P ARKE I' S NATURAL AND EXPERIMENTAL P H I L O S O P H Y. gE' 3~~~~~~~~~~7" - ----— * -— f' —-__ - — 6 t E do p'gO a 6 P/Xsh-r's ya68mg A=gOGGS I - N-~== — __=~=~~ — See page3~3, Paker's Ntural Pilosoph aa~~~~~l~~~'!Ji;;'1;'P17-~~~~~~~~~~~~~~~~~~ SC-OOL COMPENDIUM OF NATURAL AND EXPERIMIEINTAL PHILOSOPHY, EMBRACING THE ELEMENTARY PRINCIPLES OF MECHANICS, HYDROSTATICS, HYDRAULICS, PNEUMATICS, ACOUSTICS, PYRONOMLICS, OPTICS, ELECTRICITY, GALVANISM, MAGNETISIM, ELECTRO-5MAGN ETIS.M, MAGN ETO-ELECTRICITY, AND ASTRONO0MY. CONTAINING ALSO A DESCRIPTION OF THE STEAM AND LOCOMOTIVE ENGINES, AND OF THE ELECTRO-MAGNETIC TELEGRAPH. BY RICHIARD GREEN PARKER, A.M. LATR PRINCIPAL OF THE JOHNSON GRAMMAR SCHOOL, BOSTON; AUTHOR OF "AIDS TO ENGLISH COMPOSITION," A SERIES OF "SCHOOL READMRS,"' ~ GEOGRAPHICAL QUESTIONS," ETC. ETO. Delectando pariter que monendo Prodesse quam conspici. CORRECTED, ENLARGED AND IMPltOIYLE. NEW YORK: A. S. BARNES & CO., 51 & 53 JOHN-STREET. 1856. BY RICHARD G. PARKER, A. M. JUVENILE PHILOSOPHY, or PiIrL6soPIr aN FAMILIAR 7ICVERSATIONS. Designed to teach Young Children to Think. II. FIRST LESSONS IN NATURAL PHILOSOPHY. Designed to teach the Elemente of the Science. Arranged from the Compendium of School Philosophy. III. A SCTTOOL COMPENDIUM OF NATURAL AND EXPERIMENTAL PHILOSOPHY. Embracing thle Elementary Principles of Mechanics, Iydrostatics. tlydraulics, Pneumlr1atics, Acoustics, Pyronomnics, Opltics, Electricity, Gaivanism, Mantmtismn, Ele(trl-Magnetisnm, Magneto-Electririty. and Astronomy. Containing also a Dlescri)tion of the Steam and Locomotive Engines, and of tile Electro-Magnetic Telegraph. A new Edition, corrected, enlarged, and improved. ollcge t1il oop l)l BY PROF. W. II. C. BARTLETT. W. H C. BARTLETT'S COLLEGE COURSE OF NATURAL PHIILOSOPHY. Used at the United States Milit:ry Academy, and designed for Colleges and Universities. VOL. I. ELEMENTS OF MECHANICS. II. ELEMENTS OF ACOUSTICS AND OPTICS. " III. ELEMENTS OF ASTRONOMY. ANALYTICAL MECHANICS, treated by the Aid of the Calculus. Is one volume 8vo. Designed for advanced Students..-9 1ool []clcmislr, BY JOHN A. PORTER, Professor of Chemistry in Yale College. Entered according to Act of Congress, in the year 18553, By A. S. BARNES & CO., ID the Clerk's Office of tho District Coulrt of the United States for the Southern District of New York. PREFACE. IN the year 1837, the school-committee of the city of Boston ordered a few articles of philosophical apparatus to be furnished for each of the grammar-schools of that city; and the author of this work, who for many years had been at the head of one of those schools, finding no elementary work, unencumbered with extraneous matter, suitable to explain the apparatus, attempted to supply the deficiency. The result was the first edition of this work. A few years afterwards, the philosophical apparatus was exchanged for one of better construction, and much more extended application, and an enterprising publishing house in New York induced the author to revise and extend his work. This was done in the year 1848. Since that time the progress of science has been so great that another revision is imperatively demanded; and the author, anxious not to be " behind the age," has made another careful revision, in which he is conscious of no omission in the notices of the present state of science, in the departments embraced in this volume, suitable for a work designed to be strictly elementary, ana designed for those only whose progress in " the exact sciences" must necessarily be limited. The " Questions" which have ap. poared in previous editions he had no hand in preparing. Indeed, in his opinion, such appendages to school-books, in the hands of experienced teachers, are of very questionable expediency. But, as it is a custoim rWost ulwwred in " olservance," le has, in this 1* VI PREFACE. edition of 1854, complied with that custom, and prepared themr with his own hands. If he is not deceived in the result of his labors, his work will commend itself by the following 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 experiments which can be performed by means of the apparatus. 3. It is enriched by a representation and a description of the Locomotive and the Stationary Steam Engines, and the various forms of the Electric Telegraph now in operation in this country. 4. The subjects of Pyronomics, Electricity, Magnetism, ElectroMagnetism, and Magneto-Electricity, as well as Astronomy, have large space allotted to them. Most of the latest discoveries in physical science have also received their due share of attention. 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 illustrations, and then repeated on separate leaves at the end of the voluime. The number is also given, where each principle may be found to which allusion is made throughout the volume. Suitable questions, also prepared by the author himself, and obnoxious to no objection as " leading questions," have been placed in immediate connection with the most important principles contained in 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 contained in a smaller letter. Much useful and PREFACE. VI] interesting matter is also 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 elementary 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 quotation. Being at a loss, therefore, whonm to credit for some of the expressions which he has borrowed, he makes this general acknowledgment, in the hope that it may be said of him, as it was once said of the Mantuan bard, that "he has adorned his thefts, and polished the diamonds which he Zas stolen." BoSTON OCTOBER 1853. ADVERTISEMENT TO THE NEW STEREOTYPE OF 1854 IN the revision of this work the apthor has endeavored to present his materials under a better classification. The omission of seventyfive pages of Questions, prepared by another hand, found at the end of the book in previous editions, has given room for a large collection of new facts and principles which the present improved state of science has revealed, without materially enlarging the size of the volume. The author now gives it to the world, in confidence that it is much more deserving of the unexpected favor it has received. All changes in a text-book are necessarily attended with inconve niences to teachers; but they who would keep pace with the progress of science must submit to such inconvenience, or be behind the age. The present is emphatically the age of "progress," and they who profess to record the triumphs of science must keep a blank page in their journals for the record of new conquests. So much of apology seems to be due for the appearance of a new revision of this volume so soon after the former revision. The author indulges the belief that no advance has been'made in fact, in principle, or in physical law, which has not received its due share of attention so far as is consistent with the plan of a work professing to be strictly elementary. 4 KNEELAND-PLACE, 1853. LIST OF WORKS WHICH HAVE BEEN CONSULTED, OR FROM WHICH EXTRACTS HAVE BEEN TAKEN, IN THE PREPARATION OF THIS VOLUME. Annals of Philosophy; Arnott's Elements of Physics; Bartlett s Philosophy; Bigelow's Technology; Cambridge Physics; Chambers' Dictionary; Enfteld's, Olmsted's, Smith's, Blair's, Bakewell's, Dra per's, Grund's, Johnson's, Jones', Comstock's, and Conversations on, Natural Philosophy; Davis' Manual of Magnetism; Encyclopedia Americana; Franklin's Philosophical Papers; Henry's Chemistry, King's Manual of Electricity; Lardner's Works; Library of Useful Knowledge; Orbs of Heaven; Paxton's Introduction to the Study of Anatomy; Pambour on Locomotive Engines on Railways; Penny Cyclopedia; Peschel's Elements of Physics; Philips' Astronomy; Sir John Herschel's Astronomy, SiUliman's Journal of Science; Singer's Electricity; Scientific Class Book; Scientific Dialogues; Smith's Explanatory Key; The Year Book; Turner's Chemistry; Wilkins' Astronomy; Worcester's and the American School Geography; Lathrop, McIntire and Keith, on the Globes; World's Progress; Annual of Scientific Discovery; Webster's Dictionary; Treasury of Knowledge; Gregory's Chemistry; Science of Familiar Things; Loomis' Elements of Geology; Chambers' Educational Course; Brande's Encyclopedia; Ure's Dictionary; McCulloch's Commercial Dictionary; Patent Office Reports. SCHEDULE OF PHILOSOPHICAL APPARATUS USED IN THE GlRAMMAR-SCHOOLS OF THE CITY OF BOSTON.* Adopted by the School Committee, Aug. 1817. 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 Levers, 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 intermittent 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. * The cost of this apparatus is about two hundred and sixty dollars. It was made by Mr. Joseph MI. Wightman, importer and manufacturer of Philosophical Apparatus, No. 83 Cornhill, Boston, and in an eminent degree unites beauty with durability. Messrs. Chamberlain & Ritchie, also, in Washington-street, excel in their manufacture of Philo. sophical Instruments of all kinds. In the department of Electricity and Magnetism, Messrs. Palmer & Itall, successors of Daniel Davis, 428 Washington-street, have malny articles of excellent design and execution. PHILOSOPHICAL APPARATUS. XI Fountain Glass, Cock, and Jet for Vacuum. Brass NMitgdeburg Hlernispheres. [InproNed Weighit-lifter for upward pressure. lion Weight of fifty-six pounds, and Strap, for Weight-lifte. Flexible Tube anrd Connectors, Brass Plate and Sliding Rod. Boluit Head and Jar. Tatll Jatr andl Ba3lloon. H;ndl and Bladder Glasses. Wood Cylinder and Plate. India-rubber Barg for expansion of air. Guinea and Feather Apparatus. Glass Flask and Stop-cock for weighing air. ELECTRICITY. Pla te Electrical Machine. Pith-bMill Electrometer. Electrical B tttery of four Jars. Electricdl DischaLrger. Imag e Plltes and Figure. Insulated Stool. Chime of Bells. Miser's Plate for shocks. Tissle Figure, Ball and Point. Electrical Flyer and Tellurian. Electrical Sportsman, Jar and Birds. MfIhogany 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. Fair of Magnetic Swans. Glass Vase for Magnetic Swans. Horseshoe Magnet. ASTRONOMY. Improved School Orrery. Tellurian, or Season Machine. ARITHMETIC AND GEOMETR'. 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. C o N T E N T S DIVISIONS OF THE SUBJECT,.7.......... 7 OF MATTER AND ITS PROPERTIES,..1.......... 19 OF GRAVITY,....................... 833 MECHANICS, OR TIIE LAWS OF MOTION,............. 41 THE MECHANICAL POWERS................... 70 REGULATORS OF MOTION.................. 100 IIYDROSTATICS,...................... 108 IHYDRAULICS,....1.................. 128 PNEUMIATICS, ~..................... 138 ACOUSTICS....1...................173 PYRONOMICS, S..................... 185 THE STEAM-ENGINE,...1..96 OPTICS......................... 210 ELECTRICITY,................ 258 GALVANISM, OR VOLTAIC ELECTRICITY,.... e... 283 IMAONE.TIS.I.....................298 ELECTRO-MNAGINETIS-I,. 8......................308 THE ELECTRO-MAG^NETIo TELEGRAPH,..............19 THE E1,ECTROTYPE PROCESS,..... 31 IA.-CNETO. ELEC'rRICITY............... 382 TIIER.Im(-EL,ECTRICITY,.....................8. 3 ASTRONOIY,......................335 Tll Inlex at tile clfse of thle vo:ume, being ill alLnd comuprlrelclive, will bE rtulid 1ore LnlveVie:iL fur referenle. INTRODUCTION. THE term Philosophy 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 phenomena, both of mind and of matter. When applied 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 perfections, 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 Intellectual Philosophy, or Metaphysics. The word Theology is derived from two Greek words, the former of which (~som) signifies God, and the latter (no?7os) means a discourse; and these two words, combined in the term Theology, literally imply a discourse about God. The latter of these two Greek words (Aoyoo 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 mineralogy, the science of minerals; meteorology, the science which treats of meteors; ichthyology, the science of fishes; entomology, the science of insects; lithology, of stones; conchology, of shells, &c. The word Metaphysics is composed of two Greek words, MIeta (or iFTX), which signifies beyond, and phusis (or vual), which signifies nature, and in composition these words imply something 2 XIV INTRODUCTION. beyond nlatutre. From the latter of these words, phusis (Rrri.q), we obtain the term physics, which in its most extended sensoe implies the science of nature and 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. Mian, as a mere animal, is included in the science of Physics; but, as a being possessed of a soul, of intellect, of the powers of perception, consciousness, volition, reason, and judgment, he becomes a subject of consideiration in the science of Metaphysics. 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'appropriate 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 voluntary action. Stones, the various kinds of earth, metals, and many minerals, are in-.stances of unorganized matter. Fossils, that is, substances dug out of the earth, are frequently instances of a combination of organized and unorganized matter. Unorganized matter also enters into the composition of organized matter. Thus, the bones of animals contain lime, which by itself is unorganized matter. Physical Science, or Physics, with its subdivisions of Natural Itistory (including Zoology, Botany, Mineralogy, Conchology, Entomology, Ichthyology, &c.) and Natural Philosophy, including its own appropriate subdivisions, embraces the whole field of organized and unorganized matter. The term Natural Philosophy is considered by some authors 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. INTRODUCTION. XV 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 acquire by such changes. The operations of chemistry may be described under the heads of Analysis or decomposition, and Synthesis or combination. Natural Philosophy makes us acquainted with the condition and relations of bodies as they spontaneously arise, without any agency of our own. Chemistry teaches us how to alter the natural arrangement of elements to bring about some particular condition that we desire. To accomplish these objects in 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 philosophical knowledge proceeds either from observation or experiment, 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 according to the extent of the subjects which they respectively embrace. Thus, it is a general law that all bodies attract each other in proportion to the quantity of matter which they contain. 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 Geometry, the science of Natural Philosophy, &c. 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 examination of general laws, or of abstract and speculative principles. The theory of music is a science; the practice of it is all art. XVI TNTRODUCTION. Science differs friom art in the same manner that knowledge 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 knowledge, though he have not the least skill to perform any operation 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 explains the principles on which tools and machines are constructed, 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. DIVISIONS OF THE SUBJECT. i 1. NATURAL PHILOSOPHY, or PHYSICS, is the WMat is iNatural science which treats of the powers, properties and Philoso- mutual action of natural bodies, and the laws and phy? operations of the material- world. 2. Some of the principal branches of Natural Philosophy are Mechanics, Electricity, Pneumatics, Galvanism, Hydrostatics, Magnetism, Hydraulics, Electro-Magnetism, Acoustics, Magneto-Electricity, Py ronomics, Astronomy. Optics, NOTE. - This list of branches might be considerably enlarged, but perhaps a rigid classification would rather suggest the omission of some of them, as pertaining to the department of chemistry. What is 3. MECHANICS. - Lechanics is that branch of Mechan- Natural Philosophy which relates to motion and cs? the moving powers, their nature and laws, with their effects in machines. 4. Mechanics is generally considered under two divisions, called Statics and Dl)ynamics. 2s 18 NATURAL PHILOSOPHY. 5. The word Statics is derived from a Greek word implying rest, and it is applied.to that department of mechanics which treats of the properties and laws of bodies at rest. 6. Dynamics, from a Greek word signifying power or force, treats of the properties and laws of bodies in mnolion. 7. Pneunzatics treats of the mechanical properties and effects of air and similar fluids, called elastic fluids or gases. 8. Hydrostatics treats of the gravity and pressure of fluids in a state of rest. 9. Hydraulics treats of fluids in motion, and of the instruments and machines by which their motion is guided or controlled. 10. Acoustics treats of the laws of sound. 11. Pyronomics treats of the laws and effects of heat. 12. Optics treats of light, color and vision. 13. Electricity treats of an exceedingly subtle agent, called the electric fluid. 14. Galvanism (sometimes called chemical electricity) is a branch of Electricity. 15. Magnetism treats of the properties and effects of the magnet or loadstone. 16. Electro-Magnetism treats of magnetism induced by electricity. 1 7. Magneto-Electricity treats of electricity induced by magnetism. 18. Astronomy treats of the heavenly bodies, -the sun, moon, stars, planets, comets. 19. The agents whose effects or operations are described in Natural Philosophy are divided into two classes, called respectively Ponderable and Imponderable Agents. NOTE. - Some writers on Philosophy have suggested a different classification, into Bodies and Agents, calling bodies ponderable, and agents imponderable. 20. Ponderable agents are those which have weight, as water, air, steam. 21. Imponderable agents are those which have no weight such as light, heat, magnetism and electricity. OF MATTER AND ITS PROPERTIES. 19 What is 22. MATTER. - Matter is the general name of Maiter? everything that occupies space. 23. Matter exists in four different states or forms, namely, in the solid, liquid, gaseous and vesicular forms. 24. Matter exists in a solid form when the particles of which it is composed cohere together. The different degrees of cohesion which different bodies possess causes them to assume different degrees of hardness. 25. Matter exists in a liquid state Zhen the component parts do not cohere with sufficient force to prevent their separation by the mere influence of their weight. The surface of a fluid at rest always conforms itself to the shape of the portion of the earth's surface over which it stands. 26. Matter exists in a -gaseous or a/riform state when the particles of which it is composed have a repulsion towards each other which causes them to separate with a power of expansion to which there is no known limit. Of this, smoke presents a familiar instance. As it ascends it expands, the particles repelling each other until they become wholly invisible. NOTE. - The word aeriformn means, in the form of air. 27. The vesicular form of matter is the form in which we see it in clouds. It consists of very minute vesicles, resembling bubbles, and it is the state into which many vapors pass before they assume a fluid condition. 28. Some substances are capable, under certain conditions, of assuming all these different forms. Water, for instance, is solid in the form of ice, fluid as water, in the gaseous state when converted into steam, and vesicular in the form of clouds. 29. All matter, whether in the solid, liquid, gaseous, or vesicular form, is either simple or compound in its nature. But this consideration of matter pertains more properly to the science of chemistry. It is proper, however, here to explain what is meant by a simple or homogeneous and a compound or heterogeneous substance. 30. All matter is composed of very minute particles or atoms, united together by diffirent degrees of cohesion. When all the atous are of the same kind, the body is a simple or homogeneous substance. Thus, for instance, pure iron, pure gold, &c., consists of very minute particles or atoms, all of which are pure iron or pure gold. But water, and many other substances, are compound substances, composed of atoms of two or more different substances, combined by chemical affinity. NOTE. - The ancient philosophers supposed that all material substances wore composed of Fire, Air, Earth and Water, and these four substances were called the four elements, because they were supposed to be the simple :20 NATURAL PHILOSOPHY. substances of which all things are composed. But modern science has shown that not one of these is a simple substance. Water, for instance, is momposed of two invisible gases, called Hydrogen and Oxygen, united in the proportion of one part, in weight, of hydrogen to eight of oxygen; or, by measure, one part of oxygen to two of hydrogen. In like manner air, or, rather, what the ancients understood by air, is composed of oxygen united with another invisible gas, called nitrogen or azote, in the proportion of seventy-two parts of the latter to twenty-eight of the former. The enumeration of the elementary substances, which, either by themselves or in union with one another, make up the material world, properly belongs to the science of chemistry. As this work may fall into the hands of some who will not find the information elsewhere, a list of the simple substances or elements is here presented, so far as modern science has investigated themn. They are sixty-one in number, forty-nine of which are metallic and twelve are non-metallic. The forty-nine metals are Gold, Manganese, Potassiu/n, Didynium, Silver, Cadmlium, Sodium, Erbiunm, Iron, u rani um, Lithium, 7Terbium, Copper, Palladium, Barium, um, Ruthenium, Tin, Rhodium, Strontium, Pelopium,.Mlercury, Iridium, Calcium, Nliobziun. Lead, Osmium, Magnesium, Selenium. Zinc, Titaniuml, Aluminum, [T'his substance is of 7ues Nickel, ColumLbium, Glucinum, tionable nature, sonme of Cobalt, Tellurium, Yttrium, its pr,operties indicating a Bismuth, Tungsten, Zirconium, metallic and some a nonP'latinum, Molybdenum, Thorium, metallic character.] Antimony, Vanadium, Cerium, [The last seven, in ltalic, Arsenic, Chromium, Lantanium, have not yet been fully investigated. The non-metallic elements are Oxygen, Sulphur, Chlorine, Fluorine, Hydrogen, Phosphorus, Bromine, Borax, Nitrogen, Carbon, Iodine, Silica. Of the elementary substances now enumerated, about fourteen constitute the great mass of our earth and its atmosphere. The remainder occur only in comparatively small quantities, while nearly a third of the whole number is so rare that their uses in the great economy of nature are not underitood, nor have they as yet admitted of any useful application. The science of Geology reveals to us the fact that raCnite appears to be the foundation of the crust of the earth; and in the granite, either in its original formation, or in veins or seams which have been thrown up by subterranean forces into the granite, all of the elementary substances which have been enumerated are to be found. A chart is presented below in which the materials composing the strata of the crust of the earth are enumerated, together with a tabular view of the composition of these materials. It is not contended that this chart is perfectly accurate in all its details; but, as it affords an interesting alld extensive subject of investigation, and as it is not to be found elsewhere in print, it is thought that it will be well worth the space which it occupies, although a rigid classification would exclude it from this work. OF MATTER AND ITS PROPERTIES. 21 Dr. Boynton's Chart of Materials that enter into the Composition of Granite. _:t 0 02 h 0 Quartz....... 100 Feldspar...... 65 19 14 1 Albite..... 70 20 10 Mica.... 46 261 1 5 8 1 2 2 Fluor. Acid. Prot. Ox. Hornblende.. 48 12 14 19 7 Auite... 4 1 24 17 4 Diallage...... 47 4 13 25 8 3 M1. Chlorite.... 27 18 2 15 31 7 Talc.r.. 5... 7 1 4 27 8 3 Hyllpersthene.... 56 2 2 14 25 1 Actynolite...... 56 2 12 13 17 M1. Steatite...... 62 1 1 28 2 6 Serpentine..... 42 5 33 7 13 Schorl...... 36 36 1 2 5 14 2 4 B. Acid. Prot. Garnet..... 40 20 1 36 3 Prot. PIot. M. Garnet..... 36 181 15 31 Clay....... 75 10 5 2 3 Prot. Green Sand... 48 7. 8 26 11 Carbonate of Lime.. 56 44 Carb. Acid. Carbonate of Magnesia 48 2 50 " " What are 31. There are seven essential $ properties bethe essen- longing to matter, namely, 1. Impenetrability; lial Propertiesof 2. Extension; 3. Figure; 4. Divisibility; 5. InMatter? destructibility; 6. Inertia; 7. Attraction. What is 32. IMPENETRABILITY. - Impenetrability is the l]mpenetrability? power of occupying a certain portion of space, so * An essential property of a body is that which is necessary to the absolute existence of the body. All matter in common possesses these essential properties, and no particle of matter can exist without any one of them. Different bodies possess other different properties which are not essential to their existence, such as color, weight, brittleness, hardness, &c. These are called accidental properties, as they depend on circum stances not essential to the very existence of a body. 22 NzATURAL PHILOSOPHY. that where one body is another cannot be without displacing it. 33. This property, Impenetrability, belongs to all bodies and forms of matter, whether solid, fluid, gaseous, or vesicular. The impenetrability of common air may be shown by immers: ng an inverted tumbler in a vessel of water. The air prevents the water from rising into the tumbler. An empty bottle, also, f.lr;ilbly held horizontally under the water, will exhibit the same property; for the bottle, apparently empty, is filled with air, which escapes in bubbles from the bottle as the water enters it. But, if the bottle be inverted, the water cannot enter the bottle, on account of the impenetrability of the air within.* $ Tlis circumstance explains the reason why water, or any other liquid, poured into a tunnel closely inserted in the mouth of a decanter, will run over the sides of the decanter. The air filling the decanter, and having no means of escape, prevents the fluid from entering the decanter; but, if the tunnel be lifted from the decanter but a little, so as to afford the air an opportunity to escape, the water will then flow into the decanter in an uninterrupted stream. When a nail is driven into wood or any other substances, it forces the particles asunder and makes its way between them. An experiment was made at Florence, many years ago, to show the impenetrability of water. A hollow globe of gold was filled with water and subjected to great pressure. The water, having no other means of escape, was seen to exude from the pores of the gold. The reason why fluids appear less impenetrable than solids is that the particles which compose the fluids move easily among themselves, on account of their slight degree of cohesion, and when any pressure is exerted upon a fluid the particles move readily into the unoccupied space to which they have access. But, if the fluid be surrounded on all sides, and have no means of escape, it will be found to possess the property of impenetrability in no less a degree than solid bodies. A well-known fact seems, at first view, to be at variance with this statement. When a vessel is filled to the brim with water or other fluid, a considerable portion of salt may be dropped into the fluid without causing the vessel to overflow. And, when salt has been added until the water can hold no more in solution, a considerable quantity of sugar can be added in a similar manner. The explanation of this familiar fact is as follows. The particles of the sugar are smaller than the particles of the salt, and the particles of the salt are smaller than the particles which compose the water. Now, supposing all of these particles to be globular, they will arrange themselves as is represented in Fig. 1, in which the particles of the water are indicated by the largest circles, those of the salt by the next in size, and those of the sugar by the smallest. Familiar Experiment. - Fill a bowl or tumbler with peas, then pour on Mie peas mustard-seed or fine grain, shaking the vessel to cause it to fill the vacant spaces between the peas. In like manner add, successively, fine sanld, water, salt and sugar. This will afford an illustration of the apparenlt paradox mf two bodies occupying the same space, and show that it is only apporent. OF MATTER AND ITS PROPERTIES. 23 Wha/t 2 34. EXTENSION. -Extension is but another Exten- name for bulk or size, and it is expressed by the Sionl terms length, breadth or width, height, depth and thickness. NOTE. - Length is the extent from end to end. Breadth or width is the extent from side to side. Height, 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. What is 35. Figure is the form or shape of a body. Figure? 36. Figure and Extension are separate properties, although both may be represented by the same terams, length, breadth, &c. But they differ as the words shape and size differ. Two bodies may be of the same figure or shape, but of vastly different size. A grape and an orange resemble each other in shape, but differ widely in size. The limits of extension constitute figure, but figure has no other connexion with extension. What is 37. DIVISIBILITY.- Divisibility is susceptibility Divisi- i bility? of being divided. 38. To the divisibility of matter there is no known limit, nor can we conceive of anything so small that it is not made up of two halves or four quarters. It is indeed true that our senses are quite limited in their operation, and that we cannot perceive or take cognizance, by means of our senses, of many objects of the existence of which we are convinced without their immediate and direct testimony. 39. Sir Isaac Newton has shown that the thickest part of a soapbubble does not exceed the two-millionth part of an inch. 40. The microscopic observations of Ehrenberg have proved that there are many species of little creatures, called lnfusoria, so small that millions of them collected into a single mass would not exceed the bulk of a grain of sand, and thousands of them might swim side by side through the eye of a small needle. 41. In the slate formations in Bohemia these little creatures aro found in a fossil state, so small that it would require a hundred and eighty-seven millions of them to weigh a single grain. 42. A single thread of the spider's web has been found to be composed of six thousand filaments. 43. 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-millionth part of a grain of gold, and yet it may be seen with the naked eye. 24 NATURAL PHILOSOPHY. 44. The particles which escape from odoriferous objects also affbrd instances of extreme divisibility. What is Inde- 45. INDESTRUCTIBILITY. — By the Indestructistructi- bility of matter is meant that it cannot be destroyed. lility? 46. A body may be indefinitely divided or altered in its form. color, and other unessential properties, but it can never be destroyed by man. It must continue to exist in some form, with all its essential properties, through all its changes of external appearance. HE alone " who can create can destroy." 47. When water disappears, either by boiling over afire or by evaporation under the beat of the sun, it is not destroyed, but merely changed from a liquid to a fluid form, and becomes steam or vapor. Some of its unessential properties are altered, but its essential properties remain the same, under all the changes which it undergoes. In the fiorm of water it has no elasticity * and but a limited degree of' compressibility.* But when " it dries up" (as it is called) it rises in the form of steam or vapor, and expands to such a degree as to become invisible. It then assumes other properties, not possessed before (such as elasticity and expansibility); it ascends in the air and forms clouds; these clouds, affected by the temperature of the air and other agents, again fall to the earth in the form of rain, hail, snow or sleet, and form springs, fountains, rivers, &c. The water on or in the earth, therefore, is constantly changing its shape or situation, but no particle of it is ever actually destroyed. 48. Substances used as fuel, whether in the form of wood, coal, or other materials, in like manner undergo many changes by the process of combustion. Parts of them rise in the form of smoke, part ascends in vapor, while the remainder is reduced to the form of ashes; but no part is absolutely destroyed. Combustion merely disunites the simple substances of which the burning materials are composed, forming them into new combinations; but every part still continues in existence, and retains all the essential t properties of bodies. What is 49. INERTIA. -InertiaS is the resistance of Inertia? matter to a change of state, whether of motion or of rest. * Late writers assert that water has a slight degree both of elasticity and expansibility. t The retader will be careful to carry in his mind what is meant'by the terrn an essential property. It is explained in the note to No. 31, page 21. t The literal meaning of inertia is inactivity, and implies inability to change a state of' rest or of motion. A clear and distinct understanding of this property of all matter is essential in all the departments of material philosopliy. All matter, mechanically considered, must be in a state either OF MATTER AND ITS PROPERTIES. 25 50. A body at rest cannot put itself in motion, nor can a body In motion stop itself. This incapacity to change its state from rest to motion, or from motion to a state of rest, is what is implied by the term inertia. 51. It follows, therefore, from what has just been stated, that when a body is in motion its inertia will cause it to continue to movo until its motion is destroyed by some other force. 52. There are two forces constantly exerted around us which tend to destroy motion, namely, gravity and the resistance of the air. All motion caused by animal or mechanical power is affected by these two forces. Gravity (which will presently be explained) causes all bodies, whether in motion or at rest, to tend towards the centre of the earth, and the air presents a resistance to all bodies moving in it. Could these and all other direct Fig. 2. obstacles to motion be set aside, a body when once put in motion would always remain in motion, and a body at rest, unaffected by any external force, would always remain at rest.* 53. Experiment to illustrate Inertia. — Fig. 2 represents the simple apparatus of Mr. Wightman for illustrating the inertia of a body. A card is placed on the top of a stand, and a ball is balanced on the card. A quick motion is of motion or rest; and, in whatever state it may be, it must remain in that state until a change is effected by some efficient cause, independent of the body itself. A body placed upon another body in motion partakes of the motion of the body on which it is placed. But, if that body be suddenly stopped, the superincumbent body will not stop at the same time, unless it be securely fastened. Thus, if a horse moving at a rapid rate be suddenly stopped, the rider will be thrown forward, on account of this inertia of his body, unless by extra exertion he secures himself on the saddle by bracing his feet on the stirrups. On the contrary, if the horse, from a state of rest, start suddenly forward, the rider will be thrown backwards. For the same reason, when a person jumps from a vehicle in motion to the ground, his body, partaking of the motion of the vehicle, cannot be suddenly brought to a state of rest by his feet resting on the ground, but will be thrown forward in the direction of the motion which it has acquired from the vehicle. This is the reason that so many accidents happen from leaping from a vehicle in motion. *- In the absence of all positive proof from the things around us of the statement just made, we may find from the truths which astronomy teaches that inertia is one of the necessary properties of all matter. The heavenly bodies, launched by the hand of their Creator into the fields of Infinite space, with no opposing force but gravity alone, have performed their stated revolutions in perfect consistency with the character which this property gives them; and all the calculations which have been made with respect to them, verified as they have repeatedly been by observation, have been predicated on their possession of this necessary property of allmatter..g 26 NATURAL PHILOSOPHY. then given to the card by means of a spring, and the card flies off, leaving the ball on the. top of the stand.* 54. Nature seems to have engrafted some knowledge of mechanical laws on the instinct of animals. When an animal, and especially a large animal, is in rapid motion, he cannot (on account of the inertia of his body) suddenly stop his motion, or change its direction; and the larger the animal the more difficult does a sudden stoppage become. The hare pursued by the hound often escapes, when the dog is neatly upon him, by a sudden turn, or changing the direction of its flight, thus gaining time upon his pursuer, whose inertia is not so readily overcome, and who is thus impelled forward beyond the spot where the hare turned. 55. Children at play are in the same manner enabled "to dodge" their elder playmates, and the activity of a boy will often enable him to escape the pursuit of a man. 56. It is the effect of inertia to render us sensible to motion. A person in motion would be quite unconscious of that state, were it not for the obstacles which have a tendency to impede his progress. In a boat on smooth water, motion is perceptible only by the apparent change in the position of surrounding objects; but, if the course of the boat be interrupted by running aground, or striking against a rock, the person in the boat would feel the shock caused by the sudden change from a state of motion to a state of rest, and, unless secured to his seat in the boat, he would be precipitated forward What is At- 57. ATTRACTION.-Attraction is the tendency traction? which different bodies or portions of matter have to approach or to adhere to each other. What is the 58. Every portion of matter is attracted by every law of At- other portion of matter, and this attraction is the traction? stronger in proportion to the quantity and the distance. The larger the quantity and the less the distance, the stronger is the attraction.t * The ball remains on the pillar in this case not solely from its inertit, but because sufficient motion is not communicated to the ball by the friction of the card-to counteract the effect of gravity on the ball. If the ball, therefore, be not accurately balanced on the card, the experiment will not be successful, because the card cannot move without communicating at least a portion of its motion to the ball. t IN. B. This subject will be more fully treated under the head of Grawity. See page 33.] OF MATTER AND ITS PROPERTIES. 27 ftlow many 59. There are two kinds of attraction, namely, trationinds of At- the Attraction of Gravitation and the Attraction there z of Cohesion. The former belongs to all matter, whatever its form; tho latter appears to belong principally to solid bodies. What zs the 60. The Attraction of Gravitation is the Attraction of Gravi- reciprocal attraction- of separate portions of tation 2 matter. What is tne 61. The Attraction of Cohesion is that which Attraction causes the particles of a body to cohere together. of Cohesion? [See No. 31.] 62. The attraction of cohesion appears to exist but in a very slight degree, if at all, in liquids and fluids. Exemplify the two kinds 63. The attraction of gravitation causes a body, of Attrac- when unsupported, to fall to the ground. The tion; nameIy, Gravity attraction of cohesion holds together the particles and Cohesive of a body, and causes them to unite in masses.* Attraction? 64. Having described the essential properties of bodies, we come now to the consideration of other properties belonging respectively to different kinds-of matter; such as Porosity, Density, Rarity, Compressibility, Expansibility, Mobility, Elasticity, Brittleness, Flexibility, Malleability, Ductility, Tenacity. 65. It has already been stated that matter consists of minute particles or atoms, united by different degrees of cohesive attraction. These atoms are probably of different shapes in different bodies, and the different degrees of compactness with which they unite give rise to certain qualities, which differ greatly in different substances. These qualities or properties are described under the names of Porosity, Density and Rarity, which will presently be described. * Besides these two kinds of attraction, there seem to be other kinds of attractive force, active in vegetation and in animal life, known by the names of Endosmose and Exosmose, terms applied to the transmission of gaseous matter or vapors through membranous substances. See note to Capillary Attraction, under the head, of Hydrostatwcs, on page 11. Other kinds of attraction, called Electrical and Magnetical Attraction, will hereafter be considered under their appropriate head. The subject of Chemical Attraction or Affinity belongs distinctly to the subject of Chemistry and will not, therefore, be considered in this work 28 NATUR L PHILOSOPHY. 66. Besides the property of attraction possessed by the particles or atoms of which a body is composed, there seems to be another property, of a nature directly opposite to attraction, which exerts itself with a repulsive force, to prevent a closer approximation of the particles than that which by'the law of their nature they assume. This property is called repulsion. This repulsion prevents the particles or atoms from coming into perfect contact, so that there must be small spaces between them, where they do not absolutely touch one another. [See Figure lst.] These spaces are called pores, and where they exist give rise to that property or quality described under the name of Porosity. What is 67. POROSITY.- Porosity implies, therefore, that Porosity? there are spaces, or pores, between the particles or atoms which form the mass of a body. 68. DENSITY. - When the pores are few, so that a large number of particles unite in a small mass, the body is called a dense body. What is 69. Density, therefore, implies the closeness or Density? compactness of the particles which compose any substance. 70. RARITY. — When the pores in any substance are numerous so that the particles which form it touch one another in only a few points, the body is called a rare body. What is 71. Rarity, therefore, is the reverse of density, Rarity. and implies extension of bulk without increase in the quantity of matter. 72. From what has now been stated it appears [See No. 67] that the particles of a body are connected together by a system of attractions and repulsions which give rise to distinctions which have already been described. It remains to be stated that these attractions and repulsions differ much in degree in different substances, and this difference gives rise to other properties, which will now be explained, under their appropriate names. 73. COMPREssIBILITY. —When the repulsion of the particles of any substance can be overcome and the mass can be reduced within narrower limits of extension, it is said to possess the property of Compressibility.* * Compressibility differs from Contractibility rather in cause than in effect. Contractibility implies a change of bulk caused by change of temperature, or any other agency not mechanical. Compressibility implies that the diminution of bulk'is caused by some external mechanical force. OF MATTER AND ITS PROPERTIES. 29 Wha~t is 74. Compressibility, therefore, may be defined, the Cornpres-.susceptibility of a reduction of the limits of exsibility? tension. 75. This property is possessed by all known substances, but in Very different degrees,- some substances requiring but little force to compress them, others resisting very great forces; but it is not known that there is any substance unsusceptible of compression, if a sufficient force be applied.* 76. Liquids in general are less easily compressed than solid bodies; so much so, indeed, that in practical science they are generally considered as incompressible. Under a very considerable mechanical force, a slight degree of compression has been observed.j 77. EXPANSIBILITY.-The system of attractions and repulsions among the particles of a body are sometimes so equally balanced that thev exist, as it were, in an equilibrium. In other cases the repulsive energy is so great as to predominate when the attractive force is unaided. When the repulsive energy is permitted to act without restraint, it forces the particles asunder and increases the limits of extension, giving rise to another property of matter possessed by many bodies, but in an eminent degree by matter in a gaseous form. This property is called Expansibility. What is 78. Expansibility,$ therefore, may be defined F xpansi- as that property of matter by which it is enabled to increase its limits of extension. 79. ELASTICITY. - When the atoms or particles which constitute a body are so balanced by a system of attractions and repulsions that they resist any force which tends to change the figure of the * Sir Isaac Newton conjectured that if the earth were so compressed as to be absolutely without pores, its dimensions might not exceed a cubic inch. t Under a pressure of fifteen pounds on a square inch, water has been diminished in bulk only by about forty-nine parts in a million. Under a pressure of fifteen thousand pounds on a square inch, it was compressed by about one-twentieth of its volume. The experiment was tried in a cannon, and the cannon was burst. t Expansibility and Dilatability are but different names for the same property; but expansion implies an augmentation of the bulk or volume, dependent on mechanical agency, while dilatation expresses the same condition produced by some physical cause not properly falling under the denomination of mechanical force. Thus heat dilates most substances, while cold contracts them. It is on this principle that the thermometer is constructed. [See page 149, No. 546.] All gaseous bodies are invested with the property of dilatability to an unlimited degree, by means of which, when unrestrained, they will expand spontaneously, and that without the application of any external agency, to a degree to which there is no known limit. 30 NATURAL PHILOSOPHY. body, they will possess another property, known by the name of Elasticity. Wlat is 80, Elasticity, therefore is the property which Elastic- causes a body to resume its shape after it has been'ty? compressed or expanded.@ 81. Thus, when a bow or a steel spring is bent, its elasticity causes it to resume its shape. 82. India rubber (or caoutchouc) possesses the property of elasticity in a remarkable degree, but steam and other bodies in a gaseous form in a still greater.t 83. Ivory is endowed with the property of elasticity in a remarkable degree, but exhibits it not so much by the mere force of pressure, but it requires the force of impact to produce change of form41 What is 84. BRITTLENESS. - Brittleness implies aptness Brittleness.? to break into irregular fragments.~ $ This property is possessed, in at least some small degree, by all substances; or, at least, it cannot be said that any substance is wholly destitute of elasticity. Even water and other liquids, which yield with difficulty to compression, recover their volume with a force apparently equal to the compressing force. But, for most practical purposes, many substances, such as putty, wet paste, moist paper, clay, and similar bodies, afford examples of substances possessing the property of elasticity in so slight a degree that they are treated as non-elastic bodies. t The gases or adriform bodies afford the most remarkable instances of elasticity. When water is converted into steam it occupies a space seventeen hundred times greater than the water from which it is formed, and its elasticity causes it to expand to still larger dimensions on the application of heat. It is this peculiar property of steam, modified, as will be explained in a future part of this work, which is the foundation of its application in the movement of machinery.'All gaseous bodies are equally elastic. i The metals which are best adapted to produce sound are those which are most highly elastic. It sometimes happens that two metals, neither of which have any great degree of hardness or elasticity, when combined in certain proportions, will acquire both of these properties. Thus tin and copper, neither of which in a pure state is hard or elastic, when mixed in a certain proportion, produce a compound so hard and elastic that it is eminent for its sonorous property, and is used for making bells, &c ~ Brittleness and hardness are properties which frequently accompany each other, and brittleness is not inconsistent with elasticity. Thu glass, for instance, which's the most- brittle of all known substances, is highly elastic. The same body may acquire or be divested of its brittleaess according to the treatment which it receives. Thus iron, and some other metals, when heated and suddenly plunged into cold water, become brittle; but if, in a heated state, they are buried in hot sand, and thus be OF MATTER AND ITS PROPERTIES. 1 What is 85. FLEXIBILITY. - Flexibility implies a disFlexiFeitiy- position to yield without breaking when bent. ility i What is 86. MALLEABILITY. - Malleability implies that iMallea- property by means of which a body may be rebility? duced to the form of thin plates by means of the hammer or the pressure of rollers. 87. This property is possessed in an. eminent degree by some of the metals, especially gold, silver, iron and copper, and it is of vast importance in the arts. A knowledgeof the uses of iron, and of its malleability, is one of the first steps from a savage to a civilized state of life. 88. The most malleable of the metals is gold, which may be hammered to such a degree of thinness as to require three hundred and sixty thousand leaves to equal an inch in thickness.* 89. DUCTILITY.-Some substances admit of being extended simultaneously both in length and breadth. Others can be extended to a greater degree in length alone; and this property gives rise to another name, called Ductility. What is 90. DUCTILITY. -Ductility is that property Ductility? which renders a substance susceptible of being Irawn out into wire. 91. The same metals are not always both ductile and malleable to the same degree. Thus iron may be beaten into any form, when heated, but not into very thin plates, but it can be drawn into extremely fine wire. Tin and lead, on the contrary, cannot be drawn out into small wire, but they are susceptible of being beaten into extremely thin leaves. 92. Gold and platinum have a high degree both of ductility and malleability. Gold can be beaten (as has already been stated) into permitted to cool very gradually, they will lose their brittleness and acquire the opposite quality of flexibility. This process in the arts is called annealing. * The malleability of the metals varies according to their temperature. Iron is most malleable at a white heat. Zinc becomes malleable at the temperature of 300; or 400~. Some of the metals, and especially antimony, arsenic, bismuth' and cobalt, possess scarcely any degree of this property. The familiar process of welding is dependent on malleability. The two pieces of metal to be welded are first heated to that temperature at which they are most malleable, and, the ends being placed together, the particles are driven into such intimate connexion by the welding-haimmer that they cohere. Different metals may in some cases be thus welded together. -32 INATURAL PHILOSOPHY. leaves so thin that it would require many thousands of them to equal an inch in thickness. It has also been drawn into wire so attenu ated that one hundred and eighty yards of it would not weigh more than a single grain. An ounce of such wire would be more than fifty miles in length. But platinum can be drawn even to a finer wire than this. What is 93. TENACITIY. - Tenacity implies the adhesion Tenacity? of the particles of a body. 94. Tenacity is one of the great elements of strength. It is the absence of tenacity -which constitutes brittleness. Both imply strength, but in different forms. Thus glass, the most brittle of all substances, has a great degree of tenacity. A slender rod of glass, which cannot resist the slightest lateral pressure; if suspended vertically by one end will sustain a very considerable weight at the other end.*. A knowledge of the tenacity of different substances is of great use in the arts. The tenacity of metals and other substances has been ascertained by suspending weights from. wires of the metals, or rods and cords of different materials. The following table presents very nearly the weights sustained by wires of different metals, each having the diameter of about one-twelfth of an inch. Lead, 27 pounds. Silver, 187 pounds. Tin, 34 " Platinum, 274 Zinc, 109 " Copper, 302 Gold, 150 " Iron, 549 " Cords of different materials, but of the same diameter, sustained the following weights: Common flax, 1175 pounds. New Zealand flax, 2380 pounds Hemp, 1633 " Silk, 3400 The following table presents a more extended list of materials. The area of a transverse section of the rods on which the experiment was tried was one square inch. Pounds Avoirdupois. Pounds Avoirdupois. English Oak, 8,000 to 12,000 Tin, 7,129 Fir, 11,000 Lead, 3,146 Beech, 11,500 Rope, 1 inch in circumMahogany, 8,000 ference, 1,000 Teak, 15,000 Whale line, 2 inches in Cast Steel, 134,256 circumference, spun by Iron Wire, 93,964 hand, 2,240 Swedish Bar-iron, 72,064 Do., by machinery, 3,520 Cast-iron, 18,656 Rope, 3 inches in circumWrought Copper, 33,792 feronce, 5,628 Platinum Wire, 52,987 Do., 4 inches, 9,988 Silver Wire, 38,257 Cable, 141 inches, 89,600. Gold, 30,888 Do., 23 inches, 255,360 Zinc, 22,551 A more particular account of the tenacity of various substances will be OF GRAVITY. 33 95. The tenacity of metals is much increased by uniting them.. A compound consisting of five parts of gold and one of copper has a tenacity of more than double that of the gold or copper alone; and brass, which is composed of copper and zinc, has a tenacity more than double that of the copper, and nearly twenty times as great as that of the zinc alone. A mixture of three parts of tin and one of lead has a tenacity more than double that of the tin; and a mixture of eight parts of lead and one of zinc has a tenacity nearly double that of the zinc, and nearly five times that of the lead alone.* 96. GRAVITY.-It has already been stated that matter in all its forms, whether solid, fluid or gaseous, possesses the property of attraction7. This property, with its laws, is now to be particularly considered, under the name of Gravity. What is 97. Gravity is the reciprocal attraction of sepGravity? arate portions of matter. -With what All bodies attract each other with a force proforce do all bodies at- portionate to their size, density and distance from other ach each other. [See No. 59.] other? 98. This law explains the reason why a body which is not supported falls to the earth. Two bodies existing in any portion of space mutually attract each other, and would rush together were they not prevented by some superior force. Let us suppose, for instance, that two balls made of the same materials. but one weighing 11 pounds and the other weighing only one pound, were ten feet apart, but both were a hundred feet above the surface of the earth. According to this law, the two balls would rush together, the lighter ball passing over nine feet of the distance, and the heavier ball over one foot; and this they would do, were they not both prevented by Ia superior force. That superiorforce is the earth, which, being a much lar ger body, attracts them both with a superiorforce. This superior force they 3will both obey, and both will therefore fall to the earth. As the attraction of the earth and of the balls is mutual, the earth will also move towards the balls while the balls are falling to the earth; but the size of the earth is so much greater than that of the balls, that the distance that the earth would move towards the balls would be too small to be appreciated.t found in Barlow's Essay on the Strength of Timber, Rennie's Treatise (in Phil. Trans. 1818), Tredgold's Principles of Carpentry, and the 4th vol. of Manchester Memoirs, by Mr. Hodgkinson. # There are many other specific properties of bodies besides those that have now been enumerated, the consideration of which belongs to the science of Chemistry. t The earth is one quatrillion, that is, one thousand million millions times larger than the largest body which has ever been known to fall 34 NATURAL PHILOSOPHY. 99. The attraction of the earth is the cause of what we call weight. When we say that a body weighs an ounce, a pound, or a ton, we express by these terms the degree of attraction by which it is drawn towards the earth. Therefore, What is 100. Weight is the measure of the earth's atWeight? traction.* 101. As this attraction depends upon the quantity of matter which a body contains, it follows that What' bodies have t-he Those bodies will have the greatest weight greatest which contain the greatest quantity of matter.t weight? 102. TERRESTRIAL GRAVITY. - It has already been stated [see No. 97] that the attraction which one mass of matter has for another is in proportion to the quantity and the distance; and that, the larger the quantity of matter and the less its distance, the stronger will be the attraction. The law of this attraction may be stated as follows: What is the 103. Every portion of matter attracts every law of at- other portion of matter with a force proportioned traction? directly to the quantity, and inversely as the square of the distance. through our atmosphere. Supposing, then, that such a body should fall through a distance of one thousand feet, the earth would rise no more than the hundred billionth part of an inch, a distance altogether imperceptible to ojur senses. The principle of mutual attraction is not confined to the earth. It ex. tends to the sun, the planets, comets and stars. The earth attracts each of them, and each of them 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, summer and winter, and all the grand operations which are described in astronomy.' 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. t The weight of a body is not dependent solely on its size or bulk; its density must also be considered. If we take an equal quantity, by measure, of two substances,- lead and cork, for instance,- we shall find that, although both are of the same size, the lead will weigh much more than the cork. The cork is more porous than the lead, and, consequently, the particles of which it is composed must be further apart, and therefore there must be fewer of them within a given bulk; while, in the lead, the pores are much smaller, and the particles will, therefore, be crowded into a much smaller space OF' GRAVITY. 35 104.- Let us now apply this law to terrestrial gravity -that is, to the earth's'attraction; and, for that purpose, let us suppose four balls, of the same size and density, to be placed respectively as follows, namely: The first at the centre of the earth. The second on the surface of the earth. The third above the earth's surface, at twice the distance of the surface from the centre (that distance being four thousand miles). The fourth to be half way between the surface and the centre. To ascertain the attractive force of the earth on each of these balls, we reason thus: fhe first ball (at the centre) will be surrounded on all sides by an equal quantity of matter, and it will remain at rest. The second ball will be attracted downwards to the centre by the whole mass below it.'The third ball, being at twice the distance from the surface (gravity decreasing as the square of the distance increases), will be attracted by a force equal to only one-fourth of that at the surface. The fourth ball, being attracted downwards by that portion of the earth which is below it, and upwards by that portion which is above it, will be influenced only by the difference between these two opposite attractions; and, as the downward attraction is twice as great as the upward, the downward attraction will prevail with half its original force, the other half being balanced by the upward attrac-, tion. 105. As weight is the measure of the earth's attraction, we may represent this principle by the weight of the balls, as follows (supposing the weight of each ball, at the surface of the earth, to be one pound): The first ball will weigh nothing. The second will weigh one pound. The third will weigh one-quarter of a pound. The fourth will weigh one-half of a pound. The law of terrestrial gravity, then, may be stated as follows: What is the 106. The force of gravity is greatest at the surlaw of Ter- face of the earth, and it decreases upwards as the restrial square of the distance from the centre increases, Gravity? and downwards simply as the distance from the centre decreases. 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. 1000 miles from the centre it will weigh ~ of a pound. 2000 " " " " " of a pound. 3000 " " " " J" " " ofa pound. 4000 " " " " " " 1 pound. 86 NATURAL PHILOSOPHY. 8000 miles from the centre it will weigh ~ of a pound. 12000 " 4 16000 " " " " " " 20000 " " " " " ". 24000'' " " " " ~. 28000 " 32000' " If the principles that have now been stated have been understood, the solution of the following questions will not be difficult. 107. Questions for Solutzon. [N. B. We use the term weight in these questions in its philosophical sense, as " the measure of the earth's attraction at the surface."] (1.) Suppose that a body weighing 800 pounds could be sunk 500 miles deep into the earth, - what would it weigh? Solution. 500 miles is A of 4000 miles; and, as the distance from the centreo is decreased by i, its weight would also be decreased in the same proportion, and the body would weigh 700 pounds. (2.) Suppose a body weighing 2 tons were sunk one mile below the surface of the earth, what would it weigh? Ans. 1.99951T. (3.) If a load of coal weighs six tons at the surface of the earth, what would it weigh in the mine from which it was taken, supposing the mine were at a perpendicular distance of half a mile from the surface? Ans. 5.99925T. (4.) If the fossil bones of an animal dug from a depth of 5228 feet from the surface, weigh four tons, what would be their weight at the depth where they were exhumed? Ans. 3 T. 19cwt. 981b. + (5.) If a cubic yard of lead weigh 12 tons at the- surface of the earth, what would it weigh at the distance of 1000 miles from the centre? A ns. 3 T. (6.) If a body on the surface of the earth weigh 4 tons, what would be its weight if it were elevated a thousand miles above the surface? Solution. Square the two distances 4000 and 5000, &c. Tons. cwt. qrs. lbs. Answer. 2 11 0 20. (7.) Which will weigh the most, a body of 3000 tons at the distance of 4 millions of miles from the earth, or a body of 4000 tons at the distance of 3 millions of miles Ans..003T. and.0071T. + (8.) How far above the surface of the earth must a pound weight be carried to make it weigh one ounce avoirdupois? Ans. 12000 mi. (9.) If a body weigh 2 tons when at the distance of a thousand miles above the surface of the earth, what would it weigh at the surface? Ans. 3 T. 2cwot. 501b. (10.) Suppose two balls ten thousand miles apart were to approach each other under the influence of mutual attraction, the weight of one being represented by 15, that of the other by 30. How far would each move? Ans. 66662 mi. and 3333 mni. 3 Y~ VVS7Y~QLC VV;2 OF GRAVITY. 37 (11.) Which would have the stronger attraction on the earth, a body at the distance of 95 millions of miles from the earth, with a weight represented by 1000, or a body at the distance represented by 95, and a weight represented by one? Ans. As -67,T- to 2. (12.) Supposing the weight of a body to be represented by 4 and its distance at 6, and the weight of another body to be 6 and its distance at 4, which would exert the stronger power of attraction? Ans. The second, as - to -. 108. THE CENTRE OF GRAVITY.-As every part of a body possesses the general property of attraction, it is evident that the attractive force of the mass of a body must be concentrated in some point; and this point is called the centre of gravity of the body. What is the 109. The Centre of Gravity of a body is the Centre of Gravity ofa point about which, all the parts balance each body? other. 110. This point in all spherical bodies of uniform density willbe the centre of sphericity. 111. As the earth is a spherical body, its centre of gravity is at the centre of its sphericity. 112. When bodies- approach each other under the effect of mutual attraction, they tend mutually to approach the centre of gravity of each' other. 113. For this reason, when any body falls towards the earth its motion will be in a straight line towards the centre of the earth. No two bodies from different points can approach Fig. 3. the centre of a sphere in a parallel direction, and no two bodies suspended from different points can hang parallel to one another. 114. Even a pair of scales hanging perpendicularly ", to the earth, as represented in Fig. 3, cannot be i exactly parallel, because they both point to the same spot, namely, the centre of the earth. But their convergency is too small to be perceptible. What is a 115. The direction in which a falling body apVertical preaches the surface of the earth is called a Vertical Line? Line. No two vertical lines can be parallel. 116. A weight suspended from any point will always assume a vertical position.* ~ Carpenters, masons and other artisans, make use of a weight of lead suspended at rest by a string, for the purpose of ascertaining whether their work stands in a vertical position. To this implement they give the name of plumb-line, from the Latin word pltumbum, lead. 4 88 NATURAL PHILOSOPHY. 117. All bodies under the influence of terrestrial gravity will fall to the surface of the earth in the same space of time, when ht an equal distance from the earth, if nothing impede them. But the air presents by its inertia a resistance to be overcome. This resistance can be more easily overcome by dense bodies, and therefore the rapidity of the fall of a body will be in proportion to its density. To what is the resist118. The resistance of the air to the fall of a ance of the air to afall- body is in direct proportion to the extent of its ing body surface. proportioned? 119. Heavy bodies can be made to float in the air, instead of falling immediately to the ground, by making the extent of their surface counterbalance their weight. Thus gold, which is one of the heaviest of all substances, when spread out into thin leaf is not attracted by gravity with sufficient force to overcome the resistance of the air; it therefore floats in the air, or falls slowly. A sheet of paper also, for the same reason, will fall very slowly if spread open, but, if folded into a small compass, so as to present but a small surface t6 the air, it will fall much more rapidly. 120. This principle will explain the reason why a person can with impunity leap from a greater height with an expanded umbrella in his hand. The resistance of the air to the broad surface of the umbrella checks the rapidity of the fall. 121. In the same manner the aeronaut safely descends from a balloon at a great height by means of a parachute. But, if by any accident the parachute is not expanded as he falls, the rapidity of the fall will not be checked. [See Fig. 4.] 122. EFFECT OF GRAVITY ON TIIE DENSITY OF THE AI. - The air extends to a very considerable distance above the surface of the earth.* Chat portion which lies near the surface of the earth has to sustain the weight of the portions above; and the pressure of the upper parts * We have no means of ascertaining the exact height to which the air extends. Sir John Herschel says: "Laying out of consideration all nice questions as to the probable existence of a definite limit to the atmosphere, beyond which there is, absolutely and rigorously speaking, no air, it is clear that, 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 it turbid, as mud does water). It seems probable, from many indications, that the greatest height at which visible clouds ever 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 whioh the atmosphere extends has never been ascertained, it ceases to reflect the sun's rays at a greater height than forty-five miles oF GRAVITY. 89 of thle atmosphere on those beneath renders the air near the surface of the earth much more dense than that in the upper regions. Fig. 4. Wvhat effect 123. The air or atmosphere exists in a state. has Gravity poni the of compression, caused by Gravity, which inair? creases its density near the surface of the earth. 124. Gravity causes bodies in a fluid or gaseous form to move in a direction seemingly at variance with its own laws. Thus smoke and steam ascend, and oil poured into a vessel containing a heavier fluid will first sink and then rise to the surface. This seemingly anomalous circumstance, when rightly understood, will be found to be in perfect obedience to the laws of gravitation. Smoke and steam are both substances less dense than air, and are therefore less forcibly attracted by gravitation. The air being more strongly attracted than steam or smoke, on account of its superior density, falls into the space o(cupied by the 40 NATURAL PHILOSOPHY. steam, and forces it upwards. The same reasoning applies in the case of oil; it is forced upwards by the heavier fluid, and both phenomena are thus seen to be the necessary consequences of gravity. The rising of a cork or other similar light substances from the bottom of a vessel of water is explained in the same way. This cilcumstance leads to the consideration of what is called specific gravity. What is 125. SPECIFIC GRAVITY.- Specific Gravity meant by ISpecific is a term used to express the relative weight of Gravity? equal bulks of different bodies.* 126. 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. HIence 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. 127. From what has now been said with respect to the attrac tion of gravitation and the specific gravity of boolies, it appears that, although the earth attracts all substances, yet this very attraction causes some bodies to rise and others to fall. 128. Those 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 - 6 and other light substances will not sink in water, be- j:-\ cause, the specific gravity - of water being greater, the water is more strongly at- Ii! tracted, and will be drawn down beneath them. [For a table of the specific gravity of'bodies, see Hlydrostatics.] 129. The principle which causes balloons to rise is the same which occasions the ascent of smoke, steam, &c. The materials of which * 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 light; 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 the earth, -that is, they weigh the samle. MECHANICS. 41 a balloon is made, are heavier than air, but their extension is greatly increased, and they are filled with an elastic fluid of a different nature, specifically lighter than air, so that, on the whole, the balloon when thus filled is much lighter than a portion of air of the same dimensions, and it will rise. 130. Gravity, therefore, causes bodies which are lighter than air to ascend, those which are of equal weight with air to remain stationary, and those which are heavier than air to descend. But the rapidity of their descent is affected by the resistance of the air, which resistance is proportioned to the extent of surface in the falling body. 131. MECHANICS.- Mechanics treats of moWhat is Mechanics? tion, and the moving powers, their nature and laws, with their effects in machines. M7tat is 132. Motion is a continued change of place. Motion? 133. On account of the inertia of matter, a body at rest cannot put itself in motion, nor can a body in motion stop itself. What is meant by 134. That which causes motion is called a Force. a Force. What is meant b~y 135. That which stops or impedes motion is Resist- called Resistance.@ ance. Whato ~things 136. In relation to motion, we must consider are to be considered in re- the force, the resistance, the time, the space, lation to mo- the direction, the velocity and the momentum. tion? What is the 137. The velocity is the rapidity with which a v.elocityand to what is it body moves; and it is always proportional to the proportion- force by which the body is put in motion. al? 138. 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 # A force is sometimes a resistance, and a resistance is sometimes a force The tnv terms are used merely to denote opposition. d=* 42 NATURAL PHILOSOPHI. in the same time, the velocity of the latter is double that of th. former. What is the rulefor 139. To find the velocity of a body, the space findingy the passed over must be divided by the time employed velocity of a moving in moving over it. body? Thus, if a body move 100 miles in 20 hours, the velocity is found by dividing 100 by 20. The result is five miles an hour.* 140. Questions for Solution. (1.) If a body move 1000 miles in 20 days, what is its velocity 1 Ans. 60 miles a day. (2.) If a horse travel 15 miles in an hour, what is his velocity? Ans. l of a mile in a minute. (3.) Suppose one man walk 300 miles in 10 days, and another 200 miles in the same time, - what are their respective velocities? Ans. 30 & 20. (4.) If a ball thrown fiom a cannon strike the ground at the distance of 3 miles in 3 seconds from the time of its discharge, what is its velocity! A. 1. (5.) Suppose a flash of lightning come from a cloud 3 miles distant from the earth, and the thunder be heard in 14 seconds after the flash is seen; how fast does sound travel? Ans. 11313 ft. per sec. (6.) The sun is 95 millions of miles from the earth, and it takes 8. minutes for the light from the sun to reach the earth; with what velocity does light move! t Alzs. 191919 + sni. 1per see. * Velocity is sometimes called absolute, and sometimes relative. Velocity 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 as two to one. t From the table here subjoined, the velocities of the objects enumerated may be ascertained in miles per hour and in feet per second, fractions omitted TABLE OF VELOCITIES. Miles per hour. Feet per second. A man walking..4............... A horse trotting..... 7................ 10 Swiftest race-horse.... 60................. 88 Railroad train in England. 32.................47 cc" " America 18................. 26 " Belgium. 25................36 "- France. 27................40 cc " Germany 24.................35 English steamboats in 1 14......... *. 26 channels........ American on the Hudson..1. 18......26 Fast-sailing vessels.... 10......14.. MECHANICS. 43 How is thte 141. The time employed by a body in motion time employed by a mov- may be ascertained by dividing the space by the ing body as- velocity. certained? Thus, if the space passed over be 100 miles. and the velocity 5 miles in in hour, the time will be 100 divided by 5. Ans. 20 hours. 142. Questions for Solution. (1.) If a cannon-ball, with a velocity of 3 miles in a minute, strike the ground at the distance of one mile, what is the time employed. Ans. X of a minute, or 20 seconds. (2.) Suppose light to move at the rate of 192,000 miles in a second of time, how long will it take to reach the earth from the sun, which is 95 millions of miles distant. Ans. 8 min. 14.07 sec. + (3.) If a railroad-car run at the rate of 20 miles an hour, how long will it take to go from Washington to Boston,-distance 432 miles? Ans. 21.6 hr. (4.) Suppose a ship sail at the rate of 6 miles an hour, how long will it take to go from the United States to Europe, across the Atlantic Ocean, a distance of 2800'miles. Avs. 19 da. 10 hr. 40 min. (5.) If the earth go round the sun in 365 days, and the distance travelled be 540 millions of miles, how fast does it travel? An&s. 1,479,452 4 Mi2,. (6.) Suppose a carrier-pigeon, let loose at 6 o'clock in the morning from Washington, reach New Orleans at 6 o'clock at night, a distance of 1200 miles, how fast does it fly? Acts. 100 sni. per hr. low may the space passed 143. The space passed over may be found by over by a body in motion be multiplying the velocity by the time. ascertained? Miles per hour. Feet per second. Slow rivers........ 3........... 4 Rapid rivers.... 7............... 10 Moderate wind...... 7............... 10 A storm......... 36................. 52 A hurricane....... 80............... 117 Common musket-ball.. 850............... 1,240 Rifle-ball........ 1,000............ 1,466 24 lb. cannon-ball.... 1,600.............. 2,316 Air rushing into a vacuum 884............. 1,296 at 32?... 884.... 1,296 Air gun bullet, air com- pressed to' 01 of its 466............. 683 volume.)..... Sound.......... 743..... 1,142 A point on the surface of 1, the earth.......... 1,520 Earth in its orbit.... 67,374..............98,815, The velocity of light is 192,000 miles in a second of time. The velocity of the electric fluid is said to be still greater, and some ut'loricies state it to be at the rate of 288 000 miles in a. second of time. 44 NATURAL PHILOSOPHY. Thus, if the velocity be 5 miles an hour, and the time 20 hours, the space will be twenty multiplied by 5. Ans. 100 miles. 144. (1.) If a vessel sail 125 miles in a day for ten days, how far will it sail in that time'! - A-is. 1250 mni. (2.) Suppose the average rate of steamers between New York and Albany be about 11 miles an hour, which they traverse in about 14 hours, what is the distance between these two cities by the river. Alms. 154 mi. (3.) Suppose the cars going over the railroad between these two cities -travel at the rate of 25 miles an hour and take 8 hours to go over the distance, how far is it from New York to Albany by railroad. Ans. 200 mi. (4.) If a man walking from Boston at the rate of 24 miles in an hour reach Salem in 6 hours, what is the distance from Boston to Salem? Atns. 15 mi. (5.) The waters of a certain river, moving at the rate of 4 feet in a second, reach the sea in 6 days from the time of starting from the source of the river. What is the length of that river - Ans. 392-_ mi. (6.) A cannon-ball, moving at the rate of 2400 feet in a second of time, strikes a target in 4 seconds. What is the distance of the target? A. 9600ft. 145. The following formula embrace the several ratios of the time, space and velocity: (1.) The space divided by the time equals the velocity, or = v. (2.) The space divided by the velocity equals the time, or - t. v (3.) The velocity multiplied by the time equals the space, or vXt s. How many kinds of 146. There are three kinds of motion, namely, motion are Uniform, Accelerated and Retarded. there? What is Uni- 147. Uniform Motion is that by which a form Motion? body moves over equal spaces in equal times. What is -Accel- 148. Accelerated Motion is that by which the erated Motion? velocity increases while the body is moving. Vhat is Re- 149. Retarded Motion is that by which the tarded Motion? velocity decreases while the body is moving. How are uni- 150. Uniform Motion is produced by the form, accelerated and re- momentary action of a single force. Accelertarded motion ated Motion is produced by the continued action respectlively produced of one or more forces. Retarded Motion is produced by some resistance. 151. A ball struck by a bat, br a stone thrown from the hand is MECHANICS. 45 in theory an instance of uniform motion; and, if the attraction of gravity and the resistance of the air could be suspended, it would proceed onwards in a straight line, with a uniform motion, forever. But, as the resistance of the air and gravity both tend to deflect it, it in fact becomes first an instance of retarded, and then of accelerated motion. 152. A stone, or any other body, falling from a height, is an instance of accelerated motion. The force of gravity continues to operate upon it during the whole time of its descent, and constantly increases its velocity. It begins its descent with the first impulse of attraction, and, could the force of gravity which gave it the impulse be suspended, it would continue its descent with a uniform velocity. But, while falling it is every moment receiving a new impulse from gravity, and its velocity is constantly increasing during the whole time of its descent. 153. A stone thrown perpendicularly upward is an instance of retarded motion; for, as soon as it begins to ascend, gravity immediately attracts it downwards, and thus its velocity is diminished. The retarding force of gravity acts upon it during every moment of its ascent, decreasing its velocity until its upward motion is entirely destroyed. It then begins to fall with a motion continually accelerated until it reaches the ground. What time does a body 154. A body projected upwards will occupy the occupyj in its scent and its same time in its ascent and descent. ascent and descent? This is a necessary consequence of the effect of gravity, which uniformly retards it in the ascent and accelerates it in its descent. How canper- 155. PERPETUAL MOTION. - Perpetual Mopetual motion be produced? tion is deemed an impossibility in mechanics, because action and reaction are always equal and in contrary directions. Whby Act ison meant 156. By the action of a body is meant the by Action and Rediction? effect which it produces upon another body. By rediction is meant the effect which it receives from the body on which it acts. Thus, when a body in motion strikes another body, it acts upon it, or produces motion; but it also meets with resistance from the body which is struck, and this resistance is the reaction of the body. 46 NATURAL PHILOSOPHY. Illustration of Action and Reiction by means of Elastic and Non-elastic Balls. (1.) Figure 6 represents two ivory * balls, A and B, Fig. 6. of equal size, weight, &c., suspended by threads. If the ball A be drawn a little on one side and then let go, it will strike against the other ball B. and drive it off A B 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 by 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.) Fig. 7 represents six ivory balls of equal weight, suspended by threads. If the ball A be drawn out of the perpendicular and let fall against B, it will communicate its mo- Fig. 7 tion to B, and receive a reaction from it which will stop its own motion. But the ball B cannot move 7T7 without moving C; it will therefore communicate the motion which it received from A to C. and receive from (I a reaction, which will stop its motion. In like manner the motion and reaction are received by each ol the balls D, E, F; but, as there is no ball beyond F to act upon it, F will fly off. N. B. This experiment is to be performed with elastic balls only. (3). Fig. 8 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 mo- Fig. 8. tion 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 fiom the vertical line than the ball ID was before it fell. Still, E * It will be recollected that ivory is considered highly elastic. MECHAICS. 47 nowever, action and reaction are equal, for the action on E is only enough to make it move through a smaller space, but so much of D's motion is now also destroyed. * 157. 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.t 158.. It is likewise upon the same principle of action and reaction that fishes swim, or, rather, make their way through the water, namely, by striking the water with their fins. J 159. 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. How may 160. Motion may be caused either by action ox caused?be reaction. When caused by action it is called Incident, and when caused by reaction it is called Reflected Motion. ~ F Figs. 6 and 7, as has been explained, show the effect of action andt reaction in elastic bodies, and Fig. 8 shows the same effect in non-elastic bodies. When the elasticity of a body is imperfect, an intermediate 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 -,this, as in the preceding cases, the whole quantity of motion destroyed in +th striking ball will be equal to that produced in the ball struck. Condected with " the Boston school apparatus " is a stand with ivory balls, to give a visible illustration of the effects of collision., t 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 muscular power to put them in motion. J The power possessed by fishes, of sinking or rising in the water, is greatly assisted by a peculiar 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. ~ 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. Reflected 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 is turned back. This return of the ball is called reflected motion. As reflected motion is caused by reaction, and reaction is increased 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. 48 NATURAL PHILOSOPHY. What is 161. The angle * of incidence is the angle formed an angle of Inci- by the line which the incident body makes in its dence? passage towards any object, with a line perpendicular to the surface of the object. * 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, every Fig. 9. part of the outer edge of which, called the ciroumference, is equally distant from a point T within, called the centre. [.See Fig. 9.1 ] 3. The straight lines drawn from the centre to the circumference are called radii. [The C singular number'of this word is radius.] Thus, D A in Fig. 9, the lines C D, C O, C R,c and C A, 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. 6. All angles are measured by the number of degrees which they contain Thus, in Fig. 9, 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 angles R C 0 and 0 C D are angles of 45 degrees. 7. Alngles 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. 9, R C A is a right angle, 0 C R an acute, and 0 C A an obtuse 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. 9, R C is perpendicular to D A. 9. The tangent of a circle is a line which touches the circumference, with. out cutting it when lengthened at either end. Thus, in Fig. 9, the line RT 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. 11.] 11. A parallelogram is a figure whose opposite sides are equal and parallel [SeeFigs. 12 and 13.] A square is also a parallelogram. 12 A rectangle is a parallelogram whose angles are right angles. [N. B. It will be seen by these definitions that both a square and a rectangle are parallelograms, but all parallelograms are not rectangles nor squares. A square is both a parallelogram and a rectangle. Three things are essential to a square; namely, the four sides must all be equal, they must also be parallel, and the angles must all be right angles. Two things only are essential to a rectangle; namely, the angles must all be right angles, and the opposite sides must be equal and parallel. One thing only is essential to a parallelogram; namely, the opposite sides must be equal and parallel.] 13. The diagonal of a square, of a parallelogram, or a rectangle, is a line MECHANICS. Explain 162. Thus, in Fig. 10, the line Fig. 10. Fig. 10 A B C represents a wall, and P B o A a line perpendicular to its surface. O is a - ball moving in the direction of the dotted. line, O B. The angle O B P is the angle of R incidence. What is 163. The angle of reflection is the angle formed thfeanle by the perpendicular with the line made by the tion? reflected body as it leaves the surface against which it struck. Thus, in Fig. 10, the angle P B R is the angle of reflection. What is the proportion of the angle 164. The angles of incidence and reof incidence to the flection are always equal to one another.@ angle of rcfleetion? (1.) Thus, in Fig. 10, the angle of incidence, O B P, and the angle of reflection, P B R, are equal to one another; that is, they contain an equal number of degrees. What will be the 165. From what has now been stated with course of a body in motion which regard to the angles of incidence and reflecstrikes against tion, it follows, that when a ball is thrown anothe? fixed body? perpendicularly against an object which it cannot penetrate, it will return in the same direction,; but, if it be thrown obliquely, it will return obliquely on the opposite side of the perpendicular. The more obliquely the ball is thrown, the more obliquely it will rebound. t drawn through either of them, and terminating at the opposite angles. Thus, in Figs. 11, 12, and 13, the line A C is the diagonal of the square, parallelogram, or rectangle. An understanding of this law of reflected motion is very important, because it is a fundamental law, not only in Mechanics, but also in Pyronomics, Acoustics and Optics. t 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 reaoh an object out of the range of a direct shot. 50 NATURAL PIIPHILOSOPHY. What is the 166. MOMENTUM. -The Momentums of a Momentum body is its quantity of motion, t and it expresses of a body? the force with which it would strike against another body. How is the Mow s entum The Momentum of a body is ascertained by of a body multiplying its weight by its velocity. calculated? 167. Thus, if the velocity of a body be represented by 5 and its weight by 6, its momentum will be 30. ouow can a 168. A small or a light body may be made small or a light body to strike against another body with a greater be made to force than a heavier body simply by giving it do as much da.mage as suficient velocity,-that is, by making it have a large one? greater momentum. Thus, a cork weighing 1 of an ounce, shot from a pistol with the velocity of 100 feet in a second, will do morLe damage than a leaden shot weighing I of an ounce, thrown from the hand with a velocity of 40 feet in a second, because the momentum of the cork will be the greater. The momentum of the cork is j X 100 = 25. That of the leaden shot is J X40 =5. 169. Questionsfor Solution. (1.) What is the momentum of a body weighing 5 pounds, moving with the velocity of 50 feet in a second? Ans. 250. (2.) What is the momentum of a steam-engine, weighing 3 tons, moving with the velocity of 60 miles in an hour. Anes. 180. [N. B. It must be recollected that, in comparing the momenta of bodies, the velocities and the time of the bodies compared must be respectively of the same denomination. If the time of one be minutes and of the other be hours, they must both be considered in minutes, or both in hours. So, with regard to the spaces and the weights, if one be feet all must be expressed in feet; if one be in pounds, all must be in pounds. It is better, however, to express the weight, velocities and spaces, by abstract numbers, as follows:1 (3.) If a body whose weight is expressed by 9 and velocity by 6 is in motion, what is its momentum? AAns. 54. (4.) A body whose momentum is 63 has a velocity of 9; what is its weight 1 Ans. 7. * The plural of this word is momenta. t The quantity of motion communicated to a body does not affect the duration of the motion. If but little motion be communicated, the body will move slowly. If a great degree be impaited, it will move rapidly. But in both cases the motion will continue until -it is destroyed' by soine external force. MECHANICS. 51 [N. B. The momentum being the product of the weight and velocity, the weight is found by dividing the momentum by the velocity, and the velocity is found by dividing the momentum by the weight.] (5.) The momentum is expressed by 12, the weight by 2; what is the velocity? Ans. 6. (6.) The momentum 9, velocity 9, what is the weight? Ans. 1. (7.) Momentum 36, weight 6, required the velocity. Ans. 6. (8.) A body with a momentum of 12 strikes another with a momentum of 6; what will be the consequence 3 Ans. Both have momn. of 6. [N. B. When two bodies, in oppostte directions, come into collision, they each lose an equal quantity of their momenta.] 4 (9.) A body weighing 15, with a velocity of 12, meets another coming in the opposite direction, with a velocity of 20, and a weight of 10; what will be the effect 3. Ans. Both move with mom. of 20. (10.) Two bodies meet together in opposite directions. A has a velocity of 12 and a weight-of 7, B has a momentum expressed by 84. What will be the consequence 3 Ans. Both mom. destroyed. (11.) Suppose the weight of a comet be represented by 1 and its velocity by 12, and the weight of the earth be expressed by 100 and its velocity by 10, what would be the consequence of a collision, supposing them to be moving in opposite directions 3 Als. Both have monm. of 988. (12.) If a body with a weight of 75 and a velocity of 4 run against a man Whose weight is 150, and who is standing still, what will be the consequence, if the man uses no efflrt but his weight? Abes. Mian has vel. of 1i. (13.) With what velocity must a 64 pound cannon-ball fly to be equally effective with a battering-ram of 12,000 pounds propelled with a velocity of 16 feet in a second 3. Ans. 3000,ft. 170. ATTRACTION- LAW OF FALLING BODIES. - When one body strikes another it will cause an effect proportional to its own weight and velocity (or, in other words, its momentum); and the body which receives the blow will move on with a uniform velocity (if the blow be sufficient to overcome its inertia) in the direction of the motion of the blow. But, when a body moves by the force of a constant attraction, it will move with a constantly accelerated motion. 171.- This is especially the case with falling bodies. The earth attracts them with a force sufficient to bring them down through a certain number of feet during the first second of time. While the body is thus in motion with a velocity, say of sixteen feet, the earth still attracts it, and during the second second it communicates an additional velocity, and every successive second of time the attrae — tion of the earth adds to the velocity in a similar proportion, so that during any given time, a falling body will acquire a velocity which, in the same time, would carry it over twice the space through which it has already fallen. Hence we deduce the following law: llat ais the 172. A body falling from a height will fall ing bodies? sixteen feet in the first second of time,* three * This is only an approximation to the truth; it actually falls sixteen feet and one inch during the first Second, three times that distance in the second, &c The questions proposed to be solved assume sixteen feet only. 52 NATURAL PHILOSOPIIY. times that distance in the second, five times in the third, seven in the fourth, its velocity increasing during every successive second, as the odd numbers 1, 3, 5, 7, 9, 11, 13, &c.* The laws of falling bodies are clearly demonstrated by a mechanical arrangement known by the name of i" Attwood's Machine," in which a small weight is made to communicate motion to two others attached to a cord passing over friction-rollers (causing one to ascend and the other to descend), and marking the progress of the descending weight by the oscillations of a pendulum on a graduated scale, attached to one of the columns of the machine. It has not been deemed expedient to present a cut of the machine, because without the machine itself the explanation of its operation would be unsatisfactory, with the machine itself in view the simplicity of its construction would render an explanation unnecessary. * The entire spaces through which a body will have fallen in any given number of seconds increase as the squares of, the times. This law was discovered by Galileo, and may thus be explained. If a body fall sixteen feet in one second, in two seconds it will have fallen four times as far, in three seconds.nine times as far, in four seconds sixteen times as far, in the fifth second twenty-five times, &c., in the sixth thirty-six times, &c. ANALYSIS OF THE MOTION OF A FALLING BODY. Number of Seconds. Spaces. Velocities. Total Space. 1 1 2 1 2 3 4 4 3 5 6 9 4 7 8 16 5 9 10 25 6 11 12 36 7 13 14 49 8 15 16 64 9 17. 18 81 10 19 20 100 From this statement it appears that the spaces passed through by a falling body, in any number of seconds, increase as the odd numbers 1, 3, 5, 7, 9, 11, &c.; the velocity increases as the even numbers 2, 4, 6, 8, 10, 12, &c.; and the total spaces passed through in any given number of seconds increase as the squares of the numbers indicating the seconds, -thus, 1, 4, 9, 16, 25, 36, &c. Aristotle maintained that the velocity of any falling body is in direct proportion to its weight; and that, if two bodies of unequal weight were let fall from any height at the same moment, the heavier body would reach the ground in a shorter time, in exact proportion as its weight exceeded that of the lighter one. Hence, according to his doctrine, a body weighing two pounds would fall in half the time required for the fall of a body weighing only one pound. This doctrine was embraced by all the followers of that distinguished philosopher, until the time of Galileo, of Florence, who flourished about the middle of the sixteenth century. Ile maintained that the velocity of a falling body is not affected by its weight, and challenged the adherents of the Aristotelian doctrine to the test of experiment. The leaning tower of Pisa was selected for the trial, and there the experiment was tried which proved the truth of Galileo's theory. A distinguished writer thus describes the scene. "On the appointed day the disputants MECHANICS. 53 173. The height of a building, or the depth of a well, may thus be estimated very nearly by observing the length of time which a stone takes in falling from the top to the bottom. 171. Exercisesfor Solution. (1.) If a ball, dropped from the top of a steeple, reaches the ground in 5 seconds, how high is that steeple. 16+48+80+ —112+144 —400 feet; or, 5X5 —25, square of the number of seconds, multiplied by the number of feet it falls through in one second, namely, 16 feet; that is, 25X16=400 feet. (2.) Suppose a ball, dropped from the spire of a cathedral, reach the ground in 9 seconds, how high is that spire. 16 —48 — 80+- 112+- 144+-176-208 +2L0+272129 6 feet. Or, squaring the time in seconds, 92=81, multiplied by 16l-1296. Ans. [It will hereafter be shown that this law of falling bodies applies to all bodies, whether falling perpendicularly or obliquely. Thus, whether a stone be thrown from the top of a building horizontally or dropped perpendicularly downwards, in both cases the stone will reach the ground in the same time; and this rule applies equally to a ball projected from a cannon, and to a stone thrown from the hand.] (3.) If a ball, projected from a cannon from the top of a pyramid, reach the ground in 4 seconds, how high is the pyramid? Anss. 256ft. (4.) lIow deep is a well, into which a stone being dropped, it reaches the water 6 feet from the bottom of the well in 2 seconds. Ames. 70et. (5.) The light of a meteor bursting in the air is seen, and in 45 seconds a meteoric stone falls to the ground. Supposing the stone to have proceeded from the explosion of the meteor perpendicularly, how far from the earth, in feet, was the meteor? 452X16= —32,400 feet. (6.) What is the difference in the depth of two wells, into one of which a stone being dropped, is heard to strike the water in 5 seconds, and into the other in 9 seconds, supposing that the water be of equal depth in both, and making no allowance for the progressive motion of sound 3 A. 896.ft, repaired to the tower of Pisa, each party, perhaps, with equal confidence. It was a crisis in the history of human knowledge. On the one side stood the assembled wisdom of the universities, revered for age and science,:venerable, dignified, united and commanding. Around them thronged the multitude, and about them clustered the associations of centuries. On the other there stood an obscure young man (Galileo), ivith no retinue of followers, without reputation, or influence, or station. But his courage was equal to the occasion; confident in the power of truth, his form is erect and his eye sparkles with excitement. But the hour of trial arrives. The balls to be employed in the experiments are carefully weighed and scirt tinized, to detect deception. The parties are satisfied. The one ball is exactly twice the weight of the other. The followers of Aristotle maintain that, when the balls are dropped from the tower, the heavy one will reach the ground in exactly half the time employed by the lighter ball. Galileo -isserts that the weights of the balls do not affect their velocities, and that the times of descent will be equal; and here the disputants join issue. The balls are conveyed to the summit of the lofty tower. The crowd assemble round the base; the signal is given; the balls are dropped at the same instant; and, swift descending, at the same moment they strike the earth. Again and again the experiment is repeated, with uniform results Galileo's triumph was complete; not i shadow of a doubt remained." [" The Orbs of Heaven."] 53 54- NATURAL PHILOSOPRY. (7.) A boy raised his kite in the night, with a lantern attached to it. Unfortunately, the string which attached the lantern broke, and the lantern fell to the ground in 6 seconds. How high was the kite i Ans. 576ft. 175. RETARDED MOTION OF BODIES PROJECTED UPWARDS. - All the circumstances attending the accelerated descent of falling bodies are exhibited when a body is projected upwards, but in a reverised order. row can we 1176. To determine the height to which a determine the height to which body, projected upwards, will rise, with a a bodypro- given velocity, it is only necessary to deterjected upwards wilh a given mine the height from which a-body would fall velocity, will to acquire the same velocity. ascend? 177. Thus, if it be required to ascertain how high a body would rise when projected upwards with a force sufficient to carry it 144 feet in the first second of time, we reverse the series of numbers 16+ 48+80+112 144 [see table on page 52], and, reading them backward, 144 + 112 + 80 + 48 + 16, we find their sum to be 400 feet, and the time employed would be 5 seconds. How does the time of the as- 178. The time employed in the ascent and cent of a body descent of a body projected upwards will, compare with the time of its therefore, always be equal. descent Questionsfor Solution. (1.) Suppose a cannon-ball, projected perpendicularly upwards, returned to the ground in 18 seconds; how high did it ascend, and what is the velocity of projection? Ans. 1296ft.; 272ft. lst sec. (2.) How high will a stone rise which a man throws upward with a force sufficient to carry it 48 feet during the first second of time. Ans. 64ft. (3.) Suppose a rocket to ascend with a velocity sufficient to carry it 176 feet during the first second of time; how high will it ascend, and what time will it occupy in its ascent and descent? Ans. 576ft.; 12 sec. (4.) A musket-ball is thrown upwards until it'reaches the height of 400 feet. How long a time, in seconds, will it occupy in its ascent and descent, and what space does it ascend in the first second?. Ans. 10 see.; 144ft. (5.) A sportsman shoots a bird flying in the air, and the bird is 3 seconds in falling to the ground. How high up was the bird when he was shot? Ans. 144ft. (6.) How long time, in seconds, would it take a ball to reach an object 6000 feet above the surface of the earth, provided that the ball be projected, with a force sufficient only to reach the object? Ants. 17.67 sec. + 179. COMPOUND MOTION. -Motion may be produced either by a single force or by the operation of two or more forces. MECHANICS. 55 In what direc- 180. Simple Motion is the motion of a body tion is the mo- impelled by a single force, and is always in a tion of a body impelled by a straight line in the same direction with the singleforce? force that acts. Wlhat is Com- 181. Compound Motion is caused by the poundMotion? operation of two or more forces at the same time. When a body is struck by two 182. When a body is struck by two equal equalforcesin forces, in opposite directions, it will remain at opposite directions, how will rest. it move? 183. If the forces he unequal, the body will move with diminished force in the direction of the greater force: Thus, if a body with a momentum of 9 be opposed by another body with a momentum of 6, both will move with a momentum of 3 in the direction of the greater force. How will a 184. A body, struck by two forces in difbody move ferent directions, will move in a line between when strucskby them, in the direction of the diagonal of a two forces in different direc- parallelogram, having for its sides the lines tions? through which the body would pass if urged by each of the forces separately. How will the body move, if 185. When the forces are equal and at the forces are,equal and at right angles to each other, the body will right angles to move in the diagonal of a square. each other? 186. Let Fig. 11 represent a ball struck by Fig. 11. the two equal forces X and Y. In this figure D x the forces are inclined to each other at an angle Y of 90~, or 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, A 56 NATURAL PHILOSOPHY. through which it passes, is the diagonal of the square, A B C D. This line also represents the resultant of the two forces. The time occupied in its passage from C 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. low will a body move 187. If two unequal forces act at right under the influ- angles to each other on a body, the body will ence of two unequalforces at move in the direction of the diagonal of a right angles to rectangle. each other? Explain Fig. 188. Illustration.-In Fig. 12 the ball C is 12. represented as acted upon by Fig. 12. X two unequal forces, X and Y. The force X n c would send it to B, and the force Y to D. As [' Y it cannot obey both, it will move in the direc' tion C A, the diagonal of the rectangle A B C D. B How will the body move Hor will the boy move *189. When two forces act in the if t/he forces act in the direction of any other direction of an acute or an obtuse than a right angle? owan a willht abodnge angle, the body will move in the diH1ow will a bodyv move if theforces act in the di- rection of the diagonal of a parallelorection of an acute or obtuse angle? gram. Expldin 190. Illustration. — In figure 13 the ball C is Fig. 13. supposed to be influenced by two ig,.13. forces, one of which would send it to B and D the other to D, the forces acting in the direction of an acute angle. The ball will, therefore, move between them in the line A B C A, the longer diagonal of the parallelogram A B C D. 191. The same figure explains the motion of a ball when the two forces act in the direction of an obtuse angle. 192. Illustration. - The ball D, under the influence of two MECHANICS. 57 forces, one of which would send it to C, and the other to A, which, it will be observed, is in the direction of an obtuse angle, will proceed in this case to B, the shorter diagonal of the parallelogram A B C D. [N. B. A parallelogram containing acute and obtuse angles has two diagonals, the one which joins the acute angles being the longer.] What is Re-? 193. Resultant Motion is the effect or resultant MIotion? suit of two motions compounded into one. 194. If two men be sailing in separate boats, in the same direction, and at the same rate, and one toss an apple to the other, the apple would appear to pass directly across from one to the other. in a line of direction perpendicular to the side of each boat. But its real course is through the air in the diagonal of a parallelogram, formed by the lines representing the course of each boat, and perpendiculars drawn to those lines from the spot where each man stands as the one tosses and the Explain other catches the apple. In Fig. 14. Fig. 14. the lines A B and C-D represent the G F course of each boat. E the spot where the man A B stands who tosses the apple; while the apple is c D in its passage, the boats have passed from E and G to H and F respectively. But the apple, having a motion, with the man, that would carry it from E to HL, and likewise a projectile force which would carry it from E to G, cannot obey them both, but will pass through the dotted line E F, which is the diagonal of the parallelogram E G F H.* How can we 195. When a body is acted upon by three or asection of the more forces at the same time, we may -take any direction of the * On the principle of resultant motion, if two ships in an engagement be sailing before the wind, at equal rates, the aim of the gunners will be exactly as though they both stood still. But, if the gunner fire from a shil: sLanding still at another under sail, or a sportsman fire at a bird on the wing, each should take his aim a little forward of the mark, because the ship and the bird will pass a little forward while the shot is passing to them.' 58 NATURAL PHILOSOPHY. motion when two of them alone, and ascertain the resultant of the body is in- those two, and then employ the resultant as a new fluenced by three or more force, in conjunction with the third,* &c. forces? What is Cir- 196. CIRCULAR MOTION. — Circular Mocular Motion? tion is motion around a central point. ~What causes 197. Circular motion is caused by the conCircular Mo- tinued operation of two forces, by one of which tion? the body is projected forward in a straight line, while the other is constantly deflecting it towards a fixed point. [See NVo. 184.] 198. 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. 184) 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 circumference of the circle. How many 199. There are three different centres which centres recentres re- require to be distinctly noticed; namely, the quire to be noticed in Me- Centre of Magnitude, the Centre of Gravity, chanics? and the Centre of Motion. " The resultant of two forces is always described by the third side of a triangle, of which the two forces may be represented, in quantity and direction, by the other two sides. When three forces act in the direction of the three sides of the same triangle, the body will remain at rest. When two forces act at right angles, the resultant will form the hypothenuse of a right-angled triangle, either of the sides of which may be found, when the two others are given, by the common principles of arithmetic or geometry. From what has now been stated, it will easily be seen, that if any number of forces whatever act upon a body, and in any directions whatever, the resultant of them all may easily be found, and this resultant will be their mechanical equivalent. Thus, suppose a body be acted upon at the same time by six forces, represented by the letters A, B, C, D, E, P. First find the resultant of A and B by the law stated in No. 184, and call this resultant G. -Iih the same manner, find the resultant of G and C, calling it H. Then find the resultant ofIH and D, and thus continue until each of the forces be found, and the last resultant will be the mechanical equivalent of the whole MECHANICS. 59 lWhat is the 200. The Centre of Magnitude is the central Centre of Magnitude? point of the bulk of a body. What is the 201. The Centre of Gravity is the point Centre of about which all the parts balance each other. WVhat is the 202. The Centre of Motion is the point Centre of lMotion? around which all the parts of a body move. What is the - 203. When the body is not of a size nor Axis of Mo- shape to allow every point to revolve in the haon? same plane, the line around which it revolves is called the Axis of Motion.* Does the cenDoes or the axicens 204. The centre or the axis of motion is jf motion re- generally supposed to be at rest. olve? 205. 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. TVhat are Cen- 206. The two forces by which circular tral Forces? motion is produced are called Central Forces. Their names are, the Centripetal Force and the Centrifugal Force.t What is the 207. The Centripetal Force is that which Centripetal confines a body to the centre around which it revolves. What is the 208. The Centrifugal Force is that which Centrifugal Force?. impels the body to fly off from the centre.. o 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. IJ The word centripetal means seeking the centre, and centrifugal means flying from the centre. In circular motion these two forces constantly balance each other; otherwise the revolving body will either approach the centre, or recede.tom it, according as the centripetal or centrifugal force is the stronger. 60 NATURAL PHILOSOPHY. l/Thatfollows 209. If the centrifugal force of a revolving if the centrip- body be destroyed, the body will immediately etal or centrifugalforce approach the centre which attracts it; but if be destroyed? the centripetal force be destroyed, the body will fly off in the direction of a tangent to the curve which it describes in its motion.* 210. Thus, when a mop filled with water is turned swiftly round by the handle, the threads which compose the head will fly off from the centre; but, being confined to it at one end, they cannot part from it; while the water they contain, being unconfined, is thrown off in straight lines. TVhen a body 211. The parts of a body which are furthest is revolving around its from the centre of motion move with the csre whaits greatest velocity; and the velocity of all the parts move with parts diminishes as their distance from the the greatest axis of motion diminishes. velocity? Explain 212. Fig. 15 represents the vanes of a windmill. Fig. 15. The circles denote the paths in which the different parts of the vanes move. M is the'centre Fig. 15. or axis of motion around which all the parts revolve. The outer part revolves in the circle D E F G, another part revolves b ( in the circle H I J K, and the inner part in. J( M ID the circle L N 0 P. Consequently, as they \ j all revolve around MI in the same time, the velocity of the parts which revolve in the outer circle is as much greater than the velocity of the parts which revolve in the inner circle, L N (0 P, as the diameter of the outer circle is greater than the diameter of the inner. * The centrifugal force is proportioned to the square of the velocity of a moving body. Hence, a cord sufficiently strong to hold a heavy body revolving around a fixed centre at the rate of fifty feet in a second, would require to have its strength increased four-fold, to hold the same ball, if its velocity should be doubled. MECHANICS. 61 In the daily revolu- 213. As the earth revolves round its tion of the earth around its own axis, axiS, it follows, from the preceding illuswhat parts of the tration, that the portions of the earth earth move most slowly, and what which move most rapidly are nearest to the partsmost rapidly? equator, and that the nearer any portion of the earth is to the poles the slower will be its pmotion. What is re- 214. Curvilinear motion requires the action quired in order to produce of two forces; for the impulse of one single curvilinear force always produces motion in a straight motion? and why? oline. What effect 215. A body revolving rapidly around its has the centrf- longer axis, if suspended freely, will gradually ugal force on a body revolv- change the direction of its motion, and revolve ing around its longer axis? around its shorter axis. This is due to the centrifugal force, which, impelling the parts from the centre of motion, causes the most distant parts to revolve in a larger circle.* * This law is beautifully illustrated by a simple apparatus, in which a hook is made to revolve rapidly by means of multiplying wheels. Let an oblate spheroid, a double cone, or any other solid having unequal axes, be suspended from the hook by means of a flexible cord attached to the extremity of the longer axis. If, now, it be caused rapidly to revolve, it will immediately change its axis of motion, and revolve arouid the shorter axis. The experiment will be doubly interesting if an endless chain be suspendedc from the hook, instead of a spheroid. So soon as the hook with the chain suspended is caused to revolve, the sides of the chain are thrown outward by the centrifugal force, until a complete ring is formed, and then the circular chain will commence revolving horizontally. This is a beautiful illustration of the effects of the centrifugal force.'An apparatus, with a chain and six bodies of different form, prepared to be attached to the multiplying wheels in the manner described, accompanies most sets of philosophical apparatus. Attached to the same apparatus is a thin hoop of brass, prepared for connexion with the multiplying wheels. The hoop is made rapidly to revolve around a vertical axis, loose at the top and secured below. So soon as the hoop begins to revolve rapidly, the horizontal diameter of the ring begins to increase and the vertical diameter to diminish, thus exhibiting the manner in which the equatorial diameter of a revolving body is lengthened, and the polar diameter is shortened, by reason of the centrifugal force. The daily revolution of the earth around its axis has produced this effect, so that the equatorial diameter is at least twenty-six miles longer than the polar. In those planets that revolve faster than the earth the tffect is still & 62 NATURAL PHILOSOPHY. What is Pro- 216. PROJECTILES. - Projectiles is a branch jectiles I )f Mechanics which treats of the motion of bodies thrown or driven by an impelling force above the surface of the earth. What is a 217. A Projectile is a body thrown into the Projectile? air, — as a rocket, a ball from a gun, or a stone from the hand. The force of gravity and the resistance of the How are projectiles affected air cause projectiles to form a curve both in their in their mo- ascent and descent; and, in descending, their tiaon? motion is gradually changed from an oblique towards a perpendicular direction..Explain 218. In Fig. 16 the force of projection would carry Fig. 16. a ball from A to D, while gravity would bring it to 0. If these two forces alone prevailed, the fig. 16. ball would proceed in the dotted line to B. D A 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 B will fall somewhere about E. What is the 219. When a body is thrown Fig. lx. course of a in a horizontal direction, or upbody thrown. body thrown awards or downwards, obliquely, its obliquely in a horizontal course will be in the direction of direction? a curve-line, called a parabolat more striking, as is the case withbthe planet Jupiter, whose figure is nearly that of an oblate spheroid. The developments of Geology have led some writers to the theory that the earth, during one period of its history, must have had a different axis of motion; but it will be exceedingly difficult to reconcile such a theory to the law of rotations which has now been explained, especially as a much more rational explanation can be given to the phenomena on which the theory was built. * 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. t The science of gunnery is founded upon the laws relating to projectilee. MECHANICs. 63 (see Fig. 17); but when it is thrown perpendicularly upwards or downwards, it will move perpendicularly, because the force of projection and that of gravity are in the same line of direction. 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 instrument called the Ballistic penduluLm. 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 communicate 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 Dommon 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 cannonball is 2000 feet per second, and this only at the moment of its leaving the gun. In order to increase the velocity from 1650 to 2000 feet, one-half more 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 obtained is only 1200 or 1300 feet per second. Great charges of powder are, therefore, not only useless, but dangerous; for, though they give little additional force to the ball, they hazard the lives of many by their liability to burst the gun. Experiment has also shown that, although long guns give a greater velocity to the shot than short ones, still that, on the whole, short ones are preferable; and, accordingly, armed ships are now almost invariably furnished with short guns, called carronades. The length of sporting guns has also been greatly reduced of late years. Formerly, the barrels were from four to six feet in length; but the best fowling-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 employed 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 64 NATURAL PHILOSOPHY. What forces 220. A ball thrown in a horizontal direction affect a horiaontal pro- is influenced by three forces; namely, first, the izctile, and jactile, and force of projection (which gives it a horizontal What effect do theyproduce? direction); second, the resistance of the air through which it passes, which diminishes its velocity, without changing its direction; and third, the force of gravity, which' finally brings it to the ground. Hotw is the force oaf 221. The force of gravity is neither increased fected by the nor diminished by the force of projection.* force of projection? Explain 222. Fig. 18 represents a Fig. 18 Fig. 18. cannon, loaded with a ball, C and placed on the top of a tower, at such a height as to require just three I seconds for another ball to descend per- jl 6 pendicularly. Now, suppose the can- Jl L 3 a non 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 the base of the tower at the same instant. In this figure C a represents the perpendicular line of the filling ball. C b is the curvilinear path of the projected ball, 3 the horizontal line 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 of the * The action of- gravity being always the same, the shape of the curve of every projectile 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 saine 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 passed over in the same time will be greater;. but in both cases the descent of the ball towards the earth, in the saine time, will be the same number of feet, whether it moves fast cr slow, or even whether it movo forward at all, or not. VMECHANICS. 65 chird second it strikes the ground. Meantime, that projected 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 downwards exactly as fast as the other, since it meets the line 1 4, which is parallel to the horizon, 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 projected 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. What effect 223. Hence it appears that the horizontal jectile jbrce. motion does not interfere with the action of on gravity? gravity, but that a projectile descends with tihe same rapidity while moving forward that it would if it were acted on by gravity alone. This is the necessary result of the action of two forces. What is the 224. The Random of a projectile is the horizontal Random of a projectile? distance from the place whence it is thrown to the place where it strikes. At what angle 225. The greatest random takes place at an doestheeatn angle of 45 degrees; that is, when a gun is take place? pointed at this angle with the horizon, the ball is thrown to the greatest distance. What will be Let Fig. 19 represent a gun or Fig. 19. the eect if a ball be thrown a carronade, from which a ball 90 at any angle is thrown at an angle of 45 deabove 45 de- grees with the horizon. If the ball be thrown at any angle above 45 degrees, the random will be the same., as it would be at the same number of degrees below 45 degrees.* * 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. 6* 66 NATURAL PHILOSOPHY. What is the, 226. CENTRE OF GRAVITY. - It has already Centre of been stated [see Nos. 109 & 110] that the Gravity of a body? CeGntre of Gravity of a body is the point around zwhich all the parts balance each other. It is, in other words, the centre of the weight of a body. What is the 227. The Centre of Magnitude is the central Centre of Magnitude? point of the bulk of a body. Where is the 228. When a body is of uniform density, the Centre Of centre of gravity is in the same point with the gravity of a body? centre of magnitude. But when one part of the body is composed of heavier materials than another part, the centre of gravity (being the centre of the weight of the body) no longer corresponds 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. If a body be composed of different materials, not united in chemical combination, the centre of gravity will not correspond with the centre of magnitude, unless all the materials have the same specific gravity. When will a 229. When the centre of gravity of a body is body etands supported, the body itself will be supported; and tbhen will - it fall? but when the centre of gravity is unsupported, the body will fall.@ WVhat is the 230. A line drawn from the centre of gravLine of Direction? ity, perpendicularly to the horizon, is called the Line of Direction. 231. The line of direction is merely a line indicating the path which the centre of gravity would describe, if the body were permitted to fall freely. * The Boston School Apparatus contains a set of eight Illustrations for 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. MECHANIJS. 61 t47len will a 232. When the line of direction falls within body stand: the base * of any body, the body will stand; but and when will it fall? when that line falls outside of the base, the body will fall, or be overset. Explain 233. (1.) Fig. 21 represents a loaded Fig. 21. Fig. 21. wagon on the declivity of a hill.. The line C F represents a horizontal line, D E the base of the wagon. If the wagon be loaded in such a manner that the centre of gravity be at B, the per- pendicular 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 will fall outside of the base, and the wagon will be overset. From this it follows -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 wheels; 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. 234. So long as we stand uprightly, the line of direction falls within this base. When we lean on one side, the centre of gravity not being supported, we no longer stand firmly. How does a 235. A rope-dancer performs all his feats of agilrope-dancer rope-dancer ity by dexterously supporting the centre of gravity. perform his feats ofagil- For this purpose, he carries a heavy pole in his ity? 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, in changing the situation of the centre of gravity, he keeps the line of direction within the base, and he will not fall.t * Tha base of a body is its lowest side. The base Fig. 20. of a body standing on wheels or legs is represented by - -- lines drawn from the lowest part of one wheel or leg to the lowest part of the other wheel or leg. Thus, in Figs. 20 and 21, D E represents the base of the wagon and of the table. D t The shepherds in the south of France afford an interesting instance of the application of the art of balancing to the common business of life. These men walk on stilts from three to four feet high, and their children, 68 NATURAL PHILOSOPHY. 236. A spherical body will roll down a slope, because the centre of gravity is not supported.* 237. Bodies, consisting of but one kind of substance, as wood, stone or lead, and whose densities are consequently uniform, will stand more firmly than bodies composed of a variety of substances, of different densities, because the centre of gravity in such cases more nearly corresponds with the centre of magnitude. 238. When' a body is composed of different materials, it will stand most firmly when the parts whose specific gravity is the greatest are placed nearest to the base. When will a 239. The broader the base and the nearer body stand most firmly? the centre of gravity to the ground, the more firmly a body will stand. 240. For this reason, high carriages are more dangerous than low ones. 241. A pyramid also, for the same reason, is the firmest of all Fig. 22. structures, because it has a broad base, and but little elevation. when quite young, are taught to practise the same art. By means of these odd additions to the length of the leg, their feet are kept out of the water, or the heated stnd, and they are also enabled to see their sheep at a greater distance. The-y use these stilts with great skill and care, and run, jump. and even dance on them, with great ease. - A cylinder 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. Now, in order that the cyl inder may roll down the plane, as it is here situated, the centre of gravity must rise, which is impossible; the centre of gravity must always descend in moving, and will descend by the nearest and readiest means, which will be by forcing the cylinder up the slope, until the centre of gravity is sup ported, and then it stops. A body also in the shape of two cones united at their bases can be made to roll up an inclined plane formed by two bars with their lower ends inclined towards each other. This is illustrated by a simple contrivance in the " Boston School Set, " and the fact illustrated is called " the mechanical paradox." MErCHANICS. 69 242. A cone has also the same stability; but, mathematically considered, a cone is a pyramid with an infinite number of sides. 243. Bodies that have a narrow base are easily overset, because, if they are but slightly inclined, the line of direction will fall outside of the base, and consequently their centre of gravity will not be supported. Why caL3/n a 244. A person can carry two pails of water more person carry two pails of easily than one, because the pails balance each zwater noroe other, and the centre of gravity remains supported easily than one? by the feet. But a single pail throws the centre of gravity on one side, and renders it more difficult to support the body. Where is the 245. COMIMON CENTRE OF GRAVITY OF TWO centre ofgrav- BODIES. -When two bodies are connected, they ity of two bodies connect- are to be considered as forming but one body, and ed together? 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. Explain. Fig. 23. Figures 23, 246. Fig. 23 represents a A 24, and 25. bar with an equal weight fastened at each end; the centre of gravity is at A, the middle of the bar, and whatever supports this centre will support both the bodies and the pole. 247. Fig. 24 represents a bar with an Fig. 24. unequal weight at each end. The centre of gravity is at C, nearer to the larger body. 248. Fig. 25 represents a bar with un- Fig. 25. equal weights at each end, but the larger w weight exceeds the less in such a degree - that the centre of gravity is within the larger body at C.* * There are no laws connected with the subject of Natural Science so grand and stupendous as the laws- of attraction. Long before the sublime fiat, " Let there be light, " was uttered, the Creator's voice was heard amid 70 NATURAL PHILOSOPHY. What things 249. THE MECHANICAL POWERS. — There in Mechanics require dis- are five things in mechanics which require a tinct consid- distinct consideration, namely: eration? First, the power that acts. Secondly, the resistance which is to be overcome -by the power. Thirdly, the centre of motion, or, as it is sometimes called, the fulcrum.lt Fourthly, the respective velocities of the power and the resistance; and, the expanse of universal emptiness, calling matter into existence, and sub jecting it to these laws. Obedient to the voice of its Creator, matter sprang from " primeval nothingness, " and, in atomic embryos, prepared to cluster into social unions. Spread abroad in the unbounded fields of space, each particle felt that it was "not good to be alone. " Invested with the social power, it sought companionship. The attractive power, thus doubled by the anion, compelled the surrounding particles to join in close embrace, and thus were worlds created. Launched into regions of unbound space, the new-created worlds found that their union was but a part of a great social system of law and order. Their bounds were set. A central point controls the Universe, and in harmonious revolution around this central point for ages have they rolled. Nor can one lawless particle escape. The sleepless eye of Nature's law, vicegerent of its God, securely binds them all. "Could but one small, rebellious atom stray, Nature itself would hasten to decay." With this sublime view of Creation, how can we escape the conclusion that the very existence of a law necessarily implies a Law-giver, and that Law-giver must be the Creator? Shall we not then say, with the Psalmist, " It is the FOOL who hath said in his heart that there is no God" Who, then, will not see and admire the beautiful language of Mr. Alison, while his heart burns with the rapture and gratitude which the sentiments are so well fitted to kindle: "W;Vhen, in the youth of Moses,' the Lord appeared to him in Horeb,' a voice was heard, saying,' Draw nigh hither, and put off thy shoes from off thy feet, for the place where thou standest is holy ground.' It is with such a reverential awe that every great or elevated mind will approach to the study of nature, and with such feelings of adoration and gratitude that he will receive the illumination that gradually opens upon his soul." " It is not the lifeless mass of matter, he will then feel, that he is examining; it is the mighty machine of Eternal Wisdom, - the workmanship of Him' in whom everything lives, and moves, and has its being.' Under an aspect of this kind, it is impossible to pursue knowledge without mingling with it the most elevated sentiments of devotion — it is impossible to perceive the laws of nature without perceiving, at the same time, the presence and the providence of the Law-giver:-and thus it is that, in every age, the evidences of religion have advanced with the progress of true philosophy; and that SCIIENCE, IN ERECTING A MONUMSENT TO HERSELF, HAS, AT THE SAME, ERECTED AN ALTAR TO THE DEITY." ~ The wordfulcrult means a prop, or support. THE mECHAICAL POWERS. 71 Fifthly, the instruments employed in the construction of the machine. 250. (1.) The power that acts is the muscular strength of men or animals, the weight and momentum of solid bodies, the elastic force of steam, springs, the pressure of the air, the weight of water and its force when in motion, &c. (2.) The resistance to be overcome is the attraction bf gravity or of cohesion, the inertness of matter, friction, &c. (3;) The centre of motion, or-the fulcrum, is the point about which all the parts of the body move. (4.) The -velocity is the rapidity with which an effect is produced. (5.) The instruments are the mechanical powers which enter into the construction of the machine. 251. The powers which enter into the construethe Me- struction of a machine are called the Mechanical chanical Powers. They are contrivances designed to inPowers? crease or to diminish force, or to alter its direction.!What is 252. All the Mechanical Powers are constructed the fundamental on the principle that what is gained in power is principle lost in time. This is the fundamental law of of Mechanics? Mechanics. 253. If 1 lb. is required to overcome the resistance of 2 lbs., the 1 lb. must move over two feet in the same time that the resistance takes to move over one. Hence the resistance will move only half as fast as the power, or, in other words, the resistance requires double the time required by the power to move over a given Space. Erzplain 254. Fig. 26 illustrates the principle as applied to the Fig. 26. lever. W represents the weight, Fig. 26. F the fulcrum, P the power, and the bar W F P the lever. To raise the weight W to w, 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, 72 NATURAL PHILOSOPHY. the arc P p is double the are W w; or, in other words, the dis. tance P p is double the distance of W w. Now, as these distances 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 me, chanical principles. How many 3Me- 255. There are six Mechanical Powers: chanical Powers are there, and the Lever, the Wheel and Axle, the Pulley, their names? the Inclined Plane, the Wedge and the Screw. All instruments and machines are constructed on the principle of one or more of the Mechanical Powers. All the Mechanical Powers may be reduced to three classes, namely: 1st, a body revolving on an axis; 2d, a flexible cord; and, 3d, an inclined surface, smooth and hard. To the first belongs the lever, and the wheel and axle; to the second, the pulley; to the third, the- inclined plane, the wedge and the screw. What is the 256. The Lever is an inflexible bar, movaLever, and how is it used.? ble on a fulcrum or prop. It is used by making one part to rest on a fulcrum, applying the power to bear on another part, while a third part of the lever opposes its motion to the resistance which is to be overcome. 257. In every lever, therefore, whatever be its form, there are three things to be distinctly considered, namely: the position of the fulcrum, of the power, and of the weight, respectively. It is the position of these which makes the distinction between the different kinds of levers. How many kinds 258. There are three kinds of levers, of levers are there'? called the first, second and third, according to the respective position of the fulcrum, the power, and the weight. These'may be represented thus: Power, Fulcrum, Weight. Power, Weight, Fulcrum. Weiglht, Power, Fulcrum. TIIE MECHANICAL POWERS. 73 What is the That is,- (1.) The power* is at one end, the postont of the weight at the other, and the fulcrum between them. weight, and (2.) Power at one end, the fulcrum at the the fulcrum, other, and the weight between them. respectively, in the three kinds (3) The weight is at one end, the fulcrum at of lever? the other, and the power between them. Describe a lever 259. In a lever of the first kind the fulcrum of the first kind is placed between the power and the weight. by figure 27, and tell the ad- Fig. 27 represents a lever of the first kind, vantage gained resting on the fulcrum by it. Fig. 27. by it. F, and movable upon it. W is the weight to be moved, and B P is the power which moves it. The advantage gained in raising a weight, by the use of this kind of lever, is in proportion as the distance of the power from the fulcrum exceeds that of the weight from the fulcrum. Thus, in this figure, if the distance between P and F be double that between W and F, then a man, by the exertion of a force of 100 pounds with the lever, can move a weight of 200 pounds. From this it follows that the nearer the power is applied to the end of the lever, the greater is the advantage gained. Thus, a greater weight can be moved by the same power when applied at B than when it is exerted at P. On what prin- 260. The common steelyard, an instrument for ciple is the com- mon steelyard weighing articles, is constructed on the principle constructed? of the lever of the first kind. It consists of a Describe the rod or bar, marked with notches to designate the steelyard. pounds and ounces, and a weight, which is mova* It is to be understood, in the consideration of all instruments and machines, that some effect is to be produced by some power. The names power and weight are not always to be taken literally. They are terms used to express the cause and the effect. Thus, in the movement of a clock, the weight is the cause, the movement of the hands is the effect. The cause of motion, whether it be a weight or a resistance, is technically called the power; the effect, whether it be the raising-of a weight, the overcoming of resistance or of cohesion, the separation of the parts of a body, compression or expansion, is technically called the weight. 7 74 NATURAL PHILOSOPHY. ble along the notches. The bar is furnished with three hooks, on the longest of which the article to be weighed is always to be hung.. The other two hooks serve for the handle of the instru Fig. 28. ent when in use. The piot of each of these two hooks seres ment when in use. The pivot of each of these two hooks serves for the fulcrum. 261. When suspended by the hook 0, as in Fig. af What re se 28, it is manifest that a pound weight at E will hooks in the balance as many pounds at W as the distance besteelyard? tween 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. The same instrument may be used to weigh heavy articles, by using the middle hook for a handle, where, as will be seen in Fig. 29, the space between the pivot of F (which in this case is the fulcrum) and the pivot of D (from which the weight is suspended) being lessened, is contained a greater number 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 * Qf Equilibrium. - In the calculations of the powers of all machines it is THE MECHANICAL POWERS. 75 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. Fig. 29. c F A balance, or pair of scales, is a lever of the first kind, with equal arms. Steelyards, scissors, pincers, snuffers, and a poker used for stirring the fire, are all levers of the first kind. The longer the handles of scissors, pincers, &c., and the shorter the points, the more easily are they used. 262. The lever is made in a great variety of forms and of many different materials, and is much used in almost every kind of mechanical operation. Sometimes it is detached from the fulcrum, necessary to have clearly in mind the difference between action and equi. librium. By equilibrium is meant an equality of forces; as, when one force is opposed by another force, if their respective momenta are equal, an equilibrium is produced, and the forces merely counterbalance each other. To produce any action, there must be inequality in the condition of one of the forces. Thus, a power of one pound on the longer arm of a lever will balance a weight of two pounds on the shorter arm, if the distance of the power from the fulcrum be exactly double the distance of the weight from the fulcrum; and the reason why they exactly balance is, because their momenta are equal. No motion can be produced or destroyed without a difference between the force and the resistance. In calculating the mechanical advantage of any machine, therefore, the condition of equilibrium must first be duly considered. After an equilibrium is produced, whatever is added upon the one side or taken away on the other destroys the equilibrium, and causes the maciiue to move. 76 NATURAL PHILOSOPHY. but most generally the fulcrum is a pin or rivet by which the lever is permanently connected with the frame-work of other parts of the machinery. 263. 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. Ho1w is power 264. To gain power by the use of the gained by the 1 use of the lever, the fulcrum must be placed near the lever? 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 fulcrum. What is a 265. A Com- Fig. 30. Compound pound Lever, rep-? D nc Lever? resented in Fig.1a Am I H 30, 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. Describe the 266. In a lever of the second kind, the ful-,dver ofthese.th crum is at one end, the power at the other, and Fib. 31. the weight between them. (1.) Let Fig. 31 represent a lever of the second kind. F is the fulcrum, P the power, and W the weight. Fig. 31. The advantage gained by a lever of this kind is in proportion as the distance of the power from,. the fulcrum exceeds that of the weight from the fulcrumn. Thus, in this figure, if the distance w # This being the case, it is evident that the shape of the lever will not influence its power, whether it be straight or bent. The direct distance between the fulcrum and the weight, compared with the same distance between the fulcrum and the power, being the only measure of the mechanical advantage which it affords. TUIE MICICHA1CAL POWErS. 77 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, carrying a heavy burden suspended on a bar, may be made to bear unequal portions of it; by placing it nearer to the one than the other. 267. Two horses, also, may be made to draw unequal portions of a load, by dividing the bar attached to the carriage in such a manner that the weaker horse may draw upon the longer end of it. 268. Oars, rudders of ships, doors turning on Fig. 32. hinges,'and cutting-knives. which are fixed at one end, are constructed upon the principle of levers of the second kind.* Describe the 269. In a lever of the third kind the fulcrum lever oj'the third kindeby is at one end, the weight at the other, and the Fig. 33. power is applied between them. In levers of this kind the power must always exceed the weight in the same proportion as the distance of the weight from the fulcrum exceeds that of the power from the fulcrum. In Fig. 33 F is the fulcrum, W the weight, Fig. 33. and P the power between the fulcrum and the F weight; and the power must exceed the weight r in the same proportion that the distance between W and F exceeds the distance between P and F. w 270. A ladder, which is to be raised by the strength of a man's arms, represents a lever of this kind, where the fulcrum is that end which is fixed against the wall; the weight may be considered as at the top part of the ladder, and the power is the strength applied in raising it. 271. The bones of a man's arm, and most of the movable bones of animals, are levers of the third kind. But the loss of power in limbs of animals is compensated by the beauty and compactness of * It is on the same principle that, in raising a window, the hand should be applied to the middle of the sash, as it will then be easily raised, whereas, if the hand be applied nearer to one side than the other, the centre of gravity being unsupported, will cause the further side to bear against the frame, and obstruct its free motion. ~,.. 78 NATURAL PHILOSOPHY. the limbs, as well as the increased velocity of their motion. The wheels in clock and watch work, and in various kinds of machinery, may be'considered as levers of this kind, when the power that moves them acts on the pinion, near the centre of motion, and the resistance to be overcome acts on the teeth at the circumference. But here the advantage gained is the change of slow into rapid motion. 272. PRACTICAL EXAMPLES OF LEVERAGE. Questions for Solution (1.) Suppose a lever, 6 feet in length, to be applied to raise a weight of 50 pounds, with a power of only 1 pound, where must the fulcrum be placed? Ans. 1.41 in. + (2.) If a man wishes to move a stone weighing a ton with a crow-bar 6 feet in length, he himself being able, with his natural strength, to move a weight of 100 pounds only, what must be the greatest distance of the fulcrum from the stone?,Ans. 3.42 n. + (3.) If the distance of the power from the fulcrum be eighteen times greater than the distance of the weight from the fulcrum, what power would be required to lift a weight of 1000 pounds? Ans. 55.55 lb. + (4.) If the distance of the weight from the fulcrum be only a tenth of the distance of the power from the fulcrum, what weight can be raised by a power of 170 pounds? Ans. 1700 lb. (5.) In a pair of steelyards the distance between the hook on which the weight is hung and the hook by which the instrument is suspended is 2 inches; the length of the steelyards is 30 inches. How great a weight may be suspended on the hook to balance a weight of 2 pounds at the extremity of the longer arm. - Ans. 28 lb. (6.) Archimedes boasted that, if he could have a place to stand upon, he could move the whole earth. Now, suppose that he had a fulcrum with a lever, and that his weight, compared with that of the earth, was as I to 270 millions. Suppose, also, that the fulcrum were a thousand miles from the earth; what must be his distance from the fulcrum? Ans. 270,000,000,000 mi. (7.) Which will cut the more -easily, a pair of scissors 9 inches long, with the rivet 5 inches from the points, or a pair of scissors 6 inches. long, with the rivet 4 inches from tloe points? Ans. The first. (8.) Two persons, of unequal strength, carry a weight of 200 pounds suspended from a pole 10 feet long.- One of them can carry only 75 pounds, the other must carry the rest of the Weight. How far from the end of the pole must the weight be suspended 2 Ans. 3.75 ft. (9.) How must the whiffle-tree * of a carriage be attached, that one horse may draw but 3 cwt. of the load, while the other draws 5 cwt. 2 Ans. At Z. (10.) On the end of a steelyard, 3 feet long, hangs a weight of 4 pounds. Suppose the hook, to which articles to be weighed are attached, to be at the extremity of the other end, at. the distance of 4 inches from the hook by which the steelyards are held up. How great a weight can be estimated by the steelyard 2 Ans. 32 lb. What is the 273. THE WHEEL AND AXLE. - The Wheel and Axle? Wheel and Axle consists of a cylinder with a wheel attached, both revolving around the same axis of motion. * The whiffle-tree is generally attached to a carriage by a hook or leather band in the centre, so that the draft shall be equal on both sides The hook or leather band thus becomes a fulcrum. TiUh MECHANICAL POWERS. 79 How are the 274. The weight is supported by a rope or powe and tlhe chain wound around the cylinder; the power is weight applied to the wheel applied to another rope or chain wound around and axle? the circumference of the cylinder. Sometimes projecting spokes from the wheel supply the place of the chain.* 275. The place of the cylinder is'sometimes supplied by a small wheel. Explain the 276. The wheel and axle, though made in construction of the wheel and many forms, will easily be understood by inaxle by Fig. specting Figs. rig.34. 34 and 35. In P 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. What is the The advantage advantage gained is in gained by the use of the wheel proportion as and axle? the circumfer- w ence 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. How does the 277. Some- Fig. 35 wheel and axle times the axle C described in Figdescr. 3be5 differ is constructed from that de- with a winch or A\\ scribed in Fig. handle, as in 34? Fig. 35, and sometimes the wheel has pro- 3 1 jecting spokes, as in Fig. 34. * A cylinder is a long circular body of uniform diameter, with extremitiet. forming equal and parallel circles. 80 NATURAL PHILOSOPHY. 278. The principle upon which the wheel and On what principle is the axle is constructed is the same with that of the wrheel and axle other Mechanical Powers, the want of power constructed? 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 fromthe centre of motion; for the wheel describes a great circle in the same time that the axle describes a small one; therefore the power is increased in the same proportion as the circumference 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. 279. The wheel and axle are sometimes called "the perpetual lever," the diameter of the wheel representing the longer arm, the diameter of the axle representing the shorter arm, the fulcrum being at the common centre. 280. The capstan,* on board of ships and other vessels, is con structed on the principle of the wheel and axle. It consists of an axle placed uprightly, with a head a(r drum, pierced with holes for the lever, or levers, which supply the place of the wheel. 281. Windmills, lathes, the common windlass, used for drawing water from wells, and the large wheels in mills, are all constructed on the principle of the wheel and axle. 282. 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. 283. CRANKS. - Cranks are sometimes conWhat are Cranks, and nected with the axle of a wheel, either to give or how are they to receive its motion. They are made?..ig. 36. made by bending the axle in such a manner.as to form four right angles facing in different directions, as is represented in Fig. 36. TAey are, infact, nothing more than a double winch. * 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 capstan; if vertically, a windlass. THE MECIHANICAL POWERS. f1 284. A rod connects the -crank with other parts of the machinery, either to communicate motion to or from a wheel. When the rod which communicates the motion stands perpendicular to the crank, which is the case twice during each revolution, it is at what is commonly called the dead point, and the crank loses all its power. But, when the rod stands obliquely to the crank, the crank is then effective, and turns or is turned by the wheel. 285. Cranks are used in the common foot-lathe to turn the wheel. They are also common in other machinery, and are very convenient for changing rectilinear to circular motion, or circular to rectilinear. 286. When they communicate motion to the wheel they operate like the shorter arm of a lever; and, on the contrary, when they communicate the motion from the wheel they act like the longer arm. 287. FLY-WHrEELS are heavy rims of metal iWhat are Flywheels, and secured by light spokes to an axle. They are what is their used to accumulate power, and distribute it use.? 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 with regularity. 288. Fly-wheels are particularly useful in connexion with cranks, especially when at the dead points, as the momentum of the flywheel, received from the cranks when they acted with most advantage, immediately carries the crank out of the neighborhood of the dead points, and enables it to again act with advantage. 289. There are two ways in which the wheel and axle is supported, namely, first on pointed pivots, projecting into the extremities of the axle,* and, secondly, with the extremities of the axle resting on gudgeons. As by the former mode a less extensive area is subjected to friction, it is in many cases to be preferred. flow many 290. WATER-WHEELS. - There are three kinds of kinds of Water-wheels. called, respectively, * The terms axle, axis, arbor and shaft, are synonymously used by mechanics to express the bar or rod which passes through the centre of a wheel. The terminations of a horizontal arbor are called gudgeons, and of an upright one frequently pivots; but gudgeons more frequently denote the beds on which the extremities of the axle revolve, and pivots are either the pointed extremities of an axle, or short pins in the frame of a machine which receive the extremities.of the axle. The term axis, in a more exact sense, may mean merely the longest central diameter, or a diameter about which motion takes place. 82 NATURAL PHILOSOPHY. Water-wheels ~ the Overshot, the Undershot and the Breast are there? Wheel. 291. The Overshot Wheel receives its motion from the weight of the water flowing in at the top. Describe the Fig. 37 represents the Overshot Wheel. It conOvershot sists of a wheel turning on an axis (not repreWhseel. sented in the figure), with Fig. 37. compartments called buckets, a b c d, &c., d at the circumference, 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 opposite or ascending part are always empty until they are again presented to the stream. This kind of wheel is the most powerful of all the water-wheels. 292. The Undershot Wheel is a wheel which is moved by the motion of the water. It receives its impulse at the bottom. Describe the Fig. 38 rep- Fig. 38. Undershot resents the UnWheel. dershot Wheel. A Instead of buckets at the cir- d cumference, it is furnished 0 with plane surfaces, called float-boards, a b c d, &c., which receive the impulse of the water, and cause the wheel to revolve. Describe the 293. The Breast Wheel is a wheel which receives Breast;Wheel. the water at about half its own height, or at the THE MECHANICAL POWERS. 83 level of its own axis. It Fig. 39. is moved both by the weight and the motion of the water. Fig. 39 represents a Breast Wheel. It is furnished either with buckets, or with float-boards, M. II fitting the water-course, receiving the weight of the water with its force, while in motion it turns with the stream. 294. In the water-wheels which have now been described, the motion is given to the circumference of the larger wheel, either by the weight of the water or by its force when in motion. 295. All wheels used in machinery are connected with the different parts of the machine by other parts, called gearing. Sometimes they are turned by the friction of endless bands or cords, and sometimes by cogs, teeth, or pinions. When turned by bands, the motion may be direct or reversed by attaching the band with one or two centres of motion respectively. 296. When the wheel is intended to revolve in Fig. 40. the same direction with the one from which it receives its motion, the band is attached as in Fig. 40; but when it is to revolve in a contrary direction, it is crossed as in Fig. 41. In Fig. 40 Fig. 41. the band has but one centre of motion; in Fig. 41 it has two. 297. Instead of the friction of bands, the rough surfaces of the wheels themselves are made to communicate their motion. The wheels and axles thus rubbing together are sometimes coated with rough leather, which, by increasing the friction, prevents their slipping over one another withoutcommunicating motion. 298. Figure 42 represents such a combination of wheels. As the wheel a is turned by the weight S, its axle ig. 42 presses against the circumference of the wheel b, ig. 42. causing it to turn; and, as it turns, its axle rubs d against the circumference of the wheel c, which in like -manner communicates its motion to d. Now, as the circumference of the wheel a is equal to six times the circumference of its axle, it is evident that when the wheel a has made one rev- F S olution b will have performed only one-sixth of a revolution. The wheel a must therefore turn round six times to cause b to turn once. In like manner b must perform six revolutions to cause c to turn once, and c must turn as many times to cause d to 84 NATURAL PHILOSOPHY. revolve once.- Hence it follows that while d revolves once on its axis c must revolve'six times, b thirty-six times, and a two hundred and sixteen times.. 299. If, on the contrary, the power be applied at F, the conditions will all be reversed, and c will revolve six times, b thirty-six, and a two hundred and sixteen times. Thus it appears that we may obtain rapid or slow motion by the same combination of wheels. How may rapid or 300. To obtain rapid motion, the power slow motion be obtained at pleasure must be applied to the axle; to obtain by a combination of slow motion, the power must be applied to wheels with their awhles? the circumference of the wheel. 301. Wheels are sometimes moved by means of cogs or teeth articulating one with another, on the circumference of the wheel and the -axle. 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. 302. Fig. 43 represents a connexion of cogged wheels. The wheel B, being moved by a Fig. 43. string around its circumference, is a simple wheel, without teeth. Its axle, being furnished with cogs or leaves, to which the teeth of the wheel D are fitted, communicates its B H motion to D, which, in like manner, moves the wheel C., The power P and the weight W must be attached to the circumference of the wheel or P of the axle, according as a slow bI or a rapid motion is desired. 303. Wheels with teeth or cogs are of three kinds, according to Fig. 44. Fig. 45. A the position of the teeth. When the teeth are raised perpendicular to the axis, they are called spur wheels, or spur gear. When the teeth are parallel withi the axis, they are called crown wheels. When TEE MhEGOHANICAL POWERS. 5'they are raised on a surface inclined to the axis, they are called bevelled wheels. In Fig. 43 the wheels are spur wheels. In Figs. 44 and 45 the wheels are bevelled wheels. 304. Different directions may be given to the motion produced by wheels, by varying the position of their axles, and causing them to revolve in different planes, as in Fig. 44; or by altering the shape and position of the cogs, as in Fig. 45. How may the 305. The power of toothed wheels may be eelsr ef tthed e estimated by substituting the number of teeth mated? in the wheel and the number of leaves in the pinion for the diameter or the circumference of the wheel and axle respectively. 306. SUSPENSION OF ACTION. - In the arrangement of machinery, it is often necessary to cut off the action of the moving power from some parts, while the rest continues in motion. This is done by causing a toothed wheel to slide aside in the direction of its axis to and from the cogs or leaves into which it articulates, or, when the motion is communicated by a band, by causing the band to slip aside from the wheel to another wheel, which revolves freely around the axle, without communicating its motion. 307. Wheels are'used on vehicles to diminish the friction of the road. The larger the circumference of the wheel, the more readily it will overcome- obstacles, such as stones or inequalities in the surface of the road. 308. A large wheel is also attended with two additional advantages; namely, first, in passing over holes, ruts and excavations, a large wheel sinks less than a small one, and consequently causes less jolting and expenditure of power; and, secondly, the wear of large wheels is less than that of small ones, for, if we suppose a wheel six feet in diameter, it will turn round but once while a wheel three feet in diameter will turn round twice, its tire will come twice as often to the ground, and its spokes will twice as often have to bear the weight of the load. 309. But wheels must be limited in size by two considerations: first, the strength of the materials; and secondly, the centre of the wheel should never be higher than the breast of the horse, or other animal by which the vehicle is drawn; for otherwise the animal would have to draw obliquely downward, as well as forward, and thus expend part of his strength in drawing against the ground.* # 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 sliding 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. 8 86 NATURAL PHILOSOPHY. 310. PRACTICAL EXAMPLES OF POWER APPLIED TO THE WHEEL AND AXLE. Questions for Solution. (1.) With a wheel 5 feet in diameter and a power of 6 pounds, what must be the diameter of the axle to support 3 cwt.. Ans. 1.2 in. (2.) How large must be the diameter of the wheel to support with 10 lbs. a weight of 5 cwt. on an axle 9 inches in diameter? Ans. 3T.5ft. (3.) A wheel has a diameter of 4 feet, an axle of 6 inches. What power must be applied to the wheel to balance 2 cwt. on the axle? A??s. 25 lb, (4.) There is a connexion of cogged wheels having 6 leaves on the pinion and 36 cogs on the wheel. What is the proportion of the power to the weight in equilibrium? Anas. As 1 to 6. (5.) Suppose a lever of six feet inserted in a capstan 2 feet in diameter, and six men whose united strength is represented by e of a ton at the capstan, how heavy an anchor can they draw up, allowing the loss of I of their power from friction? A s. 2 T. (6.) What must be the proportion of the axle to the wheel, to sustain a weight 30 cwt. with a power of 3 cwt.? Ans. As 1 to 10. (7.) The weight is to the power in the proportion of six to one. What must be the proportion of the wheel to the axle? Ans. 6 to 1. (8.) The power is represented by 10, the axle by 2. How can you represent the wheel and axle. Ans. 10: weight:: 2:,wheel. (9.) The weight is expressed by 15, the power by 3. What will represent the wheel and axle? Ans. 5 and 1. (10.) The axle is represented by 16, the power by 4. Required the proportion of the wheel and axle. Ans. 4: weight:: 16: wheel. (11.) What is the weight of an anchor requiring 6 men to weigh it, by means of a capstan 2 feet in diameter, with a lever 8 feet long, 2 feet of its length being inserted in the capstan; supposing the power of each man to be represented by 2 cwt., and a loss of ~ the power by friction? Ans. 56 cwt. (12.) A stone weighing 2 tons is to be raised by a windlass with spokes 2 feet in length, projecting from an axle 9 inches in diameter. How many men must be employed, supposing each man's power equal to 2 cwt., including the loss by friction 1 Ans. 2.5 ment. What is a 311. THE PULLEY. - The Pulley is a small Pulley' wheel turning on an axis, with a string or rope in a groove running around it. How many kinds There are two kinds of pulleys -the of pulleys are there? fixed and the movable. 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. Explain 312. Fig. 46 represents a fixed pulley. P is a Fig. 46. small wheel turning on its axis, with a string running round it in a groove. W is a weight to be raised, F is the force or power applied. It is evident that, by pulling the string at F, the weight must rise just as much as the string is drawn THE MECHANICAL POWERS. s1 down. As, therefore, the velocity of the weight and the Fig. 46. power is precisely the same, it is manifest that they balance each other, and that no mechanical advantage is gained.* But this pulley is very useful for changing the direction of motion. If, for instance, we wish to: W raise a weight to the top of a high building, it can be done F with the assistance of a fixed pulley, by a man standing below. 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. On what prin- 313. The fixed pulley operates on the same ciple does the principle as a lever of the first kind with equal fixed Fulley act? 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. 314. The movable pulley differs from Hkow does the movable pulley the fixed pulley-by being attached to Fig. 47. differ from the the weight; it therefore rises and fixed? falls with the weight. Explain 315. Fig. 47 represents a movable pulley, B Fig. 47. with the weight W attached to it by a hook M 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 w weight one inch, it is evident that both sides of the string * Although the fixed pulley gives no direct mechanical advantage, a man may advantageously use his own strength by the use of it. Thus, if he seat himself on a chair suspended from one end of a rope passing over a fixed pulley, he may draw himself up by the other end of the rope by exerting a force equal only to one-half of his own weight. One half of his weight is supported by the chair and the other half by his hands, and the effect is the same as if he drew only one half of himself at a time; for, the rope being doubled across the pulley, two feet of the rope must pass through his hands before he can raise himself one foot. In this manner laborers and others frequently descend into wells, and from the upper floors of stores, by means of a rope passing over a fixed wheel or pulley. 88 NATURAL PHILOSOPHY. 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 movable pulley of two pounds.* What is the ad- 316. The power gained by the use of pulvantage gained in the use of the leys is ascertained by multiplying the nummovable pulley? ber of movable pulleys by 2.t 317. A weight of 72 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 movable 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. Explain 318. Fig. 48 represents a system of fixed and Fig. 48. movable pulleys. In the block F there Fig. 48. are four fixed pulleys, and in the block M there A are four movable pulleys, all turning on their common axis, and rising and falling with the weight W. The movable pulleys are connected with the fixed ones by a string attached to the hook H, passing over the alternate 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 I t Thus, it is seen that pulleys act on the same principle with the lever and the wheel and axle, the deficiency of the strength of the power being compensated by superior velocity. Now, as we cannot increase our 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. t This rule applies only to the movable pulleys in the same block, or when the parts of the rope which sustains the weight are parallel to each other. The mechanical advantage, however, which the pulley seems to possess in theory, is considerably diminished in practice by the stiffness of the ropes and the friction of the wheels and blocks. When the parts of the cord, also, are not parallel, the pulley becomes less efficacious; and when the parts of the cord which supports the weight very widely depart from parallelism, the pulley becomes wholly useless. There are certain arrangements of the cord and the pulley by which the effective power of- the THE MECHANIOAL POWERS. 89 shortened one foot, and, consequently, that the power P must descend eight times that distance. The power, therefore, must pass over eight times the distance that the weight moves. 319. The movable pulley, as well as the fixed, acts on the same principle with the lever, the deficiency of the strength of the power with the movable pulley being compensated by its superior velocity. On what princi- 320. The fixed pulley acts on the principle of ple is the mov- a lever with equal arms. [See No. 313.] The able pulley constrblucted cn movable pulley, on the contrary, by giving a superior velocity to the power, operates like a lever with unequal arms. 321. Practical use of Pulleys.- Pulleys are used to raise goods into warehouses, and in ships, &c., 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 deck by means of the fixed pulleys, while the labor is facilitated by the mechanical power of the movable ones. 322. Both fixed and movable pulleys are constructed in a great variety of forms, but the principle on which all kinds are constructed is the same. What is generally called a tackle and fall, or a block and tackle, is nothing more than a pulley. Pulleys have likewise lately been attached to the harness of a horse, to enable the driver to govern the animal with less exertion of strength. 323. It may be observed, in relation to the MeWhat law applies to all the chanical Powers in general, that.power is always Mechanical gained at the expense of time and velocity; that Powers? 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, 4c.: and that the same quantity of force used to raise two pounds one foot will raise one pound two feet, 4c. And, further, it masy 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 multiplied by the velocity of the power. pulley may be augmented in a three-fold instead of a two-fold proporti )n But, when such an advantage is secured, it must be by contriving to make the power pass over three times the space of the weight. o * 90 NATURAL PHILOSOPHY. In what propor- Hence we have the following rule: Tihe tion is the power to the weight power is in the same proportion to the when the mov- weight as the velocity of the weight is to able pulley is used p:the velocity of the power.* 3 24. PRACTICAL EXAMPLES OF APPLICATION OF THE PULLEY. Questions for Solution. (!.) Suppose a power of 9 lbs. applied to a set of 3 movable pulleys. Al lowing I loss for friction, what weight can be sustained by them 1 A. 36 lb. (2.) Six movable pulleys are aBtached to a weight of 1800 lbs.; what power will support them, allowing a loss of two-thirds of the power from friction?' Azs. 450 lb. (3.) Six men, with a block and tackle containing nine movable pulleys, are required to raise a sail. Suppose each man's strength to be represented by two cwt. and two-thirds of the power lost by friction, what is the weight of the sail, with its appendages? Ans. 72 cwt. (4.) If a stone weighing 3 tons is to be raised by horse power to the wall of a building in process of erection, by means of a derrick fiom which are suspended 3 movable pulleys, how many horses must be employed, supposing each horse capable of drawing as much as eight men, each of whom can lift 2 cwt., making an allowance of two-thirds for friction? Ans. 1. (5.) A block contains 5 movable pulleys, connected with a beam containing 5 fixed pulleys. A weight of halfa ton is to be raised. Allowing a loss of two-thirds for friction, what power -must be applied to raise it? A. 8 cwt. (7.) The power is 3, the weight is 27; how many pulleys must be used, if friction requires an allowance of two-thirds? Ans. 27. (8.) Friction one-third of the power, power 6, weight 72, - how many pulleys? Ans. 18. (9;) Weight 84, friction nothing, pulleys, 3 fixed, 3 movable; required the power. Ans. 14. (10.) Power 12, friction 8, four pulleys, two of them fixed; required the.weight. Ans. 16. (11.) Six movable and six fixed pulleys. The weight is raised 3 feet. How far has the power moved Ans. 36 ft. (12.) The power has moved 12 feet; how far has the weight moved under two pulleys, one fixed, the other movable. Ans. 6ft. (13.) The weight, suspended from a fixed pulley, has moved 6 feet. How far has the power moved. Ans. 6ft. (14.) The power has moved 20 feet under a fixed pulley; how far has the weight moved. Ans. 20ft. What is the In- 325. THE INCLINED PLANE.- The Inclined Plane? dined Plane consists of a hard plain surface, inclined to the horizon. 326. The principle on which the inclined plane acts as a mechanical power is simply the fact that it supports part of the weight. If a body be placed on a horizontal plane, its whole weight will be * The stiffness of the cords and the friction of the blocks frequently require large deduction to be made from the effective power of pulleys. The loss thus occasioned will sometimes amount to two-thirds of the power. THE MECHANICAL POWERS. 91 supported; but, if the plane be elevated at one end, by degrees, it will support less of the weight in proportion to the elevation. until the plane becomes at right angles to the horizon, when it will support no part of the weight, and the body will fall perpendicularly. 327. A body, in ascending or descending an inclined plane, will have a greater space to traverse than if it should rise or fall perpendicularly. The time, therefore, of its ascent or descent will be longer, and thus it will oppose less resistance, and thus, also, a less force will be required to cause its ascent. Hence, we see that the fundamental principle of Mechanics, " What is gained in power is lost in time," applies to the Inclined Plane as well as to the Mechanical Powers that have already been described. What is the ad- 328. The advantage gained by the use of vantage gained by the use of the the inclined plane is in proportion as the inclined plane? length of the plane exceeds its perpendicular height. Fig. 49 represents an inclined plane. C A its height, C B its length, and W a weight which is to be Fig. 49. 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 weight of four -- pounds on the inclined plane C B. 329. The greater the inclination of the plane, the greater must be its perpendicular height, compared with its length; and, of course, the greater must be the power to elevate a weight along its surface. 330. Instances of the application of the inclined plane are very common. Sloping planks or pieces of timber leading into a cellar, and on which casks are rolled up and down; a plank or board with one end elevated on a step, for the convenience of trundling wheelbarrows, or rolling barrels into a store, &c., are inclined planes. 331. Chisels and other cutting instruments, which are chlamfered, or sloped only on one side, are constructed on the principle of the inclined plane.* 332. Roads which are not level may be considered as inclined planes, and the inclination of the road is estimated by the height corresponding to some proposed length. To raise a load up an inclined plane requires a power sufficient to carry it along the whole distance of the length of the base, and then to lift it up to * Chisels for cutting wood should have their edges at an angle of about 30; for cutting iron from 500 to 600, and for cutting brass at,about 800 or 90(. Tools urged by pressure may be sharper than those'which, like the wedge, are driven by percussion. 92 NATURAL PHILOSOPHY. the elevation; but in the inclined plane a feebler force will accomplish the desired object, because the resistance is spread equally over the whole distance.* What is the 333. THE: WEDGE.- The Wedge consists Wedge? - of two inclined planes united at their bases. What is the ad- 334. The advantage gained by the wedge vantage gained bytheuseof the is in proportion as its length exceeds the wedge? thickness between the converging sides. In what proportion is the It follows that the power of the wedge is in propower of the portion to its sharpness. wedge? 335. Fig. 50 represents a wedge. The line a b Fig. 60. represents the base of each of the inclined planes of which it is composed, and at which they are united. V b 336. The wedge is a very important mechanical power, used to split rocks, timber, &c., which could not be effected by any other power.f 337. Axes, hatchets, knives, and all other cutting instruments, chamfered, or sloped on both sides, are constructed on the principle of the wedge; also pins, needles, nails, and all piercing instruments. On what does 338. The effective power of the wedge depends oe ffectiofthe on friction; for, if there were no friction,, the power of the wedge depend? wedge would fly back after every stroke. * Mention has already been made of the sagacity of animals in a former page [see No. 541], and a sort of intuitive knowledge which they appear to possess of philosophical principles. In ascending a steep hill, a common dray-horse will drag his load from side to side, as if he were conscious that he thus made the plane longer in proportion to its height, and thereby made his load the lighter. t The wedge is an instrument of exceedingly effective power, and is frequently used in presses for extracting the juice of seeds, fruits, &c. It is used especially in the oil mill, by which the oil is extracted from seeds. The seeds are placed in hair bags, between planes of hard wood, which are pressed together by wedges. The pressure thus exerted is so intense that the seeds, after the extraction of the oil, are converted, into masses as hard and compact as the most dense woods. Wedges are used also in the launching of vessels, and also for restoring buildings to the perpendicular which have been inclined by the sinking of the foundation. THE MECHANICAL POWERS. 93'339. The wedge derives much of its power from the force of percussion, which in its nature is so diffirent from continued force, such as the pressure of weights, the force of springs, &c., that it would be difficult to submit it to numerical calculation; and, therefore, we cannot properly represent the proportion which a blow bears to the weight. What isthe 340. THE SCREW. -- The Screw is an inScrew? dined plane wound around a cylinder, thus producing a circular inclined plane, forming what is called the threads of the screw. 341. Cut a piece of paper in the shape of an inclined plane, as represented by Fig. 49, and, beginning, with the end represented by the, height C A, in that Figure, wind it around a pencil, or a round ruler. The edge of the paper will be a circular inclined plane, and will represent the threads of the screw. The distance between any two threads on the same side of the rule will represent the perpendicular height of the inclined plane that extends once around the cylinder, and the advantage gained in the use of the; screw (when used without a lever) will be the same as in the inclined plane; namely, as the length of the plane exceeds the perpendicular height. But the screw is seldom used alone. A lever is generally attached to the screw, and it is with this attachment the screw will now be considered. 342. The Screw is generally accompanied What appendage generally attends by an appendage called the nut, which consists the Screw? of a concave cylinder or block, with a hollow spiral cavity cut so as to correspond exactly with the threads of the screw. When thus fitted together, the screw and the nut form two inclined' planes, the one resting on the other. 343. Sometimes the screw is movable and Is the screw, or the nut mov- the nut is stationary, and sometimes the screw able? is stationary and the nut is movable. 344. ~At every revolution the screw or the nut advances or retreats through a space equal to the distance between the threads of the screw. Ta wcha~t manne 345. The power applied to a screw generally does the power describes a circle around the screw, perpendicr'plied to the ular to the plane in which the screw or nut screw move? moves. 94 NATURAL -PHILOSOPHY. What is the advan- 346. The advantage gained by the tage gained by the screw is in proportion as the circumferscrew? ence described by the power exceeds the distance between the threads of the screw. VWhat is meant by 347. The cylinder with its threads is called the Convex and the Convex Screw, and the nut is called the Concave Screw? Concave Screw. The lever is sometimes attached to the screw, and sometimes to the nut. Explain 348. Fig. 51 represents a fixed screw Fig. 51. Fig. 51. S, with a movable nut N, to which is attached the leverL. By turning the lever in one direction the nut descends, aid by turning it in the opposite direction the nut ascends, at every revo\ution of the lever, through a space equal to the distance between the threads of the screw; to accomplish which, the hand or power applied to the end of the lever L will describe a circle around the screw S, of which the radius is L S. The power thus passes over a space represented by the circumference of this circle, and the advantage gained is in the same proportion as the space exceeds the distance between each thread of the screw Explain 349. Fig. 52 represents a movable Fig. 52. Fig. 52. screw, with a nut fixed in a frame, and Lconsequently immovable. As the lever L is turned, the screw ascends or descends at every revolution of the lever through a space equal to the distance between the threads of the screw, and the advantage gained is in the same proportion as in the case of the movable nut in Fig. 51. 350. It will thus be seen that, although the screw is usually considered distinctly as a mechanical power, it is in fact a compound power, consisting of two circular inclined planes, moved by a lever. 351. The power of the screw being estimated by the distance between the threads, it follows that the closer the threads are together, the greater will be the power, but the slower will be the motion produced; for, every revolution of the lever advances the THl MECHANICAL tPOWEiS. 95 screw or the nut only through a space as great as the distance of the threads from each other. 352. The screw is applied to presses and engines of all kinds where great power is to be applied, without percussion, through small distances. It is used in bookbinders' presses, in cider and Fi. 53. wine presses, in raising buildings. It is also used for coining, and for punching square or circular holes Fi,. 54. through thick plates of metal. When used for this purpose, the lever passes through the head of the screw and terminates at both ends with heavy balls or weights, the momentum of which adds to the force of the screw, and invests it with immense power. 353. HUNTER'S ScREw. —The ingenious contrivance known by the name of Hunter's Screw consists of two screws of different threads playing one within the other; and such will be the effect, that while one is advancing forward the other will retreat, and the resistance will be urged forward through a distance equal only to the difference between the threads of the two screws. An indefinite increase in the power is thus obtained, without diminishing the thread of the screw.* * From what has been stated with regard to the Mechanical Powers, it appears that by their aid a man is enabled to perform works to which his unassisted natural strength is wholly inadequate. But the power of all machines is limited 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 destroyed, and it can withstand no resistance which is stronger than its cohesive attraction. Besides the strength of the materials, it is necessary, also, to consider the time which is expended in the application 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 million of years, in order to move the earth the twenty. seven millionth part of an inch. 96 NATURAL PHILOSOPHY. 354. PRACTICAL EXAMPLES OF THE APPLICATION OP THE INCLINED PLANE AND THE SCREW. Questions for Solution. (1.) With an inclined plane the power moves 16 feet, the power is to the weight as 6 to 24. How far does the weight move? Ans. 4ft. (2.) The length of an inclined plane is 5 feet, the proportion of the power to the weight is as 2 to 10. What is the height of the plane? A. 1ft. (3.) An inclined plane is 4 feet high, a power of 6 lbs. draws up 30 lbs. What is the length of the plane? Ans. 20ft. (4.) The length of a plane is 12 feet, the height is 3 feet. What is the proportion of the power to the weight to be raised? Ans. As 1 to 4. (5 ) The distance between the threads of a screw is 1 inch, the length of the lever is 2 feet. What is the proportion Ants. 1 to 150.79 + (6.) Which will exert the greater force, a lever 3 feet long with the fulcrum 6 inches from one end, or a screw with a distance of 1 inch between the threads and a lever one foot long I Anzs. The screuw. (7.) A screw with the threads 2 inches apart, and a lever 6 feet long, draws a ship of 200 tons up an inclined plane whose length is to the height in the proportion of 1 to 16. What power must be applied to the lever of the screw. Avs. 11.05 lb. + (8.) If a man can lift a weight of 150 lbs., how much can he draw up an inclined plane whose length is to its height as 24 to 3? AAns. 1200 lb. (9.) A Hunter's screw has a lever four feet long. The distance between the threads of the larger screw is 1 inch, between those of the smaller i of an inch. How much weight can a man whose power is represented by 175 lbs. move with such a screw? Ans. 211115.52 lb. (10.) A screw with a lever of 2 feet in length, and a distance of k of an inch between its threads, acts on the teeth or cogs of a wheel whose diameter is to that of the axle as 4 to 1. Fastened to the axle is a rope, one end of which is attached to a weight at the bottom of an inclined plane, the length of which is to the height as 12 to 3. Suppose this weight to require the strength of a man who can lift 200 lbs. to be applied to the lever of the screw to move it. What is the weight? Ans. 9650995200 lb. W/hat is the 355. THE KNEE JOINT, OR TOGGLE TOggle Joint? 99 ggll JOINT.- The Toggle Joint, or Knee Joint, consists of two bars united by a hinge or ball and socket, which, being urged by a power perpendicular to the resistance, acts with rapidly-increasing force, until the bars form a straight line; The toggle (or knee) joint affords a very useful mode of converting velocity into power, the motion produced being very nearly at right angles with the direction of the force. It is a combination of levers, and the same law applies to it as to all machinery, namely, that the power is to the resistance inversely as the space of the power is to the space of the resistance. THE MECHANICAL POWERS. 97 Explain 356. Fig 55 represents a toggle joint, rig 65. R!~ 55 A C -and B C are the two rodscon- A nected by a joint at C. A moving force applied at C, in the direction CI D, acts with great and D." constantly increasing power to separate the parts B A and B. 357. The operation of the toggle Fig. 56. joint is seen in the iron'oints 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 impression.* 358. MEDIA.- The motion of all bodies is affected by the substance or element i which they move, and by H I which they are on all sides surrounded. Thus the bird flies in the air, the fish swims in the water. Air therefore is the medium in which the foirmer moves, while water is the medium in which the motion of the latter is made. What is a 359. A Medium is the substance, solid or fluid, Medium? Medium which surrounds a body, and which the body must displace as it moves. 360. - When the fish swims or the bird flies, each must force itb way through the air or the water; and the element thus displaced must rush into the spot vacated by the body in its progress. It has already been stated that the body of the fish or of the bird is propelled in its motion in the one case by the reaiction of the air on the wings of the bird, and in the other of the water on the fins of a fish. The fish moves in the denser medium and needs therefore to present a less surface for the reiction of the water; while the bird, living in a comparatively rare medium, presents in his wings a much larger extent of surface to receive the reaction of the air. In making the fins of a fish, therefore, so much smaller, in proportion to its size, than the wings of a bird, nature herself has taught us that, In wat -proiportion 361. The resistance of a medium is is the resistance of a medium? in exact proportion to its density. * A similar effect, but with a reversed action, is produced when a long rope tightly strained between two points, is forcibly pulled in the middle. 9 098 -NATUItAL PHILOSOPHY. 362. A body falling' through water will move more slowly than one falling in the air, because it meets with more resistance from the inertia of the water, on account of the greater density of the water. What is a 363. A VACUUM. — A Vacuum is unoccuKV~acuum? pied space; that is, a space which contains absolutely nothing. 364. From this definition of a vacuum, it appears that it does not mean a space which to our eyes appears empty.:What we call an empty bottle is, in fact, full of air, or some other invisible fluid. If we sink an empty bottle in water or any other liquid, neither the water nor arny other liquid can enter until some portion of the air is expelled. A small portion of water enters- the bottle immersed, and the air issues in bubbles from the mouth of the bottle. Other portions of water then enter the bottle, expelling the air in similar manner, until the water entirely fills the bottle, and then the airbubbles cease to rise. 365. From this statement of the meaning of the term " a vacuum," it will be seen that if a machine be worked in a vacuum (or, as it is more commonly expressed in Latin, "in vacuo ") its motion will be rendered easier, because the parts receive no resistance from a surrounding medium. What is Fric- 366. FRIcTIO. — Friction is the resistance tion, and how which bodies meet with in rubbing against manzy kinds of each other. friction are there? De- There are two kinds of friction, namely, scribe each. the rolling and the sliding friction. The rolling friction is caused by the rolling of a circular body. 367. The sliding friction is produced by the sliding or dragging of one surface over another. 368. Friction is caused by the unevenness of the surfaces which come into contact.* It is diminished in proportion as the surfaces are:smoothed and well polished. The sliding friction is overcome with more difficulty than the rolling. ~ All bodies, how well soever they may be polished, have inequalities ilt their surfaces, which may be perceived by a microscope. When, therefore, the surfaces of two bodies come into -contact, the prominent parts of the one will: often fall into' the hollow partsof:the other, and cause more o0 less resistance to motion. THE IMECHANICAL POWERIS. 99 What portion 369. Friction destroys, but never can generate, of the power of motion. It is iusually computed that fiiction a machine is lost byfric- destroys one-third of the power of' a machine. tion? In calculating the power of a machine, therefore, an allowance of one-third must be made for loss by friction.* What is used 3370. Oil, grease, black-lead or powdered soapto lessenfric- stone, is used to lessen friction, because they act zion? and as a polish by filling up the cavities of the why? rubbing surfaces, and thus make them slide more easily over each other. How doesfric- 371. Friction increases: tion increase? (1.) As the weight or pressure is increased. (2.) As the extent of the surfaces in contact is increased. (3.) As the roughness of the surface is increased. How may fric- 372. Friction may be diminished: tion be dimin- (1.) By lessening the weight of the body in ishled 2? motion. (2.) By mechanically reducing the asperities of the sliding surfaces. (3.) By lessening the amount of surface of homogeneous bodies in contact with each other. (4.) By converting a sliding into a rolling motion. (5.) By applying some suitable unguent.t * 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 propared 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 continues its motion. t From the experiments made by Coulomb, it appears that the friction of heterogeneous; bodies is. generally less than that of homogenous that is, that if a body rub against another composed of the same kind of wood or metal, the friction is greater than that of different kinds of metal, or of wood. Ferguson's experiments go to prove that the friction of polished steel sgainst polished steel is greater than that of polished steel on copper or on 1[00 NATURAL PHILOSOPHY. What are thp 373. Friction, although it retards the motion uses of fric- of machines, and causes a great loss of power, lion? performs important benefits in full compensation. Were there no friction, all bodies on the surface of the earth would be clashing against each other. Rivers would dash with unbounded velocity, and'we should see little'but motion and collision. But, whenever a body acquires a great velocity, it soon loses it by friction against the surface of the earth. 374. The friction of water against the surfaces it runs over soon reduces the rapid torrent to a gentle stream; the fury of the tempest is lessened by the friction of the air on the- face of the earth; and the violence of the ocean is soon subdued by the attrition of its own waters. Our garments, also, owe their strength to friction; and the strength of ropes, cords, sails and various other things, depends on the same cause, for they are all made of short fibres pressed together by twisting, and this pressure causes a sufficient degree of friction to prevent the fibres sliding one upon another. Without friction it would be impossible to make a rope of the fibres of hemp, or a sheet of the fibres of flax; neither could the short fibres of cotton have ever been made into such an infinite variety of forms as they have received from the hands of ingenious workmen. Wool also, has been converted into a thousand textures of comfort and luxury, and all these are constituted of fibres united by friction. What is the 375. REGULATORS OF MOTION. - THE PendZulum? PENDULUM. The Pendulum $ consists of a brass. In a combination where gun-metal rubs against steel, the same weight may be moved with a force of fifteen and a half pounds that it would require twenty-two pounds to move when cast-iron moves against steel. * The pendulum was invented by Galileo, a great astronomer of Florence, in the beginning of the seventeenth' century. Perceiving that the chandeliers suspended from the ceiling of a lofty church vibrated long and with great uniformity, as they were moved by the wind or by any accidental disturbance, he was led to inquire into the cause of their motion, and this inquiry led to the invention 6f the pendulum. From a like apparently insignificant circumstance arose the great discovery of the principle of gravitation. During the prevalence of the plague, in the year 1665, Sir Isaac Newton'retired into the country to avoid the contagion. Sitting in his orchard, one day, he observed an apple fall from a tree. His inquisitive mind was immediately led to consider the cause which brought the apple to the ground, and the result of his inquiry was the discovery of that grand principle of gravitation which may be considered as the. first and most im. portant law of material nature. Thus, out of what had been before the eyes of men, in one shape or another, from the creation of the world, did these philosophers bring the most important results. REGULATORS OF MOTION. 101 weight or ball suspended by a rod, and made to swing backwards and forwards. What are the motions of a 376. The motions of a pendulum are called pendulum. call- its vibrations or oscillations, and they are ed, and how are they caused by gravity.+ caused? Whfat is the The part of a circle through which it moves arc of a pendlaroaulum? iS called its arc. What differ- 377. The vibrations of pendulums of equal ence is there in the time of the length are very nearly equal, whether they vibrations of move through a greater or less part of their pendulums of ares.t equal length? 378. In Fig. 57 AB represents a pendulum, Fig. 57. I) F E C the are in which it vibrates. If the T& pendulum be raised to E it will return to F, if it be raised to C it will return to D, in nearly the D c same length of time, because that, in proportion H as the arc is more extended, the steeper will be its beginnings and endings, and, therefore, the more rapidly will it fall.: * When a pendulum is raised from a perpendicular position, its weight will cause it to fall, and, in the act of falling, it acquires a degree of motion which impels it to a height beyond the perpendicular almost as great as that to which it was raised. Its motion being thus spent, gravity again acts upon it to bring it to its original perpendicular position, and it again acquires an impetus in falling which carries it nearly as high on the opposite side. It thus continues to swing backwards and forwards, until the resistance of the air wholly arrests its motion. It will be understood that gravity affects every part of the length of the pendulum. A ball or flattened weight is attached to the lower end of the pendulum to concentrate the effects of gravity in a single point. In the construction of clocks, an apparatus connected with the weight or the spring is made to act on the pendulum with such a force as to enable it to overcome the resistance of the air, and keep up a continued motion. t It has already been stated that a body takes the same time in rising and falling when projected upwards. Gravity brings the pendulum down, and inertia causes it to continue its motion upwards.: The length of the arc in which a pendulum oscillates is called its amnplitude. 9* 1.02 NATURAL PHILOSOPHY. On tihat does 3'79. The time occupied in the vibration of the timte of the a pendulum depends upon its length. The oscillations of a pendulum longer the pendulum, the slower are its videpentd? brations.A What is the 380. The length of a pendulum which length of a vibrates sixty times in a minute (or, in other pedtlum that words, which vibrates seconds) is about thirtyvibrates Once y every second nine inches. But in different parts of the of time? earth this length must be varied. Whichimust be the A pendulum, to vibrate seconds at the longer, to vibrate seconds, apendulum equator, must be shorter than one which at the equator or one vibrates seconds at the poles.t it the poles? (fow is a clock 381. A clock is regulated by lengthening'egulated.? or shortening the pendulum. By lengthening the pendulum, the clock is made to go slower; by shortening it, it will go faster.t * The weight of the ball at the end of a pendulum does not affect the duration of its oscillations. t The equatorial diameter of the earth exceeds the polar diameter by about twenty-six miles; consequently the poles must be nearer to the centre of the earth's attraction thin the equator, and gravity must also operate with greater force at the poles than at the equator. Hence, also, the length of a pendulum, to vibrate in any given time, must vary with the latitude of the place. t The pendulum of a clock is made longer or shorter by means of a screw beneath the weight or ball of the pendulum. The clock itself is nothing more than a pendulum connected with wheel-work, so as to record the number of vibrations. A weight is attached in order to counteract the retarding effect of friction and the resistance of the air. The wheels show how many swings or beats of the pendulum have taken place in a given time, because at every beat the tooth of a wheel is allowed to pass. Now, if this wheel have sixty teeth, it will turn round once in sixty vibrations of the pendulum, or in sixty seconds; and a hand, fixed on the axis'of the wheel projecting through the dial-plate, will be the second-hand of the clock. Other wheels are so connected with the first, and the number of teeth in them is so proportioned, that the second wheel turns sixty times slower than the first, and to'this is attached the minute-hand; and the third wheel, moving twelve times slower than the second, carries the hourhand. On account of the expansion of the pendulum by heat, and its contraction by cold, clocks will go slower in summer than in winter, because the pendulum is thereby lengthened at that season. REGULATORS OF MOTION. 103 1i wnhat pro- 382. The lengths of pendulums are to each portion are t/e lengths of other as the square of the time of their pendulums? vibration. 383. According to thlis law, a. pendulum, to vibrate once in two seconds, must be four times as long as one that vibrates once in one second; to vibrate once in three seconds, it must be nine times as long; to vibrate once in four seconds, it must be sixteen times as long'; once in five seconds, twenty-five times as long, &c. The seconds employed in the vibrations being 1, 2, 3, 4, 5, 6, 7, 8, 9, the length of the pendulums would be as 1, 4,. 9, 16, 25, 36, 49, 64, 81. A pendulum, therefore, to vibrate once in five seconds, must be over eightyfeet in length. 384. As the oscillations of a pendulum are dependent upon gravitation, the instrument becomes useful in ascertaining the force of gravity at different distances from the centre of, the earth. 385. It has already been stated that the centrifugal force at the equator is greater than in those parts of the earth which are near the poles. As the centrifugal force operates in opposition to that of gravity, it follows that the pendulum must also be affected by it; and this affords additional reason why a pendulum, to vibrate seconds at the equator, must be shorter than one at the poles. It has been estimated that, if the revolution of the earth around its axis were seventeen times faster than it is, the centrifugal force at the equator would be equal to the force of gravity, and, consequently, neither could a pendulum vibrate, nor would bodies there have any weight. 386. As every part of a pendulum-rod tends to vibrate in a different time, it is necessary that all pendulums should. have a weight attached to them, which, by its inertia, shall concentrate the attractive force of gravity. 387.` Pendulums are subject to variationn in'warm and cold weather, on account of the. dilatation and contraction of the materials of which the rod is composed, by heat and cold. For this reason, the same pendulum is always longer in summer than it is in winter; and a clock will, therefore, always be slower in summer than in winter, unless some means are employed by which the effects of heat and cold on the length of the pendulum can be counteracted. This is sometimes effected in what is called'the gridiron pendulum by combining bars or rods of steel and brass, and in the mercurial pendulum by enclosing a quantity of quicksilver in a tube near the bottom of the pendulmn. 388. In order to secure a continuous motion to the pendulum (or, in other words, to keep a clock in motion), it is necessary that the pendulum should hang in a proper position. A practised ear can easily detect any error in this respect by the irregularity in the 104 NATURAL PHILOSOPHY. ticking, or (as it is called) by its being " out of beat." To remedy this fault, it is necessary either to incline the clock to the one side or the other, until the tickings are synchronous; or, in other words. are made at equal intervals of time. It can sometimes be done without moving the clock, by slightly bending the upper appendage of the pendulum in such a manner that the two teeth, or projections, shall properly articulate with the escapement-wheel. [See No. 303.] 329. Table of the Lengths of Pendulums to vibrate Seconds in different latitudes. Inches. Inches. At the equator, 39. At the equator, 39. Lat. 10~ North, 39.01 Lat. 10~ South, 39.02 20 " 39.04 20 " 39.04 30 " 39.07 30 " 39.07 40 " 39.10 40 " 39.10 50'" 39.13 50 " 39.13 60 " 39.16 60 " 390. The observations have been extended but little further, north or south of the equator. Different observers have arrived at different results; probably on account of their different positions in relation to the level of the sea in which the observations were made. In such a work as this, a table of this kind, without pretending to ex. treme accuracy, is useful, as showing that theory has been con. frmed by observation. 391. The moving power of a clock is a weight, which, being wound up, makes a constant effort to descend, and is prevented by a small appendage of the pendulum, furnished with two teeth, or projections, which the vibrations of the pendulum cause alternately to fall between the teeth of a wheel called the escapement-wheel. The escapement-wheel is thus permitted to turn slowly, one tooth at a time, as the pendulum vibrates. If the pendulum with its appendage be removed from the clock, the weight will descend very rapidly, causing all the wheels to revolve with great velocity, and the clock becomes useless as a time-piece. 392. The moving power of a watch * is a spring, called the mainspring, which being tightly wound around a central pin, or axis, its elasticity makes a constant effort to loosen. This power is communicated to a balance-wheel, acted upon by a hair-spring, and having an escapement similar- to that of the clock. If the hair-spring, with the escapement, be removed, the main-spring, being unrestrained. A watch differs from a clock in having a vibrating wheel, instead of a pendulum. This wheel is moved by a spring, called the hair-spring. The place of the weight is supplied by another larger spring, called the,nainspring. REGULATORS OF MOTION. 105 will cause the wheels to revolve with great rapidity, ant the watch, also, becomes useless as a time-piece.* What is a Bat- 393. TiE BATTERING RAM.- The Battering tering Ranm? Ram was a military engine of great power, used to beat down the walls of besieged places. Explain 394. Its construction, and the principle on which it Fig, 58. was worked, may be understood by inspection of Fig 58, in which A B represents a large beam, heavily loaded, witlh Fig. 58. a head of iron, A, resembling the head of a ram, from which it takes its name. The beam is accurately balanced, and suspended by a rope or chain C, hanging from another beam, supported by the frame D E F G. At the extreme end B, ropes or chains were attached, by which it could be drawn upwards through the arc of a circle, like a pendulum. The frame was sometimes mounted on wheels. 395. Battering rams were frequently from fifty to a hundred feet in length, and, moving with a force compounded of their weight and velocity, were almost irresistible.t * As a regulator of motion, the pendulum of the clock is to be lengthened or shortened, and the hair-spring of a watch is to be tightened or loosened. This is to be done in the former case in the manner already explained in the text; in the latter, by turning what is called the regulator, which tightens or loosens the hair-spring. t The ram used by Demetrius Poliorcetes at the siege of Rhodes wayB 106 NATURAL PHILOSOPHY. 396. The force of a battering ram is estimated by its momentum; that is its weight multiplied by its velocity. 397. Questionsfor Solution. (1.) Suppose a battering ram weighing 5760 lbs., with a velocity of 11 feet in a second, could penetrate a wall, with what velocity must a cannon-ball weighing 24 lbs. move to do the same execution? 5760 X 11 = 63360 - 24 - 2640 feet, or one half of a mile in a second. (2.) If a battering ram have a momentum of 58,000 and a velocity of 8. what is its weight 1 AAs. 7T250 (3.) If a ran have a weight of 90,000 and a momentum 81,000, what is its velocity? Aslc..9 (4.) What is the weight of a ram with a velocity of 12 and a momentum 60,0(}? Ans. 5000. (5.) Will a cannon-ball of 9 lbs. and a velocity of 3,000, or a ram with a weight of 15,000 and a velocity of 2, move with the greater force? An7.8. The ram. What is the 398. THE GOVERNOR. The Governor is an Governor? ingenious piece of mechanism, constructed on the principle of the centrifugal force, by means of which the supply of power in machinery is regulated.* Explain 399. Fig. 59 represents a governor. A B and Fig. 59. A C are two levers, or arms, loaded with heavy one hundred and six feet long. At the siege of Jerusalem Vespasian employed a ram fifty feet long, armed with an iron butt, with twenty-five projecting points, two feet apart, each as thick as the body of a man. The counter weight at the hindmost end amounted to 1075 cwt., and 1500 men were required to work the machine. I' This very useful appendage to machinery, though long used in mills and other mechanical arrangements, owes its happy adaptation to the steamengine to the ingenuity of Mr. James Watt. In manufactures, there is one certain and determinate velocity with which the machinery should be moved, and which, if increased or dimuinished, would render the machine unfit to perform the work it is designed to execute. Now, it frequently happens that the resistance is increased or.liminished 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. But, besides the alteration in the resistance just noticed, there is, also, frequently, greater changes in the power. The heat by which steam is generated cannot always be perfectly regulated. At times it may afford an excess, and at other times too little expansive power to the steam. Water, also, is subject to change of level, and to consequent alteration as a moving power. The wind, too, which impels the sails of a wind-mill, is subject to great increase andl diminution. To remedy all these inconveniences is the duty assigned to the governor. REGULATORS OF MOTION. 107 Fig. 59' balls at their extremities B and C, and AN tsuspended by a joint at A upon the extremity of a revolving shaft A D. A a is a collar, or sliding box, connected with the levers by the rods b a and c a, B a C with joints at their extremities. When the shaft A D revolves 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 b a and c a. On the contrary, when the shaft A D revolves slowly, the weights B and o( will fall by their own weight, and the rods b a and c a will cause the collar a to descend. The steam-valve in a steamengine, 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. Whlat is t~he 400. The Main-spring of a, watch consists of a Main-spring long ribbon of steel, closely coiled, and contained of a watch? in a round box. It is employed instead of a weight, to keep up the motion. 401. As the spring, when closely coiled, exerts a stronger force than when it is partly loosened, in order to correct this inequality the chaiiu through w-hich it acts is wound upon an axis surrounded by a spiral groove (called af/)see), gradually increasing in diameter from the top to the bottoni so ttiat, in proportion as the strength of the spring is diminished, it maly act on a larger lever, or a larger wheel and axle. Expleain 40)2. Fig. 60 represents a spring coiled in a round box. Fig. 60. A B is the fatsee, Fig. 60. surrounded by a spiral groove, on which the chain C is wound. WVhen the watch is recently wound, the spring is in the greatest state of tension, and will, therefore, turn the fusee l08 NATURAL PHILOSOPHY. by the smallest groove, on the principle 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 want of strength in the spring is compensated by the mechanical 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. Of what does 403. HYDROSTATICS.*- Hydrostatics treats Hydrostatics treat? of the nature, gravity and pressure of fluids. WVhat is the Whdaterence s the 404. Hydrostatics is generally confined to the tween Hy- consideration of fluids at rest, and Hydraulics to draulics and fluids in motion. Hydrostatics? What is a 405. A Fluid is a substance which yields to Flui"d? the slightest pressure, and the particles of which, having but a slight degree of cohesion, move easily among themselves.t * The subjects of Hydraulics and Hydrostatics are sometimes described under the general name of Hydrodynamics. The three terms are from the Greek language, compounded of VJuJq (hidor), signifying water, and vu.ax!tr; (dunamis),force or power; asatzrxo~ (staticos), standing, and uveoc (aulos), a tube or pipe. Hience Hydrodynamics would imply, the science which treats of the properties and relations of water and other fluids, whether in a state of motion or rest; while the term IHydrostatics would be confined to the consideration of fluids in a state of rest, and Hydraulics to fluids in motion through tubes or channels, natural or artificial. t There is this remarkable difference between bodies in a fluid aun bodies in a solid form, namely, that every particle of a fluid is perfectly independent of every other particle. They do not cohere in masses, like the parti'cles of a solid, nor do they repel one another, as is the case with the particles composing a gas. They can move among one anotheir with the least degree oy friction, and, when they press down upon one another in virtue of their own weight, the downward pressure is communicated in als directios, causing a pressure upwards, sideways, and in every possible manner. Herein the particles of a fluid differ from the particles of a solid, even when reduced to the most impalpable powder; and this it is which comnstitutesJfluidity, namely, the oower of transmitting pressure in every direction, and that, too, svith the least degree of friction. The particles which compose a fluid must be very much smaller than the finest grain of an im palpable powder. HYDROSOA'TICS. 109 low does a 406. A liquid differs from a fluid in its, liquid differ degree of compressibility and elasticity. Fluids from a fluid? are highly compressible and elastic. Liquids, on the contrary, have but a slight degree either of compressibility or of elasticity.* 407. Another difference between a liquid and a fluid arises from the propensity which fluids have to expand whenever all external pressure is removed. Thus, whenever a portion of air or gas is removed from a closed vessel, the remaining portion will expand, and, in a rarer state, will fill the whole vessel. Liquids, on the contrary, will not expand without a change of temperature. Liquids, also, have a slight degree of cohesion, in virtue of which the particles will form themselves into drops; but the particles of fluids seemn to possess the opposite quality of repulsion, which causes them to expand without limit, unless confined within the bounds of some vessel, or restricted within a certain bulk by external pressure. 408. The fluid form of bodies seems to be in great measure, if not wholly, attributed to heat. This subtle agent insinuates itself between the particles of bodies, and forces them asunder. Thus,,for instance, water divested of its heat becomes ice, which is a solid. In the form of water it is a liquid, having but in a very slight. degree the properties either of compressibility or elasticity. An additional supply of heat converts it into steam, endowed with a very great degree both of elasticity and compressibility. But, so soon as steam loses its heat, it is again converted into water. Again, the metals become liquid when raised to certain temperatures, and it is known that many, and supposed that all, of them would be volatilized if the required supply of heat were applied. *. The celebrated experiment made at Florence, many years ago, to test the compressibility of water, led to the conclusion that water is wholly incompressible. Later experiments have proved that it may be compressed, and that it also has a slight degree of elasticity. In a voyage to the West Indies, in the ycar 1839, an experiment was made, at the suggestion of the author, with a bottle filled with fresh water from the tanks on the deck of the Sea Eagle. It was hermetically, sealed, and let down to the depth of about seven hundred feet. On drawing it up, the bottle was still full, but the water was brackish, proving that the pressure at that great depth had forced a portion of the deep salt water into the bottle, previously compressing the water in the bottle to make room for it. As it rose to the surface, its elasticity restored it to its normal state of density. At great depths in the sea the pressuie of the superincumbent mass increases the density by compression, and it has been calculated that, at'a depth of about ninety miles, water would be compressed into one-half of it; volume, and at a depth of 360 miles its density would be nearly eqtfal to that of mercury. Under a pressure of 15,000 lbs. to a square inch, Mr Perkins, of Newburyport, subsequently of London, has sh:wn that water it reduced in-bulk one part.in. twenty-four. 10 1IO ATUIRAT PHILOSOPHY. The science of Geology furnishes sufficient reasons for believing that all known substances were once not only in the liquid form, but also previously existed in the form of gats.* HOW do fluids 409. GRAVITATION OF FLUInDS. -- Fluids gravigravitate. tate in a more perfect manner than solids, on account of their want of cohesive attraction.:The'particles of a solid body cohere so strongly that, when the centre of gravity is supported, the whole mass will be supported. But every particle of afluid gravitates independently of every other particle. WivtLy cannot 410. On account of the independent gravitafluids be tion and want of cohesion of the particles of a mioulded into fluid, they cannot be formed into figures, nor prefigures! served in heaps. Every particle makes an effort to descend, and to preserve what is called the level. or equilibrium. What is the 411. The level or equilibrium'of fluids is equilibrium of the tendency of the particles' so to arrange themselves that every part of the surfact shall'be equally distant from the centre of the earth; that is, from the point towards which gravity tends. What is the 412. Hence the surface of all fluids, when in a urfaoe of theall state of rest, partakes the spherical form of the fluids? earth. 413. For the same reason, a fluid immediately conforms itself to the shape of the vessel in which it is contained. The plarticles of a solid body being united by cohesive attraction, if any one of the. be supported it. will uphold those also with which it is united. But, when any particles of a fluid is unsupported, it is attracted down to the level of the surface of the fluid; and.the readiness with which fluids yield to the slightest pressure will enable the particle, by its own weight, to penetrate the surface of the fluid, and mix with it. * The science of Chemistry unfolds the fact that all the great changes in the constitution of bodies are aompanied by the exhibition of heat either iW a free or latent ounditioni I YDROS' TATI(;'S. 111 What is Ca- 414. CAPILLARY ATTRACTION.- Capillary pillary Attrac- Attraction is that attraction which causes ton? W;hat are Ca- fluids to ascend above their level in capillary pillary Tubes? tubes. Capillary * tubes are tubes with very fine bore. 415. This kind of attraction exhibits itself not only in tubes. but also between surfaces which are very near together. iThis may be beautifully illustrated by the following experiment. Take two pieces of flat glass, and, having previously wet them, separate their edges on one side by a thin strip of wood, card or other material; tie them together, and partly immerse them perpendicularly in colored water. The water will then rise the highest on that side where the edges of the glass meet, forming a beautiful curve downwards towards the edges which are separated by the card. 416. Immerse a number of.tubes with fine bores in a glass of colored water, and the.wa-ter will rise above its equilibrium in all, but.highest in the tube with the finest bore. 417. The. cause of this seems to be nothing more than the ordinary attraction of the particles of matter for each other. The sides of a small orifice are so near to each other as to attract the particles of the fluid on their 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 the length of the tube. On the outside of the tube, the opposite surfaces cannot act on the same column of water, and, therefore, the influence of attraction is here imperceptible in raising the fluid. 418. All' porous substances, such as sponge, bread, linen, sugar, &c., may be considered as collections of capilla-y tubes; and, for this reason, water and other liquids will rise in: them when they are partly immersed. 41lJ9. It is on the same principle that the wick of a lamp will carry up the oil to supply the flume, although the flame is several inches above the level of the oil.t If the end of a towel happen to ~ The word capillary is derived from the Latin word capilla (hair), and it is applied to this kind of attraction because it is exhibited most prominently in tubes the bores of which are as fine as a hair, and hence called capillary tubes. t 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 capillary attraction is impeded by the compression. The reinedy 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 attention, 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 wicks and prevents tre 112 NATURAL PHILOSOPHY. 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 rain falls upon it, it will absorb the water, swell, and sometimes split the rock. In this manner millstone quarries are worked in Germany. 420. ENDOSMOSE AND ExosMOSE. -In addition to the capillary attraction just noticed as peculiar to fluids, another may be mentioned, as yet but imperfectly understood, which seems to be due partly to capillary and partly to chemical attraction, known under the names endosmose and exosmose.-* These phenomena are manifested in the transmission of thin fluids, vapor and gaseous matter, through membranes and porous substances. The ascent of the sap in vegetable, and the absorption of nutritive matter by the organs of animal life, are to be ascribed to these causes. 421. When two liquids of different densities are separated by a membranous substance or by porcelain unglazed, endosmose will carry a current inwards, and exosmose will force one outwards, thus causing a partial mixture of the fluids. 422. Experiment.-Take a glass tube, and, tying a piece of bladder or clean leather over one end for a bottom, put some sugar into it, and having poured a little water on the sugar, let it stand a few hours in a tumbler of water. It will then be found that the water has risen in the tube through the membranous substance. This is due to endosmose. If allowed to stand several days, the liquid will rise several feet. If the experiment be reversed, and pure water be put into the tube, and the moistened sugar into the tumbler, the tube will be emptied by exosmose. 423. The liquid that has the less density will generally pass to the denser liquid and dilute it. What peculi- 424. GRAVITATION OF FLUIDS OF DIFFERENT arity is there DENSITIES. - When solid bodies are placed one in the grauids- above another, they will remain in the position in tation ofJiuids of different which they are placed so long as their respective densities? centres of gravity are supported, without regard to their specific gravity. With fluids the case is different. 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 n-ot 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, although the wick when first adjusted may be of the proper size, the glutinous 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 a~s has already been noticed in cases where the wick is too large to allow the free operation of capillary attraction. * Endosmose, from eJUt', within, and wo(uoS, inmpulsion. Exosmose, fron: Et, ouiwai d, and cwoao, imrpul3sion. HYIDROSTATICS. 113 Fluids of different specific gravity will arrange themselves in the order of their density, each preserving its own equilibrium. 425. Thus, if a quantity of mercury, water, oil and air, be put into the same vessel, they will arrange themselves in the order of their specific gravity. The mercury will sink to the bottom, the water will stand above the mercury, the oil above the water, and the air above the oil; and the surface of each fi;id will partake of the spherical form of the earth, to which they. all respectively gravitate. What is a Spirit 426. A Water or Spirit Level is an inLevel, or a Water strument constructed on the principle of the Level? equilibrium of fluids. It consists of a glass tube, partly filled 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. 427. Fig. 61 represents a Water Level. A B is a FigE 61. g lass tube partly filled with water. l[i. O. C is a bubble of air occupying the A C B space not filled by the water. When both f ends of the tube are on a level, the air-bubble will remain in the centre of the tube; but, if either end of the tube be depressed, the water will descend and the air-bukble will rise. The glass tube, when used, is generally set in a wooden or a brass box. It is an instrument much used by carpentper masons, surveyors, &c. [N. B. The tube is generally filled with spirit, instead of water, o, account of the danger that the water will freeze and burst the glass. HIence the instrument is called indifferently the Spirit Level or the Water Level.] 428. EFFECT OF THE PECULIAR GRAVITATION fluids do less OF FLUIDS.- Solid bodies gravitate in masses, damage than their parts being so connected as to form a falling solids whole, and their weight may be regarded as concentrated in a point, called the centre of gravity; while each 10O 114 NATURAL PHILOSOPHY. particle ff 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 water be converted into ice, the particles losing their fluid form, and being united by cohesive attraction, gravitate unitedly in one mass. In what direc- 429. PRESSURE OF FLUIDS. - Fluids not tion do fluids press, on ac- only press downwards like solids,'but also count of their upwards, sidewise,* and in every direction. weight? 430. So long as the equality of pressure is undisturbed, every particle will remain at rest.: If the fluid be disturbed by agitating it, the equality of' pressure will be disturbed, and the fluid will not rest until the equilibrium is restored. How are the 431. The downward pressure of fluids is downward, lat- shown by making an aperture in the bottom of eral anrd up- a vessel of water. Every particle of the fluid ward pressure of fluids shown? above the aperture will run downwards through the: opening. 432. 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. 433. Thq upward pressure is shown by taking a glass tube, open at both ends, inserting a cork in one end (or stopping it with the finger), and immersing the other in the water. The water will not rise in the tube. But the moment the cork is taken out (or the finger removed), the fluid will rise in the tube to a level with the surrounding water. Fig. 62. * If the particles of fluids were arra.nged in Fig. 63. regular columns, as in Fig. 62, there would be no lateral pressure; for when one particle is perpendicularly above the other, it can press only downwards. But, if the'Jarticles be arranged as in Fig. 63, where a particle presses between two particles beneath, these last must suffer a lateral pressure. In whatever manner the particles are arranged, if they be globular,-as is supposed, there imubt be spaces between them, [See Fig. l,page 22.] HYDROSTATICS. 11.5 What is the 434. The pressure of a fluid is in proporlaw of fluid tion to the perpendicular distance from the pressure? surface; that is, the deeper the fluid, the greater will be the pressure. This pressure is exerted in every direction, so that all the parts at the same depth press each other with equal force. 435. A bladder, filled with air, being immersed in water, will be contracted in size, on account of the pressure of the water in all directions; and the deeper it is immuersed, the more will it be contracted.* 436.; An empty bottle, being corked, and, by means of a weight, let down to a certain depth in the sea, will either be broken by the pressure, or the cork will be driven into it, and the bottle be filled with water. IThis will take place even if the cork be secured with wire and seadod. 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.t $ The. weight of a cubic inch of water at the temperature of 620 of Fahrenheit's -thermometer is 36066 millionths of a pound avoirdupois. The pressure of a column of water of the height of one foot will therefore be twelve times this quantity, or.4328 (making allowance for the repeating decimal), and the pressure upon a square foot by a column one foot high will be found by multiplying this last quantity by 144, the number of square inches in a square foot, and is therefore 62.3332. Hence, at the depth of lbs. lbs. 1 foot the pressure on a square inch is 4328, on a square foot, 62.3232 2 feet................. 8656, " 2 " feet, 124.6464 3 "...............1.2984, " 3 " " 186.9696 4 ".............;. 1.7312, " 4 ";." 249.2928 5 "............... 2.1640, " 5 "'" 311.61C0 6'................ 2.5968, " 6 " " 373.9392 7 "........ 3.0296, " 7 " "c; 436.2624 8 ".-.......... 3.4624, " 8'" " 498.5856 9 "............... 3.8952, " 9 " " 560.9088 10 "....... 4.3280, "c 10 " " 623.2320 100"............. 43.2800, " 100 " " 6232.3200 From this table, the pressure on any surface at any depth may easily be found. It will thus be seen that there is a certain limit beyond which divers cannot plunge with impunity, nor fishes of any kind live. Wood that has been sunk to,reat depths in the sea will ha've its pores so filled with water, and its:specific gravity so increased, that it will no longer float. t "Experiments at Sea. — We are indebted-to a friend, who has just arrived from Europe, says; the' Baltimore Gazette, for the following experiments made on board the Charlemagne:; 26th of September, 1836, the weather being calm, I corked an empty 116 NATURAL PHILOSOPHY. 437. Questions for Solution. (1.) What pressure is sustained by the body of a fish having a surface of 9 square feet at the depth of 150 feet. Ans. 84186.82 lb. (2.) What is the pressure on a square yard of the banks of a canal, at the depth of four feet AMns. 2243.6852 lb. (3.) What pressure is exerted on the body of a man, at the depth.of 30 feet, supposing the surface of his body to be 21 sq. yd.? Ais. 42068.16 lb. (4.) Suppose a whale to be at the depth of 200 feet, and that his body presents a surface of 150 yards. What is the pressure? Arts. 1682T264 lb. (5.) How deep may a glass vessel containing 18 inches of square surface be sunk without being broken, supposing it capable of resisting an equal pressure of 1500 lbs. Asns. 192.54ft. + (6.) What is the pressure sustained on the sides of a cubical water-tight box at the depth of 150 feet below the surface, supposing the box to rest on the bed of the sea, and each side to be 8 feet square? Ans. 299151.36 lb. (7.) How deep can a glass vessel be sunk without breaking, supposing that it be capable of resisting a pressure of 200 pounds on a square inch! Ans. 462.1ft, + 438. The lateral pressure of a fluid proceeds What causes the entirely from the pressure downwards, or, in lateral pressure of fluids? other words, from the weight of the liquid above; consequently, the lower an orifice 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. wine-bottle, and tied a piece of linen over the coYrk; 1 then sank it into the sea six hundred feet; when drawn immediately up again, the cork was 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 bottle six hundred feet. Upon drawing it up imlnediately, 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 bottom 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 was 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 doubt that it would have been filled with water." [See also note on page 109.] It is the opinion of some philosophers that the pressure at very great depths of the sea is so great that the water is condensed into a solid state; %nd that at or near the centre of the earth, if the fluid could extend s, deeply, this pressure would convert the whole into a solid mass of fire. HYDROSTATICS. 117 439 Fig. 64 represents a vessel of water, with oriExplain fiees at the side at different distances from the surface. The Fig. 64. different curves in the figure, described by the liquid in running out of the vessel, show the action of gravity, and the effects pro- C duced by the force of the pressure on the liquid at different depths. 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 the force will be strongest at C. Wl hat effect has 440. As the lateral pressure arises solely the length and from the downward pressure, it is not affected the width of a body of fluid by the width nor the length of the vessel in upon its lateral which it is contained, but merely by its depth; pressure.? for, as every particle acts independently of the rest, it is only the column of particles above the orifice that can weigh upon and press out the water. To what is the 441. The lateral pressure on one side of a lateral pressure cubical vessel will be equal only to half of the equal? pressure downwards; for every particle at the bottom of a vessel is pressed upon by 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. What causes the 442. The upward pressure of fluids, although upwardpressure apparently in opposition to the principles of of ajfluid? gravity, is but a necessary consequence of the operation of that principle; or, in other words, the pressure upwards, as well as the pressure downwards, is caused by gravity. 443. When water is poured into a vessel with a g.65. 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 118 NATURAL PHILOSOPHY. particles above them. As they cannot descend Big. 5. through the bottom of the vessel, they will change their direction and rise in the spout. Fig. 65 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 Repeat the law d d t f ofeat the lares depends entirely on the height, and not on the.of-fluid pressure. length or breadth, of the column of fluid. [See No. 434.] 444. Any quantity of fluid, however small, Vhat is tle imay be made to balance any other quantity, Hydrostatic lParadox? however large. This is what is called the Hydrostatic Paradox.@ Explain 445. The principle of what is called the hydro5Fig. 66. static paradox is illustrated by the hydrostatic bellows represented in Fig. 66. A B is a long tube, one-inch square. C D E F are the bellows, consisting of two boards-, eight inches square, connected by broad pieces of leather, or india-rubber cloth, in the manner of a pair of common bellows. One pound * A paradox is something which is seemingly absurd, but true in fact. But in what is called the Hydrostatic Paradox there is in reality no paradox at all. It is true that a small quantity of fluid will balance any quantity, however large,' but it is on the same principle as that with which the longer arm of the lever acts. In order to raise the larger quantity of fluid, the smaller quantity must be elevated to a height in proportion as the bulk of the larger quantity exceeds the smaller. Thus, to raise 500 lbs. of water by the descending force of one pound, the latter must descend 500 inches while the former is rising one inch; and hence, what is called the hydrostatic paradox is in strict conformity with the fundamental principle of Meehanics, that what' is gained in power is lost in time, or in spaclq. HYDROSTATICS. 119 of water poured into the tube will raise sixty- Fig. 66. four 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, which c causes the pressure. The hydrostatic bellows may be constructed in a variety 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 manner as to allow the upper board to rise and fall like the common bellows. A perpendicular tube is sQ adjusted to this apparatus that water poured into the tube, passing between the boards, will separate them by its upward 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 separate the surfaces before they are loaded with the weight.] How is the 446. The force of pressure exerted on the bel. force ofpres- lows by the water poured into the tube is estisure on the hydrostatic mated by the comparative size of the tube and bellows esti- the bellows. Thus, if the tube be one inch square, Pmated? 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 Whattfunda- inches long. It will thus be perceived that the mental law of fundamental principle of the laws of motion is 120 NATURAL PHILOSOPHY. Mechanis ap- Ithere also in full force, namely, that what is plies also to gained in power is lost either in time or in hydrostatic space; for, while the water in the bellows is pressurel? rising to the height of one inch, that in the tube passes over 144 inches. Explain 447. Another form of apparatus, by mea-ns of Fig. 67. which it can be proved that fluids press in proportion to their perpendicular height, and not their quantity, is seen in Fig. 67. This apparatus unites simplicity with convenience. Instead of two boards, connected with leather, an india-rubber bag is placed between two boards, connected by crossed bars with a board below, loaded with weights, and the upper boards are made to rise or fall as the water runs into or out of the bag. It is an apparatus easily repaired, and the bag may also be used for gas, or for experiments in Pneumatics. A and B are two vessels of unequal size, but of the same length. These may sucm, 67. cessively be screwed to A B a the apparatus, and filled with- water. Weights may then be added to the suspended scale until the pressure is counterbalanced. It will then be perceived that, although A is ten times larger than B, the water will stand at the same height in both, because they are of the same length. If C be used instead of A or B, the apparatus may be used as the hydrostatic bellows.* * If a cask be filled with water, and a long pipe be fitted to it, by pouring water into the pipe it will exert so great a pressure as to burst the cask. In the same manner a mountain would be rent asunder by hydrostatio pressure, if a deep crevice, oommunicafing with a small fountain below, be filled with water by the riun. HYDROSTATICS. 121 In what man- 448. HYDROSrATIC PRESSURE USED AS A ner may hy- MECHANICAL POWER. -If water be confined rostatiur e be e- i any vessel, and a pressure to any amount sure be em-a ployed as a be exerted on a square inch of that water, a Mechanical Powearical pressure to an equal amount will be transmitted to every square inch of the surface of the vessel in which the wrater is confined: 449. This property of fluids seems to invest us with a power of increasing the intensity of a pressure exerted by a comparatively sriitll firce, without any other limit than that of the strength of the materials of which the engine itself is constructed. it also enables us with great facility to transmit the motion and force of one machine to another, in cases where local circumstances preclude the possibility of instituting any ordinary mechanical connexion 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. On what prin- 450. It is on the principle of hydrostatic press. cipie is Bra- ure that Bramah's hydrostatic press, represented mais hydro- in Fig. 68, is constructed. The main features of static press constructed? this apparatus are as follows: a is a narrow, and Explain Fig. A a large metallic cylinder, having communi68. cation one with the other. Water stands in both the cylinders. The piston S carries a ig. 68. strong head P, which - works in a frame opposite to a similar plate R. Between the two plates the d substance W to be i compressed is placed. [n the narrow tube, 2 is a piston p, worked by'a leyer.bd, its short arm 11 122 NATURAL PHILOSOPHY. c b 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, while 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 latter on the piston S will be equal to 400 times that on p. 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 man works with a force of 50 pounds, the piston p will consequently descend on the water with a force of 2500 pounds. Deducting one-fourth for the loss of power caused by the different impediments to motion, and one man would still be able to exert 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, &c.; also in uprooting trees, testing tiha strength of ropes, &c. lVzen will one fluidfloat on 451. A fluid specifically lighter than another the surface of fluid will float upon its surface.* another fluid? [N. B. This is but another way of stating the law mentioned in Nos. 409 and 410.] 452. If an open bottle, filled with any fluid specifically lighter than water, be sunk in water, the lighter fluid will rise from the oottle, and its place will be, supplied with the heavier water. Whr~en zuill a 453. Any substance whose specific gravity is body rise, sink greater than any fluid will sink to the bottom of or float, in a that fluid, and a body of the same specific gravity fluid? with a fluid will neither rise nor fall in the fluid, but will remain in whatever portion of the fluid it is placed. I The slaves in the West Indies, it is said, steal rum by inserting the long neck of a bottle, full of water, through the top aperture of the rum oask. The water falls out of the bottle into the cask, while the lighten rum ascends ia is stead. HYDROSTATICS. 123 But a body whose specific gravity is less than tlha- of a fluid will float. This is the reason why some bodies will sink and others float, and still others neither sink nor fioat.* iow deep will 454. A body specifically lighter than a fluid a bod'y sink in will sink in the fluid until it has displaced a porajiuldi? tion of the fluid equal in weight to itself: 455. If a piece of cork is placed in a vessel of water, about onethird part of the cork will sink below, and the remainder will sta;nd above, the surface of the water; thereby displacing a portion of water equal in bulk to about a third part of the cork, and this quantity of walter is equal in weight to the whole of the cork, because the specific gravity of water is about three times as great as that of cork. 456. It is on the same principle that boats, ships, &c., although composed of materials heavier than water, are made to float. From their peculilar shape, they are made to rest lightly on the water. The extent of the surface presented to the water counterbalances the weight of the manterials, and the vessel sinks to such a depth as will cause it to displace a portion of water equal in weight to the whole weight of the vessel. From a knowledge of the Epecific gravity of water, and the materials of which a vessel is composed, rules have been formed by vwhich to estimarte the tonnage of vessels; that is to say, the weight which the vessel will sustain without sinking. stdat is tefr 457. The standard which has been adopted to estimating the estimate the specific gravity of bodies is rain or specific gra)- distilled water, at the temperature of 60~.t ity of bodies?, The bodies of birds that firequent the water, or that live in the water, are generally much lighter than the fluid in which they move. The feathers and down of water-fowl contribute much to their buoyancy, but fishes have the power of dilating and contracting their bodies by means of an internal air-vessel, which they can contract or expand at pleasure. The reason that the bodies of persons who have been drowned first sink, and, after a number of days, will float, is, that when first drowned the air, being expelled from the lungs, makes the body specifically heavier than Water, and it will of course sink; but, after decomposition has taken place, the gases generated within the body distend it, and render it lighter than water, and they will cause it to rise to the surface. t As heat expands and cold condenses all metals, their specific gravity cannot be the sacme in summer that it is in winter. For this reason, they will not serve as a standard to estimate the specific gravity of other bodies The- reason that distilled water is used is, that spring, well, or river water is seldom perfectly pure, and tlhe various substanecs mixed with it &fIec;t it 124 NATURAL PHILOSOPHY. This is found to be a very convenient standard, because a cubic foot of water at that temperature weighs exactly one thousand ounces. 458. Taking a certain quantity of rain or distilled water, we.find that a quantity of gold, equal in bulk, will weigh nearly twenty times as much as the water; of lead, nearly twelve times as much; while oil, spirit, cork, &;c., will weigh less than water.* weight. The cause of the ascent of steam or vapor may be found in its specific gravity. It may here be stated that rain, snow and hail, are fornmed by the condensation of the particles of vapor in the upper regions of the atmosphere. Fine, watery particles, coming within the sphere of each other's attraction, unite in the form of a drop, which, being heavier than the air, falls to the earth. Snow and hail differ from rain only in the different degrees of temperature at which the particles unite. When rain, snow, or hail falls, part of it reascends in the form of vapor and forms clouds, part is absorbed by the roots of vegetables, and part descends into the earth and forms springs. The springs form brooks, rivulets, rivers, &c., and descend to the ocean, where, being again heated by the sun, the water, rising in the form of vapor, again forms clouds, and again descends in rain, snow, hail, &c. The specific gravity of the watery particles which constitute vapor is less than that of the air near the surface of the earth; they will, therefore, ascend until they reach a portion of the atmosphere of the same specific gravity with themselves. But the constant accesrion of fresh vapor from the earth, and the loss of heat, cause several particles to come within the sphere of each other's attraction, as has been stated above, and they unite in the form of a drop, the specific gravity of which being greater than that of the atmosphere, it will fall in the form of rain. Water, as it descends in rain, snow or hail, is perfectly pure; but, when it has fallen to the earth, it mixes with the various substances through which it passes, which gives it a species of flavor, without affecting its transparency. $ TABLE OF SPECIFIC GRAVITIES. Temperature about 40~ Fahrenheit, Distilled Water, 1. Palladinum, 11.500 Mercury, 13.596 Iridium, 18.'650 Sulphuric Acid, 1.841 Copper, 8.850 Nitric Acid, 1.220 Lead, 11.250 Prussic Acid,.696 Bismuth, 9.822 Alcohol (pure),.792 Tellurium, 6.240 Ether,.715 Antimony, 6.720 Spirits of Turpentine,.869 Chromium, 5.900 Essence of Cinnamon, 1.010 Tungsten, 17.500 Sea Water, 1.026 Nickel, 8.:270 Milk, 1.030 Cobalt, 7.810 Wine,.993 Tin, 7.293 Olive Oil,.915 Cadmium, 8 687 Naphtha,.847 Zinc, 7.190 Iodine, 4.946 Steel, 7.820 Platinum, 22.050 Iron, 7.788 Gold, 19.360 Cast-iron, 7.200 Silver, 10.500 Manganese, 8.012 Rhodium, 11.000 SOdium, 972 HYDROSTATICS. 125 Hlow is the 459. The specific gravity of bodies that will specificgravity sink in water is ascertained by weighing them of a body ascertained when first in water, and then out of the water, and it is greater dividing the weight out of the water by the loss than that of waler? of weight in water. Potassium,.875 Elm,.800 Diamond, 3.530 Yew,.807 Arsenic, 5.670 Apple Tree,.733 Graphite, 2.500 Yellow Fir,.657 Phosphorus, 1.770 Cedar,.561 Sulphur, 2.086 Sassafras,.482 Lime, 3.150 Poplar,.383 Galena, 7.580 Cork Tree,.240. Marble, 2.850 Flint Glass, 3.330 White Lead, 6.730 Pearls, 2.750 Plaster of Paris, 2.330 Coral, 2.680 Nitrate of Potash, 1.930 China-ware, 2.380 Emerald, 2.700 Porcelain Clay, 2.210 Garnet, 3.350 Flint, 2.600 Feldspar, 2.500 Granite, 2.700 Serpentine. 2.470 Slate, 2.825 Alum, 1.700 Alabaster, 2.700 Topaz, 3.500 Brass, 8.300 Bituminous Coal, 1.250 Ice,.865 Anthracite, 1.800 Common Air,.001 Pulverized Charcoal, 1.500 Hydrogen Gas,.000105 Woody Fibre, 1.500 Living Men,.891 Lignum Vits, 1.350 Brandy,.820 Boxwood, 1.320 Mahogany, 1.003 Beech,.852 Chalk, 1.733 Ash,.845 Carbonic Acid Gas,.001527 By means of this table the weight of any mass of matter can be ascertained, if we know its cubical contents. A cubic foot of water weighs exactly 1000 ounces. If we multiply this by the number annexed to any substance in this table, the product will be the weight of a cubic foot of that substance. Thus anthracite coal has a specific gravity of 1.800. A thousand ounces, multiplied by this sum, produces 1800 ounces, which is the weight of a cubic foot of anthracite coal. The bulk of any given weight of a substance may also readily be ascertained by dividing that weight in ounces by the number of ounces there are in a cubic foot. The result will be the number of cubic feet. The cube root of the number of cubic feet will give the length, depth and breadth, of the inside of a square box that will contain it. It is to be understood that all substances whose specific gravity is greater than water will sink when immersed in it, and that all whose specific gravity is less than that of water will float in it. Let us, then, take a quantity of water which will weigh exactly one pound; a quantity of the substances specified in the table, of the same bulk, will weigh as follows: Platinum, 23. lbs. Silver, 11.091 lbs. Fine (Gold, 19.640 " Copper, 9.000 ";Mercury, 14.019 " Iron, 7.645" Lead, 11.525 " Glass, 3.000" 11* 126 NATURAL PHILOSOPHY. 460. Fig. 69 represents the scales for ascerDescribe tht;. scales used for taniing the specific gravity Fig. 69..finding the. of bodies. One scale is secy lica body. shorter than the other, and a hook is attached to the bottom of the scale, to which substances whose specific gravity is sought may be attached and sunk in water. 461. Suppose a cubic inch of gold weighs nineteen ounces-when weighed out of the water, and but eighteen ounces * when weighed Marble, 2.705 lbs. Brandy,.820?Is. Chalk, 1.793 Living Men,.891 " Coal,. 1.250 " Ash,.800 " Mahogany, 1.0(3 " Beech,.700" _Milk, 1.034 Elmr,.600" Boxwood, 1.030" Fir,.500" Rain Water, 1.000 " Cork,.240 " Oil,.920 " Common Air,.00 11 Ice,.908 " y Idrogen Gas,.000105" A cubic foot of water weighs one thousand avoirdupois ounces. By multiplying the number opposite to any substance in the above ti;bl.e by one thousand, we obtain the weight of a cubic foot of that substance in ounces. Thus, a cubic foot of platinum is 23,000 ounces in weight. In the above table it appears that the specific gravity of liviig -men is about one-ninth less than that of common water. So long, therefore, as the lungs can be kept free from water, a person, although unacquainted with. the art of swimming, will not completely sink, provided the hands and arms be kept under water. The specific gravity of sea-water is greater than that of the water of lakes and rivers, on account of the salt contained in it. On this account, the water of lakes and rivers has less buoyancy, and it is more difficult to swim in it. * The gold will weigh less in the water than out of it, on account of the upward pressure of the particles of water, which in some measure supports it, and, by so doing, diminishes its weight. Now, as the upward pressure of these particles is exactly sufficient to balance the downward pressure of a quantity of water of exactly the saime dimensions with the gold, it follows thalt the gold will lose exactly as much of its weight in water as a quantity of water of the same dimensions with the gold will weigh. And this rule applies to all bodies, heavier than water, that are immersed in it. Thei will lose as much of their weight int water as a quantity of water of their own dimensions weighs. All bodies, therefore, of the samie size, lose the same quantity of their weight in water. Hence, the specific gravity of a body is the weight of it rompared with that of water. As a body loses a quantity of its weight when immersed in water, it follows that when the body is lifted from the water that portion of its weight which it had lost will be restored. This is the reason that a bucket of water, drawn from a well, is heavier when it rises above the surface of the water in the well than it is while it remains below the surface. For the same reason our limbs feel heavy in leaviag a bath YEIDROSTATICS. 12 i inw-ater, thle loss in water is one ounce. The weight out of water, nineteen ounces, being divided by one (thle loss in water), gives nineteen. The specific graivitv of gold, then, wvould be nineteen or, in other words, gold is nineteen tillles heavier than water. iow is/ Me spe- 462. The specific gravity of a body that will cific g'rarity o not sink in water is ascertained by dividing its a body tligter weight by the sum of its weight added to the Ithan water foand waer loss of weight which it occasions in a heavy body previously balanced in water.* 463. If abody lighter than water weiglrs six ounces, and, on being attached to a heavy body, balanced in water, is found to occasion it to lose twelve ounces of its w'eight, its specific gravity is determined l:y dividing its weigoht (six ounces) by the suIn of its weight added to the loss of weight it occasions in the heavy body; namnely, 6 added to 12, which, in other words, is 6 divided by 18, or, 6 which is I. 4G64. Questionsfor Solution. (1.) A body lighter than water caused the loss of 10 lbs. to a heavier body immiersed in water. In air the same body weighed 30 lbs. What wtas its specific giavity? Solution.- 30 lbs., its weight, divided by (30+10=) 40 (the.sum of its weight added to the loss of weight which it caused in another body preoviously balanced in the water). Ans..75. (2.) A body tha.t weighed 15 lbs. in air weighed but 12 in water. What was its specific gravity I An.. 5. (3.) If ta cubic foot of water weich 1000 ounces, what is the weight of an equatl bullk of' goldl? A7s. 122 lb. 8 ozs. (4.) The weight of an equal bulk of lead? Ans. 720 lb. 5 oz. (5.) The weight of an equal bulk of cork? Asi. 15 lb. * The method of ascertaining the specific gravities of bodies was dis. covered accimdentally by Archimedes. Ile had been employed by the King of Syracuse to investigate the iletals of a golden crown, which he suspected had been adulterated by the workmneni. The philosopher labored at the pioblemn in vain, till, going one dacy into the bath, lie perceived that the water rose in the bzath in proportion to the bulk of his body. I-le instantly perceirved that any other substatnce of equal size would raise the vwater just as much, thou h one of eyual weight and less bulk could not produce the samne effect. lie then obtained two masses, one of gold and one of silver, each equal in weight to the crown, arid having filled a vessel very accurately ith winter, he first plunged the silver Imass into it, and observed the quantity of wanter that flowed over; he then did the same with the gold, ard found th at a less quantity ha1d passed over than before. lience he inferred tha:t, tholgh of equal weight, the bulk of the silver was greater than that of the gold, and that the qualntity of water displaced was, in each experimllent, equal to the bulk of the ilietal. lie next made trial with the crown, and found that it displaced more water than the gold, and leds than the silver, which led him to conclude that it was neither pure gold nor pure silver. 128 NATURAL PHILOSOPHY. (6.) The weight of an equal bulk of iron I An,.,477 I. 137 z. (7.) What is the weight of a cubic foot of mahogany 7 n4ls. 66 lb. 7 oz. (8.) The weight of a cubic foot of marble i Ans. 169 lb. 1 os. (9.) What is the weight of an iceberg 6 miles long, i mile wide, and 400 feet thick 2 A1is. 949.259,520 toans. (10.) W)hat is the weight of a marble statue, supposing it to be exactly a yard and half of cubic measure 2 Ans. 6S47.C3 lb. + (11.) If a cubical body of cork exactly 9 inches on each side be placed in water, how deep will it sink 2 Ans. 2.16 in.'(12.) Suppose that 4 boats were made each out of one of the following kinds of wood, namely, ash, beech, elm and fir, which would carry the greatest weight without sinking? A n. Th'at v elm. 465. An Hydrometer is an instrument to Whlat nse an?ascertain the specific gravity of liquids. Hydrometer? and on whsat 466. The hydrometer is constructed on the principle is it constructed? principle that the greater the weight of a liquid the greater will be its buoyancy. How is Gan hi 467. The hydrometer is made in a variety of How is an h-Theh drometer con- forms, but it generally consists of a hollow ball.structed? of silver, glass, or other material, with a graduated scale rising from the' upper part. A weight is attached telow 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 remains above the surface of the fluid. The greater the specific gravity of the fluid, the less will the scale sink. Of what use 468. The hydrometer is a very useful instruis the hydrom- ment for ascertaining the purity of many articles eter? in common use. It sinks to a certain determinate depth in various fluids, and if the fluids be adulterated the hy. drometer will expose the cheat. Thus, for instance, the specific gravity of sperm oil is less than that of whale oil, and of course has less buoyancy. If, therefore, the hydrometer does not sink to the proper mark of sperm oil, it will at once be seern that the article is not pure. Of what does 469. HYDRAULICS. - Hydraulics treats of Hydraulics fluids in motion, and the instruments by which treat their motion is guided or controlled. their motion is guided or controlled. tIi Y.) A U L ic-t.. 129 470. This branch of Hydrodynamics describes the effects of liquids issuing from pipes and tubes, orifices or apertures, the motion of rivers and canals, and the forces developed in the action of fluids with solids. 471. The quantity of a liquid discharged in a What quantity given time through a pipe or orifice is equal to a of a liquid will be discharged column of the liquid having for its base the orifice fr'om an oriJice or the area of the bore of the pipe, and a height or pi8 of a given size? equal to the space through which the liquid would pass in the given time. 472. Hence, when a fluid issues from an orifice in a vessel, it is discharged with the greatest rapidity when the vessel from which it flows is kept constantly full.* This is a necessary consequence of the law that pressure is proportioned to the height of the column above. From what orifice 473. When a fluid spouts from several orifices will a fluid spout In the side of a vessel, it is thrown with the to the greatest greatest random from the orifice nearest to the distance? centre. 474. A vessel filled with any liquid will discharge a greater quantity of the liquid through an orifice to which a short pipe of peculiar shape is fitted, than through an orifice of the same size without a pipe. 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 liquid.; But, if the pipe project into the vessel, the quantity discharged will be diminished, instead of increased, by the pipe. 475. The quantity of a fluid discharged through a pipe or an orifice is increased by heating the liquid; because heat diminishes the cohesion of the particles, which exists, to a certain degree, in all liquids. 476. Water, in its motion, is retarded by the What par t of friction of the bottom and sides of the channel a current of water flows through which it passes. For this reason, the most rapidly, velocity of the surface of a running stream is and why,? always greater than that of any other part. 0' The velocity with which a liquid issues from an infinitely small orifice in the bottom or sides of a vessel that is kept full is equal to that which a heavy body would acquire by falling from the level of the surface to the level of the orifice. -[Brlande.] 180 NATURAL PHILOSOPHY. 477. 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 imlpediments, the velocity which the waters would acquire would produce very disastrous consequences.* An inclination of three inches in a mile, in the bed of a river, will give the current a velocity of about three miles an hour. 478. To measure the velocity of a stream at its surface, hollow floating bodies are used; as, for example, a glass bottle filled with a sufficient quantity 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 caused 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 revolutionas in a given time. How may the 479. The velocity of a current of water at any velocity of a current at any portion of its depth may be Fig. 7O. depth be ascer- ascertained by immersing in tained? it a bent tube, shaped like a tunnel at the end which is immersed. 480. Fig. 70 is a tube shaped like a tunnel, 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. Howvare waves 481. Wavds are caused, first, by the friction caused? between air and water, and secondly, and on a much grander scale, by the attraction of the sun and moon exerted on the surface of the ocean, producing the phenomena of the tides. 482. The contriving hand of a benevolent Creator is qeen more clearly in nothing, than in the laws and operations of the material world. Were it not for the almost ceaseless motion of the water the ocean itself would become a putrid mass. Decayed and decay* See what is stated with regard to friction in Nos. 373 and 374. I YDRAULICS. i ii.r.ntter would he constantly emitting pestilential vapors, poisonillng tile atlnlosp!iere, iand spreading contagion and death to every breltnlinr inhabitlant of thle earth. The ceaseless motion" mixes up the poisonous ingredients, and prevents their floating on the surf'ace.* 483. The equilibrium of a fluid,,ccording to recent discoveries, cannot be disturbed by waves to a greater depth than about three hundred and fifty times the altitude of the wave. 484. When oil is poured on the windward side of a pond, the whole surface w-ill become smooth. The oil protects the water from the fiictiofi 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 baIrrel of oil on the water. What are the 485. The instruments or machines for principal bhy- raising or drawning water are the common dralits or a- pump, the forcing-pump, the chain-pump, the:hines? siphon, the hydraulic ram, and the screw of Archimedes. [The common pump and the forcing-pump will be Fig. 71. noticed in connexion with Pneumatics, as their operntion is dependent upon principles explained in that department of Philosophy. The fire-engine is nothing more than a double forcing-pump, andwill be noticed in,ile salue connexion.] 486. TheChain-pump is Fhaipt is the a machine by which the water is lifted through a box or ll channel, by boards fitted to the channel and attached to a chain. It has been used ll principally on board of ships. 487. Fig. 71 represents a Chain- i I Fgpl. pump. It consists of a square box through which a number of square e 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,' The undulations of large bodies of water have also produced material,jhanges on the face of the globe, purposely designed by Creative:Wisdomn working by secondary causes, the uses of which are described in the science of Geology. 182 NATURAL PHILOSOPHY. So as to move with little friction. 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 enters the box, lifts up the water above it, and discharges it at the top. 488. The screw of Archimedes is a maWhat is the chine said to have been invented by the phichimEdes: losopher Archimedes, for raising water and draining the" lands of Egypt, about two hundred years before the Christian era. Fig. 72 repre- Fig. 72 Explain7 sents the screw of Fig. 72. Archimedes. A single tube, or two tubes, are wound in the form of a screw around a shaft or cylinder, supported 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. 489. The Siphon is a tube bent in the form What is the of the letter U, one side being a little longer S~iphon? than the other, to contain a longer column of the fluid. 490. Fig. 73 represents a Siphon. A Fig. 73. igpla.n siphon is used by filling it with water or some other fluid, then stopping both ends, and in this state immersing the shorter leg or side into a vessel containing a liquid. The ends being thenunstopped, the liquid will run through the siphon until the vessel is-emptied. In performing this experiment, the end of the siphon whzch is out of the water manst always be below the surface of the water I-YDRAiLIXoS. 1A, 33 On what prin- 491. The principle on which the siphon acts Pciple does the is, that the longer column having the greater siphon act? hydrostatic pressure, the fluid will run down in the direction of that column. The upward pressure in the smaller column will supply a continued stream so long as that column rests below the surface of the water. IN. B. This principle will be better understood after the principle is explained on which the operation of the common pump depends; for the upward and downward pressure both depend-'on the pressure of the atmos phere.] 492. The siphon may be used in exemplifying the equilibrium ot fluids; for, if the tube be inverted and two liquids of different density poured into the legs, they will stand at a height in an inverse proportion to their specific gravity. Thus, as the specific gravity of mercury is thirteen times greater than that of water, a column of mercury in one leg will balance a column of water in the other thirteen times higher than itself. But, if but one fluid be poured into both legs, that fluid will stand at equal height in both. Ezplain the toy 493. The toy called Tantalus' * Cup consists called Tantalus' of a goblet containing a wooden figure, with a Cup. siphon concealed within. The water being poured into the cup until it is above the bend of the siphon, rises in the shorter leg, which opens into the cup, and runs out at the longer end, which pierces the bottom. Fig. 74. 494. Fig. 74 represents the cup with the siphon, the figure of the man being omitted, in order that the position of the siphon may be seen. 495. TIrE HYDRAULIC RAMt iS an ingeW. tat is the Hydraulic Ram? nious machine, constructed for the purpose of raising water by means of its own impulse or momentum. * Tantalus, in Heathen mythology, is represented as the victim of perpetual thirst, although placed up to the chin- in a pool of water; for, as soon as he attempts to stoop to drink, the water flows away from his grasp; hence our English word tantalize takes its origin. In the toy described above, the siphon carries the water away before it reaches the mouth of the figure. t* The Hydraulic Ram, sometimes called by its French name, Belier fly12 134 NATURAL PHILOSOPHY. 496. In the construction of an hydraulic ram, there must be, in tie first place, a spring or reservoir elevated at least four or live feet above the horizontal level of the machine.* Secondly, a pipe must conduct the water from the reservoir to the machine with a descent at least as great as one inch for every six feet of its length. Thirdly, a channel must be provided by which the superfluous water may run off. 497. The ram itself consists of a pipe having two apertures, both guarded by valves of sufficient gravity to fill by their own weight, one of which opens downwards, the other opening upwards into an air-tight chamber. An air-vessel is generally attached to the chamber, for the purpose of causing a steady stream to flow from the chamber, through another pipe, to the desired point where the water is to be discharged. Explain the con- 498. Fig. 75 represents the hydraulic ram. struclion of the A B represents the tube, or body of the ram, Hydraulic Rain having two apertures, C and D, both guarded by by/F]g. 75. r valves; C opening downwards, D opening updrauligqe~, in its present form, was invented by Montgolfier, of Montpelier. An instrument or machine of a similar construction had been previously constructed by Mr. Whitehurst, at Chester, but much less pertect in its mode of action, as it required to be opened and shut by thoe hand by means of a stop-cock. Montgolfier's machine, on the contrary, is set in motion by the action of the water itself. * Such an elevation may easily be obtained in any brook or stream of running water by a dam at the upper part of the stream, to form a reservoir. It has been calculated that for every foot of fall in the pipe running from the reservoir to the ram sufficient power will be obtained to raise about a sixth part of the water to the height of ten feet. With a fall of only four feet and a half, sixty-three hundred gallons of water have been raised to the height of one hundred and thirty-four feet. But, the higher the reservoir, the greater the force with which the hydraulic ram will act. The ope ration of the principle by which the hydraulic ram acts is familiar to those who obtain water for domestic purposes by means of pipes fromn an elevated reservoir, as is the case in many of our large cities. A sudden stoppage of the flow, by turning the cock too quickly, causes a jarring of the pipes, which is distinctly perceived, and often loudly heard all over the building. This is due to the su.lden changre from a state of rapid motion to a state of rest. The inertia of the fluid, or its resistance tt, a change from a state of rapid motion-to a state of rest, a property which it possesses in common with all other kinds of matter, explains the cause of-the violent jarring of the pipes, the stopping of which arrests the motion of the fluid; and the violence, which is in exact proportion to the momentum of the fluid, is sometimes so great as tJ burst the pipes HYDRAULICS. 135 wards, and both falling by their own weight. Let us now suppose the valve C to be open and D shut. The water, descending through the tube A B with a force proportionate to the height of the Fig. 71. _____ C reservoir, forces up the valve C and closes the aperture, thus suddenly arresting the current, and causing, by its reaction, a pressure throughout the whole length of the pipe; this pressure forces up the valve D, and causes a portion of the water to enter the chamber above D. The current having thus spent its force, the valve C immediately falls by its own weight, by which means the current is again permitted to flow towards the aperture C. The pressure at D thereby being removed, that valve immediately falls, and closes the aperture. When this takes place, everything is in the same state in which it was at first. The water again begins to flow through the aperture at C, again closing that valve, and again opening D; and the same effects are repeated at intervals of time, which, for the same ranm, undergo but little variation. The water being thus forced into the chamber E, as it cannot return through the valve ID, it mumt proceed upwards through the pipe G, and is thus carried to any desired point of discharge. An air-vessel is frequently attached to the chamber 186 NATURAL PHIILOSOPHY. of the ram, which performs the same office as it does in the forcing-pump, namely, to cause a steady stream to flow from the pipe G. The action, both of the ram and the forcing-pump, without the air-vessel, would be spasmodic.* How a~re Sprzings 499. SPRINGS. AND RIVULETS.- Springs and and Rivulets Rivulets are formed.by the water from rain, formed? snow, &c., which penetrates the earth, and descends until it meets a substance which it cannot penetrate. A reservoir is then formed by the union of small streams under ground, and the water continues to accumulate until it finds an outlet. Jig. 76. 2PL Fig. 76 represents a vertical section of the crust of the earth. a, c, and e are strata, either porous, or full of cracks, which permit the water to flow through, while b, d and f, are impervious to the water. Now, according to the laws of hydrostatics, the water at b will descend and form a natural spring at g: at i it will run with considerable force, forming a natural jet; and at 1, p and g, artesian wells may be dug, in which the water will rise to the respective heights g h, p k, and 1 m, the water not' The simplicity and economy of this mode of raising water have caused it to be quite extensively adopted in the Northern States. When well constructed, an hydraulic ram will last for years, involving no additional trouble and expense, more than occasionally leathering the valves when they have been too much worn by friction. The origin of the name will be readily perceived from the mode of its action. "Et'potum pastas age, Tityre et inter agendum, Occursare capro, cornuferit ille, caveto."- Virg. Bucdlic 9, v. 25 HYDRAULICS. 137 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'i obtained by digging wells. How high will 500. A spring will rise nearly as high, but the water of a cannot rise higher than the reservoir from spring rise? whence it issues. Friction prevents the water from rising quite as high as the reservoir. ro what height 501. Water may be conveyed over hills and valmay water be leys in bent pipes and tubes, or through natura] conveyed in passages, to any height which is not greater than tubes? the level of the reservoir from whence it flows. 502. 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, metallic, or stone pipes. 503. Fountains are formed by water carried Hlow are foun- through natural or artificial ducts from a resertains formed? voir. The water will spout from the ducts to dearly the height of the surface of the reservoir. 504. In Fig. 76.a fountain is represented at i, Explain the issuing from the reservoir, the height of which is fougntin by represented by a c. The jet at i will rise nearly Fig.. 76. as high as c. 505. A simple method of making an artificial eig. S7. fountain may be understood by Fig. 77. A glass siphon a b c is immersed in a vessel of water, and the air being exhausted from the siphon, a jet will be produced at a, proportioned to the fineness of the bore and the length of the tube. [N. B. The force of this kind of artificial jet is in a great measure dependent on a pneumatic principle.] 12* 138 NATURAL- 1HILOSOPHY. 506. lHERo's FOUNTAIN. —The hydraulic instrument called ihero's Fountain is an apparatus for projecting water by means of the pressure of confined air. Fig. 78 represents Itero's Fountain:. It consists of two vesFig. 78 sels, both air-tight, and communicating by a pipe, which, being inserted into the top of the:!i''l' } lower vessel, reaches nearly to the top of the upper vessel, which is in two parts, the upper l part being filled with water, which descends in _ a pipe seen on the right in the figure to the lower vessel, and, as it fills the lower vessel, condenses the air, forcing it up through the lefthand pipe, and. causing it to press on the sur-'face of the water in the lower part of the upper vessel. The water in the upper vessel is thus forced through the central pipe in a jet, to a height nearly as great as the length of the pipe on the right. The supply of water is furnished in the upper part of the upper vessel, which may always be kept full by any external supply. 507.: MAECHANICAL AGENCY OF FLUIDS.How does'water becomne a me- Water becomes a mechanical agent of great chanicalagent? power by means of its weight, its momentum and its fluidity. It is used as the moving power of presses. to raise portions of itself, and to propel or turn wheels of different constructions, whrlich, being connected with machinery of various kinds, form mills and other engines, capable of exerting great force. Wl4hat is Pneu- 508. PNEUMATICS. — Pneumatics treats of matics? the mechanical properties and effects of air and similar fluids, called elastic fluids and gases, or -a"riform fluids. 4hirat is meant 509. A/riform fluids are those which have the for o fl airsJormre yluid? M form of air. IMany of them are invisible,* or * Gases are all invisible, except when colored, which happens only in a few instanoes. PNEUMATICS. 139 nearly so, and all of them perform very important 6perations in the material world. But, notwithstanding that they are in most instances imperceptible to our sight, they are really material, and possess all the essential properties of matter. They possess, also, in an eminent degree, all the properties which have been ascribed to liquids in general, besides others by which th 3y are distinguished from liquids. 510. Elastic fluids are divided into two classes, WhVat is the difference be- namely, permanent gases and vapors. The gases tween a permna- cannot be converted into the liquid state by any nent gTs and known process of art;*but the vapors are readily reduced to the liquid form either by'pressure or diminution of temperature. There is, however, no essential difference between the mechanical properties of both classes of fluids. 511. As the air which we breathe, and which What subjects are embraced surrounds us, is the most familiar of all this class in the science of bodies, it is generally selected as the subject ofPneumatics? of Pneumatics. But it must be premised that the same laws, properties and effects, which belong to air, belong in common, also, to all a'rziform fiuids or gaseous bodies. 512. There are two principal properties of air, What are the namely, gravity and elasticity. These are called two principal the principal properties of this class of bodies, properties qf because they are the means by which their presair, and other gaseousbodies? ence and mechanical agency are especially exhibited. What degree 513. Although the aeriform fluids all have trctohhiave t - weight, they appear to possess no cohesive atgaseous bodies? traction. 514. The great degree of elasticity possessed by all abriform fluidls, rendlers them susceptible of comnpression and expansion to an almost unlimited extent. The repulsion of their particles causes them to expand, while within certain limits they are easily coin* Carbonic acid gas forms an exception to this remark. Water also is the union of oxygen and hydrogen gas. 140 NATURAL PHILOSOPHY. pressed. This materially affects the state of density and rarity under which they are at times exhibited.* ~What lacws 515. It may here be stated, that all the laws pertaintoaeri- and properties of liquids (which have been deformbodies in scribed under the heads of Hydrostatics and general? Hydraulics) belong also to aeriform fluids. The chemical properties of both liquids and fluids belong peculiarly to the science of Chemistry, and are not, therefore, considered in this volume. WMhat is the 516. The air which we breathe is an elastic air'which we fluid, surrounding the earth, and extending breathe? to an indefinite distance above its surface, and constantly decreasing upwards in density. 517. It has already been stated that the air Where is the air in its most near the surface of the earth bears the weight of condensed that which is above it. Being compressed, thereforhy and fore, 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.t 518. 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 expansion which takes place in the more dense air contained within the body is often painful. It * The terms-" rarefaction " and " condensation," and " rarefied " and " con densed," must be clearly understood in this connexion.- They are applied respectively to the expansion and compression of a body. t 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 nitrogen, or azote; of the former of which there are twenty-one parts and of the latter seventy-nine, in a hundred. The air is not visible because it is perfectly transparent. It may be felt when it moves in thi form of wind, orby swinging the hand rapidly backwards and forwards. PNEUMATICS. 141 occasions distension, and sometimes causes the bursting, of the smaller blood-vessels in the nose and ears. Besides, in such situations we are more exposed both to heat and cold; for, though the atmosphere 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. 519. Besides the two principal properties, gravity * and elasticity, the operations of which produce most of the phenomena of Pneumatics, it will be recollected that as air, although an invisible is yet a material substance, possessing all the common properties of matter, it possesses also the common property of impenetrability. This will be illustrated by experiments. Where is the 520. The pressure of the atmosphere caused pressure of the by its weight is exerted on all substances, interair felt? nally and externally, and it is a necessary conseWhat pressure. does a man of quence of its fluidity. The body of a man of common stat- common stature has a surface of about 2000 ure experience from the square inches, whence the pressure, at 15 pounds weight of the per square inch, will be 30,000 pounds. The air? 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. What effect 521. Heat insinuates itself between the particles has heat upon of bodies and forces them asunder, in opposition air and other to the attraction of cohesion and of gravity'; it elastic fluids? therefore exerts its power against both the attraction of gravitation and the attraction of cohesion. But, as the attraction of cohesion does not exist in a/riform fluids, the expansive power of heat upon them has nothing to contendl with *: It has been computed that the weight of the whole atmosphere is equal to that of a globe of lead sixty miles in diameter, or to five thousand billions of tons. 142 NATURAL PHILOSOPHY. but gravity. Any increase of temperature, therefore, expands an elastic fluid prodigiously, and a diminution of heat condenses it. T'ialat is the 522. A column of air, having a base an inch weight of a square, and reaching to the top of the atmocolumrn of air sphere, weighs about fifteen pounds. This presswith a base of " square inch? ure, like the pressure of liquids, is exerted equally in all directions. WVhat is meant 523. The elasticity of air and other a/riform by the elasticity fluids is that property by which they are inof air and other anrdorm creased or diminished in extension, according as fluids? they are compressed. IWAhat effect 524. This property exists in a much greater has art increase degree in air and other similar fluids than in any or oa dirinu- other substance. In fact, it has no known limit; tion ofpressure upon an aeri- for, when the pressure is removed friom any porfiorm body? tion of air, it immediately expands to such a degree that the smallest quantity will diffuse itself over an indefinitely large space. And, on the contrary, when the pressure is increased, it will be compressed into indefinitely small dimensions. What is Ma- 525. The elasticity or pressure of air and riotte's Law? all gases is in direct proportion to their density; or, what is the same thing, inversely proportional to the space which the fluid occupies. This law, which was discovered by Mariotte, is called "lMariotte's Laov." This law may perhaps be better expressed in the following language; namely, the density of an elastic fluid is in direct proportion to the pressure vwhick. it sutstaills. Houw does air 526. Air becomes a mechanical agen' by become a mechanical means of its weight, its elastinity, its inetia, agent? and its fluidity. Wi~th wlawt 527. The fuid ity of.ur invets Z, as it invests power does all other liquids, wit'h tR porwer of trans:itting PNEUMATICS. 143 fluidlty invest pressure. But it has already been shown, under a fluid? the head of Hydrostatics, that fluidity is a necessary consequence of-the independent gravitation of the particles of a fluid. It may, therefore, be included among the effects of weight. 528. The inertia of air is exhibited in the resistance which it opposes to motion, which has already been noticed under the head of Mechanics.* This is clearly seen in its efficts upon falling bodies, as will be exemplified in the experiments with the air-pump. What is a 529. A Vacuum is a space from which air Vactcunl u? and every other substance have been removed. 530. The Torricellian vacuum was discovered ~Vhzat is the mnost peJfect by Torricelli, and was obtained in the following vacuumn Ithat manner: A tube, closed at one end, and about has been obhasnbeen? ci- thirty-two inches long, was filled with mercury; tained? the open end was then covered with the finger, so as to prevent 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, leaving a vacuum at the top of about two inches. This vacuum, called from the discoverer the Torricellian vacuum, is the most perfect that has been discovered.t * Thefly, as it is called, in the mechanism of a clock by which the hours are struck, is an instance of the application of the inertia of the air in MIechanics. t Torricelli was a pupil of the celebrated Galileo. The Grand Duke of Tuscany having had a \deep well dug, the workmen found that the water would rise no higher than thirty-two feet. Galileo was applied to for an explanation of the reason without success. Torricelli conceived the idea of substituting mercury for water, arguing that if it was the pressure of the atmosphere that would raise the water in the pump to the height of thirtytwo feet, that it would sustain a column of mercury only one-fourteenth as high, or thirty inches only, on account of its greater specific gravity. lIe therefore determined to test it by experiment. He accordingly filled a small glass tube, about four feet long, with merculry, and, stopping the open end with his finger, he inverted it into a basin of mercury. On retmoving his finger, the mercury immediately descended in the tube, and stood at the height of about thirty inches; thus demonstrating the fact that it was the pressure of the air on the surface of the mercury in the one case, and of the water in the other, that sustained the oolumn of mercury Sn the tube, and oaf the water iu the pmuwsp 144 NATURAL PHILOSOPHY. 531. As this is one of the most important discoveries of the science of Pneumatics, it is thought to be deserving of a labored explanation. The whole phenomenon is the result of the equilibrium of fluids. The atmosphere, pressing by its weight (fifteen pounds on every square inch) on the surface of the mercury in the vessel, counterpoised the column of mercury in the tube when it was about thirty inches high, showing thereby that at column of the atmosphere is-equal in weight to a column of mercury of the same base, having a height of thirty inches. Any increase or diminution in the density of the air produces a corresponding alteration in its weight, and, consequently, 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 thirty-three 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 greater than that-of mercury as the specific gravity of mercury exceeds that of the fluid employed. 532.- This discovery of Torricelli led to the construction of the barometer,* for it was reasoned that if it was the weight of the atmosphere which sustained the column of mercury, that on ascend. ing any eminence the column of mercury would descend in proportion to the elevation. What is a Ba- 533. The Barometer is an instrument to rometer? measure the weight of the atmosphere, and thereby to indicate the variations of the weather.t 534. Fig. 83 represents a barometer. It Big. 79. Fig lain consists of a long glass tube, about thirtythree inches in length, closed at the upper o end, and filled with mercury. The tube is then in- 9 verted in a cup or leather bag of mercury, on which the pressure of the atmosphere is exerted. As the tube is closed at the top, it is evident that the mercury cannot descend in the tube without producing a vacuum. The pressure of the atmosphere (which is capable of supporting a column of mercury of about thirty inches in height) prevents the descent of the mercury; and ~ Among those to whom the world is indebted for the invention of the Darometer, and its applications in science, may be mentioned the names of Descartes, Pascal, Morienne and Boyle. The original idea is due to Torriaelli's experiment. t The word barometer is from the Greek, and signifies "a nmeasure of the weight," that is, of the atmosphere. PNEUMATICS. 145 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 mercury in the tube, which will rise or fall in exact proportion with the pressure. 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. 535. The barometer, as thus constructed, only required the addition of an index and a weather-glass, as seen Fig. 80. in Fig. 80, to give a fair and true announcement of the state and weight of the atmosphere. The instruments are now manufactured in several different forms. The different forms of the barometer in general use are the common Mercurial Barom- 28 eter, the Diagonal, and the Wheel Barometer, all 27 of which are constructed with a column of mercury. The Aneroid or Portable Barometer is a new instrument, in which confined air is substituted for mercury. This is a convenient form of the instrument for portable purposes. But the principle is the same in all, and repeated observations during the ascent of the loftiest mountains in Europe and America have confirmed the truth of barometrical announcements; for, by its indications, the respective heights of the acclivities in high regions can now be ascertained by means of this instrument better than by any other course, - with this advantage, too, that no proportionate height need be known to ascertain the altitudes. * 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 diminution 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. t F]rom the explanation which has now been given of the barometer, it 13 146 NATURAL PHILOSOPHY. n uhat tprzn- 536. The pressure of the atmosphere on the ciple is the ba- mercury, in the bag or cup of a barometer, being rem.cter conl- exerted on the principle of the equilibrium of structed~ 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 ascertain the height of mountains and other places above the level of the sea. WSthen is the 537. The air is the heaviest in dry weather, atmosphere and consequently the mercury will then rise Ihermiest? highest. In wet weather the dampness renders will readily be seen that a column of any other fluid will answer as well as mercury, provided the tube be extended in an inverse proportion to the specific gravity of the fluid. But mercury is the most convenient, because it requires the shortest tube. In navigation the barometer has become an important element of guidance, and a most interesting incident is recounted by Captain Basil Hall, indicative of its value in the open sea. While cruising off the coast of South America, in the Medusa frigate, one day, when within the tropics, the commander of a brig in company was dining with him. After dinner, the conversation turned on the natural phenomena of the region, when Captain Hall's attention was accidentally directed to the barometer in the state-room where they were seated, and, to his surprise, he observed it to evince violent and frequen't alteration. His experience told him to expect bad weather, and he mentioned it to his friend. Ilis companion, however, only laughed, for the day was splendid in the extreme, the sun was shining with its utmost brilliance, and not a cloud specked the deep-blue sky above. But Captain Hall was too uneasy to be satisfied with bare appearances. lie hurried his friend to his ship, and gave immediate directions for shortening the top hamper of the frigate as speedily as possible. IHis lieutenants and the men looked at him in mute surprise, and one or two of the former ventured to suggest the inutility of the proceeding. The captain, however, persevered. The sails were furled, the top-masts were struck; in short, everything that could oppose the wind was made as snug as possible. His friend, on the contrary, stood in under every sail. The wisdom of Captain Hall's proceedings was, however, speedily evident; just, indeed, as he was beginning to doubt the accuracy of his instrument. For -hardly had the necessary preparations been made, and while his eye was ranging over the vessel to see if his instructions had been obeyed, a dark, hazy hue was seen to rise in the horizon, a leaden tint rapidly overspread the sullen waves, and one of the most tremendous hurricanes burst upon the vessels that ever seaman encountered on his ocean home. The sails of the brig were immediately torn to ribbons, her masts went by the board, and she was left a complete wreck on the ternmpestuous surf which raged around her, while the frigate was driven wildly along at a furious rate, and had to scud under bare poles across the wide Pacific, full three thousand miles, before it could be sai4 that phe wg4 in safety from the blast PNEUMATICS. 147 the air less salubrious, and it appears, therefore, more heavy then, although it is, in fact, much lighter. A,' whagt time 538. The greatest depression of the barometer of the day is occurs daily at about four o'clock, both in the mornthe highest. a i ahed lowest ing and in the afternoon; and its highest elevation state of the at about ten o'clock, morning and night. In sum baromreter? mer these extreme points are reached an hour or two earlier in the morning, and as much later in the afternoon. 539. Rules have been proposed by which the changes of the weather may be predicted by means of the baroneter. Hence the graduated edge of the instrument is marked with the words' rain," "Jair," " changeable." "firost," &c. These expressions are predicated on the assumption that the changes of the weather may correctly be predicted by the absolute height of the mercury. But on this little reliance can be placed. The best authorities a.gree that it is rather the change in the height on which the predications must be made. 540. As the barometer is much used at the present day, it has been thought expedient to subjoin a few general and special rules, from different authorities, by which some knowledge of the uses of the instrument may be acquired. 541. General Rules by which Changes of the Weather may be prognosttcated by means of the Barometer.* (1.) Generally the rising of the mercury indicates the approach of fair weather. (2.) In sultry weather the fall of the mercury indicates coming thunder. In winter the rise of the mercury indicates frost. In frost, its fall indicates thaw, and its rise indicates snow. (3.) Whatever change of weather suddenly follows a change in the barometer, may be expected to last but a short time. Thus, if fair weather follow immediately the rise of the mercury, there will be very little of it, and, in the same way, if foul weather follow the fall of the mercury, it will last but a short time. (4.) If fair weather continue for several days, during which the mercury continually falls, a long succession of foul weather will probably ensue; and again, if foul weather continue for several days, while the mercury con4tinually rises, a long succession of fair weather will probably succeed. (5.) A fluctuating and unsettled state in the mercurial column indicates changeable weather. - Lardner, page 75, Pneumatics. W51 Special Rules by which we may know the Changes of the Weather by means of the B1arometer.t (1.) The barometer is highestof all during a long frost, and it generally rises with a north-west wind. * These rules, says Dr. Lardner, from whose work they are extracted, may to some extent be relied upon, but they are subject to some uncertainty. t These rules are from a (difirent authority. 148 NATURAL PHILOSOPHY. (2.) The barometer is lowest of all during a thaw which follows a long frost, and it generally falls with a south or east wind. (3.) While the mercury in the barometer stands above 30', the air must oe very dry or very cold, or perhaps both, and no rain may be expected. (4.) When the mercury stands very low indeed, there will never be much rain; although a fine day will seldom occur at such times. (5.) In summer, after a long continuance of fair weather, the barometer will fall gradually for two or three days before rain falls; but, if the fall of the mercury be very sudden, a thunder-storm may be expected. (6.) When the sky is cloudless and seems to promise fair weather, if the barometer is low, the face of the sky will soon be suddenly overcast. (7.) Dark, dense clouds will pass over without rain when the barometer is high; but if the barometer be low it will often rain without any appearance of clouds. (8.) The higher the mercury, the greater probability of fair weather. (9.) When the mercury is in a rising state, fine weather is at hand; but when the mercury is in a falling state, foul weather is near. (10.) In frosty weather, if snow falls, the mercury generally rises to 30, where it remains so long as the snow continues to fall; if after this the weather clears up, very severe cold weather may be expected. It will be observed that the barometer varies more in winter than in summer. It is at the highest in May and August; then in June, March, September and April. It is the lowest in November and February; then in October, July, December and January. [These rules are from Dr. Brewer's work called " The Science of Familiar Things."] 543. OF THE DIFFERENT STATES OF THE BAROMETER. - Of the Fall of the Barometer. - In very hot weather the fall of the Barometer indicates thunder. Otherwise, the sudden fall of the barometer leads to the expectation of high wind. In frosty weather the fall of the barometer denotes a thaw. If wet weather follow soon after the fall of the barometer, but little of such weather may be expected. In wet weather, if the barometer falls, expect much wet. In fair weather, if the barometer falls and remains low, expect much wet in a few days, and probably wind. The barometer sinks lowest of all for wind and rain together; next to that for wind, except it be an east or north-east wind. 544. Of the Rise of the Barometer. - In winter the rise of the barometer presages frost. In frosty weather, the rise of the barometer presages snow. If fair weather happens soon after the rise of the barometer, expect but little of it. In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two. In wet weather, if the mercury rises suddenly very high, fine weather will not last long. The barometer rises highest of all for north and west winds; for all other winds, it sinks. 545. The Barometer in an Unsettled State. - If the motion of the mercury be unsettled, expect unsettled weather. PNEUMATICIS. 149 If it stand at " much rain," and rise to, " changeable," expe:zt fair weather zf short continuance. If it stand at "fair," and fall to " changeable," expect foul weather. Its motion upwards indicates the approach of fine weather; its motion downward indicates the approach of foul weather. What is the 546. THE THERMOMETER.- The TherThermometer, mometer * is an instrument to indicate the temand on what principle is it perature of the atmosphere. It is constructed constructed? on the principle that heat expands and cold contracts most substances. 547. The thermometer consists of a capillary tube, closed at the top and terminating downwards in a bulb. It is filled with mercury, which expands and fills the whole length of the tube or contracts altogether into the bulb, according to the degree of heat or cold to which it is exposed. Any other fluid may be used which is expanded by heat and contracted by cold, instead of mercury.'ig. 81. 548. On the side of the thermometer is a scale to indicate the rise and fall of the mercury, and consequently the temperature of the weather. What scale is 549. There are several different scales adoptedfor the applied to the thermometer, of which those therm ometer of Fahrenheit, Reaumur, Delisle and Celin this country? sius, are the principal. The thermometer 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 30 degrees. The scales of Reaumur and Celsius fix zero at the freezing point of water; and that of Delisle at the boiling point. W4'at is the 550. THE HYGROMETER. - The HygromHygrometer? eter is an instrument for showing the degree of moisture in the atmosphere. * The word " Thermometer" is from the Greek, and means C" a measure of heat." ", Hygrometer" means " a measure of moisture." 13* 150 NATURAL PHILOSOPHY. Tow is it con- 55]. The hygrometer may be constructed of struqted? any material which dryness or moisture expands or contracts; such as most kinds of wood, catgut, twisted cord, the beard of wild oats, &c. It is sometimes also composed of a scale balanced by weights on one side, and a sponge, or other substance which readily imbibes moisture, on the other. 552. By 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 in the-various 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 gallons of water in the space of twelve hours. His experiment was thus made': le put a- glass, mouth downwards, on a grass-plot, on which it had not rained fbr 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 of 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 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. 553. 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 surtices. 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. WVhat is the 554. THE DIVER'S BELL OR DIVING-BELL. Diving-bel, The Divinoaiving-bell Wh The Divino-bell is a large vessel shaped like and on uwhat principle is it an inverted goblet, in which a person may constructed? safely descend to great depths in the water. It is constructed on the principle of the impenetrability of air. PNEUMATICS. 151 555. It has already been stated that air, being a material substance, possesses all the given essential properties of matter, and among theiii the property of imlpenetrability. The weight of the air giving it a pressure in every directio)n or the property of' fluidity, it penetrates and fills all things around us, unless by mechanical means it be cairefillly excluded. An open vessel, of whatever kind, is alw ays full either of air or of some other substance, and unless the air is first permitted to escape no other substance can take the plLace of the air. 556. If a tumbler be inverted and immersed in water, the cwater will not rise in the tumbler, because the air in the tumbler fills it. If the tumbler be inclined so as to let the air ascend in obedience to the laws of the equilibrium of fluids, the,w-ater will rush in and dis-;place the air, while the lighter air. ascerinding, rises to the surfitce of the water. If this experiment be made with a bottle, the air will rise in bubbles with a gurgling sound. The samne experiment may be made with a tube closed at one end by the finger; the w1ater will not enter the tube until by the removal of the finger the air be permitted to escape. It is on this principle that the diving-bell is constructed. 557. Fig. 82 represents a Fig. 82. Explain the construction of the diving-bell. It consists of a diving-bell by large heavy vessel, formed P -F7ig~. 82.! like a bell (but maybe made of any other shape), with the mloulth open. It descends into the water with its mouth downwards. The air within it having no outlet, it 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 d 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 recover 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. 0 is a bent metallie tube attached to one side and reaching the air within; and P is the forcing-pulrp through which air is forced into the bell. The for cing-pump is attached to the tube by a joint at D. When the bell descends to a great depth, the pressure of the water 152 NATURAL PHILOSOPHIY. 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, supplied 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 endangered. The heated and impure air is allowed to escape through a stop-cock in the upper part of the bell. 558. THIE COMMON WATER PUMP. —How is water raised in a corn. Water is raised in the common pump by mon pump? means of the pressure of the atmosphere How high may water be raised on the surface of the water. A vacuum by a. common being produced by raising the piston or pump-box,@ the water below iS Fig. 83. forced up by the atmospheric pressure, on the principle of the equilibrium 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. P 559. Fig. 83 represents the common Ei. 83. pump, improperly called the sucking- -. V Fig.8. 83. pump. The body consists of a large tube, Y or pipe, the lower end of which is immersed in the water which it is designed to raise. P is the piston, V a valve t in the piston, which, opening upwards, admits the water to rise through it, but prevents its return. Y is a similar valve in the body of the. * In order to produce such a vacuum, it is necessary that the piston or box should be accurately 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 piston'is generally worked by a lever, which is the handle of the pump, not represented in the figure. t 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, according 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. PNE;UMATICS. 153 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 V, and on the ascent of the piston is lifted up by the piston, and a vacuum is again formed below, 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 uninterrupted stream. 560. In the description here given of the common pump, as well as in the figure, it will be observed that the comnmon form of the handle of the pump is not noticed. The handle of the pump is merely a lever of the first kind; the fulcrum is the pin which attaches it to the pump, and the iron rod connected with the upper valve of the pump is raised or depressed by means of the handle. 561. Although water can be raised by the atmospheric pressure only to the height of thirty-three feet above the surface, the common pump is so constructed that after the pressure of the atmosphere. has forced the water through the 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 upper valve from the surface of the water must never exceed thirty-two feet; and in practice it must be much less. 562. THE FORCING-PUMP. The ForcingHow does the Forcing-punlp pump differs from the common pump in dlifferfrom the having a forcing power added, to raise the common pump? water to any desired height. 563. Fig.. 84 represents the forcing-pump. The,iog. 84 body and lower valve V are similar to those in the common pump. The piston P has no valve, but is solid; when, therefore, the vacuum'is produced above the a154 NATURAL PHILOSOPHY. FIg. 84 lower valve, the water, on the descent 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 elasticity, forces the water out -V through the jet J in a continued stream,:-: and with great force. "It is on this principle that fire-engines are constructed. 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. 564. TiE FIRE-ENGINE consists of two forcingHow is the Fire-enginthe pumps, worked successively by the elevation and constructed? depression of two long levers of the second kind, called "Brakes." Fig. 85. 565. THE AIR-PUMP. — The Air —pump is What is the Air-pump, and a machine constructed on the principle of the on what prin- elasticity of the air, for the purpose of exciple is it constructed? hausting the air from a vessel prepared for the purpose. This vessel is called a receiver, and is made of glass, in order that the effects of the removal of the air may be seen. 566. Air-pumps are made in a great variety of forms; but all are constructed on the principle that, when any portion of confined PNEUMATICS. 155 air is removed, the residue, immediately expanding, by its elasticity fills the space occupied by the portion that has been withdrawn. 567. Fig. 86 represents a single-barrel airE1rplain the C0on struction of the pump, used both for condensing and exhausting. air-pump by A D is the stand or platform of the instrurFi g. 86. Fzig. 86. ment, which is screwed down to the table by means of a clamp, underneath, n1 which is not represented in the figure. -R is the glass vessel, or bulbed receiver, from which the air is to be exhausted. P is a solid piston, accurately fitted to the bore of the cylinder, and H the handle by which it is moved. The dotted line T - T represents the communication ~ \ between the receiver 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 r v. c v is the condensing valve, communicating with the barrel B by means of an aperture near a, and opening outwards through the condensing pipe p. Explacin the op- 568. The operation of the pump is as follows; eration of the The piston P being drawn upwards by the hanair-purnp by dle H, the air in the receiver CR, expanding 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 rc.eiver R more and more 156 NATURAL PHILOSOPHY. 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. 569. From this statement it will appear that a perfect vacuum can never be obtained by the air-pump as at present constructed. 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. The nearest approach made to a perfect vacuum is the famous experiment of Torricelli, which has been explained in No. 530. That would be a perfect vacuum, were there not vapor rising from the mercury. 570. From the explanation which has been How may the airbe condensed given of the operation of this air-pump, it will by means of the readily be seen that, by removing the receiver pump which has R, and screwing any vessel to the pipe p, the air may be condensed in the vessel. Thus the pump is made to exhaust or to condense, without alteration. What is a con- 571. Air-pumps in general are not adapted densing syr- for condensation; that office being performed by inge? P an instrument called "a condensing syringe," which is an azr-pump reversed, its valves being so arranged as to force air into a chamber, instead of drawing it out. For this purpose, the valves open inwards in respect to the chamber, while in air-pumps they open outwards. 572. A guage, constructed on the principle of the barometer, is sometimes adjusted to the air-pump, for the purpose of exhibiting the degree of exhaustion. How does the 573. The double air-pump differs from the double air-pump single air-pump, in having'two barrels and two differ from. the pistons; which, instead of being moved by the single? hand, are worked by means of a toothed wheel. playing in notches of the piston-rods. Fig. 87 represents an air-pump of a different construction. In this pump the piston is stationary, 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. PNEUMATICS. 157 Fig. 87. 574. By means of the air-pump many interesting experiments may be performed, illustrating the gravity, elasticity, fluidity, and inertia of air. 575. EXPERIMENTS ILLUSTRATING THE GRAVITY OF AIR.- Having adjusted the receiver to the plate of the air-pumnp, 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 pressure to contend with, presses with a force of nearly fifteen pounds on every square inch of the external surface of the receiver. 576. The exact amount of pressure depends on the d6gree of exhaustion, being at its maximum of fifteen pounds when there is a perfect vacuum. On readmitting the air, the receiver may be readily removed.* 577. THE MAGDEBURGH CUPS, OR HEMIMWha a re the SPHERES. — Fig. 88 represents the Magdeburgh Magdeburgh Cups, and what Cups, or Hemispheres. They consist of two holdo they illus- low brass cups, the edges of which are accurately fitted together. They each have a handle,. The air is readmitted into the receiver by turning a screw which is inserted into the receiver, in which there is an aperture, through which the (aternal air rushes with considerable force. 14 158 NATURAL PHILOSOPHY. pig. 88. to one of which a stop-cock is fitted. The stopcock, 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 within them, turn the stop-cock to prevent its readmission, and screw -the handle that had been removed to the stop-cock. Two persons 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 between them, by turning the cock, they will fall asunder by their own weight. When the air is exhausted from within them, the pressure of the surrounding air upon the outside keeps them united. This pressure being equal to a pressure of fifteen pounds on every square' inch of the surface, it follows that the larger the cups; or hemispheres, the more difficult it will be to separate them. 578. The Magdeburgh Cups derive their name from the city where the experiment was first attempted. Otto Guericke constructed two hemispheres which, when the air was exhausted, were Fig. 89. held together by a force of about three-fourths of a ton. Fig. 89 shows the manner in which such an experiment may be tried. Fig. 9 What principle 579. TIlE HAND-GLASS.- Fig. does the Hand- 90 is nothing more than a tunrglassillustrate? bler, 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, cover it closely with the palm of the hand, and work the pump. The air PNEXJUMAh'1' TS. 159 within the glass being thus exhausted, the hand will be pressed down by the weight of the air above it: on readmitting the air, the hand may be easily removed. Whzat principle 580. THE BLADDER-GLASS. Fig. 91. is illustrated by Fig. 91 is a bell-shaped glass, the Bladder- covered with a piece of bladglass? der, 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 weight of the external air on the bladder will burst it inwards. with a loud explosion. Pig 92. What does the 581. THE INDIA-RUBBER GLASS. q~;~7nJ I~India-rubber — Fig. 92 is a glass similar to Glass show? 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 different results. Instead of bursting, the india-rubber will-be pressed inwards the whole depth of the glass. What is illus- 582. TirE FOUNTAIN-GLASS AND JET. —Fig. trated by means S3 represents the jet, which is a small brass of the Fobuntaii- tibe. Fig. 94 is the fountain-glass. The ex. glass and Jet? periment with these instruments is designed to ig. 93. show the pressure of the atmosphere on Fig. 94. the surface. of liquids. Screw the straight Hi\ 2 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 stopcock, remove the glass from the. pump, and, immersing 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 like a fountain. 160 NATURAL PHILOSOPHY. o are the 583. PNEUMATIC SCALES FOR WEIGHING Ar.Pneumatic Fig. 95 represents the flask, Fig. 95. Scales used? or glass vessel and scales for weighing air. Weigh the flask when full of air; then exhaust the air and weigh the flask again. The difference between its present and former weight is the weight of the air that was contained in the flask. Whadt princi- 584. THE SUCKER. A pie does "the circular piece of wet leather, with a string;Sucker"' illus- attached to the centre, being pressed upon a *trate? smooth surface, will adhere with considerable tenacity, when drawn upwards by the string. The string in this case must be attached to the leather, so that no air can pass under the leather. what is the What is the 585. TIE MERCUJRIAL OR WATER TUBE.object of the Exhaust the air from a glass tube three feet MercurialT or long fitted with 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 thirty inches; or, if immersed in water, the water will rise and fill the tube, and-would fill it were it thirty feet long. This experiment shows the manner in which water is raised to the boxes or valves in common water-pumps. How is the elas- 586. EXPERIMENTS SHOWING THE ELASTICITY ticity of the air OF THE AIR. — Place an india-rubber bag, or a illustrated? bladder, partly inflated, and tightly closed, under the receiver, and, on exhausting the air, the air within the bag or bladder, expanding, will fill the bag. On reaidmitting the air, the bag will collapse. The experiment may also be made with some kinds of shrivelled fruit, if the skin be sound. The internal air, expanding, will give the fruit a fresh and plump appearance, which will disappear on the reiidmission of the air. 587. The same principle may be illustrated by the india 1i AUUMAI'CS. 161 rubber and bladder glasses, if they have stop-cocks to confine the air. 588. A small bladder partly filled with air may be sunk in a vessel of water by means of a weight, and placed under the receiver. On exhausting the air from the receiver, the air in the bladder will expand, and, its specific gravity being thus diminished, the bladder with the weight will rise. On readmitting the air, the bladder will sink again. How can the 589. AIR CONTAINED IN WATER AND IN WOOD. presence of air - Place a vessel of water under the receiver, and, in wood be de- on exhausting the air from the receiver, the air tected n the water, previously invisible, will make its appearance in the form of bubbles, presenting the semblance of ebullition. 590. A piece of light porous wood being immersed in the water below the surface, the air will be seen issuing in bubbles from the pores of the wood. 591. THE PNEUMATIC BALLOON.- Fig. 96. ciple of the Pneu- Fig. -96 represents a small glass balmatic Balloon. loon, with its car immersed in a jar of water, and placed under a receiver. On exhausting the air, the air within the balloon, expanding, gives it buoyancy, and it will rise in the jar. On readmitting the air, the balloon will sink. 592. The experiment may be performed without the air-pump by covering the jar with some elastic substance, as india-rubber. By pressing 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, expanding, 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 pleasure without resort to the pump. 593. The following is a full explanation: — The pressure on the top of the vessel first condenses the air between the covel 14* 162 NATURAL PHILOSOPHY. 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 prebsure, 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, however, be made to rise, if the perpendicular height of the water above it be diminished by inclining the vessel to one side. 594. This experiment proves many things; namely: First. The Miateriality of air, by the pressure of the hand on the top being communicated to the water below through the air in the upper part of the vessel. Secondly. The compressibility of air, by what happens in 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 in 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 further the globe has descended in the water. Sevent/hly. It exemplifies many circumstances of fluid support. A person, therefore, who is familiar with this experiment, and csl explain it, has learned the principal truths of Hydrostatics and Pneumatics. 595. The Pneumatic Balloon also exhibits the principle on which the ^well-known glass toy, cailled the Cartesian Devil, is constructed and it ma.y be thus explained: Several irnages of glass, hollow Mwithin, and each having a small opening at the heel by which Avater may pass in and out, mnay be made to mancmuvre in a vessel of water. Place them in a. vessel in the same manner with the balloon, but, by allowing difttrent quantities of water to enter the rPNEUlA'TICS. 163 apertures in the images, cause them 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 fillow in the order of their specific gravity; and they will stop or return to the surface in reverse order, when the pressure ceases. A person exhibitirng 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 movements 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 comnmand. 596. EXPERIIMENTS WITH CONDENSED AIR.What is the use othe Con- HE CONDENSING AND EXIIAUSTING SYRINGE.densing and The Condensing Syringe is the air-pump reversed. Exhausting The Exhausting Syringe is the simple air-pump Syringe? without its plate or stand. These implements are used respectively with such parts Fig. 97. of 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. For what purpose 597. TE AIRis the Air-cham- CHAMBER.- The airber used?.1 chamber, Fig. 97, is a hollow brass globe prepared for the reception of a stop-cock, and is designed for the reception of condensed air. It is made in different forms in different sets, and is used by screwing it to a condensing pump or a condensing syringe. IW~hat prin- 598. STRAIGIIT AND REVOLVING JETS FROM iple of principle of Pneu- CONDENSED Amt. — Fill the air-chamber (Fig. 164 NATURAL PHILOSOPHY. matics is illus- 97) partly with water, and then condense the trated by the air. Then confine the air by turning the cock; straight and after which, unscrew it from the air-pump, and revolingjets? 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 rig. 98. rig. 99. a straight continued stream, in the other in'the form of a wheel. Figs. 98 and 99 represent a view of the straight and the revolving jets. In the revolving jet the water is thrown frQm 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 opposite sides of the arms of 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 relieved on one side by the jethole, 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. Explain the 599. THE PRINCIPLE OF THE AIR-GUN. - With principle of the air-chamber, as in the last experiments, a the Air-gun.' small brass cylinder or gun-barrel, Fig. 100, may be substituted for the jets, and loaded with a small shot Fig. 100. or paper ball. On turning the cock quickly, the condensed air, rushing out, will throw the s'hot 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 small portion only of the condensed air is admitted to. escape at a time; so that the chamber. being once filled, will afford two or three dozen discharges. The force 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 us great in proportion to its force as that of the common gun. PNEUMATICS. 165 600. Condensed air may be weighed in the In iweighing air what must air-chamber, but, in estimating its weight, the always be temperature of the room must always be taken taken into the into consideration, as the density of air is maaccount? terially affected by heat and cold. WAat does the 601. EXPERIMENTS SIIOWING TIIE INERTIA or Guinea and AIR. - THE GUINEA AND FEATIIER DROP. - The Feather Drop inertia of air is shown by the guinea and feather illustrate? drop, exhibiting the resistance which the air opposes to falling bodies. This apparatus is made in different i orim, some having shelves on which the Fig. 102. Fig. 101 guinea and feather rest, and, when the air is exhausted, they are made to fall by the turn/ ing of a handle. A better form is that represented in Fig. 101, 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 shown 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. 101 and 102, one of which, Fig. 101, is furnished with a stop-cock,* the other, Fig. 102, with shelves. What prin- 602. EXPERIMENTS SHOWING THE FLUIDITY OF ciple is explain- AIR. - THE WVEIGHT-LIFTER. -- The upward presstbe weight-ns oure of the air, one of the properties of its fluidity, lifter? may be exhibited by an apparatus called the * 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 posFible. This precaution is suggested by econolny, as well as by convenience 166 NATURAL PHILOSOPHY. weight-lifter, made in different forms, but all on the same principle. The one represented in Fig. 103 consists of a glass tube, of large bore; set in a strong case or stand, supported by three legs. A piston is accurately fitted to the bore of the tube, and a book is attached to the bottom of the piston, fiom 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 number of pounds' weight that can be raised by this instrument may be estimated by multiplying the number of square inches in the bottom of the piston by fifteen. Explain the 603. THE PNEUMATIC SHOWER-BATH. - On the Pneumatic principle of the upward pressure of the air the Shower-bath. 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 fiom 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.i 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 lifted 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. l~Whact two 604. MISCELLANEOUS EXPERIMENTS DEPENDING prope-ties of ON TWO OR MORE OF TIHE PROPERTIES OF ATit. - PNKEUMATJCS. 167 air are illus- THE BOLT-HEAD AND JAR. — Fig. 104, a glass trated by meansof/heb globe with a long neck, called a bolt-head (or mieans of the BJolt-head and any long-necked bottle), partly filled with water, Jar? is inverted in a jar of water (colored with a few drops of red ink or any coloring matter, in order Fig. 104. that 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 bolthead, 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 readmission of the air, the water will nearly fill the bolt-head, affording an accurate test of the degree of exhaustion. Wl~hat tuwo 605. THE TRANSFER OF FLUIDS FROM ONE principles are VESSEL TO ANOTHER.- The experiment may be concerned in made with two bottles tightly closed. Let one the transfer of.fluids ronm be partly filled with water, and the two conone vessel to nected by a bent tube, connecting the interior of another? 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 reiadmitting the air, the water will return to its original position, so long asithe lower end of the bent tube is below the surface. What e 606. EXPERIMENTS WITH THE SIPHON. -Close What experiments are per- the shorter end of the siphon with the finger or fJrrmed with with a stop-cock, and pour mercury or water into the siph on? 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 168 NATURAL PHILOSOPHY. outwards for the air, the fluid will rise to an equilibrium in both arms of the siphon. 607. Pour any liquid into the longer arm of the siphon until the shorter arm is filled. Then close the shorter end, to prevent 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. What effect is 608. AIR ESSENTIAL TO ANIMAL LIFE. - If produced on an animal placed an animal be placed under the receiver, and the under an ex- air exhausted, it will immediately droop, and, if hausted reIausted re-? the air be not speedily readmitted, it will die. ceiver? 609. AIR ESSENTIAL TO COMBUSTION.-Place How is it shown that air a lighted taper, cigar, or any other substance that is essential to will produce smoke, under the receiver, and excombustion? haust the air; the light will be extinguished, and the smoke will fall, instead of rising. If the air be readmitted, the smoke will ascend. What effect is 610. THE PRESSURE OF THE AIR RETARDS eproduced on EBULLITION. -Ether, alcohol, and other distilled exhausted re- liquors, or warm water, placed under the receiver, ceiver? will appear to boil when the air is exhausted. What effecthas 611. The existence of many bodies in a liquid the pressure of form depends on the weight or pressure of the the air on the atmosphere upon them. The same force, likeform of bfodiems wise, prevents the gases which exist in fluid and solid bodies from disengaging themselves. If, by rarefying 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 experiment proves this: Place a quantity of lukewarm water, milk or alcohol, under a receiver, and exhaust the air, and the liquid * EBULLITION.- The operation of boiling. The agitation of liquor by heat, which throws it up into bubbles. PNEUATIrICS. 169 will either. pass off in vapor, or will have the appearance of boiling. 612. An experiment to prove that the pressure WVhat experiment shows of the atmosphere preserves some bodies in the that the liquid liquid form may thus be performed. Fill a long form of some bodies is de- vial, or a tube closed at one end, with water, and pdedent on invert it in a vessel of water. The atmospheric atmosspheric pressure will retain the water in the vial. Then, pressure? 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 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. 613. A simple and interesting experiment con How nay Ca water befi ozen nected with the science of chemistry may thus be under a r6- performed by means of the air-pump. A watchceiver?; 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. 614. 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 ki effects, namely, the freezing of the water, depend upon two different principles which pertain to the science of chemistry. What is the 615. THE PNEUMATIC PARADOX.- An interPneumatic Paradoxs esting experiment, illustrative of the pneumatic 15 170 NATURAL PHILOSOPHY. 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 passing 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 tne pin projecting into the tube to prevent the upper card from sliding off. It will then be impossible to displace the upper card by blowing through the quill, on account of the adhesion produced by the current passing between the discs. On this principle smoky chimneys have been remedied, and the ofice' of ventilation more effectually performed.'Whlat is 616. WIND. - Wind is air put in motion. Wind? 617. There are two ways in which the motion In what two ways may the of the air may arise. It may be considered as motion of the an absolute motion of the air, rarefied by heat air be ex- and condensed by cold; or it may be only an plained? apparent motion, caused by the superior velocity of the earth in its daily revolution. 618. 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. 619. The portions north of the rarefied spot wind caunsedth produce a north wind, those to the south produce a south-wind, while those to the east and west in like 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 frequently in the torrid zone, where the heat is greatest. Tlhe air, being more rarefied there than in any other PNrEU-MA>tTCS. 171 part of the globe, is lighter, and, consequently, ascends; that about the polar regions is continually flowing from the poles to the equator, to restore the equilibrium; while the air rising from the equator flows in an upper current towards the poles, so that the polar regions may not be exhausted. What wind 620. A regular east wind prevails about the prevails in the equator, caused in part by the rarefaction of the equatorial air produced by the sun in his daily course from regions? east to west. This wind, combining with that from the poles, causes a constant north-east wind for about thirty degrees north of the equator, and a south-east wind at the same distance south of the equator. 621. 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 the 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 certain 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 different seasons of the 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. WVhen the place of the sun with respect to the different positions of the earth at the different seasons of the year is understood, it will be seen that they all depend upon the same principle. The reason that the wind'generally subsides 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 declines, and, consequently, the force of the wind abates. The great variety of winds in the temperate zone is thus explained. The air is an exceedingly 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 suffer more or less perturbation, according to the situation of the country, the position of mnpuntains, valleys, and a variety of other causes. Hence every climate must be liable to variable winds. The quality of winds is affected by the countries over which they pass; and 172 NATURIA I PHILOSOPtPHY. they are sometimes rendered pestilential by the heat of deserts or the putrid exhalations of marshes and lakes. Thus, from the deserts of Africa, Arabia and the neighboring countries, a hot wind blows, called Samiel, or Simoon, which sometimes produces instant death. A similar wind blows fiom the desert of Sahara, upon the western coast of Africa, called the Ilarmattan, producing a. dryness and heat which is almost insupportable, scorching like the blasts of a furnace. d 622. WHIRLWINDS AND WATERSPOUTS. -The H07o is wind soetirmes af- direction of winds is sometimes influenced by the fected by the form of lofty and precipitous mountains, which, f~aceof~taq resisting their direct course, causes them to descend with a spiral and whirling motion, and with great force. 623. A similar effect is produced by two winds meeting at an angle, and then turning upon a centre. If a cloud happen to be between these two winds thus encountering each other, it will be condensed and rapidly turned round, and all light substances will'be carried up into the air by the whirling motion thus produced. W;rhat is sup- 624. The whirlwind, occurring at sea, occaposed to be the sions the singular phenomenon of the watercause of waterspouts? spout. Fig..105....=-B r aCOUSTICS. 173 nWghat does 625. Fig. 105 represents the several forms in Fig. 105 represent? which water-spouts are sometimes seen. 626. From a dense cloud a cone descends in the form of a trumpet, with the small end downwards. At the same tihme, the surface of the sea under it is agitated and whirled round, the waters are converted into vapor, and ascend with a spiral motion, till they unite with the cone proceeding from the cloud. Frequently, however, they disperse befobre the junction is effected. Both columns diminish towards their point of contact, where they are sometimes not more than three or four feet in diameter. In the centre of the waterspout there is generally a vacant space, in which none of the small particles of water ascend. In this, as well as around the outer edges of the water-spout, large drops of rain fall. Water-spouts sometimes disperse suddenly, and sometimes continue to move rapidly over the surface of the sea, continuing sometimes in sight for the space of a quarter of an hour When the water-spout breaks, the water usually descends in the form of heavy rain. It is proper here to observe that by some authorities the phenomena of water-spouts are considered as due to electrical causes. 627. A notion has prevailed that water-spouts are dangerous to shipping. It is true that small vessels incur a risk of being overset if they. carry much sail, because sudden gusts of wind, from all points of the compass, are very common in the vicinity of waterspouts; but large vessels, under but a small spread of canvas, encounter, as is said, but little danger. 628. PneLmnatics forms a branch of physical science which has been entirely created by modern discoveries. Galileo first demonstrated that air possesses weight. His pupil, Torricelli, invented the barometer; and Pascal, by observing the difference of the altitudes of the mercurial column at the top and the foot of the Puy de Dome, proved that the suspension of the mercury is caused by the pressure of the atmosphere. Otto Guericke, a citizen of Magdeburg, invented the air-pump about the year 1654; and Boyle and lapariotte soon after detected, by its means, the principal mechanical properties of atmospheric air. Analogous properties have been proved to belong to all the other adriform fluids. The problem of determining the velocity of their vibrations was solved by Newton and Euler, but more completely by Lagrange. The theoretical principles relative to the pressure and motion of elastic fluids, fiom which the practical formulae are deduced, were established by Daniel Bernoulli in his Hydrodynamica (1738), but have been rendered more general by Navier. Weat zs 629. AcousTics. - Acoustics is the science Acoustics? which treats of the nature and laws of sound. It includes the theory of musical concord or harmony. 15* 174 NATURAL PUILOSOPTY. tVhat is 630. Sound is the sensation produced in the sound? organs of hearing by the vibrations or undulations transmitted through the air around.@ 631. 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. 632. Again, if the experiments 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.t Vlhy is a sound 633. Sounds are louder when the air surlouder in cold rounding the sonorous body is dense than when weather? it is in a rarefied state, and in general the intensity of sound increases with the density of the medium by which it is propagated. 634. 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 top of mountains, where the air is rare, the human 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. What are So- 635. Sonorous bodies are those which pronorous bodies? duce clear, distinct, regular, and durable sounds, such 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. * " The sensation of sound is produced by the wave of air impinging on the membrane of the ear-drum, exactly as the momentum of a wave of the 6ea would strike the shore." - [Lardner.] t In performing these experiments, the bell must be placed in such a manaer that whatever supports it will, rest on a soft cushion of wool, so as to prevent the vibrations from being communicated to the plate of the air pump, or any other of the solid parts of the apparatus. AOOUSTICS. 175 To what do 636. Bodies owe their sonorous property bodies owe their ons owe their op-to their elasticity. But, although it is unerties? doubtedly the case that all sonorous bodies are elastic, it is not to be inferred that all elastic bodies are sonorous. 637. The vibrations of a sonorous body give a tremulous or undulatory motion to the air or the medium by which it is surrounded, similar to the motion communicated to smooth water when a stone is thrown into it. What are the 638. Sound is communicated more rapidly best conductors and with greater power through solid bodies of sound? 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. 639. If a person lay his head on a long piece of timber, he can hear the scratch of a pen at the other end, wmlle it could not be heard through the air. 640. 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. 641. It is on the principle of the greater power of solid bodies to communicate sound that the instrument called the Stethoscope * is constructed. What is the 642. The Stethoscope is a perforated cylinStethoscope, der, of light, fine-grained wood, with a funneland on what principle is it shaped extremity, which is applied externally to constructed? the cavities of the body, to distinguish the sounds within. MWat is the use 643. By means of the stethoscope the phyof the stetho- sician is enabled to form an opinion of the healthy scope? action of the lungs, and other organs to which the'ear cannot be directly applied. ~ The word Stethoscope is derived from two Greek wurds, ore9os, the breast, and onesu),, to examine, and is given to this instrument because it is applied to the breast of a person for the purpose of ascertaining the condition of the lungs and other internal organs. Dr. Webster suggests that the term Phonophorus, or Sound-conductor, would be a preferable name for the instrument. 176 NATIURAL PHILOSOPHY. With what rapidity 644. Sound passing through the air does sound move? moves at the rate of 1120 feet in a second of time; and this rule applies to all kinds of sound, whether loud or soft.* Vhat kind of 645. The softest whisper, therefore, flies as fist sounds move as the loudest thunder; and the force and direction fastest? of the wind, although they affect the continuance of a sound, have but slight effect on its velocity. 646. Were it not for this uniform velocity of all kinds of sound, the music of a choir, or of an orchestra, at a short distance, would be but a strange confusion of discordant sounds; for the different instruments or voices, having different degrees of loudness, could not simultaneously reach the ear. 647. The air is a better- conductor of sound when it is humid than when it is dry. A bell can be more distinctly heard just before a rain; and sound is heard better inthe night than in the day, because the air is generally more damp in the night. 648. The distance to which sound may be heard depends upon various circumstances, on which no definite calculations can be predicated.,Volcanoes, among the Andes. in South America, have been heard at the distance of three hund:ied miles; naval engagements have been heard two hundred; and oven the watchword "All's well," pronounced by the unassisted human voice, has been heard from Old to New Gibraltar, a distance of twelve miles. It is said that the cannon fired at the battle of Waterloo were heard at Dover. 649. A clear and frosty atmosphere is favorable to the transnission of sound, especially. where the surface over which it passes is smooth and level. Conversa:tion in the polar regions has been carried on between persons more than a mile apart. The cannon in naval engagements in the English. Channel have been heard in the centre of England. 650;. A blow struck under the water of the Lake of Geneva was heard across the whole breadth of the lake, a distance of nine miles. The earth itself is a good conductor of sound. The trampling of horses can be heard at a great distance by putting the ear to the ground, and the approach of railroad-cars can be ascertained when very far off by applying the ear to the rail.' The velocity ofsound has sometimes been estimated as much as eleven hundred and forty-two feet in'a second. The state of the air must, however, be taken into consideration. The higher the temperature,.the greater the velocity; and it has been ascertained that within certain limits the velocity is increased about one foot for every degree that the thermometer rises. Experimlents made with a cannon at midnight by'Arago, Gay Lassac, and others, when the thermometer stood at Go61, gave 1118.39 feet per second as the velocity of sound. The rate state-d in No. 644 will not therefore be far from the truth. The experiments which gave a result,f eleven hundred and forty-two feet in a second were probably made when the weamthet was oxtremely witam. ACooUSTICo. 1 t7 To what prac- 651. This uniform velocity of sound enab es us tical nse is the velocity of to ascertain, with some degree of accuracy, the sound applied? distance of an object from which it proceeds. If, for instance, the flash of a gun at sea is seen a half of a minute before the report is heard, the vessel must be at the distance of about six miles. 652. In the same manner the distance of a thunder-cloud may be estimated by counting the seconds that intervene between the flash of the lightning and the roaring of the thunder, and multiplying them by 1120. What is the 653.'THE ACOUSTIC PARADOX. - Sound, as has Acoustic Para- already been stated, is propagated by the undulations of the air. Now, as these undulations or waves are dox? precisely analogous to the case of two series of waves formed upon the surface of a liquid, there is a point where the elevation of a wave, produced by one cause, will coincide with the depression of another wave produced by another cause, and the consequence will be neither elevation nor depression of the liquid. Explain the, 654. Whben, therefore, two sounds are produced acoustic para- in different places, there is a point between them dox. where the undulations will counteract each other, and the two sounds may produce silence. 655. A simple illustration of this fact may be made with a tuning-fork. If this instrument be put into vibration and held up to the ear and rapidly turned, the sound, instead of being continuous, will appear to be pulsative or interrupted; but, if slowly caused to revolve at a distance from the ear, a position of the forks will be found at which the sound will be inaudible. 656. A similar experiment may be made with the tuning-fork held over a cylindrical glass vessel. Another glass vessel of similar kind being placed with its mouth at right angles to the first, no sound will be heard; but, if either cylinder be removed, the sound will be distinctly audible in the other. The silence produced in this way is due to the interference of the undulations. This seeming paradox, when thus explained, like the paradox mentioned under the heads of Hydrostatics and Pneumatics, and another to be mentioned under the head of Optics, will be found to be perfectly consistent with the laws of sound. What is an 657. An echo is produced by the vibrations echo? of the air meeting a hard and regular surfiace, such as a wall, a rock, a mountain, and being reflected back to the ear, thus producing the salme sound a second. and sometimes a third and fourthl tiine. 178 NATURAL PHILOSOPHY. Why are there 658. For this reason, it is evident that no echo no echoes at sea, or on a can be heard at sea, or on an extensive plain, where plain there are no objects to reflect the sound. 1By wuhat law 659. Sound, as well as light and heat, is reis sound re- flected in obedience to the same law that has fiected? already been stated in Mechanics, namely, the angles of incidence and of reflection are always equal. GG0. It is only necessary, therefore, to know how sound strikes against a reflecting surface, to know how it will be reflected. It is related of Dionysius, the tyrant of Sicily, that he had a dungeon (called the ear of Dionysius) in which the roof was so constructed as to collect the words, and even the whispers, of the prisoners confined therein, and direct them along a hidden conductor to the place where he sat to listen; and thus he became acquainted with the most secret expressions of his unhappy victims. Onr what principle 661. Speaking-trumpets are constructed are speaking-trumpets constructed? on the principle of the reflection of sound. 662. The voice, instead of being diffused in the open air, is con6ned within the trumpet; and the vibrations which spread and fall against the sides of the instrument are reflected according to the angle of incidence, and fall in the direction of the vibrations, which proceed straight forward. The whole of the vibrations are thus collected into a focus; and, if the ear be situated in or near that spot, the sound will be prodigiously increased. How is a hear- 663. Hearing-trumpets, or the trumpets used ing trumpet by deaf persons, are also constructed on the same constructed? principle; but, as the voice enters the large end of the trumpet, instead of the small one, it is not so much confined, nor so much increased.* 664. The musical instrument called the trumpet acts also on the same principle with the speaking-trumpet, so far as its form tends to increase the sound. 665. The smooth and polished surface of the interior parts of certain kinds of shells, particularly if they be spiral or undulating, * In this connexion the author cannot refrain from giving publicity to the value of a pair of acoustic instruments worn by one of the members of his family. They consist of two small hearing-trumpets of a peculiar construction, connected by a slender spring with an adjusting slide, which, passing over the head, keeps both trumpets in'their place. They are concealed from observation by the head-dress, and enable the wearer to join in con. versation of ordinary tone, from which without them she is wholly debarred. The instruments were ma-de by B. S. Codmau & Co. 57 Tremont st., Boston. ACOUSTICS. 179 fit them to collect and reflect the various sounds which are taking place in the vicinity. Hence the Cyprias, the Nautilus, and some other shells, when held near the ear, give a continued sound, which resembles the roar of the distant ocean. On what prin- 666. Sound, like light, after it has been reflectciple are ed from several surfaces may be collected into one whisperinggalleries con- point, as a focus, where it will be more audible structed? than in any other part; and on this principle whispering-galleries may be constructed. 667. The famous whispering-gallery in the dome of St. Paul's church, in London, is constructed on this principle. Persons at very remote parts of the building can carry on a conversation in a soft whisper, which. will be distinctly audible to one another, while others in the building cannot hear it; and the ticking of a watch may be heard from side to side. 668. There is it church in the town of Newburyport, in Massa chusetts, which, as was accidentally discovered, has the same property as a whispering-gallery. Persons in opposite corners of the building, by facing the'wall, may carry on a conversation in the softest whisper, unnoticed by others in any other part of the building. It is the building which contains in its cemetery the remains of the distinguished preacher, Whitefield. W.Vhat is an 669. AcousTic TUBEs.- Sounds may be conAcoustic Trube? veyed to a much greater distance through continuous tubes than through the open air. The tubes used to convey sounds are called Acoustic Tubes. They are much used in public houses, stores, counting-rooms, &c., to convey communi; cations from one room to another. 670. The quality of sound is affected by the furniture of a room, particularly the softer kinds, such as curtains, carpets, &c.; because, having little elasticity, they present surfaces unfavorable to vibrations. 671. For this reason, music always sounds better in rooms with bare walls, without carpets, and without curtains. For the same reason, a crowded audience increases the difficulty.of speaking. 672. As'a general rule, it may be stated that plane and smooth urJ'aces reflect sound without dispersing it; convex surfaces disperse it, 2nd concave surfaces collect it. [low is the 673. The sound of the human voice is prosound of the duced by the vibration of two delicate membranes human voice situated at the top of the windpipe, between which the air from the lungs passes. 180 NATURAL PHILOSOPHY. 674. The tones are varied from grave to acute, by opening or contracting the passage; and they are regulated by the muscles belonging to the throat, by the tongue, and by the cheeks. The management of the voice depends much upon cultivation; and although many persons can both speak and sing with ease, and with great power, without much attention to its culture, yet it is found that they who cultivate their voices by use acquire a degree of flexibility and ease in its management, which, in a great measure, supplies the deficiency of nature.* 675. Ventriloquism t is the art of speaking in hat is Ven- such a manner as to cause the voice to appear triloquism? to proceed from a distance. 676. The art of ventriloquism was not unknown to the ancients, and it is supposed by some authors that the famous responses of the oracles at Delphi, at Ephesus, &c., were delivered 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 considered as little short of miracles. were performed by means of ventriloquism. Thus houses have been made * Dr. Rush's very valuable work on " The Philosophy of the Human Voice " contains much valuable matter in relation to the human voice. Dr. Barber's " Grammar of Elocution," and the " Rhetorical Reader," by the author of this volume, contain useful instructions in a practical form. To the work of Dr. Rush both of the latter works are largely indebted. t The word Ventriloquism literally means, "speaking from the belly," and it is so defined in Chambers' Dictionary of Arts and Sciences. The ventriloquist, by a singular management of the voice, seems to have it in his power " to throw his voice " in any direction, so that the sound shall appear to proceed from that spot. The words-are pronounced by the organs usually employed for that purpose, but in such a manner as to give little or no motion to the lips, the organs chiefly concerned being those of the throat and tongue. The variety of sounds which the human voice is capable 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 of corks, and the gurgling of the flow. ing liquor, the sound of air rushing through a crevice on a wintry night, and -a great variety of other noises of the same kind, are imitated by the voice so exactly as to deceive any hearer who does not know whence they proceed. ACOUSTICS. 181 to appear haunted, voices have been heard from tomts, and the dead have been made to appear to speak, to the great dismay of the neighborhood, by means of this wonderful art. Ventriloquism is, without doubt, in great measure the gift of nature; but many persons can, with a little practice, utter sounds and pronounce words without opening the lips or moving the muscles of the face; and this' appears to he the great secret of the art. hGow is the 677. MUSICAL SOUNDS, OR HIARMIo NY. -- Th soundof a mu- sound produced by a musical string is caused by sical string its vibrations; and the height or depth of the caused? 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 instrument proced from the long strings, and the high tones from the short ones. 678. Fig. 106. A B represents a musical string. Eig.ila, If it he Fig. 106. Fig. 106. da drawn...; — up to G, its elas- -- ticity will not on- a. B y carry it back a -n —- again, but will give it a momentum which will carry it to H, fiom whence it will successively return to T, F, 0, D, &c., until the resistance of the air entirely destroys its motion. On what does 679. The quality of the sound produced by the quality of strings depends upon their length, thickness, the tone of a weight, and degree of tension. The quality string depend? of the sound produced by wind instruments depends upon their size, their length, and their internal diameter. 680. 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. 681. Icong and large strings, when loose, produce the lowest 16 182 NATURAL PHILOSOPHY. tones; but different tones may be produced from the same string, according to the degree of tension. Large wind instruments, 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. flzow does the 682. The quality of the sound of all musical temperature of the weather af- instruments is affected by the changes in the fect thetone of temperature and specific gravity of the atmosa musical instrument phere. 683. As heat expands and cold contracts the materials of which the instrument is made, it follows that the strings will have a greater degree of tension, and that pipes and other wind instruments will be contracted, or shortened, in cold weather. For this reason, most musical instruments are higher in tone (or sharper) in cold weather, and lower in tone (or more flat) in warm weather. On what is the 684. The science of harmony is founded on science of har- the relation which the vibrations of sonorous mony founded? bodies have to each other. 685. 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 produced. When the vibrations of two strings frequently coincide, they produce a musical chord; and when the coincidence of the vibrations is unfrequent, discord is produced. 686. A simple instrument, called a monochord, contrived for the purpose of showing the length and degree of tension of a string to produce the various musical tones, and to show their relations, may thus be made. A single string of catgut or wire, attached at one end to a fixed point, is carried over a pulley, and a weight is sus-'ended to the other end of the string. The string rests on two bridges, between the fixed point and the pulley, one of which is fixed, the other movable. A scale is placed beneath the string by which the length of the vibrating part between the two bridges may be measured. By means of this simple instrument, the respective lengths required to produce the seven successive notes of the gamut will be as follows: it being premised that the longer the string the slower will be its vibrations. 687. Let the length of the string required to produce the note called C be I; the length of the string to produce the successive notes will be c D E F G A B C A2~ ACOUSTICS. 183 688. Hence, the octave will require only half of the length of the fundamen- i( tal note, and the vibrations that produce it will be as two to one. The vibrations of the string in producing the successive 1 I notes of the scale will be as follows:' 1 C D E F G A B C 14ii s ~ y 2. I I That is,to produce the note D nine vibra- 1.' _ _ I: iz, tions will be made in the same time that I eight are made by C, five of E to four of 1 C, four of F to three of C, three of G to two of C, five of A to three of C, li fifteen of B to eight of C, and two of i kt i [ i the octave C to one of the fundamen- ~ 1 IJ J.. tal C. — t — H 689. The same relations exist in each I. I I I successive octave throughout the whole t nusical scale. i l l I 690. As harmony depends upon the coincidence of vibrations, it follows that | the octave produces the most perfect har- * iTh I mony; next in order is the fifth, as every third vibration of the fifth corre- l [-| sponds with every second vibration of the I I i fundamental. Next to the fifth in the order of' harmony follows the fourth, and *~.+ - after the fourth the third.I I'1 691. The following scale, containing I three octaves, exhibits the proportions hl I I I which exist between the fundamental and I I all the other notes within that compass. g1 I I I 692. In the lowest line of this scale i 1 I I the numbers show the intervals. The 1 i figures abdve express the number of I I 1'ibrations of the fundamental or tonic, and the upper line of figures represents bI I j the proportionate vibrations of each suc- cessive interval. J I 693. It is found in practice that when r two sounds are caused by vibrations - | || which are in some simple numerical pro- I 1iI portion.to each other, such as 1 to 2, or 1 WI b | 2 to 3, or 3 to 4, &c., they are pleasing.* 1- 1 to the ear; and the whole science of har- 1 i| mony is founded on such relations.. 694. The principal harmonies are the N i i I octave, fifth, fourth, major third, and [ 1i~ 184 NATURAL PHILOSOPHY. minor third; and the relations between them and the fundamental or tonic are as follows Octave, 2 to 1. Fifth, 3 " 2. Fourth. 4 " 3. Major Third, 5 " 4. Minor Third, 6 " 5. 695. The following Rules may serve as the basis of interesting calculations. (1.) Strings of the same diameter and equal tension vibrate in times in an inverse proportion to their lengths. (2.) The vibrations of strings of equal length and tension are in an inverse proportion to their diameters. (3.) The vibrations of strings of the same length and diameter are as the square roots of the weights causing their tension. (4.) The vibrations of cylindric tubes closed at one end are in an inverse proportion to their length. (5.) The sound of tubes open at both ends is the same with that of tubes of half the length closed at one end. [The limits of this work will not admit the further consideration of the subject of Harmony. It constitutes of itself a science, involving principles which require deep study and investigation; and they who would pursue it advantageously will scarcely expect, in the pages of an elementary work of this kind, that their wants will be supplied.] 696. Questions for Solutzon. (1.) A rocket was seen to explode, and in two seconds the sound of the explosion was heard; how far off was the rocket? Ans. 2240ft. (2.) The flash from a cloud was seen, and in five seconds the thunder was heard; what was the distance of the cloud? Ans. 5600./t. (3.) A musical string four feet long gave a certain tone; what must be the length of a string of similar size and tension to produce the note of a fifth A? Ans. 2ft. S in. (4.) A certain string vibrates 100 times in a second; how many times must a string of the same kind vibrate to produce the octave? the fifth? the minor third. the major third. Ans. 200; 150; 120; 120. (5.) Supposing that two sounds, with all their attending circumstances similar, reach an ear situated at the point of interference of the undulations, -what will be the consequence. [See Nos. 653 and 654.] (6.) The length of a string being 36, what will be length of its octave'! fifth? fourth? major and minor thirds? Ans. 18; 24; 27; 28.8; 31). (7.) A stone, being dropped into a pit, is heard to strike the bottom in 7 seconds; how deep was the pit! Ans. By Algebra, 660ft. [N. B. In estimating the velocity of sound, it is generally reckoned in practice as only at 1090 feet per second, supposing the thermometer at the freezing point; and one foot per second is added for every degree of temperature above the freezing point, or 32~. The average rate of 1120 feet has been assumed in the text.] PYRONOMICS. 1 85 (8.) Suppose the length of a music-string to be five feet; what will be the length of the L5th, all other circumstances being equal? Ans. 4 in. (9.) The length of the fifth being four, what will be the length of the fundamental, or tonic? Ans. 6. (10.) WVhat must be the length of a pipe of an open diapason to produce the same tone with four foot C of the stopped diapason 1 AiLs. 4ft. [N. B. The open diapason consists of pipes open at both ends; the stopped diapason has its pipes closed at one end. [See No. 695 (5).] (11.) In what proportion are the vibrations of two strings of equal length and diameter, one stretched with a weight of twenty-five pounds, the other with a weight of fifty pounds? rSee.no. 695 (8).] Aqss. 1 to 1.41 + (12.) In what proportion are the vibrations of two strings of equal length and tension, but having diameters in the proportion of 3 to 5? Ans. 5 to 3. What is Py- 697. PYRONOMICS, OR THE LAWS OF ronomic s? HEAT. — Pyronomics is the science which treats of the laws, the properties and operations of heat. What is heat, 698. The nature of heat is unknown, but it and what is its has been proved that the addition of heat to any weight? substance produces no perceptible alteration in the weight of that substance. Hence it is inferred that heat is imponderable. 699. heat is undoubtedly a positive substance, What is cold? or quality. Cold is merely negative, being only the absence of heat. What effect 700. Heat pervades all bodies, insinuating has heat on all itself, more or less, between their particles, bodies? and forcing them asunder. Heat and the aftraction of cohesion constantly act in opposition to each other; hence, the more a body is heated, the more its particles will be separated. 701. Heat causes most substances to dilate or expand, while cold (which is merely the absence of heat) causes them to contract.# Since there is a continual change in the temperature of all bodies on the surface of the earth, it necessarily follows that there will be a constant corresponding change in their magnitude as they are affected by heat and cold. They expand their bulk in a warm day, and contract it in a cold one. In warm weather the flesh swells, * The exceptions to this remark are water and clay. Water expands when it freezes; elay contracts when heated. 16* 186 NA URAL PHILOSOPIY. the blood-vessels are well filled, the hands and the feet, as well as other parts of the body, expand or acquire a degree of plumpness, and the skin is distended; while, on the contrary, in cold wea.ther the flesh appears to contract, the vessels shri'nk, and the skin appears shrivelled. Hence a glove or a shoe which is too tight in the summer will often be found to be easy in cold weather. 702. 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 within 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. What kindt of 703. Heat passes through some bodies with bodies arrest more difficulty than through others, but there is the progres3 no kind of matter which can completely arrest its of heat? progress. What is 704. Of all the effects of heat, that produced upon steam? water is, perhaps, the most familiar. 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 the form of drops of water. This fiact can be seen by holding a cold plate over boiling water. The steam rising from the water will be condensed 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. I-eat also produces most remarkable 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. 705. The attraction of cohesion causes the How is rain produced? is in small watery particles which compose mist or produced? vapor to unite together in the form of drops of water. It is thus that rain is produced. The clouds consist of PYRONOMICs. 87 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. 706. The thermometer, an instrument designed to measure degrees of heat, has already been described, in connexion with the barometer, under the head of Pneumatics. Heat, under the name of caloric, is properly a subject of consideration in the science of Chemistry. 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 sulphuric acid, or vitriol, is poured into a vial of cold -water, the vial and the liquid immediately become hot. A further illustration of the existence of latent. or concealed heat is given at the firl side every day. A portion of cold fuel is placed upon the grate or hearth, and a spark is applied 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 Pyronomzics. The subject of latent heat belongs more properly to the science of Chemistry. 707. The terms heat and cold, as they are generally used, are merely relative 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 warm hand and warm to the cold one. What are the 708. SOURCES OF HEAT. -The four prinprincipal cipal sources of the development of heat are sources of the Sun, Electricity, Chemical Action and Meheat? chanical Action. The heat produced by fire and flame is due to chemical action. 188 NATURAL PHILOSOPHY. What is the 709. But, of all the sources from which heat source of the has b.een developed by human agency, that progreatest degree duced by electrical action, and especially the of heat? galvanic battery, is by far the most eminent in its degree and in its effects. It can reduce the most refractory substances to a fluid state, or convert them to their original elements. 710. The heat generally ascribed to the sun is attended by peculiar phenomena, but imperfectly understood. It may, perhaps, be questioned whether there be any absolute heat in the rays of that luminary, for we find that the heat is not in all cases proportionate to his proximity. Thus, on the tops of high mountains, and at great elevation, it is not found that the heat -is increased, but, on the contrary, diminished. But there are other phenomena which lead to the conclusion that his rays are accompanied by the development of heat, if they are not the cause and the source of it. 7.11. All mechanical operations are attended by heat. Friction, sudden compression, violent extension, are all attended by heat. The savage makes his fire by the friction of two pieces of dry wood. Air, suddenly and violently compressed, ignites dry substances; * and India-rubber especially, when suddenly extended, shows evident signs of heat; arid an iron bar may be made red -hot by beating it quickly on an anvil. Even water, when strongly compressed, gives out heat. What are the 712. The principal effects of heat are principal ef- three, namely: feels qf leat 7 (1.) Heat expands most substances. (2.) It converts them from a solid to a fluid state. (3.) It destroys their texture by combustion. 713. There are many substances on which ordinary degrees of heat, and, indeed, heat of great intensity, seems to produce no sensible effects; and they have, therefore, received the name of incombustible bodies. Bodies usually called incombustible are generally mineral substances, such as stones, the earths, &c. All vegetable substances, and most animal substances, are highly combustible. The metals also all yield to'the electrical or galvanic battery. But there is sufficient evidence that all bodies were once in a fluid or gaseous state, and that the solid forms that they have assumed are due to the loss of heat. Could the same degree of i Syringes have been constructed on this principle A solid piston being forcibly driven downward on dry tinder, ignites it PYRONOMI>CS. 189 intensity of heat be restored, it- is presumed that they would resume their liquid or gaseous form. Wihat 4is the 714. Heat tends to diffuse itself equally through first ladlw of fieat? all substances. If a heated body be placed near a cold one, the temperature of the former will be lowered, while that of the latter will be raised. 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 he-at. Thus, if the hand be'successively applied to a woollen garment, a mahogany table, and a marble slab, all of which have been for some time in the same room, the woollen garment will appear the warmest, and the marble slab the coldest, of the three articles; but, if a thermomleter be applied to each, no difference in the temperature will be observed. What is the 715. From this it appears that some substances.reason that conduct heat readily, and others with great difsome sub- ficulty. The reason that the marble slab seems stancesfeel warm and the coldest is, that marble, being a good conothers cold in ductor of heat, receives the heat from the hand the same room? so readily that the loss is instantly felt by the hand; while the woollen garment, being a bad conductor of heat, receives the heat from the hand so slowly that the loss is imperceptible. What is the 716. The different power of receiving and cause of the conducting heat, possessed by different substances, difference in the warmth of is the cause of the difference in the warmth of different gar- various substances used for clothing. ments? WhI-y are 717. Thus, woollen garments are warm garwoollen gar- ments, because they part slowly with the heat ments warm, which they acquire from the body, and, conseandlinen, cool? quently, 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 readily receives fresh heat from the body. It is, therefore, constantly receiving heat from the body and throwing it out into 190 NATURAL PHILOSOPHY. the air, while the woollen garment retains the heat which it receives, and thus encases the body with a warm covering. 718. 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. How is heat 719. Heat is propagated in two ways, namely, propagated? 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. 720. Different bodies conduct heat with differWhat are the ent degrees of facility. The metals are the best best conductors of heat? conductors; and with regard to their conducting power, stand in the following order, namely: Gold, platinum,, silver, copper, iron, zinc, tin, lead. 721. Any liquid, therefore, may be more readily heated in a silver vessel than in any other of the same thickness, except one of gold, or of platinum, on account of its great conducting power. Why are the 722. Metals, on account of their conducting 4andles of tea power, cannot be handled when raised to a tempeand coffee pots made of wood? rature above 120 degrees of Fahrenheit. For this reason, the handles of metal tea-pots and coffeepots 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. 723. Wood conducts heat very imperfectly. 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. But an iron bar, being a good conductor of heat, cannot be handled near the heated end. 724. 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. Water becomes scalding hot at 150 degrees; but air, heated far beyond the temperature 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 PYRONOMIoS. 191 money, &e. Eggs, placed on a tin frame, were roasted hard in twenty minutes; and a beef-steak was overdone in thirty-three minutes. 725. Chantrey, the celebrated sculptor, had an oven which he used for drying his plaster cuts and moulds. The thermometer generally stood at 300 degrees in it, yet the workmen entered, and remaiped 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 contact with the skin. 726. The air, being a bad conductor, never radiates heat, nor is it ever made hot by the direct rays of the sun. The air which comes in contact with the surface of the earth ascends, and warms the air through which it passes in its ascent. Other air, heated in the same way, also ascends, carrying heat, and this process is repeated till all the air is made hot. 727. In like manner, in cold weather, the air resting on the earth is. made cold by contact. This cold air makes the air above it cold, and cold currents (or wind) agitate the mass together till a uniform temperature is produced. 728. Heat is reflected by bright substances, and How is heat reflected? the angle of reflection will be equal to the angle of incidence. 729. 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 inclined towards the fire in such a manner as to reflect the heat downwards. 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." 730. This power of reflecting heat, possessed by bright substances, is the reason why andirons and other articles that are kept bright, although standing very near the fire, never become hot; while other darker substances, further from the fire, become hot. But, if they are not bright, heat will penetrate them. 731. The reflecting power of bright and light-colored substances accounts also for the superior coolness of white and light-colored fabrics for clothing. Why are dark 732. Black and dark-colored surfaces absorb garments heat. This is the reason why black and darkwarmer than colored fabrics are warmer when made into garlight ones? ments than those of light color. 733. Snow or ice will melt under a piece of black cloth, while it would remain perfectly solid under a white one. The farmers in some of the mountainous parts of Europe are accustomed to spread 192 NATURAL PHILOSOPHY. black earth, or soot, over the snow, in the spring, to hasten its melting. and enable them to commence ploughing. What effect has heat 734. The density of all substances is augupon the density of su~bstances? Y S mented 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 liquid freezes in them. For the same reason, ice floats instead of sinking in water; for, as its density is diminished, its specific gravity is consequently diminished. Were it not for this remarkable property of water, large ponds and lakes, exposed to intensecold, would become solid masses of ice; for. if the ice, when formed on the surface, were more dense (that is, more heavy) than the watter 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 creatures in the water. But the specific gravity of ice causes it to continue on the surface, protecting the water below from congelation. 735. Cold is merely the absence of heat; or rather, What is cold? more properly speaking, inferior degrees of heat are termed cold. 736. The effect of heat and cold, in the expansion and contraction of glass, is an object of common observation; for it is this expansion and contraction which cause so many accidents with glass articles. Thus, when hot water is suddenly poured into. a cold glass of any form, the glass, if it have any thickness, will crack; and, on the contrary, if cold water be poured into a heated glass vessel, the same effect will be' produced. The reason of which is this; Hieat makes its way but slowly through glass; the inner surface, therefore, when the hot water is poured into it, becomes heated, and, of course, distended 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. 737. -'l'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 fracture may be prevented (it is said) by making a minute notch on the bottom of the tube with a diamond. This precaution has been used in an establishment where six lamps were lighted every day, and not a single glass has been broken in nine years. What bodies retain 738. Different bodies require different quanheat the longest? tities of heat to raise them to the same temn PYRONOMICS. 193 perature; and those which are heated with most difficulty retain their heat the longest. Thus, oil becomes heated more speedily than water, and it likewise cools more quickly. 739. The most obvious and direct effect of heat on a body is to increase its extension in all directions. 740. Coopers, wheelwrights and other artificers, avail themselves 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. 741. 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 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 application of the expansive power of heat 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 tho 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 terminated 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 suffired to cool. The powerful 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 gradually and steadily restored to their upright position. 742. The Pyrometer is an instrument to What is the Pyrometer? 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 expands, and turns the index to show the degree of expansion. 743. Wedgewood's pyrometer, the instrument. commonly used for high temperatures, measures beat by the contraction of clay. 17 194 N ATURAL PHILOSOPHY. WThat effect 744 - The expansion caused by heat in has heat on solid and liquid bodies differs in different subbively, rin the stances; but aeriform fluids all expand alike, solid, liquid and and undergo uniform degrees of expansion at airiform state? various temperatures. 745. The expansion of solid bodies depends, in some degree, on the cohesion of their particles; but, as gases and vapors are desti tute of cohesion, heat operates on them without any opposing power. 746. When heat is applied to water or other What effect liquids, it converts them into steam or vapor. The has heat on s he form of deprivation of heat reconverts them into the. liquid liquid bodies? form. It is on this principle that distillation takes place. What is a 747. The vessel employed for distillation is called Still? a Still.* Fig. 107. Explain 748. Fig. 107 represents a Still. A liquid being poured Fig. 107. into the large vessel a, heat is applied. below, which converts the liquid gradually into steam or vapor, which, having no other outlet, passes through the spiral tube, called the worm, in vessel b, and from b through another worm, in c. The worm, being surrounded with cold water, condenses the vapor in the tube or worm, and reconverts it to a fluid state, and it flows out # The subject of distillation properly belongs to the science of Chemistry, but it is here introduced for the benefit of those who cannot readily refer to a treatise on thiat subject. PYRONOMICS. 193 at e in a tepid stream. The worm is of different lengths, and its only use is to present a large extent of surface to the cold water, so that the vapor may readily be condensed. 749; The process of distillation is sometimes used to purify a liquid, as the vapors which rise are unmixed with the impurities of the fluid. Important changes are thus made, and the still becomes highly useful in the arts. At what tern- 750. When water is raised to the temperaperature is ture of 212~ of Fahrenheit's thermometer, it water convert- ed into steam? is converted into steam. It is then highly elastic and compressible. What effect 751. The elastic force of steam is increased has heat upon by heat; and decrease of heat diminishes it. steclam? The amount of pressure which steam will exert depends, therefore, on its temperature. 752. The temperature of steam is always the What is the temperature same with that of the liquid from which it is of confined formed, while it remains in contact with that liquid; steam? and when heated to a great degree, its elastic force will cause the vessel in which it is contained to burst, unless it is made sufficiently strong to resist a prodigious pressure. 753. It has already been stated that water is converted into steam at the temperature of 2120. When closely confined it may be raised to a higher temperature, and it will then emit steam of greatly increased elastic force. How is steam 754. When any portion of steam comes in condensed? contact with water, it instantly parts with its heat to the water, and becomes condensed into water. The whole mass then becomes water, increased in temperature by the amount of heat which the steam has lost. On what prop- 755. This is the great and peculiar property erty do the of steam, on which its mechanical agencies demechanical agencies of pend, namely, its power of exertinigf a high steam depend? degree of elastic force, an d losing it instantaneously. 196 NATURAL PHILOSOPHY. Howm y the 756. There are two ways in which steam mechanical iS made instantly to lose its mechanical force; force of steam be instantly namely, first, by suddenly opening a passage destroyed? for its escape into the open air, where it immediately becomes visible,* by a sudden loss of part of its heat, which it gives to the air; and secondly, by conveying it to a vessel called a condenser, where it comes directly into contact with a stream of water, to which it instantly gives up its heat and is condensed into water. 757. Steam occupies a space about sevenMl/at.pace does steam oc- teen hundred times larger than when it is concupy? verted into water. But the space that a given quantity of water converted into steam will occupy depends upon the temperature of the steam. The more it is heated the greater space it will fill, and the greater will be its expansive force. I~What is the 758. THE STEAM-ENGINE. -The SteamSteam-engine engine is a machine moved by the expansive force of steam. In what man- 759. The mode in which steam is made to act ner is steam is by causing its expansive force to raise a solid made to act? piston accurately fitted to the bore of a cylinder, like that in the forcing-pump. The piston 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 by the admission of cold water, or withdrawn from under * Steam in a highly elastic state - that is, when at a high temperature - is perfectly dry and invisible. The reason that we are able to see it after it has performed its work and issues from the steam-engine is, that as soon as it comes in contact with the air it immediately parts with a portion of its heat (and, because air is not a good conductor, only a portion), and is condensed into small vesicles, which present a visible form, resembling smoke. Its expansive force, however, is not wholly destroyed; for the vesicles themselves expand as they rise, and soon become invisible, mingling with other vapors in the air. Could we look into the cylinder, filled with highly elastio steam, we should be able to see nothing. But, that the steam is there, and in its invisible form exerting a prodigious force, we know by the mc vements of the piston STEAM-ENGINE. 197 it, a vacuum will be formed, and the pressure of the atmosphere on the piston above will drive it down. The admission of more steam below will 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. How was the 760. This is the mode in which the engine of soNewom engn Newcomen and Savery, commonly called the atand Savery mospheric engine, was constructed. It was called constructed? the atmospheric engine because half of the work was done by the pressure of the atmosphere, namely, the down. ward motion of the piston. What improve- 761. The celebrated Mr. James Watt introments did duced two important improvements into the steam. Watt make in the steam-en- engine. Observing that the cooling of the cylinder gine? by the water thrown into it to condense the steam lessened the expansibility of the steam, he contrived a method to withdraw the steam from the principal cylinder, after it had performed its office, into a condensing-chamber, where it is reconverted into water, and conveyed back to the boiler. The other improvement, called the double action, consists in substituting the expansive power of steam for the atmospheric pressure. This was performed by admitting the steam into the cylinder above the raised piston, at the same moment that it is removed from below it; and thus the power of steam is exerted in the descending 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 above, 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 doubleacting condensing steam-engine. [See also, No. 766.] Explain 762. Fig. 108 represents that portion of the steamFig. 108. engine in which steam is made to act, and propel such machinery as may be connected with it. It also exhibits two 17* 1.98 NATURAL PHILOSOPHY. improvements of BMr. Watt. Fig. 108. The principal parts are the J boiler, the cylinder and its piston, the condenser, the G Q M air-pump, the steam-pipe, ] B I the eduction-pipe, and the H cistern. In this figure, A represents the boiler, C E - c H I N the cylinder, with IH the piston, B the steamn-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, when open, to the cylinder, both above and below the piston. K is the eductionpipe, 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. L is the condenser, and O a stop-cock for the admission of cold water. N is the 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, expelling the air. This process is called blowing out, and is heard when a steamboat is about starting. 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 attached, which, rising and falling with the beam and the. piston, open the stop-cock 0, ad* The steam and the eduction pipes are sometimes made in forms differing from those in the figure, and they differ much in different engines. ATEM-ENGINE. 191, mitting 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 alternately opened and closed; the steam passing from the boiler drives the piston alternately 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 pump M, the rod of which is connected with the workingbeam, carries the water from the condenser back into the boiler, by a communication represented in Fig. 109. The safety-valve R, connected with a lever of the second kind, is made to open when the pressure 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. IHfow is the 763. The power of a steam-engine is genPower of a steam-engine erally expressed by the power of a horse, estimated? 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. What are the 764. The steam-engine is constructed in vatwo kinds of rious forms, and no two manufacturers followsteam-engines and how do they ing exactly the same pattern; but the two prindiffer? cipal kinds are the high and the low pressure engines, or, as they are sometimes called, the non-condensing and the condensing engines. 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, 200 NATURAL PHILOSOPHY. is let off into the open air. As this kind of engine occupies less space, and is much less complicated, it is generally used on railroads. 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. 765. The steam-engine, as it is constructed Who were the principal im- at the present day, is the result of the inventions provers of the and discoveries of a number of distinguished indi. steam-engine? steam-engine 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, Newecomen, Fulton, and especially Mr. James Watt. 766. To the inventive genius of Watt the engine is indebted for the condenser, the appendages for parallel motion, the 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 become a thing alike stupendous for its force and its flexibility; for the prodigious power it can exert, and the ease and precision and ductility with wlhich it can be varied, distributed, and applied. The trunk of an elephant, 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. " Explain 767. Fig. 109 represents Watt's double-acting condensFig. 109. ing steam-engine, in which A represents the boiler, containing a large quantity of water, which is constantly replaced as fast as portions are converted into steam. B is the steam-pipe, conveying the steam to the cylinder, having a steam-cock b to admit or. exclude the steam at pleasure. o is the cylinder, surrounded by the jacket c c, a space kept constantly supplied with hot steam, in order to keep the cylinder from being cooled by the external air. D is the eduction-pipe, communicating between the cylinder and the condenser. E is he condenser, with a valve e, called the injection-cock, admitting STEABI-ENGINE. 201 a jet of cold water, which meets the steam the instant that the steam enters the condenser. F is the air-pump, which is a common suction-pump, but is here' called the air-pump because it removes from the condenser not only the water, but also the air, and the steam that escapes 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 Fig. 109. by H. 1 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 channel between the steam-pipe, cylinder, eduction-pipe, and condenser; which levers are raised or depressed by projections attached to the piston-rod of the pump. M M is an apparatus for changing the circular motion of thle working-beam into par 202 NATUI.AL PHILO$OPIIY, 0o W11I tmi.2~'' r:ii I~~~~~~~~~~~~~~~~I STEAM-ENGINE. 203 allel 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 falling 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 0 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 recede further from the shaft, and by partly closing a valve connected with the boiler, will diminish the supply of steam to the cylinder, and thus reduce the speed to the rate required. The steam-engine thus constructed is applied to boats to turn wheels having paddles attached to their circumference, which answer the purpose of oars. [See Fig. 110.] It is used also in work-shops, factories, &c.; and different directions and velocities may be given to the motion produced by the action of the steam on the piston, by connecting the piston to the beam with wheels, axles and levers, according to the principles stated under the head of Mechanics. Steamboats are used principally on rivers, in harbors, bays, and on the coast. They are made of all sizes, and carry engines of different power, proportioned to the size of the boat. The steamship [See Fig. 111], in addition to its steam-engines Fig. 11. 204 NATURAL PHTILOSO PHY. and paddles, is rigged with masts and sails to increase the speed, or to make progress if the engines get out of order. The Propeller differs from a steam-boat or steam-ship, by having an immense screw projecting from under the stern of the ship, instead of paddle-wheels. The screw is caused to revolve by means of steamengines, and forces the vessel forward by its action on the water. What is the 768. The locomotive engine is a highlocomotive pressure steam-engine, -mounted on wheels, steam-eng ine? steam-egne and used to draw loads on a railroad, or other level road.- It is usually accompanied by a large wagon, called a tender, in which the wood and water used by the engine are carried. Explain 769. Fig. 112 represents a side view of the internal Fig. 112. construction of a locomotive steam-engine; in which F represents the fire-box, or place where the fire is kept; D the door through which the fuel is introduced. The spaces marked B are the interior of the boiler, in which the water stands at the height indicated by the dotted line. The boiler is closed on all sides, all its openings being guarded by valves. The tubes marked p p conduct the smoke and flame of the fuel through the boiler to the chimney C C, serving, at the same time, to communicate the heat to the remotest part of the boiler. By this arrangement, none of the heat is lost, as these tubes are all surrounded by the water. S S S is the steam-pipe, open at the top V S, having a steam-tight cock, or regulator, V, which is opened and shut by the lever L, extending outside of the boiler, and managed by the engineer. The operation of the machine is as follows: The steam being generated in great abundance in the boiler, and being unable to escape out of it, acquires a considerable degree of elastic force. If at that moment the valve V be opened, by the handle L, the steam, entering the pipe S, passes in the direction of the arrow, through the tube, and enters the valve-box at X. There a sliding-valve, which moves at the same time with the machine, opens for the steam a communication successively with each end of the cylinder below. Thus, in the figure, the entrance on the right hand of the sliding-valve is represented as being open, and Fig. 112 C S z~~~~~~s C~~~~~~~ B 31 W O T IE CN COY —S L O S —---— TM — -NI D~~~~ VIEW OF THE INTERNAL CONSTRUCTION OF HINKLEY a DRURY S LOCOMOTIVE STEAMI-ENGIINF 206 NATURAL PHILOSOPHY.,iIl "If',l',~/f111!,,i! CP Fig. 114. 0~~~~~ ~,FSSTTOAYTA-NIEWTHEC TUFTS' STATIONARY STEAM-ENGINE, WITH SECTIONS. 208 NATURAL PHILOSOPHY. w~~~y A~ll.?s, ean'ii11~~ -~~ — i - -A~A~~i~' jllllj(/l~//l/;j lli llllii li'~ STEAM-ENGINE. 209 the steam follows in the direction of the arrows into the cylinder, where its expansive force will move the piston P in the direction of the arrow. The steam or air on the other side of the piston passes out in the opposite direction, and is conveyed by a tube passing through C C into the open air. The motion of the piston in the direction of the arrow causes the lever N to close the sliding-valve on the right, and open a communication for the steam on the opposite side of the piston P, where it drives the piston back towards the arrow, at the same time affording a passage for the steam on the right of the piston to pass into the open air. Motion being thus given to the piston, it is communicated, by means of the rod R and the beam G, to the cranks K K, which, being connected with the, axle of the wheel, causes it to turn, and thus move the machine. -Thus constructed, and placed on a railroad, the locomotive steam-engine is advantageously used as a substitute for horse power, for drawing heavy loads. The apparatus of safety-valves, and other appliances for the management of the power produced by the machine, are the same in principle, though differing in form, with those used in other steam-engines; for a particular description of which, the student is referred to practical treatises upon the subject. What is the 770. THE STATIONARY STEAM-ENGINE.best form of This engine is generally a high-pressure or ghein stea-en- non-condensing engine, used to propel machinery in work-shops and factories. As it is designed for a labor-saving machine, it is desirable to combine simplicity and economy with safety and durability in its construction; and that form of this engine is to be preferred which in the greatest degree unites these qualities. Describe the Sta- 771. The figure on page. 207 represents tionary Steam- Tufts' stationarysteam-engine,* with sectionsof engzne. the interior. Like the double-acting condense * This engine was constructed by Mr. Otis Tufts, of East Boston, Mas18* 210 NATURAL PHILOSOPHY. ing engine of Mr. Watt, described in Fig. 109, it is furnished with a governor, by which the supply of steam is regulated; and, like the locomotive, Fig. 112, the cylinder, with its piston, has a horizontal position. The steam is admitted into the valvebox through an aperture at E, in the section, and from thence passes info the cylinder through a sliding-valve, alternately to each side of the piston P, as is represented by the direction of the arrows, the -sliding-valve being moved by the rod V, communicating with an " eccentric" apparatus attached to the axis of the fly-wheel. The direction of the current of steam to the valve-box is represented by the arrow at I, and its passage outward from the cylinder, after, it has moved the piston, is seen at O. In this engine there is no working-beam, as in Watt's engine, Fig. 109, -but the motion is communicated from the piston-rod to a crank connected with the fly-wheel, which, turning the wheel, will move all machinery connected either with the axle or the circumference of that wheel. Fig. 115 represents the Locomotive Steam-engine in one of its most perfect forms, as used on railways at the present day. 772. OPTICS.- Optics is the science which What is Optics? W -ha i treats of light, of colors, and of vision. How are all sub- 773. The science of Optics divides all substances consid- stances into the following classes: namely, ered in Optics? luminous, transparent, and translucent; reflecting, refracting, and opaque. 774. Luminous bodies are those which What are luminous bodies? shine by their own light; such as the sun, the stars, a burning lamp, or a fire. sachusetts. It is the engine used to propel the machinery at a late Fair of the Massachusetts Mechanic Association, where it was very highly and justly commended for its beauty and simplicity of construction, and the perfectly " noiseless tenor ofits way." The' figure which represents it is an electrotype copy of a steel plate, designed' by'Brown & Harbrys, under the direction of Mr. Tufts. The electrotype copy was taken by Mr. A. Wilcox, Washington-street. Boston. The electrotype process will be noticed in a subsequent page of this volume.' OPTICS. 211 What are trans- 775. Transparent substances are those parent sub- which allow light to pass through them freely, so that objects can be distinctly seen through them; as glass, water, air, &c.i 776. Translucent bodies are those which Whaucent arbodies trns-permit ai: portion of light to pass through lucent bodies? z them, but render the object behind them indistinct; as horn, oiled paper, colored glass, &c. What are re- 777. Reflecting substances are those which flecting sub- do not permit light to pass through them; stances? but throw it off in a direction more or less oblique, according as it falls on the reflecting surface; as polished steel, looking-glasses, polished metal, &c. 778. Refracting substances are those which TWhat are refracting sub- turn the light from its course in its passage stances? through them; and opaque substances are those which permit no light to pass through them, as metals, wood, &c. What is light? 779. It is not known what light is. Sir Whlat asre the Isaac Newton supposed it to consist of two theories respecting the na- exceedingly small particles, moving from ture of light? luminous bodies; others think that it consists of the undulations of an elastic medium, which fills all space.t These undulations (as is supposed) produce the * No substance that exists on our earth is perfectly transparent, and light must, therefore, necessarily be impaired in its passage through all transparent media, and the diminution it suffers will vary as the medium is more or less transparent, and as the passage it makes is of greater or less length. The exact ratio in which light is diminished has not yet been determined; it is, however, an established fact, that even those bodies which approach most nearly to perfect transparency become opaque when their thickness is considerably increased. t These two theories of light are called respectively the corpuscular and the undulatory theory. By the former the reflection of light is supposed tc take place in the same manner as the reflection of solid elastic bodies, as has been explained under the head of Mechanics [see No-. 165, page 49]. By the latter the propagation of light takes place from every luminous point, by means a f the undulatory movements of the ether. On this hypoth 212 NATURAL PHILOSOPEY. sensation of light to the eye, in the same manner as the vibrations of the air produce the sensation of sound to the ear. The opinions of philosophers at the present day are inclining to the undulatory theory. What is a ray 780. A ray of light is a single line of of linght? light proceeding from a luminous body. 781. Rays of light are said to diverge When are rays said to diverge? when theyseparate more Fig. 116. widely as they proceed from a luminous body. Fig. 116 represents the E116. rays of light diverging as they proceed from the luminous body, from F to D. 782. It will be seen by this figure that, as light is projected in every direction, its intensity must decrease with the distance, and this decrease is determined by a fixed law. The light received upon any surface decreases as the square of the distance increases. Thus, if a portion of light fall on a surface at the distance of two feet from any luminary, a surface twice that distance will receive only one-fourth as much light; at three times that distance, oneninth; at four times the distance, one-sixteenth, &c. Hence a person can see to read at a short distance from a single lamp much better than at twice the same distance with two lamps, &c. When are rays 783. Rays of light are said to converge of light said to converge.? when they approach each other. The point esis, the waves of light follow the general laws of the reflection of all elasticfluids, and, accordingly, every wave from every point, when it irmpinges on any resisting object so as to be reflected, forms a new wave in its course back, having its centre as much on the other side of the obstacle as the centre of the original wave was on this side. In the case of light the centre of the original wave is, obviously, the luminous point. There is a remarkable similarity, therefore, between the reflection of light, and echo, or the reflection of sound. It has been shown, under the head of Acoustics, that when two waves meet under certain circumstances, the elevation of one wave exactly filling up the depression of another wave, produces what is called the acoustic paradox, namely, two sounds producing silence. It will readily be seen that the same undulatory movements in Optics will produce the same analogous effect; or, in other words, that two rays of light may produce darkness; and this may, with equal propriety, be termed the optzcal paradox. But a clear understanding of the principles involved in what is called respectively the hydrostatic, pneumatic, acoustic and optical para dox, shows that there is no paradox at all, but that each is the necessary result of certain fixed and determinate laws. OPTICS. 213 at which converging rays meet is called Fig. 117. the focus.. ig. 117 represents con117. verging rays of light, of which the point F is the focus. What is a beam 784. A beam of Fig. 118. Ojf ight? light consistsofmany - rays running in parallel lines. ~/. Explain Fig. Fig. 118 represents a beam of light. 118. 785. A pencil of rays is a collection of What is a pencil of rays? diverging or converging rays. [See Figs. 116 and 117.] 786. Light proceeding from a luminous In what direction, and with body is projected forward in straight lines in what rapidity, every possible direction. It moves with a does light move? rapidity but little short of two hundred thousand miles in a second of time. 787. Every point of a luminous body is From what part of a luinou a centre, from which light radiates in every body does light direction. Rays of light proceeding from proceed? different bodies cross each other without interfering. The rays of light which issue from terrestrial bodies continually diverge, until they meet with a refracting substance; but the rays of the sun diverge so little, on account of the immense distance of that luminary, that they are considered parallel. 788. A shadow is the darkness produced by What is a shadow? the intervention of an opaque body, which prevents the rays of light from reaching an object behind the opaque body. 789. Shadows are of different degrees of Why are shad-arkness becaus ows of different darkness,because the light from other lumi 214 NATURAL PHILOSOPHY. degrees of dark- nous bodies reaches the spot where the hess? shadow is formed. Thus, if a shadow be formed when two candles are burning in a room, that shadow will be both deeper and darker if one of the candles be extinguished. The darkness of a shadow is proportioned to the intensity of the light, when the shadow is produced by the interruption of the rays from a single luminous body. What produces 790; As the degree of light and darkness the darkest can be estimated only by comparison, the shadow? strongest light will appear to produce the deepest shadow. Hence, a total eclipse of the sun occasions a more sensible darkness than midnight, because it is immediately contrasted with the strong light of day. Hence, also, by causing the shadow of a single object to be thrown on a surface, - as, for instance, the wall, -from two or more lights, we can tell which is the brightest light, because it will cause the darkest shadow. 791. When a luminous body is larger than WVhat is the shape of the an opaque body, the shadow of the opaque shadow of an body will gradually diminish in size till it opaqueody? terminates in a point. The form of the shadow of a spherical body will be that of a cone. Fig. 119. A repre119. A 119. lain Wisents the sun, and B Fig.119. the moon. The sun being much larger than the moon, e (l causes it to cast a converging shadow, which terminates at E. 792. When the luminous body is smaller than the opaque body, the shadow of the opaque body will gradually increase in size with the distance, without limit. OPTICS. 215 In Fig. 120 the shadow Fig. 120. of the object A increases i. c in size at the different dis- I tances B, C, D, E; or, in i other words, it constantly diverges. 793. When several luminous bodies shine upon the same object, each one will produce a shadow. What is it the Fig. 121 represents a ball A, illuminated by object of Fig. the three can-. 121 to show? dies B, C, and D. The light B produces the shadow b, the light C the shadow c, and the light D the shadow d; l e but, as the light from each of the candles shines upon all the shadows except its own, the shadows will be faint. What becomes of 794. When rays of light fall upon an falls on an opaque body, part of them are absorbed, and opaque object'? part are reflected. When is lightP Light is said to be reflected when it is said to be re- thrown off from the body on which it falls; flected? and it is reflected in the largest quantities from the most highly polished surfaces. Thus, although most substances reflect it in a degree, polished metals, looking-glasses, or mirrors, &c., reflect it in so perfect a manner as to convey to our eyes, when situated in a proper position to receive them, perfect images of whatever objects shine on them, either by their own or by borrowed light. 795. That part of the science of Optics'rics Catop- which relates to reflected light is called Catoptrics. 216 NATURAL PHILOSOPHY. What is thefun- 796. The laws of reflected light are the damental law of same as those of reflected motion. Thus, Catopirics? when light falls perpendicularly on an opaque body, it is reflected back in the same line towards the point whence it proceeded. If it fall obliquely, it will be reflected obliquely in the opposite direction; and in all cases the angle of incidence will be equal to the angle of reflection. This is the fundamental law of Catoptrics, or reflected light. 797. The angles of incidence and reflection have already been described under the head of Mechanics [see explanation of Fig. 10, No. 162]; but, as all the phenomena of reflected light depend upon the law stated above, and a clear idea of these angles is necessary in order to understand the law, it is deemed expedient to repeat in this connection the explanation already given. An incident ray is a ray proceeding to or falling on any surface; and a reflected ray is the ray which proceeds from any reflecting surface. Fig. 122 is designed to show Fig. 122. 122.lain ig. the angles of incidence and of reflection. In this figure, M A M is a mirror, or reflecting surface. P is a line perpendicular to the surface. I A represents an incident ray, falling on the mirror in such a manner as to form, with the perpendicular P, the angle I A P. This is called the angle of incidence. The line R A is to M be drawn on the other side of P A in such a manner as to have the same inclination with P A as I A has: that is, the angle R A P is equal to I A P. The line R A will then show the course of the reflected ray; and the angle R A P will be the angle of reflection. From whatever surface a ray of light is reflected. - whether it be a plain surface, a convex surface, or a concave surface, - this OPTICS. 217 law invariably prevails; so that, if we notice the inclination of any incident ray, and the situation of the perpendicular to the surface on which it-falls, we can always determine in what ma.nner or to what point it will be reflected. This law explains the reason why, when we are standing on one side of a mirror, we can see the reflection of objects on the opposite side of the rofnm, but not those on the same side on which we are standing. It also explains the reason why a person can see his whole figure in a mirror not more than half of his height. It also accounts for all the apparent peculiarities of the reflection of the different kinds of mirrors. How are lu- 798. Opaque bodies are seen only by reminous and flected light. Luminous bodies are seen by opaque bodies respectively the rays of light which they send directly to seen? our eyes. What effect 799. All bodies absorb a portion of the light has reflection on the inten- which they receive; therefore the intensity of sity of light? light is diminished every time that it is reflected. What does 800. Every portion of a reflecting surface every portion reflects an entire image of the luminous body surface relect? shining upon it. W~rhy do we When the sun or the moon shines upon a not see many sheet of water, every portion of the'surface reflects images of the an entire image of the luminary; but, as the image same thing reflectedby a can be seen only by reflected rays, and as the reflecting sur- angle of reflection is always equal to the angle of face? incidence, the image from any point can be seen only in the reflected ray prolonged. Why do objects 801. Objects seen by moonlight appear fainter appearfainter than when seen by daylight, because the light by by moonlight? which they are seen has been twice reflected; for, the moon is not a luminous body, but its light is caused by the sun shining upon it. This light, reflected from the moon and falling upon any object, is again reflected by that object. It 19 218 NATURAL PHILOSOPHY. suffers, therefore, two reflections; and since a portion is absorbed by each surface that reflects it, the light must be proportionally fainter. In traversing the atmosphere, also, the rays, both of the sun and moon, suffer diminution; for, altho:?.gh pure air is a transparent medium, which transmits the rays of light freely, it is generally surcharged with vapors and exhalations, by which some portion of light is absorbed. 802. All objects are seen by means of the When is an object invisi- rays of light emanating or reflected from them; ble? and therefore, when no light falls upon an opaque body, it is invisible. This is the reason why none but luminous bodies can be seen in the dark. For the same reason, objects in the shade or in a darkened room appear indistinct, while those which are exposed to a strong light can be clearly seen. We see the things around us, when the sun does not shine directly upon them, solely by means of reflected light. Everything on which it shines directly reflects a portion of its rays in all possible directions, and it is by means of this reflected light that we are enabled to see the objects around us in the day-time which are not in the direct rays of the sun. It may here also be remarked that it is entirely owing to the reflection of the atmosphere that the heavens appear bright in the day-time. 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 twilight, but a sudden transition from the brightest sunshine to darkness immediately upon the setting of the sun. 803. When rays of light, proceeding from How do rays of light enter any object; enter a small aperture, they cross a small aper- one another, and form an inverted image of the ture.? object. This is a necessary consequence of the law that light always moves in straight lines. EFplain 804. Fig. 123 represents the rays from an object, Fig. 123. a c, entering an aperture. The ray from a passes OPTICS. 219 town through the aperture to d, and the ray from c passes up to b, and thus these rays, crossing at the aper- rig. 123. ture, form an inverted image on the wall. The b room in which this experiment is made should be \7 darkened, and no light permitted to enter, excepting through the* aperture. It then becomes a camera obscura. 805. 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 eve. 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 presented on the plate in an erect position. What is the 806. The angle of vision is the angle formed angle of at the eye by two lines drawn from opposite parts of an object. What is the 807. The angle C, in Fig. 124, represents the object of Figures 124 angle of vision. The line A 0C, proceeding from and 125? one extremity of the object, meets the line B (I from the opposite extrem- Fig. 124. ity, and forms an angle C A at the eye;-this is the angle of vision. 808. Fig. 125 represents the different angles made I A by the same object at different distances. From an inspection of rig. 125. the figure, it is evident that the nearer D the object is to the eye, the wider must c be the opening of the lines to admit the extremities of the object, and, consequent- B ly, 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 figure, the three crosses F G, D E, and A B, are 220 NATURAL PHILOSOPHY. 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. 809. The apparent size of an object depends upon On what does the apparent the size of the angle of vision. But we are accussize of an ob- tomed to correct, by experience, the fallacy of apject depend? pearances; and, therefore, since we know that real objects do not vary in size, but that the angles under which we see them do vary with the distance, we are not deceived by the variations in the appearance of objects. Thus, a -ouse at a distance appears absolutely smaller than the window through which we look at it; otherwise we could not see it through the window; but our knowledge of the real size of the house prevents our alluding to its apparent magnitude. In Fig. 124 it will be seen that the several crosses, A B, D E, F G, and H I, although very different in size, on account of their different distances, subtend the same angle A C B; they, therefore, all appear to the eye to be of the same size, while, in Fig. 125, the three objects A B, D E, and F G, 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 to a correct observance of the angle of vision that the art of perspective drawing is indebted for its accuracy. When is an 810. When an object, at any distance, does object invisible on account of not subtend an angle of more than two seconds es distance? of a degree, it is invisible. At the distance of four miles a man of common stature will thus become invisible, because his height at that distance will not subtend an angle of two seconds of a degree. The size of the apparent diameter of the heavenly bodies is generally stated by the angle which they subtend. 811. When the velocity of a moving body When is motion irnper- does not exceed twenty degrees in an hour, its ceptible? motion is imperceptible to the eye. It is for this reason that the motion of the heavenly bodies is invisible, notwithstanding their immense velocity. 812. The real velocity of a body in motion round a point depends on the space comprehended in a degree. The more dis OPTICS. 221 tant the moving body from the centre, or, in other words, the larger the circle which it has to describe, the larger will be the degree. 813. In Fig. 126, if the man at A, and the Fi,. 126. man at B, both start together, it is manifest that A must move more rapidly than B, to - A arrive at C at the same time that B reaches J B D, because the arc A C is the arc of a larger /, circle than the are B D. But to the eye at E L 1 the velocity of both appears to be the same, C D f E because both are seen under the same angle of vision. 814. A mirror' is a smooth and polished surWhat are mirrors, and face, that forms images by the reflection of the how are they rays of light. Mirrors (or looking-glasses) made? are made of glass, with the back covered with an amalgam, or mixture of mercury and tin foil. 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 an alloy of copper and tin, called speculum metal. W/Vhat are the 815. There are three kinds of mirrors, different kinds namely, the plain, the concave, and the conqojt nrrors? vex mirror. Plain mirrors are those which have a flat surface, such as a common looking-glass; and they neither magnify nor diminish the image of objects reflected from them. 816. The reflection from plain mirrors is always B-y what law are objects re- obedient to the law that the angles of incidence and flectedfrom a reflection are equal. For this reason, no person looking gla9ss? can see another in a looking-glass, if the other cannot see him in return. 19* '222 NATURAL PHILOSOPHY. How do look- 817. Looking-glasses or plain mirrors cause in g- lasses ke al osjects everything to appear reversed. Standing before a appear? looking-glass, if a person holds up his left hand it will appear in the glass to be the right. 818. A looking-glass, to reflect the whole person, needs be but half of the length of-the person. 819. When two plain mirrors stand opposite to each other, the reflections of the one are cast upon the other, and to a person between them they present a long-continued vista. 820. When two reflecting surfaces are inclined at an angle, the reflected objects appear to have a common centre to an eye viewing them obliquely. It is on this principle that the kaleidoscope is constructed. What is a 821. The Kaleidoscope consists of two reflecting Kaleidoscope? surfaces, or pieces of looking-glass, inclined to each other at an angle of sixty degrees, and placed between the eye and the objects intended to form the picture. The two plates are enclosed in a tin or paper tube, and the objects, consisting of pieces of colored glass, beads, or other highly-colored fragments, are loosely confined between two circular pieces of common glass, the outer one of which is slightly ground, to make the light uniform. On looking down the tube through a small aperture, and where the ends of the glass plates nearly meet, a beautiful figure will be seen, having six angles, the reflectors being inclined the sixth part of a circle. If inclined the twelfth part or twentieth part of a circle, twelve or twenty angles will be seen. By turning the tube so as to alter the position of the colored fragments within, these beautiful forms will be changed; and in this manner an almost infinite variety of patterns may be produced. The word Kaleidoscope is derived from the Greek language, and means " the sight of a beautiful form." The instrument was invented by Dr. Brewster, of Edinburgh, a few years ago. 822. A convex mirror is a portion of the external surface of a sphere. Convex mirrors have therefore a convex surface. 823. A concave mirror is a portion of the inner surface :OPTICS. 223 of a hollow sphere. Concave mirrors, have therefore a concave surface. Explain 824. In Fig. 127, M N represents both a. convex Fig. 127. and a concave mirror. They are both a portion of a sphere of which O-is the centre. The outer part of M N is a convex, and the inner part is Fig. 127. a concave mirror. Let A B, G C D; E F, represent rays P falling on the convex mirror A MN. As the three rays are A parallel, they would all be per- D pendicular to a plane or flat. mirror; but no ray can fall perpendicularly on a concave H or convex mirror which is not directed towards the centre of the sphere of which the mirror zs a portion. For this reason, the ray C D is perpendicular to the mirror, while the other rays, A B and E F, fall obliquely upon it. The middle ray therefore, falling perpendicularly 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 E F T will be equal to the angles of reflection P B G and T F HI; 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 not really unite at that point, but only appear to do so; for the rays do not pass through the mirror, since they are reflected by it. 825. The image of an object reflected from a convex mirror is smaller than the object. 2824 NATURAL PHILOSOPHY. ~'What is the 826. This is owing to the divergence of the reobject of flected rays. A convex mirror Converts, by rej ecFig. 128 tion, parallel rays into divergent rays; rays that fall upon the mirror divergent are rendered still more divergent by reflection, and convergent rays are reflected either parallel, or less con- Fig. 128. vergent. If, then, an A object, A B, be placed before any part of a convex mirror, the g two rays A and B, proceeding from the A extremities, falling \ convergent on the mirror, will be reflected less convergent, and will not come to a focus until they arrive at 0; then an eye placed in the direction of the reflected rays will see the image formed in (or rather behind) the mirror at a b; and, as the image is seen under a smaller angle than the object, it will appear smaller than the object. What is the 827. The true focus of a concave mirror is truefocus of a point equally distant from the centre and the a concave mnirror? surface of the sphere of which the mirror is a portion. When will 828. When an object is further from the conthe image re- cave surface mirror than its focus, the imaoe will be flected from a concave be inverted; but when the object is between the upright, and mirror and its focus, the image will be upright, when inverted? and grow larger in proportion as the object is placed nearer to the focus. What pe- 829. Concave mirrors have the peculiar propculiar prop- erty of forming images in the air. The mirror erty have, and the object being concealed behind a screen, concave or a walland the object being stronr-gly illui,ars or a wall, and the object being strongly illumi OPTICS. 225' nated, the rays from the object fall 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 apparitions, 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. When is the 830. The image reflected by a concave image from a mirror is larger than the object when the concave mirror larger than the object is placed between the mirror and its object focus. VThat is the de- 831. This is owing to the convergent prop-s sign of Fig. erty of the concave mirror. If the object A, 129. B be placed between the concave mirror and its focus f, the rays rig. 129. A and B from its _ extremities will fall divergent on the mirror, and, on being reflect- f ed, become less divergent, as if they proceeded from C. To an eye placed in that situation, namely, at C, the image will appear magnified behind the mirror, at a b, since it is seen under a larger angle than the object. 832. There are three cases to be considered with regard to the effects 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. Ist. In the first case, the rays of light diverging after reflection but in a less degree than before such reflection took place, the im 226 NATURAL PHILOSOPHY. age will be larger than the object, and appear at a greater or smaller distance from the surface 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 concavity, where, as will be 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, because at this point the object and the image coincide. 3d. In the cases just considered, the images will appear inverted; and in the case where the object is further from the mirror than its centre of concavity, 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 coincide when the latter is stationed exactly at the centre. 833. The following laws flow from the fundamental law of Catoptrics, namely, that the angles of incidence and reflection are always equal. In estimating these angles, it must be recollected that no line 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. 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. [It is recommended that the learner be required to draw a figure to represent each of these laws.] 834. LAWS OF REFLECTION FROM CONVEX MIRRORS.- (i.) Parallel rays reflected from a CONVEX surface are made to diverge. (2.) Diverging rays reflected from a CONVEX surface are made more diverging. (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 * For the sake of distinction, the principal focus is called " the focus of parallel rays."- Peschel. OPTICS. 227 than the focus, they will converge less when reflected from a CONVEX surface. (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, further from it than the point towards which they converged. (6.) 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 converged. (7.3 If converging rays tend to the centre, when reflected they will proceed in a direction as if from the centre. 835. LAWS OF REFLECTION FROM CONCAVE MIRRORS. - (1.) Parallel rays reflected from a CONCAVE surface are made converging. [See Note'to No. 837.] (2.) Converging rays falling upon a CONCAVE surface are made to converge more. (3.) Diverging rays falling upon a CONCAVE surface, if they diverge from the focus of parallel rays, become parallel. (4.) If from a point nearer to the surface than that focus, they diverge less than before reflection. (5.) If from a point between that focus and the centre, they converge, after reflection, to some point on the contrary side of the centre, and further from the centre than the point from which they diverged. (6.) If from a point beyond the centre, the reflected rays will converge 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 back to thb same point from which they proceeded. How are objects 836. As a necessary consequence of the laws seenfromacon- which have now been recited, it may be stated, vex mirror? First, in regard to cONVEX MIRRORS, the im. ages of objects invariably appear beyond the mirror; in other words, they are virtual images. Secondly, they are seen in 228 NATURAL PHILOSOPHY. their natural position, and, Thirdly, they are smaller than the objects themselves; the further the object is from the mirror, and the less the radius of the mirror, the smaller the image will be. If the object be very remote, its image will be in the virtual focus of the mirror. 837. Secondly, in regard to CONCAVE MIRRORS. (1.) The image of an object very remote from a concave mirror, as that of the sun, will be in the focus of the mirror, and the image will be extremely small.* (2.) Every object which is at a distance from the mirror greater than its centre produces an image between this point and the focus smaller than the object itself, and in an inverted position. (3.) If the object be at a distance from the mirror equal to the length of its radius, then the image will be at an equal distance from the mirror, and the dimensions of the image will be the same as those of the object, but its position will be inverted. (4.) If the object be between -the focus and the centre of curvature, the image will be inverted, and its size will much exceed that of the object. These four varieties of inverted images, produced by the reflection of the rays of light from concave mirrors, are sometimes called "physical spectra." * This is the manner in which concave mirrors become burning-glasses The rays of the sun fall upon them parallel [see No. 835], and they are all reflected into one point, called the focus, where the light and heat are as much greater than the ordinary light and heat of the sun as the area of the mirror is greater than the area of the'focus. It is related of Archimedes, that he employed burning-mirrors, two hundred years before the Christian era, to destroy the besieging navy of Marcellus, the Roman consul. His mirror was, probably, constructed from large numbers of flat pieces. M. de Vilette constructed a burning-mirror in which the area of the mirror was seventeen thousand times greater than the area of the focus. The heat of the sun was thus increased seventeen thousand times. M. Dufay made a concave mirror of plaster of Paris, gilt and burnished, twenty inches in diameter, with which he set fire to tinder at the distance of fifty feet. But the most remarkable thing of the kind on record is the compound mirror constructed by Buffon. He arranged one hundred and sixty-eight 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 two hundred and nine feet, to melt lead at a hull dred feet, and silver at fifty feet. OPTICS. 229 The existence and position of these spectra may easily be shown experimentally thus: Ea~periment.- Hold a candle opposite to a concave mirror, at the distances named in the last four paragraphs respectively. The spectrum can, in each case, be received on a white screen, which must be placed at the prescribed distance from the mirror. Different optical instruments, especially reflecting telescopes, exhibit the application of these spectra. (5.) If a luminous body, as, for instance, the flame of an argand lamp, or a burning coal, be placed in the focus of a concave mirror, no image will be produced, but the whole surface of the mirror will be illuminated, because it reflects in parallel lines all the rays of light that fall upon it. This may be made the subject of an experiment so simple as not to require further explanation. The reflectors of compound microscopes, magic lanterns and lighthouses, by means of which the light given by the luminous body is increased and transmitted in some particular direction that may be desired, are illustrations of the practical application of this prin3iple. (6-.) Lastly, place the object between the mirror and the focus, and the image of the object will appear behind the mirror. It will noi be inverted, but its proportions will be enlarged according to the proximity of the object to the focus. It is this circumstance that gives to concave mirrors their magnifying powers, and, because by collecting the sun's rays into a focus they produce a strong heat, they are called burning-mirrors. 838. MEDIA, OR MEDIUMS, AND REFRACWhat is a Me- TION.- A Medium,* in Optics, is any subdiurT in Optics? stance, solid or fluid, through which light can pass. WIhat is refrac- 839. When light passes in an oblique tion? direction from one medium into another, it is turned or bent from its course, and this is called refrac* The proper plural of this word is media, although mediums is frequently used. 20 230 NATURAL PHILOSOPHY. tionz. The property which causes it is called refrangibility. 840. DIoPTRICS.- That part of the sciWhat is Dioptrics - ence of Optics which treats of refracted light is called Dioptrics. ~W~lhat is meant 841. A medium, in Optics, is called dense or by a denser and rare according to its refractive power, and not rarer medium according to its specific gravity. Thus, alcohol, in Optics.~ and many of the essential oils, although Qf less specific gravity than water, 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. 842. There are three fundamental laws of What are the fundamental Dioptrics, on which all its phenomena delaws of Diop- pend, namely: trics? (1.) 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. (2.) When light passes in an oblique direction, from a rarer to a denser medium, it will be turned from its course, and proceed through the denser medium less obliquely, and in a line nearer to a perpendicular to its surface. (3.) When light passes from a denser to a rarer medium in an oblique direction, 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. 843. In Fig. 130, the line A B represents a Explain Fig. ip30.) Fig. ray of light passing from air into water, in a perpendicular direction. According to the first OPTICS. 231 law stated above, it will continue on in the Fig. 130. same line through the denser medium to E. If the ray were to pass upward through the denser medium, the water, in the same per-, pendicular direction to the air, by the same law it would also continue on in the same straight line to A. E R D 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 A B E than D is. Again, if the ray proceed from the denser medium, the 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 from the perpendicular E B A than G is. The refraction is more or less in all In what proporion is rerac- cases in proportion as the rays fall more or less tion in all cases? obliquely on the refracting surface. 844. From what has now been stated with When are we in danger *of mis regard to refraction, it will be seen that many danger of mis-. takingthe depth interesting facts may be explained. Thus, an of water, and oar, or a stick, when partly immersed in water, why? 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. For the same reason, when we look obliquely upon a body of water it appears more shallow than it really is. But, when we look perpendicularly downwards, we are liable to no such deception, because there will be no refraction. 845. Let a piece of money be put into a cup or a bowl, and'the 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, keeping the eye 232 NATURAL PHILOSOPHY. directed to the same spot, let the cup be filled with water, —the money will become distinctly visible. Why do we, not 8j846. The refraction of light prevents our see the sun, moon seeing the heavenly bodies in their real situaandstars, in their tion. true places? The light which they send to us is refracted in passing through the atmosphere, and we see the sun, the stars, &c., in the direction of the refracted ray. In consequence of this atmospheric refraction, the sun sheds his light upon us earlier 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 twilight. The greater the density of the air, the higher is its refractive power, and, consequently, the. longer the duration of twilight. It is proper, however, here to mention that there is another reason, why we do not see the heavenly bodies in their true situation. Light, though it moves 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, consequently, in a situation which he abandoned eight minutes and a half before. The refraction of light does not affeict 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. 847. When a ray of light passes from What effect is produced when one medium to another, and through that light suffers two into the first again, if the two refractions be equal refractions? equal, and in opposite directions, no sensible effect will be produced. This explains the reason why the refractive power of flat windowglass produces no effect on objects seen through it. The rays suffer two. refractions, which, being in contrary directions, produce the same effect as if no refraction had taken place. 848. LENSES. -A Lens is a glass, which, What is a Lensform, causes the rays owing to its peculiar form, causes the rays OPTICS. 2.33 of light to converge to a focus, or disperses them, according to the laws of refraction. Explain the dif- 849. There are various kinds of lenses, ferent kinds of named according to their focus; but they lenses. are all to be considered as portions of the internal or external surface of a sphere. 850. A single Fig. 131. convex lens has A one side flat and the other convex; e as A, in Fig. 131. 851. A single concave lens is flat on one side and concave on the other, as B in Fig. 131. 852. A double convex lens is convex on both sides, as C, Fig. 131. A double concave lens is concave on both sides, as ID, Fig. 131. A meniscus is convex on one side and concave on the other, as E, Fig. 131. What is the 853. The word meniscus is derived from the meaning of a Greek language, and means literally a little Meniscus? 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. What is the axis 854. The axis of a lens is a line passing of a lens? through the centre: thus F G, Fig. 131, is the axis of all the five lenses. 20* 234 NATURAL PHILOSOPHY. 85$. The peculiar form of the various lenses? kinds of lenses causes the light which passes through them to be refracted from its course according to the laws of Dioptrics. It will be remembered that, according to these laws, light, in passing from a rarer to a denser medium, is refracted towards the perpendicular; and, on the contrary, that in passing from a denser to a rarer medium it is refracted further How must we from the perpendicular. In order to estimate estimate the effect of a lens? 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 attentive 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 convex and con- focus, and magnify objects at a certain discave lenses re- tance. spectively? (2.) That concave lenses disperse the rays, and diminish objects seen through them. Vhat is the fo- 856. The focal distance of a lens is the cal distance of distance from the middle of the glass to the a lens? 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. 857. When parallel rays * fall on a conWhat ralys will pass through a vex lens, those only which fall in the direclens without re- tion of the axis of the lens are perpendicular fractions?:7 to its surface, and those only will continue % The rays of the sun are considered parallel at the surface of the earth. They are not so in reality, but, on account of the great distance of that luminary, their divergency is so small that it is altogether inappreciable. OPTICS. 235 on in a straight line through the lens. The other rays, falling obliquely, are refracted towards the axis, and will meet in a focus. 858. It is this property of a convex lens On whlat prin- which gives it its power as a burning-glass, or ciple are sunglasses, or sun-glass. All the parallel rays of the sun burning-glasses, which pass through the glass are collected toconstructed? gether in the focus; and, consequently, the heat at the focus is to the common heat of the sun 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. 859. 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 two hundred and twelve pounds, made by Mr. Parker, in England, melted the most refractory substances. Cornelian was fused in seventy-five seconds, a crystal pebble in six seconds, and a piece of white agate in thirty seconds. This lens was presented by the King of England to the Emperor of China. 860. If a convex lens have its sides ground What is a Mul- down into several fiat surfaces, it will present tiplying-glass? as many images of an object to the eye as it has flat surfaces. It is then called a Multiplying-glass. Thus, if one lighted candle be viewed through a lens having twelve flat surfaces, twelve candles will be seen through the lens. The principle of the multiplying-glass is the same with that of a convex or concave lens. 861. The following effects result from the laws of refraction 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.) Diverging rays will be made to diverge less, to become par 236 NATURAL PHILOSOPHY. allel, or to converge, according to the degree of divergency before refraction, or the convexity of the surface. (3.) Converging rays towards the centre of convexity will suffei no refraction. (4.) Rays converging to a point beyond the centre of convexity will be made more converging. (5.) Converging rays towards a point nearer the surface than the centre of convexity will be made less converging by refraction. [When the rays proceed out of a denser into a rarer medium, the reverse occurs in each case.] 862. 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 suffer 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 converging, parallel, or diverging, according to their degree of convergency before refraction. 863. 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 in 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, having a flat surface. [When the rays proceed out of a denser into a rarer medium, the reverse takes place in each case.] 864. Double convex, and double concave What kinds of glasses are used glasses, or lenses, are used in spectacles, to in spectacles, remedy the defects of the eye: the former, and for what purpose? when by age it becomes too flat, or loses a What kinds of portion of its roundness: the latter, when glasses are generally worn be by any other cause it assumes too round a old petrsons? form, as in the case of short-sighted (or, as What kind by young? Y they are sometimes called, near-sighted) persons. Convex glasses are used when the eye is too flat, and concave glasses when it is too round. These lenses or glasses are generally numbered, by opticians, according to their degree 6f. 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. OPTICS. 237 865. THE EYE. - The eyes of all animals are constructed on the same principles, with such modifications as are necessary to adapt them to the habits of the animal. The knowledge, therefore, of the construction of the eye of an animal will give an insight of the construction of the eyes of all. 866. The eye is composed of a number of of what is the eye comrn- coats, or coverings, within which are enclosed posed?- a lens, and certain humors, in the shape and performing the office of convex lenses.* What are the different 867. The different parts of the eye 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. E~plain 868. Fig. 132 represents Fig. 132. Fig. 132. a front view of the eye, in which a a represents the Cornea, or, as it is commonly called, the white of the eye; e e is the Iris, having a circular a opening in the centre, called the pupil, o e) a p, which contracts in a strong light, and expands in a faint light, and thus regilates the quantity which is admitted Fig. 133.,o the tender parts in the interior of the eye. 7b F.xplain 869. Fig. 133 rep- L Fig. 133. resents a side view of l k 3 the eye, laid open, in which b b represents the cornea, e e the iris,. h d d the pupil,ffthe aqueous hu- i \T( mor, g g the crystalline lens, h h ~ The following description of the eye is taken principally from Paxton's Introduction to the Study of Anatomy, edited by Dr. Winslow Lewis, of this eity. 238 NATURAL oPHILOSOPHY. the vitreous humor, i i i i i the retina, c c the choroid, a a a a a the sclerotica, and n the optic nerve. Describe the 870. The Cornea forms the anterior portion Cornea. 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 different periods of life. As it covers the pupil and the iris, it protects 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 finelypolished surface, and causes the brilliancy of the eye. Describe the 871. The Iris is so named from its being Iris. 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 involuntarily enlarges or diminishes. 872. It is on the color of the iris that Vhat causes a the color of the eye depends. Thus a person person's eyes to beblack, blue or iS said to have black, blue, or hazel eyes, gray, 4c.?. according as the iris reflects those colors respectively. What is the 873. The Pupil is merely the opening in the Pupil? iris, through which the light passes to the lens behind. It is always circular in the b uman eye, 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. 874. In cats, and other animals which are said How can some animals to see in the dark, the power of dilatation and consee in the traction is much greater -;it is computed that their dark? 2 pupils may receive one hundred times more light OPTICS. 239 at one time than at another. That light 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. When we come from a dark place into a strong light, our eyes suffer pain, because the pupil, being expanded, admits a larger quantity of light to rush in, before it has had time to contract. And, when we go from a strong light into a faint one, we at first imagine ourselves in darkness, because the pupil is then contracted, and does not instantly expand. Describe the 875. The Aqueous Humor is a fluid as clear Aqueous Hu- as the purest water. In shape it resembles a mo'. meniscus, and, being situated between the cornea and the crystalline lens, it assists in collecting and transmitting the rays of light from external objects to that lens. Wrhat is the 876. The Crystalline Lens is a transparent Crystalline body, in the form of a double convex lens, Lens? placed between the aqueous and the 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. What is tlhe 877. The Vitreous Humor (so called from its Vitreous Hu-resemblance to melted glass) is a perfectly mor? transparent mass, occupying the globe of the eye., Its shape is like a meniscus, the convexity of which greatly exceeds the concavity. 878. In Fig. 134 the shape of the Fig. 134. aqueous and vitreous humors and the crystalline lens is presented. A is the aqueous ___-a humor, which is a meniscus, B the crystal- A l B line lens, which is a double convex lens, and C the vitreous humor, which is also a meniscus, whose concavity has a smaller radius than its convexity. 240 NATURAL PHILOSOPHY. What is the 879. The Retina is the seat of vision. The Retina? 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. What is the 880. The Choroid is the inner coat or coverChoroid? ing of the eye. Its outer and inner surface is covered with a substance called the pi gmentum nigrum (or black paint). Its office is, apparently, to absorb the rays of light immediately after they have fallen 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. Describe the 881. The Sclerotica is the outer coat of the Sclerotica. eye. It derives its name from its hardness. Its office is to preserve the globular 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 in 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. 882. The Optic Nerve is the organ which pat is thi e carrie s the impressions 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. What optical 883. The eye is a natural camera obscura instrument does the ye [see No. 805], and the images of all objects resemble? seen by the eye are represented on the retina in the same manner as the forms of external objects are delineated in that instrument. Explain 884. Fig. 135 represents only those parts of the eye Fig. 135. which are most essential for the explanation of the oivrcosO 241 Fig. 15. phenomenon of 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 crystalline lens d, by which they are made to converge on the retina, where they form the inverted imagef e. Holw is the 885. The convexity of the crystalline humor is convexity of increased or diminished by means of two muscles, the crystalline to which it is attached. By this means, the focus lens altered, andfor what of the rays which pass through it constantly falls purpose? on the retina; and an equally distinct imageis formed, both of distant objects and those which are near. How can you 886. Although the image is inverted on the reaccount for tina, we see objects erect, because all the images the apparent position of formed on the retina have the same relative posiobjects? tion which the objects themselves have; and, as the rays all cross each other, the eye is directed upwards to receive the rays which proceed from the upper part of an object, and downwards to receive those which proceed from the lower part. 887. A distinct image is also formed on the reWhy do we not see double tina of each eye; but, as the optic nerves of the with two eyes? 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 description now 21 2442 NATURAL PHILOSOPHY. given of the eye, it may be seen what are the defects wnich are remedied by the use of concave and convex lenses, and how the use of these lenses remedies them. Wlhat defects 888. When the crystalline humor of the eye is of the eye are too round, the rays of light which enter the eye spectacles designed to converge to a focus before they reach the retina, remedy? 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 crystalline lens, brings the rays to a focus on the retina, and produces distinct vision. 889. The eye is also subject to imperfection by For what defects of the reason of the humors losing their transparency, eye is there either by age or disease. For these imperfections no remedy? 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 puncture 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 immediately diminishes in size, and total blindness is the inevitable result. WMhat is a 890. A single microscope consists simply of single micro- a convex lens, commonly called a magnifyingscope? glass; in the focus of which the object is placed, and through which it is viewed. 891. By means of a microscope the rays of light from an object are caused to diverge less; so that when they enter the pupil of the eye they fall parallel on the crystalline lens, by which they are refracted to a focus on the retina. Explain 892. Fig. 136 represents a convex lens, or single ig. 136. microscope, C P. The diverging rays from the object A B are refracted in their passage through the lens C P, and OPTICS. 243 made to fall parallel"on Fig. 136. the crystalline lens, by which they are refracted to a focus on the retina Ri R; and the image is' thus magnified, because the divergent rays are B collected by the lens and a? carried to the retina. What glasses 893. Those lenses or microscopes which have the have the shortest focus have the greatest magnifying power; greatest magnifying and those which are the most bulging or convex powers? have the shortest focus. Lenses are made small because a reduction in size is necessary to an increase of curvature. What is a 894. A double microscope consists of two double micro- convex lenses, by one of which a magnified scope? image is formed, and by the other this image is carried to the retina of the eye. Explain 895. Fig. 137 represents the effect produced by the Fig. 137. lenses of a double microscope. The rays which diverge from the object A B are collected by the lens L M (called the object-glass, because it is nearest to the object), and form an Fig. 137. inverted magnified image at C D. The rays which diverge 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 244 NATURAL PHILOSOPHY. the single microscope, and forms a still more magnified image on the retina R R. What is the 896. The solar microscope is a microscope solar micro- with a mirror attached to it, upon a movable scope. joint, which can be so adjusted as to receive the sun's rays and reflect them upon the object. It 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. 897. 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 object; the image is then formed by light reflected from the object, instead of being transmitted through it. 898. The magnifying power of a single miHowis thepma- croscope is ascertained by dividing the least nfying power of single and distance at which an object can be distinctly coublpe micro- seen by the naked eye by the focal distance of scopes ascertained? the lens. This, in common eyes, is about seven inches. Thus, if the focal distance of a lens be only J of an inch, then the diameter of an object will be magnified 28 times (because 7 divided by: is the same as 7 multiplied by 4), and the surface will be magnified 784 times. The magnifying power of the compound microscope is found in a similar manner, by ascertaining the magnifying power, first of one lens, and then of the other. The magnifying power of the solar microscope is in proportion as the distance of the image from the object-glass is greater than that of the object itself from it. Thus, if the distance 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. OPTICS. 246 -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, it the object be placed near the focus, the image will be magnified. WhatistheMag. The Magic Lantern is an instrument conic Lantern? structed on the principle of the solar microscope, but the- light is supplied by a lamp instead of the sun. 899. The objects to be viewed by the magic lantern are generally painted with transparent colors, on glass slides, which are Fig. 138. 0 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 slides through the lenses, by means of which a magnified image is thrown upon the wall, on a white surface prepared to receive it. Fig. 138 represents the magic lantern. The Describe Fig. 1 rays of light from the lamp are received upon the concave mirror e, and reflected 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 slide, inserted at b. The rays from the illuminated object at b are carried divergent through the lens a, forming an image on the screen at f. The image will increase or diminish in size, in proportion to the distance of the screen from the lens a. 21* 246 NATURAL PHILOSOPHY. 900. DISSOLVING VIEWS. The exhibition How are "Dis- called " Dissolving Views " is made by means solving Views" represented.? of two magic lanterns of equal power, so as to throw pictures of. the same magnitude in the same position on the screen. By the proper adjustment of sliding tubes and shutters, one picture on the screen is made brighter while the other becomes fainter, so that the one seems to dissolve into the other. In the hands of a skilful artist * this is an exhibition of the most pleasing kind. 901. TELESCOPES. —A Telescope is an IArhat is a Tel escoTpe? instrument for viewing distant objects, and causing them to appear nearer to the eye. How are tele- 902. Telescopes are constructed by placing scopes construct- lenses of different kinds within tubes that slide ed? within each other, thus affording opportunity of adjusting the distances between the lenses within. 903. They are also constructed with mirrors, in addition to the lenses, so that, instead of looking directly at an object, the eye is directed to a magnified image of the object, reflected from a concave mirror. This has given rise to How many kinds the two distinctions in the kinds of telescopes of telescopes are in common use, called respectively the Refract. ing and the Reflecting Telescope. flow is the Re- 904. The Refracting Telescope is confracling Telescope construct- structed with lenses alone, and the eye is ed? directed toward the object itself. 905. The Reflecting Telescope is conHow does a Reflecting Tele- structed with one or more mirrors, in addi~ Mr. John A. Whipple, of this city, has given several exhibitions of this kind, with great success. A summer scene seemed to dissolve into the same scene in mid-winter; a daylight view was gradually made to faint successively into twilight and moonshine; and many changes of a most interesting nature showed how pleasing an exhibition might be made by a skilful combination of science and art OPTICS. 24!7 scopedifferfrom tion to the lenses; and the image of the a Refractinrg. a Refracting object, reflected from a concave mirror, is seen, instead of the object itself. 906. Each of these kinds of telescope has its respective advantages, but refracting telescopes have been so much improved that they have in some degree superseded the reflecting telescopes. Whlat is an 907. Among the improvements which have Achromatic T ble- been made in the telescope, may be mentioned, scope? as the most important, that peculiar construction of the lenses by which they are made to give a pencil of white light, entirely colorless. Lenses are generally faulty in causing the object to be partly tinged with some color, which is imperfectly refracted. The fault has been corrected by employing, a double object-glass, composed of two lenses of different refracting power, which will naturally correct each other. The telescopes in which these are used are called Achromatic. Common telescopes have a defect arising from the convexity of the object-glass, which, as it is increased, has a tendency to tinge the edges of the images. To remedy this defect, achromatic lenses were formed by the union of a convex lens of crown glass with a concave lens of flint glass. Owing to the difference of the refracting power of these two kinds of glass, the images lecamefreefromz color and more distinct; and hence the glasses which produce them were called Achromatic, that is,freefrom color. Lenses are also subject to another imperfection, called spherical aberration, arising from the different degrees of thickness in the centre and edges, which causes the rays that are refracted through them respectively, to come to different focuses, on account of the greater or less refracting power of these paits, consequent on their difference in thickness. To correct this defect, lenses have been constructed of gems and crystals, &c., which have a higher refractive power than glass, and require less sphericity to prlduce equal effects. What is the sitm- 908. The simplest form of the telescope conplest form of the sists of two convex lenses, so combined as to telescove? increase the angle of vision under which the 248 NATURAL PHILOSOPHY. object is seen. The lenses are so placed that the distance between them may be equal to the sum of their focal distances. VVhich is the Object-glass, and 909. The lens nearest to the eye is called which the Eye- the Eye-glass, and that at the other extremglass, of a telescope. ity is called the Object-glass. 910. Objects seen through telescopes of this How are objects construction (namely, with two glasses only) seen through telescopes of the are always inverted, and for this reason this simplest con- kind of instrument is principally used for asstruction? tronomical purposes, in which the inversion of the object is immaterial. Hence, this is also called the Nightglass. What is the daf 911. The common day telescope, or spyference between glass, is an instrument of the same sort, with a day and a the addition of two, or even three or four night telescope? glasses, for the purpose of presenting the object upright, increasing the field of visior, and diminishing the aberration caused by the dissipation ot' the rays. 912. Fig. 139 represents a night-glass, or Ezpilain Figo. astronomical telescope. 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 from a very Fig. 139. K O C ry B N distant body, as a star, and which may be considered 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 niany times as the focal length of the eye-glass is contained in the focal length of the object-glass. Thus, if the focal length of the eye-glass D 0 be contained 100 times in that of OPTICS. 249 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 O, and the ray N B in the direction D P. 913. Fig. 140 represents a day-glass, or terExplain Fg restrial telescope, commonly called a spy-glass. This, likewise, consists of a tube A B H G, containing four lenses, or glasses, namely, A B, C D, E F, and G H. The lens A B is the object-glass, and G II the eye-glass. The two additional eye-glasses, E F and C D, are of the same size and shape, and placed at equal distances from each other Fig. 140.,I G I C - - in such a manner that the focus of the one meets that of the next lens. These two eye-glasses E F and C D are introduced for the purpose of collecting the rays proceeding from the inverted image M N, into a new upright image, between G H and E F; and the image is then seen through the last eye-glass G H, under the angle of vision P 0 Q. Opera Glasses are constructed on the prineraGlassres O- ciple of the refracting telescope. They are, in fact, nothing more than two small telescopes, united in such a manner that the eye-glasses of each may be moved together, so as to be adjusted to the eyes of different persons. Of what does the 914. THE REFLECTING TELESCOPE. -The ReReflecting Tel- fleeting Telescope, in its simplest form, conescope consist? sisted of a concave mirror and a convex eye-glass. The mirror throws an image of the object, and the eye-glass views that image under a larger angle of vision. 26,50 NATURAL PHILOSOPHY. This instrument was subsequently improved by Newton, and since him by Cassegrain, Gregory, Hadley, Short, and the Herschels. 915. Fig. 141 represents the Gregorian TExplain Fig. Telescope. It consists of a large tube, containing two concave metallic mirrors, and two plano-convex eye-glasses. The rays from a distant object are received through the open end of the tube, and proceed from r r Fig. 141. A. B to r r, at the large mirror A B, which reflects them to a focus at g, whence they diverge to the small mirror C, which reflects them parallel to the eye-glass F, through a circular aperture in the middle of the mirror A B. The eye-glass F collects those reflected rays into a new image at I, and this image is seen magnified through the second eye-glass G. It is thus seen that the mirrors bring the object near to the eye, and the eye-glasses magnify it. Reflecting telescopes are attended with the advantage that they have greater magnifying power, and do not so readily decompose the light. It has already been stated that the improvements in refractors have given them the greater advantage. How does the 916. The Cassegrainian telescope differs front Cassegrain- that which has been described, in having the ian telescope differfrom smaller mirror convex. This construction is atthe Gregorian? tended with two advantages; first, it is superior in distinctness of its images, and, second, it dispenses with the necessity of so long a tube. OPTICS. 251 917. The telescopes of Herschel and of Lord MYrat peculiaritics are Rosse dispense with the smaller mirror. This is there in the done by a slight inclination of the large mirror, so telescopes of Herschel and as to throw the image on one side, where it is viewed the Earl of by the eye-glass. The observer sits with his back Rosse? towards the object to be viewed. Herschel's gigantic telescope was erected at Slough, near Windsor, in 1789. The diameter of the speculum or mirror was four feet, and the mirror weighed 2118 pounds; its focal distance was forty feet. 918. The telescope of Lord Rosse is the largest that has ever been constructed. The diameter of the speculum is six feet, and its focal distance fifty-six feet. The diameter of the tube is seven feet, and the tube and speculum weigh more than fourteen tons. The cost of the instrument was about sixty thousand dollars. The telescope lately imported for Harvard University is a refractor. It is considered one of the best instruments ever constructed. What is 919. CHROMATICS. — Thatpart of the science Chromatics? of Optics which relates to colors is called Chromatics. oJf,lht is 920. Light is not a simple thing in its ight composed? nature, but is composed of rays of different colors, each of which has different degrees of refrangibility, ind has also certain peculiarities with regard to reflection. 921. Some substances reflect some of the Of what color are bodies rays that fall upon them and absorb the others, composed? some appear to reject all of them and absorb none, while others again absorb all and reflect none. Hence, bodies in general have no color of themselves, independent of light, but every substance appears of that color which it reflects. 922. White is a due mixture of all colors in What are white and nice and exact proportion. When a body reblack? flects all the rays that fall upon it, it will appear white, and the purity of the whiteness depends on the perfectness of the refection. 252 NATURAL PHILOSOPHY. 923. Black is the deprivation of all color, and, when a body reflects none of the rays that fall upon it, it will appear black. 924. Some bodies reflect two or more colors either partially or perfectly, and they therefore present the varied hues which we perceive, formed from the mixture of rays of different colors.* What are the 925. The colors which enter into the composicogtos of tion of light, and which possess different degrees of refrangibility, are seven in number, namely. red, orange, yellow, green, blue, indigo, and violet. What is a 926. A Prism is a solid, triangular piece of Prism? highly-polished glass. 927. A prism which will answer the same purpose as a solid onc may be made of three pieces of. plate glass, about six or eight inches long and two or three broad, joined together at their edges, and made water-tight by putty. The ends may be fitted to a triangular piece of wood, in one of which an aperture is made by which to fill 0 When the eye has become fatigued by gazing intently on any object, of a red or of any other color, the retina loses, to some extent, its sensitiveness to that color, somewhat in the same manner that the ear is deafened for a moment by an overpowering -sound. If that object be removed and another be presented to the eye, of a different color, into the composition of which red enters, the eye, insensible to the red, will perceive the other colors, or the compound color which they would form by the omission of the red, and the object thus presented would appear of that color. The truth of this remark may be easily tested. Fix the eye intently for some time on a red wafer on a sheet of white paper. On removing the wafer, the white disk beneath it will transmit all the colors of white light; but the. eye, insensible to the red, will perceive' the blue or green colors at the other end of the spectrum, and the other spot where the red wafer was will appear of a bluish-green, until the retina recovers- its sensibility for red light. The colors thus substituted by the fatigued eye are called the accidental color. The accidental colors of the seven prismatic colors, together with black and white, are as follows: Accidental Color. Red.....................Bluish Green. Orange..................Blue. Yellow........... Indigo. Green.......... Violet reddish. Indigo...... Orange red. Vialet. Orange yellow. Black......... White. White..................lauk OPTICS. 253 it with water, and thus to give it the appearance and the refractive power of a solid prism. What effect 928. When light is made to pass through a has aprism prism, the different-colored rays are refracted on the light or separated, and form an image on a screen or that passes through it? wall, in which the colors will be arranged in the order just mentioned. Explain 929. Fig. 142 represents rays of light passing from Fig. 142. the aperture, in a window-shutter A B, through the prism P. Instead of continuing in a straight course to E, and there forming an image, they will be refracted, in their passage through the prism, and form an image on the screen C D. But, Fig. 142. A violet.... indigo. Blue... Yellow Orange I Red.... D E B as the different-colored rays have different degrees of refrangi. bility, those which are refracted the least will fall upon the lowest part of the screen, and those which are refracted the most svill fall upon the highest part. The red rays, therefore, sufferng the smallest degree of refraction, fall on the lowest part of he screen, and the remaining colors are arranged in the order if their refraction. 930. It is supposed that the red rays are refracted the least, on ccount of their greater momentum; and that the blue, indigo and violet, are refracted the most, because they have the least momentum. the same reason, it is supposed, will account for the red appearance of the sun through a fog, or at rising and setting. The increased quantity of the atmosphere which the oblique rays must traverse, and its being loaded with mists and vapors. which are usually formed at those times, prevents the other rays from reaching us. A similar reason will account fur the blue appearance of the sky, 22 254 NATURAL PHILOSOPHtY. As these rays have less momentum, they cannot traverse the atmomphere so readily as the other rays, and they are, therefore, reflected hack to our eyes by the atmosphere. If the atmosphere did not reflect any rays, the skies would appear perfectly black. 931. If the colored rays which have been sepaHow can the rays refract- rated by a prism fall upon a convex lens, they ed by a prism will converge to a focus, and appear white. Hence be reignited? it appears that white is not a simple color, but is produced by the union of several colors. 932. The spectrum formed by a glass prism being divided into 360 parts, it is found that the red occupies 45 of those parts, the orange 27, the yellow 48, the green 60, the blue 60, the indigo 40, and the violet 80. By mixing the seven primitive colors in these proportions, a white is obtained; but, on account of the impurity of all colors, it will be of a dingy hue. If the colors were more clearly and accurately defined, the white thus obtained would appear more pure also. An experiment to prove what has just been said may be thus performed: Take a circular piece of board, or card, and divide it into parts by lines drawn from the centre to' the circumference. Then, having painted the seven colors in the proportions above named, cause the board to revolve rapidly around a pin or wire at the centre. The board will then appear of a white color. From this it is inferred that the whiteness of the sun's light arises from a due mixture of all the primary colors. 933. The colors of all bodies are either the simple colors, as refracted by the prism, or such compound colors as arise from a mixture of two or more of them. 934. From the experiment of Dr. Wollaston, What are the three simple it appears that the seven colors formed by the prism colors? may be reduced to four, namely, red, green, blue, and violet; and that the other colors are produced by combinations of these, but violet is merely a mixture of blue and red, and green is a mixture of blue and yellow. A better division of the simple colors is blue, yellow, and red. 935. Light is fumnd to possess both heat and chemical action. O)PTICS. 25 The prismatic spectrum presents some remarkable phenomena with regard to these qualities; for, while the red rays appear to be the seat of the maximum of heat, the violet, on the contrary, are the apparent seat of the maximum of chemical action. 936. Light, from whatever source it proceeds, is of the same nature, composed of the various-colored rays; and although some substances appear differently by candle-light from what they appear by day, this result may be supposed to arise from the weakness or want of purity in artificial light. 937. There can be no light without colors, and there can be no colors without ligh/st. 938. That the above remarks in relation to the colors of bodies are true, may be proved by the following simple experiment. Place a colored body in a dark room, in a ray of light that has been refracted by a prism; the body, of whatever color it naturally is, will appear of the color of the ray in which it is placed; for, since it receives no other colored rays, it can reflect no others. 939. Although bodies, from the arrangement of their particles, have a tendency to absorb some rays and reflect others, they are not so uniform in their arrangement as to reflect only pure rays of one color, and perfectly absorb all others; it is found, on the contrary, that a body reflects in great abundance the rays which determine its color, and the others in a greater or less degree in proportion as they are nearer or further from its color, in the order of refrangihility. Thus, the green leaves of a rose will reflect a few of the red rays, which will give them a brown tinge. Deepness of color proceeds frols a deficiency rather than an abundance of reflected rays. Thus, if a body reflect only a few of the green rays, it will appear of a dark green. The brightness and intensity of a color shows that a great quantity of rays are reflected. That bodies sometimes change their color, is owing to some chemical change which takes place in the internal arrangement of their parts, whereby they lone their tendency to reflect certain colors, and acquire the power of reflecting others. How is a rain- 940. The rainbow is produced by the rebowproduced? fraction of the sun's rays in their passage through a shower of rain; each drop of which acts as a prism in separating the colored rays as they pass through it. 941. This is proved by the following considerations: First, a rainbow is never seen except when rain is falling and the sun shining at the same time; and that the sun and the bow are always in opposite parts of the heavens; and, secondly, that the same appearance may be produced artificially, by means of water thrown into the air, when the spectator is placed in a proper 256 NATURAL PHILOSOPHY. position, with his back to the sun; and, thirdly, that a similar bow is generally produced by the spray which arises from large cataracts or waterfalls. The Falls of Niagara afford a beautiful exemplification of the truth of this observation. A bow is always seen there when the sun is clear and the spectator's back is towards the sun. 942. As the rainbow is produced by the refraction of the sun's rays, and every change of position is attended by a corresponding change in the rays that reach the eye, it follows that no two persons can see exactly the same rainbow, or, rather, the same appearance from the same bow. 943. POLARIZATION OF LIGHT.-The Polarization of Light is a change produced on light by the action of certain media, by which it exhibits the appearance of having polaritv, or poles possessing different. properties. This property of light was first discovered by Huygens in his investigations of the cause of double refraction, as seen in the Iceland crystal. The attention of the scientific world was more particularly directed to it by the discoveries of Malus, in 1810. The knowledge of this singular property of light has afforded an explanation of several very intricate phenomena in Optics, and has afforded corroborating evidence in favor of the undulatory theory; but the lilmits of this volume will not allow an extended notice of this singular property. 944. OF THE THERMAL, CHEMICAL, AND OTHER NON-OPTICAL EFFECTS OF LrGHT. - The science of Optics treats particularly of light as the medium of vision. But there are other effects of this agent, which, although more immediately connected with the science of chemistry, deserve to be noticed in this connexion. -945. The thermal effects of light, that is, its agency in the excitation of heat when it proceeds directly from the sun, are well known. But it is not generally known that these effects are extremnely unequal in the difierently colored rays, as they are refracted by the prism. It has already been stated that the red rays appear to possess the thermal properties in the greatest degree, and that in the other rays in the spectrum there is a decrease of thermal power towards the violet, where it ceases altogether. But, on the contrary, that the chemical agency is the most powerful in the violet, friom which it constantly decreases towards the red, where it ceases altogether. Whether these thermal and chemical powers exist in all light, from whatever source it is derived, remains yet to be ascertained. Tile chromatic intensity of the colored spectrum is greatest in the yellow, from whence it decreases both ways, terminating almost abruptly in the red, and decreasing by almost imperceptible shades towards the violet, where it becomes faint, and then wholly indistinct. Thus it appears that the greatest heating power resides where the chemical power is feeblest, and the greatest chemical OPTICS. 265 power where the heating power is feeblest, and that the optical power is the strongest between the other two. 946. The chemical properties of light are shown in this, that the light of the sun, and in an inferior degree that of day when the sun is hidden from view, is a imeans of accelerating chemical combinations and decompositions. The following experiment exhibits the chemical effects of light: Place. a mixture of equal parts (by measure) of chlorine and hydrogen gas in a glass vessel, and no change will happen so long as the vessel be kept in the dark and at an ordinary temperature; but, on exposing -it to the daylight, the elements will slowly combine and form hydrochloric acid; if the glass be set in the sun's rays, the union will be accompanied with an instantaneous detonation. The report may also be produced by transmitting ordinary daylight through violet or blue glass to the mixture, but by interposing a red glass between the vessel and the light all combination of the elements is prevented. What is 947. The chemical effects of light have recently meant by Pho- been employed to render permanent the images obtography, or tained by means of convex lenses. The art of thus HIeliography? fixing them is termed Photography, or Heliography. These words are Greek derivatives; the former meaning "writing or drawing by means of light," the latter, " writing or draw. ing by the aid of the sun." Who is the 948. The thode in which the process is performed author of Pho is essentially as follows: The picture, formed by a togrPhy? camera obscura, is received on a plate, the surface of tography 2? which has been previously prepared so as to make it as susceptible as possible of the chemical influence of light. After the lapse of a longer or shorter time, the light will have so acted on the plate that the various objects the images of which were projected upon it will. appear, with all their gradations of light and shade, most exactly depicted in black and white, no color being present. This is the process commonly known by the name of DIaguerreotype, from M. Daguerre, the author of the discovery. Since his original discovery. he has ascertained that by isolating an/d electrlifying the plate it acquires such a sensibility to the chelmical influence of light that oine-tenth of a second is a sufficient timie to obtain the requisite luminous impression for the formation of the picture. 949. The chemical effects of light are seen in the varied colors of the vegetable world. Vegetables which grow in dark places are either white or of a palish-yellow. The sunny side of fruits is of a richer tinge than that which grows in the shade. Persons whose daily employment keeps them much within doors are pale, and more or less sicklyj in consequence of such confinement. 22* 258 NATURAL PHILOSOPHY. From what has now been detailed with regard to the nature, the effects, and the importance of light, we may see with what reason the great epic poet of our language has apostrophized it in the words: " Hail, holy Light! offspring of Heaven, first born, Bright effluence of bright essence increate;" and why the author of the " Seasons " has in a similar manner addressed it in the terms: "Prime cheerer, Light! Of all material beings first and best! Efflux divine! Nature's resplendent robe! Without whose vesting beauty all were wrapt In unessential gloom; and thou, 0 Sun! Soul of surrounding worlds, in whom best seen Shines out thy Maker! may I sing of thee? " 950. ELE 4RICITY.- Electricity is the Wrhat is Electricity? name given to an imponderable agent which pervades the material world, and which is visible only in its effects. 951. It is exceedingly elastic, susceptible of sMhat are its high degrees of intensity, with a tendency to simplest effects? equilibrium unlike that of any other known agent. Its simplest exhibition is seen in the form of attraction and repulsion. 952. If a piece of amber, sealing-wax, or smooth glass, perfectly clean and dry, be briskly rubbed with a dry woollen cloth, and immediately afterwards held over small and light bodies, such as pieces of paper, thread, cork, straw, feathers, or fragments of goldleaf, strewed upon a table, these bodies will be attracted, and fly towards the surface that has been rubbed, and adhere to it for a' certain time. 953. The surfaces that have acquired this power of attraction are said to be excited; and the substances thus susceptible of be-ing excited are called electrics, while those which cannot be excited in a similar manner are called non-electrics. What are Mtie 954. The science of Electricity, therefore, electrical divis- divides all substances into two kinds, namely, ions of all sub- Electrics, or those substances which can be stances? excited, and Non-electrics, or those substances which cannot be excited. ELECTRICITY. 259 955. The word Electricity is derived from a Greek word, which signifies amber, because this substance was supposed to possess, in a remarkable degree, the property of producing the fluid, when excited or rubbed. The property itself was first discovered by Thales of Miletus, one of the seven wise men of Greece. The word is now used to express both the fluid itself and the science which treats of it, What are the 956. The nature of electricity is entirely pre vailing theo- unknown. Some philosophers consider it a ries of' electric- fluid; others consider it as two fluids of opposite qualities; and others again deny its materiality, and deem it, like attraction, a mere property of matter. The theory of Dr. Franklin was; that it is a single fluid, disposed to diffuse itself equally among all substances, and exhibiting its peculiar effects only when a body by any means becomes possessed of more or less than its proper share. That when any substance has more than its natural share it is positively electrified, and that when it has less than its natural share it is negatively electrified; that positive electricity implies a redundancy, and negative electricity a deficiency, of the fluid. The prevalent theory at the present day is that it consists of two fluids, bearing the names of positive and negative. 957. Professor Faraday has proposed a nomenclature of electricity, which has been adopted in some scientific treatises. From the Greek words 2,.Ef.rqolo, (electricity, or amber, from which it was first produced), and l;6s (a way or path), he formed the word electrodes, that is, ways or paths of electricity. The course of positive electricity he called the anode (from the Greek;,voios, an ascending or entering way), and the course of the negative electricity the cathode (from the Greek Y.,.9%oh;, a descending way, or path of exit). The terms positive and negative are, however, more frequently employed to designate the extremities of the channels through which electricity passes. Positive electricity is sometimes expressed by the term plus, or its character +-; and negative electricity by the term minus, or its character -. How may elec- 958. Electricity may be excited by sevcity be e eral modes -as, 1st, byfriction, whence it is called Frictional Electricity; 2dly, by chemical action, called, from its discoverers, Galvanic, or- -Voltaic Electricity; 3lly, by the action of hc t, whence it is called 260 NATURAL PHILOSOPHY. Tlzermo-Electricity; 4thly, by Magnetism. Frictional Electricity forms the subject of that branch of Electricity usually treated under the head of Natural Philosophy; Electricity excited by chemical action forms the subject of Galvanism; and Electricity produced by the agency of heat, or by Magnetism, is usually considered in connection with the subject of Electro-Magnetism. The intimate connection between these several subjects shows how close are the links of the chain by which all the departments of physical science are united. 959. The electric fluid is readily commuWhat is meant by a Conductor nicated from one substance to another. Some and a Non-con- substances, however, will not allow it to pass ductor of electricuiy elec through or over them, while others give it a free passage. Those substances through which it passes without obstruction are called Conductors, while those through which it cannot readily pass are called Non-conductors; and it is found, by experiment, that all electrics* are non-conductors, and all non-electrics are good conductors of electricity. 960. The following substances are electrics, or non-conductors of electricity; namely, Atmospheric air (when dry), Feathers, Glass, Amber, Diamond, Sulph-;r, All precious stones, Silk, All gums and resins, Wool, Tfie oxides of all metals, Hair, Beeswax, Paper, Sealing-wax, Cotton. All these substances must be dry, or they will become more or klss conductors. * The terms"electica; " sad "non-eleotrica" have fallen into uisse; ELECTRICITY. 261 961. The following substances are non-electrics, or conductors of electricity; namely, All metals, Living animals, Charcoal, Vapor, or steam. 962. The following are imperfect conductors (that is, they conduct the electric fluid, but not so readily as the substances above mentioned); namely, Water, Common wood, Green vegetables, Dead animals, Damp air, Bone, Wet wood, Horn, &c. All substances containing moisture. When is a con- 963. When a conductor is surrounded on iuctor said to all sides by non-conducting substances, it is,e insulated? said to be insulated. 964. As glass is a non-conducting substance, any conducting substance:surrounded with glass, or standing on a table or stool with glass legs, will be insulated. 965. As the air is a non-conductor when dry, a substance which rests on any non-conducting substance will be insulated, unless it communicate with the ground, the floor, a table, &c. 966. When a communication is made beHow is a conuowricharcond tween a conductor and an excited surface, the electricity from the excited surface is immediately conveyed by the conductor to the grouind; but, if the conductor be insulated, its whole surface will become electrified, and it is said to be charged. PWhat is the 967. The earth may be considered as the grand reservoir principal reservoir of electricity; and when a of electricity? communication exists, by means of any conducting substance, between a body containing more than its natural share of the fluid and the earth, the body will immediately lose its redundant quantity, and the fluid will escape to 262 NATURAL PHILOSOPHY. the, earth. Thus, when a person holds a metallic tube to an excited surface, the electricity escapes from the surface to the tube, and passes from the tube through the person to the floor; and the floor being connected with the earth by conducting substances, such as the timbers, &c., which support the building, the electricity will finally pass off, by a regular succession of conducting substances, from the excited surface to the earth. But, if the chain of conducting substances be interrupted,-that is, if any non-conducting substance occur between the excited surface and the course which the fluid takes in its, progress to the earth, — the conducting substances will be insulated, and become charged with electricity.. Thus, if an excited surface be connected by a long chain to a metallic tube. and the metallic tube be held by a person who is standing on a stool with glass legs, or on a cake of sealing-wax, resin, or any other non-conducting substance, the electricity cannot pass to the ground, and the person, the chain and the tube, will all become electrified. What is the simWhlest mohe of 968. The simplest mode of exciting elecexciting electric- tricity is by friction. ity? Thus, if a thick cylinder of sealing-wax, or sulphur, or a glass tube, be rubbed with a silk handkerchief, a piece of clean flannel, or the fur of a quadruped, the electric fluid will be excited, and may be communicated to other substances from the electric thus excited. Whatever substance is used, it must be perfectly dry. If, therefore, a glass tube be used, it should previously be held to the fire, -and gently warmed, in order to remove all moisture from its surface. 969. The electricity excited in glass is by Vitreous and called the Vitreous or positive electricity; Resinous elec- and that obtained from sealing-wax, or other tricity? resinous substances, is called Resinous, or negative electricity. ELEOTRICITY. 263 W970. The vitreous and resinous, or, in What are the effects when a other words, the positive and hegative elecbody is charged tricities, always accompany each other; for, with either kind of electricity? if any surface become positive, the surface with which it is rubbed will become negative, and if any surface be made positive, the nearest conducting surface will become negative; and, if positive electricity be communicated to one side of an electric, (as a pane of glass, or a glass vial), the opposite side will become negatively electrified, and the plate or the glass is then said to be charged. 971. When one side of a metallic, or other conductor, receives the electric fluid, its whole surface is instantly pervaded; but when an electric is presented to an electrified body, it becomes electrified in a small spot only. What is the 972. When two surfaces oppositely electrified are effect when united, their powers are destroyed; and, if their two bodies oppositely union be made through the human body, it proelectrified are duces an affection of the nerves, called an electric united? shock. What is the law of 973. Similar states of electricity repel electrical attraction each other; and dissimilar states attract and repulsion? each other. Thus, if two pith-balls, suspended by a silk thread, are both positively or both negatively electrified, they will repel each other; but if one be positively and the other negatively electrified, they will attract each other. What is the 974. The Leyden jar is a glass vessel used Leydenjar? for the purpose of accumulating the electrid fluid, procured from excited surfaces. Explain 975. Fig. 143 represents a Leyden jar. It Fig. 143. is a glass jar, coated both on the inside and the outside with tin-foil, with a. cork, or wooden stopper, through 264 NATURAL, PHILOSOPHY which a metallic rod passes, terminating upwards m a brass knob, and connected by means of a wire, at the other Fig. 143. end, with the inside coating of the jar. The coating extends both on the inside and outside only to within two or three. inches of the top of the jar. Thus prepared, when an excited surface is applied to the brass knob, or connected with it by any conducting surface, it parts with its electricity, the fluid enters i the jar, and the jar is said to be charged. Wihen a jar is 976. When the Leyden jar is II icsted whectiee charged, the fluid is contained on the ity? surface of the glass. The coating serves only as a conductor to the fluid; and, as this conductor within the glass is insulated, the fluid will remain in the jar until a communication be made, by means of some conducting substance, between the inside and the outside coating of the jar. If then a person apply one hand or finger to the brass knob, and the other to the outside coating of the-jar, a communication will be formed by means of the brass knob with the inside and outside of the jar, and the jar will be discharged. A vial or jar that is insulated cannot be charged. What is an Elec- 977. An electrical battery is composed of trical Battery? a number of Leyden jars connected together. The inner coatings of the jars are connected together by chains or metallic bars attached to the brass knobs of each jar; and the outer coatings have a similar connection established by placing the vials on a sheet of tin-foil. The whole battery may then be charged like a single jar. For the sake of convenience in discharging the battery, a knob connected with the tin-foil on which the jars stand projects from the bottom of the box which contains the jars. What is the joint- 978. The jointed discharger is an instrued discharger? ment used to discharge a jar or battery. Explain Fig. 144 represents the jointed discharger. It Fig. 144. consists of two rods, generally of brass, terminating ELECTRICITY. 265 at one end in brass balls, and connected Fig. I" together at the other end by a joint, like that of a pair of tongs, allowing them to be opened or closed. It is furnished with a glass handle, to secure the person who holds it from the effects of a shock. When opened, one of the balls is made to touch the outside coating of the jar, or the knob connected with the bottom of the battery, and the other is applied to the knob of the jar or jars. A communication being thus formed between the inside and the outside of the jar, a discharge of the fluid will be produced. Where must 979. When a charge of electricity is to be a body be sent through any particular substance, the placed, in or- substance must form a part of the circuit of der to receive a charge of electricity; that is, it must be placed in such electricity? a manner that the fluid cannot pass from the inside to the outside surface of the jar, or battery, without passing through the substance in its passage. What effect have sharp 980. Metallic rods, with sharp points, metallic points? silently attract the electric fluid. If the balls be removed from the jointed discharger, and the two rods terminate in sharp points, the electricity will pass off silently, and produce but little effect. How may a 981. A Leyden jar, or a battery, may be silently Leyden jar or discharged by presenting a metallic point, even that battery be silently dis- of the finest needle, to the knob; but the point must charged? be brought slowly towards the jar. 982. It is on this principle that lightning-rods ciple are light- are constructed. The electric fluid is silently ning-rods drawn from the cloud by the sharp points on the constructed? rods, and is thus prevented from suddenly exploding on high buildings. W~hat is 983. Electricity of one kind or the other is genmeat:tby ~ erally induced in surrounding bodies by the vicin. 23 266 NATURAL PHILOSOPHY. ity of a highly-excited electric. This mode of comrnElectricity by municating electricity by approach is styled inducInduction? tion. 984. A body, on approaching another body powerfully electrified, will be thrown into a contrary state of electricity. Thus, a feather, brought near to a glass tube excited by friction, will be attracted to it; and, therefore, previously to its touching the tube, negative electricity must' have been induced in it. On the contrary, if a feather be brought near to excited sealing-wax, it will be attracted, and, consequently, positive electricity must have been induced in it before contact. What is 985. When electricity is communicated from Electricityby one body to another in contact with it, it is Transfer? called electricity by transfer. What zs an 986. The electrical machine is a machine Electrical Machine, andconstructed for the purpose of accumulating or on what prin- collecting electricity, and transferring it to other ciple is it constructed? substances. 987. Electrical Machines are made in various forms, but all on the same principle, namely, the attraction of metallic points. The electricity is excited by the friction of silk on a glass surface, assioted by a mixture or preparation called an amalgam, composed of mercury, tin, and zinc. That recommended by Singer is made by melting together one ounce of tin and two ounces of zinc, which are to be mixed, while fluid, with six ounces of mercury, and agitated in an iron or thick wooden box, until cold. It is then to be reduced to a very fine powder in a mortar, and mixed with a sufficient quantity of lard to form it into a paste. The glass surface is made either in the form of a cylinder or a circular plate, and the machine is called a cylinder or a plate machine, according as it is made with a cylinder or with a plate. Explain 988. Fig. 145 represents a plate electrical maFig. 145. ohine. A D is the stand of the machine, L L L L ELECTRICITY. 267 are the four glass legs, or posts, which support and insulate the parts of the machine. P is the glass plate (which in some machines is a hollow cylinder) from which the electricity is excited. and H is the handle by which the plate (or cylinder) is turned. R is a leather cushion, or rubber, held closely to both sides of the glass plate by a brass clasp, supported by the post G L, which is called the rubber-post. S is a silk bag, embraced by the same clasp that holds the leather cushion or rubber; and it is connected by strings S S S attached to its three other corners, and to the legs L L and the fork F of the prime conductor. 0 is the prime conductor, terminating at one end with a movable Fig. 145. brass ball, B, and at the other by the fork F, which has one prong on each side of the glass plate. On each prong of the fork there are several sharp points projecting towards the plate, to collect the electricity as it is generated by the friction of the plate against the rubber. V is a chain or wire, attached to the brass ball on the rubber-post, and resting on the table or the floor, designed to convey the fluid from the ground to the plate. When negative electricity is to be obtained, this chain is removed from the rubber-post and attached to the prime conductor, and the electricity is to be gathered from the ball on the rubberpost. Explain the 989. OPERATION OF THE MACHINE. - By turning operation of the handle H, the glass plate is pressed by the rub 268 NATURAL PIILOSOPIHY. the Electr. her. The friction of the rubber against the glass cal Iachine, plate (or cylinder) produces a transfer of the electric fluid from the rubber to the plate; that is, the cushion becomes negatively and the glass positively electrified. The fluid which thus adheres to the glass, is carried round by the revolution of the cylinder; and, its escape being prevented by the silk bag, or flap, which covers the plate (or cylinder) until it comes to the immediate vicinity of the metallic points on the fork F, it is attracted by the points, and carried by them to the prime conductor. Positive electricity is thus accumulated on the prime conductor, while the conductor on the rubber-post, being deprived of this electricity, is negatively electrified. The fluid may then be collected by a Leyden jar from the prime conductor, or conveyed, by means of a chain attached to the prime conductor, to any substance which is to be electrified. If both of the conductors be insulated, but a small portion of the electric fluid can be excited; for this reason, the- chain must in all cases be attached to the rubber-post, when positive electricity is required, and to the prime conductor when negative electricity is wanted. What is an 990. On the prime conductor is placed an Electrom- Electrometer, or measurer of electricity. It is eter, and on made in various forms, but always on the prinwhat principie is it con- ciple that similar states of electricity repel each structed? other. It sometimes consists of a single pith-ball, attached to a light rod in the manner of a pendulum, and behind is a graduated arc, or circle, to measure the repulsive force by degrees. Sometimes it is more simply made (as in the figure), consisting of a wooden ball mounted on a metallic stick, or wire, having two pith-balls, suspended by silk, hair, or linen threads. When the machine is worked, the pith-balls, being both similarly electrified, repel each other; and this causes them to fly apart, as is represented in the figure; and they will continue elevated until the electricity is drawn off. But, if an uninsulated conducting substance touch the prime conductor, the pith-balls will fall. The height ELOCCTRIrITY. 269 to which the balls rise, and the quickness with which they are elevated, afford some test of the power of the machine. This simple apparatus may be attached to any body the electricity of which we wish to measure. The balls of the electrometer, when elevated, are attracted by any resinous substance, and repelled by any vitreous substance that has j]een previously excited by friction. 991.. If an electric, or a non-conductor, be presented to the prime conductor, when charged, it will produce no effect on the balls; but if a non-electric, or any conducting substance, be presented to the conductor, the balls of the electrometer will fall. This shows that the conductor has parted with its electricity, and that the fluid has passed off to the earth through the substance, and the hand of the person presenting it. 992. An Electroscope is an instrument, of more Bennett's delicate construction, to detect the presence of Electroscope. electricity. The most sensitive of this kind of apparatus is that called Bennett's Gold-leaf Electroscope, improved by Singer. It consists of two strips of gold-leaf suspended under a glass covering, which completely insulates them. Strips of tin-foil are attached to the sides of the glass, opposite the gold-leaf, and when the strips of gold-leaf diverge, they will touch the tin-foil, and be discharged. A pointed wire surmounts the instrument, by which the electricity of the atmosphere may be observed. 993. An Electrophorus is a simple apparatus by which small portions of electricity may be generated by induction. It consists of a disc, or circular cake of resinous substance,* on which is laid a smaller circular disc of metal, with a glass handle. Rub the resinous disc with hair or the fur of some animal, and the metallic disc, being pressed down on the resin by the finger, may then be raised by the glass handle. It will contain a small portion of electricity, which may be communicated to the Leyden jar, and thus the jar may slowly be charged. * A mixture of Shell-lao, resin and Venice-turpentine, cast in a tin mould. 23* 270 NATURAL PHILOSOPHY. 994. EXPERIMENTS WITH THE ELECTRICAL MACHINE. -In peforming experiments with the Electrical Machine, great care must be taken that all its parts be perfectly dry and clean. Moisture and dust, by carrying off the electricity as fast as it is generated, prevent successful action.- Clear and cold weather should be chosen, if possible, as the machine will always perform its work better then. 995. When the machine is turned, if a person touch the prime conductor; the fluid passes-off through the person to the floor without his feeling it. But if he present his finger, his knuckle, or any part of the body, near to the conductor, without touching it, a spark will pass from the conductor to the knuckle, which will produce a sensation similar to the pricking of a pin or needle. 996. If a person stand on a stool with glass legs, or any other non-conductor, he will be insulated. If in this situation he touch the prime conductor, or a chain connected with it, when the machine is worked, sparks may be drawn from any part of the body in the same manner as from the prime conductor. While the person remains insulated, he experiences no sensation from being filled with electricity; or, if a metallic point be presented to any part of his body, the fluid may be drawn off silently, without being perceived. But if he touch a blunt piece of metal, or any other conducting substance, or if he step from the stool to the floor, he will feel the electric shock; and the shock will vary in force according to the quantity of fluid with which he is charged. 997. THa TISSUE FIGURE. Fig. 146 is a. 146. figure with a dress of fancy paper cut into X;, narrow strips. When placed on the prime:: conductor, or, being insulated, is connected ~ with it, the strips being all electrified will < / recede and form a sphere around the head. On presenting a metallic point to the electrified, strips, very singular combinations will take place. If the electrometer be ELECTRICITY. 271 removed from the prime conductor, and a tuft of feathers, or hair, fastened to a stick or wire, be put in its place, on turning the machine the feathers or hair will become electrified, and the separate hairs will rise and repel each other. A toy is in this way constructed, representing a person under excessive fright. On touching the head with the hand, or any conducting substance not insulated, the hair will fall. How is the 998. The Leyden jar may be charged by preLeyden jad senting it to the prime conductor when the machine is worked. If the ball of' the jar touch the prime conductor it will receive the fluid silently; but, if the ball of the jar be held at a small distance from the prime conductor, the sparks will be seen darting from the prime conductor to the jar with considerable noise. 999. The jar may in like manner be filled with negative elec tricity by applying it to the ball on the rubber-post, and connecting the chain with the prime conductor. 1000. If the Leyden jar be charged from the prime conductor (that is, with positive electricity), and presented to the pith-balls of the electrometer, they will be repelled; but if the jar be charged from the brass ball of the rubber-post (that is, with negative electricity), they will be attracted. 1001. If the ball of the prime conductor be removed, and a pointed wire be put in its place, the current of electricity flowing from the point when the machine is turned may be perceived by placing a lighted lamp before it; the flame will be blown from the point; and this will be the case in what part soever of the machine the point is placed, whether on the prime conductor or the rubber; or if the point be held in the hand, and the flame placed between it and the machine, thus showing that in all cases the fluid is blown from the point. Delicate apparatus may be put in motion by the electric fluid when issuing from a point. In this way electrical orreries, mills, &c., are constructed. 1002. If the electrometer be removed from the prime con 272 NATURAL PHILOSOPHY. ductor, and a pointed wire be substituted for it, a wire with sharp points bent in the form of an S, balanced on it, will be made to revolve rapidly. In a similar manner the motion of the sun and the earth around their common centre of gravity, together with the motion of the earth and the moon, may be represented. This apparatus is sometimes called an Electrical Tellurium. It may rest on the prime conductor or upon an insulated stand. Describe 1003. A chime of small bells on a stand, Fig. 147. Fig. 147. Fig. 147, may also be rung by means of brass balls suspended from the revolving wires. The principle of this revolution is similar to that mentioned in connection with the revolving jet, Fig. 98, which is founded on the. law that action and reaction are equal and in opposite directions. 1004. If powdered resin be scattered over dry cotton-wool, loosely wrapped on one end of the jointed discharger, it may be infalllled by the discharge of the battery or a Leyden jar. Gunpowder may be substituted for the resin. 1005. The universal discharger is an instrument for directing a charge of electricity through any substance, with certainty and precision. Explain 1006. It consists of two sliding rods, A B and { Fig. 148. D, terminating at the extremities, A and B, with blas balls, and at the other ends which A ig. 148. rest upon the ivory table or stand E, having a fork, to which any small substance may be attached. The whole is insulated by glass, legs, or pillars. The rods slide - through collars, by which means their distance from one anothe may be adjusted. 1.007. In using the universal discharger one of the rods or slides must be connected by a chain, or otherwise, with the out. ELECTRICITY. 27,q side, and the other with the inside coating of the jar or battery. By this means the substance through which the charge is to be sent is placed within the electric circuit. 1008. By means of the universal discharger, any small metallic substance may be burnt. The substance must be placed in the forks of the slides, and the slides placed within the electric circuit, in the manner described in the last paragraph. In the same manner, by bringing the forks on the slides into contact with a substance placed upon the ivory stand of the discharger, such as an egg, a piece of a potato, water, &c., it may be illuminated. 1009. Ether or alcohol may be inflamed by a spark communicated from a person, in the following manner: The person standing on the insulating stool receives the electric fluid from the prime conductor by touching the conductor or any conducting substance in contact with it; he then inserts the knuckles of his hand in a small quantity of sulphuric ether, or alcohol, held in a shallow metallic cup, by another person, who is not insulated, and the ether or alcohol immediately inflames. In this case the fluid passes from the conductor to the person who is insulated, and he becomes charged with electricity. As soon as he touches the liquid in the cup, the electric fluid, passing from him to the spirit, sets it on fire. 1010. The electrical bells are designed to show the effects of electrical attraction and repulsion. 1011. In some sets of instruments, the bells are insulated on a separate stand; but the mode here described is a convenient mode of connecting them with the prime conductor. 1012. They are Fig. 149. Eplain Fig. thus to be ap149 plied: The ball B of the prime conductor, with i its rod, is to be unscrewed, and the rod on which the- bells are A B suspended is to be screwed in its 274 NATURAL PHILOSOPHY. place. The middle bell is to be connected by a chain with the table or the floor. When the machine is turned, the balls suspended between the bells will be alternately attracted and repelled by the bells, and cause a constant ringing. If the battery be charged, and connected with the prime conductor, the bells will continue to ring until all the fluid from the battery has escaped. It may be observed, that the fluid from the prime conductor passes readily from the two outer bells, which are suspended by chains; they, therefore, attract the two balls towards them. The balls, becoming electrified by contact with the outer bells, are repelled by them, and driven to the middle bell, to which they communicate their electricity; having parted with their electricity, they are repelled by the middle bell, and again attracted by the outer ones, and thus a constant ringing is maintained. The fluid which is communicated to the middle bell, is conducted to the earth by the chain attached to it. Explain twhat 1.013. SPIRAL TUBE. —The passage of the Fig. 150 rep- electric fluid from one conducting substance to resents. another, is beautifully exhibited by means of a glass tube, having a brass ball at each end, and coated in Fig. 150. the inside with small pieces of tin-foil, placed at small distances from each other in a spiral direction, as represented in Fig. 150. 1014. In the same manner various figures, letters and words, may be represented, by arranging similar pieces of tin-foil between two pieces of flat glass. These experiments appear more brilliant in a darkened room. 1015. THE HYDROGEN PISTOL.- The hydrogen Explain Fig. 151. pistol is made in a variety of forms, sometimes in the exact form of a pistol, and sometimes in ELECTRICITY. 275 the.form of a piece of ordnance. The form Fig. 151. in Fig. 151 is a simple and cheap contrivance, and is sufficient to explain the manner in which the instrument is to be used in any of its forms. It is to be filled with hydrogen gas, and a cork inserted, fitting tightly. When thus prepared, if the insulated knob K be presented to the prime conductor, it will immediately explode. 1016. A very convenient and economical Explain Fig. way of procuring hydrogen gas for this and other experiments, is by means of the hydrogen gas generator, as represented in Fig. 152. It consists of a glass vessel, with a brass cover, in the centre of which is a stop-cock; from the inside of the cover another glass vessel is suspended, with its open end down- tion of Le Verrier, of Paris. * To the last two asteroids in the list no names have as yet been given. It is proper to be observed that tile asteroids are frequenltly known better by their numbers than by their names. Thus O represents Polhymnia, and C Calliope, &c. 340 NATURAL. PHILOSOPHY. What is th7e 1226. The name planet properly means difference be — a wandering star, and was given to this tween aplanet class of the heavenly bodies because they and ac star? are constantly moving, while those bodies which are called fixed stars preserve their relative positions. The planets may likewise be distinguished from the fixed stars by the eye by their steady light, while the fixed stars, on the contrary, appear to twinkle. 1227. The sun, the moon, the planets, and the fixed stars, which appear to us so small, are supposed to be large worlds, of various sizes, and at different but immense distances fiom us. The reason that they appear to us so small is, that on account of their immense distances they are seen under a small angle of vision. What univer- 1228. It has been stated, in the early pages sal law keeps of this book, that every portion of matter is atthe planets tracted by every other portion, and that the and other heavenly bodies it force of the attraction depends upon the quantity their places? of matter and the distance. As attraction is mutual, we find that all of the heavenly bodies attract the earth, and the earth likewise attracts all of the heavenly bodies. It has been proved that a body when actuated by several forces will be influenced by each one, and will move in a direction between them: It is so with the heavenly bodies; each one of them is attracted by every other one; and these attractions are so nicely balanced by creative wisdom, that, instead of rushing together in one mass, they are caused to move in regular paths (called orbits) around a central body, which, being attracted in different directions by the bodies which revolve around it, will itself revolve around the centre of gravity of the system. Thus, the sun is the centre of what is called the solar system, and the planets revolve around it in different times, at different distances, and with different velocities. 1229. The paths or courses in which the Wat iorbitea planets move around the sun are called their orbits. ASTRONOMY. 341 All of the heavenly bodies move in conic sections,* namely, the circle, the ellipse, the parabola and the hyperbola. Wlhlat is meant 1230. In obedience to the universal law of by a year? gravitation, the planets revolve around the sun as the centre of their system; and the time that each one takes to perform an entire revolution is called its year. Thus, the planet Mercury revolves around the sun in 87 of our days; hence a year on that planet is equal to 87 days. The planet Venus revolves around the sun in 224 days; that is, therefore, the length of the year of that planet. Our earth revolves around the sun in about 365 days and 6 hours. Our year, therefore, is of that length. 1231. The length of time that each planet takes in performing its revolution around the sun, or, in other words, the length of the year on each planet, is as follows. (The fractional parts of the dasy are omitted.) In the same connexion will also be found the mean distance of each planet from the sun, and the time of revolution around its axis; or, in other words, the length of the day on each. Mean Distance from Length of the Length of the Year the Sun in millions Day in Hours in Days. of Miles. and Minutes. MnRCURY..................... 8 86 24 5' VENUS...................... 224 858 23 21' EARTH....................... 865 95 24 00' MARS........................ 686 145 24 89' 1. Ceres............. 1,680 2. Pallas....................,683 3. Juno.................... 1,592 4. Vesta..................... 1,325 5. Astrea................. 6. HIebe................... T. Iris..........'.......... 8. Flore................... 7. Iriis 8- Flora. I w About 266 9. Metis...................Between 1,400 Between 1,4080 10. Hygeia........... and.2,100 11. Parthenope............. 12. Clio............ 13. Egeria................. 14. Irene................ 15. Eunomia............... 16. Psycle................. 1,885 * Conic sections are curvilinear figures, so called because they can all be fornned by cutting a cone in certain directions. If a cone be cut perpendicular to its axis, the surface cut will be a circle. If cut oblique to the axis, the surface cut will be an ellipse. If cut parallel to the slope of the cone, the section will be a parabola. If 3iut parallel to the axis, the bection will be an hypeerbola. 29* 842 NATURAL PHILOSOPHY. Mean Distance from Length of the Length of the Year the Sml in millions Day in Hours in IDiys. of Miles. and Minutes. 17. Thetis.................... 1,430 18. Melpomene............... 1,269 19. Fortuna........... 1,896 20. Massilia... 1,359 About 266 21. Lutetia................. 1,387 22. Calliope.................. 1,815 23. Thalia........- 1,511 24. Themis.... 2,037 25. Phocoea..... 26. Proserpina. 27. Euterpe.................. 28. Bellona................... 29. Amphitrite............... 80. Urania.................... 31. Euphrosyne............... 82. Pomona................. 83. Polhymnia............... 35. Unnamed................ JUPITER...................... 4,32 494 9 55 SATURN. - 10,759 906 10 16' URANUS.................... 80,686 1,824 NEPTUNE.................... 60,126 2,856 The sun turns on its axis in about 25 days and 10 hours. Bive an ac- 1232. There is a very remarkable law, discount of covered by Professor Bode, founded, it is true, Bode's law. on no known mathematical principle, but which has been found to accord so exactly with other calculations, that it is recognized as Bode's law for estimating the distances of the planets from the sun. Thus: Write the arithmetical progression, 0, 3, 6, 12, 24, 48, 96, 192, 384. To each of the series add 4, and we have the sums, 4, 7, 10, 16, 28, 52, 100, 196, 388, which will represent very nearly the comparative distance of each planet. Now, the distance of the earth from the sun is 95 millions of miles, and as that distance is represented in the progression by 10, it follows that the distance of Mercury is -4 of 95 millions, of Venus -7,O &c. What led to 1233. It is to be observed, however, that before the discovery the discovery of the minor planets, there was a of the minor very-remarkable interval between the planets 7pJlc~net8 e Mars and Jupiter, and that Bode's law, which seemed to accord with the distance of all the other planets, ASTRONOMY. 34t2 appeared here to fail in its application. Kepler had suspected that an undiscovered planet existed in the interval; but it was not certainly known until a number of distinguished observers assembled at Lilienthal, in Saxony, in 1800, who resolved to direct their observations especially to that part of the heavens where the unknown planet was supposed to be. The result of the labors of these observers, and others who have followed:themn, has been the discovery of the thirty-five minor planets, all situated between the planets Mars and Jupiter. But these What opinion minor planets are so small, and their paths or has beenform- orbits vary so little, that it has been conjectured ed in relation. to the minor that they originally formed one large and resplenplanets? dent orb, which, by the -operation of some un-.known cause, has exploded and formed the minor planets that revolve in orbits very near that of the original planet. 1234. Of these thirty-five small bodies, which are quite invisible without the aid of a good telescope, ten were discovered by Mr. Hind, of Mr. Bishop's private observatory, Regent's Park, London; seven by De Gaspairis, of Naples; three by Chacornac, at Marseilles; three by Luther, at Bilk, Germany; two by Olbers, of Bremen; two by Hencke, of Driessen, Germany; two by Goldschmidt, at Paris; and one each by Piazzi, of Palermo; Harding, of Lilienthal, Germany; Graham, at Mr. Cooper's private observatory, Markree Castle, Ireland; Marth, of London; and Ferguson, of Washington. TWhat is the 1235. The paths or orbits of the planets shape of the are not exactly circular, but elliptical. orbits of the They are, therefore, sometimes nearer to planets? the sun than at others. The mean distance is the medium between their greatest and least distance. Those planets which are nearer to the sun than the earth are called inferior planets, because their orbits are within that of the earth; and those which are further from the sun are called superior planets, because their orbits are outside that of the earth. Give the rela- 1236. The relative size of the sun, the tive size of the sun, moon, moon and the larger planets, as expressed by and primary the length of their diameters, is as follows: planets. 344 NATURAL PHILOSOPHY. Sun.. 882,000 Mars... 4,100 Moon.. 2,153 Jupiter. 88,640 Mercury.. 2,950 Saturn. 75,000 Venus.. 7,800 Uranus. 34,500 Earth... 7,912 Neptune.. 37,500 How klargeare 1237. The size of the minor planets has been the minor so variously estimated, that little reliance can be planets? placed on the calculations. Some astronomers estimate them as a little over 1000 miles, while others place them much below that standard. Vesta has been described as presenting a pure white light; Juno, of a reddish tinge, and with a cloudy atmosphere; Pallas is also stated as having a dense, cloudy atmosphere; and Ceres, as of a ruddy color. These four undergo various changes in appearance, and but little is known of any of them, except their distance and time of revolution. E xplain 1238. Fig. 182 is a representation of the comFig. 182. parative size of the larger planets. Fig. 182. Jupiter. ercuy ers Yenus.Earth. 0o Uranus. Sir J. F. W. Herschel gives the following illustration of the comparative size and distance of the bodies of the solar system. " On a well-levelled field place a globe two feet in diameter, to represent the Sun; Mercury will be -represented by a grain of mustaLrd-seed, on the circumference of a circle 164 feet in diameter for its orbit Venus, a pea, on a circle 284 feet in diameter-; the Earth. also a pea, on a circle of 430 feet; Mars, a rather large pin's head; on a ASTRONOMY. 345 circle of 654 feet; Juno, Ceres, -Vesta, and Pallas, grains of sand, in orbits of from 1,000 to 1,200 feet; Jupiter, a moderate-sized orange, in a circle nearly half a mile in diameter; Saturn, a small orange, on a circle of four-fifths of a mile; Uranus, a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half; and Neptune, a good-sized plum, on a circle about two miles and a half in diameter. "'To imitate the motions of the planets in the above-mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus, in 4 minutes and 14 seconds; the Earth, in 7 minutes;:Mars, in 4 minutes and 48 seconds; Jupiter, in 2 hours, 56 minutes; Saturn, in 3 hours, 13 minutes; Uranus, in 12 hours, 16 minutes; and Neptune, in 3 hours, 30 minutes." What is the 1239. The Ecliptic is the apparent path Ecliptic, and of the sun, or the real path of the earth. why is it so It is called the ecliptic, because every called P eclipse, whether of the sun or the moon, must; be in or near it. 1240. The Zodiac is a space or belt, sixWZodihat is the teen degrees broad, eight degrees each side of the ecliptic. It is called the zodiac from a Greek word, which signifies an animal, because all the stars in the twelve parts into which the ancients divided it were formed into constellations, and most of the twelve constellations were called after some animal. 1241. Sir J. F. W. Herschel, in his excellent treatise on Astronomy, says: "Uncouth figures and outlines of men and monsters are usually scribbled over celestial globes and maps, and serve, in a rude and barbarous way, to enable us to talk of groups of stars, or districts in the heavens, by names which, though absurd or puerile in their origin, have obtained a currency from which it would be difficult to dislodge them. In so far as they have really (as some have) any slight resemblance to the figures called up in imagination by a view of the more splendid' constellations,' they have a certain convenience; but as they are otherwise entirely arbitrary, and correspond to no natural subdivisions or groupings of the stars, astronomers treat them lightly, or altogether disregard them, except for briefly naming remarkable stars, as'Alpha Leonis,''Beta Scorpii,' &c., by letters of the Greek alphabet attached to them. " This disregard is neither supercilious nor causeless. The constellations seem to have been almost purposely named and delineated `46 NATURAL PHILOSOPHY. to cause as much confusion and inconvenience as possible. Innumerable snakes twine through long and contorted areas of the heavens( where no memory can follow them; bears, lions, and fishes, large and small, northern and southern, confuse -all nomenclature, &c. A better system of constellations might have been a material help as an artificial memory." What are the 1242. The zodiac is divided into twelve signs of the signs, each sign containing thirty degrees of zodiac, and how many degrees in the great celestial circle. The names of these each? signs are sometimes given in Latin, and sometimes in English. They are as follows: Latin. English. Latin. English. (1) Aries, The Ram. (7) Libra, The Balance. (2) Taurus, The Bull. (8) Scorpio, The Scorpion. (3) Gemini, The Twins. (9) Sagittarius, The Archer. (4) Cancer, The Crab. (10) Capricornus, The Goat. (5) Leo, The Lion. (11) Aquarius, The Water-bearer. (6) Virgo, The Virgin. (12) Pisces, Tile Fishes. 1243. The signs of the zodiac and the various bodies of the solar system are often represented. in almanacs and astronomical works, by signs or characters. In the following list the characters of the planets, &c., are represented. ~ The Sun. CD The Earth. 3 Ceres. (C The Moon. < Mars. 9 Pallas.: Mercury. ~ Vesta. -2 Jupiter. y Venus. ~ Juno. 2 Saturn. i Uranus. The following characters represent the signs of the Zodiac. co Aries. S. Leo. t Sagittarius. 8 Taurus. ug Virgo. Xl Capricornus. i Gemins. _ _ Libra. - Aquarius. _a Cancer. li. Scorpio. X Pisces. From an inspection of Fig. 183 it appears that when the earth ASTRONOMY. 3i7 as seen from the sun, is in any particular constellation, the sun, as viewed from the earth, will appear in the opposite one. Have the signs 1244. The constellations of the zodiac do not of the zodiac now retain their original names. Each conalvays remained the same, stellation is about 30 degrees eastward of the and why? sign of the same name. For example, the constellation Aries is 30 degrees eastward of the sign Aries, and the constellation Taurus 30 degrees eastward of the sign Taurus, and so on. Thus the sign Aries lies in the constellation Pisces; the sign Taurus, in the constellation Aries; the sign Gemini, in the constellation Taurus, and so, on. Hence the importance of distinguishing between the signs of the zodiac and the constellations of the zodiac. The cause of the difference is the precession of the equinoxes, a phenomenon which will be explained in its proper connexion. How are the 1245. The orbits of the other planets orbits of the planets situated are inclined to that of the earth; or, in with respect to other words, they are not in the same that of the earth? plane. Explain Fig. 183 represents an oblique view of the plane Eig. 183. of the ecliptic, the orbits of all the primary planets, and of the comet of 1680. That part of each orbit which is above the plane is shown by a white line; that which is below it, by a dark line. That part of the orbit of each planet where it crosses the ecliptic, or, in other words, where the white and dark lines in the figure meet, is called the node of the planet, from the Latin nodus, a knot or tie. E1.rplain 1246. Fig. 184 represents a section of the plane Fig. 184. of the ecliptic, showing the inclination of the orbits of the planets. As the zodiac extends only eight degrees on each side of the ecliptic, it appears from the figure that the orbits of some of the planets are wholly in the zodiac, while those of others rise above and descend below it. Thus the orbits of Juno, Ceres and Pallas, rise above, while those of all the other planets are confined to the zodiac. ,:s -S Fa321lnP D Libra. Sahl1The S e I m' H~~~~~~~~~~~~~~~~~~~~ a 4-oeung X ~~~~~~ oMN0 Ps ~ ~ ~ ~ ~ ~ - a ft 44r.i[ 4 1~~~, ~~~~~~~~~~4~~~- Ia~an cu;i~~~~~~WNNR W45~~~~~~~~~~~~t:24 ~~~~iic p; t~~~~~~~4g4 ~~~~~~~~~~~~~~3~~~~~~~~~~~4 coo ASTRONOMY. 349 When is a 1247. When a planet or heavenly body 4eavenly body said to be in any is in that part of its orbit which appears to constellation? be near any particular constellation, it is said to be in that constellation. Thus, in Fig. 147, the comet of 1680 appears to approach the sun from the constellation Leo.'What is me-ant 1248. The perihelion* and aphelion* by the perihelion of a heavenly body express its situation with and aphelion, the perigee aind ao- regard to the sun. When a body is nearest gee, of a heavenly to the sun, it is said to be in its perihelion. body? When furthest from the sun, it is said to be in its aphelion. 1249. The earth is three millions of miles nearer to the sun in its perihelion than in its aphelion. The apogee * and perigee * of a heavenly body express its situation with regard to the earth. When the body is nearest to the earth, it is said to be in perigee; when it is furthest from the earth, it is said to be in apogee. VWhere is the 1250. The perihelia of the planets, as seen from perihelionand the sun, are in the following'signs of the zodiac, aphelion ofthe namely: Mercury in Gemini, Venus in Leo, the earth lionofthEarth in Cancer, Mars in Pisces, Vesta in Sagittarius, Juno in Taurus, Ceres in Leo, Pallas in Leo, Jupiter in Aries, Saturn in Cancer, Uranus in Virgo, and Neptune in Taurus. What is meant 1251. When a planet is so nearly on a by the inferior and superior line with the earth and the sun as to pass conjunction and between them, it is said to be in its inferior opposition of a conjunction; when behind the sun, it is said * The plural of Perihelion is Perihelia, and of Aphelion is Aphelia. The words perihelion, aphelion, apogee, and perigee, are derived from the Greek language, and have the following meaning: Perihelion, near the sun. Aphelion, from the sun. Perigee, near the earth. Apogee, from the earth 30 350 NATURAL PHILOSOPHY. to be in its superior conjunction; but when behind the earth, it is said to be in- opposition. What is the in- 1252. The axes of the planets, in their clination of the clinationxes of the revolution around the sun, are not perpenplanets to the dicular to their orbits, nor to the plane of the lane of theirs ecliptic, but are inclined in different degrees. What causes 1253. This is one of the most remarkable the seasons? What causes circumstances in the science of Astronomy, the differences because it is the cause of the different seasons, in the length of the days and spring, summer, autumn and winter; and nights? because it is also the cause of the difference in the length of the days and nights in the different parts of the world, and at the different seasons of the year. 1254. The motion of the heavenly bodies is not uniform. They move with the greatest velocity when they are in perihelion, or in that part of their orbit which is nearest to the sun; and slowest when in aphelion. 1255. It was discovered by Kepler, and proved by Newton, that if a line is drawn fiom the sun to either of the planets, this line Fig.185. passes over or describes equal areas in equal times. This line is called the radius-vector. This is one of | Kepler's great laws. Explain In Fig. 185, Fig. 185. let S represent the sun, and E the earth, and the ellipse or oval, be the earth's orbit, or path around the sun. A By lines drawn from the sun at S to the outer edge of the G... figure, the orbit is divided E ASTRONOMY. 351 into twelve areas of different shapes, but each containing the same quantity of space. Thus, the sjaces E S A, A S B, D S C, &c., are all supposed to be equal. Now, if the earth in the space of one month will move in its orbit from E to'A, it will in another month move from A to B, and in the third month from B to C, &c., and thus its radius vector will describe equal areas in equal times. The reason why the earth (or any other heavenly body) moves with a greater degree of velocity in its perihelion than in its aphelion may likewise be explained by the same figure. Thus: The earth, in its progress from F to L, being constantly urged forward by the sun's attraction, must (as is the case with a falling body) move with an accelerated motion. At L, the sun's attraction becomes stronger, on account of the nearness of the earth; and consequently in its motion from L to E the earth will move with greater rapidity. At E, which is the perihelion of the earth, it acquires its greatest velocity. Let us now detain it at E, merely to consider the direction of the forces by which it is urged. If the sun's attraction could be destroyed, the force which has carried it from L to E would carry it off in the dotted line from E to GC, which is a tangent to its orbit. But, while the earth has this tendency to move towards G, the sun's attraction is continually operating with a tendency to carry it to S. Now, when a body is urged by two forces, it will move between them; but, as the sun's attractionW is constantly exerted, the direction of the earth's motion will not be in a straight line, the diagonal of one large parallelogram, but through the diagonal of a number of infinitely small parallelograms; which, being united, form the curve line E A. It is thus seen that while the earth is moving from L to E the attraction of the sun is stronger than in any other part of its orbit, and will cause the earth to move rapidly. But in its motion from E to A, from A to B, from B to C, and from o to F, the attraction of the sun, operating in an opposite direction, will cause its motion from the sun to be retarded, until, at F, the direction of its motion is reversed, and it begins again to 352 NATURAL PHILOSOPHY. approach the sun. Thus it appears that in its passage from the perihelion to the aphelion the motion of the earth, as well as that of all the heavenly bodies, must be constantly retarded, while in moving from their aphelion to perihelion it is constantly accelerated, and at their perihelion the velocity will be the greatest. The earth, therefore, is about seven days longer in performing the aphelion part of its orbit than in traversing the perihelion part; and the revolution of all the other planets, being the result of the same cause, is affected in the same manner as that of the earth. What are the 1256. The other two great laws disthree laws of covered by Kepler, on which the discoveries Kepler? of Newton, as well as the whole modern theory of the planets, are based, are - 1257. (1.) That the planets do not move in circles, but in ellipses, of which the sun is in one of the foci. 1258. (2.) In the motion of the planets, the squares of the times of revolution are as the cubes of the mean distances from the sun. It was by this law that, in the want of other means, the distance of the planet Uranus from the sun was estimated. How much nearer 1259. The earth is about three millions is the earth to the is the earth to the of miles nearer to the sun in winter than sun in summer than in the win- in summer. The heat of summer, thereter? [Be careful fore, can be only partially affected by not to be caught in this question.] the distance of the earth from the sun. The sun is nearest to the earth in the summer of the southern hemisphere, and the heat is more intense there than in corresponding latitudes of the north. This is due to the greater amount of land in the northern hemisphere, which by its radiating power diffuses the heat more equally. When is the heat 1260. On account of the inclination of of the sun the the earth's axis, the rays of the slln fall greatest? more or less obliquely on different parts ASTRONOMY. 353 of the earth's surface at different seasons of the year. The heat is always the greatest when the sun's rays fall vertically; and the more obliquely they fall, the fewer of them fall on any given space. This is the reason why the days are hottest in summer, although the earth is further from the sun at that time. Explain 1261. Fig. 186 represents the manner in which Fig. 186 the rays of the sun fall upon the earth in summer and in winter. The north pole of the earth, at all seasons, constantly points to the north star N; and, when the earth is nearest to the sun, the rays from the sun fall as indicated by W in the figure; and, as their direction is very oblique, and they have a larger Fig. 186. portion of the atmosphere to traverse, much of their power is lost. Hence we have cold weather when the earth is nearest te the sun. But when the earth is in aphelion the rays fall almost vertically or perpendicularly, as represented by S in the figure and, although the earth is then nearly three millions of milet further from the sun, the heat is greatest, because the rays fall more directly, and have a less portion of the atmosphere to traverse. This may be more familiarly explained by comparing summer rays to a ball or stone thrown directly at an object, so as to 3O: 854 NATURAL PHILOSOPHY. strike it with all its force; and winter rays to the same ball or stone thrown obliquely, so as merely to graze the object. Why is it cooler 1262. For a similar reason we find, even in ealrly inr the summer, that early in the morning and late in morning than in the middle the afternoon it is much cooler than at noon, of the day? because the sun then shines more obliquely. The heat is generally the greatest at about three o'clock in the afternoon; because the earth retains its heat for some length of time, and the additional heat it is constantly receiving from the sun causes an elevation of temperature, even after the rays begin to fall more obliquely. What causes the 1263. It is the same cause which occasions different cli- the variety of climate in different parts of the mates in different parts of the earth. The sun always shines in a direction world? nearly perpendicular, or vertical, on the equator, and with different degrees of obliquity on the other parts of the earth. For this reason, the greatest degree of heat prevails at the equator during the whole year. The further any place is situated from the equator, the more obliquely will the rays fall at different seasons of the year, and, consequently, the greater will be the difference in the temperature. hat places will 1264. If the axis of the earth were perpenAThat places will have the coolest dicular to its orbit, those parts of the earth temperature? which lie under the equator would be constantly opposite to the sun; and as, in that case, the sun would, -at all times of the year, be vertical to those places equally distant firom both poles, so the light and heat of the sun would be dispersed with perfect uniformity towards each pole; we should have no variety of seasons; day and night would be of the same length, and the heat of the sun would be of the same intensity every day throughout the year. What effects are 1265. It is, therefore, as has been stated, produced by the inclinationYof owing to the inclination of the earth's the earth's axis? axis that we have the agreeable variety ASTRONOMY. 3 5 of the seasons, days and nights of different lengths, and that wisely-ordered variety of climate which causes so great a variety of productions, and which has abforded so powerful a stimulus to human industry. 1266. The wisdom of Providence is frequently displayed in apparent inconsistencies. Thus, the very circumstance which, to the short-sighted philosopher, appears to have thrown an insurmountable barrier between the scattered portions of the human race, has been wisely ordered to establish an interchange of blessings, and to bring the ends of the earth in communion. Were the same productions found in every region of the earth, the. stimulus to exertion would be weakened, and the wide field of human labor would be greatly diminished. It is our mutual wants which bind us together. 1267. In order to understand the illustration of the causes of the seasons, &c., it is necessary to have some knowledge of the circles which are drawn on the artificial representations of the earth. It is to be remembered that all of these circles are wholly imaginary; that is, that there are on the earth itself no such circles or lines. They are drawn on maps merely for the purpose of illustration. Explain 1268. Fig. 187 represents the earth. N S is the Fig. 187. axis, or imaginary line, around which it daily turns; N is the north pole, S is the south pole. Fig. 187. These poles, it will be seen, are the extremities of the axis N S. C D represents the equator, which is a circle around the earth, at an equal dis- - tance from each pole. The curved lines proceeding from N to S are meridians. They are all circles surrounding, the earth, and passing through the poles. These meridians may be multiplied at pleasure. The lines E F, I K, L M, and G H, are designed to represent circles all of them parallel to the equator, and for this reason they are called parallels of latitude. These also may be multiplied at pleasure. But in the figure these lines, which are parallel to the equator, 856 NATURAL PHILOSOPHY. and which are at a certain distance from it, have a different name, derived from the manner in which the sun's rays fall on the surface of the earth. Thus the circle I K, 231 degrees from the equator; is called the tropic of Cancer, and the circle L M is called the tropic of Capricorn. The circle E F is called the Arctic Circle. It represents the limit of perpetual day when it is summer in the northern hemisphere, and of perpetual night when it is winter. On the 21st of March the rays of the sun fall vertically on the equator, and on each succeeding day on places a little to the north,. until the 21st of June, when they fall vertically on places 23. degrees north of the equator. Their vertical direction then turns back again towards the equator, where the rays again fall vertically on the 23d of September, and on the succeeding days a little to the south; until the 21st of December, when they fall vertically on the places 231 south of the equator. Their vertical direction then again turns towards the equator Hence the circles I K and L M are called the tropics of Cancer and Capricorn. The word tropic is derived from a word which signifies to turn. The tropics, therefore, are the boundaries of the sun's apparent path north and south of the equator, or the lines at which the sun turns back. The circle G H is the Antarctic Circle, and represents the limit of perpetual day and night in the southern hemisphere. The line L K represents the circle of the ecliptic, which, as has already been stated, is the apparent path of the sun, or the real path of the earth. This circle, although it is generally drawn on the terrestrial globe, is, in reality, a circle in the heavens; and differs from the zodiac only in its width, - the zodiac extending eight degrees on each side of the ecliptic. Explain 1269. Fig. 188 represents the manner in which the l'g. 188. sun shines on the earth in different parts of its orbit; or, in other words, the cause of the change in the seasons. S represents the sun, and the dotted oval, or ellipse, A B C D, the orbit of the earth. The outer circle represents the zodiac, with ASTRONOM(Y. 357 the rosition of the twelve signs or constellations. On the 21st of June, when the earth is at D, the whole northern polar region is continually in the light of the sun. As it turns on its axis, therefore, it will be day to all the parts which are exposed to the light of the sun. But, as the whole of the Antarctic circle is within the line of perpetual darkness, the sun can shine on no part of it. It will, therefore, be constant night to all places within that circle. As the whole of the Arctic circle is within Fig. 188..,ne -in nC (Ae1rrirZz S,tre7nn th i * the line of perpetual light, no part of that circle will be turned from the sun while the earth turns on its axis. To all places, therefore, within the Arctic circle, it will be constant day. On the 22d of September, when the earth is at 0, its axis is neither inclined to nor from the sun, but is sidewise; and, of course, while one-half of the earth, from pole to pole, is enlightened, the other half is in darkness, as would be the case if its axis were perpendicular to the plane of its orbit; and it is this 858 NATURIAL PHILOSOPHY. which causes the days and nights of this season of the year to be of equal length. On the 23d of December the earth has progressed in its orbit to B, which causes the whole space within the northern polar circle to be continually in darkness, and more of that part of the earth north of the equator to be in the shade than in the light of the sun. Hence, on the 21st of December, at all places north of- the equator the days are shorter than the nights, and at all places south of the equator the days are longer than the nights. Hence, also, within the Arctic circle it is uninterrupted night, the sun not shining at all; and within the Antarctic circle it is uninterrupted day, the sun shining all the time. On the 20th of March, the earth has advanced still further, and is at A, which causes its axis, and the length of the days and nights, to be the same as on the 20th of September. What is meant 1270. From the explanation of figure 198, by the Equinoxes it appears that there are two parts of its or-i:t and the Sol- in which the days and nights are equal all over stices 7 the earth. These points are in the sign of Aries and Libra, which are therefore called the equinoxes. Aries is the vernal (or spring) equinox, and Libra the autumnal equinox. 1271. There are also two other points, called solstices, because the sun appears to stand at the same height in the heavens in the middle of the day for several days. These points are in the signs Cancer and Capricorn. Cancer is called the summer sole stice, and Capricorn the winter solstice. How are day 1272. Day and night are caused by the rota. and night caus- tion of the earth on its axis every 24 hours. ed, and what is It is day to that side of the earth which is thle reason of the difference in towards the sun, and night to the opposite side. their length? The length of the days is in proportion to the inclination of the axis of the earth towards the sun. It may be seen, by the above figure, that in summer the axis is most inclined towards the sun, and then the days are the longest. As the north ASTrONOMY. 359 pole becomes less inclined, the days shorten, till on the 21st of December it is inclined 23A degrees from the sun, when the days are the shortest. Thus, as the earth progresses in its orbit, after the days are the shortest, it changes its inclination towards the sun, till it is again inclined as in the longest days in the summer. Which of the 1273. As the difference in the length of the planets has the days and the nights, and the change of the ence in its sea- seasons, &c., on the earth, is caused by the insons?, clination of the earth's axis, it follows that all the planets whose axes are inclined must experience the same vicissitude, and that it must be in proportion to the degree of the inclination of their axes. As the axis of the planet Jupiter is nearly perpendicular to its orbit, it follows that there can be little variation in the length of the days and little change in the seasons of'that planet. 1274. There can be little doubt that the sun, the planets, stars, &c., are all of them inhabited; and, although it may be thought that some of theni, on account of their immense distance from the sun, experience a great want of light and heat, while others are so near, and the heat consequently so great, that water cannot remain on them in'a fluid state, yet, as we see, even on our own earth, that creatures of different natures live in different elements,- as, for instance, fishes in water, animals in air, &c.,- creative wisdom could, undoubtedly, adapt the being to its situation, and with as little exertion of power form a race whose nature should be adapted to the nearest or the most remote of the heavenly bodies, as was required to adapt the fowls to the air, or the fishes to the sea. Vhat is the Sun, 1275. OF THE SUN. -The Sun is a and what is its spherical body, situated near the centre of diameter? gravity of the system of planets of which our earth is one. FHow much larger 1276. Its diameter is 882,000 English is the earth than miles, which is equal to 100 diameters of the sun? FAnswer care- the earth; and, as spheres are to each f/ully.] ~ other in the proportion of the cube of their respective diameters, therefore his cubic magnitude must exceed that of the earth one million of times. It revolves 860 NATURAL PHILOSOPHY. around its axis in 25 days and 8 hours. This has been ascertained by means of several dark spots which have been seen with telescopes on its surface. 1277. Sir. Winm. Herschel supposed the spots on the sun- to be the dark body of the sun, seen through openings in the luminous atmosphere which surrounds him. 1278. It is probable that the sun,* like all the other heavenly bodies (excepting, perhaps, comets), is inhabited by beings whose nature is adapted to their peculiar circumstances. 1279. Many theories have been advanced with regard to the nature of the sun. By some it has been regarded as an immense ball of fire; but the theory which seems most in accordance with facts is, that the light and heat are communicated from a luminous atmosphere, or atmosphere of flame, which surrounds the sun, at a considerable distance above the surface. What is the zo- 1280. The zodiacal light is a singular phediacal light, and nomenon, accompanying the sun. It is a faint its cause? light which often appears to stream up from the sun a little after sunset and before sunrise. It appears nearly in the form of a cone, its sides being somewhat curved, and generally but ill defined. It extends often from 500 to 1000 in the heavens, and always nearly in the direction of the plane of the ecliptic. It is most distinct about the beginning of March, but is constantly visible in the torrid zone. The cause of this phenomenon is not known. 1281. The sun, as viewed from the different planets, appears of different sizes according to their respective distances. Fig. 189 affords a comparative view of his apparent magnitude, as seen from all except the last twenty of the minor planets. * In almanacs the sun is usually represented by a small circle, with the face of a man in it, thus: ) ASTrIONOMY. 3 Fig. 189. Apparent Magnitude pf the Sun as seen from the Planlets 31 362 NATURAL PHILOSOPHY. Describe the 1282. OF MERCURY. - Mercury is the planet 3ler- nearest planet to the sun, and is seldom seen; cury. because his vicinity to the sun occasions his being lost in the brilliancy of the sun's rays. 1283. The heat of this planet is so great How many seas are there that water cannot exist there, except in a on theplanet state of vapor, and metals would be melted. Mercury? The intensity of the sun's heat, which is in the same proportion as its light, is seven times greater in Mercury than on the earth, so that water there would be carried off in the shape of steam; for, by experiments made with a thermometer, it appears that a heat seven times greater than that of the sun's beams in summer will make water boil. How late at 1284. Mercury, although in appearance night may only a small star, emits a bright white light, Mercury be by which it may be recognized when seen. seen? It appears a little before the sun rises, and again a little after sunset; but, as its angular distance from the sun never exceeds twenty-three degrees, it is never to be seen longer than one hour and fifty minutes after sunset, nor longer than that time before the sun rises. How does Mer- 1285. When viewed through a good telecury appear scope, Mercury appears with all the various wher1 seen through a phases, or increase and decrease of light, with telescope? which we view the moon, except that it never appears quite full, because its enlightened side is turned directly towards the earth only when the planet is so near the sun as to be lost to our sight in its beams. Like that of the moon, the crescent or enlightened side of Mercury is always towards the sun. The timre of its rotation on its axis has been estimated at about-24 hours ArTRONOMY. 363 Describe the 1286. OF VENUS. - Venus, the second planet Venus. planet in order from the sun, is the nearest to the earth, and on that account appears to be the largest and most beautiful of all the planets. During a part of the year it rises before the sun, and it is then called the morning star; during another part of the year it rises after the sun, and it is then called the evening star. The heat and light at Venus are nearly double what they are at the earth. 1287, By the ancient poets Venus was called Phosphor, or Lucifer, when it appeared to the west of the sun, at which time it is morning star, and ushers in the light of day; and Hesperus, or Vesper, when eastwvard of the sun, or evening star. Why-is Venus 1288. Venus, like Mercury, presents to us never seen late all the appearances of increase and decrease at night? of light common to the moon. Spots are also sometimes seen on its surface, like those on the sun. By reason of the great brilliancy of this planet, it may some-;imes be seen even in the day-time by the naked eye. But t is never seen late at night, because its angular distance from the sun never exceeds forty-five degrees. In the absence of the moon it will cast a shadow behind an opaque body. ~What is meant 1289. Both Mercury and Venus sometimes by the transit pass directly between the sun and the earth. of a planet? As their illuminated surface is towards the sun, their dark side is presented to the earth, and they appear like dark spots on the sun's disk. This is called the transit of these planets. 1.290. The reason why we cannot see the stars and planets in the day-time is, that their light is so faint compared with the light of the sun reflected by our atmosphere. Describe the 1291. OF THE EART. -The Earth on Earth as a which we live is tile next planet in the sorar planet. 364 NATURAL PHILOSOPHY. system, in the order of distance, to Venus. It is a large globe or ball, nearly eight thousand miles in diameter, and about twenty-five thousand miles in circumference. It is known to be round, — first, because it casts a circular shadow, which is seen on the moon during an eclipse; secondly, because the upper parts of distant objects on its surface can be seen at the greatest distance; thirdly, it has been circumnavigated. It is situated in the midst of the heavenly bodies which we see around us at night, and forms one of the number of those bodies; and it belongs to that system which, having the sun for its centre, and being influenced by its attraction, is called the solar system. How much It is not a perfect sphere, but its figure is longer is the that of an oblate spheroid, the equatorial polar than the equatorial diameter being about twenty-six miles longer diameter of the than its polar diameter. earth? [Think before you It is attended by one moon, the diameter speak.] of which is about two thousand miles. Its mean distance from the earth is about 240,000 miles, and it turns on its axis in precisely the same time that it performs its revolution round the earth; namely, in twentyseven days and seven hours. 1292. The earth, when viewed from the Describe the earth as a moon, exhibits precisely the same phases that moon. the moon does to us, but in opposite order. When the moon is full to us, the earth will be dark to the inhabitants * of the moon; and when the moon is dark to us, the earth will be full to them. The earth appears to them about thirteen times larger than the moon does to us. * This observation should be qualified by the condition that the moon is inhabited. Although there is abundant reason for the belief that the planets are "the green abodes Qf life," there are many reasons to believe that the moon, in its present state, is neither inhabited nor habitable. ASTRONOMY. 365 As the moon, however, always presents nearly the same side to the earth, there is one-half of the moon which we never see, and from which the earth cannot be seen. 1293. As this book may possibly incite the inquiry how it is that the astronomer is Able to measure the size and distances of those immense bodies th3 consideration of which forms the subject of Astronomy, the process will here be described by which the diameter of the earth may be ascertained. 1294. All circles, as has already been stated, are divided into 360 degr es, and, by means of instruments prepared for the purpose, the ra imber of degrees in any are or part of a circle can be correctly ascertained. Let us now suppose that an observer, standing upon any fixed point, should notice the position of a particular star, - the north or.polar star, for instance. Let him then advance from his station, and travel towards the north, until he has. brought the star exactly one degree higher over his head. Let him then measure the distance over which he has travelled between the two points of observation, and that distance will be exactly the length of one degree of the earth's circumference. Let him multiply that distance by 360, and it will give him the circumference of the earth. Having thus found the circumference, the diameter may readily be found by the common rules of arithmetic. This calculation is based on the supposition that the earth is a perfect sphere, which is not the case, the equatorial diameter being about twenty-six miles longer than the polar. But it is sufficiently near the truth for the present purpose. The design of this work not admitting rigid mathematical demonstrations, this instance of the commencement of a calculation is given merely to show that what the astronomer and the mathematician tell us, wonderful as it may appear, is neither bare assertion nor unfounded conjecture. Wl7hat motions 1295. It has been stated that the earth rehave the inhabit- volves upon its axis every day. Now, as the nts of the earth a earth is about 25,000 miles in circumference, planet? See, it follows that the inhabitants of the equator also, No. 1296. are carried around this whole distance in about twenty-four hours, and every hour they are thus carried through space in the direction of the diurnal motion of the earth at the rate of 1th of 25,000 miles, which is more than 1000 miles in an hour. 1296. But this is not all. Every inhabitant travels with the earth through its immense orbit, the diameter of which is about 190 millions of miles, or through a space of more than 570 mil31* 366 NATURAL PHILOSOPHY. lions of miles every year. This will give him, at the same time, a motion of more than 68 000 miles in an hour in a different direction. If the question be asked, why each individual is not sensible of these tremendously rapid motions, the answer is, that no bne ever knew what it is to be without them. We cannot be sensible that we have moved without feeling our motion, as when in a boat a current takes us in one direction, while a gentle wind carries us, at the same time, in another direction It is only when our progress is arrested by obstacles of some kind that we can perceive the difference between a state of motion and a state of rest. What would 1297. The rapid motion of a thousand miles in be the conse- an hour is not sufficient to overcome the centriquence if the petal force caused by gravity; but, if the earth earth should revolve on its should revolve around its axis seventeen times in axis once in a day, instead of once, all bodies at the equator an hour? would be lifted up, and the attraction of gravitation would be counterbalanced, if not wholly overcome. 1298. Certain irregularities in the orbit of the earth have been noticed by astronomers, which show that it is deviating from its elliptical form, and approaching that of a circle. In this fact, it has been thought, might be seen the seeds of decay. But Laplace has demonstrated that these irregularities proceed from causes which, in the lapse of immensely long periods, counterbalance each other, and give the assurance that there is no other limit to the present order of the universe than the will of its great Creator. Describe the 1299. OF MARS. -Next to the earth is planet Mars. the planet Mars. It is conspicuous for its fiery-red appearance, which is supposed by Sir John Herschel* to be caused by the color of its soil. * Sir John Herschel is the son of Sir William Herschel, the disccverer of the planet Uranus. ASTRONOMY. 367 The degree of heat and light at Mars is less than half of that received by the earth. 1300. OF THE MINOR PLANETS. —It has already been mentioned that between the orbits of Mars and Jupiter thirty-five small bodies have been discovered, which are called the minor planets. It is a remarkable fact, that before the discovery of Bode's law (see No. 1232) certain irregularities observed in the motions of the old planets induced some astronomers to sup. pose that a planet existed between the orbits of Mars and Jupiter. The opinion has been advanced that these small bodies originally composed one larger one, which, by some unknown force or convulsion, burst asunder. This opinion is maintained with much ingenuity and plausibility by Sir David Brewster. (See Edin. Encyc., art. ASTRONOMY.) Dr. Brewster further supposes that the bursting of this planet may have occasioned the phenomena of meteoric stones; that is, stones which have fallen on the earth from the atmosphere. Describe the 1301. OF JUPITER. -Jupiter is the largest planet Jupiter. planet of the solar system, and the most brilliant, except Venus. The heat and light at Jupiter are about twenty-five times less than that at the earth. This planet is attended by four moons, or satellites, the shadows of some of which are occasionally visible upon his surface. 1302. The distance of those satellites from the planet are two, four, six and twelve hundred thousand miles, nearly. The nearest revolves around the planet in less than two days; the next, in less than four days; the third, in less than eight days; and the fourth, in about sixteen days. These four moons must afford considerable light to the inhabitants of the planet; for the nearest appears to them four times the size of our moon, the second about the same size, the third somewhat less, and the fourth about one-third the diameter of our moon. 8368 NATURAL PHILOSOPHY. 1303. As the axis of Jupiter is nearly perpendicular to it, orbit, it has no sensible change of seasons. 1304. The satellites of Jupiter often pass be. What use has been made of hind the body of the planet, and also into its the eclipses of shadow, and are eclipsed. These eclipses are of Jupiter's satellitupter s sae use in ascertaining the longitude of places on the earth. By these eclipses, also, it has been ascertained that light is about eight minutes in coming from the sun to.the earth; for an eclipse of one of these satellites appears to us to take place sixteen minutes sooner when the earth is in that part of its orbit. nearest Jupiter than when in the part furthest from that planet. Hence, light is sixteen minutes in crossing the earth's orbit, and of course half of that time, or eight minutes, in coming from the sun to the earth. What is the ap- 1305. When' viewed through a telescope, pearance ofn Ju- several belts or bands are distinctly seen, somepiter as seen through a tele- times extending across his disk, and sometimes scope. interrupted and broken. They differ in distance, position, and number. They are generally dark; but white ones have been seen. On account of the immense distance of Jupiter from the sun, and also from Mercury, Venus, the Earth and Mars, observers on Jupiter, with eyes like ours, can never see either of the above named planets, because they would always be immersed in the sun's rays. Describe the 1306. OF SATURN.-Saturn is the seeplanet Saturn. ond in size, and the last but two in distance from the sun. The degree of heat and light at this planet is eighty times less than that at the earth. How is Saturn 1307. Saturn is distinguished from the particularly other planets by being encompassed by distinguished from the other three large luminous rings. They reflect planets? the sun's light in the same manner as his moons. They are entirely detached firom each other, ard ASTIhONO(MY. 369 friom the body of the planet. They turn on nearly the same axis with the planet, and in nearly the same time. 1308. These rings move together around the planet, but are about three minuztes longer in performing their revolution about him than Saturn is in revolving about his axis. The edge of these rings is constantly at, right angles with the axis of the planet. Stars are said to have been seen between the rings, and also between the inner ring and the body of the planet. The breadth of the two outer rings is about 27,000 miles, and the distance of the second ring from the planet is about 19,000 miles. As they cast shadows on the planet, Sir Wm Herschel thought them solid. 1309. The surface of Saturn is sometimes diversified, like that of Jupiter, with spots and belts. Saturn has eight satellites, or moons, revolving around him at different distances, and in various times, from less than one to eighty days. 1310. Saturn may be known by his pale and steady light. The eight moons of Saturn revolve at diffeient distances around the outer edge of his rings. Sir William Herschel saw them moving along it, like bright beads on a white string. They do not often suffer eclipse by passing into the shadow of the planet, because the ring is in an:oblique direction. Describe the 1311. OF URANUS.-Uranus, the fourth planetUiranus. in size, ismthe most remote of all the old planets. It is scarcely visible to the naked eye. The light and heat at Uranus are about 360 times less than that of the earth. 1312. This planet was long known by the name of IHerschel, the discoverer, who, in announcing his discovery, named it the " Georgium Sidus," in honor ol King George III. The name of Uranus was given to it by the continental astronomers. 370 NATURAL PHILOSOPHY. It was formerly considered a small star, but Sir Win. Herschel, in 1781, discovered, from its motion, that it is a planet. By hovw many 1313. Uranus is attended by six moons, moons is Uranus or satellites, all of which were discovered attended? by Sir Wm. IIerschel, and all of them revolve in orbits nearly perpendicular to that of the planet. Their motion is retrograde. W7hat is the gen- 1314. It appears to be a general law of sateral law of the ellites, or moons, that they turn on their axis rotation of sat- in the same time in which they revolve around ellites? their primaries. On this account, the inhabitants of secondary planets observe some singular appearances, which the inhabitants of primary planets do not. Those who dwell on the side of a secondary planet next to the primary will always see that primary; while those who live on the opposite side will never see it. Those who always see the primary will see it constantly in very nearly the same place. For example, those who dwell near the edge of the moon's disk will always see the earth near the horizon, and those in or near the centre will always see it directly or nearly overhead. Those who dwell in the moon's south limb will see the earth to the northward; those in the north limb will see it to the southward; those in the east limb will see it to the westward; while those in the west limb will see it to the eastward; and all will see it neater the horizon in proportion to their own distance from the centre of-the moon's disk. Similar appearances are exhibited to the inhabitants of all secondary planets. These observations are predicated on the supposition that the moon is inhabited. But it is not generally believed that our moon is inhabited, or in its present condition fitted for the residence of any class of beings. 1315. It is a singular circumstance, that before the discovery of ASTROIOMkY. 371 Urallus some disturbances and deviations were observed by astronomers in the motions of Jupiter and Saturn, which they could accuLnt for only on the supposition that these two planets were influenced by the attraction of some more remote and undiscovered planet. The discovery of Uranus completely verified their opinions, and shows the extreme nicety with which astronomers observe the motions of planets. What led to the 1316. OF NEPTUNE. — The discovery of the discovery of the planet Neptune (named originally Le Verrier, planet Neptune? from its discoverer, in 1846) is one of the greatest triumphs which the history of science records. As certain perturbations of the movements of Saturn led astronomers to suspect the existence of a remoter planet, which suspicions were fully confirmed in the discovery of Uranus, so also, after the discovery of Uranus, certain irregularities were perceived in his motions, that led the distinguished astronomers of the day to the belief that even beyond the planet Uranus still another undiscovered planet existed, to reward the labors of the discoverer. Accordingly Le Verrier, a young French astronomer, urged by his friend Arago, determined to devote himself to the attempt at discovery. With indefatigable industry he prepared new tables of planetary motions, from which he determined the perturbations of the planets Jupiter, Saturn, and Uranus, and as early as June, 1846, in a paper presented to the Academy of Sciences in Paris, he pointed out where the suspected planet would be on the 1st of January, 1847. He subsequently determined the mass and the elements of the orbits of the planet, and that, too, before it had been seen by a human eye. On the 18th of Septemnber of 1846, he wrote to his friend, M. Galle, of Berlin, requesting him to direct his telescope to a certain point in the heavens, where he suspected the stranger to be. His fiiend complied with his request, and on the first evening of examination discovered a strange star of the eighth magnitude, which had not been laid down in any of the maps of that portion of the heavens. The following evening it was found to have moved in a direction and with a velocity very nearly like that which Le Verrier had pointed out. The planet was found within less than one degree of the place 13'72 NATURAL PHILOSOPHY. where Le Verrier had located it. It was subsequently ascertained that a young English mathematician, Mr. Adams, ol Cambridge, had been' engaged in the same computations, and had arrived at nearly the same results with Le Verrier. 13817. What shall we say of science, then, that enables its devoted followers to reach out into space, and feel successfully in the dark for an object more than twenty:eight hundred millions of miles distant 1318. In conclusion of this brief notice of the planets, a plate is here presented showing the relative appearance of the planets as viewed through a telescope. It will be observed that the planets Mercury and Venus have similar phases to those of our moon. Fig 190 Relative Telescopic appearance of the Planets. What zs a 1319. OF COMETS. - The word Comet is deComet? rived from a Greek word, which means hair; and this name is given to a numerous class of bodies, which occasionally visit and appear to belong to the solar system. These bodies seem to'consist of a nucleus, attended with a lucid haze, sometimes resembling flowing hair;' from whence the name is derived. Some comets appear to consist wholly ASTRONoMY. 873 of this hazy or hairy appearance, which is frequently called the tail of the comet. Fig. 191. Comet of:1811, one of the most brilliant of modern times. Period, 2888 years. 1320. In ancient times the appearance of comets was regarded with superstitious fear, in the belief that they were the forerunners of some direful calamity. These fears have now been banished, and the comet is viewed as a constituent member of the system, governed by the same harmonious and unchanging laws which regulate and control all the other heavenly bodies. 1321. The number of comets that have occasionally appeared within the limits of the solar system is variously stated from 350 to 500. The paths or orbits of about 98 of these have been calfculated from observation of the times at which they most nearly approached the sun; their distance from it and from the earth at those times; the direction of their movements, whether tromn east to west, or frolm west to east; and the places in the 32 374 NATURAL PllILOSOPIHY. starry sphere at which their orbits crossed that of the earth and their inclination to it. The result is, that, of these 98, 24 passed between the sun and Mercury, 33 passed between Mercury and Venus, 21 between Venus and the Earth, 16 between the Earth and Mars, 3 between Mars and Ceres, and 1 between Ceres and Jupiter: that 50 of these comets moved from east to west; that their orbits were inclined at every possible angle to that of the earth. The greater part of them ascended above the orbit of the earth when very near the sun; and some were observed to dash down' from the upper regions of space, and, after turning round the sun, to mount again. 1322. Comets, in their revolution, describe What is the shape of the or bits of long narrow ovals. They approach very near comets? the sun in one of the ends of these ovals, and when they are in, the opposite end of the orbit their distance from the sun is immensely great. 1323. The extreme nearness of approach to the sun gives to a comet, when in perihelion, a swiftness of motion prodigiously great. Newton calculated the velocity of the comet of 1680 to be 880,000 miles an hour. This comet was remarkable for its near approach to the sun, being no further than 580,000 miles from it, which is but little more than half the sun's diameter. Brydone calculated that the velocity of a comet which he observed at Palermo, in 1770, was at the rate of two.millions and a half of miles in an hour. 1324. The luminous stream, or tail, of a comet, follows it as it approaches the sun, and goes before it when. the comet recedes from the sun. Newton, and some other astronomers, considered the tails of comets to be vapors, produced by the excessive heat of the sun. Others have supposed them to be caused by a repulsive influence of the sun. Of whatever substance they may be, it is certain that it is very rare, because the stars may be distinctly seen through it. 1325. The tails of comets differ very greatly in length, ASTRONO MY. 3 and some are attended apparently by only a small cloudy light, while the length of the tail of others has been estimated at front 50 to 80 millions of miles. Fig. 192. The comet of 1680, observed by Newton. Rapidity of its motion around the sun, a umillion of miles ip ali hour. Length of tail, 100 millions of miles Period, 600 years. It has never reappeared 1326. It has been argued that comets consist of very little solid substance, because, although they sometimes approach very near to the other heavenly bodies, they appear to exert no sensi ble attractive force upon those bodies. It is said that in 1454 the moon was eclipsed by a comet. The comet, must, therefore, have been very near the earth (less than 240,000 miles); yet it produced no sensible effect on the earth or the moon; for it did not cause theim to mlake any perceptible deviation friom their 376 NATURAL PHILOSOPHY. accustomed paths round the sun. It has been ascertained that comets are disturbed by the gravitating power of the planets; but it does not appear that the planets are in like manner affected by comets. Some comets have exhibited the appearance of two or more tails, and the great comet of 1744 had six Fig 193 The great. comet of 1744 ASTRONOMlY~. 377 1327. Many comets escape observation because they traverse that part of the heavens only which is above the horizon in the day-time. They are, therefore, lost in the brilliancy of the sun, and can be seen only when a total eclipse of the sun takes place. Seneca, 60 years before the Christian Era, states that a large comet was actually observed very near the sun, during an eclipse. 1328. Dr. Halley, Professor Encke and Gambart, are the first astronomers that ever successfully predicted the return of a comet. The periodical time of Halley's comet is about 76 years. It appeared last in the fall of 1835, and presented different apFig. 194. lIalley's comet, as seen by Sir John Herschel, Oct. 29th, 1835. Very changeable in its appearance First recognized by Halley in 1682. Period, 76 years. 32* 878 NATURAL PHILOSOPHfY. pearances from different points of observation. That of Encke is about 1200 days; that of Biela, about 63 years. This last comet appeared in 183'2 and in 1838. Fig. 195. Halley's comet, as seen by Struve, Oct. 12th, 183.5. First seen by Halley in 1682. Period, 75 years. 1329. The comet of 1758, the return of which was predicted by Dr. Halley, was regarded with great interest by astronomers, because its retztrn was pIedicted. But four revolutions before, in 1456, it was looked upon with the utmost horror. Its long tail spread consternation over all Europe, already terrified by the rapid success of the Turkish arms. Pope Callixtus, on this occasion, ordered a prayer, in which both the comet and the Turks were included in one anathema. Scarcely a year or a ASTRONOM3Y. 3179 month now elapses without the appearance of a comet in our system. But it is now known that they are bodies of such extreme rarity that our clouds ale massive in comparison with them. They have no more density than the air under an exhausted receiver. Herschel saw stars of the 6th magnitude through a thickness of 30,000 miles of cometic matter. The number of comets in existence within the compass of the solar system is stated by some astronomers as over seven millions. 1330. Fig. 194 represents Halley's comet, as seen by Sir John Herschel, while Fig. 195 represents the same comet as seen only a few days before by Struve. 1331. THE COMET OF 1856.-The following interesting details in relation to a comet expected in 1856 are given by Babinet, an eminent French astronomer. It is translated from the Courier des Etats Unis. " This comet is one of the grandest of which historians make mention. Its period of revolution is about three hundred years. It was seen in the years 104, 392, 683, 975, 1264, and the last time in 1556. Astronomers agreed in predicting its return in 1848; but it failed to appear, and continues to shine still unseen by us. Already the observatories began to be alarmed for the fate of their beautiful wandering istar, when a learned calculator of Middlebourg, M. Bonume, reassured the astronomical world of the continued existence of the venerable and magnificent comet. "Disquieted, as all other astronomers were, by the non-arrival of the comet at the expected time, M. Bomme, aided by the preparatory labors of Mr. hlind, has revised all the calculations and estimated all the actions of all the planets upon the comet for three hundred years of revolution. The result of this patient labor gives the arrival Qf the comet in August, 1858, with an uncertainty of two years, more or less; so that firom 1856 to 1860 we may expect the great comet which was the cause of the abdication of the Emperor Charles V., in 1556. "It is known that, partaking of the general superstition, which interpreted the appearance of a comet as the forerunner of some fatal event, Charles V. believed that this comet addressed its menaces particularly to him, as holding the first rank among sovereigns. The grealt and once wise but now wearied and shattered monarch, had been for sonie tiume the victim of cruel reverses. There were threatening indications in the political, if not in the physical horizon, of a still greater tempest to come. lie was left to cry in despair,' Fortune abandons old men.' The appearance of the blazing star seemed to him an admonition fromn Heaven that he must cease to bh a sovereign if he would avoid a fatality from which one without author 380 NATUlAL PH-ILOSOPHIY. ity might be spared. It is known that the emperor survived his abdication but a little more than two years. " Another comet, which passed near us in 1835, and which has appeared 25 times since the year 13 before the Christian Era, has been associated by the superstitious with many important events which have occurred near the periods of its visitation. "In 1066, William the Conqueror landed in England at the head of a numerous army about the time that the comet appeared which now bears the name of Halley's comet. The circumstance was regarded by the English as a prognostic of the victory of the Normans. It infused universal terror into the minds of the people, and contributed not a little towards the submission of the country after the battle of Hl-astings, as it had served to discourage the soldiers of Harold before the conibat. The comet is represented upon the famous tapestry of Bayeux, executed by Queen Matilda, the wife of the conqueror. " This celebrated tapestry is preserved in the ancient episcopal palace at Bayeux. It represents the principal incidents, including the appearance of the comet, in the history of the conquest of England by William, Duke of Normandy. It is supposed to have been executed by Matilda, the conqueror's wife, or by the Empress Matilda, daughter of Henry I. It consists of a linen web, 214 feet in length and 20 inches broad; and is divided into 72 compartments, each having an inscription indicating its subject. - The figures are all executed by the needle. "The same comet, in 1451, threw terror among the Turks under the command of Mahomet II., and into the ranks of the Christians during the terrible battle of Belgrade, in which forty thousand Mussulmans perished. The comet is described by historians of the time as' immense, terrible, of enormous length, carrying in its train a tail which covered two celestial signs (60 degrees), and producing universal terror.' Judging from this portrait, comets have singularly-degenerated in our day. It will be remembered, however, that in 1811 there appeared a comet of great brilliancy, which inspired some superstitious fears. Since that epoch science has noted nearly 80 comets, which, with few exceptions, were visible only by the aid of the telescope. Kepler, when asked how many comets he thought there were in the heavens, answered,' As many as there are fish in the sea.' " Thanks to the progress of astronomical science, these singular stars are no longer objects of terror. The theories of Newton, Halley, and their successors; have completely destroyed the imaginary empire of comets. As respects their physical nature, it was for a long time believed that they were composed of a compact centre, surrounded by a luminous atmosphere. On this subject the opinion of M. Babinet, who must be regarded as good authority on such questions, is as follows:' Comets cannot exercise any material influence upon our globe; and the earth, should it traverse a comet in its entire breadth, w uld perceive it no more than if it should cross ASTRONOMY. 3 51 a cloud a hundred thousand millions of times lighter than our atmosphere, and which could no more make its way through our air than the slightest puff of an ordinary bellows could make its way through an anvil.' It would be difficult to find a comparison more reaissuring." * What are the 1332. OF THE FIXED STARS. The Fixed Fixed Stars Stars are all supposed to be immensely large supposed tobe? bodies, like our own sun, shining by their own light, which they dispense to systems of their own. Hlow are the 1333. They are classed by their apparent fixed stars magnitudes, those of the sixth magnitude being classified? the smallest that can be seen by the naked eye. Stars which can be seen only by means of the telescope * THE COMET OF 1853.- Mr. Hind, in a letter to the London Times, gives the following particulars with regard to the comet which appeared during the year now closing (1853): " The comet which has been so conspicuous during the last week was very favorably seen here on Saturday, and again on Sunday evening. On the latter occasion, allowing for the proximity of the comet to the horizon, and the strong glow of twilight, its nucleus was fully as bright as an average star of the first magnitude; the tail extended about three degrees from the head. When viewed in the comet-seeker, the nucleus appeared of a bright gold color, and about half the diameter of the planet Jupiter, which was Shining at the same time in the southern heavens, and could be readily compared with the comet. The tail proceeds directly from the head in a single stream, and not, as sometimes remarked, in two branches. The distance of this body from the earth at 8 o'clock last evening, was 80,000,000 miles; and hence it results, that the actual diameter of the bright nucleus was 8000 miles, or about equal to that of the earth, while the tail had a real length of 4,500,000 miles, and a breadth of 250,000, which is rather over the distance separating the moon from the earth. It is usual to assume that the intensity of a comet's light varies as the reciprocal of the products of the squares of the distance from the earth and sun; but the present one has undergone a far more rapid increase of brilliancy than would result from this hypothesis. The augmentation of light will go on till the 3rd of September, and itwill be worth while to look for the comet in the day-time about that date; for this purpose an equatorially mounted telescope will be required, and I would suggest the addition of a light green or red glass, to take off the great glare of sunlight, the instrument being adjusted to a focus on the planet Venus. This comet was discovered on the 10th of June, by Mr. Klinkenfues, of the Observatory at Gottingen, but was not bright enough to be seen without a telescope until about August 13. In a letter, copied into the'inmes a few days since, Sir William Hamilton hints at the possibility of this being the comet I had been expecting; but I avail myself of the present opportunity of stating that such is not the case, the elements of the orbits having no resemblance. The comet referred to will probably reappear between the years 1858 and 1861; and, if the perihelion passage takes place during the summer months, we may expect to seeca body of far more imposing aspect than the one at present visible." 382 wATURAL PHILOSOPHY. are called telescopic stars. They, also, are classified; the classes reaching even to the seventeenth or twentieth magnitude. How many 1334. The number of the stars of the stars are there first magnitude is about twenty-four; of of the first and second mani- de, fifty; of the third, tude? two hundred. The numbel of the smallest, visible without a telescope, is from twelve to fifteen thousand. How many of 1335. Within a few years the distances the fixed stars of nine of the fixed stars have been calcnhave had their lated. This distance is so immense, that distances very nearly ascer- light, travelling with the inconceivable tained? velocity of nearly two hundred thousand miles in a second of time, from Siriu8s, is more than fourteen years in reaching the earth; from Arcturus, more than twenty-five years; and from the Pole Star, more than forty-eight years. 1336. Tens of thousands of years must roll away before the most swiftly-flying of all the fixed- stars can complete even a small fragment of its mighty orbit; but such has been the advance of science, that if a star move so slowly as to require five millions of years to complete its revolution, its motion could be perceived in one year; and in ten years its velocity can be compute(l, and its period will become known in the lifetime of a single observer. Who first di- 1337. The stars are the fixed points to vided the stars which we must refer in observations of the into constella- motions of all the heavenly bodies. Hence tions? the stars were grouped in the earliest ages, (but by whom we know not), numbered and divided into constellations. the names of which have survived the fall of empires. ASTRONOMY. 3 83 Whatprobably 1338. It is generally supposed that part, causes the dif- if not all of the difference in the apparent ference in the apparent size magnitudes of the stars is owing to the difof the stars? ference in their distance. 1339. The distance of the stars, according to Sir J. Herschel, cannot be less than 19,200,000,000,000 miles. How much greater it really is, we know not, except in a few cases. 1340. Although the stars generally appear fixed, they all have motion; but their distance being so immensely great, a rapid motion would not perceptibly change their relative situation in two or three thousand years.. Some have been noticed alternately to appear and disappear. Several that were mentioned by ancient astronomers are not now to be seen; and some are now observed which were unknown to the ancients. 1341. Many stars which appear single to the naked eye, when viewed through powerful telescopes, appear double, treble, and even quadruple. Some-are subject to variation in their apparent magnitude, at one time being of the second or third, and at another of the fifth or sixth magnitude. What is the 1342. The Galaxy, or Milky Way, is a Galaxy? remarkably light, broad zone, visible in the heavens, passing from north-east to south-west. It is known to consist of an immense number of stars, which, from their apparent'nearness, cannot be distinguished from each other by the naked eye. 1343. Sir Wm. Herschel saw, in the course of a quarter of an hour, the astonishing number of 116,000 stars pass through the field of his telescope, while it was directed to the milky way. 1344. The ancients, in reducing astronomy to a science, formed the stars into clusters, or constellations, to which they gave particular names. 1345. The number of constellations among the ancients was about 50. The moderns have added about 50 more. 1346. Our observations of the stars and nebula are confined principally to those of the northern hemisphere. Of the coustellations near the south pole we know but little. 384 NATUIRAL PHILOSOPHY. What effect 1347. In- determining the true place of any hsthe atmonte of the celestial bodies, the refractive power of length of the the atmosphere must always be taken into cday? consideration. This property of the atmosphere adds to the length of the days, by causing the sun to appear before it has actually risen, and by detaining its appearance after it has actually set. 1348. On a celestial globe, the largest star in each constellation is usually designated by the first letter of the Greek alphabet, and the next largest by the second, &c. When the Greek alphabet is exhausted, the English alphabet, and then numbers, are used. TSi~hy are the 1349. The stars, and other heavenly bodies, stars never are never seen in their true situation, because seen zn their the motion of light isoprogressive; and, during true position? the time that light is coming to the earth, the earth is constantly in motion. In order, therefore, to see a star, the telescope must be turned somewhat before the star, and in the direction in which the earth moves. 1350. Hence, a ray of light passing through What is meant' O by the aberra- the centre of the telescope to the observer's tion of light? eye does not coincide with a direct line from his eye to the star, but makes an angle with it; and this is termed the aberration of light. What is the 1351. The daily rotation of the earth on its Polar Star? axis causes the whole sphere of the fixed stars, &c., to appear to move round the earth every twentyfour hours from east to west. To the inhabitants of the northern hemisphere, the immovable point on which the whole seems to turn is the Pole Star. To the inhabitants of the southern hemisphere there is another and a corresponding point in the heavens. What is the 1352. Certain of the stars surrounding the circle of per- north pole never set to us. These are inpetual apparilio and of eluded in a circle parallel with the equator, ASTRONOMY. 385.erpetual oc- and in every part equally distant from the cultation? north pole star. This circle is called the circle of perpetual apparition. Others never rise to us. These are included in a circle equally distant from the south pole; and this is called the circle of perpetual occultation. Some of the constellations of the southern hemisphere are represented as inimitably beautiful, particularly the cross. What is par- 1353. The parallax of a heavenly body allax? is the angular distance between the true and the apparent situation of the body. Describe 1354. In Fig. 196, A G B represents the earth, Fig. 196. and 0 the moon. To a spectator at A, the moon Fig. 198. D A Bwould appear at F While to another, at B, the moon would would appear at F; while to another, at- B, the moon would appear at D; but, to a third spectator, at G, the centre of the earth, the moon would appear at E, which is the true situation. The distance from F to E is the parallax of the moon when viewed from A, and the distance from E to D is the parallax when viewed from B. 1355. From this it appears that the situation of the heavenly bodies must always be calculated from the centre of the earth; and the observer must always know the distance between the place of his observation and the centre of the earth, in order to make the necessary calculations to determine the true situation of the body. Allowance, also, must be made for the refraction of the atmosphere. 33 386 NATUiRAL PHILOSOPHY. Describe the 1356. OF THE MooN.-The Moon is a Aioon. secondary planet, revolving about the earth in about twenty-seven days, seven hours. Its distance from the earth is about 240,000 miles. It turns on its axis in precisely the same time that it performs its revolution abdut the earth. Consequently it always presents the same side to the earth; and as its apparent diameter in different parts of its orbit is different, it follows that it must be sometimes nearer to the earth than at others. 1357. The surface of the moon appears to be volcanic and very mountainous. Occasional volcanoes have been seen in action on the dark side. No heat has been detected in the moon's rays, even when most powerfully concentrated, that will affect the most delicate thermometer; and hence it has been inferred that the heat is absorbed in traversing the upper regions of our atmosphere. What is one of the 1358. One of the most common errors most commonerrors with regard to the moon is that which aswith regard to the moon, and how has cribes to it an influence over the weather. it been proved'an Tables of the weather have been compared error? with the lunar phases for a period of a hundred years, and over a thousand lunations, during which time about 491 new or full moons have been attended by a change of the weather, and 509 have not. 1359. The moon is equally innocent of putrefaction, notwithstanding the popular belief that it hastens that process, especially'n fish. The same cause which produces dew causes moisture on substances exposed to it, and this moisture is the real cause of putrefaction. 1360. Dr. Olbers, of Bremen, by a comparison of a great number of cases, arrived at the conclusion that the moon has no effect on insanity; although the popular belief is that the fits are aggravated or affected by the lunar phases. ASTRONOMY. 387 What is the 1361. The force of gravity at the surdensity of the face of the moon is about one-fifth that of moon compared with thatofthe the earth; hence ten pounds on the earth earth? will be equal to two on the moon. The days and nights on the moon are each equal to fourteen of our days. The axis of the moon is perpendicular to its orbit, and therefore the moon can have no variety of seasons. The moon likewise has no atmosphere,s and therefore it cannot be inhabited; for there can be no vegetation, no clouds, no ocean, no liquids, no light in dwellings, no twilight; in short, nothing that could fit it for the habitation of any order of beings with which we are acquainted. 1362. In connexion with what has now been stated with regard to the moon and its volcanic appearances, it will be proper to notice the subject of aerolites, or meteoric stones; because, according to the opinion of some, they are of lunar origin. Three theories have been broached with regard to them: 1st, that they are formed in the. air, from-materials existing there, in a sublimated state; 2d, that they are parts of an exploded- planet; 3d, that they are thrown fromrnthe volcanoes in the moon. To the first of these theories there is a material objection in the fact that gases, when in contact, must.mix, and gases necessary to form these substances cannot, therefore, remain in the air unmixed. To the second hypothesis it may be objected, that if they were parts of a broken planet they would probably be composed of more heterogeneous materials. But it is well known that all of them are composed of the same constituent parts, namely, sulphur, magnesia, manganese, iron, nickel, chromium, and, in one recorded instance only, charcoal. In favor Of the third supposition, which refers them to a lunar origin, it may be remarked that a body thrown seventy miles from the moon would escape from the moon's attraction; and that a velocity six times greater than that of a cannon-ball would be sufficient to throw a body beyond the moon's attraction.. As terrestrial volcanoes have thrown bodies with this velocity, it is not improbable that lunar volcanoes may do the same. What is the 1363. The most obvious fact in relation to mi~ost oiVOUs the moon is that its disc is constantly changing fact in relation to the its appearance: sometimes only a semi-circular moon? 1 edge being illuminated, while the rest is dark; 388 NATURAL PHILOSOPHY. at another time, the whole surface appearing resplendent. This is caused by the relative position of the moon with regard to the sun and the earth. The moon is an opaque body, and shines only by the light of the sun. When, therefore, the moon is between the earth and the sun, it presents its dark side to the earth; while the side presented to the sun, and on which the sun shines, is invisible to the earth. But when the earth is between the sun and the moon, the illuminated side of the moon is visible at the earth. Describe 1364. In Fig. 197, let S be the sun, E the earth, Fig. 197. and A B C D the moon in different parts of her Fig. 197. a c orbit. When the moon is at A, its dark side will be towards the earth, its illuminated part being always towards the sun. Hence the moon will appear to us as represented at a. But when it has advanced in its orbit to B, a small part of its illuminated side coming in sight, ittappears as represented at b, and is said to be horned. When it arrives at 0, one-half its illuminated side is visible, and it appears as at c. At C, and in the opposite point of its orbit, the moon is said to be in quadratzure. At D its appearance is as represented at d, and it is said to be gibbous. At E all the illuminated side is towards. us, and we have a full moon. During the other half of its ASTRONOMY. 389 revolution, less and less of its illuminated side is seen, till it again becomes invisible at A. What is the 1365. The mean difference in the rising of the mean differ- moon, caused by its daily motion, is a little less ence in the rising of the than an hour. But, on account of the different moonfrom day angles formed with the horizon by different parts to day? of the ecliptic, it happens that for six or eight nights near the full moons of September and October the moon rises nearly as soon as the sun is set. As this is a great convenience to the husbandman and the hunter, inVVhat is meant asmuch as it affords them light to continue their by the Harvest and the Hunt- occupation, and, as it were, lengthens out their er's Moon, and day, the first is called the harvest moon, and the swhen do they occur? d second the hunter's moon. These moons are always most beneficial when the moon's ascending node is in or near Aries. 1366. The following signs are used in our common almanacs to denote the different positions and phases of the moon. D or D) denote the moon in the first quadrature, that is, the quadrature between change and full; C or ( denotes the moon in the last quadrature, that is, the quadrature between full and change. O denotes new moon; O denotes full moon. 1367. When viewed through a telescope, the surface of the moon appears wonderfully diversified. Large dark spots, supposed to be excavations, or valleys, are visible to the eye; some parts also appear more lucid than the general surface. These are ascertained to be mountains, by the shadows which they cast. Maps of the moon's surface have been drawn, on which most of these valleys and mountains are delineated, and names are given to them. Some of these excavations are thought to be four miles deep, and forty wide. A high ridge generally surrounds them, and often a mountain rises in the centre. These immense depressions probably very much reeemble what would be the appearance of the earth at the moon 33* 890 NATURAL- PHILOSOPHY. were all the seas and lakes dried up. Some of the mountains are supposed to be volcanic. What are the 1368. OF THE TIDEs.-The tides are the Tides? regular rising and falling of the water of the ocean twice in about twenty-five hours. They are occasioned: by the attraction of the moon upon the matter of the earth, and they are also affected by that of the sun.,pqZai~n 1369. Let M, Fig. 198, be the -moon revolving in Fie. 198. her orbit; E, the earth covered with water; and 8, Fig. 198. the sun. Now, the point of the earth's surface, which is nearest to the moon, will gravitate towards it more, and the remoter point less, than the centre, inversely as the squares of their respective distances. The point A, therefore, tends away from the centre, and the,centre tends away from the point B; and in each case the fluid surface. must rise, and in nearly the same degree in both cases., The effect must be diminished in proportion to the distance from these points in any direction; and at the points C and. D, ninety degrees distant, it ceases. But there the level of the waters must be lowered, because of the exhaustion at those places, caused by the overflow elsewhere. Thus the action of the moon causes the ocean to assume the form of a spheroid elongating it iu the direction of the moon. ASTRONOMY. 391 Thus any particular place, as A, while passing from under the moon till it comes under the moon again, has two tides. But the moon is constantly advancing, in its orbit, so that the earth- must a little more than complete its rotation before the place A comes under the moon. This causes high water at any place about fifty minutes later each successive day. As the moon's orbit varies but little from the ecliptic, the moon is never more than 299 from the equator, and is generally much less. Hence the waters about the equator, being nearer the moon, are more strongly attracted, and the tides are higher than towards the poles. 1370. The sun attracts the waters as well as the moon. When the moon is at full or change, being in the same line of direction, *(see Fig. 198), the sun acts with it; that is, the sun and moon tend to raise the tides at the same place, as seen in the figure. The tides are then very high, and are called spring tides. Ex~plain IFig. But when the moon is in its quarters, as in 199. Fig. 199, the sun and moon being in lines at Fig. 199. right. angles tend to raise tides at different places; namely the moon at C and D, and the sun at A and B. Tides that are produced when the moon is in its quarters, are low, and are called neap tides. 1371. There are so many natural difficulties to the free progress of the tides, that the theory by which they are accounted for is, in fact, and necessarily, the most imperfect of all the theories connected with astronomy. It is, however, indisputable that the moon has an effect upon the tides, although it be not 892 NATURAL PHILOSOPHY. equally felt in all places, owing to the indentations of the coast, the obtructions of islands, continents, &c., which prevent the free motion of the waters. In narrow rivers the tides are frequently very high and sudden, from the resistance afforded by their banks to the free ingress of the water, whence what would otherwise be a tide, becomes an accumulation. It has been constantly observed, that the spring tides happen at the new and full moon, and the neap tides at the quarters. This circum stance is sufficient in itself to prove the connexion between the influence of the moon and the tides. 1372. An Eclipse is a total or partial obWhat is an Eclipse? scuration of one heavenly body by the intervention of another. The situation of the earth with regard to the MThen does an eclipse of the moon, or rather of the moon with regard to the sun or of the earth, occasions eclipses both of the sun and moon take place? moon. Those of the sun take place when the moon, passing between the sun and earth, intercepts his rays. Those of the moon take place when the earth, coming between the sun and moon, deprives the moon of his light. Hence, an eclipse of the sun can take place only when the moon changes, and an eclipse of the moon only when the moon fulls; for, at the time of an eclipse, either of the sun or the moon, the sun earth, and moon, must be in the same straight line. If the moon revolved around the earth in the Why is there nt an eclise at same plane in which the earth revolves around every newl and the sun, that is, in the ecliptic, it is plain that full moon? the sun would be eclipsed at every new moon, and the moon would be eclipsed at every full. For, at each of these times, these three bodies would be in the same straight line. But the moon's orbit does not coincide with the ecliptic, but is inclined to it at an angle of about 50 20'. Hence, since the apparent diameter of the sun is but about ~ a degree, and that of the moon about the same, no eclipse will take place at ASTRONOMY. 393 new-or full moon, unless the moon be within ~ a degree of the ecliptic, that is, in or near one of, its nodes. It is found that if the moon be within 16l~ of a node at time of change, it will be so near the ecliptic, that the sun will be more or less eclipsed; if within 12~ at time of full, the moon will be more or less eclipsed. Why are there 1373. It is obvious that the moon will be snore eclipses of oftener within 16~~ at the time of new moon, t.e s than of than within 12~ at the time of full; consethe mnoon in a given course of quently, there will be more eclipses of the sun years? than of the moon in a course of years. As the nodes commonly come between the sun and earth but twice in a year, and the moon's orbit contains 360~, of which 16~~, the limit of solar eclipses, and 12~, the limit of lunar eclipses, are but small portions, it is plain there must be many new and full moons without any eclipses. Although there are more eclipses of the sun Expolain Fig. than of the moon, yet more eclipses of the 200. moon will be visible at a particular place, as Boston, in a course of years, than of the sun. Since the sun is very much larger than either the earth or moon, the shadow of Fig. 200. these bodies must always terminate in a point; that is, it must always be a cone. In Fig. 200, let S be the sun, m the moon, and E the earth. The sun constantly illuminates half the earth's surface, that is, a hemisphere; and consequently it is visible to all in this hemisphere. But the moon's shadow falls upon a part only of this hemisphere; and hence the sun appears eclipsed to a part only of those to whom it is visible. Sometimes, when the moon is at its greatest distance, its shadow, O 394 NATURAL PHILOSOPHY. m, terminates before it reaches the earth. In eclips'es of this kind, to an inhabitant directly under the point 0, the outermost edge of the sun's dise is seen. forming a bright ring around the moon; from which circumstance these eclipses are called annular, from annulus, a Latin word for ring. Besides the dark shadow of the moon, m 0, in which all the light of the sun is intercepted (in which case the eclipse is called total), there'is another shadow, r C D S, distinct from the former, which' is called the penumbra. Within this, only a part of the sun's rays are intercepted, and the eclipse is called partial. If a person could pass, during an eclipse of the sun, from O to D, immediately on emerging from the dark shadow, O m, he would see a'small part of the sun; and would continually see more and more till he arrived at D, where all shadow would cease, and the whole sun's disc be visible. Ap pearances would be similar if he'went from O to C. Hence the penumbra is less and less dark (because a less portion of the sun is eclipsed), in proportion as the spectator is more remote from 0, and nearer C or D. Though the penumbra be continually increasing in diameter, according to its length, or the distance of the moon from the earth, still, under the most favorable circumstances, it falls on but about half of the illuminated hemisphere of the earth. Hence, by half the inhab itants on this hemisphere, no eclipse will be seen. 1374. Fig. 201 represents an eclipse of the Ecl~plain Fig. moon. The instant the moon enters the earth's 201. shadow at x, it is deprived of the sun's light, rig. 201. ~~-~~~5~ ~ ~ Bl \~~s~ —-- ASTRONOMY. 395 and is eclipsed to all in the unilluminated hemisphere of the earth. Hence, eclipses of the moon are visible to at least twice as many inhabitants as those of the sun can be; generally the proportion is much greater. Thus, the inhabitants at a particular place, as Boston, see more eclipses of the moon than of the sun. The reason why a lunar eclipse is visible to all to whom the moon at the time is visible, and a solar one is not so to all to whom the sun at the time is visible, may be seen from the nature of these eclipses. 0 We speak of the sun's being eclipsed; but, properly, it is the earth which is eclipsed. No change takes place in the sun; if there were, it would be seen by all to whom the sun is visible. The sun continues to diffuse its beams as freely and uniformly at such times as at.others. But these beams are intercepted, and the earth is eclipsed only where the moon's shadow falls, that is, on only a part of a hemisphere. In eclipses of the moon, that body ceases to receive light from the sun, and, consequently, ceases to reflect it. to the earth. The moon undergoes, a change in its appearance; and, consequently, this change is visible at the same time to all to whom the moon is visible; that is, to a whole hemisphere of the earth. 1375. The earth's shadow (like that of the moon) is encompassed by a penumbra,' C R S D', which is faint at the edges towards R and S, but becomes darker towards F and G. The shadow. of the earth is but little darker than the region of the penumbra next to it. -Hence it is very difficult to determine the exact time when the moon passes from the penumbra into the shadow, and from the shadow into the penumbra; that is, when the eclipse begins and ends. But the beginning and ending of a solar eclipse may be determined almost instantaneously. 1376.. The diameters of the sun and moon What is meant t igs meant are: supposed to be divided into twelve equal by digits, as applied to eclipses parts, called digits. These bodies are said to of. the sun and have as lmany digits eclipsed as there are of f te mpars involoon those parts involved'in darkness. 396 NATURAL PHILOSOPHY. 1377. There must be an eclipse of the sun as often, at least, as the moon, being near one of its nodes, comes between the sun and the earth. The greatest number of both solar and lunar eclipses that can take place during the year is seven. The usual number is four, two solar and two lunar. 1378. A total eclipse of the sun is a very remarkable phenonmenon. June 16, 1806, a very remarkable total eclipse took place at Boston. The day was clear, and nothing occurred to prevent accurate observation of this interesting phenomenon. Several stars were visible; the birds were greatly agitated; a gloom spread over the landscape, and an indescribable sensation of fear or dread pervaded the breasts of those who gave themselves up to the simple effects of the phenomenon, without having their attention diverted by efibrts of observation. The first gleam of light, contrasted with the previous darkness, seemed like the usual meridian day, and gave indescribable life and joy to the whole creation. A total eclipse of the sun can last but little more than three minutes. An annular eclipse of the sun is still more rare than a total one. 1379. OF TIME.-When time is calcuehate is the dif lated by the sun, it is called solar time, and the solar and the the year a solar year; but when it is calcusidereal year? lated by the stars, it is called sidereal time, and the year a sidereal year. The sidereal year is 20 minutes and 24 seconds longer than the solar year. 1380. The. solar year consists of 365!Vhicl, is the longer, a'solar days, 5 hours, 48 minutes, and 48 seconds; or a sidereal but our common reckoning gives 365 days year, and by how'much? only to the year. As the difference amounts to nearly a quarter of a day every year, it is usual every fourth year-to add a day. Every fourth year the Romans reckoned the 6th of the calends of March, and the following, day as one day; which, on that account, they called bissextile, or twice the 6th day; whence we derive the name of bissextile for the leap year, ASTRONOMY. 397 in which we give to February, for the same reason, 29 days every fourth year. 1381. A solar year is measured from the time the earth sets out from a particular point in the ecliptic, as an equinox, or solstice, until it returns to the same point again. A sidereal year is measured by the time that the earth takes in making an entire revolution in its orbit; or, in other words, from the time that the sun takes to return into conjuction with any fixed star. 7zat is the pre- 1382. Every equinox occurs at a point, cession of the 50" of a deg. of the great circle, preceding equinoxes, emoes the place of the equinox, 12 months before; and this is called the precession of the equinoxes. It is'this circumstance which has caused the change in the situation of the signs of the zodiac, of which mention has already been made. 1383. The earth's diurnal motion on an inclined axis, together with its annual revolution in an elliptic orbit, occasions so much complication in its motion as to produce many irregularities; therefore, true equal time cannot be measured by the sun. A clock which is always perfectly correct will, in some parts of the year, be before the sun, and in other parts after it. There are When do the but four periods in which the sun and a sun and clock perfect clock will agree. These are the agree? 15th of April, the 15th of June, the 1st of September, and the 24th of December. What is the 1384. The greatest difference between greatest dif. true and apparent time amounts to between fee7e e- sixteen and seventeen minutes. Tables of tween t~'ue and apparent equation are constructed for the purpose of time? pointing out and correcting these differences 34 898 NATURAL PHILOSOPHY. between solar time and equal or mean time, the denomination given by astronomers to true time. 1385. As it may be interesting to those who have access to a celestial globe to know how to find any particular star or constellation, the following directions are subjoined. There is always to be seen, on a clear night, a beautiful cluster of seven brilliant stars, which belong to, the constellation "Ursa Major," or the: Great Bear. Some have supposed that they will aptly represent a plough; others say that: they are more like a wagon and horses, the four stars representing the body of the wagon, and' the other three the horses. Hence they are called by some the plough, and by others they are called Charles' wain, or wagon. Fig. 202 represents these seven stars; Fig. 202. a b d g represent the four, and e z B P the other three stars. Perhaps they may more properly be called a large I! dipper, of which e z B represent the handle. If a line be drawn through the stars b and a, and carried upwards, it + I will pass a. little to the left, and nearly + I', + touch a star represented in the figure by B g P. This is the:'polar star, or the north / pole star; and the stars b and a, which ~4 appear to point to it, are called the pointers, because they appear to point to the polar star. The polar star shines with a steady and rather dead kind of light. It always appears in the same position, and the north pole of the earth always points to it at all seasons of the year. The other stars seem to move round it as a centre. As this star is always in the north, the cardinal points may at any time be found by starlight. By these stars we. can also find any other star or constellation. Thus, if we conceive a line drawn from the star z, leaving B ASTRONOMY. 399 a little to the left, it will pass through the very brilliant star A. By looking on a celestial globe for the star z, and supposing the line drawn on the globe, as we conceive it done on the heavens, we shall find the star and its name, which is Arcturus. Conceiving another line drawn through g and b, and extended some distance to the right, it will pass just above another very brilliant star. On referring to the globe we find it to be Capella, or the goat. In this manner the student may beccme acquainted with the appearance of the whole heavens. TABLE T. Distance Time of Revolution from the Mercury 3,140.062.47 6.680 Mercury....... 24 hrs. 87.98 ds. 37 Vents70......... 7,800.952.93 1.911 Venus......... 23.36 " 224. 7 a68 Venosf. 1W78ile.93s. Earth......... 7,926 1.000 1.00 -1.000 Earth.. 24 " 365.25 " 95 Mars.......... 4,100.138.50.431 Mars...... 24.64 "L 687. " 144 Jupiter........ 87,000 1233.412 2.85.037 Jupiter........ 9.94 " 4332.60 " 494 o Saturn........ 79,1.60 900.000 1.03.011 Saturn....... 10.27 " 10759. " 906 Uranus........ 34,500 82.759.76.003 Uranus........ 9.5 " 30688. "| 1,822 Neptune.. 41,500 144.008.69.001 Neptune....... 2,850 Sun. | 887,870 1410366.376 28.65 Moon......... Moon......... 2,163.020.15 Sun 25.3 " T'ABLE II. - Of Secondary Planets. OF THE MOON. OF THE SATELLITES OF JUPITER. Inclination Inclination of Distance from the Earth. of Orbit Revolution around Distance from Jupiter. Orbits to the Orbit Revolution around to the Ecliptic. the Earth. of Jupiter. Jpir. Miles. Deo. Min. Dass. HIours. Min. Miles. Dey. Mil. Sec. Days. Hours. Min. 240,000 5 5 27 7 43 I 269,800 3 18 38.1 18 27 I1 426,500 3 18 O0 3 14 58 Moon's diameter, 2159. Bulk (that of the earth being l 680,000 3 13 58 7 1 42 1), 1-49. ill 680,000 3 13 58 7 3 42 Period from change to change, 29 days, 12 hours, 44 IV 1,152,000 2 36 00 1 16 32 minutes.. OF THE SATELLITES OF SATURN. OF THE SATELLITES OF URANUS. Inclination of Inclination of Distance from Saturn. Orbits to the Orbit evolution Distance from Uranus. Orbits to the Orbit aro olution of Saturn. arund Saturn. of Uranus. around Uranus. lVM;'s. Deg. Min. Days. MHotrs. Min. Miles. De,. Min. Sec. Days. Hours. Min. VII 126,020 30 0 22 37 9 5 VII 161,720 30 1 8 53 I 2or80 16 7 I 200,235 30 1 21 18 II 293,600 do. 8,16 57 11 256,449 30 2 17 44 III 342,416 do. 10 23 4 III 358,225 30.4 12 25 IV 392,450 do. 13 10 56 IV 830,440 30 15 22 41 V 785,060 do. 38 1 48 V 2,414,660 42 45 79 7 54 VI 1,570,000 do. 107 16 42 TABLE III. Latitude. Longitude. Latitude. Longitude. 0 t,O O, London..... 51 30 49 N. 0 5 48 W. Washington (U. S.).... 38 53 34 N. 77 1 30 W. Greenwich....... 51 28 39 N. 0 0 0 New York 40 42 40 N. 74 1 8 W. Paris. 48 50 13 N. 2 20 24 E. Philadelphia.... 39 56 59 N.. 75 9 54 W. Berlin 52 31 13 N. 13 23 52 E. Boston....... 42 21 23 N. 71 4 9 W. Dublin 52 23 13 N. 6 20 30 W. Baltimore...... 39 17 23 N. 76 37 30 W. Edinburgh....... 55 57 23 N. 3 10 54 W. Albany......... 42 39 3 N. 73 44 49 W. Bremen.. 53 4 36 N. 8 48 58 E. Brunswick (Maine)... 43 53 0 N. 69 55 1 W. t Copenhagen.......... 55 40 53 N. 12 34 57 E. Charleston (S. C.)..... 32 46 33 N. 79 57 27 W. Dorpat (Russia)..'..... 58 22 47 N. 26 43 45 E. Cincinnati............ 39 5 54 N. 84 27 0 W. Milan,. 45 28 1 N. 9 11 48 E. New Haven.... 41 18 30 N. 72 56 45 W. Marseilles..... 43 17 50 N. 5 22 15 E. New Orleans.. 29 57 45 N. 90 6 49 W. Naples....... 40 51 47 N. 14 15 4 IE. Princeton (N. J.)... 40 20 41 N. 74 39 33 W. Petersburgh........ 59 56 31 N.. 30 18 57 E. Providence.......... 41 49 22 N. 71 24 48 W. Rome.... 41 53 52 N. 12 28 40 E. Richmond (Va.).. 37 32 17 N..7727 28 W. Turin. 45 4 6 N. 7 42 6 E. Charlottesville(Va.).... 38 2 3 N. 78 31 29 W. Vienna.......... 48 12 35 N. 16 23 0 E. Savannah (Ga.). 32 4 56 N. 81 8 16 W. Wardhas (Lapland).... 70 22 36 N. 31 7 54 E. Schenectady (N. Y.)....42 48 0 N. 73 55 0 W. Canton.............. 23 8 9 N.. 113 16 54 E. Cape Horn............ 55 58 41 S. 67 10 53 W. Cape of Good Hope.... 33 56 3 S. 18 28 45 E. ILLUSTRATIONS. Fig. 2. Fig. 3. Fig 1. Fig.'4. = _~S —=-~=-;-;Lx 404 NATURAL PHILOSOPHY. Fig. 5. Fig. 6. Fig. 7. A BCDE F "~ames~~~!~~ ~~~~Fig. 8. Fig. 9. iD~~~~~~~~~~~ AD p U. 0.0 AXBA B D E H A'B Fig. 15. ac~ l~~Fig. 16. Fig. 17. {K~~~~DA B E: c A ILLUSTRATIONS 405 Fig. 18. Fig. 19. C GO Fig. 20. Fig. 21. C F Fig. 22. Fig. 23 Fig. 24. Fig. 25. Fig. 26 Fig. 27. I' $ 406 NATURAL PHILOSOPHIIY. Pig. 28. C Fig. 29. Ln Fig.S30. Fig. 31. WORM.......... M. wf ILLUSTRATIONS. 407 Fig. 32. Fig. 34. P Fig. 33 w Fig. 35 Ing... Fig. 37. Fig. 38. Fig. 39 -Pig. 40. Fig. 41. "9i 408 NATURAL PHILOSOPHY. Fig. 43 Fig. 42. i_ B P \V/ p WI Fig 4 Fig. 45. Fig. 48. Jig. 49. Fig. 46. F ~ F PB b Fig. 61. Figme ~~~~~Big. a Fig. 52. ILLUbI'tATION$S., 409 Iig..4. Fig. i. 6Fig. 56 ~ B B Fig. 58. Fig o9 Fig 60 Fig. 62. ID Fig. 61. F 6. Fig. 49. A C B 35 410 NATURAL PHILOSOPHY. Fig. 64. Fig. 68. A A Fig. 65. Fig 67. A Fig. 68.'w~,'~i~~f~~~zS ~'~Fig. 69. ab r$ R ^ E\\w\\\\\ | "e tiDn ILLUSTRATIONS. 411 Fig. 70. Big. 71. lig. 7 2. Fig. 73. Fig. 74. ig. 75. ~~-~~- ~_ II _ E —LL~~~~~~~~~~~~~~~~ 412 NATURAL P/IILOSOpty.....Fig. 76. ----------— (-j~~~~~ ST (1 6 K-~~- e!141 Fg ig.. f:.......... -rig. 78 ii~~~~~~~~~~~7 Fig. 80...... a A FAIR ~.'' ~I"'::. ---— ~,1-.....:::'"'' ~ Y,' ~ ~.':",:" 28"~"~;~':"~~~~~P:'.';:~'~:':'::~?:'.. 27 RAIN Fig. 82 Fig. 83 ~ig~ ~~Fi. 83. Fig. 81 p~~~~ Y.'~`~~~~~~~~:I Eig~.~. $3`~`'~`~':~'~:~ ~I.'''~;`~~:tC'''''' ": ~-.~;~ ~ ~ S ILLUSTRATIONS. 413 Fig. 84. A Fig. 85. Fig 86 It B5 121 35@ 414 NATURAL PHILOSOPHY. Fig. 87.! s ILLUSTRATIONS. 415 Pig. 90. Fig. 91. Fig. 92. Fig. 96, pig. 93. Fig. 94. Fig. 95. Fig. 97. Fig. 98. Fig. 99. Fig. 100. Pig. 101. 416 NATURAL PHILOSOPHY. Big. 102. L, " w ADFig. 103. Fig. 10. ii~~~~~~~'I Fig. 105. --— 1 -- -----— t' - - — i Fig. 106.. >,~~~~~c'. — ILLUSTRATIONS. 417 Fig. 107. Fig. 108. Fig. 108. T, AA. - ~ ~~~ 418 NATURAL PHILOSOPHY. H ilr - ll I ~'~ i "~~~~~~~~~~~~~~~~'. isl','.II J! ILLUSTRATIONS. 419 V ~~~~~ ~z 4" 0- W~~~~~~~~~~~~ I' z I'r z r~~~ 4-1 li~~ 0 I-~~~~~~~~~i'1~~~~~ ii~ ~~ H 420) NATURAL PHIILOSOPHY.'i/ ] II.8 I /1 /~~~~~~~/~ II'',,' a i~~~~rliiliiIII ii g~ ~~~~IZ' / /I I' i',~,'I/t/l "'f,!J!,!Il/,/:t':i!,,! b,!Ll,?! U,;:!l!'/..ti. ~ M I IN' I;/ ":iiP:1 i:',:!!! i il!i~ iill,it -?,1''y' lialI ILLUSTRATIONS. 421 P;~4: —'080~~~~~~~~~~E ~ OX d X~~~~~~~~~~~~u'-~~~~~~ d~~~~~~~~~~~~~~~~~~~~~u ~k g 86 Fig. 11. ci Itz ~~~~~~~~~~~~~~~_ ~ I-A~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~s ~~~~~~f! t~~~~~~~~~~~~~~uul~...... ap~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~sw ILLUSTRATIONS. 423 Fig. 117. Fig. 118. *Fig. 120. Fig. 119. Fig. 121. Fig. 122. Fig. 123. a Fig. 124. A Fig 125. 1= I XC I 9 A E B Fig. 127. Fig. 126. B i 2 C D IT 424 NATURAL PHILOSOPHY. Fig. 128. Fig. 129. Fig 130. Fig. 131. G A B D E A Pig, 132. Fig. 133.!11 _- (~l4 a, e ILLUSTL'{^ATIONS. 125 Fig. 134. Fig. 135. Fig..136. A. mg. 137.: 6 426 NATURAL PHILOSOPHY. Fig 138. Fig. 139. B N Fig. 140. A E CM Fig. 141. A I c Fig. 142. ilet.. Indigo........... Blue... Greea. i Yellow \ Red..".... ]": J) ILLUSTRATIONS. 427 Fig. 144. Big. 14,' Fig. 145. Flg. 146. Fig. 147. - ~ ij ~. Fig. 148. C3 c Fig. 149. E A _ Fig. 150. _~~~~o ra _ = _ n %aQ _ 428 NATURAL PHILOSOPHY. Pig. 152. Fig. 153. DFEig. 15F7. F~ig. 15 Fig. 155.160. t7p~~~~ rl ~Fig. 156. Fig. 157. Hi Fig. 159. Fig. 160. ILLUSTRATIONS. 429 Fig. 161. Fig. 12.ig. 163. Fig. 164. ccl,'IL\O I _f~~~~~~~~~t rr ~~~~~~~~~~~~~~~~~~~~~N t jI' 1 i~~~~~~~~~~~~~~~~~~~illl CJL ~xl~~~~~~~~~~~~~~~~~~~~~~~ ~~~~ ~s=~ ~ ~ ~ ~' i~~~~~~~i,,,,,I, I,,I,, i~~~~~~~~~~~~~~~~~~~It ILLUSTRATIONS. 431 Fig. 168. Fig. 169 432 NATURAL PHIJL-SOPIY. Fig. 170. F'ig. 171. Filg. 17. Fig. 172. I174 be.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. Fig, 174..I *'." ~l ~ ~}~$~[~ [~~~~~~~l~l i~l " l iul~w iIlI 1111? Ii -~~ — -—:~: MORSE'S TELEGRAPUIC ALPHABET.1 __ ALPHABET. U a —o- -, NUBMAL9. C —~- -~ C = —-— r=l= —= —~.3 —-—;;-~ e s --- 3 ------- 67- - - - - - w 7 k 9~ 17b - - - 434 NATURAL PHILOSOPHY. Fig. 176 Fig. 177. cL~,i_7,ce, ILLUSTRATIONS. 435 Fig. 178. Fig. 179. /4E 436 NATURAL PHIL OSOPHY. Fig. 182. Jupiter Ircury Mars Venus Earth. O O 0 0 Herschel 35: '- gnmO Aa " ir' 0 ~ ~.,a ~,/~ Pi P~Paay? s lu n..~p.:fST 21'~s - Libra O va' " -,, F e_ Bagl 4P W. Om~~~~~~~n ea H ~k-.?_l FZi.'4 o'." N~P, G LYPI= —-~-sltg. " ~ "J'ggI'gleIs 41F ~ ~ ~ fif~% n ~ a ~ r c~~~~~~~~~~Il csY~~~~~~~~~~~~~~~~~~~~~~~~~4 438 NATURAL PHILOSOPHY. Fig. 185. B3 E Fig. 186. Fig. 187. *L\ \ \I/ / Yd4 U~~ ILLUSTRATIONS. 4389 Fig. 188..rv / bzumel'...;.h..'Er Sme re i, P r t / h eiLrthera Ni DCL'Y 440 NATURAL PHILOSOPHY. Fig. 189 KI ILLUSTRATIONS. 441 Fig. 190. Fig. 191. 442. NATURAL PHILOSOPHY - Fig. 192, Z-~~~~~~~~~~~ ILLUSTRATIONS. 443 Fig. 193. 1 __ _ 4-44 NATURAL PHILSOPHY. Fig. 194. ILLUSTRATIONS. 445 Fi~. 195. A lPig.I95. 13, __ g8 446 NATURAL PHILOSOPHY. Fig. 197. C atII AN Fig. 198. Mf M Fig. 200o. abL~~~~~~.7re th - ILLUSTRATION' 447 Fig. 201. Fig. 202 + + + +b B._ __..a + A. Air, compressibility of the.162 Air, condensation of at surface of Aberration of light... 384 the earth........ 140 " spherical.. 247 Air, condensed, experiments with 163 Accidental colors.......252 Air contained in wood and water, Achromatic........247 experiments to show.... 161 Acid, carbonic........ 21 Air diminishes upwards in densAcid, sulphuric, effects of on ity........... 140 water......... 187 Air, elasticity of the... 142, 162 Acoustic paradox..... 177 Air, elasticity of the, experiments Acoustics.........173 showing........... 160 " definition of... 18 Air, effect of gravity on density of 38 Acoustic tubes.....179 Air essential to animal life, exAction........... 45 perirnent to prove..... 168 Action and reaction, illustration Air essential to combustion, exof.......... 46 periments to prove.....168 Action, suspension of..... 85 Air, fluidity of........ 142 Actynolite....... 21 Air, fluidity of, experiments showAeriform, definition of... 19 ing............. 165 I' fluids........ 138 Air, gravity of the, experiments Aeriform fluids compressed and illustrating....... 157 expanded without limit... 139 Air-gun.......... 164 Airiform fluids have no cohesive Air, how a mechanical agent. 142 attraction..........139 " impenetrability of...22, 141 A'6riform fluids have all the prop- " inertia of..... 28, 143 erties of liquids....... 140 Air, inertia of, experiments showAeriform fluids have weight... 139 ing.......... 165 Aeronaut, how he descends from Air, lightness of the...... 162 a balloon......... 38 " materiality of the..... 162 AMrolites.......... 387 Air, miscellaneous experiments Affinity, chemical....19, 27 with.......... 166 Agents... 18 Air necessary to animal life and' imponderable...... 18 ta combustion.....140 " ponderable.... 18 Air, of what composed..... 20 Air.140 Air, pressure of the as the depth 162 Air, a bad conductor of heat.. 191 " pressure of in all directions 162 Air, as an element...... 19 Air, pressure of the on a baromAir-bladder of fishes...... 47 eter........... 14 Air-chamber..........163 Air, pressure of the on a square Air, component parts of the.. 140 inch....... 141 281 note Air, pressure of the on the body 141 Air, compression of, caused by Air, pressure of the preserves the gravity........ 39 liquid form of some bodies.. 169 38* 450 INDEX. Air, pressure of the retards ebul- Aristotle's opinion of the velocity lition.......... 168 of a falling body....... 52 Air-pump.......... 154 Arsenic............ 20 Air-pump, experiments performed " not malleable..... 31 by the.....157 Asteroids........... 339 Air-pump of steam-engine.. 201 Astrea.'....... 339 Air-pump, the double.. 156 Astronomy, definition of. 17, 18, 335 Air, resistance of the.... 25, 38 Astronomers, distinguished... 336 Air, resistance of the to a cannon- Astronomy, father of.. 336 ball...... 62 Atmosphere, weight of the... 141 Air, scales for weighing.... 160 Atmospheric telegraph. 331 Air, two principles, properties of 139 Attraction........ 25, 26, 33 " when heaviest.. 146 " capillary..... 111 Air, when the best conductor of " chemical...... 27 sound'....'176'" kinds of...... 27 Air; why not visible..... 140 " law of falling bodies. 51 Albite........... 20 " mutual....... 34 Alison, extract from. 70 "c of all bodies.... 34 " All's well," how far heard.. 176 " of cohesion.... 27 Alumina......... 21 " of gravitation.... 27 Aluminum......... 20 " of the earth....... 33 Ampere's discoveries in electro- " on what dependent.. 34 magnetism......... 309 Attwood's machine... 52 Ampere's electro-magnetic appa- Augite........... 21 ratus......... 314 Austral polarity 302 Analysis of the motion of a fall- Axes of the planets, inclination ing body.......... 52 of.............350 Angle.... 48 Axis, exact sense of........ 81 Angles, how measured..... 48 Axis, longer, a body revolving Angle of vision....... 219 around........... 61 Angles of incidence and'of reflec- - Axis of motion........ 59 tion..48,-49, 216 Axis of the earth, effects of its Angles, right, obtuse and acute 48 inclination.........354 Animal electricity... 282 Axis of the earth, geological theAniiials, sagacityof..... 92 ory of... 62 Annrealing..... 31' Axis, what bodies revolve around Antiniony.' 20 an............. 59 "'I not malleable.... 31 Axle.......8.....81 Aphelion........... 349 Azote.......... 20, 140 Apogee. 349 Apparatus for illustrating the B. tendency of a body to revolve around its shorter axis.'. 61 Babbit's metal.......... 99 Apparition', circle of, perpetual.385 Bain's telegraph....... 326 Apparitions, deceptive. 225 Baker, the Connecticut..... 191 Aqueous humor...... 237, 239 Balaince-wheel......... 104 Arago's experiments on velocity Balance............ 75 of sound.. 176 _Ballittic pendulum...... 63 A rbbr'.'..8......... 81 Bsalloon, how to descend from... 38 Archimedes' boast to Hiero 95 " the pneumatic....161 Al;chiiiedes, burning mirrors of. 228 Ball, thrown in a horizontal diArchimedes discovers the method rcction............ 64 of ascertaining the specific gravy Ball, force of, how. estimated. 63 ity of bodies. [: 127 note Bands with one and two centres Archimedes, screw of......132 of motion....... 83 Arc of a pendulum......101 Banks,"Sir Joseph...... 190 Arcturus.,... 399 Barber's Grammar of Elocution. 180 INDEX. 451 Barium......... 20 Bodies, what will rise and what Barometer......144 and note will fall in air...... 40 Barometer, the aneroid or porta- Body acted upon by three or more ble...... 145 forces....................57 Barometer, the diagonal..... 145 Body, parts of.which move with Barometer, of the different states greatest velocity. 60 of the............ 148 Bodies, what ones will float and Barometer, greatest depression of what sink in water...... 123 the...... 147 Body, when it will fall..... 66 Barometer, its importance 146 note Bohemia-slate, formations of.. 23 " rules of the... 147 Bolt-head, and jar..167 " the mercurial... 145 Bomme M.. 379 Base of a body.... 67 Bones of a man's arm, levers of Batteries, thermo-electric... 335. third kind.......... 77 " galvanic....... 287 Borax.20........... 20 Battering ram........105 Boreal polarity............302 Battering ram, force of, how es- Bottle, effect of pressure of the timated.........105 sea upon.......... 115 Battery, electrical....... 264 Boyle............144 Battery,.Groye's............293 Boynton's, Dr., chart of materi-' how discharged silently 265 als which form granite.... 21 Battery of the le'ctro-magnrieti Bramah's hydrostatic press.. 121 telegraph.......... 321 Brass, how made brittle.... 30 Battery,; protected sulphate of Breadth......, 23 copper..........293 Breast-wheel........ 82, 83 Battery, Smee's..... 290 Brittleness........ 27, 30 " sulphate of copper. 292 Brittleness, how acquired by iron, Beam pflight..'..... 213. steel, opper and brass.... 30 Belgrade, battle of, and the comet380 Bromine.. 20 Bellows,; hydrostatic, -how con- Brooks, how formed....124 structed.........1 19 Buckets of water-wheels.... 82 Bell, the diver's or the diving. 150 Bpuckets of water,, why heavier Bevelled wheels........ 85 when lifted from the well.. 126 Birds, bodies of....... 123 Bulk of a body, how ascertained, how.they fly.:.:.... 47 from its weight... 125 cc muscular power of.;.. 47 Burdens, how made unequal. 77 Bismuth......... 20 Burning-glasses..228, 235 " not malleable..... 31 Bissextile,.meaning of-.....396 C. Black. 252 Black lead, uses of in overcom- Cadmium......... 20 -inglfriction........ 99 Calcium...... 20 Bladder-gla'ss........159 Calliope..339 Bladder, inflated, why compress- Caloric......... 187 ed in water...... 115 Calorimotor.......... 297 Boats, how propelled. 47 Camera obscura...... 219, 240 Boats, on what principle they Camera obscura, portable, how float................ 123.made.219 Boats, motion in, why impercep- Cannon-ball, greatest velocity tible............26 that can be given to..... 63 Bode's law......... 342 Cannon-ball, force of the resistBodies........ 18 ance of the air to...... 62 " attraction of...... 33 Cannon, how far heard... 176.Bodies of drowned persons, why Caoutchouc, or India-rubber.. 30 they sink and afterwvards rise. 123 " balls, elasticity of. 47 Bodies, what are easily overset. 69 Capillary attraction.... 111 " what stand most firmly. 68 c " cause of.. 111 452 INDEX. Capillary tubes........ 111 Chimneys, glass, how preserved Capstan.......... 80 from cracking........ 192 Capstan and windlass, difference Chisels, on what principle con-.between.....80.... 80 structed.......... 91 Carbon......... 20 Chlorine.......... 20 Carbonate of lime... -... 21 Chlorite........... 21 Carbonate of magnesia.... 21 Chord, musical, how produced.182 Carbonic acid. 21 Choroid...... 237, 240 Carriages, high, why dangerous. 68 Chromatics........ 251 Carronades......... 63 Chromium..........20 Cartesian devil........162 Circle........... 48 Cask, how burst by hydrostatic Circle of perpetual apparition.385 pressure....... 120 note Circles. 59 Cassegranian telescope.... 250 Circles, circumference of, how diCastors, why applied to legs of - vided......... 48, 365 tables, i&c.......... 85 Circular motion...... 58 Catoptrics.. 215 Circular motion changed to recCelestial bodies, true place of. 384 tilinear by cranks.... 81 Celsius' thermometer..... 149 Circular motion, how caused. 58 Centraliforces...... 69 5Clay. 21 Centre of gravity.. 57, 58, 59, 66 Climates, cause of....... 354 Centre of gravity, illustrations Clock, before and after the sun. 397 of......... 66 note " how regulated..... 102 Centre of magnitude.. 58, 59, 66 "- moving power of... 104 Centre of motion.... 58, 59, 71 Clock, periods when it agrees with Centre of sphericity..... 37' the sun.......... 397 Centrej what bodies revolve Clocks, why they go fastest in around a...... 59 winter......-...102 Centres.......... 58 Clock, what it is........102 Centrifugal force 59'" wheels of, their use.. 102 Centrifugal force, effect of on a Clothing, cause of warmth of. 189 body revolving around its longer Clouds.. 24 axis............ 61 " of what composed.... 186 Centrifugal force, to-what propor- Cobalt........ 20, 298 tioned..........60 " not malleable..... 31 Centrifugal force, where greatest 103 Coffeepots, why with wooden han" meaning of.... 59 dies....... 190 Centripetal force........9Cgs. 5.......... 83, 84 " meaning of.... 59 Cohesion, attraction of... 27 Ceres...........339 Cohesion, attraction of, its effects Cerium........20 on watery particles..... 186 Chain-pump....... 131 Cold........ 185,192 Chaises, tops of, toggle-joint. 97 Cold, its effects on the density of Chamfered......... 91 bodies......... 192 Chantrey, the sculptor....191 Colors.......... 254 Charged, meaning of.....261 " accidental... 252 "Charlemagne," experiment on Columbium.20 board of the......115 Comets..........372 Charles V. and the comet..379 " density of.....379 Charles' wain or wagon....398 Comet, Halley's, as seen by Sir Chart of materials forming the John Herschel, and by Struve. erust of the earth...... 20 377, 378, 379 Chemical affinity...... 19, 27 Comet, Halley's, periodical time Chemical attraction...... 27 of.... 377 Chemiical effects of light.. 256, 257 Comets, how regarded formerly. 373 Chemical electricity....259 Comets in the solar system, numChemistry....... 19, 110 her of........... 379 INDEX. 453 Comets, Kepler's opinion of their Convex mirrors, effects of... 224 number........... 380 Convex screw......e.. 94 Comets, number of.. 373 Convex surfaces, facts with regard Comet of 1680...... 375 to..235 el " 1744......... 376 Copernicus........336 1811.........373 Copper.20 Comet of 1853, Mr. Hind's ac- Copper and tin, sonorous propercount of the...381 ties of....... 30 Comet of 1856........379 Copper, how made brittle.. 30 Comets, orbits of.......374 Cords, tenacity of...... 32 Comets, return of, first predicted Cork, how deep it will sink... 123 by Halley, Encke, and Biela. 377 " why lighter than lead.. 34 Comets, tails of........374 Cornea..... 237, 238 " velocity of.... 374 Corpuscular theory of light... 211 Common centre of gravity of two Couronne des tasses.290 or more bodies....... 69 Crank, dead point of.....- 81 Complex wheel-work..... 83 Cranks......... 80 Compound battery..... 290 Crown-wheel......... 84 " lever...... 75 Crust of the earth, materials com" motion....... 55 posing the....... 20 " " how produced. 54 i Crystalline humor, convexity, how Compressibility..... 27, 28, 29 l increased or diminished... 241 Concave mirrors....... 222 Crystalline humor, effect of when " " effects of.. 225 too round......... 242 Concave mirrors, laws of refiec- Crystalline lens....... 237 tion fromi... 227 Cup of Tantalus........ 133 Concave mirrors, peculiar prop- Cups, the Magdeburgh..... 157 erty.of........... 224 Current, velocity of a, how measConcave mirror, true focus of.. 224 ured............ 130 " screw.........:94 Curve of a projectile, on what deConcave. surfaces, facts with re. pendent........ 64 gard to....... 236 Curvilinear motion..... 61 Condensation.140 Cutting instruments...... 91 Condensed.........140 Cylinder, definition of a..... 79 Condenser..... 198 Cylinder, how made to roll up a " of steam-engine... 200 slope........... 68 Condensing syringe..... 156, 163 Cylinder, wheel substituted for. 79 Conduction of heat..... 190 Conductors of the galvanic fluid. 285 s......... 258,260 D. " of.heat.189 Cone...'........ 69 Daguerreotype proofs... 257 Conic sections 341 Darkness produced by, two rays Conjunction, inferior and supe- of light........ 212 note rior.............. 349 Davies' Treatise on Magnetism. 316 Connecticut baker..191 Day and night, cause of.... 358 Conservatory of arts and trades, Days and nights, cause of differhow restored to perpendicular.193 ence in length of......350 Constellations.383 Dead point of a crank..... 81 " of the zodiac.. 347 Delisle's thermometer... 149 Contractibility........ 281 Delphi, oracle of.1.....1 80 Converging rays.......212 Demetrius Poliorcetes.105 Conversation in polar regions Density.......... 27, 28 heard at great distances... 176 Density of air, effect *of gravity Convex mirrors......... 222 on......... 38 Convex mirrors, laws of reflection Depth of a well, how estimated. 53 from............226 Descartes..........144 454 INDEX. Devil, the Cartesian'... 162 Earth, a good conductor of sound 176 Dew and fog, difference-between. 150 ".as viewed from the moon. 364 Dew, how produced.......150 "attraction of the.... 33 Diagonal.......... 48 " centre of gravity of. 37 "of a parallelogram.. 55 Earth, consequences of a more " of a square...... 55 rapid rotation of the...... 366 Diallage........... 21 Earth, constituent elements of the 20 Diameter........... 48 Earth, crust of the, materials cormDiameter, equatorial, how length- posing,...... 20 ened..... 61 Earth, diameter of, how ascerDiameter, equatorial of the earth, tained.......... 365 longer than polar, and why.. 61 Earth, figure of the...... 364 Diameter of the earth, equatorial " how known to be round. 364 and polar......... 102 Earth, how much larger than any Diameter of the earth, how ascer- fallinghbody 33 tained..........365 Earth, motions of its inhabitants 365 Didynium.......... 20 " nearer the sun'in winter. 352 Digits............. 395 Earth, parts of which move most Dilatability........... 29 rapidly............ 61 Dionysius, ear of........ 178 Earth, strata of the.....2... 20 Dionysius, how he overheard his Earth, the principal reservoir of prisoners..........178 electricity. 261 Dioptrics. 230 Ebullition retarded by pressure " laws of.......230 of the air........ 168 Dipping of a magnet..... 303 Echo.............177 Dipping of a magnet, how reme- " why never heard at sea..178 died........... 303 Eclipse........... 392 Direction.......... 41 " annular.3.......394 line of. 66 Eclipses, greatest number of in a Discharge, the jointed.. 264 year....... 396 Dissolving views...... 246 Eclipse, lunar, to whom visible. 395 Distance at which a man- is in- " solar, to whom visible.395 visible........ 220 " totalof 1806...... 396 Distance, greatest which can be Eclipses, why more of the sunestimated...... 382 than of the moon.-..... 393 Distances measured by velocity Eclipse, why not at every new and of sound............. 177 full'moon.........392 Distillation.......... 194 Eclipse, partial...... 394 Distilled water, why used as stand- " total..3.......94 ard of specific gravity.... 123 Ecliptic. 345 Diverging rays........212 Egeria...339 Divers, limit to the depth of.. 115 Ehrenberg's microscopic observaDiving bell, or diver's bell.. 150 tions... 23 Divisibility....... 21 Elastic fluids..139 " extent of...... 23 Elasticity........27, 29, 30 " definition of.... 23 " of air........ 142 "Dodge," how children.... 26 " of gaseous bodies.. 30 Double action of the steam-engine 200 " of ivory...... 46 Drowned persons, why they sink Electrical battery.... 264 and afterwards rise..... 123 " bells. 273 Ductility......27, 31 " fire-alarm...... 330 Dynamics..... 17 " machine....266 " meaning of.... 18 Electrical machine, experiments with 2........ 270 E. Electrical sportsman......275 Electric current, direction of, how Earth.................368 ascertained. 310 INDEX. 455 Electrical tellurium..... 272 Electro-plastie process..... 331 Electric fluid, velocity of... 43 Electro plating and gilding... 331 Electricity.... 17, 18, 258 Electroscope......... 269 Electricity acquired by induction 278. Electrotype process... 231 Electricity and magnetism, resem- Elementary substances, enumerablance between.......302 tion of........... 20 Electricity, animal...... 282 Elements, the four........ 19 " by induction.... 266 Ellipse........... 341 " circuit of.... 265 Elocution, Barber's Grammar of. 180 Electricity as excited by galvan-. "Empty," common meaning of. 98 ism and by friction, difference Endosmose... 27, 112 between........ 283 Engineer, how enabled to direct Electricity by transfer... 266 his guns.......... 65 Electricity, effects of similar. Engine, the fire........ 154 states........ 263 " thesteam....... 196 Electricity, frictional... 282, 283 Equilibrium...... 74, 75 Electricity, frictional and chemi- " of fluids... 110 cal, how they differ..... 294 Equilibrium of fluids, exemplified Electricity, galvanic, quantity of.295 by means. of the. siphon..:. 133 " nature of.. 259 Equilibrium of fluids, how disturbElectricity, quantity of excited by ed by waves.. 131 chemical action... -. 284 note Equilibrium of fluids of different Electricity, simplest mode of ex- densities......... 113 citing...........262 Equilibrium of mercury, water, Electricity, the vitreous or posi- oil, air, &c.......... 113 tive, the resinous or negative. 262 Equinoxes......... 358 Electricity, three states of... 335 " precession of the.. 397 " voltaic........283 Equivalent, mechanical.... 58 Electrics........... 258, 260 Erect, why objects are seen.. 241 Electric telegraph, history of the 329 Erbium.. 20 Electrqo-magnet.. 317 Escapement-wheel.... 104 Electro-magnet, communication Essential property, meaning of 21 of magnetism to steel by means Essential properties of matter. 21 of............318 Eunomia..........339 Electro-magnetic multiplier.. 313 Evaporation, Dr. Watson's experElectro-magnetism.....17, 260, 308 iment............150 Electo-magnet, the U or horse- Eye............237 shoe............ 319 " a camera obscura.....240 Electro-magnetism, definition of. 18 " different parts of the... 237 Electro-magnetism, discoveries of Eye-glass..........248 ~ rsted, Faraday, Ampere, Ara- Eye, imperfections of, how caused 242 go, and Sir H. Davy...308, 309 c ofwhat composed..... 237 Electro-magnetism, facts of.. 309 Eyes, two, why they do not cause Electro-magnetic induction.. 312 double vision.......241 Electro-magnetic rotation. 313, 316 Exercises for solution.... 53 note Exhausting syringe.....163 Electro-magnetic telegraph, sig- Exosmose. 27, 112 nal-key and registering appa- Expansibility.27, 29 ratus of the...... 322 Expansion,. how it differs from Electro-magnet of Prof. Henry dilatation.29... 29 and Dr. Ten Eyck..... 317 Experiments showing inertia of Electro-magnetic telegraph.. 319 air........ 165 Electro-magnetic telegraph, how Extension........ 21, 23 put into operation......324 Electro-metallurgy...... 331 F. Electrometer.... 268 Eleetrolphorus.......29 ahrenheit's termoeter.. 140 456 INDEXI. Falling bodies, law of..... 51 Fluids, particles of, how arranged 114 Faraday, announcement of in re- " pressure of.114 lation to solar spots and mag- Fluids, pressure of, according to netic variation....... 304 height.........119, 120 Faraday's discoveries in electro- Fluids, pressure of, on what demagnetism..........308 pendent..........118 Faraday's electro-magnetic appa- Fluids, pressure of, to what proratus........313 portional.. 115 Faraday's nomenclature of elec- Fluids, surface of....... 110 tricity.......... 259 Fluids, upward pressure of. 114, 117 February, why 29 days every Fluids, why unsusceptible of formfourth year..... 397 ation into figures.... 110 Feldspar.......... 21 Fluorine.......... 20 Fire-alarm, the electrical... 330 Fly..... 143 Fire, as an element... 19 Flying of birds, how effected.. 47 Fire-engine.... 154: Fly-wheels....... 81 Fifth........... 184 Fly-wheels and the dead points of " how produced.....182 cranks........... 81 Figure..... 21, 23 Fly-wheel in the steam-engine. 203 Fishes, how they swim, rise or. Focus of concave mirrors... 224 sink, &c.... - 47 Bog and dew, difference between 150 Fixed pulley, mechanical advan- Fog, how produced......150 tage of........... 87 Force............41 Fixed pulley, operation of the. 87 Forces, at an angle..... 56 Flavio de Melfi, inventor of mari- " effects of.... 55 ner's compass........ 306 " three or more in action. 57 Flexibility....... 27, 31 " unequal at right angles. 56 Float, how heavy bodies can be Forcing-pump.......153 made to.......... 38 Formulhe..........44 Float-boards of water-wheels.. 82 Fortuna......... 339 Flora.......-.339 Fountain, glass and jet... 159 Florence, experiment made at on Fountain, Hero's.......138 impenetrability of water. 22, 109 Fountains, artificial, how conFluid and solid bodies, difference structed.,....... 137 between........... 108 Fountains, how formed.... 137 Fluid, definition of...;.. 108 Fourth............184 Fluidity of air...... 142,165 Fowling-pieces, length of... 63 C" what constitutes... 108 Franklin, inventur of lightningFluid pressure, law of.....115 rods...........281 Fluids, a6riform.......138 Free heat.187 Fluids, a6riform, expanded and Frictional electricity... 259,283 compressed without limit.. 139 Friction..... 90 note, 98 Fluids and liquids, how different 109 " cause of....... 99 Fluids, effects of their peculiar " how diminished.... 99 gravitation........ 113 " how increased..... 99 Fluids, equilibrium of... 122,133 CC loss of power caused by. 99 Fluids; downward pressure of, " important uses of... 100 how shown.........114 Friction of the beds and banks of Fluids, gravitation of..... -110 rivers...........130 "i how different from liquids 109 Friction, particles of fluids desti" how they gravitate... 113 tute of.......... 108 " lateral pressure of. 114, 116 Friction-wheels........ 99 117 Fuel, combustion of...... 24 Fluids, level or equilibrium of.110 Fulcrum........ 70, 71, 72 " mechanical agency of.. 138 " generally a pin or a rivet 76 Fluids of different densities, grav- Fulcrum in levers of different itation of. 112. kinds....... 77 INDEX. 457 Fulcrum of steelyards..... 74 Glass, the bladder....... 159 Fulton, Robert.........200 " the fountain and jet... 159 Fundamental law of mechanics. 71 " the hand...... 158 F'see of a watch.......107 " the I;dia-rubber.... 159 Glass, why easily cracked when G. suddenly heated......192 Glass, why used in mirrors... 221 Galaxy...... 383 Governor........106, 200 Galileo..... 100, 143, 337 Governor applied to steam-engine Galileo's experiment at Pisa to by James Watt..... 106, 203 prove his law of falling bodies 52 Governor, explanation of the.. 106 Galileo's law of falling bodies. 52 " uses of the...... 106 Galvanic action, three elements Grain of hammered gold.... 23 necessary for........ 285 Grand law of nature.... 69 note Galvanic batteries.......287 Granite............. 20 " battery.......289 Gravitation, attraction of.... 27' circle........ 286 " of fluids... 110,112 Galvanic circle, effects of, how in- Gravity......... 25, 33 creased.. 2871 Gravity causes pressure of fluids Galvanic circle, essential parts of upwards as well as downwards.117 a.................286 Gravity, centre of. -.. 37, 59, 66 Galvanic circle, simplest, of what Gravity, effect of on density of air 38 composed.286 Gravity, effects of on different Galvanic electricity.....259 bodies.... 41 Galvanic electricity, process for Gravity, force of, not affected by obtaining.......... 286 projection.... 64 Galvanic fluid, how excited.. 284 Gravity, force of on projectiles. 62 " piles..... 287." " where greatest 35 Galvanism...... 17, 18, 283 " how it increases and deC6 facts explained by.. 296 creases.......... 35 Galvano-plastic process....331 Gravity, law of terrestrial... 35 Galvanotype......... 331 Gravity, specific... 40, 126 note Garments, light-colored why cool 191 Gravity, specific, scales for ascer-' linen, why cool... 189 tamining.......... 126 Garments, to what they owe their Gravity, specific, standard of. 123 strength.......... 100 " terrestrial...... 34 Garments, woollen, why warm.189 Great Bear....... 398 Garnet............ 21 Greensand.........21 Gaseous bodies, elasticity of.. 30 Gregorian telescope.....250 Gaseous bodies, to what degree Gridiron pendulums...103 they may be dilated..... 29 Grove's battery..... 293 Gases............139 Gudgeons........... 80 Gases, how prevented from rising Guericke, Otto.1... 158 from a fluid........ 168 Guinea and feather drop..165 Gay Lussac's experiments on the Gunnery, science of..!.. 62 velocity of sound...... 176 Gunpowder, force of....... 63 Gearing......... 83 Gunpowder, great charges of useGeology........ 62 less and dangerous. 63 Georgium Sidas........ 369 Guns, how tested....... 63 Gibbous.......... 388 Guns, short ones, why preferable 63 Glucinum......... 20 Gun, the air..... 164 Gold.20 Gymnotus electricus...... 282 Gold, both ductile and malleable 31 " divisibility of..... 23 H Goid, the most malleable of all metals...... 31 Hail, how formed..... 124, 150 Glas, its brittleness.2 " how it differs from snow. 124 458 INDEX. Hair-spring.14........ 14 Herschel sees stars through a Hall. Captain Basil......146 comet........379 Halley's comet as seen by Sir Herschel, Sir J. F. *W.'s illustraJohn HIerschel.......379 tion of the size and distance of Hand-glass...... 138 the planets....... 344 Handiles of tea-pots, &c., why of Herschel, Sir John's opinion of wood.........190 the height of the atmosphere. 38 Hare's calorimot;or...... 297 Herschel's telescope and its powHarmony.......... 181 er........... 251,?37 how produced....183 Ileterogeneous........ 19 Harmony, science of,- on what Iliero employs Archimedes to defounded....... 182 tect the adulteration of a crown.12' Barvest-moon..... 389 Hind's account of the comet of Beat accompanies all great chang- 1853........... 381 es in bodies...... 110 Ilipparchus, father of astronomy. 336 Beat, application of its expansive lomuogefeneous.... 1.. ]9 power as a mechanical agent. 193 HEornblende.... 21 Heat and cold........ 187 Hiorizontal motion does not affect " conductors of......189 that of gravity..... 65 effects of........ 188 Horse-power as applied to the " effects of on bodies.. 185, 188 steam-engine meaning of.. 199 Heat, effects of on density of sub- Horses, how made to draw unequal stances.......... 192 portions of a load...... 77 Heat, effects of on water.. 186, 194 House's printing telegraph... 328 Beat, free...... 187 hIuman voice, powers of the.. 180 " first law of....... 189 Humor, the vitreous.... 237,259 " imperfect conductors of.190 " the aqueous.... 237, 239 " its effects on a body... 141 Hunter's moon........ 388 " most obvious effects of.. 193 "6 screw. 95 "' how propagated.....190 Hydraulics.... 17, 18, 1)9 128 "' latent.......187 Hydraulic-ram....... 133 " law of the reflection of. 191 Hydrodynamics...... 108,129 " laws of...1...... 185 Hydro-electric........ 334 " nature of........ 185 Hydrogen........... 20 " of the sun... 188 ". gas generator.... 275 Heat produced by electrical ac- " pistol........ 274 tion......188 Hydrometer.. 128 Heat, sources of...... 187 Hydrostatic bellows, how conHIearing trumpets...... 178 structed....... 119 Heavenly bodies, motion of the Hydrostatic paradox....118 when the most rapid.... 350 Hydrostatic press, Bramah's.. 121 Heavenly bodies, why not seen in Ihydrostatic pressure, as a metheir true place.......232 chanical power.1.....121 Heavens, why bright in the day- Hydrostatic pressure, caused by time........218 height, not by quantity.. 119 Hebe............ 339 Hydrostatics..... 17, 18, 108 Height of a building, how esti- HIIygeia......... 339 mated........... 53 IIygrometer....... 149, 150 Height to which a body projected Hyperbola...... 4..341 upward will rise.. 54 flypersthene..... 2. 21 Heliacal ring..........318 Heliography.... 257 IRelix.............316 Henry's and Dr. Ten Eyck's electro-magnet.......317 Ice formed under a receiver..169 fIero's fountain..... 1388 " how made to melt rapidly.. 191 INDEX. 459 eIe, why wrapped in woollen or J. packed in shavings.....190 Ice, why wooden spoons and forks Jansen....... 337 are used for....1...1 90 Jerusalem, siege of..... 106 Image from concave mirrors.. 225 Jet, the straight and revolving 163 " " convex mirrors.. 223 Jointed discharger..... 264' inverted....... 218 Juno............339 Impenetrability..... 21, 22 Jupiter....... 367, 368 Imponderable agents.... 18 Jupiter, a prolate spheroid, and Incidence, angle of...... 48 why............ 62 Incident motion........ 47 Jupiter's belts.3...... 368 Incident rays.........216 Jupiter, satellites of.....367 Inclination of earth's axis, effects of............. 354 K. Inclined plane...... 90 1" IC advantage of.. 91 Kaleidoscope........... 222 " " application of the 91 Kepler........... 337 (I "4 principle of the. 90 "c laws of...337,350,352 Incombustible bodies..... 188 Kepler's opinion of the number of Indestructibility.....21, 23 comets...........380 India rubber........ 30 Klinkenfues........ 381 "' " 1 balls, elasticity of. 47 Knee-joint.. 96 " " glass..... 158 Induction, electricity by.. 266, 278 L. " electro-magnetic.. 312 Inertia...... 21, 24, 26, 41 Ladder a lever........ 77 "' experiment to illustrate. 25 Lakes, why more difficult to swim of air.....38, 143, 165 in............. 126 " of a fluid, effects of the. 134 Lamp, defects of, how remedied. 112 of fly-wheels..... 81 Lamps, why they will not burn. 111 " of water....... 98 Lamp, wick of, how it supplies the Inferior conjunction......349 flame...........11 " planets........ 343 Lantanium........ 20 Infusoria......... 23 Latent heat......... 187 Instruments for raising water.. 131 Lathes............ 80 Insulated, meaning of... 261, 270 Law, Bode's......... 342 Intensity as applied to electricity, Law, fundamental of mechanics, meatsing of.........295 pyronomics, acoustics and op"In vacuo"........ 98 tics........... 49 Iridium......... 20 Law, Mariotte's. 142 Iodine... 20 " of falling bodies..... 51 Irene...... 339 Lawsof heat........ 185 Iris of the eye.... 237, 238 " of reflected sound.... 178 Iris, the planet or asteroid.... 339 Laws of reflection from concave Iron............. 20 mirrors...... 226, 227 Iron, a knowledge of the uses of Law of the heavenly bodies. *. 340 the first step towards eiviliza- Lead.20 tion.. 31 " not ductile.. 31 Iron, ductile but not malleable " why heavy...... 34 into thin plates....... 31 Le Verrier...........371 Iron, how made brittle..... 30 Leap-year......... 396 " oxide of........ 21 Leaves of a wheel...... 84 " when most malleable... 31 Length........... 23 Ivory, elasticity of..... 30,46 Lens, axis ofa.......233 460 INDEX. Lens, coneavo-convex..... 233 Light, velocity of..... 43 convex as a burning-glass. 235 " zodiacal........ 360 " double concave..... 233 Lightning, how caused... 278 " double convex...... 233 Lightning-rods........265 Lenses.........232 " by whom invented 281 Lens, effect of how estimated. 234d Lightning-rods, the best, how con" focal distance of a.... 234 structed.......... 280 Lenses in spectacles....... 236 Lime.......... 21 Lens, single concave...... 233 Lime, carbonate of...... 21 ".single convex...... 233 Linen garments, why cool... 189 " the.crystalline....... 237 Line.of direction....... 66 Level, how ascertained.... 113 Liquid, how it differs from a fluid 109 " or equilibrium of fluids.110 Liquids have a slight degree of Levels, spirit or water.... 113. ohesion..........109 Lever............ 93 Liquids not easily compressed. 29 " advantage in use of.. 73 Liquid, quantity of discharged Lever, force of the, on what de- from an orifice. 129' pendent....... 76 Lithium..... 20 Lever, how used. 72 Load-stone.298 C' kinds of........ 72 Locomotive steam-engine... 208 " many forms of the... 75 Looking-glasses. -.....221 "' of first kind...... 73 Looking-glass, length of to reflect' " of second kind.... 76 the whole person.. 222 " of third kind... 78 Lucifer.......... 363 " perpetual, the..... 80 Luminous bodies... 210 Lever, power of not dependent on Lutetia......... 339 its shape.:........ 76 Lever, principle of the.... 71 M.. the bent........ 76 Lever, things to be considered in Machine.......... 71 the... 72 Machinery, propelled by electriciLeyden-jar...... - 263 ty.............. 279 cc. how charged..:-.. 271 Machine,.Attwood's...... 52 Leyden-jar, how discharged silent- Machines, velocity of, how reguly...........265 lated........... 106 Light, aberration of...... 384 Magazine, magnetic...307 " absorbed by'all bodies..217 Magdeburgh cups......157 " beam of.2......213 Magnesia.......... 21 " color of....... 251, 252 ".. carbonate of.... 21 Light, corpuscular and undulatory Magnesium.......... 20 theories of.. 211 Magnet, attraction and repulsion Light, heat and. chemical action of.........300,301 of..... 254 Magnet, attractive power of, where Light, how projected..... 213 greatest.300 Light, intensity of, law of de- Magnet, broken........ 302 crease.......... 212 Magnet communicates its propLight, passing.into different medi- erties.......301 urns....... 230 Magnet, dipping. of a..... 303 Light, polarization of. 256 " effect of heat upon.. 302 " reflected....... 215 Magnet, horse-shoe or U, how'.". laws of...216 armed.....308 " reflection of... 211 Magnetic influence, all bodies susLight, Sir Isaac Newton's opinion ceptible of...301 of.. 211 Magnetic magazine...... 307: Light, theories of...... 211 Magnetic needle...304 Light, thermal, chemical and non- Magnet, keeper of a....302, 308 optical effects of...... 256 Magnet, properties of.. 299 IN DEX. 461 Magnetic poles...... 300 Mechanical operations always at"power on surface... 302 tended by heat.......188 Magnetism........ 17, 18, 298 Mechanical paradox...... 68 Magnetism and electricity, re- power. 70 semblances of....... 302 " powers...... 71 Magnet, modes of supporting.. 300 Mechanical powers, enumeration Magnetic poles, where strongest 304 of the........ 72 Magnet, north and south poles of, Mechanical powers, on what prinwhere most powerful.... 306 ciple constructed...... 71 Magneto-electricity.... 17, 332 Mechanical powers, principal law Mag,neto-electricity, most power- of the......... 89 ful effects of, how obtained.. 332 Mechanical powers, reducible to mIagneto-electric machine.. 333 three classes......... 72 Magnet, polarity of...... 299 Mechanical properties of gases, Magnet, poles of changed by elec- vapors, &c....... 139 tricity......... 303 Mechanics........ 17, 41 Magnet, powers of, how increased 301 " fundamental law of. 71, 91 " kinds of.... 299 118 " artificial, how made.306, 307 Mechanics, fundamental law of, " the receiving...327 its application to hydrostatic " U or horse-shoe.....301 pressure...... ~ 119 " variation of. 303, 304 note Media.... 97, 229 Magnitude, centre of... 59, 66 Medium.......... 97 Mlain-spring of a watch.... 104, 107 Mediums....... 97, 229 MIajor third.......... 184 Medium in optics....... 230 Malleability..... 27, 31 Melpomene..... 339 Malleability, dependent on tem- Meniscus........... 233 perature............ 31 Mercurial pendulum..... 103 Manganese........... 20 " tube..........160 Marco Paolo......... 306 Mercury............... 20 Mariner's compass...... 304 " theplanet, transit of.. 363 Mariner's compass, inventor of Mercury, the planet, why not often the... 306 seen.............. 362 Mariner's compass, needle of, how Metallic points..... 265 placed..........305 Metals, good conductors of heat. 190 Mariner's compass, how mounted 305 " names of the..... 20 "' " C points of the 305 Metals, order of their conducting Mariotte's law.... 142 power of heat... 190 Mars............ 366 Metals, tenacity of...... 32 [Massila...339 Meteoric stones...387 Materials, strength of of.. 95 Meteoric stones, Dr. Brewster's Materials which compose the crust opinion of........ 367 of the,earth.... 20 Metes.... -....339 Materials, tenacity of. 3.. 2 Mica................ 21 Matter, attractive....; 34 Microscope, a double.... 243 "' definition of.... 19 " a single.. 242 " essential properties of. 21 Microscope, compound magnify" gaseous form of.... 19 ing power of, how ascertained 244 Matter, its different states or Microscope, magnifying power of, forms.............. 19 how ascertained... 244 Matter, liquid form of..... 19 Microscope, the solar.....244 Matter, quantity of, how estimat- Microscope, the solar,'magnifying ed............... 40 power of...... 244 Materiality of air....... 162 Microscopes, what have theMatter, solid form of. 19 greatest magnifying power. 24' Mechanical agency of fluids.. 138 Milk, why affectedI by thunde " equivalent.,...... 58 and lightning........ 281 891 462 INDEX. Milky-way......... 383 Mountain, how burst by hydroMIinor third......... 184 static pressure....... 120 Mirror............ 221 Musical scale.........183 " concave........222 " sounds........ 181 " convex........ 222 Multiplier, electro-magnetic.. 313 p" lain.......... 221 Multiplying-glass.......235 Mirrors of half the he.ght show a Musical chord, how produced.. 182 whole-length figure.... 217 Musical instruments, why affected Mirrors reverse all images.. 222 by the weather....... 182 " use of glass in.... 221 Music of a choir dependent on the Miscellaneous experiments with uniform velocity of sound.. 176 air............ 166 Music of strings, how caused.. 181 Mobility..... 27 Mutual attraction....... 34 Molybdenum........ 20 Momenta........... 50 N. Momentum.... 41, 50 Momentum of a body, how ascer- Natural Philosophy, definition of 17 tained........... 50 Neap tides......... 391 Monochord......... 182 Needle, the magnetic.....304 Moon.......... 386 Needle, how placed in a mariner's " as cause of tides.. 391 compass........ 305 " as seen through a telescope 389 Negative electricity.... 259, 262 Moon, common errors in respect " (galvanic) pole.... 287 to the............ 386 Neptune........... 371 Moon, density of the....387 Newcomen and Savary's steam" difference in daily rising 389 engine.197 " gibbous........ 388 Newton, Sir Isaac. 23, 337' harvest and hunter's.. 389 Newton, Sir Isaac, discovery of " horned........ 388 gravitation......100 in quadrature.....388 Newton's discoveries, on what Moon-light, objects seen by, why based...........352 faint............ 217 Newton's (Sir Isaac) opinion of Moon, surface of the.....386 light.......... 211 IC uninhabitable.... 364 Newton, Sir Isaac's, opinion of the Morienne..... 144 -earth's compressibility -.. 29 Morse's telegraph.......320 Nickel.......... 20, 298 " telegraphic alphabet..323 Niobium........... 20 Motion...........41 Nitrogen.......... 20 Motion, accelerated, retarded and Non-conductors.. 258, 260 uniform.......... 44 Non-electrics... 258, 260 Motion, axis of........ 59 Nut and screw..... 93 " centre of....... 59 Motion, how transmitted by hy-. drostatic pressure......121 Motion, incident and reflected. 47 Oars, on what principle constructMotion impelled by two or more ed. 77 forces.......55 Object, apparent size of, on what Motion of the heavenly bodies, dependent..2...... 20 Pause of the....... 34 Objects, when invisible. 218, 220 Motion, perpetual....... 45 Octave............184 " regulators of.... 100 " how produced.....182 " reversed....... 83 (Ersted's discoveries in electroMotion, slow or rapid produced a4 magnetism......... 308 pleasure by machinery.... 84 Oil, effects of in smoothing the Alotion, when imperceptible.. 220 surface of water...... 131 Moving power in machines, how Oil, glutinous matter in...111 stoppe d........., 85 Oil-mills. 9....92 INDEX. 463 Oil, why it floats....... 39 Pendulum, length of to vibrate Olber's, Dr., opinion on lunacy. 386 two seconds.......... 103 Opaque bodies........ 217 Pendulum, length of varies with Opera-glasses........ 249 the latitude....1...02 Opposition..........350 Pendulums, table of the lengths Optical paradox......... 212 of to beat seconds in different Optic-nerve....... 237, 240 latitudes......... 104 Optics......... 17, 210 Pendulum, the ballistic... 63 " definition of..... 18 " the gridiron.... 103 Oracles of Delphi, Ephesus, &c.. 180 " the mercurial.... 103 Orbit, meaning of.......340 Pendulums, to what variations Orbits of the planets, inclination subject........... 103 of........... 347 Pendulum, use of the ball of.101 note Orbits of the planets, not circular 343 Penumbra....... 394 Otto Guericke........ 158 Percussion, force of.93 "Out of beat," meaning of.. 104 Perigee....... 349 Overshot-wheel........ 82 Perihelion...... 349 Osmium.......... 20 Permanent magnets......301 Oxyde of iron......... 21 Perpendicular....... 48 Oxygen......... 20 Perpetual lever....... 80 " motion....... 45 Perpetual motion, approximation P. to...........288 Phocea...... 339 Pails, why two can be carried Phosphor..........363 umore easily than one.... 69 Phosphorus......... 20 Palladium.......... 20 Photography...........257 Pallas..........339 Physical spectra......228 Parabola......... 62, 341 Physics, definition of..... 17 Parachute..... 38 Piazzi.......... 343 Paradox.......... 118 Pincers........... 75 " acoustic....... 177 Pinions........... 83 hydrostatic...... 118 Pipes, tonesof, on what dependent 181 s, mechanical...... 8 Pivots............ 81 " optical........212 Plane, the inclined...... 90 c pneumatic...... 169 Planet, meaning of... 339 Paradox,optical, pneumatic, acous- Planet and star, difference betic, &c., no paradox.. 212 note tween..........339 Parallax........... 385 Planets, characters by which they Patrallel motion, appendages for 200 are represented....... 346 Parallelogram.48 Planets, inferior and superior.. 343 Parthenope......... 339 " minor....... 339, 367 Pascal........... 144 " how discovered. 342 Pelopiumn....... 20 Planets, minor,,by whom discovPendulum......... 100 ered............ 343 Pendulum, cause of slowness and Planets, minor, size of..... 344 rapidity of vibrations.... 102 " names of the... 338, 339 Pendulums, continuous motion Planets, relative appearance of, of, how preserved.. 103 as seen through a telescope.372 Pendulum, how lengthened or Planets, the primary..... 338 shorteneid...... 102 Planmets, when in a particular conPendulum, how to be suspended 103 stellation.349 " its tumotion, how caused 101 Platinumu......... 20 Pendulums, length of, proportion Platinum, both ductile and malleof............. 103 able........... 31,32 Pendulum, length of to vibrate Plough, constellation of the.. 398 seconds.........102 Plumb-line... 37 464 INDEX. Pneumatics...... 17, 18, 138 Propeller...... 204 Pneumatic balloon..... 1. 61 Properties, essential and accidenPneumatic paradox...... 169 tal, of matter....... 21 " shower-bath.... 166 Properties, essential and unessen" scales..1......160 tial............ 23 Pointers......... 398 Prussian blue......... 327 Poker........ 75 Psyche........339 Polarity......... 299 Ptolemy.. 336 " boreal and austral,. 302 Pulley............ 86 Polarization of light.....256 " fixed and movable... 86 Polar or pole star..... 384, 398 " fixed, use of...... 87 Poles, magnetic.....-. 300, 304 Pulleys, mechanical principle of Poles, magnetic, where strongest 304 same as that of levers.... 88 Ponderable agents...... 18 Pulley, movable,: how it differs Pope Callixtus and the comet of from a fixed....... 87 Halley..378 Pulley, movable, principle of the 89 Pores............ 28 Pulley, power of, how ascertained 88 Porosity...27, 28 Pulleys, practical use of... 89 Positive electricity..... 259, 262 Pump; the chain....... 131 Positive (galvanic) pole... 287 " the common, for water. 152 Potash...... 21 " the forcing.153 Potassium...... 20 " the air.l......154 Power............. 72 Pupil.......... 237, 238 Power, how gained by use of the - Pyramid, why the firmest of struclever..76 tures... 68 Power, how to be-understood 73 note Pyrometer 193 Powers, mechanical..... 70, 72 " Wedgewood's.. 193 Power that acts...... 7 Pyronomics... 17, 18, 185, 187 Power, weight and velocity, pro- Pyt hagoras.336 portion of......... 90 Precession of the equinoxes.. 397 Q Press, Bransah's hydrostatic.. 121 Presses, screws applied to... 95 Quadrature..... 388 Pressure at any depth, how esti- Quartz......... 21 mated..1......115 Questions for solution 36, 42, 43, 50 Pressure, fluid, law of.... 115 53, 54, 78, 86, 90, 96, 106, 116; 127, Pressure, hydrostatic, as a me- 184 chanical power.......121 Pressure, hydrostatic, caused by R. height, not by quantity... 119 Pressure of fluids....... 114 Radiation of heat...... 190 Pressure of fluids in proportion to Radii.-..... 48 height of column...... 120 Radius......... 48 Pressure of the air.... 141, 162 " vector.3..:.. 350 " of water at great depths 109 Rain, how formed... 12L, 150, 186 Pressure on hydrostatic bellows, Rainbow, how produced....255 how estimated.. 119 Ram, the battering...... 105 Primary planets..338 " the hydraulic..... 133 Principle of all machines... 72 Random of a projectile... 65 Principle of the mechanical pow- Rarefaction.....] 40 ers............ 71 Rarefiedl.......... 0 Prism 252 Rarity...... 27, 28 Projectiles........ 62 Ray of light....... 212 Projectile, random of..... 65 Rays of light absorbed... 215 Projection, force of..... 62 " " converging..212 Projection, force of, has no effect Rays, converging and diverging, on gravity.........64 laws of......... 227 IND)E 465 Rays of light, diverging.... 2 Rudders, on what principle conRays of light from terrestrial ob- structed......... 77 jects...... 213 Rules relating to musical strings 184 Reader, The Rhetorical.... 180 Rules by which changes of' the Reauimur's thermometer... 149 weather'may be prognosticated Receiver.............154 by means of the barometer. 147 Rectangle... 48 Rules relating to musical pipes. 184 Rectilinear motion converted to Rush's Treatise on the Voice..180 circular -....... 81 Rutheniumn.......... 20 Reflected motion....... 47 Reflecting substances..... 211 S. " telescope....246, 249 Refraction.......... 229 Safety-valve.199 Refracting substances.....211 Sagacity of animals...... 92 " telescope..... 246 Sap; ascent of, to what due. 112 Refrangibility. 230 Satellites, general:law of..370 Registering apparatus of the tel- Saturn....368 egraph..322 Saturn's rings..... 368 Regulators of motion. 100 Scales for ascertaining specific Rein, F. C., hearing trumpets or gravity.......... 126 cornets.... 178 note Scale, the musical....... 183 Repulsion........ 28 Scales, the pneumatic... 160 Resinous electricity......262 Schorl.21 Resistance....-41 Science of harmony, on what Resistance of a medium, to what founded......... 182 proportioned.... 97 Scissors. 75 Resistance of the air-..... 38 Sclerotica.......237 ".. to be overcome... 71 Screw........ 93 Resultant.......... 58 " a compound power.. 94 ".'. motion....... 57 " advantage of the.... 94 " of two forces.... 56 " convex and concave. 94 Resultant of two forces, how de- " power of, how estimated. 94 scribed.... 58 " Hunter's....... 95 Retarded motion of bodies pro- " of Archimedes.....132 jected upwards.54 " uses of the.. 95 Retina.'...... 237, 240 "Sea-Eagle," experiment made Reversed motion........ 83 on board of the.109 Revolving-jet.......163 Seasonscause of the..... 350 Revolution of the planets, length " explanation of the cause 355, of..4''.. 341 356 Rhetorical Reader..... 180 Sea-water, cause of its increased Rhodium.......... 20 specific gravity....126 Rhodes, siege of....... 105 Seebeck, Professor, discoveries of Rifles, how tested.... 63 in thermo-electricity... 334 Rivers, how formed..'....124 Selenium......... 20 Rivers, why difficult to swim in. 126 Serpentine......... 21 Rivulets, how formed... 124, 136 Shadow............ 213 Roads, inclined planes... 91 Shadows, darkest, how produced 214 Rolling friction.98 Shadows from several luminousRomans, the ancient, how they bodies..215 conveyed water......137 Shadows, increasing and diminish Rope-dancer, how enabled to per- ing......214 form his-feats........ 67 Shadow of a spherical body, form Ropes, strength of, on what de- of........214 pendent........100 Shadows, why of different degrees Rosse's telescope.......251'of darkness......... 213 Rotation, electro-magnetic.-.. 313 Shaft. 81 466 INDEX. Sheiherds, balancing of in south Sounds, distance to which they of Frane......... 67 may be conveyed.... 176 Ships, on what principle they float 123 Sounds, musical...... 181 Sidereal time...........396 cc producing silence... 177 " year......... 396 Sound, velocity of.... 176 note Silence produced by two sounds 177 Sounds, what pleasing to the ear 183 Silica......... 20, 21 " when loudest.... 174 Silver, best conductor of heat.. 190 Sources of heat...... 187 Simple motion.55 Space......... 41 Sidereal year, how measured.. 397 c how estimated... 43 Signal-key of the electric tele- Speaking-trumpets......178 graph.......... 322 Specific gravity.... 40, 126 note Signs of the zodiac.... 346 Specific gravity of bodies, how asSigns used in almanacs.... 389 certained. 125,127 Silurus electricus...... 282 Specific gravity, scales for ascerSilver............ 20 taining -..126 Siphon........... 132 Specific gravity, standard of 123 Siphon, equilibrium of fluids ex- gravities, table of.. 124 emplified by means of the. 133 Sphericity, centre of...... 37 Siphon, experiments with the.167 Spectacles..........236 c" principle of the....133 Spectrum of a prism...... 254 Sky, why blue........ 253 Spherical aberration.....247 Slate formations in Bohemia.. 23 Spherical body, how made to roll Slaves in West Indies, how they down a slope........ 68 steal rum......... 122 Spider's web......... 23 Steel, how made brittle.... 30 Spiral tube......... 274 Sliding friction........ 98 Spirit level...... 113 Smee's battery........ 290 Spirit or water level, with what Smoke, why it ascends..... 39 filled......... 113 Snow, how formed.... 124,150 Spots in the sun.....304 " how it differs from hail. 124: Sportsman aiming at abird... 57 Snow and ice, how made to melt Spring, how high it will rise.. 137 rapidly......... 191 Springs, how formed.....136 Snuffers..75 Spring-tides...391 Soap-bubble, thickest part of.. 23 Spur-gear........ 84 Soda..21 Spur-wheel.. 84 Sodium.......... 20 Square............ 48 Solar microscope....... 244 Square rods, why better than round Solar system, account of the 337,338 as conductors of electricity. 279 " time......... 396 Standard of specific gravity. 123 " year, how measured. 396,397 Stars, distance of the..... 382 Solstices....... 358 Stars, distance of the, Sir John Sonorous bodies......174 Herschel's opinion of.... 383 Sonorous property of bodies, to Stars, how distinguished from what due........ 175 planets........339 Sound.1....... 174 Stars, the fixed........381 Sound affected by the furniture Stars, why not seen in the dayof a room.........179 time............363 Sound, by what laws reflected.. 178 Stars, why not seen in their true Sound, by what reflected and dis- place...........384 persed........... 179 Statics........... 17,18 Sound, focus of.1....... 179 Stationary steam-engine. 209 Sound, how communicated most Steam...........195 rapidly.......... 175 Steamboats.......... 203 Sou. d of the human voice... 179 Steam, cause of the ascent of.. 1241 " of strings, how caused.. 181 " dry and invisible.... 196 rapiclity of.......176 Steam-engine applied to boats. 203 INDEX. 467 S3,mtn-engine, power of, how esti- Sun, planets and stars, inhabited 359 hmated............. 199 Sun, ired appearance of tle, how Steam-engine, the.. 6 caused........... 253 (" imllprovers of the. 200 Sun's heat, efftct (,f on the eart}.l150 Steamn-engine, Nwomnllen and Sa- Superior conjunction.....349 vary's....197 " plintets..343 Steamll-engitne, Watts' double act- Surinam eel......... 2~s2 ingll, condensillng.. 197 Suspension of actin.. 5 Steaul-elnglie, \Vatts' improve- | Synchronous ticklnhgs of a clock. 104 ultelts o tale......... 197 l Syracuse, King of', eumploys ArSteam-engine, the locomotive 2)4, chimedes to detect the adultera208 tion of a crown..... 127 noteo Steam-engine, the stationary.. 209 Syringes for striking fire... 188 Steaum-elngine, T'ufts' stationary 207 Syringe, the condellsingl. 156 163 Steam, fouuldation of its application to achinery...... 30 T Steam, how condensed into water 195T 6' how mnade to act.... 196 Table of specific gravities... 124 Steam, on what its mechanical Table of the lengths of pendulums 104 agency depends... 195 " of velocities....... 42 Steam, pressure of, on what de- Tackle and fall........ 89 pendent.......... 195 Talc............ 21 Steam-ship.......... 203 j Tangent.......... 48, 60 Steam, space occupied by.. 196 Tantalus...... 133 note temlperature of.....195 Tantalus' cup. 133 " why it ascends..... 3 Tantalize, origin of the word. 133 Steatite........... 21 Tapestry of Bayeux......380 Steelyards...... 75 Tea-pots, why they have handles " how to be used. 7 of wood.190 Steelyards, mechanical principle Teeth............ 83 of the.......... 73 Telegraph, atmospheric. 331 Stereo-electric current..... 334 " Bain's...... 326 Stethoscope.. 175 " electric, history of the 329 Still............. 194 " electrc-magrnetic.. 319 Stilts used in south of France. 67 Telegraph, electro-magnetic, repStraight jet.......... 163 resentation of the.3.323 Strata of the earth...... 20 Telegraph, electric, principles of Streamn, velocity of, how measured 130 its construction...... 320 Strings, musical sounds of, how Telegraph, House's printing.. 328 produced..... 181 Telegraphic battery...... 321 Strings, musical quality of the Telegraph, meaning of... 319 note sounds of........181 Telescopes..........246 Strontium...... 20 Telescope, achromatic..... 247 Struve's opinion of the distance " Cassegrainian... 250 of the stars......... 382 " day and night... 248 Substance, heterogeneous... 19 " Gregoiian... 250 " homogeneous.... 19 " Herschel's..... 251 Sucker........... 160 " power of.337 Sulphate of copper battery... 292 " Lord Rosse's.... 251 Sulphate of copper battery (pro- " reflecting.... 246 tected).......... 293 " refracting..... 246 Sulphur........... 20 ", simplest form of the. 247 Sfun, as cause of tides..... 391 Tellurium.......'20 " as viewed from the planets. 360 Tenacity........ 27, 32 " its size., &c........ 359 " of cords....... 32 SMun, moon and planets, relative " of the metals..... 32 size of the....... 343 " of mletals, how increased 33 468 INDEX. Tenacity of various substances. 32 Trumpet, speaking.... 175 Tender of a steam-engine... 204 Tubes, capillary...... 111 Terbium......... m... 20 mercurial.......160 Terrestrial gravity....... 34 Tufts' stationary steam-engire. 207 Thermal effects of light.... 256 Tune.......... 41 Thermometer........ 149 Tungsten...... 20 " Celsius' -..... 149 Tycho Brahe.........336 c" Delisle's.....149 "- Fahrenheit's...149 Thermometer, on what principle constructed......... 29 Therlmrometer, Reaumur's. 149 Umbrella, use of in leaping from Thermo-electric........ 334 high places. 38 c" b~atteries..Un335 Undershot wheel....... 82 Thermo-electricity... 20, 334 Undultions of light.211 Thetis...........33 " of water, effects of. 131 Thorium................... 20 VUndulatory theory of light..211 Threniversal discharger.....272 Thunder-clouds, distance of, how Uania 339 measured.. 177 Uranium. 20 Thunder-house........ 277 Uranus.. 369 cc moons of........37 Thunder-storm, safest position in.281 Tides-..... 390 Ursa ajor......... 398 " neap and spring.....391 Time, apparent and true, differ- V. ence between........ 397 Time as kept by clock and by the Vacuum......... 98, 143 sun............397 Vacuum, a perfect, not to be proTime employed in the ascent and cured by means of the air-pump 156 descent of a body equal.. 54 Vacuum, the Torricellian... 143 Time,- how estimated. 43 Valve........... 152 " sidereal and solar... 396 Vanadium........... 20 Time of ascent and descent of a Vapor, cause of ascent of.... 124 body........... 45 Vapors..... 139 Tin....... 20 Vegetables, why white or yellow Tin and. copper, sonorous proper- when growing in dark places. 256 ties of........ 30 Vehicle in motion, cause of acciTin, not ductile........ 31 dents from......... 25 Tissue figure........ 270 Velocities, table of...... 42 Titanium....... 20 Velocity.......... 41, 71 Toggle-joint....... 96 " absolute and relative.. 42 " operation of the.. 97 " how estimated... 42 Tones of the voice, -how varied.180 Velocity of balls thrown by gunTonic...... 183 powder.......... 63 Tonnage of vessels, how estimated 123 Velocity of light and of the elecTorpedo......... 282 tric fluid.......... 40 Torricelli...143 Velocity of parts of a body, how Torricellian vacuum...... 143 diminished........ 60 Towns and fortifications, attacks Velocity of sound... 176 and note on..63 Velocity of sound, distances Transfer of fluids.......167 measured by the...... 177 Transit of Mercury and Venus.363 Velocity of sound, experiments of Translucent bodies...... 211 Arago, Gay Lussac and others 176 Transparent bodies....... 211 Velocity of a stream, how measTropic... 356 ured......... 130 Trumpet......... 178 Velocity of the surface of a Trumpeits, hearing.178 stream, greatest..... 129 INDEX. 469 Velocity required in machines, Water, not destitute of compress-'how regulated. 106 ibility-....... 109 Ventriloquism........180 Water, of what composed... 20 Venus.363 Water, pressure of at great - transit of......363 depths.........109, 116 Venus, why never seen late at Water, pressure of at any depth, night.-.....363 how estimated.......115 Vertical line........ 37 Water, pressure of at different Vesicular form of matter, defi- depths.1.......... 15 nition- of;..19 W. ater-pump. 152 Vespasian, batte'ring-ram of.. 106 Water-spouts..-....... 172 Vesper.......... 363 Water, weight of a cubic foot of. 126 Vessels, tonnage of, how esti- " weight of a cubic inch of 115 mated........... 123 Water when falling, whliy less inVesta.....'. 339 jurious than ice.......114 Vision, angle of...... 219 Water, when perfectly pure.. 124 Victoria...........339 Water, why it appears more shalVitreous electricity..... 262 low than it is........ 231 Vitreous humor..237,239 Water-wheels;....... 81 Vitriol, effects of on water... 187 " most powerful.. 82 Voice, Dr. Rush's Treatise on the 180 Watson,' Dr., experiment of, to " sound of the...... 179 show degree of evaporation. 150 Voice, the human, imitative pow- Watt, James...106 er of the.......... 180. Watt, James, his improvements Voice; tones of the, how varied. 180 of the steam-engine.... 197 Voltaic battery.......289 Waves, how caused...... 130 " electricity... 259, 283 Waves of light, laws of.. 212 note pile'. 288 Wedge...........92 " advantage of the.... 92. Wedge, effective power of, on what dependent..92 War, how it has been elevated to Wedge, power of the..... 92 a science.......... 63 Wedges, use of. 92 Warmth of clothing, cause of.. 189 Wedgewood's pyrometer....193 Watch, how it differs from a clock.104 Weight.........34, 72 " how regulated..... 105 " cause of....... 34 " moving power of a... 104 " lifter......... 165 Water............21 Weight, loss of in bodies weighed Water, converted into steam, space in water.......... 126 occupied by...... 30 Weight of any body, how ascerWater, distilled, the standard of tained by its cubical contents. 125 specific gravity......123 Weight raised by wheel and axle, Water, elasticity and compressi- hox' supported....... 79 bility of...... 24 Weight, what bodies have the Water expands when freezing.. 192 greatest.......... 34 Water-fowl, buoyancy of.... 123 Welding.. 31 Water frozen under the air-pump.169 Wheel and axle..... 78 Water, how applied to move ma- " " advantage of. 7'9 chinery.........83 c" "; construction of. 79 Water, how converted into steam.195 " " how supported. 81 Water, how high raised by means " " principle of the. 80 of common pump...... 153 Wheel, escapement.... 104 Water, how much diminished in' Wheels, friction....... 99 bulk by pressure...... 29 Wheels in machinery acting as Water, instruments for raising. 131 levers........... 78 Water-level........ 113 Wheels, large and small, advanWater, motion of, how retarded. 129 tages of each........ 40 470 INDEX. Wheels, locked, how and why. 85 Wind, why it subsides at sunset. 171 Wheels of a clock, their use..101 WTindlfass......... 80 " power of....... 80 Windlass and capstan, difference " size of limited by what. 85 between.......... 80 " tires of how secured.. 193 Wind-mills....... 80 Wheels, toothed, method of ascer- Window, where the hand should taining power of...... 85 be applied to raise.77 Wheels, use of on roads.... 85 Wollaston, experiments of...254 Wheel with teeth, of three kinds 84 Wooden spoons and forks, why Whirlwinds..........172 preferred for ice......190 Whispering-gallery...... 179 Woollen garments, why warm. 189 Whispering-gallery in Newbury- Worcester, Marquis of..... 200 port............ 179 Worm of a still.1.......'95 Whisper, motion of a, rapidity of the............ 176 Y. White...........251 Whitefield..........179 Year.......... 341 Wick of a lamp, principle of the 111 Year, leap 396 Width........... 23 Year, sidereal and solar....396 Wightman's apparatus for inertia 25 Yttrium.......... 20 William, Duke of Normandy.. 380 William the Conqueror...380 Winch applied to wheel and axle 79 Z. " double......... 80 Wind........... 170 Zodiac....... 345 Wind, cause of the different direc- Zodiacal light........ 360 tions of the...... 171 Zodiac, constellations of the, Wind, east, cause of at the equa- change of.........347 tor......... 171 Zodiac, signs of the......346 Wind instruments, sound of, on Zinc............ 20 what lependent....... 181 Zinc, at what temperature malleWinds quality of the, how affect- able. 31 ed..... 171 Zirconinum.........21 DAVIES' COURSE OF MATHEMATICS. DAVIES' FIRST LESSONS IN ARITHMETIC-For'eginnes DAVIES' ARITIIMETIC.-Designed for the use of Academies and Schools. KEY TO DAVIES' ARITHMETIC. 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