Part 1

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CONVERSATIONS ON NATURAL PHILOSOPHY,

IN WHICH THE ELEMENTS OF THAT SCIENCE ARE FAMILIARLY EXPLAINED.

_Illustrated with Plates._

BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY, &c.

WITH CORRECTIONS, IMPROVEMENTS, AND CONSIDERABLE ADDITIONS IN THE BODY OF THE WORK;

_Appropriate Questions, and a Glossary:_

BY DR. THOMAS P. JONES, PROFESSOR OF MECHANICS, IN THE FRANKLIN INSTITUTE OF THE STATE OF PENNSYLVANIA.

Philadelphia: Published and Sold by John Grigg, No. 9 North Fourth Street. Stereotyped by L. Johnson. 1826.

_Eastern District of Pennsylvania, to wit:_

Be it remembered, that, on the twenty-fourth day of April, in the Fiftieth year of the Independence of the United States of America, A. D. 1826, John Grigg, of the said District, hath deposited in this office the title of a book, the right whereof he claims as proprietor, in the words following, to wit:

"Conversations on Natural Philosophy, in which the Elements of that Science are familiarly explained. Illustrated with Plates. By the Author of Conversations on Chemistry, &c. With Corrections, Improvements, and considerable Additions, in the Body of the Work; appropriate Questions, and a Glossary: By Dr. Thomas P. Jones, Professor of Mechanics, in the Franklin Institute, of the State of Pennsylvania."

In conformity to the Act of the Congress of the United States, entitled "An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Authors and Proprietors of such Copies, during the times therein mentioned;"--And also to the Act, entitled, "An Act supplementary to an Act, entitled, 'An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Authors and Proprietors of such Copies during the times therein mentioned,' and extending the benefits thereof to the arts of designing, engraving, and etching, historical and other prints."

D. CALDWELL, Clerk of the Eastern District of Pennsylvania.

PREFACE.

Notwithstanding the great number of books which are written, expressly for the use of schools, and which embrace every subject on which instruction is given, it is a lamentable fact, that the catalogue of those which are well adapted to the intended purpose, is a very short one. Almost all of them have been written, either by those who are without experience as teachers, or by teachers, deficient in a competent knowledge of the subjects, on which they treat. Every intelligent person, who has devoted himself to the instruction of youth, must have felt and deplored, the truth of these observations.

In most instances, the improvement of a work already in use, will be more acceptable, than one of equal merit would be, which is entirely new; the introduction of a book into schools, being always attended with some difficulty.

The "Conversations on Chemistry," written by Mrs. Marcet, had obtained a well-merited celebrity, and was very extensively adopted as a school-book, before the publication of her "Conversations on Natural Philosophy." This, also, has been much used for the same purpose; but, the observation has been very general, among intelligent teachers, that, in its execution, it is very inferior to the former work.

The editor of the edition now presented to the public, had undertaken to add to the work, questions, for the examination of learners; and notes, where he deemed them necessary. He soon found, however, that the latter undertaking would be a very unpleasant one, as he must have pointed out at the bottom of many of the pages, the defects and mistakes in the text; whilst numerous modes of illustration, or forms of expression, which his experience as a teacher, had convinced him would not be clear to the learner, must, of necessity, have remained unaltered. He therefore determined to revise the whole work, and with the most perfect freedom, to make such alterations in the body of it, as should, in his opinion, best adapt it to the purpose for which it was designed. Were the book, as it now stands, carefully compared with the original, it would be found, that, in conformity with this determination, scarcely a page of the latter, remains unchanged. Verbal alterations have been made, errors, in points of fact, have been corrected; and new modes of illustration have been introduced, whenever it was thought that those already employed, could be improved; or when it was known, that, from local causes, they are not familiar, in this country.

The editor feels assured, that, in performing this task, he has rendered the book more valuable to the teacher, and more useful to the pupil; and he doubts not that the intelligent author of it, would prefer the mode which has been adopted, to that which was at first proposed.

The judicious teacher will, of course, vary the questions according to circumstances; and those who may not employ them at all, as questions, will still find them useful, in directing the pupil to the most important points, in every page.

The Glossary has been confined to such terms of science as occur in the work; and is believed to include all those, of which a clear definition cannot be found in our common dictionaries.

CONTENTS.

CONVERSATION I.

ON GENERAL PROPERTIES OF BODIES. 9

INTRODUCTION. General Properties of Bodies. Impenetrability. Extension. Figure. Divisibility. Inertia. Attraction. Attraction of Cohesion. Density. Rarity. Heat. Attraction of Gravitation.

CONVERSATION II.

ON THE ATTRACTION OF GRAVITY. 22

Attraction of Gravitation, continued. Of Weight. Of the Fall of Bodies. Of the Resistance of the Air. Of the Ascent of Light Bodies.

CONVERSATION III.

ON THE LAWS OF MOTION. 32

Of Motion. Of the Inertia of Bodies. Of Force to produce Motion. Direction of Motion. Velocity, absolute and relative. Uniform Motion. Retarded Motion. Accelerated Motion. Velocity of Falling Bodies. Momentum. Action and Reaction equal. Elasticity of Bodies. Porosity of Bodies. Reflected Motion. Angles of Incidence and Reflection.

CONVERSATION IV.

ON COMPOUND MOTION. 46

Compound Motion, the result of two opposite forces. Of Curvilinear Motion, the result of two forces. Centre of Motion, the point at rest, while the other parts of the body move round it. Centre of Magnitude, the middle of a body. Centripetal Force, that which impels a body towards a fixed central point. Centrifugal Force, that which impels a body to fly from the centre. Fall of Bodies in a Parabola. Centre of Gravity, the point about which the parts balance each other.

CONVERSATION V.

ON THE MECHANICAL POWERS. 54

Of the Power of Machines. Of the Lever in general. Of the Lever of the first kind, having the Fulcrum between the power and the weight. Of the Lever of the second kind, having the Weight between the power and the fulcrum. Of the Lever of the third kind, having the Power between the fulcrum and the weight. Of the Pulley. Of the Wheel and Axle. Of the Inclined Plane. Of the Wedge. Of the Screw.

CONVERSATION VI.

ASTRONOMY.

CAUSES OF THE MOTION OF THE HEAVENLY BODIES. 70

Of the Earth's annual motion. Of the Planets, and their motion. Of the Diurnal motion of the Earth and Planets.

CONVERSATION VII.

ON THE PLANETS. 80

Of the Satellites and Moons. Gravity diminishes as the Square of the Distance. Of the Solar System. Of Comets. Constellations, signs of the Zodiac. Of Copernicus, Newton, &c.

CONVERSATION VIII.

ON THE EARTH. 91

Of the Terrestrial Globe. Of the Figure of the Earth. Of the Pendulum. Of the Variation of the Seasons, and of the Length of Days and Nights. Of the Causes of the Heat of Summer. Of Solar, Siderial, and Equal or Mean Time.

CONVERSATION IX.

ON THE MOON. 108

Of the Moon's Motion. Phases of the Moon. Eclipses of the Moon. Eclipses of Jupiter's Moons. Of Latitude and Longitude. Of the Transits of the inferior Planets. Of the Tides.

CONVERSATION X.

HYDROSTATICS.

ON THE MECHANICAL PROPERTIES OF FLUIDS. 118

Definition of a Fluid. Distinction between Fluids and Liquids. Of Non-Elastic Fluids, scarcely susceptible of Compression. Of the Cohesion of Fluids. Of their Gravitation. Of their Equilibrium. Of their Pressure. Of Specific Gravity. Of the Specific Gravity of Bodies heavier than Water. Of those of the same weight as Water. Of those lighter than Water. Of the Specific Gravity of Fluids.

CONVERSATION XI.

OF SPRINGS, FOUNTAINS, &c. 128

Of the Ascent of Vapour and the Formation of Clouds. Of the Formation and Fall of Rain, &c. Of the Formation of Springs. Of Rivers and Lakes. Of Fountains.

CONVERSATION XII.

PNEUMATICS.

ON THE MECHANICAL PROPERTIES OF AIR. 136

Of the Spring or Elasticity of the Air. Of the Weight of the Air. Experiments with the Air Pump. Of the Barometer. Mode of Weighing Air. Specific Gravity of Air. Of Pumps. Description of the Sucking Pump. Description of the Forcing Pump.

CONVERSATION XIII.

ON WIND AND SOUND. 146

Of Wind in General. Of the Trade Wind. Of the Periodical Trade Winds. Of the Aerial Tides. Of Sound in General. Of Sonorous Bodies. Of Musical Sounds. Of Concord or Harmony, and Melody.

CONVERSATION XIV.

ON OPTICS. 157

Of Luminous, Transparent, and Opaque Bodies. Of the Radiation of Light. Of Shadows. Of the Reflection of Light. Opaque Bodies seen only by Reflected Light. Vision Explained. Camera Obscura. Image of Objects on the Retina.

CONVERSATION XV.

OPTICS--_continued._

OF THE ANGLE OF VISION, AND REFLECTION OF MIRRORS. 168

Angle of Vision. Reflection of Plain Mirrors. Reflection of Convex Mirrors. Reflection of Concave Mirrors.

CONVERSATION XVI.

ON REFRACTION AND COLOURS. 179

Transmission of Light by Transparent Bodies. Refraction. Refraction by the Atmosphere. Refraction by a Lens. Refraction by the Prism. Of Colour from the Rays of Light. Of the Colours of Bodies.

CONVERSATION XVII.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS. 195

Description of the Eye. Of the Image on the Retina. Refraction by the Humours of the Eye. Of the use of Spectacles. Of the Single Microscope. Of the Double Microscope. Of the Solar Microscope. Magic Lanthorn. Refracting Telescope. Reflecting Telescope.

GLOSSARY, 205

CONVERSATION I.

ON GENERAL PROPERTIES OF BODIES.

INTRODUCTION. GENERAL PROPERTIES OF BODIES. IMPENETRABILITY. EXTENSION. FIGURE. DIVISIBILITY. INERTIA. ATTRACTION. ATTRACTION OF COHESION. DENSITY. RARITY. HEAT. ATTRACTION OF GRAVITATION.

EMILY.

I must request your assistance, my Dear Mrs. B., in a charge which I have lately undertaken: it is that of instructing my youngest sister, a task, which I find proves more difficult than I had at first imagined. I can teach her the common routine of children's lessons tolerably well; but she is such an inquisitive little creature, that she is not satisfied without an explanation of every difficulty that occurs to her, and frequently asks me questions which I am at a loss to answer. This morning, for instance, when I had explained to her that the world was round like a ball, instead of being flat as she had supposed, and that it was surrounded by the air, she asked me what supported it. I told her that it required no support; she then inquired why it did not fall as every thing else did? This I confess perplexed me; for I had myself been satisfied with learning that the world floated in the air, without considering how unnatural it was that so heavy a body, bearing the weight of all other things, should be able to support itself.

_Mrs. B._ I make no doubt, my dear, but that I shall be able to explain this difficulty to you; but I believe that it would be almost impossible to render it intelligible to the comprehension of so young a child as your sister Sophia. You, who are now in your thirteenth year, may, I think, with great propriety, learn not only the cause of this particular fact, but acquire a general knowledge of the laws by which the natural world is governed.

_Emily._ Of all things, it is what I should most like to learn; but I was afraid it was too difficult a study even at my age.

_Mrs. B._ Not when familiarly explained: if you have patience to attend, I will most willingly give you all the information in my power. You may perhaps find the subject rather dry at first; but if I succeed in explaining the laws of nature, so as to make you understand them, I am sure that you will derive not only instruction, but great amusement from that study.

_Emily._ I make no doubt of it, Mrs. B.; and pray begin by explaining why the earth requires no support; for that is the point which just now most strongly excites my curiosity.

_Mrs. B._ My dear Emily, if I am to attempt to give you a general idea of the laws of nature, which is no less than to introduce you to a knowledge of the science of natural philosophy, it will be necessary for us to proceed with some degree of regularity. I do not wish to confine you to the systematic order of a scientific treatise, but if we were merely to examine every vague question that may chance to occur, our progress would be but very slow. Let us, therefore, begin by taking a short survey of the general properties of bodies, some of which must necessarily be explained before I can attempt to make you understand why the earth requires no support.

When I speak of _bodies_, I mean substances, of whatever nature, whether solid or fluid; and _matter_ is the general term used to denote the substance, whatever its nature be, of which the different bodies are composed. Thus, the wood of which this table is made, the water with which this glass is filled, and the air which we continually breathe, are each of them _matter_.

_Emily._ I am very glad you have explained the meaning of the word matter, as it has corrected an erroneous conception I had formed of it: I thought that it was applicable to solid bodies only.

_Mrs. B._ There are certain properties which appear to be common to all bodies, and are hence called the _essential or inherent properties_ of bodies; these are _Impenetrability_, _Extension_, _Figure_, _Divisibility_, _Inertia_ and _Attraction_. These are also called the general properties of bodies, as we do not suppose any body to exist without them.

By _impenetrability_ is meant the property which bodies have of occupying a certain space, so that where one body is, another can not be, without displacing the former; for two bodies can not exist in the same place at the same time. A liquid may be more easily removed than a solid body; yet it is not the less substantial, since it is as impossible for a liquid and a solid to occupy the same space at the same time, as for two solid bodies to do so. For instance, if you put a spoon into a glass full of water, the water will flow over to make room for the spoon.

_Emily._ I understand this perfectly. Liquids are in reality as substantial or as impenetrable as solid bodies, and they appear less so, only because they are more easily displaced.

_Mrs. B._ The air is a fluid differing in its nature from liquids, but no less impenetrable. If I endeavour to fill this phial by plunging it into this bason of water, the air, you see, rushes out of the phial in bubbles, in order to make way for the water, for the air and the water can not exist together in the same space, any more than two hard bodies; and if I reverse this goblet, and plunge it perpendicularly into the water, so that the air will not be able to escape, the water will no longer be able to fill the goblet.

_Emily._ But it rises some way into the glass.

_Mrs. B._ Because the water compresses or squeezes the air into a smaller space in the upper part of the glass; but, as long as it remains there, no other body can occupy the same place.

_Emily._ A difficulty has just occurred to me, with regard to the impenetrability of solid bodies; if a nail is driven into a piece of wood, it penetrates it, and both the wood and the nail occupy the same space that the wood alone did before?

_Mrs. B._ The nail penetrates between the particles of the wood, by forcing them to make way for it; for you know that not a single atom of wood can remain in the space which the nail occupies; and if the wood is not increased in size by the addition of the nail, it is because wood is a porous substance, like sponge, the particles of which may be compressed or squeezed closer together; and it is thus that they make way for the nail.

We may now proceed to the next general property of bodies, _extension_. A body which occupies a certain space must necessarily have extension; that is to say, _length_, _breadth_ and _depth_ or thickness; these are called the dimensions of extension: can you form an idea of any body without them?

_Emily._ No; certainly I can not; though these dimensions must, of course vary extremely in different bodies. The length, breadth and depth of a box, or of a thimble, are very different from those of a walking stick, or of a hair.

But is not height also a dimension of extension?

_Mrs B._ Height and depth are the same dimension, considered in different points of view; if you measure a body, or a space, from the top to the bottom, you call it depth; if from the bottom upwards, you call it height; thus the depth and height of a box are, in fact, the same thing.

_Emily._ Very true; a moment's consideration would have enabled me to discover that; and breadth and width are also the same dimension.

_Mrs. B._ Yes; the limits of extension constitute _figure_ or shape. You conceive that a body having length, breadth and depth, can not be without form, either symmetrical or irregular?

_Emily._ Undoubtedly; and this property admits of almost an infinite variety.

_Mrs. B._ Nature has assigned regular forms to many of her productions. The natural form of various mineral substances is that of crystals, of which there is a great variety. Many of them are very beautiful, and no less remarkable by their transparency or colour, than by the perfect regularity of their forms, as may be seen in the various museums and collections of natural history. The vegetable and animal creation appears less symmetrical, but is still more diversified in figure than the mineral kingdom. Manufactured substances assume the various arbitrary forms which the art of man designs for them; and an infinite number of irregular forms are produced by fractures and by the dismemberment of the parts of bodies.

_Emily._ Such as a piece of broken china, or glass?

_Mrs. B._ Or the masses and fragments of stone, and other mineral substances, which are dug out of the earth, or found upon its surface; many of which, although composed of minute crystals, are in the lump of an irregular form.

We may now proceed to _divisibility_; that is to say, a susceptibility of being divided into an indefinite number of parts. Take any small quantity of matter, a grain of sand for instance, and cut it into two parts; these two parts might be again divided, had we instruments sufficiently fine for the purpose; and if by means of pounding, grinding, and other similar methods, we carry this division to the greatest possible extent, and reduce the body to its finest imaginable particles, yet not one of the particles will be destroyed, but will each contain as many halves and quarters, as did the whole grain.

The dissolving of a solid body in a liquid, affords a very striking example of the extreme divisibility of matter; when you sweeten a cup of tea, for instance, with what minuteness the sugar must be divided to be diffused throughout the whole of the liquid.

_Emily._ And if you pour a few drops of red wine into a glass of water, they immediately tinge the whole of the water, and must therefore be diffused throughout it.

_Mrs. B._ Exactly so; and the perfume of this lavender water will be almost as instantaneously diffused throughout the room, if I take out the stopper.

_Emily._ But in this case it is only the perfume of the lavender, and not the water itself that is diffused in the room.

_Mrs. B._ The odour or smell of a body is part of the body itself, and is produced by very minute particles or exhalations which escape from the odoriferous bodies. It would be impossible that you should smell the lavender water, if particles of it did not come in actual contact with your nose.

_Emily._ But when I smell a flower, I see no vapour rise from it; and yet I perceive the smell at a considerable distance.

_Mrs. B._ You could, I assure you, no more smell a flower, the odoriferous particles of which did not touch your nose, than you could taste a fruit, the flavoured particles of which did not come in contact with your tongue.

_Emily._ That is wonderful indeed; the particles then, which exhale from the flower and from the lavender water, are, I suppose, too small to be visible?

_Mrs. B._ Certainly: you may form some idea of their extreme minuteness, from the immense number which must have escaped in order to perfume the whole room; and yet there is no sensible diminution of the liquid in the phial.

_Emily._ But the quantity must really be diminished?

_Mrs. B._ Undoubtedly; and were you to leave the bottle open a sufficient length of time, the whole of the water would evaporate and disappear. But though so minutely subdivided as to be imperceptible to any of our senses, each particle would continue to exist; for it is not within the power of man to destroy a single particle of matter: nor is there any reason to suppose that in nature an atom is ever annihilated.

_Emily._ Yet, when a body is burnt to ashes, part of it, at least, appears to be effectually destroyed: look how small is the residue of ashes in the fire place, from all the fuel which has been consumed within it.

_Mrs. B._ That part of the fuel, which you suppose to be destroyed, evaporates in the form of smoke, and vapour, and air, whilst the remainder is reduced to ashes. A body, in burning, undergoes no doubt very remarkable changes; it is generally subdivided; its form and colour altered; its extension increased: but the various parts, into which it has been separated by combustion, continue in existence, and retain all the essential properties of bodies.

_Emily._ But that part of a burnt body which evaporates in smoke has no figure; smoke, it is true, ascends in columns into the air, but it is soon so much diffused as to lose all form; it becomes indeed invisible.