UC-NRLF B 30b 52? GIFT OF Dr. Horace Ivie EDUCATION DEFT PLYMPTON'S PARKER'S PHILOSOPHY, A SCHOOL COMPENDIUM NATURAL AND EXPERIMENTAL PHILOSOPHY: EMBRACING THE ELEMENTARY PRINCIPLES OF MECHANICS, HYDROSTATICS, HYDRAULICS I&tipFICfe, ACOFS.TTTS, PYRONOMICS, OPTICS, ELECTRICITY-,' GALVANISM, 'MAGNETI6li,' ELECTRO-MAGNETISM, CONTAINING ALSO ,A DESCRIPTION OF THE STEAM AND LOCOMOTIVE ENGINES, .jfllND OF THE ELECTRO-MAGNETIC TELEGEAPH. BY RICHARD GREEN PARKER, A.M., AUTH0R OP "AIDS TO ENGLISH COMPOSITION," A' SERIES OF "SCHOOL READERS," ETC. Delectando pariier que monendo. Prodesse quam conspici. A NEW EDITION, REVISED AND ENLARGED, BY GEO. W. PLYMPTON, A.M., PROFESSOR OF PHYSICAL SCIENCE, BROOKLYN POLYTECHNIC INSTITUTE. NEW YORK: - COLLINS & BROTHER, PUBLISHERS, 370 BROADWAY. G1FTOF -TE> -2 ' "P CONTENTS. DIVISIONS OF THE SUBJECT, . . . . . . .17 OP MATTER AND ITS PROPERTIES, * . ; . . 19 OF GRAVITY, . . . .-* 1 || .... 33 MECHANICS, on THE LAWS OF MOTION, . . . . 41 THE MECHANICAL POWERS, . . . . ' 70 REGULATORS OF M^oSt, .... ^ .. 100 S, V/ ?"" ; ., . .108 -; .t A, ?\ . ' 128 ? " " \* . . . . . . .138 ACOUSTICS, . . . ... . . " '.* '- .' . 173 PYltONOMICS, - . 185 THE STEAM-ENGINE, 196 OPTICS, . . . . .* . . . , . .210 ELECTRICITY, 258 GALVANISM, OR VOLTAIC ELECTRICITY, ..... 283 MAGNETISM, . 298 ELECTRO MAGNETISM, . 308 THE ELECTROMAGNETIC TELEGRAPH, .... 319 THE ELECTROTYPE PROCESS, 331 MAGNETO-ELECTRICITY, . . . . . . . 332 THERMO-ELECTRICITY, 334 ASTRONOMY, , 335 APPENDIX, . ... . 403 The Index at the close of the volume, being full and comprehensive, will be found more convenient for reference. Entered according to Act of Congress, in the year 1871, by . COLLINS & BROTHER, In tho Office of the Librarian of Congress, at Washington. EDUCATION DEPT P E E. F A O E TO THE REVISED AND ENLARGED EDITION. THE favor with which this book has, from its first appearance, been received by the teachers of this country, has induced the pub- lishers to offer yet another edition to the schools of the United States. It is presented as a revision and an enlargement of the previous edition. The revision of the book has led to such corrections of the text of the older work as the recent progress in physical science demanded. This has been accomplished without changing the numbering of the paragraphs or their distribution on the pages. Where a more extended correction seemed necessary than this plan permitted, the reader has been referred by note to the Appendix for the supple- mentary portion. It was deemed an exceedingly desirable object by the publishers that the new work should be presented in such shape that, when introduced to classes using the old edition, the exchange might be effected with the least possible inconvenience to teacher and pupil. The principal emendations have been made in the subjects of Mechanics, Heat, Hydrodynamics, and Optics. In Mechanics par- ticularly, the progress of ideas within a short period demands that the rudimentary conceptions of Force, Power, and work in the mind of the learner should be more sharply defined. The first paragraphs of the Appendix, giving the distinction between these terms, and also introducing the term Energy, have been prepared in accordance with this demand. The mechanical theory of Heat, the practical relation of Hydro- statics and Hydraulics to Mechanical Engineering, the later uses of compressed air, and the theory of the Spectroscope, have received a due share of space in the additional pages. A large number of new illustrations have been added, which, it is hoped, will aid the necessarily concise Appendix. GEO. W. PLTMPTON. POLYTECHNIC INSTITUTE, December, 1871. 934218 INTRODUCTION. THE term Philosophy literally signifies, the loye 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 knowl- edge, 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 natural division of all things that exist is into body and mind things material and immaterial, spiritual and corporeal. Physics relates to material things, Meta- physics to immaterial. Man, as a mere animal, is includ- ed in the science of Physics ; but, as a being possessed of a soul, of intellect, of the powers of perception, conscious- ness, volition, reason, and judgment, he becomes a sub- ject of consideration 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 mat- ter, on the contrary, possesses no such organs, and is con- INTRODUCTION. V sequently incapable of life and voluntary action. Stones, the various kinds of earth, metals, and many minerals, are instances 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 History (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 a's embracing the whole extent of physical science, while others use it in a more restricted sense, including only the general properties of unorganized matter, the forces which act upon it, the laws which it obeys, the results of those laws, and all those external changes which leave the substance unaffected. It is in this sense that the term is employed in this work. Chemistry, on the contrary, is the science which inves- tigates the composition of material substances, the inter- nal 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 con- dition 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 ac- complish 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 experi- ment, or from both. It is a matter of observation that water, by cold, is converted into ice ; but if, by means of freezing mixtures, or evaporation, we actually cause water to freeze, we arrive at the same knowledge by experiment. VI INTRODUCTION. 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 Philo- sophy, &c. The terms art and science have not always been em- ployed 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 an art. Science differs from 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 OP THE SUBJECT. _ . 1. NATURAL PHILOSOPHY, or PHYSICS, is the \\ hat is . , Natural science which treats of the powers, properties and Philoso- mu tual action of natural bodies, and the laws and operations of the material world. 1. Some of the principal branches of Natural Philosophy are Mechanics, Electricity, Pneumatics, Galvanism, Hydrostatics. Magnetism, Hydraulics, Electro-Magnetism, Acoustics, Magneto-Electricity. Pyronomics, Astronomy. Optics, NOTE. This list of branches might be considerably enlarged, but per- haps a rigid classification would rather suggest the omission of some ol tbem, as pertaining to the department of chemistry. What is ^' MECHANICS. Mechanics is that branch of Mechan- Natural Philosophy which relates to motion and the moving powers, their nature and laws, with their effects in machines. 4. Mechanics is generally considered under two division*, culled Stfities uud Dynamics. 18 NATURAL PHILOSOPHY. 5. The word Statics is derived from a Greek word implying test 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- q $he prapeYtie^ |vn>l i&ws of bodies in motion. S J "V,,^ 7 ''I, 'I C^S "y? I. Pneumatics treats of t,h<^ mechanical properties and effecta o^)^,gr\aii^\KlEaiIjapj3didiiL ^ill.ec^'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 instru- ments and machines by which their motion is guided or con- trolled. 10. Acoustics treats of the laws of sound. II. 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, jailed the electric fluid. 14 Galvanism (sometimes called chemical f l.ectricity) is a branch of Electricity. 15. Magnetism treats of the properties and effects of the magnet or loadstone. 16. Electro-Magnetism treats of magnetism inc uced by elec- tricity. 17. Magneto-Electricity treats of electricity indu-sed by mag- netism. 18. Astronoiny 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 classi- fication, into Bodies and Agents, calling bodies ponderable, and agenta /- ponderable. 20. Ponderable agents are those which have weight, as air, steam. 21. Imponderable agents are those which have no weight sueli as light heat, magnetism and electricity. OF MATTEE AND ITS PEOPEETIES. 19 What is 22. MATTEE. Matter is the general name of Matter? everything that occupies space. 23. Matter exists in three different states or forms namely, in the solid, liquid, and gaseous 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 when 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 aeriform state when the par- ticles 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 in- stance. As it ascends it expands, the particles repelling each other until they become wholly invisible. NOTE. The word aeriform means, m 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, arid 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 consider- ation of matter pertains more properly to the science of chemistry. It is proper, however, here to explain what is meant by a simple 01 homogeneous and a compound or heterogeneous substance. 30. All matter is composed of very minute particles or atoms united together by different degrees of cohesion. When all the atoms 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 substance* *ere composed of Fire, Air, Earth arid AVater, and these four substanow *re called the f^ur elements, because they were supposed to be the fiuiHt 1* 20 NATUKAL 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 composed 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 mixed with another invisible gas, called nitrogen or azote, m the proportion of seventy-two pa rts of the latter to twenty-eight of the former. The enumeration of the elementary substances, which, either by them- selves 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 herejpresented, so far as modern science has in- vestigated them. They are sixty-three in number, forty-nine of which are metallic, and fourteen are non-metallio. The forty-nine metals are Gold, Silver, Iron, Copper, Tin, Mercury, Lead, Zinc, Nickel, Cobalt, Bismuth, Platinum, Antimony, Arsenic, Manganese, Cadmium, Uranium, Palladium, Ehodium, Iridium, Osmium, Titanium, Cesium, Tungsten, Molybdenum. Vanadium, Chromium, The non-metallic elements are Oxygen, Sulphur, Hydrogen, Phosphorus, Nitrogen, Carbon, Chlorine, Potassium, Sodium, Lithium, Barium, Strontium, Calcium, Magnesium, Aluminum, Glucinum, Yttrium, Zirconium, Thorium, Cerium, Lanthanium, Bromine, Iodine, Selenium, Fluorine, Didynium, Tantalum^ Erbium, Thallium, Ruthenium, Rubidium, Niobium, Indium. Boron, Silicon, Tellurium. 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 understood, nor have they as yet admitted of any useful application. The science of Geology reveals to us the fact that granite 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 and extensive subject of inves- tigation, 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 classifies* tion would exclude it from this work. OF MATTER AND ITS PROPERTIES. 21 Dr. Boyntoris Chart of Materials that enter into the Composition of Granite. Quartz . . 1 100 65 70 46 48 54 47 27 57 56 56 62 42 3(1 40 36 75 48 < i J K M O 5 7. X 1 2 3 Prot 31 i * 1 3 7 3 1 6 13 11 o 1 | i 2 Fluor AciJ 4 B. Acid 44 Carb. Acid 60 " iy 20 20 12 A 18 1 2 2 1 30 20 18 10 7 14 10 2 1 8 10 2 1 1 14 24 13 4 '2 12 1 5 1 5 56 5 19 17 25 15 27 u 13 28 33 5 2 48 1 8 Prot. Ox. 7 4 8 ' M. 31 Prot. 8 25 17 M. 2 7 14 Prot. 36 Prot. 15 3 Prot. 26 Feldspai Albite Mica . . Hornblende .... Chlorite Talc .... Hypersthene .... Actrnolite Steatite Serpentine Schorl Garnet M. Garnet . . Clay . Green Sand Carbonate of Lime . . Carbonate cf Magnesia What are 31. There are seven essential * properties be- the essen- i on g m g to matter, namely, 1. Impenetrability; crties of 2. Extension ; 3. Figure ; 4. Divisibility ; 5. In* Matter? dostructibility ; 6. Inertia; 7. Attraction. What is 32. IMPENETRABILITY. Impenetrability is th Impene- * . , . ,. /. trabilityl power ot occupying a certain portion 01 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 ane of them. Different bodies possess other different properties which are not essential to their existence, such as color, weight, brittleness, hardness. Ac. These are called accidental properties, as they depend ou circum- stances not essential to the very existence of a body. J2 NATURAL PHILOSOPHY. that where one body is another cannot be without Jig- placing 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 immersi n g an inverted tumbler in a vessel of water. The air prevents the water from rising into the tumbler. An empty bottle, also, forcibly held horizontally under the water, will exhibit the same property , for the bottle, apparently empty, is tilled 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.* * This circumstance explains the reason why water, or any other liquid, poured into a tunnel closely inserted in the mouth of a decanter, will rua 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 tunne] be lifted from the decanter but a little, so as to afford the air an opportunity to escape, the water will then flow iutc the decanter in an un- interrupted 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 r to show the im- penetrability of water. A hollow globe of gold was filled with water and subjected to great pressure. The water, having no other means of escape, teas seen to exude from the pores of the gold. The reason why fluids appear less impenetrable than solids is that th-j 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 fiuid 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 state- ment. When a ve-'sel is filled to the brim with water or other fluiu, a con- siderable portion 01 salt may be dropped into the fiuid 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 arc 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 indi- cated by the largest circles, those of the salt by the pext in size, and those of the sugar by the smallest. Familiar Experiment. Fill a bowl or tumbler with peas, then pour on Uie peas mustard-seed or fine grain, shaking the vessel to cau?e it to fill tho racar.t spaces between the peas. In like manner add, successively, fine sand, cater, salt and sugar. This will afford an illus -ration of the apparent paraduy i i;vo bodies occupying the same space, and ; how that it is ouJy OF MATTEE AND ITS PROPEETIES. 23 What is 34: ' EXTENSION. Extension is but another Extern name for bulk or size, and it is expressed by the 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 frona the top to the bottom. The measure of a body frun the bottom to the top is f-allei 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. 14- iiat zs gg Figure is the form or shape of a body. 36. Figure and Extension are separate properties, although both may be represented by the same terms, .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 eaph 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 Dh'isi- , . , . . , , 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 soap- bubble does not exceed the two-millionth part of an inch. 40. The microscopic observations of Ehrcnberg have proved that there are many species of little creatures, called Infusoria, so small tnat 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 tornposed 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 bo divided into two hundred strips ; and each strip into two hundred rarts. One of these parts is only OIK two-millionth part of a graiw :f gold, and yet it may be seen with the naked eye 24 NATURAL PHILOSOPHY. 44. The particles which escape from odoriferous objects also aflbrd instances of extreme divisibility. \Vhat is l nde . 45. INDESTRUCTIBILITY. By the Indestructi- siructi- bility of matter is meant that it cannot be destroyed. 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 ove* a fire or by evaporation under the heat 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 under- goes. In the form of water it has no elasticity * and but a limited degree of compr-*^ bility.* 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 (sucb 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 ar composed, forming them into new combinations ; but every part still continues in existence, and retains all the essential] properties of bodies. What is 49. INERTIA. Inertia J 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 reader wil] bo careful to carry in his mind what is meant by the terra an essential property. It is explained in the note to No. 31, page 21 } The lite-rid meaning of inertia is inactivity, and implies inability tc change a ttat* of rest or of motion. A clear and distinct understanding 0) this property of all matter is essential in all the departments cf material philosophy. All matter, mec. v mically considered, must be in a state either OF MATTER AND US PROPERTIES. 25 50 A body at rest cannot put itself in motion, nor can a 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 ino-ve 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) eausf* all bodies, whether in motion or at rest, to tend towards the ceutn of the earth, and the air presents a resistance to all bodiea moving in it. Could these and all other direct Fig. 2. obstaJes to motion be set aside, a body when once put in motion would always remain in *\ M. r*HJ 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 employed 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 of motion o/ rest ; and, in whatever state H may be, it must remain in that state until a change is effected by some r flicient 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 placid. 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 hors" moving at a rapid rate be suddenly stopped, the rider will be thrown fofyard, 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 vehfcle 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 throwr forward in the direction of the motion which it has acquired from the vehicle. This is the reason that bo many accidents happen from leaping from a vehicle in motion. * In the absence of all positive proof from the things around us ol the statement just made, we may find from the truths which astronomy teaches that inertia is one of the necessary properties of all matter. Th* heavenly bodies, launched by the hand of their Creator into the fields of, infinite space, with no opposing forca but gravity alone, have performed their stated revolutions in perfect consistency with the character which '.his property gives them ; and all the calculations which have been made with respect to them, verified as they Lave repeatedly been by v - '"-ration, havo been predicated on their possess ; on of this necessary piv, .-> of iJI #6 NATURAL PHILOSOPHY. is then given to the card by means of a spring, and the card flies off, leaying the ball on the top of the stand.* 54. Nature seems to have engrafted some knowledge of mechan- ical 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 nearly 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 beyo^V the spot where the hare turned. 55. Children at play are in the same manner enabled " ti ^oitge^ 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 mention. A person in motion would be quite unconscious of that state, were it aot 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 ^' Every portion of matter is attracted by everj MW of At- other portion of matter, and this attraction is the ractwn . g t r0 nger in proportion to the quantity and the dis- tance. The larger the quantity and the less the distance, thw stronger is the attraction.! * The ball remains on the pillar in this case not solely from its tncrtif but because sufficient motion is not communicated to the bll by the fi Lo- tion of the card to counteract the effect of gravity on the ball. If ihe bail, therefore, be not accurately balanced on the card, the experiment vilJ not be successful, because the card cannot move without communicating at least a portion of its motion to the ball. f [N. B. This subject will be more fully treated under the head of 7rawfy See page 33.] OF MATTER AND ITS PROPERTIES. 27 7 . , 59. There are two kinds of attraction How many kinds of Attraction namely, the Attraction of Gravitation and are there ? the Attraction of Cohesion. (See par. 1388.) The former belongs to all matter, whatever its form, the more thai fifty miles in length. But platinum can be drawn oven to a fine wire than this. What is 93. TENACITY. Tenacity implies, the cohesion Tenacity ? f t h e particles of a body. 94. Tenacity is one of the great elements of strength. It is th* absence of tenacity which constitutes brittleness. Both implj strength, but in different forms. Thus glass, the moct brittle of aL substances, has a great degree of tenacity. A slender r'/d 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 ic of great use in the arts. The tenacity of metals and other substances has oeen ascertained oy suspending weights from wires of the metals, or rc*8 and cords of different materials. The following table presents very nearly the weights sustained by wires of different metals, each having the diameter of aDout 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 diametek, sustained the foi lowing 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 iraa one square inch. Pounds Avoirdupois. Pounds Avoirdupois English Oak, 8,600 to 12,000 Tin, . 7,129 Fir, 11,000 Lead, 3,146 Beech, 11,500 Rope, 1 inch in circum Mahogany, 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, 18,655 Rope, 3 inches in sircum- Wrought Copper, 33,792 ference, 6,628 Platinum Wire, 62,987 Do., 4 inches, 9/J88 Silver Wire, 38,257 Cable, 144 inches, 89,60C ttold, 30,888 Do., 23 inches, 'Z55.36C /inc, 22,551 A more particular aeooui t cJ the tenacity of various substances will b< OF GKAV1TY. '-*3 95. The tenacity of metals is much increased by uniting then?. 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 sclid, fluid or gaseous, possesses the property of attraction. 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 sej, Gravity? arate portions of matter. A11 bo(iies attract each other with a force pro- bodics at- portionate to their size, density and distance from eachother - [See No. 59.] 98. This law explains the reason why a body which is not sup ported 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 weigh- ing 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 a superior force. That superior force is the earth, which, being a much larger body, attracts them both with a superior force. This superior force they will 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. f found in Barlow's Essaj on the Strength of Timber, Rennie's Treatis* (in Phil. Trans. 1818), Tredgold's Principles of Carpentry, and the 4th vol. oi' Manchester Memoirs, by Mr. Hodgkinson. * There are many other specific properties of bodies besides those thai have now been enumerate I, the consideration of which belongs to th* eiieuce of Chemistry. | The earth is one quatrillijn, that is. one thousand million miJiiom times larger than the largest body which had ever been known to t'al 84 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 toward^ the earth. Therefore, What is 100. Weight is the measure of the earth's Weight? attraction* 101. As this attraction depends upon the quantity of matter which a body contains/it follows that What bodies Those bodies will have the greatest weight have the greatest which contain the greatest quantity of mat- "*" ter.f 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 : }Vhat is 1^' ^ vei 7 portion of matter attracts every the law of other portion of matter with a force propor- attraction? , . , ,. ., ,, ... , . , tioned directly to the quantity, and inversely as the square of the distance. through our atmosphere. Supposing, then, tnat 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 our 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 aie 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 hundroJ pounds, we express, by these terms, the degree of attraction by which it i* drawn towards the earth. f 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 panicles of ace OF GKAVITY. 35 104. Let us now apply this law to terrestrial gravity that is, to tLe earth's attraction ; and, for that purpose, let us suppose four balls of the same size and density, to be placed respectively as fol lows, 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 : The first ball (at the centre} will be surrounded on all sides by at 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 oppo site 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 (sup posing the weight of each ball, at the surface of the earth, tv 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 ' th ^^' ^ e f rce f gravity * s greatest at the sur law of Ter- face of the earth, and it deer oases upwards as the restrial square of the distance from the centre increases, and downwards simply as the distance from the centre decreases. According to the principles just stated, a body which at th sur- face of the earth weighs a pound at the centre of the earth wiT ireigh nothing. 1000 miles from the centre it will weigh i of a pound 2000 '-' !*' " " of a pound. 3000 " " " " | of a pound 4000 " " " " " " 1 pound. NATUIUL PHILOSOPHY. 8000 miles from the centre it will weigh 4 of a pound 12000 ' . " " 16000 " " ' ' r ^. 20000 ' " " ' 24000 * " " ' 28000 < tt M t 32000 ' tt tt t 64> If the priniiples that have now been stated have been understood, the solution of the following questions will not be difficult. 107. Questions jor Solution. [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 | of 4000 miles ; and, as the distance from the centre is decreased by $ , 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. 1.999571 (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, sup- posing the mine were at a perpendicular distance of half a mile from the surface ? Ans. 5. 99925 T 7 . (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. ST. IScwt. 98lb. + (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? Ans. ST. (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 qre. Ibs. Answer. 2 11 20. (7.) Which will weigh the most, a body of 3000 tons at the dis- tance of 4 millions of miles from the earth, or a body of 4000 tons at the distance of 3 millions of miles ? Ans. .003 T 7 . and .0077 7 . + (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, whai vt; Id it weigh at the surface? Ans. 3T. Zcwt. 50Z5. (10.) Suppose two balls ten thousand miles apart were to ap- proach each other under the influence of mutual attraction, t.h weight of one being represented by 15, that of the other by 3j dow far ^vould each move? Ans. 6666f mi. and $38?i *. OP GRAVITY 37 (11.) ^ hich would have the stronger attraction on ine eartn, 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 ^^l^m to ^Vs- (12.) Supposing the weight of a body to be represented by 4 ana 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 attrac- tion? Ans. The second, as to . 108. THE CENTRE OF GRAYITY. 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 Gravity of a P oint about which, all the parts balance each *^y - ? other. 110. This point in all spherical bodies of uniform density will be the centre of sphericity. Ill As the earth is a spherical body, its centre of gravity la 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 Fi 3 the centre of a phere in a parallel direction, and no two bodies suspended from different points can hang parallel to one anotherj* 114. Even a pair of scales hanging perpendicularly to the earth, as represented in Fig. 3, cannot be 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 ^^- ^ke Direction in which a falling body ap- Vertical preaches the surface of the earth is-called a Vertical l ' ine? Line. No two vertical lines can be parallel. 116. A weight suspended from any point will always assume *f plumb -line, from the LatUi woH jjiwf-nm , lead 88 NATURAL PHILOSOPHY. 117 All bodies under the influence of terrestrial gravity will full to the surface of the earth in the same space of time, when at an equal distance from the earth, if nothing impede them. But the air presents by its inertia a resistance to be overcome. This resist- ance can be more easily overcome by deLse bodies, and therefore the rapidity of the fall of a body will be in proportion to its density. To what is a^ofthe 118 ' The resistance of the air to toe fall of a air to a fall- body is in direct proportion to the extent of its ing body surface. propor- tioned * 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 to 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 um- brella 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 befalls, the rapidity of the r all will not be checked. [See Fig. 4.] 122. EFFECT OF GRAVITY ON THE DENSITY OF THE Aia. The air ixtends to a verv considerable distance above the surface of the earth.* Chat portion which lies near the surface of the earth has to sustain she 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 txtends. 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 risible vapors, diffused and floating in the air, sustained by it, and render- ing it tur.Hid, as mud does water). It seems probable, from many indica tions, that the greatest height at which visible clouds ever exist does nui 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 ceasti tr f eUec f Ihe sun's rajs at a greater height than forty-five uules OF GRAVITY. f the atmosphei-e on those beneath renders the air near the surfaw of the earth much more dense than that in the upper region*. Fig. 4. What effect 123. The air or atmosphere exists in a state upon the ^ compression, caused by Gravity, which in- air ? 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 con- taining 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 gravi- tation. 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 fccoount of its superior density, falls into the space occupied by th NATURAL PHILOSOPHY. fteam, and forces it upwards. The same reasoning applies in tl>* case of oil ; it is forced upwards by the heavier fluid, and both phb nomena are thus seen to be the necessary consequences of gravity The rising of a cork or other similar light .substances from the hot torn of a vessel of water is explained hi the same way. This circum- stance leads to the consideration of what is called specific gravity What is 125. SPECIFIC GRAVITY. Specific Gravity S^ed/fc^ k a term use( * * ex P ress th 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. Hence we say that the specific gravity of cork is greatei 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, &o. 127. From what has now been said with respect to the attrac tion of gravitation and the specific gravity of bodies, it appears that, although ihe 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 and other light substances will not sink in water, be- cause, the specific gravity of water being greater, the water is more strongly at- tracted, and will be drawn down beneath them. [For a table of the specific gravity of bodies, see Hy- drostatics.] 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, 'ertainedf J Thus, if the space passed over be 100 miles, and the velocity 5 miles in an 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 1 Ans. 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 1 Ans. 8 tnin. 14.07 we. + (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 ? Ana. 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 1 Ans. 19 da. 10 hr. 40 min. (5.) [f the earth go round the sun in 365 days, and the distance travelled be 540 millions of miles, how fast does it travel 1 Ans. 1,479,452 s 4 w mi. (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 1 Aw. 100 mi. per hr. How may the *rL,Sy 148 - The P P as8ed over ma y bo foi]nd fe y in motion be multiplying the velocity by the time. ascertained ? Miles per hour. Feet } ft second Slow rivers 3 ... 4 Rapid rivers 7 10 Moderate wind 7 1C) A storm . 36 52 A hurricane 80 117 f'.rnmon musket-ball ... 850 1,240 Rifle-ball 1,000 1,466 '24 lb. cannon-ball .... 1,600 2,346 Air rushing into a vacuum ) dt 32^ F 5 W ' ' ' 1 2% \ir-gun bullet, air com- S pressed to '01 of its V 466 . . . 683 volume ) Sound , 743 ..... . 1,142 A point on the surface of > , n 7 , , ' the earth \ 1)037 1 ' 520 Earth in its orbit .... G7,374 98,815. The velocity of light is 186,000 miles in a second of time. The veJo:ity of the electric fluid is said to be still greate dad sow* luthonciei .sti-te it to be at tho rate of 288 000 miles in a second *x time. 9* 44 NATURAL PHILOSOPHY Thus, if the velocity be 5 miles an hour, and the time 20 hours she space will oe 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 &iil in that time '? Ans. 1250 mi. (2.) Suppose the average rate of steamers between New York and Aloan? be about 11 miles an hour, which they traverse in about 14 hours, what is the distance between these two cities by the river 1 Ans. 154 mi. (3.) Suppose the cars going over the railroad between these two citie? travel at the rate of 25 miles an hour and take 8 hours to go over the dis- tance, how far is it from New York to Albany by railroad 1 Ans. 200 mi, (4.) If a man walking from Boston at the rate of 2 miles in an hour reach Salem in 6 hours, what is the distance from Boston to Salem 1 Ans. 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 froin the time of starting from the source of the river. What is the length of that river 1 Ans. 392-jiy mi. (6.) A cannon-ball, moving at the rate cf 2400 feet in a second of time, strikes a target in 4 seconds. What is the distance of the target! A. 9600 ft 145. The following formulae 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, r - = t. (3.) The velocity multiplied by the time equals the space, How many _ 146. There are three kinds of Motion are there ? namely, Uniform, Accelerated and Retarded. What is 147. When a body moves over equal spaces in Uniform , .. ., '. . . , , _ _ . Motion? equal times, the motion is said to be uniform. What is 1^' Wh en ^ ne spaces or distances over which Accelerated a body moves in equal times are successively greater, the motion is said to be Accelerated. What is -^. Wh en the spaces for equal times are Retarded successively less, the motion is called Retarded Motion? ,.. ,. Motion. How are Uni- 150. Uniform Motion is produced by the form,Acceler- ,. , ated and jRe- momentary action of a single force. Accel- tarded Motion era t e d Motion is produced by the continued respectively produced ? action of one or more forces. Retarded Mo- tion is produced by some resistance. MECHANICS. 45 151. A ball struck by a bat, or a stone thrown from the hand, is 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 onward 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 accel- erated 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 con- stantly 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 immedi- ately 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 acceler- ated until it reaches the ground. What time does a body ^54. A body projected upwards will occupy the occupy in its . .. , , ascent and same time m lts ascent an( l descent. descent ? This is a necessary consequence of the effect of gravity, which uniformly retards it in the ascent and accelerates it in its descent. 155 ; P*PKTnAL MOTION. - Perpetual Mo- be produced? tion is deemed an impossibility in mechanics, because action and reaction are always equal and in con- trary directions. 156 ' B J the aCti H f a bod 7 is meant thc Reaction ? effect which it produces upon another body. By reaction 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 whicb ip struck, and this resistance is the reaction of the body. 46 NATURAL PHILOSOPHY. Uliistratioji of Action and Reaction by tieaiis of Elastic and Non-elastu Balls. (1.) Figure 6 represents two ivory * balls, A and B, 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 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 los 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- tion to B, and receive a reaction from it which will stop its own motion. But the ball B cannot move without moving ; it will therefore communicate the motion which it received from A to C. and receive from C a reaction, which will stop its motion. In like manner the motion and reaction are received by each ol the balls D, E, F ; but, as there is n ball beyond F to act upotj it, F Will fly off. N. B. Thi experiment is to bt performed -vith elastic balls . i ly. (3). Fig. 8 represents two tails of clay (which are r ot elastic* of equal weight, suspended by s* rings. If the ball D be raised and let fall against E, oTily part of the mo- Fg. 8. tion of 1) will be destroyed by it (because the Dodies ai > non- elastic), and the two balls will move on togeth- er to and e, which are less distant from the ver- tical line than the ball D was before H foil. Still, * It will be recollected that ivory is considered highly elastic. MECHANICS. 47 however, action and reaction are equal, for the action on E ia only enough to make it move through a smaller space, but sc 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. | 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 com pletely to prevent any reaction in a backward direction. ^How may 160. Motion may be caused either by action ox motion be . ^ TT1 i , .. ... -MI caused? reaction. When caused by action it is callecf Incident, and when caused by reaction it is called Reflected Motion. * Figs. 6 and 7, as has been explained, show the effect of action ancr re- action 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 bo produced ; that is, the ball which is struck will rise higher than in case of non-elastic bodies, and less so than in that of perfectly elastic bodies; and the striking ball will be retarded more than in the former case, but not stopped completely, as in the latter. They will, therefore, both move onwards after the blow, but not together, or to -the same distance ; but in this, as in the preceding cases, the whole quantity of motion destroyed in the striking ball will be equal to that produced in the ball struck. Con- nected with " the philosophical apparatus " is a stand with ivory balls, to give a visible illustration of the effects of collision. f The muscular power of birds is much greater in proportion to their wei-ght than that of man. If a man were furnished with wings sufficiently large to epable him to fly, he would not have sufficient strength or muscular power to put them in motion. J.The power possessed by fishes, ot sinking or rising in the water, ia greatly assisted by a peculiar apparatus furnished them by nature, called aii air-bladder, by the expansion or contraction of which they rise or fall, an 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 mo- tion 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 mo- tion is caused by reaction, and reaction is increased by elasticity, it follows that reflected motion is always greatest in those bodies which are most elas- tic. 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. NATURAL PHILOSOPHY. What, u 161. The angle * of incidence is the angle formed of a jlna- by the line which the incident body makes in its detue? passage towards any object, with a line perpendic- ular 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 tht length of the lines. 2. A circle is a perfectly round figure, every part of the outer edge of which, called the cir- cumference, is equally distant from a point within, called the centre. [See Fig. 9.] 3. The straight lines drawn from the centre to the circumference are called radii. [The singular number of this word is radius.} Thus, in Fig. , the lines CD, C 0, C R, and C A, are radii. 4. T^e 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 th 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 and C D are angles of 45 degrees. 7. Angles of 90 degrees are right angles ; angles of less than 90 degrees, acute angles; and angles of more than 90' degrees are called obtuse angles, Thus, in Fig. 9, RC A is a right angle, C R an acute, and 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 [See Figs. 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 u rectangle are parallelograms, but all parallelograms are not rectangles nor equares. A square is both a parallelogram and a rectangle. Three thing* ure essential to a square; namely, the four sides must all be equal, they must ttlso be parallel, and the angles must all be right angles. Two things only lire essential lo a rectangle ; namely, the angles must all be right angles, and the opposite sides must be equal and parallel. One thing only is essen- tial to a parallelogram; namely, the opposite sides must be equal and . 13 The diagonal of & square, of a parallelogram, or a rectangle, \a e liu MECHANICS. Explmn 162. Thus, in Fig. 10, the line Fig. 10. Fig. 10 ABC represents a wall, and P B -^ a line perpendicular to its surface. O is a _ """". ball moving in the direction of the dotted ^.--** line, B. The angle O B P is the angle of R - "" incidence. What is 163. The angle or reflection is the angle formed tfr^fiec- ty ^ e P er P en dicular with the line made by the iion ? reflected body as it leaves the surface against which it struck. Thus, in Fig. 10, the angle P t R is the angle of reflection. 164. The angles of incidence and re- of incidence to the flection are always equal to one another.* angle of reflection ? (1.) Thus, in Fig. 10, the angle of incidence, 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 gtate a w jth course of a body . . in in motion which regard to the angles of incidence and renec- strikes against t it f u owg tna t when a ball is thrown unothen fixed . )ody ? perpendicularly against an object whick it cannot penetrate, it will return in the same direction , but, if it be thrown obliquely, it will return obliquely on f ,he opposite side of the perpendicular. The more 06- liquely the ball is thrown, the more obliquely it will rebound. \ 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, parallelo- gram, or rectangle. * An understanding of this law of reflected motion is very import? nt, because it is a fundamental law, not only in Mechanics, but also in Pyro- :iomics, 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, ^ 173. The height of a building, or the depth of a well, may thus be estimated very nearly by observing the length of time wnich stone takes in /ailing; from the top to the bottom. 174. Exercises far Solution. (1.) If a ball, dropped from the top of a steeple, reaches the ground in 5 seconds, how high is that steeple 1 16-+-48-l-80-f-112-f 144=400 feet ; or, 5><5=25, square of the nuinbe* 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 1 16-f-48-|-80-(-l 12+144+176+208+240+272=1296 feet. Or, squaring the time in seconds, 92=81, multiplied by 16=sl2v6. 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 perpen- dicularly 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 1 Ans. 256ft. (4.) How 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 1 Ana. 10ft. (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 pro- ceeded from the explosion of the meteor perpendicularly, how far from the earth, in feet, was the meteor 1 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 6 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 1 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 th multitude, and about them clustered the associations of centuries. On the other there stood an obscure young man (Galileo), with no retinue of fol- lowers, 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. Thf balls to be employed in the experiments are carefully weighed and scru- tinized, to detect deception. The parties are satisfied. The one ball is exactly twice the weight of the other. The followers of Aristotle maintaip 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. Galilee asserts that the weights of the balls do not affect their velocities, and that the tunes of descent will be equal ; and here the disputants join issue The balls are conveyed to the summit of the lofty tower. The crowd at^ Bemble round the base ; the signal is given ; the balls are dropped at the frame instant ; and, swift descending, at the same moment they strike the earth. Again and again the experiment is repeated, with uniform result?; Galileo's triumph was complete ; not shadow of a doubt remained I ["The Orbs of Heaven."} 54 NATURAL PHILOSOPHY. ("?.} A boy raised his kice in the night, with a lantern attached to it Ihifoiunately, the string which attached the lantern broke, and the lanton foil U the ground in 6 seconds. How high was the 'rite 1 Ans. r >76/i5. 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 reversed order. 176 ' To determine the height to which a height to which body, projected upwards, will rise, with a ^etfed upwards iven velocit J> Jt is onl J necessary to deter- with a given mine the height from which a body would fall t0 aC( l uire the Same velo( % 177. Thus, if it be required to ascertain how high a body would rise when projected upwards with a force sufficient to carry it It* feet in the first second of time, we reverse the series of numbers 1G-4- 48 + 804-112-[-144 [see table on page 52], and, reading them backward, 144 -\- 112 -4- 80 -j- 48 -f- 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 ^m{rewith descent of a bod y projected upwards will, the time of its therefore, always be equal. descent 1 Questions for Soliition (1.) Suppose a cannon-ball, projected perpendicularly upwards, returneo to the ground in 18 seconds ; how high did it ascend, and what is the velocity of projection 1 Ans 1'296/i. ; 272 ft. 1st sec. (2.) How high will a stone rise which a man throws upward with a forea (Sufficient to carry it 48 feet during the first second of time 1 Ans. $4Jt. (3.) Suppose a rocket to ascend with a velocity sufficient to carry it 17 feet during the first second of time ; how high will it ascend, and what tiiue will it occupy in its ascent and descent 1 Ans. 576.A ; 12 sec. (4.) A musket-ball is thrown upwards until it reaches the height of 400 ftet. How long a time, in seconds, will it occupy in its ascent and descent, and what space does it ascend in the first second T Ans. 10 sec. ; 144 A (5.) A sportsman shoots a bird flying in the air, and the bird is 3 .'cconds in falling to the ground. How high up was the bird when he was ahot ? -Aw* 144A (G.) How long time, in seconds, would it take a ball to reach an object 3000 feet above the surface of the earth, provided that the ball be projected with a force sufficient only to reach the object ] Ans. 17.67 sec. + 179. COMPOUND MOTION. Motion may be produced either by a single force or by the operation of two or mow MECHANICS. 55 *n what direc- 180. Simple Motion is the motion of a body 'ion is the mo- i m p e ll e( } by a single force, and is always in a f ton of a body r * 7 . impelled by a straight line in the same direction with the single force? force that acts. What is Com- 181. Compound Motion is caused by the pound Motion? O p era ti on O f two or more forces at the same time. When a body is struck by two 182. When a body is struck by two equal equalforces, m f orces< j n opposite directions, it will remain at opposite direc- tions, h)w will rest. it move ? 183. If the forces be unequal, the body will move with dimin- ished force in the direction of the greater force. Thus, if a body with a momentum of 9 be opposed by another body with a momen- tum of 6, both will move with a momentum of 3 in the direction of the greater force. How will a 184 - A bod J> struck by two forces in dif- body move ferent directions, will move in a line between ^for r ces k in y them > in the direction of the diagonal of a different direc- parallelogram, having for its sides the lines 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 an s les to each ther > tha forty wil1 right angles to move in the diagonal of a square. each other ? 186. Let Fig. 11 represent a ball struck by Fig n the two equal forces X and Y. In this figure the forces are inclined to each other at an angle 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, 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 th same as the force X would require to send it to B, or the force Y to send it to D. How will a -ir>fr Tf body move 187- If two unequal forces act at right under the influ- angles to each other on a body, the body will enceoftwoun- . . ,. . i . f T / r equal forces 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 ifi represented as acted apon by two unequal forces, X and Y. The force X would send it to B, and the force Y to D. As it cannot obey both, it will move in the direc- tion C A, the diagonal of the rectangle A B C D. ^ ' B . act in ** direction of any other direction of an acute or an obtuse than a right angle? j th bodjr will move m the di _ How will a body move if t ' ^ J the forces act in the di- rection of the diagonal of a para'Jlelo- rection of an acute or ffram obtuse angle ? Explain 190. Illustration. In figure 13 the ball C ib Fig. 13. supposed to be influenced by two Flg 13> forces, one of which would send it to B and the other to D, the forces acting in tho direction of an acute angle. The ball will, therefore, move between them in the line C A, the longer diagonal of the paralleiogm^ A B C D. 191. The same figure explains the motion of a ball when ihc two forces act in the direction of an obtuse angle. 192. Illustration. ."he ball D, ui.der tbe intlucnce of twc 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 lw diagonals, the one which joins the acute angles being the longer.] What is Re- 193. Resultant Motion is the effect or re suit ant Mo- ,^ P , . , , . tion ? su ^ * 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 diag- onal 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 course of each boat. E the spot where the man A stands who tosses the apple ; while the apple is c in its passage, the boats have passed from E and G to II and F respectively. But the apple, having a motion, with the man, that would carry it from E to H, 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 na*it, axi9 3 ^ follows, from the preceding illus- what parts of the tration, that the portions of the earth earth move most , . , . -,, .-> slowly, and what which move most rapidly are nearest to the purls most rapidly ? equator, and that the nearer any portion of the earth is to the poles the slower will be its motion. What is re- 214. Curvilinear motion requires the action two f rces ; f r tne impulse, of one single curvilinear force always produces motion in a straight motion? and .. why? lme ' What effect 215. A body revolving rapidly around its ugal ^ force on longer axis, if suspended freely, will gradually 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 ex- tremity of the longer axis. If, now, it be caused rapidly to revolve, it will immediately change its axis of motion, and revolve around the shorter axis. The experiment will be doubly interesting if an endless chain be sus- pended 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 out- ward 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 u chain and six bodies of different form, prepared to be attached to the multi- plying whoels in the manner described, accompanies most sets of philo- sophical apparatus. Attached to the same apparatus is a thin hoop of brass, prepared for con nexion 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 tho 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, go that the equatorial diameter is at least twenty-six miles longer than thu polar, lii those planets that revolve faster thau the earth the effect is still ()2 NATURAL PHILOSOPHY. What is Pro- 216. PROJECTILES. Projectiles is a brancl jectiles. D 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 *nto the Projectile? a j rj 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 pro- . , f J . . iectiles affected air cause projectiles to lorm a curve both in their in their mo- ascent and descent ; and, in descending, their 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 ball would proceed in the dotted line to B. D 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. 11. course of a i n a horizontal direction, or up- J&ZgZa rds or downwards, obUyuely, its horizontal course will be in the direction of direction? a curve -lme, called a parabola* A more striking, as is the case with the planet Jupiter, whose figure is nearlj 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 axir of motion ; but it will be exceedingly difficult to reconcile such a theory i the law of rotations which has now been explained, especially as a muc more rational explanation can be given to the phenomena on which tl theory was built. * It is calculated that the resistance of the air to a cannon-ball of Im- pounds' weight, with the velocity of two thousand feet in a second, is moi than equivalent to sixty times the weight of the ball. \ The science of gunnery is founded upon the laws relating to project! let MECHANICS. 68 (see Fig. 17; ; but when it is thrown perpendicularly upwards or downwai Is, it will move perpendicular!} 7 , 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 pendulum. It consists of a large block of wood, suspended by a rod in the manner of a pendulum. Into this block the balls are fired, and to it they 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 atmo- sphere 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 eommon 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 cannon- ball is 2000 feet per second, and this only at the moment of its leaving Ue gun. In order to increase tht 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 v the greatest velocity that can be ob- tained is only 1200 or 1300 feet per second. Great charges of powder are, therefore, not only useless, but dangerous ; for, though they give little 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 Fcrmerly, 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 duckg aud wild geese, which require and heavier guns 64 NATURAL PHILOSOPHY. A bal1 thr Wn ln a h01 Z ntal directlr)n izontal pro- is influenced by three forces ; namely, first, the whateffect^do ^ orce ^ P ro j ec ^ on (which gives it a horizontal they produce? direction) ; second, the resistance of the air through which it passes, which diminishes its velocity, with- out changing its direction ; and third, the force of gravity, which finally brings it to the ground. How is the gravity af- 221. The force of gravity is neither increased fectedby the nor diminished by the force of projection.* / rce of pro- jection " Explain 222. Fig. 18 represents a Fig. 18 l ' ' cannon, loaded with a ball, and placed on the top of a tower, at such a height as to require just three seconds for another ball to descend per- pendicularly. Now, suppose the can- non to be fired in a horizontal direc- tion, 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 falling 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 "ailing ball reaches 1, the next second 2, and ajfc 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 eleva- tion, will reach the ground at the same instant. Thus, a ball from a cannon, with a charge sufficient to throw it half a mile, will reach the ground at the same instant of time that it would had the charge been sufficient to throw it one, two, or six miles, from the same elevation. The distance to which a ball will be projected will depend entirely on the force with which it ig 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 sa.ine time will be greater ; but in both cases the descent of the ball towards the earth in the same time, will be the same number of feet, whether it UK ?es fast or slow, <>r eveu whether it uiovo forward at all or nut. MECHANICS. 65 second it strikes the ground. Meantime, that projected from the cannon moves forward with such velocity as to reach 4 at the saoe time that the falling ball reaches 1. But the projected ball falls downwards exactly as fast as the other, since ife meets the line 1 4, which is parallel to the norizon, 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 de- scent. 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 horizonta* has the pro- mo f lon ^ oes not interfere with the action of jectile jorce J . on gravity* gravity, but that a projectile descends with the same rapidity while moving forward that it would if it were acted on by gravity alone. This is the neces- sary result of the action of two forces. What is the 224. The Random of a projectile is the horizontal Random of a ,. . . . ,, vrojsctile ? distance from the place whence it is thrown to the place where it strikes. At what angle 225. The greatest random takes place at au est random ~ an ^ e f 45 degrees; that is, when a gun ia 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 a carronade, from which a ball is thrown at an angle of 45 de- above 45 de- grees w i tn 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 with it tue range of shot. 66 NATURAL PHILOSOPHY. What is tfu 226. CENTRE OF GRAVITY. It has already Centre of been stated ^ Nos> 10 g & 11Q j that tht Gravity of a body? Centre of Gravity of a body is the point around win -h all the parts balance each other. It is in other words, the centre of the weight of a body. (See Appendix, par. 1404.) e 227< Tlie Centre of Magnitude is the central 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 oj a body ? centre of magnitude. But when one part of the body is cor \posed 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 no. 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 stand sup ported, the body itself will be supported ; and when will ' J ' H fall ? but when the centre of gravity is unsupported, the body will fall. What is the 230. A line drawn from the centre of tfrav- L-neofDirec- m , . . . tion? ity, perpendicularly to the horizon, is called tli3 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 per mitted to fall freely. MECHANICS. 07 Wen will a 232. When the line of direction falls within b andwhmwill the baSG * f ai ^ b d ^' the b d ^ Vri11 Stand 5 but it fall? when that line falls outside of the base, the body will fall, or be overset. E^lain 2*3. (1.) Fig. 21 represents a loaded Fig. 21. Pt 8- 2L 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- c p 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 agil r erform Ce his ^ r b ^ dexterously supporting the centre of gravity feats of agil- For this purpose, he carries a heavy pole in his *ty * hands, which he shifts from side to side as he alters hi& 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 ^8' 2Q> of a bod/ 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. 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 Tiieso men walk on stilts from three to four feet high, and their children 3* NATURAL PHILOi: : PI Y. 236. A spherical body will roll down a slope, because thu 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 it;inrl 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 tho greatest are placed nearest to the base. 239. The broader the base and the nearer When will a 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 lias a broad base, and but little elevation. vben 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 01 the heated 'sand, and they are also enabled to see their sheep at a greater distance. They use these stilts with great skill and care, and run, jump, and even dance on them with great ease. * 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 gra vity is removed from the middle of the body to some point in the le*d, as t hat substance is much heavier than wood. Now, in order that the cyl- iud the Wkeel and Axle ' the Pulle y> their names f 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 whee] 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, mova- Lever , and how ,, c , is it used ? "le 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 cpposes 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 2 58. There are three kinds of levers, \)j levers are here J 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, Weight, Power, Fulcrum * More properly called simple machines. THE MECHANICAL POWERS. the position of the voider, the the fiiicrum, respectively, hi the tnree kinds Describe a ~ever oj the first kind by figure 27, and tell the ad- fig. 27 That is, (1.) The poyer^ is at one end, the weight at the other, and the fulcrum between them. (2.) Power at one end, the fulcrum at tho other, andthe weight between them. (3) Th^weight is at one end, the fulcrum at the other, and the power between them. 259. In a lever of the first kind the fulcrum is placed between the power and the weight. Fig. 27 represents a lever of the first kind vantage gained resting on the fulcrJn l jy it. T, , , , r , and movable upon it. W is the weight to be moved, and 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 fol- lows that tine, 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 13 than when it is exerted at P. On what prin- ciple is the com- mon steelyard Constructed? Describe the steelyard. 260. The common steelyard, an instrument for weighing articles, is constructed on the principle of the lever of the first kind. It consists of a rod or bar, marked with notches to designate the pounds and ounces, and a weight, which is inova- * It is to be understood, in the consideration of all instruments and ma- chines, that some effect is to be produced by some power. The names fM>wer and weight are not always to be taken literally. They are terms usefl to express the cause and the effect. Thus, in the movement of a clock, *;he weight is the cause, the movement of the hands ib the effect. The cilice 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, couiprea uu tir expansion, is technically called the 74 NATURAL PHILOSOPHY. ole along the notches. The bar is furnished with throe hoc**, on the longest of which the article to be weighed is always to bf fang. The other two hooka serve for the handle of the instru Fig. 28. ment when in use. The pivot of each of these two hooks serves for the fulcrum. 261. When suspended by the hook C, as in Fig. are 'th^tfoee 28 > {i is manifest that a P ound wei g ht at E wil1 hooks in the balance as many pounds at W as the distance be- stedyard? tween tne p j vot O f j) an( j tne p i vot O f Q J s con . tained in the space between the pivot of C and the ring front 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 apposite sides of the bar. An equilibrium * will always be Of Equilibrium. In the calculations of the powers of all machines it if THE MECHANICAL POWERS. produvA,<3 when the product of the weights on the opposite sides of the fulcrum into their respective distances from it are to one another. Fig. 29. 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. (See Appendix, par. 1415.) 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 inind 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 equi- librium 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 bal ance a weight of two pounds on the shorter arm, if the distance of tlie 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 me- chanical advantage of any machine, therefore, the condition of equilibrium must first be duly considered. After an equilibrium is produced, whatevei is added upon the one side or taken away on fie other destroys the equi- librium, and causes the machine to move 76 NATURAL PHILOSOPHY. nut most generally the fulcrum is a pin or rivet by which the 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 w r ill 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. HM is power 264. To gain power by the use of the usTofthe l ^ver, 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, there- fore, depends on its length, together with the power applied, and the distance of the weight from, the ful- crum.* Wha'isa 2t > 5 - A Com- ^ ^ Compound pound Lever, rep- resented in Fig. 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- lever of the sec- crum [ a a t one enc i the power at the other, and ond fand,with Fig. 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 31t The advantage gained by a lever of this kind is J/ in proportion as the distance of the power from pu,,,,,,,,, , ,.. the fulcrum exceeds that of the weight from the fulcrum. 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 distanr? 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 *hieh it afford* THE MECHANICAL POWEKS. 77 from P to F is four times the distance from W to F, then & power of one pound at P will balance a weight of four pounds at W. (2.) On the principle of this kind oflever, two persons, carrying a heavy burden suspended on a bar, may be made to bear unequal portions of it, by pi icing it nearer to the one than the other. 267. Two horses also, may be made to draw unequal portuns of a load, by dividing the bar attached to the carriage in euch a manner chat the weaker horse may draw upon the longer end of it.. 208. Oars, rudders of ships, doors turning on 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 l thirdkindl is at One 6nd ' the we S ut at the otlier ' 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 irom t he 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 weight ; and the power must exceed the weight in the same proportion that the distance between W and F ex2eeds the distance between P and P. 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 consid- ered 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 easify 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 hinbs, as well as the increased velocity of their motion. Tna wheels in clock and watch work, and in various kirds 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 ? An*. 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 u weight of 100 pounds only, what must be the greatest distance of the ful- crum from the stone 1 Ana. 8.42 in. -f- (3.) If the distance of the power from the fulcrum be eighteen timei greater than the distance of the weight from .he fulcrum, what^power wiauld be required to lift a weight of 1000 pounds 1 Ann. 55.55 Ib. + (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 1 Ans. 1700 Ib. (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 1 Ans. 28 Ib. (G.) Archimedes boasted that, if he could have a place to stand upon, he oould 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 1 to 270 millions. Suppose, also, that the fulcrum were a thousand miles from the earth ; wh;it mut 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, tvith ihe rivet 5 inches from the points, or a pair of scissors 6 inches long, with the rivet 4 inches from the points 1 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 7 Ans. 8.75ft. (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. 1 Ans. At j. (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 1 Ans. 32 Ib. What is the 273. THE WHEEL AND AXLE. The tek? ' Wheel and Axle consists of a cylinder with a wheel attached, both revolving around the same axis of motion. * The whiffle-tree is gererally attached to a carriage by a huok or -scribe the Bread Wheel 293. The Breast Wheel is a wheel which receives the water at about half its own height, or at the THE MECHANICAL POWERS. 83 rig level of its own axis. It is moved by the weight and acquired force of the water. Fig. 39 represents a Breast Wheel. It is fur- nished either with buck- ets or with float-boards, fitting the water-course, receiving the weight of the water with its force, while in motion it turns with the stream. (See Appen- dix, par. 1442.) 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 differ- ent parts of the machine by other parts, called gearing. Sometimes they are turned by the friction of endless bands or cords, and some- times by cogs, teeth, or pinions. When turned by bands, the motion may b* direct or reversed by attaching the band with one or two centres of motion respectively. 296. When the wheel is intended to revolve in 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 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 com- municate their motion. The wheels and axles thus rubbing to gether are sometimes coated with rough leather, which, by increas- ing the friction, prevents their slipping over one another without communicating motion. 298. Figure 42 represents suoii a combination of wheels the wheel a is turned by the weight S, its axle presses against the circumference of the wheel b, causing it to turn ; and, as it turns, its axle rubs 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- olution b will have performed only one-sixth of a revolution. The wheel a must therefore turn round six times tc cause b to turn once. In like manner b must perform six revolutions Fig. 40 84 NATURAL PHILOSOPHY. to cause c to turn once, and c must turn as many times to cause d to revolve once. Hence it follows that while d revolves once on its axis c must revolve six times, 6 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 sljw motion be ob- . , , . -, , , t , . trine* at pleasure must be applied to the axle ; to obtain by a combination of slow motion, the power must be applied to the circumference of the wheel. C wheels with their axles 'f 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 string around its circumfer- ence, is a simple wheel, with- out teeth. Its axle, being fur- nished with cogs or leaves, to which the teeth of the wheel D are fitted, communicates its 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 of the axle, according as a slow or a rapid motion is desired. 303. Wheels with teeth or cogs are of three kinds, according tf W Fig. 44 Fig. 45. the position of the teeth. When the teoth are raised perpendicular to the axis, they are called spur wheels or spur gear. When the THE MECHANICAL POWEKS. 85 teetli are parallel with the axis, they are called crown wheels. When 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 ^wheds f be 0t e^i estimated b J 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 an.'l 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 tho surface of the road. 308. A large wheel is also attended with two additional advan- tages , 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 awheel 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 th/: 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, aiid 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 tha rolling friction. 86 NATURAL PHILOSOPHY. 310. PRACTICAL EXAMPLES OF POWER APPLIED TO THE \A IIREL AND AJL. Questions for Solution. (1.) With a wheel 5 feet in diameter and a power of 6 pounds, whaJ nust be the diameter of the axle to support 3 cwt. 1 Ans. 1.2 in. (2.) How large must be the diameter of the wheel to support with 10 lb~ a weight of 5 cwt. on an axle inches in diameter 1 Ans. 81.5ft. (3.) A wheel has a diameter of 4 feet, an axle of 6 inches. What power must be applied tf the wheel to balance 2 cwt. on the axle ? Ans. 25 Ib. (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 * Ans. 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 i of a ton at the capstan, how heavy an anchor can they draw up, allowing the loss of of their powe? from friction } Ans. 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. 1 Ans. As 1 to 10. (7.) The weight is to the power in the proportion of six to one. What must bo the proportion of the wheel to the axle 1 Ans. 6 to 1. (8.) The power is represented by 10, the axle by 2. How can you repre- sert the wheel and axle 1 An*. 10 : weight:: 2 : wheel. (9.) The weight is expressed by 15, the power by 3. What will repre- sent the wheel and axle 7 Ans. 5 and 1. (10.) The axle is represented by 16, the power by 4. Required the pro portion 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 ita 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. b&cwt. (12.) A stone weighing 2 tons is to be raised by a windlass with spoked 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., and the re- bistance increased j by friction ? Ana, 5 men. What is a 311. THE PULLEY. The Pulley is a small Pulley 7 wheel turning on an axis, with a string or rope in a groove running around it. How many kinds There ftre twQ kindg Q f pu lleys the of pulleys are ? there i 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 "# small wheel turning on its axia, with a string running round it in a groove. W is a weight to be raised, F is the force t>r 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 FOWU128. S7 down. As, therefore, the velocity of the weight and the m%. 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 raise a weight to the top of a high building, it can be done with tbe 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 tiple does the principle as a lever of the first kind with equal fixed pulley act? armgj where the f u]crum 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 How does the * movable pulley the fixed pulley by being attached to Fig. 47. differ from the tne we ight ; it therefore rises and fixed ? falls with the weight. Explain 315. Fig. 47 represents a movable pull ny, Fig. 47. w ith the weight W attached to it by a hook below. One end of the rope is fastened at F ; and, as the power P draws the weight upwards, the pulley rises with the weight. Now, in order to raise the weight one inch, it 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 exert- ing 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 tne 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 Ihrough 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 meaue of a rope passing over a Cxei' wheel ( f pulley. 88 NATURAL PHILOSOPHY. must be shortened ; in order to do which, the power P rnunt 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 bal- ance a weight on the movable pulley of two pounds. 1 * What is the ad- 316. The power gained by the use of pul- vantage gained , . t . , , , . , . . in the use of the ^ e J s 1S ascertained by multiplying the num- movable pulley ? ber of movable pulleys by 2.f 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 pul- leys, or by a power of 3G 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 pa^s over two feet, with two pulleys four feet, with four pulleys eight feet. Explain 318. Fig. 48 represents a sy.stem of fixed and 8 ' movable pulleys. In the block F there are four fixed pulleys, and in the block M there are four movable pulleys, all turning on their com- mon 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 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. f 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 aud the fricMon 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 par- allelism, the pulley becomes wholly useless. There are certain arrange- ments of the Jord aud the pulley by which the effective power of tb THE MECHANICAL POWEKS. 89 shortened one foot, and, consequently, that the power I* 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- 32 - The fixed P ulle J acts on the principle of pie is the mov- a lever with equal arms. [See No. 313.] The structed? ^ ' mova kl e P u ^ e y> on * ne 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 kind? of pulleys are in these causes advantageously applied : for the sails are raised up to the xuasts 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 con- structed 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 Me- What law ap- . J , plies to all the chanical Powers in general, that power is always Mechanical* gained at the expense of time and velocity ; thai 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 pomids in sixty minutes, The rolling friction is caused by the rolling of a circular body. 36T. 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 soevr they may bo polished, have inequalities in 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 parts of the other, aud cause more w l.'ss ret Stance to motion. THE MECHANICAL POWERS. 99 What portion 369. Friction destroys, but never can gen- of the power of r ^ motion. It is frequently computed a machine is lost J r by friction f that friction destroys one-third of the power of a machine. In calculating the power of a machine, therefore, an allowance of one- third must be made for loss by friction.* 370. Oil, grease, black-lead or powdered soap- What is used to lessen fric- stone, is used to lessen friction, because they act tion? and ag a polish by filling up the cavities of the 9 ' rubbing surfaces, and thus make them slide more easily over each other. How does fric- 371. Friction increases : lion increase ? (1.) A.S 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 ma J be diminished : lion be dimin- (1.) By lessening the weight of the body in ished ? motion. (2.) By mechanically reducing the roughness of the sliding Kurfaces. (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 pre- pared a composition for the wheel-boxes of locomotive engines and other machinery, which, it is said, has still further reduced the amount of fric- t'on. 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 v>r 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 Pte\ against polished $Uel \s greater than ttmt of rolished steel on cupper or on 100 NATURAL PHILOSOPHY. What cure the 373. Friction, although it retards the motion uses offnction f o f machines, and causes a great loss of power, performs important benefits in full compensation. Were there no friction, all bodies on the surface of the earth would be clash- ing against each other. Kivers 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 soun 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 ivaters. 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 henip, 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. TlIE Pendulum ? PENDULUM. The Pendulum * consists of a brass. la 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 chan deliers 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 of 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 iCtio, 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 inind was immediately led to consider the cause 'vhich 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 arid 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 w>rjkl, di*, ulum? 1S called lts arc ' '' : ' V . . , *' What differ- 377. The vibrations of. yenfo lions pf < ^ke time^of the l en g tn are ver j nearly equal, '^fcetner* ^^ vibrations of move through a greater or less part of thcii pendulums of i e^ual length? arCS 't 378. In Fig. 57 A B represents a pendulum 5 K. 57. DFEC the arc in which it vibrates. If the which Crates seconds) is about thirty- inches. But in different parts of the i engtn must be varied. ' to vibrate sec nds at the seconds, a pendulum equator, must be shorter than one which vibrates seconds at the oles- How is a clock 381. A clock is regulated by lengthening regulated? or shortening the pendulum. By lengthening the pendulum, the clock is made to go slower ; by shortening it, it will go faster. J * 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 than 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. j: The pendulum of a clock is made longer or shorter by means of a scre-w 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 sh^w 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 hour- baud. On account of the expansion of the pendulum by heat, and its con- traction by cold, clocks will go slower in summer than in winter, the pendulum is thereby lengthened at that season, REGULATORS OF MOTION". 103 In what pro- 382. The lengths of pendulums are to portion are the eadl other ag the re of the time ()f tlieir l@7l(Jv/1S OT pendulums f vibration. 383. According to this 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 eighty feet in length. 384. As the oscillations of a pendulum are dependent upon gra- vitation, 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, conse- quently, 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 dif- ferent time, it is necessary that all pendulums should have a weight attached to them, which, by its inertia, shall concentrate the attract- ive force of gravity. 387. Pendulums are subject to variation in warm and cold weather, on account of the dilatation and contraction of the mate- rials 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 coun- teracted. 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 pendulum. 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 L04 NATURAL PHILOSOPHY. ticking, or (as it is called) by its being " out of beat." 1 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 pro- jections, shall properly articulate with the escapement-wheel. [-See No. 303.] Table of the Lengths of Pendulums to vibrate Seconds in different latitude* Inches. Inches At the equator, 39. Lat. 10 North, 39.01 At the equator, Lat. 10 South, 39. 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 whicfi 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- firmed 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 projec- tions, 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 main- spring, which being tightly wound around a cetitral pin, or axis, its elasticity makes a constant effort to loosen. This power is commu- nicated to a balance-wheel, acted upon by a liair-spring, and having an escapement similar to that of the .;lock. 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 i> pendulum. This wheel is moved by a spring, called the hair-spring. Th place of the weight is supplied by another larger spring, called the main- spring. REGULATORS OF MOTION. Ho wUl cause the wheels to revolve with great rapidity, ana the ,v a \ ^ also, becomes useless as a time-piece.* What is a Bat- 393. THE BATTERING RAM. The Batter'ng Ram was a military engine of great power, ined to beat down the walls of besieged places. Explain 394. Its construction, and the principle on which it Hg>5' was Corked, may be understood by inspection of > ig. 58, in which A B represents a large beam, heavily loaded / ith Fig. 58. a noad of iron, A, resembling the head of a ram, from which it takes its name. The beam is accurately balanced, and sus- pended by a rope or chain C, hanging from another beam, sup- ported by the frame D E F Gr. 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 wah 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.! * 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 Khodes was 106 NATURAL PHILOSOPHY. 096. The force of a battering rain is estimated by its momentum ihat is its weight multiplied by its velocity. 397. Questions for Solution. (1.) Suppose a battering rant weighing 5760 Ibs., with a velocity of 11 feet in a second, could penetrate a wall, with what velocity inudt a can- non-ball weighing 24 Ibs. move to do the same execution 1 57GO X ll = 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 Ans. 7250 (3.) If a ram have a weight of 90,000 and a momentum 81,000, what is its velocity 1 Ana. .9 (4.) What is the weight of % ram with a velocity of 12 and a momentum 60,000? Ans. 5000. (5.) Will a cannon-ball of 9 Ibs. 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? . Ans. The ram, What is the 398. THIS GOVERNOR. The Governor is an Governor? ...,, A , ingenious piecfc 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 rig.m. ^ Q are ^ WQ i everSj or arm s, loaded with heavy one hundred and six feet long. At the siege of Jerusalem Vespasian em- ployed a ram fifty feet long, armed with an iron butt, with twenty-five pro- jecting 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. * This very useful appendage to machinery, though long used in mills and other mechanical arrangements, owes its happy adaptation to the steam engine to the ingenuity of Mr. James Watt. In manufactures, there is one certain and determinate velocity with tfhich the machinery should be moved, and which, if increased or dimin- ished, would render the machine unfit to perform the work it is designed to execute. Now, it frequently happens that the resistance is increased or diminished by some of the machines which are worked being stopped, or others put on. The moving power, having this alteration in the resistance, would impart a greater or less velocity to the machinery, were it not for the regulating power of the governor, which increases or diminishes the supply of water or of steam, which is the moving power. 13ut, 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 and diminution To remedy all these inconveniences is th< duty assigned to the governor. itEGULATO-RS (F MOTION. 107 balls at their extremities B and C, and suspended by a joint at A upon the ex- tremity of a revolving shaft AD. A a is a collar, or sliding box, connected with the levers by the rods It a and c a : with joints at their extremities. When the shaft A D revolves rapidly, the cen- trifugal force of the balls B and will cause them to diverge in their attempt to fly off, and thus raise the collars, by means of the rods b a and c a. On +he con- trary, when the shaft A D revolves slowly, the weights B and C 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 steam- engine, or the sluice-gate of a water-wheel, being connected with the collar a, the supply of steam or water, which puts the works in motion, is thus regulated. What is the ^' ^ ne Main-spring of a watch consists of a Main-spring long ribbon of steel, closely coiled, and contained *f a watch? in a round box> j t ig em pi y e( i i ns tead 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 chain through which it acts is wound upon an axis surrounded by a spiral groove (called a fusee] , gradually increasing in diameter from the top to the bottom ; so that, in proportion as the strength 9f the spring is diminished, it may act on a larger lever, or a larger wheel and axle. Explain 402. Fig. 60 represents a spring coiled in a round box Fig. 60. A B is the fusee, surrounded by a spiral groove, on which the chain C is wound. When the watch is recently wound, the spring is in the greatest state of tension, ana will, therefore, turn the fusee 5 Fig. 60. 108 NATUKAL PHILOSOPHY. by the smallest groove, on tlie principle of the wheel and axle. As the spring loses its force by being partly un- wound, 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 com- municated from the fusee by a cogged wheel, which turns with the fusee. Of what does 403. HTDEOSTATICS.* Hydrostatics treats Hydrostatics ., ., n . , treat ? f the nature, gravity and pressure of fluids. What is tTie dif- 404. Hydrostatics is generally confined to &%% the consideration of fluids at rest, and Hy- Hydrostatics f draulics to fluids in motion. What is a 405. A Fluid is a substance which yields Fluid f fa the slightest pressure, and the particles of which, having but a slight degree of cohesion, move easily among themselves.! * The suijects of Hydraulics and Hydrostatics are sometimes descrioea under the general name of Hydrodynamics. The three terms are from the Greek language, compounded of nJop (hudor), signifying water, and Svrums (dunamis) , force or power ; oraTtxog (staticos), standing, and uuXog (aulos), a tube or pipe. Hence 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 Hydrostatics 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 and 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 particles of a solid, nor do they repel one another, as is the case with the particles composing a gas. They can move among one another with tho least degree of friction, and, when they press down upon one another ir virtue of their own weight, the downward pressure is communicated in aU directions, 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 con, ttitutes fluidity, namely, the power of transmitting pressure in every direction. and that, too, with the least degree of friction. The particles whioh compos* a fluid must be very much smaller than the finest gnan of OD iiuyal t >.i')le pow ier. HYDROSTATICS. . 109 Sow does a 406. A liquid differs from a gas in its de- l froma^or ^ Qe of compressibility and elasticity. Gases \japor ? are highly compressible and elastic. Liquids, on the contrary, have but a slight degree either of com- pressibility or of elasticity.* 407. Another difference between a liquid and a gas arises from the propensity which gases 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 gases seem to possess the opposite quality of repulsion, which causes them to ex- pand without limit, unless confined within the bounds of some ves- sel, 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 itfeelf 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 tempera- tures, 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 com pressed, and that it also has a slight degree of elasticity. In a voyage to the West Indies, in the year 1839, an experiment was made, at Vne sugges- tion 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 pressure of the superincumbent mass increases the density by compression, and it has been calculated that, at H depth of about ninety miles, water would be compressed into one-half of ite- volume, and at a depth of 360 miles its density would be nearly equal t< that of mercury. Under a pressure of 15,000 Ibs. to a square inch, .Mr. Perkins, of Newburyport, subsequently of London, has sh:wn that witer ia reduced in bulk one part iu twenty-four. 110 NATUKAL 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 gag.* How do fluids 409. GRAVITATION OF FLUIDS. Fluids gravi- gramtatef ^ a ^ e j n a more perfect manner than solids, oc account of their want of cohesiv e attraction. The particles of a solid body cohere so strongly that, when the centre of gravity is supported, the whole mass vill be supported. But every particle of a fluid gravitates independently of every other par- ticle. yy, 410. On account of the independent gravita- fluids be tion and want of cohesion of the particles of a moulded into fl u i ( j > they cannot be formed into figures, nor pre- served in heaps. Every particle makes an effort to descend, and to preserve what is called the level or equi- librium. What is the ^11. The level or equilibrium of fluids ia equilibrium of the tendency of the particles so to arrange themselves that every part of the surfao 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 Surface* of all state of rest) ' P artakes tlie spherical form of the fluids ? earth. 413. For the same reason, a fluid immediately conforms itself tc> the shape of the vessel in which it is contained. The particles of a solid body being united by cohesive attraction, if any one of them be supported it will uphold those also with which it is united. But, when any particle 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, >y its own weight, to penetrate the surface of the fluid, and mis jrith it. * The science of Chemistry unfolds the fact that all the great changes in the constitution of bodies are accompanied by the exhibition of heat either in a free or latent condition. * HYDROSTATICS. 11 J mat is Ca- 414. OAPILLAKY ATTRACTION. Capillary pMaryAttrac- Attraction is that attraction which causes What 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. This 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 vhere the edges of the glass meet, forming -a beautiful curve down- wards towards the edges which are separated by the card. 416. Immeree a number of tubes with fine bores in a glass of colored water, and the water 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 ordi- nary attraction of the particles of matter for each other. The sides of a small oiifice are so near to each other as to attract the 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 capillary tubes; and, for this reason, water and other liquids will rise in them when they are partly immersed. 419. It is on the same principle that the wick of a lamp will carry up the oil to supply the flame, although the flame is several inches above the level of the oil.f If the end of a towel happen to * The ^ord 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 borex of which are as fine as a hair, and hence called capillary kabes. t The reason why well-filled lamps will sometimes fail to give light is, lhat the wick is too large for its tube, and, being thus compressed, the japillary attraction is impeded by the compression. The remedy is to reduce the size of the wick. Another cause, also, that prevents a clear light, is that the flame is too far from the surface of the oil. As capillary ittraction acts only at short distances, the surface of the oil should always fc 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 wick, arid prevents the 1- 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 diiven. 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 mill- stone quarries are worked in Germany. 420. ENDOSMOSE AND EXOSMOSE. In addition to the capillary attraction just noticed as peculiar to fluids, another may be men- tioned, 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 mani- fested 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, thup causing a partial mixture of the fluids. . 422. Eocperimsxt. Take a glass tube, and, tying a piece of bladder 01 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 or DIFFERENT arity is there DENSITIES. When solid bodies are placed one l tation of'fluids a ^ ove an other, they will remain in the position in 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 not of such size as, by its length, as well as its thickness, to fill the cup, and thereby leave no room for the oil. It must also be remembered that, 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 Fame difficulty as has already been noticed in cases where the wick is too large to allow the free operation of capillary attraction, * Endosmose, from evdov, within, and ua^og, impulsion Exosmose, from i,'?, uulwai d, aud ujjuo{, impulsion HYDKOSTATICS. 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 fluid will partake of the sphericaJ form of the earth, to which they all respectively gravitate. What is a Spirit 426. A Water or Spirit Level is an in- Level, or Water strument constructed on the principle of the 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 Fig 61* S^ ass * u ^ e partly filled *ith water. C is a bubble of air occupying the space not filled by the water. When both 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-bubble will rise. The glass tube, when used, is generally set in a woode^ or a brass box. It is an instrument much used by carpenten 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 gliss. Henct the instrument is called indifferently the Spirit Level or the Water Level.] 428. EFFECT OF THE PECULIAR GRAVITATION Why do falling _. . fluids do less OF -FLUIDS. bond bodies gravitate in masses, damage than their parts being so connected as to form a whole, and their weight may be regarded as concentrated in a point, called the centre of gravity; while each 114 NATUKAL PHILOSOPHY. - particle of a fluid may be considered as a separate mass, gravi- tating 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- 4-29. PEESSUEE OF FLUIDS. Fluids not ^SL* onl y P ress downwards like solids, but also of their weight f upwards, sidewise,* and in every direction. (See Appendix, par. 1418.) 430. So long as the equality of pressure is undisturbed, every particle will remain at rosv. If the fluid be disturbed by agitating it, the equality of pressuie will be disturbed, and the fluid will not rest until the equilibrium io restored. TT ffo 431. The downward pressure of fluids is downward, lat- shown by making an aperture in the bottom of eral and up- a yesse i O f wa t cr . Every particle of the fluid ward pressure . . . , 1,1 qf 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. The 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. Pig. 62. * If the particles of fluids were arranged in Fig. 63. regular columns, as in Fig. 62, there would be no lateral pressure ; for when one particle is per- pendicularly above the other, it can press only downwards. But, if the particles be arranged as in Fig. 63, where a particle presses between tw particles beneath, these last must suffer a lateral pressure. In whatever manner the particles are arranged, if they be globular, as is supposed, there muht be spaces between them \See Fig. I, page 22.] HYDROSTATICS. 115 What is the ' P ressure a u s in P r OP r " law of fluid tion to the perpendicular distance from the 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. (See par. 1423.) 435. A bladder, filled with air, being immersed in water, will te contracted in size, on account of the pressure of the water in all Hrectiong ; and the doeper it is immersed, the more will it be con- tacted.* 436. An empty bottle, being corked, and, by means of a weight, iet down to a certain depth in the sea, will either be broken by the pressure, or t>tO cork will be driven into it, and the bottle be filled with wetrr. This will take place even if the cork be secured with wire and peeled. But a bottle filled with water, or any other liquid, may be iet down to any depth without damage, because, in this case, the internal pressure is equal to the external. f * T'ae weight of a cubic inch of water at the temperature of 62o of Fah- lonheit's thermometer is 36066 millionths of a pound avoirdupois. The pr&asure 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 Ibs. Ibs. 1 foot 2 feet 3 " 4 " 5 " 6 " 7 " 8 " 9 " 10 " 100" the pressure on a square in-ih is .4328, on a square foot, 62.3232 .8656, 124.6464 1.2984, 186.9696 1.7312, 249.2928 2.1640, 311.6160 2.5968, 373.9392 3.0296, 436.2624 3.4624, 498.5856 3.8952, ' 560.9088 4.3280, 623.2320 43.2800, " " 6232.3200 Fiom this table, the pressure on aty ..-u^face 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 great depths in the sea will have its pores so filled with water, and its specific gravity so increased, that it will no longer float. f " Experiments at Sea We are indebted to a friend, who has just arrived from Europe, says the Baltimore Gazette, for the fol'owing experiments made on board the Charlemagne : - 26th of September, 1836, tko weather being calm, I corked an cuiptj 5* lit) NATURAL PHILOSOPHY. 437. Questions for Solution. (1.) What pressure is sustained by the body of a fi**h having a surface of i> square feet at the depth of 150 feet 1 -4ns. 8-4136.32 Ib. (2.) What is the pressure on a square yard of the banks of a canal, at the depth of four feet \ . Ana. ii243.tis52 tt>. (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 2j sq. yd.1 Ans. 4206S.1<5#>. (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 ? Ans. 16827264 Ib. (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.1 Ans. 192.54/5. + (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 'he bed of the sea, and each side to be 8 feet square? An*. 299151.36/6. (7.) How deep can a glass vessel be sunk without breaking, srjpposing that it be capable of resisting a pressure of 200 pounds on a square inch \ Ans. 462.1 /t + 438. The lateral pressure of a fluid proceeds 14 hat causes the en tirely from the pressure downwards, or. in lateral pressure , . _ of fluids ? other words, from the weight or 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 cork ; 1 then sank it intf the sea six hundred feet ; when drawn immediately up again, the cork waf inside, the linen lemained 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 necb i)l' the bottle, and sank the bottle six hundred feet. Upon drawing it up immediately, the cork was found inside, having forced its way by the twine, and in so doing had broken itself in two pieces ; the bottle was filled with water. " I then made a stopper of white pine, long enough to reach to the bot- tom of the bottle; after forcing this stopper into the bottle, I cut it ofif 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 place'd 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 j *nd that at or near the centre of the earth, if the fluid could extend so deeply, this pressure would convert the whole into a solid mass of firo. HYDROSTATICS. 11? Fig. 64. 489 Fig. 64 represents a vessel of water, with ori fices at the side at different dis- tances from the surface. The different curves in the figure, described by the liquid in running out of the vessel, show the action of gravity, and the effects pro- duced by the force of the pressure on the liquid at different depths. At A the press- ure is the least, because there is less weight of fluid abo\e. At B and C the fluid is driven outwards by the weight of that portion above, and the force will be strongest at C. 440. As the lateral pressure arises solely from the downward pressure, it is not affected by the width nor the length of the vessel in What effect has the length and the width of a body of fluid upon its lateral which it is contained, but merely by its depth ; pressure ? ^ ag everv p ar ti c le acts independently of the rest, it is only the column of particles above the orifice that cap weigh upon and press out the water. To what is the 441. The lateral pressure on one side of a lateral pressure cu bical vessel will be equal only to half of the 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, althougl upward pressure apparently in opposition to the principles of 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 8 P out (like a tea -P ot > f r instance), the water rises in the spout to a level with that in the body of the ves- sel. The particles of water at the bottom of the vessel are pressed upon by the particles above them, and to tins pressure they will yield, if there is any mode of making way for the 118 NATURAL PHILOSOPHY. particles above them. As they cannot descend R 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 magni- fied. From an inspection of the figure, it appears that the par- tide 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, ana 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 de P ends entirely on the height, and not on the ture. length or breadth, of the column of fluid. [Sen No. 434.] 444, Any quantity of fluid, however small, What is the m j^ ma( j e ^ k a l ance an y other quantity Hydrostatic * J ^ J Paradox ? however large. This is what is called the Hy- drostatic Paradox.* Explain 445. The principle of what is called the hydro- Fig. 66. static paradox is illustrated by the hydrostatic bellows represented in Fig. 66 A B is a long tube, one inch square C D EF are the bellows, consisting of two boards, eight inches square, connected by broad pieces of leather, or india-rubber ploth 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 Ibs of watei by the descending force of one pound, the latter must descend 500 inohes while the former is rising one inch ; and hence, what is called the hydro- rtatic paradox is in strict conformity with the fundamental principle of Me p'lauics, that what is gained in power is lost iu time, or hi space HYDROSTATICS. 119 of wator pour ,<1 iiito the tube will raise sixty- Pig. 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, con- sequently, will not raise so great a weight, because it is the height, not the quantity, which 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 bj 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 so 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 may be poured into the "bellows to separate the surfaces before they are loaded with the weight.] How is the force 446. The force of pressure exerted on lettoics estimated f tube is estimated by the comparative size of the tube and the bellows. Thus, if the tube be one inch square, and the top of the bellows twelve inches, thus con- taining 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 What f undo- fill the tube were it 144 inches long. It will mental law of tlms be perceived that the fundamental prin- 120 NATURAL PHILOSOPHY. Fig. 67. Mechanics ciple of the laws of motion is here also in full 'hwirostatfc * f orce namely, that what is gained in power pressure ? is lost either in time or in space j for while the water in the bellows is rising to the height of one inch, that in the tube passes over 144 inches. Explain 447. Another form of apparatus, by means 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 suc- cessively be screwed to the apparatus, and filled with water. Weights may then be added to the suspended scale until the pressure is counter- balanced. It will then be perceived that, al- though 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, water poured into the pipe will exert so great hydro static pressure as to burst the cork. HYDROSTATICS. /// ner man- hy- ployed as a 4i8. HYDROSTATIC PRESSURE USEI> AS A MECHANICAL POWER. If water be confined * u an ^ vesse ^ an( * a pressure to any amount be exerted on a square inch of that water, a pressure to an equal amount will be trans- mitted to every square inch of the surface of the vessel in which the water 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 small force, 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 pre- clude the possibility of instituting any ordinary mechanical con- nexion between the two machines. Thus, merely by means of water-pipes, very great pressures may be transmitted to any dis- tance, and over inequalities of ground, or through any other ob- structions. (See par. 1423.) On what prin- ^ ^ * s on ^ e P r i n ip le OI> hydrostatic press- cipJe is Bra- ure that Bramah's hydrostatic press, represented in Fi ~ gg j g cons t ruc ted. The main features of . , this apparatus are as iollows : a is a narrow, and mah's hydro- static press constructed? Explain Fig. A a large metallic cylinder, having communi- cation one with the other. Water stands in both the cylinders. The piston S carries a strong head P, which works in a frame op- posite to a similar plate R. Between the two plates trie substance W to be compressed is placed. In the narrow tube, z is a piston p, worked by a lever cf)d, its short arm 122 NATURAL H1IL030PHY. j& 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 , of the force-pump has an area of half an inch, while the greal 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 impedi- ments 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, gun- powder, &c. ; also in uprooting trees, testing ths strength of ropes, &c. (See pars. 1425, 1426.) When will one fluid float on 451. A fluid specifically lighter than another the surface of fl u id w {\\ fl oat Up0 n 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. j .,, 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 with a fluid will neither rise nor fall in the fluid but will remain in whatever portion of the fluid it is placed * The slaves in the West Indies, it is said, steal rum by inserting th long neck of a bottle, full of water, through the top aperture of the rum aask. The water falls out of the bottle i uto the cask, while the light* rum ascends in ite stead HYDROSTATICS. 123 But a body whose specific gravity is less than that of a fluid will float. This is the reason why some bodies will sink and others float, and still others neither sink nor float.* (See par. 1427.) How deep will 454 - A bod y specifically lighter than a fluid a body sink in will sink in the fluid until it has displaced a por- a fluid? tion of the fluid eual in we ih t to 455. If a piece of cork is placed in a vessel of water, about one- third part of the cork will sink below, and the remainder will stand 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 water 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 peculiar shape, they are made to rest lightly on the water. The extent of the surface presented to the water counterbalances the weight of the materials, 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 specific gravity of water, and the materials of which a vessel is composed, rules have been formed by which to estimate the tonnage of vessels ; that is to say, the weight which the vessel will sustain without sinking. standard for ^"' ^ e standard which has been adopted to estimating the estimate the specific gravity of bodies is rain or specific grav- distilled water at tho temperature of 60.t ity of bodies ? * The bodies of birds that frequent 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 thaa 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 same in summer that it is in winter. For this reason, the} will not serve as a standard to estimate the specific gravity of other bodies The reason that distilled water is used is, that spring, wel', or river water u eldom perfectly pure, aud the various substances -mixed with it affect itf 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 formed by the condensation of the particles of vapor in the upper regions of the atmosphere. Fine, watery particles, coming within the sphere of each aiher'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 forma clouds, part is absorbed by the roots of vegetables, and part descends into the earth and forms springs. The springs form brooks, rivulets, rivers, . -f- (11.) If a cubical body of cork*exactly 9 inches on each side be placed in water, how deep will it sink 1 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 tfhe greatest weight without sinking 1 Ans. That ojfir. What is an 465. An Hydrometer is an instrument to ascer- Hydrometer? tain the specific gravity of liquids. (See par. and on what -[Aof\\ principle is it constructed? 466> T^ hydrometer is constructed on the principle that the greater the weight of a liquid, the greater will be its buoyancy. How is an hy- ^67. The hydrometer is made in a variety of drometer con- foras, but it generally consists of a hollow ball of silver, glass, or other material, with a gradu- ated scale rising from the upper part. A weight is attached Otslow the ball. When the instrument thus constructed is im- mersed 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 ^68. The hydrometer is a very useful instru- 1*5 the hydrom- ment for ascertaining the purity of many articles 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 seen that the Article is not pure. Of what does Hy- 469. HTDEAULICS. Hydraulics treats of draulics treat? liquids in motion, and the instruments by whicli their motion is guided 01 controlled. (See par. 1437.) HYL>KAUL1CS. 1L5J* 470 This branch of Hydrodynamics describes tLj ejects ot liquids issuing from pipes and tubes, orifices or apertuies, 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 & iven time tlm >ugh a P^P 6 or orifice is equal to a be discharged column of the liquid having for its base the orifice from an orifice or the area of the bore of the pip6j and a helght 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 ia 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 ^73. When a fluid spouts from several orifices will a fluid snout * n * ne 8 ^ e ^ a vesse l> it is thrown with the to the greatest greatest random from the orifice nearest to the distance ? centre the random being measured horizon- tally from the bottom of the vessel. 474. A vessel filled with any liquid will discharge a greater quan- tity of the liquid through an orifice to which a short pipe of pecu- liar shape is fitted, than through an orifice of the same size without a pipe. _ (See par. 1457.) 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 pro- ject 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 a current of ^^ion f tne bottom and sides of the channel water flows through which it passes. For this reason, the most rapidly, ve i oc i ty O f tne surface of a running stream is and why ? J always greater than that of any other part. * The velocity with which a liquid issues from an infinitely small orifice in ihe 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 o? khe surface to thf of the orifice. Brndt. 180 NATUKAL 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 impediments, the velocity which the waters would acquire would produce very dis- astrous consequences.* An inclination of three inches in a mile, in the bed of a river, will give the current a velocity of about three uiiles 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 rev- olutions in a given time. How may the 479. The velocity of a current of water at any portion of its depth may be depth be ascer- ascertained by immersing in iained? ft a ^nt 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 _jj 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. How are waves 481. Waves 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 hand of a wise and benevolent Creator is seen in nothing more clearly than in the laws and operations of the material world. Were it not for the almost ceaseless motion of the water, the ocea? * See what is stated with regard to fr ction in Nos. 373 and 374. HYDRAULICS. What are the principal hy- draulic instru- ments or ma- :hines ? itself would become unbearable. Decayed and decaying matter would be constantly emitting pestilential vapors, poisoning the at- mosphere, and spreading contagion and death far beyond the borders of the ocean. The " ceaseless motion " distributes the poisonous in- gredients, and aids tliat change which renders them harmless. 483. The equilibrium of a fluid, according 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 will become smooth. The oil protects the water from the friction of the wind or air. It is said that boats have been pre- served in a raging surf, iu consequence of the sailors having emptied a barrel of , oil on the water. 485. The instruments or machines for raising or drawing water arethe common pump, the forcing-pump, the chain-pump, the siphon, the hydraulic ram, and the screw of, Archimedes. [The common pump and the forcing-pump will be Fig. 71. aoticed in connexion with Pneumatics, as their opera- tion is dependent upon principles explained in that department of Philosophy. The fire-engine is nothing more than a double forcing-pump, and will be noticed in *ne same connexion.] 486. The Chain-pump is a machine by which the water is lifted through a box or channel, by boards fitted to the channel and attached to a chain. It has been used principally on board of ships. 487. Fig. 71 represents a Chain- pump. It consists of a square box through which a number of square ooards or buckets, connected by a chain, is ir^de to pass. The chain passes over the wheel C and under the wheel D, which is under crater. 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 T ~~ working by secondary causes, the uses of which are described in the of Oeologj Wliat is the nhain-pump ? 132 NATUEAL 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, aj5 it enters the box, lifts up the^vater above it, and discharges it at the top. 488. The screw of Archimedes is a ma- What is the c hj ne ggj^ have been invented by the plr- chimldes ? losopher Archimedes, for raising water and draining the lands of Egypt, about two hun- dred years before the Christian era. Fig. 72 repre- Ezplain gents tne screw O f Archimedes. A single tube, or two tubes, are wound in the form of a screw around a shaft or cylinder, supported by the gs 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. What is the 489. The Siphon is a tube bent in the form Siphon f O f fo Q i e ^ er "TJ, one side being a little longer than the other, to contain a longer column of the fluid. Explain 490. Fig. 73 represents a Siphon. A siphon Kg. 73. Fig. 73. i s use d by fining it with water or some other fluid, then stopping both ends, and in this state immers- ing the shorter leg or side into a vessel containing a liquid. The ends being then unstopped, the liquid will run through the siphon until the vessel is emptied. In performing this experiment, the end, of the siphon which is out of the water must always be below the surface of the water in the vessel. HYDRAULICS. 133 On what prin- 491 The principle on which the siphon acts ciple does the j s tnat tne i on g er co lumn having the greater siphon act? ' & hydrostatic pressure, the fluid will run down in the dhection 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. [N. B. This principle will be better understood after the principle is ex- plained 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 ol 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 in- verse 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 Explain the toy ^93. The toy called Tantalus' * Cup consists called Tantalus' of a goblet containing a wooden figure, with a ^ U P' 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. THE HYDRAULIC RAM + is an i What is the Hy- . ,. L , f ' draulic Ram ? nious machine, constructed for the purpose of raising water by means of its own im- pulse or momentum. * Tantalus, in Heathen mythology, is represented as the victim of per- petual 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. f The Hydraulic Ram, sometimes called by its French name, Better Hy- 134 NATURAL PHILOSOPHY. 496 In the construction of an hydraulic ram, there musst no, in the first place, a spring or reservoir elevated at least four 01 five 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 superflu- ous water may run off. 497. The ram itself consists of a pipe having two apertures, both guarded by valves of sufficient gravity to fall by their own weight, one of which opens downwards, the other opening up- wards 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 ' Fi g' 75 re P resent s the hydraulic ram. struction of the A B represents the tube, or body of the ram, havin g two apertures, C and D, both guarded by valves ; C opening downwards, D opening up- draulique, 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 perfect in ita mode of action, as it required to be opened and shut by the 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 reser- voir. It has been calculated that for every foot of fall in the pipe running from the reservoir to the ram sufficient power wjll 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 res- ervoir, 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 from 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 sudden change from a state of rapid motion to a state of rest. The ineitia of the fluid, or its resistance to a change from a state of rapid mo- tion 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 po jjreal as to burst the pipes HYDRAULICS. 135 vrards, 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. 75. 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 immediately falls by its own weight, by which means the current is again permitted to flow towards the aper- ture 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 ram, undergo but little variation. The water being thus forced into the chamber E, as it cannot return through the. valve D, it must proceed upwards through the pipe G, aad is thus carried to any desired point of dis- charge. An air-vessel is frequently attached to the chamber 136 NATURAL PHILOSOPHY. 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 Gr. The action, both of the ram and the forcing-pump, without the air-vessel, would be spasmodic.^ How are Springs 499 ' SPRINGS AND RIVULETS. Springs and and Rivulets Rivulets are formed by the water from rain, foi-med? 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. Flf. 76. Fig. 76 represents a vertical section of the crust of the eart* ", c, and e are strata, -either porous, or full of cracks, which per niit the water to flow through, while b, d and /, 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 I, p and g, artesian wells may be dug, in which the water will rise to the respective heights g h, p k, and I 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 con- structed, 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, Ocoursare capro, curnuferit ille, caveto " Virg. Bucolic 3, r. 2i faYDft AH LICS. 137 b?ing allowed to come in contact with the porous soil through which the bore is made, but being brought in pipes to the sur- face ; 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 50 - A s P rin 8 wil1 rise nearl 7 as hi 8 h ' 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 reser- voir. Co what height 501. Water maybe conveyed over hills and val- may water be leys in bent pipes and tubes, or through natural 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 con- vey water over them. The moderns effect the same object by meana of wooden, metallic, or stone pipes. How arefoun- 503. Fountains are formed by water carried tains formed? through natural or artificial ducts from a reser- voir. The water will spout from the ducts to nearly the height of the surface of the reservoir. (See par. 1456.) 504. In Fig. 76 a fountain is represented at i, issuing from the reservoir, the height of which is represented by a c. The jet at i will rise nearly as high as c. 505. A simple method of making an artificial 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 is exertcd equally in all directions. What is meant 523. The elasticity of air and other aeriform by tlie elasticity fl^g j s t h at property by which they are in- of air and ' . . * , . ' other aeriform creased or diminished in extension, according as fluids ? they are compressed. What effect ' 524. This property exists in a much greater. has an increase degree in air and other similar fluids than in any tionof n r ensure other substance - In fact > Jt has no known limit upon an aeri- for, when the pressure is removed from any per- form 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 press- ure is increased, it will be compressed into indefinitely smal dimensions. What is Ma- 525. The elasticity or pressure of air and riotte's Law ? a }j g ase s is in direct proportion to their dens- ity ; or, what is the same thing, inversely proportional to the space which the fluid occupies. This law, which was discovered by MariottP, is called " Mariotte's Law" This law may perhaps be better expressed in the following language ; namely, the density of an . elastic fluid is i-ti direct proportion to the pressure which it sustains. IIow does 526. Air becomes a mechanical agent by m T ecMM a means of its wei gH its elasticity, its inertia, agent ? and its fluidity. With what 527. The 'fluidity of air invests it, as it invests power does %\\ o ther fluids, with the power of tranmittinf TN K UMATICS . 1 4 '{*, fluidity invest pressure. But it has already been shown, under a fluid? the head of Hydrostatics, that fluidity is a neces- sary consequence of the independent gravitation of the particle? 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 }f Mechanics.* This is clearly seen in its effects upon foiling bodies, as will be exemplified in the experiments with the air-pump What is a 529. A Vacuum is a space from which aii Vacuum ? an( j ever v O ther substance have been removed 530. The Torricellian vacuum was discovered Vihat is the ,-,.. , i * i .1 * n t most perfect D y Torncelh, and was obtained m the following vacuum that manner : A tube, closed at one end, and about thirty-two inches long, was filled with mercury ; 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 dis- joverer the Torricellian vacuum, is the most perfect that has been discovered.! * The fly, 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 Mechanics. 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 thirty- two feet, that it would sustain a column of mercury only one-fourteenth as hifh, or thirty inches only, on account of its greater specific gravity. He therefore determined to test it by experiment. He accordingly filled a linall glass tube, about four feet long, with mercury, and, stopping the open end with his finger, he inverted it into a basin of mercury. On removing his finger, the mercury immediately descended in the tube, and rtood 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 >ase, and of the water in the other, that sustain* ! the column of mercury \n tlic tube, and of the water in the puuip. 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 a column of the atmo sphere 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 mer- cury, 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 Tomcelli led to the construction of the Darometer,* for it was reasoned that if it was the weight of the atmosphere which sustained the column of mercury, that on ascend- ' >ng any eminence the column of mercury would descend in pro- portion 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, f 534. Fig. 83 represents a barometer. It ^79. &?P la congists O f a long glass tu k ej . ^0^ thirty- three inches in length, closed at the upper end and filled with mercury. The tube is then in- ver /ed 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 barometer and its applications in science,rnay be mentioned the names of Descartes, Pascal, Morienue, and Boyle. The original idea is due to Torri- telli's experiment. t The word barometer is from the Greek, and signifies "a measure oftk< weight" that is, of the atmosphere. PNEUMATICS. 145 Fig. 80. 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.* 1 At the side of the tube there is a scale, marked inched 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 in Fig. 80, tc give a fair and true announcement of the state and weight of the atmosphere. The instruments are now manufactured in several dif- ferent forms. The different forms of the barometer in general use are the common Mercurial Barom- eter, the Diagonal, and the Wheel Barometer, all of which are constructed with a column of mer- cury. The Aneroid or Portable Barometer is a new instrument, in which confined air is substi- tuted for mercury. This is a convenient form of the instrument for portable purposes. But the principle is the same in all, and repeated observa- tions during the ascent of the loftiest mountains in Europe and America have confirmed the truth of barometrical announcements ; for, by its indi- cations, 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 altitude.! * The elasticity of the air causes an increase or diminution of its bulk, according as it is affected by heat and cold; and this increase and diminu- tion of bulk materially affect its specific gravity. The height of a column of mercury that can be sustained by a column of the atmosphere must, therefore, be affected by the state of the atmosphere. t From the explanation whieh has now been given of the barometer, it 146 NATURAL PHILOSOPHY. On hat ^^' ^ e P ressure f *^ e atmosphere on th< principle is mercury, in the bag or cup of a barometer, being the barometer exerted on the principle of the equilibrium of constructed? fluids, must vary according to the situation in which the barometer is placed. For this reason, it will be the greatest in valleys and low situations, and least on the top of high mountains. Hence the barometer is often used to ascer- tain the height of mountains and other places above the level of the sea. Wlien is the ^37. The air is the heaviest in dry weather, atmosphere and consequently the mercury will then rise 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 liall, 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 Vthe conversation turned on the natural phenomena of the region, when .'Captain Hali's attention was accidentally directed to the barometer in the *tate-room where they were seated, and, to his surprise, he observed it to ' evince violent and frequent alteration. His experience told him to expect bad weather, and he mentioned it to his friend. His 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 appear- ances. He hurried his friend to his ship, and gave immediate directions for shortening the top hamper of the frigate as speedily as possible. His lieutenants and the men looked at hiir, in mute surprise, and one or two of the former ventured to suggest the inucility of the proceeding. The cap- tain, however, persevered. The sails were furled, the top-masts were struck; -in short, everything thi?,t 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 evi- dent ; just, indeed, as he was beginning to doubt the accuracy of his in- strument. 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 hur- ricanes 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 tempestuous 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 said that she was in safety from the PNEUMATICS. 147 the air less salubrious, and it appears, therefore, more heavy then, although it is, in fact, much lighter. A. what time ^38. The greatest depression of the barometer of the day is occurs daily at about four o'clock, both in the morn- 'and lowest * n & an( ^ * n *k a ft em0011 5 an ^ ^8 highest elevation state of the at about ten o'clock, morning and night. In sun* barometer ? mer these extreme points are reached an hour or two earlier in the morning, and as much later in the afternoon, 589. Rules have been proposed by which the changes of th weather may be predicted by means of the barometer. Heno the graduated edge of the instrument is marked with the words "ram," "fair," "changeable." "frost," .&c. These expressions are predicated on the assumption that the changes of the weathsr may correctly be predicted by the absolute height of the mercury.- But on this little reliance can be placed. The best authorities agree 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 hag 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 prognost* cated 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 indicate? 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, along succession of foul weather will probably ensue; and again, if foul weather continue for several days, while the mercury con- tinually rises, a long succession of fair weather will probably succeed. (5.) A fluctuating and unsettled state in the mercurial column indicate* changeable weather. Lardner, page 75, Pneumatics. 542. Special Rules ly which we may know the. Changes of the Weather ly means of the Barometer^ (1.) The barometer is highest of 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 uncer- tainty. t These rules are from a different 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 vhe air must oe very dry or very cold, or perhaps both, and no rain may be ixpected. (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 #ill 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. (G.) 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 appeal ance 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 wrhen the mercury is in a falling state, foul weather is near. (10.) In frosty weather, if snow falls, the mercury generally rises to 30 J , 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 Bummer. 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. OP THE DIFFERENT STATES OF THE BAROMETER. Of the Fall of tk Barometer, In very hot weather the fall of the Barometer indicates thun- ier. Otherwise, the sudden fall of the barometer leads to the expectation )f 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 oi uch 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. 54i. 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. 645. The Barometer in an Unsettled State. If the motion of the mercury b* unsettled, expect unsettled weather. PNEUMATICS. 149 If it stand at "much rain" and rise to "changeable," expect fair weather of short continuance. If it stand at "fair" and fall to "changeable," expect foul weather. Its motion upwards indicates the approach of fine weather j its motion downward indicates the approach of foul weather. Wlt.at is ike ^46. THE THERMOMETER. The Ther- 1 Thermometer, mometer * is an instrument to indicate the tem- ^rindple is it perature of the atmosphere. It is constructed Constructed? O n 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. Fig . 81 b48. On the side of the thermometer is a scale to ^\ indicate the rise and fall of the mercury, and conse- quently the temperature of the weather. WJiat scale is ^49. There are several different scales adopted for the applied to the thermometer, of which those ^ihis^oun of ^ahrenbeit, Reaumur, Delisle and Gel- fry * sius, are the principal. The thermometer in common use in this country is graduated by Fahren- neit'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 Reau- mur and Celsius fix zero at the freezing point of water ; and that of Delisle at the boiling point. What is the 550. THE HYGROMETER. The Hygrom- Hygromete*- ? e t e r is an instrument for showing the degree of moisture in the atmosphere. * The word "Thermometer" is from the Greek, and means "a meaaurt of heat." " Hygrometer " means "a measure of moisture." 150 NATURAL PUTT/)SOPUY. How is it con- 551. The hygrometer may be constructed of tt7-u$ted ? any material which dryness or moisture expand* 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 sMe, and a sponge, or ot 1 er 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 bail. Experiments have been made to show the quantity of moist- ure thus raised from the ground by the heat of the sun. Dr. Wat- son 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 : He put a glass, mouth downwards, on a grass-plot, on which it had net rained for above a month. In less than two minutes the inside was covered with vapor ; and in half an hour drops began to trickle down its inside. The mouth of the glass was 20 square inches. There are 12% square inches in a square yard, and 4840 square yards in ,m 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 equa' to 1600 gallons, -from an acre, in 12 Lours. 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 darth is colder than the atmosphere, the moisture in the atmosphere condenses in the form of dew, on the ground, or other surfaces. Clouds are nothing more than vapor condensed by the cold of the upper regions of the atmosphere. Rain is produced by the sudden cooling of large quantities of watery vaj>or. Snow and hail are produced in a similar manner, and differ from rain only in the de- gree of cold which produces them. What is the 554. THE DlVER ; S BELL OR DlVING-BELL. Diving-bell, The Diving-bell is a large vessel shaped like and on what . , b , . f. . principle is it an inverted goblet, in which a person may constructed? safely descend to great depths in the water. It is constructed on th'e principle of the impenetrability of air. PNEUMATICS. 151 555. It has already been stated that air, being a material sub stance, possesses all the given essential properties of -wcer, and among them the property of impenetrability. The weight of the air giving it a pressure in every direction, or the property of fluidity, it penetrates and fills all things around us, unless by mechanical means it be carefully excluded. An open vessel, of whatever kind, is always full either of air or of some other substance, and unless the air is first permitted to escape no other substance can take the place of the air. 556. If a tumbler be inverted and immersed in water, the water will not rise in the tumbler, because the air in the tumbler fills it. [f the tumbler be inclined so as to let the air ascend in obedience to the laws of the equilibrium of fluids, the water will rush in and dis- place the air, while the lighter air. ascending, rises to the surface of the water. If this experiment be made with a bottle, the air will rise in bubbles with a gurgling sound. The same experiment may be made with a tube closed at one end by the finger ; the water will not enter the tube until bv 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 con- ... ... . _ struction of the diving-bell. It consists of a diving-bell by large heavy vessel, formed Fi. 82. of various shapes), with the mouth open. It descends into the water with its mouth down- wards. The air within it having no outlet, it is compelleu by the order of specific grav- ities to ascend in the bell, and thus (as water and air cannot occupy the same space at the same time) prevents the water from rising in the bell. A person, therefore, may de- scend 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. P represents the bell with the diver in it. rif tube attached to one side and reaching the air within ; and P is the forcing-pump through which air is forced into the bell. The forcing-pump is attached to the tube by a joint at D. When die bell descends to a great depth, the pressure of the water C is a bent metal- NATURAL PHILOSOPHY. condenses the air within the bell, and causes the water to asoen;! in the bell. This is forced out bv constant accessions of fresh air, supplied as above mentioned. Great care mast 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. (See par. 1462.) 558. THE COMMON WATER PUMP. Hew is water , Tr ^ . . , . ,, , raised in a com- Water is raised in" the common pump by mon pump How high may water be raised by a pump ? Fig. 83. means of the pressure of tho atmosphere on the surface of the water. A vacuum 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. 559. Fig. 63 represents the common pump, generally called the suction- pump. The body consists of a large tube, 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 bodj. of the * In order to produce such a vacuum, it is necessary that the piston 01 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 i* produced by the raising of the piston, the water will not ascend The pis ton is general'j worked by a lever, which is the handle of the pump, not represented in the figure. f A valve is a lid, or cover, so contrived as to open a communication ic 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 pumj they are generally made of thick leather partly covered with wood I.i the air-pump they are made of oiled silk, or thin leather softened wit* oil. The clapper of a pair of bellows is a familiar specimen "i v 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 sol*d piston, accurately fit- ted to the bore of the cylinder, and H the handle by which it is moved. The dotted line T represents the communication between the receiver H and the barrel B ; it is a tub hrough which the air, entering at the opening I, on the plate of the pump, passes into the barrel through the exhausting valve E v. c v is the condensing valve, communicating with the barrel B by means of an aperture near E, and opening outwards through the condensing pipe p. Explain the op- 568 ' The operation of the pump is as follows eration of the The piston P being drawn upwards by the han- air-pump by ,jle H, the air in the receiver R, expanding bv its elasticity, passes by the aperture I througt the tube T, and through the exhausting valve E v, into the bar- rel. 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 j9, where it passes out into the open air.* It cannot return through the con- densing valve c v, because that valve opens outwards only. By continuing this operation, every ascent and descent of the piston P must render the air within the receiver B. more and wore 7 156 NATURAL PHILOSOPHY. rare, until its elastic power is exhausted. Tne receiver L< tbeo said to be exhausted; and, although it stiii contains a smaiJ quantity of air, yet it is in so rare a state that the space within the receiver is considered a vacuum. 569. Prom 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. Prom the -explanation which has been &be cwLfed S iven of the operation of this air-pump, it will by means tf the readily be seen that, by removing the receiver pump which has ft an( j screw i n g any vessel to the pipe p, the l >een described? . J m, air may be condensed in the vessel. Thus the pump is made to exhaust or to condense, without alteration. (Vtifii is a con- *^1. Air-pumps in general are not adapted densing syr- for 3ondensation ; that office being performed by tn S e an instrument called " a condensing syringe," which is an air-pump reversed^ its valves being so arranged as to force air into a chamber, instead of drawing it out. For this purpose, the valves open inwards in respect to the chamber, while in air-pumps they open outwards. 572. A guage, constructed on the principle of the barometer, ia sometimes adjusted to the air-pump, for the purpose of exhibiting ihe degree of exhaustion. How does the ^^' ^ ne Double air-pump differs from tho double air-pump single air-pump, in having two barrels and two differ from the pjgtons ; which, instead of being moved by the single ? hand, are worked by means oi a toothed wheel, pla-ying 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. 151 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 OP AIR.- -Having adjusted the receiver to the plate of the air-pump, exhaust the air end the receiver will be held firmly on the" plate. The forcn 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 degree or ex- haustion, being at its maximum of fifteen pounds when there is a perfect vacuum. On readmitting the air, the receiver may be readily *enioved.* 577. THE MAGDEBURGH CUPS, OR HEMI- SPHERES. Fig. 88 represents the Magdeburgb Cups, or Hemispheres. They consist of two hol- low brass cups, the edges of which are accu- rately fitted together. They each have a handle, * The air is readmitted into the receiver by turning a screw which is in- serted into the receiver, in which there is an aperture, through which the external air rushes with considerable force. What are the Magdeburgh Cups, and what Jo they illus- trate ? 158 NATURAL PHILOSOPHY. ** 88 - to one of which a stop-cock is fitted. The stop- cock, being attached to one of the cups, is to be screwed to the plate of the air-pump, and left open. Having joined the other cup to that on the pump, exhaust the air from 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 press- ure 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 Magdeburg Cups derive their name from the city where the experiment was first attempted. Otto Guericke con- structed two hemispheres which, when the air was exhausted, were helc 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 ' 90 ' What principle 579 - THE HAND-GLASS. Fig. does the Hand- 90 is nothing more than a tuui- glass illustrate? 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. Tbo a ; j PNEUMATICS. 15U 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. What principle . 580 ' p BLADDER-GLASS.- is illustrated by Fig. 91 is a bell-shaped glass, the ^Bladder- COV ered with a piece of blad-. 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. *** 92 ' What does the 581. THE INDJA-RUBBER GLASS. India-rubber Fig. 92 is a glass similar to the one represented in the last figure, covered with india-rubber. The same experiments may be made with this as were mentioned in tke 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- ^' ^ HE FOUNTAIN-GLASS AND JET. Fig. trated by means 93 represents the jet, which is a small brass of the Fountain- tllbc pi 94 ig the fountain-glass. The ex- elass and Jet ! periment with these instruments is designed to Kg. 93. ghow the pressure of the atmosphere on ri e- 94 - the surface of liquids. Screw the straight jet to the stop-cock, the stop-cock to the fountain-glass, with the straight jet inside of the fountain-glass, and the lower end of the stop-cock to the plate of the air-pump, and then open the stop-cock. Having ex- nausted the air from the fountain-glass, close the stop- sock, 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 glas? like a fountain. 160 NATURAL PHILOSOPHY. How are the 583. PNEUMATIC SCALES FDR WEIGHING AIR. Pneumatic Fig. 95 represents the flask, rig. 95. Scales used? QJ . glagg yeggel and gcaleg for vveighing air. Weigh the flask when full of air ; then exhaust the air and weigh the tiusk again. The difference between its present and former weight is the weight of the air that was contained in the flask. What princi- . 584 ' THE SUCKER. A pie does " the cL-cular piece of wet leather, with a string Sucker "illus- attached to the centre, being pressed upon a 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 **^* -^ HE MERCURIAL OR WATER TUBE. : object of the Exhaust the air from a glass tube three feet long fitted With a st P- 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- *^6. 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, un- der the receiver, and, on exhausting the air, the air within the bag or bladder, expanding, will fill the bag. On readmitting 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 readmission of the air 587. The same principle may be illustrated by the india- PNEUMATICS. lt>i ruober and bladder glasses, if they have stop-cocks tc 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 thua diminished, the bladder with the weight will rise. On read- mitting the air, the bladder will sink again. TT 589. AlR CONTAINED IN WATER AND IN WOOD. now can trie presence of air Place a vessel of water under the receiver, and, in wood be de- on exhausting the air from the receiver, ihe air tected? . .,...,, . , in 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 bubble* from tho pores of the wood. Explain che prin- 59L THE PNEUMATIC BALLOON. "* ciple of the Pneu- Fig. 96 represents a small glass bal- matic Balloon. loonj witt itg car i mmerse d in a jar of water, and placed under a receiver. On exhaust- ing the air, the air within the balloon, expanding, gives it buoyancy, and it will rise in the jar. On readmit- ting the air, the balloon will sink. 692. The experiment may be performed without the air-pump by covering the jar with some elastic sub- stance, as india-rubber. By pressing on the elastic Jm, 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 conve- nient 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 covei 162 NATURAL PHILOSOPHY. and the surface of the water; this condensation presses upo& the water below, and, as this pressure affects every portion of the water throughout its whole extent, the water, by its upward pressure, compresses the air within the balloon, and makes room for the ascent of more water into the balloon, so as to alter the specific gravity of the balloon, and cause it to sink. As soon as the pressure ceases, the elasticity of the air in the balloon drives out the lately-entered water, and, restoring the former lightness to the balloon, causes it to rise. If, in the commence- ment of this experiment, the balloon be made to nave 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 materiality 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 direC' tions, because the effects happen in whatever position the jar be held. Sixthly. It shows that pressure z> as the depth, because less press- ure of the hand is required the further the globe has descended in the water. Seventhly. It exemplifies many circumstances of fluid support A person, therefore, who is familiar with this experiment, and can 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, called the Cartesian Devil, is constructed ; and it may be thus explained: Several images of glass, hollow within, and each having a small opening at the heel by which water may pass in and out, may be made to manoeuvre in a vessel of water. Place them in a vessel in the same manner with the bal- loon. Out, by allowing different quantities of water to enter thf PNEUMATICS. 1(>3 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 follow 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 exhibiting these figures to spectators who do not understand them, while appearing carelessly to rest his hand on the cover of the ves- sel, 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, press- ure can be made by a rod rising through the hole, and obeying the foot of the exhibitor, the most surprising evolutions may be pro- duced among the figures, in perfect obedience to the word of com- mand. 596. EXPERIMENTS WITH CONDENSED AIR. What ts the m ^ T-. use of the Con- -"- HE CONDENSING AND EXHAUSTING SYRINGE. densing and The Condensing Syringe is the air-pump reversed. The* Exhausting Syringe is the simple air-pump without its plate or stand. These implements are used respectively with such parts pig. 97. of the apparatus as cannot conveniently be attached to the a ; .r-puinp, 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 exhaust- ing syringes are united, and are made io perform each office respectively, by merely reversing the part which con- tains the valve. For what purpose 597 - THE AlR ' is the Air-cham- CHAMBER. The air- krused? 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. What prin- **98. STRAIGHT AND REVOLVING JETS FROM tiple <*f Pneu- CONDENSED AIR. Fill the air-chamber (Fig. 7* 164 NATURAL PHILOSOPHY. . . 97) partly with water, and then eondense the mattes is illus- ' " J trated by the air. Then confine the air by turning the cock ; straight and a ft er which, unscrew it from the air-pump, and revolving jets? ^ QW ^ ^ Btra ; ght Qr ^ revolving j et Then open the stop-cock, and the water will be thrown from the chamber in the one case in a straight continued stream, in the other in the form of a wheel. Figs. 98 and 99 represent a view of the straight and the revolving jets. In the revolving jet the water is thrown from two small apertures made at each end on opposite sides, to assist the revolution. The circular motion is caused by the reaction of the water on the 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 jet- hole, the arm is caused to revolve in a contrary direction. This experiment, performed with the straight jet, illustrates the principle on which "Hero's ball" and "Hero's fountain" are constructed. % Explain the ^9. THE PRINCIPLE OF THE AIR-GUN. With principle of the air-chamber, as in the last experiments, a the Air-gun. gmall bragg cylinder or gim . barr ei 5 Fig. 100, may be substituted for the jets, and loaded with a small shot f ig . 10 o or paper ball. On turning the cock quickly, the* con- densed air, rushing out, will throw the shot to a consider- able 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 tilled, will afford two or three dozer* discharges. The force of the air-gun has never been eqial to more than a fifteenth of the force of a common charge of powder, and. the loudness of the report made in its discharge is always as great in proportion to its force as that of the comuou gun PNEUMATICS. 1W. 600. Condensed air may be weighed in the iir 'tv/tat must air-chamber, but, in estimating its weight, the \lways he temperature of the room must always be taken into consideration, as the density of air is ma- terially affected by heat and cold. What doc th ^^' ^ XPERIMENTS SHOWING THE INERTIA OF Guinea and AIR. THE GUINEA AND FEATHER DROP. The Feather Drop inertia of air is shown by the guinea and feather illustrate? ,.,. A . J . . ,. , ,, drop, exhibiting the resistance which the air opposes to falling bodies. This apparatus is made in different forms, some having shelves on which the Kg. 102. guinea and feather rest, and, when ihe air is exhausted, they are made to fall by the turn- ing of a Jiandle. A better form is that repre- sented 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 ex- hibited in Figs. 101 and 102, one of which, Fig. 101, is fur- nished with a stop-cock,^ the other, Fig. 102, with shelves. What prin- 602. EXPERIMENTS SHOWING THE FLUIDITY OP ciple is explain- AIR. THE WEIGHT-LIFTER. The upward press- erf by means of ^ . ,, ^, A . *'.. the weight- ure * Q air ' one * tne P r P ertie s of its fluidity, liftsr? 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 pos- sible This precaution is suggested by economy, as well as by co iveuieuc* 166 NATURAL PHILC SOPHY. 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, sup- ported by three legs. A piston is accu- rately fitted to the bore of the tube, and a hook is attached to the bottom of the piston, from which weights are to be suspended. One end of the elastic tube is to be screwed to the plate of the pump, and the other end attached to the top of this instrument. The air being then exhausted from the tube, the weights will ba 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 pneumatic shower-bath is constructed. It con- sists of a tin vessel perforated with holes in the bottom for the shower, and having an aperture at the top, which is opened or closed at pleasure by means of a spring-valve. [Instead of the spring-valve, a bent tube may be brought round from the top down the side of the vessel, with an aperture in the tube below the bottom of the vessel, which may be covered with the thumb.] On immersing the vessel thus constructed in a pail of water, with the valve open, and the tube (if it have one) on the outside of the pail, the water will fill the vessel. The aperture then being closed with the spring or with the thumb, and the vessel being lifted out of the water, the upward pressure of the an will confine the water in the vessel. On removing the thumb or opening the valve, the water will descend in a shower, untiJ the vessel is emptied. What tw ^^' -M- ISCELLANEOU8 EXPERIMENTS DEPENDING properties ON TWO on MOKE OF THE PROPERTIES OF AIR. PNEUMATICS. 167 of air are THE BOLT-HEAD AND JAR. Fig. 104, a glasb illustrated ly globe with a long neck, called a bolt-head (or means of the ,, ,, , .,, Bolt-head and an J long-necked bottle), partly tilled with water, 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 bolt- head, expanding, expels the water, showing the elas- ticity 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. What two 605. ^ HE TRANSFER OF _ FLUIDS FROM ONH principles are VESSEL TO ANOTHER. The experiment may bo concerned in made with two bottleg tightly closed. Let one the transfer of 6 J fluids from De partly tilled with water, and the two con- one vessel to nected by a bent tube, connecting the interior of the empty bottle with the water of the other, and extending nearly to the bottom of the water. On exhausting the air from the empty bottle, the water will pass to the other, and, on readmitting the air, the water will return to its original position, so long as the lower end of the bent tube is below the surface. What ex eri- m EXPERIMENTS WITH THE SIPHON. Close msnts are per- the shorter end of the siphon with the finger or formed with w ith a stop-cock, and pour mercury or water into the longer side. The air contained in the shorter side will prevent the liquid from rising in the shorter side. But, if the shorter end be opened, so as to afford free passage lt>& NATURAL PHILOSOPHY. outwards for the air, the fluid will rise to an equilibrium m both arms of the siphon. 607. Pour any liquid into the longer arm of the siphon until *ne shorter arm is filled. Then close the shorter end, tc pre- vent the admission of the air ; the siphon may then be turned in any direction arid 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> AlR ESSENTIAL T0 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 cewer? ~ the air be not s P eei % readmitted, it will die. 609. AIR ESSENTIAL TO COMBUSTION. Place How is it shown that air & lighted taper, cigar; or any other substance that is essential to w iu produce smoke, under the receiver, and ex*- 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 OP THE AIR RETARDS ^herundlran EBULLITION.* Ether, alcohol, and other distilled exhausted re- liquors, or warm water, placed under the receiver, caver ? w j}} appear to boil when the air is exhausted. a. , 611. The existence of many bodies in a liquid the pressure of f rm depends on the weight or pressure of the the air on the atmosphere upon them. The same force, like- lodies * 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 experi- ment 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 bj teat, which throws it up iuto bubbles. PNEUMATICS. 169 will either pass off in vapor, or will have the appearance of boiling. 612. An experiment to prove that the pressure What expert- r . ment shows of the atmosphere preserves some bodies in the that the liquid liquid form may thus be performed. Fill a long form of some . -. , v , , T .,, bodies is de- v ' or a closed at one end, with water, and pendent on invert it in a vessel of water. The atmospheric atmospheric pressure w m re tain the water in the vial. Then, pressure *. 5 i "" i by means or a bent tube, introduce a lew 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 ex- naust 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 lip the entire space itself. On readmitting the ar, the ether becomes condensed, and the water will re- ascend into the vial. a 613. A simple and interesting experiment con water be frozen nected with the science of chemistry may thus b^ under a rt- performed by means of the air-pump. A watch- glass, containing water, is placed over a small vessel containing sulphuric acid, and put under the bulbed receiver. When the air is exhausted, vapor will freely rise from the water, and be quickly absorbed by the acid. An intense degree of cold is thus produced, and the water will freeze. 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 ha 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 inter- Pneumatic . ..-* *\i Paradox? esting experiment, illustrative or the pueaiuatic 170 NATURAL PHILOSOPHY. paradox, may be thus performed : Pass a small open tube (yjs 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 ur^en remedied, and the office of ventilation mpre effectually performed. 61G ' WlND ' Wind i 8 ^ P ut in motion. 617. There are two ways .in which the motion In what two . . . ways may the * tne air mav anse - ma j be considered as motion of the an absolute motion of the air, rarefied by heat a / r 5T an< ^ condensed by cold ; or it may be only an 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 pushes into the rarefied spot from all directions. This is what we call wind. 619. The portions north of the rarefied spot v^ndlav^ed? P ro ^ uce 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 direc- tions, 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. The air, being more rarefied there than in any other PNEUMATICS. 171 part jf 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. 620. A regular east wind prevails about the What wind prevails in the equator, caused in part by the rarefaction of the equatorial air produced by the sun in his daily course from ' east to west. This wind, combining with that xrom 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 re- marked, 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 raav 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. When 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 mountains, valleys, and a variety of other causes Hence every climate must be liable to variable winds. The (fuality of winds is affected by the countries over which they pass ? uud 172 NATURAL PHILOSOPHY. ciiey fire sometimes rendered Destilential by the heat of deserts or the imtrid 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 from the desert of Sahara, upon the western coast of Africa, called the Harmattan, producing a dryness and heat which is almost insupportable, scorching like the blasts of a furnace. 622. WHIRLWINDS AND WATERSPOUTS. The How is wind sometimes af- direction ot winds is sometimes influenced by the form of lofty and precipitous mountains, which, 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. What is sup- 624. The whirlwind, occurring at sea, occa fected by the face of a country ? to *e the cause oj vater- tpouts ? spout iong ^ singular phenomenon of the water Fig. 3U6. ACOUSTICS. 173 What doe* ^25. Fie. 105 represents the several forma in Fig. 105 rep- . . resent * which water-spouti 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 time, the surface of the sea under it is agitated and whirled round, the waters are con- verted into vapor, and ascend with a spiral motion, till they unite with the cone proceeding froin the cloud. Frequently, however, they disperse before 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 water- spout 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, lurge 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 wate^-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 water- spouts ; but large vessels, under but a small spread of canvas, encounter, as is said, but little danger. 628. Pneumatics forms a branch of physical science which has been entirely created by modern discoveries. Galileo first demon- strated that air possesses weight. His pupil, Torricelli, invented the barometer; and Pascal, by observing the difference of the alti- tudes 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 Magde- burg, invented the air-pump about the year 1654 ; and Boyle und Manotte soon after detected, by its means, the principal mechanical properties of atmospheric air. Analogous properties have been proved to belong to all the other aeriform fluids. The problem of determining the velocity of their vibrations was solved by Newton and Euler, but more completely by Lagrange. The theoretical prin- ciples relative to the pressure and motion of elastic fluids, from which the practical formulae are deduced, were established by Daniel Bernoulli in his Hydrodynamica (1738), but have bee& rendered more general by Navier. What is 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. '74 NATURAL PHILOSOPHY. \Vhat is 630. Sound is the sensation produced in the *>mnd? or g ans O f 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. 032. 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.f Why is a sound ^^' Bounds are louder when the air sur- louder in cold rounding the sonorous body is dense than when weather? ft j s m 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 iring of a gun produces a sound scarcely louder than the crack- ing of a whip. What are So- 635. Sonorous bodies are those which pro- norous bodies ? ^ uce c i earj 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 soM 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 sea would strike the shore." [ Lardner.] t In performing these experiments, the bell must be placed in such a man- aer thac 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 putup, or any other of tUe solid parts of the apparatus. ACOUSTICS. 175 To what do 636. Bodies owe their sonorous property bodies owe their , ,,. , .., -r, ,, ,.. sonorous prop- io thelr elasticity. But, although it is un- erties ? 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 un- dulatory motion to the air or the medium by which it is surrounded, similar to the motion communicated to smooth water when a stono is thrown into it. What are the 638. Sound ' l & communicated more rapidly best conductors, and with greater power through solid bodies than through the air, or fluids. It is conducted by water about four times quicker than by air, and by solids about twice as rapidly as by water. 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, wnile 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 tivice, because the wall will convey it with greater rapidity than the air, though each will brin^ it to the ear. 641. It is on the principle of the greater power of solid bodies ti communicate sound that the instrument called the Stethoscope * is constructed. What is the 642. The Stethoscope is a perforated cylin- Stethoscope, fe r O f li^ht, fine-drained wood, with a funnel- and on what ' . . . ; principle is it shaped extremity, which is applied externally to constructed? fa cav ities of the body, to distinguish the sounds within. T YJ t ,7 643. By means of the stethoscope the phy- of the stetho- sioian 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 words, art &oS, the breast, and i leven hundred and forty-two feet in a second were probably made when the weather was e*- trmufcl warm ACOUSTICS. 177 To what prao 651. This uniform velocity of sound enab es us velocit l of h " 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 thi.t intervene between the flash of the lightning and the roaring of the thunder, and multiplying them by 1120. . 7 653, THE ACOUSTIC PARADOX. Sound, as has i is me a ] reac i y 5 een s t a ted, is propagated by the undulations Acoustic Vara- of ^ ^ Now? ag these undu]ation8 or waves are 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 con- sequence will be neither elevation nor depression of the liquid. Explain the ^' ^ ien ' therefore, two sounds are produced acoustic para- in different places, there is a point between them dox- where the undulations will counteract each other, r.nd 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 tc 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 iield over a cylindrical glass vessel. Another glass vessel of similai 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 657. An echo is produced by the vibrations of the an echo? a i r meeting a hard and regular surface, such as a wall, a rock, a mountain, and being reflected back to the ear, thus producing the same sound a second and sometimes a third and fourth time. 178 NATUKAL PHILOSOPHY. Why are there ^58. For this reason, it is evident tbat no echo no echoes sea can be heard at sea, or on an extensive plain, where there are no objects to reflect the sound. Bywhatlawis 659. Sound, as well as light and heat, is re- sound reflected ? fleeted in obedience to the same law that has already been stated in Mechanics namely, the angles of inci- dence and of reflection are always equal. 660. 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 con- fined 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. What is said 661. Speaking-trumpets do not depend for of speaking- trumpets? . their efficiency upon the reflection of sound. 662. The voice, instead of being diffused in the open air, is con- fined within the trumpet, and the vibrations imparted by the lips to the column of air within the trumpet produce better waves in the open air than the lips alone would be able to do. Speaking- trumpets are chiefly used by naval officers to aid the voice, so that the word of command may be heard above the sound of winds and waves. How is a hear- 663 - Hearing-trumpets, or the trumpets used ing-trumpet by deaf persons, are also constructed on the same e ' principle ; but as the voice enters the large end of the trumpet instead of the small one, it is not so much con- fined, 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 connection 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 con- struction, connected by a slender spring with an adjusting slide, which, passing over the head, keeps both trumpets in their place. They are con- cealed from observation by the head-dress ? and enable the wearer to join in conversation of ordlc&ry tone, from which without them she is wholly debarred. ACOUSTICS. |7'' dt them to collect ar4 reflect the various sounds which are taking place in the vicinity. Hence the Cyprias, the Nautilus, and som-o other shells, when held near the ear, give a continued sound, whici resembles the roar of the distant ocean. On what prin- 666. Sound, like light, after it has been reflect wAis erin *- e( ^ ^ rom severa ^ surfaces may be collected into one galleries cot,- point, as a focus, where it will be more audibk appear haunted, voices have been heard from tomls, and the iead 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 mus- cles of the face ; and this appears to be the great secret of the art. How is the ^' ^ USICAL SOUNDS, OR HARMONY. The soundofamu- sound produced by a musical string is caused by steal ^string its vibrations ; and the height or depth of the tone depends upon the rapidity of these vibra- tions. Long strings vibrate with less rapidity than short ones; and for this reason the low tones in a musical instrument pro- ceed* from the long strings, and the high tones from the short ones. That character of sound depending upon rapidity of vi- bration is called pitch. 678. Fig. 106. AB represents a musical string. Explain T f f r, p Fig. 106. " tt b Fi g- m drawn __ ___ G-- ...... up to G-, its elas- ^-'"'.-- ----- ^ '""---IT"^^ ticity will not on- x^ '^*- - _ ~ ly carry it back *x**>~7 -------- & _________ l again, but will *"**-*.. F ------- ~,-'''* *~-~ iy ____ --~ give it a momen- tum which will carry it to H, from whence it will successively return to T, F, C, D, &c., until the resistance of the air entirely destroys its motion. , 679. The pitch ^of the sound produced by the pitch of the strings depends upon their length, thickness, tone of a string we ight, and degree of tension. The pitch oi depend f the sound produced by wind instruments de- pends 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 re- action are equal, the effect is the same in both cases. 681. Long and large strings, when loose, produce the lowest NATURAL PHILOSOPHY tones but differet-t tones may be produced from the saii.e 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. How does the 682- Tb. e qu dity of the sound of all musical temperature of , the weather of- instruments is anected by the changes in the feet the tone of temperature and specific gravity of the atmos- a musical in- , ttrument? P nere ' 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 instru- ments 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 ^^' ^ ne sc i ence of harmony is founded on science of har- the relation which the vibrations of sonorous many founded? bodieg have to each other> 685. Thus, when the vibrations of one string are double those ol 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 pro- duce 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 tft 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- pended 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 respect- ive lengths required to produce the seven successive notes of the gamut will be as follows : it being premised that the longer the gtring the slower will be its vibrations. 637. Let the length of the string required to produce the note called C be 1 ; the length of the string to produce the successive aotes will be CDEFGA B C J I t J I A -4- ACOUSTICS. 183 688. Hence, the octave will require only lialf of the length of the fundamen- tal 'note, and the vibrations that produce it will be as two to one. The vibrations of the string in producing the successive notes of the scale will be as follows : C D E F O A 1 * t I i"f That is, to produce the note D nine vibra- tions will be made in the same time that eight are made by C, five of E to four of 0, four of F to three of C, three of G to two of C, five of A to three of C, fifteen of B to eight of C, and two of the octave C to one ol the fundamen- tal C. 689. The same relations exist in each successive octave throughout the whole nusical scale. 690. As harmony depends upon the coincidence of vibrations, it follows that the octave produces the most perfect har- mony ; next in order is the fifth, as every third vibration of the fifth corre- sponds with every second vibration of the fundamental. Next to the fifth in the order of harmony follows the fourth, and after the fourth the third. 691. The following scale, containing three octaves, exhibits the proportions which exist between the fundamental and all the other notes within that compass. 692. In the lowest line of this scale the numbers show the intervals. The figures above express the number of 'ibrations of the fundamental or tonic, and the upper line of figures represents the proportionate vibrations of each suc- cessive interval. 693. It is found in practice that when two sounds are caused by vibrations which are in some simple numerical pro- portion to each other, such as 1 to 2, or <2 to 3, or 3 to 4, &c., they are pleasing to the ear ; and the whole science of har- oiony is founded on such relations. 694 The principal harmonies are the Kituvc, fifth, fourth, major third, and - ! | ORB ~f1 - iffl INI s lljTj - ilWIi s 184 NATURAL PHILOSOPHY. minor third ; and the relations between them and the fundamental or tunic are as follows : Octave, 2 to 1. Fifth, 3 " 2. Fourth. 4 " 3. Major Third, 5 " 4. Minor Third, 6 " 5. 695. Tht following Rules may serve as the basts 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, in- volving 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 Solution. (1.) A rocket was seen to explode, and in two seconds the sound of the explosion was heard ; how far off was the rocket 1 Ann. 2240 ft. (2.) The flash from a cloud was seen, and in five seconds the thunder was heard ; what was the distance of the cloud 1 Arts. 5600./Z. (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 7 Ans. 2ft. 8 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 1 the fifth 1 the minor third 1 the major third 1 Am. 200; 150; 120; 125. (5.) Supposing that two sounds, with all their attending circumstances similar, reach an ear situated at the point of interference of the undula- tions, what will be the consequence 1 [See Nos. 653 and 054.] (6.) The length of a string being 36, what will be length of its octave ! fifth 'J fourth 1 major and minor thirds ] Ans. 18; 24; 27; 28.8; 30. (7.) A stone, being dropped into a pit, is heard to strike the bottom in 7 seconds ; how deep was the pit 1 Ans. By Algebra, 600,/i. [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 tem perature above the freezing point, or 32. The average rate of 1120 fetyj Kaa been assumed in the text.] PYRONOMICS. 185 (8.) Suppose the length of a music-string to be five feet ; what will bo the length of the 15th, 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.) What must be the length of a pipe of an open diapason to produce the same tone with four foot C of the stopped diapason ? Ans. ^ft. [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 ? [See No. 695 (3).] Ans. 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 697. PYRONOMICS, OK THE LAWS OF Pyronomicsf J EAT . Pyronomics is the science which treats of the laws, the properties and operations of heat. What is 698. Heat is now known to be a motion of the Heat? minutest particles (or molecules) of a body. The molecules of all known bodies are continually in motion. This motion may be transmitted from one body to another. What is 699. Cold is therefore only an absence or partial Cold ? absence of this motion of the molecules. We say a body is cold when the motion of its molecules is less than usual, or less than that of surrounding bodies. What effect has 700. When a body is heated to a high tem- heat on bodies ? perature, the motion of the molecules becomes greater and greater, and the whole body becomes larger. Heat and the attraction of cohesion constantly act in opposition to each other ; hefice, the more a body is heated, the more its par- ticles will be separated. (See par. 1463.) 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, * Two exceptions to this remark are water and clay. Water expand* when it freezes ; clay contracts when heated. 1 86 NJ TURAL PHILOSOPHY. {he blocl vessels are well filled, the hands arid the feet, as will an othei parts of the body, expand or acquire a degree of plumpness ,md the skin is distended ; while, on the contrary, in cold weathei the flesh appears to contract, the vessels shrink, 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, &o. The heat insinuates itself between the particles, ;md forces them asunder. These particieo thn are removed from that degree of proximity to each other within which coxitsive attrac ii(n exists, and the body is reduced to a fluid form. When the heat is removed the bodies return to their former solid state. 147. t k' d f ^^' -^ eat P asses through some bodies with 'at lies arrest diore difficulty than through others, but there is 'he progress no kind of matter which can completely arrest its v,f hf.at ? progress. (See par. 1465.) What is 704. Of all the effects of heat, that produced upou fteam? ,oter is, perhaps, the most familiar. The particles are totally separated, and converted into steam or vapor, and {heir extension is wonderfully increased. The steam wlrch arises from boiling water is nothing more than portions of the water heated. The heat insinuates itself between the par- ticles of the water, and forces them asunder. When deprived of the heat, the particles will unite in the form of drops of A r ater. This fact can be seen by holding a cold plate over boiling water. The steam rising from t ho water will be condensed into drops on the bottom of the plate. The air which we breathe generally con tains 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 weathor. Heat also produces most remarkable effects upon air, causing it to expand to a wonderful extent, while the absence of heat causes it r.'j "shrink or contract into very small dimensions. 705. The attraction of cohesion causes the How is 'M' small watery particles which compose mist or vapor to unite together in the form of drops of water. It is thus that rain is produced. The clouds consist of PYKO^OMICS. 187 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 barom- eter, 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 per- ceptible 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 fireside 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, noi can we perceive it in the fuel ; it must, therefore, have existed somewhere in a latent state. It is, however, the effects of free heat, or free caloric, which are embraced in the science of Pyronomics. The subject of latent heat belongs more properly to the science of Chemistry. (See par. 1470.) 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 ' SOURCES OF HEAT. The four prin- itrindpal cipal sources of the development of heat are twras of the gunj Electricity, Chemical Action and Me- chanical Action. The heat produced by fire rvul flame is due to chemical action. IH8 . NATURAL PHILCSuPHY. I*T._, ,L 709. But, of all the sources from which heat V\ hat is the ource of the has been developed by human agency, that pro- greatest degree duced by electrical action, and especially the 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 ascri jeu to the sun is attended by peculiar phenomena, but imperfectly understood. It may, perhaps. De questioned whether there be any absolute heat in the rays of that luminary, for we find that the heat is not in all cases propor- tionate 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. 711. 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, whan suddenly extended, shows evident signs of heat ; and 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 P rinci P al effects of heat arft principal ef- three, namely : f-cts of heat i ^ Heat expan( is mos t substances - (2.) It converts them from a solid to a liquid state. (3.) It converts them from the liquid to the gaseous state. 713. There are many substances on which ordinary degiees 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 com- bustible. 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 * Syringes have been constructed on this principle A solid piston being forcibly driven downward or dry tinder, ignites it PYKONOMICS. 189 > a tensity of heat be restored, it is presumed that they would resume tktiir liquid or gaseous form. What is the 714 Heat tends to diffuse itself equally through fast law of . ' heat ? a ^ 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 heat. 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 ther- mometer be applied to each, no difference in the temperature will be observed. What is the ^'^' -^ rom tn is ^ appears that some substances reason that conduct heat readily, and others with great dif- wme sub- faculty. The reason that the marble slab seems stances feel y i , . warm and tae coldest is, that marble, being a good con- others cold in ductor of heat, receives the heat from the hand thesameroom? &Q rea dil y 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 los^ is imperceptible. What is the 716. The different power of receiving and difference in conducting heat, possessed by different substances, the warmth of is the cause of the difference in the warmth of different gar- various substances used for clothing. ments ? Why are 1^-1 ^^us, woollen garments are warm gar woollen gar- ments, because they part slowly with the heat [G}i they ac( l uire from the bod y> and ' cons - 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 read- ily receives fresh heat from the body. It is, therefore, con- >iantly receiving heat fron- the body arid throw : ng it out into 190 NATURAL PHILOSOPHY. the air, while the woollen garment retains the heat which it re- ceives, 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 Jieat 719. Heat is propagated in two ways namely, propagated? by conduction and by radiation. Heat is "propa- gated 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. (See par. 1469.) 720. Different bodies conduct heat with differ- What are the en t degrees of facility. The metals are the best orsofheat* 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 he more readily heated in a diver vessel than in any other of the same thickness, except one oJ gold, or of platinum, on account of its great conducting power. Why are the 722. Metals, on account of their conducting handles of tea power, cannot be handled when raised to a tempe- made'ofwoodi rature above 12 de g rees of Fahrenheit. For this reason, the handles of metal tea-pots and coffee- pots are commonly made of wood ; since, if they were made of metal, they would become too hot to be grasped by the hand, soon after the vessel is filled with heated fluid. 723. Wood conducts heat very imperfectly. For this reason, wooden spoons and forks are preferred for ice. Indeed, so imper- fect 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 efficacv in preserving the warmth of the body. Water becomes scalding hot at 150 degrees ; but air, heated far beyond the tempe- rature 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 D 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 PYKOJNOMIOS. 1'Ji money, ,i^ produce darkness ; and this may, with equal propriety, be termed the optical paradox. T3ut a clear understa tiding of the principles involved in what is called respectively the hydrostatic, pneumatic, acoust*'' nd optical para dox, shows that there is no paradox at all, but that each ia the necessarj result of oertai- fixed and determinate laws OPTICS. :tt which converging rays meet is called F; K n?. the focus. Fig. 117 represents con- Erplain fig. verging rays of light> of which the point F is the focus. What is abeam 784. A beam of Fig ' 118 - of light ; light consists of many rays running in parallel lines. Explain Fig. Fig> 118 repr esents a beam of light. 785. A pencil of rays is a collection of What ts a pen- , . . . r r , T . ci/ of rays? diverging or converging rays. [&* F.#, 116 and 117.] 786. Light proceeding from a luminous In what dtrec- . , . . , . . tion, and with body is projected forward m straight lines in what rapidity, every possible direction. It moves with a does light mo vet . ,. , 1-1, i 111 rapidity but little short of two hundred thou sand miles in a second of time. 787. Every point of a luminous body ia From what part , . , , . , , . of a luminous a Centre 5 trom which light radiates in every body does ligkt direction. Rays of light proceeding from different bodies cross each other witliO'U interfering. The rays of light which issue from terrestrial bodies continually diverge, until they meet with a refract- ing substance , out the rays of the sun diverge so little, on account of the immense distance of that luminary, that they are considered parallel. What is a 788. A shadow is the darkness produced shadow ? ky the intervention of an opaque body, which prevents the rays of light from reaching an object behind the opaque body. Why are shad- 789 ' Sliaclows are of different degrees of ' oic of different darkness, because the light from other luml- 214 NATURAL PHILOSOPHY. degrees oj dark- nous bodies reaches the spot where the 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 can dies be extinguished. The darkness of a shadow is propor- tioned to the intensity of the light, when the shadow is produced by the interruption of the rays froui a single luminous body. What produces 79 - As the de g ree of H g ht and darkness the darkest can be estimated only by comparison, the strongest light will appear to produce the deepest shadow. Hence, a total eclipse of the sun occa- sions 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 mor* lights, we can tell which is the brightest light, because it will cause the darkest shadow. 791. When a luminous body is larger than What is the , , , , , f ,, thape of the an opaque body, the shadow of the opaque shadow of an body will gradually diminish in size till it ( ' ue terminates in a point. The form of the shadow of a spherical body will be that of a cone. Fig. 119. A repre- Kxplain Fig. gents th(J ^ ^ B the moon. The sun being much larger than the moon, 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. 9,15 IK Fig. 120 the shadow * 12 - of the- object A increases ui size at the different dis- tances B, C, D, E; or, in 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- dles B, C, .and D. The light B produces the shadow 3, the light C the shadow c, and the light D the shadow d ; but, as the light from each of th^ candles shines upon all the shad- ows except its own, the shadows will be faint. Fig. 121. What becomes of the light which falls on an upaque object? When is light said to be re- flected? 794. When raya of light fall upon an opaque body, part o.^ them are absorbed, and part are reflected. Light is said to be reflected when it is thrown off from the body on which it falls ; 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, look- ing-glasses, or mirrors, &c., reflect it "in so perfect a man- ner 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 lio;ht. 795. That part of the science of Optics which relates to reflected light is called Outuptricx. What is Catop- NATURAL PHILOSOPHY. Wnat is the fun- T96 > The laws of r^cted light are the damenfal law of same as those of reflected motion. Thus, atop no, 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 ta 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 beer described under the head of Mechanics [see explanation of /<%. 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 sur- face ; and a reflected ray is the ray which proceeds frorr any reflecting surface. Fig. 122 is designed to show 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 rep- resents an incident ray, falling on the mirror in such a manner as to form, with the perpen- dicular P, the angle I A P. This is called the angle of incidence. The line R A is to be drawn on the othor 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 K A P is equal to I A P. The line R A will then show the course of the reflected ray ; and the angle RAP will be fche 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 Explain Fig. 122. Fig. 122. OPTICS. 217 law invariably prevails ; so that, if we notice the inclination of any incident raj, and the situation of the perpendicular to the surface on which it falls, we can always determine in what man- ner or to what point it will be reflected. This law explains thg 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 room, 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 re- minous and fl ec t e d light. Luminous bodies are seen by opaque bodies ' . respectively the rays of light which they send directly to seen ? Qur What effect 799. All bodies absorb a portion of the light ontlwinten- 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 ^a reflecting reflects an entire image of the luminous body surface reflect ? shining upon it. T , r , , Whon the sun or the moon shines upon a H- liy do we 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 n . reflected by a can " e seen on v "J reflected rays, and as the reflecting sur- angle of reflection is always equal to the angle of incidence, the image from any point can be ~,een only in the reflected ray prolonged. Why do objects 801. Objects seen by moonlight appear faiutc; tppear fainter than when seen by daylight, because the light by ' which they are seen has been twice reflected ; for, the moon is not a luminous body, but its light is caused by thu Hun shining upon it. This light, reflected from the moon u.nd fulling upon any object, is again reflected by tba* object. If 218 NATURAL PHILOSOUiY. Buffers, therefore, two reflections ; and since a portion is absorbed by each surface that reflects it, the light must be proportion*- illy fainter. In traversing the atmosphere, also, the rays Loth of the sun and moon, suffer diminution ; for, although 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 . J J 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 direc- tions, 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 sunsb?ne to darkness immediately upon the setting of the sun. 803. When rays of light, proceeding from liht enter an 7 object, enter a small aperture, they cross enter a small aper- one another, and form an inverted image of the object. This is a necessary consequence of the law that light always moves in straight lines. K.rj>/am ^04. Fig. 123 represents the rays from an object ttV. 123 a c, entering an aperture. The ray from a passes OPTICS. down through the aperture to d, and the ray from c up to f>, and thus these rays, crossing at the aper- Fi Jv!3 - ture, form an inverted image on the wall. The room in which this experiment is made should be darkened, and no light permitted to enter, except- ing through the aperture. It then becomes a camera obscura. 805. These words signify a darkened chamber. In the future de Bcription which will be given of the eye, it will be setn that the camera obscura is constructed on the same principle as the eye. If a convex lens be placed in the aperture, an inverted picture, not only of a single object, but of the entire landscape, will be found on the wall. A portable camera obscura is made by admitting the light into a box of any gize, 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 dimin- ished image of the landscape, or. of any group of objects, is present- ed on the plate in an erect position. What is the 806. The angle of vision is the angle formed angle of a t the eye by two lines drawn from opposite vision? f J parts ot an object. What is the 807. The angle C, in Fig. 124, repiesents the "figures 124 an o^ e ^ v i s i n - The line A C, proceeding from and 125 ? one extremity of the object, meets the line B C from the opposite extrem- Fig. 124. ity, and forms an angle G at the eye ; this is the angle of vision. 808. Fig. 125 represents the different angles made by the same object at dif- ferent distances. From an inspection of the figure, it is evident that the nearer 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- E B ly, the larger the angle under which it is seen ; and, on the con trary, that objects at a distance will form small angles of vision Thug, in this figure, the three crosses F G, D E, and A B. are NATURAL PHILOSOPHY. all of the same size ; but A B, being the most distant, subtesdr 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 DOE and FOG. 809. The apparent size of an object depends upon On what docs , . ., , ,. . . the apparent tne size * tne an g le * vision. But we are accus- size of an ob- tomed to correct, by experience, the fallacy of ap- yect depend ? p earances j ari & t 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 house 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 in will be seen that the several crosses, A B, D E, F G, and II 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 dif- ferent 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 of not subtend an angle of more than two seconds its distance ? O f 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. Wh 811. When the velocity of a moving body tion imper- does not exceed twenty degrees in an hour, its teptible / motion is imperceptible to the eye. It is for this reason that the motion of the heavenly todies is invisible, notwithstanding their immense velocity. 812. The real velocity of a body in motion round a point de- pends on the spuee comprehended in a degree. The more dis- OPTICS. C ^i tun j 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 ** 126 - man at B, both start together, it is manifest that A must move more rapidly than B, to arrive at C at the same time that B reaches D, because the arc A C is the arc of a larger circle than the arc B D. But to the eye at E the velocity of both appears to be the same, C ^ B because both are seen under the same angle of vision. uri 814. A mirror is a smooth and polished sur- Wfiat are mirrors, and face, that forms images by the reflection of the ^ ^ n k Mirrors (or looking-glasses) 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 cover- ing, 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. What are the 815> There are three kinds of mirrors, different kinds namely, the plain, the concave, and the con- of mirrors f 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 By what law are objects re- obedient to the law that the angles of incidence and fleeted from a reflection are equal. For this reason, no person ' can see another in a looking-glass, if the other can- not see him in return. BTATUBAL PHILOSOPHY. How do looUng- 817 - Looking-glasses or plain mirrors cause glasses make all everything to appear reversed. Standing before Ejects appear? & i oo king-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 mirroTS stand opposite to each other, the reflections of the one are cast upon the other, and to a person be- tween them they present a long-concinued 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 v 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 cir- cular 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 in- clined 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 forma 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 in- dented by Dr. Brewster, of Edinburgh, a few years ago. 822. A convex mirror is a portion of the external sur- face of a sphere. Convex mirrors have therefore a convex surface. 823 A concave mirror is a portion of the inner surface 01TICS. The outer part of M N is a Fig. 127. of a hollow sphere. Concave mirrors have therefore a con- cave surface. Exj-ilain 824. In Fig. 127, M N represents both a convex rig. 127. and a concave mirror. They are both a portion of a sphere of which is the centre, convex, and the inner part is a concave mirror. Let A B, C D, E F, represent rays falling on the convex mirror M N. As the three rays are parallel, they would all be per- pendicular to a plane or flat mirror ; but no ray can fall perpendicularly on a concave or convex mirror which is not directed tmvards the centre of the sphere of which the mirror is 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 EFT will be equal to the angles of reflection P B G and T F H ; and, since we see objects in the direction of the reflected rays, we shall see the image at L, which is the point at which the reflected rays, if continued through the mirror, would unite and form the image. This point is equally distant from the surface and the centre of the sphere and is called the imaginary focus of the mirror. It is called the imaginary focus, because the rays do 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 oairror is smaller than the object 224: NATUKAL PHILOSOPHY. What is the ^' This is owing to the divergence of the re- ooject of fleeted rays. A convex mirror converts, try reflec- TTV/y "1 9Q 9 ff ' tion, parallel rays into divergent rays; rays that fall upon the mirror divergent are rendered still more diver- gent by reflection, and convergent rays are reflected either parallel, or less con- Oig. 128. vergent. If, then, an object, A B, be placed before any part of a convex mirror, the two rays A and B, proceeding from the extremities, falling convergent on the mirror, will be re- flected less convergent, and will not come to a focus until they arrive at C ; then an eye placed in the direction of the reflected rays will see the image formed in (or rather behind) the mirror at a b ; and, as the image is seen under a smaller angle than the object, it will appear smaller than the object. What is the $27. The true focus of a concave mirror is true focus of a point equally distant from the centre and the 'mirror? surface of the sphere of which the mirror is a portion. When will 828. When an object is further from the con- ike image re- cav e surface mirror than its focus, the image will be aconcaveTe inverted; but when the object is between the upright, and mirror and its focus, the image will be upright, 'ed T " an ^ S row l ar g er * n proportion as the object is placed nearer to the focus. What pe- 829. Concave mirrors have the peculiar prop- culiar prop- er ty of forming images in the air. The mirror ^oncar^mir- an( * tne OD J ect being concealed behind a screen, rnrs? or a wall, and the object being strongly illumi- OPTIOJ. Dated, the ra^ from the object fail upon the minor, 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 pre- sented with great distinctness and beauty, by raising a fine trans- parent 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 m i rror j s larger than the object when the concave mirror .T larger than the object is placed between the mirror and its <**"*' focus. Fig. 129. What is the de- 831 - This is owin g to the convergent prop- iign of Fig. erty of the concave mirror. If the object A B be placed between the concave mirror and ifr focus /, the rays A and B from its extremities will fall divergent on the mirror, and, on being reflect- 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 I 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 princi pal focns. 2. When it is situated between its centre of concavity and that focus. 3. When it is more remote than the centre of concavity. 1st. In the first case, the rays of light diverging after reflection but in a less degree than before such reflection took place, the iui 2^6 NATURAL PHILOSOPHY age will be larger than the object and appear at a greater 01 smaller distance from the surface oi the mirror, and behind it. Tha Image in this case will be erect. 2d. When the object is between the principal focus and the cen- tre 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 im- age 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 Catop- trics, 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 cf the sphere of which the mirror is a portion. The truth of these state- ments may be illustrated by simple drawings ; always recollecting, in drawing the figures, to make the angles of incidence and reflec- tion equal. The whole may also be shown by the simple experi- ment of placing tho 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. (1.) Par- allel 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 jbcus 0' OITIOS. 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 con- verged. (7.) 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 Kirface 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 ra>& 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 tht same point from which they proceeded. How are objects 836. As a necessary consequence of the laws teen from a con- which have now been recited, it may be stated, First, in regard to CONVEX MIRRORS, the im- ages of objects invariably appear beyond the mirror ; in other . they are virtual images. Secondly, they are seen in 10 NATURAL PHILOSOPHY. their natural position, and, Thirdly, they are smaller than the objects themselves ; the further the object is from the mir- ror, 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 MIRKORS. (1.) The image of an object very remote from a concave mir- ror, as that of the sun, will be in the focus of the mirror, and the image will be extremely small.* 1 (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 o'Mect be at a distance from the mirror equal to the length of its radius, then the image will be at an equal dis- tance 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 th* reflection of the rays of light from concave mirrors, arc some- times called "physical spectra." * This is the manner in which concave mirrors become burning-glasses. The rays of the sun fail 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 Butfon. 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 U>aJ at a liun- dreu feet, and silver at fifty feet. OPTICS. 1329 The existence and position of these spectra may easily be shown experimentally thus : Experiment. Hold a candle opposite to a concave mirror, at ths 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 telesccpes, 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 con- cave 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 reflectois of compound microscopes, magic lanterns and light- houses, by means of which the light given by the luminous bodj is increased and transmitted in some particular direction that maj be desired, are illustrations of the practical application of this prin- ciple. (6.) Lastly, place the object between the mirror and the focus, and the image of the object will appear behind the mir- ror. It will not 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 REFRAO What is a Me- TTON - A Medium,* in Optics, is any sub- dium in Optics? stance, solid or fluid, through which light can pass. What is refrac- 839. When light passes in an oblique 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 OM<1. 230 NATURAL PHILOSOPHY. twn. The property which causes it is called bility 840. DIOPTRICS. That part of the sci- tri~.9? l P " ence of Optics which treats of refracted light k called Dioptrics. What is meant ^^" ^ me dium, in Optics, is called dense or by a denser and rare according to its refractive power, and not r Ot>ti T lUm accordin g to its specific gravity. Thus, alcohol, and many of the essential oils, although of less specific gravity than water, have a greater refracting power, and are, therefore, called denser media than water. In the fol- lowing 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 th densest, and the first the rarest, namely : air, ether, ice, water, alcohol, alum, olive oil, oil of turpentine, amber, quartz, glass, molted sulphur, diamond. 842. There are three fundamental laws of What are the T .. . . , . , ,, ., , , fundamental Dioptrics, on which all its phenomena de- Imos of Diop- pend, namely : (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 *' ray of light passing from air into water, in a perpendicular direction. According to the first OPTICS. [aw stated above, it will continue on in the ** 1SO 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. But, if the ray proceed from a rarer to a denser medium, in an oblique direction, as from C to B, when it enters the denser medium it will not continue on in the same straight line to D, but, by the second law, stated above, it will be refracted or bent out of its course and proceed in a less oblique direction to F which is nearer the perpendicular ABE than D is. 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 tion is ^refrac cases m proportion as the rays fall more or less lion in all cases ? obliquely on the refracting surface. 844. From what has now been stated with f regard to refraction, it will be seen that many taking the depth interesting facts may be explained. Thus, an if water, and oar? or a s ti c k, w hen partly immersed in water, appears bent, because we see one part in one medium, and tht 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 down- wards, we are liable to no such deception, because there will be no refraction. 845. Let a piece of money be put iito a cup or a bowl, and the cup and the eye * be placed in such a position that the side of the *:}< will just hfde the money from the sight; then, keeping the ev 232 NATURAL PHILOSOPHY. directed to the same spot, let the cup be filled with water, th monoy will become distinctly visible. Why do we not 846< The refraction of H g ht prevents our see the sun, moon seeing the heavenly bodies in their real situa- and stars, intheir ^j on 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 conse- quence 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 dissi- pated 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, conse- quently, the longer the duration of twilight It is proper, however, here to mention that there is another rea- son, why we do not see the heavenly bodies in their true situ- ation. 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 \vhen 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 und a half before". The refraction of light does not affect the appear- ance 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 ,. , if produced when one medium to another, and through that light suffers two into the first again, if the two refractions be iijual refrac- , , . ,. f ' on t ? J equal, and in opposite directions, no sen- sible effect will be produced. This explains the reason why the refractive power of flat window- glass produces no effect on objects seen through it. The rays suffer two refractions, which, being in contrary directions, produce the game effect as if no refraction had taken place. 848. LENSES. A Lens is a glass, which, What is a Lens? . .. ,. f ' owing to its peculiar form, causes the rays OPTICS. of light to converge to a focus, or disperses them, according to the laws of refraction. Explain the dif- &49. There are various kinds of lenses, ferent kinds of named according to their focus ; but they are all to be considered as portions of the internal or external surface of a sphere. (See par. 1480.) 850. A single convex lens has one side flat and the other convex ; as A, in Fig. 131. 851. A single concave lens is flat on one side and concave on the other, aa B in Fig. 131. 852. A double convex lens is convex on both sides, as C, Fig. 131. A double concave le.ns is concave on both sides, as D, 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 f a Greek language, and means literally a little moon. This term is applied to a concavo-convex lens, from its similarity to a moon in its early appearance. To this kind of lens the term periscopic has recently been applied, from the Greek language, meaning literally viewing on all sides. When the concave and convex sides of periscopic glasses are even, or parallel, they act as plane glasses ; but when the sides are unequal, or not parallel, they will act as concave or convex lenses, according as the concavity or the convexity is the greater, What is the axis 854. The axis of a lers is a line passing yf a lens? through the centre : thu? F G, Fig. 131, is the axis of all the five lenses. Meniscus ? NATURAL PHILOSOPHY. 85D. 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 th) 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 c. ti T U fh W f ^ rom ^ e P er P en dicular. I* 1 order to estimate feet of a lens J the effect of a lens, we must consider the situa- tion of the perpendicular with respect to the surface of the leas. 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 observa- tion, 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 **>**,* magnify objects at a certain dis- cave lenses re- tance. *P ec (2.) That concave lenses disperse the rays, and diminish objects seen through them. What is the fr- 856. The focal distance of a lens is the cal distance of distance from the middle of the glass to the focus. This, in a single convex lens, is equal to the diameter of the sphere of which the lens is a portion, and in a double convex lens is equal to the radius of a sphere of which the lens is a portion. 857. When parallel rays * fall on a corx^ What rays will , , , , . , . ., . . .. pass through a vex ^ ens ? those only which fall in the direc- lens without re- tion of the axis of the lens are perpendicular to its surface, and those only will continue * The rays of the sun are considered parallel at the surface of the earth. They aie not so in reality, but, on account of the great distance of that luminary, their divergency is SJ 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. (See par. 1484) 858. It is this property of a convex lens ci ^/e W are ^sun w ^ c ^ gives it its power as a burning-glass, or glasses, or sun-glass. All the parallel rays of the sun burning-glasses, w hich pass through the glass are collected to- conslructed ? ,, . ,, , . 7 7 gether in the focus ; and, consequently, the heat at the focus is to the common heat of the sun as tJie 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 timeb greater at the focus than at the lens. 859. The following effects were produced by a large lens, or burn- ing-glass, two feet in diameter, made at Leipsic in 1691. Pieces of lead and tin were instantly melted ; a plate of iron was soon ren- dered 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 down into several flat surfaces, it will present as many images of an object to the eye as it has flat surfaces. It is then called a Multiplying-glass. Thus, if cne 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 crnvex or concave lens. 801. 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 sur- face , will become converging. '2.) Divevginor rays will be made to diverge less, to become por- 10* 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 sufibr 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 thac tne 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 boyond the centre, from the centre, or between the centre and the (3.) Don verging rays are made less converging, parallel, or diverg- ing, accosting to their .degree of convergency before refraction. 803. 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 paral- lel ; and one invariable result is produced by the rays when jpaes- ing 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 -. . a , i purpose? when by age it becomes too flat, or loses a What kinds of portion of its roundness: the latter, when erall * worn^b V an J otner cause it assumes too i ound a old persons? form, as in the case of short-sighted (or, as "I \ Tl 7 * J 7, O\? young ? m y tne y are sometimes called, near-tighted) 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 of convexity or. concavity ; so thai, by knowing the number that fits the eye, the purchaser can generally bo accommodated without the trouble af trying many glajaes. uracs. "231 806 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 con- struction of the eyes of all. ~? 866. The eye is composed of a number of U/ what is \ ... tke eye com- coats, or coverings, within which are enclosed posed? a j enSj an( j certain humors, in the shape and performing the office of convex lenses. What are the different 86 ^. The different parts of the eye par Is of the eye ? are : (1.) The Cornea. (2.) The Iris. (3.) The Pupil. (4.) The Aqueous Humor. (6.) The Vitreous Humor (7.) The Retina. (8.) The Choroid. (9.) The Sclerotica. (5.) The Crystalline Lens. | (10.) The Optic Nerve. Explain 868 ' Fi S' 132 re P resents 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 opening in the centre, called the pupil, p, which contracts in a strong light, and expands in a faint light, and thus reg- ulates the quantity which is admitted to the tender parts in the interior of the eye. Explain 869 ' *ig- 13S re P" Fig. 133. resents a side view of the eye, laid open, in which b b represents the cornea, e e the iris, i d the pupil, //the aqueous hu- aaor, g g the crystalline lens, 'i h NATbRAL PHILOSOPHY. the vitreous humor, i i i i i the retina, c c the choroid, a a a a a the scl erotica, and n the optic nerve. Describe the $7 '0. The Cornea forms the anterior portion Cornea. the eye. It is set in the sclerotica in the same nanner 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 finely - polished surface, and causes the brilliancy of the eye. Describe the 871. The Iris is so named from its being l> of different colors. It is a kind of circular curtain, placed in the front of. the eye, to regulate the quan- tity of light passing to the back part of the eye. It has a circular opening in the centre, which it involuntarily en- larges or diminishes. 872. It is on the color of the iris that What causes a the color of the eye depends. Thus a person person's eyes to . .,"< ^ t ^ i ^ ^ i be black blue or ls sai( * * nav ^ black, blue, or hazel eyes gray, <%c. .-' according as the iris reflects those colors respectively. What is the ^73. 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 human eye, but in quadrupeds it is of different shape. W nen the pupil i& expanded to its utmost extent, it is capable of admitting ten times the quantity of light that it does when most con- tracted. 874. In cats, and other animals which are paid Borne 'animals to see * n ^ e dark, *he power of dilatation and eon- M* in the traction is much greater; it is computed that their pupils may receive one hundred times more ligh< 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 eyee suffer pain, because the pupil, being expanded, admits alarger quan tity 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. 875. The Aqueous Humor is a fluid as clear Describe the Aqueous Hu- as the purest water. In shape it resembles a meniscus, and, being situated between the cor- nea and the crystalline lens, it assists in collecting and transmitting the rays of light from external objects to that lens. 876. The Crystalline Lens is a transparent [Wiat ts the J r 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. T*-/, , ,; 877. The Vitreous Humor (so called from its \\hat is the ^ Vitreous Hu- resemblance to melted glass) is a perfectly transparent mass, occupying the globe of tho eye. Its shape is like a meniscus, the convexity of which greatly exceeds the concavity. 878. In Fig. 134 the shape of the aqueous and vitreous humors and the crys- tal] ine lens is presented. A is the aqueous Humor, which is a meniscus, B the crystal- line lens, which is a double convex lens, and C the vitreous humor, which is aide a meniscus, whose concavity has a small ir radius than its con- vexity. 240 flATUKAL PHILOSOPHY. What is tto &7S. The Retina is the seat of vision. The Retina ? ra j s O f 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 ^80. The Choroid is the inner coat or cover- Choroid? j n g O f the eye. Its outer and inner surface is covered with a substance called the pigmentum 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 $81. The Sclerotica is the outer coat of the Sclerotica. e ye. 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 sclero- tica are attached the muscles which move the eye. It re- ceives 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 what is the carr j es the impressions made by the ravs of Optic Nerve? J * 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 r ? o n r i 11 f-n-i- foes the eye i see o. 805J, and the -images of all objects resemble? seen by the eye are represented on the 'retina in the same manner as fne forms of external objects are delineated in that instrument. Explain 884. Fig. 135 represents only those parts of the eye l **' ' which are most essential foi the explanation of the phenomenon of vision. The image is formed thus : The : ay? 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 image / e. (See par. 1488.) Hew i.s the 885. ^ ue convexity of the crystalline humor is convexity of increased or diminished by means of two muscles, thecryttaltinefo M h it at t ac hed. By this means, the focus lens altered, m J and for what of the rays which pass through it constantly falls purpose? on the retina; and an equally distinct image is fcrmed, both of distant objects and those which are near. How can you 886. Although the image is inverted on the re- "he^wrent tma ' we see ^J ects erec ^ because all the images position of formed on the retina have the same relative posi- oojccts ? 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 re- Win! do we not "see double tma 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 a?e seen single. Although an object nay be distinctly seen with only one eye, it has been calculated that the use of both eyes makes u difference >f about one-twelfth. From the description now NATURAL PHILOSOPHY. given of the eye, it may be seen what are the defects wnich art remedied by the use of concave and convex lenses, and how the use of these lenses remedies them. What defects 888. When the crystalline humor of the eye is of the eye are too roun a tne rays of light which enter the eye spectacles de- or signed to converge to a focus before they reach the retina, remedy ? an d ) 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 converg- ency 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 b\ For what de- J , A- fects of the reason 0* the humors losing their transparency eye is there either by age or disease. For these imperfection? no remedy ? nQ g] asses O g- er a reme dy, without the aid of surgi- cal 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 inevi table result What is a ^' ^ g i n o^ e microscope consists simply of singlemic.ro- a convex lens, commonly called a magnifying- scope. 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 &' ' microscope, C P. The diverging rays from tbe object A B are refracted in their passage through the leu* C P. aii'J OPTICS. 243 made to fall parallel on the crystalline lens, by which they are refracted to a focus on the retina R R, ; and the image is thus magnified, because the divergent rays are collected by the lens and carried to the retina. 893. Those lenses or microscopes which have What glasses ,,,.' . - . have the great- tne shortest focus have the greatest magnifying est magnifying power ; and those which are the most bulging or convex have the shortest focus. Lenses are made small because a reduction in size is necessary to an increase of curvature. What is a double 894. A compound microscope consists microscope? O f wo convex lenses, by one of which a magnified 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 compound 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. 1ST. invertcd magnified image at C D. The rays which diverge from this image are collected by the lens N O (called the eye-glass, it is nearest to the eye), wWch acts- on the principle of 24A NATURAL PHILOSOPHY. the single microscope, and forms still another image on the retina RR. IW / ' fk &$$ The solar microscope is a microscope solar micro- with a mirror attached to it, upon a movable wye ; joint, which can be so adjusted as to receive the sun's rays and reflect them upon the object. It con- sists 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 ' t 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 mi- %? them *S~ croscope is ascertained by dividing the least of singie and distance at which an object can be distinctly double micro- geen j^y tne na k e( i eyc b y t h e f oca l distance of scopes ascer- . . mi . . , tained? ie ^ Gns ' This, in common eyes, is about seven inches. Thus, if the focal distance of a lens be only of an inch, then the diameter of an object will be magnified 28 times (because 7 divided by 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 propor- tion as the distance of the image from the object-glass is greater than that of the object itself from it. Thus, if the dis- tance of the object from the object-glass be of an inch, and the distance of the image, or picture, on the screen, be *en feet. .ir 120 inches, the object will be magnified in length 4bO times or in surface 280,000 times, OPTICS. 245 A lens may be caused to magnify or to aiminish an object. If the jbject be placed at a distance from the focus of a lens, and the im- age be formed in or near the focus, the image will be diminished ; but, if the object be placed near the focus, the image will be mag- nified. What is the Mag- The Magic Lantern is an instrument con- k Lantern? structed on the principle of the solar micro- scope, but the light is supplied by a lamp instead of the sun. 899. The objects to be viewed by the magic lantern are gener- ally painted with transparent colors, on glass slides, which are Fig. 138. 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 rays of light from the lamp are received upon the concave mirror e, and reflected to the con- vex lens c, which is called the condensing lens, because it con- centrates 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 imagi on the screen at/. The image will increase or diminish in sizo in proportion to the distance of the screen from the lens a. Describe Fig. 138. 1Mb NATURAL PHILOSOPHY. 900. DISSOLVING VIEWS. The exhibition Hair are" Dis- ca i} e d Dissolving Views " is made by means srlving Views , . , , t "presented? * * wo rna g i c lanterns of equal power, so as to throw pictures of the same magnitude in the Kime 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 What is a Tel- . t ,. . . ,. x , / tscope? instrument tor viewing distant objects, and causing them to appear nearer to the eye. How are tele- ^2. Telescopes are constructed by placing scopes construct- lenses of different kinds within tubes that slide *"' 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 ^ ne t wo distinctions in the kinds of telescopes there? m common USG > called respectively the Refract- ing and the Reflecting Telescope. How is the Re- 904. The Refracting Telescope is con- scopT^onstruci- structed with lenses alone, and the eye is d? * directed toward the object itself. 905. The Reflecting Telescope is con- How does a Re- -. . , . -, > fleeting Tele- structed with one. or more mirrors, in addi- * Mr. John A. Whipple, of this city, has given several exhibition!* of this kind, with great success. A summer scene seemed to dissolve into tlie same scene in mid-winter ; a daylight view was gradually made to faint successively into twilight and moonshine; and many changes of a most in- teresting nature showed how pleasing an exhibition might be made by o skiiful combination of science and art OPTICS. scope differ j rorr. tion tc the lenses; and the image oi the object, reflected from a >concave mirror, is seen, instead of the object itself. 906. Each of these kinds of telescope has its respective advan- tages, but refracting telescopes have been so much improved that they have in some degree superseded the reflecting telescopes. What is an ^07. Among tn e improvements which have Achromatic Tele- been made in the telescope, may be mentioned, sc P e - as the most important, that peculiar construc- tion 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 employ- ing 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. Com- mon 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 became free from color and more distinct; and hence the glasses which produce them were called Achromatic, that is, free from color. (See pars. 1509-1511.) Lenses are also subject to another imperfection, called spheri- cal aberration, arising from the different degrees of thickness in the "/nitre and edges, which causes the rays that are refracted through them respectively, to come to different focuses, on ac- count of the greater or less refracting power of these parts, con- sequent on their difference in thickness. To correct this defect, tenses have been constructed of gems and crystals, &c., which have a higher refractive power than glass, and require less sphericity to produce equal effects. What is the sim- 908 - T ^ e simplest form of the telescope coii- pkst form of the sists of two convex lenses, so combined as to ttkscove? increase the angle of vision under which th 248 NATURAL PIIILOSC PIIY. Object-glass, and which the Eye- glass, of a iele- object is seen. The lenses are so placed that the distance between them may be equal to the sum of their focal distances Which is the f\ n ^ 909. The lens nearest to the eye is culled the Eye-glass, and that at the other extrem- ity is called the Object-glass. 910. Objects seen through telescopes of tlip construction (namely, with two glasses only) are always inverted, and for this .reason this kind of instrument is principally used for as- tronomical purposes, in which the inversion of the object is immaterial. What is the dif- 91L ThQ common day telescope, or spy- ference between glass, is an instrument of the same sort, with a day and a fo e addition of two, or even three or four night telescope? glasses, lor the purpose ol presenting the object upright, increasing the field of vision, and diminishing the aber- ration caused by the dissipation of the rays. 912. Fig. 139 represents the parts of an Explain Fig. as t rO nomical telescope. It consists of a tube lot/* 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 Pig. 139. scope , How are objects teen throu ^h tel- escopes of the simplest con- struction * 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, magni- fied as many times as the focal length of the eye-glass is con- tained in the focal length of the object-glass. Thus, if the focal length of the eye-glass D C be contained 100 times in that o f OPTICS. the object-glass A B, the star will be seen magniLed 100 times. It will be seen, by the figure, that the image is inverted ; for the ray M A, after refraction, will bs seen in the direction O, and the ray N B in the direction D P. (See par. 1508.) 913. Fig. 140 represents a day-glass, or ter- 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 H 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. Explain Fig. 140. H 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 in- verted 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 Q. (See par. 1511.) Opera Glasses are constructed on the prin- era Classes * ^~ c ^ e ^ ^ e 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. (See par. 1512.) Of what does the ^14. THE REFLECTING TELESCOPE. The Re- fafating Tel- fleeting Telescope, in its simplest form, con- escope consist? gisted of ft concave mirror and a convex eye-glass. The mirror throws an image of the object, and the s views that image under a larger angle of vision. J50 NATURAL PHILOSOPHY. This instrument was subsequently improved by Newton, and since him by Cassegrain, Gregory, Hadley, Short, and th< Herschels. 915. Fig. 141 represents the Gregorian l & Telescope. It consists of a large tube, con taining two concave metallic mirrors, and two plano-convex eye-glass 3S. The rays from a distant object are received through the open end of the tube, and proceed from r t Fig. 141. Explain 141. > B to r r, at the large mirror A B, which reflects them to a focus at <7, whence they diverge to the small mirror C, which re- flects them parallel to the eye-glass F, through a circular aper- ure in the middle of the mirror A B. The eye-glass F col- lects 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. (See par. 1514.) How does the 916. The Cassegrainian telescope differs from iaTteSco n e that which haS been described ' in havin g tkp differ from smaller mirror convex. This construction is at- the 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 talescopes of Herschel and of Lord WJiat peat- liaritus are -Kosse dispense with the smaller mirror. This is there in the done by a slight inclination of the large mirror, so Herschel and as * tnrow tne i ma g e on on e 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 gigan- tic telescope was erected at Slough, near Windsor, in 1789. The diameter of the speculum or mirror was four feet, and the mir- ror weighed 2118 pounds ; its focal distance was forty feet. (See par. 1514.) 918. The telescope of Lord Rosse is the largest that has ever been constructed. The diameter of the speculum is six feet, aud 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 $60,000. The telescope now belonging to Harvard University is a refractor. It is considered one of the best instruments ever constructed. What is 919. CHROMATICS. That part of the sci- Chromatics? ence O f Optics which relates to colors is called Chromatics. Of what is light 920. Light is not a simple thing in its composed? nature, but is composed of rays of different colors, each of which has different degrees of refrangibility, and has also certain peculiarities with regard to reflection. Of what color ^^' ^ ome substances reflect some of the are bodies rays that fall upon them and absorb the others, composed? gome appear to reflect 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 tint color which it- reflects. What are ^^* White ^ s a ^ ue mixture of all colors in white and nice and exact proportion. When a body re- flects all the rays that fall upon it, it will ap- pear white, and the purity of the whiteness depends on the perfcc tiiess of the reflection. 11 NATURAL PHILOSOPHY. 923. Black is the deprivation of all col( r, and, body reflects none of the rays that fall upan 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 composi- 2^??i f tion of light, and which possess diiferent 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 one 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 till * 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 sensitive- ness 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 ligh 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. Th colors thus substituted by the fatigued eye are called the accidental color. The accidental colors of the seven prismatic colo -8, together with blacfc and white, are as follows : Accident il Coltr Red . Bluish Green. Orange Blue. Yellow Indigo. Green Violet reddisn. Indigo Orange red. Violet Orange yellow. Black White. White . . Bkick OPTICS. 25?. it with water, and thus to give it the appearance and the refractive power of a solid prism. 928. When light is made to pass through a prism, the different-colored rays are refracted or separated, and form an image on a screen or wall, in which the colors will be arranged in the order just mentioned. 929. Fig. 142 represents rays of light passing from f* m ' 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 G D. But, Fig. 142. A What effect has a prism on the light that passes through it ? Explain 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 will fall upon the highest part. The red rays, therefore, suffer- ing the smallest degree of refraction, fall on the lowest part of the screen, and the remaining colors are arranged in the order of their refraction. (See par. 1491.) 930. It is supposed that the red rays are refracted the least, on account 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 appoar- ance of the sun through a fog, or at rising and setting. Ihe in- creased 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 reach- ing us. A similar reason will account fir the blue appearance of the ?!;/. 254 NATURAL PHILOSOPHY. As these rays hive less momentum, they cannot traverse the atino*- Ehere so readily as the other rays, .and they are, therefore, reflected ack 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 sepa- How can the . r rar* refract- rated by a pnsi: fall upon a convex lens, they ed by a prism will converge to a focus, and appear white. Hence it appears that white is not a simple color, but if- produced by the union 01 several colors. 932. The spectrum formed by a glass prism being divided in+o 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 ir, 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 mixtiu-e of all the primary colors. (See par. 1492.) 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. (See par. 1498.) 934. From the experiment of Dr. Wollaston, What are the . three simple xt appears that the seven colors formed by the prism colors? ma y be reduced to four, namely, red, green, blue, and violet ; and that the other colors are produced by combina- tions 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. (See par. 1502.) 935. Light is found to possess both heat and chemical udioa. OPTICS. 255 The prismatic spectaim presents some remaikable phenomena with regard to these qualities : for, while the red rays appear to be tna seat of the maximum of heat, the violet, on the contrary, are the apparent se,\t of the maximum of chemical action. 036. 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 rtault may be supposed to arise from the weakness or want of purity in artificial light. 037. There can be no light without colors, and there can be no colors without light. 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 re- fracted 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 con- trary, that a body reflects in great abundance the rays which deter- mine its color, and the others in a greater or less degree in propor- tion as they are nearer or further from its color, in the order of refrangibility. 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 from a deficiency rather than an abundance of reflect- ed 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 8 great quantity of rays are reflected. That bodies sometimes change -their color, is owing to some chemical change which takes pla^e in the internal arrangement of their parts, whereby they looe their tendency to reflect certain colors, and acquire the power of reflecting others. How is a rain- 940. The rainbow is produced by the re- bow produced ? 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 pcver 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 game appearance may be produced artificially, by means of water thrown into the air, when the spectator fc placed in a proper ICATUllAL PHILOSOPHY. position, with his back to the sun ; and, thirdly, that a simitar 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. (See par. 1501.) 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 persona can see exactly the same rainbow, or, rather, the same appearance from the same bow. 943. The Polarization of Light is a change effected during reflec- tion or refraction, by which the etlier vibrations on one side of the ray are stopped. (See par. 1478, Appendix.) 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 intri- cate phenomena in Optics, and has afforded corroborating evidence in favor of the undulatory theory ; but the limits of this volume will not allow an extended notice of this singular property. 9-14. OF THE THERMAL, CHEMICAL, AND OTHER NON-OPTICAL EFFECTS OF LIGHT.* 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 sci- ence of Chemistry, deserve to be noticed in this connection. 945. The thermal effects of light, that is, its agency in the excita* tion of heat when it proceeds directly from the sun, are well known. But it is not generally known that these effects are extremely un- oqual in the differently 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 vi'olet, from which it constantly decreases towards the red, where it ceases alto- gether. Whether these thermal and chemical powers exist in all light, from whatever source it is derived, remains yet to be ascer- tained. The 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 '* See also pars. 1492-1494. OPTICS. 25' jw)v\pr \vht-re tha heating power is feeblest, and tnat the optiooJ 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 means of accelerating chemical combina- tions and decompositions. The following experiment exhibits the chemical effects of light : Place a mixture of equal parts (by measure) of chlorine and hy- drogen 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 dayliglif through violet or blue glass to the mixture, but by interposing a re^ glass between the vessel and the light all combination of the elements is prevented. 947. The chemical effects of light have recently What is , . x . . meant by Pho- " een employed to render permanent the images ob- tograpky., or tained by means of convex lenses. The art of thua Heliographytfc^ thera is terraed Photography, or Heliography. These words are Greek derivatives ; the former meaning " writing or draining by means of light," the latter " writing or draw- ing by the aid of the sun." (See par. 1491.) I/I/A 4i 948. The mode in which the process is performed Who is the . ,. !-. c ,, m, r . , c f , , th f PI ls essen * ;ia ^y as follows: ihe picture, formed by a it & h i camera obscura, is received on a plate, the surface of 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 pro- jected upon it will appear, with all their gradations of light and shade, most exactly depicted in black and white, no color being Kesent. This is the process commonly known by the name of iguerreotype, from M. Daguerre, the author of the discovery Since his original discovery, he has ascertained that by isolating and electrifying the plate it acquires such a sensibility to the chemical influence of light that one-tenth of a second is a sufficient time to Dbtain 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 with a tendency to equilibrium unlike tha-l; 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 olean and dry, be briskly rubbed with a dry woollen cloth, and im- mediately afterwards held over small and light bodies, such as pieces of paper, thread, cork, straw, feathers, or fragments of gold- leaf, 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 being excited are called electrics, while those which cannot be excited in a similar manner are called non-electrics. 954. The science of Electricity, therefore, What are the ,. . , _. -,*' Metrical divis- divides all substances into two kinds, namely, ions of all sub- Electrics, or those suostances which can be excited, and Non-electrics, or those sub stances which cannot be excited. OPTICS. ^51 SIOWPI where tha heating power is feeblest, and tnut the optical Bovver is the strongest between the other two. 940. 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 means of accelerating chemical combina- tions and decompositions. The following experiment exhibits the chemical effects of light : Place a mixture of equal parts (by measure J of chlorine and hy- drogen 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 re glass between the vessel and the light all combination of the elements is prevented. 947. The chemical effects of light have recently What is , . meant by Pho- been employed to render permanent the images ob- tography., or tained by means of convex lenses. The art of thua Heliography? fo in ^ them ig termed p hotograpnVj 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." (See par. 1491.) Who is the ^" ^ he mO( * e * n wn * cn tne process is performed , f pi * s essentially as follows: The picture, formed by a ^o- T h i l ~ camera obscura, is received on a plate, the surface of grap y . w hich 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 pro- jected 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 Daguerreotype, from M. Daguerre, the author of the discovery Since his original discovery, he has ascertained that by isolating and electrifying the plate it acquires such a sensibility to the chemical influence of light that one-tenth of a second is a sufficient time 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 vhite 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 aickly, in consequence of such confinement. 260 NATURAL PHILOSOPHY. Tlier n no-Ehctricity ; 4thly, by Magnetism. Frictiona^ Electricity forms the subject of that branch of Electricitj 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 connec- tion with the subject of Electro-Magnetism. The intimate connection between these several subjects shows ho'/r close arc the links of the chain by which all the departments of physical science are united. 95U. The electric fluid is readily coinmu- by a Conductor n ^ cate ^ fr m one substance to another. Some and a Non-con- substances, however, will not allow it to pass tricitl i tbrough or over them, while others give it a free passage. Those substances through which it pa?EC& without obstruction are called Conductors while those through which it cannot readily pass are called Non-conduct?) s ; and it is found, by experiment, that all electrics* are non-conductors, and all non-electrics ars good conductors of electricity. 960. The following substances are electrics, or non-conductors vF electricity ; namely, Gntta Percha. Atmospheric air (when dry), Feathers, Glass, Amber, Diamond, Sulpb.r, All pvccious stones, Silk, All *roms and resins, Wool, The r&ides of all metals, Hair, Tiopswax, Paper, Soal ing-wax, Cotton. All these substances must be dry, or they will beccran mor* nea-/it by erally induced in surrounding bodies by the vie-in- NATUltAL PHILOSOPHY. ity of a highly-excited electric. This mode of com- raunicating electricity by approach is styled induc- tion. 984. A body, on approaching another body powerfully elec- trified, 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 Electricity by one body to another in contact with it, it is Transfer* ^^ electricity by transfer. W/,attsan 986 The e i ectr i ca i mac hine is a machine Electrical Maeki we, and constructed for the purpose of accumulating or Ir^it'con collectia S electricity, and transferring it to other structed? 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 sur- face, assisted 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 w r ooden 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 macTe either in the form of a cylinder or a circular plate, and the machine is called a cylinder or a plata maohine, according as it is made with a cylinder or with a plate. Explain 988. Fig, 145 represents a plate electrical m.v Fig. 145. chine. A D is the stand of the machine, L L L L 207 are the four glass legs, or posts, which support and insulate the parts of the machine. P is the glass plate (which in some ma- chines is a hollow cylinder) from wh ; ch 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. G IP the prime conductor, terminating at one end with a movable Fig. 145. B O 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 fioor, designed to convey the fluid from the ground to the plate When negative electricity is to be obtained, this chain is re moved from the rubber-post and attached to the prime conductor and the electricity is to be gathered from the ball on the rubber post. Explain the ^89. OPERATION OF THE MADHINE. By turning operation of the handle H, the glass plate is pressed by the rub- Ii68 NATURAL PHILOSOPHY. the Electri- ber. The friction of the rubber against the glas* cal Machine. pl a ^ e ( or cylinder) produces a transfer of the elec- tric fluid from the rubber to the plate; that is, the cushion be- comes negatively and the glass positively electrified. The fluid which thus adheres to the glass, is carried round by the revolu- tion of the cylinder ; and, its escape being prevented by the silk oag, 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 con- veyed, by means of a chain attached to the prime conductor, to any substance which is to be electrified. If both of the conduc- tors 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 rubier-post, when positive electricity is required, and to the prime conductor when negative electricity is wanted. What is an ^90. ^ n ^ e P r ^ me conductor is placed an Electron.- Electrometer, or measurer of electricity. It ia *what*mincl ma( ^ e ^ n various forms, but always on the prin- ple is it con- ciple that similar states of electricity repel each < /rucied - 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 lineif threads. When the machine is worked, the pith-balls, being both similarly electrified, repel each other ; and this caus is them to fly apart, as is represented in the figure;- and they will continue elevated until the electric- ity is drawn off. But, if an uninsulated conducting substance tou".h the prime conductor, the pith-balls will fall. The height KLECT1CIU1TY. 2(59 k 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 been 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, im- proved by Singer. It consists of two strips of gold-leaf suspended under a glass covering, which completely insulates them. Strip? 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 con- sists 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 resii 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-lac resin and Venice- turpentine, aast in a tin mcuH Z70 NATURAL PHILOSOPHY. 994. EXPERIMENTS WITH THE ELECTRICAL MACHINE In peforming experiments with the Electrical Machine, great ear* inurt be taken that all its parts be perfectly dry and clean Moisture arid 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 pre- sented 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. THE TISSUE FIGURE. Fig. 146 is a dgure with a dress of fancy paper cut into 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 elec- trified strips, very singular combinations will take place. If the electrometer be ELECTRICITY. 271 removed from the prime conductor, and a tuft of feathers, 01 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 pre- Leydenjar gen ting it to ^he 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 con- necting 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 fl uid when issuing from a point. In this way electrical orreries, mills, &c., are constructed. 1002. If the electrometer be removed from the prime con- NATURAL PHILOSOPHY. Fig. 147. ductor, and a pointed wire be substituted for /t, a wire with sharp points bent in the form of an S, balanced on it, will ba 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 insu- lated stand. Describe 1003. A chime of small bells on a stand, 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, j? 1 ig. 98, which is founded on the law that action and reaction are equal and in opposite directions. 1004. If powdered resin be scattered over cotton-wool, loosely wrapped on one end of the ; ointed discharger, it may be inflamed by the discharge of the battery or a Leyden jar. Gunpowder may be substituted for thf 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 C tg. 14. j^ terminating at the extremities, A and B, with b r ass balls, and at the other ends which 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 anoth* r may be adjusted. 1007. In using the universal discharger one of the rods 01 Slides must be connected by a chain, or otherwise, with the out Fig. 148. ELECTRICITY* JJY'j mde, and the other with the inside coating of ftie jar or battery. By this means the substance through which tke charge is to b first rise to the electrified top of the stool, and thus becoming themselves electrified, will be repelled, and fall to the ground, the floor, or the table ; where, parting with their electricity, they will again be attracted by the stool, thus rising and falling with considerable rapidity. In order to conduct this, experiment successfully, the images, &c., must be placed within a short distance of the bottom of the stool. 1020. On the same principle light figures may be made to dance when placed between two discs, the lower one being placed upon a sliding stand with a screw to adjust the distance, and the upper one being suspended from the prime conductor, as in Fig. 154. 1021. A hole may be perforated through a quire of paper, by charging the battery, resting the paper upon the briss ball of the battery, and making a communication, by means of the jointed discharger, between the ball of one of the jars, and the brass ball of the box. The paper, in this case, will be between the ball of the battery and the end of the discharger. KLECTK1UITY. 277 1022. Gold-leaf may be forced into the pores of glass by placing it between two slips of window-glass, pressing the slips of glass firmly together, and sending a shock from a battery through tdem. If gold-leaf be placed between two cards, and a strong charge be passed through them, it "will be completely fused. 1023. When electricity enters at a point, it appears in the form of a star ; but when it goes out from a point, it puts on the appearance of a brush. 1024. The thunder-house, Fig. 155, is de- /)escnei Fig. s i gne< j to s h ow the security afforded by light- ning-rods when lightning strikes a building This is done by placing a highly-combustible material in th inside of the house, and passing a charge of electricity through it. On the floor of the house is a surface of tin-foil. The hydrogen pistol, being filled with hydrogen gas from the gasometer, must be placed on the floor of the thunder-house, and connected with the wire on the opposite side. The house being then put together, a chain must be connected with the wire on the side opposite to the lightning-rod, and the other end placed in contact either with a single Leyden jar or with the battery. When the jar, thus situated, is charged, if a connection be formed between the jar and the points of the lightning-rod, the fluid will pass off silently, and produce no effect. But, if a small brass ball be placed on the points of the rod, and a charge of ele^tncity be sent to it from the jar or the battery, the gas in the pistol will explode, and throw the parts of the house asunder with a loud noise. 1025. The success of this experiment depends upon the proper con- nection of the jar with the lightning-rod and the electrical pistol. On the side of the house opposite to the lightning-rod there is a wire, passing thrrugh the side, and terminat?iig on the outside in a 278 NATURAL x-HILOSOPilY. hook ^ 'hen the house is put together, this wire, ir the inside- must touch the tin- foil on the floor of the house. The hydrogen pistol must stand on the tin-foil, and its insulated knob, or wire, pro- jecting from its side, must be connected with the lower end of the lightning-rod, extending into the inside of the house. A communi- cation must then be made between the hook on the outside of the house and the outside of the jav, or battery. This is conveniently done by attaching one end of a chain to the hook, and holding the other end in the hand against the side of a charged jar. By pre- senting the knob of the jar to the points of the lightning- rod no effect is produced ; but if a brass ball be placed on the points at P, and the knob of the jar be presented to the ball, the explosion will take place. If the charged jar be very suddenly presented to the points, the explosion may take place ; and the jar may be silently discharged if it be brought very slowly to the ball. The thunder- house is sometimes put together with magnets. What is light- 1026. The phenomena of lightning are ning and thun- caused by the rapid motion a; vast quanti- ties of electric matter. Thunder is the noise which accompanies the passage of electricity through the air. What is sup- 1027. The aurora borealis (or northern posed to be the lights) is supposed to be caused by the electric cause of the fluid pass i ng through highly-rarefied air ; and northern lights? 9 . J most of the great convulsions of nature, suca as earthquakes, whirlwinds, hurricanes, water-spouts, &c., are generally accompanied by electricity, and often depend upon it 1028. The electricity which a body manifests by being brought near to an. excited body, without receiving a spark from it, is said to be acquired by induction. When an insulated but un. electrified conductor is brought near an insulated charged con- ductor, the end near to the excited conductor assumes a state of opposite electricity, while the farther end assumes the same kind of electricity, that is, if the conductor be electrified positively, the unelectrified conductor will be negative at the nearer end, and positive at the further end, while the middle point evinces neither positive nor negative electricity. [See No. 993. 1029. The experiments which have now been described exem- ELKOTKIOITY. all the elementary principles of the science of electricity hese experiments may be varied, multiplied, and extended in innu- merable forms, by an ingenious practical electrician. Among other things with which the subject may be made interesting, may be mentioned the following facts, &c. 1030 A number of feathers, suspended by strings from an insu- lated conducting substance, will rise and present the appearance of a flight of birds. As soon as the substance is discharged, the feathers will fall. The experiment may be varied by placing the sportsman on the prime conductor, without the use of the Leyden jar, to which the birds are attached. 1031. Instead of the Leyden jar, a plate of common glass (a pane of window-glass, for instance) may be coated on both sides with tin- foil, leaving the edges bare. A bent wire balanced on the edge of the glass, to the ends of which balls may be attached, with an image at each end, may be made to represent two persons tilting, on the same principle by which the electrical bells are made to ring. 1032. Miniature machinery has been constructed, in which the power was a wheel, with balls at the ends of the spokes, situated within the attractive influence of two larger balls, differently electri- fied. As the balls on the spokes were attracted by one of the larger balls, they changed their electrical state, and were attracted by the otb^r, which, in its return, repelled them, and thus the motion being ghen to the wheel was communicated by cranks at the end of the axle to the saws above. 1033. When the hand is presented to the prime conductor, a spark is communicated, attended with a slightly painful sensation. But, if a pin or a needle be held in the hand with the point towards the conductor, neither spark nor pain will be perceived, owing tc the attracting (or, perhaps, more properly speaking, the receiving.) power of the point. 1034. That square rods are better than round ones to conduct electricity silently to the ground, and thus to protect buildings ; may be proved by causing each kind of rod to approach the prime conductor when charged. It will thus be perceived that, while little effect is produced on the pith-balls of the electrom- eter by the near approach of the round rod, on the approach of the square one the balls will immediately fall. The round rr.d, also, will produce an explosion and a spark from the ball r.f the prime conductor, while the square one will draw off the fluid silently. 103.^. The effects of pointed conductors upon clouds charged with electricity may be familiarly exemplified by suspending a small fleece of cotton-wool from the prime conductor, and 280 NATURAL rillLOSOPilY. other smaller fleeces from the upper one, by small filaments. On presenting a point to them they will be repelled, and all drawn together ; but, if a blunt conductor approach them, they will be attracted. 1036. From a great variety of facts, it has been ascertained, that lightning-rods afford but little security to any part of a building beyond twenty feet from them ; and that when a rod is painted it loses its conducting power. What are the 1037. The lightning-rods of the most ap- best kinds of proved construction, and in strictest accordance with philosophical principles, are composed of *maR square rods, similar to nail-rods. They run over the building, and down each of the corners, presenting many elevated points in their course. At each of the corners, and on the chimneys, the rods should be elevated several feet above the building. If the rods are twisted, it will be an improvement, as thereby the sharp surfaces presented to collect the fluid will point in more varied directions. 1038. The removal of silk and woollen garments, worn during the day in cold weather, is often accompanied by r. slight noise, resem- bling that of sparks issuing from a- fire. A similar effect is pro- duced on passing the hand softly over the V,ck of a cat. These effects are produced by electricity. 1039. It may here be remarked, that the verms positive and nega- tive, are merely relative terms, as applied to the subject of electric- ity. Thus, a 'body which is possessed of its natural share of electricity, is positive in respect to one that has less, and negative in respect to one that has more than its natural share of the fluid. 3o, also, one that has more than its natural share is positive with regard to one that has only its natural share, or less *lian its natu- ral share, ,and negative in respect to one having a larger share than itself. 1040. The experiments with the spiral tube connected with Fig 150 may be beautifully varied by having a collection of such tubes placed on a stand ; and ajar coated with small strips, resembling a brick wall, presents, when it is charged, a beautiful appearance ir, she dark. 1041. The electric fluid occupies no perceptible space of time in its passage through its circuit. The rapidity of its motion ha* been estimated as high as 288,000 miles in a second of time. B always seem? to prefer the shortest passage, when the conductors ELECTKIC1TY. 2bl ire equally good. Thus, if two, ten, a hundred, or a thousand or more persons, join hands, and be made part of the chcuit of the fluid in passing from the inside to the outside of a Leyden jar, they will ill feel the shock at the same moment of time. But, in its passage, the Quid always prefers the best conductors. Thus, if two clouds, iifterently electrified, approach one another, the fluid, in its passage iTom one cloud to the other, will sometimes take the earth in its course, because the air is a bad conductor. 1042. In thunder-storms the electric fluid sometimes passes from the clouds to the earth, and sometimes from the earth to the clouds and sometimes, as has just been stated, from one chnid to the earth, and from the earth to another cloud.* What are 1043. It is not safe, during a thunder-storm, to comparatively take shelter under a tree, because the tree attracts safe and un- the fluid and the human body being a better con . safe positions . * during a ductor than the tree, the fluid will leave the tree thunder-storm? an d p ass into the body. It is also unsafe to hold in the hand edge-tools, or any sharp point which will attract the fluid. The safest position that can be chosen during a thunder-storm is a recumbent posture on a feather bed ; and in all situations a recumbent is safer than an erect position. No danger is to be apprehended from lightning when the interval between the flash and the noise of the explosion is as much as three or four sec- onds. This space of time may be conveniently measured by the beatings of the pulse, if no time-piece be at hand. 1044. Lightning-rods were first proposed by Dr. Franklin, to whoa, is also ascribed the honor of the discovery that thunder and light- ning are the effects of electricity. He raised a kite, constructed of a silk handkerchief adjusted to two light strips of cedar, with a pointed wire fixed to it ; and, fastening the end of the twine to a key, and the key, by means of a piece of silk lace, to a post (the silk lace serving to insulate the whole apparatus), on the approach of a * Lightning appears under several different forms. That which appears when the discharge takes place between two clouds at some distance apart, or between a cloud and the earth, exhibits a bright zigzag narrow band of light, and is called popularly o/iam-lightning. The irregular path is prob- ably caused by the resistance offered Toy the air to the current or electrical impulse. When two clouds slightly charged with different electrieitlos ap- proach each other, the discharge is from a great many points at the same time, and the appearance of a broad flash has entitled it to the name of s7^-lightning. A third form of lightning is called JaZMightning ; no satis- factory explanation of it has yet been given. It appears like a bright ball of flame, moving no faster than a man can walk ; it explodes violently after a few seconds. 282 NATURAL PHILOSOPHY. thunder- cloud, he was able to collect sparks from the key, to charge Leyden jars, and to set fire to spirits. This experiment established the identity of lightning and electricity. The experiment was a dangerous one, as was proved in the case of Professor Richman, of St. Petersburgh, who fell a sacrifice to his zeal for electrical science by a stroke of lightning from his apparatus. What are the 1045. Among the most remarkable facts con- Electrical nected with the science of electricity, may be men- Animals. tioned the power possessed by certain species of fishes of giving shocks, similar to those produced by the Leyden jar. There are three animals possessed of this power,, namely, the Torpedo, the Gymnotus Electricus (or Surinam Eel), and the Silurus Electricus. But, although it has been ascertained that the Torpedo is capable of giving shocks to the animal sys- tem, similar to those of the Leyden jar, yet he has never been made to afford a spark, nor to produce the least effect upon the most delicate electrometer. The Gymnotus gives a small but perceptible spark. The electrical powers of the Silurus are in- ferior to those of the Torpedo or the Gymnotus, but still sufficient to ghe a distinct shock to the human system. This power seems to have been bestowed upon these animals to enable them to secure their prey, and to resist the attacks of their enemies. Small fishes, when put into the water where the Gymnotus is kept, are generally killed or stunned by the shock, and swallowed by the animal when he is hungry. The Gymnotus seems to be possessed of a new kind of sense, by which he perceives whether the bodies presented to him are conductors or not. The consid- eration of the electricity developed by the organs of these ani- mals of the aquatic order, belongs to that department called Animal Electricity. 1046. It will be recollected that the phenomena which have now been described with the exception of what has just beer stated as belonging to animal electricity, belong to the subject of frictional electricity. But there are other forms in which this subtle agent presents itself, which are yet to be described, which show that its operations are not confined to beautiful GALVANISM. experiments, such as have already been presented, nor to the terrific and tremendous effects that we witness in tho storm and the thunder-gust. Its powerful agency works unseen on ihe intimate relations of the parts and properties of bodies of every description, effecting changes in their constitution and character so wonderfully minute, thorough and universal, that it may almost be considered as the chief agent of nature, the prime minister of Omnipotence, the vicegerent of creative power. What is 1047. GALVANISM, OR VOLTAIC ELECTRIC- Gilvanism ? ITY . Galvanism, or Voltaic Electricity, is a branch of electricity which derives its name from Galvani, who first discovered the principles which form its basis. 1048. Dr. Aloysius Galvani was a Professor of Anatomy in Bolog- na, and made his discoveries about the year 1790. His wife, being consumptive, was advised to take, as a nutritive article of diet, some soup made of the flesh of frogs. Several of these animals, recently skinned for that purpose, were lying on a table in his laboratory, near an electrical machine, with which a pupil of the professor was amusing himself in trying experiments. While the machine was in action, he chanced to touch the bare nerve of the leg of one of the frogs with the blade of a knife that he held in his hand, when sud- denly the whole limb was thrown into violent convulsions. Galvani, being informed of the fact, repeated the experiment, and examined minutely all the circumstances connected with it. In this way he was led to the discovery of the principles' which form the basis of this science. The science was subsequently extended by the discov eiies of Professor Volta, of Pavia, who first constructed the galvanic or voltaic pile, in the beginning of the present century. To produce electricity mechanically (as has been stated under the head of frictional electricity), it is necessary to excite an electric or non-conducting substance by friction. But galvanic action is pro- duced by the contact of different conducting substances having a chemical action on one another. How does gal- 1049. Frictional electricity is produced by the vanism differ mechanical action of bodies on one another ; but %lkicityl~ alvanism or galvanic electricity, is produced by their chemical action. What is the 1050. The motion of the electric fluid, excited difference in b y ga l van i c power diff ers f rom tna t explained (he effects of J _ . ... frictional and und er the head of frictional electricity in its in- 12* 284 NATURAL PHILOSOPHY. rJiemtcal elec- tensity and duration ; for, while the xatter exhibits tricity ? itself in sudden and intermitted shocks and explo- sions, the former continues in a constant and uninterrupted cui- rent so long as the chemical action continues, and is interrupted only by the separation of the substances by which it is produced.* 1051. The nerves and muscles of animals are What is most sensitive to mos t easily affected by the galvanic fluid ; and the the galvanic voltaic or galvanic battery possesses the most sur- prising powers of chemical decomposition. How is the 1052 The galenic faid. or influence, is ex- galvanic jluia excited ? cited by the contact of pieces of different metal, and sometimes by different pieces of the same metal. 1053. If a living frog, or a fish, having a slip of tin-foil on its back, be placed upon a piece of zinc, spasms of the muscles will be ex- cited whenever a communication is made between the zinc and the tin-foil. 1054. If a person place a piece of one metal, as a half-dollar, above his tongue, and a piece of some other metal (as zinc) below the tongue, he will perceive a peculiar taste ; and, in the dark, will * The different action of gravity on the particles of water while in the liquid state, and the same particles in the solid state in the form of ice, has been explained in the early pages of this volume. In the one case each particle gravitates independently, while in the form of ice they gravitate in one mass. The fall of a body of ice would therefore produce more serious injury than the fall of the same quantity of water in the liquid form. There is a kind of analogy (which, though not sufficient for a philosophical expla- nation, may serve to give an insight into the difference between the effects produced by frictional electricity and that obtained by chemical means.) between the gravitation of water and ice, respectively, and the motion of frictional and chemical electricity. If the water be dropped in an infinitely narrow stream, its effects, although mechanically equal, would be so gradual as to be imperceptible. So, also, if a given portion of electricity be set in motion as it were in one mass, and an equal quantity move in an infinitely narrow current, there will be a corresponding difference in its apparent results. The difference in intensity may perhaps be partially understood by this illustration, although a strict analogy may fail to have been made out, owing in part to the nature of an imponderable agent. A strict analogy cannot exist between the operations of two agents, one of which is pondera- ble and the other imponderable. But, that there is something like ar analogy existing in the cases cited, will appear from statements which have been made on good authority, namely, that there is a greater quantity of electricity developed by the action of a single drop of acid on a very niiuuto portion of zinc, than ib usually brought 'f to action in the darkest cloud thai, shroud? the hot j SOD. GALVANISM. seo a flash of light whenever the outer edges of the metals are in contact. 1055. A faint flash may be made to appear before the eyes by putting a slip of tin-foil upon the bulb of one of the eyes, a piece ot silver in the mouth, and making a communication between then? In these experiments no effect is produced so long as the metals are kept apart ; but, on bringing them into contact, the effects above described are produced. T;ir7 1056. It is essential in all cases to have three What is es- sential lo pro- elements to produce galvanic action. In the ex- duce galvanic periments which have already been mentioned in the case of the frogs, the fish, the mouth and the eye, the moisture of the animal, or of the mouth, supplies the place of the acid, so that the three constituent parts of the circle are completed. What is said of 1057. The conductors of galvanic electric- conductors of ity, like those of frictional electricity, are of Galcanism? -., 1 m . . _ , all degrees ol excellence. The metals, char- coal, plumbago, and solutions of acids and salts, are good conductors ; while gutta-percha, rubber, glass, resin, sul- phur, dry wood, air, etc., are poor conductors, or, as they are generally termed, non-conductors. Insulators may be made of any of the above solid non-conductors. . 1058. The acid employed in the galvanic cir- H hat kind of . acid must be cult must always be one that has a strong am nit y employed in for one of the metals in tho circuit. When zinc is employed, sulphuric acid may form one of tbe three elements, because that acid has a strong affinity for zino. What is a law 1059 ' ^ certain quantity of electricity is always of chemical developed whenever chemical action takes plac*. action? Between a fluid and a solid body. This is a gen- eral law of chemical action ; and, indeed, it has been ascertained that there is so intimate a connection between electrical and chemical changes, that the chemical action can proceed only to a certain extent, unless the electrical equilibrium, which has disturbed, be again restored. Hence, we find that in the 280 NATURAL PHILOSOIHY. simple, as well as in the compound galvanic circle, the oxydatioa of the zinc proceeds with activity whenever the galvanic circle is completed ; and that it ceases, or at least takes place very slowly, whenever the circuit is interrupted. What is neces- 1060. To produce any galvanic action it it ITexdte'gaL necessary to form what is called a galvanic vanic action ? circle , that is, a certain order or succession of substances capable tf exciting electricity. Of what 15 the 1061. The simplest galvanic circle is com- simplest gal- posed of three conductors, one of which must be solid 5 and ne fluid ; tne third ma y be either solid or fluid. What is the 1062. The process usually adopted for obtam- "fo^ohaininir * n & g alvam ' c electricity is, to place between two galvanic eke- plates of different kinds of metal a fluid capable tricity ? O f exerting some chemical action on one of the plates, while it has no action, or a different action, on the other A communication is then formed between the two plates. Explain 1063. Fig. 156 represents a *&' ' simple galvanic circle. It con- sists of a vessel containing a portion of diluted sulphuric acid, with a plate of zinc, Z, and of copper, C, immersed in it. The plates are separated at the bottom, and the circle is completed by connecting the two plates on the outside of the vessel by means of wires. The same effect will be pro- duced, if, instead of using the wires, the metallic plates come into direct contact. 1064. In the above ar- What are the essential parts rangement, there are three of a galvanic elements or essential parts, namely, the zinc, the copper, end the acid The acid, acting chemical 1 ,) upon the zinc, pro- Fig. 156. GALVANISM. V&l duces an alteration in the electrical state of the metal. The zinc, communicating its natural share of zhc electrical fluid to the acid, becomes negatively electrified. The copper, attracting the same fluid from the acid, becomes positively electrified. Any conducting substance, therefore, placed within the line of com- munication between the positive and negative points, will re- ceive the charge thus to be obtained. The arrows in Fig. 156 show the direction of the current of positive electricity, namely, from the zinc to the fluid, from the fluid to the copper, from the copper back through the wires to the zinc, passing from zinc to copper in. the acid, and from copper to zinc out of the acid. The substance submitted to the action of the electric cur- Where must a rent must be placed in the line of communication substance be between the copper and the zinc. The wire con- affected by *al- necte d w ^ tn tae copper is called the positive pole r vanic action ? and that connected with the zinc the negative pole, and in all cases the substance submitted to galvanic action must be placed between the positive and negative poles. 1065. The electrical effects of a simple galvanic circle, such as has now been described, are, in general, too feeble to be perceived, except by very delicate tests. The muscles of animals, especially those of cold-blooded animals, such as frogs, &c., the tongue, the eye, and other sensitive parts of the body, being very easily affected, afford examples of the operation of simple galvanic circles. In these, although the quantity of electricity set in motion is exceedingly small, it is yet sufficient to produce verj considerable effects ; but it produces little or no effect on the most delicate electrometer. 1066. The galvanic effects of a simple circle H^w may gat- r , . vanic action be may be increased to any degree, by a .repetition increased? of t j ie game s i m pi e combination. Such repe- titions constitute compound galvanic circles, and are called galvanic piles, or galvanic batteries, according to the mode in which they are constructed. 288 NATURAL PHILOSOPHY. 1.0G7. It appears a \ first view to be a singular fact, that, in a simple galvanic circle, composed of zinc, acid and copper, the zinc enr will always be negative, anil the copper end positive ; while, in all compound galvanic circles composed of the same elements, the zinc will be positive, and the copper negative. This apparent difference arises from the compound circle being usually terminated by two superfluous plates. What is the 1068. The voltaic pile consists of alternate Voltaic pile ? places of two different kinds of metal, sepa- rated by woollen cloth, card, or some similar substance. Explain 1069. Fig. 157 represents a voltaic Fig. 157. Fig. 157. pile. A voltaic pile may be con- structed in the following manner : Take a number of plates of silver, and the same num- ber of zinc, and also of woollen cloth, the oloth having been soaked in a solution of sal ammo- niac in water. With these a pile is to be formed, in the following order, namely : a piece of silver, a piece of zinc, a piece of cloth, and thus repeated. These are to be supported by three glass rods, placed perpendicularly, with pieces of wood at the top and bottom, and the pile will then be complete, and will afford a constant current of electric fluid through any conducting sub- stance. Thus, if one hand be applied to the lower plate, and the other to the upper one, a shock will be felt, which will be repeated as often as the contact is renewed. Instead of silver, copper plates, or plates of other metal, may be used in the above arrangement. The arrows in the figure show the course of the current of electricity in the arrangement of silver, zinc, &c. 1070. Voltaic piles have been constructed of layers of gold and silver paper. The effect of such piles remains undisturbed for years. With the assistance of two such piles, an approxi- mation to perpetual motion, in a self-moving clock, has been in- vented by an Italian philosopher. The motion is produced by the attraction and repulsion of the piles exerted on a pith-ball, on the principle of the electrical bfclls. The ton of one of tht GALVANISM. piles was positive, and the bottom negative. T\e other pile was in an opposite state ; namely, the top negative, and the bottom positive. W\aih the 1071. The voltaic, or galvanic battery, is a galvanic bat- combination of metallic plates, immersed in tcry pairs in a fluid which exerts a chemical action on one of each pair of the plates, and no action, or, at least, a different action, on the other. What is the 1072. The electricity excited by the battery diction of the i j th Ud t ^ fl id M h t current in the r * galvanic bat- upon it chemically. Thus, in a battery composed ter y * of zinc, diluted sulphuric acid and copper, the acid lets upon the zinc, and not on the copper. The galvanic fluid proceeds, therefore, from the zinc to the acid, from the acid to the copper, &c. Instead of using two different metals to form the galvanic circuit, one metal, in different states, may be em- ployed ; the essential principle being, that one of the elements shall be more powerfully affected by some chemical agent than the other. Thus, if a galvanic pair be made of the same metal, one part must be softer than the other (as is the case with cast and rolled zinc) ; or a greater amount of surface must be exposed to corrosion on one side than on the other ; or a more powerful chemical agent be used on one side, so that a current will be sent from the part most corroded, through the liquid, to the part least corroded, whenever the poles are united, and the circuit thereby completed. " Explain 1073. Fig. 158 represents Fig. 158. Fig. 158. a voltaic battery. It con- sists of a trough made of baked wood, wedgewood-ware, or some other non- conducting substance. It is divided into grooves, or partitions, for the re- ception of the acid, or a saline solution) and the plates of zinc or copper (or other metal) are iniinered by pairs in the grooves. Ti'cse 290 NATURAL PHILOSOPHY. pairs of plates are united by a slip of metal passing from the one and soldered to the other ; each pair being placed so aj to enclose a partition between them, and each cell or groove in tho trough containing a plate of zinc, connected with the copper plate or the succeeding cell, and a copper plate joined with the ainc plate of the preceding cell. These pairs must commence with copper and terminate with zinc, or commence with zinc and terminate with copper. The communication between the firsl and last plates is made by wires, which thus complete the gal- vanic circuit. The substance to be submitted to galvanic action is placed between the points of the two wires. How can a 1074. A compound battery of great power is compound bat- obtained by uniting a number of these troughs. tery of great _ . ./ power be ob- ^ a similar manner, a battery may be produced tained? by uniting several piles, making a metallic com- munication between the last plate of the one and the first plate of the next, and so on, taking care that the order of succession of the plates in the circuit be preserved inviolate. Describe the 1075 - The ^ouronm Kg . 159> Couronne des des tosses, represented in iasses. Fig 159) ig anotacr f orm of the galvanic battery. It consists of a number of cups, bowls, or glasses, with the zinc and copper plates im- mersed in them, in the order represent- ed in the figure ; Z indicating the zinc, and C the copper plates ; the arrows denoting the course 1 , of the electric fluid. 1076. The electric shock from the voltaic battery may bo received by any number of persons, by joining hands, having previously wetted them. Describe Smee's 1077. SMEE'S GALVANIC B ATI ERY 13 represented Battery, i n Fig. 160, and affords an instance of a battery in its simplest form. It consists of a glass vessel (AS a tumbler), i>n \viiich rest* the frame that supports the apparatus GALVANISM. Two screw-v3ups rise from the frame, to which Fig. ieo. wires may be attached for the conveyance of the electric current in any direct ice. One of the screw-cups communicates with a thin strip of platinum, or platinum-foil, which is sus- pended within the glass vessel between two plates of zinc, thus presenting each surface of the platinum to a surface of zinc ; and the gal- vanic action is in proportion to the extent of the opposite sur faces of the two metals, and their nearness to each other. The other screw-cup is connected with the two zinc plates. The screw-cup connected with the platinum is insulated from the metallic frame which supports it, by rosewood, and a thumb- screw confines the zinc plates, so that they can be renewed when necessary. The liquid employed for this battery is sulphuric acid, or oil of vitriol, diluted with ten parts of water by measure. To prevent the action of the acid upon the zinc plates, their sur- faces are commonly amalgamated, or combined with mercury which prevents any chemical action of the acid with the zino until the galvanic circuit is established, when the zino is imme- diately attacked by the acid. Explain 1078. Fig. 161 represents a series of three pairs Fig. 161. of this battery, in which it will be observed that the Fig. 161. platinum of one is connected with the zinc of the next, and that the terminal wires proceed, consequently, one from a platinum jjhte, and the other from a zinc plate, as iii a single pair. 2D2 NATURAL PHILOSOPHY. Describe tht sulphate of topper bat- tery by Figures 162 and 163 1078. SULPHATE OF COPPER BATTERY. Fig 162 represents a sulphate of copper battery, and Fig. 163 a vertical section of the same battery. It consists of a double cylinder of copper, C C, Fig. 163, with a bottom of the same metal, which Fig. 162. Fig. 163. serves the double purpose of a gal- panic plate and a vessel to contain the exciting solution. The solu- tion is contained in the space be- tween the two copper cylinders. A movable cylinder of zinc, Z, is let down into the solution whenever the battery is to be used. It rests on three arms of wood or ivory at the top, by means of which it is in- sulated. Thus suspended in the solution, the surfaces of zinc and copper, respectively, face each other. A screw-cup, N, is at- tached to the zinc, and anoth- er, P, to the copper cylinder, to receive the wires. When a communication is made be- tween the two cups, electricity is excited. The liquid em- ployed in this battery is a solution of sulphate of copper (common blue vitriol) in water. A saturated solution is first made, and to this solution as much more water is added. 1079. A pint of water will dissolve about a quarter of a pound oi blue vitriol. The solution described above will therefore contain about two ounces of the salt to the pint. The addition of alcohol in small quantities increases the permanency of the action of the oolution The zinc cylinder bhould always betaken out of the solu- tion when the battery is not in use ; but the solution may remain in the battery The battery will keep in good action ft r tw?uty or thirty minutes at a time GALVANISM. 29.S 1080. The sulphate of copper battery, although not so ener- getic as Smee's, is found very convenient ir. a large class of experiments, and is particularly recommended to those who are inexpert in the use of acids ; because the sulphate of copper, being entirely neutral, will not injure the color nor the texture of organic substances. Describe the 1081. There is another form of the sulphate of protected sul- copper battery, called the Protected Sulphate of phate of cop- Copper Battery, which differs from the one described ' in having a porous cell 01 earthenware, or leather, interposed between the zinc and the copper, thus forming two cells, in the outer of which sulphate of copper may be used, and in the inner one a solution of sulphate of soda (Glauber salt), or chloride of sodium (common salt), or even dilute sulphuric acid. This battery will continue in use for several days, and it is therefore of great use in the electrotype process. 1082. GROVE'S BATTERY. This is the most * energetic battery yet known, and is the one most generally used for the magnetic telegraph. The metals employed are platinum and zinc, and the solutions are strong nitric acid in contact with the pla- tinum, and sulphuric acid diluted with ten or Fig> twelve parts of water in contact with the zinc. This battery must be used with great care, on account of the strength of the acids used for the solutions, which send out injurious fumes, and which are destructive to organic sub- stances. Fig. 164 represents Grove's bat- tery. The containing vessel is glass ; within this is a thick cylinder of amalgamated zinc, standing on short legs, and divided by a longitudinal opening on one side, in order to allow the acid to circulate freely. Inside of this is a porous cell of unglazed porcelain, containing the nitric acid, and strip uf platinum. The platinum is supported by a strip of bras? fixed by a thumb-screw and an insulating piece of ivory to the 294 NATURAL PHILOSOPHY. arm proceeding from the zinc cylinder. The amalgamated z:nc if not acted upon by the diluted sulphuric acid until the circuit of the battery is completed. But, as the nitric acid will filter through the porous cell, and act upon the zinc, it is advisable to remove the zinc from the acid when the battery is to remain inactive. The action of Grove's battery may be considered as three times greater than that of the sulphate of copper battery. What are the 1083. The spark from a powerful voltaic bat- effecls of a pow- tery acts upon and inflames gunpowder, char- erful voltai- bat- coalj cotto n , and other inflammable bodies, fuses tery? . ; .. .. , , all metals, ourns up or disperses diamonds and other substances on which heat in other forms produces little or no effect. 1084. The moat striking effects of Galvanism on the human frame, aftor death, were exhibited at Glasgow, a few years ago. The subject on which the experiments were made was the body of the murderer Clydesdale, who was hanged at that city. He had Deen suspended an hour, and the first experiment was made in about ten minutes after he was cut down. The galvanic battery employed consisted of 270 pairs of four-inch plates. On the appli- cation of the battery to different parts of the body, every muscle was thrown into violent agitation ; the leg was thrown out with great violence, breathing commenced, the face exhibited extraordi- nary grimaces, and the finger seemed to point out the spectators. Many persons were obliged to leave the room from terror or sick- ness ; one gentleman fainted, and some thought that the body had really come to life. 1085. The wires, by which the circuit of the How are the ' J hands protected battery is completed, are generally covered with when using a gutta-percha, in order that they may be held or lattery ? directed to any substance. (n what respects 1086 ' There are three F inci P al circum - loes the electric- stances in which the electricity produced by ity produced by the ga i va nic or voltaic battery differs from the galvanic bat- , , iery differ from tnat obtained by the ordinary electrical ma- ihat obtained by chine ; namely, the machine ? ^ The yery bw degree of intenslfy of tnat produced by the galvanic battery, compared with that obtained h\ the machine GALVANISM. 1087 By inte7isity is here meant something analogous to what is implied by density as applied to matter ; but in the ono case it is a ponderable agent, in the other an imponderable, so that a strict analogy cannot be made out between them. The term density cannot be applied to any of the imponderable agents, light, sound, heat or electricity. We speak of the in- tensity of light, an intensity of heat, &c. Hence, the word intensity is properly applied to electricity, and we speak of its tension, instead of its density. Which will de- The quant i t y O f electricity obtained by gal- velop the great- ..... , er quantity of vamc action is much greater than can be electricity, the obtained by the machine; but it flows, as it ft3 were, in narrow stream, ' The action of the electrical machine may be compared to a mighty torrent, dashing and exhausting itself in one leap from a precipitous height. The galvanic action may be compared to a steady stream, supplied by an inexhaustible fountain. In other words, the mo- mentum of the electricity excited by galvanism is less than that from the electrical machine ; but the quantity, as has been stated. is greater. (2.) The very large quantity of electricity which is set in mo- tion by the voltaic battery ; and, (3.) The continuity of the current of voltaic electricity, and its perpetual reproduction, even while this current is tending to restore the equilibrium. 1088. Whenever an electrical battery is charged, how great soever may be the quantity that it contains, the whole of the power is at once expended, as soon as the circuit is completed. Its action may be sufficiently energetic while -it lasts, but it is exerted only for an instant, and, like the destructive operation of lightning, can effect during its momentary passage only sud- den and violent changes, which it is beyond human power to regulate or control. On the contrary, the voltaic battery con- tinues, for an indefinite time, to develop and supply vast quan- tities of electricity, which, far from being lost by returning to their source, circulate in a perpe f ual tream and with uudiiuiu- '296 NATUHAL PHlLUSOPIiY ished force. The effects of this continued current on the bodie>s subjected to its action will therefore be more definite, and will be constantly accumulating ; and their amount, in process of time, will be incomparably greater than even those of the ordi- nary electrical explosion. It is therefore found that changes ii the composition of bodies are effected by galvanism which car be accomplished by no other means. The science of galvanism therefore, has extended the field and multiplied the means ot investigation in the kindred sciences, especially that of Chem istry. 1089. A common electrical battery may bo char g ed from a voltaic battery of sufficient sion manifested size ; but a battery constructed of a small num- in the galvanic b er O f pairs, even though the plates are large, furnishes no indication of attraction or repul- sion equal to that which is given by the feeblest degree of excitation to a piece of sealing-wax. A galvanic battery con- sisting of fifty pairs of plates will affect a delicate gold-leaf electrometer; and, with a series of one thousand pairs, even pith balls are made to diverge. 1090. The effect of the voltaic pile on the On what does ,11 . the effect of the animal body depends chiefly on the number of voltaic battery plates that are employed; but the intensity of e ? en the spark and its chemical agencies increase more with the size of the plates than with their number. 1091. Galvanism explains many facts in Mention some of the familiar ef- common life. fects ofgalvan- Porter, ale, or strong beer, is said to have a peculiar taste when drunk from a pewter ves sel. The peculiarity of taste is caused by the galvanic circle formed by the pewter, the beer, &c., and the moisture of the under lip. Works of metals the parts of which are soldered together soon tarnish in the places where the metals are joined. Ancient coins composed of a mixture of metal have cruiu- GALYAJS'ISM. bled to pieces, while those composed )i pure metal have been uninjured. The nails and the copper in sheathing of ships are soon corroded about the place of contact. These are all the effects of galvanism. There are persons wno profess to be able to find out seams in brass and copper vessels by the tongue which the eye cannot discover ; and, by the same means, to distinguish the base mix- tures which abound in gold and silver trinkets. 1092. From what has now been stated, it will be seen that the effects of galvanic action depend on two nrcumstances ; namely, 1st, the size of the plates employed ii i the circuit ; and, 2dly, the number of the pairs constituting a battery. But there is a remarkable circumstance to be noticed in this con- nexion ; namely, that there is one class of facts dependent on the extension of the size of the plates, and On what does another on the increase of their number. The the power of a f deve i op fa at and 7nagne tism is de- battery to pro- e auce heat and to pendent on the size of the plates, that is, on the affect the animal ex t en t of the surface acted upon by the chem- system respect- . .. . ively depend ? lca * a g en ^ J while the power to decompose chemical compounds, and to affect the animal system, is affected in a greater ratio by the increase of the number of the pairs. 1093. The name Color imotor (that is, the heat ^ WaS a PP lied bv Dr ' Hare ' of Philadelphia, to a very powerful apparatus which he constructed, with large plates, and which he found possessed of a very remarkable power in producing heat. Batteries con- structed for this purpose usually consist of from one to eight pairs of plates. They are made in various forms; sometime? the sheets of copper and zinc are coiled in concentric spirals, sometimes placed side by side ; and they may be divided into a great number of small plates, provided that all the zinc plates are connected together, and all tlie copper plates together, and tfATUKAL PHILOSOPHY. then tho.t the experiments are performed in a channel oj com* munication, opened between the SETS OF PLATES, and not between PAIIIS, as in the common battery ; for it is immaterial whether one large surface be used, or many small ones electrically con- nected together. The effect of all these arrangements, by which the metallic surface of a single pair is augmented, is to increase the quantity produced. 1094. The galvanic or voltaic battery is one of the most valuable acquisitions of modern science. It has proved in many instances the key by which science has entered into the innermost recesses of nature, and discovered the secret of many of her operations. It has, in great measure, lifted the hitherto impenetrable veil that has concealed the mysterious workings in the material world, and has opened a field for investigation and discovery as inviting as it is boundless. It has strengthened the sight and enlarged the view of the philosopher and the man of science, and given a degree of cer- tainty to scientific inquiry hitherto known to be unreached, and sup- posed to be unattainable ; and, if it has not yet satisfied the hopes of the alchemist, nor emulated the gold-converting touch of Midas, it has shown, almost to demonstration, that science may yet achieve wonders beyond the stories of mythology, and realize the familial adage that " truth is stranger than fiction 1095. MAGNETISM. Magnetism treata netiim " aff ~ f tnc properties and effects of the magnet, or loadstone. 1096. The term loadstone, or, more properly, leadstone, was ap- plied to an ore of iron in the lowest state of oxidation, from its attractive properties towards iron, -and its power of communicating its power to other masses of iron. It received the name of Magnet from Magnesia, in Asia Minor (now called Guzelhizar) , about fif- teen miles from Ephesus, where its properties were first well known. The term magnet is now applied to those substances which, natu- rally or artificially, are endowed either permanently or temporarilv with the same attractive power. 1097. Certain ores of iron are found to be naturally pos- sessed oi magnetic properties, and are therefore called natural or native magnets, or loadstones. Besides iron and some of the compounds nickel, and, perhaps, cobalt, also possess magnetic properties. But al. conductors of electricity are capable of exerting the magnetic properties of attraction and rcpultdon while conveying a current of electricity, as will be shown uncLr the head of Electro-Magnetism. 1098. That part of science which relates to the development of magnetism by means of a current of electricity will be noticed ufi- der.the head of Electro-Magnetism, in which connexion will also be mentioned the development of electricity by magnetism, to which the term Magneto-Electricity has been applied. What are the 1099. There are two kinds of magnets, two kinds of namely, the native or natural magnet, an<3 the artificial. 1100. The native magnet, or loadstone, is an ore of iron, found in iron mines, and has the property of attracting *ron, and other substances which contain it. What is a per- 1101. A permanent artificial magnet is a manent magnet? piece of iron to which perm anent magnetic properties have been communicated. f permanent periment, the artificial is to be preferred to or the artificial ^ u magnet magnet ? 1103. If a straight bar of soft iron be held in a vertical posi- tion (or, still better, in a position slightly inclined to the perpen- dicular, the lower end deviating to the north), and struck several smart blows with a hammer, it will be found to have acquired, by this process, all the properties of a magnet; or, in other words, it will become an artificial magnet. What are the 1104. The properties of a magnet are, properties of a polarity ; attraction of unmagnetic iron ; at- traction and repulsion of magnetic iron ; the power of communicating magnetism to other iron. Beside* these properties, the magnet has recently been discovered to be possessed of electrical properties. These will be considered it another connexi Dn. What is the po- 1105. By the polarity of a magnet is meant larity of a mag- the property of pointing or turning to the north and south poles. The end which points 13 800 NATUKAJ to the north is called the north pole of the magnet, and the other the south pole. 1106. The attractive powor of a magnet is generally stated to be greatest at the poles ; but the actual poles, or points of greatest magnetic intensity, in a steel magnet, are not exactly at the ends, but a little witnm them. How willa mag- 1107 - When a magnet is supported in net move when such a manner as to move freely, it will eeysuspen spontaneously assume a position directed nearly north and south. 1108. The points to which the poles of a What are the ., 7 ml magnetic poles / magnet turn are the magnetic poles. These do not exactly coincide with the astronomical poles of the earth ; but, although the value of the magnetic needle has been predicated on the supposition that its polar- ity is a tendency to point exactly to the north and south poles of the earth, the recent discovery of the magnetic poles, as the points of attraction, has not depreciated the value of the compass, because the variation is known, and proper allowances can be made for such variation. 1109. There are several ways of supporting How are mag- ma g rie t, so as to enable it to manifest its nets supported] polarity, first, b,y suspending it, accurately balanced, from a string. Secondly, by poising it on a sharp point. Thirdly, by attaching it to some buoyant substance, and allowing it to float freely on water. of magnetic "at- 1110. Different poles of magnets attract, traction and re- and similar poles repel each other. pulsion ? There is here a close analogy between the attractive and repul- sive powers of the positive and the negative forms of electricity, and the northern and southern polarities of the magnet. The same law obtains with regard to both ; namely, between like ;>cu?m there ff rt-pu/siitn, bfjiovn inlikc there is attraction MAGNETISM:. 301 1111. A magnet, whether native or artificial attracts iron or which has no magnetic properties ; but it both attracts and reptls those substances when they are magnetic : that is the oorth pole of one magnet will attract the south pole of another, and the south pole of one will attract the north of another ; but the north pole of the one repels the north pole of the other, and the south pole of one repels the south pole of another. 1112. If either pole of a magnet be brought near any small piece of soft iron, it will attract it. Iron filings will sdso adhere in clusters to either pole. To what bod- 1113. A. magnet *nay communicate its ies are the ma - ,. ,, . , v netic properties properties to other unmagnetized bodies. most easily com- But these properties can be generally con- municatcd? > . ,, , . veyed to no otter substances than iron. nickel or cobalt, without the aid of electricity. Coulomb has discovered that " all solid belies are sus- ceptible of magnetic influence" But the " influence," is perceptible only by the nicest tests, and under peculiar circumstances. What are per- 1H4. All permanent natural and artificial manent mag- magnets, as well as the bodies on wt icli they act, are either iron in its pure state or such compounds as contain it. What effect has 1115 - The powers of a magn t are in- the use of a ma g- creased by action, and are impaired and net on its -power? 1^.1.1 j- even lost by long disuse. TJ-, . . 1116. When the two poles of a magnet are V\ fiat is a horse-shoe or brought together, so that the magnet resembles u ****** in shape a horse-shoe, or the capital letter U, it is called a horse-slioe magnet, or a U magnet ; and it may be made to sustain a considerable weight, by suspending substances from a small iron bar, extending from one pole 302 NATURAL PHILOSOPHY. to tho other. This bar is called the keeper. A small adr- ditiui may be made to the weight every d&y. 1117. Soft iron acquires the magnetic power very readily, And also loses it as readily ; hardened iron or steel acquires the property with difficulty, b^t retains it permanently. MTT. < f 77 1118. When a magnet is broken or divided. What follows when a mag- each part becomes a perfect magnet, having ivide net is divided? b()th a north and gouth pole This is a remarkable circumstance, since the central part of a, magnet appears to possess but little of the magnetic power; out, when a magnet is divided in the centre, this very part as- sumes the magnetic power, and becomes possessed in the one part of the north, and in the other of the south polarity. 1119. The magnetic power of iron or steel appears to reside wholly on the surface, and is independent of its mass. In what do 1120. In this respect there is a strong resem- magnetism blance between magnetism and electricity. Elec- and electricity . . A , . ; , .. resemble each tricity, as has already been stated, is wholly con- other? fined to the surface of bodies. In a few words, magnetism and electricity may be said to resemble each other in the following particulars : (U) Each consists of two species, namely, the vitreous and the resinous (or, the positive and negative) electricities ; and the northern or southern (sometimes called the Boreal and the Austral) polarity. (2.) In both magnetism and electricity, those of the same aamc repel, and those of different names attract each other. (3.) The laws of induction in both are similar. (4.) The influence, in both cases (as has just been stated) resides at the surface, and is wholly independent of their mass. What effect H21. Heat weakens, and a great degree of has heat )n heat destroys the power of a magnet ; but the magnetic attraction is undiminished by the in- terposition of any bodies, except iron, steel, &c. MAGNETISM. '60% ftiuai jtiwr 1122. Electricity frequently changes the -'"uses will aj poles of a magnet ; and the explosion of a small feet the polar- ityofa mag- quantity of gunpowder, on one of the poles. "d f produces the same effect. Electricity, also, sometimes renders iron and steel magnetic, which were not so before the charge was received. What is the 1123. The effect produced by two magnets, e foutlemag- use( l together, is much more than double that net ? of either one used alone. What is meant 1124. When a magnet is suspended freely by "the dip- f roin fts centre, the two poles will not lie in ping of a mag- net, and hav the same horizontal direction. This is called ts it corrected? ^ j nc ii nat i on or t he dipping of the magnet. 1125. The tendency of a magnetic needle to dip is corrected, in the mariner's and surveyor's comp-asses, by making the south ends of the needles intended for use in northern latitudes some- what heavier than the north ends. Compass-needles, intended to be employed on long voyages, where great variations of lati- tude may be expected, are furnished with a small sliding-weight, by the adjusting of which the tendency to dip may be counter- acted. The cause of the dipping of the needle is the superior attraction caused by the closer proximity of the pole of the mag- net to the magnetic pole of the earth. In north latitude, the north pole of the needle dips ; in south latitude, the south pole. l n w hat direc- 1126. The magnet, when suspended, does not Han does a invariably point exactly to the north and south nag-net point . , . ... when free/y points, but varies a little towards the east or ' l Vended > t h e west This variation differs at different places, at different seasons, and at different times in the day. 1127. T? e variation of the magnetic needle from what has been supposed its true polarity was a phenomenon that for centuries uad baffled the science of the philosopher to explain. Recent discoveries have given a satisfactory explanation of this apparent 54 NATURAL PHILOSOPHY. anomaly.* The earth has, in fact, four magnetic poles, two of which are strong and two are weak. The strongest north pole is in America, the weakest, in Asia. The earth itself is consid- ered as a magnet, or, rather, as composed in part of m^gnetie substances, so that its action at the surface is irregular. The variation of the needle from the true geographical meridian ii therefore subject to changes more or less irregular, t What gift has 1128. The science of Magnetism has rendered Magnetism i mmense advantages to commerce and navigation, bestowed on by means of the mariner's compass. The Mari- navigaticm ? ner's Compass consists of a magnetized bar of steel What is the . , . fi , Mariner's called a needle; having at its centre a cap fitted to ? it, which is supported on a sharp-pointed pivot * The following statement has been made in the National Intelligencer on the authority of its London correspondent : Mr. Faraday, in a late lecture before the Royal Institution upon the Magnetic Forces, made the following important announcement . " A German astronomer has for many years been watching the spots on the sun, and daily recording the result. From year to year the groups of spots vary. They are sometimes very numerous, sometimes they are few. After a while it became evident that the variation in number followed ?. descending scale through five years, and then an ascending scale through five subsequent years, so that the periodicity of the variations became 8 visible fact. " While our German friend was b%jr with his groups of sun-spots, a* Englishman was busy with the variations of the magnetic needle, lie, too. was a patient recorder of patient observation. On comparing his tabular results with those of the German astronomer, he found that the variationi of the magnetic ne>,Jle corresponded with the variations of the sun-spots, that the years when the groups were at their maximum, the variations of the needle were at their maximum, and so on through their series. Thia relation may be coincident merely, or derivative ; if the latter, then do we connect astral and terrestrial magnetism, and new reaches of science are open to us." t The northern magnetic pole on the western continent is in latitude 70 N. and longitude 97 VV. On the eastern continent the pole is about at the point where the Lena River crosses the Arctic circle. The south poles are nearly on the Antarctic circle, one in 130 E. longitude, and the other 120 W. from Greenwich. The poles are doubtless slowly swinging about the poles of the earth. The direction of the needle for the northeast portion of the North American continent is west of north. A line on winch the needle points clue north runs through Lake Erie, the eastern pint of Ohio, a c< rner of Pennsylvania, the District of Columbia, and North Carolina. West of this line the needle points to the e.ist of north. At San Francisco the pres- ent direction of the needle (1871) is nearly 17 east of north. The line of no variation is slowly moving westward, and the direction of the needle over the whole continent is slowly changing in the same direction. MAGNETISM. 303 Sxe<\ in the base of the instrument. A. circular plate, or card, the circumference of which is divider into degrees, is attached to the needle, and turns with it. On an inner circle of the card tho thirty -two points of the mariner's compass are inscribed Fig; 165. 1129. The needle is generally placed under the card of a mariner's compass, so that it is out of sight; but small needles, used on land, are placed above the card, not attached to it, and the card is permanently fixed to the box. ] 130. The compass is generally fitted by two sets of axes to an outer box, so that it always retains a horizontal position, even when the vessel rolls. When the artificial magnet or necdU is kept thus freely suspended, so that it may turn north or south, the pilot, by looking at its position, can ascertain in what direc- tion his vessel is proceeding j and, although the needle varies a little from a correcjjrpolarity, yet this variation is neither so great, nor ro irregular as seriously to impair its use as a guide to the vessel in its course over the pathless deep. JOO N>.IUKAL PHILOSOPHY 1131. The invention of the mariner's ompass is usual iv ascribed to Flavio de Melfi, or Flavio Gioia, a Neapolitan, about the year 1302. Some authorities, however, assert that it was brought from China by Marco Paolo, a Venetian, in 1260. The invention is also claimed both by the French and English. ^ 1132. The value of this discovery may be esti- *as the mar- mated from the consideration that, before the use .ncr's com- of the compass, mariners seldom trusted themselves out of sight of land ; they were unable to make long or distant voyages, as they had no means to find their way back. This discovery enabled them to find a way where all is trackless ; to conduct their vessels through the mighty ocean, out of the sight of land ; and to prosecute those discoveries, and perform those gallant deeds, which have immortalized the names of Cook, of La Perouse, Vancouver, Sir Francis Drake, Nelson, Parry, Franklin and others. Which pole of 1133. Xhe north pole of a magnet is more "ke^nore * powerful in the northern hemisphere, or north powerful ? of the equator, and the south pole in the south- ern parts of the world. 1134. When a piece of iron is brought sufficiently near to a magnet, it becomes itself a magnet ; and bars of iron that have ptood long in a perpendicular situation are generally found te be magnetic. How are arti- 1135> Artificial magnets are made by ap- fieial magnets plying one or more powerful magnets to pieces of steel. The end which is touched by the north pole becomes the south pole of the new mag- net, and that touched by the south pole becomes the north pole. The magnet which is employed in magnetizing a steel bar loses none of its power by being thus employed ; and as the effect is increased when two or more magnets are used, with one magnet a number of bars may be mag- netized, and then combined together; by which means MAGNETISM. 307 fcheir power may be indefinitely increased. Such an ap- paratus is called a compound magnet. 1136. There are several methods of making artificial magnets. One f the most simple and effectual consists in passing a strong horse-shoe magnet over bars of steel. In making bar (or straight) magnets, the bars must be laid lengthwise, on a flat table, with the marked end of one bar against the unmarked end of the next ; and in making horse- shoe magnets, the pieces of steel, previously bent into their proper form, must be laid with their ends in contact, so as to form a figure like two capital U's, with their tops joined together, thus, c3^ ; observing that the marked ends come opposite to those which are not marked ; and then, in either case, a strong horse-shoe magnet is to be passed, with moderate pressure, over the bars, taking care to let the marked end of this magnet pre- cede and its unmarked end follow it, and to move it constantly over the steel bars, so as to enter or commence the process at a mark, and then to proceed to an unmarked end, and enter the next bar at its marked end, and so proceed. After having thus passed over the bars ten or a dozen times jn each side, and in the same direction as to the marks, they will be converted into tolerably strong and permanent magnets. But if, after having continued the process for some time, the exciting magnet be moved over the bars in a contrary direc- tion, or if its south pole should be permitted to precede aftei the north pole has been first used, the previously-excited mag- netism will disappear, and the bars will be found in their original state. This mode of making artificial magnets is likely. to be wholly superseded by the new mode by electrical aid vthich will be noticed in connexion with Electro-magnetism. How is a com- 1137 - A compound magnet may be made by pound magnet taking several horse-shoe magnets of equal size, constructed? ^ after liayillg magnet i zed them, uniting them together by means of screws. 13* #08 NATURAL PHILOSOPHY. 1138. A magnetic needle is made by fastening tho steel on a piece of ooard, and drawing magnets over it frjm the oentre outwards. 1139. A horse-shoe magnet should le kepi How should a 7 r ,, , r horw-ihoe armed, by a small bar ol iron or steel, connect- magnet be kept ? ing the two poles. The bar is called " the keeper" Interesting experiments may be made by a magnet, even of no great power, with steel or iron tilings, email needles, pieces of fer- ruginous substances, and black sand which contains iron. Such substances may be made to assume a variety of amusing forms and positions by moving the magnet under the card, paper or t:ble, on which they are placed. Toys, representing fishes, frogs, aquatic birds, &c., which are made to appear to bite at a hook, birds floating on the water, &c., are constructed on magnetic principles, and sold in the shops. What is Eke- 1140. Electro-magnetism relates to magnet- tro'-nagnetism? j sm w hi c h j s induced by the agency of electricity. 1141. The passage of the two kinds of electricity (namely, the positive and the negative) through their circuit is called the elec- tric currents ; and the science of Electro-magnet' sin explains the phenomena attending those currents. It has already been stated that from the connecting wires of the galvanic circle, or battery, there is a constant current of electricity passing from the zinc to the copper, and from the copper to the zinc plates. In the single circle these currents will be negative from the zinc, and positive from the copper ; but in the compound circles, or the battery, the current of positive electricity will flow from the zinc to the copper, and the current of negative electricity from the copper to the zinc. From the effect produced by electricity on the magnetic needle, it had been conjectured, by a number of eminent philosophers, that magnetism, or magnetic attraction, is in some manner caused b^ electricity. In the year 1819, Professor (Ersted, of Copenhagen, made the grand discovery of the power of the electric current to induce magnetism ; thus proving the connexion between magnetism and electricity. In a short time after the discovery of Professor (Ersted, Mr. Faraday discovered that an electrical spark could be taken from a magnet ; and thus the relation between magnetism and electricity was fully proved. In a paper'published a few years ago, this distinguished philosopher has very ably maintained ihe identity of common electricity, voltaic electricity, magnetic electric- ity (or electro-magnetism), thermoelectricity, and animal electric- ity. The phenomena exhibited in all these five kinds of electricity (Jifler merely in degree, and the state of intensity in t'^c action cf tlu; tti.&UTKO -MAC NkTIbM. 30;) fluid. Thi discovery of Professor Oersted has been followed uut by Ampere, who, by his mathematical and experimental researches, has presented a theory of the science less obnoxious to objections than that proposed by the professor. The discovery of CErstcd was limited to the action of the electric current on needles previously magnetized ; it was afterwards ascertained by Sir Humphrey Davy and M. Arago that magnetism maybe developed in steel not pre- viously possessing it, if the steel be placed in the electric current. Both of these philosophers, independently of each other, ascertained that the uniting wire, becoming a magnet, attracts iron filings and collects sufficient to acquire the diameter of a common quill ; but the moment the connexion is broken, all the filings drop off, and the attraction diminishes with the decaying energy of the pile. Filings af brass or copper, or wood-shavings, are not attracted at all. 1142. All the effects of electricity and galvanism that have hitherto been described have been produced on bodies inter- posed between the extremities of conductors, proceeding from the positive and negative poles. It was not known, until the discoveries of Professor (Ersted were made, that any effect could be produced when the electric circuit is uninterrupted What is the It will presently be seen that this constitutes the difference be- g rea t distinction between electricity and electro- \ricity and magnetism, namely, that one describes the effect electro-mag- of electricity when interrupted in its course, and that the other more especially explains the effect of an uninterrupted current of electricity. What are Jie 1143. The principal facts in connexion with the prtnctpa science of electro-magnetism are, i a c ts of elcc- iro-magnet- (1.) That the electrical current, passing uninter. tsm ? ruptedly through a wire connecting the two ends of a galvanic battery, produces an effect upon the magnetic needle. (2.) That electricity will induce magnetism. (3.) That a magnet, or bundle of magnets, will induce elec- tricity. (4.) That the combined action of electricity and magnetism, as described in this science, produces a rotatory motion of certain kinds of bodies, in a direction pointed out by certain laws. (5.) That the periodical variation of the magnetic needle lilO NATURAL PHILOSOPHY. from the true meridian, or, in other words, the variation of thf compass, is caused by the influence of the electric currents. (6.) That the magnetic influence is not confined to iron, steel &c., but that most metals, and many other substances, may be converted into temporary magnets by electrical action. (7.) That the magnetic attraction of iron, steel, &c., may b prodigiously increased by electrical agency. (8.) That the direction of the electric current may, in ah cases, be ascertained. (9.) That magnetism is produced whenever concentrated elec- tricity is passed through space. (10.) That whila in common electrical and magnetic attrac- tions and repulsions those of the same name are mutually repulsive, and those of different names attract each other, in the attractions and repulsions of electric, currents it is precisely the reverse, the repulsion taking place only when the wires are so situated that the currents are in opposite direction. The consideration of the subject of electricity induced by magnetism properly belongs to the subject of Magneto-elec- tricity, in which connexion it will be particularly noticed. How is the 1144. The direction of the electric current is ^rrenTo/ ascertained by means of the magnetic needle. If electricity a sheet of paper be placed over a horse-shoe mag- ascertained? net> an( j fi ne 1,1^ sau( l, or steel filings, be dropped loosely on the paper, the particles will be disposed to arrange themselves in a regular order, and in the direction of curve lines. This is, undoubtedly, the eSect of some influence, whether that ot electricity, or of magnetism alone, is not material at present to decide. How will a 1145 A magnet freely suspended tench ^magnTplace to assume a position at right angles to the itself in relation direction of a current of electricity passing to the electrical Current? near jt ' 11UJ. If a wire, which connects the extremities of a voltaie "* LSM. 311 Buttery, be brought over and parallel with a magnetic neecTie ut rest, or with its poles properly directed north and south, that end of the needle next to the negative pole of the battery wi.'l move towards the west, whether the wire be on one side of the needle or the other, provided only that it be parallel with it. 1147. Again, if the connecting wire be lowered on either side of the needle, so as to be in the horizontal plane in which the needle should move, it will not move in that plane, but will have a tendency to revolve in a vertical direction; in which, however, it will be prevented from moving, in consequence of tjie attrac tion of the earth, and the manner in which it is suspended When the wire is to the east of the needle, the pole nearest to the negative extremity of the battery will be elevated ; and when it is on the west side, that pole will be depressed. 1148. If the connecting wire be placed below the plane in which the needle moves, and parallel with it, the pole of the needle next to the negative end of the wire will move towaida the east, and the attractions and repulsions will be the reverse of those observed in the former case. How does the 1149. The action of the conducting-wire in ekctrt-magnetic these cases exhibits a remarkable peculiarity. current act? ^11 other known forceL exerted between two points act in the direction of a straight line connecting these points, and such is the case with electric and magnetic actions, separately considered; but the electric current exerts its mag- netic influence laterally, at right angles to its own course. Nor does the magnetic pole move either directly towards or directly from the conducting-wire, but tends to revolve around it without changing its distance. Hence the force must be considered as acting in the direction of a tangent to the circle in which the magnetic pole would move. What effect has H5Q. The two sides of an unmagnetized a voltaic bat- . ,. -i i i ^i ' fry on unrru.g- stee ^ needle will become endued with the r-m/ v/.i nor th and south polaiity, if tlie needle be itti NATURAL PHILOSOPHY. placed parallel with the connecting wire of a voltaic battery, and nearly or quite in contact with it. But, if the needle be placed at right angles with the connecting wire, it will become permanently magnetic ; one of its extremities point- ing to the north pole and the other to the south, when it is finely suspended arid suffered to vibrate undisturbed. To what may 1151. Magnetism maybe communicated Communicated to n ' on all( ^ stee ^ by means of electricity by the voltaic, from an electrical machine ; but the effect battery, and . , . . . uhat is the pro- can be more conveniently produced by means cess called ? O f the voltaic battery. This phenomenon is called electro-magnetic induction. What is a 1152. A Helix is a spiral line, or a line wound Helix ? into the shape of a cork-screw. What use is H53. If a helix be formed of wire, and a made of a helix , , , , , . , . , . . . in conne.rhm " ar * stee ^ " e enclosed within the helix, on irith the battery ? applying the conducting- wires of the battery to the extremities of the helix, the steel bar will immediately become magnetic. The electricity from a common electrical machine, when passed through the helix, will produce the same effect. The wire which forms the helix should And what must first be done be coated with some non-conducting substance, uitfi the wire of Slic h as silk wound around it : as it may then the helix * in M n- - i be formed into close coils, without suffering trio electric fluids to pass from surface to surface, which would im- pair its effect. 1155. If such a helix be so placed that it may move freely as when made to float on a basin of water, it will be attracted and repelled by the opposite poles of a common magnet. 1156. If a magnetic needle be surrounded by coiled wire. covered with silk, a very minute portion of electricity through ELECT K< >- M A (T fcTISM . 3 1 it the wire will cause the needle to deviate from it.s proper lirectiori. Wh tisar Elec H&7. A needle thus prepared is called an ro-mag-nf.tic Electro-magnetic Multiplier. It is, ia fact, a Muliipier very delicate electroscope, or rather galvanom- eter, capable of pointing out the direction of the electric cur rent in all cases. 1158. Among the most remarkable of the What is meant A , .., ,, . , )>/ the Electro- ^ acts connected with the science of electro- magnetic Rota- magnetism is what is called the Electro- magnetic Rotation. Any wire through which a current of electricity is passing has a tendency to revolve around a magnetic pole in a plane perpendicular to the current, and that without reference to the axis of the magnet the pole of which is used. In like manner a mag netic pole has a tendency to revolve around such a wire. 1159. Suppose the wire perpendicular, its upper end posi- tive, or attached to the positive pole of the voltaic battery, and its lower end negative ; and let the centre of a watch-dial rep- resent the magnetic pole : if it be a north pole, the wire will rotate round it in the direction that the hands move ; if it be a south pole, the motion will be in the opposite direction. From these two, the motions which would take place if the wire were inverted, or the pole changed, or made to move, may be readily ascertained, since the relation now pointed out remains constant. 1160. Fig. 166 represents the ingenious ap- ^XVM i Fig. p ara t us> invented by Mr. Faraday, to illustrate the electro-magnetic rotation. The central pil- .ar supports a piece of thick copper wire, which, on the one side, dips into the mercury contained in a small glass cujr 'A To a pin at the bottom of this cup a small cylindrical magnet is attached by a thread, so that one pole shall rise a little above i.he surface of the mercury, and be at liberty to move around Mi bottom of the cup is perforate!, nnd hup a cop* NATURAL HHCLOSOPliY. 16Q - per pin passing through it, which, touching the mercury on the inside, is also in contact with the wire that proceeds outward?. ' n that side of the in- i .{.rumen t. On the other bide of the instrument t>, the thick copper wire, soon after turning down, terminates, but a thinnci piece of wire forms a communication between it and the mercury on the cup beneath. As freedom of motion is re- garded in the wire, it is made to communicate with the formci by a ball and socket-joint, the ball being held in the socket by a thread ; or else the ends are bent into hooks, and the one is then hooked to the other. As good metallic contact is required, the parts should be amalgamated, and a small drop of mercury placed between them; the lower ends of the wire should also be amalgamated. Beneath the hanging wire a small circular mag- net is fixed in the socket of the cup , so that one of its poles is a little above the mercury. As in the former cup, a metallic connexion is made through the bottom, from the mercury to the external wire. If now the poles of a battery be connected with the horizon- tal external wires c c, the current of electricity will be through the mercury and the horizontal wire, on the pillar which con nects them, and it will now be found that the movable part of the wire will rotate around the magnetic pole in the cup b, and the magnetic pole round the fixed wire in the other cup a, in the direction before mentioned. By u-.ing a very delicate apparatus, the magnetic pole oi the earth may be made to put the wire in motion. Kjrtfoiii Pi* 1161. Fig. 167 represents another ingenious Ki<. contrivance, invented by M. Ampere for illus- ELECTKO-MAGNETISK. 315 tratiiig the electro-magnetic rotation ; and it has the advantage of comprising within itself the voltaic combination which is employed. It consists of a cylinder of copper about two inches high, and a little less than two inches internal diameter, within which is a small cylinder, about one inch in diameter. The two cylinders are connected together by a bottom, having an aperture in its centre the size of the smaller cylinder, leaving a circular cell, which may be filled with acid. A piece of strong cop- per wire is fastened across the top of the inner cylinder, and from the middle of it rises, at a right angle, a piece of copper wire, supporting a very small metal cup, containing a few globules of mercury. A cylinder of zinc, open at each end, and about an inch and a quarter in diameter, completes the voltaic combination. To the latter cylinder a wire, bent like an inverted U, is soldered at opposite sides ; and in the bend of this wire a metallic point is fixed, which, when inserted in the little cup of mercury, sus- pends the zinc cylinder in the cell, and allows it a free circular motion. An additional point is directed downwards from the contra! part of the stronger wire, which point is adapted to a small hole at the top of a powerful bar magnet. When the apparatus with one point only is charged with diluted acid, and set on the magnet placed vertically, the zinc cylinder revolves 'n a direction determined by the magnetip pole which is upper- most. With two points, the copper revolves in one direction and the zinc in a contrary direction. 1162. If, instead of a bar magnet, a horse-shoe magnet be employed, with an apparatus on each pole similar to that which has now been described, the cylinders in each will revolve in apposite directions. The small cups of mercury mentioned in the preceding description are sometimes omitted, and the points are inserted in an indentation on the inverted U. NATURAL PHILOSOPHY. How is the mag 1163. The magnetizing power of the con- o/lhe n Kry r ductin g * of a Battery is very greatly in- increased f creased by coiling it into a helix, into which the body to be magnetized may be inserted. A single cir- cular turn is more efficient than a straight wire, and each turn adds to the power within a certain limit, whether the whole forms a single layer, or whether each successive turn encloses the previous one.* How is a Mix 1164 ' When a helix of S reat P wer is of great power required, it is composed of several layers of obtained? . mu **". -i t v wire. Ihe wire terming the coil must be insulated by being wound with cotton, to prevent any lat- eral passage of the current. 1165. Fig. 168 represents a helix on a stand. A bar of soft iron, N S, being placed within tbe helix, is connected with the battery- bv Fig. 168. Explain Fig. 168. * The amount of power dereloped by electro-magnets depends, when the size of the magnet and its envelope of wire is suitable, upon the amount of zinc consumed in the battery. Much controversy has been held about the possibility of so employing electro-magnetism as to substitute it economic- ally as a motive force for those agents now employed viz., steam, water, and air. The attempts thus far have been, unsuccessful. Although many machines have been devised whose propelling force was electricity or olec- tro-magnetiRm, the cost of their propulsion was far greater than by any other plan in use. ELE.OTKO-MAGNETISM. H 1 7 tncans of the screw-cups on the base of the stand. The two extremities of the bar instantly become strongly magnetic, and keys, or pieces of iron, iron filings, nails, &c., will be held up so Jong as the connexion with the battery is sustained. But, so soon as the connexion is broken, the bar loses its magnetic power, and the suspended articles will fall. The bar can be made alternately to take up and drop such magnetizable articles as are brought near it, as the connexion with the battery is made or broken. 1166. A steel bar placed within the helix acquires the polar- ity less readily, but retains it after the connexion is broken. Small rods or bars of steel, needles, &c., may be made perma nent magnets in this way. 1167. A bar temporarily magnetized bv What is an Etec- . , , . . . n , r ,, tro-ma'rnet? " ie electric current is called an Electro- magnet. 1168. To ascertain the poles of an elec- ffow ran the poles . . , , , J , of an electro- tro-magnet, it must be observed that the magnet be dis- north pole will be at the furthest end of the helix when the current circulates in the direction of the hands of a watch. 1169 Magnets of prodigious power have been formed by mean- 1 of voltaic electricity. \Vliat was the H70. An electro-magnet was constructed by ptnt-KT of the Professor Henry and Dr. Ten Eyck which was electro-magnets capal)]e O f supporting a weight of 750 pounds. constructed .v/ L Prof. Henry They have subsequently constructed another, and ^ Dr. Ten w hi c h will sustain 2063 pounds. It consists of a bar of soft iron, bent into the form of 2 horse-shoe, and wound with twenty-six strands of copper bell- wire, covered with cotton threads, each thirty-one feet long ; about eighteen inches of the ends are left projecting, so that only twenty-eight feet of each actually surround the iron. The aggregate length of the coils is therefore 728 feet. Each strand NATURAL PHILOSOPHY. is ^ound on a little less than an inch ; in the middle of the horse-shoe it forms three thicknesses of wire ; and on tho ends, or near the poles, it is wound so as to form six thicknesses. Bo ing connected with a battery consisting of plates, containing u little less than forty-eight square feet of surface, the ipagnet supported the prodigious weight stated above, namely, 2063 pounds. 1171. HE- Fig- 169. Explain Fig. LIACAL RlNO> Fig. 169 rep- resents a heliacal ring, or ring of wire bent in the form of a helix, with the ends of the * wire left free to be inserted in the screw-cups of a bat- tery. Two semicircular pieces of soft unmagnetized iron, furnished with rings, the upper one for the hand, the lower one for weights, tire prepared to be inserted into the helix, in the manner of the links of a chain. As soon as the ends of the helix are inserted into the screw- cups of the battery, the rings will be held together, with gre;;t force, by magnetic attraction. 1172. That the attraction is caused, or that the magnetism is in- duced, by the circulation of electricity around the coils, may be proved by the following interesting experiment. Hold the heliacal ring horizontally over a plate of small nails, and suspend an unmag- netized bar perpendiculai ly on the outside of the ring, over the nails, and there will be no attraction. Suspend the bar perpendicularly through the helix, and the nails will all attach themselves to it in the form of tangents to the circles formed by the coils of the helia cal ring. H ehors&- 1-173. Communication of Magnetism to Steel shoe magnets by tfie Electro-magnet. Bars of the U fora? ELECTKO-MAGNET1G TELEGKAPH . most readily are most readily magnetized by drawing them from the bend to the extremities, across the poles of the U electro-magnet, in such a way that both halves of the bar may pass at the same time over the poles to which they are applied. This should be repeated several times, recol- lecting always to draw the bar in the same direction. 1174. Fig. 170 represents the U electro- magnet with the bar to be magnetized. Whec the bar is thick, both surfaces should be drawn across the electro-magnet, keeping each half applied to the same pole. To remove the magnetism, it is only necessary to Fig. 170. Explain Fig. 1TO. On what funda- mental principle is the Electric Telegraph con- constructed 1 reverse the process by which it was magnetized, that is, to draw the bar across the electro-magnet in a contrary direction. 1175. THE ELECTRO-MAGNETIC TELEGRAPH.* From the description which has now been given of the electro-magnetic power, it will readily be perceived that a great force can be made to act simply by bringing a wire into contact with another conductor, and that the force can be in- stantly arrested in its operation by removing the wire from the contact ; in other words, that by connecting and disconnecting a helix with a battery, a prodigious power can be made succes * The word telegraph is compounded of two Greek words, rtfts (tele), sig- nifying at a distance, and ynayo (grapho), to write, that is, to signify or to write at a distance. The word telescope is another compound of the word fijit with the word ox^nia: (scrpio), to see, an instrument to see at a di* tartce. B"0 NATURAL PHILOS01 IIY eively to act and cease to act. Advantage has been taktii o' this principle in the construction of the American electro-mag; netic telegraph, which was matured by Professor Morse, and first put into operation between the cities of Baltimore and . Washington, in 1844> It was not, however, ies 'rendereTl-he unti * Professor Henry, of Princeton, New Jer- magnetic tele- sey, had discovered the mode of constructing tfraph possible? the power f ul electro-magnets which have been noticed, that this form of the telegraph became possible. 1176. The principles of its construction may Explain the man- b b { fl d fon ^. ner m which the J electric telegraph An electro-magnet is so arranged with its performs its armature that when the armature is attracted it communicates its motion to a lever, to which a blunt point is attached, which marks a narrow strip of paper, drawn under it by machinery resembling clock-work, whenever the electro-magnet is in action. When the electro-magnet ceases to act, the armature falls, arid, communicating its motion to the lever, the blunt point is removed from its contact with the paper. By this means, if one of the wires from the battery is attached to the screw-cup, whenever the other wire is attached to the remaining cup the armature is powerfully attracted by the magnet, and the point on the lever presses the paper into the groove, of a roller, thereby making an indentation on the paper, corresponding in length to the time during which the contact with the battery is maintained, the paper being drawn slowly under the roller. 1177. In the construction of the electric tele- \Vhat is the agent in the graph the first object of consideration is the de- electrti tele- yelopment of the agent. The agent is the electric fluid, which is brought into action by a battery. The forms of batteries chiefly employed for this purpose are Grove's, Bunsen's, Daniel's, Snell's or its modifications, and Le- clanche's. If the telegraph is in constant use, and the battery is to be renewed at intervals of two or three days, one of the first above mentioned is used. EJ,KCTKO-MAGNETIO TELEGKAPH. Fig. lYl represents a battery composed of ffxplatn Fig. twe j ve Cll p s> on t ^ e p r i nc ip] e o f Grove's battery, each cup containing a thick cylinder of zinc, with a porous cell, two acids, and a strip of platinum, as described in Fijr. 104. The chemical action of the acids on the Fig. 171 Fig. 172. zinc generates a powerful current of electricity towards the screw-cups at A B. 1178. The second step in the construction of Explain Fig. ^ telegraph is rep rescntcd by Fig. 172. The wires from the battery represented in Fig. 171 are carried to the screw-cups in the apparatus represented by Fig. 172, called the sig- nal-key, A to A and B to B, respectively. It will be observed that the cups of the signal-key are insulated, and that the electric fluid can finish its circuit only when the fin- ger depresses the knob and makes it come in con- tact with the metallic strip below, thus forming a communication between the screw-cups. The signal -key thus regulates the com- pletion of the circuit, and the flow of the current of electricity, ut the will of the operator. 822 NA1UKAL PHILOSOPHY. 1179. The signal-key is made in several Explain Fig. forms in the different telegraphs, and in Fig. 173 is represented in its more perfect construc- tion. It coir Ists of a lever, mounted on a horizontal axis, with a, knob of ivory for the hand at the extremity of the long arm. wrhich is at the left in the cut. This lever is thrown up by a spring, so as to avoid contact with the button on the frame below, except when the lever is depressed for the purpose of com- pleting the circuit. A regulating screw is seen at the extremity of the short arm of the lever, which graduates precisely the amount of motion of which it is at any time susceptible. 1180. The third and last part of the tele- Explain Fig. g rap h i s the registering apparatus, represented in Fig. 174. Here are two screw-cups, for the insertion of the wires from i distant battery. An iron in the shape of a U magnet stands Fig 174 at the left of the screw-cups, each arm of which is surrounded by a helix or coil of wire, the ends- of which, passing down through the stand, are connected below with the screw-cups. It will then be seen that when the signal-key is depressed the electrir circuit is completed, and that the electricity, passing through fhe noils of wire, renders the U-shaped iron highly magnetic iU JSUTKO- UAUNJET 1C TELL ./ .*A PH. Fig. 17fc. 321 NATURAL PHILOSOPHY. and it attracts the armature down. The armature IK fixed to the shorter arm of a lever, and when the shorter arm is attracted down, the longer arm, with a point affixed, is forced upward and makes an indentation upon a strip of paper. The length of the indentation on the paper will depend on the length of time that the signal-key is depressed. When the signal-key is permitted again to rise, the electric current is broken, the U-shaped iron ceases to be a magnet, and, the armature being no longer attracted, the weight of the longer arm will cause that arm to fall, and no mark is made on the paper. When the telegraph was first constructed, it was thought nec- essary to have two wires in order to form the circuit. It has since been found that the earth itself will serve for one-half the circuit, and that one wire will alone be necessary to perform the work of the telegraph. 1180. Fig. 175 represents the manner in which Explain Fig. tne e ] ec t r i c telegraph is put into operation. -On the left of the figure is seen the operator, with the battery at his feet and his finger on the signal-key. From one screw-cup of the battery extends a wire which traverses the whole distance between two cities, elevated on posts for security. In the distant city the wire reaches another screw- cup to which it is attached, while from another screw-cup at 'the same station another wire is attached, which extends back to the operator first mentioned. The depression of the signal-key forms a connexion between the two poles of the battery by means of the wire, and the fluid will traverse the whole distance between the two stations in preference to leaping over the space between the two screw-cups. The right of the figure represents the receiver of the information, reading the message which has thus been imprinted by the point. 1182. In the preceding figures the mere out- ExpUun fig. j- neg ^ ave h een given, in order that they may be distinctly understood. To present the strip of paoer bo that it may readily receive the impression, addi- ELECTRO-MAGNETIC TELEGRAPH. 325 tional machinery becomes necessary. The complete registering machine is shown in Fig. 176, in which S represents a large spool Pig. 176. on which the paper is wound, and clock-work with rollers to give the paper a steady motion toward the point by which the marks are to be made. A bell is sometimes added, which is struck by a hammer when the lever first begins to move, in order to draw the attention of the operator. 1183. .It will be recollected that this form of the magnetic telegraph is familiarly known as Morse's, the machine making nothing but straight marks on the slip of paper. But these straight marks may be made long or short, at the pleasure of the operator. If the key be pressed down and instantly be per- mitted to rise, it will make a short line, not longer than a hyphen. By means of a conventional alphabet, in wJijch the tetters are expressed by the repetition and combination of marks varying in length, any message may be conveniently spelt out, so as to be distinctly understood at the distant station. Thesa are the essential features of Morse's Telegraph. ISO NATURAL PHILOSOPHY. 1184. It is necessary, in long lines of telegraphs, to combine tlia efiects of several batteries to supply the loss of power in traversing long circuits. This is done by local batteries or relays, as they are sometimes called, familiarly known in connexion with Morse's tele- graph. The use of the relays may be dispensed with by increasing the power of the batery, or distributing it in groups along the line It is sometimes divided by arranging one-half at each end of the line For every twenty miles an addition of one of Grove's pint sups should be made. The expense of acids for each cup for two days does not much exceed one cent. For a line of telegraph extending around the earth, twelve hundred Grove's cups would be required, distributed at equal distances, fifty in a group. 1185. BAIN'S TELEGRAPH. The telegraph known by the name of Bain's telegraph, the simplest now in use, differs from Morse's principally in its mode of registering. It performs its work by the decomposition of a saline solution. The pen or point is stationary. A circular tablet, moved by clock-work, under the point, receives the point in concentric grooves, and the writing is arranged in spiral lines, occupying but little space. Explain Fig. 177 represents Bain's telegraph. The pen Fig. 177. holder is connected with the positive wire of the battery, and the tablet with the negative. The circuit is COIB Fig. 177. ELECTROMAGNETIC TELEGRAPH. Fig. 178. Dieted by paper moistened with a solution of the yellow prus- siate of potash, acidulated with nitric or sulphuric acid. The pen-wire is of iron. When the circuit is completed, the solution attacks the pen, dissolves a portion of its iron, and forms the color known as Prussian blue, which stains the paper. The alphabet used by this line is the same in principle as that used m the telegraph of Morse. The advantage of this telegraph consists in^ the rapidity with which the disks at both ends are made to revolve, by which a message may be communicated at the rate of a thousand letters in a minute. Explain H86. The Fig. 178. CC H commonly used in connexion with Bain's telegraph is rep- resented in Fig. 178. It consists of a U magnet, each arm surrounded by a helix of wires, which, when the current passes, causes the armature to be attracted and give mo- tion to machinery, by which a bell or a glass is rung. Explain 1187. Fig. rig. 179. 179 represents the receiving magnet in its improved form. The armature is mounted on an upright bar, directly before the poles of the U magnet, which is surrounded by many coils of insulated wire. In this magnet the points of contact are preserved from oxidation by the use of platinum. Fig. 179. 3z8 NATURAL PHILOSOPHY. 1188. HOUSE'S PRINTING TELEGRAPH. This telegraph differ? from the other principally in its printing with great rapidity the letters which form the message. Explain 1189. Fig. 180 represents the mechanical part of Fig. 180. House's telegraph. The operator sits at a key-board gimilar to that of a pianoforte or organ, and, by depressing a Vig. 180. key, the letter corresponding with the key ie made to appear at a little window at the top of the instrument, while it is at the same time printed on a strip of paper below. The principle by which this exceedingly ingenious operation is performed is simply this : A given number of electrical impulses are given for each letter. These impulses give motion to a wheel, so that on the depression of a key the circuit will be broken at precisely the point which corresponds with the letter. The machinery by which this is effected is necessarily complicated and it falls not within the province of this work to g6 fu ther into the explanation. The whole process is described in Davis' Book of the Telegraph, to which this volume is indebted for most of l he particulars which have been giveii in relation to the subject. ELECTRO MAGNETIC TELEGRAPH. 'j The following history of the electric telegraph in this country is extracted fex'iu. the Portland Advertiser, and deserves a place in this connexion : " The electric telegraph, being used solely for the o^nveyance of news and communications, is so intimately connected with posts and post-offices, *hat a brief sketch of its rapid progress in the United States is here given. " It is to American ingenuity that we owe the practical application of the magnetic telegraph for the purpose of communication between distant points, and it has been perfected and improved mainly by American scienc and skill. While the honor is .due to Professor Morse for the practical application and successful prosecution of the telegraph, it is mainly owing to the researches and discoveries of Professor Henry, and other scientific Americans, that he was enabled to perfect so valuable an invention "The first attempt which was made to render electricity available for the transmission of signals, of which we have any account, was that of Lesage, a Frenchman, in 1774. From that time to the present there have been numerous inventions and experiments to effect this object ; and, from 1820 to 1850, there were no less than sixty-three claimants for different varieties of telegraphs. We will direct attention only to those of Morse, Bain and House, they being the only kinds use.d in this country. " During the summer of 1832, Professor S. F. B. Morse, an American, con- ceived the idea of an elective or electro-magnetic telegraph, and, after numerous experiments, announced his invention to the public in April, 1837. " On the iOth of March, 1837, Hon. Levi Woodbury, then Secretary of the Treasury, issued a circular requesting information in regard to the propriety of establishing a system of telegraphs for the United States, to which Professor Morse replied, giving an account of his invention, its pro- posed advantages and probable expense. At that time * he presumed five words could be transmitted in a minute.' " In 1838, the American Institute reported that Morse could telegraph the words ' steamboat Caroline burnt ' in six minutes. Now, a thousand such words are telegraphed in two minutes. " In 1844, Congress built an experimental line from Baltimore to Wash- ington, to test its practical operation. That line was soon continued on to Philadelphia and New York, and reached Boston the following year. Two branches diverge from this line, one from Philadelphia to St. Louis, 1000 miles, the other from New York, via Buffalo, to Milwaukie, 1300 miles long. One also, 1400 miles in length, goes from Buffalo to Lockport, and from thence through Canada to Halifax, N. S., whence there is a continuous line through Portland to Boston. The great Southern line, from Washing- ton to New Orleans, is 1700 miles long. Another, 1200 miles, running to New Orleans from Cleveland, Ohio, via Cincinnati. The best paying line, it is said, is that between Washington and New York, which, during six months of last year, transmitted 154,514 messages, valued at $68,499 ; and the receipts for the year ending July, 1852, were $103,060. The average performance of the Morse instruments is from 8000 to 9000 letters per hour. The cost 01 construction, including wire, posts, labor, -) ; and reacheJ its destination at a few minutes before noon (New Crleans time), and was published in the evening papers of both cities at the same hour. This is now a daily occurrence. " Through its instrumentality (we mean no pun) Webster's deatb was simultareuusly made known throughout the length and breadth of our land, and the next morning the pulpits from Maine to New Orleans were echoing in e'u'ogies to his greatness, and mourning his departure. " The great extent of the telegraph business, and its importance to the community, is shown by a statement of the amount paid for despatches by the associated press of New York, composed of the seven principal morning papers, the Courier and Knijuirer, Tribune., Herald, Journal of Commerce, Sun, Times and Express. During the year ending November 1, 1852, these papers paid nearly $50,000 for despatches, and about $14,000 *or special and exclusive messages, not included in the expenses of the association. "The difference between Morse's and House's telegraph is, principally, that the first traces at the distant end what is marked at the other ; while House's does not trace at either end, but makes a signal of a letter at the distant end which has been made at the other, and thus, by new machinery, and a new power of air and axial magnetism, is enabled to print the signal letter at the last end, and this at the astonishing rate of sixty or seventy strokes or brakes in a second, and at once records the information, by its own machinery, in printed letters. Morse's is less complicated, and more easily understood; while House's is very difficult to be comprehended in its operations in detail, and works with the addition of two more powers, one air, and the other called axial magnetism. One is a tracing or writing telegraph, the other a signal and printing telegraph. " The telegraphs in England are next in importance and extent to those in this country. They were first established in 1845, and there are about 4000 miles of wire now in operation. " The charge for transmission of despatches is much higher than in America, one penny per word being charged for the first fifty miles, and one farthing per mile for any distance beyond one hundred miles. A message of twenty words can be sent a distance of 500 miles in the United States for one dollar, while in England the same would cost seven dollars " 1190. THE ELECTRICAL FIRE ALARM. The principle of the electric telegraph has recently been applied to a very ingenious pie?e of mechanism, by which an alarm of fire may be almost instantly com- municated to every part of a large city. Wires, extending from the towers of the principal public buildings in which large bells are suspended, unite at a central point, where the operator is in constant attendance. On an alarm of fire in any locality, the watch or police of the district goes to a small box, kept in a -onspicuous place, which he opens, and makes a telegraphic communication to the central operator, who, immediately recognizing the signal and the district from which it came, gives the alarm, by making each bell in connexion with the telegraph strike the number corresponding with the district in which the alarm commenced. By this means the alarm is communicated simultaneously to all parts of the city. ThitJ ingenious application of scientific principles hap been in suc- cessful operation In the city of Boston long enough to prove its gieat value. ELECTKOTY PIS PKOCESS. 331 * 1191. THK ATMOSPHERIC TELEGRAPH. An ingenious appar atus, called " The Atmospheric Telegraph" has recently been constructed by Mr. T. S. Richardson-, of Boston, designed to send packages through continuous tubes by means of atmos* pheric pressure An air-tight tube being laid between two places, either under or above ground, a piston, called by Mr. R. a plunger, is accurately fitted to its bore, behind which the package designed to be sent is attached. The air having been exhausted from the tube by engines at the opposite end, the pressure of the atmosphere will drive the piston, or plunger with its load, forward to its proposed destination. This ingenious application of atmospheric pressure operates with entire success in the model, and 'has been also successfully tested in tubes that have been laid to the extent of a mile. Pa- tents have been secured for the invention in England, Franje, find other countries of Europe, as well as in this country ; and a company is now forming for testing the principle between the cities of Boston and New York. The air is to be exhausted from the tubes by means of steam-engines, and there are to be intermediate stations between those two cities. 1192. THE ELECTROTYPE PROCESS. This process, known by the various names electrotype, electro plating and gilding, gal- vanotype, galvano-plastic, electro-plastic and electro-metallurgy, is a process by which a coating of one metal is made to adhere to and take the form of another metal, by electrical agency. 1193. It is a process purely chemical and electrical, and the con- sideration of the subject pertains more properly to the science of Chemistry. As this volume has not professed to pursue a rigid classification, it may not be amiss to give this brief notice of the process. 1194. It consists in subjecting a chemical solution of one metal to electrical action with another metal. A solution of a salt or oxide, having a metallic base, forms part of the electric circuit, and, by the electrical action, the oxygen or acid will be drawn to the positive end of the circuit, while the pure meta) vill be forced to the negative pole, where it will either combine 14* 382 NATURAL PHILOSOPHY. with the metal or adhere to it, taking its exact form. The thickness of the coating of the pure metal will depend on tiw length of time that the body to be coated is subjected to the combined action, chemical and electrical. Hence a mere film or a solid crust may be attached to any conducting substance. When a substance not in itself a conductor is to be coated, it must first be made a conductor by covering its surface with some substance which will impart the conducting power. This is usually effected by means of finely-powdered black lead. 1195. When a part only of a body is to be coated by tho electrotype process, the parts which are to remain uncoate^ must previously be protected by means of a thin covering of wax, tallow or some other non-conducting substance. 1196. MAGNETO-ELECTRICITY. Mag- What is Mag- , A . .^ f . , , neto-electridty? neto- electricity treats of the development of electricity by magnetism. How is Mag- 1197. Electric currents are excited in a neto-electridty conductor of electricity by magnetic changes developed? . , . , . . . . . m , ?L taking place in its vicinity. Thus, the movement of a magnet near a metallic wire, or near an iron bar enclosed in a wire coil, occasions currents in the wire. 1198. When an armature, or any piece of soft iron, ia brought into contact with one or both of the poles of a magnet, it becomes itself magnetic by induction, and by its reaction adds to the power of the magnet : on the contrary, when removed from the contact, it diminishes the power of the mag- net, and these alternate changes in its magnetic state induce a current of electricity. 1199. The most powerful effects are obtained b ? cauain g a bar of soft iron ' enclosed in a ffects of mag- helix, to revolve by mechanical means near the wto-ekctncity p i eg O f a stee j ma g ne t. As the iron approaches the poles in its revolution, it becomes mag- netic; as it recedes from them, its magnetism disappear* ; *md MAGNETO- ELECTRICITY . this alternation of magnetic states causes the flow of a current of electricity, which may be directed in its course to screw- cups, from which it may be received by means of wires con- nected with the cups. 1200. TIIE MAGNETO-ELECTRIC MACHINE.- * Fig. 181 represents the magneto-electric ma- chine, in which an armature, bent twice at right angles, is made to revolve rapidly in front of the poles of a compound steel magnet of the U form. The U magnet, whose Explain 181. aorth pole is seen at N, is fixed in a horizontal position, with its poles as near the ends of the armature as will allow the latter to rotate without coming into contact with them. The armature is mounted on an axis, extending from the pillar P to a small pillar between the poles of the magnet. Each of its legs is enclosed in a helix of fine insulated wire. The upper part of the pillar P slides over the lower part, and can be fastened in tiny position by a binding screw. In this way the band con- necting the two wheels may be tightened at pleasure, by in- creasing the distance between them. This arrangement aLso renders the machine more portable. By means of the multiply- ing- wheel W, which is connected by a band with a small wheel on the axis, the armature is made to revolve rapidly, so that the magnetism induced in it by the steel magnet is alternately ;*&! NATUKAL PHILOSOPHY. ilfi&troyed and renewed in a reverse direction to the previous jne. When the legs of the armature are approaching the mag- net, the one opposite the north pole acquires south polarity, and the other north polarity. The magnetic power is greatest while the armature is passing in front of the poles. It gradually diminishes as the armature leaves this position, arid nearly dis- appears when it stands at right angles with the magnet. A* each leg of the armature approaches the other pole of the U magnet, by the continuance of the motion magnetism is again induced in it, but in the reverse direction to the previous one. These changes in the magnetic state of the armature excite electric currents in the surrounding helices, powerful in proper tion to the rapidity with which the magnetic changes are pro- duced. 1201. Shocks may thus be obtained from the machine, and, if the motion is very rapid, in a powerful machine the torrent of shocks becomes insupportable the muscles of the hands which grasp the handles are involuntarily contracted, so that it is impossible to loosen the hold. The shocks, however, are instantly suspended by bringing the metallic handles into contact. 1202. THERMO-ELECTRICITY. Thermo- What is Thar- , , . . f , - . . no-electricity? electricity expresses a form of electricity developed by the agency of heat. 1203. In the year 1822, Professor Seebeck, of Lorlin, dis- covered that currents of electricity might be produced by the partial application of heat to a circuit composed exclusively of soJ.id conductors. The electrical current thus excited has been termed Thermo-electric (from the Greek Thermos, wbich signi- fies heat), to distinguish it from the common galvanic current; which, as it requires the intervention of a fluid element, was denominated a Hydro-electric current. The term Stereo-electric current has also been applied to the former, in order to mark its being produced in systems formed of solid bodies alone. It is evident that if, as is supposed in the theory of Ampere, mag- nets owe their peculiar properties to the continual circulation of electric currents in their minute parts, these currents will <\)iue under the description of the stereo-electric currents TH UXMO ELECTRICITY.- -A8TKOM OMY. rfSfi 1204. From the views of electricity which have now been given, it appears that there are, strictly speaking, three state* of electricity. That derived from the common electrical ma- chine i& in the highest degree of tension, and accumulates until it is able to force its way through the air, which is a perfect non-conductor. In the galvanic apparatus the currents have a smaller degree of tension; because, although they pass freely through the metallic elements, they meet with some impedimenta in traversing the fluid conductor. But in the thermo-electric currents the tension is reduced to nothing ; because, throughout the whole course of the circuit, no impediment exists to its free and uniform circulation. 1205. If the junction of two dissimilar metals be heated, an electrical current will flow from the one to the other. 1206. Instead of two different metals, one metal in differen* Conditions can be used to excite the current. 1207. Metals differ greatly in their power to excite a cur- rent when associated in thermo-electric pairs. A current may be excited with two wires of the same metal, by heating the end of one, and bringing it into contact with the other. Thi? experiment is mostr-successful when metals are used that have the lowest conducting power of heat. 1208. Thermo-electric batteries have been constructed with sufficient power to give shocks and sparks, and produce various magnetic phenomena, indicative of great magnetic power ; but the limits of this volume will not allow a further consideration of the subject. 1209. ASTRONOMY. Astronomy treats What is Aslron- r> L , -\ i i -i ^i i omy i of the heavenly bodies, the sun, moon, plan- ets, stars and comets, and of the earth as a member of the solar system. 1210. The study of astronomy necessarily involves an acquaint- ance with mathematics, but there are many interesting facts, which have Leen fully established by distinguished astronomers, whicb ought to be familiar to those who huve neither the opportunity nor d8(.' NATURAL PHILOSOPHY. tJio leisu/e to pursue the subject by the aid of mathematical light. To such the following brief notice of the subject will not be devoid Df interest 1211. Some of the most distinguished men who Who are some have contributed to the great mass of facts and tf the most dis- laws which make up the science of Astronomy tinguished As- were Hipparchus, Ptolemy, Pythagoras, Coperni- tronomers ? cus, Tycho Brahe, Galileo, Kepler and Newton, The present century has added to this list many other* whose fame will descend to posterity with great lustre. 1212. Hipparchus is usually considered the father of Astronomy. He was born at Nicaea, and* died about a hundred and twenty-five years before the Christian era. He divided the heavens into con- stellations, twelve in the ecliptic, twenty-one in the northern, and sixteen in the southern hemisphere, and gave names to all the stars. He discovered the difference of the intervals between the utinn- nal and vernal equinoxes, and, likewise, by viewing a tree on a plain, and noticing its apparent position from different places of observation, he was led to the discovery of the parallax of the heav- enly bodies ; that is, the difference between their real and apparent position, viewed from the centre and from the surface of the earth. He determined longitude and latitude, fixing the first degree of lon- gitude at the Canaries. 1213. Ptolemy nourished in the second century of the Christian era. He was a native of Alexandria, or Pelusium. In his system he placed the earth in the centre of the universe, a doctrine univer- sally adopted and believed until the sixteenth century, when it was confuted and rejected by Copernicus. Ptciemy gave an account of the fixed stars, and computed the latitude and longitude of one thou- sand and twenty-two of them. 1214. Pythagoras was born at Samos, and his death is supposed to have taken place about five hundred years before the Christian era. He supposed the sun to be the centre of the universe, and that the planets revolved around him in elliptical orbits. This doc- trine, however, was deemed absurd until it was established by Co- pernicus in the sixteenth century. 1215. Tycho Brahe, a Danish astronomer, flourished about the middle of the sixteenth century. His astronomical system was sin- gular and absurd, but the science is indebted to him- for a more cor- rect catalogue of the fixed stars, and for discoveries respecting the motions of the moon and the comets, the refraction of the rays of light, and for many othei important improvements. To him, also, was Kepler indebted for the principal facts which were the basis of nis astronomical labors. 1216. Copernicus v~ ^ tre of & system O f heavenly now adopted. J bodies, called planets, which revolve around him as a centre. Secondly. The earth is one of these planets. Thirdly. Some of these planets are attended by satel- lites or moons, which revolve around their respective planets, and with them around the sun. 338 KATUBAL PHILOSOPHY. Fourthly. The size, distance and rapidity of motion of each of these planets is known to be different. Fifthly. The stars are all of them suns, with systems of their own, and probably many, if not all of them, having planets, with their moons revolving around them as centres. Sixthly. There is a central point of the universe, around which all systems revolve. Whatismeant 1221 F THE S LAR SYSTEM.-By the by the Solar Solar System is meant the sun and all the System? heavenly bodies which revolve around it. These are the planets with their satellites or moons, our earth with its moon, together with an unknown number of comets. What are 1222 - OF THE PRIMARY PLANETS. Those Primary bodies which revolve around the sun, with- Planete? out rev olving, a t the same time, around some other central body, are called Primary Planets. 1223. For many years the planets were con- Gwethenames u i of the eight sidered to be six in number only, and they were primary all, except our earih, named after the gods Oi planets. heathen mythology, Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In the year 1781, Sir William Herschel discovered another, to which the name of Uranus has been given; an<^ in the year 1846 an eighth was discovered, to which the nan e of Le Verrier was at first given, from a dis- tinguished Fr-^i ch astronomei, by moans of whom it was pointed out. It is now known by the name of Neptune. How many 1224. Besides these primary planets, it was minor pri- discovered, between the years 1800 and 1807, mary planets , / have been dis- that between Mars and Jupiter there were TOUT covered? smaller planets, of such diminutive size, compared with the others, that they were called Asteroids. Since the year 1845 one hundred and nine more have been discovered, so that ASTRONOMY. 339 there are now known to be no fewer than one hundred and thirteen asteroids, or minor planets, between the orbits of Mars and Jupiter. 1225. THE MINOR PLANETS. The following is a catalogue of the minor planets at present known, arranged in the order of their discovery, together with the other known planets of out solar system : Nime and Number by which the Minor Planets are known. Date of Discovery. Names of Discoverers. Sira. MERCUKY VENUS. THE EAETU. MARS. 1 Ceres . . . 1801.. Jan. 1 . Piazzi, of Sicily. 2. Pallas 1 802.. March 28... Gibers, of liromoii. 8. Juno 1804. .Sept. 1 Harding. 4 Vesta 1807.. March 29.. Olbers. 5. Astrea 6. Hebe 1845.. Dec. 8 1847. .July 1 1 1 en eke, of Germany. Ilencke. 7. Iris 8. Flora 1847.. August 13... 1847. .Got 18 Hind, of London. Hind. 9. Metis 1848.. April 26 Graham, of Ireland 10 Hy'eia 1849 April 12 11. Parthenope 1850.. May 11 ... De Gasparis. 12 Clio 1850 Sept 18... Hind 13. Egeria 1850.. Nov. 2 De Gasparis. 14. Irene 1851.. May 19... Hind. 15. Eunomia 1851.. July 29 De Gasparis. 16 Psyche 1852 March 17 De Gusparis. 17. Thetis 1852.. April 17 Luther, of Germany. 18 Melpomene 1852 June 25 Hind 19. Fortuna 20. Massilia 21 Lutetia 1852.. August 22... 1852.. Sept. 19 1852 Nov 15 Hind. Do Gasparis. 22. Calliope .. lS52..Nov 16 . Hind. 23. Thalia *.... 24. Themis 1852.. Deo. 15 1853 April 5 Hind. De Gasparis. 25 Phoca-a 1S53 April 6 26. Proserpina 1853 May 5 Luther. 27. Euterpe 28. Bellona lS53..Nov. 8 1 854.. March 1 Hind Luther. 29 Aiiiphitrite 1S54 March 1 80. Urauia 81. Euphrosyne 32 Pomona 1854.. July 22 1854.. Sept 1..:... 1854 Get 9 8 Hind. Ferguson, of Washington. Goldschmidt 33. Polhymnia 1854 .Get 28 ... Chacornac. 84 85 JUPITER. SATURN. URANUS 1855.. April 14 1855.. April 27. 1131 Chacornac. Sir William Ilerschel. NKPTUNE 1 1846.. Sept 23... -j Dr. Galle, of Berlin, by d'reo tion of Le Verrier, of Paris. The 112th asteroid was discovered in September, 1870. The 113th, in March ; the 114th, in July, 1871. The honor of many late discoveries of these bodies rests with Frof. Watson of this country. NATDKAL 1H1J/)S< >l'H V. What is the 1%2G. The name planet properly means tiifftrence be- a wandering star, and was given to this tween a planet c l ass o f the heavenly bodies because they and a star? ,, . , ., t , . ,. are constantly moving, while those bodies wfcich are called fixed stars preserve their relative posi- tions. The planets may likewise be distinguished from the fixed stars by the eye by their steady light, while ilic 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 ; arious sizes, and at different but immense distances from 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. Whatuniver- 1228. It has been stated, in the early pages sallow keeps of this book, that every portion of matter is at- ^r a ^f tracted b y y other p rtion ' and * tha bodies in their force of the attraction depends upon the quantity places? O y ma f( er an d the distance. As attraction is mutual, we find that all of the heavenly bodies attract the earth, and the earthr likewise attracts all of the heavenly bodies. It hr.s 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 BO nicely balanced by creative wisdom, that, instead of rushing together in one mass, they are caused to move in regular paths Ccalled orbits) around a central body, which, being attracted in different directions by the bodies which revolve around it, will its^f 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 dis- tances, and with different velocities. 1229. The paths or courses in which the P lanets move arounJ the sun are called their orbits. ASTKONOMY. All of the heavenly bodies move in conic sections,* namely, itf circle, the ellipse, tins parabola and the hyperbola. What 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 ^65 days and 6 hours. Our year, therefore, is of that length. 1231. The length of time that each planet take.* in perform- ing its revolution around the sun, or, in other words, the length of the year on ea>a planet, is as follows. (The fractional parts of the day are omitted.} In the same connexion will also be found the mean distance of each planet from the sun. -ind the time of revolution arcur.d its axis , or, in other words the length of the day on each. Length of the Year in Days. Mean distance from the Sun in millions of Miles. Length of lb Day in Hours and UfentM. 8T 86* 24 & 224 SQl 23 <\ EAKTH 365 95 24 00 MARS 686 145 24 89' 1 Ceres . .. 1.6^0 2. Pallas 1.6S8 8 Juno \ .5H2 4 Vosta 1,825 6. Hebe 1 Iris 8 Flora 9 Metis About 266 10 Hygeia Between 1,400 11 Parthenope and 2,100 12 Clio 13. Egeria 14. Irene 15. Eunomla _ 16 Psyche 1886 * Conic sections are curvilinear figures, so called because they can all be formed by cutting a cone in certain directions. If a cone be cut perpendicu- lar 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 cut parallel to the axis, the section will ba ' an NATUKAL PHILOSOPHY. Length of the Year in Days. Mean Distance from the Sun in millions of Miles. Lentil of iba Dav in HOM* aud A!i,ini 17. Thetis 1,430 "1 18. Mel ( H.nione 1,269 19. Fortima 1,896 20. Ma-^ilia . . 1,859 21. Lutetia 1,887 About 266 22 (;allinpe 1 S15 23. Thalia 1,571 24. Themis 2,037 25 Pliocaea 26. Proserpina 27. Euterpe 28. Bellona 29. Arnnliitrite 80. Urania 81. Enphrasyne 82. Pomona 83. Polhymnih. 85. f - JUPITER 4882 494 9 56' SATURN 10.759 906 10 1 URANUS 80 (JS6 1 S24 NEPTUNE 60 126 2856 Give an ac- count of Bode's law. The sun turns on its axis in about 26 days and 10 hours. 1232. There is a very remarkable law, dis- covered by Professor Bode, founded, it is true, 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 distance 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 92 millions of miles, and as that distance is represented in the progression by 10, it follows that the distance of Mercury is 7 * 6 of 92 millions, of Venus ^g, &c. Wh tied t 1233. It is to be observed, however, that before the discovery tne discovery of the minor planets, there was a of the minor very remarkable interval between the planet. 1 Jupiterj and that Bode > s law/ planets f to accord with the ^distance of all the other planets, ASTRONOMY. 343 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 them, has been the discovery of the one hundred and thirteen minor planets, all situated between the planets Mars and Jupiter. What opinion ^ ut tnese m i nor planets are so small, and their has been formed paths or orbits vary so little, that it has been con J ectured that the y originally formed one large and resplendent orb, which, by the opera- tion of some unknown 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 Gasparis, of Naples ; three by Chacornac, at Marseilles ; three by Luther, at Bilk, Germany ; two by f)lbers, of Bremen ; two by Hencke, of Dries^en, Germany ; two by Goldschmidt, at Paris ; and one each by Piazzi, of Palermo; Harding, of Lilienthal, Ger- many; Graham, at Mr. Cooper's private observatory, Markree Castle, Ireland ; Marth, of London ; and Ferguson, of Washington. 1235. The paths or orbits of the planets What ts the i -. iv ^ i shape of the * re no * exactly circular, but elliptical. orbits of the They are, therefore, sometimes nearer to p a 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. Qwe the rda- 1236 The relatiye size o f t h e srm t he live size of the , -, tun, moon, moon and the larger planets, as expressed by and primary the length of their diameters, is as follows : ylo/uets. 344 NATURAL PHILOSOPHY. Sun . . Moon . . Mercury . Venus . . Earth . . 852,000 . 2,153 . 2,962 . 7,510 7,912 Mars . . Jupiter . Saturn . Uranus . Neptune . 4,920 85,390 71,904 33,024 36,620 How large are the minor planets ? 1237. The size of the minor planets has been so variously estimated, that little reliance can be 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 arid time of revolution. Explain 1238. Fig. 182 is a representation of the com /<%. 182. parative size of the larger planets. Fig. 182. .Sir J. F. W. Herschel gives the following illustration of the com- parative 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 nmstafd-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 u |V:a, on a eirvle of 4ijO feel , Mars, a. rather largo pin's head, on a ASTKO.NUMY.
or tne rca * P atn * the earth. why is it so It is called the ecliptic, because every eclipse, whether of the sun or the moon, must be in or near it. 1240. The Zodiac is a space or belt, six- Zoctiac? ^ een degrees broad, eight degrees eacli side of the ecliptic. It is called the zodiac from a Greek word, which sig- nifies 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. Herso.hel, in his excellent treatise on As- tronomy, says : " Uncouth figures, and outlines of men and iiion- sters 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 alv-uird 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 ar- bitrary, 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, Le&nis? 'Beta Scorpii,' &c., by letters of the Greek alphabet attached to them. " This disregard is neither supercilious nor causeless. The con- stellations seem to have been almost purpose b nained and delineated 546 NATURAL PHILOSOPHY. to cause as much confusion and inconvenience as possible. Ir* numerable snaked twine through long and contorted areas of tho heavens, where no memory can follow them ; bears, lions, and fishes, large and small, northern and southern, confuse all nomen- clature, &c. A better system of constellations might have been a material help as an artificial memory." What arc the 1242. The zodiac is divided into twelve sign* of i he signs, each sign containing thirty degrees of many degrees in tuc g reat celestial circle. The names of these wch ? signs are sometimes given in Latin, and sometimes in English. They are as follows : Latin. English. I) Aries, The Ram. 2) Taurus, The Bull. 3) Gemini, The Twins. : I) Cancer, The Crab. |5) Leo, The Lion. '()) Virgo, The Virgin. Latin. English. (7) Libra, The Balance. (8) Scorpio, The Scorpion. (9) Sagittarius, ^ The Archer. (10) Capricornus, The Goat. (11) Aquarius, The Water-bearer. (12) Pisces, The Fishes. 1243. The signs of the zodiac and the various bodies of the solar system arc often represented, in almanacs and astronomical works, by signs or characters. In the following list the characters of the planets, &c.. are I ('presented. The Sun. The Earth. Ceres. < The Moon. Mara. $ Pallas. Mercury. g Vesta. % Jupiter. 9 Venus. cj> Juno. \i Saturn. $ Uranus. The following characters represent the signs of the Zodiac* \, Leo. $ Sagittarius. TTJJ Virgo. >J Capricornus. Libra. zz Aquarius. TH. Scorpio. X Pisces. Prom an inspection of Fig. 183 it appears that when the earth K Aries. y Taurus. U Gemins. IB Cancer. ASTRONOMY. ,54'7 a? rieeu from the SUD, is in any particular constellation, the sun as viewed from the earth, will appear in the opposite one. Hare the signs 1244. The constellations of the zodiac do not of the zodiac now reta j n ^ c { r original names. Each con- 3/ioayt remain- ;-',. . , ^/T i sd the same, stellation is about 30 degrees eastward of the and why? sign of the same name. For example, the con- stellation 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 dis- tinguishing 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 propei connexion. t 1245. The orbits of the other planets orbits of the T ' i i ,1 , c ^ ^ planets situated are inclined to that of the earth ; or, m with respect to other words, they are not in the same that of the earth? P lane ' Explain Fig. 183 represents an oblique view of the plane Fig. 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. Ejcplain 1246. Fig. 184 represents a section of the plane Fig. 184. O f 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 arc 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. 15 848 NATURAL PIJILOSOPHY. ASTRONOMY. When is a 1247. When a planet or heavenly body heavenly body . . ,, , ,, .^ ,., , . , w m tnat P art < lts orDlt which appears to constellation f fa near ail y p ar ticular constellation, it is said to be in that constellation. This, in Fig. 147, the comet of 1680 appears to approach the aun from the constellation Leo. . mat is meant 1248 - The perihelion* and aphelion* by the perihelion of a heavenly body express its situation with ? re ard to the sun - When a **% is nearest gee, of a heavenly to the sun, it is said to be in its perihelion- When furthest from . the sun, it is said to be in its aphelion. 1249. The earth is three millions of miles nearer to the flun 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. Where i the 1^50. The perihelia of the planets, an seen from 77- j the sun, are in the following signs of the zodiac, perineiionana namel Mercury in Gemini, Venus in Leo, the Irthf Earth in Cancer, Mars in Pisces, Vesta in Sagitta- rius, 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 V the inferior ,. .11 i , ^ and superior une wl ^" * ne eai> th and the sun as to pass conjunction and between them, it is said to be in its inferior opposition of a . ^ . , 11-1,1 . i vianet ? conjunction ; when behind the sun, it is said * The jhiral 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, fr<,m the earth 350 NATURAL PHILOSOPHY. to be in its superior conjunction ; but when behind the earth, it is said to be in opposition. 1252. The axes of the planets, in their revolution around the sun, are not perpen- dicular to thtir orbits, nor to the plane of the ecliptic, but are inclined in different degrees 1253. This is one of the most remarkable circumstances in the science of Astronomy, because it is the cause of the different seasons, What is the in- clination of the axes of the planets to the plane of their orbit t ? What causes the seasons ? What causes the differences in the length of the days and nights ? spring, summer, autumn and winter ; and 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 from the sun to either of the planets, this line passes over or describes equal areas in equal times. This line is called the ra- dius-vector. This is one of Kepler's great laws. Explain In Fig. 185, Fig. 185. let g represent the sun, and E the earth, and the ellipse or oval, be the earth's orbit, or path around the sun. By lines drawn from the sun at S to the outer edge of the .. figure, the orbit is divided ASTRONOMY. 351 into twelve areas of different shapes, but each containing the same quantity of space. Thus, the spaces 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, taat earl j in tne morning and late in 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 tht additional heat it is constantly receiving from tho 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 mUtfes^n Differ- the variet J of climate in different parts of the ent 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. What places will 1264 ' lf the axis f the earth were P er P en - nave 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 from 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, inclination of *"' owing" to the inclination of the earttis the earth's axis? a3: l s (/iaf we have, the agreeable variety ASTRONOMY. of the seasons, days and nights of different lengths ^ and that wisely -ordered variety of climate which causes S'O great a variety of productions^ and which has afford- ed so powerful a stimulus to human industry. 12GG. The wisdom of Providence is frequently displayed in appar- ent 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 ^f 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. Krp/ain 1268. Fig. 187 represents the earth. N S is the axis, or Imaginary line, around which it daily * ' ' Fig. 187. N is the north pole, S is the south pole. These poles, it will be seen, aie the extremities of the axis N S. CD represents the equator, which is a cir- cle around the earth, at an equal dis- tance from each pole. The curved lines proceeding from N to S are me- ridians. They are all circles sur- rounding the earth, and passing through ! he poles. These meridians may be multiplied at pleasure. The lines E F, I K, L M, and G H, are designed to reprcsei t Circles all of them parallel to the equator, and for this reason '.hey are called parallels of latitude. These also may be mul t'ipliei at pleasure. Bu*, in the figure the.se lines, which are parallel to the equator, 15* 556 ^ATURAL 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, 23^ degrees from the equator, is called the tropic of Cancer, and the circle L M is called the tropic oi 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 or 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 23^ south of the equator. Their verti- cal direction then again turns towards the equator Hence the circles I K and L M are called the tropics of Cancer and Cap- ricorn. The word tropic is derived from a word which signifies 10 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 thu 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 rea. 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 ex- tending eight degrees on each side of the ecliptic. Explain 1269. Fig. 188 represents the manner in which the . tg. lo . gun s h} nes 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, ABC I), the whit of the earth. The outer circle represents the ?odiac with ASTRONOMY. the position of the twelve signs or constellations. On the of June, when the earth is at D, the whole northern polar reg : on 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 Pig. 188. the line of perpetual light, no parr of that circle will be turned >rom the sun while the earth turns on its axis. To all placew, therefore, within the Arctic circle, it will be constant day. On the 22d of September, when the earth is at C, its axis is aeither inclined io nor from the sun, but is sidewise ; and, o! course, while one-half of the earth, from pole to pole, is enlight- ened, 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 NATURAL 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 th* 1 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. 1270. From the explanation of figure 198. What is meant by the Equinoxes lfc appears that there are two parts of its ore-it and the Sol- J n which the days and nights are equal all ovei 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 autumna? 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 sol- stice, and Capricorn the winter solstice. 1272 ' Da and niht are Caused b the r ta ' How are day and night cans- tion of the earth on its axis every 24 houi s. ed and what is It is d to that gide of the earth which k the reason of the . difference in towards the sun, and night to the opposite side. 'heir 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. A& the uortl* ASTitOA'UMY. 359 pole becomes less inclined, the days shorten, till on the 21st of De< eember it is inclined 23 J- degrees frmn the sun, when the day. are the shortest. Thus, as the earth progresses in its orbit, after Lhe days are the shortest, it changes its inclination towai ds the sun, till it is again inclined as in the longest days in the summer. Which of the 1:>73. As the difference in the length of the ^test^ffer- da ? S and the ni g hts > and the chan g e f the ence in its sea- seasons, &e., on the earth, is caused by the in- sons clination of the earth's axis, it follows that al] 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 some of the other planets and satellites are inhabited; and although it may he thought that some of them, on account of their immense distance from the dun, experience a great want of light and heat, while others are so near, and the heat consequently so great, that water cannot remain un 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 re- quired to adapt the fowls to the air, or the fishes to the sea. WhatiitheSton, 1275 ' F THB SUN. The Sun is a e-nd what is its spherical body, situated near the centre of gravity of the system of planets of which our earth is one. H^w much larger -1276. Its diameter is 853,000 English I* the earth tJian m ji eg w hich j s equa j to ^Qg diameters of the sun .-,., \Answer care- the earth ; and, as spheres are to each f uit y-] 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 $60 NATURAL PHILOSOPHY. around its axis in 25 days and 3 hours. This has been ascertained by means of several dark spots which have been seen with telescopes on its sui face. 127T. Sir Win. Herschel supposed the spots on the eun to be the dark body of the sun, seen through open- ings in the luminous atmosphere which surrounds him. 1278. It is probable that the sun,* like all the other heavenly bodies (excepting, perhaps, comets), is in- habited 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 at- mosphere of flame, which surrounds the sun, at a con- siderable distance above the surface. What is the zo- 1280. The zodiacal light is a singular phe- diacal light, and nomeuoii, accompanying the sun. It is a faint light which often appears .x> stream up from the sun a little after sunset and before sunrise. It appears nearly in the form of a cone, its bides being somewhat curved and generally but ill defined. It extends often from 50 to 100 in the heavens, and always nearly in the direction of the place of che 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 >f different sizes according to their respective distances. Fig. 189 affords a comparative view of his apparent magnitude, as seen from all except the smaller of the minor planets. * In almanacs the sun is usually represented by a -small circle, with the face of a raun in it : thus, ASTliONOMY. 3(51 Fig. 189. Apparent Magnitude uf the Sun as seen from thr> Platiclf NATUBAL PHILOSOPHY. Describe the 128& ^ F MERCURY. Mercury is the planet Mer- Dearest planet to the sun, and is seldom seen; ' ury ' because his vicinity to the sun occasions his being lost in the brilliancy of tie sun's rays. How many 1283. The heat of this planet is so great ^thlplawt that water cannot exist th ere except in a Mercury ? state of vapor, and metals would be melted. The intensity of the sun's heat, which is in the same pro- portion 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. ,, iii 1284. Mercury, although in appearance night may only a small star, emits a bright white iight, ^een? Ty ** ^ which it may be recognized when 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 tele- cury appear scope, Mercury appears with all the various when seen J . through a phases, or increase and decrease of light, with tslescope ? 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 time of its rotation on its axis has been estimated at about twenty-four hours. A8TKONOMY. #U# 1286. OF VENUS. Venus, the second planet Venus. p] anet j n or( J er f rom t h e smij j g fa Q nea reat 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 Luci- fer, 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 eastward of the sun, or evening star. Why is Venus 1288. Venus, like Mercury, presents to us never seen late a ll the appearances of increase and decrease at night ? ,, , . , , , n , 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 Ven us sometimes by the transit pass directly between the sun and the earth. ofapa ^ & t k e j r 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. 1290. The reason why we cannot see the stars and planets- in the day-time is, that their light is so faint compared with the 'ight of the sun reflected by our atmosphere. Describe the 1291. OP THE EARTH. The Earth OB Earth as a . . , .. . . . . , which we live is the next planet in the solar #64 NATURAL PHILObOPHY. system, in tkc 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 th t of an Mate sp heroid, the equatorial polar than the . ., , equatorial diameter being about twenty -six miles longer diameter of the than its olar diameter. earth? [Think before you It is attended by one moon, the diameter speak.] O f w hi c h 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 per- forms its revolution round the earth ; namely, in twenty- seven days and seven hours. 1292. The earth, when viewed from the Describe the earth as a moon, exhibits precisely the same phases that moon. t j ie moon d oes 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 mhitbitod. Although there is abundant reason for the belief that the pl'uiets are " the green abodes of life," there are many reasons to bel'eve thai" the moon, in Us i-rent-nt state, i? noitltor inhabits! n.r hubitaUc ASTitONOMY. 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, arid from which the earth cannot be seen. 1293. As this book may possibly incite the inquiry how it is th.it the astronomer is ible to measure the size and distances of those immense bodies tl.3 consideration of which forms the subject of Astronomy, the process will "here be described by which the diam- eter of the earth may be ascertained. 1294. All circles, as has already been stated, are divided into 36(1 degr es, and, by means of instruments prepared for the purpose, the c imber of degrees in any arc 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 bo exactly the length of one degree of the earth's circumference. Let him multiply that dis- tance by 3GO, 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 poJar. 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^he astronomer and the mathematician tell us, wonderful as it may appear, is neither bare assertion nor unfounded conjecture. What motions 1295. It has been stated that the earth re- have the inhabit- volves upon its axis every day. Now, as the ants of t/ie earth ^ . , t -r A AA -i c tl a~tl sa ear ^ n 1S about 25,000 miles in circumierenct\ planet? Sec, it follows that the inhabitants of the equator also, No. 1296. are carr i e( j around this whole distance in about twenty-four hours, and every hour they are thus cariied through space in the direction of the diurnal motion of the earth at the rate of ^ th 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 moons is Uranus or satellites, all of which were discovered by Sir William Herschel, and all of them revolve in orbits nearly perpendicular to that of the planet. Their motion is retrograde. w , . 1314. It appears to be a general law of sat- general law of ellites, or moons, that they turn on their axis the rotation of ^ n t j ie same ^ me i n w ]ii c fi they revolve around satellites f their primaries. On this account, the inhabit- ants 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 Fide 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 gee 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 nearer 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 Tnoon 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 tf ASTRONOMY. 37l CTraims some d.sturbances and deviations were observed hy astron. jniers in the motions of Jupiter and Saturn, which they could account for onl y on the supposition that these two planets were in duenced 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 ^^ ^ F NEPTUNE. The discovery of the discovery of the planet Neptune (named originally Le Verrier, planet Neptune ? fi . om itg discovererj in 1846) is one of the g rea test triumphs which the history of science records. As certain per- turbations of the movements of Saturn led astronomers to sus- pect the existence of a remoter planet, which suspicions were fully confirmed in the discovery of Uranus, so also, after the dis- covery of Uranus, certain irregularities were perceived in his motiong, that led the distinguished astronomers of the day to the belief that even beyond the planet Uranus still another undis- covered planet existed, to reward the labors of the discoverer. Accordingly Le Verrier, a young French astronomer, urged by his friend A tag, determined to devote himself to the attempt at discovery. With indefatigable industry he prepared new tables of planetary motions, from which he determined the pertur- bations 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, be- fore it had been seen by a human eye. On the 18th of September 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 friend 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 follow- ing evening it was found to have moved in a direction and with a velocity very nearly like that which Le Verrier had pointed out Che planet wa t s found within less than one degree of the place 16 J72 NATUKA.L PHILOSOPHY. where Le Verrier had located it. It was subsequently ascer- tained that a young English mathematician, Mr. Adams, of Cambridge, had been engaged in the same computations, and had arrived at nearly the same results with Le Verrier. 1317. 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 iniJes distant ? 1318. In conclusion of this brief notice of the planets, a plate is here presented showing the relative appearance of the planets is viewed through a telescope. It will be observed that the planets Mercury and Venus have similar phases to those of our uioon. Pig 190 Relative Telescopic appearance of the Planets. , 5 a 1319. OF COMETS. The word Comet is de- ? rived from a Greek word, which means hair; and this name is given to a numerous class of bodies, which occa- sionally 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 aame is derived. Some comets appear to consist wholly ASTKONOMY. 373 af tftis hazy or hairy appearance, which is frequently ealloci the tail of the comet. Fig. 191 Comet of 1811, one of the most brilliaut of modern times. Period, 2888 years. 1320. In ancient times the appearance of comets was regard- ed with superstitious fear, in the belief that they were the fore- runners 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 a 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 calculated 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 from cae-t to wet?t, or from we?t to east ; and the places in tli3 374 NATURAL P11ILOSOPII1. 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 Japiter : that 50 of these comets moved from east to west ; that fcheir 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 orbits 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 ob- served at Palermo, in 1V70, 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 aa it approaches the sun, and goes before it when the 3omet recedes from tho sun. Newton, and some other astronomers, considereo the tails of comets to be vapors, produced by the excessive heat of the sun. Others have supposed them to be caused by a re- pulsive influence of the sun. Of whatever substance they may be, it is certain that it is very rare, because the stars may bo distinctly seen through it. 1325 The tails of comets differ very greatly in length, &8T1CONOM1'. and some are attended apparently by only a small cloudy light, while the length of the tail of othsrs has been esti- mated at from 50 to 80 millions of miles. Kg. 192 The comet of 1680, observed by Newton. Eapidity of its motion around the sun, a million of miles in an hour. Length of tail, 100 millions of miles. Period, 600 years. It has never re- appeared. 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 aoi cause them to make any perceptible deviation from their 37<3 NATURAL PHILOSOPHY. accustomed paths round the sun. It has been ascertained tlt&i 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 raoiv tails, and the great comet of 1744 had six Fig 193 The great comet of 1744. ASTRONOMY. 377 J8 4 27. Many comets escape observation because tb-.y traverse fhat part of the heavens only which is above the borizon in the Jay-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 sim, 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 appealed last in the fall of 1835, and presented diilererit aj> Halley's comet, as seen by (Sir Joiiu iierscheJ, October 29th, 1835. Da- changeable in its appearance. First recognized by Halley in 1682. Period, 16 years. 378 ^"ATUKAL PHILOSOPHY. pearances from (113*61601 points of observation. That of Enck is about 1200 days ; tliat of Biela, about 6 years. This last comet appeared in 1832 and in 1838. Pig. 195. Halley's comet, as seen by Struve, Oct. 12th, 1835. in 1682. Period, 75 years. First seen by Halley 1329. The comet of 1758, the return of which was predicted oy Dr. Halley, was regarded with great interest by astronomers, because its return was predicted. 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 ASTRONOMY. 37 y month now elapses without the appearance of a comet in our system. But it is now known that they are bodies of such ex- treme rarity that our clouds are massive in comparison with them. They have no more density than the air under an ex- nausted 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 solai 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 or 1856. ^ The following interesting details in relation to a comet expected in 1856 were given by Babinet, an em- inent 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 ia 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 star, when a learned calculator of Middlebourg, M. Bomrne, 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 prepar- atory labors of Mr. Hind, has revised all the calculations and esti- mated 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 of the comet in August, 1858, with an uncertainty of two years, more or less ; so that from 1856 to 1860 we may expect the great comet which was the cause of the abdication of the Em- peror Charles V., in 1556. " It is known that, partaking of the general superstition, which interpreted the appearance of a comet as the forerunner of som** fatal event, Charles V. believed that this comet addressed its menaces particularly to him, as holding the first rank among sovereigns. The great and once wise but now wearied and shattered monarch, had been for some time the victim of cruel reverses. There were threat- ening indications in the political, if not in the physical horizon, of a still greater tempest to come. lie was left to cry in despair, ( For- tune abandons old men.' The appearance of the blazing star seemed Co him an admonition from Heaven that he must cease to bo a sov- if he would avoid a fatality from which one without author- 16* 380 NATURAL PHILOSOPHY. 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, hari 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 Nor- mans. It infused universal terror into the rninds of the people, and contributed not a little towards the submission of the country after the battle of Hastings, as it had served to discourage the soldiers of Harold before the combat. 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 Eng- land by William, Duke of Normandy. It is supposed to have been executed by Matilda, the conqueror's wife, or by the Empress Ma- tilda, 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 ara 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 Mus- sulmans 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 singu- larly 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 30 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 8tars are no longer objects of terror. The theories of Newton, Halley, and their successors, have completely destroyed the imag- inary 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 materiaJ 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 jrosw a cloud A hundred thousand millions of tmis lighter than our at- mosphere, 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 reassuring. ' ' * What are the 1332 ' F TIIE FlXED S ' rARS ' ~ The Fixed Fixed Stars Stars are all supposed to be immensely large suppose*/ to be? bodieg; like our Qwn ^ shining by their c.wn liglit, which they dispense to systems of their own , 1383. They are classed by their apparent fixed stars magnitudes, those of the sixth magnitude being classified? the smallest tnat can be geeu by the naked eye. Stars which can be seen only by means of the telescope * THE COMET OP 1853. Mr. Hind, in a letter to the Dmdon Times, give? 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 bead. 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 cou'd be readily com- pared with the comet. The tail proceeds directly from the head in a single stream, an] not, as sometimes remarked, in two branches. The distance of this body irom 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 'ength 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 Sep- tember, and it will be worth while to look for the comet in the day-time about that date ; for this purpose an equatorially mounted telescope will bo 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 i'ccus on the planet Venus. This comet was discovered on the 10th of June, by Mr. Kliukenfues, 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 Times a few days since, Sir William Hamilton hints at the possibility of this being the comet I had been expecting; but I avail myself oi the present opportunity of stating that such is not the case, the elements of tho orbits having no resemblance. The comet referred to will probably reappeir between the years 1858 and 1861 ; and, if the perihelion passage takes place during the summer months, we may expect to see a body of tkr u\-re imposing aspect than the one at present visible." ,W2 NATURAL PHU are caLed telescopic stars. Th >y, 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 ., ", . , . ~ A v, -, . , second magni- th e second magnitude, fifty ; of the third. tude f two hundred. The number 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 calcu- have had their lated> rpj^ distance is so immense, that distances very .11. nearly ascer- light, travelling with the inconceivable tainedf velocity of nearly two hundred thousand miles in a second of time, from Sirius, is more than twenty 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 revo- lution, its motion couid be perceived in one year ; and in ten years its velocity can be computed, and its period wiU become known in the lifetime of a single observer. Who first di 1337. The stars are the fixed points k tided the stars which we must refer in observations of the niotions of a11 the heavenly bodies. Hence 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 fal] of empires. ASTRONOMY. ^Scl What probably iS38. It is generally supposed that part, causes the dif- ^ llot a u of t } ie c iiff ereilce i n the apparent fcrenceinthe . ' , . . * {_ ... ^parent size magnitudes ol the stars is owing to the dif- ofthe stars? ference in their distance. 1339. The distance of the stars, according to Sir J. ilerschel, 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 mo- tion would not perceptibly change their relative situation in two or three thousand years. Some have been noticed alternately to ap- pear and disappear. Several that were mentioned by ancient as- tronomers are not now to be seen ; and some are now observe* 1 , which were unknown to the ancients. 1841. Many stars which appear single to the naked eye, when viewed through powerful telescopes, appear double, treblo, and evei. quadruple. Some are subject to variation ir.. their apparent magni- tude, 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 noith-east to south-west. It is known to consist of an immense number of stars, which, from their apparent nearness, cannot be distin- guished 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 deld of his telescope, while it was directed to the milky way. 1344. The ancients, in reducing astronomy to a sci- ence, formed the stars into clusters, or constellations, to which they gave particular names. 1345. The number of constellations among the ancienta \vas about 50. The moderns have added about 50 more. 1346. Oar observations of the stars and nebulae are confined principally to those of the northern hemisphere. Of the tious near the south yole we know but little. NATUliAL PHILOSOPHY. What effect 1847. In determining the true place of any ] a$ h^on r the of the celestial bodies > the refractive power of length oj tht the atmosphere must always be taken into **y* consideration. This property of the atmo- sphere 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 celrstial 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. Wh ar the 1349. The stars, and other heavenly bodies stars never are never seen in their true situation, because seen in their ^ motion of light is progressive ; and. during true position ? . 6 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. Wliat is meant ^ 35 * ^ ence ' a ra y ^ ^8 nt P assin g through by the aberra- the centre of the telescope to the observer's Hon of light * e ^ Q ( j oeg not comc i(j e w j t h 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 P^lar Star ? ax j g causes the whole sphere of the fixed stars, &c., to appear to move round the earth every twenty- four 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 r/f the southern hemisphere there is another and a cor- responding point in the heavens. What is the 1852. Certain of the stars surrounding the dtde- of per- UOT t\ l po le never set to us. These are in- petual appar- , 1,7.11 ition and of eluded hi a circle parallel with t!;o equator, ASTKONOMY. 380 to ^nd in every part equally distant from tiro Citation? 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 oo- cullation. Some of the constellations of the southern hemisphere are represented as inimitably beautiful, par- ticularly the cross. What is par- 1353. The parallax of a heavenly body allax* ig the angular distance between the true and the apparent situation of the body. Describe 1354. la Fig. 196, A G B represents the earth f \g. 196. am i (j the moon. To a spectator at A, the moon i would appear at F; while to another, at B, the moon woul;! 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 vicvTed 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 thf 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 tc make the necessary calculations to determine the true situation of the body. Allowance, also, must be made for the refraction jl the atmosphere. <*6fc NATURAL PHILOSOPHY. lions of miles every year. This will give him, at the same time, *: 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 one ever knew what it is to be without them. We can- not 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 oi motion and a state of rest. What would 1297. The rapid motion of a thousand miles in le the conse- an hour is not sufficient to overcome the centri- yutnce if the peta i f orce cause( i by gravity ; but, if the earth sarth should r J \ ? revolve on its should revolve around its axis seventeen times in axis once in a day, instead of once, all bodies at the equator would be lifted up, and the attraction of gravita- tion 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 flatiet Mars, the planet Mars. It is conspicuous for its fiery-red appearance, which is supposed by Sir Johi) llersclier* to be caused by the color of its soil. * Sir John Herschel is the son of Sir "William Herschel, the discov^rr oi' *he piauet Uranu. ASTRONOMY. 36? The degree of heat and light at Mars is less than half of that received by the earth. 1300. OP THE MINOR PLANETS. It. has already been mention ed that between the orbits of Mars and Jupiter one hundred and thirteen 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 aLd Jupi- tei. 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 Ed'm. Encyc., art. ASTRONOMY.) Dr. Brewster further supposes that the bursting of this planet may have occasioned "the phenomena of meteoric atones ; that is, stones which have fallen on the earth from the atmosphere. Describe the 1301. OF JUPITER. Jupiter is the largest planet Jupiter. p i anet O f tm3 so i ar gj ste m, and the most bril- liant, except Venus. The heat and light at Jupiter art 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 aiford considerable light to the inhab- itants 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. 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. an d A B C D the moon in different parts of hei Via, 1*7 orbit When the moon is at A, its dark side will be towards the earth, its illuwvtated 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 fide coming in sight, it appears as represented at b. and is said to be horned. When it arrives at C, 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 quad' At D its appearance is as represented at d, and it is to be gibboits. At E all the illuminated side is towards .is, aud wo have a full moon. During the other half of it* ABi'KO^OMY. revolution, less and less of its illuminated side is seen, till it again becomes invisible at A. Wfiat is (he 1365. The mean difference in the rising of the mean differ- moon, caused by its daily motion, is a little lusr ruing of the ^ an an ^ our - But, on account of the different moon from day angles formed with the horizon by different parts to day . Q f tne ec iipti C) 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 con- venience to the husbandman and the hunter, in- ^the l Harvest asmuca as li affords tn ein light to continue their and the Hunt- occupation, and, as it were, lengthens out thei/ er'sMoon, and day, the first is called the harvest moon, and the occur t 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 tle different positions and phases of the moon. } or }) denote the moon in the first quadrature, that is, the quad- rature between change and full ; C or < denotes the moon in the last quadrature, that is, the quadrature between full and cnange. $ denotes new moon ; Q denotes full moon. 1367. When viewed through a telescope, the surface of the moon appears wonderfully diversified. Large dark spots, sup- posed 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 whicV- 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 re- semble what would be the appearance of the earth at the m >OP 390 NATURAL PHILOSOPHY. were all the seas and lakes dried up. Some of the mountain* are supposed to be volcanic. What are the 1368. OF THE TIDES. The tides are the Tides f 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. Explain 1369. Let M, Fig. 198, be the moon revolving in Fig. 198. her orbit ; E, the earth covered with water ; and S, Fig. 196. 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 re- spective 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 propoi- tion 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 in the direction of the IDOOD. ASTEOKOMT. 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 29 from the equator, and is generally much less. Hence the waters about the equator, being nearer the moon, are more strongly attracted, and fche 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. Explain But when the moon is in its quarters, as in Fig. 199, Fig. 199. the sun and moon being in lines at right angles, tend Pig. 199. 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 prog- ress 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 392 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 fre- quently 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 con- stantly 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 ob- What is an ,. ,, , 1111,1- Eclipse ? scuration of one heavenly body by the interven- tion of another. The situation of the earth with regard to the When does an ,1^.1 . , -, 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< Thoge of fhe gun take p]ace wheQ ^ 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 . . , . , , , not an eclipse at samc plane m which the earth revolves around every new and the sun, that is, in the ecliptic, it is plain that full moon? ^ gun wou i ( j b e eo iip sec i a t every new moon, anl the moon would be eclipsed at every full. For, at each of these times, these three bodies would be in the same stiaight line. But the moon's orbit does not coincide with the ecliptic, but is inclined to it at an angle of about 5" 20'. Hence, since the apparent diameter of the sun is but about a degree, and that of the moon about the same, n } eclipse will take place at ASTKONOMT. 393 new or full moon, unless the moon be within ^ a deg/ee of the ecliptic, that is, in or near one of its nodes. It is found tbit if the moon be within 16 of a node at time of change, it will be so near the ecliptic, that the sun will be more or le&a 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 more eclipses of oftener within 16^ at the time of new moon, ^moo^ini than withiu 12 at the time of ful1 ; cons 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 E*P lain Ft 8- than of the moon, yet more eclipses of the inoon 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 a') 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 whoa it is visible, Some- times, when the moon is at its greatest distance, its shadow, 394 NATURAL PHILOSOPHY. m., terminates before it reaches the earth. In eclipses of this kind, to an inhabitant directly under the point 0, the outermo*'. edge of the sun's disc is seen, forming a bright ring around the moon ; from which circumstance these eclipses are called annu- lar, from anrndus, a Latin word for ring. Besides the dark shadow of the moon, m O, 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 to D, immediately on emerging from the dark shadow, m, he would see a small part of the sun ; and would con- tinually 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 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 re mote from 0, and nearer G or D. Though the penumbra be continually increasing in diameter, according to its length, 01 the distance of the moon from the earth, still, under the most favorable circumstances, it falls on but about half of the illu minated hemisphere of the earth. Hence, by half the inhab tants on this hemisphere, no eclipse will be seen. 1374. Fig. 201 represents an eclipse of th Explain Fig. mooilt The j nstant tlie moon enters the earth's shadow at x, it is deprived of the sun's light Fig. 201. A.Sl'JtiUNOMY. Ji and is eclipsed to all in the un illuminated 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 par- ticular placo, as Boston, see more eclipses of the moon than of the SUD, 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 tho nature of these eclipses. 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 appear- ance ; and, consequently, this change is visible at the same time to all to whom the moon is visible ; that is, to a whole hemis- phere of the earth. 1375. The earth's shadow (like that of the moon) is encom- passed by a per^jaDra, C R S D, which is faint at the edges towards R and S, but becomes darker towards F and Gr. 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 end- ing of a solar eclipse may be determined almost instantaneously. 1376. The diameters of the sun and moon KgJ* ^ap- are su PP sed to be divided into twelve P* ] plied to 'eclipses parts, called digits. These bodies are said tc of the sun and have as many digits eclipsed as there are of H'ttenuion? those parts involved m darkness NAIUKAL PiiILO3OPHl. 1377. There must be an eclipse of the sun fk sften, al least, as the moon, being near one of its nodes, comes between the aun 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 phe nomenon. June 16, 1806, a very remarkable total eclipse took place at Boston. The day was clear, and nothing occurred to prevent accu- rate 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 efforts of observation. The tiret 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 calcu- ference 'between ^ ate( ^ *>y tne 8un ' ' lt is calle( l solar time ? an( ^ the solar and the the year a solar year ; but when it is calcu- ndereal ear ? gidereal and the year a sidereal year. The sidereal year is 20 min- utes and 24 seconds longer than the solar ^ :ar. 1380. The solar year consists of 365 days, 5 hours, 48 minutes, and 48 seconds; sidereal but our common reckoning gives 365 days by 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 ^e derive the name of bissextile for the leap v^>. ASTKONOMY. b97 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 equi- nox, 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. What is the pre- 1382 - Ever J equinox occurs at a point, cession cj the 50" of a deg. of the great circle, preceding the place of the equinox, 12 months before j and this is called the precession of the equinoxes. It is this circumstance which has caused the change in the situ- ation 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 pro- duce 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 ^ ut * ur P eri ds m which the sun and a sun and dock perfect clock will agree. These are the agree? 15th of ^prf], the 15th of June, the 1st oi September, and the 24th of December. What is the 1384. The greatest difference between greatest dif- true and apparent time amounts to between s i xteen an d seventeen minutes. Tables of apparent equation are constructed for the purpose of * f pointing oat and correcting these differences 398 NATURAL PHILOSOPHY. between solar time and equal or mean time, tne denomina- tion 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 con- stellation, the following directions are subjoined. There is always to be seen, on a clear night, a beautiful clus- ter 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 1 wain, or wagon. Fig. 202 represents these seven stars ; K B- aoa - b a g represent the four, and e z B \ the other three stars. Perhaps they may more properly be called a large F \\ dippei of which e z B represent the j \ handle. If a line be drawn through the I '* . a stars I *nd a, and carried upwards, it ( j will pass a little to the left, and nearly +/z e i + touch a star represented in the figure by / 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 & steady and rather dead kind ol 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 constella- tion. Thus, if we conceive a line drawn from the star z, leaving B ASTKONOMT. 399 a little to the left, it will pass through the very brilliant star A. By looking on a celestial globe for the star 2, 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 &, and extended some distance to the right, it wiU pass just above another very brilliant star. On referring to the glcue, we find it to be Capella, or the goat. In this manner the student m.ay "W~n->. acquainted with th appearance of the whole heavens. 400 NATURAL PHILOSOPHY. i 1.2 l! CO O . 00 CO . t^ 10 [9 1 1 i I i 1 1 s s S g i 1 I S | 8? S S W rf III I 00 l^ c* ot t- t- 'ji co *- s ^ J s e i H S ASTRONOMY 401 - & s il pu jo t- eo b ?H ?-i c* cb t- lo. o> ab S O t; CO I I I I I I I I e s 111 8 &:" S 8 -* 5 55 g S ; b 9- S I I 8 40V NATURAL "HILOSOPHY. I .000* 5 'CO 10 CO T*H J(MrHlOlOlO rHC The same weight lying upon the Energy differ ground, although it exerts a pressure of 100 from pressure? ,, , _, n IDS., has no Energy, because, until it is lifted, it has no power to perform work. 1396. Power in Mechanics signifies ability Define Power. J to perform a certain amount of work in a given time. The most common unit for measurement of power is the Horse Power. It is employed in comparing the efficiency of engines, water-wheels, and other motors. Define Horse 1397. An engine of one-horse-power is capa- Power. big O f ra i s i n g 33 ? 000 pounds one foot high in one minute, or, in other words, of performing 33,000 units of work in a minute. An engine which can perform twice as much work in a minute, or the same amount in hah 01 a minute, is a two- horse-power engine. 1398. This unit of measure for engines was first employed by James Watt when steam-engines were first employed in the place of horses in working pumps. It was then supposed that this was an average of the power of working-horses. It is now known to be much too high. A careful series of experiments by better methods has made known the fact that two-thirds of the above amount, or 22,000 units of work in a minute, is more nearly the power of a horse of average strength ; but the term, when applied to motors, implies the amount as first estimated in the time of Watt. The strength of a man being much less than that of a horse, 'his power to perform work is of course proportionately less. The power of a man varies considerably with the method by which he applies his strength. A man of ordinary strength lifting weights with his hands can perform about 1500 units of work per minute, and continue it eight hours a day ; by working at a vertical crank or windlass, he can perform 2500 units of work per minute ; when in good position for rowing, so that the muscles of the back, arms, and legs are all well employed, he can perform 4000 units of work per minute, and continue it for several hours. In trials of strength 406 MECHANICS. which are to continue but for a few minutes, this amount of work is greatly exceeded. Steam-engines vary from less than TOO to more than 1000 horse-power. g ow ft 1399. The number of Horse Power of any Horse Power engine or other motor is calculated by mul- tiplying together the number of pounds of force exerted by the distance in feet per minute, and dividing by 33,000. EXAMPLES FOR SOLUTION. 1400. (1) What must be the power of an engine to raise a block of stone weighing 2000 pounds to a height of 40 feet in one minute ? Ans. 2000 x 40 -r- 33000 - 2 T 4 is ihQ wei S hfc of an e( l ual bulk of water. Bepeat the Rule 1434 To find its s P ecific f aYU 7> then > for specific weigh it in the manner described on page 126, and divide its true weight by its loss in water. What is 1435. A body lighter than water is pressed Buoyancy? upward when immersed by force equal to the difference between the weight of the body and the weight of an equal bulk of water ; this force is called the buoyancy of the body. What is the 1436. The specific gravity of such a body ^{%ofK$t is found by dividing the weight f the body solids ? . by its weight added to its buoyancy. The buoyancy is found by determining how much weight is required to sink the body. (See page 127.) Define 1437. Hydraulics treats of the laws gov- Hydraulws. erning liquids in motion, and of the useful application of these laws in the employment of water as a motive power, the water supply and drainage of cities, and the improvement of rivers and harbors. How is water em- !438. Water is employed as a motive ployed for power* power k y utilizing the energy of its fall as it descends the rivers on its way to the sea. MECHANICS. 419 1439. The water power of running streams is made useful by selecting a place on the stream where the fall is sufficiently rapid, and, if necessary, building a dam to secure a vertical fall. From the store of water thus held back in the stream, an artificial channel or flume conducts the water to the water-wheel. Mention the ' different forms of Water-wheels. When is an Overshot Wheel used f 1440. Water-wheels are of four different kinds: Overshot, Undershot, Breast and Turbine. (See page 82.) 1441. The Overshot Wheel is employed when the stream is small and the fall high. The flume is continued Fig. 215. out over the wheel, as represented in the figure, and the buckets are filled in succession. The wheel being overloaded on one side, turns with a force proportioned to this extra weight. The whole force of the falling water can never be util- ized by this means, even if all the water of the stream falls into the buckets, for, as may be seen from the figure, a por- tion of the water falls from the bucket again before it has reached its lowest point. Fig. 216. 1442. The Breast Wheel is adapted to a larger supply of water and a lower fall. The water is re- ceived at about half the height of the wheel in buckets or upon floats. If the water comes upon the buckets above the centre, it is called a high-breast wheel ; if below, a loiv- 18 420 MECHANICS. breast wheel. The loss of power occurs here as in the Overshot Wheel neither of them affording more than 75 per cent, of the real power of the stream. The Undershot Wheel is a ruder What is said of the Undershot kind of motor ; it is furnished with floats, Wheel? n . . .. , or flat projecting surfaces only, against Fig. 217. which the running water impinges, and, by aid of the momentum acquired in its previous descent, affords power to turn the wheel. Only 20 per cent, of the power of the water is ren- dered effective by this kind of wheel. What is said of 1444. The Turbine Wheel is by far the the Turbine? best of all the hydraulic motors. It 13 readily made to utilize from 80 to 90 per cent, of the power of the stream. 1445. It is placed hori- zontally, and is entirely immersed in the water. Fig. 218. Fig. 218 represents a plan of the wheel. Fig. 219 is an elevation or verti- cal section of the same wheel. The flume conducts Describe Figs, the water vertically upon the wheel ; the size 218 and 219. O f |h e fl ume \ s equal to the size of the cen- tral portion of the wheel marked a a a in Fig. 218 and MECHANICS. 421 G G in Fig. 219. The buckets are shown at vvv. The spiral lines marked a a a represent curved partitions or guide-curves fixed in the flume, and form no part of the wheel ; they serve to give such direction to the descending current as to enable it to act to the best advantage on the buckets. 1446. It will be seen by examining Fig. 219 that the water de- scending the flume D G is prevented from passing directly down- ward by the lower plate HUH, and has no other outlet than the space between P and H, where the buckets are placed. The shaft E E turns with the wheel, and affords the means of communication with such machinery as the wheel is designed to carry. The above figures represent the Fourneyron Turbine, so called from the inventor. 1447. There is another variety known as Centrevent Turbines, from the fact that the water flows in at the circumference, and escapes at the centre. Fig. 220 represents one of this class. The water is conducted from the vertical flume C by a horizontal con- duit at the bottom, entirely around the wheel. The only escape 422 MECHANICS. for the water is between the buckets and cut at the central open- ings marked / at the top and bottom of the wheel. Fig. 220. 1448. Turbine Wheels are always made of iron, and are so accu- rately made that no sensible leakage occurs between the fixed and movable portions (as at G, Fig. 219). 1449. Turbines are much smaller than other water-wheels de- signed for the same streams. It is not uncommon to replace an overshot wheel twenty feet in diameter and six feet wide by a tur- bine only three feet in diameter and six inches deep, and obtain greater efficiency by the change. What is the 1450. It should be carefully remembered U yidded^by e a that no Und of hydraulic motor can afford stream? any more power than is due to the weight of the water yielded ly the stream descending through the height of the fall. How is the power of a 1451. The theoretical horse-power of stream calculated? a stream is found by a survey which MECHANICS. Fig. 221. measures the amount of water flowing by a particular point in one minute, and the amount of fall that can be made available. The number of pounds per minute multiplied by the number of feet fall, and the product divided by 33,000, determines the entire horse-power developed by the stream ; the percentage of this amount utilized depends upon the kind of wheel used. Describe 1452. Barker's Mill, represented in Fig. Barker's Mitt. ^21, is frequently employed as a motor. If water be supplied so fast as to keep the central tube DC filled, there will result a hydrostatic pres- sure within the curved arms at the bottom. This pressure is in all directions ; the arms be- ing opened so as to dis- charge the water in one direction from each arm, the pressure is relieved on that side, but remain- ing on the opposite, it gives motion to the arms. 1453. It is popularly be- lieved that Barker's Mill runs by aid of a push of the outflowing jet against the air ; but the fact that it runs better under an exhausted receiver than in the open air, serves to dispel this impression! Small Barker's Mills are made in order to show this experiment. (See Fig. 98) are cities sup- 1454. The water supply of cities is ac- uiih water? complished by providing a reservoir of 424 MECHANICS. water higher than the dwellings, and from this reservoir distributing the water through the streets in iron pipes. In some cities, as Boston and New York, the water comes from natural sources several miles off, but high enough to answer the purpose of distribution. In Brooklyn, Philadel- phia, and Chicago the water is pumped up into reservoirs which have the required height. 1455. The distribution-pipes are laid about the streets at a suffi- cient depth to provide against freezing. The sizes for different dis- tricts are fixed in accordance with the known principles of Hydrau- lics. The water can, of course, rise no higher in pipes in the houses than the surface of the water in the reservoir. It very rarely rises as high, in consequence of the constant flow of water out of the pipes at lower levels. The friction of the sides of the pipe and the loss of force by change of direction at the bends, are serious impediments to the flow of water through pipes. Fig. 222. 1456. Fig. 222 represents a reservoir and a foun- Fig. 222. tain supplied by it. If a pipe were extended high enough, from the centre of the pond, in place of the foun- MECHANICS. 425 tain, the water would slowly rise to the height of the water in the reservoir. The fountain jet will not rise so high, both on account of the friction of the pipe and resistance of the air. The farther the fountain is from the reservoir, the less will the height of the jet be, because the friction is proportioned to the length of the pipe. 1457. When a liquid is allowed to flow through an ori- fice in the side or bottom of a vessel, it is found by experi- ment that the shape of the outlet influences very largely the rapidity of the discharge. 1458. Fig. 223 represents different forms of orifices, and aids to explain the difference in their action. Fig. 223. tl! I '// Explain I n the first, it will be seen that the currents (rc- Fig. 223. presented by broken lines), coming from the oppo- site sides of the outlet, oppose each other, so that the velo- city is somewhat checked ; the result is a contraction of the outflowing stream to a size considerably less than the ori- fice. The contraction is found to occur at a distance from the orifice varying from half its diameter to the whole diameter. In the second form of orifice the currents do not oppose each other, and a better flow of liquid is the result ; the stream is the whole size of the orifice, and flows with great emoothness. 426 MECHANICS. The third is the most unfavorable form for a free dis- charge, as currents are formed in the vessel having a direc- tion nearly opposite to the discharge. The discharge is also influenced by nearness to the side of the vessel. In No. 4 it will be seen that the current would be deflected by the momentum of particles from one side. ox,- /* .*... 1459. To calculate the velocity and hence How is the quantity th tit of n id discharge d from any ori- falcula^ 9 fice ' we employ the principle (explained on page 129, note) that the velocity of the stream is equal to that acquired by a body falling through a height equal to the depth of the liquid. The velocity is therefore calculated by multiplying the square root of the depth in feet by 8. The result expresses the velocity in feet per second ; but by reason of the various resistances, the velocity is generally only about 76 per cent, of this calculated amount. PNEUMATICS. 1460. Recent applications of the principles of Pneumatics invest this branch of Philosophy with a new interest. TP-T , , . The compressibility and perfect elasticity of ^reoTsmd'l the air all ^ of its e nom * cal use in th e pro- i/ * " 9 pulsion of engines, wherever the means of con- use in Mechanics? g ensation are g readilv obtained. The drilling machinery of the Mont Cenis and Hoosac Tunnels were operated by the expansive property of compressed air. The condensation was effected by power obtained from water-wheels outside of the tunnel, and conducted through tubes to the machines to be operated. The work could not have been as efficiently done by any other known source of power. The air, after doing its work, served to ventilate the tunnel ; if steam power had been employed, a large excess of the power neces- sary to perform the work would have been required to afford the proper ventilation to the workmen. 1461. Recent experiments have demonstrated the practicability of conveying the compressed air fifteen or twenty miles through pipes from the locality where the air is compressed to the engines which it is to be employed to drive. The project of employing a portion of the enormous store of waste power of Niagara to com- press air to be used at Buffalo, twenty miles distant, has been seri- ously entertained. 1462. Diving-bells of larger dimensions than ever before em- ployed have played an important part in the work of preparing the foundations for the East River Bridge, between New York and MECHANICS. 427 Brooklyn. Instead of the ordinary diving-bell described on page 151, an enormous box, or ccvisson, was made, 164 feet long and 102 feet wide, and launched into the water open side downward. After being floated to the proposed site of the bridge pier, it was gradu- ally sunk to the bottom by the masonry laid on top. Ir.on tubes or shafts had been previously prepared projecting through the top of the caisson, which was several feet thick ; to provide for the en- trance of the workmen and discharge of the material, air was forced in by air-pumps (worked by steam on shore) to such a pressure that the water was forced out, and the workmen stood on the bottom. Seventy or eighty men at a time worked with ease in this caisson. The material dug from the bottom allowed the caisson to sink by degrees until a depth of forty-live feet from the surface of high water was reached. The masonry upon the top was built fast enough to keep the surface above water. When its final resting- place was reached, the interior was filled with concrete, and the caisson left to form the base of the granite pier. Many interesting phenomenon were noticed belonging to the denser atmosphere in the chamber. Sounds were heard more dis- tinctly ; a painful pressure on the drum of the ear gradually passed away ; breathing seemed at first slightly difficult ; candle flames burned with a great deal of smoke ; a workman who had occasion to go under water in a pit in the bottom of the caisson found he could remain for an unusual length of time without the least in- convenience. At St. Louis, where an iron caisson was sunk to the depth of 110 feet, it was found necessary to change workmen at the greatest depth at intervals of two hours, as longer stay induced temporary paralysis. At this caisson the compressed air was applied to an- other purpose. Tubes reaching from the sand and water at the bottom were extended upwards to the outer air ; another set of tubes conducted the compressed air just into the lower open ends of the first-mentioned tubes ; the strong upward current of air drew with it sand and water at a rate that precluded the necessity of any other means of dredging, and all of the material for fifty feet of doscent was removed by this means. This dragging action of a current of air, when urged over liquids, or even over bodies of air, has been lately utilized in several ways. Currents in the ocean under the influence of strong winds are a phenomena of the same kind. It seems to be an exhibition of the property of adhesion. 1463. Heat is a motion of the minute par- ticles or molecules of a body. All kinds of bodies, whether animal, vegetable, or mineral, and in any condition of matter solid, liquid, or gaseous possess this motion in their molecules. When this motion is less than usual in any particular body, we say the body is cold ; but we know of no instance of the entire absence of heat. Hot lb* 428 MECHANICS. bodies are those whose molecules are vibrating with great rapidity. Define 1464. When a hot body is placed in con- Conduction. ^ ac ^ w ith a colder one, the molecular motion of the former is gradually imparted to the latter, begin- ning at the point of contact, and gradually extending throughout the mass. This communication of heat from particle to particle is termed Conduction. Explain 14G5. The rapidity of conduction is very differ- Fig. 224. en ^ j n different solid bodies. This fact is ex- hibited by the apparatus Pigi 224> represented in Fig. '#24. Rods of different sub- stances are fitted in the side of a water-tight ves- sel ; to the end of each rod is attached a marble, held by a bit of common wax. The box being filled with hot water, the different conducting powers of the several rods is approximately shown by the shortness of the time required in each case to release the marble. 146C. Liquids and Gases are poor conductors of heat. The molecular motion is not easily communicated from one molecule to another on account of the ease with which each moves over or away from the other. How are liquids 1467. Heating is generally brought and gases heated? about j n t ] iege two c l asses O f bodies by contact with heated solid bodies. Explain 1468. A vessel of water is made hot throughout, Fig. 225. jf ] iea t ^g applied at the bottom of the vessel, as represented in Fig. 225. The heat at the bottom of the MECHANICS. 429 Fig. 225. vessel extending to the nearest molecules of liquid, they, by reason of the vibrations imparted to them, are pushed a little further apart, so that this portion of the liquid is made lighter than the sur- rounding portions, and rises to the top ; the particles tak- ing the place thus vacated are in turn heated, and con- tinuous currents are thereby established. These currents are made visible in a class-room experi- ment by adding a little fine sulphur to the water. What is the pro- This process of heating by circulation is cess called ? called convection. How is heat 1469. Heat is transmitted to great dis- transmitted ^ tances and in all directions from hot bodies by the vibrations of the ether which is sup- posed to fill all space ; this kind of transmission of heat is called radiation, and the vibrations pass through space with the velocity of 186,000 miles per second. Define Latent 1470. When a body receives heat from any Heat, also source whatever, two effects are immediately Sensible Heat. , , , . , , . , . produced; besides being raised in tempera- ture, it is expanded in volume. That portion of the heat which is concerned in expanding the body is said to be latent, as it cannot be detected or measured at once by the usual methods. That portion of the heat which raises the temperature is said to be sensible. 1471. The amount of heat necessary to raise the same 430 MECHANICS. amount of different substances to the same temperature varies considerably. It requires thirty times as much heat to raise a certain weight of water one degree in tempera- ture as is required to raise the same weight of mercury through the same temperature. What is a 1472. A heat unit is the amount necessary Heat Unit f ra i se O ne pound of water one degree Fahrenheit. Define *1473. The heat necessary to raise one Specific Heat. p 0un d O f an y substance one degree in tem- perature is (when measured in heat units) called its spe- cific heat. The specific heat of water is therefore 1 ; of mercury, ^'o ; of iron, \ ; of copper, about T x r ; and of nearly all known substances, whether solid, liquid, or gaseous, the specific heat is less than that of water. 1474. In all practical applications of heat the laws of latent heat are of the highest importance. Illustration of some of the leading facts is afforded in the following account of the behavior of water when heated from below the freezing to above the boiling point. Place a pound of ice in a suitable vessel for the application of heat. Upon heating it will be found that the melting begins ex- actly at 32' Fahrenheit, and that the water will remain at that tem- perature until all the ice has disappeared. This shows that heat must be expended to convert a pound of ice at 32 to a pound of water at the same temperature. By careful experiment it has been found that this expenditure of heat is enough to raise 143 pounds of water 1. The latent heat of water is therefore said to be 143 Fahrenheit. If now the pound of water be heated to the boiling point (212 Fahrenheit), the escaping steam will be found to be of the same temperature ; and although a long-continued application of the heat be necessary to boil the water away, no rise of temperature above 312 J will be found in either water or steam. Heat has been neces- sary again to effect the change of state, and in this change 967 heat units have been employed. In other words, it requires 180 units of heat to raise 1 lb. of water from 32 fo 212. and it requires 5-V times as much heat to simply convert the same water into steam, without raising its temperature above 212". It should be remembered that the latent heat becomes sensible when steam is condensed to water, or water converted to ice. MECHANICS. 431 prn, . . ,, 1475. Heat is employed in various ways to pro- \r a ! / duce motive power chiefly, however, in the steam- Mecnanicat engilie> The relation between heat and work has J^qmrnient been carefullv determined, and is found to be as follows : " The heat required to raise one pound of water one degree is equivalent to a force necessary to raise 772 pounds one foot high ;" or, more briefly, " one heat unit is equiva- lent to 772 units of work." By reason of various losses in our heat motors, we do not re- alize more than -^ of this amount even in our best steam-engines. OPTICS. What is 1476. Light is the result of vibrations in the Light? ether which fills all space. These vibrations are of different degrees of rapidity ; the slowest, which affect our senses, we recognize as heat, as already explained in a previous section ; those which are capable of producing vision vary also in their rate of vibration ; the slowest that can affect the eye being those that produce a dull red, and the most rapid a violet color. These waves all move for- ward at a rate of 186,000 miles in a second. Even the longest of these waves (the red) are so minute that 39,000 are included in a single inch, while of the violet 57,500 are contained in the same length. When the waves are high, the light is said to be intense. 1477. A Eay is only the direction along Define a Ray. . . .. which the wave is moving. Describe the dif- 1478. Light and heat waves are essen- {ouTd^andl^ht tiall F different in their character from sound waxes. waves. The undulations which produce sound are only backward and forward motions in the air or other medium which transmit them. The light and heat waves are vibrations on all sides of the line along which they are propagated. 1479. The number of these vibrations that enter the eye in a second is found by multiplying the number of inches 432 MECHANICS. in 186,000 miles by the number of vibrations in a single inch ; these for extreme violet are 660 millions of millions.* How many 1480. Lenses are transparent bodies with forms of lenses curved surfaces. They are generally made are there? , n , .~ , ,, -. of glass, and are classified generally under gix varieties. Fig. 226. M IT The double convex (M), the plano-convex (N), the men- iscus (0), the double concave (P), the plano-concave (Q), and the concavo-convex (E). 1481. Lenses thickest in the middle magnify; those thickest at the edges diminish. This property leads to their being classified under two classes ; the first three in the above list being convex, or magnifying glasses, and the last three concave, or diminishing glasses. Explain. 1482. The action of a double convex lens upon Fig, 227. parallel rays of light is represented in Fig. 227. Fig. 227. All the rays that strike the glass obliquely are bent from * For the laws of Reflexion of Light, see 816-838. The principles of Kefraction, as defined in 842, are best understood hy con- sidering the properties of lenses. MECHANICS. 433 their course both on entering and leaving the lens. The ray X keeps its direction, as it is perpendicular to both surfaces. All the rays meet at a common point, F, called the focus. 1483. Practically, this result is rarely realized. When the sur- faces are spherical, the rays that fall upon the lens nearest the edge meet at a focus nearer the lens than rays that are nearer the centre. The highest degree of skill is required in the optician to bring the glass to such a shape that all the rays having the same direction shall meet at the same point. Explain 1484. Fig. 228 shows how the magnifying effect Fig. 228. O f a double convex lens is produced. The insect Fig. 228. A B is seen by the light reflected from it, which passes through the lens. The ray from A to (7, instead of passing directly on, is bent in accordance with the laws of refrac- tion to D ; at this point it is again bent downward, so that the eye which receives it sees this portion of the insect in the direction D a. In like manner, the ray of light from B so reaches the eye us to appear to come from #. Other rays from all parts of the insect show the corresponding points to the eye, so that the complete insect appears to extend from a to I. Explain 1485. Fig. 229 shows how the rays from any Fig. 229. "bright object, when allowed to pass through the lens and fall upon a flat surface properly placed, will form 434 MECHANICS. an inverted image of the object. Eays from the point of the candle flame at A, diverging to different points on the Fig. 239. lens, are converged to a focus at a. The point B, in like manner, sends forth rays that find a focus at I ; the foci of rays from other points of the flame will fall in their proper places between a and #, and the image represents the true shape of the flame. 1486. Iij this case the image is smaller than the object^ but it will be sufficiently evident upon reflection, from the method by which the image is produced, that if a bright object were placed at a b, a large inverted image would be formed at A B. These rela- tive positions of image and object are easily found for any good lens by experiment. Fig. 230. 1487. The diminishing effect of a concave lens is illus- MECHANICS. 435 Explain trated by Fig. 230. The rays of light from top Fig. 230. an( j bottom of the vase A I>, when passing through the lens, are rendered less convergent, and appear when reaching the eye to proceed from a much smaller object, as a b. This lens cannot be made to form an image on a screen. Explain Fig. 231. 1488. The use of the convex lens of the eye is in Fig. 231. The rays from A B, after Fig. 221. 233. passing through the aqueous humor, the crystalline lens and vitreous humor form a small but wonderfully accurate image (a ~b) on the retina. Explain the 1489. In some eyes the convex lens is too defect in near- far from the retina, and consequently the image formed is very indistinct. For such cases spectacles of con- cave lenses are needed to check the too rapid convergence of the ray:. People needing such aids to distinct vision are said to be near- sighted. Fig. 232 represents the position of the crystalline lens in such cases. 436 MECHANICS. Ffc. 233. Explain 1490. Fig. 233 represents the opposite condi- Fig.23'3. tion ; here no distinct image is formed, because the crystalline lens is too near the retina. Tho rays require to be converged more rapidly. Spectacles of convex lenses are therefore required. People needing them to see distinctly are called far- sighted'. Fig. 234. Explain the 1491. The Prism, whose construction and Prism. properties is explained on page 252, has be- come of late one of the most important optical instruments. MECHANICS. 43? When a beam of light falls upon the prism, as repre- sented in Fig. 234, the slowest waves are deflected from their course mucli less than the more rapid ones ; hence what would be a bright spot on the wall, if the prism were removed, becomes by the different amount of refraction of different waves an elongated band cf several colors, called the spectrum. The red ray is refracted least, and the violet most of the visible rays. Experiment shows that other waves pass through the prism, and are refracted some less than the red, and others more than the violet. The waves below the red are heat waves, and may be detected by the thermometer ; while those beyond the violet pro- duce chemical effects, and are detected by such sensitive preparations as the photographer uses. 1493. If the aperture admitting the light be exceedingly narrow, and parallel to the direction of the prism, dark ' lines are seen across the spectrum lying between the bands of color. These have long been known as the Fraunhofer lines, and are accounted for as follows, viz. : Different gases and vapors intercept certain of the waves of light, and permit others to pass. When such waves are wanting in the spectrum, the places where they would fall, not being illuminated through the slit, are left dark. The instrument used in the study of these lines is called the Spectroscope. The spectrum in this instrument, instead of falling upon a screen, is directed by means of lenses to the eye. The lenses serve to magnify the lines and the spaces between them, so that a far greater number are seen. In the forms of the spectro- scope designed for the higher scientific uses, several prisms are em- ployed, and also measuring scales to determine the exact relative position.of the lines. On what facts 1493. The following facts form the foun- ^ectr U ^e k0 dation of spectroscopic analysis : founded? (i) Every gas or vapor possesses the property of intercepting certain waves of light while permitting others to pass freely through. (2) Each gas or vapor has this property in a degree peculiar to itself, so that the dark lines of the spectrum afford a means for its detection. (3) Solid or liquid luminous bodies give spectra without dark lines (continuous spectra) when no gas or vapor is in the path of the light. 438 MECHANICS. (4) Luminous gases never give continuous spectra ; they yield only bright lines or bands exactly in those places where dark lines appear Vv-heii the gases are permitted to intercept the light from a luminous solid or liquid body. 1494. The inference drawn from these facts respecting sunlight is, that the sun itself is a Bell-luminous solid or liquid body, and that it is surrounded with numerous vapors, which we recognize for the most part as familiar sub- stances. Well-known metals, when vaporized by intense heat, can be made to produce the lines we find in the solar spectrum. 1495. Fraunhofer discovered and mapped five hundred or six hundred of these lines in 1814. As many as six thousand are now observed and so located by philosophers as to be recognized. A portion of these lines as they appear in the solar spectrum, with the letters by which Fraun- hofer designated the larger ones, is represented in Fig. 235. &$ s^rswjffi2*?fcS new metals not previously known. The knowl- edge of the existence in the sun of substances familiar to us as forming essential portions of the earth. That the fixed stars are similar to our sun in constitution. That many of the neb- ulas are only gaseous bodies. 1497. In addition to these it forms by far the most delicate method in the laboratory for the detection of the chemical elements. The problems at which philosophers are now engaged with this wonderful instrument are the determination of the source of the light in the Zodiacal light, the Aurora Borealis, and the Solar Corona, all of which afford one or two lines not familiar, among substances of the earth. Comets hereafter will be objects of the closest scrutiny ; and when bright enough to afford a spectrum, the question of their consti- tution will probably be satisfactorily settled. Why are red, Colors ? 1498. The spectrum is S enerall y Considered to be made up of seven different VIOLET. MECHANICS. 430 colors, each forming a separate band. It is found, how- ever, that having pigments of red, yellow, and blue colors only, all other colors may be formed by mixing these in proper proportions. These three have consequently been called the Primary Colors, and many have supposed that the remaining colors of the spectrum were caused simply by the overlapping of the bands of these three colors. What is said 1499. On the other hand, it is found that of Secondary the so -called Secondary Colors orange, Colors ? , . , . j -i i i green, and violet cannot be decomposed by the prism, as they should be if composed only of mixed light. Explain 1500. A green band from a spectrum being Fig.23Q. p asS8( j through a second prism, gives the same Fur. 230. Explain the Eainlow. color again. (See Fig. 236.) The same result is obtained with each of the colors of the spectrum. 1501. The Rainbow is formed by the re- fraction and reflection of sunlight in the rain-drops each drop acting as a prism to decompose the white light. The inner or primary bow is the brightest, being formed by two refractions and one reflection, while the secondary bow is formed by two refractions and two reflections. As some light is lost at each reflection, the secondary bow is not so bright as the primary. 440 MECHANICS. The Fig. 237 represents the courses of the rays in form- ing both rainbows. Fig. 237. What are 1502. Colors are said to be complementary Complementary to each other when, taken together, they contain all the constituents of white light. This recognizes the theory of three primary colors. Red and green are complementary, because green may be com- posed of yellow and blue. Yellow and purple are comple- mentary to each other ; also blue and orange. Why are they 1503. Complementary colors are also called called Contrast- . , , , . , ing Colors f contrasting colors, because, when seen side by side, each heightens the effect of the other. Explain 1504. Fig. 238 exhibits the pairs of contrasting Fig. 238. co lors. The primary colors are joined to the cen- tre by heavy lines ; midway between these are the second- ary colors, so placed that each lies between the primaries that compose it. Each color about the circle is complementary to the color that is exactly opposite. The colors about the circle MECHANICS. 441 Fig. 238. GREEN ELLOWISH GREEN VIOLET ORANGE RED having double names, as yellowish-green, reddish-orange, etc., are supposed to be composed of equal parts of the colors that lie ad- jacent. 1505. The principles gov- erning the use of contrasting colors are of great import- ance to decorators and all who employ colors in per- manent ornamentation. Har- monies and discords are rec- tfioiET^ ognized in color compositions as well as in music ; but we have at present no nomen- clature of colors which will enable us to identify by name any but the simplest. The rules to be observed in the use of colors cannot, therefore, be given, until names are devised by which, they can be recognized. 1506. Color Blindness is simply inability distinguish between colors, and does not imply any imperfection in the eye, so far as seeing form is concerned. To a person entirely color-blind, all objects appear either black or white or gray. 1507. People partially color-blind usually distinguish yellow without difficulty, although they may be unable to see any differ- ence between red and green. In an examination of over three hun- dred persons by Dr. Wilson, of Edinburgh, one in every fifty-five was color-blind to this extent. A slight approach to color-blindness is manifested in an inability to see the slight purple tint in com- binations of purple and blue. This is very common indeed. All attempts to cure this defect in the eye by training have failed. Describe the 1508. TELESCOPES. The achromatic tele- a^hromatic' sc P e descri bed in P ar - 90 ? is represented in lens. section in Fig. 239. It will be observed that the object-glass is composed of two different lenses, and of different forms ; the halves of the lenses, as, for instance, the parts between M and D, appear in the section like two prisms. If either were em- What is Color Blindness? 442 MECHANICS. ployed alone, it would act like a prism, and refract the colors to the eye somewhat separated. An indistinct view Fig. 239. is the result of such action. But when two prisms are em- ployed, and so used that one corrects the color separation of the other, the opposite surfaces will be parallel, and the rays leave the prisms with the same direction in whicli they come to it, provided the prisms are of the same den- sity ; otherwise the denser prism may be the thinnest one. Fig. 240. vr Describe 1509. This is shown in Fig. 240. The ray I, Fig.24Q. under the action of the prism C, alone would form a spectrum whose violet color would be at v, and red at r ; by interposing a thinner prism of material having higher dispersive power, the ray is directed to v r, and the colors are combined. 1510. In the case of the object-glass of the telescope, the compound lens thus formed is thickest in the middle hence belongs to the class that magnify. The rays of light from the distant object form an image a I c which is viewed with the eye-lens P Q. MECHANICS. 443 1511. The above is the Astronomical Telescope. The arrangement of lenses in the Terrestrial Telescope is shown in Fig. 241. Fig. 241. The first image is formed at n m, but if viewed through the lens CD would appear as through the astronomical telescope, inverted ; hence the rays are allowed to cross each other at L, and form another image at m' n', which being seen through the" eye-piece C H, is in the desired position. 1512. The Galilean Telescope employs only one object- glass and one eye-glass to see objects erect. This arrange- ment forms the common "opera glass" and the "mariner's Fig. 242. glass." The magnifying power is low, but the amount of light it collects and brings to the eye adapts it for use at night. 1513. It will be seen by the figure that the rays, after passing through the object-glass, converge in such a man- ner as would form an image at m n ; but the concave eye- 19 444 MECHANICS. piece checks this convergence, and brings the rays to the eye nearly parallel. 1514. The Beflecting Telescope, represented in section in Fig. 141, is sometimes constructed so as to be used by looking in at the side. This is accomplished by setting the reflector, represented at C in Fig. 141, at such an angle that the collected rays are directed into the eye-piece in the side of the tube. 1515. The largest Reflecting Telescope ever constructed is that of the Earl of Rosse. The great speculum is six feet in diameter, and the tube is sixty feet long. The largest Refracting Telescopes yet completed have object- glasses of only twenty-three inches diameter. INDEX Aberration of light 384 " spherical . . 247 \ccidental colors ... . 252, 442 , Achromatic . 247 Acid, carbonic 21 Acid, sulphuric, effects of on water 187 Acoustic paradox 177 Acoustics 173 " definition of . 18 Acoustic tubes 179 Action 45 Action and reaction, illustration of 46 Action, suspension of 85 Actynolite ... 21 Aeriform, definition of . ... 19 fluids 138 Aeriform fluids compressed and expanded without limit . . . 139 Aoriform fluids have no cohesive attraction 139 Aeriform fluids have all the prop- erties of liquids 140 Aeriform fluids have weight. . . 139 Aeronaut, how he descends from a balloon 38 Aerolites 387 A flinity, chemical 19,27 Agents 18 " imponderable 18 " ponderable 18 Air 140, 426 Air, a bad conductor of heat . . 191 Air, as an element 19 Air-bladder of fishes ....... 47 Air-chamber 163, 42C Air, component parts of the . . 140 281 note Air, compression of, caused by gravity 39 Air, compressibility of the . 162 Air, condensation of at srrface of the earth 140 Air, condensed, experiments with 103 Air contained in wood and water, experiments to show ..... 161 Air diminishes upwards in dens- ity 14C Air, elasticity of the ... 142, 102 Air, elasticity of the, experiments showing 160 Air, effect of gravity on density of 38 Air essential to animal life, ex- periment to prove 16fc> Air essential to combustion, ex- periments to prove lOf Air, fluidity of 142 Air, fluidity of, experiments show- ing 1'io Air, gravity of the, experiments illustrating .... ... 1^7 Air-gun 164 Air, how a mechanical agent . . 142 " impenetrability of ... 22, 141 inertia of 28, 143 Air, inertia of, experiments show- ing 1C5 Air, lightness of the 162 " materiality of the 162 Air miscellaneous experiments Wl th . . . 166 Air necessary to animal life and to combustion 140 Air, of wnat composed 20 Air, pressure of the as the depth 102 ** pressure of in all directions 162 Air, pressure of the on a barom- eter HO Air, pressure of the on a square inch Hi Air, pressure of the on the body 141 Air, pressure of the preserves the liquid form of some bodies . . 19 446 INDEX. Air, pressure of the retards ebul- lition 168 Air-pump 154 Air-pump, experiments performed by the 157 Air-pump of steam-engine . . . 201 Air-pump, the double 156 Air, resistance of the . . . .25,38 Air, resistance of the to a cannon- ball 62 Mr, scales for weighing . . . .160 Air, two principles, properties of 139 '* when heaviest 140 Air, when the best conductor of sound 176 Air, why not visible 140 Albite 20 Alison, extract from 70 All's well," how far heard . . 176 Alumina 21 Aluminum 20 Ampere's discoveries in electro- magnetism 309 Ampere's electro-magnetic appa- ratus 314 Analysis of the motion of a fall- ing body 52 Angle 48 Angles, how measured 48 Angle of vision 219 Angles of incidence and of reflec- tion 48, 4 ( .i, 216 Angles, right, obtuse and acute . 48 Animal electricity 282 Animals, sagacity of 92 Annealing 31 Antimony 20 " not malleable 31 Aphelion 349 Apogee 349 Apparatus for illustrating the tendency of a body to revolve around its shorter axis .... 61 Apparition, circle of, perpetual . 385 Apparitions, deceptive 225 Aqueous humor 237, 239 Arago's experiments on velocity of sound 176 Arbor 81 Archimedes' boast to Hiero . . 95 Archimedes, burning mirrors of. 228 Archimedes discovers the method of ascertaining the specific grav- ity of bodies 127 note Archimedes, screw of 132 Arc of a peiilulun 101 4rctur\i . . . 309 Aristotle's opinion of the verity of a falling body 53 Arsenic 20 " not malleable 3? Asteroids 33S Astnea 339 Astronomy, definition of . 17, 18, 33 Astronomers, distinguished . . . 330 Astronomy, father of 336 Atmosphere, weight of the . . .141 Atmospheric telegraph 331 Attraction . 25, 26, 33 capillary Ill chemical 27 kinds of 27 law of falling bodies . 51 mutual 34 of all bodies 34 of cohesion '27 of gravitation .... 27 " of the earth 33 " on what dependent . . 34 Attwood's machine 52 Augite 21 Austral polarity 30 2 Axes of the planets, inclination of 3->0 Axis, exact sense of 81 Axis, longer, a body revolving around . . . t'l Axis of motion 59 Axis of the earth, effects of its inclination 334 Axis of the earth, geological the- ory of 62 Axis, what bodies revolve around an 59 Axle. . . .' 81 Azote 20, 140 B. Babbit's metal 99 Bain's telegraph 326 Baker, the Connecticut 191 Balance-wheel 104 Balance 75, 411 Ballistic pendulum 63 .balloon, how to descend from . . "58 " the pneumatic .... 161 Ball, thrown in a horizontal di rcction ........... 64 BailH, force of, how estimated . 63 Bands with one and two centres of motion 83 Banks, Sir Joseph ,190 Barber'? Grammar of Elocution 180 INDEX. 44? Barium 20 Barometer ..... 144 ind note Barometer, the aneroid or porta- ble 145 Barometer, the diagonal . . . 145 Barometer, of the different states 01 the 148 Barometer, greatest depression of the 147 barometer, its importance 146 note * rules of the . . . . . 147 " the mercurial . . . .145 Base of a body 67 Batteries, thermo-electric . . . 335 " galvanic 287 Battering ram 105 Battering ram, force of, how es- timated 105 Battery, electrical 264 Battery, Grove's 293 " how discharged silently 2G5 Battery of the electro-magnetic telegraph 321 Battery, protected sulphate of copper 293 Battery, Sraee's 290 " sulphate of copper . . 292 Beam of light . 213 Belgrade, battle of, and the cornet 380 Bellows, hydrostatic, how con- structed 119 Boll, the diver's or the diving . 150 Bevelled wheels 85 Birds, bodies of 123 how they fly 47 * muscular power of .... 47 Bismuth 20 not malleable . ... 31 Bissextile, meaning of 396 Black 252 Black lead, uses of in overcom- ing friction 99 Bladder-glass 159 Bladder, inflated, why compress- ed in water 115 Boats, how propelled 47 Boats, on what principle they float 123 Boats, motion in, why impercep- tible 26 Bode's law 342 Bodies 18 " attraction of 33 Bodies of drowned persons, why they sink and afterwards rise . 123 Bodies, what are easily overset . 69 " what stand most firmly . 68 Bodies, what will rise aud what will fall in air 4C Body acted upon by three or more forces 57 Body, parts of which move with greatest velocity 60 Bodies, what ones will float and what sink in water 123 Body, when it will fall 66 Bohemia slate, formations of . . 23 Bolt-head, and jar 167 Bomine M 370 Bones of a man's arm, levers of third kind 77 Borax . 20 Boreal polarity 302 Bottle, effect of pressure of the sea upon 115 Boyle 144 Boynton's, Dr., chart of materi- als which form granite .... 21 Bramah's hydrostatic press . . 121 Brass, how made brittle .... 30 Breadth 23 Breaoc-wheel 8 '2, 83 Brittleness 27, 30 Brittleness, how acquired by iron, steel, copper and brass .... 30 Bromine 2(. Brooks, how formed 124 Buckets of water-wheels .... 82 Buckets of water, why heavier when lifted from the well . . 12fc Bulk of a body, how ascertained from its weight 125 Burdens, how made unequal . . 77 Burning-glasses 228, 23-' C. Cadmium 20 Calcium 20 Calliope 339 Caloric 187,427 Calorimotor 297 Camera obscura 219, 240 Camera obscura, portable, how made 21y Cannon-ball, greatest velocity that can be given to 03 Cannon-ball, force of the resist- ance of the air to 62 Cannon, how far heard 17 -.? Chisels, on what principle coii structed 91 Chlorine 20 Chlorite 21 Chord, musical, how produced . 182 Choroid 237, 240 Chromatics 251, 436 Chromium 20 Circle 48 Circle of perpetual apparition . 385 Circies 59 Circles, circumference of, how di- vided 48, 365 Circular motion 5S* Circular motion changed to rec- tilinear by cranks 81 Circular motion, how caused . . 58 Clai 21 Climates, cause of 354 Clock, before and after the sun . 397 " how regulated 102 " moving power of .... 104 Clock, periods when it agrees with the sun 397 Clocks, why they go fastest in winter 103 Clock, what it is 102 " wheels of, their use ... 102 Clothing, cause of warmth of . . 189 Clouds 24 " of what composed .... 186 Cobalt 20, 298 " not malleable 31 Coffee-pots, why with wooden han- dles .............. 190 Cogs 83, 84 Cohesion, attraction of 27 Cohesion, attraction of, its effects on watery particles 186 Cold 185, 192 Cold, its effects on the density of bodies 192 Colors 254 " accidental 252 Columbium 20 Comets 372 " density of 379 Comet, Halloy's, as seen by Sir John Herschel, and by Struve. 377,378,379 Comet, Halley's, periodical time of 371 Comets, how regarded former y . 373 Comets in the' solar system, num- br of . .379 INDEX. 449 Comets, Kepler's opinion of their , number 380 ; Comets, number of ...... 373 j Comet of 1080 375 I " " 1744 376 ! " " 1811 373 Comet of 1853, Mr. Hind's ac- count of the . 381 Comet of 1856 379 Comets, orbits of .374 Comets, return of, first predicted by Halley, Encke, and liiela . 377 Cornets, tails of 374 " velocity of 37 ~ Common centre of gravity of two or more bodies 69 Complex wheel-work 83 j Compound battery 290 " lever 75 " motion 55 " " how produced . 54 Compressibility 27, 28, 29 Concave mirrors 222 " " effects of ... 225 Concave mirrors, laws of reflec- tion from 227 Concave mirrors, peculiar prop- erty of 224 Concave mirror, true focus of . . 224 " screw 94 Concave surfaces, facts with re- gard to . 236 Condensation 140 Condensed 140 Condenser 1 ( J8 " of steam-engine . . . 200 Condensing syringe .... 156, 103 Conduction of heat . . . .190,428 Conductors of the galvanic fluid . 285 258, 260 " of heat 189 Cone 69 Conic sections 341 Conjunction, inferior and supe- rior 349 Connecticut baker 191 Conservatory of arts and trades, how restored to perpendicular .193 Constellations SS3 " of the zodiac . . 347 Oontractibility 28 Converging rays 212, 227 Conversation in polar regions heard at great iistances . . . 176 Convex mirrors 222 Convex mirrors, laws of reflection from . . 220 Convex mirrors, effects of . . 224 Convex screw 94 Conv-ex surfaces, facts with regard to ^35 Copernicus 33 G Copper 20 Copper and tin, sonorous proper- ties of 30 Copper, how made brittle ... 30 Cords, tenacity of 32 Cork, how deep it will sink ... 123 " why lighter than lead . . 34 Cornea 237, 238 Corpuscular theory of light . . .211 Couronne des tasses 290 Crank, dead point of 81 Cranks 80 Crown-wheel . 84 Crust of the earth, materials com- posing the 20 Crystalline humor, convexity, how increased or diminished . . . 241 Crystalline humor, effect of when too round 242 Crystalline lens 237, 435 Cup of Tantalus 133 Cups, the Magdeburgh 157 Current velocity of a, how meas- ured 130 Curve of a projectile, on what de- pendent 64 Curvilinear motion 61 Cutting instruments 91 Cylinder, definition of a .... 79 Cylinder, how made to roll up a slope 68 Cylinder, wheel substituted for . 79 1). Daguerreotype proofs 257 Darkness produced by two rays of light 212 note Davies' Treatise on Magnetism .316 Day and night, cause of .... 358 Days and nights, cause of differ- ence in length of ...... 350 Dead point of a crank 81 Delisle's thermometer 149 Delphi, oracle of 180 Demetrius Poliorcetes . . . . 105 Density 27,28 Density of air, effect of gravity . on. 38 Depth of a well, how estimated . 53 De.?caitcs H* 450 INL>EX. Devil, the Cartesian 162 Dew and fog, difference between. 150 Dew, how produced 150 Diagonal 48 " of a parallelogram . . 55 " of a square 55 Diallage 21 Diameter 48 Diameter, equatorial, how length- ened 61 Diameter, equatorial of the earth, longer than polar, and why . . 61 Diameter of the earth, equatorial IUM polar 102 Diameter of the earth, how ascer- tained 365 Didyniuin 20 Digits 31)5 Dilatability 29 Dionysius, ear of 178 Dionysius, how he overheard his prisoners 178 Dioptrics 230 " laws of 230 Dipping of a magnet 303 Dipping of a magnet, how reme- died 303 Direction 41 " line of 66 Discharge, the jointed 264 Dissolving views 246 Distance at which a man is in- visible 220 Distance, greatest which can be estimated 382 Distances measured by veU<;ty of sound . 177 Distillation 194 Distilled water, why used as stand- ard of specific gravity . . . . 123 Diverging rays 212 Divers, limit to the depth of 115,426 Diving bell, or diver's bell . . . 160 Divisibility 21 " extent of 23 definition of .... 23 " Dodge," how children . . . . 26 Double action of the steam-engine 200 Drowned persons, why they sink and afterwards rise 123 Ductility. . 27,31 Dynamics 17 " meaning of 18 R. Earth . . 363 Earth, n good conductor of 8>r a rivet 1>J | Fulcrum in levers of different kinds . 71 Pxilorum of steelyard; 74 Pulton, Robert 200 Fundamental law of mechanics . 71 Fusee of a watch . . 107 Galaxy 383 Galileo 100, 143, 337 Galileo's experiment at Pisa to prove his law of falling bodies 52 Galileo's law of falling bodies . 52 Galvanic action, three elements necessary for 285 Galvanic batteries 287 " battery 289 circle 286 Galvanic circle, effects of, how in- creased 287 Galvanic circle, essential parts of a 286 Galvanic circle, simplest, of what composed 286 Galvanic electricity 259 Galvanic electricity, process for obtaining 286 Galvanic fluid, how excited . . 284 piles 287 Galvanism . 17,18,283 " fact? explained by . . 296 Galvano-phustie process .... 331 Galvanotype 331 GarrnentSj light-colored why cool 191 " linen, why cool . . . 189 Garments, to what they owe their strength 100 Garments, woollen, why warm . 189 Garnet 21 Gaseous bodies, elasticity of . . 30 Gaseous bodies, to what degree they may be dilated 29 Gases 139 Gases, how prevented from rising from a fl'-iid 168 Gay Lussa"'s experiments on the velocitj /f sound 176 Gearing 83 Geology 62 Georgiurv Bidas 368 Gibbous 388 Glucinuia 20 Gold 20 G Id, both ductile and malleable 31 divisibility of 23 Gclit. the most malleable of all metals 31 m&t \ it? l.rittleuese . ... 32 Gla^x, the bladder l:<4 " the fountain anJ jet . . 169 " the hand ,158 " the India-rubber . . . 159 Glass, wh^ easily cracked whea suddenly heated 1 M Glass, why used in mirrors . . .221 Governor 106. 2\>0 Governor applit 1 to steam-engine by James Watt 106, 203 Governor, explanation of the . . 106 " uses of the 106 Grain of hammered gold .... 23 Grand law of nature . ... 69 note Granite 20 Gravitation, attraction of ... 27 of fluids ... 110,112 Gravity 25, 33 Gravity causes pressure of fluids upward as well as down U7, 418 Gravity, centre of .... 37, 59, 66 Gravity, effect of on density of air 38 Gravity, effects of on different bodies , U Gravity, force of, not affected bv projection t',4 G rarity, force of on projectiles . *'>2 " " where greatest 35 " how it increases and de- creases 35 Gravity, law of terrestrial . . . 35 Gravity, specific ... 40, 126, 41ft Gravity, specific, scales for ascer- taining 126 Gravity, specific, standard of 123, 416 " terrestrial 34 Great Bear 398 Green sand 21 Gregorian telescope 2")0 Gridiron pendulums 103 (j rove's battery ........ 2iJ3 Gudgeons 8C Guericke, Otto 158 Guinea and feather drop .... 165 Gunnery, science of 62 Gunpowder, force of 63 Gunpowder, great charges of use- less and dangerous 63 Guns, how tested . ..... (u 1 Guns, short ones, why preferable 63 Gun, the air !*>' ijymnotuselectricus ..... 28; H. Hail, how formed 124, 16V * Low it differs from snow . lt^ 154 INDEX. Hair -spring 104 flail, Captain Basil 146 Bblley's comet as seen oy Sir JohnHerschel 379 Hand-glass 158 Handles of tea-pots, tc. why of wood . . 190 Hare's caloriinotor 297 Harmony 181 " how produced .... 183 Harmony, sciance of, on what founded 182 Harvest-moon 389 Heat accompanies all great chang- es in bodies 110 Heat, application of its expansive power as a mechanical agent 193, 431 Heat and cold 187, 4 Hiero employs Archimedes to de- tect the adulteration of a crown. 127 Hind's account of the comet of 1853 381 Hipparchus, father of astronomy . 336 Homogeneous 19 Hornblende. 21 Horizontal motion docj not affect that of gravity 65 Horse-power as applied to the steam-engine, meaning of . . 199 Horses, how made to draw unequal portions of a load 77 House's printing te'egraph . . . 328 Human voice, powers of the . . 180 Humor, the vitreous .... 237, 259 " the aqueous . . . .237,239 Hunter's moon 383 " screw 95 Hydraulics . . 17, 18, 108, 128, 418 Hydraulic-ram 133 Hydrodynamics 108,129 Hydro-electric . . 334 Hydrogen '20 " gas generator .... 275 " pistol 274 Hydrometer 128,417 Hydrostatic bellows, how con- structed 119 Hydrostatic paradox 118 Hydrostatic press, Bramah's 121, 415 Hydrostatic pressure, as a me- chanical power 121 Hydrostatic pressure, caused by height, not by quantity . . .119 Hydrostatics ... 17, 18, 108, 412 Hygeia 339 Hygrometer 149, 150 Hyperbola 3-11 Hypcrsthene 21 I. Ice formed under a receiver . 1C9 " Low mode to melt rapidly . . 1W1 INDEX. 455 l(e, why wrapped in woollen or i packed in shavings 190 I foe, why wooden spoons and forks are used for 190 [mage from concave mirrors . . 225 " ' convex mirrors . . '223 " inverted 218 Impenetrability 21, 22 Imponderable agents .... 18 Incidence, angle of 48 Incident motion . ....... 47 Incident rays 216 Inclination of earth's axis, effects of 354 Inclined plans 90 * " advantage of . . 91 " " application of the 91 " " principle of the . 90 Incombustible bodies 188 Indestructibility 21, 23 India rubber 30 " " balls, elasticity of . 47 " " glass 158 fndustioa, electricity by . . 266, 278 " electro-iaagnetic . .312 Inertia ...... 21, 24, 26, 41 " experiment to illustrate . 25 " of air 38, 14'3, 165 ** of a fluid, effects of the . 134 " of fly-wheels ...... 81 " of water 98 Inferior conjunction 349 " planets 343 Infusoria 23 Instruments for raising water . .131 Insulated, meaning of ... 201, 270 Intensity as applied to electricity, meaning of 295 '" In vacuo" 98 "ridium 20 Iodine 20 Irene 3-3'J Iris of the eye . . . . . . 237,238 Iris, the planet or ast eroid . . .339 Iron 20 Iron, a knowledge of the uses of the first step towards civiliza- tion 31 too. ductile but not malleable into thin plates 31 i. n\ t how made brittle 30 " oxide of 21 " when most malleable ... 31 , elasticity rf 30, 46 ! Jansen ... 33* Jerusalem, siege of . . . . 106 Jet, the straight and revolving 163 Jointed discharger 2J1 Juno 339 Jupiter .367,368 Jupiter, a prolate spheroid, and why 62 Jupiter's belts 368 Jupiter, satellites of 367 Kaleidoscope 22i Kepler 337 " laws of .... 337,350,352 Kepler's opinion of the number of comets 380 Klinkenfues . 381 Knee-joint 96 Ladder a lever . . 77 Lakes, why more difficult to swim in 126 Lamp, defects of, how remedied .112 Lamps, why they will not burn . Ill Lamp, wick of, bow it supplies the flame Ill Lantanium 20 Latent heat 187 Lathes 80 Law, Bode's .342 Law, fundamental of mechanics, pyronomics, acoustics and op- tics 49 Law, Mariotte's 142 " of falling bodies 51 Laws >j{ heat -185 " of reflected sound . . . . 178 Laws of reflection from concave mirrors . 226, 227 Law of the heavenly bodies . . 340 Lead 20 " not ductile 31 " why heavy 34 Le Verrier 371 Leap-year 3!>6 Leaves of a wheel ... .84 Length 23 Lens, axis of a .... 23? 456 INDEX. Lene, ooicavo-convex .... 233 convex as a burning-glass . 235 " double concave . . . . .233 " double convex 233 Lenses 232, 482 Lens, effect of how estimated . . 234 " focal distance of a . . . .234 Lenses in spectacles 230 Lens, single concave 233 " single convex 233 " the crystalline 237 Level, how ascertained 113 " or equilibrium of fluids . 110 Levels, spirit or water 113 Lever 93 " advantage in use of ... 73 Lever, force of the, on what de- pendent 70 Lever, how used 72 k< kinds of 72 " many forms of the . ... 75 " of first kind 73 " of second kind 7(5 of third kind 78 perpetual, the 80 Lever, power of not dependent on its shape 76 Lever, principle of the .... 71 " the bent 76 Lever, things to be considered in the 72 Leyden-jar 263 " how charged .... 271 Leyden-jar, how discharged silent- ly " .... 265 Light, aberration of 384 " absorbed by all bodies . .217 " beam ol 213 " color of .... 251, 252, 438 Light, corpuscular and undulatory theories of 211, 431 Light, heat and chemical action of 254 Light, how projected 213 Light, intensity of, law of de- crease 212 Light, passing into different medi- ums 230 Light, polarization of ..... 256 " reflected 215 " " laws of .... 216 * reflection of 211 l/ght, Sir Isaac Newton's opinion of . . . . 211 Light, theories of 211 Light, thermal, chemical and non- tfleotn..f ...... 2&C Light, velocity of . . 41 " zodiacal 3GC Lightning, how caused 278 Lightning-rods 265 " by whom invented 281 Lightning-rods, the best, how con- structed 280 Lime 21 Lime, carbonate of 21 Linen garments, why cool . . . 189 Line of direction 66 Liquid, how it differs from a fluid 109 Liquids have a slight degree of cohesion 109 Liquids not easily compressed . 2S/ Liquid, quantity of discharged from an orifice 129 Lithium 20 Load-stone 298 Locomotive steam-engine . . . 208 Looking-glasses 221 Looking-glass, length of to reflect the whole person 223 Lucifer 363 Luminous bodies 210 Lutetia 339 M. Machine 71 Machinery, propelled by electrici- ty 278 Machine, Attwood's 52 Machines, velocity of, how regu- lated 106 Magazine, magnetic .307 Magdeburgh cups . . . . . ... 157 Magnesia 21 " carbonate of .... 21 Magnesium 20 Magnet, attraction and repulsion of 300,301 Magnet, attractive power of, where greatest 300 Magnet, broken . 302 Magnet communicates its prop- erties 301 Magnet, dipping of a ..... 303 " effect of heat upon . . 302 Magnet, horse-shoe or U, how armed . . . .' 308 Magnetic influence, all bodies sus- ceptible of . 301 Magnetic magazine 307 Magnetic needle ....... 304 Magnet, keeper of a . . . 302, SOS Magnet, 1'iMj.crtifg of . 29V 457 Magrtic poles .... . 300 " power on surface . . 302 Magnetism 1". \8, 298 Magnetism and electricity re- semblances of 302 Magnet, modes of supporting . . 300 Magnetic poles, where strongest 304 Magnet, north and south poles of, where most powerful .... 306 Magneto-electricity . . . . 1*,332 Magneto-electricity, most power- ful effects of, how obtained . .332 Magneto-electric machine . . . 533 Magnet, polarity of 299 Magnet, poles of changed by elei tricity JQ3 Magnet, powers of, how increased 30 \ " kinds of 29? " artificial, how made. 306, 30\ " the receiving 32* " U or horse-shoe .... 30] " variation of . 303, 304 note Magnitude, centre of ... 59, 66 Main-spring of a watch . . 104, 107 Major third 184 Malleability 27,31 Malleability dependent on tem- perature . 31 Manganese 20 Marco Paolo 306 Mariner's compass 304 Mariner's compass, inventor of the 306 Mariner's compass, needle of, how placed 305 Mariner's compass, how mounted 305 " " points of the 305 Mariotte's law 142 Mars 366 Massila 339 Materials, strength of 95 Materials which compose the crust of the earth 20 Materials, tenacity of 32 Matter, attractive 34 " definition of 19 " essential properties of . 21 " gaseous form of .... 19 .Matter, its different states or forms 19 Matter, liquid form of 19 Matter, quantity of, how estimat- ed 40 Materiality of air 162 Matter, solid form of 19 Mechanical agency of fluids . . 1 18 " equivalent 58 Mechanical operations always at- tended by heat .188 Mechanical paradox 68 " power 70 " poAvers ...... 71 Mechanical powers, enumeration of the ... , 9 72 Mechanical powers, on what prin- ciple constructed 71 Mechanical powers, principal law of the 89 Mechanical powers, reducible to three classes 72 Mechanical properties of gases, vapors, &G. 139 Mechanics 17,41,403 " fundamental law of . 71, 91 118 Mechanics, fundamental law of, its application to hydrostatic pressure 119, 413 Media 97, 229 Medium 97 Mediums 97, 229 Medium in optics 230 Melpomene 339 Meniscus 233 Mercurial pendulum 103 " tube 160 Mercury 20 " ths planet, transit of . . 363 Mercury, the planet, why not often seen 362 Metallic points 265 Metals, good conductors of heat . 190 " names of the 20 Metals, order of their conducting power of heat 190 Metals, tenacity cf 32 Meteoric stones . , 387 Meteoric stones, J*-. Brewster's opinion of 367 Metes 339 Mica 21 Microscope, a double 24S " a single 242 Microscope, compound nrp^nify- ing power of, how ascertained 244 Microscope, magnifying power of, how ascertained 244 Microscope, the solar 244 Microscope, the solar, 3it,guify'ug power of 241 Microscopes, what hjvve ti^ greatest magnifying powr . 2-1 r , Milk, why aiusctjd by and lightufcg . 458 INDEX. Milky-way . 383 I Minor thirl 134 Mirror 221 j " concave 222 j " convex 222 ; " plain 221 Mirrors of half the he ght show a whole-length figure . . . .2171 Mirrors reverse all images . . 222 " use of glass in . . . . 221 Miscellaneous Ciperiments with air 166 i Mobility 27 j Molybdenum 20 ! Momenta 50 j Momentum 41,50! Momentum of a body, how ascer- tained 50 ; Monochord 182 Moon . 386 " as cause of tides 391 " as seen through a telesoope 389 Moon, common errors in respect to the 386 Mooa, density of the 387 " di Here nee in daily rising 389 " gibbous 388 " harvest and hunter's . . 389 " horned 388 " in quadrature 388 Moon-light, objects seen by, why faint 217 I Moon, surface of the 386 i " uninhabitable 3(i4 | Morienne 144 j Morse's telegraph 320! " telegraphic alphabet . . 323 | Motion 41 Motion, accelerated, retarded and uniform 44 Motion, axis of 59 " centre of 59 Motion, how transmitted by hy- drostatic pressure 121 Motion, incident and reflected . 47 Motion impelled by two or more forces 55 Motion of the heavenly bodies, cause of the 34 Motion, perpetual 45, 407 " regulators of 100 " reversed 83 Motion, slow or rapid proluced at pleasure by machinery .... 84 Motion, when imperceptible . . 220 Moving p< wer in machines, how stopped . 8fi Mountain, how burst by hydro- static pressure 12$ Musical scale 183 " sounds 181 Multiplier, electro-magnetic . .313 Multiplying-glass 235 Musical chord, how produced . . 182 Musical instruments, why affected by the weather 182 Macic of a choir dependent on the uniform velocity of sound . .176 Music of strings, how caused . . 181 Mutual attraction . . 34 N. Natural Philosophy, definition of 17 Neap tides 391 Needle, the magnetic 304 Needle, how placed in a mariner's compass 305 Negative electricity .... 259, 202 " (galvanic) pole ... 287 Neptune 371 Newcomen and Savary's steaiu- engine 197 Newton, Sir Isaac .... 23, 337 Newton, Sir Isaac, discovery of - gravitation 100 Newton's discoveries, on what based 352 Newton's (Sir Isaac) opinion of light 211 Newton, Sir Isaac's, opinion of the earth's compressibility ... 29 Nickel 20, 298 Niobium 20 Nitrogen 20 Non-conductors . . . 15?, 2l)fl Non-electrics 258, 260 Nut and screw . 95 Oars, on what principle construct ed . .71 Object, apparent size of, on what dependent 220 Objects, when invisible . 218, 220 Octave 184 " how produced 182 CErsted's discoveries In electro magnetism 108 Oil, effects of in smoothing the surface of water ^31 Oil, glutinous matter in . . Ill Ji'-ujillB . US INDEX. 459 Oil, why it fiVata 39 Giber's, Dr., opinion on lunacy . 386 Opaque bodies 217 Opera-glasses 249 Opposition 350 Optical paradox 212 Optis-nerve 237, 240 Optics 17, 210 " definitiun uf 18 Oracles of Delphi, Ephesus, Ac. . 180 Orbit, meaning of . 340 Orbits of the planets, inclination of 347 Orbits of the planets, not circular 343 Otto Guericke 158 " Out of beat," meaning of . . .104 Ovorshot-wheel 82 Osmium .... 20 Oxyde of iron . . 21 Oxygen 20 Pails, why two can be carried more easily than one .... 69 Palladium 20 Pallas 339 Parabola 62, 341 Parachute 38 Paradox 118 acoustic 177 hydrostatic 118 mechanical 68 optical 212 pneumatic 169 Paradox, optical, pneumatic, acous- tic, Ac., no paradox . . 212 note Parallax 385 Parallel motion, appendages for 200 Parallelogram 48 Parthenope 839 Pascal 144 Pelupium 20 Pendulum 100 Pendulum, cause of slowness and rapidity of vibrations . . . .102 Pendulums, continuous motion of, how preserved 103 Pendulum, how lengthened or shortened 102 Pendulum, how to be suspended 10:i " its motion, how caused 101 Pendulums, length of, proportion of 103 Pondulu'Ji, length of to vibrate seconds 102 Pendulum, length of to ei orate two seconds 103 Pendulum, length of varies with the latitude 102 Pendulums, table of the lengths of to beat seconds in different latitudes 104 Pendulum, the ballistic . . . . 6J " the gridiron .... 10U " the mercurial .... 103 Pendulums, to what variations subject 103 Pendulum, use of the ball of. 101 note Penumbra 394 Percussion, force of 93 Perigee 349 Perihelion 349 Permanent magnets 301 Perpendicular 48 Perpetual lever .80 ". motion 45 Perpetual motion, approximation to 288 Phocea 339 Phosphor 363 Phosphorus 20 Photography 257 Physical spectra 228 Physics, definition of 17 Piazzi . 343 Pincers 75 Pinions 83 Pipes, tones of, on what dependent 181 Pivots 8] Plane, the inclined 90 Planet, meaning of 339 Planet and star, difference be- tween 339 Planets, characters by which they are represented 34fi Planets, inferior and superior . . 343 " minor 339, 367 " " how discovered . 342 Planets, minor, by whom discov- ered 343 Planets, minor, size of 344 " names of the ... 338, 339 Planets, relative appearance of, as seen through a telescope . 372 Planets, the primary 338 Planets, when in a particular con- stellation 349 Platinum 20 Platinum, both ductile and malle- able 31,32 Plough, constellation of the ", 398 Pluuib-lu-e 3" 460 INDEX. Pneumatics 17 18, 138 Pneumatic balloon 1(51 Pneumatic paradox 169 " shower-bath .... 166 " scales 160 Pointers 3H8 Poker 75 Polarity 299 boreal and austral . . ,. 302 Polarization of light 256 Polar or pole star 384,398 Poles, magnetic 300,304 Poles, magnetic, where strongest 304 Ponderable agents 18 Pope Callixtus and the comet of Halley 378 Pores 28 Porosity 27, 28 Positive electricity .... 259, 262 Positive (galvanic) polo .... 287 Potash 21 Potassium 20 Power 72, 405 Power, how gained by use of the lever 76 Power, how to be understood 73, 405 Powers, mechanical 70, 72 Power that acts 7 Power, weight and velocity, pro- portion of 90 Precession of the equinoxes . . 397 Press, Bramah's hydrostatic 121, 415 Presses, screws applied to ... 95 Pressure at any depth, how esti- mated 115 Pressure, fluid, law of 115 Pressure, hydrostatic, as a me- chanical power 121 Pressure, hydrostatic, caused by height, not by quantity . . . 119 Pressure of fluids 114 Pressure of fluids in proportion to height of column 129 Pressure of the air .... 141, 162 " of water at great depths 109 Pressure on hydrostatic bellows, how estimated 119 Primary planets 338 Principle of all machines ... 72 Principle of the mechanical pow- ers 71 Prism 252 Projectiles 62 Projectile, random of 65 Projection, force of 62 Projection, force of, has 10 effect on gravity . 64 Propeller ...... 204 Properties, essential aud acciden- tal, of matter ........ 21 Properties, essential and unesssu- tial ............ 23 Prussian blue ...... 3 '27 Psyche ......... 539 Ptolemy .......... .536 Pulley ........... 8C " fixed and movable ... 86 " fixed, use of ...... 87 Pulleys, mechanical principle of same as that of levers .... 88 Pulley, movable, how it differs from a fixed ........ 87 Pulley, movable, principle of the 89 Pulley, power of, how ascertained 88 Pulleys, practical use of .... 89 Pump, the chain ....... 131 " the common, for water . .152 the forcing ...... 153 l the air ........ 154 Pupil .......... 237, 238 Pyramid, why the firmest of struc- tures ........... 68 Pyrometer .......... 193 " Wedgewood's ... 193 Pyronomics .... 17, 18, 185, 187 Pythagoras ....... 336 Q. . .388 Quadrature. . ... Quartz ..... ....... ^1 Questions for solution 36, 42, 43, 50 53, 54, 78, 86, 90, 96, 106, 116, 127, 184 U. Radiation of heat 190 Radii 48 Radius 48 " vector 350 Rain, how formed . . . 121,150, 186 Rainbow, how produced .... 255 Ram, the battering 105 " the hydraulic 133 Random of a projectile ... 65 Rarefaction . .- 140 Rarefied 140 Rarity ......... . 27, 28 Ray of light 21 1 Rays of light absorbed . . . . 215 " " converging . . . 212 Rays, converging and diverging, laws of , . 22" INDEX 461 Rays of light, diverging . . . ^i2 Rays of light from terrestrial ob- jects . , 213 Reader, The Rhetorical .... 180 Reaumur's thermometer . . . 149 Receiver 154 Rectangle 48 Rectilinear motion converted to circular 81 Reflected motion 47 Reflecting substances ..... 211 " telescope . . 246, 249, 444 Refraction 229 Refracting substances 211 " telescope . . . 246, 443 Refrangibility 230 Registering apparatus of the tel- egraph 322 Regulators of motion 100 Rein, F. C., hearing trumpets or cornets 178 note Repulsion f . . . . 28 Resinous electricity 262 Resistance 41 Resistance of a medium, to what proportioned 97 Resistance of the air 38 " to be overcome ... 71 Resultant 58 " motion 57 " of two forces .... 56 Resultant of two forces, how de- scribed 58 Retarded motion of bodies pro- jected upwards 54 Retina ... 237, 240 Reversed motion 83 Revolving-jet 163 Revolution of the planets, length of 341 Rhetorical Reader ....... 180 Rhodium 20 Rhodes, siege of 105 Rifles, how tested 63 Rivers, how formed 124 Rivers, why difficult to swim in . 126 Rivulets, how formed . . . 124, 136 Roads, inclined planes .... 91 Rolling friction 98 Romans, the ancient, how they conveyed water ...... 137 Rope-dancer, how enabled to per- form his feats 67 Ropes, strength of, on what de- pendent 100 Rosse's telescope 251,444 Rotation, electro-magnetic . . 313 Rudders, on what principle con- structed 77 Rules relating to musical strings 184 Rules by which changes of' the weather may be prognosticated by means of the barometer . . 147 Rules relating to musical pipes . 184 Rush's Treatise on the Voice . . 180 Ruthenium 20 S. Safety-valve 199 Sagacity of animals 92 Sap, ascent of, to what due .112 Satellites, general law of .... 370 Saturn 368 Saturn's rings 368 Scales for ascertaining specific gravity 126 Scale, -the musical 183 Scales, the pneumatic 16C Schorl 21 Science of harmony, on what founded 1S2 Scissors 75 Sclerotica .'237 Screw 93 a compound power .... 94 advantage of the .... 94 convex and concave ... 94 power of, how estimated . 94 Hunter's 95 of Archimedes 132 uses of the 95 "Sea-Eagle," experiment made on board of the 109 Seasons, cause of the 350 " explanation of the cause 355, 356 Sea-water, cause of its increased specific gravity 12G Seebeck, Professor, discoveries of in thermo-electricity .... 334 Selenium 20 Serpentine 21 Shadow 213 Shadows, darkest, how produced 214 Shadows from several luminous bodies 215 Shadows, increasing and diminish ing 214 Shadow of a spherical body, form of 2U Shadows, why of different degreed of darkness 213 Sli aft . 81 462 Shet herds, balancing of in south of Frame GT Ships, on what principle they float 123 Sidereal time 3 ( J6 " year 39G Silence produced by two sound? 177 Silica 20, 21 Silver best conductor of heat . . IS ' "Simple motion 55 Sidereal year, how measured . . 397 Signal-key of the electric tele- graph 322 Signs of the zodiac 346 Signs used in almanacs .... 389 Silurus electricus . . 282 Silver 20 Siphon 132 Siphon, equilibrium of fluids ex- emplified by means of the . . 133 Siphon, experiments with the . 167 " principle of the . . . .133 Sky, why blue 253 Slate formations in Bohemia . . 23 Slaves in West Indies, how they steal rum 122 Steel, how made brittle .... 30 Sliding friction 98 Soiee's battery 290 Smoke, why it ascends 39 Snow, how formed .... 124, 150 " how it differs from hail . I'z4 Snow and ice, how made to melt rapidly 191 Snuffers 75 Soap-bubble, thickest part of . . 23 Soda . . . 21 Sodium 20 Solar microscope 214 Solar system, account of the 337,338 time 396 " year, how measured .396,397 Solstices 358 Sonorous bodies 174 Sonorous property of bodies, to what due 175 Sound 174 Sound affected by the furniture of a room 179 Sound, by what laws reflected . .178 Sound, by what reflected and dis- persed 179 Sound, focus of 179 Round, how communicated most rapidly 175 Soui.d of the human voice . . . 179 " of strings, ho*y causoc 1 . .181 * rapidity of . ... . . 176 Sounds, distance to which the^ may be conveyed ...... 17^ Sounds, musical . 18? " producing silence . . . 177 Sound, velocity of . . . .176 note Sounds, what pleasing to tne ear 183 " when loudest . ... 174 Sources of heat 187 Space ............ 41 " how estimated 43 Speaking-trumpets 178 Specific gravity . . 40, 126 note, 410 Specific gravity of bodies, how as- certained 125, 127, 417 Specific gravity, scalos 'or ascer- taining 126 Specific gravity, standard of . . 123 " gravities, table of . . .124 Sphericity, centre of 37 Spectacles . . 236 Spectrum of a prism 254 Spherical aberration 247 Spherical body, how made to roll down a s.'ope 68 Spider's web ?3 Spiral tube 274 Spirit level 113 Spirit or water level, with what filled 113 Spots in the tun 304 Sportsman aiming at a bird ... 57 Spring, how high it will rise . . 137 Springs, how formed ..... 136 Spring-tides .... , . 391 Spur-gear 84 Spur-wheel 84 Square .... 48 Square rods, why bi tter than round as conductors of electricity . . 279 Standard of specific gravity . . 123 Stars, distance of the 382 Stars, distance of the, Sir John HerscheFs opinion of .... 383 Stars, how distinguished from planets . 339 Stars, the fixed 381 Stars, why not seen in the day- time 363 Stars, why not seen in their true place . 384 Statics 17,18 Stationary steam-engine .... 209 Steam 195 Steamboats 205 Steam, cause of the ascent of . .124 " dry and invisible .... 19G Steam engine applied to boat* . 103 INDEX. 463 Steam-engine, power of, how esti- ] Sun, planets and stars, inhabited 359 mated 199 Steam-engine, the ...... 1UG " improvers of the . 2UO Steam-engine, Neweoinen and Sa- vary's 197 Sleaiu-engine, Watts' double act- ing, condensing 197 S.eam-engine, Watts' improve- Sun, rei appearance of the, how caused 253 Sun's heat, effect of on the earth.150 Superior conjunction 349 " planets 343 ourinam eel 282 Suspension of action 85 Synchronous tickings of a clock . 104 uients of the 197 j Syracuse, King of, employs Ar- Steain-engine, the locomotive 204, 208 Steam-engine, the stationary . . 209 Steam-engine, Tufts' stationary 207 Steam, foundation of its applica- tion to machinery 30 Steam, how condensed into water 195 how made to act 196 chimedes to detect the adultera- tion of a crown 127 note . . 188 156, 163 Steam, on what its mechanical agency depends 195 Bteatn, pressure of, on what de- pendent 195 Steam-ship 203 Steam, space occupied by ... 196 " temperature of 195 " why it ascends '39 Steatite 21 Steelyards 75 " how to be used . . 74 Steelyards, mechanical principle of the 73 Stereo-electric current 334 Stethoscope 175 Still 194 Stilts used in south of France . 67 Straight jet 163 Strata of the earth 20 Stream, velocity of, how measured 130 Strings, musical sounds of, how produced 181 .Strings, musical quality of the sounds of 181 Strontium 20 ftruve's opinion of the distance of the stars 382 Substance, heterogeneous ... 19 " homogeneous .... 19 Sucker 100 Sulphate of copper battery . . . 292 Sulphate of copper battery (pro- tected) 293 Sulphur 20 Sun, as cause of tides 391 " as viewed from the planets 360 " its size, &c 359 Sun, moon aud planets, relative use of the 343 Syringes for striking fire Syringe, the condensing T. Table of specific gravities . 124 Table of the lengths of pendulums 104 " of velocities 42 Tackle and fall 89 Talc 21 Tangent 48, 60 Tantalus 133 note Tantalus' cup 133 Tantalize, origin of the word . .133 Tapestry of Bayeux 380 Tea-pots, why they have handles of wood 190 Teeth 83 Telegraph, atmospheric . . . .331 " Bain's 326 " electric, history of the 329 " electro-magnetic . . 319 Telegraph, electro-magnetic, rep- resentation of the 323 Telegraph, electric, principles of its construction 320 Telegraph, House's printing . 328 Telegraphic battery 321 Telegraph, meaning of . . 319 note Telescopes 246 Telescope, achromatic . . . 247, 442 " Cassegrainian . . . 250 " day and night . . . 248 " Gregorian . . . 250, 443 liersehePs 251 " " power of .337 " Lord Rosse's . . 251, 444 " reflecting .... 240, 444 refracting ... 24C, 443 " simplest form of the . 247 Tellurium 20 Tenacity 27,32 " of cords 32 " of the metals . . . . 3 " of metals, how iiioroo^Hl 3? 464 INDEX. Tenacity of various substances . 32 | Trumet, speaking . . . fender of a steam-engine . . . 204 I Tubes, capillary ^erbium 20 1 " inercuru.1 Terrestrial gravity 34 i Tufts' stationary steain-engice pi ! .&?,, .* i ; .!, A ne/> m _ Tungsten Thermal effects of light .... 256 Thermometer 149 Celsius' 149 " Delisle's 149 " Fahrenheit's . . .149 Thermometer, on what principle constructed 29 Thermometer, Reaumur's . . . 149 Thermo-electric 334 " batteries. . . .335 Thermo-electricity .... 2CO, 334 Thetis 339 Thorium 20 Threads of a screw 93 Thunder-clouds, distance of, how measured 177 Thunder-house 277 Thunder-storm, safest position in.2l Tides 390 " neap and spring 391 Time, apparent and true, differ- ence between 397 Time as kept by clock and by the sun 397 Time employed in the ascent and descent of a body equal ... 54 Time, how estimated 43 " sidereal and solar .... 396 Time of ascent and descent of a body 45 Tin 20 Tin and copper* sonorous proper- ties of 30 Tin, not ductile 31 TLs.-'ue figure 270 Titanium 20 Toggle-joint 96 " operation of the . . 97 Tones of the voice, how varied . 180 Tonic 183 Tonnage of vessels, how estimated 123 Torpedo . . ." 282 Torricelli 143 Torricellian vacuum ..... 143 Towns and fortifications, attacks on Transfer of fluids 167 Transit of Mercury and Venus . 303 Translucent bodies 211 Transparent bodies 211 Tropic 356 Trumpet . 178 Titiujpets, hearing 178 Tycho Brahe . 17N 111 160 20'i 41 20 336 U. Umbrella, use of in leaping fioiu high places 38 Undershot wheel 82 Undulations of light 211 " of water, effects of . 131 Undulatory theory of light . . . 211 Universal discharger ... 272 Urania 339 Uranium 20 Uranus 369 " moons of 370 Ursa Major 398 Unit of Heat 4.80 Unit of Work 404 V. Vacuum 98, 143 Vacuum, a perfect, not to be pro- cured by means of the air-pump 156 Vacuum, the Torricellian . . . 143 Valve 152 Vanadium 20 Vapor, cause jf ascent of .... 12-1 Vapors 139 Vegetables, why white or yellow when growing in dark places . 256 Vehicle in motion, cause of acci- dents from 25 Velocities, table of 42 Velocity 41,71 " absolute and relative . . 42 " how estimated 42 Velocity of balls thrown by gun- powder 63 Velocity of light and of the elec- tric fluid 40 Velocity of parts of a body, how diminished 60 j Velocity of sound . . .176 and note 63 j Velocity of sound, distances measured by the 177 Velocity of sound, experiments of Arago, Gay Lussac and others 170 Velocity of a stream how meas- ured 130 Velocity of the surface of a stream, greatest I2i ! INDEX. 405 Velocity required in machines, bow regulated 106 Ventriloquism 180 Venus 363 " transit of 3C3 Venus, why never seen late at night 363 Vertical line 37 Vesicular form of matter, defi- nition of 19 Vespasian, battering-ram of , . 108 Vesper . . 363 Vessels, tonnage of, how esti- mated 123 Vesta 339 Vision, angle of 219 Victoria 339 Vitreous electricity 262 Vitreous humor 237,239 Vitriol, effects of on water . . . 187 Voice, Dr. Rush's Treatise on the 180 " sound of the 179 Voice, the human, imitative pow- er of the 180 Voice, tones of the, how varied . 180 Voltaic battery 289 " electricity . . . 259,283 pile . .' 288 War, how it has beeu elevated to a science 63 Warmth of clothing, cause of . . 189 Watch, how it differs from a clock. 104 " how regulated 105 " moving power of a . . . 104 Water 21 Water, converted into steam, space occupied by 30 Water, distilled, the standard of specific gravity .123 Water, elasticity and compressi- bility of 24 Water expands when freezing . .192 Water-fowl, buoyancy of . . . .123 Water frozen under the air-pump. 169 Water, how applied to move ma- chinery 83, 419 Water, how converted into steam. 195 Water, how high raised by means of common pump 153 Water, how much diminished in bulk by pressure .... 29, 412 Water, instruments for raising . 131 Water-level 113 Water, motion of, how retarded . 129 Water, not destitute ol compress- ibility 109 Water, of what composed . . 20 Water, pressure of at great depths 109, 116 Water, pressure of at any depth, how estimated 115 Water, pressure of at diflereat depths 115 Water-pump 152 Water-spouts 172 Water, weight of a cubic foot of . 126 , " weight of a cubic inch of 115 Water when falling, why less in- jurious than ice 114 Water, when perfectly pure . . 124 Water, why it appears more shal- low than it is^ 231 Water-wheels 81,419 " most powerful . . 82 Watson, Dr., experiment of, to show degree of evaporation . .150 Watt, James 106 Watt, James, his improvements of the steam-engine 19? Waves, how caused 130 Waves of light, laws of 212 note, 437 Wedge 92 " advantage of the .... 92 Wedge, effective power of. on what dependent 92 W T edge, power of the 92 Wedges, use of 92 Wedgewood's pyrometer .... 193 Weight 34,72 " cause of 34 " lifter 165 Weight, loss of in bodies weighed in water 126 Weight of any body, how ascer- tained by its c'ibical contents . 125 Weight raised by wheel and axle, how supported 79 Weight, what bodies have the greatest 34 Welding 31 Wheel and axle 78 " " advantage of . 79 " " construction of . 79 " " how supported . 81 " principle of the . 80 Wheel, escapement 104 Wheels, friction . 99 Wheels in machinery acting as levers 78 Wheels, large and small, advan- tages of each 85 466 INDEX. Wheels, locked, how and wo> . 85 Wheels of a clock, their U30 101 " power of 80 " size of limited bv iftat 85 " tires of how secured . 1 ( J3 Wheels, toothed, method ut ascci taining power of 85 Wheels, use of on roads .... 85 \Vheol with teeth, of three kinds 34 Whirlwinds 172 Whispering-gallery 1/0 Whispering-gallery in Newbury- port 179 Whisper, motion of a, rapidity of the 170 White '251 Whitefield 179 Wick of a lamp, principle of the 111 Width 23 Wightuian's apparatus for inertia 25 William, Duke of Normandy . . 380 William the Conqueror .... 380 Winch applied to wheel and axle 79 " double 80 Wind . . 170 Wind, cause of the different direc- tions of the 171 Wind, east, cause of at the equa- tor 171 find Instruments, sound of, on what iependent 181 Wind* juality oi the, ho\\ affect- ed . 171 Wind, why it subsides at sunset . !TJ Windlass 80 Windlass and capstan, difierence between 80 Wind-mill? 80 Window, where the hand should be applied to raise . . . . 77 Wollaston, experiments of ... 254 Wooden spoons and forks, why preferred for ice 190 Woollen garments, why warm . 189 Worcester, Marquis of ... 20 Worm of a still . . ... 19.5 Work, Unit of, 404 Y. Year 341 Year, leap 396 Year, sidereal and solar . . . 396 Yttrium . . 20 Zodiac . . 345 Zodiacal light 360 Zodiac, constellations of the, change of 347 Zodiac, signs or the 34< Zinc 20 Zinc, at what temperature malle- able S3 Zirconium & / '. YB 360CO THE UNIVERSITY OF CALIFORNIA LIBRARY