrors? What facts are stated with regard to convex mirrors, as resulting from the fundamental law of Catoptrics 1 166 NATURAL PHILOSOPHY. 2. Diverging rays reflected from a convex surface, are made more diver oino-. 3. When converging rays tend towards the focus of parallel rays, they will become parallel when reflected from a convex surface. 4. When converging rays tend to a point nearer the surface than the focus, they will converge less when reflected from a CONVEX smface. 5. If converging rays tend to a point between the focus and the centre, they will diverge as from a point on the other side of the centre, farther from it than the point towards which they converged. G. If converging rays tend to a point beyond the centre, they will diverge as from a point on the contrary side of the centre, nearer to it than the point towards which they con- verged. 7. If converging rays tend to the centre, when reflected, they will proceed in a direction as far from the centre. FACTS WITH REGARD TO COXCAVE MIRRORS. 1. Parallel rays, reflected from a concave surface, are made converging. 2. Converging rays, falhng upon a conca^-e surface, are made to converge more. 3. Diverging rays, falhng upon a concave surface, if they diverge from the focus of parallel rays, become parallel. 4. If from a point nearer to the sui'face than that focus, they diverge less than before reflection. o. If from a point between that focus and the centre, they What is said of parallel rays ? What is said of diverging rays ? W^hat is said of converging rays, when they tend towards the focus of parallel rays ? What is said of converging ra3^s, when they tend to a point nearer the surface than the focus 1 What is said of converging rays, when they tend to a point between the focus and the centre? What is said of con- verging rays, when they tend to a point beyond the centre? What is said of converging rays, when they tend to the centre? NVhat is said with regard to parallel rays, when reflected from a con- cave surface ? What is said of converging rays ? What is said of di- verging rays, if they diverge from a focus of parallel rays ? What, if from a point nearer to the surface than that focus ? OPTICS. 1G7 converge, after reflection, to some point on the contrary side of the centre, and fartlier from the centre than the point from which they diverged. G. If from a point beyond the centre, the reflected rays will convei-ge to a point on the contrary side, but nearer to it than the point from which they diverged. 7. If from the centre, they will be reflected thither again. REFRACTION OF LIGHT. 221. That part of the science of Optics which treats of refracted light is called Dioptrics. 222. By the refractionf of light is meant its being turnt?d or bent from its course ; and this always takes place when it passes obliquely from one medium to another. 223. By a medium, J in Optics, is meant any substance through which light can pass. Thus, air, glass, water, and other fluids, are media. * Concave mirrors, by the property which they possess of causing par- allel rays to converge to a focus, are sometimes used as burning-glasses. M. Dufay made a concave mirror of plaster of Paris, gilt and burnished, 20 inches in diameter, with which he set fire to tinder at the distance of 50 feet. But the most remarkable thing of the kind on record, is the com- pound mirror constructed by Buffon. He arranged 168 small plane mirrors in such a manner as to reflect radiant light and heat to the same focus, like one large concave mirror. With this apparatus, 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. t The power of being refracted is called refrangihility. X The plural number of this word is media, although mediums is some- times used. A medium is called dense or rare, in optics, according to its refractive power, and not according to its specific gravity. Thus, alcohol, and many of the essential oils, although of less specific gravity than water. What, if from a point between that focus and the centre ? If from a point beyond the centre? If from the centre ? 221. What is Dioptrics? 222. What is meant by the refraction of light? When does this take place ? 223. What is a medium, in Optics? Give some examples of media. Note. In what proportion is a medium dense or rare ? 168 NATURAL PHILOSOPHY. 224. There are three fundamental laws of Dioptrics, on which all its phenomena depend, namely : 1st. When light passes from one medium to another, in a direction perpendicular to the surface, it continues on in a straight line without altering its course. 2d. When light passes in an oblique direction, from a 7-arer to a denser medium, it will be turned from its course, and proceed through the denser medium less obhquely, and in a line nearer to a perpendicular to its surface. 3d. When light passes from a denser to a rarer medium, it passes through the rarer medium in a more oblique direction, and in a line further from a perpendicular to the surface of the denser medium. 1. In Fig. 110, the line AB repre- sents a ray of light passing from air into water, in a perpendicular direction. Ac- cording to the fiist law, stated above, it will continue on in the same line through the denser medium to E. If the ray were to pass upward through the denser medium, the water, in the same perpen- dicular direction to the air, by the same law it would also continue on in the same straight line to A. 2. But if the ray proceed from a rarer to a denser medium, in an oblique direction, as from C to B, when it enters the denser medium it will not continue on in the same straight line to D, but, by the second law, stated above, it will be refracted or bent out of its course, and proceed in a less oblique direction to F, which is nearer the perpendicular ABE than D is. 3. Again, if the ray proceed from the denser medium, the have a greater refracting power, and are, therefore, called denser media than water. In the following list, the various substances are enumerated in the order of their refractive power, or, in other words, in the order of their density, as media ; the last-mentioned being the densest, and the first the rarest, namely: air, ether, ice, water, alcohol, alum, olive oil, oil of turpentine, amber, quartz, glass, melted sulphur, diamond. 224. What are the three fundamental laws of Dioptrics? Illustrate the first law by the line A B, in Fig. 110. Illustrate the second law by the line C B. Illustrate the third law by the Ime FB. Fig. 110. G E F OPTICS. 100 water, to the rarer medium, the air, namely, from F to B, — instead of pursuing its straight course to G, it will be refracted according to the third law above stated, and proceed in a more oblique direction to C, which is further fi'om the pei'pondiculcir E B A than G is. 4. The refraction is more or less in all cases in proportion IS the rays fall more or less obliquely on the refracting sur- I'ace. 5. From what has now been stated, with regard to refrac- tion, it will be seen that many interesting facts may be explain- ed. Thus, an oar or a stick, when partly immersed in water, appears bent, because we see one part in 'one medium, and the other in another medium : the part which is in the water appears higher than it really is, on account of the refraction of the denser medium. 6. For the same reason, when we look obliquely upon a body Df water it appears more shallow than it really is. But when we look imyendkularly downwards, we are liable to no such deception, because there will be no refraction. 7. Let a piece of money be put into a cup or a bowl, and ^he cup and the eye be placed in such a position that the side of the cup will just hide the money from the sight ; then keep- ing the eye directed to the same spot, let the cup be filled with water, — the money will become distinctly visible. 225. The refraction of light prevents our seeing the heavenly bodies in their real situation.* * There is another reason, also, why we do not see the heavenly bodies in their true situation. Lis^ht, thou^rh it move with great velocity, is about eight and a half minutes in its passage from the sun to the earth, so that when the rays reach us, the sun has quitted the spot he occupied on their departure ; yet we see him in the direction of those rays, and, consequent- ly, in a situatioii which he abandoned eight minutes and a half before. The refraction of light does not affect the appearance of the heavenly bodies when they are vertical, that is, directly over our heads, because the rays then pass vertically, a direction incompatible with refraction. In what proportion does the refraction increase or diminish ? Why does an oar or a stick, when partly immersed in water, appear bent ? Why does the part which is in the water appear higher than it really is? Why does a body of water, when viewed obliquely, appear more shallow than it really is? In what direction can we look so as to cause no refraction? What experiment is here related ? 225. Why do we not see the heavenly bodies in their real situation? 8 170 NATURAL PHILOSOPHY. The light which they send to us is refracted in passing through the atmosphere, and we see the sun, the stars, &c., in the dii ection of the refracted ray. In consequence of this at- mospheric refraction the sun sheds his hght upon us earher in the morning and later in the evening, than we should otherwise perceive it. And when the sun is actually below the horizon, those rays which would otherwise be dissipated through space, are refracted by the atmosphere towards the surface of the earth, causing twihght. The greater the density of the air the higher is its refractive power, and, consequently, the longer the duration of twilight. 226. When a ray of light passes from one medium to another, and through that into the first again, if the two refractions be equal, and in opposite directions, no sen- sible effect will be produced. This explains the reason why the refractive power of flat window-glass produces no effect on objects seen through it. The rays sufl'er two refractions, which, being in contrary di- rections, produce the same effect as if no refraction had taken place. 227. A lens is a glass, which, owing to its peculiar form, causes the rays of light to converge to a focus, or disperses them according to the laws of refraction. It may here also be remarked, that it is entirely owing to the reflection • of the atmosphere that the heavens appear bright in the daytime. If the atmosphere had no reflective power, only that part would be luminous in which the sun is placed ; and on turning our back to the sun, the whole heavens would appear as dark as in the night ; we should have no twi- light, but a sudden transition from the brightest sunshine to darkness, im- mediately upon the setting of the sun. In what direction do we see them ? What causes twilight ? Upon what does the duration of twilight depend ? What other reason is given, in the note, why we do not see the heavenly bodies in their true situation? When does the refraction of light not affect the appearance of the heaven- ly bodies? Why do the heavens appear bright in the daytime? 226. What effect is produced when a ray of light passes from one me- dium to another, and through that into the first again? Why does the refractive power of flat window-glass produce no effect on objects seen through it? 227. What IS a lens? OPTICS. J71 There are various kinds of lenses, named according to their focus ; but they are all to be considered as por- tions of the internal or external surface of a sphere. 1. A single con- vex lens has one side flat and the other convex ; as A in Fig. 111. 2. A single con- cave lens is flat on one side and concave on the other, as B in Fig. 111. ' 3. A double convex lens is convex on both sides, as C, Fig. 111. 4. A double concave lens is concave on both sides, as D, Fig. 111. 5. A meniscus^ is convex on one side and concave on the other, as E, Fig. 111. 6. The axis of a lens is a line passing through the centre ; thus, F G, Fig. Ill, is the axis of all the five lenses. 228. The peculiar form of the various kinds of lenses causes the light which passes through them to be re- fracted from its course, according to the laws of Di- optrics. * The word meniscus is derived from the Greek language, and means literally a little moon. This term is applied to a concavo-convex lens, from its similarity to a moon in its early appearance. To this kind of lens the term periscopic has recently been applied, from the Greek language, meaning literally viewing on all sides. When the concave and convex sides of periscopic glasses are even or parallel, they act as plane glasses ; but when the sides are unequal, or not parallel, they will act as concave or convex lenses, according as the concavity or the convexity is the greater. How are all lenses to be considered? What is a single convex lens? What part of Fig. Ill represents a single convex lens? What is a single concave lens? What part of Fig. Ill represents a single concave lens? What is a double convex lens? What part of Fig. Ill represents a double convex lens? What is a double concave lens? What part of Fig. Ill represents a double concave lens? What is a meniscus? What part of Fig. Ill represents a meniscus? What is the axis of a lens? What line, in Fig. Ill, represents the axis of all the five lenses? 228. What is stated with egard to the form of the lenses? 172 NATURAL THILOSOPHY. It will be remembered that, according to these laws, light, in passing from a rarer to a denser medium is refracted to- wards the perpendicular ; and, on the contrary, that in pass- ing from a denser to a rarer medium, it is refracted further from the perpendicular. In order to estimate the effect of a lens, we must consider the situation of the perpendicular, with respect to the surface of the lens. Now, a perpendicular, to any convex or concave surface, must always, when prolonged, pass through the centre of sphericity ; that is, in a lens, the centre of the sphere of which the lens is a portion. By an at- tentive observation, therefore, of the laws above stated, and of the situation of the perpendicular on each side of the lens, it will be found in general, — 1. That convex lenses collect the rays into a focus, and mag- nify objects at a. certain distance. 2. That concave lenses disjyerse the rays, and diminish objects seen through them. 229. The focal distance of a lens is the distance from the middle of the glass to the focus. This, in a single convex lens, is equal to the diameter of the sphere of which the lens is a portion ; and in a double convex lens is equal to the radius of a sphere of which the lens is a portion. 230. When parallel rays* fall on a convex lens, those only which fall in the direction of the axis of the lens are perpendicular to its surface, and those only will continue on in a straight line through the lens. The other rays, falling obliquely, are refracted towards the axis and will meet in a focus. * The rays of the sun are considered parallel at the surface of the earth. How is light refracted in passing from a rarer to a denser medium ? How, in passing from a denser to a rarer? What must be considered in estimating the effect of lenses? Through what must a perpendicular, to any convex or concave surface, always, when prolonged, pass ? What is stated with regard to convex lenses? What, with regard to concave lenses ? 229. What is the focal distance of a lens? To what is this equal in a single convex lens ? To what is it equal in a double convex lens ? 230. When parallel rays fall on a convex lens, which one is perpen- dicular to its surface ? How are the other rays, falling obliquely, refracted ? OPTICS. 173 It is tins property of a convex lens which gives it its power as a buniiiig-o-lass. All tlie parallel rays of the sun which pass through tlie glass, are collected together in the focus; and, consequently, tJie heat at the focus is to the comm,on heat of the SU71, as the area of the glass is to the area of the focus. Thus, if a lens, four inches in diameter, collect the sun's rays into a focus, at the distance of twelve inches, the image will not be more than one-tenth of an inch in diameter ; the surface of this little circle is 1600 times less than the surface of the lens, and, consequently, the heat will be 1600 times greater at the focus than at the lens.^ 231. The following effects result from the laws of re fraction. FACTS WITH REGARD TO CONVEX SURFACES. 1. Parallel rays passing out of a rarer into a denser medium, through a convex surface, will become converging. 2. Diveiging rays will be made to diverge less, to become parallel, or to converge, according to the degree of divergenc}?- before refraction, or the convexity of the surface. 3. Converging rays, towards the centre of convexity, will suffer no refraction. 4. Rays converging to a point beyond the centre of con- vexity, will be made more converging. * The following effects were produced by a large lens, or burning-glass, two feet in diameter, made at Leipsic in 1691. Pieces of lead and tin were instantly melted ; a plate of iron was soon rendered red-hot, and afterwards fused, or melted ; and a burnt brick was converted into yellow glass. A double convex lens, three feet in diameter, and weighing 212 pounds, made by Mr. Parker, in England, melted the most refractory sub- stances. Cornelian was fused in 75 seconds, a crystal pebble in 6 seconds, and a piece of white agate in 30 seconds. This lens was presented by the king of England to the emperor of China. What property of a convex lens gives it its power as a burning-glass ? Where are all the parallel rays of the sun, which pass through the glass, collected? How does the heat at the focus compare with the common heat of the sun ? What is related in the note with regard to the effects of lenses produced by burning-glasses? 231. What is the first effect related as resulting from the laws of re- fraction with regard to convex surfaces? What is said of diverging rays? What is said of rays converging towards the centre of convexity ? What of rays converging to a point beyond the centre of convexity? 174 NATURAL PHILOSOPHY. 5 Convemno- rays towards a point nearer the surface than the centre of convexity, will be made less converging by refrac- *'^[When the rays proceed out of a denser into a ra^'er medium, the reverse occurs in each case.] FACTS WITH REGARD TO CONCAVE SURFACES. 1. Parallel rays, proceeding out of a rarer into a denser medium, through a concave surface, are made to diverge. 2 Diverging rays are made to diverge more,— to suher no refraction,— or to diverge less, according as they proceed from a point beyond the centre, from the centre, or between the centre and the surface. . 3 Converging rays are made less convergmg, parallel, or diverging, according to their degree of convergency before re- fraction.^ . ,. [When the rays proceed out of a denser mto a rarer medmm, the reverse takes place in each case.] 232. Double convex, and doable concave glasses, or lenses, are used in spectacles, to remedy the defects ol the eve ; the former, when by age it becomes too iiat, or loses' a portion of its roundness ; the latter, when by * The above eight principles are all the necessary consequence of the operation of the three laws mentioned as the fundamental laws of Dioptrics.. The reason that so many different principles are produced by the operation of those laws, is, that the perpendiculars to a convex or concave surface are constantly varying, so that no two are parallel. But m flat surfaces the perpendiculars are parallel; and one invariable result is produced by the rays when passing from a rarer to a denser, or from a denser to a rarer medium, havuig a flat surface. What of ravs converging to a point nearer the surface than the centre of convexity? ' When the rays proceed out of a denser into a rarer me- dium, what occurs? . What is stated of parallel rays, proceeding from a rarer mto a denser medium, through a concave surface? What is said of diverging rays? What is said of converging rays? Of what are the above eight prmci- ples the necessary consequence ? What is the reason that so -many dif- ferent principles are produced by the operation of these laws ? ^ 232. For what are double convex and concave glasses, or lenses, used in spectacles ? OPTICS. 175 any other cause it assumes too round a form, as in the case of short-sighted (or, as they are sometimes called, near-sighted) persons. Convex ghvsses are used when the eye is too flat, and concave glasses w^hen it is too round.* Fig. 112. THE EYE. 233. The eye is composed of a number of coats, or coverings, w^ithin which are enclosed a lens, and certain humors, in the shape, and performing the office of con vex lenses. 1. The different parts of the eye, are: 1. The Cornea. 6. The Vitreous Humor. 2. The Iris. 7. The Retina. 3. The Pupil. 8. The Choroid. 4. The Aqueous Humor. 9. The Sclerotica. 5. The Crystalline Lens. 10. The Optic Nerve. 2. Fig. 112 represents a front view of the eye, in which a a rep- resents the Go nea, or, as it is commonly called, the white of the eye ; eei^ the Iris,t having a cir- cular opening in the centre, called the pupil, 2h which contracts in a strong light, and expands in a faint light, and thus regulates the quan- tity which is admitted to the ten- der parts in the interior of the eye. ^ These lenses or glasses are generally numbered, by opticians, accord- ing to their degree of convexity or concavity ; so that by knowing the number that fits the eye, the purchaser can generally be accommodated, without the trouble of trying many glasses. t It is the iris which gives the peculiar color to the eye. What glasses are used when the eye is too flat ? What are used when the eye is too round? 233. Of what is the eye composed? What are the different parts of the eye? First? Second? Third? Fourth? Fifth? Sixth? Seventh? Eighth? Ninth? Tenth? What does Fig. 1 12 represent ? Explain tho figure. 176 NATURAL PHILOSOPHY. 3. Fig. 113 represents a side view of the eye, laid open, in which h b represents the cor- nea, e e the his, d d the pupil, // the aqueous humor, gg the crystalline lens, hh the vitreous liumor, i i i i i the retina, c c the choroid, aaaaa the scle- ] -otic a, and n the optic nerve. 4. The cornea forms the anteiior portion of the eye. It is set in the sclerotica in the same manner as the crystal of a watch is set in the case. Its degree of convexity varies in different individuals and in differ- ent periods of life. As it covers the pupil and the iiis, it pro- tects them from injury. Its principal office is to cause the light which reaches the eye to converge to the axis. Part of the light, however, is reflected by its finely pohshed surface, and causes the brilliancy of the eye. 5. The iris is so named from its being of different colors. It is a kind of circular curtain, placed in the front of the eye to regulate the quantity of light passing to the back part of the eye. It has a circular opening in the centre, which it involun- tarily enlarges or diminishes. 6. " The pupil is merely the opening in the iiis, through which the light passes to the lens behind. It is always circular in the human eve, but in quadrupeds it is of different shape. When the pupil is expanded to its utmost extent, it is capable " of admitting ten times the quantity of light that it does when most contracted.^ In cats and other animals, which are said * When we come from a dark place into a strong our eyes snfier pain, because the pupil being expanded, admits a larger quantity of light to rush in, before it has bad time to contract. And when we ofo from a strong hght into a faint one, we at first imaofine ourselves in dark- ness, because the piipil is then contracted, and does not inxtaniJy expand. What does Fig. 113 represent? Explain the figure. \Vnat part of the eye does the cornea form ? Is its degree of convexity the same m ail per sons and all periods of life ? What is its principal office ] From what does the iris take its name ? What is the use of the iris ? What is the pupil ? W^hat is its form in the human eye ? How much more light is the pupil capable of admitting, when expanded to its utmost extent, thai when most contracted ? OPTICS. 177 to see in the dark, the power of dikitation and contraction is much i^: eater; it is computed, tliat their pupils may receive one bundled times more light at one time than at another. Tbat bo bt only, which passes the pupil, can be of use in vision ; that which falls on the iris being reflected, returns through the cornea, and exhibits the color of the iris. 7. The aqueous humor is a fluid, as clear as the purest water. In shape it resembles a meniscus, and, being situated between the cornea and the crystalline lens, it assists in collect- ing and transmitting the rays of light from external objects to that lens. 8. The crystalline lens is a transparent body, in the form of a double convex lens, placed between the aqueous and vitreous humors. Its office is not only to collect the rays to a focus, on the retina, but also to increase the intensity of the light which is directed to the back part of the eye. 9. The vitreous humor (so called from its resemblance to melted glass) is a perfectly transparent mass, occupying the globe of the eye. Its shape is like a meniscus, the convexity of which greatly exceeds the concavity. 10. In Fig. 114 the shape of the Fig. 114. aqueous and vitreous humors and the crystalline lens is presented, a is the aqueous humor, which is a meniscus, h the crystalhne lens, which is a double " convex lens, and c the vitreous humor, which is, also, a meniscus, whose con- cavity has a smaller radius than its convexity. 11. The retina is the seat of vision. The rays of light being refracted in their passage by the other parts of the eye, are brought to a focus in the retina, where an inverted image of the object is represented. 12. The choroid is the inner coat or covering of the eye. Its outer and inner surface is covered with a substance called the What is said of those animals which are said to see in the dark? What light, only, is of use in vision? What becomes of the light which falls on the iris? What is the aqueous humor? What is its form? Of what use is it? What is the crystalhne lens? What is its office? What is the vitreous humor ? Why do persons sometimes experience pain when pass- ing from a dark place into strong light ? What is the shape of the vitreous humor? Explain Fig 114. What is the retina? What is the choroid? 8* 178 NATURAL PHILOSOPHY. mqmentum nigrum, (or black paint.) Its office is, apparently, to absorb the rays of light immediately after they have fal en on the retina. It is the opinion of some philosophers, that it is the choroid and not the retina, which conveys the sensation produced by rays of light to the brain. _ 13 The sclerotica is the outer coat of the eye. It derives its name from its hardness. Its office is to preserve the lobular figure of the eye, and defend its more delicate internal structure To the sclerotica are attached the muscles which move the eye. It receives the cornea, which is inserted m it somewhat like a watch-glass in its case. It is pierced by the optic nerve, which, passing through it, expands over the inner surface of the choroid, and thus forms the retina. 14 The optic nerve is the organ which carries the impres- sions made by the rays of light, (whether by the medium of the retina, or the choroid,) to the brain, and thus produces the sensation of sight.^ 234. The eye is a natural camera ohscura,\ and the images of all objects seen bv the eye are represented on the retina, in the same manner as the forms of external objects are delineated in that instrument. 1 Fio- 115 represents only those parts of the eye which are mos't essential for the explanation of the phenomena of Fig. 115. * For the above description of the eye and its parts, the author is main- ly indebted to Paxton's Introduction to the Study of Anatomy, edited by Dr. Lewis of this city. t The camera obscura is explained in a note on page 157. Bv what is its outer and inner surface covered? What is its office? What is the opinion of some philosophers with regard to the cnoroid? What is the sclerotica ? From what does it derive its name ? What is its office ? What are attached to the sclerotica ? What is the optic nerve ? OPTICS. 179 vision. The image is formed thus. The rays from the object c d, diverging towards the eye, enter the cornea c, and cross one another in their passage through the crystaUine lens d, by which they are made to converge on the retina, where they form the inverted* image, fe. 2. The convexity of the crystalhne humor is increased or * Although the image is inverted on the retina, we see objects erect, because all the images formed on the retina have the same relative position which the objects themselves have ; and as the rays all cross each other, the eye is directed upwards, to receive the rays w^hich proceed from the upper part of an object, and downwards, to receive those which proceed from the lower part. A distinct image is also formed on the retina of each eye ; but as the optic nerves of the two eyes unite, or cross each other before they reach the brain, the impressions received by the two nerves are united, so that only one idea is excited, and objects are seen single. Although an object may be distinctly seen with only one eye, it has been calculated that the use of both eyes makes a difference of about one-twelfth. From the de- scription now given of the eye, it may be seen what are the defects which are remedied by the use of concave and convex lenses, and how the use of these lenses remedies them. When the crystalline humor of the eye is too round, the rays of light which enter the eye converge to a focus before they reach the retina, and, therefore, the image will not be distinct ; and when the crystalline humor is too flat, (as is often the case with old persons,) the rays will not converge on the retina, but tend to a point beyond it. A convex glass, by assisting the convergency of the crystal- line lens, brings the rays to a focus on the retina, and produces distinct vision. The eye is also subject to imperfection by reason of the humors losing their transparency, either by age or disease. For these imperfections no glasses offer a remedy without the aid of surgical skill. The operation of couching and removing cataracts from the eye, consists in making a punc- ture or incision through which the diseased part may escape. Its office is then supplied by a lens. If, however, the operator, by accident or want of skill, permit the vitreous humor to escape, the globe of the eye imme- diately diminishes in size, and total blindness is the inevitable result. 234.' What philosophical instrument does the eye resemble in its con- struction? Explain Fig. 115. Note. Why do the objects appear erect when the images are inverted ? Why do we see only one image when an image is formed on both eyes? What are the defects which are remedied by the use of concave and convex lenses ? In what other way is the eye subject to imperfection ? Is there any remedy for this ? ISO NATURAL PHILOSOPHY. diminished bv means of two muscles, to yvhich. it is attacbed. Bv this means the focus of the rays which pass through it, constantlv falls on the retina ; and an equally disimct image is formed, both of distant objects and those which are near. 235. A single microscope consists simply of a convex lenN commonly called a magnifying-glass ; in the locus of which the object is placed, and through which it is viewed. 1. By means of a microscope the rays of hght from an ob- ject are caused to divero-e less ; so that when they enter the pupil of the eye. they Ml parallel on the crystalhne lens, by which thev are 'refracted to a focus on the retina. 2. Fig. 'lie represents a convex lens, or single microscope, C P. The diverging rays from the ob- Fig. lie. ject AB are refract- ed in their passage throuo'h the lens C P, Imd made to fall parallel on the crystalline lens, by which they are re- fracted to a focus on the retina R R ; and the image is thus magnified, because the divergent rays are collected by the lens and carried to the retina. 3. Those lenses or microscopes which have the shortest focus, have the greatest m:i unifying power; and those which are the most bulging or convex, have the shortest focus. Lenses are made small because a reduction in size is necessary to an increase of curvature. •236. A double microscope consists of two convex lenses, bv one of which a magnified image is iormed, By what is the convexity of the crystalhne humor increased or dimin- ished ? ^^'hat is effected by this means 1 235. What is a single microscope ' What is the use of this microscope ? What fi2:ure represents a microscope ? Explain the figure. What lenses have the greatest magnifying power' What lenses have the shortest focus ? Q36. Of v>'hat does a double microscope consist ? OPTICS. 181 and by the other this image is carried to the retina of the eye. Kig\ 117 represents the effect produced by the lenses of a double microscope. The rays wliich diverge from the object A 13 are collected by the lens L M, (called the object-glass, be- cause it is nearest to the object,) and form an inverted magni- Fig. 117. fied image at C D. The rays which di veige from this image are collected by the lens N 0, (called the eye-glass, because it is nearest to the eye,) which acts on the principle of the single microscope, and forms a still more magnified image on the retina R R. 237. The solar microscope, is a microscope with a mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays and reflect them upon the object. Jt consists of a tube, a mirror or looking-glass, and two convex lenses. The sun's rays are reflected by the mirror through the tube upon the object ; the image of which is thrown upon a white screen, placed at a distance to receive it. 1. The microscope, as above described, is used for viewing transparent objects only. When opaque objects are to be viewed, a mirror is used to reflect the light on the side of the What is the use of these two lenses? What does Fig. 117 represent? Explain the figure. 237. What is the solar microscope? Of what does it consist? By what, in this microscope, are the sun's rays reflected, and upon what? For viewing what objects, only, is the microscope, above described, used? How do those microscopes, used for viewing opaque objects, differ from these ? 182 NATURAL PHILOSOPHY. object ; the image is then formed by light reflected from the object, instead of being transmitted through it. 2. The mao-nifying power of a single microscope is ascertain- ed by dividing the least distance at which an object can be dis- tinctly seen by the naked eye, by the focal distance of the lens. This, in common eyes, is about 7 inches. Thus, if the focal distance of a lens be only \ of an inch, then the diameter of an object will be magnified 28 times, (because 7, divided by i, is the same as multiplying 7 by 4,) and the mrface will be magnified 784 times. 3. The magnifying power of the compound microscope found in a similar manner, by ascertaining the magnifying power, first of one lens, and then of the other. 4. The magnifying power of the solar microscope is in pro- portion as the distance of the image, from the object-glass, is greater than that of the object itself from it. Thus, if the dis- tance of the object from the object-glass be \ of an inch, and the distance of the image> or picture, on the screen, be ten feet, or 120 inches, the object will be magnified in length 480 times, or, in surface, 230,000 times.^ 238. The magic lantern is an instrument constructed on the principle of the solar microscope, but the light is supplied by a lamp instead of the sun. 1. The objects to be viewed by the magic lantern are generally painted with transparent colors, on glass shdes, which are received into an opening in the front of the lantern. The light from the lamp, in the lantern, passes through them, and carries the pictures, painted on the shdes, through the lenses, by means of which a magnified image is thrown upon the wall, on a white surface prepared to receive it. * A lens may be caused to. magnify or to diminish an object. If the object be placed at a distance from the focus of a lens, and the image be formed in or near the focus, the image will be diminished ; but if the ob- ject be placed near the focus, the imago will be magnified. How is the image then formed? How is the magnifying power of a single microscope ascertained? Illustrate this. How is the magnifying of the compound microscope ascertained? In what proportion is the mag- nifying power of the solar microscope ? Illustrate this. iYo^e. How may a lens be made to magnify or diminish an object ? 238. What is the magic lantern ? How are objects, viewed by the magic lantern, generally represented ? OPTICS. 183 2. Fig. 118 represents the magic limtern. The rays of hght from the lamp are received upon the concave mirror e, and re- Fig. 118. fleeted to the convex lens c, which is called the condensing lens, because it concentrates a large quantity of light upon the object painted on the shde, inserted at h. The rays from the illu- minated object at h, are carried divergent through the lens a, forming an image on the screen at /. The image will increase or diminish in size, in proportion to the distance of the screen from the lens a. 239. A telescope is an instrument for viewing distant objects. There are two kinds of telescopes, namely, the refract- ing telescope and the reflecting telescope. A refracting telescope is one in which the object itself is viewed, through the medium of a number of lenses. A reflecting telescope is one in which the image of the object is reflected from a concave mirror, within the tube of the telescope, and viewed through a number of lenses.* * The image of the object seen through a refracting telescope is never so clear and perfect as that obtained by the reflecting telescope ; because What figure represents a magic lantern? Explain the figure. In what proportion will the size of the image increase or diminish ? 239. What is a telescope? How many kinds of telescopes are there? What are they ? What is a refracting telescope ? What is a reflecting telescope? Note. Why is the image of an object, seen through a refract- ing telescope, less clear and perfect than when seen through a reflecting telescope ? 184 NATURAL PHILOSOPHY. 1. There are two kinds of refracting telescopes, called the astronomical telescope, or night-glass, and the terrestrial tele- scope, or day-glass.^ In the former, or night-glass, there are but two lenses or glasses, but the object is viewed in an invert- ed position. As the glass is used principally for viewing the heavenly bodies, the inversion of the image produces no incon- venience. In the latter, or day-glass, two additional lenses are introduced to give the image its natural position. 2. Fig. 119 represents a night-glass, or astronomical tele- scope. It consists of a tube, A B C D, containing two glasses, or lenses. The lens, A B, having a longer focus, forms the object-glass ; the other lens, D C, is the eye-glass. The rays Fig. 119. ..___A M from a very distant body, as a star, and which may be con- sidered parallel to each other, are refracted by the object-glass A B to a focus at K. The image is then seen through the eye- glass D C, magnified as many times as the focal leng^.h of the eye-glass is contained in the focal length of the object-glass. Thus, if the focal length of the eye-glass D C, be contained 100 times in that of the object-glass A B, the star will be seen magnified 100 times. It will be seen by the figure, that the image is inverted ; for the ray M A, after refraction, will be seen in the direction C 0, and the ray N B, in the direction D P. 3. Fig. 120 represents a day-glass or terrestrial telescope, commonly called a spy-glass. This, likewise, consists of a tube, A B H G, containing four lenses, or glasses, namety, the dispersion of colors which every lens produces, in a greater or less de- gree, renders the image dull and indistinct, in proportion to the number of lenses employed. * Some glasses or telescopes are marked " Night and Day." These have four glasses, two of which maybe removed when the heavenly bodies are viewed. How many kinds of refracting telescopes are there ? What are they ? How do they differ the one from the other? What does Fig. 119 repre- sent ? Explain the figure. OPTICS. 185 A 1^ C 1), K \'\ iun\ Ct If. The Iciis A 15 is the, ()l)ject-