BERKELEY LIBRARY UNIVERSITY OF CALIFORNIA ' tf CONVERSATIONS IN WHICH THE ELEMENTS OF THAT SCIENCE ARE FAMILIARLY EXPLAINED, AND ADAPTED TO THE COMPREHENSION OF YOUNG PUPILS. ILLUSTRATED WITH PLATES. BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY, AND CONVERSATIONS ON POLITICAL ECONOMY. IMPROVED BY APPROPRIATE QUESTIONS, FOR THE EXAMINATION OF SCHOLARS ; ALSO BY AND A DICTIONARY OF PHILOSOPHICAL TERMS. BY REV. J. L, BLAKE, A. M. Rector of St. Matthew's Church, and Principal of a Literary Seminary, Boston, Mass. BOSTON STEREOTYPE EDITION. BOSTON: GOULD, KENDALL & LINCOLN. SUCCESSORS TO LINCOLN & EDMANDS. AND SOLD BY THE PRINCIPAL BOOKSELLERS IN THE UNITED STATES. 1835. EDUC-PSYCH DISTRICT OF MASSACHUSETTS, to wit. < District Clerk's Office. BE IT REMEMBERED, that on the fourth day of December, A. D. IfesM, in th forty-ninth year of the independence of the United States of America. JOHN LAURIS BLAKE, of the said district, has deposited in this office the title of a book, the right whereof he claims as author, in the worriu following, to wit: "Conversations on Natural Philosophy, in which the elements of that science are familiarly explained, and adapted to the comprehension of young pupils. illustrated with plates. By the author of Conversations on Chemistry, and Con- versations on Political Economy. Improved by appropriate Questions, for the examination of Scholars ; also by Illustrative Notes, and a Dictionary of Philoso- phical Terms. By J. L. BLAKE, A. M. Rector of St. Matthew's Church, and Principal of a Literary Seminary. Boston, ft1a.=s." 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 o'f such copies during the times therein mentioned ;" and also to an 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 de- signing, engraving, an\2 etching historical, and other prints." JNO. W. DAVIS, \Clerk of the District ' ^ of Massachusetts. NOTICE. This fascinating and familiar treatise on Natural Philosophy has probably contributed more to excite, in the minds of the young, a fondness for studying the science, than all other works together. The easy, natural, and striking illustrations, with which it abounds, awaken a deep interest, and rivet the atten- tion of the pupil. This edition has been introduced into the Female Depart- ment of the Publick Schools in Boston, with the happiest effect. Instructors have assured the publishers, that no study is pursued with equal gratification to the scholars. From numerous testimonials, the following recommendation is selected : From the Rev. Jasper Adams, D.D., Principal and Professor of Mathematics and Natural Philosophy, in Charleston College, South Carolina. " I have been highly gratified with the perusal of your edition of Conversations on Natural Philosophy. The Questions, Notes and Explanations of Terms are valuable additions to the work, and make this edition superior to any other with which I am acquainted. I shall recommend it wherever I have opportu- nity." Charleston, Jan. 10, 1826. M3 PREFACE. -*- PSYCH. UBRAfflT THE following work does not probably contain so much of the science of Natural Philosophy as might be crowded into a volume of equal size, on some different plan. The author seems to have been influenced chiefly by other considerations ; and, in the opinion of the editor, with the most happy success. Mrs. Marcet did not profess to prepare a work suited to the highest stages of education. Her aim was to accommodate an important science to the literary taste and intellectual apprehensions of persons, within whose reach Natural Philosophy had not previously been placed to accommo- date to the use of schools generally a science, which had hitherto been considered too abstruse and uninteresting for any, whose minds had not been disciplined and invigorated by long and regu- lar habits of study. Instead of exhausting the intellectual ener- gies of youth in committing to memory definitions and mathemati- cal demonstrations, which would not be understood, she proposed to illustrate the great principles of Natural Philosophy by compari- sons of the most familiar kind ; and, it is believed, Mrs. Marcet has done more, in this way, towards giving youth a taste for the study of philosophy than all others who havepnblished treatises on the sub- ject. In her preface she remarks: u It is with increased diffidence that the author offers this little work to the publick. The encou- raging reception which the Conversations on Chemistry and Politi- cal Economy have met with has induced her to venture on publish- ing a short course on Natural Philosophy. They are intended, in a course of elementary science, to precede the Conversations on Chemistry, and were actually written previous to either of her other publications." The Conversations on Natural Philosophy were introduced into the editor's Seminary about three years since, then at Concord, N. H.; but it was soon found that his pupils were often embar- rassed in not knowing to what particular parts they were chiefly to direct the attention, committing to memory what was not neces- sary and omitting what was, thereby causing great loss of time as well as of improvement. This induced him to' prepare, as they were needed, day after day, Questions for their Examination. When questions were thus prepared upon the whole work, it was 581 IV PREFACE. judged expedient to have them published in a pamphlet, which was accordingly done ; but being prepared in haste and without thought of their being published, they were of course imperfect ; nor was there opportunity to revise them, when afterwards printed with notes in connexion with the work itself But as successive editions were required, and as the demand is still increasing, ne has been induced to revise and write them anew, placing tlietn at the bottom of the several pages to which they relate; and, also to in- crease the number ofNoies, and to add to the volume a Dictionary of Philosophical Terms. As the work is now presented to the publick, the Editor has full confidence in recommending it to Instructors, well persuaded it will lessen their own labour and facilitate^he improvement of their pupils. It Is perfectly obvious, that, instead of embodying, the questions at the close of the book, as in former impressions great convenience will be found, both by instructors and scholars, in having them printed on the pages from which they are to be an- swered ; nor is the labour of finding the answers to be given so les- sened, as to enable scholars to select those answers without read- ing ami studying the whole book. It lias been thought best to place the Plates at the end of the volume. If interspersed throughout the work, as in former edi- tions, it is evident that no more than one page could face each Plate, while a very considerable number of pages would have re- ference to it, so that the object contemplated could only in a smaU degree be accomplished. Besides, it is judged advisable by the editor, that the plates should not face the explanations in the Text if practicable. Many of the Questions are to be answered from the Plates ; but if the several Plates were placed opposite tne different portions of the work to which they relate, the answers might be read from the explanations there given instead of being recited from the figures as intended. J. L. BLAKE. Boston, December, 1624. CONTENTS. CONVERSATION I. On General Properties of Bodies. INTRODUCTION; General Properties of Bodies ; Impenetrability; Extension ; Figure ; Divisibility ; Inertia ; Attraction ; Attrac- tion of Cohesion ; Density ; Rarity ; Heat ; Attraction of Gra- vitation. Page 9. CONVERSATION II. On the ^traction of Gravity. Attraction of ^Gravitation continued ; Of Weight ; Of the fall of Bodies; Of the resistance of the Air ; Of the Ascent of Light Bodies. Page 24. CONVERSATION III. On the Laws of Motion. Of Motion ; Of the Inertia of Bodies; Of Force to produce Mo- tion ; Direction of Motion ; Velocity, absolute and relative ; Uniform Motion ; Retarded Motion ; Accelerated Motion ; Ve- locity of Falling Bodies ; Momentum ; Action and Re-action equal ; Elasticity of Bodies ; Porosity of Bodies ; Reflected Mo- tion ; Angles of Incidence and Reflection. Page 36. CONVERSATION IV. On Compound Motion. Compound Motion the result of two opposite forces; Of Circular Motion, the result of two forces, one of which confines the body to a fixed point ; Centre of Motion, the point at rest whilo the other parts of the body move round it ; Centre of Magnitude the middle of a body ; Centripetal Force, that which confines a body to a fixed central point ; Centrifugal Foree,that which im- pels a body to fly from the centre ; Falfof Bodies in a Parabola ; Centre of Gravity, the Centre of Weightj or point about which the narts balance each other. Page 51. 1 * 'VI CONTENTS. CONVERSATION V. On the Mechanical Powers. Of tiie Power of Machines ; Of the Lever in general ; Of the Le- ver 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. Pages 60, 68. CONVERSATION VI. ASTRONOMY. Causes of the Earth's Annual Motion. Of the Planets, and their motion ; Of the Diurnal motion of the Earth and Planets. Page 78. CONVERSATION VII. On the Planets. Of the Satellites or Moons ; Gravity diminishes as the square of the Distance; Of the Solar System; Of Comets; Constellations, signs of the Zodiack ; Of Copernicus, Newton. . What is divisibility in natural philosophy ? 17. What aro instances of practical divisibility of matter to a jri Mttt e*. tent ? 18. On what principle is it that we can smell odorife- rous objects <* 14 GENERAL PROPERTIES OF BODIES. Emily. But when I smell a flower, I see no vapour rise from it ; and yet I can perceive the smell at a con- siderable 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 par- ticles 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 na- ture an atom is ever annihilated. Emily. Yet, when a body is burnt to ashes, part of it, at least, appears to be effectually destroyed 1 Look how small is the residue of ashes beneath the ornte, from all the coals which have been consumed within it. Mrs. B. 'That part of the coals, which you suppose to be destroyed, evaporates in the form of smoke and va- pour, 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 t but the various parts, into which it has t been separated by combustion, continue in exist- ence, and retain all the essential properties of bodies. Emily. But that part of a burnt body which evapo- rates in smoke has no figure ; smoke, it is true, ascends 19. If we inhale particles of odoriferous objects, why cnnnot we see these particles: 120. If the particles of fragrant liquid in a phial escape from the phial in order to perfume a room, why can we not see them escape ? 21. Is not the matter, of which wood is composed, destroyed or annihilated, when burnt to ashes ? GENERAL PROPERTIES OF BODIES. 15 in columns into the air, but it is soon so much diffused as to lose all form ; it becomes indeed invisible. Mrs. B. Invisible, I allow ; but we must not imagine that what we no longer see no longer exists. Were every particle of matter that becomes invisible annihilated, the world itself would in the course of time be destroyed. The particles of smoke, when diffused in the air, continue still to be particles of matter, as well as when more closely united in the form of coals : they are really as substantial in the one state as in the other, and equally so when by their extreme subdivision they become invisible. * No particle of matter is ever destroyed j this is a principle you must constantly remember. Every thing in nature decays and corrupts in the lapse of time. We die, and our bodies moulder to dust ; but not a single atom of them is lost ; they serve to nourish the earth, whence, while living, they drew their support.* The next essential property of matter is called inertia; I this word expresses the resistance which inactive matter makes to a change of state./ Bodies appear to be equally incapable of changing their actual state, whether it be of motion or of rest. You know that it requires force to put a body which is at rest in motion ; an exertion of strength is also requisite to stop a body which is already in motion. The resistance of the body to a change of state, in either case, is called its inertia. Emily. In playing at base-ball I am obliged to use all my strength to give a rapid motion to the ball ; and when I have to catch it, I am sure 1 feel the resistance * As a further illustration of the great practical divisi- bility of matter, it may he added, that a single grain or* gold may be hammered by a gold-beater until it will cover fifty square inches. Each square inch roay then be divided into two hundred strips, and each strip into two hundred parts, which may be seen with the naked eye ; consequently^a^square inch contains forty thousand visible parts, which mujWSJjjed by 50, the number of square inches which a grain of gold will make, give two million parts, which may be seen with the naked eye. It has also been calculated, that sixteen ounces of gold, which, in the form of a cube, would not measure one inch and a quarter in its side, will completely gild a quantity of silver wire sufficient to surround the globe. 22. Is it a principle in natural philosophy that no particle of matt : can be destroyed ? 23. What is meant by the term inertia? 24. What instances of great practical divisibility of matter are given in the note ? , 16 GENERAL PROPERTIES OF BODIES. it makes to being stopped. But if I did not catch it, it would soon fail to the ground and stop of itself. Mrs. B. Inert matter is as incapable of stopping of it- self, as it is of putting itself in motion: when the ball ceases to move, therefore, it must be stopped by some other cause or power ; but as it is one with which you are yet un- acquainted, we cannot at present investigate its effects. The last property which appears to be common to all bodies/is attraction, j All bodies consist of^infinitely small particles of matter, each of which possesses the power of ' attracting/or drawing towards it, and uniting with any other particle sufficiently near to be within the influence of its attraction ; but in minute particles this power ex- tends to so very small a distance around them that its effect is not sensible, unless they are (or at least appear to be) in contact ; it then makes them stick or adhere together, and is hence called the/ attraction of cohesion. Without this power, solid bodies would fall in pieces, or rather crumble to atoms. Emily. I am so much accustomed to see bodies firm and solid, that it never occurred to me that any power was reqiiisite to unite the particles of which they are composed. But the attraction of cohesion does not, I suppose, exist in liquids ; for the particles of liquids do not remain to- gether so as to form a body, unless confined in a vessel ? Mrs. B. I beg your pardon; it is the attraction of cohesion which holds this drop of water suspended at the end of my finger, and keeps the minute watery particles of which it is composed united. / But as this power is stronger in proportion as the particles of bodies are more closely united, the cohesive attraction of solid bodies is much greater than that of fluids. The thinner and lighter ^ fluid is, the less is the cohesive attraction of its particles, because they are further apart ; and in elastic fluids^such as air, there is no cohesive attraction among the particles. 2"). What would be the consequence, if a body were put in motion and no resistance should be offered ? 26. What is the property common to all bodies? 27. Of what do all bodies consist :' 28. What is the power called which binds these strall particles together ? 2i). What would be the conse- quence if the power of cohesive attraction were destroyed ? 30. Do^s the power of cohesion exist also in liquids ? 3.1. Hew would you prove that it exists in liquids r 3*2. Why are some bodies hard and others soft ? GENERAL PROPERTIES OF BODIES. 17 Emily. That is very fortunate ; for it would be im- possible to breathe the air in a solid mass ; or even in a liquid state. But is the air a body of the same nature as other bodies ? Mrs. B. [Undoubtedly^ in all essential properties. Emily. Yet you say that it does not possess one of the general properties of bodies cohesive attraction 1 Mrs. B. The particles of air (are not destitute of the power of attraction} but they are too far distant from each other to be influenced by it x and the utmost efforts of human art have proved ineffectual in the attempt to com- press them, so as to bring them within the sphere of each other's attraction, and make them cohere, Emily. If so, how is it possible to prove that they are endowed with this power ? Mrs. B. The \air is formed of particles precisely of the same nature as those which enter into the composi- tion of liquid and solid bodies^ in which state we have a proof of their attraction. Emily. It is then, I suppose, owing to the different degrees of attraction of different substances, that they are hard or soft ; and that liquids are thick or thin ? Mrs. B. Yes ; but you would express your meaning better by the term density, which denotes the degree of closeness and compactness of the particles of a body 3 thus you may say, both of solids, and of liquids, that the stronger the cohesive attraction the greater is the den- sity of the body. In philosophical language, density is said to be that property of bodies by which they con- tain a certain quantity of matter, under a certain bulk or magnitude. Rarity s the contrary of density y it denotes the thinness and subtlety of bodies : thus you would say that mercury or quicksilver was a very dense fluid ; ether, a very rare one, &,c. Caroline. But how are we to judge of the quantity of matter contained in a certain bulk ? , 33. Does the attraction of cohesion exist in the air ? 34. But are the particles of the air actually under tlie influence of this attraction ? 35. Why are they not, if attraction^ belong to them? 3G. How do we know that 'attraction does belong to the air if no influence is exerted upon it ? 37. What is meant by the term density ? 38. What is meant by the term rarity ? J8 GENERAL PROPERTIES OF BODIES. Mrs. B. By\the weight i under the same bulk bodies are said to be dense in proportion as they are heavy. Emily. Then we may say that metals are dense bodies, (wood x comparatively a rare one, &)c. But, Mrs, B., when the particles of a body are so near as to attract each other, the effect of this power must increase as they are brought by it closer together ; so that, one would suppose that the body would gradjally augment in density, till it was im- possible for its particles to be more closely united. Now we know that this is not the case ; for soft bodies, such as cork, sponge, or butter, never become, in consequence of the in- creasing attraction of their particles, as hard as iron ? Afrs. B. In such bodies as cork and sponge, the parti- cles which come in contact are so few as to produce but a slight degree of cohesion ;jthey are porous bodies, which, owing to the peculiar arrangement of their particles, abound with interstices which separate the particles ; and these vacancies are filled with air, the spring or elasticity of which prevents the closer union of the parts. But there is another fluid much more subtle than air, which pervades all bodies, this is\heat.) Heat insinuates itself more or less be- tween the particles of all bodies, and /forces them asunder ; you may therefore consider' heat and the attraction of co- hesion,) as constantly acting in opposition to each other. Emily. The one endeavouring to rend a body to pieces, the other to keep its parts firmly united. Mrs. B. And it is this struggle between the contend- ing forces of heat and attraction, which prevents the ex- treme degree of density which would result from the solo influence of the attraction of cohesion. Emily. The more a body is heated then, the more its particles will be separated. Mrs. B. Certainly ; we find that bodies swell or dilate by heat : this effect is very sensible^in butter) for instance, which expands by the application of heat : till at length 39. How are we to judge of the quantity of matter in bodies? 40. In what proportion are bodies dense of the same bulk ? 41. What bodies are usually said to be dense ? 42. What ones are said to be rare ? 43. Why are not sponge and cork and other similar substances hard since their particles come in contact ? 44. What fluid is named mere subtle than air ? ^o. What effect has heat on bodies ? 46. What two forces are said to act always on bodies in opposition to each other ? 47. In what cases may we see the effect of heat in the expan- sion of bodies, or ip 1!< onnrtion of thmr particle? ' GENERAL PROPERTIES OF BODIES. 19 the attraction of cohesion is so far diminished that the par- ticles separate, and the butter becomes liquid. A similar effect is produced by heat on metals, and all bodies sus- ceptible of being melted. Liquids, you know, are made to boil by the application of heat : the attraction of soher sion then yields entirely to the expansive power ; the particles are totally separated and converted into steam or vapour. But the agency of heat is in no body more sen- sible than in air, which dilates and contracts by its in- crease or diminution in a very remarkable degree.* Emily. The effects of heat appear to be one of the most interesting parts of natural philosophy. Mrs. B. That is true ; but heat is so intimately con- nected with chemistry, that you must allow me to defer the investigation of its properties till you become ac- quainted with that science. To return to its antagonist, the attraction of cohesion ; it is this power which restores to vapour its liquid form, which unites it into drops when it falls to the earth in a shower of rain, which gathers the dew into brilliant gems on the blades of grass. Emily. And I have often observed that after a shower, the water collects into large drops on the leaves of plants ; but I ca.nnot say that I perfectly understand how the Attraction of cohesion)produces this effect. Mrs. B. Rain does not fall from the clouds in the form of drops^but in that of mist or vapour, which is composed of very small watery particles ; these in their descent, mutually attract each other, and those that are sulficient- Jy near in consequence unite and form a drop, and thus *\The expansive power of heat 'produces some of the most in- teresting phenomena in nature. J The boiling of liquids, is the im- mediate result of this power ; and the operation, although simple, is peculiarly worthy of notice. As the numerous particles become expanded or rarified, they are continually rising to, and escaping from the surface, which occasions an agitation of the liquid, pro- portioned, in its violence, to the degree of heat operating on it.-^And on exposing our hands or other limbs to the fire, the internal fluid becomes expanded) which causes them to appear swollen; whereas, when exposea to the cold, the abstraction of the heat causes them to be compressed. 48. Huw^re liquids made to boil by heut ; or how is the mo- tion or agitation of boiling liquids produced 9 49. Why are our hands and fingers swollen or larger on being held near the fire, than when exposed to the cold ?- 50. In what state does r,v,r. aJ: from t!,r "Viffc* > - ~-K1 What collects this mist or wp ur into -Jrops ' 20 GENERAL PROPERTIES OF BODIES. the mist is transformed into a shower. The dew also was originally in a stale of vapour, but is, by fthe mutual at- traction of the particles,)formed into small globules on the blades of grass: iu a similar manner the rain upon the leaf collects into large drops, which, when they become too heavy for the leaf to support, fall to the ground. Emily. AM this is wonderfully curious 1 I am almost bewildered with surprise and admiration at the number of new ideas I have already acquired. Mrs. B. Every step that you advance in the pursuit of natural science, will fill your mind with admiration and gratitude towards its Divine Author. In the study of natural philosophy, we must consider ourselves as read- ing the book of nature, in which the bountiful goodness and wisdom of God is revealed to all mankind ; no study can then tend more to purify the heart, and raise it to a religious contemplation of the Divine perfections. There is another curious effect of (|he attraction of co- hesionj which I must point out to you. It enables liquids to rise above their level in capillary tubes ; these are tubes, the bores of which are so extremely small that li- quids ascend within them, from the cohesive attraction between the particles of the liquid and the iuteriour sur- face of the tube. Do you perceive the water rising above its level in this small glass tube, which I have immersed in a goblet full of water ? Emily. Oh yes ; I see it slowly creeping up the tube, but now it is stationary ; will it rise no higher ? Mrs. B. No ; because the cohesive attraction be- tween the water and the internal surface of the tube is now balanced by the weight of the water within.. it : if the bore of the tube were narrower, the water would rise higher ; and if you immerse several tubes of bores of dif- ferent sizes, you will see it rise to different heights in each of them. In making this experiment, you should colour the water with a little red wine, in order to render the effect more obvious. All porous substances, such as sponge, bread, linen, y be considered as collections of capillary tubes j if you dip one end of a lump of sugar intovvater, the 5'2. What causes the dew on leaves and blades of 'grn.es to collect into drops ? 53. Why will liquids rise above their level in capillary tubes ? 54. On what principle do sponge, and other porous substances absorb liquids? GENERAL PROPERTIES OP BODIES. 21 water will rise in it; and wet it considerably above the surface of that into which you dip it. Emily. In making tea I have often observed that effect without being able to account for it. Mrs. B. Now that you are acquainted with the at- traction of cohesion, I must endeavour to explain to you that of {Gravitation, which is a modification of the same power; {the first is perceptible only in very minute par- ticles, and at very small distances; \the other acts on the lorgest bodies, and extends to immense distances/- Emily. You astonish me: surely you do not mean to say that large bodies attract each other. Mrs. B. Indeed I do: let us take, for example, one of the largest bodies in nature, and observe whether it does not attract other bodies. What is it that occasions the fall of this book, when I no longer support it? Emily. Can it be fhe attraction of the earth $ I thought that all bodies had a natural tendency to fall. Mrs. B. They have a natural tendency to fall, it is true; but that tendency is produced entirely by the at- traction of the earth; the earth being so much larger than any body, on its surface, forces every body, which is not supported, to fall upon it. Emily. If the tendency which bodies have to fall re- sults from the earth's attractive power, the earth itself can have no such tendency, since it cannot attract it- self, and therefore it requires no support to prevent it from falling. Yet the idea that bodies do not fall of their own accord, but that they are drawn towards the earth by its attraction, is so new and strange to me, that I know not how to reconcile myself to it. Mrs. B. When you are accustomed to consider the fall of bodies as depending on this cause, it will appear to you as natural, and surely much more satisfactory, than if the cause of their tendency to fall were totally unknown. Thus you understand, that all matter is at- tractive, from the smallest particle to the largest mass; and that bodies attract each other with a force (propor- tional to the quantity of matter they contain.^ Emily. I do not perceive any difference between the attraction of cohesion and that of gravitation : is it not be- 55. What is the difference between cohesive attraction and gravitation ? 56. What causes bodies to fall to the earth ? 57. In what proportion do bodies gravitate towards or attract each other? 22 GENERAL PROPERTIES OP BODIES. cause every particle of matter is endowed with an attrac- tive power, .that large bodies, consisting of a great num- ber of particles, are so strongly attractive 1 Mrs. B. True. There is, however, this difference between the attraction of particles and that of masses, that the former is stronger than the latter, in proportion to the quantity of matter. Of this you have an instance in the attraction of capillary tubes, in which liquids ascend by the attraction of cohesion, in opposition to that of gravity. It is on this account that it is necessary that the bore of the tube should be extremely small ; for if the column of water within the tube is not very minute, (^the attraction would not be able either to raise or support its weight, in opposition to that of gravity. You may observe, also, that all solid bodies are enabled by the force of the cohesive attraction of their particles to resist that of gravity, which would otherwise disunite them, and bring them to a level with the ground, as it does, in the case of liquids, the cohesive attraction of which is not sufficient to enable it to resist the power of gravity.* """"~~~~~~ * (The power of gravitation is greatest at the surface of the earth, "whence it decreases both upwards and downwards ; but not in the same proportion. The force of fjravity 'upwards is as the square of the distance from the centre. That is, giavity at the surface of the earth, which is about(4000 miles from Hie cen- tre, is four times more powerful than it would be at double ihat distance, or 8000 miles from the centre^ Gravity and weight may be taken, in particular circumstances, as synonymous terins. We say, a piece of lead weighs a pound, or sixteen ounces ; but if by any means it could be carried 4000 miles above the surface of the earth, it would weigh only one fourth of a pour d, or four ounces ; and if it could be transported to 8000 miles above the earth, which is three times the distance from the centre that the surface is, it would weigh only one ninth of a pound, ur something less than two ounces. And it is demonstrated, that the force of gravity downwards de- creases, as the distance from the surface increases, ^o that at one half the distance from the centre to the surface, the same weight 58. What example is given to s'low that cohesive attraction is stronger than gravitation ? 59. Why must tbe bore of capil- lary tubes be exceedingly small for water to rise in them ? 60. What would be the effect of gravitation on bodies, were it not for cohesive attraction ? (51. Where, is the power of gravity greatest ? 62. In what proportion does gravity decrease, Jrom the surface of the earth upwards ? 63. In what proportion does it decrease downwards ? GENERAL PROPERTIES OF P.ODIES. 23 .Emily. And some solid bodies appear to be of this nature, as sand and powder for instance : there is no at- traction of cohesion between their particles ? Mrs. B. Every grain of powder or sand is composed of a great number of other more minute particles, firmly united by the attraction of cohesion ; but amongst the separate grains there is no sensible attraction, because they are not in sufficiently close contact. JEmil. Yet they actually touch each other ? Mrs. B. The surface of bodies is in genera^ so rough and uneven, that when in actual contact, they touch each other only by a few points. \ Thus, if I lay. upon the table this book, the binding of which appears perfectly smooth, yet so few of the particles of its under surface come in contact with the table, that no sensible degree of cohesive attraction takes place ; for you see, that it does not stick, or cohere to the table, and I find no difficulty in lifting it off. It is only ijvhen surfaces perfectly flat and well polished\ are placed in contact, that the particles approach in suffi- cient number, and closely enough, to produce a sensible degree of cohesive attraction. Here are two hemispheres of polished metal, I pre^s their flat surfaces together, hav- ing previously interposed a few drops of oil, to fill up every little porous vacancy. Now try to separate them. already described would weigh on]y one half of a pound, and so on Thus, a piece of metal weighing, on the surface of the earth, one pound, will At the centre weigh ... 1000 miles from the centre, v 1-4 pound./ 2000 I J-2 3000 3-4 40uO 1 8000 1-4 12,000 1-9 And at the distance of the moon from the earth which is 240,000 miles, it would weigh only the 3, 600th part of a pound, because the d stance is 60 times further from the centre of the earth than the surface, 64. If a body weigh one pound at the surface of the earth, what will be its weight at the centre at 1000 a* 2000- at 3000 at 4000 at 8000 and at 12,000 miles from the centre of it ? 65. What is the reason that cohesive attraction doe? not ope- rate on different bodies brought into contact, as well as on the particles of the same body ? 66. When will the surfaces of different bodies adhere to each other by the force of cohesive attraction ? 24 ON THE ATTRACTION OF GRAVITY. Emily. It requires an effort beyond my strength, though there are handles for the purpose of pulling them asunder. Is the firm adhesion of the two hemispheres, merely owing to the attraction of cohesion ? Mrs. B. There is no force more powerful, since it is by this that the particles of the hardest bodies are held together. It would require a weight of several pounds, to separate these hemispheres. Emiiy. In making a kaleidoscope, I recollect that the two plates of glass, which were to serve as mirrors, stuck so fast together, that I imagined some of the gum 1 had been using had by chance been interposed between them ; but now I make no doubt but that it was their own natu- ral cohesive attraction which produced this effect. JbJrs. B. Very probably it was so ; for plate-glass has an extremely smooth, flat surface, admitting of the con- tact of a great number of particles, Between two plates, laid one over the other. Emily. But, Mrs. B. the cohesive attraction of some bodies is much greater than that of others ; thus, glue, gum, and paste, cohere with singular tenacity. Mrs. B. That is owing to the peculiar chemical pro- perties of those bodies, independently of their cohesive at- traction. There are some other kinds of modifications of attrac- tion peculiar to certain bodies ; namely, that of magnet- ism, and of electricity ; but we shall confine our attention merely to the attraction of cohesion and of gravity ; the examination of the latter we shall resume at our next meeting. CONVERSATION II. ON THE ATTRACTION OF GRAVITY. Attraction of Gravitation, continued ; Of Weight ; Of the Fall of Bodies ; Of the Resistance of the Air ; Of the Ascent of Light Bodies. EMILY. I HAVE related to my sister Caroline all that you have taught me of natural philosophy, and she has been so much delighted by it, that she hopes you will have the goodness to admit her to your lessons. ON THE ATTRACTION OF GRAVITY. 25 Mrs. B. Very willingly ; but I did not think you had any taste for studies of this nature, Caroline ? Caroline. 1 confess, Mrs. B., that hitherto I had form- ed no very agreeable idea, either of philosophy, or philo- sophers ; but what Emily has told me, has excited my curi- osity so much, that I shall be highly pleased if you will allow me to become one of your pupils. Mrs. B. I fear that I shall not find you so tractable a scholar as Emily ; I know that you are much biassed in favour of your own opinions. Caroline. Then you will have the greater merit in re- forming them, Mrs. B. ; and after all the wonders that Emily has related to me, I think I stand but little chance against you and your attractions. Mrs. B. You will, I doubt not, advance a number of objections ; but these I shall willingly admit, as they will be a means of elucidating the subject. Emily, do you recollect the names of the general properties of bodies ? Emily. Impenetrability, extension, figure, divisibility, inertia, and attraction. Mrs. B. Very well. You must remember that these are properties common to all bodies, and of which they cannot be deprived ; all other properties of bodies are called accidental, because they depend on the relation or connexion of one body to another. Caroline. Yet surely, Mrs. B., there are other proper- ties which are essential to bodies, besides those you have enumerated. Colour and weight, for instance, are com- mon to all bodies, arid do not arise from their connexion with each other, but exist in the bodies themselves ; these, therefore, cannot be accidental qualities. Mrs. B. I beg your pardon ; these properties do not exist in bodies independently of their connexion with other bodies. Caroline. What ! have bodies no weight ? Does not this table weigh heavier than this book ; and, if one thing weighs heavier than another, must there not be such a thing as weight ? Mrs. B. No doubt : but this property does not appear to be essential to bodies ; it depends upon their connex- G7. What were the names of the common or general properties of bodies given in the first Conversation ? 63. What are called the accidental properties of bodies ? 60. Are colour and weight common or accidental ^ev >erties ? 3 26 ON THE ATTRACTION OP GRAVtTV. ion with each other. Weight is an effect of the power of attraction, without which the table and the book would have no weight whatever- JEmily. I think I understand you ; is it not the at- traction of gravity, which makes bodies heavy 1 Mrs. B. You are right. I told you that the attrac- tion of gravity was proportioned to the quantity of mattei which bodies contained : now the earth consisting of a much greater quantity of matter than any body upon its surface, the force of its attraction must necessarily be greatest, and must draw every thing towards it ; in con- sequence of which, bodies that are unsupported fall to the ground, whilst those that are supported press upon the object which prevents their fall, with a weight equal to the force with which they gravitate towards the earth. Caroline. The same cause then which occasions the fall of bodies produces also their weight. It was very dull in me not to understand this before, as it is the na- tural and necessary consequence of attraction ; but the idea that bodies were not really heavy of themselves ap- peared to me quite incomprehensible. But, Mrs. B., if attraction is a property essential to matter, weight must be so likewise ; for how can one exist without the other ? Mrs. B- Suppose there were but one body existing in universal space, what would its weight be ? Caroline. That would depend upon its size ; or, more .accurately speaking, upon the quantity of matter it con- tained. Emily. No, no ; the body would have no weight, whatever were its. size ; because nothing would attract it. Am I not right, Mrs. B.? Mrs. B. You are : you must allow, therefore, that it would be possible for attraction to exist without weight ; for each of the particles of which the body was composed, would possess the power of attraction ; but they could exert it only amongst themselves ; the whole mass, hav- ing nothing to attract, or to be attracted by, would have no weight. Caroline. I am now well satisfied that weight is not essential to the existence of bodies ; but what have you 70. What is weight, or of what is it the effect ? 71. If there were but one body in the universe, would there be any such thing as weight? 72. Can cohesive attraction exist where there is no weight ? ON THE ATTRACTION OF GRAVITY. 27 to object to colours, Mrs. B. ? You will not, I think, deny that they really exist in the bodies themselves. Mrs. 13. When we come to treat of the subject of co- lours, I trust trtat I shall be able to convince you, that co- lours are likewise accidental qualities, quite distinct from the bodies to which they appear to belong. Caroline,. Oh do pray explain it to us now, I am so very curious to know how that is possible. Mrs. B. Unless we proceed with some degree of or- der and method, you will in the end find yourself but lit- tle the wiser for all you learn. Let us therefore go on regularly, and make ourselves well acquainted with the general properties of bodies, before we proceed further. Emily. To return, then, to attraction, (which appears to me by far the most interesting of them, since it belongs equally to all kinds of matter,) it must be mutual between two bodies ; and if so, when a stone falls to the earth, the earth should rise part of the way to meet the stone ? Mrs. B. Certainly ; but you must recollect that the force of attraction is proportioned to the quantity of mat- ter which bodies contain, and if you consider the differ- ence there is in that respect, between a stone and the earth, you will not be surprised that you do not perceive the earth rise to meet the stone ; for though it is true that a mutual attraction takes place between the earth and the stone, that of the latter is so very small in comparison to to that of the former, as to render its effect insensible. Emily. But since attraction is proportioned to the quantity of matter which bodies contain, why do not the hills attract the houses and churches towards them ? Caroline. You surprise me, Emily ; what an idea ! How can the houses and churches be moved, when they are so firmly fixed in the ground ? Mrs. B. Emily's question is not absurd, and your answer, Caroline, is perfectly just ; but can you tell us why the houses and churches are so firmly fixed in the ground. Caroline. I am afraid. I have answered right by mere chance ; for I be^in to suspect that bricklayers and car- penters could give but little stability to their buildings, without the aid of attraction. 73. If the attraction of gravitation is mutual between bodies, why do we not see the earth rise part way to meet the stone which is falling towards it ? 28 ON THE ATTRACTION OF GRAVITY. Mrs. B. It is certainly the cohesive attraction between the bricks and the mortar which enables them to build walls, and these are so strongly attracted &y the earth, as to resist every other impulse ; otherwise they would ne- cessarily move towards the hills and the mountains ; but the lesser force must yield to the greater. There are, how- ever, some circumstances in which the attraction of a large body has sensibly counteracted that of the earth. If, whilst standing on the declivity of a mountain, you hold a plumb-line in your hand, the weight will not fall perpen- dicular to the earth, but incline a little towards the moun- tain ; and this is owing to the lateral, or sideways attrac- tion of the mountain, interfering with the perpendicular attraction of the earth. Emily. But the size of a mountain is very trifling compared to the whole earth 1 Mrs. B. Attraction, you must recollect, diminishes with distance ; and in the example of the plumb-line, the weight suspended is considerably nearer to the mountain than to the centre of the earth ? Caroline. Pray, Mrs. B., do the two scales of a ba lance hang parallel to each other ? Mrs. B. You mean, I suppose, in other words, to in quire whether two lines which are perpendicular to the earth, are parallel to each other 1 I believe I guess the reason of your question ; but I wish you would endeavour to answer it without my assistance. Caroline. I was thinking that such lines must both tend by gravity to the same point, the centre of the earth ; now lines tending to the same point cannot be parallel, as parallel lines are always at an equal distance from each other, and would never meet. Mrs. B. Very well expl ained ; you see now the use of your knowledge of parallel lines : had you been igno- rant of their properties, you could not have drawn such a conclusion. This may enable you to form an idea of the great advantage to be derived even from a slight knowledge of geometry in the study of natural philoso- phy ; and if, after I have made you acquainted with the first elements, you should be tempted to pursue the study, 74. And why are not houses and other objects at the side of a mountain attracted or drawn away from their foundations towards it? 75. How can it be shown that mountains possess a side- ways attraction ? 7C. Would two lines suspended by weights be parallel to each other ? ON THE ATTRACTION OF GRAVITY. 29 I would advise you to prepare yourselves by acquiring some knowledge of geometry. This science would teach you that lines which fall perpendicular to the surface of a sphere cannot be parallel, because they would all meet, if prolonged to the centre of the sphere ; while lines that fall perpendicular to a plane or flat surface, are always parallel, because, if prolonged, they would never meet. Emily. And yet a pair of scales, hanging perpendicu- lar to the earth, appear parallel 1 Mrs. B. Because the sphere is so large, and the scales consequently converge so little, that their inclination is not perceptible to our senses ; if we could construct a pair of scales whose beam vvould extend several degrees, their convergence would be very obvious ; but as this cannot be accomplished, let us draw a small figure of the earth, and then we may make a pair of scales of the pro- portion we please, (fig. 1. plate 1.) Caroline. This figure renders it very clear : then two bodies cannot fall to the earth in parallel lines ? Mrs. B. Never. Caroline. The reason that a heavy body falls quicker than a light one, is, I suppose, because the earth attracts it more strongly ? Mrs. B. The earth, it is true, attracts a Heavy body more than a light one ; but that would not make the one fall quicker than the other. Caroline. Yet since it is attraction that occasions the fall of bodies, surely the more a body is attracted, the more rapidly it will fall. Besides, experience proves it to be so. Do we not every day see heavy bodies fall quickly, and light bodies slowly ? Emily. It strikes me, as it does Caroline, that as at- traction is proportioned to the quantity of matter, the earth must necessarily attract a body which contains a great quantity more strongly, and therefore bring it to the ground sooner than one consisting of a smaller quantity. Mrs. B. You must consider, that if heavy bodies are attracted more strongly than light ones, they require more attraction to make them fall, Remember that bo- 77. Whjr would they not be ? 78. Why is not their con- verorency perceptible ?- ~ - 79. What figure illustrates the con- vergency of two lines suspended perpendicularly to the surface of the earth ? 80. Do heavy and light bodies fall to the ground with equal rapidity ? 3* 30 ON THE ATTRACTION OF GRAVITY. dies have no natural tendency to fall, any more than to rise, or to move laterally, and that they will not fall un- less impelled by some force ; now this force must be pro- portioned to the quantity of matter it has to move : a body consisting of 1000 particles of matter, for instance, requires ten times as much attraction to bring it to the ground in the same space of time as a body consisting of only 100 particles. Caroline. I do not understand that ; for it seems to me that the heavier a body is, the more easily and rea- dily it fall?. Emily. I think I now comprehend it ; let me try if I can explain it to Caroline. Suppose that I draw towards me two weighty bodies, the one of lOOlbs., the other of lOOOlbs., must I not exert ten times as much strength to draw the larger one to me, in the same space of time as is required for the smaller one 1 And if the earth draw a body of lOOOlbs., v. eight to it in the same space of time that it draws a body of lOOlbs., does it not follow that it attracts the body of lOOOlbs. weight with ten times the force that it does that of lOOlbs. 1 Caroline. I comprehend your reasoning perfectly ; but if it were so, the body of lOOOlbs. weight, and that of lOOlbs. would fall with the same rapidity ; and the consequence would be, that all bodies, whether light or heavy, being at an equal distance from the ground, would fall to it in the same space of time : now it is very evident that this con- clusion is absurd ; experience every instant contradicts it ; observe how much sooner this book reaches the floor than this sheet of paper, when I let them drop together. Emily. That is an objection I cannot answer. I musl refer it to you, Mrs. B. Mrs. B. I trust that we shall not find it insurmount- able. It is true that, according to the laws of attraction, all bodies at an equal distance from the earth, should fall to it in the same space of time ; and this would actually take place if no obstacle intervened to impede their fall. But bodies fall through the air, and it is the resistance of the air which makes bodies of different density fall . with different degrees of velocity. They must 81. To what must the force of gravity be proportional neces- sary in causing bodies of different weights to faH to the ground? 82. What are the laws of attraction in regard to the falling of bodies at equal distances from the earth ? 83. But why then do heavy bodies fall quicker than light ones ? ON THE ATTRACTION OF GRAVITY. 31 all force their way through the air, but dense heavy bodies overcome this obstacle more easily than rarer and lighter ones. The resistance which the air opposes to the fail of bo- dies is proportioned to their surface, not to their weight; the air being inert, cannot exert a greater force to support the weight of a cannon-ball, than it does to support the weight of a ball (of the same size) made of leather ; but the cannon-ball will overcome this resistance more easily, and fall to the ground, consequently, quicker than the leather ball. Caroline. This is very clear, and solves the difficulty perfectly. The air offers the same resistance to a bit of lead and a bit of feather of the same size ; yet the one seems to meet with no obstruction in its fall, whilst the other is evidently resisted arid supported for some time by the air. Emily. The larger the surface of a body, then, the more air it covers, and the greater is the resistance it meets with from it. Mrs. B. Certainly : observe the manner in which this sheet of paper falls ; it floats awhile in the air, and then gently descends to the ground. I will roll the same piece of paper up into a ball : it offers now but a small surface to the air, and encounters therefore but little re- sistance : see how much more rapidly it falls. The heaviest bodies may be made to float awhile in the air, by making the extent of their surface counterbalance their weight. Here is some gold, which is the most dense body we are acquainted with, but it has been beaten into a very thin leaf, and offers so great an extent of surface in propor- tion to its weight, that its fall, you see, is still more retarded by the resistance of the air than that of the sheet of paper. Caroline. That is very curious ; and it is, I suppose, upon the same principle that iron boats may be made to float on w?ter ? But, Mrs. B., if the air is a real body, is it not also subjected to the laws of gravity ? Mrs. B. Undoubtedly. Caroline. Then why does it not, like all other bodies, fall to the ground 1 81. To what is the resistance, that the air opposes to falling bodies, proportioned ? 85. How can heavy bodies be made to float awhile in the air instead of falling immedunoly to 4 hfl srround? 86. Does the air gravitate towards the earth? 32 CX TUB ATTRACTION OF GRAVITY Mrs. B. On account of its spring or elasticity. The air is an ela stick fluid ; a species of bodies, the peculiar property of which is to resume, after compression, their ori- ginal dimensions ; and you must consider the air of which the atmosphere is composed as existing in a state of com- pression, for its particles being drawn towards the earth by gravity, are brought closer together than they would otherwise be, but the spring or elasticity of the air by which it endeavours to resist compression gives it a constant ten- dency to expand itself, so as to resume the dimensions it would naturally have, if not under the influence of gravity. The air may therefore be said constantly to struggle with the power of gravity without bejng able to overcome it. Gravity thus confines the air to the regions of our globe, whilst its elasticity prevents it from falling like other bo- dies to the ground. Emily. The air then is, I suppose, thicker, or I should rather say more dense, near the surface of the earth, than in the higher regions of the atmosphere ; for that part of the air which is nearer the surface of the earth must be most strongly attracted. Mrs. B. The diminution of the force of gravity, at so small a distance as that to which the atmosphere extends (compared with the size of the earth) is so inconsiderable as to be scarcely sensible ; but the pressure of the upper parts of the atmosphere on those beneath, renders the air near the surface of the earth much more dense than the upper legions. The pressure of the atmosphere has been compared to that of a pile of fleeces of wool, in which the lower fleeces are pressed together by the weight of those above ; these lie light and loose, in proportion as they approach the up- permost fleece, which receives no external pressure, and is confined merely by the force of its own gravity. Caroline. It has just occurred to me that there are some bodies which do not gravitate towards the earth. Smoke and steam, for instance, rise instead of falling. 87. Why then does it not fall like other bodies completely to the surface of the earth? 88. What two forces cont nually operate against each other on the air ? 89. Is the air of the same density at the surface of the earth as at a distance from it ? 90. At which is the density the greatest ? 91. Why is the air more dense at the surface of the earth than at a distance from it ? 92. To what has the pressure of the atmosphere been compared? ON THE ATTRACTION OP GRAVITY. 33 Mrs. B. It is still gravity which produces their as- cent ; at least, were that power destroyed, these bodies would not rise. Caroline. I shall be out of conceit with gravity, if it is so inconsistent in its operations. Mrs. B' There is no difficulty in reconciling this ap- parent inconsistency of effect. The air near the earth is heavier than smoke, steam, or other vapours ; it conse- quently not only supports these light bodies, but forces them to rise, till they reach a part of the atmosphere, the weight of which is not greater than their own, and then they remain stationary. Look at this basin of water : why does the piece of paper which I throw into it float on the surface ? Emily. Because, being lighter than the water, it is supported by it. Mrs. B. And now that I pour more water into the basin, why does the paper rise 1 Emily* The water being heavier than the paper, gets beneath it and obliges it to rise. Mrs. B. In a similar manner, are smoke and vapour forced upwards by the air ; but these bodies do not, like the paper, ascend to the surface of the fluid, because, as we observed before, the air being thinner and lighter as it is more distant from the earth,vapours rise only till they attain a region of air of their own density. Smoke,indeed, ascends but a very little way ; it consists of minute particles of fuel carried up by a current of heated air from the fire below : heat, you recollect, expands all bodies ; it consequently ra- refies air, and renders it lighter than the colder air of the atmosphere ; the heated air from the fire carries up with it vapour and small particles of the combustible materials which are burning in the fire. When this current of hot air is cooled by mixing with that of the atmosphere, the minute particles of coal or other combustible fall, and it is this which produces the small black flakes which render the air and every thing in contact with it, in London, so dirty. Caroline. You must, however, allow rne to make one more objection to the universal gravity of bodies ; which 93. How does gravity operate in causing smoke and steam to rise instead of falling to the earth ? 94. How high will they rise before they become stationary ? 95. What familiar illus- tration is given,of the principle upon which smoke and vapour ascend ? -96. Of what does smoke consist ? 34 ON THE ATTRACTION OF GRAVITY. is the ascent of air balloons, the materials of which are undoubtedly heavier than air : how, therefore, can they be supported by it ? Mrs. B. I admit that the materials of which balloons are made are heavier than the air ; but the air with which they are filled is an elastick fluid, of a different nature from the atmospherick air, and considerably lighter ; so that on the whole, the balloon is lighter than the air which it dis- places, and consequently will rise, on the same principle as smoke and vapour. Now, Emily, let me hear if you can explain how the gravity of bodies is modified by the effect of the air ? Emily. The air forces bodies which are lighter than itself to ascend ; those that are of an equal weight will remain stationary in it ; and those that are heavier will descend through it ; but the air will have some effect on these last ; for if they are not much heavier, they will with difficulty overcome the resistance they meet with in pass- ing through it, they will be borne up by it, and their fall will be more or less retarded. Mrs. B. Very well. Observe how slowly this light feather falls to the ground, while a heavier body, like this marble, overcomes the resistance which the air makes to its descent much more easily, and its fall is proportionally more rapid. I now throw a pebble into this tub of water ; it does not reach the bottom near so soon as if there were no water in the tub, because it meets with resistance from the water. Suppose that we could empty the tub, not only of water, but of air also, the pebble would then fall quicker still, as it would in that case meet with no resistance at all to counteract its gravity. Thus you see that it is not the different degrees of gravity, but the resistance of the air, which prevents bo- dies of different weight from falling with equal velocities ; if the air did not bear up the feather, it would reach the ground as soon as the marble. Caroline. I make no doubt that it is so ; and yet I do not feel quite satisfied. I wish there were some place void of air, in which the experiment could be made. Mrs. B. If that proof will satisfy your doubts, I can give it you. Here is a machine called an air pump, (fig. 2. pi. I.) by means of which the air may be expelled from 97. On what principle does a balloon rise, since it is made of materials heavier than the air through which it rises ? 98 How is the gravity of bodies modified by the effect of the air ? 99. What is the use of the air pump ? ON THE ATTRACTION OP GRAVITY. 35 any cfose vessel which is placed over this opening, through which the air is pumped out. Glasses of various shapes, usually called receivers, are employed for this purpose. We shall now exhaust the air from this tajl receiver v/hich is placed over the opening, and we shall find that bodies of whatever weight or size within it, will fall from the top to the bottom in the same space of time. Caroline. Oh, I shall be delighted with this experi- ment ; what a curious machine ! how can you put the two bodies of different weight within the glass, without admitting the air ? Mrs. Jb*. A guinea and a feather are already placed there for the purpose of the experiment : here is, you see, a contrivance to fasten them in the upper part of the glass ; as soon as the air is pumped out, I shall turn this Iit.de screw, by which means the brass plates which .support them will be inclined, and the two bodies will fall. Now I believe I have pretty well exhausted ib* air. Caroline. Pray let me turn the screw. I declare, they both reached the bottom ac the same instant ! Did you see, Emily, the feather appeared as heavy as the guinea ? Emily. Exactly ; and fell just as quickly. How won- derful this is! what a number of entertaining experi- ments might be made with this machine ! Sirs. B- No doubt there are a great many ; but we shall reserve them to elucidate the subjects to which they relate ; if I had not explained to you why the guinea and the feather fell with equal velocity, you would not have been so well pleased with the experiment. Emily. 1 should have been as much surprised, but not so much interested ; besides, experiments help to imprint on the memory the facts they are intended to illustrate ; it will be better therefore for us to restrain our curiosity, and wait for other experiments in their proper places. Caroline. Pray by what means is the air exhausted in this receiver 1 Mrs. B. You must learn something of mechanicks in order to understand the construction of a pump. At our next meeting, therefore, I shall endeavour to make you acquainted with the laws of motion, as an introduction to that subject. 100. Can a feather be placed in a situation to fall as quickly as a stone ? 101. In what manner can it be done ? 36 ON THE LAWS OF MOTION* CONVERSATION III. ON THE LAWS OF MOTION. On Motion; Of the Inertia of Bodies; Of Force to produce Motion ; Direction of Motion ; Velocity, Ab- solute and Relative ; Uniform Motion ; Retarded Mo- tion ; Accelerated Motion ; Velocity of Falling Bo- dies; Momentum; Action and Re-action Equal; Elasticity of Bodies ; Porosity of Bodies ; Reflected Motion ; Angles of Incidence and Rejlection. MRS. B. THE scienca of mechanicks is founded on the laws of motion ; it will, therefore, be necessary to make you ac- quainted with these laws before we examine the mecha- nical powers. Tell me, Caroline, what do you understand by the word motion ? Caroline. I think I understand it perfectly, though I am at a loss to describe it. Motion is the act of moving about, going from one place to another ; it is the contrary of remaining at rest. Mrs. B. Very well. Motion then consists in a change of place ; a body is in motion whenever it is changing its situation with regard to a fixed point. Now since we have observed that one of the general properties of bodies is Inertia, that is, an entire passiveness either with regard to motion or rest, it follows that a body cannot move without being put into motion ; the power which puts a body into motion is called force ; thus, the stroke of the hammer is the force which drives the nail ; the pulling of the horse that which draws the carriage, &/C. Force then is the cause which produces motion. Emily. And may we not say that gravity is the force which occasions the fall of bodies ? Mrs. B. Undoubtedly. I had given you the most fa- miliar illustrations in order to render the explanation clear ; but since you seek for more scientifick examples, you may say that cohesion is the force which binds the particles of bodies together, and heat that which drives them asunder. 102. On what is the science of mechanicks founded ? 103. What is to be understood by the term motion ? 104. What i' the power called that puts a body in motion ? ON THE LAWS OP MOTION. 37 The motion of a body acted upon by a single force is always in a straight line, in the direction in which it re- ceived the impulse. Caroline. That is very natural ; for as the body is in- ert, and can move only because it is impelled, it will move only in the direction in which it is impelled. The degree of quickness with which it moves, must, I suppose, also de- pend upon the degree of force with which it is impelled. Mrs. B. Yes ; the rate at which a body moves, or the shortness of the time which it takes to move from one place to another, is called its velocity ; and it is one of the laws of motion that the velocity of the moving body is proportional to the force by which it is put in motion. We must distinguish between absolute and relative ve- locity. The velocity of a body is called absolute, if we consider the motion of the body in space, without any reference to that of other bodies. When for instance a horse goes fifty miles in ten hours, his velocity is five miles an hour. The velocity of a body is termed relative, when com- pared with that of another body which is itself in motion. For instance, if one man walks at the rate of a mile an hour, and another at the rate of two miles an hour, the relative velocity of the latter is double that of the former, but the absolute velocity of the one is one mile, and that of the other two miles an hour. Emily. Let me see if I understand it. The relative velocity of a body is the degree of rapidity of its motion compared with that of another body ; thus, if one ship sail three times as far as another ship in the same space of time, the velocity of the former is equal to three times that of the latter. Mrs. B. The general rule may be expressed thus : the velocity of a body is measured by the space over which it moves, divided by the time which it employs in that motion : thus if you travel one hundred miles in twenty hours, what is your velocity in each hour ? 105. In what direction is the motion of a body acted on hy a single force ? -106. What is meant by the velocity of motion ? 107. To what is the velocity of a moving body proportional ? 103. What is called absolute velocity ? 109. When is the velocity of a moving body called relative ?- 110. What would be instances of relative velocity ? 111. What is the general rule for calculating the velocity of a moving body ? 4 38 ON THE LAWS OF MOTION. Emily. I must divide the space, which is one hundred miles, by the time, which is twenty hours, and the answer will be five miles an hour. Then, Mrs. B., may we not reverse this rule and say, that the time is equal to the space divided by the velocity ; since the space one hun- dred miles, divided by the velocity five miles, gives twen- ty hours for the time 1 Mrs. B. Certainly ; and we may say also that space is equal to the velocity multiplied by the time. Can you tell me, Caroline, how many miles you will have travelled, if your velocity is three miles an hour, and you travel six hours ? Caroline. Eighteen miles ; for the product of 3 mul- tiplied by 6, is 18. Mrs. B. I suppose that you understand what >s meant by the terms uniform, accelerated, and retarded motion. Emily. I conceive uniform motion to be that of a body whose motion is regular, and at an equal rate throughout ; for instance, a horse that goes an equal number of miles every hour. But the hand of a watch is a much better example, as its motion is so regular as to indicate the time. Mrs. 11. You have a right idea of uniform motion ; but it would be more correctly expressed by saying, that the motion of a body is uniform when it passes over equal spaces in equal times. Uniform motion is produced by a force having acted on a body once, and having ceased to act ; as for instance, the stroke of a bat on a cricket ball. Caroline. But the motion of a cricket ball is not uni- form ; its velocity gradually diminishes till it falls to the ground. Mrs. B. Recollect that the cricket ball is inert, and has no more power to stop than to put itself in motion ; if it falls, therefore, it must be stopped by some force supe- riour to that by which it was projected, and which destroys its motion. Caroline. And it is no doubt the force of gravity which counteracts and destroys that of projection ; but if there were no such power as gravity, would the cricket ball never stop 1 Mrs. B. If neither gravity nor any other force, such as the resistance of the air, opposed its motion, the cricket 112. When is the motion of a body termed uniform ? 113. How is uniform motion produced ? ON THE LAWS OP MOTION. 39 ball, or even a stone thrown by the hand, would proceed onwards in a right line, and with a uniform velocity ibr ever. Caroline. You astonish me ! I thought that it was im- possible to produce perpetual motion ? Mrs. B. Perpetual motion cannot be produced by art, because gravity ultimately destroys all motion that hu- man powers can produce. Emily. But independently of the force of gravity, I cannot conceive that the little motion I am capable of giving to a stone would put it in motion for ever. Mrs. R. The quantity of motion you communicate to the stone would not influence its duration : if you threw it with little force it would move slowly ; for its velocity, you must remember, will be proportional to the force with which it is projected ; but if there is nothing to obstruct its passage, it will continue to move with the same velocity, and in the same direction as when you first projected it. Caroline. This appears to me quite incomprehensible ; we do not meet with a single instance of it in nature. Mrs. B. I beg your pardon* When you come to study the motion of the celestial bodies, you will find that nature abounds with examples of perpetual motion ; and that it conduces as much to the harmony of the system of the universe as the prevalence of it would to the destruc- tion of all comfort on our globe. The wisdom of Provi- dence has therefore ordained insurmountable obstacles to perpetual motion here below ; and though these obstacles often compel us to contend with great difficulties, yet there,results from it that order, regularity, and repose, so essential to the preservation of all the various beings of which this world is composed. Now can you tell me what is retarded motion ? Caroline. Retarded motion is that of a body which moves every moment slower and slower: thus when I am tired with walking fast, I slacken my pace ; or when a stone is thrown upwards, its velocity is gradually di- minished by the power of gravity. Mrs. B. Retarded motion is produced by some force acting upon the body in a direction opposite to that which first put it in motion : you who are an animated being, endowed with power and will, may slacken your pace, or 114. What is the reason tint perpetual motion cannot be pro- duced ? 115. \Vhat is retarded motion? MG. How is re- tarded motion produced ? 40 ON THE LAWS OF MOTION. stop to rest when you are tired ; but inert matter is inca- pable of any feeling of fatigue, can never slacken its pace and never stop, unless retarded or arrested in its course by some opposing force ; and as it is the laws of inert bodies which mechanicks treat of, I prefer your illustra- tion of the stone retarded in its ascent. Now, Emily, it is your turn ; what is accelerated motion 1 Emily. Accelerated motion, I suppose, takes place when the velocity of a body is increased ; if you had not objected to our giving such active bodies as ourselves as examples, I should say that my motion is accelerated if I change my pace from walking to running. I cannot think of any instance of accelerated motion in inanimate bodies ; all motion of inert matter seems to be retarded by gravity. Mrs. B. Not in all cases ; for the power of gravitation sometimes produces accelerated motion ; for instance, a stone falling from a height moves with a regularly acce- lerated motion. Emily. True ; because the nearer it approaches the earth, the more it is attracted by it. Mrs. B. You have mistaken the cause of its accele- ration of motion ; for though it is true that the force of gravity increases as a body approaches the earth, the dif- ference is so trifling at any small distance from its surface as not to be perceptible. Accelerated motion is produced when the force which put a body in motion continues to act upon it during its motion, so that its motion is continually increased. When a stone falls from a height, the impulse which it re- ceives from gravity during the first instant of its fall, would be sufficient to bring it to the ground with a uniform ve- locity : for, as we have observed, a body having been once acted upon by a force, will continue to move with a uni- form velocity ; but the stone is not acted upon by gravity merely at the first instant of its fall this power continues to impel it during the whole of its descent, and it is this continued impulse which accelerates its motion. Emily. I do not quite undertand that. Mrs. B. Let us suppose that the instant after you have let fall a stone from a high tower, the force of gra- vity were annihilated, the body would nevertheless con- 117. What is accelerated motion? 118. What is in in- stance of accelerated motion ? 119. How does gravity accele- rate the motion of falling bodies ? ON THE LAWS OF MOTION. 41 tinue to move downwards, for it would have received a first impulse from gravity, and a body once put in motion will not stop unless it meets with some obstacle to impede its course ; in this case its velocity would be uniform, for though there would be no obstacle to obstruct its descent, there would be no force to accelerate it. EMly. That is very clear. Mrs. B. Then you have only to add the power of gravity constantly acting on the stone during its descent, and it will not be difficult to understand that its motion will become accelerated, since the gravity which acts on the stone during the first instant of its descent, will con- tinue in force every instant till it reaches the ground. Let us suppose that the impulse given by gravity to the stone during the first instant of its descent be equal to one, the next instant we shall find that an additional impulse gives the stone an additional velocity equal to one, so that the accumulated velocity is now equal to two ; the following instant another impulse increases the velocity to three, and so on till the stone reaches the ground. Caroline. Now I understand it ; the effects of preced- ing impulses must be added to the subsequent velocities. Mrs. B. Yes ; it has been ascertained both by expe- riment and calculations, which it would be too difficult for us to enter into, that heavy bodies descending from a height by the force of gravity, fall sixteen feet the first second of time, three times that distance in the next, five times in the third second, seven times in the fourth, and so on, regu- larly increasing their velocities according to the number of seconds during which the body has been falling. Emily. If you throw a stone perpendicularly upwards, is it not the same length of time ascending that it is de- scending ? Mrs. B. Exactly ; in ascending, the velocity is di- minished by the force of gravity ; in descending, it is ac- celerated by it. Caroline. 1 should then have imagined that it would have fallen quicker than it rose ? Mrs. B. You must recollect that the force with which it is projected must be taken into the account ; and that 1QO. What distance will a heavy body, suspended in the air, fall the first second of time ? What distance the second ? What the third ? 121. How does the time of an ascending botly al- ways compare with the time of its 'descent? 4* 42 ON THE LAWS OP MOTION. this force is overcome and destroyed by gravity before the body falls. Caroline. But the force of projection given to a stone in throwing it upwards, cannot always be equal to the force of gravity in bringing it down again, for the force of gravity is always the same, whilst the degree of im- pulse given to the stone is optional ; I may throw it up gently or with violence. Mrs. B. If you throw it gently, it will not rise high; perhaps only sixteen feet, in which case it will fall in one second of time. Now it is proved by experiment, that an impulse requisite to project a body sixteen feet upwards, will make it ascend that height in one second ; here then the times of the ascent and descent are equal. If ut sup- posing it be required to throw a stone twice that height, the force must be proportionally greater. You see then, that the impulse of projection in throw- ing a body upwards, is always equal to the action of the force of gravity during its descent ; and that. it. is the greater or less distance to which the body rises, that makes these two forces balance each other. I must now explain to you what is meant by the mo' mentum of bodies. It is the force, or power, with which a body in motion, strikes against another body. The momentum of a body is composed of its quantity of matter, multiplied by its quantity of motion ; in other words its weight and its velocity. Caroline. The quicker a body moves, the greater, no doubt, must be the force with which it would strike against another body. Emily. Therefore a small body may have a greater mo- mentum than a large one. provided its velocity be sufficient- ly greater ; for instance, the momentum of an arrow shot from a bow must be greater than a stone thrown by the hand. Caroline. We know also by experience, that the heavier a body is, the greater is its force ; it is not there- fore difficult to understand, that the whole power or mo- mentum of a body must be composed of these two pro- perties ; but I do not understand, why they should 122. To what is the impulse of projection, in throwing a body upwards, equal? 123- What is the momentum of a body? 124. Of what is the momentum of a body composed > 125. In what way can a smaller body have a greater momentum than a larger body ' ON THE LAWS OP MOTION. 43 be multiplied, the one by the other ; I should have sup- posed that the quantity of matter should have been added to the quantity of motion ? Mrs. B. It is found by experiment, that if the weight of a body is represented by the number 3, and its velocity also by 3, its momentum will be represented by 9 ; not 6, as would be the case, were these figures added, instead of being multiplied together. I recommend it to you to be careful to remember the definition of the momentum of bodies, as it is one of the most important points in mecha- nicks ; you will find, that it is from opposing motion to matter, that machines derive their powers.* The re-action of bodies is the next law of motion which I must explain to you. When a body in motion strikes against another body, it meets with resistance from it ; the resistance of the body at rest will be equal to the blow struck by the body in motion ; or to express myself in philosophical language, action and re-action will be equal, and in opposite directions. Caroline. Do you mean to say, that the action of the body which strikes, is returned with equal force by the body which receives the blow ? Mrs. B. Exactly. Caroline. But if a man strikes another on the face with his fist, he surely does not receive as much pain by the re-action as he inflicts by the blow ? Mrs. B. No ; but this is simply owing to the knuckles having much less feeling than the face. Here are two ivory balls suspended by threads, (plate 1. fig. 3.) draw one of them, A, a little on one side, now let it go ; it strikes you see against the other ball B, and drives it off, to a distance equal to that through which the * In comparing together the momenta of different bodies, we must be attentive to measure their weights and velocities, by the same denomination of weights and of spaces, otherwise the results would not agree. Thus if we estimate the weight of one body in ounces, we must estimate the weight of the rest also in ounces, and not in pounds ; and in computing the velocities, in like man* ner, we should adhere to the same standard of measure, both of space and of time ; as for instance, the number of feet in one se- cond, or of miles in one hour. 126. If the weight of a body be respresented by 3, and its ve- locity by 3, what will be its momentum ? 127. Wl-at is meant by the term re-action, in mechanicks ? 128. To xvhat is re-action equal ? 129. Whatdoes figure 3, Plate I. illustrate ? 44 ON THE LAWS OF MOTION. first ball fell ; but the motion of A is stopped, because when it struck B, it received in return a blow equal to that it gave, and its motion was consequently destroyed. Emily. 1 should have supposed that the motion of the ball A was destroyed, because it had communicated all its motion to B. Mrs. B. It is perfectly true, that when one body strikes against another, the quantity of motion communi- cated to the second body, is lost by the first ; bat this loss proceeds from the action of the body which is struck. Here are six ivory balls hanging in a row, (fig. 4.) draw the first out of the perpendicular, and let it fall against the second. None of the balls appear to move, you see, except the last, which flies off as far as the first ball fell ; can you explain this ? Caroline. I believe so. When the first ball struck the second, it received a blow in return, which destroyed its motion ; the second ball, though it did not appear to move, must have struck against the third; the re-action of which set it at rest ; the action of the third ball must have been destroyed by the re-action of the fourth, and so on till mo- tion was communicated to the last ball, which, not beinc* re-acted upon, flies off. Mrs. B. Very well explained. Observe, that it is only when bodies are elastick, as these ivory balls are. that the stroke returned is equal to the stroke given. I will show you the difference with these two balls of clay, (fig. 5.) which are not elastick ; when you raise one of these, D, out of the perpendicular, and let it fall against the other, E, the re-action of the latter, on account of its not being elastick, is riot sufficient to destroy the motion of the foi^ mer ; only part of the motion of D will be communicated to E, and the two balls will move on together to d and e which is not so great a distance as that through which D fell. Observe how useful re-action is in nature. Birds in fly- ing strike the air with their wings, and it is the re-action of the air which enables them to rise, or advance forwards ; re-action being always in a contrary direction to action. 130. How would you explain the operation of action and re- action, as illustrated by the six ivory balls in Figure 4, Plate I. : 131. Ts the re-action of all bodies equal to the action when a blow is given P 132. In what ones is it equo.l ? 133. What is the object of figure 5, Plate I. ? -134. How does this figure show that the re-action of non-elastick bodies is*not equal to the action ? 135. On what mechanical principle is it thai birds are able to fly. ON THE LAWS OP MOTION. 45 Caroline. I thought that birds might be lighter than the air, when their wings were expanded, and by trial means enabled to fly. Mrs. B. When their wings are spread, they are bet- ter supported by the air, as they cover a greater extent of surface ; but they are still much too heavy to remain in that situation, without continually flapping their wings, as you may have noticed, when birds hover over their nests : the force with which their wings strike against the air must equal the weight of their bodies, in order that the re-action of the air may be able to support that weight ; the bird will then remain stationary. If the stroke of the wings be greater than is required merely to support the bird, the re-action of the air will make it rise ; if it be less, it will gently descend ; and you may have observed the lark, sometimes remaining with its wings extended, bat mo- tionless : in this state it drops rapidly into its nest, Caroline. What a beautiful effect this is of the law of re-action ! But if flying is merely a mechanical operation, Mrs. B., why should we not construct wings, adapted to the size of our bodies, fasten them to our shoulders, move them with our arms, and soar into the air. Mrs. B. Such an experiment has been repeatedly at- tempted, but never with success ; and it is now considered as totally impracticable. The muscular power of birds is greater in proportion to their weight than that of man ; were we therefore furnished with wings sufficiently large to enable us to fly, we should not have strength to put them in motion. In swimming, a similar action is produced on the water, as that on the air in flying ; and also in rowing ; you strike the water with the oars, in a direction opposite to that in which the boat is required to move : and it is the re-ac- tion of the water on the oars which drives the boat along. Emily. You said, that it was in elastick bodies only, that re-action was equal to action ; pray what bodies are elastick besides the air. Mrs. B. In speaking of the air, I think we denned elasticity to be a property, by means of which, bodies that are compressed returned to their former state. If I bend 136. How must a bird strike the air with its wings so as to re- main stationary ? So as to rise ? So as to descend ? 137. If flying is only the effect of re-action, why could not a man be fur nished with wings so as to fly ? 138. How is swimming effect- ed ? 139. On what principle is a boat moved upon the water? 140. What is to be understood by the elasticity of a body? 46* ON THE LAWS OP MOTION. this cane, as soon as I leave it at liberty it recovers its former position ; if I press my finger upon your arm, as soon as I remove it, the flesh, by virtue of its elasticity, rises and destroys the impression I made. Of all bodies, the air is the most eminent for this property, and it has thence obtained the name of elastick fluid. Hard bodies are in the next degree elastick : if two ivory, or metallic balls are struck together, the parts at which they touch will be flattened : but their elasticity will make them instanta- neously resume their former shape. Caroline. But when two ivory balls strike against each other, as they constantly do on a billiard table, no mark or impression is made by the stroke. Mrs. B. I beg your pardon ; but you cannot perceive any mark, because their elasticity instantly destroys all trace of it. Soft bodies, which easily retain impression, such as clay, wax, tallow, butter, &,c. have very little elasticity ; but of all descriptions of bodies liquids are the least elastick. Emily. ' If sealing-wax were elastick, instead of retain- ing the impression of a seal, it would resume a smooth suriace as soon as the weight of the seal was removed. But pray what is it that produces the elasticity of bodies 1 Mrs. B. There is great diversity of opinion upon that point, and I cannot pretend to decide which ap- proaches nearest to the truth. Elasticity implies suscep- tibility of compression, and the susceptibility of compres- sion depends upon the porosity of bodies ; for were there no pores or spaces between the particles of matter of which a body is composed, it could not be compressed. Caroline. That is to say, that if the particles of bodies were as close together as possible, they could not be squeezed closer. Emily. Bodies then, whose particles are most distant from each other, must be most susceptible of compression, and consequently most elastick ; and this you say is the case with air, which is perhaps the least dense of all bodies ? Mrs. B. You will not in general find this rule hold good, for liquids have scarcely any elasticity, whilst hard bodies are eminent for this property, though the latter are certainly of much greater density than the former ; elas- 141. What bodies are most distinguished for elasticity? 142. What bodies are not elastick ? 143. On what is elasti- city supposed to depend ? ON THE LAWS OF MOTION. 47 ticity implies, therefore, not only a susceptibility of com- pression, but depends upon the power of resuming its for- mer state after compression. Caroline. But surely there can be no pores in ivory and metals, Mrs. B. ; how then can they be susceptible of compression 1 Mrs. B. The pores of such bodies are invisible to the naked eye, but you must not thence conclude that they have none ; it is, on the contrary, well ascertained that gold, one of the most dense of all bodies, is extremely po- rous, and that these pores are sufficiently large to admit water when strongly compressed to pass through them. This was shown by a celebrated experiment made many years ago at Florence. Emily. If water can pass through gold, there must certainly be pores or interstices which afford it a passage ; and if gold is so porous, what must other bodies be which are so much less dense than gold ! Mrs. B. The chief difference in this respect is, I be- lieve, that the pores in some bodies are larger than in others ; in cork, sponge, arid bread, they form considerable cavities ; in wood and stone, when not polished, they are generally perceptible to the naked eye ; whilst in ivory, me- tals, and all varnished and polished bodies, they cannot be discerned. To give you an idea of the extreme porosity of bodies, Sir Isaac Newton conjectured that if the earth were so compressed as to be absolutely without pores, its dimen- sions might possibly not be more than a cubic inch. Caroline. What an idea ! Were we not indebted to Sir Isaac Newton for the theory of attraction, I should be tempted to laugh at him for such a supposition. What insignificant little creatures we should be ! Mrs. B. If our consequence arose from the size of our bodies, we should indeed be but pigmies ; but remem- ber that the mind of Newton was not circumscribed by the dimensions of its envelope. Emily. It is, however, fortunate that heat keeps the pores of matter open and distended, and prevents the at- traction of cohesion from squeezing us into a nut-shell. Mrs. B. Let us now return to the subject of re-action, on which we have some further observations to make, 144. Is it supposed that ivory balls, metals, and other hard sub- stances are porous ? 145. Hovy has it been proved that gold is porous ? 146. What conjecture did Sir Isaac Newton form concerning the porosity of the earth ? 48 ON THE LAWS OF MOTION. It is re-action, being contrary to action, which produces reflected motion. If you throw a ball against the wall, it rebounds ; this return of the ball is owing to the re-action of the wall against which it struck, and is called reflected motion. Emily. And I now understand why balls filled with air rebound better than those stuffed with bran and wool, air being most susceptible of compression and most elas- tick, the re-action is more complete. Caroline. I have observed that when I throw a ball straight against the wall, it returns straight to my hand ; but if I throw it obliquely upwards, it rebounds still higher, and I catch when it falls. Mrs. B. You should not say straight, but perpendi- cularly against the wall ; for straight is a general term for lines in all directions which are neither curved nor bent, and is therefore equally applicable to oblique or perpendi- cular lines. Caroline. I thought that perpendicularly meant either directly upwards or downwards. Mrs. B. In those directions lines are perpendicular to the earth. A perpendicular line has always a reference to something towards which it is perpendicular ; that is to say, that it inclines neither to the one side nor the other, but makes an equal angle on every side. Do you understand what an angle is ? Caroline. Yes, I believe so : it is two lines meeting in a point. Mrs. B. Well then, let the line A B (plate II, fig. 1,) re- present the floor of the room, arid the line C D that in which you throw a ball against it : the line C D, you will observe, forms two angles with the line A B, and those two angles are equal. Emily. How can the angles be equal, while the lines which compose them are of unequal length ? Mrs. B. An angle is not measured by the length of the lines, but by their opening. Emily. Yet the longer the lines are, the greater is the opening between them. Mrs. B. Take a pair of compasses and draw a circle over these angles, making the angular point the centre. 147. What is reflected motion ? 148. What produces it ? 149." What is meant by a perpendicular line ? 150. What is an angle ? 151. What does Fig. 1 ? plate II. illustrate ? 1 52. By what is an angle measured ? ON THE I^YS OF MOTION. 49 Emily. To what extent must I open the compasses 1 Mrs. B. You may draw the circle what size you please, provided that it cuts the lines of the angles we are to measure. All circles, of whatever dimensions, are supposed to be divided into 360 equal parts, called de- grees; the opening of an angle, being therefore a portion of a circle, must contain a certain number of degrees ; the larger the angle, the greater the number of degrees, and the two angles are said to be equal when they con- tain an equal number of degrees. Emily. Now I understand it. As the dimensions of an angle depend upon the number of degrees contained between its lines, it is the opening and not the length of its lines, which determines the size of the angle. Mrs. B. Very well : now that you have a clear idea of the dimensions of angles, can you tell me how many degrees are contained in the two angles formed by one line falling perpendicular on another, as in the figure I have just drawn ? Emily. You must allow me to put one foot of the compasses at the point of the angles, and draw a circle round them, and then I think I shall be able to answer your question : the two angles are together just equal to half a circle, they contain therefore 90 degrees each ; 90 degrees being a quarter of 360. Mrs. B. An angle of 90 degrees is called a right angle, and when one line is perpendicular to another, it forms, you see, (fig. 1.) a right angle on either side. Angles containing more than 90 degrees are called obtuse angles (fig. 2 ;) and those containing less than 90 degrees are called acute angles, (fig. 3.) Caroline. The angles of this square table are right angles, but those of the octagon table are obtuse angle.s ; and the angles of sharp-pointed instruments are acute angles. Mrs. B. Very well. To return now to your obser- vation, that if a ball is thrown obliquely against the wall it will not rebound in the same direction \ tell me, have jou ever played at billiards ? 153. Into how many degrees are all circles divided ? - 154. When are two angles said to be equal ? - 155. How many de- grees are contained in the two angles formed by the figure named ? - 156. What is called a right angle ? An obtuse angle ?- -An acute angle ? 5 50 ON THE LAWS OF MOTION. Caroline. Yes, frequently ; and I have observed that when I push the ball perpendicularly against the cushion, it returns in the same direction ; but when I send it ob- liquely to the cushion, it rebounds obliquely, but on the opposite side ; the ball in this latter case describes an ang.e, the point of which is at the cushion. I have ob- served too, that the more obliquely the ball is struck against .the cushion, the more obliquely it rebounds on the opposite side, so that a billiard player can calculate with great accuracy in what direction it will return. Mrs. 13. Very well. This figure (fig. 4. plate II.) represents a billiard table ; now if you draw a line A B from the point where the ball A strikes perpendicular to the cushion, you will find that it will divide the angle which the ball describes into two parts, or two angles ; the one will show the obliquity of the direction of the ball in its passage towards the cushion, the other its ob- liquity in its passage back from the cushion. The first is called the angle of incidence, the other the angle of re* flection, and these angles are always equal.* Caroline. This then is the reason why, when I throw a ball obliquely against the wall, it rebounds in an oppo- site oblique direction, forming equal angles of incidence and of reflection. Mrs. jR. Certainly ; and you will find that the more obliquely you throw the ball, the more obliquely it will rebound. We must now conclude : but I shall have some further observations to make upon the laws of motion, at our next meeting. * The Angle, of Incidence, is that which is contained between the line described by the inciden* ray, and a line perpendicular to the surface on which the ray strikes, raised from the point of incidence. T.he Jingle of Reflection is that which is contained between the line described by the reflected ray, and a line per- pendicular to the reflecting surface at the point in which the in- cident ray strikes that surface. 157. How does the angle of incidence compare, as to size, with the angle of reflection? 158. How wouid you illustrate the angle of incidence arid reflection by Fig. 4, plate II ? 150. What is an angle of incidence ? ICO. What is an angle of re/lection ? ON COMPOUND MOTION. 51 CONVERSATION IV. ON COMPOUND MOTION. Compound Motion, the Result of two Opposite Forces ; Of Circular Motion, the Result of two Forces, one of which confines the .Body to a Fixed Point ; centre of Mo- tion, 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 confines a Body to 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 Centre of Weight, or point about which the Parts balance each other. MRS. B. I MUST now explain to you the nature of compound mo- tion. Let us suppose a body to be struck by two equal forces in opposite directions, how will it move 1 Emily. If the directions of the forces are in exact op- position to each other, I suppose the body would not move at all. Mrs. B. You are perfectly right ; but if the forces, instead of acting on the body in opposition, strike it in two directions inclined to each other, at an angle of nine- ty degrees, if the ball A (fig. 5, plate II.) be struck by equal forces at X and at Y, will it not move ? Emily. The force X would send it towards B, and the force Y towards C, and since these forces are equal, I do not know how the body can obey one impulse rather than the other, and yet I think the ball would move, be- cause as the two forces do not act in direct opposition, they cannot entirely destroy the effect of each other. Mrs. B. Very true ; the ball will therefore follow the direction of neither of the forces, but will move in a line between them, and will reach D in the same space of time that the force X would have sent it to B, and the lf>2. Of what does the fourth Conversation treat? 103. What would be the effect if two bodies were to strike each other, when movinr in opposite directions and with equal forces ? 16)4. "What would be the effect if they were to strike in directions inclined to each other, at an angle of ninety degrees ? 1G5* How would you explain Fig 5, plate II. ? 62 ON COMPOUND MOTION. force Y would have sent it to C. Now if you draw two lines from D, to join JB and C, you will form a square, and the oblique line which the body describes is called the diagonal of the square. Caroline. That is very clear, but supposing the two forces to be unequal, that the force X, for instance, be twice as great as the force Y 1 Mrs. B. Then the force X would drive the ball twice as far as the force Y, consequently you must draw the line A B (fig. 6.,) twice as long as the line A C, the body will in this case move to D ; and if you draw lines from that point to B and C, you will find that the ball has moved in the diagonal of a rectangle. Emily. Allow me to put another case ? Suppose the two forces are unequal, but do not act on the ball in the direction of a right angle, but in that of an acute angle, what will result 1 Mrs. B, Prolong the lines in the directions of the two forces, and you will soon discover which way the ball will be impelled ; it will move from A to D, in the diagonal of a parallelogram, (fig. 7.) Forces acting in the direction of lines forming an obtuse angle, will also produce motion in the diagonal of a parallelogram. For instance, if the body set out from B, instead of A, and was impelled by the forces X and Y, it would move in the dotted diagonal B C. We may now proceed to circular motion : this is the result of two forces on a body, by one of which it is pro- jected forward in a right line, whilst by the other it is confined to a fixed point. For instance, when I whirl this ball, which is fastened to my hand with a string, the ball moves in a circular direction ; because it is acted on by two forces, that which I give it which represents the force of projection, and that of the string which confines it to my hand. If during its motion you were suddenly to cut the string, the ball would fly off in a straight line : being released from confinement to the fixed point, it would be acted on but by one force, and motion produced by one force, you know, is always in a right line. 160. What is the oblique line called, which is described bv two equal forces moving in right angular directions? 167. \Vhat does Fig. 6, of that plate illustrate .' 168. What is illustrated by Fig. 7, plate II. ? 169. Of what is circular motion the re- sult 170. What simple instance of circular motion thus pro- duced could you give ? ON COMPOUND MOTION. 53 Caroline. This is a little more difficult to comprehend than compound motion in straight lilies* Mrs. B. You have seen a mop trundled, and have observed that the threads which compose the head of the mop fly from the centre ; but being confined to it at one end, they cannot part from it ; whilst the water they contain, being unconfined, is thrown off in straight lines* Emily. In the same way, the flyers of a windmill, when put in motion by the wind, would be driven straight forwards in a right line, were they not confined to a fixed point round which they are compelled to move. Mrs. B. Very well. And observe, that the point to which the motion of a small body, such as the ball with the string, which may be considered as revolving in one plane, is confined, becomes the centre of its motion. But when the bodies are not of a size or shape to allow of our considering every part of them as moving in the same plane, they in reality revolve round a line, which line is called the axis of motion. In a top, for instance, when spinning on its point, the axis is the line which passes through the middle of it, perpendicularly to the floor. Caroline. The axle of the flyers of the windmill is then the axis of its motion ; but is the centre of motion always in the middle of a body 1 Mrs. B. No, not always. The middle point of a body is called its centre of magnitude, or position, that is, the centre of its mass or bulk. Bodies have also another centre, called the centre of gravity, which I shall explain to you ; but at present we must confine ourselves to the axis of motion. This line you must observe re- mains at rest, whilst all the other parts of the body move around it; when you spin a top the axis is stationary whilst evn.-y c'lier part is in motion round it. Caroline. But a top generally has a motion forwards, besides its spinning motion ; and then no point within it can be at rest ? Mrs. B. What I say of the axis of motion relates only to circular motion ; that is to say, to motion round a line, and not to that which a body may have at the same time in any other direction. There is one circumstance 171. What is meant by the axis of motion ? 172. Is the centre of motion always in the middle of a body ? 173. What is the middle point of a body called? 174. How is the ve locity of motion at different distances from the axis of motion ? 54 ON COMPOUND MOtloM. in circular motion, which you must carefully attend to which is, that the further any part Of a body is from the axis of motion, the greater is its velocity ; as you approach that line, the velocity of the parts gradually diminish till you reach the axis of motion, which is perfectly at rest. " Caroline. But, if every part of the same body did not move with the same velocity, that part which moved quickest, must be separated from the rest of the body, and leave it behind ? Mrs. .B. You perplex yourself by confounding the idea of circular motion, with that of motion in a right line ; you must think only of the motion of a body round a fixed line, and you will find, that if the parts farthest from the centre had not the greatest velocity, those parts would not be able to keep up with the rest of the body, and would be left behind. Do not the extremities of the Vanes of a windmill move over a much greater space than the parts nearest the axis of motion f (pi. III. fig. 1.) The three dotted circles describe the paths in which three different parts of the vanes move, and though the circles are of different dimensions, the vanes describe each .of them in the same space of time. Caroline. Certainly they do ; and I now only wonder that we neither of us ever made the observation before ; and the same effect must take place in a solid body, like the top in spinning ; the most bulging part of the surface must move with the greatest rapidity. Mrs. B. The force which confines a body to a cen- tre, round which it moves, is called the centripetal force ; and that force which impels a body to fly from the centre is called the centrifugal force ; in circular motion these two forces constantly balance each other ; otherwise the revolving body would either approach tb^ centre, or re- cede from it, according as the one or the other prevailed, Caroline. When I see any body moving in a circle, I shall remember that it is acted on by two forces. Mrs. B. Motion, either in a circle, an ellipsis, or any other curve-line, must be the result of the action of two forces ; for you know, that the impulse of one single force always produces motion in a right line. 175. What figure illustrates this? 17G. What are the forces called in circular motion, that balance or act in opposition to each other? 177. What is meant by centripetal motion .* 178. What is meant by centrifugal motion .' Citt COMPOUND MOTI6N. $& Emily. And if any cause should destroy the centripetal force, the centrifugal force would alone impel the body, and it would, I suppose, fly off in a straight line from the centre to which it had been confined. Mrs. B. It would not fly off in a right line from the centre ; but in a right line in the direction in which it was moving, at the instant of its release ; if a stone, whirl- ed round in a sling, gets loose at the point A (plate III. fig. 2.) it flies off in the direction A B ; this line is called a tangent, it touches the circumference of the circle, and forms a right angle with a line drawn from that point of the circumference, to the centre of the circle C. Emily. You say, that motion in a curve-line i's owing to two forces acting upon a body ; but when I throw this ball in a horizontal direction, it describes a curve line in falling ; and yet it is only acted upon by the force of pro- jection ; there is no centripetal force to confine it, or pro- duce compound motion. Mrs. B. A ball thus thrown is acted upon by no less than three forces ; the force of projection, which you com- municated to it ; the resistance of the air through which it passes, which diminishes its velocity, without changing its direction ; and the force of gravity, which finally brings it to the ground. The power of gravity, and the resistance of the air, being always greater than any force of projection we can give a body, the latter is gradually overcome, and the body brought to the ground ; but the stronger the projectile force, the longer will these powers be in subduing it, and the further the body will go before it falls. Caroline. A shot fired from a cannon, for instance, will go much further, than a stone projected by the hand. Mrs. B. Bodies thus projected, you observed, describ- ed a curve-line in their descent ; can you account for that? Caroline. No ; I do not understand, why it should not fall in the diagonal of a square. Mrs. B. You must consider that the force of projec- tion is strongest when the ball is first thrown ; this force, 17!). What would be the consequence, if, in circular motion, the centripetal should be destroyed ? 180. Which figure il- lustrates this? 181. What is the line called in which a body would fly off, if the centripetal force were destroyed ? 182. If a ball is thrown horizontallv, how. many forces operate upon it ? 183. What are they called ? 56 ON COMPOUND MOTION. as it proceeds, being weakened by the continued resist- ance of the air, the stone, therefore, begins by moving in a horizontal direction ; but as the stronger powers pre- vail, the direction of the ball will gradually change from a horizontal to a perpendicular line. Projection alone would drive the ball A to B, (fig. 3,) gravity would bring it to C ; therefore, when acted on in different directions, by these two forces, it moves between, gradually inclining more and more to the force of gravity, in proportion as this accumulates ; instead therefore of reaching the ground at D, as you supposed it would, it falls somewhere about E. Caroline. It is precisely so ; look, Emily, as I throw this ball directly upwards, how the resistance of the air and gravity conquer projection ! Now I will throw it upwards obliquely : see, the force of projection enables it, for an instant, to act in opposition to that of gravity ; but it is soon brought down again. Mrs. B. The curve-line which the ball has described, is called in geometry, a parabola ; but when the ball is thrown perpendicularly upwards, it will descend perpen- dicularly ; because the force- of projection, and that of gravity, are in the same line of direction. We have noticed the centres of magnitude, and of mo- tion ; but I have not yet explained to you what is meant by the centre of gravity ; it is that point in a body, about which all the parts exactly balance each other ; if, there- fore, that point is supported, the body will not fall. Do you understand this ? Emily. I think so ; if the parts round about this point have an equal tendency to fall, they will be in equilibrium, and as long as this point is supported, the body cannot fall Mrs. B. Caroline, what would be the effect, were any other point of the body alone supported 1 Caroline. The surrounding parts, no longer balancing each other, the body. I suppose, would fall on the side at which the parts are heaviest. Mrs. B. Infallibly ; whenever the centre of gravity is unsupported, the body must fall. This sometimes hap- pens with an overloaded wagon winding up a sleep hill, 184. How would you explain Fig. 3. plate III. ? 185. What is a parabola? 180. Why will a stone thrown perpendicular- ly into the air descend perpendicularly :' 1^7. What is meant by the centre of gravity ? . .. 1 88. What part of a body must be supported to keep it from falling? ON COMPOUND MOTION. 57 one side of the road being more elevated than the other ; let us suppose it to slope as is described in this figure, (plate III. fig 4,) we will say, that the centre of gravity of this loaded wagon is at the point A. Now your eye will tell you that a wagon, thus situated, will over- set ; arid the reason is, that the centre of gravity, A, is not supported ; for if you draw a perpendicular line from it to the ground at C, it does not fall under the wagon within the wheels, and is therefore not supported by them. Caroline. I understand that perfectly ; but what i the meaning of the other point B ? Mrs. B. Let us, in imagination, take off the upper part of the load ; the centre of gravity will then change its situation, and descend to B, as that will now be the point about which the parts of the less heavily laden wa- gon will balance each other. Will the wagon now be upset ? Caroline. No, because a perpendicular line from that point falls within the wheels at D, and is supported by them ; and when the centre of gravity is supported, the body will not fall. Emily. Yet I should not much like to pass a wagon in that situation ; for, as you see, the point D is but just within the left wheel ; if the right wheel was merely raised, by passing over a stone, the point D would be thrown on the outside of the left wheel, and the wa- gon would upset. Caroline. A wagon, or any carriage whatever, will then be most firmly supported, when the centre of gra- vity falls exactly between the wheels; and that is the case in a level road. Pray, whereabouts is the centre of gravity of the hu- man body ? Mrs. B. Between the hips ; and as long as we stand upright, this point is supported by the feet ; if you lean on one side, you wrll find that you no longer stand finm A rope-dancer performs all his feats of agility, by dexte- rously supporting his centre of gravity ; whenever he finds that he is in danger of losing his balance, he shifts the heavy pole, which he holds in his hands, in order to throw 180. Whnt explanation would you give of Fig. 4, plate III. ? TOO. Why do persons in ascending a hill incline forward, and in descending it incline backward'? 191. How is it that rope- dancers are able to perform their feats- of agilky without fulling? 8 ON COMPOUND MOTION. the weight towards the side that is deficient ; and thus by changing the situation of the centre of gravity, he restores his equilibrium. Caroline. When a stick is poised on the tip of the finger, is it not by supporting its centre of gravity 1 Mrs. B. Yes ; and it is because the centre of gravity is not supported, that spherical bodies roll down a slope. A sphere being perfectly round, can touch the slope but by a single point, and that point cannot be perpendicularly under the centre of gravity, and therefore cannot be sup- ported, as you will perceive by examining this figure, (fig. 5. plate 111.) Emily. So it appears ; yet I have seen a cylinder of wood roll up a slope ; how is that contrived ? J/r.s. B. It is done by plugging one side of the cylin- der with lead, as at B. (fig. 5. plate III.) the body being no longer of a uniform density, the centre of gravity is removed from the middle of the body to some point in the lead, as that substance is much heavier than wood ; now you may observe that in order that the cylinder may roll down the plane, as it is here situated, the centre of gra- vity must rise, which is impossible ; the centre of gravity must always descend in moving, and will descend by the nearest and readiest means, which will be by forcing the cylinder up the slope, until the centre of gravity is sup- ported, and then it stops. Caroline. The centre of gravity, therefore, is not al- ways in the middle of a body. Mrs. B. No, that point we have called the centre of magnitude ; when the body is of a uniform density the centre of gravity is in the same point ; but when one part of the body is composed of heavier materials than another part, the centre of gravity being the centre of the weight of the body can no longer correspond with the centre of magnitude. Thus you see the centre of gravity of this cylinder, plugged with lead, cannot be in the same spot as the centre of magnitude. Emily. Bodies, therefore, consisting but of one kind 192. Why do spherical bodies roll down a slope or inclined plane? 193. By which figure is this illustrated? 194. How can a cylinder of wood be made to roll up a slope ? 195. is the centre of gravity always the centre of magnitude ? 196. When is the centre of gravity in the same point with the centre of magnitude ? 197. When will they not be in the same point ? ON COMPOUND MOTION. 59 l| of substance, as wood, stone, or lead, and whose densities are consequently uniform, must stand more firmly, and be more difficult to overset, than bodies composed of a variety of substances, of different densities, which may throw the centre of gravity on one side. Mrs. B. Yes; but there is another circumstance which more materially affects the firmness of their position, and that is their form. Bodies that have a narrow base are easily upset, for if they are the least inclined, their centre is no longer supported, as you may perceive in fig. G. Caroline. I have often observed with what difficulty a person carries a single pail of water ; it is owing, 1 suppose, to the centre of gravity being thrown on one side, and the opposite arm is stretched out to endeavour to bring it back to its original situation ; but a pail hanging on each arm is carried without difficulty, because they ba- lance each other, and the centre of gravity remains sup- ported by the feet. Mrs. B. Very well ; I have but one more remark to make on the centre of gravity, which is, that when two bodies are fastened together, by a line, string, chain, or any power whatever, they are to be considered as forming but one body ; if the two bodies be of equal weight, the centre of gravity will be in the middle of the line which unites them, (fig. 7,) but if one be heavier than the other, the centre of gravity will be proportionally nearer the heavy body than the light one. (fig. 8.) If you were to carry a rod or pole with an equal weight fastened at each end of it, you would hold it in the middle of the rod, in order that the weights should balance each other ; whilst if it had unequal weights at each end, you would hold it nearest the greater weight, to make them balance each other. Emily. And in both cases we should support the cen- tre of gravity ; and if one weight be very considerably larger than the other, the centre of gravity will be thrown out of the rod into the heaviest weight. (fig. 9.) Mrs. B. Undoubtedly. 198. What bodies stand most firmly, and what ones arc most easily upset? 199. What is the object of Fig. C, plate III.? 200. Why can a person carry two pails of water, one in each hand, easier than a single pail ? 201. Jf two bodies are connect- ed together, how are they to be considered as to their centre of gra* vity ? 202. If they are of equal weight, where will the centre of gravity be ? 203. If they are of unequal weight, where will it be ?- 204. What is the object of Fig. 7, 8, and 9, of plate 111. ? 60 ON THE MfiCHANICAL POWERS. CONVERSATION V. ON THE MECHANICAL POWERS. Of the Power of Machines ; Of the Lever in General; Of the Lever of the First Kind, having the fulcrum be- tween 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. MRS. B, WE may now proceed to examine the mechanical pow- ers ; they are six in number, one or more of which enters into the composition of every machine. The lever, the pulley, the wheel, and axle, the inclined plane, the ivedge, and the screw. In order to understand the power of a machine, there are four things to be considered, let. The power that acts : this consists in the effort of men or horses, of weights, springs, steam, &c. 2dly. The resistance which is to be overcome by the power ; this is generally a weight to be moved. The power must always be superiour to the resistance, other- wise the machine could not be put in motion. Caroline. If, for instance, the resistance of a carriage was greater than the strength of the horses employed to draw it, they would not be able to make it move. Mrs. B. 3dly. We are to consider the centre of mo- tion, or as it is termed in mechanicks, the fulcrum ; this, you may recollect, is the point about which all the parts of the body move; and lastly, the respective velocities of the power, and of the resistance. Emily. That must depend upon their respective dis- tances from the axis of motion ; as we observed in the motion of the vanes of the windmill. Mrs. B. We shall now examine the power of the lever. The lever is an inflexible rod or beam of any kind, that is to say, one which will not bend in any direction, 205. How many of the mechanical powers are there ': 206. What are the names of them ? '207. In order to un- derstand the power of a machine, how many things are to le considered ?- 303. What is the first ? the second ? the third \ 209. What is the lever ? ON THE MECHANICAL POWERS. 61 For instance, the steel rod to which these scales are sus- pended is a lever, and the point in which it is supported the fulcrum, or centre of motion; now, can you tell me why the two scales are in equilibrium ? Caroline. Being both empty, and of the same weight, they balance each other. Emily. Or, more correctly speaking, because the centre of gravity common to both is supported. Mrs. B. Very well ; and which is the centre of gra- vity of this pair of scales ? (fig. 1. plate IV.) Emily. You have told us that when two bodies of equal weight were fastened together, the centre of gravity was in the middle of the line that connected them ; the centre of gravity of the scales must therefore be in the ful- crum F of the lever which unites the two scales ; and cor- responds with the centre of motion. Caroline. But if the scales contained different weights, the centre of gravity would no longer be in the fulcrum of the lever, but removed towards that scale which contained the heaviest weight ; and since that point would no longer be supported, the heavy scale would descend and out- weigh the other. Mrs. B. True ; but tell me, can you imagine any . mode by which bodies of different weights can be made to balance each other, either in a pair of scales, or simply suspended to the extremities of the lever ? for the scales are not. an essential part of the machine, they have no me- chanical power, and are used merely for the convenience of containing the substance to be weighed. Caroline. What ! make a light body balance a heavy one ? I cannot conceive that possible. Mrs. B. The fulcrum of this pair of scales (fig. 2.) is moveable, you see ; I can take it off the prop, and fasten it on again in another part ; this part is now become the fulcrum, but it is no longer in the centre of the lever. Caroline. And the scales are no longer true ; for thai which hangs on the longest side of the lever descends. Mrs. B. The two parts of the lever divided by the ful- crum are called its arms, you should therefore say the longest arm, not the longest side of the lever. These 210. Why are the scales as seen in Fig. 1, plate IV. in equi- librium ? 211. What is the centre of gravity to two scales in equilibrium as seen in that figure ? 212. What are the arms of a lever ? 6 62 ON THE MECHANICAL POWERS. arms are likewise frequently distinguished by the appella- tions of the acting and the resisting part of the lever. Your observation is true that the balance is now de- stroyed ; but it Will answer the purpose of enabling you to comprehend the power of a lever when the fulcrum is not in the centre. Emily. This would be an excellent contrivance for those who cheat in the weight of their goods; by making the fulcrum a little on one side, and placing the goods in the scale which is suspended to the longest arm of the lever, they would appear to weigh more than they do in reality. Mrs. B. Yon do not consider how easily the fraud would be detected ; for on the scales being emptied, they would not hang in equilibrium. Emily. True ; I did not think of that circumstance. But I do not understand why the longest arm of the lever should not be in equilibrium with the other. Caroline. It is because it ig heavier than the shortest arm ; the centre of gravity, therefore, is no longer sup- ported. Mrs. B. You are right ; the fulcrum is no longer in the centre of gravity ; but if we can contrive to make the fulcrum in its present situation become the centre of gra- vity, the scales will again balance each other ; for you recollect that the centre of gravity is that point about which every part of the body is in equilibrium. Emily. It has just occurred to me how this may be accomplished ; put a great weight into the scale -suspended to the shortest arm of the lever, and a smalter one into that suspended to the longest arm. Yes/ I have disco- vered it look, Mrs. B., the scale on the shortest arm will carry 21bs., and that on the longest arm only one, to re- store the balance, (fig. 3.) Mrs. B. You see, therefore, that it is not so imprac- ticable as you imagined to make a heavy body balance a light one ; and this is in fact the means by which you thought an imposition in the weight of goods might be effected, as a weight of ten or twelve ounces might thus be made to balance a pound of goods. Let us now take 213. What is the reason that the arms of the lever, as seen Fiff. 2, plate IV. are not supported ? 214. In what way can they be made to support each other ? 215. What is illustrated by Fig. 3, plate IV. ? ON THE MECHANICAL POWERS. 63 off the scales that we may consider the lever simply ; and in this state you see that the fulcrum is no longer the cen- tre of gravity ; but it is, and must ever be, the centre of motion, as it is the only point which remains at rest, while the other parts move about it. Caroline. It now resembles the two opposite vanes of a windmill, and the fulcrum the point round which they move. Mrs. B. In describing the motion of those vanes, you may recollect our observing that the further a body is from the axis of motion, the greater is its velocity. Caroline. That I remember and understood perfectly. Mrs. B. You comprehend then, that the extremity of the longest arm of a lever must move with greater velocity than that of the shortest arm ? Emily. No doubt, because it is furthest from the cen- tre of motion. And pray, Mrs. B., when my brothers play at see-saw, is not the plank on which they ride a kind of lever ? Mrs. B. Certainly ; the log of wood which supports it from the ground is the fulcrum, and those who ride represent the power and the resistance at each end of the lever. And have you not observed that when those who ride are of equal weight, the plank must be sup- ported in the middle to make the two arms equal ; whilst if the persons differ in weight, the plank m :st be drawn a little further over the prop, to make the arms unequal, and the lightest person who represents the resistance, must be placed at the extremity of the longest arm. Caroline. That is always the case when I ride on a plank with my youngest brother ; I have observed also that the lightest person has the best ride, as he moves both further and quicker ; and 1 now understand that it is because he is more distant from the centre of motion. Mrs. B. The greater the velocity with which your little brother moves, renders his momentum equal to yours. Caroline. Yes ; I have the most gravity, he the great- est velocity ; so that upon the whole our momentums are equal. But you said, Mrs. B., that the power should be greater than the resistance to put the machine in motion ; how then can the plank move if the momentums of the persons who ride are equal -? 216. What is the velocity of the extremity of the longest arm of a lever compared with that of the shortest arm ? 64 ON THE MECHANICAL POWERS. Mrs. B. Because each person at his descent touches the ground with his feet ; the re-action of which gives him an impulse which increases his velocity ; this spring is requisite to destroy the equilibrium of the power and the resistance, otherwise the plank would not move. Did you ever observe that a lever describes the arc of a circle in its motion ? Emily. No ; it appears to me to rise and descend perpendicularly ; at least I always thought so. Mrs. B. I believe I must make a sketch of you and your brother riding on a plank, in order to convince you of your error, (fig. 4, pi. IV.) You may now observe that a lever can move only round the fulcrum, since that is the centre of motion ; it would be impossible for you to rise perpendicularly to the point A, or for your brother to descend in a straight line to the point B ; you must iu rising and he in descending describe arcs of your respective circles. This drawing shows you also how much superiour his velocity must be to yours ; for if you could swing quite round, you would each complete your respective circles in the same time. Caroline. My brother's circle being much the largest, he must undoubtedly move the quickest. Mrs. B. Now tell me, do you think that your brother could raise you as easily without the aid of a lever ? Caroline. Oh no, he could not lift me off the ground. Mrs. B. Then I think you require no further proof of the power of a lever, since you see what it enables your brother to perform. Caroline. I now understand what you meant by say- ing, that in mechanicks, motion was opposed to matter, for it. is my brother's velocity which overcomes my weight. Mrs. B. You may easily imagine, what enormous weights may be raised by levers of this description, for the longer the acting part of the lever in comparison to the resisting part, the greater is the effect produced by it ; because the greater is the velocity of the power com- pared to that of the weight. There are three different kinds of levers ; in the first the fulcruia is between the power and the weight. 217. What docs a lever in its motion describe ? 218. What is the design of Fig. 4, plate IV. ? 219. To what is the great- ness of effect produced by the lever proportional ? 220. How many kinds of levers are there ? ON THE MECHANICAL POWERS. 65 Caroline. This kind then comprehends the several levers you have described. Mrs. B. Yes, when in levers of the first kind, the ful- crum is equally between the power and the weight, as in the balance the power must be greater than the weight, in order to move it ; for nothing can in this case be gained by velocity ; the two arms of the lever being equal, the velocity of their extremities must be so likewise. The balance is therefore of no assistance as a mechanical power, but it is extremely useful to estimate the respective weights of bodies. But when (fig. 5.) the fulcrum F of a lever is not equally distant from the power and the weight, and that the power P acts at the extremity of the longest arm, it may be less than the weight W, its deficiency being compensated by its superiour velocity ; as we observed in the see-saw. Emily. Then when we want to lift a great weight, we must fasten it to the shortest arm of a lever, and apply our strength to the longest arm ? Mrs. B. If the case will admit of your putting the end of the lever under the weight, no fastening will be re- quired ; as you will perceive by stirring the fire. Emily. Oh yes ! the poker is a lever of the first kind, the point where it rests against the bars of the grate, whilst I am stirring the fire, is the fulcrum ; the short arm or resisting part of the lever is employed in lifting the weight, which is the coals, and my hand is the power ap- plied to the longest arm, or acting part of the lever. Mrs. B. Let me hear, Caroline, whether you can equally well explain this instrument, which is composed of two levers, united in one common fulcrum. Caroline. A pair of scissors ! Mrs. B. You are surprised, but if you examine their construction, you will discover that it is the power of the lever that assists us in cutting with scissors. Caroline. Yes ; I now perceive that the point at which the two levers are screwed together, is the fulcrum ; the handles, to which the power of the fingers is applied, 221. Where is the fulcrum in the first kind ? 222. How- are we to use levers of the first kind in raising large weights . J 223. What power of mechanicks do the common scissors involve ? 224. How may the scissors be explained as formed by the lever ? 6 * C6 ON THE MECHANICAL POWERS. are the extremities of the acting part of the levers, and the cutting part of the scissors, are the resisting parts of the levers : therefore, the longer the handles and the shorter the points of the scissors, the more easily you cut with them. Emily. That I have often observed, for when I cut pasteboard or any hard substance, I always make use of that part of the scissors nearest the screw or rivet, and I now understand why it increases the power of cutting ; but I confess I never should have discovered scissors to have been double levers ; and pray are not snuffers levers of a similar description ? Mrs. B. Yes, and most kinds of pincers ; the great power of which consists in the resisting part of the lever being very short in comparison of the acting part. Caroline. And of what nature are the two other kinds of levers 1 Mrs. 13. In levers of the second kind, the weight, instead of being at one "end, is situated between the power and the fulcrum, (fig. 6.) Caroline. The weight and the fulcrum have here changed places ; and what advantage is gained by this kind of lever ? Mrs. B. In moving it, the velocity of the power must necessarily be greater than that of the weight, as it is more distant from the centre of the motion. Have you ever seen your brother move a snow-ball by means of a strong stick, when it became too heavy for him to move without assistance ? Caroline. Oh yes ; and this was a lever of the second order (fig. 7.) ; the end of the stick, which he thrusts under the ball, and which rests on the ground, becomes the fulcrum ; the ball is the weight to be moved, and the power his hands applied to the other end of the lever. In this instance there is an immense difference in the length of the arms of the lever ; for the weight is almost close to the fulcrum. Mrs. B. And the advantage gained is proportional to this difference. Fishermen's boats are by levers of this description raised from the ground to be launched 2*25. How is the second kind of lever designated J 226. Which figures illustrate the use of levers of the second kind? 227. To what is the advantage gained in the use of the se- cond kind of lever proportionaJ ? ON THE MECHANICAL POWERS. 67 into the sea, by means of slippery pieces of board which are thrust under the keel. The most common example that we ha^e of levers of the second kind is in the doors of our apartments. Emily. The hinges represent the fulcrum, our hands the power applied to the other end of the lever ; but where is the weight to be moved ? Mrs. B. The door is the weight, and it consequently occupies the whole of the space between the power and the fulcrum. Nut-crackers are double levers of this kind ; the hinge is the fulcrum, the nut the resistance, and the hands the power. In levers of the third kind, (fig. 8.), the fulcrum is again at one of the extremities, the weight or resistance at the other, and it is now the power which is applied between the fulcrum and the resistance. Emily. The fulcrum, the weight, and the power, then, each in their turn, occupy some part of the middle of the lever between its extremities. But in this third kind of lever, the weight being further from the centre of motion than the power, the difficulty of raising it seems increased rather than diminished. Mrs. B. That is very true ; a lever of this kind is therefore never used, unless absolutely necessary, as is the case in lifting up a ladder perpendiculariy in order to place it against the wall ; the man who raises it cannot place his hands on the upper part of the ladder, the power, therefore, is necessarily placed much nearer the fulcrum than the weight. Caroline. Yes, the hands are the pow^r, the ground the fulcrum, and the upper part of the ladder the weight. Mrs. B. Nature employs this kind of lever in the structure of the human frame. In lifting a weight with the hand, the lower part of the arm becomes a lever of the third kind : the elbow is the fulcrum, the muscles of the /leshy part of the arm the power ; and as these are nearer to the elbow than the hand, it is necessary that their power should exceed the weight to be raised. 228. What are the most common examples of levers of the se- cond kind ? 229. How would you explain the opening of a common door, as involving the principle of the second kind of le- vers ? 230. What is the third kind of levers ? 231. What is an instance of its use ? 232. ' How docs the raising of a weight bv the hand represent this kind of lovers f 68 ON THE MECHANICAL POWERS. Emily. Is it not surprising that nature should have furnished us with such disadvantageous levers. Mrs. B. The disadvantage^ in respect to power, is more than counterbalanced by the convenience resulting from this structure of the arm : and it is no doubt that which is best adapted to enable it to perform its various functions. We have dwelt so long on the lever, that we must, re- serve the examination of the other mechanical powers to our next interview. CONVERSATION V. CONTINUED. ON THE MECHANICAL POWERS. Of the Pulley; Of the W tied and Axle; Of the Inclined Plane; Of the Wedge; Of the Screw. MRS. C. THE pulley is the second mechanical power we are to examme. You both, I suppose, have seen a pulley ? Caroline.. Yes, frequently : it is a. circular and flat piece of wood or metal, with a string which runs in a groove round it ; by means of which, a weight may be pulled up ; thus pulleys are used for drawing up curtains. Mrs. 13. Yes; but in that instance the pulleys are fixed, and do not increase the power to raise the weights, as you will perceive by this figure* (pi. V. fig. 1.) Observe that the fixed pulley is on the same principle as the lever of a pair of scales, in which the fulcrum F being in the centre of gravity, the power P and the weight W, are equally distant from it, and no advantage is gained. Emily. Certainly ; if P represents the power employ- ed to raise the weight W, the power must be greater than the weight in order to move it. But of what use then are pulleys in mechanicks '? 233 What is the second mechanical pow^r ? 234. Whs* is a pulley ? 235. How does Fig. 1. plate V. illustrate the fixed pullev ? 236. How must the power compare with the weight in order to move it, by the use of the fixed pulley ? ON THE MECHANICAL POWERS. f Mrs. B. The next figure represents a pulley which is not fixed, (fig. 2.) and thus situated you will perceive that it affords us mechanical assistance. In order to raise the weight (W) one inch, P, the power, must draw the strings B and C one inch each : the whole string is therefore shortened two inches, while the weight is raised only one. Emily. That I understand : if P drew the string but one inch, the weight would be raised only half an inch, because it would shorten the strings B and C half an inch each, and consequently the pulley, with the weight at- tached to it, can be raised only half an inch. '-\\- Caroline. I am ashamed of my stupidity ; but I con- fess that I do not understand this ; it appears to me that the weight would be raised as much as the string is short- ened by the power. Mrs. B. I will endeavour to explain it more clearly. I fasten this string to a chair, and draw it towards me ; I have now shortened the string, by the act of drawing it, one yard. Caroline. And the chair, as I supposed, has advanced one yard. Mrs. B. This exemplifies the nature of a single fixed pulley only. Now unfasten the string, and replace the chair where it stood before. In order to represent the moveable pulley, we must draw the chair forwards by put- ting the string round it ; one end of the string may be fas- tened to the leg of the table, and I shall draw the chair by the other end of the string. I have again shortened the string one yard ; how much has the chair advanced 1 Caroline. I now understand it ; the chair represents the weight to which the moveable pulley is attached ; and it is very clear that the weight can be drawn only half the length you draw the string. I believe the cir- cumstance ttafct perplexed me was, that I did not observe the difference that results from the weight being attached to the pulley, instead of being fastened to the string, as is the case in the fixed pulley. Emily. But I do not yet understand the advantage of pulleys ; they seem to me to increase rather than diminish the difficulty of raising weights, since you must draw the string double the length that you raise the weight ; whilst 237. What kind of pulley does Fig. 2, plate V. represent, and how would you explain it ? 70 ON THE MECHANICAL POWERS. with a single pulley, or without any pulley, the weight is raised as much as the string is shortened. Mrs. B. The advantage of a moveable pulley consists in dividing the difficulty ; we must draw, it is true, twice the length of the string, but then only half the strength is required that would be necessary to raise the weight without the assistance of a moveable pulley. Emily. So that the difficulty is overcome in the same manner as it would be, by dividing the weight into two equal parts, and raising them successively. Mrs. B. Exactly. You must observe, that with a moveable pulley the velocity of the power is double that of the weight, since the power P (fig. 2.) moves two inches, whilst the weight W moves one inch; therefore the power need not be more than half the weight to make their momentums equal. Caroline. Pulleys act then on the same principle as the lever, the deficiency of strength of the power being compensated by itsr superiour velocity.] Mrs. B. You will find that all mechanical power is founded on the same principle. Emily. But may it not be objected to pulleys, that a longer time is required to raise a weight by their aid than without it ; for what you gain in power you lose in time 1 Mrs. B. That, my dear, is the fundamental law in mechanicks : it is the case with the lever as well as the pulley ; and you will find it to be so with all the other mechanical powers. Caroline. 1 do not see any advantage in the mecha- nical powers then, if what we gain by them one way is lost another. Mrs. B. Since we are not able to increase our natu- ral strength, is not that science of wonderful utility, by means of which we may reduce the resistance or weight of any body to the level of our strength ? This the mechanical powers enable us to accomplish, by dividing the resistance of a body into parts which we can succes- 238. In what does the advantage of a moveable pulley consist ? 239. How do the weight and power of a moveable pulley com- pare, that their momenta be equal ? 240. On what principle are all mechanical powers founded? 241. Is there any loss of time in the use of the moveable pulley ? "242. And to what is this loss of time proportional ? 243. What then is the ad- vantage of this pulley, or of any of the mechanical powers, if thero is as much loss in time as gain in power ? ON THE MECHANICAL POWERS. 71 sively overcome. It is true, as you observe, that it requires a sacrifice of time to attain this end, but jou must be sensible how very advantageously it is exchanged for poxver ; the utmost exertion we can make adds but little to our natural strength, whilst we have a much more unlimited command of time. You can now under- stand, that the greater the number of pulleys connected by a string, the more easily the weight is raised, as the difficulty is divided among the number of strings, or rather of parts into which the string is divided by the pulleys. Several pulleys thus connected, form what is called a sys- tem, or tackle of pulleys, (fig. 3.) You may have seen them suspended from cranes to raise goods into ware- houses, and in ships to draw up the sails. Emily. But since a fixed pulley affords us no mecha- nical aid, why is it ever used ? Mrs. B. Though it does not increase our power, it is frequently useful for altering its direction. A single pulley enables us to draw up a curtain, by drawing down the string connected with it ; and we should be much at a loss to accomplish this simple operation without its as- sistance. Caroline. There would certainly be some difficulty in ascending to the head of the curtain, in order to draw it up. Indeed, I now recollect having seen workmen raise small weights by this means, which seemed to an- swer a very useful purpose. Mrs. B. In shipping, both the advantages of an in- crease of power and a change of direction, by means of pulleys, are united : for the sails are raised up the masts by the sailors on deck, from the change of direction which the pulley effects, and the labour is facilitated by the me- chanical power of a combination of pullejs. Emily, ^^the pulleys on ship-board do not appear to me to be tBp:ed in the manner you have shown us. Mrs. B. They are, I believe, generally connected as described in figure 4, both for nautical, and a variety of other purposes ; but in whatever manner pulleys are con- nected by a single string, the mechanical power is the same. 244. What is a system or tackle of pulleys, and which figure exhibits it ? 245. If there is no mechanical aid from the fixed pulley, why is it used ? 72 ON THE MECHANICAL POWERS. The third mechanical power is the wheel and axle. Let us suppose (plate V. fig. 5.) 'the weight W to be a bucket of water in a well, which we raise by winding the rope, to which it is attached, round the axle ; if this be done without a wheel to turn the axle, no mechanical as- sistance is received. The axle without a wheel is as impotent as a single fixed pulley, or a lever, whose fulcrum is in the centre ; but add the wheel to the axle, and you will immediately find the bucket is raised with much less difficulty. The velocity of the circumference of the wheel is as much greater than that of the axle, as it is further from the centre of motion : for the wheel describes a great circle in the same space of time that the axle describes a small one, therefore the power is increased in the same pro- portion as the circumference of the wheel is greater than that of the axle. If the velocity of the wheel is twelve times greater than that of the axle, a power nearly twelve times less than the weight of the bucket would be able to raise it. Emily. The axle acts the part of the shorter arm of the lever, the wheel that of the longer arm. Caroline. In raising water there is commonly, I be- lieve, instead of a wheel attached to the axle, only a crooked handle, which answers the purpose of winding the rope round the axle, and thus raising the bucket. Mrs. JK. In this manner (fig. 6.) ; now if you observe the dotted circle which the handle describes in winding up the rope, you will perceive that the branch of the han- dle A, which is united to the axle, represents the spoke of a wheel, and answers the purpose of an entire wheel ; the other branch B affords no mechanical aid, merely serving as a handle to turn the wheel. Wheels are a very essential part to most machines : they are employed in various ways ; bulJPhen fixed to the axle, their mechanical power is always the same ; that is, as the circumference of the wheel exceeds that of the -axle, so much will the energy of its power be increased. Caroline. Then the larger the wheel the greater must be its effect. 246. What is the third mechanical power ? 247. What does Fig. 5, plate V. illustrate ? 248. In what proportion is the power of the wheel increased ? 249. How may a wheel be compared to the lever ? 250. How does Fig. C, plate V. repre- sent a wheel ? ON THE MECHANICAL POWERS. Hfos. B. Certainly. If you have ever seen any con- siderable mills or manufactures, you must have admired the immense wheel, the revolution of which puts the whole of the machinery into motion ; and though so great an effect is produced by it, a horse or two has sufficient power to turn it ; sometimes a stream of water is used for that purpose, but of late years, a steam-engine has been found both the most powerful and the most convenient mode of turning the wheel. Caroline. Do not the vanes of a windmill represent a wheel, Mrs. B. ? Mrs. B. Yes ; and in this instance we have the ad- vantage of a gratuitous force, the wind, to turn the wheel. One of the great benefits resulting from the use of machinery is, that it gives us a sort of empire over the powers of nature, and enables us to make them perform the labour which would otherwise fall to the lot of man. When a current of wind, a stream of water, or the ex- pansive force of steam, performs our task, we have only to superintend and regulate their operations. The fourth mechanical power is the inclined plane ; this is nothing more than a slope, or declivity, frequently used to facilitate the drawing up of weights. It is not difficult to understand, that a weight may much more easily be drawn up a slope than it can be raised the same height perpendicularly. But in this, as well as the other mechanical powers, the facility is purchased by a loss of time, (fig. 7.) ; for the weight, instead of moving 1 directly from A to C, must move from B to O, and as the length of the plane is to its height, so much is the resistance of the weight diminished. Emily. Yes ; for the resistance, instead of being con- fined to the short line A C, is spread over the long line BC. qt Mrs. B. The wedge, which is the next mechanical power, is composed of two inclined planes, (fig. 8.) : you 251. On what mechanical force is the wind-mill operated ? 252. What is found to be the most powerful and convenient mode of turning the wheel ? 253. What is one of the great benefits resulting from the use of machinery ? 254. What is the fourth mechanical power? 255. What is an inclined plane ? 256. How would you explain Fig. 7, plate V. ? 257. How much is the resistance of the weight diminished by tho use of the inclined plane ? 238. 'Of what is the wedge com- posed ? 7 74 ON THE MECHANICAL POWERS. may have seen wood-cutters use it to cleave wood. The resistance consists in the cohesive attraction of the wood, or any other body which the wedge is employed to sepa- rate ; and the advantage gained by this power is in the proportion of half its width to its length ; for while the wedge forces asunder the coherent particles of the wood to A and JB, it penetrates downwards as far as C. Emily. The wedge, then, is rather a compound than a distinct mechanical power, since it is composed of two inclined planes. Mrs. B. It is so. All cutting instruments are con- structed upon the principle of the inclined plane, or the wedge : those that have but one edge sloped, like the chisel, may be referred to the inclined plane ; whilst the axe, the hatchet, and the knife (when used to split asun- der) are used as wedges. Caroline. But a knife cuts best when it is drawn across the substance it is to divide. We use it thus in cutting meat, we do not chop it to pieces. Mrs. B. The reason of this is, that the edge of a knife is really a very fine saw, and therefore acts best when used like that instrument. The screw, which is the last mechanical power, is more complicated than the others. You will see by this figure, (fig. 9,) that it is composed of two parts, the screw and the nut. The screw S is a cylinder, with a spiral protuberance coiled round it, called the thread ; the nut N is perforated to contain the screw, and the inside of the nut has a spiral groove made to fit the spiral thread of the screw. Caroline. It is just like this little box, the lid of which screws on the box as you have described ; but what is this handle which projects from the nut 1 Mrs. B. It is a lever, which is attached to the nut, without which the* screw is never used as^pmechanical power : the nut with a lever L attached to RIS commonly called a winch. The power of the screw, complicated as it appears, is referrible to one of the most simple of the mechanical powers ; which of them do you think it is ? 259. In what does the resistance of the wedge consist i 260. On what mechanical principles are cutting instiuments de- signed ? 2G1. ' Which is the last mechanical power ? 262. Of what is the scrow composed ? 263. What is the construc- tion of the screw and nut ? 264. How would you explain Fig. 9, plate V.? ON THE MECHANICAL POWERS. 75 Caroline. In appearance, it most resembles the wheel and axle. Mrs. B. The lever, it is true, has the effect of a wheel, as it is the means by which you wind the nut round ; but the lever is not considered as composing a part of the screw, though it is true, that it is necessarily attached to it. But observe, that the lever, considered as a wheel, is not fastened to the axle or screw, but moves round it, and in so doing, the nut either rises or descends, according to the way in which you turn it. Emily. The spiral thread of the screw resembles, I think, an inclined plane : it is a sort of slope, by means of which the nut ascends more easily than it would do if raised perpendicularly ; and it serves to support it when at rest, Mrs. B. Very well : if you cut a slip of paper in the form of an inclined plane, and wind it round your pencil, which will represent the cylinder, you will find that it makes a spiral line, corresponding to the spiral protu- berance of the screw, (fig. ]0.) Emily. Very true ; the nut then ascends an inclined plane, but ascends it in a spiral, instead of a straight line ; the closer the thread of the screw, the more easy the as- cent ; it is like having shallow, instead of steep steps to ascend. Mrs. B. Yes, excepting that the nut takes no steps, it gradually winds up or down ; then observe, that the clo- ser the threads of the screw, the greater the number of revolutions the winch must make ; so that we return to the old principle, what is saved in power is lost in time. Emily. Cannot the power of a screw be increased also, by lengthening the lever attached to the nut ? Mrs. B. Certainly. The screw, with the addition of the lever, forps a very powerful machine, employed either for compression, or to raise heavy weights. It is used by book-binders, to press the leaves of books together; it is used also in cider and wine presses, in coining, and for a variety of other purposes. All machines are composed of one or more of these six mechanical powers we have examined : I have but one 265. To which of the other mechanical powers is the screw refer rible : 266. How can the power of the screw be in- creased ? 76 ON THE MECHANICAL POWERS. more remark to make to you relative to them, which is, that friction in a considerable degree diminishes their force, allowance must therefore always be made for it in the construction of machinery. Caroline. By friction, do you mean one part of the machine rubbing against another part contiguous to it 1 Mrs. B. Yes ; friction is the resistance which bodies meet with in rubbing against each other ; there is no such thing as perfect smoothness or evenness in nature : polished metals, though they wear that appearance more than any other bodies, are far from really possessing it ; and their inequalities may frequently be perceived through a good magnifying glass. When, therefore, the surfaces of the two bodies come into contact, the prominent parts of the one will often fall into the hollow parts of the other, and occasion more or less resistance to motion. Caroline. But if a machine is made of polished metal, as a watch, for instance, the friction must be very trifling ? Mrs. B. In proportion as the surfaces of bodies are well polished, the friction is doubtless diminished ; but it is always considerable, and it is usually computed to de- stroy one-third of the power of a machine. Oil or grease is used to lessen friction ; it acts as a polish by filling up the cavities of the rubbing surfaces, and thus making them slide more easily over each other. Caroline. Is it for this reason that wheels are greased, and the locks and hinges of doors oiled 1 Mrs. B. Yes ; in these instances the contact of the rubbing surfaces is so close, and the rubbing so continual, that notwithstanding their being polished and oiled, a con- siderable degree of friction is produced. There are two kinds of friction ; the one occasioned by the sliding of the flat surface of a body, the other by the rolling of a circular body; the friction resulting from the first is much the most considerable, for great force is required to enable the sliding body to overcome 206. What diminishes the force of all machinery ? 607. What are we to understand by friction in machinery ? 203. In what proportion is the friction of machinery destroyed ? 209. How miifh of the power of a machine is reckoned to be destroyed by friction ? 270. What is commonly used to lessen the friction of machinery ? 271 . Why will oil and grease lessen the.friction of machinery ? 272. How many kinds of friction are there ? 273. What are they? 274. "Which is the most considerable ? ON THE MECHANICAL POWERS. 77 the resistance which the asperities of the surfaces in con- tact oppose to its motion, and it must be either lifted over or break through them ; whilst, in the other kind of fric- tion, the rough parts roll over each other with comparative facility ; hence it is, that wheels are often used for the sole purpose of diminishing the resistance of friction. Emily. This is one of the advantages of carriage- wheels ; is it not ? Mrs. B. Yes ; and the larger the circumference of the wheel, the more readily it can overcome any consider- able obstacles, such as stones, or inequalities in the road. When, in descending a steep hill, we fasten one of the wheels, we decrease the velocity of the carriage, by in- creasing the friction. Caroline. That is to say, by converting the rolling fric- tion into the dragging friction. And when you had casters put to the legs of the table, in order to move it more easily, you changed the dragging into the rolling friction. Mrs. B. There is another circumstance which we have already noticed, as diminishing the motion of bodies, and which greatly affects the power of machines. This is the resistance of the medium in which a machine is worked. All fluids, whether of the nature of air, or of water, are called mediums ; and their resistance is pro- portioned to their density ; for the more matter a body contains, the greater the resistance it will oppose to the motion of another body striking against it . Emily. It would then be much more difficult to work a machine under water than in the air ? Mrs. B. Certainly, if a machine could be worked in vacua, and without friction, it would be perfect; but this is unattainable : a considerable reduction of power must therefore be allowed for the resistance of the air. We shall here conclude our observations on the me- chanical powers. At our next meeting I shall endeavour to give you an explanation of the motion of the heavenly bodies. 275. Which will most readily overcome obstacles, a large or a small wheel r 276. Why is a wheel fastened on descending a hill ? 277. What besides friction diminishes the force of all machinery ? 278. What is meant by mediums .' 279. To what is their resistance proportioned ? 280. In what state would the force of machinery be perfect? 78 CAUSES OF THE EARTH. r g ANNUAL MOTION. CONVERSATION VI. CAUSES OF THE EARTIl's ANNUAL MOTION. Of the Planets, and their Motion ; Of the Diurnal J/o- ,tion of the Earth and Planets. CAROLINE. I AM come to you to-day quite elated with the spirit of opposition, Mrs. B, ; for I have discovered such a pow- erful objection to your theory of attraction, that I doubt whether even your conjuror Newton, with his magick wand of attraction, will be able to dispel it. Mrs. B. Well, my dear, pray what is this weighty objection ? Caroline. You say that bodies attract in proportion to the quantity of matter they contain : now we all know the sun to be much larger than the earth ; why, therefore, does it not attract the earth ; you will not, I suppose, pre- tend to say that we are falling towards the sun ? Emily. However plausible your objection appears, Caroline, I think you place too much reliance upon it : when any one has given such convincing proofs of saga- city and wisdom as Sir Isaac Newton, when we find that his opinions are universally received and adopted, is it to be expected that any objection we can advance should overturn them ? Caroline. Yet I confess that I am not inclined to yield implicit faith even to opinions of the great Newton : for what purpose are we endowed with reason, if we are denied the privilege of making use of it, by judging for ourselves ? Mrs. jB. It is reason itself which teaches us, that when we, novices in science, start objections to theories established by men of acknowledged wisdom, we should be diffident rather of our own than of their opinion. I am far from wishing to lay the least restraint on your ques- tions ; you cannot be better convinced of the truth of a system, than by finding that it resists all your attacks ; but I would advise you not to advance your objections 281. If bodies attract each other in proportion to the quantity of matter they contain, why does not the sun attract the earth completely to itself? CAUSES OF THE EARTH*S ANNUAL MOTION. t9 with so much confidence, in order that the discovery of their fallacy may be attended with less mortification. In answer to that you have just proposed, I can only say, that {the earth really is attracted by the sun. Caroline. Take care at least that we are not consum- ed by him, Mrs. B. Mrs. B. We are in no danger ; but our magician Newton, as you are pleased to call him, cannot extricate himself from this difficulty without the aid of some caba- listical figures, which I must draw for him. Let us suppose the earth, at its creation, to have been projected forwards into universal space : we know that if no obstacle impeded its course, (ff; would proceed in the same direction, and with a uniform velocity for evei^ In fig. 1, plate 6., A represents the earth, and S the sun. We shall suppose the earth to be arrived at the point in which it is represented in the figure, having a velocity which would carry it on to B in the space of one month; whilst the sun's attraction would bring it to C in the same space of time. Observe that the two forces of projection and attraction do not act in opposition, but perpendicu- larly, or at a right angle to each other. Can you tell me now, how the eaith will move 1 ~ Emily. I recollect your teaching us that a body act- ed upon by two forces perpendicular to each other would move in the> diagonal of a parallelogram; if, therefore, I complete the parallelogram by drawing the lines C D, B D, the earth will move in the diagonal A D. Mrs. B. A ball struck by two forces acting perpen- dicularly to each other, it is true, moves in the diagonal of a parallelogram ; but you must observe that the force of attraction is continually acting upon our terrestrial ball, and producing an incessant deviation from its course in a right line, which converts it into that of a curve line ; every point of which may be considered as constituting the diagoLal of an infinitely small parallelogram. 282. If the earth at its creation had been put in motion by a single force without resistance, what would have been its course ? 283. How would you illustrate this by the figure ? 284. What prevents the earth from proceeding on in a right line, as im- pelled by its projectile force ? 285. In what direction does the attraction of the sun operate on the projectile force of the earth ? 236 When two forces operate perpendicularly on each other, in what direction will be their compound motion ? 287. Why then is the line A D in Figure 1, circular instead of being a right line diagonal to the parallelogram, A B D C ? 80 CAUSES OF THE EARTH'S ANNUAL MOTION. Let us detain the earth a moment at the point D, and consider how it will be affected by the combined action of the two forces in its new situation. It still retains its ten- dency to fly off in a straight line; but a straight line would now carry it away to F, whilst the sun would at- tract it in the direction D S ; how then will it proceed ? Emily. It will go on in a curve line, in a direction between that of the two forces. Mrs. B. In order to know exactly what course the earth will follow, draw another parallelogram similar to the first, in which the line D F describes the force of pro- jection, and the line D S, that of attraction ; and you will find that the earth will proceed in the curve line D G. Caroline. You must now allow me. to draw a parallel- ogram, Mrs. B. Let me consider in what direction will the force of projection now impel the earth. Mrs. B. First draw a line from the earth to the sun representing the force of attraction : then describe the force of projection at a right angle to it. Caroline. The earth will then move in the curve G I, of the parallelogram G H I K. Mrs. B. You recollect that a body acted upon by two forces, moves through a diagonal in the same time that it would have moved through one of the sides of the parallelogram, were it acted upon by one force only. The earth has passed through the diagonals of these three parallelograms in the space of three months, and has performed one quarter of a circle ; and on the same principle it will go on till it has completed the whole of the circle. It will then recommence a course, which it has pursued ever since it first issued from the hand of its Creator, and which there is every reason to suppose it will continue to follow, as long as it remains in existence. Emily. What a grand and beautiful effect resulting from so simple a cause ! Caroline. It affords an example on a magnificent scale, of the circular motion which you taught us in mechanicks. (The attraction of the sun is the centripetal force, which confines the earth to a centre ; 'arid the im- 23S. How would you explain the continued motio'n of the earth about tho sun by the. use of Fig. 1, plate VI ? 289. What is the attraction of the sun called ? 290. And what is the projectile force of the earth called ? CAUSES OF THE EARTH ? S ANNUAL MOTION. 81 pulse of projection the centrifugal force) which impels the earth to quit the sun and fly off in a tangent. Mrs. B. Exactly so. A simple mode of illustrating the eifect of these combined forces on the earth, is to cut a slip of card in the form of a right angle, (fig. 2, plate VI.) to describe a small circle at the angular point re- presenting the earth, and to fasten the extremity of one of the legs of the angle to a fixed point, which we shall con- sider as the sun. Thus situated, the angle will represent both the centrifugal and centripetal forces ; and if you draw it round the fixed point, you will see how the di- rection of the centrifugal force varies, constantly forming a tangent to the circle in which the earth moves, as it is constantly at a right angle with the centripetal force. Emily. The earth, then, gravitates towards the sun without the slightest danger either of approaching nearer or receding further from it. How admirably this is con- trived ! If the two forces which produce this circular mo- tion had not been so accurately adjusted, one would ulti- mately have prevailed over the other, and we should either have approached so near the sun as to have been burnt, or have receded so far from it as to have been frozen. Mrs. B. What will you say, my dear, when I tell you that these two forces are not, in fact, so proportion- ed as to produce circular motion in the eart!i ? Caroline. You must explain to us, at least, in what manner we avoid the threatened destruction. Mrs. B. Let us suppose that when the earth is at A. (fig. 3.), its projectile force should not have given it a velocity sufficient to counterbalance that of gravity, so as to enable these powers conjointly to carry it round the sun in a circle ; the earth, instead of describing the line A C, as in the former figure, will approach nearer the sun in the line A B. Caroline. Under these circumstances, I see not what is to prevent our approaching nearer and nearer the sun till we fall into it : for its attraction increases as we ad- vance towards it, and produces an accelerated velocity in the earth, which increases the danger. 25)1. What simple illustration is given in Fig. 2, plate VI. of the combined forces, which produced the revolution of the earth about the sun ? 202. Does the earth revolve in an exact cir- cle about the sun ? 293. What is the desijm of Fig. 3, plate Vi. : 294. In Fig. 3, plate VI. why is the Dearth in the line at B instead of the line at C according to the principle of Fig. 1.? 82 CAUSES OP THE EARTfl's ANNUAL MOTION. Mrs. B. And there is yet another danger, of which you are not aware. Observe, that as the earth approaches the sun, the direction of its projectile force is no longer per- pendicular to that of attraction, but inclines more nearly to it. When the earth reaches that part of its orbit at B, the force of projection would carry it to D, which brings it nearer the sun instead of bearing it away from it. Emily. If, then, we are driven by one power and drawn by the other to this centre of destruction, how is it possible for us to escape 1 Mrs. B. A little patience, and you will find that we are not without resource. The earth continues approach- ing the sun with a uniformly increasing accelerated mo- tion, till it reaches the point E. In what direction will the projectile force now impel it 1 Emily. In the direction E F. Here then the two forces act perpendicularly to each other, and the earth is situat- ed just as it was in the preceding figure ; therefore, from this point, it should revolve round the sun in a circle. Mrs. B. No, all the circumstances do not agree. In motion round a centre, you recollect that the centri- fugal force increases with the velocity of the body, or, in other words, the quicker it moves, the stronger is its ten- dency to fly off in a right line. When the earth, there- fore, arrives at E, its accelerated motion will have so far increased its velocity, and consequently its centrifugal force, that t/e latter will prevail ovnr the force of at- traction, and drag the earth away from the sun till it reaches G. Caroline. It is thus, then, that we escape from the dangerous vicinity of the sun ; and in proportion as we recede from it, the force of its attraction, and, conse- quently, the velocity of the earth's motion are dimi- nished. Mrs. B. Yes. From G the direction of projection is towards II, that of attraction towards S, and the earth proceeds between them with a uniformly retarded motion, till it has completed its revolution. Thus you see, that the earth travels round the sun, not in a circle, but an 295. When the earth arrives at E in the figure, why does it not revolve in a small circular orbit instead of receding off in the direction G ? 296. What is the figure called that the earth de- scribes in its revolution about the sun? 83 ellipsis, of which the sun occupies one of the foci; and that in its coarse the earth alternately approaches, and recedes from it, without any danger of being either swal- lowed up, or being entirely carried away from it. Caroline. And 1 observe, that what I apprehended to be a dangerous irregularity, is the means by which the most perfect order and harmony are produced ! Emily. The earth travels, then, at a very unequal rate, its velocity being accelerated as it approaches the sun, and retarded as it recedes from it. Mrs. B. It is mathematically demonstrable, that, in moving round a point towards which it is attracted, a body passes over equal areas in equal timesy The whole of the space contained within the earth's orbit, is in fig. 4., di- vided into a number of areas, or spaces, 1, 'J, 3, 4, &,c. all of which are of equal dimensions, though of very different forms ; some of them, you see, are long and narrow, others broad and short : but they each of them contain an equal quantity of space. An imaginary line drawn from the cen- tre of the earth to that of the sun, and keeping pace with the earth in its revolution, passes over equal areas in equal times ; that is to say, if it is a month going from A to B, it will be a month going from B to C, and another from C to E, and so on. Caroline. What long journeys the earth has to per- form in the course of a month, in one part of her orbit, and how short they are in the other part ! Mrs. B. The inequality is not so considerable as ap- pears in this figure ; for the earth's orbit is not so eccen- trick as it is there described ; and, in reality differs but little from a circle ; that part of the earth's orbit nearest the sun is called its Perihelion, that part most distant from the sun -its Aphelion ; and the earth is above three millions of miles nearer the sun at its perihelion than at its aphelion. 297. What is the name of the place occupied by the sun with- in the orbit of the earth ? 298. Is the earth's motion in moving round the sun uniform ? 299. What is mathematically demon- strable in relation to a body moving round a point towards which it is attracted ? 300. What is the design of Fijj. 4, plate VI. ? 301. What is that part of the earth's orbit called which is most dis- tant from the sun ? 302. What is that part called which is nearest the sun ? 303. How much nearer is the earth to the sun in perihelion than at its aphelion ? 84 Emily. I think I can trace a consequence fiom these different situations of the earth ; is it not the cause of summer and winter ? Mrs. B. On the contrary ; during the height of sum- mer, the earth is in that part of its orbit which is most distant from the sun, and it is during the severity of (win- ter, that it approaches nearest to it. Emily. That is very extraordinary ; and how then do you account for the heat being greatest, when we are most distant from the sun ? Mrs. B. The difference of the earth's distance from the sun in summer and winter,when compared with its total distance from the sun, is but inconsiderable. The earth, it is true,V'is above three millions of miles nearer the sun in winter than in summer""; bat that distance, however great it at first appears, sinks into insignificance in comparison of 95 millions of miles, which is our mean distance from the sun. The change of temperature, arising from this diffe- rence, would scarcely be sensible, were it not completely overpowered by other causes which produce the variations of the seasons ; but these I shall defer explaining till we have made some further observations on the heavenly bodies. Caroline. And should not the sun appear smaller in summer, when it is so much further from us ? Mrs. B. It actually does when accurately .measured ; but the apparent difference in size, is, I believe, riot per- ceptible to the naked eye. Emily. Then, since the earth moves with the greatest velocity in that part of its orbit nearest the sun, it must have completed its journey through one half of its orbit in a shorter time than the other half? Mrs. B. Yes, it is about seven days longer perform- ing the summer-half of its orbit, than the winter-half. The revolution of all the planets round the sun is the re- sult of the same causes, and is performed in the same manner as that of the earth. Caroline. Pray what are the planets ? Mrs. B. They are those celestial bodies, which re- volve like our earth about the sun); they are supposed to resemble the earth also in many other respects ; and we 304. Is ihe earth nearest the sun in summer or winter ? 305. ITow much longer is the earth performing the summer-half than the winter-half of its orbit ? 306. What are the planets ? CAUSES OF THE EARTH'S ANNUAL MOTION. 85 are led by analogy to suppose them to be inhabited worlds. Caroline. I have heard so ; but do you not think such an opinion too great a stretch of the imagination 1 Mrs. 13. Some of the planets are proved to be larger than the earth ; (it is only their immense distance from us, which renders their apparent dimensions so smalK Now, if we consider them as enormous globes, instead of small twinkling spots, we shall bo led to suppose, that the Al- mighty would not have created them merely for the pur- pose of giving us a littje light in the night, as it was formerly imagined, and we should find it more consistent with our ideas of the Divine wisdom and beneficence to suppose that these celestial bodies should be created for the habitation of beings, who are, like us, blessed by his providence. Bol.h in a moral as well as a physical point of view, it appears to rne more rational to consider the planets as worlds revolving round the sun ; and the fixed stars as other suns, each of them attended by their re- spective system of planets, to whiih they impart their in- fluence. We have brought our telescopes to such a de- gree of perfection, that from the appearances which the moon exhibits when seen through them, we have very good reason to conclude, that it is a habitable globe, for though it is true, that we cannot discern its towns and people, we can plainly perceive its mountains and val- leys ; and some astronomers have gone so far as to ima- gine they discovered volcanoes. Emily. If the fixed stars are suns, with planets re- volving round them, why should we not see those planets as well as their suns ? Mrs. B. ^.* a person move in the latitude of London, in consequence ci" iii earth's motion upon its axis ? ON THE PLANETS. 101 Mrs. B. A moment's reflection would convince you of the contrary ; a person at the equator must move quicker than one situated near the poles, since they both perform a revolution in 24 hours* Emily. True, the equator is furthest from the axis of motion. But in the earth's revolution round the sun, every part must move with equal velocity 1 Mrs. B. Yes, about afthousand miles a minute. Caroline. How astonishing ! and that it should be possible for us to be insensible of such a rapid motion. You would not tell me this sooner, Mrs. B., for fear of increasing my incredulity. Before the time of Newton, was not< the earth supposed to be in the centre of the system, and the sun, moon, and stars to revolve round it)? Mrs. B. This was'the system of Ptolemy in ancient times ;/but as long ago as t>he beginning of the sixteenth century It was discarded,! and the solar system, such as I have shown you, was established by the celebrated 'as- tronomer Copernicus, and is hence called the Copernican system. But the theory of gravitation, the source from which this beautiful and harmonious arrangement flows, we owe to the powerful genius of Newton, who lived at a much later period. Emily. It appears, indeed, far less difficult to trace by observation the motion of the planets, than to divine by what power they are impelled and guided. I wonder how the idea of gravitation could first have occurred to Sir Isaac Newton ? Mrs. B. It is said to have been occasioned by a cir- cumstance from which one should little have expected so grand a theory to have arisen. During the prevalence of the plague in the year 1665, Newton retired into the country to avoid the contagion : /when sitting one day in his orchard he observed an apple f fall from a tree, and was led to consider what could be the cause which brought it to the ground. ) 304. How fast does the earth move in its revolution about the sun ? 395. What was the system of Ptolemy concerning as- tronomy ? 396. What is the present system of astronomy called f 397. When was the Copernican system of astronomy adopted ? 398. What important discovery did Newton make touching the Copernican system ? 399. What led Newton to make his discoveries ? 9* 102 ON THE EARTH. Caroline. If I dared to confess it, Mrs. B., I should say that such an inquiry indicated rather a deficiency than a superiority of intellect. I do not understand how any one can wonder at what is so natural and so common. Mrs. B. It is the mark of superiour genius to find matter for wonder, observation, and research, in circum- stances which, to the ordinary mind, appear trivial, be* cause they are common, and with which they are satis- fied, because they are natural, without reflecting that na- ture is our grand field of observation, that within it is con- tained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God,'' Thus, it was the simple cir- cumstance of the fall of an apple, which led to the discovery of the laws upon which the Copernican system is found- ed ; arid whatever credit this system had obtained before, it now rests upon a basis from which it cannot.be shaken. Emily. This was a most fortunate apple, and more' worthy to be commemorated than all those that have been sung by the poets. The apple of discord for which the goddesses contended ; the golden apples by which Ata- lanta won the race ; nay, even the apple which William Tell shot from the head of his son, cannot be compared to this ! CONVERSATION VIII. ON THE EARTH. Of the Terrestrial Globe ; Of the Figure of the Earth j Of the Pendulum; Of the Variation of the Seasons, and of the Length of Days and Nights ; Of the Canres of the Heat of Summer ; Of Solar, Sidereal, and Equal or Mean Time. MRS. B. As the earth is the planet in which we are the most particularly interested, it is my intention this morning, to explain to you the effects resulting from its annual and 400. What does Mrs. Marcet consider a mark of superiour genius ? ON THE EARTH. 10J> diurnal motions ; but for this purpose it will be necessa- ry to make you acquainted with the terrestrial globe : you have not either of you, I conclude, learnt the use of the globes ?* Caroline. No ; I once indeed learnt by heart the names of the lines marked on the globe, but as I was in- formed they were only imaginary divisions, they did not appear to me worthy of much attention, and were soon forgotten. Mrs. B. You suppose, then, that astronomers had been at the trouble of inventing a number of lines to little purpose. It will be impossible for me to explain to you the particular effects of the earth's motion without your having acquired a knowledge of these lines : in piate VIII. fig. 2. you will find them all delineated ; and you must learn them perfectly if you wish to make any profi- ciency in astronomy. Caroline. I was taught them at so early an age that 1 could not understand their meaning ; and I have often heard you say that the only use of words was to convQy ideas. M~rn. B. The names of these lines would have con- veyed ideas of the figures they were designed to express, though the use of these figures might at that time have been too difficult for you to understand. Childhood is the sea- son when impressions on the memory are most strongly and most easily made : it is the period at which a large stock of ideas should be treasured up, the application of which we may learn when the understanding is more developed. It is, I think, a very mistaken notion that children should be taught such things only, as they can perfectly under- * The earth is of a globular form. For, 1. The shadow of the earth projected on the moon in an eclipse is always circular; which appearance could only be produced by a spherical body. 2. The convexity of the surface of the sea is evident ; the mast of an approaching ship being seen before its hull. 3. The north pole becomes more elevated by travelling northward, in .proportion to the space passed over. 4. Navigators have sailed round the earth, aid by steering their course continually westward arrived, at length, at the place from whence they departed. 401. How is it proved that the earth is globnlar f 402. \Yhal is necessary to be learnt before one can understand the ef- fects resulting from the earth's motions ? 104 ON THE EARTH. stand. Had you been early made acquainted with the terms which relate to figure and motion, how much it would have facilitated your progress in natural philoso- phy ! I have been obliged to confine myself to the most common and familiar expressions, in explaining the laws of nature, though I am convinced that appropriate and scientifick terms would have conveyed more precise and accurate ideas ; but I was afraid of not being understood. Emily. You may depend upon our learning the names of these lines thoroughly, Mrs. B. ; but before we com- mit them to memory, will you have the goodness to ex- plain them to us ? Mrs. B. Most willingly. This globe, or sphere, represents the earth ; ;the line which passes through its centre, and on which it turns, is called its axis, and the two extremities of the axis A. and B, are the poles, dis- tinguished by the names of the north and south pole. The circle C D, which divides the globe into two equal parts between the poles, (is called the equator,^ or equi- noctial line ; that part of the globe to the north of the equator is the northern hemisphere ; that part to the south of the equator, the southern hemisphere. The small circle E F, which surrounds the north pole, is call- ed the arctick circle ; that G II, which surrounds the south pole, the antarciick circle. There are two inter- mediate circles between the polar circles and the equator ; that to the north, I K, called the tropick of Cancer ; that to the south, L M, called the tropick of Capricorn. Lastly, this circle, L K, which divides the globe into two equal parts, crossing the equator and extending northward as far as the tropick of Cancor, and southward as far as the tropick of Capricorn, is called the ecliptick. The delineation of the ecliptick on the terrestrial globe is riot without danger of conveying false ideas ; for the ecliptick (as I have before said) is an imaginary circle in the heavens passing through the middle of the zodiack, and situated in the plane of the earth's orbit. 403. What, in an artificial globe, represents the enrth's axis? 404. What are the extremities of the axis called ? 4' n -5. What is the equator ? 406. What line in the figure represents the equator ? What ones the Tropicks J What, ones tho Pclnr Circles ? What one the Ecliptick ? 407. By what nnmo rr the two tropicks distinguished from each other ? 408 T\v what name are the polar circles distinguished from each oll.er ? 409. Where is the ecliptick situated ? ON THE EARTH. 105 Caroline. I do not understand the meaning of the plane of the earth's orbit. Mrs. ft. A plane, or plain, is an even level surface. Let us suppose a smooth thin solid plane cutting the sun through the centre, extending out as far as the fixed stars, and terminating in a circle which passes through the middle of the zodiack ; in this plane the earth would move in its revolution round the sun ; it is therefore called the plane of the earth's orbit, and the circle in which this plane cuts the signs of the zodiack is the eclip- tick. Let the fig. 1. plate IX. represent such a plane, S the sun, E the earth with its orbit, and A B C D the ecliptick passing through the middle of the zodiack. Emily. If the ecliptick relates only to tiie heavens, why is it described upon the terrestrial globe 1 Mrs. B. It is convenient for the demonstration of a variety of problems in the use of the globes; and besides, the obliquity of this circle to the equator is rendered more conspicuous by its being described on the same globe ; and the obliquity of the ecliptick shows the inclination of the earth's axis to the plane of its orbit. But to return to fig. 2. plate VIII. The spaces between the several parallel circles on the terrestrial globe are called zones ; that which is compre- hended between the tropicks is distinguished by the name of the .torrid zone ; the spaces which extend from the tropicks to the polar circles, the north and south tempe- rate zones; and the spaces contained within the polar circles, \the frigid zonesv The several lines which, you observe, are drawn from one pole to the other, cutting the equator at right angles, are called meridians. When any one of these meridians is exactly opposite the sun it is mid-day, or twelve o'clock in the day, with all the places situated on that meridian ; and, with the places situated on the opposite meridian, it is consequently midnight. 410. What is to be understood by the plane of the earth's orbit ? 41 1. By what fijjure is it represented ? 412. If the eclip- tick relate only to the heavens, why is it described on lite tr- restrial jrl^be p -413. What are called the zones r 1 4i4. Where isf the torrid zone ? 415. Where are the temperate zones? 41f>. Where are the frigid zones ? 417. What are tiie meridian lines ? 41S. When is it twelve o'clock at noon to all places under any particular meridian ? 419. To what places will it. r>t the same time, be midnight ? 106 ON THE EARTH, Emily. To places situated equally distant from these two meridian sj it must then be six o'clock ? Mrs. B. Yes ; if they are to the east of the sun's meridian it is six o'clock in the afternoon, because the sun will have previously passed over them ; if to the west, it is six o'clock in the morning, and the sun will be proceeding towards that meridian. Those circles which divide the globe into two equal parts, such as the /equator arid the ecliptick,, are "called greater circles ; to'distinguish them from those which di- vide it into two unequal parts', as the (tropicks and polar circles, which are called lesser circles. All circles are divided into $60 equal parts!, called degrees, and degrees into 60 equal parts, called minutes, vj^he diameter of a circle is a right line drawn across it, and passing through the centre 'J for instance, the boundary of this sphere is a circle, and its axis the diameter of that circle ; the di- ameter is equal to a little less than one-third of the cir- cumference. Can you tell me nearly how many degrees it contains ? Caroline. It must be something less than one-third of 360 degrees, or nearly /'1 20 degrees* Mrs. B. Right ; now Emily you may tell me exactly how many degrees are contained in a meridian ? Emily. A meridian, reaching from one pole to the other, is half a circle, and must therefore contain J160 - degrees. Mrs. B. Very well ; and what number of degrees are there from the equator to the poles 1 Caroline. The equator being equally distant from either pole, that distance must be half of a meridian, or a quarter of the circumference of a circle, and contain 90 degrees. Mrs. B. Besides the usual division of circles into de- grees, the ecliptick is divided into 12 equal parts, 420. To what places will it be six o'clock in the morning, and to what ones six in the evening ? 421. What circles are called greater circles ? 422. What ones arc called lesser circles ? 423. Into how many parts are all circles divided ? 424. How are degrees divided ? 425. What is the diameter of a cir- cle ? 420. How mciriy degrees does the diameter of a circle contain ? 427. How many degrees are there in a meridian reaching from one pole to the other ? 428. How many de- grees are there between the equator and the poles ? 429. How is the ecliptick divided ? ON THE EARTH. 107 culled signs, which bea.r the names of the constellations through which this circle passes in the heavens. The degrees measured on the meridians from {north to south,- or south to north, are called degrees of latitude ; those measured from east to west on the equator, the ecliptick, or any of the lesser circles, are called degrees of longi- tude ; hence these circles bear the name of longitudinal circles ; they are also called parallels of latitude. Emily. The degrees of longitude must then/vary in length ^according to the dimensions of the circle on which they are reckoned ; those, for instance, at the polar cir- cles will be considerably smaller than those at the equa- tor ? Mrs. B. Certainly ; since the degrees of circles of different dimensions do not vary in number, they must necessarily vary in length. The degrees of latitude, you may observe, never vary in length ; for the meridians on which they are reckoned are all of the same dimensions. Emily. And of what length is a degree of latitude 1 Mrs-. B. Sixty geographical miles, which is equal to 69 English statute miles. Emily. The degrees of longitude at the equator must then be of the same dimensions ? Mrs. B. They would, were the earth a perfect sphere ; but its form is not exactly spherical, being somewhat protuberant about the equator, and flattened towards the poles. This form is supposed to proceed from the superi- our action of the centrifugal power at the equator. Caroline. I thought I had understood the centrifugal force perfectly, but I do not comprehend its effect in this instance. Mrs. B. You know that the revolution of the earth on its axis must give every particle a tendency to fly off from the centre, that this tendency is stronger or weaker in proportion to the velocity with which the particle moves ; now a particle situated near one of the polar circles makes one rotation in the same space of time as a 430. What is latitude ? 431. What is longitude ? 432. Are the degrees of longitude in different latitudes of the same length ? 433. What is the length ,of a degree of latitude ? 434. What is the reason that a degree of longitude on the equa- tor is not the same as a degree of latitude ? 435. What occa- sions the protuberance of the earth at tho equator ? 108 ON THE EARTH. particle at the equator ; the latter, therefore, having a much larger circle to describe, travels proportionally faster, consequently the centrifugal force is much stronger at the equator than at the polar circles : it gradually de- creases as you leave the equator and approach the poles, where, as there is no rotatory motion, it entirely ceases. Supposing, therefore, the earth to have been originally in a fluid state, the particles in the torrid zone would re- cede much further from the centre than those in the frigid zones ; thus the polar regions would become flattened, and those about the equator elevated. Carclins, I did not consider that the particles in the neighbourhood of the equator move with greater velocity than those about the poles ; this was the reason I could not understand you. Mrs. JS. You must be careful to remember, that those parts of a body which are furthest from the centre of mo- tion must move with the greatest velocity : the axis of the earth is the centre of its diurnal motion, and the equa- torial regions the parts most distant from the axis. Caroline, My head then moves faster than my feet ; and upon the summit of a mountain we are carried round quicker than in a valley ? Mrs. B. Certainly, your head is more distant from the centre of motion, than your feet ^the mountain-top than the valley : and the more distant any part of a body is from the centre of motion, the larger is the circle it will describe, and the greater therefore must be its ve- locity. Emily. I have been reflecting that if the earth is not a perfect circle Mrs. J3. A sphere you mean, my dear ; a circle is a round line, every part of which is equally distant from the centre ; Ca sphere or globe is a round body,) the surface of which is every where equally distant from the centre. Emily. If then, the earth is not a perfect sphere, out prominent at the equator, and depressed at the poles, would not a body weigh heavier at the equalor than at the polos 1 For the earth being thicker at the equator, 436. In what manner can you account for this protuberance from centrifugal motion r 437. Why does the head of a per- son move faster than his feet in the revolution of the earth upon its axis ? 438. What is a sphere or globe ? ON THE EARTH. 109 the attraction of gravity perpendicularly downwards must fee stronger. Mrs. B. Your reasoning has some plausibility, but I am sorry to be obliged to add that it is quite erroneous ; for the nearer any part of the surface of a body is to the centre of attraction, the more strongly it is attracted ; because the most considerable quantity of matter is about that centre. In regard to its effects, you might consider the power of gravity, as that of a magnet placed at the centre of attraction. Emily. But were you to penetrate deep into the earth, would gravity increase as you approacned the centre ? Mrs. B. Certainly not ; 1 am referring only to any situation on the surface of the earth. Were you to pene- trate into the interiour, the attraction of the parts above you would counteract that of the parts beneath you, and consequently diminish the power of gravity in proportion as you approached the centre ; and if you reached that point, being equally attracted by the parts all around you, gravity would cease, and you would be without weight. Emily, Bodies then should weigh less at the equator than at the poles, since they are more distant from the centre of gravity in the former than in the latter situation. Mrs. B. And this is really the case ; but the diffe- rence of weight would be scarcely sensible, were it not augmented by another circumstance. Caroline. And what is this singular circumstance which seems to disturb the laws of nature ? Mrs. B. One that you are well acquainted with, as conducing more to the preservation than the destruction of order, the centrifugal force. This we have just ob- served to be stronger at the equator ; and as il tends to drive bodies from the centre, it is necessarily opposed to, and must lessen the power of gravity, which attracts them towards the centre. We accordingly find that bo- 439. Will any body weigh the same at the equator as at the poles ? 440. Were one to penetrate deep into the earth, would the force of gravity increase ? 441. Why not ? 442. Where will bodies weigh most, at the equator or poles ? 443. What besides the protuberance at the equator causes bodies to weigh less there than at the poles ? 10 110 ON THE EARTH. dies weigh lightest at the equator, where the centrifugal force is greatest ; and heaviest at the poles, where this power is least.* Caroline. Has the experiment been made in these different situations 1 Mrs. B. Louis XIV., of France, sent philosophers both to the equator and to Lapland for this purpose ; the severity of the climate, and obstruction of the ice, have hitherto rendered every attempt to reach the pole abor- tive ; but the difference of gravity at the equator and in Lapland is very perceptible. Caroline. Yet I do not comprehend, how the diffe- rence of weight could be ascertained ; for if the body un- der trial decreased in weight, the weight which was op- posed to it in the opposite scale must have diminished in the same proportion. For instance, if a pound of sugar did not weigh so heavy at the equator as at the poles, the leaden pound which served to weigh it, would not be so heavy either : therefore they would still balance each other, and the different force of gravity could not be as- certained by this means. Mrs. B. Your observation is perfectly just : the diffe- rence of gravity of bodies situated at the poles and at the equator cannot be ascertained by weighing them ; a pendulum was therefore used for that purpose. Caroline. What, the pendulum of a clock 7 how could that answer the purpose ? Mrs. B. A pendulum consists of a line, or rod, to one end of which a weight is attached, and it is suspend- ed by the other to a fixed point, about which it is made * If the diurnal motion of the earth round its axis were about 17 times faster than it is, the centrifugal force would, at the equa- tor, be equal to the power of gravity, and all bodies there would entirely lose weight. But if the earth revolved still quicker than this, they would all fly off. 444. How much faster must the earth move than it noio does to have the centrifugal force balance that of gravity, and thereby cause bodies entirely to lose their weight ? 445. Has an at- tempt ever been made to ascertain whether bodies will weigh hea- vier at the poles than at the equator ? 446. By whom was it mac le ? 447. Could the experiment be made by the common scales ? 448. Why not ? 449. What instrument was used in the experiment ? 450. How would you describe a pendulum ? ON THE EARTH. Ill to vibrate. Without being put in motion, a pendulum, like a plumb line, hangs perpendicular to the general sur- face of the earth, by which it is attracted ; but, if you raise a pendulum, gravity will bring it back to its perpen- dicular position. It will, however, not remain stationary there, for the velocity it has received during its descent will impel it onwards, and it will rise on the opposite side to an equal height ; from thence it is brought back by gravity, and again driven by the impulse of its velocity. Caroline. If so, the motion of a pendulum would be perpetual, and I thought you said that there was no per- petual motion on the earth. Mrs. B. The motion of a pendulum is opposed by the resistance of the air in which it vibrates, and by the fric- tion of the part by which it is suspended ; were it possible to remove these obstacles, the motion of a pendulum would be perpetual, and its vibrations perfectly regular ; being of equal distances, and performed in equal times.* Emily. That is the natural result of the uniformity of the power which produces these vibrations, for the force of gravity being always the same, the velocity of the pen- dulum must consequently be uniform. Caroline. No, Emily, you are mistaken ; the cause is not always uniform, and therefore the effect will not be so either. I have discovered it, Mrs. B. : since the force of gravity is less at the equator than at the poles, the vibra- tions of the pendulum will be slower at the equator thaa at the poles. * The vibrations of pendulums are subject to many irregularities for which no effectual remedy has yet been devised. These are owing partly to the variable density and temperature of the air, partly to the rigidity and friction of the rod by which they are sus- pended, and principally to the dilatation and contraction of the ma- terials, of which they are formed. The metalline rods of pendu- lums are expanded by heat, and contracted by cold ; therefore clocks will go faster in winter, and slower in summer. The com- mon remedy for this inconvenience is the raising or lowering th.e bob of the pendulum, by means of a screw, as occasion may re- quire. 451. What causes the vibrations of a pendulum ? 452. Why are not its vibrations perpetual ? -453. To what is the irrrgu- larity in the vibrations of pendulums owing 9 454. Why wtU clocks go faster in winter than in summer ? 455. Where do pendulums of the same length vibrate fastest ? 112 ON THE EARTH. Mrs. B. You are perfectly right, Caroline ; it was by this means that the difference of gravity was discover- ed, and the true figure of the earth ascertained. Emily. But how do they contrive to regulate their time in the equatorial and polar regions ? for, since in this part of the earth the pendulum of a clock vibrates exactly once in a second, if it vibrates faster at the poles and slower at the equator, the inhabitants must regulate their clocks in a different manner from ours. Mrs. B. The only alteration required is to lengthen the pendulum in one case, and to shorten it in the other ; for the velocity of the vibrations of a pendulum depends on its length ; and when it is said, that a pendulum vi- brates quicker at the pole than at the equator, it is sup- posing it to be of the same length. A pendulum which vibrates a second in this latitude is 36 inches long. In order to vibrate at the equator in the same space of time, it must be lengthened by the addition of a few lines ; and at the poles, it must be proportionally shortened.* I shall now, I think, be able to explain to you the va- riation of the seasons, and the difference of the length of the days and nights in those seasons ; both effects result- ing from the same cause. In moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit. Supposing this round table to represent the plane of the earth's orbit, and this little globe, which has a wire passing through it, re- presenting the axis and poles, we shall call the earth ; in moving round the table, the wire is not perpendicular to it, but oblique. * What is here stated concerning the length of pendulums as connected with the force of gravity is at complete variance with fact. The force of gravitation is greater, it is well known, at the poles than at the equator ; and since the vibration of pendulums is occasioned by gravity, the lengths of pendulums vibrating in the same time must evidently be proportioned to the gravities at the places where they vibrate. Accordingly, it is found, by observa- tion, in order to vibrate, at the equator, in the same space, the pendulum must not be lengthened, as above stated, but shortened ; and at the poles, it must not be shortened, but proportionally lengthened. 456. How do the pendulums used at the equator and at the polar regions compare in length in order to vibrate in the same time f ON THE EARTH. 1 13 Emily. Yes, T understand the earth does not go round the sun in an upright position, its axis is slanting or ob- lique. Mrs. B. All the lines, which you learnt in your last lesson, are delineated on this little globe ; you must con- sider the ecliptick as representing the plane of the earth s orbit ; and the equator which crosses the ecliptick in two places-, shows the degree of obliquity of the axis of the earth in that orbit, which is exactly 23^ degrees. The points in which the ecliptick intersects the equator are call- ed nodes. But I believe I shall make this clear to you by revolv- ing the little globe round a candle, which shall represent the sun. (Plate IX. fig. 2.) As I now hold it, at A, you see it in the situation in which it is in the midst of summer, or what is called the summer solstice, which is on the 21st of June. Emily. You hold the wire awry, I suppose, in order to show that the axis of the earth is not upright ? Mrs. B. Yes ; in summer, the north pole is inclined towards the sun. In this season, therefore, the northern hemisphere enjoys much more of his rays than the south- ern. The sun, you see, now shines over the whole of the north frigid zone, and notwithstanding the earth's diur- nal revolution, which I imitate by twirling the ball on the wire, it will continue to shine upon it as long as it remains in this situation, whilst the south frigid zone is at the same time completely in obscurity. Caroline'. That is very strange : I never before heard that there was constant day or night in any part of the world ! How much happier the inhabitants of the north frigid zone must be than those of the southern ; the first enjoy uninterrupted day, while the last are involved in perpetual darkness. Airs. B. You judge with too much precipitation ; ex- amine a little further, and you will find, that the two frigid zones share an equal fate. 457. What causes the variation of the seasons and the diffe- rence of the length of the days and nights ? 458. How much is the axis of the earth inclined to the plane of its orbit ? 459. What are the points called where the ecliptick intersects the equator ? 460. When does the summer solstice take place ? 4C" 1 .. By which figure is the change of seasons illustrated r 462. When is the north pole inclined towards the sun ? 463. What is the situation of the south pole wher the north pole is inclined to the sun ? 10* ]14 ON TIJE EARTH. We shall now make the earth set off from its position in the summer solstice, and carry it round the sun ; ob- serve that the pole is always inclined in the same direc- tion, and points to the same spot in the heavens. There is a fixed star situated near that spot, which is hence called the (North Polar star. Now let us stop the earth at B, and examine it in its present situation ; it has gone through one quarter of its orbit, and is arrived at that point at which the ecliptick cuts or crosses the equator, and which is called the autumnal equinox. Emily. That is then one of the nodes. The sun now shines from one pole to the other, just as it would constantly do, if the axis of the earth were per- pendicular to its orbit. Mrs. B. Because the inclination of the axis is now neither towards the sun nor in the contrary direction ; at this period of the year, therefore, the days and nights are equal in every part of the earth. But the next step she takes in her orbit, you see, involves the north pole in dark- ness, whilst it illumines that of the south ; this change was gradually preparing as I moved the earth from sum- mer to autumn ; the arctick circle, which was at first en- tirely illumined, began to have short nights, which in- creased as the earth approached the autumnal equinox; and the instant it passed that point, the long night of the north pole commences, and the south pole begins to enjoy the light of the sun. We shall now make the earth pro- ceed in its orbit, and you may observe that as it advances, the days shorten, and the nights lengthen, throughout the northern hemisphere, until it arrives at the winter solstice, on the 21st of December, when the north frigid zone is entirely in darkness, and the southern has uninterrupted day-light. Caroline. Then after all, the sun, which I thought so partial, confers his favours equally on all. Mrs. B. You mistake : the inhabitants of the torrid zone have much more heat than we have, as the sun's rays fall perpendicularly on them, while they shine ob- 464. To what part of the heavens does the north pole always point ? 465. What part of the figure represents the earth at the autumnal equinox ? 466. How does the sun shine upon the earth at this season of the year ? 467. When is the winter solstice ? 468. Why is the heat of the sun greater at tho equator than at a distance from it ? ON THE EARTH. 115 liquely on the rest of the world, and almost horizontally on the poles ; for during their long day of six months, the sun moves round their horizon without either rising or setting ; the only observable difference is, that it is more elevated by a few degrees at mid-day, than at mid-night. Emily. To a person placed in the temperate zone, in the situation in which we are in England, the sun will shine neither so obliquely as it does on the poles, nor so vertically as at the equator ; but its rays will fall upon him more obliquely in autumn and winter, than in summer. Caroline. And therefore, the inhabitants of the tem- perate zones will not have merely one day and one night in the year as happens at the poles, nor will they have equal days and equal nights as at the equator ; but their days and nights will vary in length, at different times of the year, according as their respective poles incline to- wards or from the sun, and the difference will be greater in proportion to their distance from the equator. Mrs. B. We shall now follow the earth through the other half of her orbit, and you will observe, that now ex- actly the same effect takes place in the southern hemi- sphere, as what we have just remarked in the northern. Day commences at the south pole when night sets in at the north pole ; and in every other part of the southern hemisphere the days are longer than the nights, while, on the contrary, our nights are longer than our days. When the earth arrives at the vernal equinox, D, where the ecliptick again cuts the equator, on the 25th of March, she is situated with respect to the sun, exactly in the same position, as in the autumnal equinox ; and the only diffe- rence with respect to the earth, is, that it is now autumn in the southern hemisphere, whilst it is spring with us. Caroline. Then the days and nights are again every where equal 1 Mrs. B. Yes, for the half of the globe which is en- lightened, extends exactly from one pole to the other, the day breaks to the north pole, and the sun sets to the south pole ; but in every other part of the globe, the day and night is of twelve hours' length, hence the word equi- 469. In what direction do the rays of the sun fall upon the polar regions of the earth ? 4701 When does day commence at the south pole ? 471. When does the earth arrive at the vernal equinox ? 472. What part of the figure exhibits the earth at the vernal equinox ? 1 16 ON THE EARTH. nox, which is derived from the Latin, meaning equal night. As the earth proceeds towards summer, the days length- en in the northern hemisphere, and shorten in tha .south- ern, till the earth reaches the summer solstice, when the north frigid zone is entirely illumined, and the southern is in complete darkness ; and we have now brought the earth again to the spot from whence we first accompanied her. Emily. This is, indeed, a most satisfactory explana- tion of the seasons ; and the more I learn, the more I ad- mire the simplicity of means by which such wonderful effects are produced. Mrs. B. I know not which is most worthy of our admiration, the cause or the effect of the earth's revolu- tion round the sun. The mind can find no object of contemplation, more sublime, than the course of this mag- nificent globe, impelled by the combined powers of pro- jection and attraction to roll in one invariable course around the source of light and heat : and what can be more delightful than the beneficent effects of this vivify- ing power on its attendant planet ! It is at once the grand principle which animates and fecundates nature. Emily. There is one circumstance in which this little ivory globe appears to me to differ from the earth ; it is not quite dark on that side of it, which is turned from the candle, as is the ca'se with the earth when neither moon nor stars are visible. Mrs. B. This is owing to the light of the candle being reflected by the walls of the room on every part of the globe, consequently that side of the globe on which the candle does not directly shine, is not in total darkness. Now the skies have no walls to reflect the sun's light on that side of our earth which is in darkness. Caroline. I beg your pardon, Mrs. B. I think that the moon and stars answer the purpose of walls in reflect- ing the sun's light to us in the night. Mrs. B. Very well, Caroline ; that is to say, the moon and planets ; for the fixed stars, you know, shine by their own light. Emily. You say that the superiour heat of the equa- torial parts of the earth arises from the rays falling perpen- dicularly on those regions, whilst they fall obliquely on these more northern regions ; now I do not understand 472. Why are the points where the ecliplick cuts or crosses the equator called equinoxes ? ON THE EARTH. 117 why perpendicular rays should afford more heat than ob- lique rays. Caroline. You need only hold your hand perpendicu- larly over the candle, and then hold it sideways obliquely, to be sensible of the difference. Emily. I do not doubt the fact, but I wish to have it explained. Mrs. B< You are quite right ; if Caroline had not been satisfied with ascertaining the fact, without under- standing it, she would not have brought forward the can- dle as an illustration ; the reason why you feel so much more heat if you hold your hand perpendicularly over the candle, than if you hold it sideways, is because a steam of heated vapour constantly ascends from the candle or any other burning body, which being lighter than the air of the room, does not spread laterally but rises perpendi- cularly, and this led you to suppose that the rays were hot- ter in the latter direction. Had you reflected, you would have discovered that rays issuing from the candle side- ways, are no less perpendicular to your hand when held opposite to them, than the rays which ascend when your hand is held over them. The reason why the sun's rays afford less heat when in an oblique direction than when perpendicular, is be- cause fewer of them fall upon an equal portion of the earth ; this will be understood better by referring to plate X. fig. 1, which represents two equal portions of the sun's rays, shining upon different parts of the earth. Here it is evident that the same quantity of rays fall on the space A B as fall on the space B C ; and as A B is less than B C, the heat and light will be much stronger in the former than in the latter ; A B, you see, represents the equatorial re- gions, where the sun shines perpendicularly ; and B C, the temperate and frozen climates, where his rays fall more obliquely.* Emily. This accounts not only for the greater heat of the equatorial regions, but for the greater heat of summer ; * It is well known, that the south side of a hill, incur hemisphere, is peculiarly warm ; and the north side, peculiarly cold. This is owing to the different degrees of obliquity, with which the rays 473. Why is the heat of perpendicular rays more intense than that of oblique ones ? 474. By which figure is this illustrated ? 475. How will you explain Fig. 1, plate X. as illustrating this subject ? 476. Why is the south side of a hill warmer than the north side of it ? 118 ON THE EARTH. as the sun shines less obliquely in summer than in winter. Mrs. B. This you will see exemplified in fig. 2, in which the earth is represented, as it is situated on the 21st June, and England receives less oblique, and consequently a greater number of rays, than at any other season ; and figure 3 shows the situation of England on the 21st December, when the rays of the sun fall most obliquely upon her. But there is also another reason why oblique rays give less heat, than perpendicular rays ; which is, that they have a greater portion of the atmosphere to tra- verse ; and though it is true that the atmosphere is itself a transparent body, freely admitting the passage of the sun's rays, yet it is always loaded more or less with dense and foggy vapour, which the rays of the sun cannot easily penetrate ; therefore the greater the quantity of atmo- sphere the sun's rays have to pass through in their way to the earth, the less heat they will retain when they reach it. This will be better understood by referring to figure 4. The dotted line round the earth, describes the extent of the atmosphere, and the lines which proceed from the sun to the earth, the passage of two equal por- tions of the sun's rays to the equatorial and polar regions ; the latter, you see, from its greater obliquity passes through a greater extent of atmosphere. Caroline. And this, no doubt, is the reason why the sun in the morning and the evening gives so much less heat, than at mid-day. Mrs. B. The diminution of heat, morning and even- ing, is certainly owing to the greater obliquity of the sun's rays ; and as such they are affected by both the causes, which I have just explained to you ; the difficul- ty of passing through a foggy atmosphere is perhaps more particularly applicable to them, as mists and vapours are very prevalent about the time of sunrise and sunset. of the sun strike the different sides of a hill. And a south-western is warmer than a south exposure, because it receives the sun's rays in the warmest part of the day. 477. Why is a south-western exposure to the sun warmer than a south exposure ? 478. What is to be illustrated by Figures 2 & 3 of plate X. ? 479. What is another reason why oblique rays give less heat than perpendicular ones ? 480. By which figure is the effect that the atmosphere has on the heat of the sun's rays illustrated ? 481. Why does the sun give more heat at mid-day, than in the morning and evening? ON THE EARTH. 119 But the diminished obliquity of the sun's rays is not the sole cause of the heat of summer ; the length of the days greatly conduces to it ; for the longer the sun is above the horizon, the more heat he will communicate to the earth. Caroline. Both the longest days, and the most perpen- dicular rays, are on the ^ 1st of June \ and yet the great- est heat prevails in July'and August* Mrs. B. Those parts of the earth which are once heat- ed, retain the heat for some length of time, and the addi- tional heat they receive, occasions an elevation of tem- perature, although the days begin to shorten, and the sun's rays fill more obliquely. For the same reason, we have generally more heat at three o'clock in the afternoon, than at twelve when the sun is on the meridian.* * There are also other causes which have an effect on tempera- ture. When the sun's rays strike upon the land, they are stop- ped and accumulated at the surface. They are then reflected into the air and to surrounding objects ; so that the reflecti*! heat is often greater than the direct heat of the sun. Hence, the heat ii valleys where the rays are reflected by the hills and moun- tains, is sometimes very great, (jn an elevated valley in Switzer- land, the heat is so much increased by reflection, that in the cen- tre there is a spot of perpetual verdure, in the midst of perpetual snows and glaciers;) and there are plains on the Himmaleh moun- tains 15,000 feet above the level of the sea, which produce fine pasturage); and, at the he-i Mrs. B. Only twenty minutes; so that the variation of the equinoctial points is very inconsiderable. I have given them a greater extent in the figure in order to ren- der them sensible. In regard to time, I must further add, that the earth's diurnal motion on an inclined axis, together with its an- nual revolution in an elliptick orbit, occasions so much complication in its motion, as to produce many irregula- rities ; therefore, true equal time cannot be measured by the sun. A clock, which was always perfectly correct, would in some parts of the year be before the sun, and in other parts after it. There are but four periods in which the sun and a perfect clock would agree, which is the . This opposite tide is rather more difficult to explain, than that which is drawn up beneath the moon ; with a little attention, however, I hope I shall be able to make you understand it. You recollect that the earth and moon are mutually at- tracted towards a point, their common centre of gravity and of motion ; can you tell me what it is that prevents their meeting and uniting at this point 1 .Emily. Their projectile force, which gives them a tendency to fly from this centre. Mrs. B. And is hence called their centrifugal force. Now we know that the centrifugal force increases in pro- portion to the distance from the centre of motion. Caroline. Yes, I recollect your explaining that to us, and illustrating it by the motion of the flyers of a wind- mill, and the spinning of a top. Emily. And it was but the other day you showed us that bodies weighed less at the equator, than in the polar re- gions, in consequence of the increased centrifugal force in the equatorial parts. Mrs. B. Very well. The power of attraction, on the contrary, increases as the distance from the centre of gravity diminishes ; when, therefore, the two centres of gravity and of motion arc in the same spot, as is the case with regard to the moon and the earth, the centrifugal 552. How often do we have a high tide ? 553. What pre- vents the earth and moon from beino- drawn together in their com- mon centre of gravity ? 554. In what proportion does the centrifugal force increase? 555. In what ratio does the pow er of attraction increase ? ON THE MOON. 133 power and those of attraction, will be in inverse propor- tion to each other ; that is to say, where the one is strong- est, the other will be weakest. Emily. Those parts of the ocean, then, which are most strongly attracted, will have least centrifugal force ; and those parts which are least attracted, will have the greatest centrifugal force. Mrs. B. In order to render the question more simple, let us suppose the earth to be every where covered by the ocean, as represented in (lig. 3, pi. XII.) M is the moon, A B C D the earth, and X the common centre of gravity and of motion of these two planets. Now the waters on the surface of the earth, about A, being more strongly attracted than any other part, will be elevated ; the at- traction of the moon at B and C being less, and at D least of all. But the centrifugal force at D, will be greatest, and the waters there, will in consequence have the great- est tendency to recede from the moon ; the waters at B and C will have less tendency to recede, and at A least of all. The waters, therefore, at D, will recede furthest, at the same time that they are least attracted, and in con- sequence will be elevated in a protuberance similar to that at A. Emily. The tide A, then, is produced by the moon's attraction, and increased by the feebleness of the centri- fugal power in those parts ; and the tide D is produced by the centrifugal force, and increased by the feebleness of the moon's attraction in those parts.* * The opinion of Mrs. Marcet concerning the tide on that part of the earth furthest from the mcon is not universally, and it is be- lioved, not generally adopted by writers on this subject. The theory may be an ingenious one ; but, it seems more probable, that the centrifugal motion of the earth is only an auxiliary and not a principal cause of this tide ; and that its principal cause is the moon's attraction. For if the globe were one solid mass of matter, every part of it would be drawn alike towards the moon ; 556. How would you explain the production of the tides by Figure 3, plate XII ? 557. Is the opinion of Mrs. Bryan con- cerning the tides, universally adopted 9 558. What is thought a more probable cause of the tide upon the part of the earth furthest from the moon than the centrifugal motion of the earth ? 12 184 ON THE MOON. Caroline. And when it is high water at A and D, it is low water at B and C : now I think I comprehend the nature of the tides again, though I confess it is not quite so easy as I at first thought. But, Mrs. B., why does not the sun produce tides as well as the rnoon ; for its attraction is greater than that of the moon ? Mrs. B. It would be, at an equal distance, but our vicinity to the moon makes her influence more powerful. The sun has, however, a considerable effect on the tides, and increases or diminishes them as it acts in conjunction with, or in opposition to the moon. Emily. I do not quite understand that. Mrs. B. The moon is a month in going round the earth ; twice during that time, therefore, at full and at change, she is in the same direction as the sun, both then act in conjunction on the earth, and produce very great tides, called spring tides, as described in fig. 4. at A and B ; but when the moon is at the intermediate parts of her orbit, the sun, instead of affording assistance, weakens her power by acting in opposition to it ; and smaller tides are produced, called neap tides, as represented in fig. 5.* but as there is not a sufficient degree of cohesive attraction in the watery parts of it to preserve perfectly its form, the waters upon that part of it nearest the moon are drawn away from (he land, while the land, which is supposed to constitute the central regions of the globe, is drawn away from the waters upon that part of it most distant from the moon. * Although the spring and neap tides are produced by the con- junction and opposition of the sun and moon, yet their effects are not immediate ; the highest tides' happen not on the days of the full and change, neither do the lowest tides happen on the days 558. How could you account for this tide, if produced by the moons attraction? 559. As the sun is larger than the moon, why does not the- sun produce the chief influence in the production of the tides? 560. But does the sun exercise no influence in the production of the tides ? 501. When does it increase, and when dimmish the- tides? 502. What is mennt by the son and moon acting in conjunction on tl>f tidos ? 503. What are the spring tides ?- 564. What are the tides called when the sun and moon are in opposition ? 565. Hov/ would you explain the spring and neap tides by the Figures ? ON THE MOON. 135 Emily. 1 have often observed the difference of these tides when I have been at the sea side. But since attraction is mutual between the moon and the earth, we must produce tides in the moon ; and these mast be more considerable in proportion as our planet is larger. And yet the moon does not appear of an oval form. Mrs. B. You must recollect, that in order to render the explanation of the tides clearer, we suppose the whole surface of the earth to be covered with the ocean ; but that is not really the case, either, with the earth or the moon, and the land which intersects the water destroys the regularity of the effect. Caroline. True ; we may, however, be certain, that whenever it is high water the moon is immediately over our heads. Mrs. B. Not so either ; for as a similar effect is pro- duced on that part of the globe immediately beneath the moon, and on that part most distant from it, it cannot be over the heads of the inhabitants of both those situations at the same time. Besides, /as the orbit of the moon is very nearly parallel to that of the earth, )she is never ver- tical but to the inhabitants of the torrid zone); in that climate, therefore, the tides are greatest, and they dimi- nish as you recede from it and approach the poles. Caroline. In the torrid zone, then, I hope you will grant that the moon is immediately over, or opposite the spots where it is high water ? Mrs. B. I cannot even admit that ; for the ocean na- turally partaking of the earth's motion, in its rotation from west to east, the moon, in forming a tide, has to contend of quadratures. But on account of the continuation of motion, it is, some time after, the exercise of the sun and moon's attraction, in the manner supposed, that the effect of their forces is most to he seen, i So that the greatest spring tides commonly happen throe days after the new and full moons i)and the least neap tides three days after the fi?st and third quarters. \ n and opposn .v tdke place '.- 566. How muck after the conjun ction amLopposit ion of the sun and moon do the spring and neap tidc.s take place f 5(;7. In what parts of the earth are the tides highest ? 568. Why are they highest in the equatorial regions ? 136 ON THE MOON. against the eastern motion of the waves. /All matter, you know, by its inertia, makes some resistance to a change of state ; the waters, therefore, do not readily yield to the attraction of the moon, and the effect of her influence is not complete tiljf three hours 'after she has passed the me- ridian, where it is full tide. Emily. Pray what is the reason that the tide is three quarters of an hour later every day ? Mrs. B. Because it is twenty-four hours and three- quarters before the same meridian on our globe returns beneath the moon. Tlhe earth revolves on its axis in about twenty-four hours ; if the moon were stationary, therefore, the same part cf our globe would, every twen- ty-four hours, return beneath the moon ; but as during our daily revolution the moon advances in her orbit, tne earth must make more than a complete rotation in order to bring the same meridian opposite the moon : we are three quarters of an hour in overtaking her. The tides, there- fore, are retarded for the same reason that the moon rises later by three quarters of an hour every day.* We have now, I think, concluded the observations 1 had to make to you on the subject of astronomy ; at our next interview, I shall attempt to explain to you the ele- ments of bydrostaticks. * There are no tides in lakes,^>ecause they are generally so small that when the moon is vertical she attracts every part alike ; and by rendering all the waters equally light, no part" can be rais- ed higher than another. The Mediterranean and Baltick seas have very small elevations^because the inlets by which they com- municate with the ocean are so narrow, that they cannot in so short a time either receive or discharge enough, sensibly to raise or sink their surfaces)? 569. Why is it not high water at a place, when the moon is di- rectly over the meridian of it ? 570. How long after the moon passes the meridian of a place before the effect of her influence becomes complete ? 571. Why are the tides three quarters of an hour later every day ? 572. Why are there no tides on the Jakes ? 573. Why are the tides small in the Mediterranean and Baltick seas f m OH THE MECHANICAL PROPERTIES OP FLUIDS. 137 CONVERSATION X. ON THE MECHANICAL PROPERTIES OF FLUIDS* Definition of a Fluid; Distinction between Fluids and Liquids; Of Non-Elastick Fluids ; Scarcely suscepti- ble of Compression ; Of the Cohesion of Fluids ; Of their Gravitation ; Of their Equilibrium ; Of their Pressure; Of Specifick Gravity; Of the Specifick Gravity of Bodies heavier than Water ; Of those of the same Weight as Water ; Of those lighter than Wa- ter ; Of the Specifick Gravity of Fluids. MRS. B. WE have hitherto confined our attention to the me- chanical properties of solid bodies, which have been illus- trated, and, I hope, thoroughly impressed upon your me- mory, by the conversations we have subsequently had on astronomy. It will now be necessary for me to give you some account of the mechanical properties of fluids a science which is calledfhydrostaticks) (A fluid is a sub- stance v/hich yields to the slightest pressure.) If you dip your hand into a basin of water, you are scarcely sensible of meeting with any resistance. Emily. The attraction of cohesion is, then, I suppose, less powerful in fluids than in (solids ? Mrs. B. Yes ; fluids, generally speaking, are bodies of less density than ^olidsl From the slight cohesion of the particles of fluids, and the facility with which they slide over each other, it is inferred, that they must be small,(smooth, and globular! '4 smooth, because there ap- pears to be little or no friction among them ; and globu- lar, because touching each other but by a point would ac- count for the slightness of their cohesion.* / * If tho particles of fluids are round, there must be vacant spaces between them, in the same manner as there are vacuities between cannon balls that are piled together ; between these balls smaller 574. What is the science called that fc^^L of the mechanical properties of fluids ? 575. What is II IJLa fluid ? 576. In which is the attraction of cohesion the i^W Upverful, solids or fluids ? 577. What is inferred from the slight cohesion of the particles of fluids, and the facility with which they slide over each other ? 12* 138 ON THE MECHANICAL PROPERTIES OF Caroline. Pray what is the distinction between a fluid and a liquid ? Mrs. 13. (liquids comprehend only one class of fluids. There is another class distinguished by the name of elas- tick fluids, or gases, which comprehends the air of the atmosphere, and all the various kinds of air with which you will become acquainted when you study chemistry* Their mechanical properties we shall examine at our next meeting, and confine our attention this morning to those of liquids, or non-elastik fluids. Water and liquids in general, are scarcely susceptible of being compressed, or squeezed into a smaller space than that which they naturally occupy.) This is supposed , to be owing to the extreme minuteness of their particles, which, rather than submit to compression, force their way through the pores of the substance which confines them, j This was shown by a celebrated experiment made at Florence many years ago. ;A hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew^) Fluids gravitate in a mere (perfect manner than solid) bodies ; for the strong cohesive attraction of the particles of the latter in some measure counteracts the effects of gravity. ^In this table, for instance, the cohesion of the particles of wood enables four slender legs to support a considerable weight. Were the cohesion destroyed, or, in other words, the wood converted into a fluid, no sup* port could be afforded by the legs, for the particles no shotmay be placed. and between these, other still smaller, or gravel, or sand, may be diffused. Tn a similar manner, a certain quantity of particles of sugar can be taken up in water without increasing its bulk, and when the water has dissolved the sugar, salt may be dissolved in it, and yet the bulk remain the same : and admitting that the particles of water are round, this is easily accounted for. 578. What reason is given in the note for supposing that the particles of fluids are round? 579. What is the distinction between a liquid and a fluid ? 580. Are water and other liquids susceptible o-f coj^MAuion ? 581. What is the reason for sup- posing they ajgH Hk>82. What experiment has been made to show that ^PIMJR not compressible ? 583. How do flu- ids gravitate compared with solids ? 584. What example is given to show that solids graviiate in a less perfect manner than liquids ? m ON fllE MECHANICAL PROPERTIES OP FLUIDS. 139 longer cohering together, each would press separately and independently, and would be brought to a level with the surface of the earthy Emily. (This want of cohesion is then the reason why fluids can never be formed into figures, or maintained in heaps \j for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gra- vity, and water always finds its level. Mrs. B. Do you understand what is meant by the level, or equilibrium of fluids 1 Emily. I believe I do, though I feel rather at a loss to explain it. f Is not a fluid level when its surface is smooth and flat, as is the case with all fluids when in a state of resU Mrs. B. Smooth, if you please, but not flat ; for the definition of the equilibrium of a fluid is, that every part of the surface is equally distant from the point to which gravity tends, that is to say, from the centre of the earth; hence the surface of all fluids must be bulging, not flat, since they will partake of the spherical form of the globe. This is very evident in large bodies of water, such as the ocean, but the sphericity of small bodies of water is so trifling, that their surfaces appear flat. /This level, or equilibrium of fluids is the natural re- sult of their particles gravitating independently of each other ;/ for when any particle of a fluid accidentally finds itself elevated above the rest, it is attracted down to the level of the surface of the fluid, and the readiness with which fluids yield to the slightest impression will enable the particle by its weight to penetrate the surface of the fluid and mix with it. Caroline. But 1 have seen a drop of oil float on the surface of water without mixing with it. Mrs. B. fThat is because oil is a lighter liquid than water. } If you were to pour water over it, the oil would rise to the surface, being forced up by the superiour gravi- ty of the water. Here is an instrument called a water- level, (fig. 1, plate XIII.) which is constructed (Upon the principle of the equilibrium of fluids./ It consists of a 585. Why cannot liquids be moulded into figures like solids? 586. What is meant by the level or equilibrium of fluids ? 587. Of what is the level or equilibrium of fluids the result ? 588. Why will oil remain upon the top of water ? 589. How is a water-Jevel constructed ? 140 ON THE MECHANICAL PROPERTIES OF FLUIDS. short tube, A B, closed at both ends, and contain in a little water ; when the tube is not perfectly horizontal fire water runs to the lower end, and it is by this means thai the level of any situation to which we apply the instru- ment, is ascertained. Solid bodies you may, therefore, consider as gravitat- ing in masses, for the strong cohesion of their particles makes them weigh altogether, ) while every particle of a fluid may be considered as composing a separate mass, gravitating independently of each other. Hence the re- sistance of a fluid is considerably less than that of a solid body ; /for the resistance of the particles acting separate- ly, they are more easily overcome. ) Emily. A body of water, in falling, does certainly less injury than a solid body of the same weight. Mrs. B. The particles of fluids acting thus indepen- dently, press against each other in every direction, not only downwards but upwards, and laterally or sideways ; and in consequence of this equality of pressure, every particle remains at rest in the fluid. If you agitate the fluid you disturb this equality of pressure, and the fluid will not rest till its equilibrum is restored. Caroline. The pressure downwards is very natural ; it is the effect .of gravity, one particle weighing upon another presses on it ; but the pressure sideways, and particularly the pressure upwards. I cannot understand. Mrs. B. If there were no lateral pressure, water would not run out of an opening on the side of a vessel. If you fill a vessel with sand, it will not run out of such an opening, because there is scarcely any lateral pressure among its particles. Emily. When water runs out of the side of a vessel, is it not owing to the weight of the water above the opening 1 Mrs. B. If the particles of fluids were arranged in regular columns thus, (fig. 2.) there would be no lateral pressure, for when one particle is perpendicularly above the other, it can only press it downwards ; but as it must continually happen, that a particle presses between two particles beneaU^^^g. 3.) these last must suffer a lateral pressure. 590. Why db^oljd bodies gravitate in masses ? 591 Why is the resistance of fluids less than that of solids : 592. Why are fluids inclined to a state of rest or equilibrium ? 593. Why will liquids run out of an opening in the vessel containing them ? ON THE MECHANICAL PROPERTIES OF FLUIDS. 14) Emily. The same as when a wedge is driven into a piece of wood, and separates the parts laterally. Mrs. B. Yes. The lateral pressure proceeds, there- fore, entirely from the pressure downwards, or the weight of the liquid above ; and consequently the lower the ori- fice is made in the vessel, the greater will be the velocity of the water rushing out of it. Here is a vessel of water (fig. 5.) with three stop cocks at different heights ; we shall open them, and you will see with what different de- grees of velocity the water issues from them. Do you un- derstand this, Caroline ?* Caroline. Oh yes. The water from the upper spout receiving but a slight pressure, on account of its vicinity to the surface, flows but gently ; the second cock having a greater weight above it, the water is forced out with greater velocity, whilst the lowest cock, being near the bottom ,of the vessel, receives the pressure of almost the whole body of water, and rushes out with the greatest impetuosity. Mrs. B. Very well ; and you must observe, that as the lateral pressure is entirely owing to the pressure down- wards, it is not effected by the horizontal dimensions of the vessel, which contains the water, but merely by its depth ; for as every particle acts independently of the rest, it is only the column of particles, immediately above the orifice, that can weigh upon and press out the water. Emily. The breadth and width of the vessel then can be of no consequence in this respect. The lateral pres- sure on one side, in a cubical vessel, is, I suppose, not so great as the pressure downwards. VAn empty bottle being corked, and, by means of a weight, let down a certain depth into the sea, it will be broken, or the cork will be driven into it by the perpendicular pressure. But a bottle filled with water, or any other liquid, may be let down to any depth, without damage, because in this case the internal pressure is equal to the external ?/ 594. From what does the lateral pressure of liquids proceed ? 595. How would y u illustrate the lateral and downward pressure of fluids by the 'figures ? r59G. What fact is mentioned in the note concerning the pressure of liquids ? 597. To what is the velocity of liquids, issuing from an orifice in the side of a vessel, proportional ? 142 ON THE MECHANICAL PROPERTIES OP FLUIDS, Mrs. B. f^No, in a cubical vessel the pressure down- wards will be double the lateral pressure on one side ; for every particle at the bottom of the vessel is pressed upon by a column of the Avhole depth of the fluid, whilst the lateral pressure diminishes from the bottom upwards to the surface, where the particles have no pressure.; Caroline. And from whence proceeds the pressure of fluids upwards ? that seems to me the most unaccounta- ble, as it is in direct opposition to gravity. Mrs. B. And yet it is a consequence of their pres- sure downwards. /When, for example, you pour water into a tea-pot, the water rises in the spout to a level with the water in the pot. jThe particles of water at the bottom of the pot are pressed upon by the particles above them ^ to this pressure they will yield, if there is any mode of making way for the superiour particles, and as they can- not descend, they will change their direction and rise in the spout. /Suppose the tea-pot to be filled with columns of parti- cles of water similar to that described in fig. 4. the par- ticle 1 at the bottom will be pressed laterally by the par- ticle 2, and by this pressure be forced into the spout where, meeting with the particle 3, it presses it upwards, and this pressure will be continued, from 3 to 4, from 4 to 5, and so on till the water in the spout has risen to a level r ith that in the pot. ) family. If it were not for this pressure upwards, forc- ing the water to rise in the spout, the equilibrium of the flui'l would be destroyed. Caroline. True ; but then a tea-pot is wide arid large, and the weight of so great a body of water as the pot will contain, may easily force up and support so small a quan- tity as will fill the spout. But would the same effect be produced if the spout and the pot were of equal dimen- sions ? Mrs. B. Undoubtedly it would. You may even re- verse the experiment by pouring water into the spout, and (you will find that water will rise in the pot to a level with that in the spout i for the pressure of the small 598. How does the pressure downwards, in a cubical vessel, compare with the lateral pressure r 599. Whence proceeds the pressure of liquids upwards ? 600. How would you illus- trate, from the figure, the upward pressure of liquids occasioned by the downward pressure ? 601. What will be the effect, in relation to this subject, if water is poured into the spout If ON THE MECHANICAL PROPERTIES OF FLUIDS. 143 quantity of water in the spout will force up and support the larger quantity in the pot. In the pressure upwards, as well as that laterally, you see that the force of pressure depends entirely on the height, and is quite independent of the horizontal dimensions of the fluid. As a tea-pot is not transparent, let us try the experi- ment by filling this large glass goblet by means of this nar- row tube. (fig. 6.) Caroline. Look, Emily, as Mrs. B. fills it, how the water rises in the goblet, to maintain an equilibrium with that in the tube. Now, Mrs. B., will you let me fill the tube by pouring water into the goblet. Mrs. B. That is impossible. However, you may try the experiment, and I doubt not but that you will be able to account for its failure. Caroline. It is very singular, that if so small a co- lumn of water as is contained in the tube can force up and support the whole contents of the goblet ; that the weight of all the water in the goblet should not be able to force up the small quantity required to fill the tube : oh, I see now the reason, the water in the goblet cannot force that in the tube above its level ; and as the end of the tube is considerably higher than the goblet, it can never be filled by pouring water into the goblet. Mrs. B. And if you continue to pour water into the goblet when it is full, the water will run over instead of rising above the level in the tube. I shall now explain to you the meaning of the specif ck gravity of bodies. Caroline. What ! is there another species of gravity with which we are not yet acquainted ? Mrs. B. No ; the specifick gravity of a body, means simply its weight compared with that of another body of the same size. When we say, that substances, such as lead and stones are heavy, and that others, such as paper and feathers, are light, we speak ^comparatively 1 ; that is to say, that the first are heavy, and the latter light, in comparison with the generality of substances in nature. Would you call wood and chalk light or heavy bodies 1 602. What is the object of figure 6, plate XIII. ? 603. What is meant by the specifick gravity of bodies ? 604. When we say that such bodies as lead and stones are heavy, and that such as paper and feathers are light, how do we speak ? 144 ON THE MECHANICAL PROPERTIES OF FLUIDS. Caroline. Some kinds of wood are heavy, ceitainly, as oak and mahogany ; others are light, as deal and box, Emily. I think 1 should call wood in general a heavy body, for deal and box are light only in comparison to wood of a heavier description. I am at a loss to deter- mine whether chalk should be ranked as a heavy or a light body ; I should be inclined to say the former, if it were not that it is lighter than most other minerals. I perceive that we have but vague notions of light and heavy. I wish there was some standard of comparison, to which we could refer the weight of all other bodies. Mrs. B. The necessity of such a standard has been BO much felt, that a body has been fixed upon for this purpose. What substance do you think would be best calculated to answer this end 1 Caroline. It must be one generally known and easily obtained, lead or iron for instance. Mrs. B. fA\\ the metals expand by heat, and condense by cold. A piece of lead, let us say a cubick inch for in- stance, would have less specifick gravity in summer than in winter ; for it would be more dense in the latter season.^ Caroline. But, Mrs. B., if you compare the weight of equal quantities of different bodies, they will all be alike. You know the old saying that a pound of feathers is as heavy as a pound of lead, Mrs. B. fWhen therefore we compare the weight of different kinas of bodies, it would be absurd to take quan- tities of equal weight, we must take quantities of equal bulk ; pints or quarts, not ounces or pounds. Caroline. Very true ; I perplexed myself by thinking that quantity referred to weight, rather than to measure. It is true, it would be as absurd to compare bodies of the same size in order to ascertain which was largest, as to compare bodies of the same weight in order to discover which was heaviest. Mrs. B. In estimating the specifick gravity of bodies, therefore, we must compare equal bulks, and we shall find that their specifick gravity will be proportional to their 605. Why would not metals, as lead, or iron, answer for the standard to determine the specifick gravities of bodies ? 606, What body has been adopted as a standard of reference ? ON THE MECHANICAL PROPERTIES OF FLUIDS. 145 weights. The body which has been adopted as a stand- ard^of reference is distilled water.* Emily. I am surprised that a fluid should have been chosen for this purpose, as it must necessarily be contain- ed in some vessel, and the weight of the vessel will re- quire to be deducted. Mrs. B. In order to learn the specifick gravity of a solid body, it is not necessary to put a certain measure of it in one scale, and an equal measure of water into the other scale : but simply to weigh the body under trial in water. If you weigh a piece of gold in a glass of water, will not the gold displace just as much water., as is equal to its own bulk 1 Caroline. Certainly, where one body is, another can- not be at the same time ; so that a sufficient quantity of water must be removed, in order to make way for the gold. Mrs. B. Yes, a cubick inch of water to make room for a cubick iuch of gold ; remember that the bulk alone is to be considered, the weight has nothing to do with the quantity of water displaced, for an inch of gold does not *(The method of ascertaining the specifick gravities of bodies was discovered accidentally by Archimedes. He had been employed by the king ot Syracuse to investigate the metals of a golden crown which he suspected had been adulterated by the workmen. The philosopher laboured at the problem in vain, till going one day into the bath, he perceived that the water rose in the bath in proportion to the bulk of his body ; he instantly perceived thatanv other sub- stance of equal size would have raised the water just as much, though one of equal weight and Less bulk could not have produced the same effect. He then got two masses, one of gold and one of silver, each equal in weight to the crown, and having filled a ves- sel very accurately with water, he first plunged tjie silver mass into it, and observed the quantity of water that'flowed over ; he then did the same with the gold, and found that a less quantity had pass- ed over than before. Hence he inferred that, though of equal weight, the bulk of the silver was greater than that of the gold, and that the quantity of water displaced was, in each experiment, equal to the bulk of the metal. He next made trial with the crown, and found it displaced more wat^r than the gold, and less than the sil- ver, which led him to conclude, that it was neither pure gold nor pure silver. 607. Who discovered the method of ascertaining 1 the specifick gravities of bodies f 008. What led him to make the discovery f 13 146 ON THE MECHANICAL PROPERTIES OF FLUIDS, occupy more space, and therefore will not displace more water than an inch of ivory, or any other substance that will sink in water. Well, you will perhaps be surprised to hear that the gold will weigh less in water, than it did out of it. Emily. And for what reason 1 Mrs. B. On account of the (upward pressure of the particles of water, which in some measure supports the gold, and by o doing diminishes its weight. ) If the body immersed in water was of the same weight as that fluid, it would be wholly supported by it, just as the water which it displaces was supported previous to its making way for the solid body.) If the body is heavier than the water, it cannot be wholly supported by it ; but the water will offer some resistance to its descent. (Caroline. And the resistance which water offers to the descent of heavy bodies immersed in it, (since it proceeds from the upward pressure of the particles of the fluid,) must, in all cases, I suppose, be the same. Mrs. B. Yes ; fhe resistance of the fluid is propor- tioned to the bulk, and not to the weight of the body im- mersed in it ; all bodies of the same size, therefore, lose the same quantity of their weight in water, j Can you form any idea what this loss will be 1 Emily. I should think it would be equal to the weight of the water displaced ; for, since that portion of the wa- ter was supported before the immersion of the solid body, an equal weight of the solid body will be supported. Mrs. B. You are perfectly right : a body weighed in water loses just as much of its weight, as is equal to that of the water it displaces : so that if you were to put the water displaced into the scale to which the body is sus- pended, it would restore the balance. You must observe, that when you weigh a body in water, in order to ascertain its specifick gravity, you must not sink the basin of the balance in the water ; but either suspend the body to a hook at the bottom of the basin, or else take off the basin, and suspend it to the arm of the balance. \(fig. 7.) Now suppose that a cubick inch 609. Why does a body weigh less in the water than out of it ? 610. To what is the resistance of water to a body immersed in it proportioned ? 611. How much does a body weighed in the water lose of its weight ? 612. Which figure shows the manner of weighing a body in water ? ON THE MECHANICAL PROPERTIES OF FLUIDS. 147 of gold weighed 19 ounces out of water, and lost one ounce of its weight by being weighed in water, what would be its specifick gravity ? Caroline. The cubick inch of water it displaced must weigh that one ounce ; and as a cubick inch of gold weighs 19 ounces, gold is 19 times as heavy as water. Emily. I recollect having seen a table of the com- parative weights of bodies, in which gold appeared to me to be estimated at 19 thousand times the weight of water. Mrs, B. You misunderstood the meaning of the table. In the estimation you allude to, the weight of water was reckoned at 1000. You must observe, that the weight of a substance, when not compared to that of any other, is perfectly arbitrary ; and when water is adopted as a standard, we may denominate its weight by any number we please ; but then the weight of all bodies tried by this standard must be signified by proportional numbers. Caroline. We may call the weight of water for exam- ple, one, and then that of gold would be nineteen ; or if we choose to call the weight of water 1000, that of gold would be 19,000. In short, the specifick gravity means how much more a body weighs than an equal bulk of water. Mrs. B. It is rather the weight of a body compared with that of water ; for the specifick gravity of many substances is less than that of water.* * Specifick Gravities of Various Bodies. Fine gold Lead - Fine Silver Copper Iron Marble Glass - Chalk - Coai ] 9,640 Mahogany 31,325 Milk 11,091 Ram water 9,000 Oil 7,645 Ice 2,705 Brandy 3,000 Living men 1,793 Cork 1,250 Common air 1,063 1,034 1,000 ,920 ,908 ,920 ,891 ,240 ,113 Experiments have been made to ascertain the specifick gravity of living men, in order to know what weight of cork fastened to a per- son in the water will keep him from sinking, on the supposition that most men were specifically heavier than river water ; but, con- trary to expectation, it was found from trials made upon ten diffe- 613. What is the spccifick gravity of living men ? J48 ON THE MECHANICAL PROPERTIES OF FLUIDS. Caroline. Then you cannot ascertain the specific!* gravity of such substances in the same manner as that of gold ; for a body that is lighter than water will float on its surface without displacing any water. Mrs. B. if a body were absolutely light, it is true that it would not displace a drop of water ; but the bodies we are treating of have all some weight, however small ; and will, therefore, displace some quantity of water. If the body be lighter than water, it will not sink to a level with the surface of the water, arid therefore it will not displace so much water as is equal to its bulk ; but it will displace as much as is equal to its weight. A ship, you must have observed, sinks to some depth in water, and the heavier it is laden the deeper it sinks, as it always displaces a quantity of water equal to its weight. Caroline. But you said just now, that in the immer- sion of gold, the bulk, and not the weight of body, was to be considered. Mrs. J5. That is the case with all substances which are heavier vhan water ; but since those which are light- er do not displace so much as their own bulk, the quan- tity they displace is not a test of their specifick gravity. In order to obtain the specifick gravity of a body which is lighter than water, you must attach to it a heavy one, whose specifick gravity is known, and immerse them to- gether ; the specifick gravity of the lighter body may then be easily calculated. Emily. But are there not some bodies which have ex- actly, the same specifick gravity as water 1 Mrs. B. Undoubtedly ; and such bodies will remain at rest in whatever situation they are placed in water. rent persons, that their mean specifick gravity was about 1-Ot.h less than common water. \So long, therefore, as the lungs can be kept free from water, a person, although unacquainted with the art of swimming, will not completely sink.\ f>14. How long will n person unacquainted with swimming remain in the water without sinking ? 615. How can the spe- cifick gravity of bodies lighter than water be obtained ? GI6. How will bodies of the same specifick gravity of water remain when immersed in it ? ON THE MECHANICAL PROPERTIES OF FLUIDS. 149 Here is a piece of wood, which, by being impregnated with a little sand, is rendered precisely of the weight of an equal bulk of water ; in whatever part of this vessel of water you place it, you will find thai it will remain sta- tionary. Caroline. I shall first put it at the bottom ; from thence, of course, it cannot rise, because it is not lighter than water. Now I shall place it in the middle of the vessel : it neither rises nor sinks, because it is neither lighter nor heavier than the water. Now I will lay it on the surface of the water ; but there it sinks a little what is the reason of that, Mrs. B. ? Mrs. B. /Since it is not lighter than the water, it cannot float upon its surface ; since it is not heavier than water it cannot sink below its surface ; it will sink, there- fore, only till the upper surface of both bodies are on a level, so that the piece of wood is just covered with water. If you poured a few drops of water into the vessel, (so gently as not to increase their momentum by giving them velocity) they would mix with the water at the surface,, and not sink lower. Caroline. This must, no doubt, be the reason why in drawing up a bucket of water out of a well, the bucket feels so much heavier when it rises above the surface of the water in the well ; for whilst you raise it in the wa- ter, the water within the bucket being of the same spe- cifick gravity as the water on the, outside, will be wholly supported by the upward pressure of the water beneath the bucket, and consequently very little force will be re- quired to raise it ; but as soon as the bucket rises to the surface of the well you immediately perceive the increase of weight. Emily. And how do you ascertain the specifick gravity of fluids ? Mrs. B. /By means of an instrument called an hy- drometer,] which I will show you. It consists of a thin glass ball A, (fig. 8, plate XIII.) with a graduated tube B, and the specifick gravity of the liquid is estimated by the depth to which the instrument sinks in it. There is 617. What solid body is of the same specifick gravity of water : 618. How is the specifick gravities of fluids ascertained ? 619. How is a hydrometer constructed ? 620. Which figure represents an hydrometer ? 13* 150 OF SPRINGS, FOUNTAINS, &C. A smaller ball, C, attached to the instrument below, which contains a little mercury ; but this is merely for the pur- pose of equipoising the instrument, that it may remain upright in the liquid under trial. 1 must now take leave of you ; but there remain yet many observations to be made on fluids ; we shall, there- fore, resume this subject at our next interview. CONVERSATION XI. OF SPRINGS, FOUNTAINS, &C. Of the Ascent of Vapour and the Formation of Clouds; Of the Formation and Fall of Rain, fyc. ; Of the Formation of Springs ; Of Rivers and Lakes ; Of Fountains. CAROLINE. THERE is a question I am very desirous of asking you respecting fluids, Mrs. B., which has often perplexed me. What is the reason that the great quantity of rain which falls upon the earth and sinks into it, does not, in the course of time, injure its solidity 1 The sun and the wind I know, dry the surface, but they have no effect on the interiour parts, where there must be a prodigious accumu- lation of moisture. Mrs. B. ( Do you not know that, in the course of time, all the water which sinks into the ground rises out of it again ? It is the same water which successively forms seas, rivers, springs, clouds, rain, and sometimes hail, snow, and ice. If you will take the trouble of following it through these various changes, you will understand why the earth is not yet drowned by the quantity of water which has fallen upon it since its creation ; and you will even be convinced, that it does not contain a single drop more water now, than it did at that period. Let us consider how the clouds were originally formed. When the first rays of the sun warmed the surface of the C21. What is the reason that the great quantity of rain which falls upon the earth and sinks into it, does not, in the course of time, injure its solidity? OP SPRINGS, FOUNTAINS, &/C. 151 earth, the heat, by separating the particles of water, ren- dered them lighter than the air. This, you know, is the case with steam or vapour. What then ensues ? Caroline. When lighter than the air it will naturally rise : and now I recollect your telling us in a preceding lesson, that the heat of the sun transformed the particles of water into vapour, in consequence of which it ascended into the atmosphere, where it formed clouds. Mrs. B. We have then already followed water through two of its transformations ; from water it becomes vapour, and from vapour clouds. Emily. But since this watery vapour is lighter than the air, why does it not continue to rise ? and why does it unite again to form clouds ? Mrs. JS. /Because the atmosphere diminishes in den- sity, as it is more distant from the earth. The vapour therefore which the sun causes to exhale, not only from seas, rivers, and lakes, but likewise from the moisture on the land, rises till it reaches a region of air of its own spe- cifick gravity ; and there, you know, it will remain sta- tionary. By the frequent accession of fresh vapour it gra- dually accumulates, so as to form those large bodies of va- pour, which we call clouds ; and these at length becoming too heavy for the air to support, fall to the ground. Caroline. They do fall to the ground, certainly, when it rains ; but according to your theory, I should have ima- gined, that when the clouds became too heavy for the region of air in which they were situated to support them, they would descend till they reached a stratum of air of their own weight, and not fall to the earth ; for as clouds are formed of vapour, they cannot be so heavy as the low- e&i regions of the atmosphere, otherwise the vapour would not have risen. Mrs. B. If you examine the manner in which the clouds descend, it will obviate this objection. In falling, several of the watery particles come within the sphere of 622. What is the cause of the ascent of vapour or steam ? 623. How are the clouds formed? 624. But since vapour is lighter than the air, why does it not continue to rise ? and why does k unite again to form clouds ? 625. What prevents the cbuds remaining in the atmosphere where they are formed ? 626. Why do the clouds descend to the earth in drops of water instead of vapour, as they ascended ? 152 OF SPRINGS, FOUNTAINS, &C. each other's attraction, and unite in the form of a drop of water./ The vapour, thus transformed into a shower, is heavier than any part of the atmosphere, and consequent- ly descends to the earth. Caroline. How wonderfully curious ! Mrs. B. It is impossible to consider any part of na- ture attentively without being struck with admiration at the wisdom it displays ; and I hope you \\ ill never con- template these wonders without feeling your heart glow with admiration and gratitude towards their bounteous Author. Observe, that if the waters were never drawn out of the earth, all vegetation would be destroyed by the excess of moisture ; if, on the other hand, the plants were not nourished and refreshed by occasional showers, the drought would be equally fatal to them. If the clouds constantly remain in a state of vapour, they might, as you remarked, descend into a heavier stratum of the atmosphere, but could never fall to the ground ; or were the power of attraction more than sufficient to convert the vapour into drops, it would transform the cloud into a mass of water, which, instead of nourishing, would destroy the produce of the earth. jVVater then ascends in the form of vapour, and descends in that of rain, snow, or hail, all of which ultimately be- come water. ; Some of this falls into the various bodies of water on the surface of the globe, the remainder upon the land. Of the latter, part re-ascends in the form of vapour, part is absorbed by the roots of vegetables and part descends into the bowels of the earth, where it forms springs. mily. Is rain and spring-water then the same ? Mrs. B. Yes, originally. The only difference be- tween rain and spring water, consists in the foreign par- ticles which the latter meets with and dissolves in its pas- sage through the various soils it traverses. Caroline. Yet spring water is more pleasant to the taste, appears more transparent, and, I should have sup- posed, would have been more pure than rain water. Mrs. B. No ; excepting distilled water, rain water is the most pure we can obtain ; arid it is its purity which renders it insipid, whilst the various salts and \differ. cut 627. What are the several changes which water undergoes in its ascent and descent ? 628. What is the difference between rain and spring water ? 629. Which is the most pure ? OF SPRINGS, FOUNTAINS, &C. 153 ingredients/ dissolved in spring water, give it a species of flavour, without in any degree affecting its transparency ; and the filtration it undergoes through gravel and sand in the bowels of the earth, cleanses it from all foreign matter which it. has not the power of dissolving. 'When rain falls on the surface of the earth, it continues making its way downwards through the pores and cre- vices in the ground. When several drops meet in their subterraneous passage, they unite and form a little rivulet; this, in its progrsss, meets with other rivulets of a similar description, and they pursue their course together in the bowels of the earth, till they are stopped by some sub- stance which they cannot penetrate./ Caroline. But you said that water could penetrate even the pores of gold, and they cannot meet with a substance more dense 1 Mrs. B. But water penetrates the pores of gold only when under a strong compressive force, as in the Floren- tine experiment ; now in its passage towards the centre of the earth, it is acted upon by no other power than gra- vity, which is not sufficient to make it force its way even through ^'stratum of clay.) This species of earth, though not remarkably dense, being of great tenacity, will not admit the particles of water to pass. Whon watei en- counters any substance of this nature, therefore, its pro- gress is stopped, and the pressure of the accumulating waters forms a bed, or reservoir. This will bo more clear- ly explained by fig(9, plate XIIlJ which represents a sec- tion, or the interiour of a hill or mountain. A is a body of water such as I have described, which when filled up as high as B (by the continual accession of water it re- ceives from the ducts or rivulets , , , ,) finds a pas- sage out of the cavity, and, impelled by gravity, it runs on, till it makes its way out of the ground at the side of the hill, and there forms a spring, C. Caroline. Gravity impels downward towards the cen- tre of the earth ; and the spring in this figure runs in a horizontal direction. 630. What renders spring water more pleasant to the taste, if it is less pure than rain water ? 631. How are springs and ri vulets at first formed ? 632. Through wh;it species of earth will not water pass ? 633. Which figure represents the manner in which springs are formed ? 634. How would you explaip this figure ? 154 OP SPRINGS, FOUNTAINS, &-C. Mrs. B. Not entirely. There is some declivity from the reservoir to the spot where the Water isues out of the ground ; and gravity, you know, will bring bodies down an inclined plane, as well as in a perpendicular di- rection. Caroline. But though the spring may descend on firs* issuing, it must afterward rise to reach the surface of the earth ; and that is in direct opposition to gravity. Mrs. B. ('A spring can never rise above the level of the reservoirjwhence it issues ; it must, therefore, find a pas- sage to some part of the surface of the earth that is lower or nearer the centre than the reservoir. It is true that, in this figure, the spring rises in its passage from B to C occasionally ; but this, I think, with a little reflection, you will be able to account for. Emily. Oh yes ; it is owing to th^pressure of fluids up- wards, and the water rises in the duct upon the same prin- ciple ais it rises in the spout of a tea-pot ; that is to say, in order to preserve an equilibrium with the water in the reservoir. Now I think I understand the nature of springs ; the water will flow through a duct, whether as- cending or descending, provided it never rises higher than the reservoir. Mrs. B. Water may thus be conveyed to every part of a town, and to the upper part of the nouses, if it is ori- ginally brought from a height superiour to any to which it is conveyed. Have you never observed, when the pave- ment of the streets have been mending, the pipes which serve as ducts for the conveyance of the water through the town 1 Emily. Yes, frequently ; and I have remarked that when any of these pipes have been opened, the water rushes upwards from them with great velocity, which I suppose proceeds from the pressure of the water in the re- servoir, which forces it out. Caroline. I recollect having once seen a very curious glass, called Tantalus's cup; it consists of a/goblet, con- taining a small figure of a man,\and whatever quantity of water you pour into the goblet, it never rises higher than 635. How high may a spring rise ? 63G. On what princi- ple does water ascend as well as descend in its course, as is often the case in being carried in ducts ? C37. What is called Tan- talus's cup ? OF SPRINGS, FOUNTAINS, &C. 156 the breast of the figure. Do you know how that is con- trived ? Mrs. B. It is by means of a/syphon,Jor bent tube, which is concealed in the body of the figure. It rises through one of the legs, as high as the breast, and there turning, descends through the other leg, and from thence through the foot of the goblet, where the water runs out. (fig. 1, plate XIV.) When you pour water into the glass A,it must rise in the syphon B, in proportion as it rises in the glass ; and when the glass is filled to a level with the upper part of the syphon, the water will run out through the other leg of the figure, and will continue run- ning out, as fast as you pour it in ; therefore the glass can never fill any higher. Emily. I think the new well that has been made at our country-house, must be of that nature. We had a great scarcity of water, and my father has been at con- siderable expense to dig a well ; after penetrating to a great depth before water could be found, a spring was at length discovered, but the water rose only a few feet above the bottom of the well ; and sometimes it is quite dry. Mrs. B. This has however, no analogy to Tantalus's cup, but is owing to the very, elevated situation of your country house. Emily. I believe I guess the reason. There cannot be a reservoir of water near the summit of a hill ; as in such a situation there will not be a sufficient number of rivulets formed to supply one ; and without a reservoir, there can be no spring. In such situations, therefore, it is necessary to dig very deep, in order to meet with a spring ; and when we give it vent, it can rise only as high as the reservoir from whence it flows, which will be but little, as the reservoir must be situated at some con- siderable depth below the summit of the hill. Caroline. Your explanation appears very clear and satisfactory. But I can contradict it from experience. At the very top of a hill, near our country-house, there is a large pond, and, according to your theory, it would be impossible there should be springs in such a situation to supply it with water. Then you know that. I have crossed 638. By what means is the water prevented from rising to the head of the figure ? 639. Why must wells on high land be dug deep in order to be supplied with water ? J56 OP SPRINGS, FOUNTAINS, &C. the Alps, and I can assure you, that there is a fine lake on the summit of Mount Ceriis, the highest mountain we passed over. Mrs. B. Were there a lake on the summit of Mount Blanc, which is the highest of the Alps, it would indeed be wonderful. But that on Mount Cenis is not at all contradictory to our theory of springs ; for this mountain is surrounded by others much more elevated, and the springs which feed the lake must descend from reservoirs of water formed in those mountains. This must also be the case with the pond on the top of the hill ; there is doubtless some more considerable hill in the neighbour- hood which supplies it with water. Emily. I comprehend perfectly, why the water in our well never rises high : but I do not understand why it should occasionally be dry. Mrs. B. Because the reservoir from which it flews being in an elevated situation^ is but scantily supplied with water ; after a long drought, therefore, it may be drained, and the spring dry, till the reservoir be reple- nished by fresh rains. It is not uncommon to see springs flow with great violence in wet weather, and at other times be perfectly dry. Caroline. But there is a spring in our grounds which more frequently flows in dry than in wet weather : how is that to be accounted for ? Mrs. B. The spring probably comes from a reservoir at a great distance, and situated very deep in the ground : it is, therefore, some length of time before the rain reacnes the reservoir, and another considerable portion must elapse, whilst the water is making its way from the reservoir to the surface of the earth ; so that the dry wea- ther may probably have succeeded the rains before the spring begins to flow, and the reservoir may be exhausted by the time the wet weather sets in again. Caroline. I doubt not but this is the case, as the spring is in a very low situation, therefore the reservoir may be at a great distance from it. Mrs. B. Springs, which do not constantly flow are called intermitting, and are occasioned by the reservoir 640. How can the lake on Mou AND SOUND* his beams have produced in the equilibrium of the atmo- sphere. But I wonder how you will reconcile these va- rious winds, Mrs. B. ; you first led me to suppose there was a constant struggle between opposite winds at the equator producing storm and tempest ; but now I hear of one regular invariable wind, which must naturally be at- tended by calm weather. Emily. I think I comprehend it : do not these winds from the north and south combine with the easterly wind about the equator, and form what are called the trade- winds '? Mrs* JB. Just so, my dear. The composition of the two winds north and east, produces a constant north-east wind ; and that of the two winds south arid east, produces a regular south-east wind : these winds extend to about thirty degrees on each side of the equator, the regions fur- ther distant from it experiencing only their respective north and south winds.* Caroline. But, Mrs. B., if the air is constantly flow- ing from the poles to the torrid zone, there must be a de- ficiency of air in the polar regions 1 Mrs. JS. The light air about the equator, which ex- pands and rises into the upper regions of the atmosphere, ultimately flows from thence back to the poles, to restore the equilibrium : if it were not for this resource, the po- lar atmospherick regions would soon be exhausted by the stream of air, which, in the lower strata of the atmosphere, they are constantly sending towards the equator. Caroline. There is then a sort of circulation of air in the atmosphere ; the air in the lower strata flowing from the poles towards the equator, and in the upper strata flow- ing back from the equator towards the poles. * On the coast of America, the trade winds are felt as far as forty degrees from the equator. By the aid of these winds, vessels sailing from Mexico to the Philippine islands, often finish a voyage, nearly equal to half the circumference of the globe, in 60 days, without altering their course, or changing a sail. But in returning, they are obliged to go north, beyond the limits of the trade winds. 704. How are the trade winds occasioned ? 705. How far on each side of the equator do these winds extend ? - 70(>. What is said of the trade winds on the coast of America ? 7>7. What fact is mentioned of vessels sailing from Mexico to the Philippine islands ? 708. Why do not the polar regions become exhausted of air, if it is continually blowing from them to the equator ? ON WIND AND SOUND. 173 Mrs. B. Exactly. I can show you an example of this circulation on a small scale. The air of this room being more rarefied than the external air, a wind or current of air is pouring in from the crevices of the windows and doors, to restore the equilibrium ; but the Kght air with which the room is filled must find some vent, in order to make way for the heavy air which enters. If you set the door a-jar, and hold a candle near the upper partot it, you will find that the flame will be blown outwards, showing that there is a current of air flowing out from the upper part of the room. Now place the candle on the floor close by the door, and you will perceive, by the inclina- tion of the flame, that there is also a current of air setting into the room. Caroline. It is just so ; the upper current is the warm light air, which is driven out to make way for the stream of cold dense air which enters the room lower down. Emily. I have heard, Mrs. B., that the periodical winds are not so regular on land as at sea ; what is the reason of that ? Mrs. B. 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 that it regularly does on the coast of Guinea, and other countries in the torrid zone. Emily. I have heard much of the violent tempests oc- casioned by the breaking up of the monsoons ; are not they also regular trade winds 1 Mrs. B. They are called periodical trade-winds, as they change their course every half-year. This varia- tion is produced by the earth's annual course round the sun, when the north pole is inclined towards that lumina- ry one half of the year, the south pole the other half. Du- ring the summer of the northern hemisphere, the countries of Arabia, Persia, India, and China, are much heated, 709. What familiar illustration can you give of the circulation of the air, first from the poles to the equator, and then rising and returning to the poles? 710. Why are the periodical winds more regular at sea than on land ? 711. What winds are call- ed monsoons ? 712. How is the variation of the monsoons produced ? 15* 14 ON WIND AND SOUND* and reflect great quantities of the sun's rays into the at- mosphere, by which it becomes extremely rarefied, and the equilibrium consequently destroyed. In order to re- store it, the air from the equatorial southern regions, where it is colder, (as well as from the colder northern parts,) must necessarily have a motion towards those parts. The current of air from the equatorial regions produces the trade-winds for the first six months^ in all the seas between the heated continent of Asia, and the equator. The other six months, when it is summer in the southern hemi- sphere, the ocean and countries towards the southern iropick are most heated, and the air over those parts most rarefied : then the air about the equator alters its course, and flows exactly in an opposite direction.* Caroline. This explanation of the monsoons is very curious ; but what does their breaking up mean ? Mrs. B. It is the name given by sailors to the shifting of the periodical winds ; they do not change their course suddenly, but by degrees, as the sun moves from one he- misphere to the other : this change is usually attended by storms and hurricanes, very dangerous for shipping \ so that those seas are seldom navigated at the season of the equinox. Emily. I think I understand the winds in the torrid fcone perfectly well ; but what is it that occasions the great variety of winds which occur in the temperate zones ? for according to your theory, there should be only north and south winds in those climates. Mrs. B. Since so large a portion of the atmosphere as is over the torrid zone is in continued agitation, these agi- tations in an elastick fluid, which yields to the slightest impression, must extend every way to a great distance ; 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 it is easy to conceive, that almost every climate must be liable to variable winds. * The south-west monsoon, which blows from April to October, brings with it floods of rain, and dreadful tempests. During the rest of the year, the north-east monsoon pioduces a dry and agree- able state of the air. 713. What effect do the monsoons have on the weather ? 714. What does the breaking up of the monsoons mean ? 715. What is it that occasions the great variety of winds which occur in the temperate zones ? ON WIND AND SOUND. 176 On the sea-shore, there is almost always a gentle sea-breeze setting in on the land on a summer's evening, to restore the equilibrium which had been disturbed by reflections from the heated surface of the shore during the day ; and when night has cooled the land, and condensed the air, we ge- nerally find it, towards morning, flowing back towards the sea. 'Caroline. I have observed that the wind, whichever way it blows, almost always falls about sun-set. Mrs. B. Because the rarefaction of air in the particu- lar spot which produces the wind, diminishes as the sun declines, and consequently the velocity of the wind abates. Emily. Since the air is a gravitating fluid, is it not affected by the attraction of the moon and the sun, in the same manner as the waters 1 Mrs. B. Undoubtedly; but the aerial tides are as much greater than those of water, as the density of water exceeds that of air, which, as you may recollect, we found to be about 800 to 1. Caroline. What a prodigious protuberance that must occasion ; how much the weight of such a column of air must raise the mercury in the barometer ! Emily. As this enormous tide of air is drawn up and supported, as it were by the moon, its weight and pres- sure, I should suppose, would be rather diminished than increased ? Mrs. B. The weight of the atmosphere is neither in- creased nor diminished by the aerial tides. The moon's attraction augments the bulk as much as it diminishes the weight of the column of air ; these effects, therefore, counterbalancing each other, the aerial tides do not aflect the barometer. Caroline. I do not quite understand that. Mrs. B. Let us suppose that the additional bulk of air at high tide raises the barometer one inch ; and on the other hand, that the support which the moon's attrac- tion affords the air, diminishes its weight or pressure, so as to occasion the mercury to fall one inch ; under these 716. What are the sca-brcezes as they are termed ? 717. Wh) does the wind generally subside at the going down oi'the sun? 718. Does the moon have any effect on the wind ? 719. How much greater are the aerial tides than those of water ? 720. Why do not the aerial tides affect the barometer ? 176 ON WIND AND SOt'#f>. circumstances the mercury must remain stationary. Thus you see, that we can never be sensible of aerial tides by the barometer, on account of the equality of pressure of the atmosphere, whatever be its height. The existence of aerial tides is not, however, hypo- thetical ; it is proved by the effect they produce on the apparent position of the heavenly bodies ; but this I can- not explain to you, till you understand the properties of light.* Emily. And when shall we learn them 1 Mrs, B. I shall first explain to you the nature of sound, which is intimately connected with .that of air ; and I think at our next meeting we may enter upon the subject of opticks. We have now considered the effects produced by the wide and extended agitation of the air ; but there is ano- ther kind of agitation of which the air is susceptible a sort of vibratory, trembling motion, which, striking on the druRi of the ear, produces sound.^ Caroline. Is not sound produced by solid bodies 1 The voice of animals, the ringing of bells, musical in- struments, are all solid bodies. I know of no sound but that of the wind which is produced by the air. Mrs. B. Sound, I assure you, results from a tremu- lous motion of the air ; and the sonorous bodies you enu- merate, are merely the instruments by which that peculiar species of rnjtion is communicated to the air. * The quality of winds is affected by the countries over which they pass ; and they are sometimes rendered pestilential by the heat of deserts, or the putrid exhalations of marshes and Jakes. Thus, from the deserts of Africa, Arabia, and the neighbouring countries, a hot wind blows, called Snniicl or Simoom, which some- times produces instant death. A similar wind blows from the Sa- hara, upon the western coast of Africa, called the Htirmattan, pro- ducing a dryncss and heat which is almost insupportable, and scorching like the blasts of a furnace. t The science which treats of the nature, phenomena, nrid laws of sound, is called Jicoustichs. This science is particularly inte- resting and valuable from its extending to the theory of musical con- cord and har.mony. 721 . By irhat is the quality of winds affected ? 722. J1 hat farts arc stated in the notes illustrating the effects thus produced on the io*nd ? 723. How is sound produced ? ON WIND AND SOUND. II 77 Caroline. What ! when I ring this little bell, is it the air that sounds, and not the bell ? Mrs. B. Both the bell arid the air are concerned in the production of sound. But sound, strictly speaking, is a perception excited in the mind by the motion of the air on the nerves of the ear; the air, therefore, as well as the sonorous bodies which put it in motion, is only the cause of sound, the immediate effect is produced by the sense of hearing : for, without this sense, there would be no sound. Emily. I can with difficulty conceive that. A person born deaf, it is true, has no idea of sound, because he hears none ; yet that does not prevent the real existence of sound, as all those who are not deaf can testify. Mrs. B. I do not doubt the existence of sound to all those who possess the sense of hearing ; but it exists neither in the sonorous body nor in the air, but in the mind of the person whose ear is struck by the vibratory motion of the air, produced by a sonorous body. To convince you that sound does not exist in sonorous bodies, but that air or some other vehicle is necessary to its production, endeavour to ring the little bell, after I have suspended it under a receiver in the air-pump, from which I shall exhaust the air Caroline. This is indeed very strange : though I agi- tate it so violently, it does not produce the least sounJ. Mrs. B. By exhausting the receiver, I have cut off the communication between the air and the bell ; the lat- ter, therefore, cannot impart its motion to the air. Caroline. Are you sure that it is not the glass, which covers the bell, that prevents our hearing it 1 Mrs. B. That you may easily ascertain by letting the air into the receiver, and then ringing the bell. Caroline. Very true : I can hear it now almost as loud as if the glass did not cover it ; and I can no longer doubt but that air is necessary to the production of sound. Mrs. B. Not absolutely necessary, though by far the most common vehicle of sound. Liquids, as well as air, are capable of conveying the vibratory motion of a sono- 724. What is sound, strictly speaking ? -725. How can it be shown that air is necessary in the production of sound ? 7*26. Why cannot a bell be heard in an exhausted receiver ? 727. Is the atmosphere the only conductor of sound ? 178 ON WIND AND SOUND* rous body to the organ of hearing ; as sound can be heard under water. Solid bodies also convey sound, as I can soon convince you by a very simple experiment. I shall fasten this string by the middle round the poker ; now raise the poker from the ground by the two ends of the string, and hold one to each of your ears: 1 shall now strike the poker with a key, and you will find that the sound is conveyed to the ear by means of the strings, in a much more perfect manner than if it had no other vehicle than the air. Caroline. That it is, certainly, for I am almost stun- ned by the noise. But what is a sonorous body, Mrs. B. 1 for all bodies are capable of producing some kind of sound by the motion they communicate to the air. Mrs. B. Those bodies are called sonorous, which pro- duce clear, distinct, regular, and durable sounds, such as a bell, a drum, musical strings, wind instruments, &LC. They owe this property to their elasticity ; for an elastick body, after having been struck, not only returns to its former situation, but having acquired momentum by its ve- locity, like the pendulum, it springs out on the opposite side. If 1 draw the string A B, which is made fast at both ends, to C, it will not only return to its original po- sition, but proceed onwards to D. This is its first vibration, at the end of which it will re- lain sufficient velocity to bring it to E, and back again to F, which constitutes its second vibration ; the third vibra- tion will carry it only to G and y, and so on till the re- sistance of the air destroys its motion. The vibration of a sonorous body gives a tremulous mo- tion to the air around it, very similar to the motion com- municated to smooth water when a stone is thrown into it. This first produces a small circular wave around the spot in which the stone falls ; the wave spreads, and gradually communicates its motion to the adjacent wa- ters, producing similar waves to a considerable extent. The same kind of waves is produced in the air by the 7*28 What besides air convey the vibratory motion of sonorous bpdiRB ? 72!>. What bodies are called sonorous ? 7:W. To what do they owe their sonorous property : 781. How would you explain Fig. 6, plate XIV. as illustrating the production of sound .* 732. To what is the tremulous motion, given to the air by a sonorous body, compared ? ON WIND AND SOUND. 171) motion of a sonorous body, but with this difference, that as air is an elastick fluid, the motion does not consist of regularly extending waves, but of vibrations, and are com- posed of a motion forwards and backwards, similar to those of the sonorous body. They differ also in the one taking * place in a plane, the other in all directions. The aerial undulations being spherical. E'nily. But if the air moves backwards as well as for- wards, hjw can its motion extend so as to convey sound to a distance. Mrs. B. The first sphere of undulations which are produced immediately around the sonorous body, by pressing against the contiguous air, condenses it. The condensed air, though impelled forward by the pressure, re-acts on the first set of undulations, driving them back again. The second set of undulations which have been put in motion, in their turn communicate their motion, and are themselves driven back by re-action. Thus there is a succession of waves in the air, corresponding with the succession of Waves in the water. Caroline. The vibrations of sound must extend much further than the circular waves in water, since sound is conveyed to a great distance. Mrs. B. The air is a fluid so much less dense than water, that motion is more easily communicated to it. The report of a cannon produces vibrations of the air whiwh extend to several miles around. ' Emily. Distant sound takes some time to reach us, since it is produced at the moment the cannon is fired ; and we see the light of the flash long before we hear the report. Mrs. B. The air is immediately put in motion by the firing of a cannon ; but it requires time for the vibrations to extend to any distant, spot. The velocity of sound is computed to be at the rate of 1142 feet in a second. Caroline. With what astonishing rapidity the vibra- tions must be communicated ! But the velocity of sound varies, I suppose, with that of the air which conveys it. If the wind sets towards us from the cannon, we must hear the report sooner than if it set the other way. 7-M If the air reverberate, how can its motion extend so as to convey sound to a distance ? 734. Why is motion more eas'ly communicated to air than to water ? 73^5. Why do we see the flash of a cannon, at a distance, before we hear the report ? 736, What is the computed velocity of sound ? 180 ON WIND AND SOUND. Mrs-. B. The direction of the wind makes less diffe- rence in the velocity of sound than you would imagine. If the wind sets from us, it bears most of the aerial waves away, and renders the sound fainter ; but it is not very considerably longer in reaching the ear than if the wind blew towards us. This uniform velocity of sound enables us to determine the distance of the object from which it proceeds; as that of a vessel at sea firing a cannon, or that of a thunder cloud. If we do not hear the thunder till half a minute after we see the lightning, we conclude the cloud to be at the distance of six miles and a half. Emily. Pray how is the sound of an echo produced ? Mrs. B. When the aerial vibrations meet with an ob- stacle, having a hard and regular surface, such as a wall, or rock, they are reflected back to the ear and produce the sa:ne sound a second time ; but the sound will then appear to proceed from the object by which it is reflected. If the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line ; if obliquely, the sound returns obliquely in the opposite direction, the angle of reflection being equal to the angle of incidence. Caroline. Oh, then, Emily, I now understand why the echo of my voice behind our house is heard so much plainer by you than it is by me, when we stand at oppo- site ends of the gravel walk. My voice, or rather, I should say, the vibrations of air it occasions, fall obliquely on the walJ of the house, and are reflected by it to the opposite end of the gravel walk. Emily. Very true ; and we have observed that when we stand in the middle of the walk, opposite the house, the echo returns to the person who spoke. Mrs. B. Speaking-trumpets are constructed on the prin- ciple of the reflection of sound. The voice, instead of being diffused in the open air, is confined within the trumpet; and the vibrations which spread and fall against the sides of the instrument, are reflected according to the angle of inci- dence, and fall into the direction of the vibrations which proceed straight forwards. The whole of the vibrations arc thus collected into a focus ; and if the ear be situated in or near that spot, the sound is prodigiously increased. 737. What effect has the direction of the wind on the velocity of sound ? 738. To what practical purpose can we apply the uniform velocity of sound ? 739. How is the sound of an echo produced? 740. On what principle are speaking-trumpets constructed f ON WIND AND SOUND. 18J Figure 7, plate XIV. will give you a clearer idea of the speaking-trumpet : the reflected rays are distinguished from those of incidence, by being dotted ; and they are brought to a focus at F. The trumpet used by deaf per- sons acts on the same principle ; but as the voice enters the trumpet at the large instead of the small end of the instrument, it is not so much confined, nor the sound so much increased. Emily. Are the trumpets used as musical instruments also constructed on this principle 1 Mrs. B., So far as their form tends to increase the sound, they are ; but, as a musical instrument, the trum- pet becomes itself the sonorous body, which is made to vibrate by blowing into it, and communicates its vibrations to the air. I will attempt to give you in a few words, some notion of the nature of musical sounds, which as you are fond of musick must be interesting to yOu. If a sonorous body be struck in such a manner, that its vibrations are all performed in regular times, the vibra- tions of the air will correspond with them ; and striking in the same regular manner on the drum of the ear, will produce the same uniform sensation on the auditory nerve and excite the same uniform idea in the mind ; or, in other words, we shall hear one musical tone. But if the vibrations of the sonorous body are irregular, there will necessarily follow a confusion of aerial vibra- tions ; for a second vibration may commence before the first is finished, meet it half way on its return, interrupt it in its course, and produce harsh jarring sounds which are called discords. Emily. But each set of these irregular vibrations, if repeated at equal intervals, would, I suppose, produce a musical tone. It. is only their irregular succession which makes them interfere, and occasions discord. 741. What does Figure 7, Plate XIV. represent? 742. Where must the ear be situated iir regard to the speaking-trumpet so as to receive an increased sound ? 743. How do the speak- ing-trumpets used by deaf persons differ from that in the figure ? 744. Flow far is a trumpet used for a musical instrument con- structed on the above principle ? -745. How must a sonorous body be struck so that its vibrations produce in the mind the same uniform idea, or one musical tone ? 746. How are harsh jar- ring sounds or discords produced ? 10 182 ON WIND AND SOUND. Mrs. B. Certainly. The quicker a sonorous body vi- brates, the more acute, or sharp, is the sound produced. Caroline. But if I strike any one note of the piano- forte repeatedly, whether quickly or slowly, it always gives the same tone. Mrs. B. Because the vibrations of the same string, at the same degree of tension, are always of a similar dura- tion. The quickness or slowness of the vibrations relate to the single tones, not to the various sounds which they may compose by succeeding each other. Striking the note in quick succession, produces a more frequent repe- tition of the tone, but does not increase the velocity of the vibrations of the string. The duration of the vibrations of strings or chords de- pends upon their length, their thickness, or weight, ana their degree of tension : thus, you find, the low bass notes are produced by long, thick, loose strings ; and the high treble notes by short, small, and tight strings. Caroline. Then the different length and size of the strings of musical instruments, serve to vary the duration of the vibrations, and consequently, the acuteness of gra- vity of the notes 1 Mrs. B. Yes. Among the variety of tones, there are some which, sounded together, please the ear, producing what \ve call harmony, or concord. This arises from the agreement of the vibrations of the two sonorous bodies ; so that some of the vibrations of each strike upon the ear at the same time. Thus, if the vibrations of two strings are performed in equal times, the same tone is produced by both, and they are said to be in unison. Emily. Now, then, I understand why, when I tune my harp in unison with the piano-forte, I draw the strings tighter if it is too low, or loosen them if it is at too high a pitch ; it is in order to bring them to vibrate, in equal times, with the strings of the piano-forte. Mrs. B. But concord, you know, is not confined to unison; for two different tones harmonize in a variety of cases. If the vibrations of one string (or sonorous body whatever) vibrate in double the time of another, the se- cond vibration of the latter will strike upon the ear at the 747. On what does the acuteness or sharpness of a musical sound depend ? 748. On what does the duration of vibrations of strings or chords in musical instruments depend ? 749. Plow is harmony or concord in sounds produced ? 750. How is an octave concord produced ? ON OPT ICKS. 183 same instant as the first vibration of the former ; and this is tlie concord of an octave. If the vibrations of two strings are as two to three, the second vibration of the first corresponds with the third vi- bration of the latter, producing the harmony called a fifth. Caroline. So, then, when I strike the key-note with its fifth, I hear every second vibration of one, and every third of the other at the same time ? Mrs. B. Yes ; and the key-note struck with the fourth is likewise a concord, because the vibrations are as three to four. The vibrations of a major third with the key-note, are as four to five ; and those of a minor third, as five to six. There are othef tones which, though they cannot be struck together without producing discord, if struck suc- cessively, give us the pleasure which is called melody. Upon theso general principles the science of musick is founded ; but I am not sufficiently acquainted with it to enter any further into it.* We shall now, therefore, take leave of the subject of sound ; and, at our next interview, enter upon that of op- ticks, in which we shall consider the nature of vision, light, and colours. 751. How is that species of harmony, called a fifth, produced? CONVERSATION XIV. ON OPTICKS. Of Luminous, Transparent, and Opaque Bodies; Of the Radiation of Light ; Of Shadows ; Of the Reflec- tion of Light ; Opaque. Bodies seen only by Reflected Li*ht ; Vision explained ; Camera Obscura ; Image of Objects on the Retina. CAROLINE. I LONO to begin our lesson to-day, Mrs. B., for I ex- pect that it will be very entertaining. * When inusick is made by the" use of strings, the air is struck by the body, and the sound is excited 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. 184 . ON OPTICKS. Mrs. B. (Opticks y is certainly one of the most interest- ing branches of Natural Philosophy, but not one of the easiest to understand ; I must therefore beg that you will give me the whole of your attention. 1 shall first inquire, whether you comprehend the mean- ing of a luminous body, an opaque body, and a transparent body. Caroline. A luminous body is lone that shines^; an opaque .... Mrs. B. Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that shine are not luminous ; for a luminous body is one that shines by its own light, as the sun, the fire, a candle, &LC.* Emily. Polished metal, then, when it shines with so much brilliancy, is not a luminous body ? Mrs. B. No, for it would be dark if it did not receive light from a luminous body ; it belongs, therefore, to the class of opaque or dark bodies, which comprehend all such as are neither luminous nor will admit the light to pass through them. } Emily. And transparent bodies, are those which ad- mit the light to pass through them ; -such as glass and water. Mrs. B. You are right. Transparent or pellucid bodies are frequently called ^nediurns^ and the rays of * The direct linrht O f the sun is calculated to be equal to that, of 6500 candles, ^placed at the distance of one foot from the object. j and that of the moon to the light of{one candle at 7^ feet distance ) of Jupiter at 1O20 feet., and of Venus at(42l fet) feir Isaac New- ton supposed rays of light to consist of exceedingly small particles, infinitely smaller than sand, moving from luminous bodies; but later writers suppose then) to consist of the undulations of an elas- tick medium, which fills all space, and \vhirh produces the sensa- tion of light to the eye, just as the vibrations of the air produce the sensation of sound to the ear.) 752. What is the science called that treats of vision ? 753. What is a luminous body ? 754. To what is the direct light of the sun calculated to be equal? 755 To 'what is the lit? Jit of the moon- -of Jupiter and of Venus, respectively calculated to he equal ? 756. What was Sir Isaac. Newton's opinion concerning the nature of light ? 757. What is a modern opinion f 758. What are opaque bodies ? 750. What are transparent bodies ? 700. What are transparent bodies frequently called? ON OPTICKS. 185 light which pass through them, are said to be transmitted by them. Light, when emanated from the sun, or any other lumi- nous bbdyAs projected forwards in straight lines in every possible direction ; so that the luminous body is not only the general centre from whence all the rays proceed, but every point of it may be considered as a centre which ra- diates light in every direction.") (Fig. 1. plate XV.) Emily. But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course ? Mrs. B. Not at all. The particles of light |ire so ex- tremely minute,jt'hat they are never known to interfere with each other/ A ray of light is a^single line of light projected from a luminous body \ and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. 2.^ Caroline. Is light then a substance composed of par- ticles like other bodies 1 Mrs. B. This is a disputed point upon which; I can- not pretend to decide.) In some respects, light is obedi- ent to the laws which govern bodies ; in others it appears to be independent of them : thus,-, though its course is guided by the laws of motionjfo dbes not seem to be in- fluenced by those of gravity/) It has never been disco- vered to have weight, though a variety of interesting ex^ periments have been made with a view of ascertaining that point ; but we are so ignorant of the intimate nature of light, that an attempt to investigate it would lead us into a labyrinth of perplexity, if not of erroiir ; we shall therefore confine our attention to those properties of light which are well ascertained. Let us return to the examination of the effects of the radiation of light from a luminous body. Since the rays of light are projected in straight lines, when they meet 761. In what manner is light produced from luminous bodies ? ^762. What is the reason that the progress of rays of 4 light is not impeded by crossing each other ? 763. What is ,1 ray of light ? 764. What is a pencil of rays ? 765. Is light a sub- stTiuce composed of particles of matter like other bodies .' 706. In what respect is it subject to the laws of matter ? 7(37. In what respect is it not subjoct to- the laws of matter ' 7(58. What is the consequence when rays of light fall upon an opaque body? 16* 186 ON OPTICKS. with an opaque body through which they are unable to pass,^hey are stopped short in their course j for they can- not move in a curve line round the body. ^ Caroline. No, certainly ; for it would require some other force besides that of projection', to produce motion in a curve line. Mrs. B. The interruption of the rays of light, by the opaque body, produces, therefore,/darkness on the oppo- site side of it j and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it. forms a shadow. Emily. A shadow ihen ;is nothing more than darkness produced by the intervention of an opaque body^which prevents the rays of light from reaching an object behind the opaque body, Caroline. Why then are shadows of different degrees of darkness : for I should have supposed, from your defi- nition of a shadow, that it would have been perfectly black ? Mrs. B. It frequently happens that a shadow is pro- duced by an opaque body interrupting the course of the rays from one luminous body, while light from another reaches the space where the shadow is formed, in which case the shadow is proportionally fainter. This happens if the opaque body be lighted by two candles : if you ex- tinguish one of them, the shadow will be both deeper and more distinct. Caroline. But yet it will not be perfectly dark. Mrs. B. Because it is still slightly illumined by light reflected from the walls of the room, arid other surround- ing objects. You must observe, also, that when a shadow is pro- duced by the interruption of rays from a single luminous body, the darkness is proportional to the intensity of the light.. Emily. I should have supposed the contrary ; for as the light reflected from surrounding objects on the sha- dow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined. 700. What does this interruption produce in regard to the body ? 770. What is a shadow ? 771. Why are shadows of diffe- rent decrees of darkness ? 772. When a shadow is produced by the interruption of rays of light from a single opaque body, to what is the darkness of the shadow proportional ? ON OPTICKS. 1 Mrs. B. Your remark is perfectly just ; but as we have no means of estimating the degrees of light and of dark- ness but by comparison, the strongest light will appear to produce the deepest shadow. Hence a total eclipse of the sun occasions a more sensible darkness than midnight, as it is immediately contrasted with the strong light of noon-day.^ Caroline. The re-appearance of the sun after an eclipse, must, by the same contrast, be remarkably brilliant. Mrs. B. Certainly. There are several things to be observed in regard to the form and extent of shadows. If the luminous body A (fig. 3.) is larger than the opaque body B, the shadow will gradually diminish in size, till it terminate in a point.] Caroline. This is the case with the shadows of the earth and the moon, as the sun which illumines them, is larger than either of those bodies. And why is it not the case with the shadows of terrestrial objects, which are equally illumined by the sun ? but their shadows, far from diminishing, are always larger than the object, and in- crease with the distance from it. Mrs. B. In estimating the effect of shadows, we must consider the apparent not the real dimensions of the lu- minous body ; and in this point of view( the sun is a small object compared with the generality of the terrestrial bo- dies which it illumines j and when the luminous body is less than the opaque body, the shadow will increase^ with the distance to infinity'.X All objects, therefore, which are apparently larger than the sun, cast a magnified shadow. This will be best exemplified, by observing the shadow of an object lighted by a candle. Emily. I have often noticed, that the shadow of my figure against the wall, grows larger as it is more distant from me, which is owing, no doubt, to the candle that shines on me being much smaller than myself? Mrs. B. Yes. The shadow of a figure A, ({fig. 4.) varies in size, according to the distance of the several sur- faces B C D E, on which it is described. 773. Why does a total eclipse of the sun occasion a more sen- sible darkness than midnight ? 774. What will be; the form of the shadow when a luminous body is larger than the opaque body upon which it shinss ? 775. And why is it not the case with shadows of terrestrial objects, which are illumined by the sun ? 77H. When the luminous body is less than the opaque body, how does the shadow increase ? 777. Which figure illustrates this? ON OfTlOKS. Caroline. I have observed, that two candles produce two shadows from the same object ; whilst it would ap- pear from what you said, that they should rather produce only half a shadow, that is to say, a very faint one. Mrs. B. The number of lights (in different directions) while it decreases the intensity of the shadow, increases their number, which always corresponds with that of the lights ; for each light makes the opaque body cast a diffe- rent shadow, as illustrated by)fig. 5/ It represents a ball A, lighted by three candles f, C, D, and you observe the light B produces the shadow 6, the light C the shadow c, and the light D the shadow d. Emily. I think we now understand the nature of shadows very well ; but pray what becomes of the rays of light which opaque bodies arrest in their course, and the interruption of which is the occasion of shadows 1 Mrs. B. Your question leads to a very important pro- perty of light, Reflection. When rays of light encounter an opaque body, which they cannot traverse/part of them are absorbed by it,- and part are reflected, and rebound just as an elastick ball which is struck against a wall. Emily. And is light in its reflection governed by the same laws as solid elastick bodies 1 Mrs. B. Exactly. If a ray of light fall perpendicu- larly on an opaque body, it is reflected back in the same line, towards the point whence it proceeded. \lf it fall ob- liquely, it is reflected obliquely, but in the opposite direc- tion ;(the angle of incidence being equal to the angle of reflection. A You recollect that law in mechanicks 1 Emily. Oh yes, perfectly. Mrs. B. If you will close the shutters, we shall ad- mit a ray of the sun's light through a very small aperture, and I cart show you how it is reflected. I now hold this mirror, so that the ray shall fall perpendicularly upon it. 778. How may more shadows than ne be produced by a single opaque body ? 779. By which figure is this illustrated p 780. What is meant by the reflection of light ? 781 . Is all the light that falls upon an opaque body reflected r 782. By what laws is the reflection of light governed '783. If a ray of light fall upon an opaque body perpendicularly, how will it be reflected ? 784. How will it be reflected if it fall upon an opaque body obliquely? 785. How does the angle of incidence compare with the angle of reflection ? ON OPTICKS. 189 Caroline. I see the ray which falls upon the mirror, but not that which is reflected by it. Mrs. B. Because its reflection is directly retrograde. The ray of incidence and that of reflection both being in the same line, though in opposite directions, are confound- ed together. Emily. The ray then which appears to us single, is really double, and is composed of the incident ray pro- ceeding to the mirror, arid of the reflected ray returning from the mirror. Mrs. B. Exactly so. We shall now separate them by holding the mirror M, (^fig. 6\ in such a manner, that the incident ray A B shalliall obliquely upon it you see the reflected ray B C, is marching off in another direc- tion. If we draw a line from the point of incidence B, perpendicular to the mirror, it will divide the angle of incidence from the angle of reflection, and you will see that they are equal. Emily. Exactly ; and now that you hold the mirror so that the ray falls more obliquely on it, it is also reflected more obliquely, preserving the equality of the angles of incidence and reflection. Mrs. B. It is by^eflected rays/ only that we sec opaque objects. [ Luminous bodies send rays of light im- mediately to our eyes,' but the rays which they send to other bodies are invisible to us, and are seen only when they are reflected or transmitted by those bodies to our eyes. Emily. But have we not just seen the ray of light in its passage from the sun to the mirror, and its reflection? yet in neither case were those rays in a direction to enter our eyes. Mrs. B. No. What you saw was the light reflected to your eyes by small particles of dust floating in the air, and on which the ray shone in its passage to and from the minor. Caroline. Yet I see the sun shining on that house yonder, ar, clearly as possible. Mrs. B. Indeed you cannot see a single ray which passes from the sun to the house ; you see no rays but 78(>. Which figure illustrates the manner in which li^lit is re- flected ? 787. "By what rays do- we see opaque bodies ? 7d8. How are we able to see lio-ht that falls upon an opaque body and is reflected, but not in a direction to meet the eye ? 190 ON OPTICKS. those which enter your eyes ; therefore it is the rays which are reflected by the house to you, and not those which proceed from the sun to the house, that are visible to you. Caroline. Why then does one side of the house ap- pear to be in sunshine, and the other in the shade ? for if I cannot see the sun shine upon it, the whole of the house should appear in the shade. Mrs. B. That side of the house which the sun shines upon, reflects more vivid and luminous rays than the side which is in shadow, for the latter is illumined only by rays reflected upon it by other objects :, ; these rays are therefore twice reflected before they reach your sighti; and as light is more or less absorbed by the bodies it strikes upon, every time a ray is reflected its intensity is diminished. Caroline. Still I cannot reconcile myself to the idea, that we do not see the sun's rays shining on objects, but only those which objects reflect to us. Mrs. B. I do not, however, despair of convincing you of it. (Look at that large sheet of water ;/can you tell why the sun appears to shine on one part of it only 1 Caroline. No, indeed ; for the whole of it is equally exposed to the sun. This partial brilliancy of water has often excited my wonder ; but it has struck me more par- ticularly by moon-light. I have frequently observed a vivid streak of rnoon-shine on the sea, while the rest of the water remained in deep obscurity, and yet there was no apparent obstacle to prevent the inoon from shining on every part of the water equally. Mrs. B. By moon-light the effect is more remarkable, on account of the deep obscurity of the other parts of the water ; while by the sun's light the effect is too strong for the eye to be able to contemplate it. Caroline. But if the sun really shines on every part of that sheet of water, why does not every part of it reflect rays to my eyes 1 Mrs. B. The reflected rays are not attracted out of their natural course by your eyes. The direction of a 739. Why does one side of an opaque body appear to be in the sun-shine and the other in the shade, when by not seeing the rays that fall upon the object, both sides of it would appear shaded ? 700. What illustration is given to show that we only see the reflected light which falls upon different objects ? ON OPTICKS. 191 reflected ray, you know, depends on that of the incident rav ;/ the sun's rays, therefore, which fall with various de- grees of obliquity upon the water, are reflected in direc- tions equally various ; some of these will meet your eyes, and you will see them, but those which fall elsewhere are invisible to you. Caroline. The streak of sunshine, then, which we now see upon the water, is composed of those rays which by their reflection happen to fall upon my eyes ? Mrs. B. Precisely. 'Emily. But is that side of the house yonder, which appears to be in shadow, really illumined by the sun, and its rays reflected another way. Mrs. B. No ; that is a different case from the sheet of water. That side of the house is really in shadow ;(it is the west side, /which the sun cannot shine upon till the afternoon. Emily. Those objects, then, which are illumined by reflected rays, and those which receive direct rays from the sun, but which do not reflect those rays towards us, ap- pear equally in shadow 1 Mrs. B. Certainly ; for we see them both illumined by reflected rays. That part of the sheet of water, over which the trees cast a shadow, by what light do you see it? Emily. Since it is not by the sun's direct rays, it must be by those reflected on it from other objects, and which it again reflects to us. Caroline. But if we all see terrestrial objects by re- flected light, (as we do the moon,) why do they appear so bright and luminous ? I should have supposed that re- flected rays would have been dull and faint, like those of the moon. Mrs. B. The moon reflects the sun's light with as much vividness as any terrestrial object. (If you look at it on a clear night,) it will appear as bright as a sheet of water, the walls of a house, or any object seen by day-light and on which the sun shines. The rays of the moon are doubtless feeble, when compared with those of the 791. Why is it that the whole surfare of water on which tho sun or moon shines does not appear illumined ? 79*2. How dona the cnse of the sheet of water named, differ from that of the house on which tha sun shines? 793. How are we enabled to see the moon ? 192 ON OPT1CKS. sun ; but that would not be a fair comparison, for the for- mer are incident, the latter reflected rays. Caroline. True ; and when we see terrestrial objects by moonlight, the light has been twice reflected, and is consequently proportionally fainter. Mrs. B. In traversing the atmosphere, the rays, both of the sun and moon{ lose some of their light.) For though the pure air is a transparent medium, which transmits the rays of light freely, we have observed, that near the sur- face of the earth it is loaded with vapours and exhalations, by which some portion of them are absorbed. Caroline. I have often noticed that an object on the summit of a hill appears more distinct than one at an equal distance in a valley, or on a plain ; which is owing, I suppose, to the air being more free from vapours in an elevated situation, and the reflected rays being conse- quently brighter. Mrs. B. That may have some sensible effect ; but when an object on the summit of a hill has a back ground of light sky, the contrast with the object makes Its outline more distinct. Caroline. I now feel well satisfied that we see opaque objects only by reflected rays ; but I do not understand how these rays show us the objects from which they pro- ceed. Mrs. B. The rays of light enter at the pupil of the eye, and -proceed to the retina, or optick nerve, which is situated at the back part of the eye-ball^ and there they describe the figure, colour, and (excepting size) form a per- fect representation of the object from which they proceed. /We shall again close the shutters, and admit the light through the small aperture^ and you will see a picture on the wall, opposite the aperture, similar to that which is delineated on the retina of the eye. Caroline. Oh, how wonderful ! there is an exact pic- ture in miniature of the garden, the gardener at work, the 794. What effect is produced on the sun and moon's rays from traversing the atmosphere ? 795. What is there in the atmo- sphere that lias a tendency to absorb the rays of light ? 796. Why is it that objects on a hill appear more distinct than at an equal distance from us in a valley ? 797. How is if. that the rays of light give us an idea of the objects from which they pro- ceed ? 798. W T hat experiment illustrates the manner in which objects are delineated on the retina of the eye ? ON OPTICKS. 193 trees blown about by the wind. The landscape would be perfect, if it were not reversed ; the ground being above and the sky beneath. Mrs* B. It is not enough to admire, you must, under- stand this phenomenon, which^s called a camera obscura} from the necessity of darkening" the room, in order to ex- hibit it. This picture is produced by the rays of light reflected from the various objects in the garden, and which are ad- mitted through the hole in the window shutter. The rays from the glittering weathercock at the lop of the alcove A', (pi. XVI. fig. 1.) represent it in this spot a; for the weathercock being much higher than the aperture in the shutter, only a few of the rays, which are reflected by it in an obliquely descending direction, can find entrance there. The rays of light, you know, always move in straight lines : those, therefore, which enter the room in a descending direction, will continue their course in the same direction, and will, consequently, fall upon the low- er part of the wall opposite the aperture, and represent the weathercock reversed in that spot, instead of erect in the uppermost part of the landscape. Emily. And the rays of light from the steps (B) of the alcove, in entering the aperture, ascend, and will describe those steps in the highest instead of the lowest part of the landscape. Mrs. B. Observe, too, that the rays coming from the alcove, which is to our left, describe it on the wall to the right; while those which are reflected by the walnut tree C D, to our right, delineate its figure in the picture to the left c d. Thus the rays, coming in different directions, and proceeding always in right lines, cross each other at their entrance through the aperture : those which come above proceed below, those from the right go to the left, those from the left towards the right ; thus every object is represented in the picture, as occupying a situation the very reverse of that which it does in nature. Caroline. Excepting the flower-pot E F, which, though 700. What is this illustration called ? 800. From what cir* cnmstance does the camera obscura derive its name? 801. How would you explain Figure 1, plate XV r I, as illustrating the camera obscura ? .302. Why do the objects exhibited by the camera obscura appear inverted ? 17 194 ON OPTICKS. its position is reversed, has not changed its situation in the landscape. Mrs. B. The flower-pot is directly in front of the aperture : so that its rays fall perpendicularly upon h, and consequently, proceed perpendicularly to the wall, where they delineate the object directly behind the aperture. Emily. And is it thus that the picture of objects is painted on the retina of the eye ?* Mrs. B, Precisely. /The pupil of the eye,' through which the rays of light enter, represents the aperture in the window-shutter ; and the image delineated on the retina, is exactly similar to the picture on the wall. Caroline. You do not mean to say, that we see only the representation of the object which is painted on the retina, and not the object itself? Mrs, B. Jf, by sight you understand that sense by which the presence of objects is perceived by the mind, through the means of the eyes, we certainly see only the jmacje of those objects painted on the retinal O iroline. This appears to me quite incredible. Mrs. B. (The nerves^tare the only part of our frame capable of sensation ; they appear, therefore, to be the instruments which the mind employs in its perceptions ; for a sensation always conveys an idea to the mind. Now it is known, that our nerves can be affected only by contact ; and for this reason the organs of sense cannot act at a distance ; for instance, we are capable of smell- ing only particles which are actually in contact with the nerves of the nose. ( We have already observed, that the odour of a flower consists in effluvia, composed of very minute particles, which penetrate the nostrils, and * (Take off the sclerotica from the back part of the eye of an ox, or other animal, and place the eye in the hole of the window-shut- ter of a dark room, with its fore part towards tho external objects ; a person in the room will, through the transparent coat, see tho inverted image painted upon the retina. 803. What part of the eve is represented by the aperture in the window-shutter ? 804. And to what is the picture on the wall in the camera obscura similar ? 805. Do we receive the sensa- tion of objects before us, from the images formed on tiie retina of the eye, or direct from the objects themselves ? 806. How is an idea of visible objects conveyed to the mind ? oft opficKs. 195 e upon the olfactory nerves, which instantly convey the idea of smell to the mind. ) Emily. And sound, though it is said to be heard at a distance, is, in fact, heard only when the vibrations of the air, which convey it to our ears, strike upon the auditory nerve. Caroline. There is no explanation required to prove fhat the senses of feeling and of tasting are excited only by contact. M'-s. B. And I hope to convince you that the sense of sight is so likewise. The nerves, which constitute the sense of sight, are not different in their nature from those of the other organs ; they are merely instruments which convey ideas to the mind, and can be affected only on contact. (Now since j-eal objects cannot be brought to touch the optick nerve, the image of them is conveyed thither by the rays of light proceeding from real objects, which actually strike upon the optick nerve, and form that image which the mind perceives./ Caroline. While I listen to your reasoning, I feel con- vinced ; but when I look upon the objects around, and think that I do not see them, but merely their image painted in my eyes, my belief is again staggered. I can- not reconcile myself to the idea, that I do not really see this book which 1 hold in my hand, nor the words which I read in it. Mrs. D. Did it ever occur to you as extraordinary, that you never beheld your own face. Caroline. No ; because I so frequently see an exact representation of it in the looking-glass. Mrs. B. You see a far more exact representation of objects on the retina of your eye : it is a much more per- f jet mirror than any made by art. Emily. But is it possible, that the extensive landscape which I now behold from the window, should be repre- sented on so small a space as the retina of the eye ? Mrs. 73. It would be impossible for art to paint so small and distinct a miniature ; but nature works with a surer hand and a more delicate pencil. That power, which forms the feathers of the butterfly, and the flowerets of the daisy, can alone portray so admirable and perfect 817. How may the nerves which constitute the sense of sight be considered ? 196 ON OPTICKS. a miniature as that which is represented on the retina of the eye. Caroline. But, Mrs. B., if we see only the image of ohjects, why do we not see them reversed, as you showed us they were, in the camera obscura? Is not that a strong argument against your theory ? Mrs. B. Not an unanswerable one, I hope. The image on the retina, it is true, is reversed, like that in the camera obscura;(as the rays, unless from a very small object, intersect each other on entering the pupil,! in the same manner a? they do on entering the camera obscura. f.The scene, however, does not excite the idea of being in- verted, because we always see an object in the direction of the rays which it sends to us.\ Emily. I confess I do not understand that. Mrs. B. It is, I think, a difficult point to explain clearly. A ray which comes from the upper part of an ob- ject describes the image on the lower part of the retina ; but experience having taught us that the direction of that ray is from above, we consider that part of the object it represents as uppermost. The rays proceeding from the lower part of an object fall upon the upper part of the re- tina ; but as we know their direction to be from below, we see that part of the object they describe as the lowest. Caroline. When I want to see an object above me, I look up ; when an object below me, I look down. Does not this prove that I see the objects themselves? for if I beheld only the image, there would be no necessity for looking up or down, according as the object was higher or lower than myself. Mrs. B. I beg your pardon. When you look up to an elevated object, it is in order that the rays reflected from it should fall upon the retina of your eyes ; but the very circumstance of directing your eyes upwards con- vinces you that the object is elevated, and teaches you to consider as uppermost the image it forms on the retina, though it is, in fact, represented in the lowest part of it. W r hen you look down upon an object, you draw your conclusion from a similar reasoning ; it is thus that we see all objects in the direction of the rays which reach our eyes. 80S. If objects are seen only by their pictures on the retina of the eye, why do they not appear reversed, as in the camera obscu- ra? ON TtfE ANGLE OF VISION. 107 But I have a further proof in favour of what I have ad- vanced, which I hope will remove your remaining doubts ; I shall, however, defer it till our next meeting, as (he les- son has been sufficiently long to-day. CONVERSATION XV. OPTICKS 'CONTINUED. ON THE ANGLE OF VISION, AND THE REFLECTION OF* MIRRORS. Angle of Vision; Reflection of Plain Mirrors; Reflection of Convex Mirrors ; Reflection of Concave Mirrors* CAROLINE. WELL, Mrs. B., I am very impatient to hear what fur- ther proofs you have to offer in support of your theory. You must allow that it was rather provoking to dismiss us as you did at our last meeting. Mrs. B. You press so hard upon me with your objec- tions, that you must give me time to recruit my forces. Can you tell me, Caroline, why objects at a distance ap* pear smaller than they really are 1 Caroline. I know no other reason than their distance. Mrs. B. I do not think I have more cause to be sa- tisfied with your reasons than you appear to be with mine. We must refer again to the camera obscura to account for this circumstance $ and you will find, that the different apparent dimensions of objects at different distances; pro- ceed from our seeing, not the objects themselves,^ but merely their image on the retina. ^( Fig. 1, plat XVII. represents a row of trees, as viewed hi the camera obscura* I have expressed the direction of the rays, from the ob- jects to the image, by lines. Now, observe, the ray which conies from the top of the nearest tree, and that which comes from the fool of the same tree, meet at the aperture, 809. Why do objects appear smaller at. a distanre than they really H re ? -810. What is an angle of vision ? 811. Which figure illustrates the ancjle of vision ? 812. How would you explain that figure in reference to the effect that distance has on the apparent size of an ohiect ? 17* 108 ON f Hfc ANGLK OF VISION. forming an angle of abou/25 degrees ; \this is called the an- gle of vision, under which" we see tlte tree. These rays cross each other at the aperture, forming equal angles on each side of it, and represent the tree inverted in the camera obscura. The degrees of the image are conside- rably smaller than those of the object, but the proportions are perfectly preserved. Now let us notice the tipper and lower ray, from the most distant tree ; they form an angle of not more than twelve or fifteen degrees, and an image of proportional dimensions. Thus, two objects of the same size, as the two trees of the avenue, form figures of different sizes in the camera obscura, according to their distance ; or, in other words, according to the angle of vision under which they are seen. Do you understand this ? Caroline. Perfectly. Mrjs. B. Then you have only to suppose that the re- presentation in the camera obscura is similar to that on the retina. Now since objects in the same magnitudes appear to be of different dimensions, when at different distances from us, let nie ask you, which it is that we see ; the real objects, which we know do not vary in size, or the images, which we know do vary according to the angle of vision under which we see them ? Caroline. I must confess, that reason is in favour of the latter. But does that chair at the further end of the room form an image on my retina much smaller than this which is close to me ? they appear exactly of the same size. Mrs. B. I assure you they do not. The experience we acquire by the sense of touch corrects the errours of our sight with regard to objects within our reach. You are so perfectly convinced of the real size of objects which you can handle, that you do not attend to their apparent difference. Does that house appear to you much smaller than when you are close to it ? Caroline. No, because it is very near us. Mrs. J3. And yet you can see the whole of it through one of the windows of this room. The image of the house, on your retina, must, therefore, be smaller than that of 813. To what is the size of the angle of vision proportioned ? ON TIIEJ ANGLE OF VISION. 09 the window through which you see it. ^ It is your know- ledge of the real size of the house frhicii prevents your attending to its apparent magnitude?. If you were accus- tomed to draw from nature, you would be fully aware of this difference. Emily. And pray, what is the reason that, when we look up an avenue, the trees not only appear smaller as they are more distant, but seem gradually to approach each other till they meet in a point ? Mrs. B. ( Not only the trees, but the road which sepa- rates the twcrrows, forms a small visual angle, in propor- tion as it is more distant from us ; therefore the width of the road gradually diminishes as weU as the size of the trees, till at length the road apparently terminates in a point, at which the trees seem to meetA But this effect of the angle of vision 'will be more fully illustrated by a little model of an avenue, which I have made for that purpose. It consists of six trees, leading to a hexagonal temple, and viewed by an eye, on the retina of which the picture of the objects is delineated. I beg that you will not criticise the proportions ; for though the eye is represented the size of life, while the trees are not more than three inches high, the dispropor- tion does not affect the principle, which the model is in- tended to elucidate. Emily, l^lie threads which pass from the objects through the pupil of the eye to the" retina, are, I suppose, to represent the rays of light which convey the image of the objects to the retina 1 Mrs. B. Yes. I have been obliged to limit the rays to a very small number, in order to avoid confusion ; there are, you see, only two from each tree. Caroline. But as one is from the summit, and the other from the foot of the tree, they exemplify the diffe- rent angles under which we see objects at different dis- tances, better than if there were more. Mrs. B. There are seven rays proceeding from the temple, one from the summit, and two from each of the an- gles that are visible to the eye, as it is situated ; from 814. Why are we mt deceived as to the size of objects if the size of their images on the retina of the eve is vari-ed by the dis- tance the .objects are from us ? rS15. Why does a road or any avenue appear to diminish in width, till at length it apparently terminates in a point? >S16. What is the reason that objects viewed in front appear larger than when viewed obliquely ? 200 ON THE ANGLE OF VtSIONi these you may form a just idea of the difference of the arl* gle of vision of objects viewed obliquely, or in front ; for though the six sides of the temple are of equal dimen- sions, that which is opposite to the eye is seen under a much larger angle than those which are viewed obliquely* It is on this principle that the laws of perspective are founded. Emily. I am very glad to know that, for I have lately begun to learn perspective, which appeared to me a very dry study ; but now that I am acquainted with the princi- ples on which it is founded, I shall tind it much more in- teresting. Caroline. In drawing a view from nature, then, we do not copy the real objects, but the image they form on the retina of our eyes 1 Mrs. J3. Certainly. In sculpture, we copy nature as she really exists ; in painting, we represent her as she ap- pears to us. It was on this account that I found it diffi- cult to explain by a drawing the effects of the angle of vision, and was under the necessity of constructing a mo- del for that purpose. Emily. I hope you will allow us to keep, this model some time, in order to study it more completely, for a great deal may be learned from it ; it illustrates the na- ture of the angle of vision, the apparent diminution of distant objects, and the inversion of the image on the re- tina. But pray, why are the threads that represent the rays of liaht, coloured, the same as the objects from which they proceed 1 Mrs. B. That is a question which you must excuse my answering at present, but I promise to explain it to you in due time. I consent very willingly to your keeping the model, on condition that you will make an imitation of it, on the same principle, but representing different objects. We must now conclude the observations that remain to be made on the angle of vision^ Jf an object, with an ordinary degree of illumination, does not subtend an angle of more than two seconds of a 817. On what principle are the laws of perspective founded? 818. In drawing a picture of any object what are we to fol- low ? 819. How is nature to be exhibited in sculpture? 820. How is it, to be represented in painting ? ^ b21. When are objects invisible ? ON THE ANGLE OF VISION. 201 degree, it is invisible. There are consequently two cases in which objects may be invisible, either if they are too small, or so distant as to form an angle Jess than two se- comls of a degree. In like manner, if the velocity of a body does not ex- ceed 20 decrees in an hour, its motion is imperceptible. Caroline.. A very rapid motion may then be imper- ceptible, provided the distance of the moving body is suffi- ciently great. Mrs. B. Undoubtedly ; for the greater its distance, the smaller will be the angle under which its motion will appear to the eye. It is for this reason that the motion of the celestial bodies is invisible, notwithstanding their immense velocity. Emily. I arn surprised that so great a velocity as 20 degrees an hour should be invisible. Mrs. B. The real velocity depends allogther on the space comprehended in each degree ; and this space de- pends on the distance of the object, and the obliquity of its path. Observe, likewise, that we cannot judge of the velocity of a body in motion unless we know its distance ; for supposing two men to set off at the same moment from A and B, (fig. 2.) to walk each to the end of their respec- tive lines C and D : if they perform their walk in the same space of time, they must have proceeded at a very different rate, and yet to an eye situated at E, they will appear to have moved with equal velocity : because they will both have gone through an equal number of degrees, though over a very unequal length of ground. Sight is an extremely useful sense no doubt, but it cannot always be relied on, it deceives us both in regard to the size and the distance of objects ; indeed our senses would be very liable to lead us into errour, if experience did not set us right. _ Emily. Between the two, I think that we contrive to acquire a tolerably accurate idea of objects. Mrs. B. At least sufficiently so for the general pur- poses of life. To convince you how requisite experience 822 What must be the velocity that its motion be perceptible ? 823. Why is the motion of the celestial bodies imperceptible ? 824. What is necessary for us to know in order to judge of the velocity of a moving body ? 825. In what respects may tho sense of sio-ht deceive us ? 826. By what are the errours into which we may be led by the senses to be corrected ? 202 ON THE ANGLE OF VISION. is to correct the errours of sight, I shall relate to yoti the case of a young man who was blind from his infancy, and who recovered his sight at the' age of fourteen, by the ope- ration of couching. At first he had no idea either of the size or distance of objects, but imagined that every thing he saw touched his eyes ; and it was not till after having repeatedly felt them, and walked from one object to ano- ther that he acquired an idea of their respective dimen- sions, their relative situations, arid their distances. Caroline. The idea that objects touched his eyes, is however not so absurd as it at first appears ; for if we consider that we see only the image of objects, this image actually touches our eyes. Mrs. B. That is doubtless the reason of the opinion he formed, before the sense of touch had corrected his judgment. Caroline. But since an image must be formed on the retina of each of our eyes, why do we not see objects double ? Mrs. B. The action of the raya v on the optick nerve of each eye is so perfectly similar, that they produce but a single sensation ^ the mind therefore receives the same idea, from the retina of both eyes, and conceives the ob- ject to be single. Caroline. This is difficult to comprehend, and, I should think, can be but conjectural. Mrs. B. I can easily convince you that you have a distinct image of an object formed on the retina of each eye. Look at the bell-rope, and tell me, do you see it to the right or the left of the pole of the fire-skreen 1 Caroline. A little to the right of it. Mrs. B. Then shut your right eye, and you will see it to the left of the pole. Caroline. That is true indeed ! Mrs. B. There are evidently two representations of the bell-rope in different situations, which must be owing to an image of it being formed on both eyes ; if the action of the rays therefore on each retina were not so perfectly similar as to produce but one sensation, we should see 827. How would objects appear as to distance, to one who had always been blind, on first being made to see ' 828. Why would they seem to touch the eye ? 829. If the image of an object is formed on the retina of each eye, why does not the object appear double ? REFLECTING MIRRORS. 203 double, and we find that to be the case with many persons who are afflicted with a disease in one eye, which pre- vents the rays of light from affecting it in the same man- ner as the other. Emily. Pray, Mrs. B., when we see the image of an object in a looking-glass, why is it not inverted as in the camera obscura, and on the retina of the eye 1 Mrs. B. Because the rays do not enter the mirror by a small aperture, and cross each otherv, as they do at the orifice of a camera obscura, or the pupil of the eye. When you view yourself in a mirror, the rays from your eyes fall perpendicularly upon it, and are reflected in the same line ; the image is therefore described behind the glass, and is situated in the same manner as the ob- ject before it. Emily. Yes, I see that it is ; but the looking-glass is not nearly so tall as I am ; how is it therefore that I can see the whole of my figure in it 1 Mrs. B. It is not necessary that the mirror should be more than^half your heightJin order that you may see the whole of yolir person in it^ffig. 3.) The ray of light C D from your eye, which falls perpendicularly on the mirror B D, will be reflected back in the same line ; but the ray from your feet will fall obliquely on the mirror, for it must ascend in order to reach it ; it will therefore be re- flected in the line D A : and since we view objects in the direction of the reflected rays, which reach the eye, and that the image appears at the same distance behind the mirror that the object is before it, we must continue the line A D to E, and the line C D to F, at the termination of which, the image will be represented. Emily. Then I do not understand why I should not see the whole of my person in a much smaller mirror, for a ray of light from my feet would always reach it, though more obliquely. Mrs. B. True; but the more obliquely the ray falls on the mirror, the more obliquely it will be reflected ; 830. When we see the image of an object in a looking-glass, why does it not appear inverted, as in the camera obscura ? 831. What must be the height of a looking jylass, in order for ono to see his whole person in it ? ~3~32. How would you explain Fig. 3, of plate XVII. ? 833. Why may we not see ourselves entire, in a looking-glass less than half our height? 204 RELFECTING MIRRCRS. '.the ray would therefore be reflected above your head, and you could not see iu This is shown by the dotted line, (fig- 3.) / Now stand a little to the right of the mirror, so that the rays of light from your figure may fall obliquely on it Emily. There is no image formed of me in the glass now. Mrs. B. I beg your pardon, there is : but you cannot see it, Because the incident rays falling obliquely on the mirror will be reflected obliquely in the opposite direc- tion, Ihe angles of incidence and of reflection being equal.^ Caroline, place yourself in the direction of the reflected rays, and tell me whether you do not see Emily's image in the glass ? Caroline. Let me consider. In order to look in the direction of the reflected rays, I must place myself as much to the left of the glass as Emily stands to the right of it. Now I see her image, but it is not straight before me, but before her ; and appears at the same distance behind the glass, as she is in front of it. Mrs. B. You must recollect, that we always see ob- jects in the direction of the last rays which reach our eyes. Figure 4 represents an eye looking at the image of a vase reflected by a mirror ; it must see it in the direction of the ray A B, as that is the ray which brings the image to the eye : prolong the ray to C, and in that spot will the image appear. Caroline. I do not understand why a looking-glass re- flects the rays of light : for glass is a transparent body which should transmit them. Mrs. B. (It is not the glass that reflects the rays which form the image you behold, but the mercury behind it. The glass acts chiefly as a transparent case, through which the rays find an easy passage. 834. How is this shown by the figure ? 835. Why cannot a person see his own image in a looking-glass, if he stand to the right or left of it P- 836. If you stand obliquely to the right of the glass, why must another person stand just as much to the left of it, in order to see your image ? 837. When you stand at the right of the glass, and I stand at the left of it, why does your image appear directly opposite to yourself?- 838. How would you illustrate this by the Figure ?- 839. If glass is a transpa- rent body, why will looking-glasses reflect light? REFLECTION OF MIRRORS. 205 Caroline. Why then should not mirrors be made sim- ply of mercury ? Airs. B. ..Because mercury is a fluid.' By amalgamat- ing it with tin-foil, it becomes of the consistence of paste, attaches itself to the glass, and forms in fact a mercurial mirror, which would be much more perfect without its glass cover ; for the purest glass is never perfectly transpa- rent ; some of the rays therefore are lost during their pas- sage through it, by being either absorbed, or irregularly reflected. This imperfection of glass mirrors has introduced the use of'metallick mirrors^ for optical purposes. Emily. But since all opaque bodies reflect the rays of light, I do not understand why they are not all mir- rors. Caroline. A curious idea indeed, sister ; it would be very gratifying to see one's self in every object at which one looked. ?& Mrs. B. It is very true that all opaque objects reflect light ; but the surface of bodies in general is so rough and uneven,/that their reflection is extremely irregular, which prevents the rays from forming an image on the retina. This you will be able to understand better, when 1 shall explain to you the nature of vision, arid the struc- ture of the eye. You may easily conceive the variety of directions in which rays would be reflected by a nutmeg grater, on ac- count of the inequality of its surface, and the number of holes with which it is pierced. /All solid bodies resemble the nutmeg-grater in these respects, more or less^ and it is only those which are susceptible of receiving a polish, that can be made to reflect the rays with regularity. As hard bodies are of the closest texture, the least porous, and capable of taking the highest polish, they make the best mirrors; none therefore are so well calculated for this purpose as metals. Caroline. But the property of regular reflection is not 840. If the mercury reflect the light, why should not mirrors be mado of that material ? 841. What description of mirrors more perfect than glass have been introduced ? 842. If all opaque bodies reflect light, why cannot we see ourselves as well when looking at any other object, as when viewing a mirror? 843. What substances make the most perfect mirrors ? 18 206 REFLECTION OP CONVEX MIRRORS. confined to this class of bodies ; for T have often seen my- self in a highly polished mahogany table. Mrs. B. Certainly ; but as that substance is less du- rable, and its reflection less perfect, than that of metals, I believe it would seldom be chosen for the purpose of a mirror. There are three kinds of mirrors used in opticks ; the plain or flat, which are the common mirrors we have just mentioned; convex mirrors;^ and concave mirrors. The reflection of the two latter is very different from that of the former. The plain mirror, we have seen, does not alter the direction of the reflected rays, and forms an image behind the glass exactly similar to the object be- fore it. A convex mirror 'has the peculiar property of making the reflected rays diverge^ by which means it di- minishes the image; and a concave mirror makes the rays converge, and, under certain circumstances, magni- fies the image.,* Emily. We have a convex mirror in the drawing- room, which forms a beautiful miniature picture of the ob- jects in the room ; and I have often amused myself with looking at my magnified face in a concave mirror. But I hope you will explain to us why the one enlarges, while the other diminishes the objects it reflects. Mrs. B. Let us begin by examining ibe reflection of a convex mirror. This is formed of a portion of the ex- teriour surface of a sphere. When several parallel rays fall upon it, that ray only, which, if prolonged, would pass through the centre or axis of the mirror, is perpen- dicular to it. In order to avoid confusion, I have in fig. 1, plate XVIII. drawn only three parallel lines, A B, C D, E F, to represent rays falling on the convex mirror M N ; the middle ray, you will observe, is perpendicular to the mirror, the others fall on it obliquely. Caroline. As the three rays are parallel, why are they not all perpendicular to the mirror ? Mrs. B. They would be so to a flat mirror ; but as 844. How many kinds of mirrors arc there used in opticks ? 845. What are they ? 846. How docs a plain mirror exhibit an object' 847. How does a convex mirror exhibit an object ? 848. How does a concave mirror exhibit an ob- ject ? 849. Of what is the convex mirror formed ? 850. What does Fig. 1, plate XVI [I. represent: 851. When seve- ral rays fall upon a convex mirror, which on.e will be perpendicu- lar to it ? REFLECTION OF CONVEX MIRRORS. 207 this is spherical, no ray can fall perpendicularly upon it which is not directed towards the centre of the sphere. Emily. Just as a weight fails perpendicularly to the earth when gravity attracts it towards the centre. Mrs. B. In order, therefore, that rays may fall per- pendicularly to the mirror at B and F, the rays must be in the direction of the dotted lines, which, you may ob- serve, meet at the centre O of the sphere, of which the mirror forms a portion. Now can you tell me in what direction the three rays, A B, C D, E F, will be reflected 7 Emily. Yes, I think so : the middle ray falling per- pendicularly on the mirror, will be reflected in the bame line : the two others falling obliquely will be reflected obliquely to G H ; for the dotted lines you have drawn are perpendiculars, which divide their angles of incidence and reflection. Mrs. B. Extremely well, Emily ; and since we see objects in the direction of the reflected ray, 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 an image. This point is equally distant from the surface and centre of the sphere, and is calluJ the imagi- nary focus of the mirror. (Caroline. Pray what is the meaning of :i focus ? M~s. B. A point at which converging rays unite. And it is in this case called an imaginary focus ; be- cause the rays do not really unite at that point, but only appear to do so : for the rays do not pass through the mir- ror, since they are reflected by it. Emily. I do not yet understand why an object ap- pears smaller when viewed in a convex mirror. Mrs. B. ( It is owing to the divergence of the reflected rays. ^ Yoa have seen that a convex mirror converts, by reflection, parallel rays into divergent rays ; rays that fall upon the mirror divergent, are rendered still more so 85*J. In what direction must rays fall on the convex mirror M, N, at the points 13, F. so as to be perpendicular to it? 853. Why will the rays A, E, in Fig. 1, plar.e XVIII. bo reflected to the points G, H ? 854. Why would the image formed from these rays be seen at the point L ? 855. What is the rehtive situa- tion of the point L, and what is it called ? 856. What is a fo- cus ? 857. Why is the point L called an imaginary focus ? Tirt. Why does an object appear smaller when viewed in a Convex mirror? 208 REFLECTION OF CONCAVE MIRRORS. by reflection, and convergent rays are reflected either parallel, or less convergent. If then an object be placed before any part of a convex mirror, as the vase A B, fig. 2. for instance, the two rays from its extremities, falling convergent on the mirror, will be reflected less conver- gent, and will not come to a focus till 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. Caroline. But the reflected rays do not appear to me to converge less than the incident rays. I should have sup- posed that, on the contrary, they converged more, since they meet in a point. Mrs. B. They would unite sooner than they actually do, if they were not less convergent than the incident rays : for observe, that if the incident rays, instead of being re- flected by the mirror, continued their course in their original direction, they would come to a focus at D, which is considerably nearer to the mirror than at C ; the image is therefore seen under a smaller angle than the object ; arid the more distant the latter is from the mirror, the less is the image reflected by it. You will now easily understand the nature of the re- flection of concave mirrors. These are formed of a por- tion of the internal surface of a hollow sphere, and their peculiar property is to converge the rays of light. Can you discover, Caroline, in what direction the three parallel rays, A B, C D, E F, which fall on the concave mirror M N, (%. 3.) are reflected 1 Caroline. 1 believe 1 can. The middle ray is sent back in the same line, as it is in the direction of the axis of the mirror ; and the two others will be reflected obliquely, as they fall obliquely on the mirror. I must now draw two dotted lines perpendicular to their points of incidence, which will divide their angles of incidence and reflection ; and in order that those angles may be equal, the two oblique rays must be reflected to L, where they will unite with the middle ray. 859. How would you explain by the Figure, the manner in which a convex mirror makes an object appear smaller than it is ? 860. Of what is a concave mirror formed? 801. flow would you explain Fig. 3, plate XVIII. as illustrating the manner in which parallel rays will be reflected ? REFLECTION OP CONCAVE MIRRORS. 209 Mrs. B. Very well explained. Thus you see that fvhen any number of parallel rays fall on a concave mir- ror, they are all reflected to a focus $ for in proportion as the rays are more distant from the axis of the mirror, they fall more? obliquely upon it, and are more obliquely reflect- ed; in consequence of which they come to a focus in the direction of the axis of the mirror, at a point equally dis- tant from the centre and the surface of the sphere, and this point is not an imaginary focus, as happens with the convex mirror, but is the true focus at which the rays unite. Emily. Can a mirror form more than one focus by reflecting rays 1 Mrs. B. Yes. If rays fall convergent on a concave mirror, '(fig. 4.) they are sooner brought to a focus, L, than parallel rays ; their focus is therefore nearer to the mir- ror M N. Divergent rays are brought to a more distant focus than parallel rays, as in fig. 5. where the focus is at L ; but the true focus of mirrors, either convex or con- cave, is that of parallel rays, which is equally distant from the centre, and the surface of the sphere. I shall now show you the reflection of real rays of light, by a metallick concave mirror. This is one made of polished tin, which I expose to the sun, and as it shines bright, we shall be able to collect the rays into a very brilliant focus. I hold a piece of paper where I imagine the focus to be situated ; you may see by the vivid spot of light on the paper, how much the rays converge : but it is not yet exactly in the focus ; as I approach the paper to that point, observe how the brightness of the spot of light increases, while its size diminishes. Caroline. That must be occasioned by the rays be- coming closer together. I think you hold the paper just in the focus now, the light is so small and dazzling Oh, Mrs. B., the paper has taken fire ! 8u2. Upon what does the obliquity depend with which parallel rays fal! upon the surface of? concave mirror ? 863. What is the focus of a concave mirror ? 864. What is the relative po- sition of the focus to a concave mirror ? 865. Is the focus of a com-ave mirror real, or only imaginary as in the convex mirror ? 8(>6. Will the focus be in th same plnce whether the rays fall parallel or converging upon the mirror r 867. Which is most distant from the mirror ? 68. Which figure illustrates this J 8C9. Which will form the more distant focus from the mirror, divergent or parallel rays.-' 870. Which figures illus- trate this/ 210 REFLECTION OF CONCAVE MIRRORS. Mrs. B. The rays of light cannot be concentrated, without, at the same time, accumulating a proportional quantity of heat : hence concave mirrors have obtained the name of^burning-mirrors. Emily. I have often heard of the surprising effects of burning-mirrors, and I am quite delighted to understand their nature. Caroline. It cannot be the true focus of the mirror at which the rays of the sun unite, for as they proceed from a point, they must fall divergent upon the mirror. Mrs. B. Strictly speaking, they certainly do. But when rays come from such an immense distance as the sun, their divergence is so trifling, as to be impercepti- ble ; and they may be considered as parallel : their point of union is, therefore, the true focus of the mirror, and there the image of the object is represented. Now that I have removed the mirror out of the influence of the sun's rays, if I place a burning taper in the focus, how will its light be reflected 1 (fig. 6.) Caroline. That, 1 confess, I cannot say. Mrs. B. The ray which falls in the direction of the axis of the mirror, is reflected back in the same line ; but let us draw two other rays from the focus, falling on the mirror at B and F ; the dotted lines are perpendicular to those points, and the two rays will -therefore be reflected to A and E. Caroline. Oh, now I understand it clearly. The rays which proceed from a light placed in the focus of a con- cave mirror fall divergent upon it and are reflected pa- rallel. It is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus. Mrs. B. Yes : when the incident rays are parallel, the reflected rays converge to a focus ; when, on the con- trary, the incident rays proceed from the focus, they are reflected parallel. This is an important law of opticks, and since you are now acquainted with the principles on which it is founded, I hope that you will not forget it. 671. What are concave mirrors sometimes called? 872. Why are they called burning-glasses ? 873. Do the rays which corae from the sun, en being reflec'.e^ by a concave mirror, meet in the true focus of the mirror ? 874. If a burning taper is placed in the focus of a concave mirror, how will its lio-ht be re- flected ; 675. VVhat is illustrated by Fig. 6, plate XVIII. ? 876. What is mentioned as an important law in opticks relating to the falling of light upon- mirrors ? THE REFRACTION OF LIGHT. 21 1 Caroline. I am sure that, we shall not. But, Mrs. B., you said that the image was formed in the focus of a con- cave mirror ; yet I have frequently seen glass concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror* Mrs. B. That is the case only, when the object is placed between the mirror and its focus; the image then appears magnified behind, or, as you call it, within the mirror. Caroline. I do not understand why the image should be larger than the object. Mrs. B. It proceeds from the convergent property of the concave mirror. If an object, A B, (fig. 7.) be placed between the mirror and its focus, the rays from its extre- mities fall divergent on the mirror, and on being reflected, become less divergent, as if they proceeded from C : to an eye placed in that situation the image will appear mag- nified behind the mirror at a 6, since it is seen under a larger angle than the object. You now, I hope, understand the reflection of light by opaque bodies. At our next meeting, we shall enter upon another property of light no less interesting, which is call- ed refraction. CONVERSATION XVI. ON REFRACTION AND COLOURS. Transmission of Light by Transparent Bodies ; Rcfrno tion; Refraction of the Atmosphere; Refraction of a 'Lens; Refraction of the Prism ; Of the Colours of Rays of Light ; Of the Colours of Bodies. MRS. B. THE refraction of light will furnish the subject of to- day's lesson. Caroline. This is a property of which I have not the faintest idea. 877. Where must the .object be placed in regard to a concave mirror, in order that t!ie image appear uehind the rr.irror ? 78. Why Joes the imagre in a concave mirror appear larger than tho object? 879. How is this illustrated by the figure? 212 THE REFRACTION OF LIGHT. Mrs. B. It is the effect which transparent mediums produce on light in its passage through them. Opaque bodies, you know, reflect the rays, and transparent bodies transmit them ; but it is found, that if a ray, in passinor from one medium into another of different density, fall obliquely, it is turned out of its course. Caroline. It must then be acted on by some new power, otherwise it would not deviate from its first direction. Mrs. B. The power which causes the deviation of the ray appears to be the attraction of the denser medium. Let us suppose the two mediums to be air and water ; if a ray of light passes from air into water, it is more strong- ly attracted by the latter on account of its superiour den- sity. Emily, In what direction does the water attract the ray ? Mrs. B. It must attract it perpendicularly towards it in the same manner as gravity acts on bodies. If then a ray A B, (fig. 1, plate XIX.) fall perpendicu- larly on water, the attraction of the water acts in the same direction as the course of the ray : it will not therefore cause a deviation, and the ray will proceed straight on to E. But if it fall obliquely, as the ray C B, the water will attract it out of its course. Let us suppose the ray to have approached the surface of a denser medium, and that it there begins to be affected by its attraction ; this attraction, if not counteracted by some other powtr, would draw it perpendicularly to the water, at B ; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome ; the ray, therefore, acted on by both these powers, moves in a direction be- tween them, and instead of pursuing its original course to D, or being implicitly guided by the water to E, proceeds towards F, so that the ray appears bent or broken. Caroline. I understand that very well ; and is not this the reason that oars appear bent in water 1 880. What is meant by the refraction of light ? 881 . When does refraction in light take place ? 882. What power causes the refraction of light ? 883. How would you illustrate the re- fraction of light by an explanation of Fig. 1, plate XIX. f 884. Why does the ray C B descend to F instead of D or E in that figure ? THE REFRACTION OF LIGHT. 213 Mrs. B. It is owing to the refraction of the rays re- flected by the oar ; but this is in passing from a dense to a rare medium, for you know that the rays, by means of which you see the oar, pass from water into air. Emily. But I do not understand why a refraction takes place when a ray passes from a dense into a rare medium ; I should suppose that it would be rather less, than more attracted by the latter. Mrs. B. And it is precisely on that account that the ray is refracted. C B, fig. 2, represents a ray passing ob- liquely from glass into water : glass being the denser me- dium, the ray will be more strongly attracted by that which it leaves than by that which it enters. The attraction of the (jlass acts in the direction A B, while the impulse of projection would carry the ray to F ; it moves therefore between these directions towards D. Emily. So that a contrary refraction takes place when a ray passes from a dense into a rare medium. Caroline. But does not the attraction of the denser medium affect the ray before it touches it ? Mrs. B. The distance at which the attraction of the denser medium acts upon a ray is so small as to be insen- sible ; it appears therefore to be refracted only at the point at which it passes from one medium to the other. Now that you understand the principle of refraction, I will show you the refraction of a real ray of light. Do you see the flower painted at the bottom of the inside of this tea-cup ? (Fig. 3.) Emily. Yes. But now you have moved it just out of sight. ; the rim of the cup hides it. Mrs. B. Do not stir. I will fill the cup with water, and you will see the flower again. Emily. I do indeed ! Let me try to explain this : when you drew the cup from me so as to conceal the flow- er, the rayvs reflected by it no longer met my eyes, but were directed above them ; but now that you have filled the cup with water, they are refracted by the attraction of the water, and bent downwards so as again to enter my eyes. Mrs. B. You have explained it perfectly : Fig. 3. 385. Why does a straight stick appear crooked when one end of it is immersed obliquely in the water ? 83(4. How would you explain Fitr. 2, plate XIX. ? 8S7. Does the attraction of die denser medium affoot the ray before it touches it? 214 THE REFRACTION OF LIGHT. will help to imprint it on your memory. You must ob- serve that when the flower becomes visible by the refrac- tion of the ray, you do not see it in the situation which it really occupies, but an image of the flower higher in the cup ; for as objects always appear to be situated in the direction of the rays which enter the eye, the flower will be seen in the direction of the reflected ray at B. Emily. Then when we see the bottom of a clear stream of water, the rays which it reflects being refracted in their passage from the water into the air, will make the bottom appear higher than it really is. Mrs. B. And the water will consequently appear more shallow. Accidents have frequently been occasion- ed by this circumstance ; and boys who are in the habit of bathing should be cautioned not to trust to the appa- rent shallowness of water, as it will always prove deeper than it appears ; unless indeed, they view it from a boat on the water, which wHl enable them to look perpendicu- larly upon it ; when the rays from the bottom passing per- pendicularly, no refraction will take place. The refraction of light prevents our seeing the heaven- ly bodies in their real situation ; the light they send to us being refracted in passing into the atmosphere, we see the sun and stars in the direction of the refracted ray ; as de- scribed in fig. 4, plate XIX. ; the dotte.d line represents the extent of the atmosphere, above a portion of the earth E B E : a ray of light coming from the sun S falls ob- liquely on it, at A, and is refracted to B : then since we see the object in the direction of the refracted ray, a spec- tator at B will see an image of the sun at C, instead of the real object as S. Emily. But if the sun were immediately over our heads, its rays falling perpendicularly on the atmosphere would not be refracted, and we should then see the real sun in its true situation. Mrs. B. You must recollect that the sun is vertical only to the inhabitants of the torrid zone ; its rays, thcre- 883. How would you describe the experiment represented in Fi# 3, plate XIX. ? 889. Why does water appear more shal- low than it really is ? 890. In what situation may the bottom of water be viewed so as to appear of its real depth ? *j91. Do we see the heavenly bodies in their real situation .> H!)2. Why do we not ? 893. By which Figure is this illustrated, and how would you describe the illustration given ? 8G4. In what si- tuation may the sun be seen in its true place ? THE REFRACTION OF LIGHT. 215 fore, are always refracted in these climates. There is also another obstacle to our seeing the .heavenly bodies in their real situations ; light, though it .noves with extreme velocity, is about eight minutes and a half in its passage from the sun to the earth ; therefore, when the rays reach us, the sun mast have 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 had aban- doned eight minutes and a half before. Emily. When you speak of the sun's motion, you mean, I suppose, his apparent motion, produced by the diurnal motion of the earth ? Mrs. B. No doubt ; the effect being the same, whether it is our earth, or the heavenly bodies which move : it is more eas^ to represent things as they appear to be, than as they really are. Caroline. During the morning r then, when the sun is rising towards the meridian, we must (from the length of time the light is in reaching us) see an image of the sun below that spot which it really occupies. Emily. But the refraction of the atmosphere counter- acting this effect, we may perhaps, between the two, see the sun in its real situation. Caroline. And in the afternoon, when the sun is sink- ing in the west, refraction and the length of time which the light is in reaching the earth, will conspire to render the image of the sun higher than it really is. Mrs. B. The refraction of the sun's rays by the at- mosphere prolongs our days, as it occasions our seeing an image of the sun both before he rises and after he sets ; for below the horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth. So like- wise we see an image of the sun before he rises, the rays that previously fall upon the atmosphere being reflected to the earth.* * Tt is entirely owinor to the reflection of the atmosphere that the heavens appear bright in the day time. For without it, only that part would be luminous in which the sun is placed ; and if 805. How long is light in coming from the sun to the earth ? 39G. How would you explain the effect this has on the ap- parent situation of that luminary ? 897. What effect does the refraction of light from the atmosphere have on the length of our days ? 898. What would be the appearance of the heavens were it not for the atmosphere ? 21G THE REFRACTION OF LIGHT. Caroline. On the other hand we must recollect thai light is eight minutes and a half on its journey ; so that, by the time it reaches the earth, the sun may perhaps be risen above the horizon. Emily. Pray, do not glass windows refract the light ? Mrs. B. They do ; but this refraction is not percep- tible, because, in passing through a pane of glass, the rays suffer two refractions, which being in contrary directions, produce the same effect, as if no refraction had taken place. Emily. I do not understand that. Mrs. B. Fig. 5, plate XIX. will make it clear to you : A A represents a thick pane of glass seen edgeways. When the ray B approaches the glass at C, it is refracted by it ; and instead of continuing its course in the same di- rection, as the dotted line describes, it passes through the pane to D; at that point returning into the air, it is again refracted by the glass, but in a contrary direction to the first refraction, and in consequence proceeds to E. Now you must observe that the ray B C and the ray D E being parallel, the light does not appear to have suffered any refraction. Emily. So that the effect which takes place on the ray entering the glass, is undone on its quitting it. Or, to express myself more scientifically, when a ray of light passes from one medium into another, and through that into the first again, the two refractions being equal and in opposite directions, no sensible effect is produced. Mrs. B. This is the case when the two surfaces of the refracting n.edium are parallel to each other ; if they are not, the two refractions may be made in the same di- rection, as 1 shall show you. we could live without air, and should turn our backs to the sun, the whole heavens would appear as dark as in the night. In this case, also, we should have no twilight, but a sudden transition from the brightest sunshine to dark, immediately upon the setting of tho 899. In what manner would the changes of day and night then take place ? 900. Is light refracted in passing through com- mon window-glass ? 901. Why then is not the refraction per- ceptible ' 902. Which figure illustrates this ? TOE REFRACTION OF LIGHT. 217 When parallel rays (fig. 6.) fall or\a piece of glass hav- ing a double convex surface] and which is called a Lens, that only which falls in the direction of the axis of the lens is perpendicular to the surface ; the other rays fall- ing obliquely, are refracted towards the axis, and will meet at a point beyond the lensJcalled its focus. 1 Of the three rays( A B Cj which fall on the lens D E, the rays A and C are refracted in their passage through it, to a and c, and on quitting the lens they undergo a se- cond refraction in the same direction which unites them with the ray B at the focus F. Emily. And what is the distance of the focus from the surface of the lens ? Mrs. B. The focal distance depends both upon the form of the lens, and of the refractive power of the sub- stance of which it is made ; in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere of which the surface of the lens forms a portion ; it is at the distance, therefore, of the Ra- dius of the sphere.) There are lenses of various forms, as you will find de- scribed in fig, t, plate XX. The property of those which have a convex surface is to collect the rays of light to a focus ; and of those which have a concave surface, on the contrary, to disperse the n. For the rays A C falling on the concave lens X Y, (fig. 7, plate XIX. ,) instead of con- verging towards the ray B, which falls on the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to a, c, and by the second to r/, e. Caroline. And lenses which have one side flat and the other convex or concave, as A and B, fig. 1, plate XX, are, I suppose, less powerful in their refractions. Mrs. B. Yes ; they are called plano-convex, and plano-concave lenses ; the focus of the former is at the 903. What is a lens ? 904. In parallel rays that pass through a lens what ones will be refracted ? r905. "in what place will the refracted rays meet ? 906. Which figure illustrates this ? 907. What is the distance of the focus from the surface of the lens ? 908. What is the property of a convex lens P 909. What is the property of a concave lens ? 9 JO. What does Figure 7, Plate 19 illustrate .'-^911. What is a plano-coi- vex lens ? 19 218 ON REFRACTION AND COLOURS. distance of the diameter of a sphere, of which the convex surface of the lens forms a portion ; as represented in fig. 2, plate XX. The three parallel rays, A B C, are brought to a focus by the plano-convex lens,(X Y at F. I must now explain to you the refraction of a triangular piece of glass, called a prism. (Fig. 3.) Emily. The three sides of this glass are flat ; it can- not therefore bring the rays to a focus ; nor do I suppose that its refraction will be similar to that of a flat pane of glass, because it has not two sides parallel; I cannot therefore conjecture what effect the refraction of a prism can produce. Mrs. B. The refractions of the light, on entering and on quitting the prism, are both in the same direction. (Fig. 3.) On entering the prism P, the ray A is refracted from B to C, and on quitting it from C to D. I will show you this in nature ; but for this purpose it will be adviseable to close the window-shutters, and ad- mit, through the small aperture, a ray of light, which I shall refract by means of this prism. Caroline. Oh, what beautiful colours are represented on the opposite wall ! There are all the colours of the rain- bow, and vvrth a brightness I never saw equalled. (Fig. 4, plate XX.) Emily. I have seen an effect, in some respect similar to this, produced by the rays of the sun shining upon glass lustres ; but how is it possible that a piece of white glass can produce such a variety of brilliant colours 1 Mrs. B. The colours are not formed by the prism, but existed in the ray previous to its refraction. Caroline. Yet, before its refraction, it appeared per- fectly white. Mrs. B. The white rays of the sun are composed of coloured rays, which, when blended together, appear co- lourless or white. Sir Isaac Newton, to whom we are indebted for the most important discoveries respecting light and colours, 9121 What is a plano-concave lens? 913. Where will be the focus of a plano-convex lens ? 914. What is illustrated by figure 2, plate XX. p 915. What is a prism: 916. What does figure 3, plate XX. represent ? 917. What is the design of figure 4, plate XX. ? 918. Are the different colours exhibited in that figure formed by the prism? 919. Of what are the white rays cf the sun composed ? 9?0. To whom are we inaebt- ed for the most important discoveries respecting light and colours? 6N REFRACTION AND COLOURS. 219 Was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays, which formed an image upon the Wall, such as you now see ex- hibited, (fig. 4!) in which are displayed the following se- ries of colours : fed, orange, yellow, green, blue, indigo, and violet. Emily. But how does 'a. prism separate these coloured fays ? Mrs. B. By refraction. It appears that the coloured fays have different degrees of refrangibility ; in passing through the prism, therefore, they take different direc- tions according to their susceptibility of refraction. The violet rays deviate most from their original course ; they appear at one of the ends of the spectrum A B : contigu- ous to the violet, are the blue rays, being those which have somewhat less refrangibiiity : then follow, in succes- sion, the green, yellow, orange, and, lastly, the red, which are the least refrangible of the coloured rays. Caroline. I cannot conceive how these colours, mixed together, can become white. Mr?-. f$. That I cannot pretend to explain ; but it is a fact that, the union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a card in compartments with these seven colours, and whirl it rapidly on a pin, it will appear white. But a more decisive proof of the composition of a white ray is afforded by re-uniting these coloured rays, and form- ing with them a ray of white light.* * The same conclusion mav be drawn from the experiment of mixing together paints of the colours exhibited in the prism, and in proper proportions, which will form white. It is true the white -will not he of the resplendent kind ; but this will be owing to the colours mixed being less bright than those produced from the prism. 921. What is tiie order of the colours displayed in the prism ? 022. Ho\v does the prism separate these rays ? 023. To what is the different directions, taken by the different rays in pass- ing through a prism, owing ? 024. Which rays deviate most and winch least from their original course in passing through a prism ? 025. What fact is mentioned respecting a painted card, as proving that these seven colours united make white ? 92 ! >. What experiment relating' to this subject is mentioned in the note ? 220 ON REFRACTION AND COLOURS. Caroline. If you can take a ray of white light to pieces., and put it together again, I shall be quite satisfied. J/r.s. JS. This can be done by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus ; and if, when thus re-united, we find that they appear white as they did before refraction, I hope that you will be convinced that the white rays are a compound of the several coloured rays. The prism P, you see, (fig. 5.) separates a ray of white light into seven coloured rays, and the lens L L brings them to a focus at F, where they again appear white. Caroline. You succeed to perfection : this is indeed a most interesting and conclusive experiment. Emily. Yet, Mrs. B., I cannot help thinking, that there may perhaps be but three distinct cclours in the spectrum, red, yellow, and blue ; and that the four others may consist of two of these colours blended together ; for in painting, we find that by mixing red and yellow, we pro- duce orange ; with different proportions of red and Wue, we make violet or any shade of purple ; and yellow and blue form green. Now it is very natural to suppose, that the refraction of a prism may not be so perfect as to se- parate the coloured rays of light completely, and that those which are contiguous in order of refrangibility may en- croach on each other, and by mixing produce the inter- mediate colours, orange, green, violet, and indigo. Mrs. B. Your observation is, 1 believe, neither quite wroncr, nor quite right. Dr. Wollaston, who has refract- ed light in a more accurate manner than had been pre- viously done, by receiving a very narrow line of light on a prism, found that it formed aspeclrum, consisting of rays of four colours only ; but they were not exactly tnose you have named as primitive colours, for they consisted of red, green, blue, and violet. A very narrow line of yellow was visible, at the limit of the red and green, which Dr. Wollaston attributed to the overlapping of the edges of the red and green light. 927. How can these colours once separated be adies re- flect all the rays that fall on them ? 943. What ones absorb them ? i ON REFRACTION AND COLOURS. Mrs. ft. I mean to say, that it has a general tendon* cy to reflect rays. Pale coloured bodies reflect all the co- loured rays to a certain degree, which produces their pale- ness, approaching to whiteness ; but one colour they re- flect more than the rest ; this predominates over the white, and determines the colour of the body. Since, then, bo* die? of a pale colour in some degree reflect all the rays of light, in passing through the various colours of the spec- trum, they will reflect them all with tolerable brilliancy ; but wil? appear most vivid in the ray of their natural co- lour. The green leaves, on the contrary, are of a dark colour, bearing a stronger resemblance to black, than to white ; they have,, therefore, a greater tendency to ab- sorb, than to reflect rays; and reflecting very few of any but the blue and yellow rays, they will appear dingy in passing through the other colours of the spectrum. Caroline. They must, however, reflect great quan- tities of the green rays to produce so deep a colour. Mrs. B. Deepness or darkness of colour proceeds rather from a deficiency than an abundance of reflected rays. Remember that bodies are, of themselves, black ; and if a body reflects only a few green rays, it will appear of a d?.rk green ; it is the brightness and intensity of the colour which show that a great quantity of rays are re- flected. Emily. A white body, then, which reflects all the rays, will appear equally bright in all the colours of the spectrum. Mrs. B. Certainly; and this is easily proved by pass- ing a sheet of white paper through the rays of the spec- trum. Caroline. What is the reason that blue often appears green by candle-light 1 Mrs. B. The light of a candle is not so pure as that of the sun ; it has a yellowish tinge, and when refracted by the prism, the yellow rays predominate ; and as blue bodies reflect the yellow rays in the next proportion, (being next in order of refrangibility,) the superabundance of yellow rays gives to blue bodies a greenish hue. Caroline. Candle-light must then give to all bodies a yellowish tinge, from the excess of yellow rays ; and yet 919 To what is darkness of colour owing ? 950. What is the reason the blue often appears green by candle-light ? 951. Why do persons of a sallow complexion appear fairer or whiter by night, if the candle-light gives all objects a yellowish tingo? I ON REFRACTION AND COLOUR. 237 it is a common remark, that people of a sallow complexion appear fairer or whiter by candle-light. Mrs. B. The yellow cast of their complexion is not so striking, when every object has a yellow tinge. Emily. Pray, why does the sun appear red through a fog ? Mrs. B. It is supposed to be owing to the red rays hav- ing a greater momentum, which gives them power to tra- verse so dense an atmosphere. For the same reason, the sun generally appears red at rising and sitting : as the increased quantity of atmosphere, which the oblique rays must traverse, loaded with the mists and vapours which are usually formed at those times prevents the other rays from reaching us. Caroline. And, pray, why are the skies of a blue co- lour. Mrs. B. You should rather say, the atmosphere ; for the sky is a very vague term, the meaning of which it would be difficult to define philosophically. Caroline^ But the colour of the atmosphere should be white, since all the rays traverse it in their passage to the earth. *Mrs. B. Do not forget that we see none of the rays which pass from the sun to the earth, excepting those which meet our eyes ; and this happens only if we look at the sun, and thus intercept the rays, in which case, you know, the sun appears white. The atmosphere is a trans- parent medium, through which the sun's rays pass freely to the earth ; but when reflected back into the atmosphere, their momentum is considerably diminished ; and they have not all of them power to traverse it a second time. The momentum of the blue rays is least ; these, there- fore, are the most impeded in their return, and are chiefly reflected by the atmosphere : this reflection is performed in every possible direction ; so that whenever we look at the atmosphere, some of these rays fall upon our eyes ; hence we see the air of a blue colour. If the atmosphere did not reflect any rays, though the objects on the sur-< face of the earth would be illumined, the skies would ap- pear perfectly black. Caroline. Oh, how melancholy that would be; and how pernicious to the sight, to be constantly viewing 9c2. Why does the sun appear red in the morninir and when seen through fosr or clouds ? ^953. Why does the sky or at- mosphere appear blue ?-. r-954. How would the sky appear if the atmosphere reflected none of the rays of light ? 228 ON REFRACTION AND COLOURS. bright objects against a black sky ! But what is the reason that bodies often change their colour ; as leaves which wither in autumn, or a spot of ink which produces an iron- mould on linen ? Mrs. JB. Jt arises from some chemical change, \vliich takes place in the internal arrangement of the parts, by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. A withered leaf thus no longer reflects the blue rays ; it appears, therefore, yellow, or has a slight tendency to reflect seve- ral rays which produce a dingy brown colour. An ink-spot on linen at first absorbs all the rays ; but exposed to the air, it undergoes a chemical change, and the spot partially regains its tendency to reflect colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould. Emily. Bodies, then, far from being of the colour which they appear to possess, are of that colour which they have the greatest aversion to, which they will not in- corporate with, but reject and drive from them. Mrs. B. It certainly is so ; though I scarcely dare venture to advance such an opinion whilst Caroline is con- templating her beautiful rose. Caroline. My poor rose ! you are are not satisfied with depriving it of colour, but even make it have an aversion to it ; and I am unable to contradict you. Emily. Since dark bodies absorb more solar rays than light ones, the former should sooner be heated if exposed to the sun. Mrs. B. And they are fouud by experience to be so. Have you never observed a black dress to be warmer than a white one ? Emily. Yes, and a white one more dazzling : the black is heated by absorbing the rays, the white dazzling by reflecting them. Caroline. And this was the reason that the brown paper was burnt in the focus of the lens, whilst the white paper exhibited the most luminous spot, but did not take fire. Mrs. B. It was so. It is now full time to conclude our lesson. At our next meeting, I shall give you a de- scription of the eye. 955. What, is the reason that they often change their colour ? 956. What dress is warmest, a black or a white one ? 957. Why is a black one warmest ? 958. Why is a white more daz- zling than a black dress ? . OPTICKS. 229 CONVERSATION XVII. OPTICKS. ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRU- MENTS. Description of the Eye ; Of the Image on the Retina ; Refraction of the Humours of the Eye ; Of ike Use of Spectacles ; Of the Single Microscope ; Of the Double Microscope ; Of the Solar Microscope ; Magick Lan- tern ; Refracting Telescope ; Reflecting Telescope. MRS. 15. THE body of the eye is of a spherical form : (fig. 1, plate XXI.) It has two membraneous coverings ; the exter- nal one, a a a, is called the sclerotica ; this has a projec- tion in that part of the eye which is exposed to view, b 6, which is called the cornea, because, when dried, it has nearly the consistence of very fine horn, and is sufficient- ly transparent for the light to obtain free passage through itl The second membrane which lines the cornea, and en- velopes the eye, is called the choroid, c c c / this has an opening in front, just beneath the cornea, which forms the pupil, d d, through which the rays of light pass into the eye. The pupil is surrounded by a coloured border, call- ed the iris, c c, which, by its muscular motion, always pre- serves the pupil of a circular form, whether it is expanded in the dark, or contracted by a strong light. This you will understand better by examining fig. 2. Emily. I did not know that the pupil was susceptible of varying its dimensions. Mrs. B. The construction of the eye is so admirable, that it is capable of adapting itself, more or less, to the circumstances in which it is placed. In a faint light the 95:). What is the form of the body of the eye ? 960. Which figure represents an eye ? 961. What is the external covering of the eye called ? 962. Which part of the eye is called the cornea ? 963. From what does the cornea take its name ? 964. What part of the eye is called the choroid ? 965. What part of the figure represents the choroid f 966. What is that part of the eye called through which the light passes ? 967. By what part of the figure is -the pupil represented ? 968. By what is the pupil of the eye surrounded ? 969. What repre- sents the iris in the figure ? 970. Is the pupil of the eye al- ways of the same size ? 20 230 OPTICKS. pupil dilates so as to receive an additional quantity of rays, and in a strong light it contracts, in order to prevent the intensity of the light from injuring the optick nerve. Observe Emily's eyes, as she sits looking towards the win- dows ; her puphs appear very small, and the iris large. Now, Emily, turn from the light and cover your eyes with your hand, so as entirely to exclude it for a few moments. Caroline. How very much the pupils of her eyes are now enlarged, and the iris diminished. This is, no doubt, the reason why the eyes surfer pain, when from darkness they suddenly come into a strong light ; for the pupil be- ing dilated, a quantity of rays must rush in before it has time to contract. Emily. And when we go from a strong light into ob- scurity, we at first imagine ourselves in total darkness ; for a sufficient number of rays cannot gain admittance into the contracted pupil, to enable us to distinguish ob- jects : but in a few minutes it dilates, apd we clearly per~ ceive objects which were before invisible. Mrs. B. It is just so. The choroid c t, is imbued with a black liquor which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye into a more perfect camera obscura. When the pupil is expanded to its utmost extent, it is capable of ad- mitting ten times the quantity of light that it does when most contracted, In cats, and animals which are said to see in the dark, the power of dilatation and contraction of the pupil is still greater ; it is computed that their pu* pils may receive one hundred times more light at one time than at another. Within these coverings of the eye-ball are contained three transparent substances, called humours. The first 971. When is it dilated, and when contracted ? 972. Why does it give the eyes pain on first going into a bright light from a dark room? 973. Why does it seem much darker on first go- ing out in the night, thai/after we have been out a short time ? 974. How much more light is admitted when the pupil is extended to the utmost, than when most contracted : 975. Why can cats, horses, and some other animals, see better in tho night than we can ?- 976. How much is it thought the pupil of their eyes extend and contract ? 977. What is contained within the coverings of the eye-ball ? Of TICKS. ,". 231 the space immediately behind the Cornea, and is Called the(aqueous humour,//, from its liquidity and its resemblance to water. Beyond this is situated the crys- talline humour, g g, so called from its clearness and trans- parency : it has the form of a lens, and refracts the rays of Jicrht in a greater degree of perfection than any that have been constructed by art : it is attached by two muscles, m 01, to each side of the choroid. The back part of the eye, between the crystalline humour and the retina, is fill- ed by the vitreous humour, h A, which derives its name from a resemblance it is supposed to bear to glass or vi- trified substar/ces. The membraneous coverings of the eye are intended chiefly for the preservation of the retina, i z, which is by far the most important part of the eye, as it is that which receives the impression of the objects of sight, and con- veys it to the mind. The retina consists of an expansion of the optick nerve, of a most perfect whiteness : it pro- ceeds from the brain, enters the eye, at ?i, on the side next the nose, and is finely spread over the interiour surface of the choroid. The rays of light which enter the eye by the pupil are refracted by the several humours in their passage through them, and unite in a focus on the retina. Caroline. I do not understand the use of these refract- ing humours ; the image of objects is represented in the camera obscura, without any such assistance. Mrs. B. That is true ; but the representation would be much more strong and distinct, if we enlarge the open- ing of the camera obscura, and received the rays into it through a lens. I have told you that rays proceed from bodies in all possible directions. We must, therefore, consider every part of an object which sends rays to our eyes, as points from which the rays diverge, as from a centre. 978. Whit arc the three humours called ? 079, From what does the aqueous humour derive its name f 980. From what does the crystalline humour derive its name ? 981. Fr m wh. Why wi-1 a concave lens renirdy this defect. ? '097. What is tho design of Fig. I,pla1o XXII."? 008. What is the reason that elderly people usually lose their sight ? 009. What remedy is there for the eye<= when the humours are decayed or flattened ? 1000. Which figure illustrates this r 100]. Why do old people without convex glasses hold the objects to be seen at a distance from the eye ? OPTICKS. 'Emily. I comprehend the nature of these two oppo- site defects very well ; but I cannot now conceive, how any sight can be perfect : for if the crystalline humour is of a proper degree of convexity, to bring the image of dis- tant objects to a focus on the retina, it will not represent near objects distinctly ; and if, on the contrary, it is adapt- ed to give a clear image of near objects, it will produce a very imperfect one of distant objects. Mrs. B. Your observation is very good, Emily : and it is true, that every person would be subject to one of these two defects, if we had it not in our power to in- crease or diminish the convexity of the crystalline humour,, and to project it towards, or draw it back from the obiect, as circumstances require. In a young well constructed eye, the two muscles to \vhich the crystalline humour is attached, have so perfect a command over it, that the focus of the rays constantly falls on the retina, and an image is formed equally distinct botA of distant objects, and of those which are near. Caroline. In the eyes of fishes, which are the only eyes I have ever seen separate from the head, the cornea does not protrude, in that part of the eye which is exposed to view. Mrs. B. The cornea of the eye of a fish is not more convex than the rest of the ball of the eye ; but to supply this deficiency, their crystalline humour is spherical, and refracts the rays so much, that it does not require the as- sistance of the cornea to bring them to a focus on the re- tina. Emily. Pray, what is the reason that we cannot see an object distinctly, if we approach it very near to the eye ? Mrs. B. Because the rays fall on the crystalline hu- mour too divergent to be refracted to a focus on the retina ; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline humour : the rays reach the retina be- fore they are collected to a focus, (fig. 4.) If it were not for this imperfection, we should be able to see and distin- 1002. By what means can the same eye see distinctly distant objects and those which are near ? 1003. What peculiarity of structure is there in the eyes of fishes ? 1001. How is this seeming defect remedied ? 1005. What is the reason that we cannot see an object distinctly when it is placed very near to the eye? 1006. By which figure is this illustrated ? 236 OPTIC KS. guish the parts of objects, which are now invisible to us, from their minuteness ; for could we approach them very near the eye, their image on the retina would be so much magnified as to render them visible. Emily. And could there be no contrivance to convey the rays of objects viewed close to the eye. so that they should be refracted to a focus on the retina. Mrs. B. The microscope is constructed for this pur- pose. The single microscope (fig. 5.) consists simply of a convex lens, commonly called a magnifying-glass ; in the focus of which the object is placed, and through which it is viewed : by this means you are enabled to approach your eye very near the object, for the lens, A B, by di- minishing the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystalline humour D, by which they are refracted to a focus on the retina, at R R. Emily '. This is a most admirable invention, and no- thing can be more simple, for the lens magnifies the ob- ject merely by allowing us to bring it nearer to the eye. Mrs. B. Those lenses, therefore, which have the shortest focus, will magnify the object most, because they enable us to bring the object nearest to the eye. Emily. But a lens, that has the shortest focus, is most bulging or convex ; and the protuberance of the lens will prevent the eye from approaching very near to the object. Mrs. 13. This is remedied by making the lens ex- tremely small : it may then be spherical without occupy- ing much space, and thus unite the advantages of a short focus, and of allowing the eye to approach the object. Caroline. We have a microscope at home, which is a much more complicated instrument than that you have described. Mrs. B. It is a double microscope, (fig. 6.) in which you see not the object A B, but a magnified image of it, a b. In this microscope, two lenses are employed, the one L Mj for the purpose of magnifying the object, is 1007. In what way can objects be seen distinctly when placed ? - 1008. Of w is the o 1010. What lenses will magnify objocts most? 1011. What . near the eye ? - 1008. Of what does a single microscope con- sist ? - 1000. What is the object of Fig. 5. plate XXIt. ? - - kind of lenses has the shortest focus? - 1012. What is repre- sented by Fig. 6, plate XXII. f - 1013. How would you explain the use of the double microscope, by the aid of that figure ? OPTICKS. 237 called the objeet-gfass ; the other N O, acts on the prin- ciple of the single microscope, and is called the eye-glass. There is another kind of microscope, called the solar microscope, which is the most wonderful from its great magnifying power ; in this we also view an image formed by a lens, not the object itself. As the sun shines, I car* show you the effect of this microscope : but for this pur- pose, we must close the shutters, and admit only a small portion of light, through the hole in the window-shutter, which we used for the camera obscura. We shall now place the object A B, (plate XXIII. fig. 1.) which is a small insect, before the lens, C D, and nearly at its fo- cus ; the image E F, will then be represented on the op- posite wall in the same manner as the landscape was in the camera obscura ; with this difference, that it will be mag- nified, instead of being diminished. I shall leave you to account for this, by examining the figure. Entity. I see it at once. The image E F is magnified, because it is further from the lens, than the object A B ; while the representation of the landscape was diminished because it was nearer the lens, than the landscape was. A lens, then, answers the purpose equally well, either for magnifying or diminishing objects ? Mrs. B. Yes ; if you wish to magnify the image,you place the object near the focus of the lens ; if you wish to produce a diminished image, you place the object at a distance from the lens, in order that the image may be formed in or near the focus. Caroline. The magnifying power of this microscope is prodigious, but the indistinctness of the image for want of light, is a great imperfection. Would it not be clearer, if the opening in the shutter were enlarged, so as to ad- mit more light ? Mrs. B. If the whole of the light admitted does not fall upon the object, the effect will only be to make the room lighter, and the image consequently less distinct. Rinily. But could you not by means of another lens bring a large pencil of rays to a focus on the object, and thus concentrate the whole of the light admitted upon it I ~~IOT4~. What does Fjcr. t, plate XXIII. represent pHHlOlS. How would you describe a solar microscope by the use of this figure ? 1016. Where must an object be placed in regard to a lens, so that the object be magnified ? 1017. Where, so that the ob- ject be diminished ? 1018. Where must all the light fall, used in the sola* microscope, so that the effect be the most favourable ? 38 Mrs. B. Very well. We shall enlarge the opening and place the lens X Y (fig. 2.) in it, to converge the rays lo a focus on the object A 13. There is but one thing more wanting to complete the solar microscope, which 1 shall leave to Caroline's sagacity to discover. Caroline. Our microscope has a small mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays, and reflect them upon the ob- ject ; if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the ob- ject more perfectly ? Mrs. D. You are quite right, d f> (fig. 2.) is a small mirror placed on the outside of the window-shutter, which receives the incident rays S S, and reflects them on the lens X Y. Now that we have completed the appara- tus, let us examine the mites on this piece of cheese, which I place near the focus of the iens. Caroline. Oh ! how much more distinct the image now is, and how wonderfully magnified ; the mites on the cheese look like a drove of pigs scrambling over rocks. Emily. I never saw any thing so curious. Now an immense piece of cheese has fallen : one would imagine it an earthquake: some of the poor mites must have been crushed ; how fast they run, they absolutely seem to gallop. But this microscope can be used only for transparent objects ; as the light must pass through them to form the image on the wall. Mrs. K. Very minute objects, such as are viewed in a microscope, are generally transparent; but when opaque objects are to be exhibited, a mirror M N (fig. 3. ) is used to reflect the light on the side of the object next the wall : the image is then formed by light reflected from the object, instead of being transmitted through it. Emily. Pray is not a magick lantern constructed on the same principles ?* * The magick lantern is an instrument used for magnifying paintinffs on glass, and throwing their images upon a white screen in a darkened chamber. 1019. What does fig. 2, plate XXIII. represent ? 1020. What is the use of the mirror in the solar microscope ? J021. For what objects can the solar microscope be used ? 1022. How can opaque objects be exhibited ? 1023. Which figure illus- trates this ? 1024. What is a magick lantern ? OPTICKS. 239 Mrs. B. Yes ; with this difference, that the light is supplied by a lamp, instead of the sun. The microscope is an excellent invention, to enable us to see and distinguish objects, which are too small to be visible to the naked eye. But there are objects which, though not really small, appear so to us, from their dis- tance ; to these we cannot apply the same remedy ; for when a house is so far distant, as to be seen under the same angle as a mite, which is close to us, the effect produced on the retina is the same : the angle it subtends is not large enough for it to form a distinct image on the retina. Emily. Since it is impossible, in this case, to approach the object to the eye, cannot we by means of a lens bring an image of it nearer to us ? Mrs. B. Yes ; but then the object being very distant from the focus of the lens, the image would be too small to be visible to the naked eye. Emily. Then, why not look at the image through ano- ther lens, which will act as a microscope, enable us to bring the image close to the eye, and thus render it visi- ble ? Mrs. B. Very well, Emily ; I congratulate you on having invented a telescope. In figure 4, the lens C D, forms an image E F, of the object A B ; and the lens X Y, serves the purpose of magnifying that image ; and this is all that is required in a common refracting telescope. Emily. But in fig. 4, the image is not inverted on the retina, as objects usually are : it should therefore appear to us inverted; and that is not the case in the telescopes I have looked through. Mrs. B. When it is necessary to represent the image erect, two other lenses are required ; by which means a second image is formed, the reverse of the first and conse- quently upright. These additional glasses are used to view terrestrial objects ; for no inconvenience arises from seeing the celesti.il bodies inverted. 10^5. How does a magick lantern differ from a solar micro- scope? : 102(5. What is the reason that the solar microscope may not be used with objects at a great distance with equal effect ? 1027. What does Fig. 4, plate XXIII. represent ? 1023. How would you explain the principle of -the common refracting tele- scope by the use of that figure ? 1029. What is necessary when the image of an object, is to be exhibited erect ? 1030. Why are not these additional glasses used in viewing celestial objects ? 240 OPTICKS. Emily. The difference between a microscof e and a telescope seems to be this: a microscope produces a magnified image, because the object is nearest the lens; and a telescope produces a diminished image, because the object is farthest from the lens. Mrs. B. Your observation applies only to the lens C D, or object glass, which serves to bring an image of the object nearer the eye, for the lens X Y, or eye-glass, is in fact a microscope, as its purpose is to magnify the image. When a very great magnifying power is required, telescopes are constructed with concave mirrors, instead of lenses. Concave mirrors, you know, produce, by reflection, an effect similar to that of convex lenses by refraction. In reflecting telescopes, therefore, mirrors are used in order to bring the image nearer the eye; and a lens or eye-glass the same as in the refracting telescope to magnify the image. The advantage of the reflecting telescope, is, that mirrors whose focus is six feet, will magnify as much as lenses of a hundred feet. Caroline. But I thought it was the eye-glass only which magnified the image; and that the other lens served to bring a diminished image nearer to the eye. Mrs. B. The image is diminished in comparison to the object, it is true; but it is magnified if you compare it to the dimensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting than in re- fracting telescopes. 1031. What part of the telescope exhibited in the figure may be considered as a simple microscope ? 103*2. When a very great magnifying power is required, how must telescopes be con- structed ? -1033. In the reflecting telescopes why are mirrors used ? 1034. How great is the advantage of the reflecting tele- (scope? r ELECTRICITY. 241 CONVERSATION XVIII. ELECTRICITY. Positive and Negative Electricity ; Electrical Machine ; Btttery; Electrical Bells ; Dancing Images ; Spiral Tube; Spotted Jar; Luminous Eggs, fyc. MRS. B. I WILL take the present opportunity for giving you some account of the properties of electricity, which, of late years, has excited much interest. Caroline. I feel much interest in the subject upon which we are to converse at the present time; for al- though I have repeatedly witnessed electrical experi- ments, and have read something respecting them, elec- tricity is still to me a subject of much mystery. Mrs. B. Nor can I promise entirely to remove the veil which now obscures it. Enough is indeed known of it, to satisfy us that electricity like caloric is an agent universally diffused, and of perpetual operation, yet of its intimate nature we cannot be said to know any thing. Some philosophers have considered it a simple sub- stance others have believed it a compound; and al- though it is generally denominated a fluid, there are others eminent in science, who have doubted its being a material agent, supposing it probable, that, like at- traction, it is a mere property of matter. As, however, it is necessary to adopt some hypothesis for the explan- ation of the discoveries which this agent has enabled us to make, I have chosen the opinion, at present most prevalent, which supposes the existence of two kinds of electricity, distinguished by the names of positive and negative electricity. Caroline. Well, I must confess, I do not feel as much satisfaction in pursuing the subject, on being told 1035. What does Mrs. B. say of the importance of electricity ? 1036. What does Caroline say of the subject? 1037. What is known of the science ? 4038. What is "the hypothesis which Mrs. B. adopts. ELECTRICITY. that so many particulars in this science are unsettled. I was in hopes the new discoveries in electricity had thrown so great light upon what was formerly dark and unknown, that every thing respecting it would now have been clearly explained. Mrs. B. This is a point which we are yet far from having attained. But, in spite of the imperfection of our theories, you will be amply repaid by the impor- tance and novelty of the subject. The number of new facts which have already been ascertained, and the immense prospect of discovery which has lately been opened to us, will, I hope, ultimately lead to a perfect elucidation of this branch of natural science; but at present you must be contented with studying the effects, and in some degree explaining the phenomena, without aspiring to a precise knowledge of the remote cause of electricity. If you must have absolute certainty you must relinquish the physical sciences. As you were content to refer many phenomena to gravitation with- out knowing any thing certain of their cause, so in electricity you must be content to do the same. Caroline. Perhaps it is unreasonable to expect a more perfect knowledge of electricity. The science I believe is of modern origin, and therefore we ought not to look for as much certainty as in those which have for a much longer time been subjects of critical investiga- tion. Mrs. B. You are partially correct in this opinion. The science as now presented to our consideration is not of long standing. Still the ancients were not igno- rant of the property which now passes under this name. They were accustomed to observe, that certain bodies, by being rubbed, acquired the power of attracting and afterwards repelling light substances. One of the ar- ticles which does so in an eminent degree is a sub- stance called amber; and, from electron, the Greek name of amber, is derived the term electricity. Among 1039. Why does not Caroline feel full satisfaction in the study ? 1040. In what manner does Mrs. B. reply to her? 1041. Why does Caroline think it would be unreasonable to expect a more perfect knowledge of this science? 1042. What did the an- cients know of it ? 1043. From what is the term electricity derived. ELECTRICITY. 243 the substances in which this principle prevails, I might name, wax, sulphur, glass, silk, fur, and woollen. Have you never witnessed any thing of the kind? Caroline. I think 1 have frequently. In frosty weather particularly I have seen upon parts of my clothes such as flannel and silk a beautiful sparkling appearance, which I now suppose must have been elec- tricity. J have also seen persons rub a, glass tube with a silk handkerchief; and a piece of sealing wax with dry flannel. The effect was most curious. At first, light substances would be attracted afterwards, repulsed. If the knuckles were presented to the tube or wax, after having thus been rubbed, a sudden sen- sation was produced, with a slight hissing noise; and, if the room were dark, there was seen between the tube and wax, and the knuckles, flashes of light. Mrs. B. I am extremely gratified that you have keen thus observing. These appearances were the effect of electricity. By continuing your observa- tions, in numerous other cases, you will notice similar effects. Nature itself is the great laboratory of this subtle principle. No principle in the material creation is more general, or more active. Its agency may be seen, in bodies of every description. It is the immedi- ate cause of thunder and lightning, and scrae have sup- posed of galvanism also, an interesting study, to which I shall, at some other time, call your attention. Emily. A principle so generally diffused; so active; and so important in its results, demands a most critical investigation. Viewed as the cause of thunder and lightning, and of galvanic phenomena, gives it an impor- tant rank among the sciences. If it pervades the whole of material nature, I should like to be made acquainted with that fact, as made manifest by visible effects. Mrs. B. Such a desire is most laudable, and I will attempt to gratify you in it. Numerous experiments are made to prove the existence, and to illustrate the 1044. What substances are here named as containing the elec- tric principle I 1045. What has Caroline witnessed ? 1046, What does Mrs. B. say of the extent of this principle ? -1047. Of what is it the immediate cause? 1048. \\hat does Emily say respecting it ? 244 ELECTRICITY. nature of the electric principle. For the purpose of making such experiments philosophers possess a variety of instruments, some of which you may have seen, but which require being thoroughly explained and well un- derstood, in order to obtain distinct and correct ideas of the science which they are designed to elucidate. Caroline. 1 have frequently seen, what is called an electrical machine, an electric battery, with various other instruments apparently designed to be used in connexion with them; but, have never been made ac- quainted with the nature of their movements. I shall feel greatly obliged to Mrs. B. if she will give me some information respecting the principles by which such wonderful results are produced. Mrs. B. This I will cheerfully do; but, before the electrical machine and battery can be advantageously explained, it is necessary to advert, for a moment, to the mode in which electricity itself is known to exist. Without understanding this, the electrical machine would be altogether incomprehensible in its results. It has been supposed by philosophers, that every body in nature has a certain degree of the electric fluid that is, of electricity which may be increased or diminished, according to circumstances. When a body has more than its natural share it is said to be positively electrified; and, when it has less than its na- tural share, it is said to be negatively electrified. There is a tendency in bodies to produce an equilibrium in the quantity of electricity they contain; hence, a body approaching or becoming united with another having less electricity gives out a part. It is the object of the electrical machine to produce this accumulation of the fluid, in a Leyden phial or jar, prepared for that pur- pose, as I shall hereafter describe. If several of these jars are connected together they form the electrical 1049. What is needed in making experiments? 1050. What electrical apparatus has Caroline seen? 1051. What does Mrs. B. propose doing before explaining them? 1052. What has been supposed by philosophers respecting electricity :' 1053. When are bodies positively and when negatively elec- trified? 1054. What is the object of the electrical machine? -1055. Of what is the electrical battery composed ? ELECTRICITY. 245 battery. When the jar or battery contains more than its proper share of the electric fluid it is said to be charged. When this accumulation is permitted to es- cape, it is said to be discharged, and an equlibrium is restored. I will now explain to you how this accumu- lation of the electric fluid is produced by means of an electric machine. Caroline. I have seen two kinds of these machines the principal part of one being a glass cylinder, and the principal part of the other being a circular plate of thick glass. Can two instruments, so unlike each other, produce the same results? Mrs. B. They can. The one is called the Cylin- drical Machine, and was formerly much used. It is represented by figure, I, of plate XXIV. The other is called the Plate Machine, and is represented by figure 2, of that plate. The latter is considered the most powerful, and is beginning to supersede the use of the former. It should here be remarked, that certain substances are capable of exciting electricity. The most remark- able of them are glass and the various vitrifications, precious stones, resins, amber, sulphur, baked wood, wax, silk, cotton, hair, feathers, paper, all dry vegeta- ble substances, and all hard stones. These bodies, which we call electrics, will not convey electricity from one body to another, and, therefore, are termed Non- conductors. There are other bodies, which, when rubbed ever so much, do not exhibit electricity, arid are hence called Non-electrics. But these being capable of conveying the fluid from one body to another are denominated Conductors. Some of them conduct electricity much better than others. The principal conductors are gold, silver, copper, brass, iron, tin, quicksilver, lead, semi- metals, ores, charcoal, all fluids, except air and oils, 1056. When is it said to be charged ? 1057. What are the names of the two electrical machines used? 1058. Why are certain substances called electrics why con-conductors ; and what are they ? 1059. Why are certain substances called non-elec- trics and, why are they called conductors ? 21* 246 ELECTRICITY. most saline substances, and stony substances. Besides these, all bodies which contain more or less of the above bodies, are also conductors such as green vege- tables and raw meat, on account of the fluids which they contain. So also, woollen cloth and silk, when wet with water, will, by means of the water, conduct electricity. Caroline. I believe I now understand the principle of action" in the electrical machine. The fluid is ex- cited by rubbing against each other two or more suh- stances denominated electrics; for instance the rubbing together the glass cylinder or plate and the silk cushion of the electrical machine. The fluid is then conveyed by means of metallic substances to a vessel that is in- sulated and is a non-conductor, as a glass phial or jar, the inside of which is coated over with metal. Here the fluid is accumulated and remains till means are taken for its escape. Is it not so? Mrs. B. You are right. I am gratified that you understand the subject so well. Thus of figure 1, of plate XXIV which represents the Cylindrical Elec- trical Machine, you will observe, that the letter A, denotes the cylinder B, the prime conductor C, the cushion D, the jar suspended from the prime con- ductor to receive the charge E, the insulating pillar of the conductor F, the insulating pillar of the rub- ber, or cushion and G, the chain to connect the cush- ion with the ground. The principle of the Plate Machine is the same; and consists, as you will see from figure 2, of plate XXIV of a circulate plate of glass, A, mounted upon an axis, and rubbed by two pairs of cushions, B, the brass conductor, C, has its points directed towards the plate, and is insulated, that is, mounted on glass, by the stem D. The letter E, is a double piece of oiled silk, passing from the cush- ions to near the points. The whole is supported by a strong mahogany frame; and the plate is turned by the 1060. What are the substances called non-electrics and con- ductors ? 1061. What explanation does Caroline give of the principle of action in the electrical machine ? 1062. How does Mrs. B. explain figure 1 of plate XXIV ? 1063. How does she explain figure two of that plate ? ELECTRICITY. 247 handle, F. When the plate is quickly turned, there will be an abundant accumulation of the electric fluid. Emily. The term insulated was mentioned by Caro- line. I am not certain that I understand what is meant by it; and, should therefore like to have it explained. Mrs. B. A conducting substance placed upon glass, or upon any other non-conductor, is said to be insulat- ed, that is, its communication with bodies which would carry away any electricity communicated to it, is cut off. A person, standing upon a stool with glass legs, or upon a cake of wax, or rosin, or any body that is suspended by threads of silk, is insulated. Thus, also, the prime conductor, or the cushion, standing upon a glass pillar which is an electric or non-conductor, is insulated; and, the same would be true of the whole machine, were it placed upon legs of a non-conducting substance. Emily. It is perfectly plain now I understand what is meant, when we say a body is insulated. But I must beg the favor of an explanation upon another subject. I do not altogether comprehend the construc- tion and use of the Leyden jar. Will Mrs. B. en- lighten me respecting this also? Mrs. B. The Leyden jar is nothing but a common glass phial or bottle, covered both within and without, with a metallic coating, reaching to within two or three inches of the top, where the glass is left bare, so that there is no conducting communication between the in- side and the outside. This is called the Leyden jar, because the discovery of it was made at the city of Leyden. Through the neck of the jar passes a brass wire, which touches the inside coating, having a knob at its outer extremity. When the jar is charged with electricity by the use of the electrical machine, if a person were to place his knuckle in contact with the knob of this wire, the fluid would instantly pass through his arms and chest, and he would feel what is called an electric shock. Or, if a metallic wire were to have one J064. What is to be understood by the term insulated? JO(V). On Tsrhat ntKor enKioot r!^oo T^milY- /l/ioi^** lit f* ir inn t rm ^ - UJO& what does tins jar given of the manner in which the fluid escapes froio the jar ? j\s. vvjijb is LU ue uuutrrsujuu uy mt: leriii msuiuu'u . ). On what other subject does Emily desire information ? ). How does Mrs. B. describe the Leyden jar ? 10C7. From ,t does this iar derive its name? 10P8. Wwt account is 248 ELECTRICITY, extremity placed in contact with the knob and the other extremity against the outside coating, as seen in figure 2, plate XXV, the fluid would leave the inside, pass along the wire to the outside, and the equilibrium would at once be restored. Caroline. I believe the wire with which this opera- tion is performed is called a discharger. The handle of it is made of glass, or some other non-conducting substance, so that it may be held in the hand, without forming a channel of communication to the person who holds it. .Mrs. B. You are right Caroline. I should have said something about the discharger, but you have an- ticipated me in it. Such instances of your ready mind in comprehending the subject we are discussing, affords me the greatest pleasure. Emily. If I recollect right, you stated that an electric battery was a collection or combination of Levden jars. If so, I wish to be informed of the ad- vantage resulting from them rather than from a sin- gle jar. Mrs. B. The electric battery is represented by figure, 1, of plate XXV. The object of it is to con- centrate a large quantity of the electric fluid, by which the most striking phenomena may be exhibited. The force of such a concentration of electricity would be incredible to one who had never witnessed it. The effects produced by lightning may be imitated for in- stance, metallic wires maybe melted down; spirits of wine may be set on fire; and the strongest animals may be made to suffer instant death. Caroline. One of the most extraordinary circum- stances, respecting electricity, which have interested my mind, is the rapidity with which the fluid circulates. The electric spark moves, and the shock is felt with the quickness of thought. Mrs. B. That is the fact. Electricity, in passing 1069, Why is the handle of the discharger made of glass? 1070. What is an electric battery ? 1071. By which figure is it exhibited ? 107U. What are some of the most extraordinary effects produced by the electric battery ? 1073. What does Caroline say has most interested her respecting electricity ? r ELECTRICITY. 249 from one place to another, is so rap ; d in its motion, that in any distance in which an experiment can be made it occupies no perceptible time. Were 10,000 men to join hands, the electric shock would be felt by the whole of them in the same precise instant of time. The electrical fluid has also been conveyed, by means of iron wire, for many miles; and it has discharged pistols, filled with hydrogen gas, placed at different distances, at the same instant. JLet several persons join hands, and let the first touch the tin-foil on the outside of a jar or phial, and the last the knob, all will feel the shock. The same experiment may also be performed, and the circle lengthened by means of pieces of wire, of which one person must keep hold of the one end, and another at the other end. Caroline. I have ever considered that the science of electricity derives great importance from the elec- tric fluid being known to be the same which produces the brilliant and terrific phenomena of thunder and lightning. TJ whom did the resemblance of lightning to the electric fluid suggest the idea of drawing that fluid from the clouds. Mrs. B. Dr. Franklin, the eminent self-taught phi- losopher of whom every American should be proud. For this purpose he had a large kite made, which he, near Philadelphia, raised in the usual manner into the air. To the end of the string he fastened a key, and from the key was a silken string, which he held in his hand, and thereby the electrical matter could not come farther than the key, silk being a non-conductor. The key soon gave out sparks like an electric machine, and he charged several jars, and performed several expe- riments in precisely the same manner as if the jars had been charged by means of the electric machine. Caroline. I should have thought, he would have been afraid of receiving injury. You know lightning frequently kills persons, and destroys the most power- ful obstacles. 1074. What are instances of the rapidity with which the fluid circulates ? 1075. What discovery did Franklin make respect- ing electricity ; 1076, By what means did he do it ? 250 ELECTRICITY. Mrs. B. He was perhaps in danger; but was so intent on making this great discovery, that he did not much regard his personal safety. When experiments are made with lightning, it requires the greatest cau- tion, as many have been hurt in performing them. In this way a Professor at St. Petersburg was killed. Caroline. Did not the discovery of Franklin, res- pecting the laws by which electricity is governed, sug- gest the means of guarding against the injuries of lightning, by the use of the lightning-rod? Mrs. B. It did; and on that account he may be re- garded as one of the greatest benefactors of mankind. The theory of guarding lightning is simple, and easily understood. Lightning, it is well known, usually strikes the loftiest objects. Hence steeples of churches and high trees are often injured by lightning Masts of ships, also, in warm climates, are exposed to the light- ning, and much damage is thereby occasioned. To guard against these injuries, it is useful to have a rod of iron higher than the top of the building, or of the mast of the ships, and extending down into the earth or the water; the lightning being attracted by the rod, is carried off, and no mischief ensues. Powder maga- zines should always be provided with a lightning rod. Emily. If the laws, which regulate lightning, are so well understood, it may be supposed that other means of safety may be provided against dangers from that fluid, where the rod cannot be had. Will you give us some instructions upon that matter. Mrs. B. You are correct, in this opinion, and I will endeavor to give you the information desired. As lightning frequently strikes the tops of trees, it is not safe, in the time of a thunder storm, to take shelter under a tree from the rain. By doing this many persons have lost their lives. Carpenters have also lost their lives, by carrying in a thunder storm, edge-tools which 1077. What is said of the danger attending experiments with electricity ? 1078. On what account may Franklin be consider- ed one o? the greatest benefactors of mankind ? 1079. What is the theory of guarding against danger from lightning ? 1080. What does Emily suppose upon the subject ? ELECTRICITY. 251 attract the lightning. Persons whose timidity induces them to seek a preservative against all possible danger, may spread a feather-bed in the middle of the floor and place a chair upon it, and sit there. Feathers do not conduct electricity. And persons may relieve their fears in the time of a thunder storm by counting the beats of the pulse, or the seconds of a stop-watch or clock, between seeing the flash and hearing the sound. If only three or four beats intervene, the thunder is going on at a sufficient distance to make it impossible for them to receive any injury. Emily. I have heard persons tell of many amusing experiments in electricity; I should like to be told some- thing about them at least the most interesting ones. Mrs. B. It is a fact, that the experiments in this science may be multiplied to almost any extent, and they are of a most interesting character. There would not be time to describe to you a hundredth part of those which are enumerated in works on this science; for they can be increased, according to the ingenuity of the electrician, or the variety of instruments in his ap- paratus, at pleasure. I. will explain to you a few which will probably be the most interesting. The Electrical Bells furnish a pleasing illustration of the attraction and repulsion of the electric matter. They are variously constructed, but the form exhibited, figure 4, plate XX V, is the simplest. The two outer bells are suspended by brass chains; the middle bell and the two clappers by fine silk threads. When the bells are attached to the conductor, and the machine is turned very slowly, the fluid will pass along the chains to the two outer bells, but will not pass along the silk to the clappers and middle bell. T^ius the outer bells being charged with an extra quantity of electricity will attract the clappers, but the moment they touch the. bells, they become charged, and are repelled with such force as to cause them to strike against the middle bell, on which they deposit their electricity, and are again 1081. In what manner may persons of timidity quiet their fears in time of a thunder storm ? 1082. What does Mrs. B. say of amusing experiments in electricity ? 1083. Which figure re- presents the electrical bells ? 1084. How are they described ? 252 ELECTRICITY. attracted. By this means a constant ringing is kept up while the machine is turned. From the inside of the middle bell a brass chain passes to the table, for the purpose of conveying away the fluid deposited on it by the clappers. Figure 5, represents a more elegant form of mounting the bells. The pillar, A, is of glass, the cross on the top is of brass; the four outer bells are affixed to this by wires or chains; the clappers are sus- pended on silk threads from the cross; the middle bell communicates with the ground by the foot. To use these, the knob, A, must be in contact with the conductor. The Dancing Images, as seen in figure 3, of plate XXV, is another pleasing experiment. Suspend from the conductor, by a glass chain, a circular plate of cop- per, and reaching to within an inch and a half, or two inches of the table. Directly under this plate, place another of 4he same form and a little larger on the ta- ble. Turn the machine, and the fluid will pass from the upper to the lower plate. If now small figures fancifully cut out of pasteboard, be introduced between the plates, they will dance about with apparent vivacity, and to the no small amusement of the spectators. The Spiral Tube is a very simple instrument, but furnishes in a pleasing manner the luminous appearan- ces of the electric fluid. This tube has a spiral row of small pieces of tin-foil fixed upon its outside surface, and laying at a small distance from each other. It is to be held by one end; the other end is to be presented so as to take sparks from the prime conductor, which will be seen at all the interstices between the spots of the tin-foil. The spiral tube is represented in figure 1 , of plate XXVI; and, in figure 2, of that plate is a set of spiral tubes, exhibiting a more brilliant display of the same effect, than is seen in the use of a single one. A mere inspection of the figures will convey an accu- rate idea of the nature of this kind of electrical appa- ratus; and it is only necessary to observe, that the brilliancy of these exhibitions will depend on the dark- 1085. How is the other set of electrical bells, figure 5, describ- ed? 1086. Which figure represents the dancing images ? 1087. What account is given of these images ? 1088. What is anelectrical spiral tube? 1089. Which figure exhibits itr 1 1090. What is said of the set of spiral lubes p ELECTRICITY. 253 ness of the room in which they are made, the dryness of the apparatus, and the strength of the sparks. Spotted Jar. Figure 5, plate XXVI, represents a jar, whose coating is formed of small pieces of tinfoil, placed at a small distance from each other. Charge this jar in the usual manner, and strong sparks of elec- tricity will pass from one spot of tin-foil to the other in a variety of directions; the separation of the tin-foil making the passage of the fluid from the outside to the table, in a darkened room, beautifully visible. Dis- charge this jar, by bringing a pointed wire gradually near the knob T, and the interval between the spots will be pleasingly illuminated, and the noise will resem- ble that of small fired crackers. If the jar is discharg- ed suddenly, the whole outside surface will appear illu- minated. The glass must be very dry. To make eggs luminous. Figure 4, plate XXVI, represents a wooden stand to support three eggs, and by the sliding supports, a, a, a, to place them at any re- quired distance asunder; b, a brass sliding wire; and c, a chain or wire; both as conductors of communication. Three eggs are to be placed at about one eighth of an inch asunder, the lower extremity of the wire b, at the same distance, and one end of the chain or wire c, in communication with the bottom egg, and the other end with the coating of a charged electrical jar. With the discharging rod, one ball being placed on the ball of the wire b, and the other to the ball of the charged jar, the discharge will instantly pass through the eggs, and in a darkened room exhibit them luminous, more so, than if held to a candle in the common way. Figure 3, plate XXVI, represents a piece of twisted fine iron wire, fixed in a glass jar containing pure oxy- gen or vital air. An electrical discharge from a jar conveyed to the ball will, at its discharge within from the extremity of the wire to the small brass ball, set the wire into a most beautiful deflagration, making in a darkened room a very brilliant appearance, till the whole of the wire is fused. 1001. For what is figure 5, plate XXVI ? 1092. How i the experiment made? 1093. What- is the object of figure 1 , of that plate ? 1094. How is the process of doing it explained ? 1095. What experiment is illustrated by figure S, of plate XXVI? 22 254 ON GALVANISM, OR VOLTAIC ELECTRICITY^ CONVERSATION XIX. ON GALVANISM, OR VOLTAIC ELECTRICITY. Galvani; Voltaic battery ; Sir H. Davy; Dr. Boslockj Corronne des tasses ; Galvanic experiments; Gymno- tus Electricus, fyc. MRS. B. HAVING said as much on electricity as needful in enabling you to pursue the study without further assist- ance, I will now occupy your attention for a short time upon the subject of galvanism, frequently called volta- ic electricity. Caroline. I suppose galvanism derives its name from Galvani, of whom I have heard. Was he not a professor in the University of Bologna? I should be delighted to hear more about him, and the science which bears his name. Mrs. B. You are right in this supposition. It was a trifling and accidental circumstance which gave rise to this new branch of physical science. About the year 1790, he was engaged in a series of experiments with a view to prove, that muscular motion was inti- mately connected with electrical action. Some dead frogs, one day, intended for a soup, were upon a table near an electrifying machine. It was observed that when a piece of metal was laid on the nerve of a frog recently dead, whilst the limb supplied by that nerve rested upon some other metal,* the limb suddenly moved, on a communictaion being made between the two pieces of metal. He instituted a series of experiments, that led to the persuasion favorable to the theory he had adopted. Emily. I should like to be informed, in what manner the communication could be made, so as to produce such a result. 1096. By what other name is galvanism called ? 1 097. From whom does galvanism derive its name ? 1098. When was the discovery made ? 1099. From what circumstance ? ON GALVANISM, OH. VOLTAIC ELECTRICITY. 255 Mrs. B. Either by bringing the two metals in con- tact, or by connecting them by means of a metallic conductor. But without subjecting a frog to any cruel experiments, I can easily make you sensible of this kind of electric action. Here is a piece of zinc one of the metals belonging to the list of elementary bodies put it under your tongue, and this piece of silver unon your tongue, -very well; now make the project- ing parts of the metals touch each other, and you will instantly perceive a peculiar sensation. Emily. Indeed I did; a singular taste, and I think a degree of heat; but I can hardly describe it. Mts. B. The action of these two pieces of metal on the tongue, is, I believe, precisely similar to that made on the nerve of a frog. I shall not detain you by a detailed account of the theory by which Galvani attempted to explain this fact, as it was soon overturn- ed by subsequent experiments, which proved that Gal- vanism (the name this new power had obtained) was nothing more than electricity. Galvani supposed that the virtue of this new agent resided in the nerves of the frog; but Volta, who prosecuted this subject with much greater success, showed that the phenomena did not depend on the organs of the frog, but on the elec- trical agency of the metals, which is excited by the moisture of the animal, the organs of the frog being only a delicate test of the presence of electric in- fluence. Caroline. I suppose then the saliva of the mout'h answers the same purpose as the moisture of the frog, in exciting the electricity of the pieces of silver and zinc, with which Emily tried the experiment. Mrs. B. Precisely. It does appear, however, ne- cessary that the fluid used for this purpose, should be of an animal nature. Water, and acids very much di- luted by water, are found to be the most effectual in 1100. How is the communication made between the two metals ? 1101. What does Mrs. B. say of the action of these two pieces of metal on the tongue ? 1102. What important improvement did Volta make upon Galvani ? 1103. What does Caroline sup- pose respecting the saliva of the mouth? 1104. What does Mrs. K. say of the agency of water and acids in promoting the de- relopement of electricity ? 256 ON GALVANISM, OR VOLTAIC ELECTRICITY. promoting the developement of electricity in metals; and accordingly the original apparatus which Volta first constructed for this purpose, consisted of a pile or succession of plates of zinc and copper, each pair of which was connected by pieces of cloth or paper im- pregnated with water, figure 5, plate XXVII; and this instrument, from its original inconvenient structure and limited strength, has gradually arrived at its present state of power and improvement, such as exhibited in the Voltaic battery. An idea may be had of the battery, by reference to fig. 1. It is a wooden trough divided into cells by metallic partitions, which are cemented firmly into grooves made in the trough. The metallic partitions are each formed by soldering together, back to back, a sheet of copper and a sheet of zinc. When these are cemented into the trough, the copper sides are all made to face one way. The cells between the plates are to be nearly filled with water, having some nitric, or other acid, mixed with it. Caroline. Though you will not allow us to inquire into the precise- cause of electricity, may we not ask in what manner the fluid acts on the metals so as to produce it? Jl/rs. B. The action of the fluid on the metals, whether water or acid be used, is entirely of a chemi- cal nature. But whether electricity is excited by this chemical action, or whether it is produced by the con- tact of the two metals, is a point upon which philoso- phers do not yet perfectly agree. Family. But can the mere contact of two metals, without any intervening fluid, produce electricity? Mrs. B. Yes, if they are afterwards separated. It is an established fact, that when two metals are put in contact, and afterwards srparated, that which has the strongest attraction for oxygen exhibits signs of positive, the other of negative electricity. 1105. How was the voltaic pile originally constructed? HOC. Which figure represents the voltaic battery? 1107. How is it described?; 1108. In what manner does the fluid act on the metals so as to produce electricity.' 1100. Emily asks if the contact of two metals, without any intervening fluid can produce electricity what answer does Mrs. B. give ? ON GALVANISM, OR VOLTAIC ELECTRICITY. 257 Caroline. It seems, then, but reasonable to infer that the power of the voltaic battery, should arise from the contact of the plates of zinc and copper. Mrs. B. It is upon the principle of the contact of the two metals, that Volta and Sir H. Davy explain the action of the voltaic battery; but many philosophers entertain doubts of the truth of the theory. The prin- cipal difficulty is, that two such plates show no signs of different states of electricity whilst together, but only on being separated after contact. Now, in the voltaic battery, those plates that are in contact, always continue so, being soldered together; they cannot, therefore, receive a succession of charges. Besides, if we consider the mere disturbance of the balance of electricity, by the contact of the plates, as the sole cause of the production of voltaic electricity, it remains to be explained, how this disturbed balance becomes an inexhaustible source of electrical energy, capable of pouring forth a constant and copious supply of electri cal fluid, though without any means of replenishing itself from other sources. The theory least liable to objection, appears to be that, first proposed by Dr. Bostock, called the chemi- cal theory. This theory supposes the electricity to be excited by the chemical action of the acid upon the zinc. All metals have a strong attraction for oxygen, and this element being found both in the water and the acid. The action of the diluted acid on the zinc, con- sists in the oxygen, combining with the metal, and oxi- dating its surface. Emily. Since there is so strong a chemical attrac- tion between oxygen and the metals, I suppose they are naturally in different states of electricity. Mrs. B. Yes; it appears that all metals are united with the positive, and that oxygen is the grand source of the negative electricity. Caroline. Does not, then, the acid act on the plates of copper, as well as on those of zinc? 1110. Upon what principle do Volta and Sir H. Davy explain the action of the voltaic battery ? 1111. What is the "difficulty in this hypothesis ? 11 12. What theory on this subject is least liable to objection? 1113. What supposition does Emily here make ? 1114. How does Mrs. B. reply to her? 22* 258 ON GALVANISM, OK VOLTAIC ELECTRICITT. Mrs. B. Apparently it does not. The acid does act upon the copper, only not so strongly as upon the zinc, and, therefore, to appearance its whole energy is exerted upon the latter. It will be best, I believe, in order to render the action of the voltaic battery more intelligible, to confine our attention at first to the effect produced on two plates only. See figure 2, plate XXVII. If a plate of zinc be placed opposite to one of cop- per, or any other metal less attractive of oxygen, and the space between them, suppose of half an inch in thickness, be filled with an acid, or any other fluid ca- pable of oxidating the zinc, the oxidated surface will have its capacity for electricity diminished, so that a quantity of electricity will be evolved from the surface. This will be received by the fluid in contact; by which it will be transmitted to the opposite metallic surface, t]be copper, which is not oxidated, and is, therefore, disposed to receive it, so that the copper plate will be- come positive, while the zinc plate will be in the nega- tive state. This evolution of electric fluid, however, will be very limited; for as these two plates admit of but very little accumulation of electricity, and are supposed to have none with other bodies; the action of the acid, and further developement of electricity will be imme- diately stopped. . Emily. This action, I suppose, can no more con- tinue, than that of a common electrical machine, which is not allowed to communicate with other bodies. Mrs. B. Precisely. In order that the acid may act freely on the zinc, and the two electricities given out without interruption, some method must be devised by which the plates may part with their electricities as they receive them. If the wires connected witn either plate are made to meet, the two electricities will then 1115. Caroline asks, if the acid does not act on the plates of copner as well as on the zinc what reply does Mrs. B. ma^e ? 1116. How is the action of the voltaic battery explained by the figure ? 1117. To what does Emily compare the limited devel- opement of electricity by the operation of this figure? 1118. In what manner does Mrs. B. propose to obviate the difficulty ? OJV GALVANISM, OR VOLTAIC ELECTRICITY. 259 be brought together, and will combine and neutralize each other, as long as this communication continues; the two plates will dispose of their respective electri- cities, and the action of the acid will be continued. Emily. That is very clear, so far as the two plates only are concerned; but I cannot say I understand how the energy of the succession of plates, or rather pairs of plates, of which the Galvanic trough is composed, is propagated and accumulated throughout a battery. Mrs. B. In order to show you how the intensity of the electricity is increased by increasing the number of plates, we will examine the action of four plates, as seen in figure 3, of plate XXVII; if you understand these, you will readily comprehend that of any number whatever. In this figure, the two central plates appear united so as to form one plate only with two different surfaces the one of copper, and the other of zinc. Now a quantity of electricity being evolved from the first zinc plate, in consequence of the action of the acid, is con- veyed by the interposed fluid to the copper plate, No. 2, which thus becomes positive. This copper plate communicates its electricity to the contiguous zinc plate No. 3, in which, consequently, some accumula- tion of electricity takes place. When, therefore, the fluid in the next cell acts upon the zinc plate, electri- city is extricated from it in a larger quantity, and in a more concentrated form than before. This concentra- ted electricky is again conveyed by the fluid to the next pair of the plates, No. 4 and 5, when it is further in- creased by the action of the fluid in the third cell, and so on, to any number of plates, of which the battery may consist, so that the electrical energy will continue to accumulate in proportion to the number of double plates, the first zinc plate of the series being the most negative, and the last copper plate the most positive. Caroline. But does the battery become more and 1110. What, does Emily say she is still unable to understand? 1120. By reference to which figure does Mrs. B. further propose to instruct her ? 1121. How does Mrs. B. explain this figure ? 260 ON GALVANISM, OR VOT/TAIC ELECTRICITY. more strongly charged merely by being allowed to stand undisturbed ? Mrs. B. No; for the action will soon stop, as al- ready explained, unless a vent be given to the accumu- lated electricities. This is easily done, however, by es- tablishing a communication by means of the wires see figure 1, between the two ends of the battery; these being brought into contact, the two electricities meet and neutralize each other, producing the shock, and other effects of electricity; and the action goes on with renewed energy, being no longer obstructed by the accumulation of the two electricities which impe- ded its progress. Emily. Is it the union of the two electricities which produces the electric spark? Mrs. B. Yes; and it is, I believe, this circumstance which gave rise to Sir H. Davy's opinion, that caloric may be a compound of the two electricities. Caroline. Yes, surely, caloric is very different from the electrical spark? Mrs. B. The difference may consist, probably, only in intensity; for the heat of the electric spark is con- siderably more intense though confined to a very minute spot, than any heat we can produce by other means. Emily. It is quite certain that the electricity of the voltaic battery is precisely of the same nature as that of the common electrical machine ! Mrs. B. Undoubtedly; the shock given to the hu- man body, the spark, the circumstance of the same substance which are conductors of the one, being also non-conductors of the other, are striking proofs of it. Besides, Sir H. Davy has shown, in his lectures, that a Leyden jar, and a common electric battery, can be charged with electricity obtained from a voltaic battery, 1122. What question does Caroline ask respecting the battery? 1123. In what way does Mrs. B. reply to her? 1124. Emily asks, if it is the union of the two electricities which pro- duce the electric spark what reply does Mrs. B. give her? 1125. How does Mrs. B. explain the difference between caloric and the electric spark? 1126. Emily asks, if tue electricity of the voltaic battery and common electric machine is the same How does Mrs. B. answer her ? the effect produced being perfectly similar to that ob- tained by a common machine. Emily. What comparison is made between the use of the voltaic battery and ihe common electrical ma- chine ? Mrs. B. The great superiority of the voltaic battery consists in the large quantity of electricity that passes; but in regard to the rapidity or intensity of the charge, it i& greatly surpassed by the common electrical ma- chine. It would seem that the shock or sensation de- pends chiefly upon the intensity; whilst, on the con- trary, for chemical purposes, it is quantity which is required. In the voltaic battery, the electricity, though copious, is so weak as not to be able to force its way through the fluid which separates the plates, whilst that of a common machine will pass through any space of water. Caroline. Would it not be possible to increase the intensity of the voltaic battery till it should equal that of the common machine ? Mrs. B. It can actually be increased till it imitates a weak electrical machine, so as to produce a visible spark when accumulated in a Leyden jar. But it can never be raised sufficiently to pass through any con- siderable extent of air, because of the ready commu- nication through the fluids employed. By increasing the number of plates of a battery, you increase its intensity, whilst, by enlarging the dimen- sions of the plates, you augment its quantity arid as the superiority of the battery over the common ma- chine consists entirely in the quantity of electricity produced, it was at first supposed that it was the size, rather than the number of plates that was essential to the augmentation of power. It was, however, found upon trial, that the quantity of electricity produced by the voltaic battery, even when of a moderate size, was 1127. What, comparison does Mrs. B. msike between the use of the voltaic battery and the common electric machine ? 1128. What reply does Mrs. B. make to Caroline respecting the intensity of the voltaic battery? 1129. What was at first supposed es- sential for the augmentation of power in the battery ? 1130. But what found on trial ? 262 ON GALVANISM, OR VOLTAIC ELECTRICITY, sufficiently copious, and the chief advantage in this apparatus was obtained by increasing the intensity, which, however, still falls very far short of that of the common machine. I should not omit to mention, that a very splendid, and at the same time, most powerful battery, was a few years ago constructed under the direction of Sir H. Davy, which he repeatedly exhibited in his course of lectures. It consists of 2000 double plates of zinc and copper, of six square inches in dimensions, arranged in troughs of wedg wood-ware, each of which contains twenty of these plates. The troughs are furnished with a contrivance for lifting the plates out of them in a very convenient and expeditious manner. Caroline. I have seen an apparatus called, the cor- ronne des lasses; was not that designed to answer the same purpose, as the voltaic battery? Jtfrs. B. It is. This apparatus is represented in figure 4, of plate XXVII. It consists of a row of common tumblers, or any other glasses, containing salt and water, or any saline fluid. Into each of these one end of a metallic ore, consisting of a plate of zinc con- nected by a wire with a plate of copper is plunged. These ores are so arranged, that the copper extremity of the first is in the same glass with the zinc extremity of the second, the copper of the second with the zinc of the third, and so on in regular order. Caroline. I have been told, that galvanic experi- ments of a very curious nature have sometimes been made on dead animals. Mrs. B. That is the fact. It has been successfully employed to restore suspended animation. If a frog, soon after it is killed, be stripped of its skin, and wires, connected with the two ends of a galvanic pile, be made to touch the muscles and nerves, the animal will exert its limbs and leap as if alive. By the same means a rabbit may be made to exert great force, sufficient to overcome the strength of a man, who would try to pre- 1131. What account is given of the large battery of Sir II. Davy? 11:^2. How is the corronne des tasses described? 1133. What interesting galvanic experiments are named? ON GALVANISM, OR VOLTAIC ELECTRICITY. 263 vent the motion of its limbs. Cold-blooded animals, particularly those which are amphibious, are more af- fected by galvanic electricity than quadrupeds or birds. After the animal has become quite cold and stiff, gal- vanism has no effect upon it. Caroline. I have read of experiments being made upon the bodies of criminals, after having been exe- cuted ; was it not with the galvanic battery ? Mrs. B. That is sometimes done; and the effects are truly wonderful. One of the most remarkable cases, of which I have read, was upon the body of a man named Clydesdale, executed at Glasgow for murder. The experiments were made by Dr. Ure of that city. In the first of these experiments, although the body had been suspended from the gallows nearly an hour, every muscle was immediately agitated with convulsive movements, resembling a violent shuddering from cold. The left side was powerfully convulsed at each renewal of the electric contact; and, the knee being previously bent, the leg was thrown out with such violence as nearly to overturn one of the assistants, who in vain attempted to prevent its extension. In succeeding experiments, which occupied about an hour, full and laborious breathing was restored; the chest heaved and fell; every muscle in the countenance was thrown into fearful action; rage, horror, despair, anguish, and ghastly smiles, united their hideous expres- sion in the murderer's face. At another time, when the rod was applied to a slight incision in the tip of the forefinger, the fist being previously clenched, that finger extended instantly ; and from the convulsive agi- tation of the arm, he seemed to point to the different spectators, some of whom thought he had come to life. Caroline. I have often read of effects similar to those of the electric battery and the galvanic pile, from certain fishes when coming in contact with other animals. Mrs. B. The most remarkable species is the Gym- notus Electricus, or Electric Eel, which is frequently 1134. What account does Mrs. B. give of galvanic experiments on the criminal that had been executed ? 1135. How are the electric shocks of the electric eel described ? 264 ON GALVANISM, OR VOLTAIC ELECTRICITY. found in the marshes and stagnant pools of Guiana, and other countries in the north of South America. The shocks they give are exceedingly severe; and Mr. Humboldt mentions a road which has been totally aban- doned, because the mules, in crossing a wide ford, were, by these violent attacks, often paralysed and drowned. Even the angler on the bank was not ex- empt from danger, the shock being conveyed along his wetted rod and fishing-line. Emily. I never before heard of such a fish; I should like to know something more about it. Mrs. B. The electric eel is about six feet long. It was supposed, that on dissection the animal, in its internal structure, would be found to resemble the elec- trical battery, or galvanic pile. The similarity, how- ever, is little more than imaginary ; and the mode by which the wonderful phenomenon is produced, remains still a subject for scientific investigation. After the animal has discharged its electric matter, the next shock is weaker, and when the animal is exhausted, it has lost all the power of producing any effect for some time. 1136. What was formerly supposed respecting this fish, and what was found to be tfie fact ? A DICTIONARY OF PHILOSOPHICAL TERMS. ABERRATION, in astronomy, an ap- parent motion of the heavenly bodies, produced by the progressive motion of light and the earth's annual motion. ACCELERATION, in mechanicks, denotes the augmentation or increase of motion in accelerated bodies. ACOUSTICKS is the science which treats of the nature, phenomena, and laws of the sense of sound. It extends to the theory of musical concord and harmony, and is, therefore, a valuable and interest- ing science. AIR, a thin, elastick fluid, surrounding the globe of the earth. The air, together with the clouds and vapours that float in it, is called the atmosphere. The height to which the atmosphere extends has nev- er been ascertained; but, at a greater height than 45 miles, it ceases to reflect the rays of light from the sun. AIR-PUMPS are machines made for ex- hausting the air from certain glass vessels, adapted to the purpose of experiments on air. ANGLE is the inclination of two lines meeting one another in a point, and called the legs of the angle. Angles, in geome- try, are called right, acute, and obtuse. A right angle contains just 90 degrees, or the quarter part of a circle. Acute angles con- tain less, and obtuse angles more than 90 degrees. ANGLE OF INCIDENCE is that which is contained between the line described by the incident ray, and a line perpendicular to the surface on which the ray strikes, raised from the point of incidence. ANGLE OF REFLECTION is contain- ed between the line described by the re- flected ray, and a line perpendicular to the reflecting surface, at the point from which the ray is reflected. ANGLE OF REFRACTION is that which is contained between the line de- scribed by the refracted ray, and a line perpendicular to the refracting surface at the point in which the ray passes through that surface. ANGLE OF VISION is that which contained between lines corning from op posite parts of an object and meeting ii the eye. ANTARCT1-CK CIRCLE, in astrono my, is an imaginary line extending round the south pole, 66 1-2 degrees from "~ equator and parallel to it. APHELION, in astronomy, is that point of in any planet's orbit in which the orbit is most distant from the sun. AQUEOUS HUMOUR, or watery hu- mour of the eye; it is the first and outer- most, and that which is less dense than either the vitreous or crystalline. It is transparent and colourless like water, and fills up the space that lies between the cornea and the crystalline humour. ARCTICK CIRCLE, in astronomy, is an imaginary line extending round the north pole, 66 1-2 degrees from the equa- or and parallel to it. AREOMETER,an instrument by which the density and gravity of fluids are mea- ured. ARIES, in astronomy, a constellation of fixed stars, drawn on the globe in the figure of a ram. It is the first of the welve signs of the zodiac from which a welfth part of the ecliptick takes its lame. It consists of sixty-six stars. ASCENSION, in astronomy, the rising of the sun or star, or any part of the equinoctial with it, above the horizon. ASTERIODS, a name given by Dr. Herschel to the new planets, Ceres, Juno, Pallas, and Vesta, lately discovered. ASTRONOMY is the science which Leaches the motions of the earth, the sun, loon, planets, comets, and stars, and xplains the phenomena occasioned by those motions. ATMOSPHERE, or atmospherick air, he fluid that surrounds our earth. With- >ut this fluid no animal could exist; veg- etation would cease, and there would be neither rain nor refreshing dews to moist- en the face of the ground; and though the sun and stars might be seen as bright specks, yet there would be little enjoy - nent of light, could we exist without it. ATTRACTION, a general term, used to denote the power or principle by which bodies mutually tend towards each other, without regarding the cause or action that nay be the means of producing the effect. ATTRACTION OF COHESION takes place between the constituent particles of the same body. By this principle bodies preserve their forms and are prevented from falling to pieces. ATTRACTION OF GRAVITATION, or gravity, is the name, of that force by which distant bodies tend towards one another. AXIS of a planet is an imaginary line which passes through its centre, and on ich it turns; and it is this motion which produces day and night. With that side of the planet facing the sun it is day; and with the opposite side, which remains in darkness, it is night. AXIS OF MOT is vvl the the TIC N, in mechanicks, is line about which a revolving body moves. Philosophically speaking, the axis " motion is said to be at rest, whilst the other parts of a body move round it; and the further any part of a body is from the axis of motion, the greater is its velocity. AXIS OF THE EARTH is an imagina- ry line conceived to pass through the cen- tre of it from one pole to the other, about which is performed its diurnal rotation. AXIS, in opticks, is that ray, among all others that are sent to the eye, which falls 266 A DICTIONARY OF perpendicularly upon it, and which conse quently passes through the centre of the twelve signs of the zodiack, represented "ye. on globes in the form of a goat. AXIS OF A GLASS, OR LENS, is a CENTRE OF GRAVITY, in median- right line joining the middle points of the icks, that point about which all the parts eye. CAPRICORN, in astronomy, one of th6 two opposite surfaces of the glass. BALANCE, or BALLANCE, in me- chanicks, one of the simple powers which serves to find out the equality or difference of weight in heavy bodies. BALLOON, a machine used in naviga- tion through the air. It takes its name from the form of the machine, the word balloon signifying any spherical hollow body, of whatever matter it be composed, or for whatever purposes it be designed. BAROMETER, an instrument for mea- suring the weight or pressure of the at- mosphere; and by that means measuring heights* and depths, determining varia- tions in the state of the air, and foretell- ing the changes in the weather. BASE, in geometry, the lowest side of the perimeter of a figure. Thus, the base cf a triangle may be said of any of its sides, but more properly of the lowest, or that which is parallel to the horizon. In rectarigled triangles, the base is properly that side opposite to the right angle. BASS, in musick, that part of a concert which is most heard, which consists of the gravest and deepest sounds, and which is played on the largest pipes or strings of the instrument. BODY, in physicks, an extended solid substance, of itself utterly passive and in- active, indifferent either to motion or rest: but capable of any sort of motion, and of all figures and forms. Body, or substance, which is the same thing, is usually denot- ed by the general term matter. BREADTH, in geometry, one of the three dimensions of bodies, which, multi-lused in a more limited sense, for the curve plied into their length, constitutes a sur-!line which bounds a circle, and otherwise face. |called a periphery ; the boundary of a BUBBLE, in philosophy, small drops or right lined figure being expressed by the- 'term perimeter. CLOUDS are a collection of mistv va- of a body do, in any situation, balance each other. CENTRE OF MOTION, that point which remains at rest, while all the other parts of a body move about it. CENTRAL FORCES, the powers which cause a moving body to tend to- wards, or recede from, the centre of mo- ' )n. CENTRIFUGAL FORCE, that by which all bodies, that move round any other body in a curve, endeavour to fly oft" from the axis of their motion in a CENTRIPETAL FORCE, that force >y which a body is every where impelled, or any how tends towards some point as a centre: such as gravity, or that force whereby bodies tend towards the centre of the earth; magnetical attraction, whe.re- liy the loadstone draws iron; and that force, whatever it be, whereby the planets are continually drawn back from right- ined motions, and made to "move in :urves. CHROMATICKS is that part of op- ticks which explains the several proper- es of the colours of light and of natural bodies. CIRCLE, in geometry, a plane figure comprehended by a single curve line, call- ed its circumference, to which right lines >r radii, drawn from a point in the middle, called the centre, are equal to each other. CIRCUMFERENCE, in a general ense, denotes the line or lines bounding a plane figure. However, it is generally vesicles of any fluid filled with air, and either formed on its surface, by an addi- tion of more of the fluid, or in its sub- pours suspended in the air. Their various stance, by an intestine motion of its com- ponent parts. colours and appearances are owing to their particular situation in regard to the BURNING-GLASS, a convex or con- sun, to the different reflection of the sun's cave glass, commonly spherical, which, I rays, and to the effects produced on them being exposed directly to the sun, collectsjby wind. all the rays thereon a very COHESION, one of the species of at- small space, called the focus, where wood, traction, denoting that force by which the or any other combustible substance, being'parts of bodies stick together. put, will be set on fire. CAMERA-OBSC JRA,in opticks, a ma- made by placing a convex glass in a hole COLOUR means that property of bo- dies which affects the sight only; thus the chine representing an artificial eye. It is grass in the fields has a green colour, blood has a red colour, the sky generally of a window shutter, arid if no light en-jappears of a blue colour, and thus of oth- ters the room but through the gjass, the pictures of all objects on the outside may be distinctly seen in an inverted position, ers that might he named. The variety of colours, as t!i(-y are presented to in> by the substances that surround us, is immense, on any white surface placed at the focus and from them arises the admirable beau- of the lene. CAPILLARY TUBES, in physicks, lit- tle pipes, whose canals are extremely nar- row, used for experiments in illustrating cohesive attraction. ty of the works of nature in the animal, in the vegetable, and in the mineral king- dom, or, more properly speaking, in the universe. COLURES, in astronomy and geogra- PHILOSOPHICAL TERMS. 267 phy, two great circles, supposed to inter-! surface of such bodies, which is called sect each other at right angles in the pules concave. Thus we say a convex lens, a of the world, and to pass through the sol- convex mirror, and convex superficies. CORNEA, the second coat of the eye, so stitiul and equinoctial points of the eclip- tick. COMETS are opaque and solid bodies. A comet, at a given distance from the earth, shines much brighter when it is on compact humour, in form of a flattish con- the same side of the earth with the sun than when it is on the contrary side; from whence it appears that it owes its brightness to the sun. COMPLEMENT, in astronomy, the dis- tance of a star from the zenith ; or the arch comprehended between the place of the star above the horizon and the zenith. COMPRESSION, the act of pressing or squeezing some matter, so as to set its parts nearer to each other, and make it the rising to the setting of the sun. The possess less space. CONCAVE, an appellation used speaking of the inner surface ot'hollow bo dies, but more especially of spherical ones. tronomical day embraces the whole in- in terval which passes during a complete re- volution of the sun. DECLINATION, in astronomy, the consonance. CONCORD, in musick, the relation of distance of any celestial object from the two sounds that are always agreeable to equinoctial, either northward or south- the ear, whether applied in succession or ward. It is either true or apparent, ac- cording as the real or apparent place of the object is considered. DEGREE, in geometry, a division of a circle, including a three hundred and six- so that sometimes ten times as much air as tieth part of its circumference. Every CONDENSER, a pneumatic engine or syringe, whereby an uncommon quantity of air may be crowded'intoagiven space ; there is at the same time in the same space, without the engine, may be thrown in by means of it, and its egress prevented by and each degree divided into sixty other valves properly disposed. CON I) open UCT ORS, in electricity, are long metal rods, whose points are raised above conds. the buildings to which the conductors are affixed, for the purpose of attracting or re- ceiving the electrick fluid, and of conduct- ing it into the earrh, or into water, thereby site to rarity to prevent such buildings from being struck by lightning. CONE, in geometry, a solid figure, hav- ing a circle for its base, and its top termi- nated in a point or vertex. degree of the zodiack. system of several stars that are seen in the heavens near to one another. Astronomers diameter of the figure. not only mark out the stars, but they dis- tribute them into asterisms, or constella- one constellation : and for the better dis- tinguishing and observing them, they re- duce the constellations to the forms of ob- jects with whinh we are well acquainted. CONVERGING, or convergent lines, in geometry ,are such as continually approach nearer one another ; or whose distance be- comes still less and less. CONVERGING RAYS, in opticks, are those rays, that, issuing from diverse points of an object, incline towards one another, till, at last, they meet and cro'ss, and then become diverging rays. CONVEX, an appellation giving to the exteriour surface of gibbous or globular bodies, in opposition to the hollow inner ing to the force impressed upon it. called from its substance, which resembles the horn of a lantern. CRYSTALLINE HUMOUR, a thick vex lens, situated in the middle of the eye, serving to make that refraction of the rays of light, necessary to make them meet in the retina, and fupn an image thereon, whereby vision may be performed. CYLINDER, in geometry, u solid body, supposed to be generated by the rotation of a parallelogram. DAY. In common language, the day is the interval of time which elapses from circle is supposed to be divided into three hundred and sixty parts, called degrees, parts called minutes ; and each of these ninutes is again divide;! into sixty se- DENSITY denotes the degree of close- ness and compactnr-s of the particles of a body ; and is that property directly oppo- DEPRESSIONOFTHEPOLE. When a person sails or travels towards the equa- tor, he is said to depress the pole, because as many degrees as he approaches nearer the equator, so many degrees will the pole CONJUNCTION, in astronomy, is the he nearer the horizon. The phenomenon meeting of two stars or planets in the same arises from the spherical figure of the earth. DIAGONAL, in geometry, a right line CONSTELLATION, in astronomy, a drawn across a quadrilateral figure, from one angle to another, by some called the DIAMETER, in geometry, a right line passingthrough the centre of a circle, and tions, allowing several stars to make up terminated at each side by the circumfe- rence thereof. DIGIT, in astronomy, the twelfth part >f the diameterof the sun or moon, is used to express the quantity of an eclipse. Thus an eclipse is said to be six digits, when six of these parts are hid. DIMENSION, in geometry, is either breadth, length, orthickness; hencea line has one dimension, viz. length ; a superfi- cies, two, viz. length and breadth ; and a body, or solid, has three, to wit, length, breadth, and thickness. DIRECTION, in mechanicks, signifies the line or path of a body's motion, along which it endeavours to proceed, accord- 268 A DICTIONARY OF DISK, in astronomy, the body and face] the distance between the centre of the f the sun and moon, such as it appears loe)lipses and the focus. of the sun and us on the earth, or the body or face of thei ECHO, a sound reverberated or reflect- earth, such as it appears to a spectator in ed to the ear from some solid body, the moon. The disk in eclipses is suppos- ed to be divided into twelve equal parts. DISCORD, in musick, a dissonant and ECLIPSE, the deprivation of the light of the sun, or of some heavenly body, by the interposition of another heavenly body unharmonious combination of sounds, so bet ween it and our sight. called in opposition to concord. DIVERGENT RAYS, in opticks, are cle of the sphere, supposed to be d'uwn through the middle of the zodiack ; or it is that path among the fixed stars, that the earth appears to describe, to an eye , , those, which, going from a point of tl visible object, are dispersed, and continu- ally depart one from another,in proportion as they are removed from the object ; in which sense it is opposed to convergent. DIVISIBILITY, that property by which the particles of matter in all bodies are capable of a separation, or disunion from each other. DIURNAL, in astronomy, something fluid, that appears to pervade all nature, relating to the day, in opposition to noc turnal, which regards the night. The diurnal motion of a planet, is so many de- grees arid minutes as any planet moves in twenty-four hours. Hence the motion of the earth about its axis is called its diurna motion. DROPS, in meteorology, small spherical bodies, into which the particles of fluids spontaneously form themselves, when let with negative, come in contact with each fall from any height. DUCT denotes any tube or canal. DUCTILITY, in physicks, a property of certain bodies, whereby they are capa- ble of being expanded, or stretched forth by means of a hammer or press. ' D YN A M ICKS. This branch of mecha- nicks relates to the action of forces that give motion to solid bodies ; which forces are calculated, both by their active powers, and by the proportion of time in which those powers become efficient. EARTH, the vast mass or planet which earth flat or cylindrical ; but from the ge- neral appearance of tiie planetary system, from tlie circular shadow of the earth in eclipses of the moon, and from the fact that the earth has been circumnavigated, it is concluded by the moderns, that it is spherical. EARTHQUAKE is a sudden motion of the earth, occasioned, it is supposed, either by the discharge of some electrical power, or by large quantities of inflamma- ble air, which, on being rarefied by inter- nal fires, forces its way through the parts that surround it. EAST, one of the four cardinal points of the world ; being that point of the ho- rizon, where the sun is seen to rise when in the equinoctial. ECCENTRICK, in geometry, a term applied to circles and spheies which have not the same centre, and consequently are not parallel, in opposition to concentrick, where they are parallel, having one com- mon centre. ECCENTRICITY, in astronomy, is the distance of the centre of the orbit of a planet from the centre of the sun, that is, steams raised from water and other fluids ECLIPTICK, in astronomy, a great cir- placed in the sun. ELASTICITY, that disposition in bo- dies by which they endeavour to restore themselves to the posture from whence they were displaced by an external force ELECTRICITY is an invisible, subtile and among other interesting phenomena, is the cause of thunder and lightning. Electricity is of two kinds positive and negative. The positive is that state of a body which contains more than its due proportion ; and the negative is that state of a body which contains less than if? due proportion. When two bodies, one charg- ed with positive electricity and the other other, so much passes from the former to the latter, as to produce an equilibrium 't passes thus with a flash and an explo- ion. Thus two clouds, charged in the above manner, coming together, or one cloud coming in contact with the earth, thunder and lightning are produced. ELLIPSIS, in geometry, a curve line re- turning into itself, and produced from the section of a cone by a plane cutting both 'ts sides, but not parallel to the base. EMERSION, in astronomy, is when any planet that is eclipsed begins to emerge or we inhabit. The ancients supposed the get outoftheshadow of the eclipsing bo-dy. tt is also used when a star, before hidden :>y the sun, as being too near him, begins to re-appear or emerge out of his rays. EQUATOR is an imaginary circle equally distant from the poles, and divid- ng the earth into two equal parts, one being called the Northern hemisphere, ind the other the Southern hemisphere. EQUINOCTIAL, in astronomy, a great circle of the celestial globe, whose poles ate the poles of the world. It is so called, Because, whenever the sun comes to this circle, the days and nights are equal all over the globe ; being the same with that which the sun seems to de.( ribe at the ime of the two equinoxes of spring and autumn. EQUINOX, the time when the sun en- ers either of the equinoctial points, where he ecliptick intersects the equinoctial. EXHALATION, a general term for all he ef /ia or steams raised from the'sur- 'ace of the earth in form of vapor. Some distinguish exhalations from vapours, ex- Dressing by the former all steams emitted from solid bodies, and by the latter, the PHILOSOPHICAL TERMS. 269 EXPANSION, the enlargement or in crease of bulk in bodies, chiefly by means more usually called a sphere, bounded by of heal. one uniform convex surface, every point EXPLOSION, a sudden and violent of which is equally distant from a point expansion of an aerial or other elastick fluid, by which it instantly throws off any obstacle that happens to be in the way sometimes with incredible force, and in such a manner as to produce the most prevented by some other force or obstacle astonishing effects. EXTENSION, in philosophy ,one of the cited by the rays of light. common and essential properties of bod.\ or that by which it possesses or takes up consisting of such vapours as are united some part of universal space, which is called the lled the place of a body. FIGURE, in physicks, expresses the surface, or terminating extremities of any body ; and, considered as a property of body affecting our senses, is defined a quality which may be perceived by two of the outward senses. Thus a table is known to be square by the sight and by the touch. FLUID, in physiology, an appellation given to all bodies whose particles easily yield to the least partial pressure or force impressed. FOCUS, in geometry and conick sec- tions, is applied to certain points in the parabola, ellipsis, and hyperbola, where the force which may be applied in order the rays reflected from all parts of these to separate its parts. curves concur and meet. FOGS are clouds which float on the sur- face of the earth, and clouds are fogs in the higher regions of the atmosphere ; from many places there may be seen float- ing in the vallies, and often in the vallies they may be seen creeping along the sides of the mountains. FORCE, in mechanicks, denotes the cause of the change in the state of a body, when, being at rest, it begins to move, or has a motion which is either not uniform reduced to two sorts, one of a body at rest, the other of a body in motion. FORCING-PUMP, in mechanicks, a kind of pump in which there is a forcer or piston without a valve. FOUNTAIN, in philosophy, a spring or source of water rising out of the earth. FRICTION, in mechanicks, the rub- bing of the parts of engines and ma- chines against each other, by which means a great part of their effect is de- stroyed. earth's surface between the polar circles and the poles. or support, by which a lever is sustained. GALAXY, in astronomy, a very re- markable appearance, sometimes double, but for the most part single, surrounding the whole concave of the heavens, called thf> galaxy or milky way. GIBBOUS, in astronomy, a term used in reference to the enlightened parts of the moon, whilst she is moving from her first quarter to the full, and from the full to the last quarter. 23* GLOBE, a round or spherical body, within called the centre. GRAVITY, a term used by physical writers to denote the cause by" which all bodies move towards each other, unless GREEN, one of the original colours ex- HAIL, a compact mass of frozen water, into drops, and are frozen while they are falling. They assume various figures, be- ing sometimes round, at other times pyra- midal, cuniated, angular, thin and flat, and sometimes stellated with six radii like the small crystals of snow. HALO, in physiology, a meteor in the form of a luminous ring or circle, of vari- ous colours, appearing round the bodies of the sun, moon, or stars. HARDNESS, in physiology, is the re- sistance opposed by a body to the separa- tion of its particles. This property de- pends on the force of cohesion ; and a body is considered more hard in propor- tion as it presents a greater resistance to HARMONY, in musick, the agreea- ble result, or union, of several musical sounds, heard at one and the same time, or the mixture of divers sounds, which to- gether have an effect agreeable to the ear. As a continued succession of musical sounds produces melody, so does a contin- ued combination of these produce harmo- y HARMONY OF THE SPHERES, a sort of musick much talked of by many of the ancient philosophers, supposed to be or not direct. Mechanical forces may be produced by the sweetly tuned motionsof the stars and planets. This harmony they attributed to the various proportionate im- pressions of the heavenly slobes upon one another, acting at proper intervals. HEIGHT, in geometry, is a perpendic- ular let fall from the vertex, or top, of any right-lined figure, upon the base or side subtending it. It is likewise the per- pendicular height of any object above the horizon. HEMISPHERE, the half of a globe or sphere, when it is supposed to be cut FRIGID ZONES, the spaces on the through its centre in the plane of one of its great circles. HORIZON, in astronomy and geogra- FULCRUM, in mechanicks, the press phy, that great circle which divides the heavens and the earth into two equal parts or hemispheres, distinguishing the upper from the lower. The horizon is either sensible or rational the sensible horizon s that circle, which being discovered by )ur senses, limits our prospect. HORIZONTAL, something relating to the horizon ; or that is taken in, or on a level with the horizon. Thus, we say, a horizontal plane. HURRICANES are violent storms, fre- 270 A DICTIONARY OF quent in South America and the West In-'given to parts of bodies which are of a dies, and other hot countries, in which the similar nature with the whole. Thus, wind changes in a short time to every I filings of iron have the same nature and point of the compass, and blows with a vio- properties as bars of iron. lence which scarcely any thing can resist. INTENSITY, in pin sicks, is the de- HYADfc;S,in astmnomy, seven stars in gree or rate of power or energy of any the bull's head, famous among the poets quality, as of heat and cold, for the bringing of rain. H YURA, in astronomy, a southern con stellation, imagined to represent a water brightness, serpent. LATITUDE, the distance of a place HYDRAULICKS teach .us to ascertain from the equator, or an arc of the meridi- the velocity and impetus of fluids when an intercepted between the zenith of the in motion, and serves as the basis for com- puting the powers of various machinery acted upon by running water. HYDROMETER, measure the extent and specific gravity of fluids. HYDROSTATIC AL BALANCE, a kind of balance contrived for the easy and exact finding of the specifick gravities of bodies both liquid and solid. HYDROSTATIC A L PARADOX is this that any quantity of fluid, however small, may be made to balance, or coun- terpoise any quantity, however large. H YDROSTATICKS treat of the nature, gravity, pressure, and motion of fluids in general, and of the methods of weighing solids in them. IMAGE, in opticks, is the appearance of an object made either by reflection or refraction. In all plane mirrors, the im- age is of the same magnitude as the object, and it appears as far behind the mirror as the object is before it. In concave mir- rors the object appears larger, and in those which are convex, it appears less than the object. IMMERSION, in astronomy, is when a star or planet is so near the sun, with re- gard to our observations, that we cannot see it ; being as it were enveloped or hid- den in the raj's of that luminary. It also denotes the beginning of an eclipse of the sun or moon, when either of those bodies begins to be darkened by the shadow of the other. IMPENETRABILITY, in philosophy, that property of a body whereby it cannot be pierced by another ; thus, a body, which so fills a space as to exclude all others, is said to be impenetrable. INCIDENCE, in mechanicks, denotes the direction in which one body strikes on another. INCLINATION, is a word frequently used by mathematicians, and signifies the mutual approach, tendency, or leaning of two lines, or planes, towards each other, so as to make an angl^. INCLINED-PLANE, in mechanicks, is merely a line or plane that makes an an- instrument to on this or that side of the equator. JUPITER, in astionomy, one of the primary planets remarkable for its great place and the equator. Hence latitude is either northern or southern, according as the place, whose latitude is spoken of, is LATITUDE, in astronomy, the dis- tance of a star or planet from the ecliptick, 'n degrees, minutes, and seconds, meas- ured on a circle of latitude drawn thiough that star or planet, being neither north or south, as the object is situated eiiher on he north or south side of the ecliptick. LEE, an epithet to distinguish that lalf of the horizon to which the wind is lirected from the other part where it arises, which latter is accordingly called to windward. LENS properly signifies a small round- sh glass, of the figure of a lentil, but is extended to any optick glass, not very thick, which either collects the rays of ight into a point, in their passage through t, or disperses them further apart, accord- ng to the laws of refraction. LEO, in astronomy, one of the twelve signs of the zodiack, the fifth in order. LEVEL, an instrument constructed for he purpose of ascertaining the exact lev- the pur el of an quei to move weishts from one level to another. INERTIA, or inactivity, is that pro- perty of matter by which it would always continue in the same state of rest, or of motion, in which it was put, unless chanced by some external force. INTEGRAL, or integrant, appellations ly fluid, building, or any other ob- ject. Levels are of two kinds the hori- zontal and the poipendicular. LEVER, in mechanicks, an inflexible right line, rod, or beam, supported in a single point on a fulcrum or prop, and ised for the raising of weights ; being either void of weight itself, or at least having such a weight as may be coinino- diously counterbalanced. L[BRA,the balance, in astronomy, one of the twelve signs of the zodiack, the xth in order ; so called, because when the sun enters it, the days and nights are equal, as if weighed in a balance. LIBRATION, in astronomy, an appa- rent inequality of the moon's motion, whereby she seems to librate about her axis, sometimes from the east to the west, and now and then from the west to the east ; so that the parts in the western limb or margin of the moon sometimes recede from the centre of the disk, and some- times move towards it, by which means gle with the horizon. It is frequently used they become alternately visible nml in- visible to the inhabitants of the earth. LIGHT is that principle, or thing, by which objects are made perceptible to our sense of seeing; or the sensation occa- sioned in the mind by the view of lumin ous objects. LIGHTNING, an electrical explosion PHILOSOPHICAL TERMS. 271 LINE, in geometry, a quantity extend- in length only, without any breadth or feet of different sounds, ranged and dis thickness. LIUUID, a fluid not sensibly elastick, the parts of which move on each other, and yield to the smallest impression. LONGITUDE, in geography, is an arch star that emits a very bright white light of the equator, intercepted between the though, by reason of his always keeping d firsi meridian passing through the propos ed place ; which is always equal to the when he does make his appearance, his angle at the pole, formed by the first me ridian and the meridian of the place. LOOKING-GLASSES are nothing but plain mirrors of glass, which, being imper- vious to the light, reflect the images of things placed before them. moon ; thus we say, lunar month, lunar year, lunar dial, or lunar eclipse. LUNATION, the time or period from synodical month. MELODY, in musick, the agreeable ef- posed in succession ; so that melody is the effect of a single voice or instrument, by which it is distinguished from harmony. MERCURY, in astronomy, is a small near the sun, he is seldom to be seen ; and notion towards the sun is so swift, that he can only be discerned for a short time. MERIDIAN, in astronomy, a great cir- cle passing through the poles of the world, and both zenith and nadir, crosses the equinoctial at right angles, and divides LUNAR, something belonging to the the sphere into two hemispheres, the east- ern and the western ; it has its poles in the east and west points of the horizon. It is called meridian, because, when the , e me or pero rom s cae meran, ecause, wen e one new moon to another it is called the sun comes to the south part of this circle, it is then mid-day ; and then the sun has MAGICK LANTERN is an instrumentjhis greatest altitude for that day. used for magnifying paintings on glass.] METEOR, in physiology, a moveable and throwing their images upon a whitejigneous body, congregated in the air by means not thoroughly understood, and screen in a darkened room. MAGNETISM explains the properties of the loadstone, or natural magnet, which is a dark coloured and iiard mineral body, and is found to be an ore of iron, being generally found in iron mines. MAGNITUDE, whatever is made up of parts locally extended, or that has several dimensions; as aline, a surface, or a solid. MANOMETER, an instrument to show or measure the alterations in the rarity or density of the air. MARS, in astronomy, the planet that revolves next beyond the earth in our sys- tem, is of a red fiery color, and always gives a much duller light than Venus, though sometimes he equals her in size. MATHEMATICKS originally signified any discipline or learning; but at present, denotes that science which teaches, or contemplates whatever is capable of being numbered or measured, in so far as it is computable or measurable; and according- ly is subdivided into arithmetick, which has numbers for its object, and geometry, which treats of magnitudes. MATTER is the general name of every substance, that has length, breadth, and thickness. MECHANICKS, is the science which treats of the laws of the equilibrium and motion of solid bodies ; of the forces by which bodies, whether animate or inani- mate, may be made to act upon one an other; and of the means bv which these as are most powerful. region through which a body in motion passes to any point ; thus ether is suppos- ed to be the medium through which the heavenly bodies move ; air, the medium wherein bodies move near the earth ; wa- ter, the medium wherein fishes live and move ; and glass is also a medium of light, as it affords it a free passage. varying greatly in size and rapidity of motion. METEOROLOGY is the science of studying the phenomena of the atmo- phere, and that term by which is express- id all the observations that tend to make hern a system. MICROSCOPE, in opticks. By micro- copes are understood instruments, of whatever structure or contrivances, that ran make small objects.appear larger than they do to the naked eye. MINUTE, in geometry.the sixtieth part >f a degree of a circle. Minutes are denot- ed by one acute accent, I hus (') ; as the se- "ond, or sixtieth part of a minute, is by wo such accents, thus ('') ; and the third iy three ('"). "MIRRORS, in catopticks, any polished iody impervious to the rays of light, and which reflects them equally. Mirrors were inciently made of metal ; but at present they are generally smooth plates of glass, tinned or quick-silvered ou the back part, and called looking-glasses. The doctrine if mirrors depends wholly on that funda- nental la-.v, that the angle of reflection is always equal to the angle of incidence. MOBILITY is that property of matter which it is capable of bt-ing moved from me part of space to another. MOMENTUM, in mechanicks, signifies the same with impetus, or quantity of mo- tion in a moving body ; whicli is always may be increased, so as to overcome such equal to the quantity of matter multiplied Ho the velocity ; or, which is the same MEDIUM, in philosophy, that space or thing, it may be considered as a rectangle nrier the quantity of matter and velocity. MONSOON, in'physiolojry, a species of wind, in the East Indies, which for six months blows constantly the same way, ;uid the contrary way the othersix months. MOON, in astronomy, a satellite, or se- condary planet, always attendant on our earth. 272 A DICTIONARY OP MOTION is defined to be the continued and successive change of place. Nothing can be produced or destroyed without mo- tion, and every thing that happens de- pends on it. MUS1CK. Any succession of sounds, however much they may vary in regard to duration, or however much they may par- take of various modes or keys, provided that succession be agreeable, and excites, in a well tuned ear, certain agreeable sensations, is called musick. NADIR, in astronomy, that point of the heavens which is diametrically opposite to the zenith,or point directly overour heads. The zenith and nadir are the two poles of the horizon. NATURAL PHILOSOPHY, otherwise called physicks, is that science which considers the powers of nature, the pro- perties of natural bodies, and their actions upon one another. NEBULAE, in astronomy, luminous spots in the heavens, some of which con- sist of clusters of telescopick stars, others appear as luminous spots of different forms. Some of them form a round com- pact system, others are more irregular, of various forms, and some are long and narrow. NIGHT, that part of the natural day during which the sun is underneath the horizon ; or that space wherein it is dusky. NODES, in astronomy, the two points wherein the orbit of a planet intersects the ecliptick, whereof the node, where the node ascends northwards, above the plane of the ecliptick, is called the ascending node, and the other ; where the planet descends to the south, is called the des- cending node. OBLATE, flattened, or shortened, as an oblate spheroid, having its axis shorter than its middle diameter, being formed by the rotation of an ellipse about the shorter axis. The oblateness of the earth refer;- to the diminution of the polar axis in re- spect of the equatorial. OBTUSE, signifies blunt or dull, in op- position to sharp or acute. Thus we say an angle is obtuse if it measures more" than nmety degrees. OCClDENT'in geography, the western quarter of the horizon, or that part of the horizon where the ecliptick, or the sun therein, descends into the lower hemi- sphere, in contradistinction to orient. OCCULTATION, in astronomy, the time a star or planet is hidden from our sight, by the interposition of the moon or of some other planet. OPACITY, in philosophy, a quality of bodies which renders them impervious to the rays of light. OPTICKS, the science of vision, in- cluding Catoptricks and Dioptrkks, and even Perspective ; as also the whole doc trine of light and colours, and all the phenomena of visible objects. ORBIT, in astronomy, the path of a planet or comet, or the curve that it des ribes in its revolution round its central body. Thus the earth's orbit is the curve ch it describes in its annual course, and usually called the ecliptick. ORION, in astronomy, a constellation )f the southern hemisphere, consisting of hirty-seven stars, according to Ptolemy : jf sixty-two, according to Sycho ; and of 10 less than eighty, in the Bmannick :atalogue. ORRERY, a curious machine for repre- senting the motions and appearances of he heavenly bodies. OSCILLATION, in mechanicks, the vibration or reciprocal ascent and descent )f a pendulum. PARABOLA, in geometry, a figure aris- ng from the section of a cone, when cut >y a plane parallel to one of its sides. PARADOX, in philosophy, a proposi- tion seemingly obscure, as being contrary o some received opinion, but yet true in *act. PARALLAX, in astronomy, denotes a change of the apparent plate of any hea- enly body, caused by being seen from 3iflerent points of view ; or it is the dif- ference between the true and apparent distance of any heavenly body from the zenith. PARALLEL straight lines, whose least distances from each other a;e every where equal, are said to be parallel. PARALLELOGRAM, in geometry, a quadrilateral right lined fiiyi re, whose op- posite sides are parallel and equal to each ther. PARHELfUM, or PARHELION, in physiology, a mock sun,ormeteor,in form of a very "bright light, appearing on one tide of the sun. PEGASUS, in astronomy, a constella- ion of the northern hemisphere, in form >f a rlyinc horse. PENDULUM, in mechanicks, denotes my heavy body so suspended as that it nay vibrate or swing Backwards and for- wards, about some fixed point, by the force of gravity. The vibrations of the pendu- um are called its oscillations. PENUMBRA, in astronomy, a partial shade observed between -the perfect shadow nnd the full light, in an eclipse. PERCUSSION, in mechanicks, the im- pression a body makes in filling or strik- g upon another, or the shock of two bodies in motion. PERIHELIUM, in astronomy, that point of a planet's or comet's (.rbif where- n it is in its least distance from the sun ; n which sense it stands in opposition to aphelium. PERIMETER, in geometry, the bound? or limits of any figure or body. The peri neter of surfaces or figures are lines,thos< of bodies are surfaces. In circular figures instead of perimeter, we say circumfe renre, or periphery. PERIOD, in astronomy, the time taken ip by a star or planet in making a revolu- tion round the sun j or the duration of its PHILOSOPHICAL TERMS. 273 course till it return to the same point of its orbit. PERIPHERY, in geometry, the circum ference of a circle, ellipsis, or any other regular curvilinear figure. PERPENDICULAR, in geometry, a line, lulling directly on another line, s< as to make equal angles on each side; called also a normal line. PERSPECTIVE, the art of represent ing, upon a plane surface, the appearanc* of objects, however diversified, similar to that they assume upon a glass-pane, in- terposed between them and the eye at a given distance. PHASES, in astronomy, the several ap pearances or quantities of illumination of the Moon, Venus, Mercury, and the other planets j or the several manners wherein they appear illuminated by the sun. PHOENIX, in astronomy, one of the constellations of the southern hemisphere, unknown to the ancients, and invisible in our northern parts. It is said to consist of thirteen stars. PH YSICKS, a term made use of for na- tural philosophy, explains the doctrines o' natural bodies, their phenomena, causes, and effects, with the various effections, motions, arid operations. PISTON, in pump-work, is a short cy- linder of metal, or other solid substance, fitted exactly to the cavity of the barrel or body of the pump. There are two kinds of pistons used in pumps, the one with a valve, and the other without a valve, call- ed a forcer. PLANE, in geometry, denotes a plain surface, or one that lies evenly between its bounding lines and as a right line is the shortest extension from one point to ano ther, so a plain surface is the shortest ex tension from one line to another. PLANET, a celestial body revolving round the sun, as a centre, and continual!} changing its position, with respect to the fixed stars; whence the name planet which is a Greek word signifying wander PLEIADES, in astronomy, an assem blage of seven stars in the neck of the con stellation Taurus, the bull ; although there are now only six of them visible to the na ked eye. The largest is of the third mag nitude, called " Lucido pleiadum." PNEUMATICKS is that branch of natural philosophy which treats of the weight, pressure, and elasticity of the air with the effects arising from them. POINT, in geometry, as defined by'Eu clid, is a quantity, which has no parts, 01 which is indivisible. Points are the ends or extremities of lines. If a point be sup posed to be moved any way, it will, by its motion, describe a line. Point, in phys icks, is the least sensible object of sight marked with a pen, point of a compass, 01 the like. Of such points all physical mag nitude consists. POLAR, in general, something relat to the poles of the world, or poles of arli ficial globes. POLARITY, the quality of a thing con- sidered as having poles ; but chiefly used 11 speaking of the magnet. POLE, in astronomy, one of the extre- nitiesof the axis, on which the sphere re- solves. These two points, each ninety de- srees from the equinoctial or equator, are >y way of eminence called the poles of the vorld ; and the extremities of the axis of artificial globes, corresponding to these r>oims in the heavens, are termed the poles hereof. POLLUX, in astronomy, a fixed star of he second magnitude in the constellation gemini, or the twins. The same name is also given to the hindermost twin, or pos- ;erior part of the same constellation. POWER, in mechanicks, denotes any brce, whether of a man, a horse, a spring, the wind, or water, which being applied to a machine, tends to produce motion. PRECESSION OB' THE EQUINOX- ES is a very slow motion of them, by which they change their place, going from east to west or contrary to the order of the signs. PROJECTION, in mechanicks, the art of communicating motion to a body, from thence called projectile. PULLEY, in mechanicks, one of the mechanical powers, called by seamen a tackle. PUMP, in hydraulicks,a machine formed on the model of a syringe, for raisingwater. PYROMETER, an instrument for mea- suring the expansion of bodies by heat. QUADRANT denotes a metliematical nstrument,of great service in astronomy, and consequently, in navigation, for tak- ing the altitudes of the sun and stars, as also for taking.ansles in surveying. QUADRATURE, in geometry, denotes the squaring or reducing a figure to a square. QUADRILATERAL, in geometry, a figure whose perimeter consists of four right lines making four angles : whence it is also railed a quadrilateral figure. The quadrilateral figures are either a parallelo- gram, trapezium, rectangle, square, rhom- bus, or rhomboides. RADIATION, the act of a body emit- ting or diffusing rays of light all around, as from a centre. RADIUS, in geometry, the semi diame- ter of a circle, or a right line drawn from the centre to the circumference. RAIN. Whatever suddenly disturbs the heat or density of the a ii, or the eclectrici- r.y of the clouds, occasions the particles of vapour to rush together, and form drops of water too heavy to continue suspended in the atmosphere. They then fall in the shape of rain, and increase in size as they fall by combining with the floating va- pours as they pass through them. RAINBOW is a meteor in form of a party-coloured arch, or semicircle, exhibit g ed only at the time when it rains. It is always seen in that point of the heavens which is opposite to the sun, and is occa- 274 A DICTIONARY OF sioned by the refraction and reflection of his rays in the drops of falling rain. RAREFACTION, in physicks, is the making a body to expand, or occupy more room <>r space, without tne accession of new matter. RAY, in opticks, a beam of light, emit- ted from a radiant or luminous body. REACTION, in physiology, tire resist ance made by all bodies to the action or the light is either altogether obstructed, impulse of others, that endeavor to change its state, whether of motion or rest. 9 Shaw* witGtuoi ui mullein ui icai. 01 BUIJJG U RECEIVER, in pueumaticks, a glass luminary. vessel for containing the thing on which an experiment in the air pump is to be made. RECTANGLE, in geometry, the same with a right angled parallelogram. moving body from its direct course, occa- sioned by the different densityof the medi- um in which it moves ; or, it is a change of direction, occasioned by a body's falling obliquely out of one medium into another of a different density. REPULSION, in physicks, that proper- ty in bodies, whereby, if they are placed just beyond the sphere of each other's at- trac'ion of cohesion, they mutually fly r< a . .,,,., from each other. R ES1STANCE, in philosophy, denotes, tant is more particularly used for an astrtf- lomical instrument made like a quadrant, xcepting that its limb only comprehends sixty degrees. The use and application of the sextant is the same with that of the juadrant. SHADOW, in opticks, a privation or di- minution of light by the interposition of in opaque body ; or it is a plane, where or greatly weakened, by the interposition of some opaque body !>etween it and the SIDEREAL DAY, is the time in which any star appears to revolve from the meri- dian to the meridian again. SIGNS, in astronomy. The eciiptick is usually divided, by astronomers, into 12 REFRACTION, is the deviation of a parts called signs, each of which of course contains 30 degrees. They are usually called signs of the zodiack; and beginning at the equinox, where the Sun intersects and rises above the equator, have these names and marks : Aries, Taurus, Leo, ^ Sagittarius, Virgo, H^ Capricornus, Gemini, II Libra, ^ Aquarius, Scorpio, Tt| Pisces, Of these signs,the first six are called north- in general, any power which acts in an ern, lying on the north side of the equa- opposite direction to another, so as to de- tor ; and the last six are called southern, stroy or diminish its effects. RETINA, the expansion of the optick, nerve on the internal surface of the eye, whereupon the images of objects being into a vessel of liquor, and the other hang- painted, are impressed, and by that means conveyed to the common sensory in the liquor will run out from the first into the brain, where the mind views and contem- plates their ideas. ROTATION, in geometry, a term chief ly applied to the circumvolution of any surface round a fixed and immoveable phon. line, which is called the axis of its rotation, an-1 by such rotations it is that solids are conceived to be generated. SAGITTARIUS, the archer, in astron- omy, the ninth sign of the zodiaok. SATELLITES, in astronomy, are cer- tain secondary planets, moving round they always attend them, and make the tour about the sun with them. dATURN is a very conspicuous planet, til-nigh not so brilliant as Jupiter. SEGMENT OF A CIRCLE,iri geometry, that part of the circle contained between a nhord and an arch of the same circle. SEMICIRCLE, in geometry, half a cir- cle, or that figure comprehended between the diameter of a circle and half the cir- cumference. SEMIDIAMETER, half the diameter, or a 'ight line drawn from the centre of a circle, 01 sphere, to its circumference ; be ing the same with what is otherwise call- ed the radius. the sixth part of a circle, or an arch com- prehending sixty degrees. The word sex- being situated to the south of the equator. SIPHON, or Syphon, in hydraulicks, a bended pipe, one end of which being put ing out of the said vessel over another, the last, after the air has been sucked out of the external or lower end of the siphon, and that as long as the liquor in the upper vessel is above the upper orifice of the si- SK Y, the blue expanse of air and atmo- sphere. The azure colour of the sky is at- tributed, by Sir Isaac Newton, to vapours beginning to condense there, and which have got consistence enough to reflect the most flexible rays. SNOW, a well known substance, form- the other planets, as the Moon does round ed by the freezing of the vapours in the the Earth. They are so called, because atmosphere. It differs from hail and hoarfrost, in being as it were crystallized, which they are not. SOLID, in philosophy, a body whose parts are so firmly connected together, as not to give way or slip from each other upon the smallest impression ; in which .sense solid stands opposed to fluids. SOLAR, something belonging to the sun ; thus the solar system is^that system of thft world wherein the heavenly bodies are made to revolve round the sun as the centre of their motion. SOLSTICE, in astronomy, that time when the sun is in one of the solstitial points ; that is, when he is at his greatest distance from the equator, thus called, be- SEXTANT, in mathematicks, denotes cause he then appears to stand still, and not to change his distance from the equa- tor for some time ; an appearance owing PHILOSOPHICAL TERMS. 275 to the obliquity of our sphere, and to which those living under the equator are strangers. SOUND. The sense of hearing is affect- ed by the pulsations or vibrations of the air, which are caused by its own expan- sion, or by the vibrations of sounding bo- dies. These sensations, or vibrations in the air, are called sounds, as are also the sensations which they produce. SPECIFICK, in philosophy, that which is peculiar to any thing, and distinguishes it from all others. SPECTRUM, in opticks. When a ray of light is admitted through a small hole, and received on a white surface, it forms a luminous spot. If a dense, transparent bo- dybe interposed,the light will be refracted, iii proportion to the density of the medium; but if a triangular glass prism be inter- posed, the light is not merely refracted but it is divided into seven different rays. This image is called the spectrum, and from its being produced by the prism, the prismatick spectrum. SPH ERE is a solid contained under one uniform round surface, such as would be formed by the revolution of a circle about the diameter thereof, as an axis. SPHEROID, in geometry, a solid, ap- proaching to the figure of a sphere. SPOTS, in astronomy, certain places of the Sun's or Moon's disk, observed to be either more bright or darker than the rest, and accordingly called facula and macula. SPRAY, the sprinkling or foam of the sea, which is driven from the top of a wave in stormy weather. SQUARE, in geometry, a quadrilateral figure, both equilateral and equiangular. STAR, in astronomy, a general name for all the heavenly bodies which are dis- persed throughout the whole heavens. SUCTION, the act of sucking or draw- ing up a fluid, as air, water, milk, or the like, by means of the mouth and lungs. SUN, in astronomy, the most conspicu- ous of the heavenly bodies, which occu- pies the centre of the system which com- prehends the earth, the primary and secondary planets, and the comets. SUPERFICIES, or surface, in geome- try, a magnitude considered as having two dimensions ; or extended in length and breadth, but without thickness or depth SWIMMING, the art or act of sustain- ing and moving the body in water. Brutes swim naturally, but men attain this art by ,o make a right angle with the diameter of the circle of which that arch is a part. TANTALUS CUP, in hydrauliek*, a siphon, so adapted to a cup, that the short eg being in the cup, the long leg may go down through the bottom of it. TAURUS, in astronomy, one of the twelve signs of the zodiack, the second m order, consisting of forty-four stars, accor- ding to Ptolemy, of forty-one, according to Tycho ; and of no less than one hun- dred and thirty-five, according to the iiri- tannick catalogue. TELESCOPE, an optical instrument, which is used for discovering and view- ng distant objects, either directly by glasses, or by reflection. THERMOMETER, an instrument for measuring the degree of heat or cold in any body. THUNDER, the noise occasioned by the explosion of a flash of lightning pas- sing through the air: or it is that noise which is excited by a sudden explosion of electrical clouds which are therefore call- ed thunderclouds. TORRLD ZONE, among geographers, denotes that space of the earth's surface ncluded between the tropicks. TRADE WINDS denote certain regular winds at sea, blowing either constantly he same way, or else alternately, a certain ength of time in one direction, an.l then as long in an opposite one. They are call- edtradewindsfromtheirusein navigation, and are very common in the Indian seas. TRANSIT, in astronomy, signifies the passage of any planet just by, or over, a fixed star, or sun, and of the moon in par- ticular, covering or moving over any plan- et. TRANSMISSION, in optickx, the act uf a transparent body passing the rays of light through its substance, or suffering them to pass; in which sense .he word stands opposed to reflection. TRANSPARENCY, in physicks, aqua- ity in certnia bodies, whereby they give princ lwith practice and industry. It consists pally in striking the water alternatelywith the hands and feet, which, like oars, row a person forward. SYRINGE, an instrument serving to im- bibe or suck in a quantity of any fluid, and to squirt or expel the same with violence. S YZYG Y, in astronomy, a term equally used for the conjunction and opposition of a planet with the sun. TANGENT, in geometry, is defined, passage to t , the rays of light, in co ontradis- inction to opacity, or that quality of ho- dies which renders them impervious to ,he rays of light. TRIANGLK, in geometry, a figure of three sides ana three angles. TROPICKS, in astronomy, and geogra- phy, are two circles supposed to be drawn round the earth on each side of the equa- tor, and 2.3 de-;. 29' distant from it. TWILIGHT, that light, whether in the morning before sunrise, or in the evening after sunset, which is occasioned by the reflection of the sun's rays in passing through the atmosphere. VACUUM, in philosophy, denotes a space emptyor devoid of all matter or body, VALVE, in hydraulicks and pueuma- ticks, is a kind of lid or cover, of a tube or vessel, so contrived as to open one way ; but which the more forcibly it is pressed general, to be a right line, which touches the other way, the closer it shuts the aner- any arch of a curve, in such a manner, as ture, so that it either admits the entrance 276 A DICTIONARY OF PHILOSOPHICAL TERMS. of a fluid into the tube or vessel, and pre-lthe ocean. They unite and often move vents its return, or admits iis escape, and|with rapidity, until they meet with some prevents its re-entrance. VAPOUR, in meteorology, a thin, hu mid matter, which, being rarefied to acer opposing wind, or other cause, which de- stroys them. WAVE, in physicks, a volume of water tain degree by the action of heat, ascends|elevated by the action of the wind, upon to a particular height in the atmosphere, where it is suspended, until it returns in the form of dew, rain, snow, or hail. VELOCITY, swiftness, or that affec- tion of motion, whereby a moving body is its surface, into a state of fluctuation, and accompanied by a cavity. WEDGE, one of the mechanical pow- ers, as they are called. The wedge is a triangular prism, whose bases are equila- disposed to run over a certain space in a teral jacute angled triangles, certain time. 5 heavens, known by the names of the morning and evening star, likewise keeps near the sun, though she recedes from himi wards ^he centre of the earth. almost double the distance of Mercury. WEEK, in chronology, a division of VENUS, the most beautiful star in the time comprising seven days. WEIGHT, in physicks, is a quality in natural bodies, by which they tend to- VESTA, one of the small planetary bo- dies discovered lately to revolve between the planets Mars and Jupiter. VIBRATION, in mecnanicks, a regu- lar reciprocal motion of the body, as, for example, a pendulum, which, being freely to side WHEEL, one of the six powers of me- chanism ; and, without doubt, contrib- utes more than any of the other five to the general convenience of mankind, by the wonderful variety of purposes, from a mill to a watch, wherein it is employed. WHIRLWINDS are formed by opposite suspended, swings or vibrates from side winds meeting and moving swiftly in a circle, raising sand and light bodies into VIRGO, in astronomy, one of the signs the air. In the deserts of Africa they constellations of the zodiack, and the sometimes draw up the sand into a mov- sixth according to order. VISIBLE, something that is an object of sight or vision, or something whereby the eye is affected, so as to produce a sen- sation. VISION is the act of seeing or of per- ceiving external objects by the organ of sight. UNDULATION, in physicks, a kind of (ing pillar, which buries all in its way. When they' appear on the ocean, theydraw up the water, and produce water-spuuts. WIND. When the air over any place is more heated than that around, it is rare- fied or expanded, and rises. The surroun- ding air rushes in to supply its place, and this produces a current called wind. YEAR, the time that the sun takes to tremulous motion or vibration observable go through the twelve signs of the zo- in a liquid, whereby it alternately rises diack. and falls like the waves of the sea. UNISON, in musick, the effect of two point ; or a point in the heavens directly sounds which are equal in degree of tune, or in point of gravity and acuteness. VOLCANOES, mountains which emit ignited matter and smoke through aper- tures, communicating with cavities in the depths of the earth. WATER, a transparent fluid, without colour, smell, or taste, in a very small de- gree compressible ; and, when pure, not liable to spontaneous change. WATER SPOUT,an extraordinary me- teor, in which a column of water is seen hanging from the clouds, and descending until it meets with a column rising from found in the different parts of it ZENITH, in astronomy, the vertical >ver our heads. The zenith is called the pole of the horizon, because it is ninety degrees distant from every point of that circle. ZODIACK, in astronomy, a broad cir- cle, whose middle is the ecliptick, and its sxtremes, two circles, parallel thereto, at such a distance from it, as to bound or comprehend the excursions of the sun and planets. ZONE, in geography and astronomy, a division of the terraqueous globe, with re- spect to the different degrees of heat n. . -/. Fiq.5. /'/..i i /; in. Ha. i'. FUJ, -f. ffr.J. Jlq. 6. \ PLATE IV. \ PI ATT. V. PLATE VI. PLATE VJI. Fig. I PLATE VJH. -a- * Kg. S> XIL XZZT PJ.A T-E XVI \ mr. PlATt. \'I PLATtiXK. .. F D PLATE XX f LATE XXI. TXH PLATEXXW PL. XXIV. PL.XXV. Fig. 1. Fig. 2. Fig. 5. ri- AX\'ii Fig. 5. Fig. 2. Fig. 1. v 14 DAY USE RETURN TO DESK ^ OM ^^ D EDUCATION - PSYCHOLOGY LIBRARY This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. 7 DAY USE DURING! SUMMER SESSIONS LD 2lA-15m-4,'63 (D6471slO)476 General Library e University of California Berkeley